Published on MATSE 81: Materials In Today's World (https://www.e-education.psu.edu/matse81)

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Lesson 1: Materials Classification

Overview

In today’s society, virtually every segment of our everyday life is influenced by the limitations, availability, and economic considerations of the materials used. In this lesson you will be introduced to the interconnectivity of processing, structure, properties, and performance of the design, production, and utilization of materials; the role of materials scientists and engineers; and the three important criteria in materials selection. You will also be introduced to the classical classification of materials: metals, ceramics, and polymers, as well as, composites and the advanced materials classification used in modern high-tech applications.

Learning Outcomes

By the end of this lesson, you should be able to:

  • Describe, with specific examples, the role of materials in human development during the Stone Age.
  • List the six different property classifications of materials that determine their applicability.
  • Cite the four components that are involved in the design, production, and utilization of materials, and briefly describe the interrelationships between these components.
  • Describe the way in which scientists and engineers differ in their utilization of materials.
  • Cite three criteria that are important in the materials selection process.
  • List the three primary classifications of solid materials, and then cite the distinctive features of each.
  • Briefly define smart material/system.
  • Note the four types of advanced materials and, for each, its distinctive feature(s).
  • Judge which material is most likely to be a promising candidate for utilization when given the primary or advanced material classifications of a list of candidate materials and one design selection criteria.

Lesson Roadmap

Lesson 1 will take us 1 week to complete. Please refer to Canvas for specific due dates.

Lesson Roadmap
To Read

Read pp 7-24 (Ch. 1) in Introduction to Materials ebook

Reading on course website for Lesson 1

To Watch Secrets of the Terracotta Warriors
To Do Lesson 1 Quiz

Questions?

If you have general questions about the course content or structure, please post them to the General Questions and Discussion forum in Canvas. If your question is of a more personal nature, feel free to send a message to all faculty and TAs through Canvas email. We will check daily to respond.

Why Study Materials?

Picadilly circus in London. Road junction with a building featuring large curved screens on the corner
Piccadilly Circus
Credit: Jimmy Baikovicius via Wikimedia Commons [1]

When materials scientist and narrator of The Secret Life of Materials videos (used in this course), Mark Miodownik, opens up the video on metal, he is at Piccadilly Circus in London, England. He marvels at how strange but wonderful it is that everything around him is man-made. This is not unique to London. A visit to the center of New York, Tokyo, Hong Kong, Beijing, Dubai, Paris, or any other 21st-century modern city would yield a similar situation. It might seem like a cliché but we are surrounded by materials. And with the range of materials available - whether it be in our professional or personal lives - we are constantly being asked to make choices about materials.

Something as routine and everyday as purchasing carbonated beverages is an example where materials choice could come into play. As we will see in the textbook, carbonated beverages can be purchased in glass, metal, or plastic containers. What factors drive manufacturers of carbonated beverages to offer their products in a range of different materials? What are the advantages and disadvantages when comparing the different materials choices for carbonated beverage containers? When selecting a material for a product there are many factors that must be taken into account, including properties, performance, and lifetime of the material; availability of raw materials; costs and energy usage in all steps of the processing; sustainability; waste disposal, etc.

Carbonated beverages packaged in cans, plastic bottles, and glass bottles.
An assortment of carbonated beverages in glass, plastic, and metal containers.
Credit: Francois Schnell via Flickr (CC BY 2.0)

Why is it important for you to understand materials? Products, devices, and components that you purchase and use are all made of materials. To select appropriate materials, and processing techniques for specific applications, you must have knowledge of the material properties and understand how the structure affects the material properties.

Throughout history, material advancement has gone hand-in-hand with societal advancements. The Stone Age, Bronze Age, and Iron Age were all significant materials and societal periods in humankind's development. One question I would pose to you: what is today's materials age? Is it the polymer age? Or perhaps we have already advanced past that one. Are we in the age of silicon, i.e., the electronic materials age? Or, are we possibly moving into a nanomaterials age? A biomaterials age? Some might suggest that we moved into the information age or the digital age. In any of these cases, it is clear that the materials and the capability of the materials underlying these technologies are integral to the current and future capabilities in these areas.

Now let us explore how deep-seated materials are in our culture by looking back at materials in antiquity.

Materials in Antiquity

Three of the greatest ‘cultural’ revolutions occurred in antiquity, and they are named for the material use associated with these revolutions. They were predominantly bloodless, occurred over a millennium, and were revolutionary, not evolutionary. These three revolutions occurred during the Neolithic Age (part of the Stone Age), the Urban Age (Bronze Age), and the Iron Age.

Before we look at the Neolithic Age revolution, let’s take a look at the pre-Neolithic Age. If we look at the human timeline below we can see that the usage of stone tools began about 3.4 million years ago. This marks the beginning of the Stone Age, which lasted until the advent of metalworking and ended at different regions from ~9000 BCE to 2000 BCE. The genus Homo emerged during the Stone Age. The earliest usage of cooking, clothes, and fire occurred during this pre-Neolithic Age, with their earliest known dates shown in the figure below. In addition to cooking, fire was particularly important from a materials point of view. Fire was used for the tempering of wood arrowheads, annealing flint, and creating charcoal before the Neolithic Age, and has been an important component of materials processes throughout all ages of human existence.

The first of these revolutions was the Neolithic Revolution, which was highlighted by the transformation from a hunter/gatherer population to a farmer/skilled artisan population. It has been argued that three steps were required for the Neolithic Revolution: 1) hunter/gatherer population increase, 2) food production in marginal areas, and 3) several communities at similar stages of development. Near the end of the Stone Age,  six civilizations emerged that satisfied these requirements.

Human timeline over millions of years.
Human Timeline
Click here to see a text alternative of the Figure.
Human timeline with an axis scale in millions of years.
Human Timeline
Millions of Years Stage Description
-10 Hominids Nakalipithecus (earlier apes)
-9 Hominids Ouranopithecus
-7 Hominids Sahelanthropus (earliest bipedal)
-6 Hominids Orrorin
-5 Hominids Human-like apes
-4.5 Hominids Ardiphithecus
-4 Hominids Early bipedal
-3.5 Hominids Australopithecus (earliest stone tools)
-3 to -2 Homo habilis
-2 to -.5 Homo erectus -2: Earliest exit from Africa, -1.5: Earliest fire use, -.6 Earliest cooking
-.5 to -.8 Neanderthal -.4: Earliest clothes
0 Home sapiens Modern Humans
Credit: Wikipedia [2]

Now we will take a closer look at the materials used during the Stone Age.

Flint (and Obsidian)

large, old, brown flint knife with golden handle
Ceremonial Flint Knife
Credit: Ceremonial Flint Knife by Keith Schengili-Roberts [3] is licensed under CC BY-SA 3.0 [4]

Flint and obsidian were very important Stone Age materials. Commonly found with chalk and limestone, flint is a form of the mineral quartz. Obsidian is a naturally occurring volcanic glass. Both were widely used in weapons and tools. As we will learn in this lesson, flint and obsidian are classic examples of ceramics. Both are hard and can be worked to produce a sharp edge, but both materials are prone to breakage. Slowly heating flint to 150 to 260 °C (300 to 500 °F), holding the temperature there for 24 hours (annealing), and then slowly cooling it back to room temperature, can relieve internal stresses which can improve the ability to produce flint tools or weapons with a sharper cutting edge. As discussed later, since flint is typically found with chalk and limestone, it is possible that the annealing of flint led to the discovery of lime mortar.

Charcoal

Charcoal is perhaps the greatest invention of the Paleolithic (Stone) Age. Charcoal is produced by partially burning organic matter (wood, bone, etc.) while limiting the supply of oxygen. One way of producing charcoal is to pile a large amount of wood, as shown in the figure, and covering it with soil to limit the amount of oxygen feeding the fire. During the burning process, considerable water is released, and at the completion of the burn, the wood is reduced to black brittle lumps of carbon (charcoal).

log pile to be burned to become charcoal
Woodpile before covering it with turf or soil, and firing it.
Credit: VargaA via Wikimedia Commons [5]

Charcoal played an important role throughout the Stone Age, the Bronze Age, and the Iron Age. Why? Very few elements (noble metals and copper in very limited quantities) occur naturally in their pure form. Elements usually occur bound with other elements forming compounds, and typically occur in a mixture with other compounds. Heat is usually applied to break the compounds or melt the element to produce the raw material needed for manufacturing, such as copper and iron.

The temperatures required depend on the compounds and elements involved and can vary considerably. The temperatures obtainable by fire depend on the fuel used and the supply of air. If wood is used as the fuel in an open fire, temperatures in the fire might range from 350 to 500° C. Charcoal, being a denser and drier fuel source, can provide temperatures up to 800 °C under similar conditions. If the fire is confined, such as in a kiln or a furnace, and air is forced into the fire, it is possible to obtain even higher temperatures. For charcoal, it is possible to reach temperatures above 1000 °C.

Later, in our lesson on metals, we will see that this temperature is insufficient to melt pure iron, which is why the processing of impure iron (iron plus carbon) was developed first. Impure iron has a much lower melting temperature than pure iron. We will see that an advanced design furnace coupled with a hotter burning fuel source (coke, a form of coal) was needed to obtain pure molten iron.

black, burned chunks of wood - charcoal
Charcoal.
Credit: JM  [6]

Lime Mortar

When annealing flint, you can expect chalk or limestone to be present. Chalk and limestone are composed primarily of calcium carbonate (CaCO3), which is the same mineral present in hard water. It often shows up as a white residue on plumbing fixtures. If chalk or limestone is heated above 800° C (obtainable with charcoal), the gas carbon dioxide is released from the calcium carbonate leaving lime (CaO). Lime produced in this matter is referred to as quicklime or burnt lime. If water is added, this quicklime or burnt lime hydrates to form a white pasty substance known as slaked lime.

It is quite possible that an observant fire tender or cook could have noticed that, after encountering rain, this material would dry and form a hard substance. We refer to the substance as lime mortar, a type of cement. It is common to confuse the term cement with concrete. Cement is a binder or material that glues things together. Concrete, on the other hand, is a combination of cement and aggregate (sand, stone, etc.). Concrete is one example of a composite material. As we will see in this lesson a composite material is a material that is composed of two or more distinct materials in combination. Cement is the material within concrete that binds the stone and sand together.

In addition to the development of lime mortar in the Fertile Crescent, the Incas, and the Mayans independently discovered lime mortar around 5000 BCE, and it was widely used in ancient Rome and Greece around 4000 BCE.

Limestone formations, edge of a cliff. Striated rocks with sloped horizontal lines
Limestone outcrop in the Torcal de Antequera nature reserve of Málaga, Spain.
Credit: Ngb via Wikimedia Commons [7], CC BY-SA 2.0 [8]

'Plaster of Paris'

Originally, the term ‘plaster of Paris’ was coined in the 1700s to describe plaster produced from gypsum located outside of Paris. Over time, the term ‘plaster of Paris’ has become the generic term for gypsum-based plaster. Many ancient Egyptian tomb paintings are created on plaster. It is produced in a way that is similar to lime mortar, except gypsum is used in place of lime and much lower temperatures are needed. The resulting plaster is not as hard as lime mortar. Plaster vessels dating from 6000 BCE have been found from ancient Egypt.

Painting of a plasterer covering a brick wall
Early 19th Century plasterer at work.
Credit: Original artwork by John Cranch (1751-1821), image is Public Domain via Wikimedia Commons [9]

Primary Civilizations

As mentioned before, near the end of the Stone Age, six civilizations that emerged that satisfied the requirements considered necessary for the Neolithic Revolution. If you look at the map below, you can see that there were two New World civilizations and four Old World civilizations, along with their names.

Map: New world: MesoAmerica (Texas, Mexico, central America) and Peru. Old world: Nile Valley, Indus Valley, Mesopotamia, Xia/Shang
New and Old World Civilizations
Credit: Wikimedia Commons [10]

The four Old World civilizations had two very important advantages over the two New World civilizations. Namely, they were situated along great river systems and, being more numerous, had a more robust trade system in place. The great river systems were very important components in trade, but possibly of equal or greater importance was the benefit of annual flooding. Annual flooding reinvigorates farmland and, before the advent of modern farming techniques, allowed for the successful growth of crops year after year over multiple decades without the need for artificial fertilizers or crop rotation management schemes.

Two of the Old World civilizations, the Nile Valley and Mesopotamia, formed what has been called the Fertile Crescent, which is widely regarded as the birthplace of civilization. As can be seen in the figure below, both locations possessed great river systems and, due to their proximity, had well-established trade routes. At the close of the pre-Neolithic age, these two civilizations were experiencing increasing populations, had extensive food production capabilities, and had several communities at similar stages of development.

Fertile Crescent includes areas in lower and upper Egypt, Mesopotamia, Elam, and Assyria.
Fertile Crescent Map
Credit: NormanEinstein via Wikimedia Commons [11], CC BY-SA 3.0 [4]

Mudbrick

The mudbrick was developed during the pre-pottery (Aceramic) Neolithic Age. Mudbricks were composed of a mixture that might have included clay, mud, loam, sand, and water mixed with a material to inhibit crumbling such as straw or rice husks. This was another example of a composite material. The ceramic material (clay, mud, loam, sand) by itself could support compressive loads but could be easily pulled apart. The second component of the composite, straw or rice husks, reinforced the first material, making it more difficult to pull the mudbrick apart. Water was used to allow the brick to be easily formed during manufacturing. 

Since the early civilizations were located in warm regions with very limited timber, early bricks were sun-dried. The bricks needed to be dried before installation. Otherwise, shrinkage and cracking would occur that would destabilize the building.

Before the usage of bricks, structures were limited to wood and piling of stone. Creation of the brick unleashed creative design of buildings, and the architect was born! Clay or mud (raw material) was readily available everywhere, as was the strengthening material, straw or rice husks. 

Later gravel and bitumen were used for stronger bricks. Bitumen is a naturally occurring (thermoplastic) polymer that, when heated, becomes a liquid and, when cooled, becomes solid. It is a black, tar-like substance with a consistency similar to cold molasses. Adding bitumen to bricks makes them both waterproof and much stronger. Bitumen is a mixture of hydrocarbons, which contains anywhere from 50 to thousands of carbon atoms. It is found in nature in rock asphalt, lake asphalt, and near other fossil fuels. In addition to being a structural improvement, bitumen, and crude oil sometimes found near bitumen deposits, provided fuel for brick kilns.

mudbrick construction at the edge of a river
Constructing a house out of mudbricks.
Credit: diasUndKompott via Wikimedia Commons [12]
Bitumen, black and shiny lumps
Natural bitumen from the Dead Sea.
Credit: Daniel Tzvi via Wikimedia Commons [13], public domain

Pottery

The development of pottery in Mesopotamia was important for the storage of food protected from moisture and insects. Pottery takes clay and water which, in the proper proportions, form a mass that can be readily shaped. Once in the desired shape, the piece is dried to remove the water and then fired to improve mechanical stability. Clay was readily available and thus an inexpensive material to use. 

Initially, unfired clay was used to line woven baskets. Although these unfired clay baskets were not particularly robust they did provide much-needed waterproofing. Possibly one of these early clay-lined woven baskets was discarded at the end of its usefulness. One could suppose that at some point the discarded basket was put into a fire to dispose of it. Later, in the cooled coals, someone could have discovered pottery shards and had the eureka moment where they realized that the firing of clay structures would produce pottery. 

The development of pottery occurred in Mesopotamia around 7000 BCE. The invention of the pottery wheel occurred in Mesopotamia sometime between 6000 and 4000 BCE. The earliest ceramic objects (figurines) known have been found in what is now the Czech Republic and have been dated as being created between 29,000 and 25,000 BCE. The earliest pottery has been found in China and dates from around 18,000 BCE. In 10,000 BCE, Japan was using roping or coiling to produce pots. In the videos of this lesson, Secrets of the Terracotta Warriors, you will see that coiling was the method used to produce these warriors.

The Near East, at the end of the Neolithic Age (c.a. 4500 BCE), had mastered fire to produce and/or modify a number of materials. They had flint tools and weapons, buildings of mudbrick with plaster finish, pottery, well-established trade routes from Mesopotamia to the Indus Valley, and a robust agrarian economy. Discussions about the Bronze Age and the Iron Age await us when we get to the lessons on metals and metal alloys. But now, let's take a look at what is materials science and engineering.

Pottery drying in the sun
Pottery drying.
Credit: STA3816 via Wikimedia Commons [14], CC BY-SA 3.0 [15]

Materials Science and Engineering

In my experience in this course, students have difficulty understanding the difference between materials scientists and materials engineers. In the reading for this lesson, materials science is defined as investigating relationships between structures and properties of materials, and concern with the design/development of new materials. Materials engineering is defined as the creation of products from existing materials in the development of new materials processing techniques. I would restate the roles of material scientists and materials engineers as:

  1. Materials scientists study materials and create new materials.
  2. Materials engineers use materials and create new processes.

Now, these statements of the roles for material scientists and engineers are, of course, oversimplifications. As I think you will see in the following video (5:34) produced by the Penn State Department of Materials Science and Engineering we believe that cutting-edge materials research and development require a thorough understanding of both materials science and engineering.

To Watch

Click for transcript of Penn State Materials Science and Engineering

GARY L. MESSING: Materials Science and Engineering at Penn State is one of the larger departments of Materials Science in the country. By virtue of that, we are able to offer a full spectrum of research and teaching in the field of materials. We're strong in all aspects of materials - ceramics, metals, polymers, composites, semiconductors. That brings a certain uniqueness to the education of a Penn State graduate.

R. ALLEN KIMEL: We've tried to basically give the students a very strong core into the fundamentals of Materials Science and Engineering which are structure, property, relationships, but because of the breadth and depth of the expertise in this department they can choose to take courses that go towards the interest which brought them here in the first place. They could choose to focus on biomaterials or they could choose to focus on energy - it really allows a student to make their own Materials Science and Engineering degree.

ANGELA LEONE: Being at Penn State you get the large university feel where you are one in thousands of students at the same time you get the experience of the small college where everyone in the Materials department knows you by name. I'm currently studying the corrosion of nuclear waste glass fibers with Dr. Pantano. I found out that he's studying glass and I'm really interested in glass. So I just set up a meeting with him and about a week later he was showing me his labs. I think the most special thing I've done is to get involved in glass blowing. I really enjoy doing it it's very unique and I don't think I'd have that experience anywhere else. A lot of the professors look for undergrads to do research for them, to give them a feel and it helps you choose if you want to go on to grad school or go into industry.

JAME ADAIR: At any given time I'll have four to six undergraduates working in my laboratory. Right now I have about six. I bring both my research into my lectures for the undergraduates and my lectures into my research. The curriculum focuses on cutting edge technology. We also run a very strong research experience for undergraduate programs - it's a summer science program where we bring in undergraduates from all over the United States, including Penn State, into our laboratories. We're at the cutting edge in terms of early detection of cancer and much more benign delivery chemotherapeutics, as well as a host of new surgical instruments based on our ceramic powder processing.

R. ALLEN KIMEL: We even take it beyond; the department and this country, have our own international internship program. We have relationships with fourteen different universities in Europe and Asia and we send our students there to join research groups for a semester and actually perform research - so it's not a study abroad, take classes, but it's actually going there with a research question in mind and then joining a research group and actually performing that research. So it's getting involved in the research enterprise - and it's global.

STEPHEN WEITZNER: I've traveled to Germany with the department's International Internship in Materials Science Program. That was great; I was in Germany for seven months doing research at the Technical University of Darmstadt, where I was also doing some work with computational modeling and provided a nice background for coming back to campus and starting my senior thesis.

GARY L. MESSING: Materials at Penn State is actually a very big enterprise not only do we have the Department of Materials Science and Engineering, but we also have the Materials Research Institute The Institute represents all of the faculty on campus that are working in the field of materials. The Institute brings the strength of the community as well as the research facilities.

MICHAEL HICKNER: At Penn State, we have great opportunities for high-level research in Materials Science. Penn State has a long history in solving real-world problems in the industry, we have over a hundred million dollars of research that the entire university does with companies per year and we work with both large companies like General Electric and Dow Chemical Company as well as small startup companies either in State College or in Silicon Valley, and so I think that the research and the ideas here are flexible. We have a lot of unique capabilities.

JOAN REDWING: The research that we are doing at Penn State in the area of low dimensional materials is impacting the field of material science by providing new routes for the synthesis of low dimensional materials and also providing new insights into how these materials behave and ultimately how we can integrate them into devices. It feeds into other activities here at Penn State that are focused on the fabrication of devices so their faculty members in Electrical Engineering who are using the low dimensional materials to fabricate transistors or other types of electronic devices.

MICHAEL HICKNER: The material science that we do at Penn State is really creating new opportunities and pushing new frontiers in material science. Our research that we do in our labs every day makes a big difference to new types of batteries or new types of medical devices or new types of structural steels that had better corrosion resistance, or are harder, or more ductile. And so I think that we both make a difference in real-world problems, but also, we open up new ways to think about science and new ways to think about materials.

 tetrahedron with characterization in the center. Structure, processing, performance, & properies are each on a corner
Materials science tetrahedron.
Credit: Public Domain

When utilizing a material, one needs to understand that the structure, properties, processing, and performance of the material are interrelated. This is represented by the materials science tetrahedron shown in the figure above. If one alters the processing, there is a direct connection with the structure, properties, and performance of the material. Adjusting any one of the factors will have varying degrees of impact on the other three factors. Characterization is the heart of the tetrahedron, signifying its role in monitoring the four components. 

In this course, we will be looking at the four components (structure, properties, processing, and performance) of materials, beginning with properties. Properties of materials can be classified into six categories: mechanical, electrical, thermal, magnetic, optical, and deteriorative. We will start by looking at mechanical properties in lesson four and electrical properties in lesson 12. Unfortunately, we will not have time in this course to look at the other four properties. In lessons 3, 5, 7, and 8 we will look at the structure, both atomic and microstructure. Lesson 10 will be concerned with the processing of materials, and the performance of material will be addressed throughout the course.

Classification of Materials

Matter is composed of solid, liquid, gas, and plasma. In this course, we are going to be looking at solids which we will break down into three classical sub-classifications: metals, ceramics, and polymers. 

In the reading for this lesson, representative characteristics of the three sub-classifications will be presented. In lesson three the chemical makeup and atomic structure will be further explored. The microstructure of the three classifications will be explored in their lessons. 

Composites is a special additional classical sub-classification. Composites are composed of two (or more) distinct materials (metals, ceramics, and polymers) to achieve a combination of properties. Composites are introduced in this lesson in the reading and we will have a later lesson devoted to them as well. (Note: composites should not be confused with alloys. We will learn later that alloys are a mixing of a metal with other elements. In an alloy the elements are blended, they are no longer distinct components.)

Advanced materials are materials that are utilized in high-tech applications. These materials are typically enhanced or designed to be high-performance materials - many times with very specific tasks in mind. 

Semiconductors are materials that can be made to switch from an insulator (off) to a conductor (on) by the application of voltage. The flow of electrons in semiconductors is somewhere between insulators, i.e., those that do not readily conduct electricity; and conductors, those materials which freely allow the flow of electrons. These materials have enabled our digital electronic age. The development of semiconductors for integrated circuits has allowed for the electronics and computer revolution that we have experienced in the last 50 years. 

Nanomaterial, whose sizes typically range from 1 to 100 nanometers, are materials in which size and/or geometry can play a significant role in the dominant materials properties. In this size range, quantum mechanical effects can dominate, as well as, chemistry due to a large number of the atoms being surface atoms instead of atoms in bulk. In addition to size effects, these materials sometimes exhibit unique functionality due to their geometry. For example, gold nanoparticles can be very chemically active, unlike bulk gold. This effect is due to a large number of unsatisfied bonds on the surface of the gold nanoparticle. 

Biomaterials are materials implanted into the body. In addition to performing their design function, they also have to have the ability to survive in the body (be biocompatible). The body can be a 'hostile' environment for materials. The body might attack the biomaterial as a foreign body (immune response) and the environment (wet and chemically active) in the body is typically one that leads to corrosion. 

Smart materials are materials that are designed to mimic biological behavior. They are materials that, like biological systems, ‘respond to stimuli.' When determining whether a material system is utilizing a smart material, it is usually useful to identify the stimuli and the response that the material will exhibit, as well as, what biological system it is mimicking.

The readings and videos in the last two lessons of this course will explore advanced materials in more detail. Now that I have set the stage it is time for you to begin the additional reading for this lesson.

Reading Assignment

Things to consider...

When you read this chapter, use the following questions to guide your reading and always remember to keep the learning objectives listed on the overview page in mind.

  • What are the materials' properties that determine materials applicability?
  • What are the differences between material science scientists and engineers?
  • What are the differences between metals, ceramics, and polymers?
  • Does ‘best of both worlds’ apply to composites?
  • Which of the materials classifications (metal, ceramics, polymers, composites, semiconductors, smart materials, biomaterials, and nanomaterials) are a distinct class of materials versus a group of materials with defining function composed of materials from distinct classes of materials?

Reading Assignment

Read pp 7-24 (Ch. 1) in Introduction to Materials ebook

Video Assignment: Secrets of the Terracotta Warriors

Now that you have read the text and thought about the questions I posed, take some time to watch this 54-minute video about determining how 8,000 terracotta warriors were manufactured in later third century BCE in China. As you watch this video, please note some of the problems that needed to be overcome and the assembly line approach that was necessary to complete everything in a two-year period.

Video Assignment

Go to Lesson 1 in Canvas and watch the Secrets of the Terracotta Warriors Video. You will be quizzed on the content of this video.

Summary and Final Tasks

Summary

Anthropologists, archeologists, and historians use the level of materials development (Stone Age, Bronze, Iron Age) to designate the stages of societal development. In today’s society, materials and materials development continue to shape development and advancement. In this lesson you were introduced to the important overarching themes of this course:

  • materials scientists investigate the relationships that exist between the structures and properties of materials
  • materials engineers design the structure of a material to produce a predetermined set of properties
  • structure and properties are interlinked
  • processing, structure, properties, and performance are interconnected
  • environment, wear, and economics are three important criteria for materials selection
  • classical classification of materials (metals, ceramics, and polymers, as well as, composites)
  • and the advanced materials classification used in modern high-tech applications.

We will utilize the important concepts introduced in this lesson throughout the rest of the course.

Reminder - Complete all of the Lesson 1 tasks!

You have reached the end of Lesson 1! Double-check the to-do list on the Lesson 1 Overview page to make sure you have completed all of the activities listed there before you begin Lesson 2.

Lesson 2: Economic, Environmental, and Societal Issues in Materials Science

Overview

Overview

In addition to fundamental materials properties, selecting which material to use in an application can be limited by a number of factors. Some of these factors include the cost of production, availability of starting materials (natural resources), level of pollution resulting from the manufacturing process, and amount of waste produced at the end of the lifecycle of the application. In this lesson, I will present relatively brief overviews of economic, environmental, and societal considerations that are important in the materials selection process.

Learning Outcomes

By the end of this lesson, you should be able to:

  • Diagram the total materials life cycle, and briefly discuss relevant issues for each stage of the cycle.
  • List the inputs and outputs for the materials life cycle analysis/assessment scheme.
  • Cite issues that are relevant to the "green design" philosophy of product design.
  • Discuss recyclability/disposability issues relative to the three primary classifications of solid materials and composite materials.
  • List and briefly discuss three controllable factors that affect the cost of a materials product.

Lesson Roadmap

Lesson 2 will take us one week to complete. Please refer to the course calendar for specific due dates.

Lesson Roadmap
To Read

Read pp 25-36 (Ch. 2) in Introduction to Materials ebook

Reading on course website for Lesson 2

To Watch Making Stuff: Cleaner
To Do Lesson 2 Quiz

Questions?

If you have general questions about the course content or structure, please post them to the General Questions and Discussion forum in Canvas. If your question is of a more personal nature, feel free to send a message to all faculty and TA's through Canvas email. We will check daily to respond.

Reading Assignment

Things to consider...

While you read the material for this lesson in your e-book and on the course website, use the following questions to guide your learning. Also, remember to keep the learning objectives listed on the previous page in mind as you learn from this text.

  • What are the relevant issues for each stage of the total materials life cycle?
  • What are the inputs and outputs for the materials life cycle analysis/assessment scheme?
  • What is the "green design" philosophy of product design and how does it differ from the total materials life cycle paradigm?
  • What are the recyclability/disposability issues relative to metals, ceramics, polymers, and composites?
  • What are the three controllable factors that affect the cost of a materials product and how do they affect the cost?

Reading Assignment

Read pp 25-36 (Ch. 2) in Introduction to Materials ebook

Introduction

In this lesson, we're going to look at the economic, environmental, and societal issues of materials science. The textbook reading for this week will introduce these topics, while the additional text on this website will supplement the reading material and explore further the topics of green design and social justice with regard to materials. The video for this lesson, Making Stuff: Cleaner, explores the science and technology of making energy production and usage cleaner and more efficient. Materials development in generating, storing, and distributing energy towards creating a more sustainable future are highlighted in the video. 

To Read

Read sections 20.1 - 20.4 in the customized e-book (answer quiz questions on those sections, and then return to this website).

Economic Considerations

First and foremost, a product must make economic sense. The price of a product must be attractive to customers, and it must return a sustainable profit to the company. To minimize product costs, materials engineers should consider three factors: component design, material selection, and manufacturing technique. Also, there could be other significant costs including labor and fringe benefits, insurance, profit, and costs associated with regulatory compliance. As the world has become more populated and that population is increasing its usage of the earth's natural resources, engineers are increasingly being asked to consider sustainable practices when developing new products. Also, since it is estimated that approximately half the energy consumed by the U.S. manufacturing industry is used to produce and manufacture materials, the efficient use of energy for manufacturing processes and utilization of sustainable energy sources when available is highly desirable.

Energy consumption of the following resources from 1970-2010: oil, coal, natural gas, hydro, nuclear, other, renewable.The consumption increased with time.
Graph showing world energy consumption.
Click here to see a text description.
Oil Consumption from 1970-2010
Year Energy, 1000 TWh per year
1970 10
1980 13
1990 14
2000 16
2010 18
Coal Consumption from 1970-2010
Year Energy, 1000 TWh per year
1970 6
1980 8
1990 10
2000 11
2010 16
Natural Gas Consumption from 1970-2010
Year Energy, 1000 TWh per year
1970 4
1980 6
1990 7
2000 9
2010 13
Hydro Consumption from 1970-2010
Year Energy, 1000 TWh per year
1970 1
1980 2
1990 2.5
2000 3
2010 4
Nuclear Consumption from 1970-2010
Year Energy, 1000 TWh per year
1970 0
1980 1
1990 2.5
2000 3
2010 3
Other Renewable Consumption from 1970-2010
Year Energy, 1000 TWh per year
1970 0
1980 0
1990 ~.1
2000 ~.1
2010 1
Credit: Wikimedia Commons

Sustainability represents the ability to maintain an acceptable lifestyle at the current level and into the future while preserving the existing environment. Your textbook discusses one approach to achieving sustainability: green product design. In the next section, we will look at some green design principles and examples of their application. Before moving on to that section, please watch the following short video. This (1:53) video on using renewable feedstock to replace nonrenewable starting raw materials highlights a green design principle used to make processes more sustainable.

To Watch

Renewable Feedstocks
Click here for transcript of Green Chemistry Principles - Renewable Feedstocks.

So far, many of our plastic products are derived from crude oil, which is a non-renewable source. We cannot grow or produce crude oil. This means we do not have an infinite supply of it. Instead, we must wait millions of years in order for dead carbon-based living organisms to be compressed by layers and layers of sediment before we get crude oil.

The term renewable feedstock the raw material that can be grown or produced by humans. The usage of renewable feedstock is attractive because it reduces the amount of harmful waste produced by the crude oil refinery and distillation processes.

Most printer inks are made from crude oil-derived pigments. If you think about the amount of printing that is done on a global scale this can be a problem in the long term. Currently in development are soy-based inks, which are derived from the oil of the soybean plant. As a plant, soybeans are a renewable resource.

The production process of these inks is overall more environmentally friendly than their petroleum-based counterparts. Also, the soy-based inks are much brighter than the petroleum-based inks. The recycling process of paper products printed with soy-based inks is also considerably more environmentally friendly. When paper products are recycled the inks need to be removed. Petroleum inks can be difficult to remove but soy-based inks can be removed with relative ease.

Credit: FuseSchool [16]

The term renewable feedstock refers to raw material that can be grown or produced by humans. The usage of renewable feedstock is attractive because it reduces the amount of harmful waste produced from the crude oil refinery and distillation processes. Most print inks are made from crude oil derived pigments. If you think about the amount of printing that is done on a global scale, this can be a problem in the long term.

Currently in development are soy-based inks which are derived from the oil of the soybean plum. As a plum, soybeans are a renewable resource. The production process of these inks is overall more environmentally friendly then their petroleum-based counterparts. Also, these soy-based inks are much brighter than the petroleum-based inks.

The recycling process of paper products printed with soy-based inks is also considerably more environmentally friendly. When paper products are recycled, the ink needs to be removed. Petroleum inks can be difficult to remove, but soy-based inks can be removed with relative ease.

Components of Green Design

Reduce, reuse, recycle symbol. Three arrows making a cycle
Reduce, Reuse, Recycle
Credit: Christopher Steer/iStockphoto

There are three primary components of green design: reduce, reuse, and recycle. The reduce concept means to redesign a product to use less material. The reuse concept means to fabricate a product using material that can be used again. Recycling refers to the concept of reprocessing a product at the end of its lifecycle into new raw material that can be processed into new products.

One green design principle is that if there is less waste produced, then there is less to clean up. Please watch the following short (2:23) video that highlights this principle.

To Watch

Click for a transcript of Green Chemistry - Principle 1.

When we say green chemistry we mean environmentally friendly chemical processes and doing chemistry with personal safety and the environment in mind. There are 12 principles designed to help make chemistry greener. We will discuss each of these 12 principles with an example.

Let's start with a simple concept. If you keep your room neat and tidy you won't have to clean up as much. If your room gets really messy then you will have a lot to clean up. The same concept is applied in green chemistry. If we prevent waste from being produced then there would be less to clean up. Now this is almost impossible to have zero waste for any given chemical reaction. On a global scale, The beauty of this principle is that it can be applied to almost every single chemical reaction conducted. For example, Polystyrene is a fossil fuel derived polymer used to make all disposable cups and packaging materials. The polymer itself needs to be expanded before it can be used in packaging. Traditionally this was done with CFCs or chlorofluorocarbons which deplete the ozone layer. Now this is done with carbon dioxide waste from other industrial processes. Instead of releasing carbon dioxide into the atmosphere and contributing to the greenhouse effect, this is contained and used to expand polystyrene. So, there is no additional carbon dioxide being released into the atmosphere.

Credit: FuseSchool [16]

Another green design principle related to producing less waste is to produce waste that is biodegradable. Please watch the following short (2:06) video that highlights this green design principle.

To Watch

Design for Degredation
Click for a transcript of Green Chemistry - Design for Degredation .

A lot of waste is produced on a global scale. Unless the waste is recycled it fills up in our landfills, destroys habitats, and will be a very serious health hazard. Imagine if one day the waste that we produce can be naturally be broken down by microbes in the environment or dissolve into safer materials. This principle explores such a concept.

Ideally, whatever we use and throw away would present no hazard to the environment and would not accumulate in landfills. If you've ever had to get stitches because of an injury, the stitches slowly dissolve over time. This is in fact a polymer called Polyglycolic acid which is broken down into its respective monomers by enzymes in our body. This is then either respired as carbon dioxide or excreted in our urine. There is a new class of plastics known as the bioplastics. These polymers are made from natural monomers such as cellulose and lactic acid and can be broken down in the environment. For more information on these, please see our Synthetic Polymers video.

As a result of these bioplastics, carrier bags can be broken apart by microbes in the environment. This is important because it reduces the amount of waste that accumulates in landfills. Egg cartons once made of polystyrene, which is derived from petroleum-based products, are now usually packaged in newspaper material which can be recycled and therefore, do not accumulate as waste.

Credit: FuseSchool [16]

Some processes result in waste that is toxic or hazardous. The following video (2:04) showcases a genetically modified bacteria that has been developed to produce an enzyme that, when used with glucose, can replace a known carcinogen in a widely used synthesis process. In addition, the replaced chemical is derived from nonrenewable fossil fuels, while glucose is readily available, non-toxic, and renewable.

To Watch

Principle 3
Click for a transcript of Green Chemistry - Principle 3.

The next time you look at tights, certain paints, or even plastics used in window frames you may want to think about the raw materials from which they are made. Do you know any of these materials? Pause the video, think, and resume when ready.

If you said Nylon for tights, you'd be right. If you said Polyurethane you'd be right for paints and coatings. Finally, plastic window frames are made of PVC. These contain a plasticizer allowing the frames to be easily molded. All of these materials require Adipic acid in their production. Usually, Benzene is used to make this compound, but unfortunately Benzene is carcinogenic or cancer-causing. Instead, glucose and a special enzyme extracted from genetically modified bacteria can be used to produce Adipic acid. What do you think the advantage is in doing the synthesis in this way? Pause the video, think about this, and resume when ready.

An answer is that glucose is harmless and is essentially sugar. So conducting the synthesis in this way protects the workers who would produce this compound by coming into contact with carcinogenic Benzene.

Credit: FuseSchool [16]

For more efficient use of energy, synthesis processes should be designed to occur near room temperature and at atmospheric pressure to reduce the amount of energy used when possible. Heating, cooling, and increasing or decreasing pressure, requires energy. The following green chemistry principle video (1:26) discusses the advantages of designing your synthesis process to occur near room temperature and at atmospheric pressure.

To Watch

Energy Efficiency
Click for a transcript of Green Chemistry Principles - Energy Efficiency

Many chemical processes are designed to be as efficient as possible. There are many reasons why this is done.

Firstly, it is very costly if the chemical synthesis is not designed to be efficient. In industry, chemists are aiming to modify or develop reactions so that they'll proceed an ambient temperature and pressure. This is because it costs energy to produce both high and very low temperatures.

The 2005 Noble Prize for chemistry was jointly awarded to a group of chemists who developed an energy efficient catalyst for a high atom economy reaction so that it was able to proceed its standard ambient temperate and pressure. Recall that a high atom economy reaction is one that uses all the atoms in the reactant to create the product or products. The reaction is called the Metathesis method and is responsible for the synthesis of many polymers with special properties, additive to polymers and fuels, and biologically active compounds such as insect pheromones, herbicides, and drugs.

Credit: FuseSchool [16]

Recycling

Recycling of used products rather than disposing of them as waste is a desirable approach for several reasons. Recycled material replaces the need to extract raw materials from the earth. The energy requirements to process recycled materials are normally less, and in the case of aluminum much less, than the energy required to process extracted raw materials from the earth. In addition, recycling conserves natural resources and eliminates the ecological impact from the extraction of raw materials from the earth. Proper product design facilitates recycling, which reduces pollution emissions and landfill deposits.

Some issues surrounding recycling include that products must be disassembled or shredded to recover materials, and collection and transportation costs are significant factors in the economics surrounding recycling. The following video examines the anatomy of a properly designed landfill. After watching the video (4:39), proceed to your textbook and read section 20.5.

To Watch

Landfill Video
Click for a transcript of the Landfill video.

Whenever you throw away your rubbish, do you ever wonder where it goes, what happens to it, and the effect this can have on the environment? The answer is that everything has not been recycled or reused ends up in a landfill site. What do you think this means? If you think that a landfill is an open hole in the ground where we bury the rubbish you'd be wrong. In fact, it is a carefully designed structure built into or on top of the ground to store your rubbish in such a way that it will be isolated from groundwater. Our freshwater supply will be kept dry preventing leaking into this groundwater and will not come into contact with the air causing horrible smells. This isolation is achieved with a bottom liner typically made from very thick PVC and a daily covering of soil. However, in many countries, open rubbish tips still exist causing environmental and health problems. If our rubbish is isolated from the surrounding environment it doesn't decompose. Landfill sites are not like a compost pile where the purpose is to bury rubbish in such a way that it will decompose quickly. Rather, landfill sites are simply a way that we cope with the vast amounts of rubbish that we produce.

Let's now take a look at the parts of make up the landfill cell and how these cells make up organized landfill sites. Groundwater has to be protected from chemicals found in our waste and so compacted clay is placed on top to act as a physical barrier. On top of this is a very thick waterproof plastic liner. This stops water that contains waste chemicals called leachate from contaminating this groundwater. A geotextile mat, typically made from polypropylene or polyesters are like a woven fabric, and this too prevents plastic sheeting from being damaged when a shark gravel layer as the rub on top is compacted. The gravel layer on top of the geotextile map filters large pieces of debris allowing only water through. this improves the drainage of the landfill cell. The gravel layer is connected to a leachate pipe. The leachate is collected through pipes where it goes into a leachate pond. This leachate can't be used as drinking water and needs to be treated like sewage before it can be moved on the sites. This must continue even when the landfill site is full. A drainage layer and the soil layer prevent firming from entering landfill cells. Finally, on top of the landfill cells old-new is where buried rubbish is compacted by heavy diggers of machinery. Bacteria in a landfill break down the trash under anaerobic conditions meaning in the absence of oxygen and so a byproduct of this breakdown is landfill gas. This presents a hazard because this gas contains methane which could explode therefore, it has to be removed.

To do this the series of pipes are embedded within the landfill to collect the gas and some landfills this gas is vented or burned. What do you think might be a better use for this gas? Pause the video and continue when you're ready.

If you said that the methane in landfill gas could be used as an energy source you'd be right. This means it could be collected and burning boilers to generate electricity.

In summary, a landfill is carefully constructed and allows waste only to be stored. It may never decompose and is not really a viable solution for dealing with waste. In addition extensive monitoring is required to ensure that no leachate escapes into the groundwater. This has to happen even once a land that is no longer in use. There aren't really any upsides to landfill where the gas produced can be collected and balance of fuel, it can also cause explosions if not vented properly. So we need ways to recycle, reuse, and reduce.

Credit: FuseSchool [16]

To Read

Read section 20.5 in the customized e-book.

In the next sections, we will be discussing the recycling of metals, glass, polymers, paper, and limits of recycling.

Recycling of Metals

As mentioned in the e-book, aluminum is the most commonly recycled nonferrous metal. (Ferrous is Latin for iron, so a nonferrous metal is a metal which does not contain iron.) Aluminum is recycled because it takes a lot less energy to recycle aluminum than it takes to extract aluminum from bauxite ore, which requires heating and electrolysis. In addition, aluminum readily forms an oxide that forms a protective surface. This protective surface protects the bulk of the aluminum from oxidizing further. This results in most of the aluminum being recovered every time it goes to the recycling phase, in contrast to iron.

In the case of iron, oxidation, i.e., rust, does not protect iron from oxygen and water, and significant amounts of iron are not recyclable because the iron has been converted to rust. Please watch the following video (5:04) which summarizes the points about recycling of metals emphasized in your e-book and this website.

To Watch

Recycling Metals
Click for transcript Recycling Metals.

Certain metals can be extracted from their ores. For example, iron is extracted from hematite in a blast furnace and copper can be extracted from malachite. The process of extracting metals from their ores can be time-consuming, costly, and harmful to the environment. Other less reactive metals such as gold and platinum can be found naturally as native metal. The issue here is that there is a finite or limited supply of these metals on earth. In this lesson we will learn about recycling metals and the advantages and disadvantages to these industrial processes.

Aluminum is the most commonly recycled metal. Aluminum is used to make soft drink cans, aluminum foil, certain food cans, and even certain packaging materials. Aluminum is also used to make the outer bodies of some cars and airplanes due to the fact that it is less dense than steel. The extraction of aluminum from its ore is done by electrolysis. It may not seem like a lot of electrical energy is needed, but keep in mind that this is done on an industrial scale worldwide. The recycling of aluminum uses only 5% of the energy used in its extraction from its ore which uses a small voltage but a large current. In fact some countries really encourage recycling by having separate sections in their public waste containers. One for waste, one for cans and other metal products, and another for paper products. Some countries also have mandatory household recycling in a sense that if aluminum cans are found in regular waste bins for pickup the fine can be issued. In fact recycling aluminum is so common that any aluminum material that you encounter today has some if not more than 50% recycled aluminum content.

Steel can be recycled and most steel mills primarily use recycled scrap steel instead of caste or pig iron from blast furnaces. Copper is also recycled. Once again this is a more energy efficient process than the extraction from its ore as well the copper ore supply on earth is being depleted at a very fast rate. As a result other methods for the extraction of copper such as phytomining and bioleaching are being used instead.

Silver and gold from jewelry are also recycled. As these metals are found as native metals the issue here is not the energy cost but rather the rarity of these metals. The main source of gold is recycled electronic goods such as computers which have their electrical contacts plated with gold so there is no corrosion and a perfect electrical connection. The main issue with recycling metals is the separation process in the waste containers. Most often not all metal products are clearly labeled as recyclable. As well, many metal products are alloys. Can you remember what an alloy is? Please pause the lesson and continue when you are ready.

An alloy is a mixture of two or more elements with at least one of the elements in the mixture as a metal. Many metal products are made into alloys for increased strength and other desirable properties, but not all alloys can be recycled such as manganese aluminum alloys. Some alloys which can be recycled need to be molten and separated into their constituent metals thereby making it a more energy costly process. As well, we have to take into consideration the fact that these metal products must first be collected and then transported to recycling facilities. All depending on the distances covered this could involve quite a bit of fuel usage, and therefore increasing CO2 and noxious gas emissions.

In summary, aluminum steel and copper are the most commonly recycled metals and the recycling process usually uses significantly less energy than extraction from their ores.

Credit: FuseSchool [16]

In the next section, we will discuss recycling of ceramics, in particular, the recycling of glass which is the most common commercial ceramic.

Recycling of Glass

Glasses are the most common commercial ceramics, however, there is little economic incentive to recycle glass. The raw materials for producing glass are inexpensive and readily available. Glass is relatively dense, which makes it expensive to transport which adds to the costs of recycling. Glass must be sorted before being processed during recycling, usually done manually which adds to costs. Not all glass is recyclable, and the glass comes in many different forms. Please watch the following video (3:29) which summarizes the points about recycling of glass emphasized in your e-book and this website.

To Watch

Recycling Glass
Click for transcript of Recycling Glass.

In this lesson we will learn about recycling glass. Although the usage of plastics is more common as it is lighter and less prone to breakage, glass is still widely used. Glass is used to make soft drink bottles, food containers such as plates and glasses, and vases. Glass is primarily silicon dioxide with a small percentage of calcium oxide and sodium oxide. The beauty about glass is that it is a hundred percent recyclable and unlike paper it does not break down into smaller components with each recycling process. For the moment, only glass bottles can be recycled not crystal glass, window glass, and windscreen glass on cars. This is because other materials are added to this type of glass and the recycling process at the moment cannot separate those materials from glass.

First, glass bottles and containers need to be collected and separated according to their color. This is important because the different colors are made due to many different substances that are added. Ideally, green glass will be recycled into green glass and brown glass into brown glass. Any the labels present must be removed and the bottles must be thoroughly washed to ensure that any residual contents and adhesives are removed. The bottles are then crushed into smaller pieces before they are melted. Why is the step important? Please pause the lesson to think about this and resume when you are ready.

This is an important step because the smaller pieces mean that melting will occur quicker due to the increased surface area. Following on the same idea if you want it to fully dissolve sugar in water, powdered sugar would dissolve faster than a cube of sugar. The smaller pieces of glass are then melted, poured into a mold, and allowed to cool and harden. The bottles are now fully recycled and ready for reuse. This recycling process uses a lot less heat energy than glassblowing and it's therefore, a more environmentally friendly option. As well as being recycled into bottles, mixed glass can be crushed to form glass aggregate and used as a cheaper substitute for gravel.

In summary, glass is fully recyclable and can be recycled a virtually infinite number of times without being degraded into other products.

Credit: FuseSchool [16]

In the next section, we will discuss some of the limitations of recycling.

Limits of Recycling

Recycling has a number of advantages. Properly done, it reduces the usage of raw materials, energy usage, air pollution, water pollution, and greenhouse gas emissions. There are, however, a number of limits to the effective implementation of recycling. Recycling can involve energy usage, hazards, labor costs, and practices by individuals and countries, which can hamper the efficient implementation of recycling plans. The biggest limit to recycling is that not all materials can be recycled and so materials can only be recycled a limited number of times due to degradation each time through the process. This degradation is referred to as downcycling.

In addition, recycling poses a number of societal and ethical issues. As highlighted in the e-book, e-waste recycling has led to electronic waste from developed countries being shipped to undeveloped countries for recycling. In many cases, this leads to low wages and terrible conditions for workers involved in the recycling process and the release of toxins which are environmental and health risks for the individuals and their surrounding communities. Please watch the following video (5:26) which summarizes the limits of recycling as discussed in your e-book and this website.

To Watch

Limits to Recycling
Click for transcript of Limits to Recycling.

A common definition is that recycling is a process which allows waste materials to be turned into new products and prevents the waste of potentially useful materials. Recycling reduces the use of fresh materials. It acts to reduce energy usage, reduces air pollution from incineration, reduces water pollution from land filling by reducing the need for conventional waste disposal, and has lower greenhouse gas emissions as compared to fresh material production. So, with all of these benefits you must be wondering if there are any limits to a process that can have such a positive effect on the environment. Limits to recycling are often considered in terms of energy, hazards, costs, and practices by individuals and countries. The biggest limits of recycling however, is that not all materials can be recycled or can only be recycled a number of times before they lose their quality. Some materials once used are always going to have to be dumped as we have no method for making them useful again. What sorts of materials do you think can be recycled? See if you can come up with five. Pause the video and continue when ready.

Commonly, these are the materials that are recycled: glass, paper, metal, plastic, textile, and electronics. How many did you get?

Now let's consider the different limitations. First up is the energy aspect. Put simply some materials like paper and aluminum metal agree to have lower processing costs when it comes to recycling them rather than using fresh materials to make new products. However, the recycling of materials like plastics is extremely energy intensive. Before the plastics can be melted and mixed together, they require sorting usually by hand as there are many different types of plastics usually indicated by special marking a number. If a mixture of plastic recycled together now ever contaminate the melt or you get a lower grade of plastic than the originals which is therefore less valuable. This reduction in the quality of recycling product is called downcycling. In addition to this there is the added complication that devices often use mixed materials. Think of a car. There are wide range of recyclable materials the copper wire the aluminum in engine some of the plastics, the glass, and the iron however, what about the alloys? These are mixtures of metals energy will be needed to separate these and so one of the most difficult problems of recycling is the separation of randomly intermixed particles.

Secondly, there are hazards to recycling including the recycling of dangerous metals. Can you think of any dangerous metals? Pause the video and name some. Continue when you are ready.

Some of the metals associated with recycling are lead and mercury. Often these metals can come from the recycling of waste electrical and electronic equipment. In India and China, a significant amount of pollution is generated where informal recycling in an underground economy of these countries has generated an environmental and health disaster. High levels of lead polybrominated diphenyl ethers, polychlorinated dioxins and furans, as well as polybrominated dioxins, have become concentrated in the air, bottom ash, dust, soil, water, and sediments in areas surrounding these underground recycling sites. Many of these chemicals become dissolved in the water that we drink called leachates. Also, plants can take up these chemicals allowing toxic chemicals to enter the food chain. Finally, if the chemicals are airborne there are own problems with us inhaling them.

There are also social issues connected to recycling whilst it may create jobs they are often jobs with low wages and terrible working conditions in developing countries. In areas without many environmental regulations or worker protections, job like ship braking can result in deplorable conditions for both workers the surrounding communities.

Thirdly, is a challenge for you what do you think the cost of recycling depends on? Pause, think, and continue when ready.

A good answer might be that the costs of recycling depend on the efficiency of the recycling program. Governments or local authorities may not recycle because it's cheaper to use landfill. Also, consumers are encouraged to recycle, but this depends on people being involved in pre-sorting their recycling. Some countries or local governments impose charges when this is not done.

Credit: FuseSchool [16]

In the next section, we will discuss the recycling of polymers, in particular, plastics.

Recycling Polymers

One way of classifying polymers is to break them up into two classes. The two classes of polymers are thermoplastic polymers and thermosetting polymers. The basic property that separates a thermoplastic polymer from a thermosetting polymer is the polymer’s response to being heated. When the thermoplastic polymer is heated, it melts, softens, and can be reformed when cooled. When the thermosetting polymer is heated, it hardens and cannot be reformed and stays hard when cooled. We will learn much more about each of these two classes of polymers and the reasons for their defining properties later in our lesson on polymer structures.

Since thermoplastic polymers can be melted and reformed, they are easily recycled. However, their properties do degrade with each reuse. Thermosetting polymers are much more difficult to recycle. Some of them can be ground up and used as filler for other processes, and, on a case-by-case basis, some can be processed to be broken down into their underlying base units which can be reused. Another approach to reducing the amount of plastic that ends up in our landfills is the development of biodegradable plastic. The idea here is that plastic can be made to breakdown (be compostable). In addition, bioplastics often come from renewable raw materials. But this leads to an ethical issue: do you use the available arable land for plastic or food production?

Now, please watch the following video (4:38) on plastics and biodegradable plastics which summarizes some of the issues around plastic recycling and bioplastics as discussed in your e-book and this website.

To Watch

Problems with Plastics and Biodegradable Plastics
Click for the transcript of Problems with Plastics and Biodegradable Plastics.

In the previous video, Limits to Recycling, we looked at the overall limits to recycling of materials. We touched on plastics in that video. You may be aware that the biggest problem in recycling plastics is they are not biodegradable so that soft drinks bottle or your wrappers that you just throw away and not biodegradable. This means the fate of most plastics is to go into landfill sites or our oceans where it will take hundreds possibly thousands of years for them to degrade. You may say what about recycling? Briefly, the problems with recycling plastics are they have to be manually sorted which is a labor and energy-intensive process. Mixed polymers are found in a range of materials so cannot be recycled and are often dumped. Once a plastic has been recycled once it can become downcycled whereby its quality is decreased. The fact is that plastic recycling rates are far below other recycling rates, but the industry has improved in recent years.

Worldwide seven groups of plastic polymers have been given a plastic identification code that people use for recycling. You can find these on most packaging or plastic materials. Different types of plastics will be recycled by different municipalities even by different countries in some instances. Generally, it's difficult to recycle plastics with numbers one and two. Plastics in these higher numbers are known as rigid plastics. So, the disposal of plastics is a tricky one. Other than reducing their use of recycling or reusing them what else do you think might be done to dispose of them? Pause the video and continue when you think you have an answer.

One way is that the plastic can be incinerated or buried. Whilst this produces useful energy, some plastics contain polymers that are halogenated When these polymers are burnt. For instance, PVC, toxic fumes like hydrochloric acid are released. This can cause severe respiratory distress and so is hugely problematic. In addition, burning of polymers produces CO2. The greenhouse gas also contributes to global warming. This is because about 10% of the oil is used as petrochemicals. If we then use the polymers again as a fuel for energy production then at least we're using the oil twice. You may have heard the idea of using bioplastics as a solution, but these two can have problems. What do you think bioplastic is? Pause the video and resume when you have an answer.

Bioplastic is a biodegradable plastic. This means it's compostable and it can be broken down by the result of bacterial action when it is disposed of. Whilst this is a benefit, there are still issues with bioplastics that need solving. Bioplastics often come from renewable raw materials like starch, maize, cellulose, and polylactic acids. These are plant-derived materials and so the ethical question arises is where the land should be used to grow crops or bioplastic manufacture instead of crops for food? Originally we said that bioplastics are readily compostable, however, bioplastics are not nearly as readily compostable as regular plant material. If you toss a bioplastic fork into your compost and assume it will be dirt in a few months you'll be disappointed. Whilst a bioplastic fork is compostable it requires high-intensity high heat commercial composting for that to happen quickly.

In summary, plastic disposal has a number of issues with regards to landfills, recycling, and incineration. However, bioplastics so slightly better do not present themselves as the better alternative because there are issues of land usage, the quality of the bioplastic, and how it is composted. The best alternative is to reduce our usage and disposal of plastics altogether.

Credit: FuseSchool [16]

In the previous video, the incineration of waste was discussed. Incineration leads to a huge volume reduction of waste, which results in less waste ending up in the landfill. Waste in the landfill is the least environmentally friendly option. However, incineration typically results in less recycling, which would be a more efficient use of recyclable material than incinerating it. This reduction of recycling due to incineration is considered the major disadvantage of incineration. Although an important concern with incineration is the production of toxins, with proper technology these toxins can be managed. A segment of the video for this week, Making Stuff: Cleaner, discusses burning waste to create electricity. Please watch the following short video (4:40) which discusses burning waste to create electricity as well as the issues regarding incineration discussed above.

To Watch

Incineration of Waste and Electricity.
Click for the transcript of Incineration of Waste and Electricity.

In this video, we're going to consider the advantages and the disadvantages to waste incineration. What do you think are the advantages of burning waste instead of committing it to the landfill? Pause the video and continue when you have an idea or some ideas.

If the waste is not burned, then it is likely to end up in a landfill site which is considered to be the least environmentally friendly option. Incineration could result in a reduction in the waste volume of around ninety percent and this could be particularly important for small cities where space is scarce and landfill is not an option. Odors and rodents are present in other methods, are not a problem when using incineration as a garbage disposal method. Nearly all of the waste that is burned could be used to generate electricity in what is called energy-from-waste schemes where households and industries could benefit from the electricity or heat produced. The electricity generated could help to pay for the startup costs of the incinerator. Additionally, the steam produced from incineration presents itself as a cost-saving energy source if recycled. Ash produced from these incinerators could be used in the construction and road-building industries. In addition, metals could be extracted from the ash and they could also be used in steel industries. The key advantage, however, is that the production of electricity and thermal energy from waste enables us to conserve conventional sources of energy such as fossil fuels? Now, what about the disadvantages of incineration? Again pause the video and think about what these could be. Resume when you have an idea or some ideas.

The main disadvantage surrounds potential pollutants found in the ash left in the incinerator and those that could be emitted from the chimney. These include dioxins, acid gases, nitrogen oxide, heavy metals, and particulates. As you may have heard from other videos, these are airborne particles that are small enough to get into the lungs of humans. They cause and aggravate respiratory problems such as asthma; however, it is the dioxins resulting from incomplete combustion contained in the gases from the chimneys that attract the most concern. This is because they are suspected of causing cancer. In addition, these emissions could be distributed through the food chains and accumulate over a long period of time impacting both ecosystems and human health. As a result, many people are opposed to having incinerators built in or near their communities.

Another disadvantage is the initial startup costs to build a waste incinerator for the production of electricity and may not be seen as a viable alternative. Where cheaper waste disposal methods exist, once built the maintenance of the machinery could be costly. The biggest disadvantage, however, is that if waste is incinerated without first being sorted this means we are incinerating waste that could have been recycled.

So in summary, the incineration of wastes presents numerous advantages. One, the energy is used the heat or to produce electricity for homes in the industry. Two, this method reduces the need for fossil fuel consumption. Three, the ash produce can be recycled for metals used in the steel industry, and finally four, this method reduces the volume of waste produced. There are also disadvantages. One, incinerators are expensive to build and maintain. Two, pollutants in the ash and emissions from incinerators are harmful to health and ecosystems and finally, three, waste that could be recycled will instead be incinerated if not properly sorted.

Credit: FuseSchool [16]

Lastly, please watch the following video (5:40) on the recycling of paper, which touches on several themes of this lesson including sustainability, downcycling, and green design principles.

To Watch

Recycling Paper
Click for the transcript of Recycling Paper.

Look around you. There are so many paper products from the sheets of paper that you write on to the pages of your textbook - to egg cartons, to newspapers, to certain grocery bags - cardboard boxes, and the list continues. The important fact here is that paper is an essential part of our lives. You may know that paper usually comes from trees. They must first be chopped down and then subjected to an industrial process called pulping which removes the lignin, the polymer that gives trees their strong hard structure. Obviously the paper production process has led to mass deforestation. The main issue here is that the trees are not being replanted, or if they are being replanted they will not grow fast enough to replace the trees that are being cut. This is a serious environmental issue because it destroys habitats, disrupts food chains and food webs, is a severe eyesore, and indirectly leads to increased carbon dioxide content in the atmosphere. It could also lead to erosion of hillsides with the consequence of flooding. An obvious solution is to recycle these paper products. In this lesson, we will learn about recycling paper and the advantages and disadvantages of this industrial process.

Paper is in fact layers of cross-linked cellulose fibers. Recall that cellulose is a polymer of glucose monomer units. If you tear a piece of paper and look closely, you will see fine hairs sticking out. These fine hairs are cellulose fibers. When paper products are recycled the cellulose furs are merely pulled apart and rearranged into new paper products. In some countries, paper recycling is highly encouraged by having specialized waste containers just for paper waste. The paper waste is then collected and separated according to their properties. Sheets of paper are recycled with other sheets of paper and cardboard boxes are recycled with other cardboard boxes.

Now, right before the paper can be recycled, any ink needs to be removed. This process is called de-inking. Most inks are petroleum-based inks which are derived from crude oil products and can be difficult to remove. Currently in development are soy-based inks which are derived from a renewable resource and makes it easier for the paper to be de-inked. Metal parts such as staples and paper clips need to be removed with a magnet and glues are removed with a chemical wash. Bleach is sometimes added to whiten the slurry and therefore, the resulting paper product will be brighter. The cellulose fibers in the slurry are mixed with new pulp and then allowed to settle on gauze and rolled to form the new sheets of paper. This step also removes any excess water from the slurry before it is air-dried.

The main disadvantage with recycling paper is that it can only be done a few times. Recycled paper is usually used to make kitchen rolls, toilet paper, and packaging materials. Each time a paper product is recycled the cellulose fibers become shorter thereby rendering the recycled paper product a little bit less useful than the original. However, this can be used for another purpose. Can you remember what cellulase is? Hint: it is an enzyme. Please pause the lesson to think about this and resume when you are ready.

Cellulase is the enzyme that breaks cellulose apart into its glucose monomers. Currently, the industrial usage of cellulase is to break down cellulose into glucose. The glucose obtained can then be fermented by certain bacteria to produce ethanol, which can then be used as a biofuel.

In summary, recycling paper products help to reduce the rate of deforestation and consumes a lot less energy. Although paper can only be recycled a few times, the cellulose fibers can be broken apart into glucose monomers which can then be used for other environmentally friendly applications.

Credit: FuseSchool [16]

Video Assignment: Making Stuff Cleaner

Now that you have read the text and thought about the questions I posed, go to Lesson 2 in Canvas and watch the Making Stuff: Cleaner video (55 minutes). This video highlights some innovations in materials science that can potentially help make our technology use cleaner in the future. In "Making Stuff: Cleaner," in contrast to the readings of this lesson, the rapidly developing science and business of clean energy is explored. Some of the latest materials developments in generating, storing, and distributing energy are investigated in the hope of creating a sustainable future.

Video Assignment

Go to Lesson 2 in Canvas and watch the Making Stuff: Cleaner Video (55 minutes). You will be quizzed on the content of this video.

Summary and Final Tasks

Summary

Producing a sustainable society is one of the greatest challenges facing our society. The supply of natural resources, the creation of pollution during the manufacture of materials, recycling issues, and materials waste all issues of concern towards creating a sustainable society. By considering a material's total life cycle, utilizing materials life cycle analysis, and implementing a ‘green design’ philosophy, engineers can work towards alleviating some of these issues. 

Reminder - Complete all of the Lesson 2 tasks!

You have reached the end of Lesson 2! Double-check the to-do list on the Overview page to make sure you have completed all of the activities listed there before you begin Lesson 3.

Lesson 3: Atomic Structure and Bonding

Overview

Electronic configuration for elements and the interatomic bonding between atoms and molecules determine some of the important properties of solid materials, including a correlation between bonding type and material classification—namely, ionic bonding (ceramics), covalent bonding (polymers), metallic bonding (metals), and van der Waals bonding (molecular solids). In this lesson, we will review briefly atomic structure, electron configurations in atoms, the periodic table, and atomic and interatomic bonding. These fundamental and important concepts will be applied to the understanding of solid materials in this and subsequent lessons of this course.

We will see in later lessons that important properties of solid materials depend on the way in which the atoms are arranged. In this lesson, we will consider some fundamental and important concepts about how the atoms are held together that compose a solid. These concepts: atomic structure, electron configuration, the periodic table, and the various types of primary and secondary interatomic bonds, are discussed with the assumption that the student has already encountered this material in a high school chemistry course. 

Learning Objectives

By the end of this lesson, you should be able to:

  • Describe and compare the Bohr and wave mechanical atomic models.
  • Describe the important quantum-mechanical principle that relates to electron energies.
  • Recognize the effect of the Pauli exclusion principle on atomic structure.
  • Produce the electronic configuration for the ground state of a given element and any ions.
  • Identify the locations of metallic, non-metallic, and intermediate elements on the periodic table.
  • Describe the general rule for electronegativity on the periodic table.
  • Contrast the behavior of valence electrons for electropositive with the valence electrons of electronegative elements.
  • Briefly describe ionic, covalent, metallic, hydrogen, and van der Waals bonds.
  • Find examples of materials with the following bond types; ionic, covalent, metallic, hydrogen, and van der Waals bonds.
  • Briefly explain the fact that water expands upon freezing to ice from its liquid phase.

Lesson Roadmap

Lesson 3 will take us 1 week to complete. Please refer to Canvas for specific due dates.

Lesson Roadmap
To Read

Read pp 37-65 (Ch. 3) in Introduction to Materials ebook

Reading on course website for Lesson 3

To Watch Chapters from Hunting the Elements, TED-Ed talks on Atoms and Periodic Table
To Do Lesson 3 Quiz

Questions?

If you have general questions about the course content or structure, please post them to the General Questions and Discussion forum in Canvas. If your question is of a more personal nature, feel free to send a message to the instructor through Canvas email. The instructor will check daily to respond.

Reading Assignment

Things to consider...

While you read the material for this lesson in your e-book and on the course website, use the following questions to guide your learning. Also, remember to keep the learning objectives listed on the previous page in mind as you learn from this text. 

  • What is the main difference between the Bohr and the wave particle atomic models?
  • How does the electronic configuration of an atom determine its materials classification?
  • What are some of the materials properties that the location of the element on the Periodic Table predicts?
  • How does bonding affect the materials properties of atoms and molecules?
  • How does bonding type determine a material's probable materials characterization?

Reading Assignment

Read pp 37-65 (Ch. 3) in Introduction to Materials ebook

The Atom

The word atom is derived from the ancient Greek adjective atomos, meaning "uncuttable" or "indivisible." The earliest concepts of the nature of the atom were debated in ancient India and ancient Greece. We now know that the atom has a nucleus composed of protons and neutrons surrounded by clouds of electrons. The protons are positively charged, electrons are negatively charged, and neutrons possess no charge. Neutrons and protons are held in the nucleus by the nuclear force, and neutrons are not simply a proton plus an electron. In fact, neutrons are required to make the nucleus stable once you have more than one proton in the nucleus.

Atoms are the fundamental building blocks of matter; they cannot be divided using chemicals. Chemical reactions to move electrons can affect how atoms bind to each other but cannot be used to divide atoms. Most of the mass of the atom is located in the nucleus, with the mass of the proton roughly equal to the larger neutron, but 1840 times the mass of the electron. In contrast, most of the volume of the atom is filled with electrons. Now please watch this brief (5:22) video on the (brief) history of atomic theory.

To Watch

TED talk, the 2,400-year Search for the Atom
Click for transcript of The 2,400-year Search for the Atom

What do an ancient Greek philosopher and a 19th century Quaker have in common with Nobel Prize-winning scientists? Although they are separated over 2,400 years of history, each of them contributed to answering the eternal question: what is stuff made of?

It was around 440 BCE that Democritus first proposed that everything in the world was made up of tiny particles surrounded by empty space. And he even speculated that they vary in size and shape depending on the substance they compose. He called these particles "atomos," Greek for indivisible. His ideas were opposed by the more popular philosophers of his day. Aristotle, for instance, disagreed completely, stating instead that matter was made of four elements: earth, wind, water, and fire, and most later scientists followed suit.

Atoms would remain all but forgotten until 1808 when a Quaker teacher named John Dalton sought to challenge the Aristotelian theory. Whereas Democritus's atomism had been purely theoretical, Dalton showed that common substances always broke down into the same elements in the same proportions. He concluded that the various compounds were combinations of atoms of different elements, each of a particular size and mass that could neither be created nor destroyed. Though he received many honors for his work, as a Quaker, Dalton lived modestly until the end of his days.

Atomic theory was now accepted by the scientific community, but the next major advancement would not come until nearly a century later with the physicist J.J. Thompson's 1897 discovery of the electron. In what we might call the chocolate chip cookie model of the atom, he showed atoms as uniformly packed spheres of positive matter filled with negatively charged electrons. Thompson won a Nobel Prize in 1906 for his electron discovery, but his model of the atom didn't stick around long.

This was because he happened to have some pretty smart students, including a certain Ernest Rutherford, who would become known as the father of the nuclear age. While studying the effects of X-rays on gases, Rutherford decided to investigate atoms more closely by shooting small, positively charged alpha particles at a sheet of gold foil. Under Thompson's model, the atom's thinly dispersed positive charge would not be enough to deflect the particles in any one place. The effect would have been like a bunch of tennis balls punching through a thin paper screen. But while most of the particles did pass through, some bounced right back, suggesting that the foil was more like a thick net with a very large mesh. Rutherford concluded that atoms consisted largely of empty space with just a few electrons, while most of the mass was concentrated in the center, which he termed the nucleus. The alpha particles passed through the gaps but bounced back from the dense, positively charged nucleus. But the atomic theory wasn't complete just yet.

In 1913, another of Thompson's students by the name of Niels Bohr expanded on Rutherford's nuclear model. Drawing on earlier work by Max Planck and Albert Einstein he stipulated that electrons orbit the nucleus at fixed energies and distances, able to jump from one level to another, but not to exist in the space between. Bohr's planetary model took center stage, but soon, it too encountered some complications. Experiments had shown that rather than simply being discrete particles, electrons simultaneously behaved like waves, not being confined to a particular point in space. And in formulating his famous uncertainty principle, Werner Heisenberg showed it was impossible to determine both the exact position and speed of electrons as they moved around an atom. The idea that electrons cannot be pinpointed but exist within a range of possible locations gave rise to the current quantum model of the atom, a fascinating theory with a whole new set of complexities whose implications have yet to be fully grasped.

Even though our understanding of atoms keeps changing, the basic fact of atoms remains, so let's celebrate the triumph of atomic theory with some fireworks. As electrons circling an atom shift between energy levels, they absorb or release energy in the form of specific wavelengths of light, resulting in all the marvelous colors we see. And we can imagine Democritus watching from somewhere, satisfied that over two millennia later, he turned out to have been right all along.

Credit: Theresa Doud, TED-Ed

To Read

Now that you have watched the video, please go to your e-textbook and read the first four sections (pages 36 to 46 in Chapter 3 of Materials for Today's World, Custom Edition for Penn State University) of this lesson's reading. When finished with the reading proceed to the next web page.

The Periodic Table

The periodic table classifies the elements according to their electron configuration. The scientist given credit for the modern periodic table is Russian chemist Dmitri Mendeleev. Please watch the following video (4:24) which explains the true genius of what Mendeleev accomplished.

To Watch

TED Talk: The Genius of Mandeleev's Periodic Table
Click for transcript of The Genius of Mendeleev's Periodic Table.

The periodic table is instantly recognizable. It's not just in every chemistry lab worldwide, it's found on t-shirts, coffee mugs, and shower curtains. But the periodic table isn't just another trendy icon. It's a massive slab of human genius, up there with the Taj Mahal, the Mona Lisa, and the ice cream sandwich -- and the table's creator, Dmitri Mendeleev, is a bonafide science hall-of-famer. But why? What's so great about him and his table? Is it because he made a comprehensive list of the known elements? Nah, you don't earn a spot in science Valhalla just for making a list. Besides, Mendeleev was far from the first person to do that. Is it because Mendeleev arranged elements with similar properties together? Not really, that had already been done too. So what was Mendeleev's genius?

Let's look at one of the first versions of the periodic table from around 1870. Here we see elements designated by their two-letter symbols arranged in a table. Check out the entry of the third column, fifth row. There's a dash there. From that unassuming placeholder springs the raw brilliance of Mendeleev. That dash is science. By putting that dash there, Dmitri was making a bold statement. He said -- and I'm paraphrasing here -- Y'all haven't discovered this element yet. In the meantime, I'm going to give it a name. It's one step away from aluminum, so we'll call it eka-aluminum, "eka" being Sanskrit for one. Nobody's found eka-aluminum yet, so we don't know anything about it, right? Wrong! Based on where it's located, I can tell you all about it. First of all, an atom of eka-aluminum has an atomic weight of 68, about 68 times heavier than a hydrogen atom. When eka-aluminum is isolated, you'll see it's a solid metal at room temperature. It's shiny, it conducts heat really well, it can be flattened into a sheet, stretched into a wire, but its melting point is low. Like, freakishly low. Oh, and a cubic centimeter of it will weigh six grams.

Mendeleev could predict all of these things simply from where the blank spot was, and his understanding of how the elements surrounding it behave. A few years after this prediction, a French guy named Paul Emile Lecoq de Boisbaudran discovered a new element in ore samples and named it gallium after Gaul, the historical name for France. Gallium is one step away from aluminum on the periodic table. It's eka-aluminum. So were Mendeleev's predictions right? Gallium's atomic weight is 69.72. A cubic centimeter of it weighs 5.9 grams. It's a solid metal at room temperature, but it melts at a paltry 30 degrees Celcius, 85 degrees Fahrenheit. It melts in your mouth and in your hand.

Not only did Mendeleev completely nail gallium, he predicted other elements that were unknown at the time: scandium, germanium, rhenium. The element he called eka-manganese is now called technetium. Technetium is so rare it couldn't be isolated until it was synthesized in a cyclotron in 1937, almost 70 years after Dmitri predicted its existence, 30 years after he died. Dmitri died without a Nobel Prize in 1907, but he wound up receiving a much more exclusive honor. In 1955, scientists at UC Berkeley successfully created 17 atoms of a previously undiscovered element. This element filled an empty spot in the periodic table at number 101, and was officially named Mendelevium in 1963. There have been well over 800 Nobel Prize winners, but only 15 scientists have an element named after them. So the next time you stare at a periodic table, whether it's on the wall of a university classroom or on a five-dollar coffee mug, Dmitri Mendeleev, the architect of the periodic table, will be staring back.

Credit: Lou Serico, TED-Eed

As mentioned in the video the true power of Mendeleev’s periodic table was the predictive ability of his table. This concept is at the heart of science. Scientists cannot just model behavior, but are required to make predictions, which later can be verified or refuted, thus, providing a test for the validity of their model or theories. It is interesting to note that Mendeleev’s work in the 1870s preceded the discovery of the atom which occurred with J.J. Thompson’s discovery of the electron in 1897 and the later work on the nucleus after 1900.

To Read

Now proceed to your e-textbook and finish reading this lesson’s reading assignment (pages 47 to 64 in Chapter 3 of Materials for Today's World, Custom Edition for Penn State University). Please proceed to the next webpage when you have completed this reading assignment.

Bonding and Bonding Type - Material Correlations

As you've recently read, there are four principal bonding types: ionic, covalent, metallic, and van der Waals. Ionic bonding involves the exchange of electrons between atoms to complete shells, either by adding or giving up electrons. The resulting atoms are oppositely charged and attract each other, resulting in an ionic bond. Covalently bonded materials have bonds in which electrons are shared between atoms. In metallic bonding, a "sea of electrons" is uniformly distributed throughout the solid and acts as a glue to hold the atoms together. Van der Waals bonds are relatively weak compared to the other three principal bond types and result when attractive forces from permanent or induced dipoles form.

diagram showing the 4 bonding types as discussed in the text above
Bonding tetrahedron.
Credit: Callister

In addition, the reading noted a correlation between materials classification and bonding time. Ionic bonding is associated with ceramics, covalent bonding is associated with polymers, metallic bonding is associated with metals, and van der Waals bonding is associated with molecular solids. As we study materials in further detail in this course we will utilize these associations to explain observed materials properties in the different materials classifications. Before we proceed to this lesson’s video assignment, there are a couple of more topics that I would like to address. Your textbook highlighted water as a material of importance and its volume expansion upon freezing. We will explore this topic further in the next section.

Water (Its Volume Expansion Upon Freezing)

Water is an extremely important molecule for life as we know it. An uncommon property that water possesses is the fact that frozen water (ice) is less dense than liquid water. This effect occurs due to the structure that occurs when water is cooled to form ice. The following video (3:55) takes a lighthearted approach to explain why ice floats.

To Watch

TED talk: Why Does Ice Float in Water?
Click for transcript of Why Does Ice Float in Water?

Water is the liquid of life. We drink it, we bathe in it, we farm, cook, and clean with it. It's the most abundant molecule in our bodies. In fact, every life form we know of would die without it. But most importantly, without water, we wouldn't have iced tea. Mmmm, iced tea.

Why do these ice cubes float? If these were cubes of solid argon in a cup of liquid argon, they would sink. And the same goes for most other substances. But solid water, a.k.a. ice, is somehow less dense than liquid water. How's that possible?

You already know that every water molecule is made up of two hydrogen atoms bonded to one oxygen atom. Let's look at a few of the molecules in a drop of water, and let's say the temperature is 25 degrees Celcius. The molecules are bending, stretching, spinning, and moving through space. Now, let's lower the temperature, which will reduce the amount of kinetic energy each of these molecules has so they'll bend, stretch, spin, and move less. And that means that on average, they'll take up less space.

Now, you'd think that as the liquid water starts to freeze, the molecules would just pack together more and more closely, but that's not what happens. Water has a special kind of interaction between molecules that most other substances don't have, and it's called a hydrogen bond. Now, remember that in a covalent bond two electrons are shared, usually unequally, between atoms. In a hydrogen bond, a hydrogen atom is shared, also unequally, between atoms. One hydrogen bond looks like this. Two look like this. Here's three and four and five, six, seven, eight, nine, ten, eleven, twelve, I could go on. In a single drop of water, hydrogen bonds form extended networks between hundreds, thousands, millions, billions, trillions of molecules, and these bonds are constantly breaking and reforming.

Now, back to our water as it cools down. Above 4 degrees Celcius, the kinetic energy of the water molecules keeps their interactions with each other short. Hydrogen bonds form and break like high school relationships, that is to say, quickly. But below 4 degrees, the kinetic energy of the water molecules starts to fall below the energy of the hydrogen bonds. So, hydrogen bonds form much more frequently than they break and beautiful structures start to emerge from the chaos.

This is what solid water, ice, looks like on the molecular level. Notice that the ordered, hexagonal structure is less dense than the disordered structure of liquid water. And you know that if an object is less dense than the fluid it's in, it will float. So, ice floats on water, so what? Well, let's consider a world without floating ice. The coldest part of the ocean would be the pitch-black ocean floor, once frozen, always frozen. Forget lobster rolls since crustaceans would lose their habitats, or sushi since kelp forests wouldn't grow. What would Canadian kids do in winter without pond hockey or ice fishing? And forget James Cameron's Oscar because the Titanic totally would have made it. Say goodbye to the white polar ice caps reflecting sunlight that would otherwise bake the planet. In fact, forget the oceans as we know them, which at over 70% of the Earth's surface area, regulate the atmosphere of the whole planet. But worst of all, there would be no iced tea. Mmmmm, iced tea.

Credit: George Zaidan and Charles Morton, TED-Ed

Now that you have watched this video, please proceed to the next section which highlights van der Waals forces and the gecko’s ability to walk on ceilings.

How Do Geckos Defy Gravity?

Close-up of gecko from underneath
Gecko feet
Credit: Magen McCrarey via Flickr [17]

Please watch the following video (4:29) which explains how geckos use van der Waals forces to walk on ceilings. While watching this video, see if you can answer the following question: how is the gecko’s ability to walk on ceilings an example of nanomaterials?

To Watch

TED talk: How do Geckos Defy Gravity?
Click for the transcript of How do Geckos Defy Gravity?

It's midnight and all is still, except for the soft skittering of a gecko hunting a spider. Geckos seem to defy gravity, scaling vertical surfaces and walking upside down without claws, adhesive glues or super-powered spiderwebs. Instead, they take advantage of a simple principle: that positive and negative charges attract. That attraction binds together compounds, like table salt, which is made of positively charged sodium ions stuck to negatively charged chloride ions. But a gecko's feet aren't charged and neither are the surfaces they're walking on. So, what makes them stick?

The answer lies in a clever combination of intermolecular forces and structural engineering. All the elements in the periodic table have a different affinity for electrons. Elements like oxygen and fluorine really, really want electrons, while elements like hydrogen and lithium don't attract them as strongly. An atom's relative greed for electrons is called its electronegativity. Electrons are moving around all the time and can easily relocate to wherever they're wanted most. So when there are atoms with different electronegativities in the same molecule, the molecule's cloud of electrons gets pulled towards the more electronegative atom. That creates a thin spot in the electron cloud where positive charge from the atomic nuclei shines through, as well as a negatively charged lump of electrons somewhere else. So the molecule itself isn't charged, but it does have positively and negatively charged patches.

These patchy charges can attract neighboring molecules to each other. They'll line up so that the positive spots on one are next to the negative spots on the other. There doesn't even have to be a strongly electronegative atom to create these attractive forces. Electrons are always on the move, and sometimes they pile up temporarily in one spot. That flicker of charge is enough to attract molecules to each other. Such interactions between uncharged molecules are called van der Waals forces. They're not as strong as the interactions between charged particles, but if you have enough of them, they can really add up.

That's the gecko's secret. Gecko toes are padded with flexible ridges. Those ridges are covered in tiny hair-like structures, much thinner than a human hair, called setae. And each of the setae is covered in even tinier bristles called spatulae. Their tiny spatula-like shape is perfect for what the gecko needs them to do: stick and release on command. When the gecko unfurls its flexible toes onto the ceiling, the spatulae hit at the perfect angle for the van der Waals force to engage. The spatulae flatten, creating lots of surface area for their positively and negatively charged patches to find complimentary patches on the ceiling. Each spatula only contributes a minuscule amount of that van der Waals stickiness. But a gecko has about two billion of them, creating enough combined force to support its weight. In fact, the whole gecko could dangle from a single one of its toes. That super stickiness can be broken, though, by changing the angle just a little bit. So, the gecko can peel its foot back off, scurrying towards a meal or away from a predator.

This strategy, using a forest of specially shaped bristles to maximize the van der Waals forces between ordinary molecules has inspired man-made materials designed to imitate the gecko's amazing adhesive ability. Artificial versions aren't as strong as gecko toes quite yet, but they're good enough to allow a full-grown man to climb 25 feet up a glass wall. In fact, our gecko's prey is also using van der Waals forces to stick to the ceiling. So, the gecko peels up its toes and the chase is back on.

Credit: Eleanor Nelsen, TED-Ed

So now that you have watched the video, can you see how this is an example of nanomaterials? A nanomaterial can utilize size and structure to perform unique abilities. The gecko utilizes van der Waals forces which operate on the scale of nanometers. In addition, the gecko utilizes the unique geometry of its feet to adhere to and release from surfaces. This is an example of using structure (or geometry) to perform a unique ability. At this time, please proceed to the lesson’s video assignment.

Video Assignment: Hunting the Elements

Video Assignment

Now please go to Lesson 3 in Canvas and watch chapters 3, 4, 5, 6, and 7 from the NOVA "Hunting the Elements" documentary. You will be quizzed on the content of these videos.

Summary and Final Tasks

Summary

Presented in this lesson were several fundamental and important concepts—namely, atomic structure, electron configurations in atoms and the periodic table, and the various types of inter-atomic bonds that hold together the atoms that compose a solid. The various types of atomic bonding, which are determined by the electron structures of the individual atoms, along with geometric atomic arrangements can determine some of the important properties of solid materials. Later in the course, we will move to the next level of the structure of materials, specifically, to some of the geometric atomic arrangements that may be assumed by atoms in the solid state.

Reminder - Complete all of the Lesson 3 tasks!

You have reached the end of Lesson 3! Double-check the to-do list on the Lesson 3 Overview page to make sure you have completed all of the activities listed there before you begin Lesson 4.

Lesson 4: Mechanical Properties

Overview

Many materials are subjected to forces or loads when in use. In such situations, it is necessary to know the characteristics of the material and to design the member in order to avoid failure during the expected life and service environment of the material. Key mechanical design properties are stiffness, strength, hardness, ductility, and toughness. Factors to be considered include the nature of the applied load and its duration, as well as the environmental conditions. The applied loads could be tensile, compressive, or shear and their magnitudes may be constant with time or may fluctuate continuously. Application time may be only a fraction of a second, or it may extend over a period of many years. Service temperature may be an important factor. In this lesson, we will introduce how the various mechanical properties are measured and what these properties represent.

Learning Objectives

By the end of this lesson, you should be able to:

  • Define stress and strain.
  • Define elastic region, plastic region, fracture point, elasticity, tensile strength, and the yield strength.
  • Given a stress-strain diagram, determine elastic region, plastic region, fracture point, elasticity, tensile strength, and the yield strength.
  • Distinguish between tensile, compressive, shear, and torsional stress.
  • Define hardness, resilience, and toughness.
  • Evaluate whether a material is ductile or brittle using a stress-strain diagram.

Lesson Roadmap

Lesson 4 will take us 1 week to complete. Please refer to Canvas for specific due dates.

Lesson Roadmap
To Read

Read pp 66-98 (Ch. 4) in Introduction to Materials ebook

Reading on course website for Lesson 4

To Watch Making Stuff: Stronger
To Do Lesson 4 Quiz

Questions?

If you have general questions about the course content or structure, please post them to the General Questions and Discussion forum in Canvas. If your question is of a more personal nature, feel free to send a message to the instructor through Canvas email. The instructor will check daily to respond.

Reading Assignment

Things to consider...

When you read the web material for this lesson and e-book material for this lesson, use the following questions to guide your reading. Also, remember to keep the learning objectives listed on the Overview page in mind.

  • What is the difference between stress and strain?
  • Given a stress-strain diagram for material, how can one identify the elastic region, plastic region, fracture point, elasticity, and the yield strength?
  • Given a stress-strain diagram for a material, how can you determine if the material is ductile or brittle?
  • How do tensile, compressive, and shear stress differ?
  • Do the terms stiffness, strength, hardness, ductility, and toughness mean the same to the general public as they do to materials scientists and engineers?

Reading Assignment

Read pp 66-98 (Ch. 4) in Introduction to Materials ebook

Tensile, Compressive, Shear, and Torsional Stress

Mechanical property terms listed such as: ductile, shear, elastic, stress, strain, and plastic.
Mechanical Property Terms
Credit: Ron Redwing

As we can see in the above graphic, there are quite a few materials terms that are used when describing the properties of materials. In this lesson, we are going to define the above terms. It turns out that many of the above terms are related to the stress-strain curve of a material. What are stress and strain, and how are they related?

Let us take a cylinder and stress it. To stress it, I would fix one end of the cylinder and pull from the other end as shown in the figure below.

The cylinder gets longer and narrower as force pulls it from either end.
Tensile Stress
Credit: Callister

According to Newton's third law, the cylinder will experience a force downward on the lower surface of the cylinder and an equal and opposite force on the upper surface of the cylinder. My cylinder has an original length of Io and surface area of Ao. As I pull on my material with the force F the cylinder will lengthen and the resulting length will be l. Stress, σ, is defined as the force divided by the initial surface area, σ=F/Ao. This pulling stress is called tensile stress. Strain is what results from this stress. Strain, ε, is defined as the change in length divided by the original length, ε=ΔI/Io. Before we proceed further with stress and strain, let's define some other types of stress. 

If instead of pulling on our material, we push or compress our cylinder we are introducing compressive stress. This is illustrated in the following figure:

The cylinder gets shorter and wider as force pushes it from either end.
Compressive Stress
Credit: Callister

If instead of applying a force perpendicular to the surface, we apply parallel but opposite forces on the two surfaces we are applying a shear stress. This is illustrated in the following figure:

A cube undergoing sheer stress. The face of cube becomes more of a rhombus.
Shear Stress
Credit: Callister

Stress related to shear is torsional stress. If we hold one end of our cylinder fixed and twist the other end as shown in the figure below, we are applying a torsional (or twisting) stress.

A cylinder twisting under torsional stress
Torsional Stress
Credit: Callister

Examples of Materials Under Stress

A ski lift with three areas highlighted.
Ski lift.
Credit: P.M. Anderson

If we look at a picture of a ski lift, we can see several different types of stress. The cable, highlighted in the box labeled A, is subject to tensile stress. The driveshaft, highlighted in the box labeled B, is experiencing torsional stress. The support pillar, highlighted in box labeled C, is subject to compressional stress. In the two figures below, the boulder is applying a compressive stress on the rock that is supporting it and the metal struts of the bridge are experiencing compressive stress while supporting the upper structure of the bridge.

First image shows a bolder on top of a thinner pillar of rock. The 2nd image shows an arch bridge with vertical struts.
Compressive stress in boulders and bridge.
Credit: P.M. Anderson

Stress-Strain Testing

The specimen is situatated in between a load cell and a moving crosshead, with an extensometer attached. The sample is thicker at the ends & thinner in the middle.
Testing stress-strain.
Credit: Callister & Rethwisch 5e. 

A typical stress-strain testing apparatus is shown in the figure above, along with the typical geometry of a tensile test specimen. During a tensile test, the sample is slowly pulled while the resulting change in length and the applied force are recorded. Using the original length and surface area a stress-strain diagram can be generated.

To Read

Now that I have introduced stress, please go to your e-textbook and read the first two sections (pages 65 to 70 in Chapter 4 of Materials for Today's World, Custom Edition for Penn State University) of this lesson's reading. When finished with the reading proceed to the next web page.

Elastic Region

What is the elastic region? It is the region where the material can be deformed and when released will return back to its original configuration. Many metals in the elastic region have a resulting strain that is proportional to the tensile load when the applied tensile load is small. Mathematically, this can be written as σ=Eε , and more generally is known as a form of Hooke's law. E is the proportionality constant and is called the modulus of elasticity or Young's modulus. Physically, the larger the value of the modulus of elasticity the stiffer the material is, i.e., the more resistant to bending the material is. If we look at a stress-strain diagram for a metal in the elastic region such as that shown in the figure below, the slope of the curve is the modulus of elasticity.

Stress-strain diagram. Epsilon on the x axis and sigma on the y axis. Line through the origin labeled linear-elastic
Stress-Strain Diagram
Credit: Callister

If we look at the figure below it is not surprising that the material listed with the highest E is diamond. Diamond has strong carbon bonds and is incredibly stiff. Larger E indicates a stronger bond. Later when we study composites in more detail, we will see that fibers are added to polymers to increase the stiffness of the material. Increased stiffness implies increased E, which you can see in the figure for the composite/fiber materials.

E value: Metal Alloys (40-400), Graphite, Cermaics & Semiconductors (10-1200), polymers (0.2-4), Composites/fibers (.6-400)
Room-temperature elastic moduli for various materials
Credit: Callister

To Read

Now that you have been introduced to elasticity, please go to your e-textbook and read section 7.3 (pages 71 to 74 in Chapter 4 of Materials for Today's World, Custom Edition for Penn State University) of this lesson's reading. When finished with the reading proceed to the next web page.

Plastic Deformation

For most metallic materials, the elastic deformation region is relatively small. At some point, the strain is no longer proportional to the applied stress. At this point, bonds with original atom neighbors start to break and reform with a new group of atoms. When this occurs and the stress is relieved, the material will no longer return to its original form, i.e., the deformation is permanent and nonrecoverable. The material has now moved into the region referred to as plastic deformation. In practice, it is difficult to identify the exact point at which a material moves from the elastic region to the plastic region. As shown in the figure below, a parallel line offset by 0.002 strain is drawn. Where that line intercepts the stress-strain curve is identified as the yield strength. The yield strength is equal to the stress at which noticeable plastic deformation has occurred.

elastic deformation(linear), plastic region(curves). parallel slope 2 elastic deformation. Distance between x intercepts is plastic strain
Stress-strain curve.
Credit: Adapted from Figure 7.10(a), Callister & Rethwisch 5e.

For many materials, the stress-strain curve looks like the curve shown in the figure below. As the stress is increased from zero, the strain increases linearly until it starts to deviate from linear at the yield strength. For increasing stress, the curve proceeds to a maximum at which point it curves downward toward the fracture point. The maximum corresponds to the tensile strength, which is the maximum stress value for the curve and is indicated by M in the figure. The fracture point is the point at which the material ultimately breaks, indicated by F in the figure.

Tensile strength equals maximum stress on engineering stress-strain curve. See text above for more details.
Maximum on stress-strain curve appears at the onset of noticeable necking.
Credit: Adapted from Fig. 7.11 Callister & Rethwisch 5e.

Resiliency and Toughness

When a person is resilient, we mean that they bounce back from change to their original personality. Resiliency in the material sense is similar. We can define resilience of the material to be the amount of energy the material can absorb and still return to its original state. If we are talking about stressing the material and having it return to its original state, we are talking about the material remaining in the elastic region of the stress-strain curve. It turns out that we can get the energy of elasticity by taking the area under the curve of the stress-strain curve. That area has been highlighted in the figure below, which is the area under the curve from the origin to the yield strength.

Stress-strain curve. See text above for description. Strain on the x-axis, Stress on the y-axis. Resiliency between the curve sigma and epsilon
Energy of elasticity shown under the curve of the stress-strain curve. 
Credit: Callister

Toughness, in contrast to resilience, is how much energy can be absorbed and still keep going. One analogy that can be used when describing toughness is that of a car in a demolition derby. The car is allowed to continue the competition as long as it is capable of moving. It does not matter how many hits and how much destruction has been done to the car, but rather as long as the car can move it can stay in the competition. The toughness of the car is based on how many hits and how much damage the car can sustain and continue in the competition. In the case of materials, the amount of energy that the material can absorb plastically before fracturing is the toughness.

In the figure below, we can see that a material can have a high tensile strength (ceramics) and yet have a small toughness. In addition, materials can be extremely ductile (unreinforced polymers) and also have a small toughness. So, a large toughness (metals) is obtained by having a high tensile strength and a high ductility.

Figure showing small(cermaics), large(metals), and very small (unreinforced polymers) toughness. See text above.
Stress-strain curve for a material with a high tensile strength (ceramics) and a small toughness.

What is a Brittle Material?

Brittle material breaks while little to no energy is absorbed when stressed. The material fractures with no plastic deformation. The material in the figure below marked with (a) shows what a brittle material will look like after pulling on a cylinder of that material. Typically, there will be a large audible snap sound when the brittle material breaks. A brittle material is also known as a material having low ductility. A stress-strain curve for brittle and ductile materials is shown in the figure below. We will talk more about ductile materials in the next section.

 1.	Brittle material break: straight &jagged. 2. Stress-strain curve, brittle: low strain, ductile material: high strain. both high stress
Stress-strain curve for brittle and ductile materials.
Credit: Image on left: Sigmund via Wikipedia [18], Image on right: Amgreen via Wikipedia [19]

You may be asking: why are ceramics so much more brittle than metals? It has to do with the type of bonding. In metals, their metallic bonds allow the atoms to slide past each other easily. In ceramics, due to their ionic bonds, there is a resistance to the sliding. Since in ionic bonding every other atom is of opposite charge when a row of atoms attempts to slide past another row, positive atoms encounter positive atoms and negative atoms encounter negative atoms. This results in a huge electrodynamic repulsion which inhibits rows of ceramic atoms from sliding past other rows. In metals, the sliding of rows of atoms results in slip, which allows the metal to deform plastically instead of fracturing. Since in ceramics the rows cannot slide, the ceramic cannot plastically deform. Instead, it fractures, which makes it a brittle material.

Malleability and Ductility

Malleability and ductility are related. A malleable material is one in which a thin sheet can be easily formed by hammering or rolling. In other words, the material has the ability to deform under compressive stress.

Malleability: Metal hammered into thin sheets, shows hammer hitting gold to make thin circle
A malleable material is one in which a thin sheet can be easily formed by hammering. Gold is the most malleable metal.
Credit: Buzzle

In contrast, ductility is the ability of a solid material to deform under tensile stress. Practically, a ductile material is a material that can easily be stretched into a wire when pulled as shown in the figure below. Recall pulling is applying tensile stress.

Ductility text showing the starting point, ductility, (how far it can stretch) and the end point, very tapered in the middle.
Ductility test.
Credit: Civil Engineering [20]

If we pull on a rod of material, some of the possible profiles of the rods at fracture are shown in the figure below.

Breaking profiles: A) Snap w/ jagged edges. B) some contour and pulling then snap, C)pull all the way into smooth contours and points
Fracture Samples
Credit: Sigmund (Own work) [CC BY-SA 3.0], via Wikimedia Commons

Profile (a) is an example of the material that fractures with no plastic deformation, i.e., it is a brittle material. Profile (b) is an example of a material that fractures after very little plastic deformation. These two profiles would be classified as having low ductility. Profile (c) in contrast is a material that plastically deforms before fracture. This material has high ductility. The stress-strain curves for the brittle, profile (a), and the ductile material, profile (c), are shown in the figure below.

brittle materials have short and tall curves. Ductile materials rise quickly but level out for a long time
Ductile and Brittle Stress-Strain Curves
Credit: Amgreen (Own work) [CC BY-SA 3.0], via Wikimedia Commons

To Read

Now that you have learned a bit about the mechanical behavior of metals, please go to your e-textbook and read pages 75 to 84 in Chapter 4 of Materials for Today's World, Custom Edition for Penn State University to learn more about this subject. When finished with the reading proceed to the next web page.

Mechanical Behavior of Ceramics

It is difficult to measure the yield strength of ceramics as they tend to fracture before they enter the plastic deformation region, i.e., they are brittle. Examples of two brittle materials that fracture before entering the plastic deformation region are aluminum oxide and glass, as shown in the figure below.

Stress (y-axis) and Strain (X-axis) Diagram. Aluminum oxide (0.0007, 240MPa) line is steeper than glass (0.0009, 55MPa)
Stress-Strain curves for two brittle materials.
Credit: Callister & Rethwisch

Tensile tests of brittle ceramics are usually not performed. It is difficult to shape these materials into the proper test structure, difficult to grab the brittle material without breaking it, and it is difficult to align the test samples to avoid bending stresses which can destroy the sample. For brittle ceramics, a three-point bending apparatus (shown in the figure below) is used determine the stress-strain behavior, and the measurement results are used to calculate an equivalent modulus of elasticity.

Three-point bending apparatus. Push up at both ends and down in the middle on the test material
Three-point bending apparatus used determine stress-strain behavior.
Credit: Adapted from Fig. 7.18, Callister & Rethwisch

To Read

Now that you have been introduced to the mechanical behavior of ceramics, please go to your e-textbook and read more on this topic on pages 84 to 86 in Chapter 4 of Materials for Today's World, Custom Edition for Penn State University. When finished with the reading proceed to the next web page.

Mechanical Behavior of Polymers

Polymers exhibit a wide range of stress-strain behaviors as shown in the figure below. The brittle polymer (red curve) elastically deforms and fractures before deforming plastically. The blue curve is a plastic polymer and is similar to curves for many metals. Its behavior begins in the linear elastic deformation region. As the curve transitions from the elastic to plastic deformation typically there is a peak stress. For polymer materials, this peak stress is identified as the yield stress. As the material is pulled further, fracture occurs. The stress value when fracture occurs is defined as the tensile strength for polymer materials. The tensile strength can be greater than, equal to, or less than the yield strength. The green curve is a class of polymers known as elastomers. These materials exhibit rubber-like elasticity and will return to their original shape and form unless they are extended to the point of fracture.

Polymer stress strain. See text above. Red: linear, steep. Blue: linear, steep, peaks and flattens out lower than red. Green: Slow increase
Mechanical properties of polymers: stress-strain behavior.
Credit: Adapted from Fig. 7.22, Callister & Rethwisch 5e.

While some of the stress-strain curves for polymers might look similar to ones for metals, polymers are mechanically different than metals (or ceramics). A highly elastic polymer may stretch over 10 times the original length before breaking, while a metal might elastically stretch 10% of the original length elastically and may stretch plastically to double the original length before reaching its fracture point. As seen in the figure below, the largest elastic modulus values for polymers are well under the values for ceramics and metals.

E value: Metal Alloys (40-400), Graphite, Cermaics & Semiconductors (10-1200), polymers (0.2-4), Composites/fibers (.6-400)
Room-Temperature Elastic Moduli for Various Materials
Credit: Callister

As shown in the figure below, the tensile strength of some polymers can rival some ceramics but are no match for even the softest of metals.

Tensile Strength (MPa): Metal Alloys (110-2000), Cermaics & Semiconductors (10-1000), polymers (6-100), Composites/fibers (2-5000)
Tensile strengths of metals/alloys, ceramics, polymers, and composites/fibers.
Credit: Callister & Rethwisch 5e

To Read

Now that you have learned a bit about the mechanical behavior of plastics, please go to your e-textbook and read pages 87 to 89 in Chapter 4 of Materials for Today's World, Custom Edition for Penn State University to learn more about this subject. When finished with the reading proceed to the next web page.

Hardness

Hardness is a measure of a material's ability to resist plastic deformation. In other words, it is a measure of how resistant material is to denting or scratching. Diamond, for example, is a very hard material. It is extremely difficult to dent or scratch a diamond. In contrast, it is very easy to scratch or dent most plastics. As shown in the diagram below, hardness increases from the very soft plastics to the incredibly hard diamond with most other materials ranging between.

Increasing hardness soft to hard: plastics, brasses Al alloys, easy to machine steels, file hard, cutting tools, nitrided steels, diamond
Hardness increases from very soft plastics to the hard diamond with most other materials ranging between.
Credit: Callister

A common method for measuring the hardness of a material is outlined in the figure below. A very hard-sphere is pushed with a set force into the material. The resulting indent is measured for width and depth. A harder material will have a smaller width and depth, i.e., smaller indentation. Larger hardness results in a high resistance to deformation from compressive loads, i.e., resistance to scratches and dents, and better wear properties.

Measurement of hardness of materials. See text above for description. Apply known force, measure indent
A common method for measuring the hardness of a material.
Credit: Callister

To Read

Now that you have been introduced to the concept of hardness, please go to your e-textbook and finish the reading for this chapter (pages 90 to 97 in Chapter 4 of Materials for Today's World, Custom Edition for Penn State University). When finished with the reading proceed to the next web page.

Video Assignment: Making Stuff Stronger

Now that you have read the text and thought about the questions I posed, take some time to watch this 53-minute video about trying to find the strongest materials in the world. As you watch this video please pay particular attention to: (1) the ways that materials can be made stronger, (2) how stronger materials can be made lighter, cheaper, or better in other ways, and (3) how new stronger materials are designed for specific applications.

Video Assignment

Go to Lesson 4 in Canvas and watch NOVA's Making Stuff: Stronger Video. You will be quizzed on the content of this video.

Summary and Final Tasks

Summary

In this lesson we discussed the stress–strain behaviors of metals, ceramics, and polymers and the related mechanical properties. Understanding and classifying the properties of materials allow us to design, produce, and utilize materials more efficiently and productively. In some cases, understanding materials allow us to utilize them for new applications. Lesson 4 provides us a language to discuss and compare different materials, while the previous lessons on the classification of materials and atomic structure along with upcoming lessons on the structure of materials will inform us regarding how geometric atomic arrangements and atomic structure can affect materials properties. In the next lesson, we will study how metal atoms arrange to form solids and some of the applications of metals.

Reminder - Complete all of the Lesson 4 tasks!

You have reached the end of Lesson 4! Double-check the to-do list on the Overview page to make sure you have completed all of the activities listed there before you begin Lesson 5.

Lesson 5: Structure and Applications of Metals

Overview

The crystal structure of a material can directly affect their properties. For example, gold and silver which share a common crystal structure are much less brittle than the metals beryllium and magnesium which possess a different crystal structure. Also, crystalline and noncrystalline materials of the same composition can possess significantly differing properties. In this lesson, we will discuss how structure can affect materials properties and also introduce imperfections, which can have major impacts on the properties of materials.

Learning Objectives

By the end of this lesson, you should be able to:

  • List and explain the contributions to material processing made by the Egyptians.
  • Explain the difference between melting and smelting.
  • Distinguish between single crystals and polycrystalline materials.
  • Describe the difference in atomic/molecular structure between crystalline and non-crystalline materials.
  • Draw unit cells for face-centered cubic, body-centered cubic, and hexagonal close-packed crystal structures.
  • Define polymorphism and allotropy.
  • Sketch the three orthogonal crystal systems (cubic, tetragonal, orthorhombic) and the hexagonal crystal system with proper lattice parameter labels.
  • Define isotropy and anisotropy with respect to material properties.
  • List and describe the different types of imperfections in a crystal.

Lesson Roadmap

Lesson 5 will take us 1 week to complete. Please refer to Canvas for specific due dates.

Lesson Roadmap
To Read

Read pp 99-120 (Ch. 5) in Introduction to Materials ebook
Read pp 121-135 (Ch. 6) in Introduction to Materials ebook

Webpages on this site for Lesson 5

To Watch Metal: The Secret Life of Materials
To Do Lesson 5 Quiz

Questions?

If you have general questions about the course content or structure, please post them to the General Questions and Discussion forum in Canvas. If your question is of a more personal nature, feel free to send a message to the instructor through Canvas email. I will check each of these daily to respond.

Things to Consider...

While you read the material for this lesson in your e-book and on the course website, use the following questions to guide your learning. Also, remember to keep the learning objectives listed on the previous page in mind as you learn.

  • What contributions did the Egyptians make in the development of materials processing?
  • What is the difference between melting and smelting?
  • How do single crystal and polycrystalline material differ in grain structure?
  • How do crystal and polycrystalline materials differ in their atomic/molecular structure?
  • What is the difference between an amorphous and crystalline material?
  • What is the difference between crystal structure and a crystal system?

Copper (Chalcolithic) and Bronze Ages

In the Neolithic Age, which was the period at the end of the Stone Age, the Egyptians were experiencing increasing population along with extensive food production capabilities, several communities in similar stages of development, and an extensive trade network with other civilizations. As the Egyptians entered the Copper and Bronze Ages, their technological advancements in gold, copper, and bronze processing were aided by their access to key natural resources. The figure below from the British Museum shows known natural resources of ancient Egypt.

Ancient Egypt's natural resources including: limestone, basalt, alabaster, carnelian, natron, gold, granite, copper, tin, and malachite.
Natural resources of ancient Egypt.
Credit: The British Museum

In the following pages, we will discuss gold, copper, and bronze processing as developed by the Egyptians.

Egyptian Gold Processing

It is not surprising that gold was the first metal processed by the Egyptians. Very few metals are found in their native state, i.e., not bound to other elements in a compound such as a mineral. Copper is very rarely found in nature as an element and iron is typically only found as an element in some meteorites. Iron from meteorites was extremely rare in Egypt and was known as metal from the gods. Gold, however, is routinely found in nature as an element unlike copper and iron, and most other metallic elements. Gold, although rare, can be found as flakes or nuggets. As shown in the illustration below from an ancient Egyptian tomb, the Egyptians used charcoal and blow pipes to reach the temperatures needed to melt gold. Also, ‘slag’ (impurities) were skimmed off the molten gold.

Illustration from an ancient Egyptian tomb at Saqqara (2500 B.C.) showing the melting and beating of gold.
Ancient tomb illustration depicting gold processing.
Credit: Scielo

The molten gold was poured into molds to form jewelry and other items. In addition, the Egyptians were able to hammer gold into very thin (5 µm) leafs. Gold is a malleable material. Malleability is a material’s ability to be deformed under compressive stress, i.e., to form a thin sheet by hammering or rolling. A ductile material (ability to be deformed under tensile stress, i.e. can be pulled into a wire) has to be a malleable material as well, but malleable materials do not have to be a ductile material. An example of this is lead. Lead is malleable but when pulled to form a wire it pulls apart. As you can see malleability and ductility are closely related but do not possess the same definition in material science.

Man holding a tile with a piece of gold layed on top
Gold leaf.
Credit: Eckhard Pecher via Wikimedia Commons [21]

The Egyptians believed gold to be a divine material which held magical powers. Electrum is an alloy of gold which is approximately 80% gold mixed with 20% silver. An alloy is a mixture of metals or a mixture of a metal with small amounts of non-metals. We will discuss metal alloys in more detail in the next lesson. In the next section, we will discuss Egyptian copper processing.

Egyptian Copper Processing

Native copper occurs in a very limited supply, so the start of the Copper Age is marked by the discovery of smelting copper from its ores which allows for a ready supply of copper. The two basic naturally occurring copper (II) carbonate minerals are pictured below.

Malachite. Green/blue sparkly rock
Malachite
Credit: JJ Harrison via Wikimedia Commons [22]
Azurite. deep blue sparkly rock
Azurite
Credit: Eric Hunt via Wikimedia Commons [23]

Copper is a very malleable material, unlike flint, which at the beginning of the Copper Age was the dominant weapon and tool material. Although copper is soft it does have a significant advantage over flint. It can be repaired. Native Americans used native copper beginning circa 6000 BCE. As mentioned, the supply of native copper is very limited and its supply was easily exhausted. Copper ores, on the other hand, were readily available. However, to extract the copper from copper ores smelting was required.

What is smelting? Smelting is a process that uses heat and chemistry to drive off other elements such as gases or slag, leaving behind only the metal. Typically, ores are impure and require a flux to separate the metal from the slag. Flux is an additive used to change the impurities from a form that is inseparable from the metal to a form that is separable. For example, adding iron ore as a flux during the smelting of copper can transform the impurity solid silicon dioxide into an iron-silicon oxide. Unlike the solid silicon dioxide which remains in the liquid copper, the iron-silicon oxide floats to the top and can be skimmed off. Smelting is different than melting in that in melting you have to be able to raise the temperature to the melting point of the material. The Egyptians did not have the ability to reach the temperatures needed to melt the copper minerals outright.

How Did Egyptians Discover that Malachite and Azurite Contain Copper?

It is unknown exactly how the Egyptians discovered that malachite and azurite contain copper. Here are a couple of possibilities.

Egyptians used malachite as a pigment and cosmetic, including as a distinctive eyeliner. While a normal open fire would not reach the temperatures required to melt bulk malachite, in powder form it is possible that accidentally putting Malachite powder on the coals of the fire could produce small balls of copper.

A second, and more likely possibility centers around the Egyptians using malachite as a pottery glaze. Small balls of copper could have been formed in the pottery kiln during firing, and then noticed by kiln workers after cooling.

Egyptian Copper Smelting Process

An ancient Egyptian depiction of 3 men working foot bellows
Egyptian use of foot bellows.
Credit: The British Museum

The Egyptian copper smelting process utilized a ’bowl furnace’ which was supplied additional air, to raise the temperature of the fire, through the usage of foot bellows. Malachite and azurite were used as a source ore for the copper, charcoal was used as the reducing agent to separate oxygen from the copper, and iron ore was used as the ‘flux’ to bind and float away impurities. The advantage that copper has over bone, wood, or flint, as a tool or weapon, is that when damaged it can be repaired. However, like most pure metals, i.e., metals with a low level of impurities, copper is soft. It turns out that intentionally or unintentionally adding another soft metal in small amounts can make the host material stronger and harder. This is a process known as alloying, which we will be discussing further in the next lesson.

Now let’s go to the next section and look at the soft metal, tin, that the Egyptians added to copper to produce a more durable and harder metal.

Egyptian Smelting of Tin

Cassiterite, a grey, reflective rock with white, feathery, plate-like crystal structures growing off it.
Cassiterite
Credit: Carles Millan via Wikimedia Commons [24]

It is not known how exactly the Egyptians discovered the smelting of tin. In some ways, it is a bit surprising. Cassiterite, SnO2, the mineral used as the ore for extracting tin, is extremely difficult to find and not particularly noteworthy, i.e., it does not stand out in the field. It is hard and, like gold, has a high specific gravity. Having a high specific gravity means that flakes or nuggets of cassiterite, like gold, would settle to the bottom of a slurry if panning for gold. So, while it is difficult to find sources of cassiterite it might have been possible to backtrack upstream by finding cassiterite flakes downstream of sources.

While it might be counterintuitive to think that the Egyptians added an even softer metal, tin, to copper to make it harder, it is possible that the Egyptians thought that whatever made cassiterite hard would be transferred to the copper. The smelting of tin is very similar to the smelting of copper. Charcoal is also used as the reducing agent. Tin, unlike copper, is too soft for practical purposes. However, when it’s mixed with copper in small amounts, typically 5 – 10% tin, it can produce a much harder metal than unalloyed copper or tin. Please proceed to the next section to learn more about this new harder metal, bronze.

Egyptian Bronze Processing

two swords made during the bronze age
Swords of the Bronze Age.
Credit: Dbachmann via Wikimedia Commons

Bronze is an alloy of copper and tin. Tin is a slightly bigger atom than copper. In bronze, typically 5 – 10% is tin and the rest is copper. The slightly larger tin atoms replace copper atoms in the copper crystalline structure as shown in the figure below. We will learn more about metal alloys in this lesson and the next. Although copper and tin are both soft metals and not ideal for tools or weapons, the combination that produces bronze is much harder than copper or tin. As we will learn later, this is due to the larger tin atoms making it harder for rows of copper atoms to move. This results in bronze being harder.

repeating circles(Cu atoms) in rows in 1st image. In 2nd, larger circles representing tin are in the lattice, making copper atoms smoosh
Tin atoms forcing copper atoms together and restricting their movement.
Credit:"Earth, Air, Fire, and Water: Elements of Materials Science," 2nd Edition, by P.R. Howell.

In the practice of producing bronze, the Egyptians placed tin with copper ingots into clay crucibles. The clay crucibles were lowered into a charcoal fire which could exceed 1100 °C through the use of blowing air using foot bellows. The Egyptians would then stir, remove the slag, and pour the melt into a mold.

Why Did it Take So Long Between the Bronze Age and the Iron Age?

The beginning of the Bronze Age occurred around 3500 BCE and the beginning of the Iron Age began around 1000 BCE. Why did it take 2000 years for bronze to be replaced by iron? Looking around us we see structural steel and concrete seemingly everywhere in our modern cities. However, the processing of iron is not a trivial process.

Due to limitations in furnace designs, i.e., the maximum obtainable temperatures, the availability and quality of iron varied greatly. As we’ll see in the next lesson’s video, Secrets of the Viking Sword, throughout history there have been legendary quality swords, i.e., Damascus and Samurai to name just a couple. These swords were produced using time-intensive and, many times, ritualistic processes. These blades were produced in areas known in the modern day as Iran, Japan, and China. Most of the iron used in weapons during the Iron Age, i.e., Roman swords, was a low-density iron sponge-like material. This sponge-like iron was then pounded to shape, densify, and remove impurities. Bronze was superior to the iron produced commonly, so why did iron ultimately replace bronze?

Bronze weapons were indeed of higher quality than the common iron weapons typically produced. However, tin, which is required for the production of bronze, is not abundantly available. As a consequence, bronze weapons were the weapons utilized by nobles, royalty, pharaohs, etc. The common foot soldier was not going to possess bronze weapons; there were not enough to go around.

Unlike tin, iron ore is readily available. So, although inferior to bronze, an army of hundreds or thousands could be equipped with iron weapons, which was not practical with bronze weapons. So, the ability to produce large numbers of iron weapons overcame the advantages of bronze. Eventually, time and further development allowed for the production of these so-called legendary swords which supplanted bronze as the weapon material of choice for the nobility. But it wasn’t until much later, during the advent of the Industrial Revolution, that advancements in furnace design and process control enabled the reliable and massive production of the iron alloy known as steel. In this lesson’s video, the importance of steel and how the production of steel was changed during the beginning of the Industrial Revolution will be showcased. We will return to this topic at the beginning of the next lesson on metal alloys.

Now, let’s take a step back from our discussion on the historical development of metal processing and begin an introduction to the structure of metals.

Structure and Application of Metals

When you mention crystal to most people, they think of fine glassware. Metal is not the first thing that comes to mind. But, in fact, most metals are crystalline, and it is rather difficult to make noncrystalline metals. Crystalline materials have their atoms arranged in a periodic, ordered 3D array. Typically, all of the metals, many ceramics, and some polymers are crystalline. Noncrystalline materials have atoms with no periodic arrangement, i.e., a random order. Noncrystalline material can result when you have complex structures or you rapidly cool from the liquid state to the solid state. Amorphous material is another name for noncrystalline material.

Why do metals form crystals? It turns out that the lowest energy for metal atoms occurs when the atoms are packed together as tightly as possible. If you’ve ever tried to put many small pieces into a large box, you know that if you put the pieces in the box in an ordered fashion you can fit much more in the box than if you just throw things into the box in a disorderly fashion. So, for metals, ordered structures tend to be nearer the minimum energy and are more stable. In addition, since metallic bonds are nondirectional it is much simpler for metal atoms to densely pack than it is for ceramics and polymers. So how do metal atoms pack together? In the next section, we will look at one of the ways that metal atoms pack together.

Simple Cubic Crystal Structure

Start by taking four atoms and arranging them in a square. Then take four more atoms and arrange them in a square. Then put the first square on the second square to form a cube with eight atoms, one at each corner. This structure is the simple cubic crystal structure. It turns out that only the metal Polonium (Po) has this crystal structure. The reason this crystal structure is so rare is that packing atoms in this way does not lead to a very high packing density. In the next section, we will add one atom to the simple crystal structure and produce a crystal structure that is much more common.

cube with spheres at the corners
Simple cubic crystal structure.
Credit: Callister

Body Centered Cubic Structure (BCC)

Let's take our simple cubic crystal structure of eight atoms from the last section and insert another atom in the center of the cube. This new structure, shown in the figure below, is referred to as body-centered cubic since it has an atom centered in the body of the cube. Some examples of metals that possess this crystalline structure include the α phase of iron, chromium, tungsten, tantalum, and molybdenum.

A cube with spheres at the corners and one in the middle. Inside the cube there is one full atom and each corner has 1/4 atom
Body-Centered Cubic Structure
Credit: Callister & Rethwisch 5e

Face Centered Cubic Structure (FCC)

If, instead of starting with a square, we start with a triangle and continue to add atoms, packing as tightly as we can, we will end up with a layer of atoms as shown in the figure below.

circles in a planar design labeled A
First layer of hexagonal structure
Credit: Callister & Rethwisch 5e

Now let me put an atom on top of that first layer over one of the 'B' positions and let it rest down into one of the valleys. I can now place two more atoms in nearby 'B' positions so that each will rest in their own valley in such a way that all three atoms will touch and form a triangle. Now let me add more atoms to the second layer, packing them in as tightly as possible. These two layers are shown in the figure below. If you look closely, you should be able to see that the second layer only covers half of the valleys produced by the first layer. The 'C' valleys are left uncovered. In fact, half of the valleys of the second layer lineup with the unoccupied 'C' valleys of the first layer.

Layer of Atoms (B) Placed on the spaces made by layer A
First and second layer of hexagonal structure
Credit: Callister & Rethwisch 5e

Now let’s put a third layer where the atoms are placed where the unoccupied valleys of the first two layers lineup, the 'C' valleys. It is a little difficult to visualize, but if one of the top layer atoms is one corner of our cube and that corner is pointing out then we obtain the cube shown in the figure below.

Layer of Atoms (C) Placed on the spaces made by layer B. Three layers repeat and make a hexagonal design
Complete three layer hexagonal structure
Credit: Callister & Rethwisch 5e

This crystal structure is known as face-centered cubic and has atoms at each corner of the cube and six atoms at each face of the cube. It is shown in the figure below. This structure, as well as the next structure we are going to discuss, has the atoms packed as tightly as theoretically possible. Metals that possess face-centered cubic structure include copper, aluminum, silver, and gold.

cube with an atom on every corner and every face. Inside the cube each face has 1/2 atom and on each corner has 1/4 atom
Face centered cubic (fcc) structure
Credit: Callister & Rethwisch 5e

In the next section, we will discuss our fourth and last crystal structure.

Hexagonal Close Packed Crystal Structure (HCP)

If you look at the figure below, you might think that hexagon close-packed crystal structure is more complicated than face-centered cubic crystal structure. In fact, it is a simpler structure.

Think back to the last section where we constructed first one layer of atoms and then a second layer of atoms for face-centered cubic structure. Now, for hexagonal close-packed crystal structure, we do not construct a third layer. Instead, the third layer is simply the first layer repeated, the fourth layer is the second layer repeated, and so on and so on as shown in the figure below.

ABABABA Structure
Hexagonal close-packed structure
Credit: Callister & Rethwisch 5e

It turns out that face-centered cubic and hexagonal close-packed crystal structures pack atoms equally tightly. Some metals with hexagonal close-packed crystal structures include cobalt, cadmium, zinc, and the α phase of titanium. A more typical representation of the hexagonal close-packed structure is shown in the figure below. In this representation a hexagon on the top and on the bottom sandwich a triangle in between the two hexagons.

Hexagons sandwiching a triangle
Hexagonal close-packed structure unit cell
Credit: Callister & Rethwisch 5e

Please proceed to your e-textbook and read the first chapter of this lesson’s assigned reading. Please return back to this website after completing that reading.

Reading Assignment 1

Now please proceed to the first reading assignment (shown below) from your e-book. After you have completed that reading please return to this page and continue the web reading.

Reading Assignment

Read pp 99-120 (Ch. 5) in Introduction to Materials ebook

Now you should be able to distinguish between single-crystal and polycrystalline materials. If you cannot draw unit cells for face-centered cubic, body-centered cubic, and hexagonal close-packed crystal structures, you should review those.

Metals routinely form crystals. However, sometimes metal is formed from many grains rather than a single crystal. A grain is a region of single crystallinity and material with many grains is a material with many crystals (grains) that are misaligned to each other. This would be termed a polycrystalline material.

Many materials, e.g. iron, titanium, and carbon, possess two or more distinct crystal structures, which is referred to as allotropy or polymorphism. We have discussed metals as though they form perfect crystals, but it turns out that in real life a perfect crystal is not possible.

In the next section, we will introduce crystal imperfections, which in many cases lead to desirable materials properties.

Imperfections in Solids

There is no such thing as a perfect crystal. Crystalline imperfections (or defects) are always present. In addition, impurity atoms are always present. Many of the properties of materials are sensitive to the presence of imperfections, and not necessarily in an adverse way.

So, what kind of imperfections exist in solids? One way to classify imperfections is by their dimensionality. Point defects exist by definition as a point (0 – dimensional) and include vacancies, interstitial atoms, and substitutional impurity atoms. These point defects are shown in the two figures below and will be discussed further in the reading.

Atoms all aligned but there is one vacancy.There is self-interstitial impurity where an atom of the same kind is in a spot it shouldn't be
Point Defects
Credit: Callister & Rethwisch 5e
a substitutional impurity atom is in a lattice of other identical atoms & an interstitial impurity atom is between the crystal lattice
Types of Impurity Atoms
Credit: Callister & Rethwisch 5e

One-dimensional or linear defects are called dislocations. An edge dislocation is when a half plane of atoms disrupts the overall crystal structure. A screw dislocation is when a half twist disrupts the overall crystal structure. A mixed dislocation is a dislocation that combines both an edge and screw dislocation together.

Grain boundaries are regions between different grains within a material. They are classified as an interfacial defect and are two-dimensional.

Now proceed to the second chapter of the Lesson 5 reading assignment and complete the reading.

Reading Assignment 2

As you do the following reading, here are some questions to keep in mind.

  • Is there really a 'pure' metal?
  • What are point defects?
  • What are edge dislocations?
  • What are screw dislocations?
  • Can you name a two-dimensional imperfection in a crystal?

Reading Assignment

Read pp 121-135 (Ch. 6) in Introduction to Materials ebook

Video Assignment: Metal: The Secret Life of Material

Now that you have read the text and thought about the questions I posed, go to Lesson 5 in Canvas and watch "Metal: The Secret Life of Materials" (51 minutes) about how science has unraveled the secrets of metal at the atomic level. In "Metal: The Secret Life of Materials," materials scientist Dr. Mark Miodownik explains the history, production, and uses of metals. Metals can be strong enough to build modern cities but soft enough to be crumbled in hand.

Video Assignment

Go to Lesson 5 in Canvas and watch the Metal: The Secret Life of Material video. You will be quizzed on the content of this video.

Summary and Final Tasks

Summary

Lesson 3 was concerned primarily with the various types of atomic bonding and how bonding is determined by the electron structures of the individual atoms. In this lesson, the structure of materials was discussed beginning with how metal atoms arrange to form solids. Within this framework, concepts of single crystal (highly ordered), polycrystalline (many unaligned regions of crystalline material), and non-crystalline (little to no order, also known as amorphous) materials were introduced. For crystalline solids, the notion of crystal structure was presented, and specified in terms of a unit cell. 

Reminder - Complete all of the Lesson 5 tasks!

You have reached the end of Lesson 5! Double-check the to-do list on the Overview page to make sure you have completed all of the activities listed there before you begin Lesson 6.

Lesson 6: Types and Applications of Metal Alloys

Overview

In this lesson, we will discuss the wide range of commercial applications of ferrous alloys, which includes steel. However, ferrous alloys do have some limitations including having low electrical conductivity compared to other metals, being heavy, and corroding in typical application environments. In addition to the ferrous alloys in this lesson we will look at a range of other (non-ferrous) metal and alloy systems: copper, aluminum, magnesium, and titanium alloys; the refractory metals; the superalloys; the noble metals; and miscellaneous alloys, including those that have lead, tin, zirconium, and zinc as base metals. Many of these non-ferrous metals and alloys have advantages over the ferrous alloys for particular applications.

Learning Objectives

By the end of this lesson, you should be able to:

  • Name four different types of steels and cite compositional differences, distinctive properties, and typical uses for each.
  • State different types of nonferrous alloys.
  • List current and historical applications of nonferrous alloys.
  • Cite distinctive physical and mechanical characteristics of nonferrous alloys.

Lesson Roadmap

Lesson 6 will take us one week to complete. Please refer to Canvas for specific due dates.

Lesson Roadmap
To Read Read pp 136-179 (Ch. 7 & 8) in Introduction to Materials ebook
To Watch The Secrets of the Viking Sword
To Do Lesson 6 Quiz

Questions?

If you have general questions about the course content or structure, please post them to the General Questions and Discussion forum in Canvas. If your question is of a more personal nature, feel free to send a message to all faculty and TAs through Canvas email. We will check daily to respond.

Types of Metal Alloys

In this lesson, we are going to take a closer look at metal alloys. First, we will define what an alloy is and how dislocations strengthen alloys. The e-textbook breaks metal alloys into two classes of metal alloys: ferrous and nonferrous alloys. Ferrous is simply the Latin name for iron, so ferrous alloys are simply iron alloys (which means that it is mostly iron mixed with lesser amounts of other metals or nonmetals) and nonferrous alloys are non-iron alloys. In the reading for this lesson, you will see the composition, properties, and applications of a wide variety of metal alloys. The video for this lesson highlights the properties and applications of some of the metal alloys and puts the materials development of the highlighted metal alloys in historical context.

What Are Metal Alloys?

An alloy is a mixture of a metal with another element, either metal or nonmetal. If we start with a base metal and we add impurity atoms there are two possible outcomes if the two mix. The two different cases are highlighted in the figure below. In the substitutional solid case, the impurity atoms replace the host atoms in the lattice. In the interstitial situation, impurity atoms squeeze between the host atoms.

Substitutional solid solution (e.g., Cu in Ni) or Interstitial solid solution (e.g., C in Fe). Refer to text above.
Solid solution of orange atoms in grey atoms
Credit: Callister

In addition to mixing, it is possible for regions of a new phase to form. An illustration of the formation of a second phase in a solid solution is shown below. The second phase can have a different composition and often a different structure.

Mix of orange and grey atoms, with a highlighted section where the orange and grey are in vertical lines
Solid solution of orange atoms in grey atoms with a second phase particle
Credit: Callister

To Watch

Now watch the following video (4:44) on alloys and how dislocations harden alloys:

Click here for a transcript of the Properties of Matter: Alloys and Their Properties video.

In this video we see how different metals bond together to form alloys which still retain the metallic properties of the starting metals but are usually stronger. Metal atoms are typified by having only a few electrons in their outer shells. This means that even when they bond there's always room in this valence shell for more electrons. Each metal atom can bond with up to 12 others in the close-packed lattice. Look at the red atom. It is surrounded by six in its plane and three on top and three underneath.

A less compact crystal structures are possible too. For example, this arrangement where each atom is bonded to eight others. Because there are still not enough electrons to complete the outer shell any of the atoms the electrons can move easily from one atom to another making metals good conductors of both electricity and heat. And because the electrons are not localized in fixed bonds, the atoms can slide past each other making them ductile allowing the metal to change shape. It also means that when you try to react metals together the atoms normally just mix into the lattice forming metallic bonds with each other and with no fixed proportions and randomly distributed. These structures are called alloys. Contrast this with compounds between metals and nonmetals or between nonmetallic elements where the proportions of each element is fixed.

The oldest example of an alloy perhaps is the way bronze took over from copper in the early human communities of Europe around 6,000 years ago. During the late Stone Age, axes began to be made of pure copper but they were fairly soft. When small amounts of tin were added to make bronze you got an ax which was twice as hard and worked well. The Bronze Age had arrived. The atoms in a metal lattice are held by non-directional bonds a sort of sea of loose electrons as we said allowing the atoms to slide past each other still touching making metals relatively easy to melt and bend but hard to vaporize. When metals change shape atoms actually slip over each other like this. However, this process does not happen all at once but bit by bit rather like trying to move a carpet by putting a rock in it.

Here is the way it happens in the metal. You see the slip moving easily one atom at a time where there's a dislocation in the lattice. It is this easy movement of atoms in the crystal lattice that makes most pure metal soft. Now put a smaller or bigger atom into the lattice and this easy movement of the dislocation is blocked. See the way the bigger atom stabilizes the dislocation which gets no further unless you put greater force meaning that it's harder to bend the alloy.

To finish let's look at some well-known alloys. Bronze, three quaters copper, quarter tin, for sculptures, boat hardware, screws, and grille work. Brass 70 percent copper, 30 percent zinc. Musical instruments, coins, door knockers. Carbon steel 99 percent iron and up to one percent carbon. The building construction, tools, car bodies, machinery rails, etc. stainless steel iron with about 18 percent chromium and eight percent nickel. Used for tableware, cookware, surgical tools, and so on. Aluminium alloys for planes contain a few percent of copper or other metals. Amalgam is mercury with silver and other metals. Once used for dental fillings. Solder lead and tin for joining electrical wires and components. Melts very easily. Gold is usually an alloy containing another metal such as silver for increasing hardness. The number of carats, k, defines how many mass parts of pure gold are found in 24 parts of the alloy.

Credit: FuseSchool [16]

After watching the video, please proceed to the next section on the development of iron smelting.

Why Did it Take So Long?

Why did it take so long (~2,000 years) for humankind to apply the concepts of smelting copper and bronze to the development of iron? And then another 3000 years to develop steel?

The major issue with the smelting of iron is that with the technology used for smelting of copper and bronze the temperature that is obtainable results in solid iron. So rather than having molten iron, the smelting of iron results in a sponge-like solid mass of impure iron.

As we will see later in the video, impurities could be pounded out of iron by hitting it. So, until the Industrial Revolution, iron could only be produced as a wrought alloy. A wrought alloy is amenable to being mechanically deformed, i.e., pounding it into a desirable shape. Since iron could not be melted it could not be cast in the molds. There were also limits to controlling impurities.

In England in 1709, Abraham Darby started to use coke instead of charcoal as his fuel source to smelt iron ore. Coke, a form of coal, allowed him to build larger and more efficient furnaces than charcoal could support. These furnaces allowed Darby to reach higher temperatures. The temperatures reached were still not high enough to melt pure iron. However, iron that has around 4.3 weight percentage of carbon has a much lower melting temperature than pure iron. Although not pure iron, the iron that he could cast (since it was molten) allowed him to manufacture cast iron pots that could compete successfully with brass.

In the 1850s, Henry Bessemer proposed an incredibly bold idea. Bessemer began using very large blast furnaces (shown below), which could produce 3 to 4 tons of molten iron in a single run. Oxygen was blasted through the furnace, which resulted in higher temperatures and the oxygen combining with carbon to form CO2 gas, which bubbled out of the iron. Initially, Bessemer’s process was not reliable. There were issues with phosphorus and sulfur contamination as well as difficulty producing iron with desired target carbon content. This latter issue was resolved by removing all carbon during the process and adding in desired amounts of carbon after purification of the iron.

Bessemer process. machine looks like large hollow egg
Bessemer blast furnace.
Credit: Public Domain, via Wikimedia Commons [25]

Now, proceed to the reading and video assignment for this lesson. We will then explore in more detail aluminum alloy and, one of my favorite alloys, metallic glass.

Reading Assignment

Things to consider...

When you read this chapter, use the following questions to guide your reading. Remember to keep the learning objectives listed on the overview page in mind as you learn from this text.

  • Often a materials problem is really one of selecting the material that has the right combination of characteristics for a specific application. Do all the ferrous alloys have the same materials properties? What are some of the differences?
  • What are the different types of nonferrous alloys?
  • How have nonferrous alloys been used in the past?
  • How are the nonferrous alloys being used currently?
  • What are the distinctive physical and mechanical characteristics of the different nonferrous alloys?
  • What are the five types of cast iron?
  • How do the microstructure and mechanical characteristics of the five types of cast iron compare?

Reading Assignment

Read pp 136-179 (Ch. 7 & 8) in Introduction to Materials ebook

Video Assignment: Secrets of the Viking Sword

Now that you have read the text and thought about the questions I posed, take some time to watch this 53-minute NOVA video about using cutting edge science, old-fashion detective work, and modern craftsmanship to reconstruct a legendary Ulfberht Viking sword. As you watch this video see if you can apply what you know about carbon content in ferrous alloys to the properties of the sword being manufactured in this video.

Video Assignment

Go to Lesson 6 in Canvas and watch the Secrets of the Viking Sword video. You will be quizzed on the content of this video.

Aluminum Alloy

Aluminum, silver rock chunk.
Aluminum
Credit: Wikimedia Commons [26]

Aluminum and its alloys were introduced in your e-textbook. The history of the development and applications of aluminum and its alloys were covered in the video for this lesson. Now I am going to expand on this material and highlight the role of aluminum in airplane development.

Aluminum is the third most abundant element in the Earth's crust, after oxygen and silicon, and is the Earth's most abundant metal. It is about 8% of the crust by mass, but it is rarely found as a native metal as it is very chemically active. Its oxide forms more readily than the oxide of iron and, unlike the oxide of iron, once formed it blocks oxygen and water from penetrating the aluminum oxide. This results in aluminum being very corrosion-resistant. Iron, on the other hand, forms rust which does not block oxygen and water, so iron pieces will rust to completion if left long enough in a wet atmospheric environment. Aluminum has a low density, which makes it a candidate for lightweight applications.

Aluminum Electrolysis

Although aluminum is abundant in nature, it occurs chemically bound to other elements, and there is no known way to smelt aluminum using traditional smelting methods. Because of this limitation, before the 19th century, pure aluminum was rarer than gold. In the 19th century, people learned how to use electrolysis to extract aluminum from aluminum oxide, AlO2. As you can see from the figure below aluminum production has continued to increase ever since.

A graph showing an exponential increase in world aluminum production from 1940 (2 millions tons/year) to 2015 (45 million tons/year)
World production trend of aluminum (in million tons per year).
Credit: By Leyo, Con-struct [CC BY-SA 3.0], via Wikimedia Commons [27]

Typically, aluminum oxide is extracted from the mineral bauxite, and then aluminum is further processed from the aluminum oxide. Visit this website to access the list of where aluminum oxide is produced [28]. Although aluminum oxide is used as an abrasive material most of the aluminum oxide is used for the production of aluminum. For more on the electrolysis process for the extraction of aluminum from aluminum oxide, please watch the fuseschool.org video linked below.

Watch Now

Please watch the following short video (3:13), How to Extract Aluminum Using Electrolysis, on the extraction of aluminum using electrolysis before proceeding to the next section on building lighter aircraft.

Click for transcript of How to Extract Aluminum by Electrolysis.

Aluminum is the most abundant metal on Earth, however, it is expensive because a lot of electricity is used to extract it. Aluminum conducts heat and electricity well, has a low density, and does not corrode. This makes it very useful for airplanes, drinks cans, electricity cables, and cooking pans. The aluminum ore is called bauxite. Bauxite is purified to yield aluminum oxide which is a white powder. Aluminum is then extracted from the aluminum oxide. The aluminum is extracted by electrolysis. In this video we are going to look at how aluminum is extracted using electrolysis. You should already know how electrolysis works. If you have forgotten, watch our video Electrolysis - How Does it Work, to refresh your memory.

In electrolysis, ions need to pass through the electrolyte and so the aluminum oxide must be made molten so that this can happen. Aluminum oxide has a very high melting point over 2000 degrees Celsius, so instead of trying to melt it the aluminum oxide is dissolved in molten cryolite. Cryolite is an aluminum compound with a much lower melting point than aluminum oxide, and so using this reduces some of the costs in extracting aluminum. The steel case is coated with graphite providing a negative cathode. The positive anodes are immersed in the molten cryolite and are also made of graphite. Remember that graphite is a form of carbon. When the battery is turned on and electricity flows the aluminum from the aluminum oxide in the cryolite forms at the negative cathode and sinks to the bottom of the tank. Here it can then be tapped off as a pure liquid metal. The aluminum sinks because it is more dense than the aluminum prior light solution. The oxygen from the aluminum oxide in the cryolite forms at the positive anodes. The oxygen reacts with the carbon of the graphite forming carbon dioxide. The positive anode therefore burns away and needs replacing regularly. This is another reason for the extraction of the aluminum being so expensive. The overall reaction is aluminum oxide to plus oxygen.

Let's have a quick look at the reactions at the electrodes. At the negative cathode where the aluminum forms the aluminum ions from the molten aluminum oxide cryolite solution are reduced. This means they gain electrons. At the positive anode, where the oxygen reacts with carbon to make carbon dioxide, the oxygen ions are oxidized. This means they lose electrons.

So, from this video you should know that extract aluminum electrolysis is used. Aluminum oxide needs to be molten for the ions to move through it, and so is dissolved in cryolite to lower the melting point. The anode is gradually ward away because the oxygen from the solution reacts with the carbon of the graphite anode producing carbon dioxide, and so the anode wears away and needs to be replaced regularly. Aluminum extraction is very expensive because a lot of electricity is needed.

Credit: FuseSchool [16]

Building a Lighter Aircraft

Double-decker airplane in flight
Airbus A380.
Credit: Sam Smith via Flickr [29]

The lighter that we can build safe aircraft the better. Reducing the operating empty weight of commercial aircraft can allow for an increase in the passengers, baggage, and cargo that the plane can safely transport. Early aircraft were made of wood and fabric. An example of an early aircraft is shown in the figure below. This provided a good combination of lightness and strength but required reinforcing struts, which added weight and drag and resulted in multiple wing designs.

Old-timey propellor plane
Reproduction of a Sopwith F.1 Camel biplane
Credit: Public Domain via Wikimedia Commons [30]

Improved airplane engine designs resulted in more powerful engines and higher airspeeds. As speed increases, drag increases nonlinearly. Single wing (less drag) airplane designs were required to take advantage of the improvements in speed.

All Metal Monoplanes

The first all metal monoplanes were developed during World War I. These were faster, but it was quickly realized that they did not climb well. Although more powerful than the initial airplane engines, the engines of World War I did not have enough power to lift the all-metal monoplanes quickly enough.

Black and white photo of airplane with one set of wings
Metal monoplane in Döberitz, Germany in 1915
Credit: Public Domain via Wikimedia Commons [31]

Wood, iron, and aluminum are possible materials for making aircraft wings. How do the densities of these materials compare? The density of water is 1 g/cm3 by definition at standard temperature and pressure. Wood floats in water so its density must be less than 1 g/cm3. Its density ranges from 0.45 to 0.85 g/cm3. Iron and aluminum do not float, so their densities must be greater than 1 g/cm3. Iron's density is equal to 7.9 g/cm3 and aluminum's density is equal to 2.7 g/cm3. So, in theory, it should be possible to reduce the weight of an airplane by utilizing aluminum instead of iron. Aluminum is about 1/3 the density of iron. But there is a problem: aluminum is not strong and alloying does not strengthen the material the way it does in bronze and iron. Aluminum needed to be strengthened, but how?

Hardened Aluminum Alloys

In 1901, German metallurgist Alfred Wilm was working to harden aluminum-copper alloys. The work was not going well so in frustration he went on holiday (vacation). Upon his return, he found a harder material and after many years of work developed a commercially viable age-hardened aluminum alloy. Age-hardened aluminum, which is about three times lighter than iron, replaced iron in aircraft manufacturing. A photo of an early aluminum-bodied aircraft is shown below.

Propeller plane in flight
Early aluminum-bodied aircraft.
Credit: Bryan Fury75 [32] at French Wikipedia [33]

Age hardened aluminum is not as strong as iron so additional aluminum is needed which does offset some of the weight savings. The video in our later Synthesis, Fabrication, and Processing of Materials lesson has more on the use of aluminum in the construction of a modern commercial jet airliner. In the photo below is a Boeing 787 Dreamliner which utilizes a composite airframe, not aluminum. Boeing claims that this airliner is 20% more fuel-efficient than previous generations of airliners.

Boeing 787-8 in flight.
Nippon Airways Boeing 787-8 (JA801A) at Okayama Airport
Credit: Spaceaero2 via Wikimedia Commons [34]

Now please proceed to the next section on one of my favorite alloys, a future star, a non-crystalline metal.

Metallic Glass

Most metals are crystalline. In fact, it is typically very difficult to make a noncrystalline metal. The following short video highlights metals that are noncrystalline, i.e., amorphous. These materials are sometimes referred to as metallic glasses.

To Watch

Ted Talk: What is Metallic Gas?
Click for transcript of What is Metallic Gas?
Steel and plastic. These two materials are essential to so much of our infrastructure and technology, and they have a complementary set of strengths and weaknesses. Steel is strong and hard, but difficult to shape intricately. While plastic can take on just about any form, it's weak and soft. So wouldn't it be nice if there were one material as strong as the strongest steel and as shapeable as plastic? Well, a lot of scientists and technologists are getting excited about a relatively recent invention called metallic glass with both of those properties, and more. Metallic glasses look shiny and opaque, like metals, and also like metals, they conduct heat and electricity. But they're way stronger than most metals, which means they can withstand a lot of force without getting bent or dented, making ultrasharp scalpels, and ultrastrong electronics cases, hinges, screws; the list goes on. Metallic glasses also have an incredible ability to store and release elastic energy, which makes them perfect for sports equipment, like tennis racquets, golf clubs, and skis. They're resistant to corrosion, and can be cast into complex shapes with mirror-like surfaces in a single molding step. Despite their strength at room temperature, if you go up a few hundred degrees Celsius, they soften significantly, and can be deformed into any shape you like. Cool them back down, and they regain the strength. So where do all of these wondrous attributes come from? In essence, they have to do with metallic glass' unique atomic structure. Most metals are crystalline as solids. That means that if you zoomed in close enough to see the individual atoms, they'd be neatly lined up in an orderly, repeating pattern that extends throughout the whole material. Ice is crystalline, and so are diamonds, and salt. If you heat these materials up enough and melt them, the atoms can jiggle freely and move randomly, but when you cool them back down, the atoms reorganize themselves, reestablishing the crystal. But what if you could cool a molten metal so fast that the atoms couldn't find their places again, so that the material was solid, but with the chaotic, amorphous internal structure of a liquid? That's metallic glass. This structure has the added benefit of lacking the grain boundaries that most metals have. Those are weak spots where the material is more susceptible to scratches or corrosion. The first metallic glass was made in 1960 from gold and silicon. It wasn't easy to make. Because metal atoms crystallize so rapidly, scientists had to cool the alloy down incredibly fast, a million degrees Kelvin per second, by shooting tiny droplets at cold copper plates, or spinning ultrathin ribbons. At that time, metallic glasses could only be tens or hundreds of microns thick, which was too thin for most practical applications. But since then, scientists have figured out that if you blend several metals that mix with each other freely, but can't easily crystallize together, usually because they have very different atomic sizes, the mixture crystallizes much more slowly. That means you don't have to cool it down as fast, so the material can be thicker, centimeters instead of micrometers. These materials are called bulk metallic glasses, or BMGs. Now there are hundreds of different BMGs, so why aren't all of our bridges and cars made out of them? Many of the BMGs currently available are made from expensive metals, like palladium and zirconium, and they have to be really pure because any impurities can cause crystallization. So a BMG skyscraper or space shuttle would be astronomically expensive. And despite their strength, they're not yet tough enough for load-bearing applications. When the stresses get high, they can fracture without warning, which isn't ideal for, say, a bridge. But when engineers figure out how to make BMGs from cheaper metals, and how to make them even tougher, for these super materials, the sky's the limit.
Credit: Ashwini Bharathula, TED-Ed

After viewing this video please proceed to the summary page of this lesson.

Summary and Final Tasks

Summary

The extremely versatile range of different metals and alloys have produced an incredible range of application for these metals and alloys. In this lesson, we have explored how ferrous metals and the many different non-ferrous metals and alloy systems are historically and currently used. Understanding the strengths and weaknesses of these materials can allow one to properly select the right material for the desired application and the environment in which the application exists. In the next lesson, we will be looking at ceramics and their role as one of the primary materials.

Reminder - Complete all of the Lesson 6 tasks!

You have reached the end of Lesson 6! Double-check the to-do list on the Overview page to make sure you have completed all of the activities listed there before you begin Lesson 7.

Lesson 7: Structure and Applications of Ceramics

Overview

As we continue to explore how the crystal structure of a material can directly affect their properties, we will turn our attention to ceramics. As an example of the role of crystal structure, noncrystalline ceramics and polymers normally are optically transparent; the same materials in crystalline (or semicrystalline) form tend to be opaque or, at best, translucent. In this lesson, we will continue our discussion of how structure can affect materials properties and look at some materials applications of ceramic materials.

Learning Objectives

By the end of this lesson, you should be able to:

  • Discuss the early development of ceramics.
  • Identify unit cells for sodium chloride, cesium chloride, zinc blende, diamond cubic, fluorite, and perovskite crystal structures.
  • Define cation and anion.
  • Name three forms of carbon and note at least two distinctive characteristics for each.
  • Describe the process that is used to produce glass-ceramics, characteristics, and list several commercial applications.
  • Name the two types of clay products and give examples of each.
  • Cite important requirements that commonly must be met by refractory ceramics and abrasive ceramics.

Lesson Roadmap

Lesson 7 will take us 1 week to complete. Please refer to Canvas for specific due dates.

Lesson Roadmap
To Read Read pp 180-214 (Ch. 9 & 10) in Introduction to Materials ebook
To Watch Ceramics: The Secret Life of Materials
To Do Lesson 7 Quiz

Questions?

If you have general questions about the course content or structure, please post them to the General Questions and Discussion forum in Canvas. If your question is of a more personal nature, feel free to send a message to the instructor through Canvas email. Email, discussion boards, internal Canvas messages are checked daily.

First Known Ceramics

The first known clay figurines date from 29,000 to 25,000 BCE. The clay figurines were found in what is now the Czech Republic and originally were fired at low temperatures. One of the figurines is shown below. Clay is composed of ceramic plates which are separated by water. In this wet form, clay is very plastic and can be molded. The water allows the plates to move past each other. When clay is dried or fired, water is driven off and, when fired, heat enables atomic bonding which locks the plates together. Done properly, the fired clay becomes hard, non-plastic, and brittle.

clay figurine of a female body
Vestonicka venuse.
Credit: che via Wikimedia Commons [35]

Around 14,000 BCE, tiles were being made in Mesopotamia (modern day Iran) and India, with pottery making beginning around 10,000 – 9,000 BCE. Around 10,000 BCE the roping or coiling method of pottery making was being used to make pots in Japan (see figure below). Recall that this technique was described in the video Secrets of the Terracotta Warriors as the method used by the Chinese to produce the terracotta warriors.

Hands shaping a clay pot made out of coils
Pottery-making using the coiling method.
Credit: Bandelier National Monument via Flickr, (CC BY-NC 2.0)

In the next section, we will look at one of the precursor steps to glass containers, Egyptian faience.

Egyptian Faience

Developed around 4,000 BCE, Egyptian faience is a glaze, or a coating used to color, decorate or waterproof an item, which is typically fused to a ceramic body through firing. Before the discovery of a process to produce glass, Egyptians used glazing to produce containers (see figures below). They combined silica (SiO2), lime (CaO), and soda (Na2O) to form their glaze. During drying, the lime, soda, and impurities move to the surface. When fired above 800 °C a glassy crust forms which ‘cements’ the piece together. The impurities provide color, while the lime protects the piece from the atmosphere. In addition, the lime, when combined with silica, lowers the melting point of silica so that firing at above 800 °C allows the glassy crust to form.

Egyptian scarab beetle, carved in clay and glazed sky blue
Commemorative marriage scarab for Queen Tiye from Amenhotep III: Walters Art Museum, Baltimore
Credit: Keith Schengili-Roberts via Wikimedia Commons [36]
sculpture of Egyptian woman in headdress with arms crossed
Shawabti of Lady Sati
Credit: David Liam Moran via Wikimedia Commons [37]

It took about 2,500 years to move from glazing to completely glass containers, which seems like a really long time when you consider that the primary raw material for glass, silica, is very readily available. It is just sand. In the next section, we will take a look at why it took so long for humankind to begin producing glass.

Silica (SiO2)

Silica (structure shown in the top figure below) has a melting temperature of 1700 °C. This is considerably higher than temperatures that are possible with charcoal and a blow pipe (800 - 1200 °C). But adding sodium changes things drastically. Sodium bonds to only one Si atom, so it breaks the ordered network of silica (see lower figure below). This results in a shortening of bond lengths which reduces the melting temperature to around 1000 °C, which is possible to reach with charcoal and a blow pipe. The effect of adding sodium to silica to lower the melting temperature appears to have been discovered around 1,500 BCE.

Structure of Quartz. Hexagonal rings with alternating Si and Oxygen. Each Si is bonded to 4 total oxygens in a tetrahedral shape
Crystalline SiO2 (Quartz)
Adapted from Fig. 3.41, Callister & Rethwisch 5e.
Structure of soda glass. Chains of Si 4+ and O 2- in a tetrahedral structure with Na+  within the chains. No longer hexagonal
Adding sodium to silica makes soda glass
Adapted from Fig. 3.41, Callister & Rethwisch 5e.

In the 1st century BCE, the Romans developed glass blowing (which has remained relatively unchanged since that time) and the production of glass products increased. Glass is easier to produce than glazing products. An interesting side note is that the Romans recycled glass. In the next section, we'll discuss the bonding of ceramics and compare it to metallic bonding.

Ceramic Bonding

Recall that the predominant bonding for ceramic materials is ionic bonding. In ionic bonding, a metal atom donates electrons and a nonmetal atom accepts electrons. This electron transfer creates positive metal ions (cations) and negative nonmetal ions (anions), which are attracted to each other through coulombic attraction. The nature of ionic bonding (creation of cations and anions) results in several differences between ionic and metallic bonding. First, ionic bonds in solids are quite directional, i.e., there are certain preferred angles. Second, to maintain charge balance the cations and anions have to be in certain ratios. Thirdly, it turns out, to form stable structures it is necessary to maximize the number of oppositely charged ion neighbors (as shown in the figure below). All of these factors make ceramic structure inherently more complex than metal structures and, as we will discuss later, also make ceramics brittle.

unstable: 4 touching - charges around a small + charge Stable:  4 touching or not charging - charges w/ around a medium + charge
Maximizing the number of oppositely charged ion neighbors to form stable structures.
Adapted from Fig. 3.5, Callister & Rethwisch 5e.

Now, please proceed to the reading for this lesson (shown on the next page).

Reading Assignment

Things to consider...

When you read these chapters, use the following questions to guide your reading. Remember to keep the learning objectives listed on the overview page in mind as you learn from this text.

  • What are the common unit cells for ceramic materials?
  • What is a cation and an anion?
  • Carbon can exist in many forms, what are they and how do the structure and properties differ between the different forms?
  • Historically, glass-ceramics have been very important, what processes have been used throughout history and what commercial applications have been produced?
  • Clays are another important class of historical materials, what are some of the clay products?
  • What are refractory and abrasive ceramics, and what are some important requirements that must be met for their use?

Reading Assignment

Read pp 180-214 (Ch. 9 & 10) in Introduction to Materials ebook

Why is Glass Transparent?

Glass is one of the noncrystalline (amorphous) forms of quartz (SiO2). Quartz is crystalline SiO2 (structure shown in figure (a) below), while fused silica is SiO2 which is amorphous SiO2 without impurities ( the structure is shown in figure (b) below).

Crystalline forms hexagonal structure. Non-crystalline is random
Crystalline and non-crystalline silicon dioxide.
Credit: Callister

In practice, impurities (such as sodium shown in the figure below) are added to the glass to lower the melting temperature and the viscosity of the glass to make it easier to work the glass at lower temperatures.

Structure of soda glass. Chains of Si 4+ and O 2- in a tetrahedral structure with Na+  within the chains. No longer hexagonal.
The addition of sodium (Na) disrupts the normal bonding structure of silicon dioxide.
Adapted from Fig. 3.41, Callister & Rethwisch 5e.

Glass's amorphous structure breaks up the band structure of SiO2 such that there are no electronic states that electrons can jump to by absorbing visible light in glass. Here is a TED-Ed video by Mark Miodownik (the host of the Secret Life of Materials videos) to explain this in more detail. In the next sections, we are going to discuss why glass is brittle and how glass is being engineered not to be so brittle.

Watch Now

Why glass is Transparent?
Click here for a transcript of Why is Glass Transparent?

Take a look out your window, put on your glasses if you wear them. You might want to grab a pair of binoculars, too, or a magnifying lens.

Now, what do you see?

Well, whatever it is, it's not the multiple layers of glass right in front of you. But have you ever wondered how something so solid can be so invisible? To understand that, we have to understand what glass actually is, and where it comes from.

It all begins in the Earth's crust, where the two most common elements are silicon and oxygen. These react together to form silicon dioxide, whose molecules arrange themselves into a regular crystalline form known as quartz. Quartz is commonly found in sand, where it often makes up most of the grains and is the main ingredient in most type of glass. Of course, you probably noticed that glass isn't made of multiple tiny bits of quartz, and for good reason. For one thing, the edges of the rigidly formed grains and smaller defects within the crystal structure reflect and disperse light that hits them. But when the quartz is heated high enough the extra energy makes the molecules vibrate until they break the bonds holding them together and become a flowing liquid, the same way that ice melts into water.

Unlike water, though, liquid silicon dioxide does not reform into a crystal solid when it cools. Instead, as the molecules lose energy, they are less and less able to move into an ordered position, and the result is what is called an amorphous solid. A solid material with the chaotic structure of a liquid, which allows the molecules to freely fill in any gaps. This makes the surface of glass uniform on a microscopic level, allowing light to strike it without being scattered in different directions.

But this still doesn't explain why light is able to pass through glass rather than being absorbed as with most solids. For that, we need to go all the way down to the subatomic level. You may know that an atom consists of a nucleus with electrons orbiting around it, but you may be surprised to know that it's mostly empty space. In fact, if an atom were the size of a sports stadium, the nucleus would be like a single pea in the center, while the electrons would be like grains of sand in the outer seats. That should leave plenty of space for light to pass through without hitting any of these particles. So the real question is not why is glass transparent, but why aren't all materials transparent?

The answer has to do with the different energy levels that electrons in an atom can have. Think of these as different rows of seats in the stadium stands. An electron is initially assigned to sit in a certain row, but it could jump to a better row, if it only had the energy. As luck would have it, absorbing one of those light photons passing through the atom can provide just the energy the electron needs. But there's a catch. The energy from the photon has to be the right amount to get an electron to the next row. Otherwise, it will just let the photon pass by, and it just so happens that in glass, the rows are so far apart that a photon of visible light can't provide enough energy for an electron to jump between them. Photons from ultraviolet light, on the other hand, give just the right amount of energy, and are absorbed, which is why you can't get a suntan through glass. This amazing property of being both solid and transparent has given glass many uses throughout the centuries. From windows that let in light while keeping out the elements, to lenses that allow us to see both the vast worlds beyond our planet, and the tiny ones right around us. It is hard to imagine modern civilization without glass. And yet for such an important material we rarely think about glass and its impact. It is precisely because the most important and useful quality of glass is being featureless and invisible that we often forget that it's even there.

Credit: Mark Miodownik, TED-Ed

The Glass Age

Let’s take a look at this introduction to the Glass Age. This video was produced by glass manufacturer Corning Incorporated and is hosted by Myth Busters Adam Savage and Jamie Hyneman.

To Watch

Click for the transcript of The Glass Age, Part 1: Flexible, Bendable Glass.

ADAM SAVAGE: Pop quiz: if you are able to look back on the present from deep in the future what age would you say we're living in?

JAMIE HYNEMAN: Is this a trick question? I mean I want to say information age, but it seems too obvious. Can I say more than one age?

ADAM SAVAGE: yeah I think it is safe to say that we are living in more than one age. From the beginning of humanity, we've seen prevailing technologies marked with milestones the Stone Age, the Bronze Age, the Iron Age, all occurring many thousands of years ago. Man's mastery of these materials has defined us, but by that metric the last couple of hundred years have seen a flurry of Ages the steam age the Industrial Age the Atomic Age the television age, the Space Age, to name, but a few but those are not the answers I was looking for.

JAMIE HYNEMAN: Is that a clue?

ADAM SAVAGE: Yes it is. I think that this age could be classified as the glass age.

JAMIE HYNEMAN: That's not what I was thinking. I know so how are we in the glass age?

ADAM SAVAGE: Well let me put it to you this way can you imagine a world without glass now I don't want a cheeky answer I want you to really think about it.

JAMIE HYNEMAN: Okay, no. I can't imagine the world without glass.

ADAM SAVAGE: Exactly. Glass is really quite extraordinary. Without it, many of our major accomplishments would never have happened. Glass has a deep and complex history and as a material, it has properties and characteristics that we are only just beginning to understand. We look right through it and think of it one dimensionally. Most of us think of glass as a fragile brittle thing that if not handled correctly will break in a spectacular fashion.

JAMIE HYNEMAN: So you're gonna break that to make a point?

ADAM SAVAGE: Indeed.

JAMIE HYNEMAN: Can I help?

ADAM SAVAGE: Yes you can and it's true our everyday common variety of glass is brittle, but it doesn't have to be that way. Glass has already altered our lives and is behaving in ways that is totally unexpected.

JAMIE HYNEMAN: Got it.

ADAM SAVAGE: Let's start with a history of glass.

JAMIE HYNEMAN: I think I can handle that. Glasses we know it is most commonly made of silica the primary ingredient of beach sand. Mix silica with a couple of other key ingredients heat it all up till it melts and bang you got glass. Humans have been making glass since ancient times starting with beads, vessels, and ceremonial accouterment. Glass making techniques spread out from Mesopotamia cultured culture changing in incremental ways for much of the last forty-five hundred years or so. The Romans even had glass windows in their important buildings as early as the 1st century AD. Glassblowing was discovered around that time and soon inexpensive and ubiquitous glass became one of the hallmarks of the Roman Empire, but no period has seen such growth in the development of glass technologies as in the last 150 years. We've been able to unlock the secrets of glass in ways that would have seemed like magic to our forebearers.

ADAM SAVAGE: Nice.

JAMIE HYNEMAN: Thanks.

ADAM SAVAGE: So tell me what's so special about the last 150 years?

JAMIE HYNEMAN: Well several things. In that period technology evolved in an exponential rate with that came tools and processes that enabled advancement across all material sciences. The leader in glass material science was and still is an upstate New York glass company that started out in the mid-1800s - Corning incorporated. One of their first products was a toughened glass lens for railroad signal lanterns that offered two radical improvements over any other lens of that time. They could be produced in a consistent color and more importantly, it didn't break when rain hit the hot glass. This helped save lives by bringing down the number of train wrecks, but it also set a course for a hundred and sixty years of innovation in glass. Of course everybody knows about Corning ware and Pyrex products - those innovations came from Corning during their early part of the last century.

ADAM SAVAGE: You know, I have tons of this in my kitchen.

JAMIE HYNEMAN: Corning no longer makes kitchen ware. They've innovated way beyond that. Let's take a look we'll start with this optical fiber right optical fiber this does two things both astonishing the first one is this.

ADAM SAVAGE: That right there is pure glass a glass strand inside the cable tightly wound around a pencil and yet not breaking. When you stop and think about it that is a mind bender. Okay, what's the second thing?

JAMIE HYNEMAN: Well it's the way the light moves through the glass when the glass is bent this way you'd expect light to leak out and get weaker and corrupt the data that it carries but that's not happening. Nearly all the light entering this optical fiber is coming out the other end.

ADAM SAVAGE: So it has a low attenuation.

JAMIE HYNEMAN: Yeah, exactly. Very low. In the late 1960s Corning figured out how to limit the attenuation or loss of light as it travels through fiber even when that fiber is bent.

ADAM SAVAGE: Nice.

JAMIE HYNEMAN: This discovery led to the practical use of fiber as a medium for voice and data communications over great distances ushering in an era of low-cost high bandwidth communications and ultimately the Internet as we know it.

ADAM SAVAGE: Wow so just how much data can these optical fibers carry?

JAMIE HYNEMAN: This video playing back right here is sucking in data at around 20 gigabits per second.

ADAM SAVAGE: It's a lot of data.

JAMIE HYNEMAN: Yeah this is Ultra High Definition raw video, but even in this case the optical fiber is not anywhere near capacity. The bottlenecks are here and here not here. The practical limit of data transport over optical fiber keeps increasing using today's technology. It's possible to transport more than a million gigabits per second about appetitive. That'd be like downloading 17,000 high-definition movies for Netflix in a single second.

ADAM SAVAGE: That's amazing. Okay tell me about this stuff.

JAMIE HYNEMAN: Well obviously it's an optical fiber as well, but instead of sending light through one end and out the other it emits light throughout its entire length.

ADAM SAVAGE: Cool. What's it good for?

JAMIE HYNEMAN: I have no idea. Okay, so I was able to seriously bend a strand of glass didn't break, but what do you think is going to happen when I try to bend a pane of glass? Ah, rhetorical question check this out.

ADAM SAVAGE: That doesn't look like it went very well.

JAMIE HYNEMAN: Well that was soda lime glass the kind of normal stuff we see around us every day, but watch what happens next. This this is glass - it's called willow glass also made by Corning and it's flexible.

ADAM SAVAGE: No way. I cannot believe that is glass.

JAMIE HYNEMAN: Well it is. There's no trickery here. This is glass, but it's as flexible as paper.

ADAM SAVAGE: So what kind of applications does that have?

JAMIE HYNEMAN: Well that's where it gets really cool. Check this out.

ADAM SAVAGE: Alright so looks like a piece of stainless steel and what is this willow glass bonded to one side is a scratch resistant coating?

JAMIE HYNEMAN: Yep.

ADAM SAVAGE: Okay, but tell me this how is the willow glass anywhere near as durable as stainless?

JAMIE HYNEMAN: Well that's a good question. Watch this.

ADAM SAVAGE: That is amazing. I cannot believe that the blade did not shatter the glass.

JAMIE HYNEMAN: It didn't and that's just half the story.

ADAM SAVAGE: All right, so what are we doing?

JAMIE HYNEMAN: Give me that. Okay take this.

ADAM SAVAGE: This is heavy man. What do you want me to do with it?

JAMIE HYNEMAN: I want you to drop that right on that piece of stainless steel with a willow glass on it.

ADAM SAVAGE: Seriously?

ADAM SAVAGE: Let's see what happens.

JAMIE HYNEMAN: Here we go. Three, two, one.

ADAM SAVAGE: No way.

JAMIE HYNEMAN: It dented it, but it didn't break the glass.

ADAM SAVAGE: That is insane.

JAMIE HYNEMAN: And you can attach this to just about any solid surface.

ADAM SAVAGE: Bendy, flexible, durable glass impressive and characteristics you wouldn't normally associate with glass, right?

JAMIE HYNEMAN: Right. I like this new glass age we're in.

Credit: Adam Savage and Jamie Hyneman, Corning Incorporated

In the next section, we will discuss why ceramics are brittle and metals are not.

So Why are Ceramics Brittle?

Why can metals be scratched and develop cracks and yet not catastrophically fail? The reason is that metals can slide along slip planes to break the crack up. Take a look at the following video showing schematically how a crack in a metal becomes a blunted crack and a void, which can effectively stop the initial crack from growing and catastrophically failing (fracture). This is in contrast with the case of ceramics (in this case, glass). As we have mentioned before in this class, the atoms cannot easily slide past one another. This is due to the fact that in a ceramic we have predominately ionic bonding, which results in positive and negative ions alternating. So, if a row of atoms attempts to slide past the next row of atoms this would move positive ions towards positive ions and negative ions towards negative ions. That is typically too costly from a free energy point of view. Instead of stress caused by the crack being relieved by slipping, the crack keeps growing, usually to fracture, as shown in the following (1:13) animation.

To Watch

Crack in Metal and Ceramics
Click for the transcript of Crack in metal and ceramic

Let's imagine that we have a surface crack in a metal. Metal atoms can easily slide along what are called slip planes to blunt the crack. So let's take a look at how this might occur. So I have a slip plane in a metal, and the crack as you see parts of the metal shift over stretching out rather than having the crack propagate all the way through the metal and thus having a fracture. So you end up with a blunted crack and a small void in this case.

Now let's look at a surface crack in a ceramic. Unlike in a metal, the atoms of the ceramic cannot move easily past one another. So instead of the material blunting the surface crack as occurs in metals, in a ceramic the stress from the crack ends up concentrated at the point of the crack. This can lead to the material fracturing as shown in this video.

Credit: Ron Redwing

So, can anything be done to prevent cracks in ceramics from growing out of control? One method is to put the surface of the glass under compressive stress (we will discuss this further in the next section). When you do this, you are building in a stress to help you with a property of the glass. This is different from annealing glass. In the case of annealed glass, the glass is heated, but not melted, and residual stress is allowed to release.

Compressive Surface Stress

When the surface of glass is under compressive stress and cracks develop on the surface, the stress acts to close the cracks and thus prevent them from growing to the point of fracture. The following video, produced by glass manufacturer Corning Incorporated and hosted by Myth Busters Adam Savage and Jamie Hyneman, discusses one commercial product called Gorilla Glass that utilizes compressive surface stress to make glass much more fracture resistant and flexible.

To Watch

Strong, Durable Glass.
Click for the transcript of The Glass Age, Part 2: Strong, Durable Glass.

ADAM SAVAGE: Let's switch gears. Your smartphone. You hold it in your hand, you put it to your ear, you keep it in your pocket or your purse, it's a great everyday use of glass that you might not think about.

JAMIE HYNEMAN: Till it breaks.

ADAM SAVAGE: Until it breaks. I, myself, have shattered over a dozen of these. But you might have noticed over the years that the display on your phone has been getting harder to break. Let's go back a few years.

[GLASS SCRATCHING]

If you'd done that to your standard 2008 phone, that's what would have happened.

JAMIE HYNEMAN: Not pretty.

ADAM SAVAGE: No, and that could easily happen just by keeping your keys and your phone in the same pocket. And if you then dropped that phone, well, the probability of the phone breaking was high.

JAMIE HYNEMAN: How high?

ADAM SAVAGE: Very high. Let me show you. Aren't you going to drop the phone? Well, sort of. Instead of dropping the phone, I'm going to drop something on the phone, this steel ball.

JAMIE HYNEMAN: For consistency's sake.

ADAM SAVAGE: Exactly. In this way, my phone drops exactly the same way, every single time. Here we go. Three, two, one.

[CRACKING]

Oh!

JAMIE HYNEMAN: I've seen you do that so many times in real life.

ADAM SAVAGE: Me, too. But that was then, this is now. Thanks to Corning, we have gorilla glass.

JAMIE HYNEMAN: I'm pretty sure that's what I've got on my phone.

ADAM SAVAGE: And it's on this phone as well. I'm about to do the same test a second time. Check this out.

JAMIE HYNEMAN: No scratch.

ADAM SAVAGE: Not at all. Now, same drop test. Ready?

JAMIE HYNEMAN: Yeah.

ADAM SAVAGE: Three, two, one.

JAMIE HYNEMAN: Wow, what a difference a few years can make.

ADAM SAVAGE: Actually, Corning came up with this method for strengthening glass long ago. Gorilla glass evolved from that process, and today is found on all the best small devices, like this. It's beginning to find its way on the larger format displays, too.

Now, it's not unbreakable. If you set out to break it, you will. But as Corning figures out ways to unlock more secrets of glass, it will continue to get more resilient. It may even get to a state where devices such as these simply don't break anymore. Oh, and by the way, Corning has just come out with a new and improved version of gorilla glass.

JAMIE HYNEMAN: Really?

ADAM SAVAGE: Yeah. Let's go deeper. Watch this.

JAMIE HYNEMAN: Hot stuff

ADAM SAVAGE: Yeah. This hot stuff is your basic, everyday soda lime glass. There's nothing remarkable about it, except that it's white hot and molten. There we go, and I'm going to drop it in cold water. It's called a Prince Rupert drop.

JAMIE HYNEMAN: Who's Prince Rupert?

ADAM SAVAGE: Some Bavarian from the 1600s, he came up with this. OK, so there's a few things going on here. The cold water rapidly cools the exterior surface of the glass, hardening it almost immediately. The interior, still molten, cools more slowly. As it cools, it contracts and attempts to put the surface in with it, but it can't.

Well, not very much. The surface has already hardened, so it gets pulled in only a little, compressing it while also creating an internal layer that remains forever under tension. It is this action that gives the glass its uncharacteristic strength. We call it compressive strength.

JAMIE HYNEMAN: Hm. It sounds like the same principles as how an arch provides strength in structural engineering.

ADAM SAVAGE: Yes, kind of. Now, Jamie, I'm going to ask for your help. We're going to attempt to destroy this Prince Rupert drop. I just want you to tip that hammer past its center point. Go ahead.

JAMIE HYNEMAN: I feel like we've been swindled.

ADAM SAVAGE: Swindled not. We have just experienced the power of compressive strength. It does, however, have an Achilles heel. Take those nippers right there and nip the backside of the tail of this Prince Rupert drop, and watch what happen.

Wait, wait, wait, cue the high-speed camera. OK, here we go. [SNAP]

Whoa! [LAUGHTER] That was even cooler than I thought it would be. That is what happens when you release the stress in compressive strength glass. The whole thing shatters.

JAMIE HYNEMAN: Spectacularly.

ADAM SAVAGE: Yes. Well, at least in that example it was. That's because the stress was so great between the outer compressive layer and the inner tension layer.

JAMIE HYNEMAN: That when released, it was a catastrophic result.

ADAM SAVAGE: This is gorilla glass. It has been refined over time, but like all gorilla glass variants that came before it, it is compressive strength glass. But it's not made in the same way as we just demonstrated, the rapid cooling method. No, instead, Corning uses an ion exchange process.

To break it down simply, the surface ion particles that naturally form during the manufacture are replaced with larger ion particles. Once exchanged, the larger ion particles create the same sort of inward pressure that we see on the Prince Rupert drop. And with this method, they are able to control and manage the resulting tension.

JAMIE HYNEMAN: I think what you're saying with this process is that they're able to tune strength in the glass by dialing in the right balance between compression and tension.

ADAM SAVAGE: Yeah, that's a great way of saying it. That, and also by adding and a few other tricks that change the molecular structure of the glass. Corning is steadily moving forward towards the holy grail.

JAMIE HYNEMAN: Then, unbreakable glass for our mobile devices?

ADAM SAVAGE: Maybe not unbreakable, but yes, thin and very tough. And by the way, the applications for this tough glass go well beyond mobile devices. Let's go over this one.

Behold, the common automobile windshield, made from regular soda lime glass. It's quite strong because it's very thick and it's laminated, which means it's two pieces of glass bonded together using resin in the middle. And the resin does two things, it gives it added strength, and it holds the glass together on impact. You've probably seen broken windshields before. Lots of crazy cracked glass, but still mostly held together in the shape of a windshield.

JAMIE HYNEMAN: Like this. That's pretty cool.

ADAM SAVAGE: Yes, it is. Windshields have been made of laminated glass for the last 100 years, and they've served us well. But there is a big drawback.

JAMIE HYNEMAN: I think I know where you're going to say. It's heavy.

ADAM SAVAGE: Yes, very heavy. And as we strive for more energy efficient cars and trucks, the weight of all that glass can be a bit of a problem.

JAMIE HYNEMAN: I have the feeling you're about to show an alternative.

ADAM SAVAGE: Yes, take a look at this. Now this is also a laminate windshield, but it's not one you'd find in production. It's experimental, which is great, because we're going to experiment with it. It has regular soda lime glass on its outside surface and resin in the center like the other one, but this windshield has gorilla glass on the inside surface.

JAMIE HYNEMAN: Sweet. It looks thinner.

ADAM SAVAGE: Yes it is. And a lot lighter, too. In fact, just by changing the one laminate, the overall weight is reduced by about a third. In a car, that adds up fast.

JAMIE HYNEMAN: Thinner, lighter, and let me guess, it's just as strong as windshield A.

ADAM SAVAGE: Well, maybe. Let's find out.

JAMIE HYNEMAN: Oh, goody.

ADAM SAVAGE: Wait, wait, no, no, no, you don't need the sledgehammer for this one, put that down. Instead, you're going to shoot this windshield with an air cannon.

JAMIE HYNEMAN: Cool.

ADAM SAVAGE: Don't get too excited, we're going to have to mount this, tie it down so it doesn't wobble, and we're going to simulate a pebble hitting this windshield at a super high speed.

JAMIE HYNEMAN: Good enough for me. When do I start shooting?

ADAM SAVAGE: Well, before we shoot the gorilla glass, let's shoot the regular soda lime laminate. Windshield A. This way. All right, here we go, perfect. You want to do the honors?

JAMIE HYNEMAN: Sure.

ADAM SAVAGE: I'll count it in. Three, two, one, go.

[GLASS SHATTERING]

JAMIE HYNEMAN: Whoa.

ADAM SAVAGE: That was a lot of damage.

JAMIE HYNEMAN: Can we see that in slow motion?

ADAM SAVAGE: The ball bearing hits the windshield at around 120 miles per hour. This could be a stone flicked up by another car. It penetrates the exterior glass, it stretches the resin. It's slowed down a bit, but it still has momentum to break the interior glass layer, causing small fragments of glass to spray out through the interior of the car.

JAMIE HYNEMAN: Not good.

ADAM SAVAGE: So let's try windshield B. [CRACKING] The gorilla glass didn't appear to break. The foil is intact.

JAMIE HYNEMAN: Let's see in slow mo.

ADAM SAVAGE: The ball bearing hits the windshield at around 120 miles per hour, just as before. It goes through the front layer of soda lime glass, stretches the resin, but doesn't have enough energy to break the gorilla glass. It's not bullet proof, I'm pretty sure if we turned up the velocity we'd breach the gorilla layer, too.

But in this test, and all things being equal, it performed a lot better than the thicker, heavier soda lime laminate windshield. That's impressive. What we're seeing here is compressive strength at its finest moment.

JAMIE HYNEMAN: OK, so it's lighter, thinner, and stronger. That's pretty good.

ADAM SAVAGE: Yes it is.

JAMIE HYNEMAN: Can I get this on my car?

ADAM SAVAGE: Not today, but soon, I hope. Come on, though. After living through the Iron Age and the Bronze Age, you can wait a little longer. Actually, when you think about it, the application for this goes way beyond the car, doesn't it? Ah yes. Feels good to be in the glass age.

JAMIE HYNEMAN: Don't touch me.

ADAM SAVAGE: Sorry.

Credit:Jamie Hyneman and Adam Savage, Corning Incorporated

You have now completed the reading for this lesson, please proceed to the next page which will introduce the video for this lesson.

Video Assignment: Ceramics: The Secret Life of Material

Now that you have read the text and thought about the questions I posed, go to Lesson 7 in Canvas and watch "Ceramics: The Secret Life of Materials" (51 minutes) about the story of how clay, concrete, and sand (ceramics) have been used to build our 21st-century cities. In "Ceramics: The Secret Life of Materials," materials scientist Dr. Mark Miodownik explains how materials from the Earth have been transformed into the building materials and technology of our modern lives.

Video Assignment

Go to Lesson 7 in Canvas and watch the Ceramics: The Secret Life of Material video. You will be quizzed on the content of this video.

Summary and Final Tasks

Summary

In this lesson, we continued to explore how the crystal structure can affect materials properties, in this case, the properties of ceramic materials. In addition to learning about ceramic crystal structure, the properties of the several forms of carbon were presented. These property combinations make carbon extremely important in many commercial sectors, including the cutting-edge field of nanotechnology that we will explore further in a later lesson. Numerous applications of ceramics, including glass, clays, refractories, and abrasives were introduced and discussed.

Reminder - Complete all of the Lesson 7 tasks!

You have reached the end of Lesson 7! Double-check the to-do list on the Overview page to make sure you have completed all of the activities listed there before you begin Lesson 8.

Lesson 8: Structure and Applications of Polymers

Overview

Although natural polymers have been used by mankind for many centuries, the use of polymers has exploded with the development of synthetic polymers within the last 100 years. Due to satisfactory properties, ease of production, and lower costs, synthetic polymers have replaced many metal, wood, rubber, and fiber parts in many materials applications. In this lesson, we look at the molecular structures of polymers and the development of numerous polymers that are synthesized from small organic molecules. Several different types of end uses of polymers in materials applications including plastics, fibers, coatings, adhesives, films, foams, and advanced materials will be discussed.

Learning Objectives

By the end of this lesson, you should be able to:

  • Describe a typical polymer molecule in terms of its chain structure and, in addition, how the molecule may be generated from repeat units.
  • Cite the differences in behavior and molecular structure for thermoplastic and thermosetting polymers.
  • Draw repeat units for polyethylene, poly(vinyl chloride), polytetrafluoroethylene, polypropylene, and polystyrene.
  • Name and briefly describe the four general types of polymer molecular structures.
  • Name and briefly describe the four types of copolymers.
  • Define hydrocarbon, unsaturated hydrocarbon, saturated hydrocarbon, and isomerism.
  • Cite the seven different polymer application types and note the general characteristics of each type.

Lesson Roadmap

Lesson 8 will take us 1 week to complete. Please refer to Canvas for specific due dates.

Lesson Roadmap
To Read

Read pp 215-231 (Ch. 11) in Introduction to Materials ebook

Read pp 232-245 (Ch. 12) in Introduction to Materials ebook

To Watch Plastic: The Secret Life of Materials
To Do Lesson 8 Quiz

Questions?

If you have general questions about the course content or structure, please post them to the General Questions and Discussion forum in Canvas. If your question is of a more personal nature, feel free to send a message to the instructor through Canvas email. The instructor will check daily to respond.

What is a Polymer?

In this lesson, we will introduce the structure, history of development, and properties of polymers. The roots of the word polymer are actually very descriptive of a polymer. The root ‘mer’ means unit, and poly means many. Taken together, the word polymer can be deconstructed as many units. Typically, ‘mer’ is referred to as a monomer. ‘Mono’, which is the root for one, literally translates as one 'mer'. A commonly used definition of polymer is a material that is composed of many monomers (from 10s to 1000s) all linked together to form chains. A monomer can be composed of one to many atoms which form the base unit which is repeated to form a polymer, as represented in the figure below.

poly means many, mer means to repeat. Polymer=many monomers
Poly means many; mer means to repeat.
Credit: Adapted from Fig. 4.2, Callister & Rethwisch 5e.
PVC tubing. Repeat units of R-CH2-CHCL-R'
PVC tubing.
Credit: uberculture via Wikimedia Commons

We will also study how chains of polymers are constructed. Polymers can resemble spaghetti noodles (linear), ladders (cross-linked), long chains with smaller chains hanging off the main chain (backbone) known as branched polymers, elaborately complex structures (network), or a mixture of some or all of these basic types. Other polymers, known as copolymers, are constructed from two distinctly different starting monomers and are classified as random, alternating, block, or graft polymers.

Now, watch this TED-Ed video titled, “From DNA to Silly Putty, the Diverse World of Polymers” (4:59), before proceeding on to the next section of our lesson.

To Watch

TED Talk: From DNA to Silly Putty, the Diverse World of Polymers
Click for transcript of From DNA to Silly Putty, the Diverse World of Polymers.

What do silk, DNA, wood, balloons, and Silly Putty all have in common? They're polymers.

Polymers are such a big part of our lives that it's virtually impossible to imagine a world without them, but what the heck are they? Polymers are large molecules made of small units called monomers linked together like the railroad cars from a train. Poly means many, and mono means one, and mers or mero means parts. Many polymers are made by repeating the same small monomer over and over again while others are made from two monomers linked in a pattern.

All living things are made of polymers. Some of the organic molecules in organisms are small and simple, having only one of a few functional groups. Others, especially those that play structural roles or store genetic information, are macromolecules. In many cases, these macromolecules are polymers. For example, complex carbohydrates are polymers of simple sugars, proteins are polymers of amino acids, and nucleic acids, DNA and RNA, which contain our genetic information, are polymers of nucleotides. Trees and plants are made of the polymer cellulose. It's the tough stuff you find in bark and stems. Feathers, fur, hair, and fingernails are made up of the protein keratin, also a polymer. It doesn't stop there. Did you know that the exoskeletons of the largest phylum in the animal kingdom, the arthropods, are made of the polymer chitin?

Polymers also form the basis for synthetic fibers, rubbers, and plastics. All synthetic polymers are derived from petroleum oil and manufactured through chemical reactions. The two most common types of reactions used to make polymers are addition reactions and condensation reactions. In addition reactions, monomers simply add together to form the polymer. The process starts with a free radical, a species with an unpaired electron. The free radical attacks and breaks the bonds to form new bonds. This process repeats over and over to create a long-chained polymer. In condensation reactions, a small molecule, such as water, is produced with each chain-extending reaction.

The first synthetic polymers were created by accident as by-products of various chemical reactions. Thinking they were useless, chemists mostly discarded them. Finally, one named Leo Baekeland decided maybe his useless by-product wasn't so useless after all. His work resulted in a plastic that could be permanently squished into a shape using pressure and high temperatures. Since the name of this plastic, polyoxybenzylmethylenglycolanhydride, wasn't very catchy, advertisers called it Bakelite. Bakelite was made into telephones, children's toys, and insulators for electrical devices. With its development in 1907, the plastics industry exploded.

One other familiar polymer, Silly Putty, was also invented by accident. During World War II, the United States was in desperate need of synthetic rubber to support the military. A team of chemists at General Electric attempted to create one but ended up with a gooey, soft putty. It wasn't a good rubber substitute, but it did have one strange quality: it appeared to be extremely bouncy. Silly Putty was born!

Synthetic polymers have changed the world. Think about it. Could you imagine getting through a single day without using plastic? But polymers aren't all good. Styrofoam, for example, is made mainly of styrene, which has been identified as a possible carcinogen by the Environmental Protection Agency. As Styrofoam products are being made, or as they slowly deteriorate in landfills or the ocean, they can release toxic styrene into the environment. In addition, plastics that are created by addition polymerization reactions, like Styrofoam, plastic bags, and PVC, are built to be durable and food-safe, but that means that they don't break down in the environment. Millions of tons of plastics are dumped into landfills every year. This plastic doesn't biodegrade, it just breaks down into smaller and smaller pieces, affecting marine life and eventually making their way back to humans.

Polymers can be soft or hard, squishy or solid, fragile or strong. The huge variation between means they can form an incredibly diverse array of substances, from DNA to nylon stockings. Polymers are so useful that we've grown to depend on them every day. But some are littering our oceans, cities, and waterways with effects on our health that we're only beginning to understand.

Credit: Jan Mattingly, TED-Ed

Natural Polymers

DNA (deoxyribonucleic acid), proteins, sugar, starches, and carbohydrates are some examples of natural polymers used by plants and animals. The corresponding monomers for these polymers are listed in the table below. In addition to these important to life polymers, natural polymers derived from plants and animals have been used by humans for many centuries. These include wood, cotton, leather, rubber, wool, and silk. One of the oldest known uses of polymers is depicted in the picture below. The Incas of South America used rubber balls in some of their competitions. In the next sections, we will begin to discuss human-made polymers known as plastics.

Polymers and Corresponding Monomers
Polymer Monomer
DNA (deoxyribonucleic acid) nucleotides
Proteins amino acids
Sugar, Starches, Carbohydrates glucose
An ulama player in action in Sinaloa. Player jumping to kick or hit incoming ball
An ulama player in action in Sinaloa.
Credit: Manuel Aguilar-Moreno / CSULA Ulama Project (Manuel Aguilar) [CC BY 2.5], via Wikimedia Commons

Plastics

Plastics are polymers derived from petroleum products. They are a perfect example of designing better, cheaper, and completely human-made materials. Plastics are inspired by nature, i.e., natural polymers, but are completely synthetic.

Most polymers are made up of carbon and hydrogen atoms, and many plastics are as well. Polymers that contain hydrogen and carbon atoms are called hydrocarbons. Carbon atoms can form single bonds to four other atoms. If the carbon atoms in a polymer are bound to four other atoms the polymer is referred to as a saturated hydrocarbon. If on the other hand, the carbon atom is not bound to four other atoms it will typically form double or triple bonds, as needed, with another carbon atom. In this case, the polymer is referred to as an unsaturated hydrocarbon. This distinction is important as unsaturated polymers are generally unstable and more reactive than their saturated cousins.

Around 1850 billiards was becoming increasingly popular, but there was a problem. The balls were made of ivory, which is in very limited supply and is, thus, very expensive. Not to mention it requires the killing of elephants to obtain. In 1856, the first human-made plastic (Parkesine) was patented by Alexander Parkes from Birmingham, England. Often called synthetic ivory it was composed of nitrocellulose – cellulose treated with nitric acid and a solvent. It was the first thermoplastic, but it failed as a commercial product due to poor product quality control. The following 10-minute video discusses the development of polymers to replace ivory billiard balls, the science behind some of the most-used plastics, and some examples of thermoplastic and thermosetting polymers.

To Watch

Crash Course: Polymers
Click for the transcript of Polymers - Crash Course Chemistry #45.

Charles Darwin was a big fan of billiards. He loved his billiard table. It was one of his prized possessions and one of the most valuable things he owned. Well - not the actual table itself. In fact, it was the balls that were so valuable. Pure ivory carved from the tusks of elephants; only the wealthiest could afford a full set. Luckily, he was married to an heiress.

A full set of billiard balls would require at least one, possibly two, full elephants worth of ivory. The idea that any bar in the world might contain a billiard table available for anyone to play, and not like run out of the bar with your pockets full of valuable ivory would have sounded insane. And the billiard industry was well aware of this problem. Billiard balls were getting more expensive, elephants were getting rarer. It was thus not with an environmental motive, that in the Phelan and Collender Pool Supply Company offered $, to anyone who could come up with a substitute material that worked as well as ivory, but could be produced more quickly and sustainably then dead elephants.

An inventor named John Wesley Hyatt took on that challenge. He used nitrocellulose, a flammable solid, created by mixing cotton with nitric acid to create a hard, shiny, white sphere. The properties were extremely similar to ivory billiard balls. The company never gave him the prize. But he did patent the technique, using it to create billiard balls, piano keys and even teeth, becoming pretty dang wealthy in the process. Also, he pretty much created the industry that made all of the polymer materials that surround you right now, and that we'll be discussing today in Crash Course Chemistry. Also elephants didn't go extinct, so that's a plus.

[Theme Music]

The polymer that John Hyatt worked on was, somewhat unsurprisingly, kinda crummy. It worked well once it was created, but the manufacturing process was dangerous because nitrated cellulose can explode in a warm breeze. So luckily, some replacements started creeping in. Replacements with some names you probably recognize, like polyvinyl chloride or PVC, bakelite, polystyrene, polyester, and nylon.

These are all polymers; huge chains or sometimes D networks of repeating organic units called monomers. Each polymer has a monomer, but they're all relatively simple at that basic one-unit level. The trick is that they bond to each other on each side potentially forever, though in reality the chains are sometimes hundreds, sometimes thousands, sometimes hundreds of thousands of units long. In order to make a polymer all you need is a molecule that can easily bond to another identical molecule at points. And the simplest of those is ethene, also known as ethylene.

It's polymer, you'll be unsurprised to hear is polyethylene, which you've probably heard about. We’ll talk more about the specifics in a second, but basically because pi bounds in the double bond are weaker than sigma bonds they can be broken and new monomers can be added. Just to note to avoid confusion polyethylene has that "-ene" sound in it, right

but it's not an alkene because all those double bonds get broken to form new sigma bonds. It's a polymerized alkene, but the molecule itself is an alkane. It's confusing so I thought it's worth pointing out. Now chemists might want a bunch of different things out of their polymers; maybe they want it to be stretchy, maybe strong, maybe transparent, maybe recyclable. Polyethylene is transparent and thermoplastic, meaning it can be melted and reformed. Making it recyclable.

Some other polymers like polyurethane or Bakelite are thermoset. Which means that they change chemically during some kind of curing process and cannot be melted down and reformed. Polyethylene can actually be converted into a thermoset polymer by introducing cross-links, basically molecular bridges between those polymer chains. Any plumbers out there probably have heard of cross-linked ethylene or PEX pipe. Which is what this is. It's extra, super strong because of those cross-links. Polyethylene is also nice because it's strength can be varied by changing the size of the molecules. If they are allowed to polymerize until they are tens of thousands of monomers long, the plastic they will form will be all knotted up in these ultra-long chains and it will be extremely strong. That's why this HDPE, high density polyethylene, is a strong bottle. Whereas this is much squishier, this is low density polyethylene. However those ultra-long chains also make it much more viscous when heated and thus more difficult to process. It also loses some of its opacity and becomes more of that milky white color. Now polyethylene is great. It's really great. So great that it's the most common plastic in the world. We produce over a million tons of it per year. But we want a lot out of our plastics - strength, color, elasticity, resilience, recyclability - we need everything from saran wrap to car tires. All of these are traits that chemists work tirelessly to create in the early to middle 20th century and continue to work on even today.

One of the earliest techniques they used to try and bring out new properties was to change the substituents on the ethylene monomer. Just, see what would happen. Like, what if we swapped out one of the hydrogens for chlorine? Well you get polychloroethene, kinda, that's not what we call it.

OK, so remember how benzene when attached to a chain is a phenyl group, and how those two words have nothing to do with each other? Well the same thing goes for the ethene functional group, which is called a vinyl group. It's an old word, super old, it comes from the Greek word for wine. And that is why chloroethene is more commonly called vinyl chloride. And polychloroethene is more commonly called polyvinyl chloride, or PVC. Which is what this little ducky is made out of and also what records are made out of, which is why we call them vinyl.

Now what happens when we change out a hydrogen for a methyl group?

Well then suddenly this molecule is a propene or, if you are using ye old ways, a propylene. And, yes, if you polymerize it, it becomes polypropylene. If one of the hydrogens is replaced with a phenyl group, well that chemical was first derived from trees in the Styrax family, so it's called styrene. Polymerize it, polystyrene. Make a foam out of it, Styrofoam. Now if you change all four of the hydrogens on the base ethylene with fluorine it becomes tetrafluoroethylene. Polymerize that and instead of hydrogens that polymer is bound entirely to fluorines. Fluorine as we could guess from its spot on the periodic table, love electrons. It is extremely electronegative. But because it holds onto its electrons so tightly, and is so satisfied in this polymerized chain, the electrons are unavailable for even minimal interactions with any other molecules. I'm not just saying this stuff is super difficult to difficult to react with, or it's really stable. It's more than that the electrons aren't even available for the sort of interactions that make things stick to each other, or cause friction. Which is why you have heard of polymerized tetrafluoroethylene, because it's super useful, either by its abbreviation, PTFE, or by its brand name Teflon.

So how do we actually make these things? Well, ethene based polymers form through a process called addition polymerization. The monomers are simply added together and no by products are formed. In order to get the process kicked off you have to introduce a free radical. To me, that always sounded like some crazy freedom-fighter diving into battle without much thought for what would come after he was consumed in the firefight. And, that's kinda what they are. Free radicals are atoms or ions that have a single unpaired electron. This is crazy unstable. It's basically like having half of covalent bond dangling off the atom. Anywhere this can form a bond, it's going to form a bond. And, in the case of addition polymerization it attacks the double bond and joins one of the carbons, while the other carbon is itself left with an unpaired electron.

The molecule itself is now a newly formed free radical, and it attack another nearby pi bond, joining with another molecule of ethene, forming another radical. This process continues until two radicals meet each other consuming both free radicals without producing any more, thus, ending polymerization. There are, of course, other sorts of polymerization, as well. Sometimes a hydroxyl group from one molecule is happy to join up with a hydrogen from another, forming water. The water will break away as a byproduct, leaving the two molecules bound together. This often occurs when an amine group, with its loosely held hydrogen, meets a carboxylic acid, with its loosely held -OH group. This is just what happens when hexamethylenediamine meets adipic acid forming another branded polymer, nylon. By dissolving hexamethylenediamine and adipic acid into two different immiscible, or un-mixable, solvents, we actually create nylon right here. The nylon forms at the interface between the two immiscible liquids. And, we can literally grab it and pull it out of the vial, twisting and spooling it until we get a nice glob of nylon. This works because hexamethylenediamine has an amine group on each end and adipic acid has a carboxylic acid on both ends. Thus, when the two monomer unit, called a dimer, is formed, there is still a carboxylic acid on one end and an amine group on the other, allowing for further polymerization. These amine acid condensation polymerizations also allow for the creation of possibly the most important polymers on the planet; natural polymers being created inside of you right now out of monomers that we call amino acids.

Did you see that one coming?

Amino acids polymerize through condensation reactions guide by the code in your DNA and some very complicated enzymes to form basically you. Other important polymers in your body include polysaccharides, which we use to store energy; and yeah DNA and RNA, which we use to encode information for the formation of proteins.

But that would be back to biology, which is a whole other Crash Course. Which to be clear is available if you'd like to watch it.

And thank you for watching this episode of Crash Course Chemistry, in it you learned: that the first commercial polymer ever saved the lives of a lot of elephants. That ethene is sometimes called ethylene. And, that a huge variety of polymers is based on the addition reaction of ethene based monomers, including Teflon. Which so friction-less because of fluorine's extreme electronegativity. You also learned how addition polymerization reactions work. And, that other polymers are formed by condensation reactions. Including the polymerization of amino acids monomers, which along with other polymers like DNA and RNA, and glycogen, make up a lot of the stuff that is you.

This episode of Crash Course was written by me, Hank Green, edited by Blake de Pastino, and our chemistry consultant was Dr. Heiko Langner. It was filmed, edited and directed by Nicholas Jenkins. Our script supervisor was Stefan Chin. Our sound designer was Michael Aranda. And, our graphics team is Thought Café.

Credit: Crash Course

After watching this video, please proceed to the first (of two) reading assignments for this lesson.

Reading Assignment 1

Things to consider...

As you do the first reading for this lesson, use the following questions to guide your learning. Remember to keep the learning objectives listed on the overview page for this lesson in mind as you learn from this text.

  • How is the basic structure of a polymer made up of monomers?
  • What are the different chain structures and how do they define polymer behavior?
  • What are the four general polymer molecular structures?
  • How important are organic polymers, in particular hydrocarbons?
  • What are the differences in behavior and structure for thermoplastic and thermosetting polymers?
  • What determines whether a polymer material is a good or bad candidate for recyclability or repurposing?

Reading Assignment

Read pp 215-231 (Ch. 11) in Introduction to Materials ebook

Polymer Formation

Polymers are formed by two main ways called addition and condensation polymerization. In addition, polymerization, an initiator (or catalyst) reacts with a starting monomer. The result of this initiation reaction is a monomer attached to the initiator with an unsatisfied bond. The unsatisfied bond is free to react with another monomer, thus adding to the chain. The process repeats over and over again until two chains combine or another initiator binds to the end of the chain, both of which will terminate the chain. In condensation polymerization, a monomer with an exposed H (hydrogen) atom binds with a monomer with exposed OH (oxygen-hydrogen) atoms. During the reaction, water is released (compensated) as the H and OH combine to form H2O (water). The following 4-minute video discusses addition and condensation polymerization.

To Watch

Condensation Polymerization.
Click for the transcript of Condensation Polymerization.

In this video, you will learn how condensation polymers form, some examples of condensation polymers and the uses of these polymers.

Unlike addition polymers where monomers react to form a single product, in a condensation polymerization reaction not only is the polymer formed but also a small molecule is eliminated or lost, normally water. Polyesters and polyamides are the two types of condensation polymer we will look at.

We will first look at polyamides. This is known as the amide link. It is formed when a carboxylic acid reacts with an amine. In the formation of nylon 6,6, we react a molecule with an amine group on each side known as hexane-1, 6-diamine and a molecule with a carboxylic acid at each end, hexanedioic acid. Since both these molecules have long chain carbons, they are only complicated the visual structure. Let us remove these and replace them with rectangular blocks. You can now see the functional groups at the ends of each molecule. A carboxylic acid and amine group positioned next to each other. These groups react to form water, which is eliminated. And a large polymer molecule is formed, held together by amide links. Hence the name Polyamide. Nylon has many uses in the textile industry but this is also structurally sound material used in engineering. Especially where low friction is required, such as in bearings or bushes.

We can now look at polyesters similarly. An ester is formed in the reaction of an alcohol with a carboxylic acid. In this example, we will show the formation of polyethylene terephthalate, more commonly known as PET. Using the carboxylic acid Benzene-1, 4-dicarboxylic acid and the alcohol ethane-1, 2-diol. Once again since those these molecules have a long carbon chain that may confuse the overall structure we will replace them with rectangular blocks. As you can see when we align the two molecules, a carboxylic acid and alcohol group can react between the molecules causing Ester links to hold the large polymer molecule together. And once again water is released. PET is commonly used as a plastic for drinks bottles and polyester is used to produce fabric for clothing.

Now at the end of this tutorial, you should understand what condensation polymers are and be able to give examples of polyesters and polyamides along with their uses.

Credit: FuseSchool [16]

Now that we have reviewed how polymers are formed, let’s discuss one of the possible ways to classify polymers, as thermoplastic or thermosetting.

Thermoplastic and Thermosetting Polymers

In the last lesson on ceramics, we saw that one way to classify ceramics is by their uses (refractories, glass, clay products, abrasives, etc.). Other possible classification categories might include crystal structure and whether they are crystalline or non-crystalline. For polymers, one useful classification is whether they are thermoplastic or thermosetting polymers. As you read in the last reading assignment, thermoplastics soften when heated and harden when cooled. This is totally reversible and repeatable. Most linear polymers and branched structure polymers with flexible chains are thermoplastics. This is in contrast to thermosetting polymers, which do not soften when heated due to strong covalent crosslinks. Thermoset polymers are generally harder and stronger than thermoplastics and have better dimensional stability.

To Watch

For more information about thermoplastic (here referred to as thermo-softening) and thermosetting polymers watch this video (4:40):

What Is Thermosetting and Thermosoftening Polymers?
Click for transcript of What Is Thermosetting and Thermosoftening Polymers?

The term polymer is used to describe a macromolecule made of many monomers or repeating units. The properties of these polymers all depend on a variety of factors. The monomer unit, the linkages between each monomer, and the intermolecular and intramolecular forces that exist between polymers. In this lesson, we will learn about two classes of polymers thermo-softening polymers and thermosetting polymers. We will also learn about their properties and how these properties arise. The term plastics is used to describe a wide range of polymers made of monomers all derived from the products obtained from the fractional distillation of crude oil. You may be familiar with polyethylene, polypropylene, and even polyvinylchloride. You can learn about the structure of these polymers, how they are made, and their real-life applications from other videos on our channel. Here we will focus on how these polymers respond to heat and why they respond the way they do.

Polyethylene, polypropylene, and polyvinyl chloride our thermo-softening polymers. This means that they soften when heated. When soft and in liquid form they can be molded into many different shapes. These plastics are used to make many everyday items such as window and door frames, pipes, wiring insulation, and waterproof clothing items just to name a few. This is made possible because polymers are not linked together. We can think of it like a bowl of noodles. Although the noodles are coiled and tangle with one another they are not linked. Like the noodles, these polymers can slide over one another making these items made from them soft and flexible. In fact, these polymers can only interact by weak intermolecular forces and can, therefore, be separated rather easily when heated giving them relatively low melting points. Some other thermo-softening polymers include polystyrene and polytetrafluoroethylene. Thermosetting polymers, on the other hand, do not soften when heated. Unlike thermo-softening polymers these thermosetting polymers are cross-linked to one another can you think about how this might affect the properties of these polymers? Pause, think, and continue when ready.

The presence of crosslinks hardens the overall structure. A good example of a thermosetting polymer is vulcanized rubber. Rubber tapped from para rubber trees is a polymer of isoprene monomers. It is a runny liquid that can be processed to make latex gloves, erasers, and party balloons. It can also be used to make car and bicycle tires though it has to be vulcanized first. For the vulcanization process, sulfur is added so the disulfide bridges link the polymers together. The presence of these cross-linkages greatly increases its strength and therefore does not soften easily when heated. Let's think about it. No matter how fast you ride your bike the tires do not change shape. Some other examples of thermosetting polymers include a substance used to make old TV sets and certain types of strong glue.

In summary, thermo-softening plastics are soft and melt when heated, whereas thermosetting plastics are hard and do not soften or change their shape when heated.

Credit: FuseSchool [16]

Now that we have discussed thermoplastic and thermosetting polymers let us review the different basic structures that polymers form and how that structure can determine whether the polymers are thermoplastic or thermosetting.

Basic Polymer Structure

There are four basic polymer structures which are shown in the figure below. In practice, some polymers might contain a mixture of the various basic structures. The four basic polymer structures are linear, branched, crosslinked, and networked.

linear, branched, cross-linked (bonded between linear), and network polymer structures. Refer to descriptions below.
Diagrams of linear, branched, crosslinked, and networked polymer structures.
Credit: Adapted from Fig. 4.7, Callister & Rethwisch 5e.

Linear polymers resemble ‘spaghetti’ with long chains. The long chains are typically held together by the weaker van der Waals or hydrogen bonding. Since these bonding types are relatively easy to break with heat, linear polymers are typically thermoplastic. Heat breaks the bonds between the long chains allowing the chains to flow past each other, allowing the material to be remolded. Upon cooling the bonds between the long chains reform, i.e., the polymer hardens.

Branched polymers resemble linear polymers with the addition of shorter chains hanging from the spaghetti backbone. Since these shorter chains can interfere with efficient packing of the polymers, branched polymers tend to be less dense than similar linear polymers. Since the short chains do not bridge from one longer backbone to another, heat will typically break the bonds between the branched polymer chains and allow the polymer to be a thermoplastic, although there are some very complex branched polymers that resist this ‘melting’ and thus break up (becoming hard in the process) before softening, i.e., they are thermosetting.

Crosslinked polymers resemble ladders. The chains link from one backbone to another. So, unlike linear polymers which are held together by weaker van der Waals forces, crosslinked polymers are tied together via covalent bonding. This much stronger bond makes most crosslinked polymers thermosetting, with only a few exceptions to the rule: crosslinked polymers that happen to break their crosslinks at relatively low temperatures.

Networked polymers are complex polymers that are heavily linked to form a complex network of three-dimensional linkages. These polymers are nearly impossible to soften when heating without degrading the underlying polymer structure and are thus thermosetting polymers.

Monomers do not have to be of a single atom type, but when referring to a specific monomer it is understood to be of the same composition structure. When building a polymer from two distinct monomers, those polymers are referred to as copolymers. Next, we will look at how copolymers are classified.

Copolymers

If a chemist is synthesizing a polymer utilizing two distinct starting monomers there are several possible structures, as shown in the figure below. The four basic structures are random, alternating, block, and graft. If the two monomers are randomly ordered then the copolymer is, not surprisingly, referred to as a random copolymer. In an alternating copolymer, each monomer is alternated with the other to form an ABABABA… pattern. In block copolymers, more complex repeating structures are possible, for example AAABBBAAABBBAAA… Graft copolymers are created by attaching chains of a second type of monomer on the backbone chain of a first monomer type.

Copolymer structures: random, alternating(red, black, red, black) , block (3 red, 3black), and graft (all red with black branches)
The four basic copolymer structures.
Credit: Adapted from Fig. 4.9, Callister & Rethwisch 5e.

Before we move on to the many uses of polymers, watch this four-minute video which will introduce the uses of polymers.

To Watch

The Uses of Polymers
Click for transcript of Uses of Polymers.

In our previous videos we have explored how polymers are formed and equations for polymerization reactions. In this video, we'll explore in more detail some different polymers and their specific uses as well as the problems associated with polymers. As you now know, polymers are a long chain of organic molecules made by repeating monetary units. There are a number of natural polymers in life such as rubber, and even in our own body we have natural polymers such as proteins, carbohydrates, and DNA to name just a few. We'll focus the rest of this tutorial on synthetic polymers. The common name to synthetically made polymers is plastics which are used very frequently in our day-to-day lives. From simple packaging to complex structural building materials. However, the increased use of plastics in our homes leads to nearly one-quarter of all the solid waste being plastic. Some of this can be recycled to minimize the effects on our environment. It's a long-term goal for many chemists is to develop more biodegradable plastics which would naturally break down in our environment. Here are some specific examples of polymers and their common uses.

Polythene used for carrier bags and plastics. High-density polyethylene is used for drain pipes, water bottles, and containers. Polystyrene is used in packaging. Polypropylene is used for bottle caps, plastic bottles, and plastic pipes. Polychloroprene Etaene, often known as PVC, is used for windows and door frames, plastic hinges, and bottles. Polly 1122 tetrafluoroethylene also known as PTFE, which is a nonstick coating on frying pans as well as being used in bearings another low friction surfaces. Kevlar is a unique polymer in that it is used for bulletproof vests and jackets. Nylon is used in textiles, clothing, and carpets. As you can see polymers play a huge role in our day-to-day lives and their use is wide and varied owing to their unique individual properties. It is important to understand the most of the alkene monomers used to make polymers are obtained in some part from crude oil and therefore, it is critical that we recycle plastics to conserve our natural resources for the future manufacture of these polymers.

There are also big problems associated with the disposal of polymers. The biggest problem as mentioned above is that polymers are non-biodegradable which means that microorganisms cannot naturally break them down. Disposal of polymers by burning or incineration is a possibility, as this generates heat which can be used to generate electricity. However, the burning of polymers produces many toxic gasses which themselves can damage the environment and cause pollution.

Now at the end of this lesson, you should have an appreciation of the importance of polymers, be able to name some key polymers along with their uses, and also describe the problems associated with polymers.

Credit: FuseSchool [16]

Now that you have watched this video, please proceed to the second (of two) reading assignments for this lesson.

Reading Assignment 2

Things to consider...

As you read the second chapter for this lesson, use the following question to guide your learning. Remember to keep the learning objectives listed on the overview page for this lesson in mind as you learn from this text.

  • Polymers can be used in a wide variety of applications, what are the seven different polymer application types?

Reading Assignment

Read pp 232-245 (Ch. 12) in Introduction to Materials ebook

What are Scientists Doing Now to Improve Polymers?

Now that you have read about the classical usages of polymers let us take a look at two short videos that discuss two areas that scientists are working in to improve or increase the usage of polymers in our daily lives.

To Watch

The first is a video about designer polymers (3:45).

What Are Designer Polymers?
Click for the transcript of What Are Designer Polymers?

Polymers have been around for a long time. Some of the commonly named examples are found in clothes. Things like nylon, polyesters, and acrylic. Others are plastics like PVC, polyethylene, and polycarbonates. Whereas some act as coatings on saucepans like PTFE, more commonly known as Teflon. The key thing is that different polymers have different properties.

More recently, chemists have developed a branch of polymers called designer polymers. A designer polymer is one that has been designed to respond to a change in environment or uses properties that are better than traditional polymers. Nylon, a traditional polymer used to make some clothes, has desirable properties he may even wear a jacket that's made from nylon. Nylon is tough lightweight and waterproof sadly it's missing a desirable property in that it doesn't allow sweat to pass through so when the person is wearing a Garmin they can become quite uncomfortable. If only there was a way of making a material breathable able to allow the sweat to pass through without losing the waterproofing on the outside of the jacket. The answer is that designers have started to use gore-tex a design a polymer. Gore-tex uses layers of different polymers they include an outer layer typically made from nylon or polyester. This makes the outer layer strong inner layers are made from polyurethane and miss provides water resistance.

Other membranes are made of PTFE which has millions of holes. These holes are small enough to allow the water vapor or sweat to pass out but does not allow larger water droplets from the outside to pass into the soft lining. Designer polymers come up in many everyday situations. Contact lenses, the traditional polymer PMMA, did not allow oxygen to pass through and touch the eye. This is because no blood visits the cornea this would block your vision. Therefore, all of the oxygen needed for the cells comes from the air. It was rigid and uncomfortable and the cells were starved of oxygen. How do you think the design of this polymer was improved? Pause and continue when you're ready.

The answer is the polymer used now is a special hydrogel. It's more flexible, softer, and is breathable. This improves the health of the eye to fillings. If you have a traditional filling the chances are it is made of silver amalgam this looks false as it contrasts against the tubes natural color. Designer polymers use a composite polymer resin which is tough, contains no dangerous chemicals like the mercury metal found in your tradition of silver amalgam fillings. The designer polymer can be matched at the tooth's natural color it's a photopolymer and when treated with lights will harden and match the color of the tooth. In summary, a designer polymer is one that has been designed to respond to a change in environment or uses properties that are better than the traditional polymers.

Some examples of these minor polymers include breathable clothing made from gore-tex, hydrogels found in contact lenses, in babies nappies, and finally tooth fillings.

Credit: FuseSchool [16]
 

To Watch

The second video (4:28) is about research into how to make flexible and lightweight electronics.

Plastic Electronics: Inventing the Future
Click for the transcript of Plastic Electronics: Inventing the Future.

Plastic electronics are electronic devices in which the active components are made out of carbon-based materials. So these are plastics, or polymers, or small molecules and the reason you want to make plastic electronics is because you want to make use of the attributes of these plastic materials. These include their mechanical flexibility, they're lightweight, they can be produced with tunable properties, and this is something you can't easily do with inorganic materials. My name is Yu Lin Lieu and I work in the field of plastic electronics.

In the field of plastic electronics, it all starts with chemistry. We need to make or synthesize new materials that are conductive or semiconductive, so they have the electrical properties that we would like so that when we incorporate them into electronic devices they're active. So in our group, some of the researchers make new materials, some of the researchers characterize the structures are these materials, and some other incorporate these materials to understand their potential in applications like transistors and solar cells.

Polyaniline is a conducting polymer that changes color. In here, this color change is triggered by applying a voltage to the sample. So the potential applications for polyaniline, in addition to being electrodes, we can use it as electrochromic displays, as well as sensors that change color when exposed to a specific chemical or reagent.

We use a process called spin coating to make thin layers of these compounds. The layers end up to be about a hundred nanometers thick that's about a thousand times thinner than my hair.

Here we examine the films we make under the microscope to see how the crystals grow during spin coating. We try to control the size of the crystals in the film. The bigger the crystals, the better the devices will turn out.

To make devices, we have to make electrical contact to the film by evaporating gold. Gold is evaporated through a mask. The pattern of the mask determines where gold is cooled. After the placement of a mask, we put the sample in the gold evaporator. Alternatively, we can evaporate gold electrodes on a clear silicone rubber-stamp, and laminate the rubber stamp onto the polymer film to make our devices. The structure of the devices depends on their function. In my opinion, their beauty derives from their functionality. Compared to inorganics like silicon, classics had unique attributes which include their lightweightness and their mechanical flexibility, their potential in their tunability in terms of their properties and soon to incorporate all these attributes into electronic devices would be really nice.

Well, the field's really exciting because it's a young field and it's growing and it's directly tied to applications that can have direct implications on the quality of our lives. Imagine electronic wallpaper that changes patterns from green stripes to pink polka dots at a click of a switch. Imagine tinted windows that can also generate power during the day. Imagine disposable sensors that would change color if the water sources contaminated or yet think of smart plastic patches that can monitor your health and deliver medication when you're sick. The possibilities are endless.

Credit: Princeton Engineering

Now that you have finished these videos please proceed to the next page of our course which will introduce the video for this lesson, Plastics: The Secret Life of Materials. This video will tie together the history, concepts, and usages of polymers that we have been discussing in this lesson, as well as, highlight some possible future usages of plastic.

Video Assignment

Now that you have read the text and thought about the questions I posed, go to Lesson 8 in Canvas and watch "Plastic: The Secret Life of Materials" (51 minutes) about the manmade and artificial materials which have changed how we live. In "Plastic: The Secret Life of Materials," materials scientist Dr. Mark Miodownik explains how we have turned our backs on nature and began to create our own better and cheaper materials.

Video Assignment

Go to Lesson 8 in Canvas and watch the Plastic: The Secret Life of Material video. You will be quizzed on the content of this video.

Summary and Final Tasks

Summary

Polymers are composed of repeating units which are repeated in four possible chain structures: linear, branched, crosslinked, and network. In this lesson, we discussed how the chemical and structural characteristics affect the properties and behavior of polymers. The seven basic end uses for polymers (plastics, fibers, coatings, adhesives, films, foams, and advanced materials) were introduced. Most polymers are not biodegradable which coupled with their heavy use in today’s society leads to a major source of waste at the end of a polymer's usage.

In the next lesson, we will look at how composites are formed from two or more distinct materials to achieve the best of two worlds.

Reminder - Complete all of the Lesson 8 tasks!

You have reached the end of Lesson 8! Double-check the to-do list on the Overview page to make sure you have completed all of the activities listed there before you begin Lesson 9.

Lesson 9: Types and Applications of Composites

Overview

A host of high-technology applications require materials that have specific and unusual properties that cannot be met by any of the monolithic conventional metals, ceramics, and polymers. Some of these requirements have been met through the judicious combination of two or more distinct materials into composite materials that possess materials properties better than those found in the monolithic classes of materials. In this lesson, we will organize the composites into four main classifications and explore the strengths, as well as many of the current applications of these materials.

Learning Objectives

By the end of this lesson, you should be able to:

  • Define and contrast the use, cost, and ease of fabrication of polymer-, ceramic-, and metal-matrix composites.
  • List and define the four main classifications of composite materials.
  • Note the three common fiber reinforcements used in polymer-matrix composites and, for each, cite both desirable characteristics and limitations.
  • Cite the desirable features of metal-matrix composites.
  • Note the primary reason for the creation of ceramic-matrix composites.
  • Name and briefly describe the two sub-classifications of structural composites.

Lesson Roadmap

Lesson 9 will take us one week to complete. Please refer to Canvas for specific due dates.

Lesson Roadmap
To Read Read pp 246-282 (Ch. 13) in Introduction to Materials ebook
To Watch Monuments to Man: The Impact and Influence of Concrete on Civilization
To Do Lesson 9 Quiz

Questions?

If you have general questions about the course content or structure, please post them to the General Questions and Discussion forum in Canvas. If your question is of a more personal nature, feel free to send a message to the course instructor through Canvas email. The instructor will check daily to respond.

Introduction to Composites

Although humans have used composite materials for millennia, the concept of composites as a distinct classification of materials was not recognized until the mid-20th century. Composite materials are formed from two or more distinct phases of materials. This is in contrast with metal alloys, which we studied in an earlier lesson. In metal alloys, additional atoms, compounds, or phases are dissolved into the base metal. This solid mixing does not result in distinct phases, which are present in composite materials. Possibly the earliest usage of a composite was by the ancient Mesopotamians (circa 3400 BCE) who realized that gluing wood at angles produced better properties than single-ply wood. Modern five-ply plywood has five plies arranged in steps of 45° (0, 45, 90, 135, and 180 degrees) for better strength. A photo of an unknown type of plywood is shown below.

piece of plywood, has several layers
Plywood.
Credit: Rotor DB, CC-BY-SA-3.0, via Wikimedia Commons

Around 1500 BCE in the Fertile Crescent, humans began adding straw to strengthen clay bricks. Human structures were no longer limited to wood or the piling of stone. Unreinforced clay bricks, like most ceramics, are strong under compression stress, but unstable when subject to tensile stresses. So, unreinforced clay bricks carry the load but will readily fall apart. Except for its unstable nature under tensile stresses, clay is otherwise an ideal building material. As a raw material, it is available almost everywhere and, before drying, it can be easily worked into the desired shape. Strengthening clay through the addition of straw, gravel, or bitumen greatly enhances its applicability as a building material. Before moving to the next section, please watch this brief introductory video (2:07) on composites.

To Watch

What is a Composite?
Click for transcript of What is a Composite?

Hello, YouTube! Welcome to the Composite HUB. What exactly is a composite material? Let's find out.

A composite is made by combining two or more different types of materials together. Sounds very simple right? Well, not really. There is actually a small catch to this. Materials that make up a composite do not dissolve in each other. They remain separate. They are just locked together. Most composites are made out of a matrix, which is a soft material. And a reinforcement, which is stiff and strong. Reinforcement usually comes in the form of particles or fibers. And it is completely surrounded by the matrix. Once combined, they give unique properties, which are superior to the individual materials on their own.

You might be surprised to hear that composite materials have been around for 1000s of years. Mesopotamians came up with the first man-made composite around 3400BC when they made plywood by sticking thin wood sticks at different angles. Around 1500 BC Egyptians used mud and straw to create strong buildings. Straw continued to provide reinforcement for ancient products such as pottery and boats. Later in the 1200s Mongols invented the first composite bow using a mixture of wood, bone, animal glue, and birch bark. Until the invention of gunpowder, this was considered one of the most powerful weapons on the earth.

The modern era of composites began in the 1900s when scientists invented plastics and glass fiber. Glass fiber, when combined with a plastic matrix, creates an incredibly strong and lightweight material. Unfortunately, many of the great advancements in composites were as a result of war, as alternative materials were required for lightweight applications in military aircraft.

In the 1970s the composite industry began to mature. Better plastic matrices and reinforcing fibers such as carbon fiber were developed. It has since been replacing metal as the new material of choice. Composite materials are now used in the aircraft industry, automobile industry, sports equipment, and many more.

Hope you enjoyed this video. If you want to know more about composites, subscribe to the Composite HUB.

Credit: Composite Hub

Composite Terms and Classifications

Composite materials are materials which are a combination of two or more distinct individual materials. These combinations are formed to obtain a more desirable combination of properties. This is called the principle of combined action. One example of this principle is the use of composites for aircraft structures. These composites are designed to be lighter weight with comparable strength to metal structural elements that they are replacing. Typically, a composite is formed with a continuous phase called the matrix. As shown in the figure below, the matrix phase surrounds another phase which is discontinuous and referred to as the dispersed phase.

Dispersed phase( looks like little 4 little separated tubes) and matrix phase (area surrounding and separating the tubes)
Matrix phase surrounding another phase which is discontinuous and is referred to as the dispersed phase.
Credit: Fig. 15.1(a), Callister & Rethwisch 5e.

The purpose of the matrix phase is to keep the dispersive phase in place, transfer stress to the dispersed phase, and protect the dispersed phase from the environment. The purpose of the dispersed phase typically depends on which material type it is composed of:

  • Metal dispersive phases are typically used to increase yield strength, tensile strength, and/or provide stability over the life of the product.
  • Ceramic dispersive phases are typically used to produce materials which resist fracture.
  • Polymer dispersive phases are typically used to increase the modulus of elasticity, yield strength, tensile strength, and/or provide stability over the life of the product.

Composites are typically classified by the type of dispersive phase used: particle reinforced, fiber reinforced, or structural. Further details on these different types of dispersive phase types will be forthcoming in the reading for this lesson, but first please watch this short four-minute video introducing composites. Note that in this video what we are calling the dispersive phase they refer to as the reinforcement phase.

To Watch

An Introduction to Composites
Click for the transcript of Intro to Composites.

From huts of mud and grasses to monuments of stone and steel. The rise of modern civilization has been paced by our development of new materials. We began with Earth, wood, and rocks. We built shelters and tools. We harnessed fire and learned to coax metal from stone. And then one day a brickmaker added straw to his clay the result was a stronger brick and the birth of manmade composites.

So what is a composite, really? Simply it's two different materials combined. The uniform substance like portland cement is called a monolithic material. Throw in a handful of gravel another monolithic material and you have concrete. A composite. In a composite you can still see the individual monolithic materials the cement in the gravel they're just locked together.

So why make composites? We combine two similar materials to create a new material that has the characteristics we need for a particular application. Portland cement is pretty tough, but you wouldn't build a bridge out of it. It's not strong enough or durable enough. Throw in some gravel and now it's durable enough for traffic, but still not strong enough to span supports. Drop in a nice grid of steel rebar and now you've got a composite material that's strong enough for bridge decking. In addition to increased strength and durability, composites also allow us to customize materials with the weight flexibility conductivity and stability we need. Although composites can have several different components they all have two things in common a matrix and reinforcement.

In our concrete bridge, the cement is the matrix and the gravel and rebar are both reinforcements. Many modern composites use resins as a matrix. Add wood or wood fibers and you have a broad family of products from plywood and particle board to high-density fiberboard and composite decking panels. Adding glass fiber or fabric as reinforcement creates fiberglass widely used everything from auto body parts and both holes to tennis rackets and swimming pool liners. Many of the most recent advances in composites have been in the field of aerospace where highly specialized fibers such as graphite or on and Kevlar are used to create incredibly strong yet amazingly lightweight materials.

Composites, they're everywhere. Building materials, furniture, toys, sports, equipment, the games we play, the roads we walk, the cars we drive, the planes we fly. Composites -  they make so much of what we do every day possible.

Credit: BioNetwork

Now that you have watched this video, please proceed to the next section.

Reading Assignment

Things to consider...

When you're reading the text for this lesson, use the following questions to guide your learning. Remember to keep the learning objectives listed on the overview page for this lesson in mind as you learn from this text.

  • What are the four main classifications of composite materials?
  • What are the common fiber reinforcements used in polymer-matrix composites, and their desirable characteristics and limitations?
  • What are metal-, ceramic-, and polymer-matrix composites?
  • What are the desirable features of metal-, ceramic-, and polymer-matrix composites, in terms of use, cost and ease of fabrication?
  • What is the primary reason for the creation of ceramic-matrix composites?
  • What are laminates and sandwich panels, and what are their typical uses?

Reading Assignment

Read pp 246-282 (Ch. 13) in Introduction to Materials ebook

Video Assignment: Monuments to Man: The Impact and Influence of Concrete on Civilization

Now that you have read the text and thought about the questions I posed, go to Lesson 9 of Canvas and watch this 45-minute video about the most effective of all building materials, composite concrete, and how humankind has discovered, developed, and utilized it throughout history. In "Monuments to Man: The Impact and Influence of Concrete on Civilization," we see how concrete creates our modern cities and how it affects how humankind works and lives in these concrete jungles.

Video Assignment

Go to Lesson 9 in Canvas and watch the Monuments to Man: The Impact and Influence of Concrete on Civilization Video. You will be quizzed on the content of this video.

Summary and Final Tasks

Summary

Composite materials give us the opportunity to combine two (or more) materials to gain the best of both materials. Many composite materials are composed of a dispersed phase which is embedded into a second phase called the matrix. The matrix completely surrounds the dispersed phase and holds them together. Most composites in use today have been created to have improved stiffness, toughness, and ambient and high-temperature strength. In this lesson, a simple scheme for the classification of composite materials which consists of four main divisions: particle-reinforced, fiber-reinforced, structural, and nanocomposites, was presented and defined. Particle reinforced composites have a dispersed phase which consists of particles whose dimensions are approximately the same in all directions. Fiber-reinforced composites have large length-to-diameter ratio particles (fibers) as the dispersive phase. Structural composites are multi-layered and designed to have low densities and high degrees of structural integrity. For nanocomposites, the dimensions of the dispersed phase particles are on the order of nanometers.

Reminder - Complete all of the Lesson 9 tasks!

You have reached the end of Lesson 9! Double-check the to-do list on the Overview page to make sure you have completed all of the activities listed there before you begin Lesson 10.

Lesson 10: Synthesis, Fabrication, and Processing of Materials

Overview

As materials are formed or processed into useful products the materials undergo changes in their materials properties. These changes can be beneficial or deleterious. Understanding these changes can enhance the performance of the material or, in some cases, prevent unanticipated materials failure. In this lesson, we discuss the common formation and processing methods for metals, ceramics, and polymers, and how these processes can effect the materials properties of the processed materials.

Learning Objectives

By the end of this lesson, you should be able to:

  • Name and describe four forming operations that are used to shape metal alloys.
  • Name and describe five casting techniques.
  • Name and briefly describe five forming methods that are used to fabricate glass pieces.
  • Briefly describe and explain the procedure by which glass pieces are thermally tempered.
  • Briefly describe processes that occur during the drying and firing of clay-based ceramic ware.
  • Briefly describe/diagram the sintering process of powder particle aggregates.
  • Briefly describe addition and condensation polymerization mechanisms.
  • Name the five types of polymer additives and, for each, indicate how it modifies polymer properties.
  • Name and briefly describe five fabrication techniques used for plastic polymers.

Lesson Roadmap

Lesson 10 will take us one week to complete. Please refer to the Syllabus or course calendar for specific due dates.

Lesson Roadmap
To Read Read pp 283-322 (Ch. 14) in Introduction to Materials ebook
To Watch Raw to Ready: Bombardier
To Do Lesson 10 Quiz

Questions?

If you have general questions about the course content or structure, please post them to the General Questions and Discussion forum in Canvas. If your question is of a more personal nature, feel free to send a message to all faculty and TAs through Canvas email. We will check daily to respond.

Reading Assignment 1

Things to consider...

As you do the following reading consider the following questions. Remember to keep the learning objectives listed on the overview page in mind as you learn from this reading.

  • What are the four forming operations that are used to shape metal alloys?
  • What are the five casting techniques?

Reading Assignment

Read pp 283-293 (Ch. 14) in Introduction to Materials ebook

Metal Forming Operations

Now that you have completed the reading assignment regarding the fabrication of metals, let us summarize some of the important points.

To Watch

One way to classify the fabrication of metals is into the categories of (mechanical) forming, casting, and miscellaneous methods. There are four types of forming processes: forging, rolling, extruding, and drawing. I like to refer to these as pounding, rolling, pushing, and pulling. Hopefully, by the end of this section, you will understand why I use those terms.

Blacksmiths have been hammering (pounding) metals into shape for some time. Today, we have large machines which pound and stamp metals into shape. Please watch this very brief (00:33) video on metal forging which shows exactly that: Forging [38]. (If that video is currently unavailable, check out this Forging video [39] instead.)

Putting metal between rollers is an effective way to create thin sheets, here is a very brief (1:23) video which shows how rolling is done: Roll Forming [40]. (If that video is currently unavailable, check out this Roll Forming video [41] instead.)

In extruding, metal is PUSHED through dies which controls the final profile of the metal piece. Please watch this brief (1:56) video on metal extruding: Extrusion [42]. (If that video is currently unavailable, check out this Extrusion video [43] instead.)

For the last process, drawing, please proceed to the next section.

How are Aluminum Cans Made?

The last of the four mechanical forming processes, drawing, is one of the processes discussed in the following video (4:45) on How are Aluminum Cans Made? While you are watching this video, please think back, way back, to Lesson 1 of this course and the reading in the textbook about different materials used for carbonated beverage containers. And remember to look for the drawing operation (hint: it is the operation that gives the can its height).

To Watch

How are Aluminum Cans Made?
Click for the transcript of How it's made - Aluminum cans.

The next time you buy a can of soft drink consider this: the aluminum tin can will always be recyclable. Unlike plastic, aluminum never deteriorates no matter how often it's melted down and used again. Aluminum cans are so lightweight, that it's hard to believe that they're made from a huge roll of our minyan sheeting that weighs nine tons. The sheet is about a meter and a half wide and a roll like this is long enough to make three-quarters of a million drink counts. The sheet feeds into a press that punches out round pieces that will be formed into counts. The punch press actually performs two operations it punches out a disc 14 centimeters in diameter then bends it into a cup. What's left of the sheet gets compacted and sent back to the aluminum factory where it's recycled into new rolls. The cup goes into a machine of the draw and iron body maker. A tool draws out the aluminum forming the body of the camp. The tool is lubricated, so it won't tear the aluminum while stretching it. The lubricant also acts as a coolant because the aluminum heats up as it's being worked. Once the body is formed a trimmer cleans and straightens the edges. Now the cans move along upside down on the conveyor belt over to the washer. The washer performs a six-stage cleaning. The first two washes are in hydrofluoric acid at 60 degrees Celsius. The last four washes are in deionized water neutral water with no pH also at 60 degrees. The cans come out of the washer and go under the hot air dryer. They're now shiny because the hydrofluoric acid wash removed a thin surface layer of aluminum.

Next, a roller passes over the cans coating the bottom rims with varnish. This coating allows the cans to slide easily on conveyor belts and in vending machines and shows up as a blue ring under an ultraviolet light. The cans are now ready to be printed this rotation printing system can apply up to five colors one at a time. Then a layer of varnish is applied to protect the ink. Even in slow motion, it's a high-speed operation, and here's the actual speed: 1800 cans per minute. Next, the cans fly through an oven that instantly hardens the ink and dries the protective varnish. Next, the Machine sprays a water-based varnish on the inside of the cans. This creates a barrier between the drink and the aluminum so the drink won't end up tasting like metal. It also prevents the aluminum from being eaten away from the inside by the acid in carbonated drinks. Next, the cans go through a machine called the Nekor which forms a 5-centimeter neck on the can. This is done gradually in 11 steps so as not to puncture the paper thin aluminum.

The next machine called the flanger forms a curved over the edge at the top of the camp which will later attach to the pull tab cover. The cans pass through a sophisticated vision system that photographs the inside of each can any can that doesn't meet standards, that has a bump or some ink inside is sent back for recycling. From here, they're shipped to the drinks company which fills them and then attaches the pull tab cover. So, now you know. You can start the whole process off by recycling your cans.

Credit: How It's Made

Now that we have reviewed the four mechanical forming processes for metals: forging, rolling, extruding, and drawing, hopefully, you also understand why I refer to them as pounding, rolling, pushing, and pulling. In the next section, we will look at how metals that cannot be mechanically formed are typically formed.

Casting

Not all metals are amenable to the mechanical deformation which occurs with mechanical forming processes discussed in the previous sections. Those metals that can undergo mechanical forming are referred to as wrought metals. For those metals that are not amenable to mechanical deformation, they are typically cast.

Casting is the process in which molten metal is poured (or cast) into molds. In the reading, you were introduced to five different casting techniques: sand, die, investment, lost foam, and continuous. Typically, it is more economical to use mechanical forming processes, since it requires more energy to heat metals until molten in the casting process. However, there are times when casting makes more sense, in addition to the obvious case of a metal not being amenable to mechanical deformation. Some of those cases include when making complicated shapes or when prototyping a part. When prototyping, the cost of making a forging die might be much more expensive than the cost of molds.

To Watch

Please watch the following video (02:54) on Metal Casting [44]. (If this video is unavailable please watch this Metal Casting [45] video.)

Now, please go to the second reading (2 of 3) of this lesson and read about how ceramics are fabricated.

Reading Assignment 2

Things to consider...

While doing the next reading, use the following questions to guide your reading. Remember to keep the learning objectives listed on the overview page of this lesson in mind as you learn from this text.

  • What are five forming methods that are used to fabricate glass pieces?
  • How are glass pieces thermally tempered?
  • What are the processes that occur during the drying and firing of clay-based ceramic ware?
  • What is the sintering process of powder particle aggregates?

Reading Assignment

Read pp 294-308 (Ch. 14) in Introduction to Materials ebook

Ceramic Fabrication Methods

The fabrication methods of ceramics are classified in three categories: glass-forming, particulate forming, and cementation. In glass-forming processes, the raw materials are heated until they melt. There are five glass-forming processes: blowing, pressing, drawing, fiber-forming, and sheet-forming. The following five-minute video highlights automated glass blowing for the production of glass bottles. Again, while you are watching this video, please think back to Lesson 1 of this course and the reading in the textbook that covered different materials used for carbonated beverage containers.

To Watch

How Glass Bottles are Made
Click for transcript of How It's Made - Glass Bottles.

Whether they're colored or clear, glass bottles and jars are green. No trees die to make this eco-friendly packaging. Glass is made of natural ingredients that are abundant. You can recycle glass endlessly, and making it uses less energy than producing metal or plastic.

The recipe for glass combines about a half a dozen natural raw materials, but the main ones are silica sand, soda ash, and limestone. Silica sand usually makes up about forty-five percent of the batch. The soda ash helps melt the silica evenly. It comprises about fifteen percent. A limestone content of about ten percent makes the finished glass more durable. They combine these ingredients with recycled glass called cullet. The factory's equipment feeds precise amounts of the materials into a furnace. Over a full day, the fiery heat two thousand seven hundred and thirty degrees Fahrenheit melts everything together producing a gooey liquid that's the consistency of honey. The molten glass pours out of the furnace. Shears cut the flow at precise intervals to produce cylindrical gobs. Each gob is the exact amount required to make one bottle or jar they dropped to a device called the scoop the scoop moves them two troughs that feed them to jar forming and bottle forming machines.

A gob of molten glass goes into a preliminary mold. In a matter of seconds, it comes out as what's called a parison a miniature version of the final bottle. Each parison and then moves into a blow mold the cavity of which is the shape of the final bottle. The equipment blows the compressed air into the parison stretching the glass outward toward the wall of the mold cavity. This process creates the final bottle shape and hollows out the inside. These are amber colored beer bottles. The color is produced by adding small amounts of iron-sulfur and carbon to the glass mix.

The factory uses a similar manufacturing process to produce other types of bottles and jars and this run the company is making 375-milliliter wine bottles out of clear glass. This run is producing 375-milliliter liquor bottles also out of clear glass, but this mold has a special feature a recessed insignia on one of the walls which produces a raised insignia on the front of the bottle.

After the bottles leave the forming machine they travel through flames. Otherwise, they would cool down too quickly and crack from thermal shock. A loader now gently pushes the bottles into what's called an annealing lehr. The bottles cool a controlled rate as they move through the lehr. This releases stress from the glass gradually. As the bottles exit the annealing lehr, a sprayer coats their exteriors with lubricant. This enables them to move smoothly through the rest of the inspection and packaging line. The bottles now line up in single file to head into the automatic inspection station. As the machine spins each bottle, cameras and probes check for imperfections such as cracks or bubbles. The inspection equipment that examines the top to check dimensions and ensure the threads for the screw cap are molded correctly. Before shipping, a worker does a final visual inspection. The proportion of cullet in glass can be as high as 90 percent. Cullet melts at a lower temperature, so for every ten percent of cullet in the mix the factory uses up to two and a half percent less energy to produce this glass. Now that's an incentive to recycle.

Credit: How It's Made

In the next section, we will discuss the important subject of heat treating glass to control stress.

Heat Treating Glass

When fabricating glass, it is usually vitally important to control the cooling of the fabricated pieces. Due to the brittle nature of ceramics, failure to remove internal stress in the glass either introduced during fabrication or due to uneven cooling will likely result in catastrophic structural failure of the piece. There are two basic types of heat treatments applied to glasses. In annealing, cooling is controlled in an effort to remove (or minimize) the internal stress in the glass. This is in contrast with tempering. In tempering, compressive stress is intentionally introduced into the surface of the piece as shown in the figure below. This compressive stress can prevent surface scratches and cracks from growing, which would likely fracture the glass.

see long description below.
Tempering: Compressive stress is intentionally introduced into the surface of the piece.
Click for a text description of image.
Compressive stress process: Before cooling the surface is very hot but once initial cooling begins the outside of the surface is cooler than the inner core. Once the material reaches room temperature there is compression on the outside surfaces and tension on the inner core. As a result surface crack growth is suppressed.
Credit: Callister

In the next section, we will discuss sintering, which is very important for particulate forming of ceramics.

Sintering

During powder press processes for the formation of ceramics, heat and pressure are used to densify and bind ceramics together as illustrated in the figure below in a process called sintering. Unlike melting, during sintering, materials are not liquefied, but instead, rely on reducing surface area effects between particles to drive the process. Ceramic materials usually have a very high melting temperature, so sintering (which is done at temperatures well below bulk melting temperatures) offers significant savings in terms of energy.

4 circles form a square. Then circles smoosh together. Pore in middle. Grain boundary where they touch, Neck at divets between circles
During powder press processes, heat and pressure are used to densify and bind ceramics together.
Credit: Callister & Rethwisch 5e

Now, please go to the third reading (3 of 3) of this lesson and read about how plastics are fabricated.

Reading Assignment 3

Things to consider...

While you do the following reading let the following questions guide your reading. Remember to keep the learning objectives listed on the overview page for this lesson in mind as you learn from this text.

  • What are the addition and condensation polymerization mechanisms?
  • What are the five types of polymer additives and, how do they modify polymer properties?
  • What are the five fabrication techniques used for plastic polymers?

Reading Assignment

Read pp 308-322 (Ch. 14) in Introduction to Materials ebook

Polymer Formation

As we discussed in the polymer lesson, there are two types of polymerization: addition (or chain) polymerization and condensation (or step) polymerization. In addition polymerization, a free radical attaches to a monomer. This results in an unsatisfied bond on the monomer, which is free to attach to another monomer. This process repeats over and over again building a polymer chain. In condensation, two chemical groups react together. Typically, one of the groups has an exposed hydrogen, while the other has an exposed oxygen-hydrogen. When the two compounds join, a monomer is formed with an exposed oxygen-hydrogen or hydrogen and releases a water molecule, H2O.

Polymers are synthesis by polymerization and the polymer properties are modified by the usage of additives. These additives are used to improve mechanical properties, processability, durability, etc. The five additive types discussed in the e-book are fillers, plasticizers, stabilizers, colorants, and flame retardants. Fillers are added to improve tensile strength, abrasive resistance, and toughness, as well as to reduce cost. Plasticizers are added to transform brittle polymers to ductile ones. Stabilizers are added to protect from degradation due to exposure to ultraviolet light. Colorants are added to provide color to the polymer. Flame retardants are added to eliminate or reduce the flammability of polymers.

Fabrication of plastic polymers can utilize one of several molding techniques: blowing, compression, injection, and transfer, or by extrusion or casting. Fibers can be spun or drawn. Films can be formed by extruding, blowing, or calendaring. The following video [46] (4:50) highlights blow molding for the production of plastic bottles. Again, while you are watching this video, please think back to Lesson 1 of this course and the reading in the textbook about different materials used for carbonated beverage containers.

To Watch

How Plastic Bottles are Made
Click for the transcript

Whether you're buying apple juice or peanut butter, you've probably noticed that fewer products come in glass containers these days. Plastic packaging is becoming more common. Plastic bottles and jars are lighter to carry and leave no shards of glass to clean up if you drop your grocery bag. Many transparent bottles and jars are made from a type of plastic called polyethylene terephthalate or PET. An automated mixer combines PET pellets with flakes of recycled PET. Reprocessed plastic loses some of its physical properties, so the recycled content cannot exceed ten percent. The PET drops from the mixture into a plastic injection machine that heats it to a piping 600 degrees Fahrenheit. The dry raw material melts into thick and gooey liquid plastic. The machine then shoots it at high pressure into a mold. This plastic injection molding process casts pieces of plastic called preforms. Starter shapes and subsequent machines will transform into bottles or jars. The molded preforms harden almost instantly thanks to a built-in cooling system. These preforms are now on their way to becoming single serving juice bottles.

This is another plastic injection molding machine. It uses the same method to make preforms for a different model: one-and-a-half to two-liter bottles. The preforms next stop is a machine called a reheat stretch blow molder. In a matter of seconds, it heats each preform just enough to make the plastic malleable, then inserts a rod to stretch the preform lengthwise while at the same time blowing in air at extremely high pressure. This forces the preform into a bottle shaped bowl. Cold water circulates within the mold to cool and set the plastic almost instantly. This lightning fast machine turns out ten thousand six hundred bottles per hour. No wonder we've had to show it to you in slow motion. A conveyor belt transports the finished bottles to the packaging area. Before blow molding, the preforms for certain models first pass through an oven.

Credit: How's it Made

You have now finished the reading for Lesson 10. Please proceed to the next page and watch the video for Lesson 10.

Video Assignment: Raw to Ready: Bombardier

Now that you have read the text and thought about the questions I posed, go to Canvas and watch this 53-minute video about how glass, titanium, fiberglass, lacquer, and aluminum alloy become a jet. In "Raw to Ready: Bombardier," we see how various components are painstakingly fabricated beginning with the raw materials to the final assembly into the regional jet aircraft.

Video Assignment

Go to Lesson 10 in Canvas and watch the Raw to Ready: Bombardier video. You will be quizzed on the content of this video.

Summary and Final Tasks

Summary

Materials are formed or manufactured into components that are incorporated into useful products. During these processes, the properties of the materials can be enhanced or adversely affected. Knowledge of these effects and the economic costs are many times needed to successfully bring a product to market. In this lesson, we looked at the most widely used fabrication and synthesis techniques for metals, ceramics, and polymers, as well as, discussed how these processes impact materials properties.

Reminder - Complete all of the Lesson 10 tasks!

You have reached the end of Lesson 10! Double-check the to-do list on the Overview page to make sure you have completed all of the activities listed there before you begin Lesson 11.

Lesson 11: Biomaterials and Smart Materials

Overview

The environment can have a large deteriorative effect on materials over time, including corrosion and degradation. For biomaterials, the materials have the additional condition for the use of having to be able to survive the unique environment of biological systems. In this lesson, we explore issues around biomaterials including structural requirements, functional requirements, biocompatibility, and ethical concerns. 

Learning Objectives

By the end of this lesson, you should be able to:

  • Distinguish between biological, biomaterials, bio-based materials, and biomimetic materials.
  • Explain the differences between structural and functional biomaterials.
  • List and discuss several examples of structural biomaterials applications.
  • List and discuss several examples of functional biomaterials applications.
  • Identify moral and societal issues involved in the usage of biomaterials.
  • Explain the connection between smart materials and biological systems.

Lesson Roadmap

Lesson 11 will take us one week to complete. Please refer to Canvas for specific due dates.

Lesson Roadmap
To Read Read the Biomaterials pages in Canvas under Lesson 11, plus the few pages included here.
To Watch Making Stuff: Smarter
To Do Lesson 11 Quiz

Questions?

If you have general questions about the course content or structure, please post them to the General Questions and Discussion forum in Canvas. If your question is of a more personal nature, feel free to send a message to all faculty and TAs through Canvas email. We will check daily to respond.

Introduction to Biomaterials

Before beginning a discussion of biomaterials there are several different terms that we should define. One way of classifying biomaterials is to use the following four materials classifications: biological materials, biomaterials, bio-based materials, and biomimetic materials.

Biological materials are materials that are produced by living organisms, such as, blood, bone, proteins, muscle, and other organic material. Biomaterials, on the other hand, are materials which are created specifically to be used for biological applications. These applications can include bone replacement, skin replacement, membranes for dialysis, artificial limbs, etc. Bio-based materials are materials that are derived from living organisms but are repurposed for other applications. One example of a bio-based material would be enzymes mass-produced by microbes to be used in the synthesis of drugs. Biomimetic materials are materials that are physically or chemically similar to materials produced by living organisms.

In the textbook reading for this lesson, materials will be classified as structural or functional and then the natural biological material will be compared and contrasted with the biomaterials designed to replace or interact with it.

Structural biomaterials, as the name implies, have as their primary function physical support and structure. Structural biomaterials are sometimes referred to as inert biomaterials. Functional biomaterials (also known as active biomaterials) have a non-structural application as their primary function. An example of a functional biomaterial would be membranes used during dialysis to filter impurities from blood. 

An example of a structural biomaterial would be a titanium steel implant with a ball and socket being used as a hip replacement. Two other terms that might be helpful to define before the reading are immune response and biocompatibility. During the body’s immune response, the body sends white blood cells to attack and destroy foreign material. Biocompatible materials are those biomaterials which typically do not elicit the body’s immune response during the operational lifetime of the biomaterial in the body.

Stainless steel hip replacement, ball is white, the rest is silver and pointed
Stainless steel and ultra high molecular weight polyethylene hip replacement.
Credit: Science Museum London / Science and Society Picture Library via Wikimedia Commons [47]

Now that we have covered a few basic terms please continue to the next section and begin the reading for this lesson.

Reading Assignment

Things to consider...

When you read this chapter, use the following questions to guide your reading. Remember to keep the learning objectives listed on the previous page in mind as you learn from this text.

  • What is the difference between biological, biomaterials, bio-based materials, and biomimetic materials?
  • What are structural biomaterials and where are they currently being used in the body?
  • What are functional biomaterials and where are they currently being used in the body?
  • Many biomaterials are available on a limited basis, who decides who gets them and why?

Reading Assignment

Read the Biomaterials pages in Canvas under Lesson 11, plus the few pages included here.

Ethics of Biomaterials

Ethical issues raised by the use of biological materials are numerous and so complex that an entire field of study known as bioethics has been created. 

Will the biomaterial be safe or potentially be harmful to the body in the near term and long term? Will data obtained during testing on animals justify the suffering and sacrifice of living creatures? Will professional and financial interests by researchers result in conflicts of interest which could taint trial data? Should supply and demand, and profit, allow biomaterials companies to charge "what the market will bear"? When evaluating a new biomaterial product what should be the balance between sustaining life versus quality of life issues? What should be the role of regulatory agencies? Should access to biomaterials be determined by medical need or ability to pay? How does society ensure that humans living in the Third World have access to current advances in biomaterial applications? How does society balance scientific advancements in the area of biomaterials with religious doctrines, which are sometimes at odds with those advancements?

Clearly, we could spend another course just on the topic of ethics in biomaterials. Hopefully, the reading in the lesson and on the website has made you aware, if you were not already, of this important subject. In the next section of our website, we will be looking at a biomaterial which is also a smart material.

Nitinol

In the lesson reading this week vascular stents were covered, including the revolutionary nitinol stents. When the body heats up this smart material ‘remembers’ its initial programmed shape. So, in addition to being a biomaterial, nitinol is a smart material as well. What are smart materials? Smart materials are materials that are designed to mimic biological behavior. They are materials that, like biological systems, ‘respond to stimuli.' More smart materials will be presented in the video for this lesson, but right now please watch this short video (1:27) on the amazing nitinol.

To Watch

WTF Paperclip!? (Nitinol)
Click for transcript

This is no ordinary paper clip. In fact, it's made from nitinol and it has some unique qualities. Let's bend it into a completely different shape. Okay, I think that should do. Now watch what happens when I drop this into some hot water. Now that is pretty darn cool. Nitinol is also known as memory metal and once it reaches a certain heat the atoms become locked into the previous arrangement that they were forged in. This is also the same material that magicians use to bend spoons. Sorry to ruin the magic. So there we have it. Nitinol. Don't forget to check out my Facebook and Twitter the links are below and as always I will see you next time. Thanks for watching.

Credit:Andy Elliott

Now proceed to the next section to watch the video for this lesson. As you watch this lesson, see if you can answer the following for each of the smart materials presented: what is the stimulus and what is the response?

Video Assignment

Now that you have read the text and thought about the questions I posed, take some time to watch this 54-minute video about one type of advanced materials (smart materials) that sense their environment and, in some cases, can even adapt to their environment. As you watch this video see if you can find the following:

  1. Smart materials are defined by their ability to sense and respond and not by the materials properties (chemistry and atomic structure) that we use for classical materials classification. Another way of stating this is, smart materials are defined by their function, not by their materials properties. As you watch this video attempt to separate the function from the materials properties by classifying the materials (metal, ceramic, polymer, composite, semiconductor, biomaterial, nanomaterial) that are utilized in the smart material examples of this video.
  2. Some materials applications are heavily dependent not only on the materials that compose their parts but also the physical structure of their parts. Hook and loop fasteners, e.g., Velcro™ tape, is an example of this. Hook and loop fasteners are composed of two parts that are typically constructed of the same synthetic polymer. However, it is the physical structure of the two parts that make hook and loop fasteners so widely used today. One side is composed of hooks, while the other is composed of loops, which combined with the material properties of polymers make hook and loop fasteners the go-to fastener where temporary bonds are required. Some of the smart materials highlighted in this video depend on the physical structure to properly function. See if you can spot which smart materials these are.

Video Assignment

Go to Lesson 11 in Canvas and watch NOVA's Making Stuff: Smarter Video. You will be quizzed on the content of this video.

Summary and Final Tasks

Summary

Biomaterials and smart materials are two of the four advanced materials that we discussed in Lesson 1 of this course. Unlike the classical classifications of materials (metals, ceramics, polymers, and composites), advanced materials are defined by their function, i.e., what role that they serve. Biomaterials can be metals, ceramics, polymers, composites, or combinations of these, that are used inside the body. They can serve structural and/or functional purposes within the body. Of course, an important consideration is how bio-compatible the material is, which determines whether the material can be used, where in the body, and the useful lifetime of the material. Smart materials can be metals, ceramics, polymers, composites, or combinations of these, that mimic life. These materials 'respond to stimuli'.

Reminder - Complete all of the Lesson 11 tasks!

You have reached the end of Lesson 11! Double-check the to-do list on the Overview page to make sure you have completed all of the activities listed there before you begin Lesson 12.

Lesson 12: Semiconductors and Nanomaterials

Overview

Conventional integrated circuit technology is approaching its theoretical limits. Scientists and engineers are turning to materials which utilize quantum dynamics to push past the conventional materials limits. Nanoelectronics is a promising replacement possibility with a wide range of potential future applications. In this lesson, we introduce the basics of semiconductor technology, as well as the basics of going nano.

Learning Objectives

By the end of this lesson, you should be able to:

  • Describe the four possible electron band structures for solid materials.
  • For a p–n junction, explain the rectification process.
  • Explain the concept of nanotechnology as it applies to materials.

Lesson Roadmap

Lesson 12 will take us one week to complete. Please refer to Canvas for specific due dates.

Lesson Roadmap
To Read

Read pp 323-347 (Ch. 15) in Introduction to Materials ebook

The following required readings can be found in Canvas:
Pages 245 to 251 (Chapter 32) of Materials in Today's World by Peter Thrower and Thomas Mason
Pages 253 to 255 (Chapter 33) of Materials in Today's World by Peter Thrower and Thomas Mason

To Watch Making Stuff: Smaller
To Do Lesson 12 Quiz

Questions?

If you have general questions about the course content or structure, please post them to the General Questions and Discussion forum in Canvas. If your question is of a more personal nature, feel free to send a message to all faculty and TAs through Canvas email. We will check daily to respond.

Energy Bands

Now that you have finished the reading for this lesson, I would like to review the four possible electron band structures for solid materials, as well as p-n junction electrical behavior. In addition, we will define nanotechnology and explore one possible application of nanotechnology which might allow for the continuing improvement in microprocessor speed in coming decades.

So, what happens when you try to shove a large number ( >1023) atoms together to make a solid? When atoms are separated, electrons will tend to occupy the lowest available discrete energy states. When atoms are brought together, the electrons are forbidden by the Pauli exclusion principle of having identical energy and quantum numbers. As shown in the figure below, as the atoms are brought closer and closer together individual allowed energy states start to spread in energy.

Graph showing 1s and 2s energy bands spreading into 12 states each as they are brought closer together. 2s is higher energy than 1s
As atoms are brought closer and closer together, individual allowed energy states start to spread in energy.
Credit: Fig. 12.2, Callister & Rethwisch 5e

When you bring ~1023 atoms together to make a solid, the separation between the allowed energy states becomes indistinguishable (too small for us to measure). Since we can no longer distinguish the individual states, the allowable energy states form a range of energies that electrons can occupy. The ranges of allowable energies are referred to as bands. These bands are shown in the figure below. In this figure allowable energy levels are plotted versus interatomic separation. However, since there are too many levels to distinguish the band splitting is represented by a shaded region, instead of the individual levels shown in the previous figure. At the equilibrium interatomic spacing, i.e. the average distance between atoms in the stable material, the lowest energy states are not spread. These are 1s electrons of the atoms which are the electrons of the atom that are close to the nucleus of the atom. As such, they do not interact with the other atoms 1s electrons and do not have to spread in energy to satisfy the Pauli Exclusion Principle. However, for this atom at the equilibrium interatomic spacing depict in the figure below, the 2s and 2p states show clear energy splitting. The range of energy splitting for the 2s and 2p states are shown to the left of the graph as energy bands along with the 1s energy level.

Graph w/ interatomic separation on x-axis & energy on y-axis. Energy bands r marked @ equilibrium interatomic spacing
Energy bands.
Credit: Fig. 12.3, Callister & Rethwisch 5e

Energy Bands (Continued)

As shown in the figures below, there are four possible band configurations.

partially filled band with a gap and then an empty band
(a) Partially filled band.
Credit: based on Callister
two filled bands. Top filled band overlaps with a smaller unfilled band
(b) Overlapping bands.
Credit: based on Callister

In the cases of (a) and (b), empty energy states are readily available and electrons (with a little bit of thermal energy) are able to speed through the material, similar to the cars pictured below.

few cars on a road
Cars driving. 
Credit: Vincent van Zeijst - Own work, CC BY-SA 3.0, Wikimedia Commons [48]
filled bands followed by a large gap & then an empty conduction band. An arrow from the filled bands can't get to the empty states & returns
(c) Insulators have a wide band gap (>2 eV) with few electrons excited across the band gap.
Credit: based on Callister
filled bands followed by a small gap & then an empty conduction band. An arrow from the filled bands goes to the empty states
(d) Semiconductors have a narrow band gap (<2 eV) with more electrons excited across the band gap.
Credit: based on Callister

In cases of (c) and (d), insulator and semiconductor, the bands are completely filled and electrons have no mobility, because like the cars in the figure below, the electrons cannot move because there are no available open spaces to move to.

traffic jam with many lanes of traffic
Traffic jam.
Credit: Australian cowboy - Own work via Wikimedia Commons [49]

In the case of semiconductors, applying a voltage can boost the electrons across the gap. This would be like kicking one of the cars from the traffic jam over a medium to an unpacked highway. Thus, the semiconductor can be changed from being an insulator (off) to a conductor (on). We will look at one aspect of this behavior in the next section.

PN Junction

In a semiconductor, it is possible to dope the material with impurities that add electrons or holes to the semiconductors. (Think of the holes as adding more open spots on the freeway so that cars can move more easily.) When atoms with extra electrons are added, they are electron donors and the semiconductor is said to be doped to n-type. When atoms that accept extra electrons are added, they are electron acceptors and the semiconductor is said to be doped p-type. When p-type material is put together with n-type material you get the basic building block of the integrated circuit industry. Please watch the following video (10:36) on how p-n junctions work.

To Watch

The PN Junction. How Diodes Work.
Click for a transcript

We can find semiconductor p-n junctions in many places. They form part of electronic and optoelectronic devices such as solar cells that transform solar energy into electrical energy; light emitting diodes, known as LEDs; rectifier diodes and transistors. To understand what semiconductor materials are and how p-n junctions are fabricated, we need to dive into the atomic world.

Currently, the most well-known semiconductor is silicon. In a silicon crystal, each atom is bonded to its neighbors by four electrons forming covalent bonds. At low temperatures, these electrons remain in the covalent-bonds. When the temperature rises, some of the electrons in the bonds are able to gain thermal energy and escape. They are now free to move and to conduct electricity. At the same time, the broken bonds can be occupied by electrons from other bonds. For these electrons to move, no additional energy is required on average. This broken-bond or new state is termed hole and behaves as a particle of positive charge and mass.

Impurities can be introduced into the semiconductor, substituting atoms of a different atomic species for the silicon atoms. If the new atom has five electrons in its outer shell, four of them will replace the four electron-bonds of silicon. The extra electron will be loosely bound to the impurity. At room temperature, this fifth electron is liberated from its original atom, becoming a conduction electron. Consequently, the impurity acquires a positive charge. This may result in the number of electrons in the doped material exceeding the number present in a pure semiconductor. The number of implanted impurities can be controlled using the fabrication technology. A semiconductor containing these impurities is called an N-semiconductor, since it has negative charge carriers. The impurities are named "donor" impurities, since they donate electrons.

An impurity with only three electrons in its outer shell can also be used. The three outer electrons complete three of the four bonds. The fourth bond remains unoccupied. However, at room temperature the electrons from other bonds can move in to occupy this free space, creating a hole in the material and a negatively charged impurity. As in the previous case, the number of implanted impurities can be controlled using the fabrication technology. So, the number of holes in this doped material can be much greater than the number of holes in a pure semiconductor. A semiconductor of this type is called a P-semiconductor, because it has positive charge carriers, and these impurities are named acceptor impurities, since they accept an electron.

A p-n junction is a structure formed by neighboring regions with different dopings, p-type and n-type semiconductors. The p-n junction is a crucial part of many devices, such as, for example, the diode. If a positive voltage drop is applied between the p terminal and the n terminal of a diode, a large current can be observed experimentally. If we change the connectors and a positive voltage drop is applied between the n terminal and the p terminal, an extremely small current, negligible for most practical applications, is observed experimentally. The p-n junction shows this asymmetric behavior.

The current can flow in one direction but not in the other. This is a peculiar behavior, which enables a wide spectrum of applications in circuits OR which can be made use of in circuits in unique ways OR which can be very useful in circuits. To understand this particular feature of the p-n junction, we must consider two mechanisms that create an electric current: the diffusion mechanism and the drift mechanism.

One way to understand the diffusion mechanism is to imagine two sets of different-colored particles concentrated in two distinct zones. If the particles are free to move in different directions, their random motion tends to equalize their concentration in the whole volume. Diffusion is the physical mechanism which gives rise to free particles trying to occupy the maximum possible volume. The drift mechanism is a movement caused by an electric field. This electric field makes the positive charge carriers move in one direction, and the negative charge carriers in the other. If there is an electric field in a region of space, there will be an electric potential associated with it. The electric field points in the direction in which the electric potential decreases.

The varying electric potential acts as a barrier, preventing the charge movement. Its effects can be understood with the following analogy: let us consider a body moving at a certain speed in the gravitational field. If the body rises, it loses kinetic energy and gains potential energy. If the initial kinetic energy is not sufficient, the body will be unable to cross the barrier, but in the event that the initial kinetic energy is enough, the body may be able to surmount it and even have sufficient kinetic energy left to enable it to continue its movement. Similarly, the electric potential behaves like a barrier to the charged particles: it allows the particles to surmount it, whenever the kinetic energy is great enough. The process of fabricating a p-n junction begins with an n-type or p-type doped semiconductor into which the opposite type of impurity is introduced. To understand how this structure works and what physical processes take place in it, a didactic model is used. The model consists of a P semiconductor perfectly matched to an N semiconductor. The P semiconductor has a much higher hole concentration than the N semiconductor. Therefore, holes from the P region will diffuse into the N region. Similarly, electrons from the N region will diffuse into the P region.

The diffusion of electrons and holes creates a region depleted of free charge particles, leaving behind the ionized impurities from which these charged particles come. Thus, a region of positively ionized impurities and a region of negatively ionized impurities appear in the p-n junction. This special distribution of charges creates an electric field. The electric potential associated with this field acts as a barrier that prevents the displacement of the electrons and holes. Equilibrium is reached when the diffusion current equals the drift current.

The potential barrier is an obstacle for the diffusion current in the device. It is possible to reduce the height of this potential barrier by the application of an external voltage. This increases the electric current. By applying an external voltage from a battery, the height of the potential barrier in the junction is modified. If a positive voltage drop is applied between the P and N regions, the barrier height is reduced. A reduced barrier cannot prevent electrons and holes from diffusing across the structure. An electric current appears in the junction due to the diffusion mechanism. Under these conditions, the p-n junction is said to operate under forward bias.

If the voltage is reversed, and becomes greater in the N than the P region, the barrier height increases, preventing the electron and hole diffusion. The electric current is then negligible. In conclusion, the p-n junction can only conduct in a single direction, giving rise to a current which increases very rapidly when the potential barrier is significantly lowered. Besides being present in countless circuits and electronic components,p-n junctions can also be found in optoelectronic applications: in devices such as LEDs, photodiodes, and solar cells.

The origin of the light emitted from an LED can be found in the physical phenomenon of recombination. Recombination is a process where an electron and a hole are annihilated, releasing energy. In the case of certain materials, and under forward bias, this energy is emitted as light. The more electron-hole pairs recombine, the more intense this light is. The operation of the photodiodes and solar cells is based on the opposite physical phenomenon: generation. Thus, a photon can create an electron-hole pair, which by its movement can generate an electric current. To summarize, p-n junctions are ubiquitous in our environment, close and distant OR near and far. It seems unbelievable that such a simple device is so useful and affects so much in our lives.

Credit:Teaching Innovation Project 11-293 of Universidad de Granada (Spain)

Now let’s define nanotechnology and explore one possible application of nanotechnology which might allow for the continuing improvement in microprocessor speed in coming decades.

Introduction to Nanotechnology

Nanotechnology involves the control of atoms and molecules to produce materials in the size range of 1 – 100 nm, whose size or geometry dominates their material properties. Nanotechnology occurs in a size range were quantum mechanics dominate, but the materials are larger than a single atom. This size range is the range where single-atom behavior is transitioning to bulk material behavior. This allows for the tuning of properties to desirable results, which allows for the creation of designer materials. An example of this is gold nanoparticles. As shown in the figure below, nano-sized gold particles range in color from bright red, pink, purple, to blue, depending on the size of the nanoparticles.

Vials of different gold particle sizes. small particle solution is red, getting larger is pinks then dark purple then largest is lavender
Solutions containing gold nanoparticles.
Credit: Aleksandar Kondinski, GFDL, via Wikimedia Commons [50]

So, how many atoms are we talking about when we say the material ranges from 1 to 100 nm in size?

A cube of 1 nm on a side would have around 100 atoms, while a cube of 100 nm on a side would have around 100 million atoms. That is quite a range. A former professor of this course, Dr. Peter Thrower, in his textbook Materials in Today’s World, calculated how many atoms would be on the surfaces of cubes of 1 nm (nanocube) and 1 cm (bulk cube). What he found was that in the nanocube, 60% of the atoms were on the surface of the cube. This was in stark contrast to the bulk cube, where only one out of 109 atoms were on the surface.

What does this mean chemically? Bulk gold is highly unreactive; it does not tarnish, it does not react with other metals, etc. It is so unreactive that it's possible to find gold in nature in its native state, gold nuggets. Nanogold, on the other hand, is extremely reactive. In bulk gold, the atoms are overwhelmingly non-surface atoms. These non-surface atoms have sufficient neighboring atoms to satisfy their bonds. In nanogold, most of the atoms are on the surface and possess unsatisfied bonds, which makes them extremely chemically reactive. This is a case where the size of the particle matters but also the geometry matters. In the case of nano gold, the geometry of possessing mostly surface atoms results in a chemically active material.

Introduction to Nanotechnology (Continued)

Nanotechnology, through the control of atoms and molecules, has the potential to create unique materials with wide-ranging applicability including the areas of medicine, smaller (faster) devices, self-assembled structures, and other designer materials applications. Nano materials are also allowing scientists to explore still unanswered fundamental materials questions. The field of nanotechnology is generally accepted to have been identified by Nobel laureate Richard Feynman during a futuristic talk entitled, "There’s Plenty of Room at the Bottom," in 1959. In that presentation, Feynman speculated about being able to write the entirety of the Encyclopedia Britannica on the head of the pin. On September 28, 1989, an IBM physicist, Don Eigler, became the first person to manipulate and position individual atoms. One example of his work is shown in the figure below utilizing 35 xenon atoms.

35 xenon atoms ordered to make the letters "I B M"
35 Xenon Atoms
Credit: nano.gov

In the next section, we will look at how carbon nanotubes might allow for faster transistors in the future.

Moore's Law

According to Wikipedia, "Moore's law is the observation that the number of transistors in a dense integrated circuit doubles approximately every two years." Gordon Moore made that observation in 1970, and as you can see from the figure below, it has been remarkably accurate over the many decades since.

Microprocessor transistor counts from 1971-2011. Curve shows transistor count doubling every two years.
Microprocessor transistor counts 1971-2011 and Moore's Law.
Credit: Wgsimon-Own work, CC BY-SA 3.0, via Wikimedia Commons [51]

But you cannot just keep making things smaller and smaller to make them faster and faster. At some point, you hit the limit of approaching zero and the fact that it becomes incredibly expensive to produce incredibly small feature sizes. As shown in the figure below, a leading trade magazine, IEEE Spectrum, has reported that transistors could stop shrinking in 2021.

Graph showing the 2013 and 2015 reports of transistors, with the 2015 report line leveling off at 10 nanometers in 2021.
Report showing that transistors could stop shrinking in 2021.
Credit: Transistors Could Stop Shrinking in 2021 article via IEEE [52]

This will mean that faster computers will not be possible based on shrinking geometries. One potential approach, instead of increasing the density of transistors is the approach of making transistors faster by using carbon nanotubes. Electrons move much faster in carbon nanotubes than conventional semiconductor materials. A picture of a research carbon nanotube bridge is shown below.

Channels in transistors with carbon nanotubes.
Research Carbon Nanotube Bridge
Credit: Moore's Law- The Rule that Really Matters in Tech article via CNET [53]

Reading Assignment

Things to consider...

When you read this chapter, use the following questions to guide your reading. Remember to keep the learning objectives listed on the previous page in mind as you learn from this text.

  • What are the four possible electron band structures for solid materials?
  • What effect does the electron band structure have on the electrical conduction of a material?
  • What is a p–n junction, and how does it rectify current?
  • What are the concepts of nanotechnology as it applies to materials?

Reading Assignment

  1. Read pp 323-347 (Ch. 15) in Introduction to Materials ebook
  2. The following readings are available in Canvas.
  • Pages 245 to 251 (Chapter 32) of Materials in Today's World by Peter Thrower and Thomas Mason.
  • Pages 253 to 255 (Chapter 33) of Materials in Today's World by Peter Thrower and Thomas Mason.

Video Assignment: Making Stuff: Smaller

Now that you have read the text and thought about the questions I posed, take some time to watch the 53-minute NOVA video about how the latest in high-powered nano-circuits and microrobots may one day hold the key to saving lives. In "Making Stuff: Smaller," we see how making materials smaller has the potential for making vastly improved and tailored materials for future materials applications.

Video Assignment

Go to Lesson 12 in Canvas and watch the Making Stuff: Smaller video. You will be quizzed on the content of this video.

 

Summary and Final Tasks

Summary

As our understanding, and control, of materials at the nanometer scale improves, we are able to manufacture materials that are: tailored for specific tasks, designed to perform at the extremes of materials properties, or can utilize structure to enable properties not achievable before. Utilizing bottom-up, self-assembly, and other novel fabrication techniques, designer materials are becoming possible for everyday usage. In fact, superior materials are known today, but their utilization is hampered by our current inability to mass produce usable quantities economically. In this lesson, we looked at the basics of electronics, as well as some of the basics of nanoelectronics, which has the potential of being the electronics of the very near future.

Reminder - Complete all of the Lesson 12 tasks!

You have reached the end of Lesson 12! Double-check the to-do list on the Overview page to make sure you have completed all of the activities listed there before you take your final.


Source URL:https://www.e-education.psu.edu/matse81/content

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