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.
By the end of this lesson, you should be able to:
Lesson 7 will take us 1 week to complete. Please refer to Canvas for specific due dates.
To Read | Read pp 180-214 (Ch. 9 & 10) in Introduction to Materials ebook |
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To Watch | Ceramics: The Secret Life of Materials |
To Do | Lesson 7 Quiz |
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.
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.
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.
In the next section, we will look at one of the precursor steps to glass containers, 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.
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 (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.
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.
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.
Now, please proceed to the reading for this lesson (shown on the next page).
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.
Read pp 180-214 (Ch. 9 & 10) in Introduction to Materials ebook
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).
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.
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.
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.
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.
In the next section, we will discuss why ceramics are brittle and metals are not.
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.
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.
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.
You have now completed the reading for this lesson, please proceed to the next page which will introduce the video for this lesson.
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.
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.
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.
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.