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.

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.

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.

Vestonicka venuse.

Credit: che via Wikimedia Commons [1]

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.

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.

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.

Commemorative marriage scarab for Queen Tiye from Amenhotep III: Walters Art Museum, Baltimore

Credit: Keith Schengili-Roberts via Wikimedia Commons [2]

Shawabti of Lady Sati

Credit: David Liam Moran via Wikimedia Commons [3]

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.

Crystalline SiO₂ (Quartz)

Adapted from Fig. 3.41, Callister & Rethwisch 5e.

Adding sodium to silica makes soda glass

Adapted from Fig. 3.41, Callister & Rethwisch 5e.

In the 1^st 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.

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).

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?

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 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.

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.

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.

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.

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.