Ceramic Materials

If you look in any introductory materials science book you find that one of the first sections describes the classification scheme. In classical materials science, materials are grouped into five categories:

Certain ancient periods of history are named after the material that was predominantly utilized at that time. The Stone Age, which began about 2.5 million years ago, is the earliest of these periods. Stone, more specifically flint, clearly satisfies our definition of a ceramic, given in Chapter 1.

The bases for understanding the structure of the atom are the quantum theory and wave mechanics, which were developed in the early 1900s. The important conclusions of these studies, particularly as they relate to materials, are:

We can divide interatomic bonds into two categories.

We begin by defining the vocabulary of the subject. Most of this section should be familiar to you from other courses.

Using Pauling’s rules, we can think of all crystal structures in terms of filling polyhedra—those we discussed in Chapter 5. Particularly simple cases are the simple cubic (sc), hexagonal close-packed (hcp), and face-centered cubic (fcc) lattices. In oxides such as Al

2

O

3

and MgO, the anion is the larger ion, which we believe forms a scaffold, with the cations then filling the interstices between the anions. This thinking has a historical bias to it. It comes from the days when ceramics were light-element oxides. Such compounds automatically have smallish cations.

In most simple metal–oxide structures,

r

C

<<

r

A

and the structures can be built up by considering a nearly close-packed arrangement of oxygen ions with cations located in interstices. The ionic radius ratios given earlier are useful and provide a means of predicting the coordination number (CN) of a particular compound, and often the predictions are in good agreement with observed values. In cases where the observed CN differs greatly from the expected value, such as 12 for K

+

in mica, KAl

3

Si

3

O

10

(OH)

2

, it is probable that the other ions present play the most important part in determining the arrangement.

As many ceramics are oxides, the oxygen partial pressure,

p

O

2

, is an important variable. There is a lot of information about many metal–oxygen systems. In part, this is due to interest in how to obtain metals by direct reduction from their oxides. A frequent way of representing free energies of formation of oxides as a function of

p

O

2

and

T

is the Ellingham diagram (Ellingham 1944) that was popularized by Richardson and Jeffes for iron and steel production (Richardson and Jeffes 1948). Much less is known about nitrides and oxynitrides or even carbides.

There are many areas in ceramics where we need high temperatures.

In characterizing a ceramic—whether it is a single crystal, polycrystalline, or a glass—there are certain types of information that we are interested in obtaining.

Until now we have only considered the ideal structures of crystals, where each atom or ion is on a regular site in the crystal. Real crystals contain a variety of imperfections or defects. In crystalline ceramics and glasses, the structure and chemistry of the material is determined by the kinetics of defect movement. For example, the kinetics of the glass-to-crystal transformation are slow if the temperature is low (typically <1,000°C) because the transformation occurs by atoms moving—in ceramics, this usually occurs by point defects moving. If point defects move too slowly, the structure with the lowest energy (the equilibrium structure) may never actually be achieved. How fast they move is determined by their structure.

Line defects in a crystalline material are known as dislocations (unless they’re disclinations, which we ignore because they’re much more difficult and not nearly as important in ceramics). In contrast to point defects, dislocations never exist in thermodynamic equilibrium because they have formation energies of ~1 eV (or more) per atom along the line and there is no significant balancing entropy contribution as there is for point defects. They are almost always present in crystals because of how the crystal grew or because it was deformed. Dislocations thus usually form due to nonequilibrium conditions, such as thermal and mechanical processing, or for thin films and single crystals, during growth. There are two special types of dislocation.

A surface is just the interface between a solid (or liquid) and a gas or vacuum. In general, the surface of a material, or any interface between materials, is a region of excess energy relative to the bulk or matrix. To maintain the lowest total energy for the system, the configuration of the surface adapts itself to minimize this excess energy. Impurities or dopants that lower the surface energy tend to concentrate in the surface. Similarly, such

defects

move to the interface if by segregating there they lower the overall energy of the system even if it raises the interfacial energy. The surface tends to orient parallel to certain crystallographic planes that have a lower energy.

Grain boundaries are internal interfaces and behave much like external surfaces; but now we have to be concerned with two crystal orientations, not one. Just as for surfaces, we have a pressure difference associated with the GB curvature and a driving force that tends to lead to an overall increase in grain size whenever possible. Grain morphology and GB topology are two aspects of the same topic. It is instructive to think of the model of soap foams: A soap film is flat when in equilibrium, and it has a finite thickness. Three soap films meet along a line—a triple junction. If you blow on a soap film (apply a pressure) it bows out until the “surface tension” balances the applied pressure.

In ceramic materials, as in other materials systems, interfaces are the most important region of the material because that’s where most of the action takes place. PBs are particularly important because they are the interfaces between dissimilar phases.

The classical view of ceramic materials is:

The onset and extent of plastic deformation is often measured when the σ − ε behavior of a material is being determined. We showed some general σ − ε curves in Chapter 16. In Figure 17.1, σ − ε curves obtained for crystals of KBr and MgO tested in bending are shown. From these curves we can identify several parameters that may already be familiar to you from discussion of the mechanical properties of metals.

Most ceramics at room temperature are brittle. That is, they fracture with very little plastic deformation. Many archeologists believe that our very existence depended on the brittleness of ceramics, particularly flint. The fracture of flint, like cubic zirconia, diamond, and glass, is termed conchoidal—producing shell-like fracture surfaces. These surfaces are very sharp and were utilized in early stone tools to cut and shape wood and to butcher animals required for food. The hides were used for clothing and were attached to wooden frames to make shelters. Stone tools were necessary for cutting vegetation and cultivating plants, allowing a change from a food-gathering economy to one of food production, which happened around the eighth millennium

bce

in southwestern Asia. This revolutionary change from hunting to farming laid the foundation of civilization.

Figure 19.1 shows a cross section of the Earth. The Earth has a mean radius of about 6,370 km and consists of three distinct concentric layers. The outermost layer is known as the crust and is relatively thin. The continental crust ranges in thickness from about 20 to 60 km, averaging approximately 30 km. It is the minerals that occur here that are important to us as raw materials for ceramics.

There are many methods that are available for the preparation of ceramic powders. They can be divided into just three basic types.

The classic definition of glass is based on the historical method of formation: This is a very unusual way of defining any material. The result is that glass is now defined in several different ways.

The sol–gel process consists of two steps. First we form a

sol

. Then we transform it into a

gel

. In ceramic synthesis, two different sol–gel routes have been identified and depend on the gel structure.

There’s a special vocabulary for shaping ceramics because it’s an ancient art. Once the constituent powders have been prepared in the desired purity and particle size, the next step in the processing of most ceramic products is fabricating them into useful shapes. Many shaping methods are used for ceramic products, and they can be grouped into three basic categories, which are not necessarily independent.

The idea of sintering is to join particles together without melting them. We may, however, use an additive that does melt. The particles can be crystalline or amorphous: We can sinter glass marbles so long as we don’t melt them; of course, the particles need not be spheres. If we go to too high a temperature, the marbles also deform.

In Chapters 14 and 15, we discussed grain boundaries (GBs) and phase boundaries (PBs), respectively. Those two chapters described the interfaces and crystal defects. In Chapter 24, we then examined how the movement of GBs can lead to sintering, grain growth, and densification. In the present chapter, we examine how the movement of PBs leads to transformations and reactions. Some examples of reactions involving the movement of a PB are given in Table 25.1: Not all of these are solid-state reactions.

More than half of the total worldwide ceramics market is glass products, accounting for over $50 billion/year. Figure 26.1 shows the distribution of glass sales.

A thick film typically has a thickness in the range 10–25μm; thin films are usually <500nm. However, what really distinguishes thick films and thin films—more than just their relative thickness—is the way in which they are produced. Thin films are often deposited using vacuum techniques such as sputtering and molecular beam epitaxy (MBE). We describe these techniques and consider acronyms and hyphens in

Chapter 28

. Thick films are deposited from a solution or paste, which must be dried and then often sintered to produce the final coating.

In the previous chapter we described how thick films of ceramics are produced. The difference between thick films and thin films is not really the thickness of the layer; it is how the layer is formed. In general, thin films are ≤500nm in thickness, whereas thick films may be several tens of micrometers in thickness or even thicker depending on the particular application. Thin films are generally prepared from the vapor phase, whereas for thick films we use a solution or slurry. Furthermore, thin films are often crystallographically oriented in a particular way with respect to the underlying substrate. This orientation relationship, known as epitaxy, is determined by the crystal structures and lattice parameters of the film and the substrate. In general, thick films and coatings have no specific orientation and contain a large number of randomly oriented crystalline grains.

For some applications, ceramic materials must be prepared as single crystals. When used as substrates for thin-film growth [e.g., silicon-on-sapphire (SOS) technology or the growth of superconductor thin films], it is the crystalline perfection of a single crystal that is the important requirement. In optical applications [e.g., the use of ruby and yttrium aluminum garnet (YAG) for laser hosts and quartz and sapphire for optical windows], single crystals are used to minimize scattering or absorption of energy. In piezoelectric materials (e.g., quartz), the optimum properties are obtained in single-domain single crystals. Table 29.1 lists some of the applications that utilize the desirable optical, electrical, magnetic, or mechanical properties of ceramic single crystals.

Ceramics are usually thought of as electrical insulators, and indeed a great many of them are. Since the first uses of electricity, it has been necessary to have good electrical insulators to isolate current-carrying wires. The expansion of the electrical industry and, in particular, the use of the electric telegraph required enormous numbers of porcelain insulators for telegraph poles. From 1888, ceramics based on steatite began to be used for the same purpose. Today, ceramics are still used to provide insulating supports in the power lines that crisscross the country.

All materials contain electrically charged particles. At a minimum, these are the electrons and protons that are part of the constituent atoms. Many ceramics also contain ions, which are charged. In a dielectric, charges have limited mobility, and they move only when they have enough energy to overcome their inertia. When an insulator receives a charge, it retains that charge, confining it within the localized region in which it was introduced. On the other hand, a conductor allows charge to flow freely and redistribute itself within the material. The distinction between conductors and nonconductors (and it’s not always a clear one) arises from the relative mobility of charge within the material.

Table 32.1 lists some of the major terms, and their units, that we meet in this chapter. It also lists the important physical constants that are needed to describe the optical properties of materials. The electromagnetic spectrum embraces a wide range of wavelengths, from the very short γ rays to the long radio waves. The portion of the spectrum that the human eye can detect is quite small. To put this in context, the full electromagnetic spectrum is shown in Figure 32.1. Radiation with a single wavelength is referred to as monochromatic; λ and

f

are related through

c

.

$$ f = \frac{c}{\lambda } $$

Applications of magnetism began with ceramics. The first magnetic material to be discovered was lodestone, which is better known now as magnetite (Fe

3

O

4

). In its naturally occurring state, it is permanently magnetized and is the most magnetic mineral. The strange power of lodestone was well known in ancient times. In c.400

bce

, Socrates dangled iron rings beneath a piece of lodestone and found that the lodestone enabled the rings to attract other rings. They had become magnetized. Even earlier (~2,600

bce

), a Chinese legend tells of the Emperor Hwang-ti being guided into battle through a dense fog by means of a small pivoting figure with a piece of lodestone embedded in its outstretched arm. The figure always pointed south and was probably the first compass. The term lodestone was coined by the British from the old English word

lode

, which meant to

lead

or

guide

.

Table 34.1 lists the important parameters we meet in this chapter and their units. The SI unit of temperature is kelvin (K), but as you have realized by now °C is often used in presenting data in materials science. As we mentioned in Chapter 1, the numerical value of a temperature difference or temperature interval expressed in °C is equal to the numerical value of the same temperature difference or interval when expressed in K. This point is worth remembering when you compare coefficients of thermal expansion or thermal conductivities for different materials.

A comprehensive definition of a biomaterial was provided at the National Institutes of Health (NIH) Consensus Development Conference on the Clinical Applications of Biomaterials in the United States.

Mining and mineral engineering are not always popular topics today. Many gemstones and mineral specimens are found during mining operations. The bulk of the minerals are then processed by physical or chemical means. Ceramists should have some knowledge of mineral processing because it can be the clue to understanding why certain impurities are present in powders used to produce high-tech ceramics (hence our discussion of raw materials in Chapter 19), but minerals and gemstones have many commercial, in addition to decorative (ornamental), uses.

Ceramic materials are an essential component of devices for production and storage of energy. Some of the topics covered in this chapter are summarized in Table 37.1. In many cases, a more efficient and cleaner process can be designed through the use of catalysts, or better catalysts. The problem is that the catalyst may change during use, and it is very difficult to see what is actually happening on the catalyst surface during the reaction.

In

Chapter 2

, we described some of the early history of ceramics and their production. The transition to a large-scale manufacturing industry occurred in Western Europe during the eighteenth century as part of the period that became known as the Industrial Revolution. The great porcelain factories established and subsidized by royal patronage at Miessen in Germany and Sèvres in France began to give way to purely commercial products being made in Staffordshire in the north of England. Later, the factories at Miessen and Sèvres began to imitate English designs. They were certainly helped in this area by immigrant workers. Emigration was a concern for the ceramics industry more than many others, such as iron production, because it relied on secret processes, such as specific body and glaze compositions. Once these became known, a worker would become valuable to a competitor.