2 Properties of Ceramics

2.1 Mechanical properties

Ceramic materials are usually ionic or glassy materials. Both of these almost always fracture before any plastic deformation takes place, which results in poor toughness in these materials. Additionally, because these materials tend to be porous, the pores and other microscopic imperfections act as stress concentrators, decreasing the toughness further, and reducing the tensile strength. These combine to give catastrophic failures, as opposed to the normally much more gentle failure modes of metals.

These materials do show plastic deformation. However, due to the rigid structure of the crystalline materials, there are very few available slip systems for dislocations to move, and so they deform very slowly. With the non-crystalline (glassy) materials, viscous flow is the dominant source of plastic deformation, and is also very slow. It is therefore neglected in many applications of ceramic materials.

These materials have great strength under compression, and are capable of operating at elevated temperatures. Their high hardness makes them widely used as abrasives, and as cutting tips in tools.

2.2 Refractory behaviour

Some ceramic materials can withstand extremely high temperatures without losing their strength. These are called refractory materials. They generally have low thermal conductivities, and thus are used as thermal insulators. For example, the belly of the Space Shuttles are made of ceramic tiles which protect the spacecraft from the high temperatures caused during reentry.

The most important requirements for a good refractory material are that it not soften or melt, and that it remain unreactive at the desired temperature. The latter requirement pertains to both self-decomposition and reaction with other compounds that might be present, either of which would be detrimental.

Porosity takes on additional relevance with refractories. As the porosity is reduced, the strength, load-bearing ability, and environmental resistance increases as the material gets more dense. However, as the density increases the resistance to thermal shock (cracking as a result of rapid temperature change) and insulation characteristics are reduced. Many materials are used in a very porous state, and it is not uncommon to find two materials used: a porous layer, with very good insulating properties, with a thin coat of a more dense material to provide strength.

It is perhaps surprising that these materials can be used at temperatures where they are partially liquified. For example, silica firebricks used to line steel-making furnaces are used at temperatures up to 1650°C (3000°F), where some of the brick will be liquid. Designing for such a situation unsurprisingly requires a substantial degree of control over all aspects of construction and use.

2.3 Electrical behaviour

One of the largest areas of progress with ceramics was their application to electrical situations, where they can display a bewildering array of different properties.

2.3.1 Electrical insulation and dielectric behaviour

The majority of ceramic materials have no mobile charge carriers, and thus do not conduct electricity. When combined with strength, this leads to uses in power generation and transmission.

Power lines are often supported from the pylons by porcelain discs, which are sufficiently insulating to cope with lightning strikes, and have the mechanical strength to hold the cables.

A sub-category of their insulating behaviour is that of the dielectrics. A good dielectric will maintain the electric field across it, without inducting power loss. This is very important in the construction of capacitors. Ceramic dielectrics are used in two main areas. The first is the low-loss high-frequency dielectrics, used in applications like microwave and radio transmitters. The other is the materials with high dielectric constants (the ferroelectrics). Whilst the ceramic dielectrics are inferior to other options for most purposes, they fill these two niches very well.

2.3.2 Ferroelectric, piezoelectrics and pyroelectric

A ferroelectric material is one that can spontaneously generate a polarization in the absence of an electric field. These materials exhibit a permanent electric field, and this is the source of their extremely high dielectric constants.

A piezoelectric material is one where an electric field can be changed or generated by applying a stress to the material. These find a range of uses, principally as transducers - to convert a motion into an electric signal, or vice versa. These appear in devices such as microphones, ultrasound generators, and strain gauges.

A pyroelectric material develops an electrical field when heated. Some ceramic pyroelectrics are so sensitive that they can detect the temperature change caused by a person entering a room (approximately 40 microkelvin). Unfortunately, such devices lack accuracy, so they tend to be used in matched pairs - one covered, the other not - and only the difference between the two used.

2.3.3 Semiconductivity

There are a number of ceramics that are semiconductors. Most of these are transition metal oxides that are II-VI semiconductors, such as zinc oxide.

Whilst there is talk of making blue LEDs from zinc oxide, ceramicists are most interested in the electrical properties that show grain boundary effects.

One of the most widely used of these is the varistor. These are devices that exhibit the unusual property of negative resistance. Once the voltage across the device reaches a certain threshold, there is a breakdown of the electrical structure in the vicinity of the grain boundaries, which results in its electrical resistance dropping from several mega-ohms down to a few hundred. The major advantage of these is that they can dissipate a lot of energy, and they self reset - after the voltage across the device drops below the threshold, its resistance returns to being high.

This makes them ideal for surge-protection applications. As there is control over the threshold voltage and energy tolerance, they find use in all sorts of applications. The best demonstration of their ability can be found in electricity sub stations, where they are employed to protect the infrastructure from lightning strikes. They have rapid response, are low maintenance, and do not appreciably degrade from use, making them virtually ideal devices for this application.

Semiconducting ceramics are also employed as gas sensors. When various gases are passed over a polycrystalline ceramic, its electrical resistance changes. With tuning to the possible gas mixtures, very inexpensive devices can be produced.

2.3.4 Superconductivity

Under some conditions, such as extremely low temperature, some ceramics exhibit superconductivity. The exact reason for this is not known, but there are two major families of superconducting ceramics.

The complex copper oxides are exemplified by Yttrium barium copper oxide, often abbreviated to YBCO, or 123 (after the ratio of metals in its stoichiometric formula YBa₂Cu₃O_7-x ). It is particularly well known because it is quite easy to make, its manufacture does not require any particularly dangerous materials, and it has a superconducting transition temperature of 90K (which is above the temperature of liquid nitrogen (77K)). The x in the formula refers to the fact that fully stoichiometric YBCO is not a superconductor, so it must be slightly oxygen-deficient, with x typically around 0.3.

The other major family of superconducting ceramics is magnesium diboride. It is currently in a family of its own. Its properties are not particularly remarkable, but it is chemically very different from all other superconductors in that it is neither a complex copper oxide nor a metal. Because of this difference, it is hoped that the study of this material will lead to fundamental insights into the phenomenon of superconductivity.

3 Processing of Ceramic Materials

Non-crystalline ceramics, being glasses, tend to be formed from melts. The glass is shaped when either fully molten, by casting, or when in a state of toffee-like viscosity, by methods such as blowing to a mould.

Crystalline ceramic materials are not amenable to a great range of processing. Methods for dealing with them tend to fall into one of two categories - either make the ceramic in the desired shape, by reaction in situ, or by forming powders into the desired shape, and then sintering to form a solid body. A few methods use a hybrid between the two approaches.

3.1 In situ manufacturing

The most common use of this type method is in the production of cement and concrete. Here, the dehydrated powders are mixed with water. This starts hydration reactions, which result in long, interlocking crystals forming around the aggregates. Over time, these result in a solid ceramic.

The biggest problem with this method is that most reactions are so fast that good mixing is not possible, which tends to prevent large-scale construction. However, small-scale systems can be made by deposition techniques, where the various materials are introduced above a substrate, and react and form the ceramic on the substrate. This borrows techniques from the semiconductor industry, such as chemical vapour deposition, and is very useful for coatings.

These tend to produce very dense ceramics, but do so slowly.

3.2 Sintering-based methods

The principles of sintering-based methods is simple. Once a roughly held together object (called a "green body") is made, it is baked in a kiln, where diffusion processes cause the green body to shrink. The pores in the object close up, resulting in a denser, stronger product. The firing is done at a temperature below the melting point of the ceramic. There is virtually always some porosity left, but the real advantage of this method is that the green body can be produced in any way imaginable, and still be sintered. This makes it a very versatile route.

There are thousands of possible refinements of this process. Some of the most common involve pressing the green body to give the densification a head start and reduce the sintering time needed. Sometimes organic binders are added to hold the green body together; these burn out during the firing. Sometimes organic lubricants are added during pressing to increase densification. It is not uncommon to combine these, and add binders and lubricants to a powder, then press. (The formulation of these organic chemical additives is an art in itself. This is particularly important in the manufacture of high performance ceramics such as those used by the billions for electronics, in capacitors, inductors, sensors, etc. The specialized formulations most commonly used in electronics are detailed in the book "Tape Casting," by R.E. Mistler, et al., Amer. Ceramic Soc. [Westerville, Ohio], 2000.) A comprehensive book on the subject, for mechanical as well as electronics applications, is "Organic Additives and Ceramic Processing," by D. J. Shanefield, Kluwer Publishers [Boston], 1996.

A slurry can be used in place of a powder, and then cast into a desired shape, dried and then sintered. Indeed, traditional pottery is done with this type of method, using a plastic mixture worked with the hands.

If a mixture of different materials is used together in a ceramic, the sintering temperature is sometimes above the melting point of one minor component - a liquid phase sintering. This results in shorter sintering times compared to solid state sintering.

4 Other applications of ceramics

A couple of decades ago, Toyota researched production of a ceramic engine which can run at a temperature of over 6000°F (3300°C). Ceramic engines do not require a cooling system and hence allow a major weight reduction and therefore greater fuel efficiency. Fuel efficiency of the engine is also higher at high temperature. In a conventional metallic engine, much of the energy released from the fuel must be dissipated as waste heat in order to prevent a meltdown of the metallic parts.

Despite all of these desirable properties, such engines are not in production because the manufacturing of ceramic parts is difficult. Imperfection in the ceramic leads to cracks. Such engines are possible in laboratory research, but current difficulties in manufacturing prevent them from being mass-produced.

Materials

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