Ceramic

Origin of ceramics

The word ceramic is derived from Greek, and strictly refers to clay in all its forms. However, modern usage of the term broadens the meaning to mean inorganic non-metalic materials. Up until the 1950s or so, the most important of these was the traditional clays, made into pottery, bricks, tiles and similar, along with cements and glass. The traditional crafts are described in the article on pottery.

The classic ceramic materials are hard, porous and brittle. The study of ceramics consists to a large extent of methods to mitigate the problems, and accentuate the strenths of the materials, as well as to offer up unsual uses for these materials.

Examples of Ceramic Materials

• Silicon nitride (Si₃N₄), which is used as an abrasive powder.
• Boron carbide (B₄C), which is used in some helicopter and tank armor.
• Silicon carbide (SiC), which is used as a succeptor in microwave furnaces, a commonly used abrasive, and as a refractory material.
• Magnesium diboride (MgB₂), which is an Unconventional superconductor.
• Zinc oxide (ZnO), which is a semiconductor, and used in the construction of varistors.
• Ferrite (Fe₃O₄), which is ferrimagnetic and is used in the core of electrical transformers and magnetic core memory.
• Steatite is used as an electrical insulator.
• Bricks (mostly aluminum silicates ), used for construction.
• Uranium oxide (UO₂), used as fuel in nuclear reactors.
• Yttrium barium copper oxide (YBa₂Cu₃O_7-x),a high temperature superconductor.

Properties of Ceramics

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 porus, the pores and other microscopic imperfections act as stress concentrators, decreasing the toughness yet further, and reducing the tensile strength. These combine to give catastrophic failures, as opposed to the normally much more gentle failure modes of metals.

It is not true to say that these materials do not show plastic deformation. However, due to the rigid structrue of the the crystaline materials, there are very few available slip systems for dislocations to move, and so they deform very slowly. With the non-crystaline (glassy) materials, viscous flow is the dominant source of plastic deformation, and is also very slow. It is therfore neglected in many applications of cermaic materials.

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

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 salient properties required for a good refreactory material is that it not soften or melt, and that it remains unreactive at the desired temperature. This latter point covers both self decomposition, and reacting with other compounds in its vicinity - either of which would be detrimental.

Porosity takes on additional relevence with refractories. As the porosity is reduced, the strength, load bearing ability and environmental resistance decreases 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 porus state, and it is not uncomon to find two matierals used - a porus layer, with very good insualting properties, with a thin coat of a more dense material to provide strength.

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

Electrical behaviour

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

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.

Powerlines 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 catagory of their insualting behaviour is that of the dielectics. 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 tend not to outmatch other options for most purpose, they fill these two niches very well.

Ferroelectric, piezoelectrics and pyroelectric

A ferroelectric material is one the 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 a microphones, ultrasound generators, strain gauges and many more.

A pyroelectric material develops an electrical field when heated. Some ceramic pyroelectrics are so sensitive they can detect the temperature change caused by a person entering a room (approximately 40 micro Kelvin). 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.

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 electrial 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 themselves 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 appreciable degrade from use, making them virtually ideal devices for this application.

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

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 it's stoichiometric formula YBa₂Cu₃O_7-x). It is particularly well known because it is quite easy to make, does not require any particularly dangerous materials to do so, and 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 all of its own, and its behavior is not particularly remarkable, other than its very different from all other superconductors (not being either a complex copper oxide, nor a metal), and thus it is hoped that studying it will reveal more information about why superconductivity exists, which will feed into high temperature superconductivity research.

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 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.

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 it very useful for coatings.

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

Sintering based methods

The principles of sintering based methods is simple. Once a roughly held together object is made (called a "green body"), it is baked in a kiln, where diffusion processes cause the green body to shrink, and close up the pores in it, 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 win 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 that can be added to this process. Some of the most common are 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, which burn out during the firing. When pressing, something organic lubricants are added, to get maximum density from pressing. It's not uncommon to combine these, and add binders and lubricants to a powder, then press.

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

If a mixture of differnt materials is used together in a ceramic, it sometimes is that the sintering temperature is above the melting point of one minor compoment - a liquid phase sintering. This results in faster sintering times over solid state sintering.

Other applications of ceramics

A couple of decades ago, Toyota researched on producing a ceramic engine which can run at a temperature of over 6000°F. Ceramic engines do not require a cooling system and hence a major weight reduction in fuel-efficient vehicles. Fuel efficiency of the engine is also higher at high temperature. In 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 the 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 manufacturing difficulties prevent them from becoming reliable products.

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