Ceramics

The word ceramic is derived from the Greek word Κεραμεικος (the name of a suburb of Athens), and in its strictest sense refers to clay in all its forms. However, modern usage of the term broadens the meaning to include all inorganic non-metallic materials. Up until the 1950s or so, the most important of these were the traditional clays, made into pottery, bricks, tiles and the like, along with cements and glass. The traditional crafts are described in the article on pottery. A composite material of ceramic and metal is known as cermet.

Properties of Ceramics

Mechanical properties

Ceramic materials are usually ionic or covalently-bonded materials, and can be crystalline or amorphous. A material held together by either type of bond will tend to 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.

Related Topics:
Ionic - Covalent - Crystal - Amorphous - Fracture - Plastic deformation - Toughness - Pore - Stress concentrators - Tensile strength - Catastrophic failure - Failure mode - Metal

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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.

Related Topics:
Plastic deformation - Slip system - Dislocation - Glass - Viscous

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Electrical properties

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.

Related Topics:
Semiconductor - Transition metal

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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

Related Topics:
LED - Zinc oxide

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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 megaohms down to a few hundred ohms. 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.

Related Topics:
Varistor - Negative resistance - Breakdown - Grain boundaries - Electrical resistance - Ohm

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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 electrical substations, 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.

Related Topics:
Surge-protection - Electrical substation - Lightning

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Ferroelectricity and subsets

Piezoelectricity, a link between electrical and mechanical response, is exhibited by a large number of ceramic materials, including the quartz resonators used as to measure time watches and other electronics. Such devices use both properties of piezoelectrics, using electricity to produce a mechanical motion (powering the device) and then using this mechanical motion to produce electricity (generating a signal). The unit of time measured is the natural interval required for electricity to be converted into mechanical energy and back again.

Related Topics:
Piezoelectricity - Quartz - Resonator - Measure time

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The piezoelectric effect is generally stronger in materials that also exhibit pyroelectricity, and all pyroelectric materials are also piezoelectric. These materials can be used to interconvert between thermal, mechanical, and/or electrical energy; for instance, after synthesis in a furnace, a pyroelectric crystal allowed to cool under no applied stress generally builds up a static charge of thousands of volts. Such materials are used in motion sensors, where the tiny rise in temperature from a warm body entering the room is enough to produce a measurable voltage in the crystal.

Related Topics:
Pyroelectricity - Motion sensor

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

In turn, pyroelectricity is seen most strongly in materials which also display the ferroelectric effect, in which a stable electric dipole can be oriented or reversed by applying an electrostatic field. Pyroelectricity is also a necessary consequence of ferroelectricity. This can be used to store information in ferroelectric capacitors, elements of ferroelectric RAM.

Related Topics:
Ferroelectric effect - Ferroelectric capacitor - Ferroelectric RAM

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The most common such materials are lead zirconate titanate and barium titanate. Aside from the uses mentioned above, their strong piezoelectric response is exploited in the design of high-frequency loudspeakers, transducers for sonar, and actuators for atomic force and scanning tunneling microscopes.

Related Topics:
Lead zirconate titanate - Barium titanate - Loudspeaker - Transducer - Sonar - Atomic force - Scanning tunneling microscope

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Positive thermal coefficient

Increases in temperature can cause grain boundaries to suddenly become insulating in some semiconducting ceramic materials, mostly mixtures of heavy metal titanates. The critical transition temperature can be adjusted over a wide range by variations in chemistry. In such materials, current will pass through the material until joule heating brings it to the transition temperature, at which point the circuit will be broken and current flow will cease. Such ceramics are used as self-controlled heating elements in, for example, the rear-window defrost circuits of most automobiles.

Related Topics:
Grain boundaries - Heavy metal - Titanate - Joule heating - Heating element

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

At the transition temperature, the material's dielectric response becomes theoretically infinite. While a lack of temperature control would rule out any practical use of the material near its critical temperature, the dielectric effect remains exceptionally strong even at much higher temperatures. Titanates with critical temperatures far below room temperature have become synonymous with "ceramic" in the context of ceramic capacitors for just this reason.

Related Topics:
Dielectric - Capacitor

~ ~ ~ ~ ~ ~ ~ ~ ~ ~