Editing Ceramic (section)

===Electrical properties===

====Semiconductors====
Some ceramics are [[semiconductor]]s. Most of these are [[transition metal oxides]] that are II-VI semiconductors, such as [[zinc oxide]]. While there are prospects of mass-producing blue [[light-emitting diode]]s (LED) 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 property that resistance drops sharply at a certain [[threshold voltage]]. Once the voltage across the device reaches the threshold, there is a [[Electrical breakdown|breakdown]] of the electrical structure{{clarification needed|date=November 2021}} in the vicinity of the grain boundaries, which results in its [[electrical resistance]] dropping from several megohms down to a few hundred [[Ohm (unit)|ohm]]s. 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 protector|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 substation]]s, 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 sensor]]s. 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====
[[File:Magnet 4.jpg|thumb|The [[Meissner effect]] demonstrated by levitating a magnet above a [[cuprate]] superconductor, which is cooled by [[liquid nitrogen]]]]
Under some conditions, such as extremely low temperatures, some ceramics exhibit [[high-temperature superconductivity]] (in superconductivity, "high temperature" means above 30 K). The reason for this is not understood, but there are two major families of superconducting ceramics.

====Ferroelectricity and supersets====
[[Piezoelectricity]], a link between electrical and mechanical response, is exhibited by a large number of ceramic materials, including the quartz used to [[crystal oscillator|measure time]] in 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.

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 inter-convert between thermal, mechanical, 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 sensor]]s, where the tiny rise in temperature from a warm body entering the room is enough to produce a measurable voltage in the crystal.

In turn, pyroelectricity is seen most strongly in materials that 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 capacitor]]s, elements of [[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 [[loudspeaker]]s, transducers for [[sonar]], and actuators for [[atomic force microscope|atomic force]] and [[scanning tunneling microscope]]s.

====Positive thermal coefficient====
Temperature increases can cause grain boundaries to suddenly become insulating in some semiconducting ceramic materials, mostly mixtures of [[heavy metals|heavy metal]] [[titanate]]s. 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 automobiles.

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.