Electrical Properties:

The electrical properties of ceramic materials vary greatly, with characteristic measures spanning over many orders of magnitude (see Table 3). Ceramics are probably best known as electrical insulators. Some ceramic insulators (such as BaTiO3) can be polarized and used as capacitors. Other ceramics conduct electrons when a threshold energy is reached, and are thus called semiconductors. In 1986, a new class of ceramics was discovered, the high Tc superconductors. These materials conduct electricity with essentially zero resistance. Finally, ceramics known as piezoelectrics can generate an electrical response to a mechanical force or vice versa.

Table 3: Electrical Resistivity of different materials.

Type	Material	Resistivity (Ω-cm)
Metallic conductors:	Copper	1.7 x 10-6
	CuO2	3 x 10-5
Semiconductors:	SiC	10
	Germanium	40
Insulators:	Fire-clay brick	108
	Si3N4	>1014
	Polystyrene	1018
Superconductors:	YBa2Cu3O7-x	<10-22 (below Tc)

Anyone who has used a portable cassette player, personal computer, or other electronic device is taking advantage of ceramic dielectric materials. A dielectric material is an insulator that can be polarized at the molecular level. Such materials are widely used in capacitors, devices which are used to store electrical charge. The structure of a capacitor is shown in the diagram.

The charge of the capacitor is stored between its two plates. The amount of charge (q) that it can hold depends on its voltage (V) and its capacitance (C).

q = CV

The dielectric is inserted between the plates of a capacitor, raising the capacitance of the system by a factor equal to its dielectric constant, k.

q = (kC)V

Using materials that have large dielectric constants allows large amounts of charge to be stored on extremely small capacitors. This is a significant contribution to the continuing miniaturization of electronics (e.g., lap top computers, portable CD players, cellular phones, even hearing aids!).

The dielectric strength of a material is its ability to continuously hold electrons at a high voltage. When a capacitor is fully charged, there is virtually no current passing through it. But sometimes very strong electric fields (high voltages) excite large numbers of electrons from the valence band into the conduction band. When this happens current flows through the dielectric and some of the stored charge is lost. This may be accompanied by partial breakdown of the material by melting, burning, and/or vaporization. The magnetic field strength necessary to produce breakdown of a material is its dielectric strength. Some ceramic materials have extremely high dielectric strengths. For example, electrical porcelain can handle up to 300 volts for every .001 inches (mil) of the material!

Table 4: Electrical property constants of different ceramic materials.

Material	Dielectric constant at 1 MHz	Dielectric strength (kV/cm)
Air	1.00059	30
Polystyrene	2.54 - 2.56	240
Glass (Pyrex)	5.6	142
Alumina	4.5 - 8.4	16 - 63
Porcelain	6.0 - 8.0	16 - 157
Titanium dioxide	14 - 110	39 - 83

Electrical current in solids is most often the result of the flow of electrons (electronic conduction). In metals, mobile, conducting electrons are scattered by thermal vibrations (phonons), and this scattering is observed as resistance. Thus, in metals, resistivity increases as temperature increases.

In contrast, valence electrons in ceramic materials are usually not in the conduction band, thus most ceramics are considered insulators. However, conductivity can be increased by doping the material with impurities. Thermal energy will also promote electrons into the conduction band, so that in ceramics, conductivity increases (and resistivity decreases) as temperature increases.

Although ceramics were historically thought of as insulating materials, ceramic superconductors were discovered in 1986. A superconductor can transmit electrical current with no resistance or power loss. For most materials, resistivity gradually decreases as temperature decreases. Superconductors have a critical temperature, Tc, at which the resistivity drops sharply to virtually zero.

Pure metals and metal alloys were the first known superconductors. All had critical temperatures at or below 30K and required cooling with liquid helium. The new ceramic superconductors usually contain copper oxide planes such as YBa2Cu3O7 discovered in 1987 with Tc = 93 K. They have critical temperatures above the boiling point of liquid nitrogen (77.4 K), which makes many potential applications of superconductors much more practical. This is due to the lower cost of liquid nitrogen and the easier design of cryogenic devices.

In addition to their critical temperature, two other parameters define the region where a ceramic material is superconducting: 1) the critical current and 2) the critical magnetic field. As long as the conditions are within the critical parameters of temperature, current, and magnetic field, the material behaves as a superconductor. If any of these values is exceeded, superconductivity is destroyed.

Applications of superconductors which rely on their current carrying ability include electrical power generation, storage and distribution. SQUIDS (Superconducting Quantum Interference Devices) are electronic devices that use superconductors as sensitive detectors of electromagnetic radiation. Possible applications in the field of medicine include the development of advanced MRI (Magnetic Resonance Imaging) units based on magnets made of superconducting coils.

The magnetic applications of superconductors are also of major importance. Superconductors are perfect diamagnets, meaning that they will repel magnetic fields. This exclusion of an applied magnetic field is called the Meissner effect and is the basis for the proposed use of superconductors to magnetically levitate trains.

Some ceramics have the unusual property of piezoelectricity, or pressure electricity. These are part of a class known as "smart" materials which are often used as sensors. In a piezoelectric material, the application of a force or pressure on its surface induces polarization and establishes an electric field, i.e., it changes a mechanical pressure into an electrical impulse. Piezoelectric materials are used to make transducers, which are found in such common devices as phonograph pickups, depth finders, microphones, and various types of sensors.

In ceramic materials, electric charge can also be transported by ions. This property can be tailored by means of the chemical composition, and is the basis for many commercial applications. These range from chemical sensors to large scale electric power generators. One of the most prominent technologies is that of fuel cells. It is based on the ability of certain ceramics to permit the passage of oxygen anions, while at the same time being electronic insulators. Zirconia (ZrO2), stabilized with calcia (CaO), is an example of such a solid electrolyte.

Fuel cells were first used in spacecraft such as the Apollo capsules and the space shuttle. At night the fuel cells were used to generate electric power, by combusting hydrogen and oxygen from gas cylinders. During the day, solar cells took over, and the excess power was used to purify and reclaim oxygen from exhaust gas and the atmosphere exhaled by the astronauts. The lambda probe in the exhaust manifold of cars works on the same principle and is used to monitor engine efficiency.