Dielectric Materials

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Dielectric Materials

Dielectric materials are substances that have very low conductivity. That is, they are electrical insulators through which an electrical current flows only with the greatest of difficulty. Technically, a dielectric can be defined as a material with electrical conductivity of less than one millionth of a mho (a unit of electrical conductance) per centimeter. A dielectric is also represented as a siemens (which is the reciprocal of its resistance in ohms). A material with a conductance of one siemens has an electrical potential difference of one volt, which produces a one-ampere current; thus, siemens = ampere/volts.

In theory, dielectrics can include solids, liquids, and gases, although in practice only the first two of these three states of matter have any practical significance. Some of the most commonly used dielectrics are various kinds of rubber, glass, wood, and polymers among the solids; and hydrocarbon oils and silicone oils among the liquids.

The dielectric constant

A common measure of the dielectric properties of a material is the dielectric constant. The dielectric constant can be defined as the tendency of a material to resist the flow of an electrical current across the material. The lower the value of the dielectric

KEY TERMS

Amplifier— A device for increasing the amount of electrical current in a system.

Capacitor— A device for receiving and storing an electrical charge, consisting of two parallel conducting surfaces separated by a dielectric material.

Conductivity— The tendency of a substance to allow the passage of an electrical current.

Polymer— A chemical compound formed by the combination of many smaller units.

Rectifier— A device that converts alternating current (AC) into direct current (DC).

Transducer— A device for converting energy from one form to another.

constant, the greater its resistance to the flow of an electrical current.

The standard used in measuring the dielectric constant is a vacuum, which is assigned the value of one. The dielectric constants of some other common materials are as follows: dry air (at one atmosphere of pressure): 1.0006; water: 80; glass: 4 to 7; wax: 2.25; amber: 2.65; mica: 2.5 to 7; benzene: 2.28; carbon tetrachloride: 2.24; and methyl alcohol: 33.1. Synthetic polymers are now widely used as dielectrics. The dielectric constants for these materials range from a low of about 1.3 for polyethylene and 2.0 for polytetrafluoroethylene (Teflon^®) to a high of about 7.2 to 8.4 for a mela-mine-formaldehyde resin.

Uses

Almost any type of electrical equipment employs dielectric materials in some form or another. Wires and cables that carry electrical current, for example, are always coated or wrapped with some type of insulating (dielectric) material. Sophisticated electronic equipment such as rectifiers, semiconductors, transducers, and amplifiers contain, or are fabricated from, dielectric materials. The insulating material sandwiched between two conducting plates in a capacitor is also made of some dielectric substance.

Liquid dielectrics are also employed as electrical insulators. For example, transformer oil is a natural or synthetic substance (mineral oil, silicone oil, or organic esters, for example) that has the ability to insulate the coils of a transformer both electrically and thermally.

Synthetic dielectrics

A number of traditional dielectric materials are still widely used in industry. For example, paper impregnated with oil is often still the insulator of choice for coating wires that carry high-voltage current. However, synthetic materials have now become widely popular for many applications once filled by natural substances, such as glass and rubber. The advantage of synthetic materials is that they can be designed to produce very specific properties for specialized uses. These properties include not only low dielectric constant, but also strength, hardness, resistance to chemical attack, and other desirable qualities.

Among the polymers now used as dielectrics are the polyethylenes, polypropylenes, polystyrenes, polyvinyl chlorides, polyamides (Nylon), polymethyl methacrylates, and polycarbonates.

Breakdown

When a dielectric material is exposed to a large electrical field, it may undergo a process known as breakdown. In that process, the material suddenly becomes conducting, and a large current begins to flow across the material. The appearance of a spark may also accompany breakdown. The point at which breakdown occurs with any given material depends on a number of factors, including temperature, the geometric shape of the material, and the type of material surrounding the dielectric. The ability of a dielectric material to resist breakdown is called its intrinsic electric strength.

Breakdown is often associated with the degradation of a dielectric material. The material may oxidize, physically break apart, or degrade in some other way that will make conductance more likely. When breakdown does occur, then, it is often accompanied by further degradation of the material.

See also Electronics; Oxidation state.

Resources

BOOKS

Huff, H.R. and D.C. Gilmer, eds. High Dielectric Constant Materials. Berlin, Germany: Springer, 2005.

Kan, Kwan-Chi. Dielectric Phenomena in Solids: With Emphasis on Physical Concepts of Electronic Processes. Amsterdam, Netherlands, and Boston, MA: Academic Press, 2004.

Raju, Gorur G. Dielectrics in Electric Fields. New York: Marcel Dekker, 2003.

Scaife, B.K. Principles of Dielectrics. New York: Oxford University Press, 1998.