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electret,solid electrically insulating, or dielectric, material that has acquired a long-lasting electrostatic polarization. Electrets are produced by heating certain dielectric materials to a high temperature and then letting them cool while immersed in a strong electric field. An electret is an analog of a permanent magnet.
a solid dielectric that retains a state of polarization for a long time after the externally applied influence that causes the polarization is removed.
If a substance whose molecules possess permanent dipole moments is melted and placed in a strong stationary electric field, the molecules become partially aligned in the direction of the field. If the melt is cooled until it solidifies and the electric field is switched off, the molecules cannot easily rotate in the solidified substance and retain their orientation for a long time. An electret so prepared may remain in a state of polarization for quite a long time, ranging from several days to many years. The first such electret was prepared from wax by the Japanese physicist M. Eguchi in 1922.
Persistent polarization of a dielectric may also be caused by the alignment of “quasi-dipoles”—for example, two vacancies of opposite sign or an impurity atom and a vacancy—in crystals, by the migration of charge carriers to electrodes, or by the injection of charge carriers from electrodes or interelectrode gaps into a dielectric during polarization. Carriers may be introduced artificially, for example, by bombarding a dielectric with an electron beam. The polarization of an electret decays with time; the decay is associated with relaxation processes (seeRELAXATION) and with the movement of charge carriers in the electret’s internal field.
Virtually all known organic and inorganic dielectrics may be used for electrets. Stable electrets may be produced from the following substances: waxes and resins, such as carnauba wax, beeswax, or paraffin; polymers, for example, polymethyl methacrylate, polyvinyl chloride, polycarbonate, or polytetrafluor-ethylene; polycrystalline inorganic dielectrics, such as titanates of alkali-earth metals, steatite, porcelain, or ceramic dielectrics other than porcelain; single-crystal inorganic dielectrics, for example, corundum or halides of alkali metals; glasses; or devitrified glasses.
A stable electret may be produced by the following methods: the heating of a dielectric to a temperature below or equal to its melting point followed by cooling in a strong electric field; the illumination of a dielectric in a strong electric field; the radioactive irradiation of a dielectric; placing a dielectric in a strong electric or magnetic field; the mechanical deformation of a polymer; friction; or placing a dielectric in the field of a corona discharge. Electrets produced by illuminating a dielectric in a strong electric field are called photoélectrets. All electrets have a stable surface charge of ~ 10 –8 coulomb/cm2.
Electrets are used as sources of a stationary electric field in such devices as electret microphones, electret headphones, vibration pickups, and signal generators. They are also used to produce an electric field in, for example, electrometers and electrostatic voltmeters. Electrets may also be employed, for example, as sensing elements in dosimetric instruments and electrostatic storage devices, as piezoelectric transducers, and as focusing devices for a beam of charged particles, as well as in barometers, hygrometers, and gas filters. Photoelectrets are used in electrophotography.
REFERENCESGubkin, A. N. Elektrety. Moscow, 1961.
Fridkin, V. M., and I. S. Zheludev. Fotoelektrety i elektrofoto-graficheskii protsess. Moscow, 1960.
Braun, W. Dielektriki. Moscow, 1961. (Translated from English.)
Fizicheskii entsiklopedicheskii slovar’, vol. 5. Moscow, 1966. Page 442.
Lushcheikin, G.A. Polimernye elektrety. Moscow, 1976.
A. N. GUBKIN
A solid dielectric with a quasi-permanent electric moment. Electrets may be classified as real-charge electrets and dipolar-charge electrets. Real-charge electrets are dielectrics with charges of one polarity at or near one side of the dielectric and charges of opposite polarity at or near the other side, while dipolar-charge electrets are dielectrics with aligned dipolar charges. Some dielectrics are capable of storing both real and dipolar charges. An example of a charge arrangement of an electret metallized on one surface is shown in the illustration.
Modern electrets used in research and in applications are often films of 5–50 micrometers thickness (foil electrets) consisting of a suitable material. They are frequently metallized on one or both sides, depending on the intended use.
Important commercial applications of real-charge electrets are in electroacoustic and electromechanical transducers, in air filters, and in electret dosimeters. Also of interest are biological applications based on the blood compatibility of charged polymers or on their favorable influence on wound or fracture healing. Commercial applications of dipolar electrets are in piezoelectric transducers and in pyroelectric detectors. See Air filter, Dielectric materials, Electrical insulation