electret

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.

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electret [i′lek‚tret]

(electricity)

A solid dielectric possessing persistent electric polarization, by virtue of a long time constant for decay of a charge instability.

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Electret

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.

Schematic cross section of an electret disk metallized on one side

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