Electret

An electret (formed as a portmanteau of electr- from "electricity" and -et from "magnet") is a dielectric material that has a quasi-permanent electrical polarisation. An electret has internal and external electric fields, and is the electrostatic equivalent of a permanent magnet. The term electret was coined by Oliver Heaviside for a (typically dielectric) material which has electrical charges of opposite sign at its extremities. Some materials with electret properties were already known to science and had been studied since the early 1700s. One example is the electrophorus, a device consisting of a slab with electret properties and a separate metal plate. The electrophorus was originally invented by Johan Carl Wilcke in Sweden in 1762 and improved by Alessandro Volta in Italy in 1775. The first documented case of production was by Mototarô Eguchi in 1925 who melted a suitable dielectric material such as a polymer or wax that contained polar molecules, and then allowed it to solidify in a powerful electric field. The polar molecules of the dielectric align themselves to the direction of the electric field, producing a dipole electret with a quasi-permanent polarization. Modern electrets are sometimes made by embedding excess charges into a highly insulating dielectric, e.g.…

An electret (formed as a portmanteau of electr- from "electricity" and -et from "magnet") is a dielectric material that has a quasi-permanent electrical polarisation. An electret has internal and external electric fields, and is the electrostatic equivalent of a permanent magnet. The term electret was coined by Oliver Heaviside for a (typically dielectric) material which has electrical charges of opposite sign at its extremities. Some materials with electret properties were already known to science and had been studied since the early 1700s. One example is the electrophorus, a device consisting of a slab with electret properties and a separate metal plate. The electrophorus was originally invented by Johan Carl Wilcke in Sweden in 1762 and improved by Alessandro Volta in Italy in 1775. The first documented case of production was by Mototarô Eguchi in 1925 who melted a suitable dielectric material such as a polymer or wax that contained polar molecules, and then allowed it to solidify in a powerful electric field. The polar molecules of the dielectric align themselves to the direction of the electric field, producing a dipole electret with a quasi-permanent polarization. Modern electrets are sometimes made by embedding excess charges into a highly insulating dielectric, e.g. using an electron beam, corona discharge, injection from an electron gun, electric breakdown across a gap, or a dielectric barrier. == Manufacture== Bulk electrets can be prepared by heating or melting the material, then cooling it in the presence of a strong electric field. The electric field repositions the charge carriers or aligns the dipoles within the material. When the material cools, solidification "freezes" the dipoles in position. Materials used for electrets are usually waxes, polymers or resins. One of the earliest recipes consists of 45% carnauba wax, 45% white rosin, and 10% white beeswax, melted, mixed together, and left to cool in a static electric field of several kilovolts/cm. The thermo-dielectric effect, related to this process, was first described by Brazilian researcher Joaquim Costa Ribeiro. Electrets can also be manufactured by embedding excess negative charge within a dielectric using a particle accelerator, or by stranding charges on, or near, the surface using high voltage corona discharges, a process called corona charging. Excess charge within an electret decays exponentially. The decay constant is a function of the material's relative dielectric constant and its bulk resistivity. Materials with extremely high resistivity, such as PTFE, may retain excess charge for many hundreds of years. Most commercially produced electrets are based on fluoropolymers (e.g. amorphous Teflon) machined to thin films. ==See also== Oliver Heaviside Corona wire Telephone Electret microphone Electromotive force Tip ring sleeve Ferroelectricity ===Patents=== Nowlin, Thomas E., and Curt R. Raschke, , "A process for making polymer electrets" ==Further reading== A discussion on polarization, thermoelectrets, photoelectrets and applications

Text derived from https://en.wikipedia.org/w/index.php?oldid=1360653500 under CC BY-SA 4.0; modifications have been made.