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Anelectret (formed as aportmanteau ofelectr- from "electricity" and-et from "magnet") is adielectric material that has a quasi-permanentelectrical polarisation. An electret has internal and externalelectric fields, and is theelectrostatic equivalent of apermanent magnet.
The term electret was coined byOliver Heaviside[1] for a (typicallydielectric) material which has electrical charges of opposite sign at its extremities.[2] Some materials with electret properties were already known to science and had been studied since the early 1700s. One example is theelectrophorus, a device consisting of a slab with electret properties and a separate metal plate. The electrophorus was originally invented byJohan Carl Wilcke in Sweden in 1762[3] and improved byAlessandro Volta in Italy in 1775.[4] The first documented case of production was by Mototarô Eguchi in 1925[5] 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-permanentpolarization. Modern electrets are sometimes made by embedding excess charges into a highly insulating dielectric, e.g. using anelectron beam,corona discharge, injection from anelectron gun, electric breakdown across a gap, or a dielectric barrier.[6][7]
There are two types of electrets:
Electrets, like magnets, are dipoles. Another similarity is the fields: they produce an electrostatic field (as opposed to amagnetic field) outside the material. When a magnet and an electret are near one another, theLorentz force acts on them: while stationary, neither has any effect on one another. However, when an electret is moved with respect to a magnetic pole, a force is felt which acts perpendicular to the magnetic field, pushing the electret along a path 90 degrees to the expected direction of "push" as would be felt with another magnet.
There is a similarity between an electret and the dielectric layer used incapacitors; the difference is that dielectrics in capacitors have an induced polarisation that is only transient, dependent on the potential applied on the dielectric, while dielectrics with electret properties exhibit quasi-permanent charge storage or polarisation. Some materials also displayferroelectricity (i.e. they react to the external fields with ahysteresis of the polarisation). Ferroelectrics can retain the polarisation permanently because they are in thermodynamic equilibrium, and thus are used inferroelectric capacitors. Although electrets are only in ametastable state, those fashioned from very low leakage materials can retain excess charge or polarisation for many years. Anelectret microphone is a type ofcondenser microphone that eliminates the need for a polarisation voltage from the power supply by using a permanently charged material.
Electret materials are quite common in nature.Quartz and other forms of silicon dioxide, for example, are naturally occurring electrets. Today, most electrets are made from syntheticpolymers, e.g.fluoropolymers,polypropylene,polyethyleneterephthalate (PET), etc. Real-charge electrets contain either positive or negative excess charges or both, while oriented-dipole electrets contain oriented dipoles. The quasi-permanent internal or external electric fields created by electrets can be exploited in various applications.
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 usuallywaxes,polymers orresins. One of the earliest recipes consists of 45%carnauba wax, 45% whiterosin, and 10% whitebeeswax, melted, mixed together, and left to cool in a static electric field of several kilovolts/cm. Thethermo-dielectric effect, related to this process, was first described by Brazilian researcherJoaquim Costa Ribeiro.
Electrets can also be manufactured by embedding excess negative charge within a dielectric using aparticle accelerator, or bystranding charges on, or near, the surface usinghigh voltagecorona discharges, a process calledcorona charging. Excess charge within an electret decays exponentially. Thedecay constant is a function of the material's relativedielectric constant and its bulkresistivity. Materials with extremely high resistivity, such asPTFE, may retain excess charge for many hundreds of years.[citation needed] Most commercially produced electrets are based onfluoropolymers (e.g.amorphousTeflon) machined to thin films.
