Electret - Similarity To Capacitors

Similarity To Capacitors

There is a similarity between an electret and the dielectric layer used in capacitors; the difference is that dielectrics in capacitors have an induced polarization that is only transient, dependent on the potential applied on the dielectric, while dielectrics with electret properties exhibit quasi-permanent charge storage or dipole polarization in addition. Some materials also display ferroelectricity; i.e. they react to the external fields with a hysteresis of the polarization; ferroelectrics can retain the polarization permanently because they are in thermodynamic equilibrium, and are used in ferroelectric capacitors. Although electrets are only in a metastable state, those fashioned from very low leakage materials can retain excess charge or polarization for many years. An electret microphone is a type of condenser microphone that eliminates the need for a power supply by using a permanently charged material.

An electret is a stable dielectric material with a quasi-permanently embedded static electric charge (which, due to the high resistance of the material, will not decay for time periods of up to hundreds of years) and/or a quasi-permanently oriented dipole polarization. The name comes from electron (Greek word for amber) and magnet and was coined by Oliver Heaviside in 1885; drawing analogy to the formation of a magnet by alignment of magnetic domains in a piece of iron. Historically, electrets were made by first melting a suitable dielectric material such as a polymer or wax that contains polar molecules, and then allowing it to re-solidify in a powerful electrostatic field. The polar molecules of the dielectric align themselves to the direction of the electrostatic field, producing a dipole electret with a permanent electrostatic 'bias'. Modern electrets are usually made by embedding excess charges into a highly insulating dielectric, e.g. by means of an electron beam, a corona discharge, injection from an electron, electric breakdown across a gap or a dielectric barrier, etc.