Electret (formed of electr- from “electricity” and -et from “magnet”) is a dielectric material that has a quasi-permanent electric charge or dipole polarisation. An electret generates internal and external electric fields and is the electrostatic equivalent of a permanent magnet. Although Oliver Heaviside coined this term in 1885, materials with electret properties were already known to science and had been studied since the early 1700s. One particular 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 and again by Alessandro Volta in Italy.
SIMILARITY TO MAGNETS
Electrets, like magnets, are dipoles. Another similarity is the radiant fields: They produce an electrostatic field (as opposed to a magnetic field) around their perimeter. When a magnet and an electret are near one another, a rather unusual phenomenon occurs: 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.
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 polarisation that is only transient, dependent on the potential applied on the dielectric, while dielectrics with electret properties exhibit quasi-permanent charge storage or dipole polarisation in addition. Some materials also display ferroelectricity; i.e. they react to the external fields with a hysteresis of the polarisation; ferro-electrics can retain the polarisation permanently because they are in thermo-dynamic equilibrium, and are used in ferro-electric capacitors. Although electrets are only in a metastable state, those fashioned from very low leakage materials can retain excess charge or polarisation 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 polarisation. 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.
There are two types of electrets:
Real-charge electrets which contain excess charge of one or both polarities, either
on the dielectric’s surfaces (a surface charge)
within the dielectric’s volume (a space charge)
Oriented-dipole electrets contain oriented (aligned) dipoles. Ferroelectric materials are one variant of these.
Cellular space charge electrets with internal bipolar charges at the voids provide a new class of electret materials, that mimic ferroelectrics, hence they are known as ferroelectrets. Ferroelectrets display strong piezoelectricity, comparable to ceramic piezoelectric materials.
Some dielectric materials are capable of acting both ways.
APPLICATIONS OF ELECTRET
Electret materials have found commercial and technical interest. For example, they are used in electret microphones and in copy machines. They are also used in some types of air filters, for electrostatic collection of dust particles, in electret ion chambers for measuring ionizing radiation or radon and in Vibration Energy Harvesting.