Electret

Electret (formed of elektr- from "electricity" and -et from "magnet") is material that has a quasi-permanent electric charge or dipole polarisation. The magnetic equivalent is a permanent magnet. Oliver Heaviside coined this term in 1885. Materials with electret properties were, however, already studied since the early 18th century. One particular example is the electrophore, a device consisting of a slab with electret properties and a separate metal plate. The electrophore was originally invented by Johan Carl Wilke in Sweden and again by Alessandro Volta in Italy.

Similarity to capacitors

There is a similarity between electrets and dielectrics in capacitors; the difference is that dielectrics in capacitors possess 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. Electrets are only in a steady state, i.e. metastable, but may still store charge or polarization for extremely long periods of time.

Materials

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 highly insulating polymers, e.g. fluoropolymers, polypropylene, polyethyleneterephthalate, etc. Real-charge electrets contain either positive or negative excess charges or both, while oriented-dipole electrets contain oriented dipoles. Electrets exhibit quasi-permanent internal or external electric fields that can be exploited in applications.

Electret types

There are two types of electrets:

Real-charge electrets contain excess charges of one or both polarities on their surfaces or in their bulk.
Oriented-dipole electrets contain oriented (aligned) dipoles. A particular variant of these electrets are ferroelectric materials.

Some dielectrics are capable of acting both ways.

Manufacture

A polarised material consists of atoms or molecules with electric dipole moment. It can be produced by cooling down material composed of long molecule chains with electric dipole moment so that so-called domains are formed.

Electrets are prepared by cooling a suitable dielectric material in a strong electric field, after heating it to a high temperature. This process repositions the charge carriers or orients the dipoles in the material, then fixes them in position. The effect is not permanent, the charge decays exponentially, but the decay process is slow, in order of decades to centuries at normal temperature.The materials used for preparation of electrets are usually waxes, polymers or resins. One of the 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.

Most of commercially produced electrets are based on fluoropolymers (eg. amorphous teflon) machined to thin films.

Applications

Electret materials have recently found commercial and technical interest. For example, they are used in one form of microphone and in copy machines. They are also used in some types of air filters, for electrostatic collection of dust particles, and in electret ion chambers for measuring ionizing radiation. See U.S. patent 6,969,484 for "Manufacturing Method and Device for Electret Processed Product"

Patents

Nowlin, Thomas E., and Curt R. Raschke, U.S. patent 4,291,245, "A process for making polymer electrets"