Electret

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Electret (formed of elektr- 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. 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 electrophorus, a device consisting of a slab with electret properties and a separate metal plate. The electrophorus was originally invented by Johan Carl Wilke in Sweden and again by Alessandro Volta in Italy.

Contents

1 Similarity to capacitors
2 Electret types
3 Materials
4 Manufacture
5 Applications
6 Further reading
7 See also
8 Patents

Similarity to capacitors

There is a similarity between electrets and the dielectric layer used 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 metastable state, but may still store excess charge or polarization for extremely long periods of time.

Electret types

There are two types of electrets:

Real-charge electrets which contain excess charges of one or both polarities on the dielectric's surfaces, or excess charge (a space charge of electrons) within a dielectric.
Oriented-dipole electrets contain oriented (aligned) dipoles. Ferroelectric materials are one variant of these.

Some dielectric materials are capable of acting both ways.

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 synthetic 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. The quasi-permanent internal or external electric fields created by electrets can be exploited in various applications.

Manufacture

Bulk electrets can be prepared by cooling a suitable dielectric material within a strong electric field, after heating it above its melting temperature. The field repositions the charge carriers or aligns the dipoles within the material. When the material cools, solidification freezes them in position. Materials used to 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.

Electrets can also be manufactured by imbedding 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 teflon, may retain excess charge for many hundreds of years. Most 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 electrostatic microphones 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 or radon. 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 4291245], "A process for making polymer electrets"

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