Electret
From Free net encyclopedia
Electret (formed of elektr- from "electricity" and -et from "magnet") is material that has a permanent electric charge polarisation. The magnetic equivalent is a permanent magnet. Oliver Heaviside coined this term.
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Similarity to capacitors
There is a similarity between electrets and capacitors; the difference is that in capacitors the polarization of the dielectric is only transient, dependent on the potential applied on the dielectric. Some materials display ferroelectric effect, react to the external field with hysteresis; these can retain the polarization over time even without external charge, and are used in ferroelectric capacitors.
Natural abundance
Electrets are quite common in nature. Quartz, for example, is a naturally occurring electret. Electrets are composed of positively charged atoms and negatively charged atoms. If arranged correctly, the formation of these atoms leads to the permanent electric field.
Electret types
There are two types of electrets:
- Real-charge electrets concentrate the positive charge carriers on one side of the object and the negative ones on the other side.
- Dipolar-charge electrets have homogeneous distribution of electric charge through the object, but their carriers, dipoles, are aligned. These are also known as 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 Template:US patent for "Manufacturing Method and Device for Electret Processed Product"
Further reading
- Jefimenko, Oleg D., and David K. Walker (illus.), "Electrostatic motors; their history, types, and principles of operation". Star City [W. Va.], Electret Scientific Co. [1973]. LCCN 73180890
- Jefimenko, Oleg D., "Electrets," (with D. K. Walker) Phys. Teach. 18, 651-659 (1980).
- Adams, Charles K., "Nature's Electricity". Tab Books, Inc., Pa. (USA). ISBN 0830627693
- Gross, Bernhard, "Charge storage in solid dielectrics; a bibliographical review on the electret and related effects". New York, Elsevier Pub. Co., 1964. (Supported by the United States Air Force through the Air Force Office of Scientific Research of the Air Research and Development Command, under grants number AF 60-6 and 61-140.)
See also
Patents
- Nowlin, Thomas E., and Curt R. Raschke, Template:US patent, "A process for making polymer electrets"de:Elektret