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((short description|Object with trapped electrical charge))
((distinguish|Electromagnet))
((Multiple issues|
((more citations needed|date=September 2013))
((Cleanup rewrite|date=January 2024))
))
An '''electret''' (formed as a [[portmanteau]] of ''electr-'' from "[[electricity]]" and ''-et'' from "[[magnet]]") is a [[dielectric]] material that has a quasi-permanent [[polarization (electrostatics)|electrical polarisation]]. An electret has internal and external [[electric field]]s, and is the [[electrostatic]] equivalent of a [[permanent magnet]]. 

The term electret was coined by [[Oliver Heaviside]]<ref>((Cite book |last=Heaviside |first=Oliver |url=https://books.google.com/books?id=HiwPAAAAIAAJ |title=Electrical Papers |date=1894 |publisher=Macmillan and Company |pages=488–493 |language=en))</ref> for a (typically [[dielectric]]) material which has electrical charges of opposite sign at its extremities.<ref>((Cite journal |last=Gutmann |first=F. |date=1948 |title=The Electret |url=https://link.aps.org/doi/10.1103/RevModPhys.20.457 |journal=Reviews of Modern Physics |language=en |volume=20 |issue=3 |pages=457–472 |doi=10.1103/RevModPhys.20.457 |bibcode=1948RvMP...20..457G |issn=0034-6861))</ref> Some materials with electret properties were already known to science and had been studied since the early 1700s. One 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 in 1762<ref>((Cite book |last=Vetenskapsakademien (Stockholm) |first=Kungliga Svenska |url=https://books.google.com/books?id=HHRJAAAAcAAJ&pg=PA202 |title=Kungliga Svenska Vetenskapsakademiens handlingar |date=1762 |publisher=Almqvist & Wiksell |language=sv))</ref> and improved by [[Alessandro Volta]] in Italy in 1775.<ref>((Cite book |last=Pancaldi |first=Giuliano |url=https://books.google.com/books?id=hGoYB1Twx4sC&pg=PA73 |title=Volta: Science and Culture in the Age of Enlightenment |date=2005 |publisher=Princeton University Press |isbn=978-0-691-12226-7 |pages=Chapter 3 |language=en))</ref> The first documented case of production was by Mototarô Eguchi in 1925<ref>((Cite journal |last=Eguchi |first=Mototarô |date=1925 |title=XX. On the permanent electret |url=https://www.tandfonline.com/doi/full/10.1080/14786442508634594 |journal=The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science |language=en |volume=49 |issue=289 |pages=178–192 |doi=10.1080/14786442508634594 |issn=1941-5982))</ref> who melting a suitable dielectric material such as a polymer or wax that contains polar molecules, and then allowing it to solidify in a powerful electric field. The polar molecules of the dielectric align themselves to the direction of the electric field, producing a dipole electret with a permanent [[Polarization density|polarization]]. Modern electrets are sometimes made by embedding excess charges into a highly insulating dielectric, e.g. using an [[electron beam]], [[corona discharge]], injection from an [[electron gun]], electric breakdown across a gap, or a dielectric barrier.<ref>((Citation |last=Gross |first=B. |title=Radiation-induced charge storage and polarization effects |date=1980 |work=Electrets |series=Topics in Applied Physics |volume=33 |pages=217–284 |editor-last=Sessler |editor-first=Gerhard M. |url=http://link.springer.com/10.1007/3540173358_12 |access-date=2024-01-22 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |doi=10.1007/3540173358_12 |isbn=978-3-540-17335-9))</ref><ref>((Cite journal |last1=Tsai |first1=Peter P. |last2=Schreuder-Gibson |first2=Heidi |last3=Gibson |first3=Phillip |date=2002 |title=Different electrostatic methods for making electret filters |url=https://linkinghub.elsevier.com/retrieve/pii/S0304388601001607 |journal=Journal of Electrostatics |language=en |volume=54 |issue=3–4 |pages=333–341 |doi=10.1016/S0304-3886(01)00160-7))</ref>

== Electret types ==
There are two types of electrets:
* '''Real-charge electrets''' which contain excess free charges such as [[Electron|electrons]] or [[Electron hole|electron holes]] of one or both polarities which can move around, either<ref>((Citation |last=Sessler |first=G. M. |title=Physical principles of electrets |date=1980 |work=Electrets |series=Topics in Applied Physics |volume=33 |pages=13–80 |editor-last=Sessler |editor-first=Gerhard M. |url=http://link.springer.com/10.1007/3540173358_10 |access-date=2024-01-22 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |doi=10.1007/3540173358_10 |isbn=978-3-540-17335-9))</ref>
** on the dielectric's surfaces (a [[surface charge]])
** within the dielectric's volume (a [[space charge]])
** Space charge electrets<ref>((Cite journal |last=Gerhard-Multhaupt |first=R. |date=2002 |title=Less can be more. Holes in polymers lead to a new paradigm of piezoelectric materials for electret transducers |url=https://ieeexplore.ieee.org/document/1038668 |journal=IEEE Transactions on Dielectrics and Electrical Insulation |language=en |volume=9 |issue=5 |pages=850–859 |doi=10.1109/TDEI.2002.1038668 |issn=1070-9878))</ref> with internal bipolar charges known as [[ferroelectret]]s.<ref>((Cite journal |last1=Zhang |first1=X. |last2=Hillenbrand |first2=J. |last3=Sessler |first3=G. M. |date=2007 |title=Ferroelectrets with improved thermal stability made from fused fluorocarbon layers |url=https://doi.org/10.1063/1.2562413 |journal=Journal of Applied Physics |volume=101 |issue=5 |pages=054114–054114–8 |doi=10.1063/1.2562413 |bibcode=2007JAP...101e4114Z |issn=0021-8979))</ref>
* '''Oriented-dipole electrets''' contain oriented (aligned) dipoles. These contain [[Bound Charge|bound charges]] at their surface, which are not free to move around.<ref>((Citation |last=Sessler |first=G. M. |title=Physical principles of electrets |date=1980 |work=Electrets |series=Topics in Applied Physics |volume=33 |pages=13–80 |editor-last=Sessler |editor-first=Gerhard M. |url=http://link.springer.com/10.1007/3540173358_10 |access-date=2024-01-22 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |doi=10.1007/3540173358_10 |isbn=978-3-540-17335-9))</ref> These are similar to [[Ferroelectrics|ferroelectric]] materials, and are always in materials which have no [[inversion symmetry]] so would also display [[piezoelectricity]].
== Similarity to magnets ==
Electrets, like magnets, are dipoles. Another similarity is the fields: they produce an electrostatic field (as opposed to a [[magnetic field]]) outside the material. 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 polarisation. Some materials also display [[ferroelectricity]] (i.e. they react to the external fields with a [[hysteresis]] of the polarisation). Ferroelectrics can retain the polarisation permanently because they are in thermodynamic equilibrium, and thus are used in [[ferroelectric capacitor]]s. 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 polarisation voltage from the power supply by using a permanently charged material.

== 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 [[polymer]]s, e.g. [[fluoropolymer]]s, [[polypropylene]], [[polyethyleneterephthalate]] (PET), 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 heating or melting the material, then cooling it in the presence of a strong electric field. The electric field repositions the charge carriers or aligns the dipoles within the material. When the material cools, solidification "freezes" the dipoles in position. Materials used for electrets are usually [[wax]]es, [[polymers]] or [[resin]]s. 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. The [[thermo-dielectric effect]], related to this process, was first described by Brazilian researcher [[Joaquim Costa Ribeiro]].

Electrets can also be manufactured by embedding excess negative charge within a dielectric using a [[particle accelerator]], or by ''stranding'' charges on, or near, the surface using [[high voltage]] [[corona discharge]]s, 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 [[polytetrafluoroethylene|PTFE]], may retain excess charge for many hundreds of years.((citation needed|date=August 2016)) Most commercially produced electrets are based on [[fluoropolymer]]s (e.g. [[amorphous]] [[Teflon]]) machined to thin films.

==See also==
((wiktionary|electret))
* [[Oliver Heaviside]]
* [[Corona wire#Charging|Corona wire]]
* [[Telephone]]
* [[Electret microphone]]
* [[Electromotive force]]
* [[Tip ring sleeve]]
* [[Ferroelectricity]]

== References ==
((reflist|25em))

===Patents===
* Nowlin, Thomas E., and Curt R. Raschke, ((US patent|4291245)), "A process for making polymer electrets"

==Further reading==
*((cite book |last1=Jefimenko |first1=Oleg D. |year=2011 |title=Electrostatic Motors: Their History, Types, and Principles of Operation  |edition=1st New Revised |publisher=Integrity Research Institute |isbn=978-1935023470))
* ((cite journal |last1=Jefimenko |first1=Oleg D. |last2=Walker |first2=David K. |year=1980 |title=Electrets |journal=[[Physics Teacher]] |volume=18 |issue=9 |pages=651–659 |bibcode=1980PhTea..18..651J |doi=10.1119/1.2340651))
* ((cite journal |last1=Walker |first1=David K.  |last2=Jefimenko |first2=Oleg D. |year=1973 |title=Volume charge distribution in carnauba wax electrets |journal=[[Journal of Applied Physics]] |volume=44 |issue=8 |pages=3459 |bibcode=1973JAP....44.3459W |doi=10.1063/1.1662785))
*((cite book |last1=Adams |first1=Charles K. |year=1987 |title=Nature's Electricity |publisher=[[TAB Books]] |isbn=978-0-8306-2769-1))
*((cite book |last1=Gross |first1=Bernhard |year=1964 |title=Charge storage in solid dielectrics; a bibliographical review on the electret and related effects |publisher=[[Elsevier]]))
*((cite journal | last1=Barker |first1= R.H.|year=1962 |title =Electrets |journal=Journal of the IEE |volume=8 |issue=93 |pages=413–416 |doi=10.1049/jiee-3.1962.0241))A discussion on polarization, thermoelectrets, photoelectrets and applications
*((cite book |editor1-last=Sessler |editor-first=Gerhard M. |year=1998 |title=Electrets |publisher=Laplacian Press |isbn=978-1-885540-07-2 |edition=3rd))

[[Category:Condensed matter physics]]
[[Category:Electrical phenomena]]
[[Category:Dielectrics]]
[[Category:Electrostatics]]

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