Electret

An electret (formed as a portmanteau of electr- from "electricity" and -et from "magnet") is a dielectric material that has a quasi-permanent electrical polarisation. An electret has internal and external electric fields, and is the electrostatic equivalent of a permanent magnet.

The term electret was coined by Oliver Heaviside^[1] for a (typically dielectric) material which has electrical charges of opposite sign at its extremities.^[2] 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^[3] and improved by Alessandro Volta in Italy in 1775.^[4] The first documented case of production was by Mototarô Eguchi in 1925^[5] 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 quasi-permanent 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.^[6]^[7]

^ Heaviside, Oliver (1894). Electrical Papers. Macmillan and Company. pp. 488–493.
^ Gutmann, F. (1948). "The Electret". Reviews of Modern Physics. 20 (3): 457–472. Bibcode:1948RvMP...20..457G. doi:10.1103/RevModPhys.20.457. ISSN 0034-6861.
^ Vetenskapsakademien (Stockholm), Kungliga Svenska (1762). Kungliga Svenska Vetenskapsakademiens handlingar (in Swedish). Almqvist & Wiksell.
^ Pancaldi, Giuliano (2005). Volta: Science and Culture in the Age of Enlightenment. Princeton University Press. pp. Chapter 3. ISBN 978-0-691-12226-7.
^ Eguchi, Mototarô (1925). "XX. On the permanent electret". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 49 (289): 178–192. doi:10.1080/14786442508634594. ISSN 1941-5982.
^ Gross, B. (1980), Sessler, Gerhard M. (ed.), "Radiation-induced charge storage and polarization effects", Electrets, Topics in Applied Physics, vol. 33, Berlin, Heidelberg: Springer Berlin Heidelberg, pp. 217–284, doi:10.1007/3540173358_12, ISBN 978-3-540-17335-9, retrieved 2024-01-22
^ Tsai, Peter P.; Schreuder-Gibson, Heidi; Gibson, Phillip (2002). "Different electrostatic methods for making electret filters". Journal of Electrostatics. 54 (3–4): 333–341. doi:10.1016/S0304-3886(01)00160-7.

[1] Heaviside, Oliver (1894). Electrical Papers. Macmillan and Company. pp. 488–493.

[2] Gutmann, F. (1948). "The Electret". Reviews of Modern Physics. 20 (3): 457–472. Bibcode:1948RvMP...20..457G. doi:10.1103/RevModPhys.20.457. ISSN 0034-6861.

[3] Vetenskapsakademien (Stockholm), Kungliga Svenska (1762). Kungliga Svenska Vetenskapsakademiens handlingar (in Swedish). Almqvist & Wiksell.

[4] Pancaldi, Giuliano (2005). Volta: Science and Culture in the Age of Enlightenment. Princeton University Press. pp. Chapter 3. ISBN 978-0-691-12226-7.

[5] Eguchi, Mototarô (1925). "XX. On the permanent electret". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 49 (289): 178–192. doi:10.1080/14786442508634594. ISSN 1941-5982.

[6] Gross, B. (1980), Sessler, Gerhard M. (ed.), "Radiation-induced charge storage and polarization effects", Electrets, Topics in Applied Physics, vol. 33, Berlin, Heidelberg: Springer Berlin Heidelberg, pp. 217–284, doi:10.1007/3540173358_12, ISBN 978-3-540-17335-9, retrieved 2024-01-22

[7] Tsai, Peter P.; Schreuder-Gibson, Heidi; Gibson, Phillip (2002). "Different electrostatic methods for making electret filters". Journal of Electrostatics. 54 (3–4): 333–341. doi:10.1016/S0304-3886(01)00160-7.

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