An electret (formed as a portmanteau of ''electr-'' 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 field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...

s, and is the electrostatic equivalent of a

permanent magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...

. Although Oliver Heaviside coined this term in 1885, materials with electret properties were already known to science and had been studied since the early 1700s. One particular example is the

electrophorus In electromagnetism, an electrophorus or electrophore is a simple, manual, capacitive, electrostatic generator used to produce charge via the process of electrostatic induction. A first version of it was invented in 1762 by Swedish professor Jo ...

, a device consisting of a slab with electret properties and a separate metal plate. The electrophorus was originally invented by

Johan Carl Wilcke Johan Carl Wilcke was a Swedish physicist. Biography Wilcke was born in Wismar, son of a clergyman who in 1739 was appointed second pastor of the German Church in Stockholm. He went to the German school in Stockholm and enrolled at the Univers ...

in Sweden and again by

Alessandro Volta Alessandro Giuseppe Antonio Anastasio Volta (, ; 18 February 1745 – 5 March 1827) was an Italian physicist, chemist and lay Catholic who was a pioneer of electricity and power who is credited as the inventor of the electric battery and the ...

in Italy. The name derives from "electron" and "magnet"; drawing analogy to the formation of a magnet by alignment of magnetic domains in a piece of iron. Historically, electrets were made by first melting a suitable dielectric material such as a polymer or wax that contains polar molecules, and then allowing it to re-solidify in a powerful electrostatic field. The polar molecules of the dielectric align themselves to the direction of the electrostatic field, producing a dipole electret with a permanent electrostatic bias. Modern electrets are usually made by embedding excess charges into a highly insulating dielectric, e.g. by means of an

electron beam Cathode rays or electron beam (e-beam) are streams of electrons observed in discharge tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to ele ...

, corona discharge, injection from an electron gun, electric breakdown across a gap, or a dielectric barrier.

Similarity to magnets

Electrets, like magnets, are dipoles. Another similarity is the radiant fields: they produce an electrostatic field (as opposed to a

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

) around their perimeter. 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 A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of a ...

; 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 dipole polarisation in addition. Some materials also display ferroelectricity (i.e. they react to the external fields with a

hysteresis Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...

of the polarisation). Ferroelectrics can retain the polarisation permanently because they are in thermodynamic equilibrium, and thus are used in ferroelectric capacitors. Although electrets are only in a

metastable state In chemistry and physics, metastability denotes an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball i ...

, 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.

Electret types

There are two types of electrets: * Real-charge electrets which contain excess charge of one or both polarities, either ** on the dielectric's surfaces (a

surface charge Surface charge is a two-dimensional surface with non-zero electric charge. These electric charges are constrained on this 2-D surface, and surface charge density, measured in coulombs per square meter (C•m−2), is used to describe the charge di ...

) ** within the dielectric's volume (a space charge) * Oriented-dipole electrets contain oriented (aligned) dipoles. Ferroelectric materials are one variant of these. Cellular space charge electrets with internal bipolar charges at the voids provide a new class of electret materials, that mimic ferroelectrics, hence they are known as

ferroelectret A ferroelectret, also known as a piezoelectret, is a thin film of polymer foams, exhibiting piezoelectric and pyroelectric properties after electric charging. Ferroelectret foams usually consist of a cellular polymer structure filled with air. Pol ...

s. Ferroelectrets display strong piezoelectricity, comparable to ceramic piezoelectric materials. Some dielectric materials are capable of exhibiting both behaviors.

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.

fluoropolymer A fluoropolymer is a fluorocarbon-based polymer with multiple carbon–fluorine bonds. It is characterized by a high resistance to solvents, acids, and bases. The best known fluoropolymer is polytetrafluoroethylene under the brand name "Teflon ...

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 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. 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 discharges, a process called ''corona charging''. Excess charge within an electret decays exponentially. The

decay constant A quantity is subject to exponential decay if it decreases at a rate proportional to its current value. Symbolically, this process can be expressed by the following differential equation, where is the quantity and (lambda) is a positive rate ...

is a function of the material's relative

dielectric constant The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insulat ...

and its bulk resistivity. Materials with extremely high resistivity, such as

PTFE Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemour ...

, may retain excess charge for many hundreds of years. Most commercially produced electrets are based on

s (e.g.

amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek ''a'' ("wi ...

Teflon) machined to thin films.

References

Patents

* Nowlin, Thomas E., and Curt R. Raschke, , "A process for making polymer electrets"