Electret

# Electret

Discussion

Encyclopedia
Electret is a dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

material that has a quasi-permanent electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

or dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...

polarisation. An electret generates internal and external electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

s, and is the electrostatic equivalent of a permanent magnet. Oliver Heaviside
Oliver Heaviside
Oliver Heaviside was a self-taught English electrical engineer, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques to the solution of differential equations , reformulated Maxwell's field equations in terms of electric and...

coined this term in 1885. Materials with electret properties were, however, already studied since the early 18th century. One particular example is the electrophorus
Electrophorus
An electrophorus is a capacitive generator used to produce electrostatic charge via the process of electrostatic induction. A first version of it was invented in 1762 by Swedish professor Johan Carl Wilcke,...

, 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 and again by Alessandro Volta
Alessandro Volta
Count Alessandro Giuseppe Antonio Anastasio Gerolamo Umberto Volta was a Lombard physicist known especially for the invention of the battery in 1800.-Early life and works:...

in Italy.

## Similarity to capacitors

There is a similarity between an electret 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
Ferroelectricity
Ferroelectricity is a property of certain materials which possess a spontaneous electric polarization that can be reversed by the application of an external electric field. The term is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic moment. Ferromagnetism was...

; i.e. they react to the external fields with a hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...

of the polarization; ferroelectrics can retain the polarization permanently because they are in thermodynamic equilibrium, and are used in ferroelectric capacitor
Ferroelectric capacitor
Ferroelectric capacitor is a capacitor based on a ferroelectric material. In contrast, traditional capacitors are based on dielectric materials. Ferroelectric devices are used in digital electronics as part of ferroelectric RAM, or in analog electronics as tunable capacitors .In memory...

s. Although electrets are only in a metastable state, those fashioned from very low leakage materials can retain excess charge or polarization for many years. An electret microphone
Electret microphone
An electret microphone is a type of condenser microphone, which eliminates the need for a polarizing power supply by using a permanently charged material....

is a type of condenser microphone that eliminates the need for a power supply by using a permanently charged material.

An electret is a stable dielectric material with a quasi-permanently embedded static electric charge (which, due to the high resistance of the material, will not decay for time periods of up to hundreds of years) and/or a quasi-permanently oriented dipole polarization. The name comes from electron (Greek word for amber) and magnet and was coined by Oliver Heaviside in 1885; 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, a corona discharge, injection from an electron, electric breakdown across a gap or a dielectric barrier, etc.

## 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
Surface charge is the electric charge present at an interface. There are many different processes which can lead to a surface being charged, including adsorption of ions, protonation/deprotonation, and the application of an external electric field...

)
• within the dielectric's volume (a space charge
Space charge
Space charge is a concept in which excess electric charge is treated as a continuum of charge distributed over a region of space rather than distinct point-like charges...

)
• 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
Ferroelectret
Ferroelectrets also known as piezoelectrets, are thin films of polymer foams, exhibiting piezoelectric and pyroelectric properties after electric charging. Ferroelectret foams usually consist of a cellular polymer structure filled with air. Polymer-air composites are elastically soft due to their...

. Ferroelectrets display strong piezoelectricity
Piezoelectricity
Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...

, comparable to ceramic piezoelectric materials.

Some dielectric materials are capable of acting both ways.

## Materials

Electret materials are quite common in nature. Quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

and other forms of silicon dioxide, for example, are naturally occurring electrets. Today, most electrets are made from synthetic polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s, e.g. fluoropolymer
Fluoropolymer
A fluoropolymer is a fluorocarbon based polymer with multiple strong carbon–fluorine bonds. It is characterized by a high resistance to solvents, acids, and bases.-History:Fluoropolymers were accidentally discovered in 1938 by Dr. Roy J...

s, polypropylene
Polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...

, 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
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing 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 for electrets are usually wax
Wax
thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...

es, polymers or resin
Resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...

s. One of the earliest recipes consists of 45% carnauba wax
Carnauba wax
Carnauba , also called Brazil wax and palm wax, is a wax of the leaves of the palm Copernicia prunifera, a plant native to and grown only in the northeastern Brazilian states of Piauí, Ceará, and Rio Grande do Norte. It is known as "queen of waxes" and usually comes in the form of hard yellow-brown...

, 45% white rosin
Rosin
.Rosin, also called colophony or Greek pitch , is a solid form of resin obtained from pines and some other plants, mostly conifers, produced by heating fresh liquid resin to vaporize the volatile liquid terpene components. It is semi-transparent and varies in color from yellow to black...

, and 10% white beeswax
Beeswax
Beeswax is a natural wax produced in the bee hive of honey bees of the genus Apis. It is mainly esters of fatty acids and various long chain alcohols...

, melted, mixed together, and left to cool in a static electric field of several kilovolts/cm. The thermo-dielectric effect
Thermo-dielectric effect
The thermo-dielectric effect is the production of electric currents and charge separation during phase transition.This interesting effect was discovered by Joaquim da Costa Ribeiro in 1944. The Brazilian physicist observed that solidification and melting of many dielectrics are accompanied by...

, related to this process, was first described by the Brazilian researcher Joaquim Costa Ribeiro.

Electrets can also be manufactured by embedding excess negative charge within a dielectric using a particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...

, or by stranding charges on, or near, the surface using high voltage
High voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...

corona discharge
Corona discharge
In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...

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
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...

and its bulk resistivity
Resistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...

. Materials with extremely high resistivity, such as Teflon, may retain excess charge for many hundreds of years. Most commercially produced electrets are based on fluoropolymer
Fluoropolymer
A fluoropolymer is a fluorocarbon based polymer with multiple strong carbon–fluorine bonds. It is characterized by a high resistance to solvents, acids, and bases.-History:Fluoropolymers were accidentally discovered in 1938 by Dr. Roy J...

s (e.g. amorphous Teflon) machined to thin films.

## Applications

Electret materials have found commercial and technical interest. For example, they are used in electret microphone
Electret microphone
An electret microphone is a type of condenser microphone, which eliminates the need for a polarizing power supply by using a permanently charged material....

s and in copy machines. They are also used in some types of air filter
Air filter
A particulate air filter is a device composed of fibrous materials which removes solid particulates such as dust, pollen, mold, and bacteria from the air. A chemical air filter consists of an absorbent or catalyst for the removal of airborne molecular contaminants such as volatile organic compounds...

s, for electrostatic collection of dust particles, and in electret ion chambers for measuring ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of uranium or thorium. Its most stable isotope, 222Rn, has a half-life of 3.8 days...

. See for "Manufacturing Method and Device for Electret Processed Product"

• Oliver Heaviside
Oliver Heaviside
Oliver Heaviside was a self-taught English electrical engineer, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques to the solution of differential equations , reformulated Maxwell's field equations in terms of electric and...

• Corona wire
• Telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

• Electret microphone
Electret microphone
An electret microphone is a type of condenser microphone, which eliminates the need for a polarizing power supply by using a permanently charged material....

• Electromotive force
Electromotive force
In physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...

• Tip ring sleeve
• Ferroelectricity
Ferroelectricity
Ferroelectricity is a property of certain materials which possess a spontaneous electric polarization that can be reversed by the application of an external electric field. The term is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic moment. Ferromagnetism was...

## Patents

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