An electret is an insulating material (or dielectric) that retains an electrostatic charge for many years and as such can be considered the electrostatic equivalent of a permanent magnet. Thus, while the magnet is a permanent source of external magnetic field, the electret is a permanent source of electric field. Electrets are usually produced in sheet or film form with one surface positively charged and the other negatively charged.
The analogy between magnets and electrets can be taken further, because both lose their effects when heated. Furthermore, a magnet will retain its magnetization longer if the magnetic path is short-circuited between the north and south poles with a bridge of iron, and similarly, an electret will retain its charge longer if the two surfaces are electrically short-circuited by sandwiching them between two connected metal plates, or keepers.
Electrical polarization
When an electret is formed, the material is said to be polarized—meaning that the positive and negative charges have been orientated in a preferred direction. Electrets are called real-charge electrets or dipolar-charge electrets, depending on the method of preparation.
In dipolar-charge electrets the charges are arranged in positive–negative pairs and aligned in the same direction throughout the material. Here the arrangement is the same as the dipoles created in a dielectric material of a capacitor, but no external electrical field is needed to keep them aligned.
In a real-charge electret, external charges are trapped near each surface of the electret, so that one surface bears a permanent positive charge and the other surface a compensating negative electric charge.
Forming electrets
The earliest electrets were formed by a Japanese scientist, Eguchi, in 1919. He found that, if a high voltage was applied to two electrodes immersed in a molten mixture of waxes and resins and the mixture was then solidified with the voltage on, the material between the electrodes retained some permanent frozen polarization. This technique is known as the thermoelectret forming process and has been extended to include forming from materials requiring only elevated temperatures (not necessarily to the molten state). Indeed, in some cases no thermal treatment is required at all.
The most common material used for early work was carnauba wax, a hard yellow Brazilian wax used in furniture polishes and cosmetics. Electrets have been formed in this way from materials including Plexiglas, ice, shellac, and anthracene, and many other solid dielectrics (insulators) can be polarized in the same manner.
Recent interest has centered on the production of real-charge electrets, where, in essence, the effect is simply the ability of some materials to retain surface electrostatic charge for a considerable period. This renewed interest coincided with the advent of pure polymers in flexible-film form. They retain charge for long periods and, because of their very good insulating properties and roll-film form, permit simpler continuous charging techniques such as knife-edge corona charging and electron-beam charging. Electrets of this type have been made from polyethylene, poly-propylene, polystyrene, polycarbonate, fluoro-carbon (including PTFE—Teflon), and many other polymer films.
