Electret

Or of the electret is an electrically insulating material which contains quasi -permanent electric charge or quasi -permanent electric dipoles oriented and thus produces a quasi- permanent electric field in its vicinity or in the interior thereof. The name is created based on the word magnet and comes from the English physicist Oliver Heaviside, who predicted the existence of electrets 1885 theoretically. The name is to show that the electret can be interpreted as an electrostatic analogue of the permanent magnet.

Already in the 18th century, there were studies on the electrical charge and the orientation of dipoles in insulating materials. A simple apparatus for the study and use of electrical effects associated was the Elektrophor, which was first reported by Wilke, then described independently by Alessandro Volta, and finally by Georg Christoph Lichtenberg ( 1778).

The modern study and application of electrets began when electrets were made around 1920 in Japan on the basis of dielectrics. A mixture of wax and carnauba wax was melted and subjected to the subsequent cooling in an external electric field, aligned by the mixture contained in the molecules with an electric dipole moment, and at the same time charges existing in the material to be separated ( a so-called Thermoelektret ). After solidification of the growing cold mixture, the mobility of the molecules is severely restricted, the orientation of the dipoles is retained practically. Such electrets, however, are stable only if they are sealed by a metallic protective layer.

Today electrets usually made of polymers (such as polytetrafluoroethylene, Polytetrafluorethylenpropylen, polypropylene, polyethylene terephthalate, polyvinylidene fluoride, and some of its copolymers ) were prepared, but partly also from inorganic dielectrics such as silicon dioxide or silicon nitride.

Technically electrets are used in some cases very large numbers as membranes in sound transducers ( electret microphone or headphones), electrically and mechanically active fibers in filter technology ( air filter), as a source of quasi-permanent electric field in radiation dosimeters, as ultrasonic transducer layers in medical technology, as infrared motion detectors, and many other applications.

Because of the bad reputation of the earlier unstable electret microphones is often hidden in that they are electret microphones. In the data sheets then is instead the words " pre - polarized ".