Electret

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The electret is an electrically insulating material that contains quasi-permanently stored electrical charges or quasi-permanently aligned electrical dipoles and thus generates a quasi-permanent electrical field in its surroundings or in its interior. The name is based on the word magnet and comes from the English physicist Oliver Heaviside , who theoretically predicted the existence of electrets in 1885. The name is intended to show that the electret can be understood as an electrostatic analogue to the permanent magnet .

As early as the 18th century there were studies on the electrical charge and the alignment of dipoles in insulating materials. A simple apparatus for investigating and using the associated electrical effects was the electrophore , which was first described by Wilke, then independently by Alessandro Volta and finally by Georg Christoph Lichtenberg (1778).

The modern study and application of electrets began with the fact that electrets were manufactured in Japan on the basis of dielectrics around 1920 . A mixture of wax and carnauba wax was melted and, during the subsequent cooling, exposed to an external electric field, through which the molecules contained in the mixture are aligned with an electric dipole moment and at the same time the charges present in the material are separated (so-called thermoelectret). After solidification, the mobility of the molecules is severely restricted and the alignment of the dipoles is practically retained. However, such electrets are only stable if they are sealed by a metallic protective layer.

Today electrets are mostly made from polymers (including polytetrafluoroethylene , polytetrafluoroethylene propylene , polypropylene , polyethylene terephthalate , polyvinylidene fluoride and some of its copolymers ), but sometimes also from inorganic dielectrics such as silicon dioxide or silicon nitride .

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

Because of the bad reputation of the previously unstable electret, the fact that microphones are electret microphones is often hidden. Instead, the specification “pre-polarized” appears in the data sheets .

literature

Individual evidence

  1. ↑ Réda Lathrache, Heinz Fißan: Fundamental studies on the separation behavior of electret filters - Part 1: Determination of the degree of separation . Dust - cleanliness. Luft , 49 (1989) No. 9, pp. 309-314.