Elektrets

An electret is a dielectric material that has a semi-permanent electric charge or dipole polarization. It is the electrostatic analogue of a permanent magnet. Unlike a capacitor, which requires a voltage source to maintain its charge, an electret retains its charge for extended periods, often years. The term "electret" was coined by Oliver Heaviside in 1892, though the concept was further developed by many researchers, notably James Curie and Marie Curie in the early 20th century.

Electrets are typically made from polymers like PTFE (Teflon) or polypropylene, though ceramics and waxes can also be used. The polarization can be induced by heating the material above its Curie temperature and then cooling it in a strong electric field. Alternatively, corona charging, where ions are sprayed onto the surface of the material, can also create electrets. The trapped charges or aligned dipoles create an external electric field.

The primary applications of electrets are in devices that require a stable, persistent electric field without requiring an external power source. These include electret microphones, where the electret material forms part of the capacitor that converts sound waves into electrical signals. They are also used in air filters, electrostatic precipitators, and some types of sensors and actuators. The persistence of their charge makes them a valuable component in low-power electronic devices and systems where continuous power supply is not feasible.