Electret

An electret is a dielectric—a material that resists electrical conduction—that maintains a persistent electric field after it has been charged or polarized. The stored state can arise from real charges trapped at a surface or within the material, or from dipoles aligned through its volume. The word describes a material state rather than one single chemical substance: polymers and other dielectrics can be engineered to retain charge or polarization for useful periods. The charge is not literally permanent; leakage, temperature and material structure affect how long it lasts.

Different electret types are prepared in different ways. A dielectric may be exposed to a strong electric field, heated and cooled under an applied field, or charged through contact with another material. These processes can trap charge carriers or orient molecular dipoles, leaving an electric field after the external charging arrangement is removed. The charge-storage mechanism and stability depend on the material and treatment. Ferroelectrets, also called piezoelectrets, use charged internal surfaces of cavities in a polymer structure; the cavity charges behave like large-scale dipoles and can produce electromechanical responses.

The most familiar electret application is a type of condenser microphone. Sound moves a thin diaphragm relative to an electrode, changing the electrical capacitance and producing a varying signal. A permanently polarized electret supplies the required electric bias, so the microphone does not need a separate high-voltage polarizing supply. Practical circuits may still use power for an internal amplifier. James West and Gerhard Sessler developed a durable foil electret microphone at Bell Laboratories; their design was finalized in 1962 and entered wider production later in the decade. Compactness and comparatively low cost helped the technology spread through consumer devices and acoustic instruments.

Electret materials are used or investigated in electrostatic applications beyond microphones, including filters and charge-based devices. Recent polymer-electret research examines energy harvesting, electrostatic motors and flexible sensing. The device performance depends on how reliably a material can retain its charge or polarization and how its geometry couples the field to moving or sensing parts. These applications use related physical principles but do not imply that every electret formulation is suitable for every device.