Foil Electrets and Their Use in Condenser Microphones

Abstract

New results are reported on polarization and depolarization characteristics of foil electrets made of polyester, fluorocarbon, polycarbonate, and polyimide. The foils (1/4 to 1‐mil thick) are polarized by exposure to d‐c fields at high temperatures. Use of a depolarization technique made it possible to measure for the first time separately the decay characteristics of homocharge and heterocharge of foil electrets. Time constants for the homocharge decay of foil electrets at room temperature were found to be about 0.1 years for polyimide, one year for polyester, and more than 100 years for fluorocarbon and polycarbonate. The relaxation times of the heterocharge on polyester and fluorocarbon electrets at room temperature vary from 0.1 to 1 year. These results are compared with other data on the net decay of both charges, derived in part from measurements at room temperature taken over a five‐year period and in part by extrapolating high‐temperature data. All time constants depend somewhat on preconditioning of the electrets after forming. Condenser microphones built with fluorocarbon and polycarbonate foil electrets are expected to show little change in sensitivity over periods of the order of 100 years.