Electrets: Dielectrics with Quasi-Permanent Charge or Polarization

The present review of very recent electret work starts with a history-oriented introduction which refers to important early studies of charge storage in insulators; a number of relevant surveys are listed for further information on previous electret research. This paper concentrates on work published during the last few years and especially on papers not covered in earlier reviews. New electret materials and novel experimental techniques are briefly discussed; here, inorganic substances and methods for the nondestructive determination of charge or polarization profiles are given special consideration. Recent developments on the subject of charge storage and transport, and of radiation effects in electrets, are reported. A major topic is the ongoing investigation of piezo-, pyro-, and ferroelectricity in polymer electrets where many new results were obtained during the last few years, The paper concludes with a discussion of a small number of interesting electret applications which may serve as examples for the large amount of device applications suggested in the literature.