@phdthesis{Mellinger2004,
  author    = {Mellinger, Axel},
  title     = {Charge storage in electret polymers: mechanisms, characterization and applications},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-5689},
  school      = {Universit{\"a}t Potsdam},
  year      = {2004},
  abstract  = {Electrets are materials capable of storing oriented dipoles or an electric surplus charge for long periods of time. The term "electret" was coined by Oliver Heaviside in analogy to the well-known word "magnet". Initially regarded as a mere scientific curiosity, electrets became increasingly imporant for applications during the second half of the 20th century. The most famous example is the electret condenser microphone, developed in 1962 by Sessler and West. Today, these devices are produced in annual quantities of more than 1 billion, and have become indispensable in modern communications technology. Even though space-charge electrets are widely used in transducer applications, relatively little was known about the microscopic mechanisms of charge storage. It was generally accepted that the surplus charges are stored in some form of physical or chemical traps. However, trap depths of less than 2 eV, obtained via thermally stimulated discharge experiments, conflicted with the observed lifetimes (extrapolations of experimental data yielded more than 100000 years). Using a combination of photostimulated discharge spectroscopy and simultaneous depth-profiling of the space-charge density, the present work shows for the first time that at least part of the space charge in, e.g., polytetrafluoroethylene, polypropylene and polyethylene terephthalate is stored in traps with depths of up to 6 eV, indicating major local structural changes. Based on this information, more efficient charge-storing materials could be developed in the future. The new experimental results could only be obtained after several techniques for characterizing the electrical, electromechanical and electrical properties of electrets had been enhanced with in situ capability. For instance, real-time information on space-charge depth-profiles were obtained by subjecting a polymer film to short laser-induced heat pulses. The high data acquisition speed of this technique also allowed the three-dimensional mapping of polarization and space-charge distributions. A highly active field of research is the development of piezoelectric sensor films from electret polymer foams. These materials store charges on the inner surfaces of the voids after having been subjected to a corona discharge, and exhibit piezoelectric properties far superior to those of traditional ferroelectric polymers. By means of dielectric resonance spectroscopy, polypropylene foams (presently the most widely used ferroelectret) were studied with respect to their thermal and UV stability. Their limited thermal stability renders them unsuitable for applications above 50 °C. Using a solvent-based foaming technique, we found an alternative material based on amorphous Teflon® AF, which exhibits a stable piezoelectric coefficient of 600 pC/N at temperatures up to 120 °C.},
  subject      = {Elektret},
  language  = {en}
}