A study of the formation conditions of electrets

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Abstract
The role played by resistivity and the importance of this variable in electret formation is demonstrated; measurements of this quantity during formation enable one to control the resulting electret characteristics and to predict future behavior. This study is concerned principally with dielectrics of moderately low (?1015 Ω cm) resistivity. It is shown that the application of the electric field during either polymerization of the monomer or evaporation of the solvent of a polymer solution results in a greater and more stable charge than was the case in normal thermoelectret formation. The effect of a number of important parameters on the maximum charges attainable and their stability is determined. These include the addition of varying amounts of plasticizers and ionic impurities to polymers, as well as polarizing field and temperature cooling speed and ageing. We show that knowledge of the polarizing field and resistivity alone are sufficient to predict a priori whether the resulting electret will be heterocharged or homocharged.

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