Dielectric Breakdown of Perspex Electrets in Nonuniform Fields

Perspex electrets having a sphere-plane electrode geometry were made by heating to 160°C, and cooling to 20°C while a positive-forming voltage of 15 kV was continuously applied to the spherical electrode. Their mean breakdown voltage for positive impulse breakdown was higher by 18 kV than that of similar specimens termed nonelectrets, which had undergone the same heat treatment but to which no forming voltage was applied, and was lower by 38 kV when the polarity of the breakdown voltage was reversed. When the electrets were depolarized by reheating to 160°C with their electrodes short-circuited, their mean breakdown own voltage was the same as that of nonelectrets. These results are consistent with a homocharge in the electret close to the electrode, which can be released during depolarization by heating. Measurement of depolarization current using electrets made from flat plates of Perspex supports this view.