High field polarisation behaviour of cross-linked polyethylene

Abstract

The presence of persistent polarisation in an electrically stressed insulating polymer may originate from a significant alignment of the dipoles (structural and impurity) and injection and subsequent localization of charge carriers. A measure of the frozen-in polarisation may be obtained by several techniques including aborption current studies, thermally stimulated discharge current studies and an analysis of thermal current transients arising from a step input of thermal radiations incident on a polymer surface with electrodes shorted. Using the last technique1–3, mentioned above, it has been shown that in low density polythene (LDPE) films, the thermal current transients may arise from a presence of impurity dipoles at low electric field stressing. As the field is increased, the injected space charge becomes the dominant contributor to such current transients which are of pyroelectric nature. It has also been shown4 that the conductivity of hexane-treated LDPE (10−18 Ω−1 m−1) at room temperature is approximately two orders of magnitude lower than that of the untreated material. The thermal current transients with LDPE show that hexane treatment seems to provide a removal of impurity ions in the original polymer in such a manner that electrode injection becomes quite dominant even at moderate charging fields.