Discharge currents of low-density polyethylene after polarization under an ac 50 Hz field

Discharge currents of low-density polyethylene after a polarization under an ac 50 Hz field have been investigated in both 100 mu m thick films and 1 mm thick slabs. These currents have been recorded against the polarization field (10-40 MV m(-1) rms), temperature (30, 50 and 70 degrees C) and the time of polarization (2-200 min). Excellent reproducibility has been observed, except at 70 degrees C, for which a physical change in the sample's morphology has been suggested. No differences were observed when the polarization time was increased from 2 min to 200 min. The voltage has been switched off after a specified polarity and the results have shown that the sign of the discharging currents can be directly related to this polarity. The effect of the injection process can be neglected as shown by a preliminary measurement made with a novel set-up. These discharge currents have been attributed to the presence of a space charge build-up during the polarization. We propose a model in which carriers are predominantly produced within the bulk. This indicates that these carriers could be activated from donor-like states or could originate from the ionization of impurities which move under the influence of the field and create microscopic charge packets. A numerical simulation has revealed that these currents can result from charge motion and diffusion. We discuss the theoretical results, which are in reasonable agreement with our discharge current measurements.