The temperature dependence of space charge accumulation in cross-linked polyethylene

This paper presents the results of laser-induced-pressure-pulse (LIPP) measurements of space charge accumulation in 0.5 mm thick planar XLPE samples with aluminium electrodes, over the temperature range 30-90 degrees C. The applied field strength was 10 kV mm(-1). The increase in transit time of the pressure pulse across the sample with increasing temperature indicated that Young's modulus decreased by 50% from 30 to 90 degrees C. It was concluded that electrons injected at the cathode are transported to the anode at temperatures above 40 degrees C, and that electron traps cause an accumulation of negative charge immediately adjacent to both cathode and anode. It appears that a concentration of impurity molecules accumulates close to the cathode, and field-assisted thermal ionisation of these molecules generates an immobile positive space charge adjoining the electronic space charge adjacent to the cathode.