Study of space charge in polyethylene by direct probing: effects of oxidation

By utilizing the laser-induced pressure-pulse technique, the authors have quantitatively studied the behavior of space charge in unoxidized and oxidized low-density polyethylene (PE) samples whose thickness ranged from 100 μm to 1 mm. The distribution of space charge changes with applied field, temperature, extent of oxidation and the thickness of the samples. Prominent negative space charge was formed near the cathode in oxidized PE, indicating that oxidation enhanced the electron injection from the cathode. The amount of negative space charge increased with applied field, and this suggests that the electron injection is enhanced by the applied field. The depth of charge centroid from the cathode became larger with increasing temperature and applied field. This indicates that electrons become more mobile as temperature increases. The complicated behavior of the space charge suggests that space charge formation is determined by the following competing processes: electron injection from the cathode, transport, trapping, detrapping, and charge exchange at the anode