Space charge profiles in low density polyethylene samples containing a permittivity/conductivity gradient

Laser induced pressure pulse space charge measurements were made on 1.5 mm thick plaques of high purity low density polyethylene equipped with vacuum-evaporated aluminium electrodes. Temperature differences up to 20 °C were maintained across the samples, which were subjected to dc fields up to 1.5×107 V m-1. Current density was also measured as a function of temperature and field. Space charge due exclusively to the temperature gradient was detected, with density of order 0.01 C m-3. The activation energy associated with the transport of electrons through the bulk was calculated as 0.09 eV. However, measurements of the temperature dependence of the current in isothermal samples yielded activation energies in the range 0.9-1.3 eV. It is deduced that most of this larger activation energy is associated with charge injection at the electrodes. Electron mobilities not less than 2×10-12 m2 V-1 s-1 were inferred by combining the space charge and current density measurements