Space charge profiles in planar LDPE with TiO2 additive and a temperature gradient

Steady state space charge profiles and dc current densities have been measured in 1 mm thick planar low density polyethylene samples, with and without TiO₂ additive (2% by weight). DC fields up to 10 kV/mm were applied. The measurements were made under isothermal conditions in the temperature range 20-60 °C, or with a temperature difference of 6°C established across the sample thickness. Activation energies of 0.75±0.05 eV and 0.50±0.05 eV respectively were obtained for isothermal samples with and without TiO ₂. The steady state current densities were lower in samples containing TiO₂. Space charge densities of order 0.05 C/m³ were observed in undoped samples well away from the electrodes. The corresponding density in doped samples was 0.2 C/m³ . In samples with a temperature gradient, the theoretically-predicted reversal of space charge polarity on reversing the direction of the temperature gradient was not observed. However, the space charge polarity reversed under applied field reversal, as expected