Space charge and steady state current in LDPE samples containing a permittivity/conductivity gradient

Electromagnetic theory predicts that a dielectric sample in which a steady DC current of density /spl epsiv/ is flowing, and in which the ratio of permittivity /spl epsiv/ to conductivity /spl sigma/ varies with position, will acquire a space charge density j/spl middot/grad(/spl epsiv///spl sigma/). A simple and convenient way to generate an /spl epsiv///spl sigma/ gradient in a homogeneous sample is to establish a temperature gradient across it. The resulting spatial variation in /spl epsiv/ is usually small in polymeric insulators, but the variation in /spl sigma/ can be appreciable. Laser induced pressure pulse (LIPP) measurements were made on 1.5 mm thick plaques of ultra pure LDPE equipped with vacuum-evaporated aluminium electrodes. Temperature differences up to 27/spl deg/C were maintained across the samples, which were subjected to DC fields up to 20 kV/mm. Current density was measured as a function of temperature and field. Negligible thermally generated space charge was observed. The charge carrier mobility in the bulk of the samples was estimated to be of order 10/sup -8/ cm/sup 2/ V/sup -1/ s/sup -1/.