Effect of space charges on the local field and mechanisms of conduction in aged PMMA

It is actually well known that the accumulated charges in insulators subjected to various external stress, involve an irreversible decrease in their performances, and present, in this way, a serious danger to the electric insulation systems. In this work, we have been interested in the effect of space charges on the local field and the mechanisms of conduction in the PMMA subjected to thermal and electric constraints. Measurements by thermal step and thermally stimulated depolarization currents techniques realized after conditioning under an electric field of 7.5 kV/mm at Tc=90°C during 2 hours show the presence of polarization phenomenon and the injected charges by the electrodes in PMMA, which influences local field value. Polarization of PMMA is confirmed by a complementary dielectric experiment. I(V) characteristics show a space charges limited conduction in PMMA. Conduction mechanisms were found to be well described by Schottky and Mott models. Indeed, at low fields ( VTFL) the bulk presents a majoritory contribution to the conduction mechanism. Comparison of the experimental results shows a good agreement.