Anomalous currents in PMMA/PS double layered films

Multilayer insulators made of two or more materials with different dielectric constants and conductivities are widely used in power apparatus such as oil impregnated cables and transformers etc.. In such insulators appreciable internal space charge accumulates at their interfaces causing polarization. The local field at the interface may be consequently increased by this polarization when a dc field in applied. Several other distinct dielectric relaxations are also usually present in a polarized insulating system. This multiplicity of polarization is seen most easily in a scan of depolarization current as a function of temperature. Double layered PMMA/PS films of average thickness 20 μm prepared by solvent evaporation technique were thermally polarized at temperatures 40, 70, 100 and 130°C with fields of 5, 25, 100 and 200 kV/cm. Short circuit TSDC measurements were carried out on these polarized films in two modes i.e. after applying a positive field to the PMMA layer. PMMA (+)/PS(-or grounded), and after applying positive field to the PS layer. PMMA(-)/PS(+). The TSDC thermograms were characterized by several peaks. Anomalous current behaviour was observed at high poling temperatures and high fields for PMMA(+)/PS(-) mode. In case of PMMA(-)/PS(+) mode such anomalous currents were observed even at low temperatures with low field; for high field values current exhibited normal behaviour. The observed phenomena has been interpreted in terms of injected homocharge and Maxwell-Wagner effect