Effect of Surface State on DC Breakdown of LDPE Films

Interface between the metal electrode and insulating material affects dielectric performance of the whole insulating system significantly. In this study, low density polyethylene (LDPE) films were treated in ozone atmosphere for different time duration to investigate the effect of interface state on breakdown performance. Fourier transform infrared spectra (FT-IR) results show that carbonyl group (C=O) is introduced into the surface of LDPE film after ozone treatment, and the amount of C=O groups increases when the ozone oxidation time extends. Surface trap distribution characterized by surface potential decay (SPD) reveals that two hole-type traps exist in the surface of LDPE and ozone treated specimens. Both trap levels become shallower and the density of deeper level traps decreases with the increasing ozone oxidation time, while that of shallower level traps increases. Space charge distribution measured by pulsed electro-acoustic (PEA) method suggests that more positive charges are injected into specimens from the anode and accumulated in the vicinity of cathode after ozone treatment. Furthermore, the dc breakdown strength declines monotonously with longer ozone treatment time, and the lowest breakdown strength of specimen occurs after 6 hours, decreasing by 26.9% compared with that of untreated LDPE. Finally, it is concluded that ozone treatment results in changing of surface chemical composition, which determines surface trap distribution (surface state), leading to the variation of charge injection and accumulation properties as well as the subsequent breakdown performance.