Breakdown and space charge formation in polyimide film under DC high stress at various temperatures

Relationship between breakdown strength and space charge formation in polyimide film under dc high stress at various temperatures is investigated using pulsed electro-acoustic (PEA) method. Some typical results of the space charge observations show that hetero space charges are always found before breakdown. The amount of the hetero charges increase with increase of temperature or increase of applied electric field. Since the enhancement of the internal electric field in the sample by the accumulation of the hetero charges is not so large, the accumulation doesn't seem to be an immediate cause of breakdown. However since it is always observed before breakdown, it may be predictive information for breakdown. In a certain case, the breakdown occurs after voltage application for few hours. However, while we give an interval of short circuit condition after observing the hetero space charge under dc stress, the total voltage application time to breakdown is almost the same to the case without the interval. It means that the hetero space charge generation may show a kind of degradation of the material.