Space charge characteristics of thermally and electrically aged polyimide films

In order to study effects of thermal and electrical aging on space charge characteristics of polyimide (PI) films, accelerated aging experiments were carried out under the conditions of 150C, -50 kV/mm up to 90 days. In addition, field emission scanning electron microscopy, X-ray diffraction, conduction current and space charge were tested to reflect the microstructure, crystalline, and charge transport characteristics, respectively. Results show that the width of surface defeat increases and the regularity degree of short-range atomic arrangement decreases under thermal and electrical aging. Besides, the conduction current monotonously increases with long-term aging. Space charge measurement results indicate that during the process of accelerated thermal and electrical aging, heterocharge firstly accumulated near the electrodes, and the amount of it gradually decreases with the increase of aging time. Meanwhile, homocharges started to accumulate and replace the heterocharge accumulation finally. It is analyzed that the imide linkage and ether linkage are broken during the long-term accelerated aging, and the surface of PI reacts with oxygen, producing gas and forming defects in the surface. On the one hand, the break of linkage forms small molecular groups, resulting in the decrease of the regularity degree of short-range atomic arrangement. On the other hand, it forms new shallow traps, resulting in the decrease of average trap depth and the increase of mobility. Both of them affect the charge transport, resulting the change of charge distribution.