Effects of water absorption on surface charge and dielectric breakdown of polyimide/Al2O3 nanocomposite films

The incorporation of inorganic nano particles (TiO
₂
, Al
₂
O
₃
and SiO
₂
etc.) with polyimide (PI) greatly improves its dielectric property. The dielectric properties of PI films are largely influenced by the absorbed water and humidity. What is more, the water absorption will be affected greatly by nano particles. In this paper, to study the effects of absorbed water on dielectric properties of moistened PI/Al
₂
O
₃
composite film, PI film specimens were prepared by incorporating 12 wt% nano Al
₂
O
₃
particles with PI. The original specimens were also prepared for contrast. Then the specimens were immersed in high purity water for 12, 24, 48, 72, 96, 120, 144 and 168 h. The water absorption and breakdown strength under different immersion time were measured. Meanwhile, given that the existence of surface charge had a critical influence on the breakdown characteristics, the corona charging tests were conducted at room temperature under pulse voltage. The pulse frequency was 300, 600 and 900 Hz, respectively. The pulse amplitude was +4 kV. The charge dissipation was investigated based on the demarcation energy theory. Obtained results show that the moisture condition affects more on the nano Al
₂
O
₃
composite films than the original ones. Compared to original PI films, the water absorption is larger and the breakdown strength falls faster for the nano ones. The decay process of surface charge and the change of detrapping charges in nanocomposite films are affected greatly by the absorbed water.