Dynamic behavior of surface potential on direct-fluorinated polyimide film under low temperature

Polyimide film has been used as an insulation material in the high temperature superconducting motor. As the High Temperature Superconducting (HTS) motor runs under low temperature, the research on surface charge accumulation and decay behaviors for the insulation material under low temperature is necessary. The surface charge has a great influence on the breakdown property. Fluorination is an effective method of polymers surface modification which can improve the properties of original material in many aspects, such as the chemical stability and electrical properties. The fluorination can influence the dynamic behaviors of surface potential when the polymers are used as an insulator. Therefore, it is necessary that the dynamic behaviors of surface potential for original and fluorinated polyimide films were measured to facilitate a thorough understanding of the mechanism of the surface charge accumulating and decaying under low temperature. Polyimide films were fluorinated at 55 °C and 0.05 MPa. The samples were cooled by liquid nitrogen for 10 s before corona charging. Then corona charging was performed with pulse voltage at a relative humidity of ~ 11% in atmospheric air. The surface potential was obtained by the device of an electrostatic voltmeter. It was found that the accumulation and decay behaviors of surface potential for polyimide films are affected by the charge polarity, sample temperature and fluorination time.