Effects of high temperature on space charge distributions in direct-fluorinated bistratal polyimide film

This Polyimide film has excellent electrical properties and it serves as an important insulating material which is widely used in the variable-frequency motor and wind generator. Since high temperature is a significant reason for degradation in insulation, the space charge distributions of polyimide (PI) films in high temperature condition is worth investigating. Under different temperature, the conductivity changes differently with different modified PI films, which influences space charges accumulating at the interface bistratal PI films. Previous studies found that surface fluorination could modify the surface layer properties of polyimide and nanocomposites in PI films could prove its corona-resistant property. In this paper, to investigate how space charges behave in bistratal PI film, double-layered samples were prepared for the experiment. The samples were made by one piece of non-fluorinated original or corona-resistant PI film and one piece of original or corona-resistant PI film which were surface-fluorinated for 15, 45 and 75 min respectively. In the experiment, the charge injecting process was held under 40, 80, 120 and 160 °C. In order to present the distributions of space charge in the 25 pm thick samples, a laser-induced thermal pulse measurement was set up. Results show that direct-fluorination and temperature change the conductivities of PI films, and that influences the space charges at the interface and the space charge behaviors in the sample. The space charges at the interface can be modulated with different fluorination time, and with the rising of the temperature, fewer space charges accumulate in the bistratal samples.