Polyimides (PIs) are widely used in many fields including aerospace and microelectronics. Due to their poor corona resistance, their practical applications were limited, especially in the field of variable frequency motors. In this study, we have achieved for the first time to increase the corona resistance by controlling the preparation process of the three-layered PI composite. A series of PI/nano-Al2O3 composite films with novel three-layer structure were prepared by in situ polymerization employing pyromellitic dianhydride and 4,4-diaminodiphenyl as raw material, N, N-dimethylacetamide as solvent, and doping of nano-Al2O3. The first layer of the PI/Al2O3 composite film was characterized by Fourier transform infrared spectroscopy, and the imidization rate under different processes was calculated. The interface structures and bonding conditions of the composite films were characterized by scanning electron microscope, and the surface morphologies of the composite films treated by different corona-resistance times were investigated. X-ray diffraction analysis was also used to study the effect of nano-Al2O3 on PIs with different imidization ratios. The corona-resistance time and breakdown field strength of the composite films prepared by different processes were also tested. The results indicated that the combination of the three-layer composite film and the corona-resistance abilities of the composite membrane surface was enhanced by increasing the imidization rate. Meanwhile, the corona-resistance time and the electrical breakdown strength of composite films were also improved by increasing the imidization rate.
Effect of interface structure regulation caused by variation of imidization rate on conduction current characteristics of PI/nano-Al2O3 three-layer composite films
