Preparation And Properties Of Nano-Al₂O₃Polyimide Composite Film With Good Corona Resistance

In recent years, with the rapid development of power electronic technology, especially the emerge of variable frequency speed control technology, higher requirement for insulation materials has been promoted. Insulation materials with high temperature resistance and corona resistance have become a major focus by researchers. Doping some certain amounts of nano-oxide particles, such as nano-alumina, silica and titania into the insulation material was proved to be a great improvement in the temperature resistance, electric breakdown strength and the corona resistance properties of insulation materials. Therefore, researches on these inorganic nano-particles, especially the effect of the addition content on the performance of polyimide composite films’has great significance.In this paper, nano-Al2O3/polyimide composite films based on Pyromellitic dianhydride(PMDA) and4,4’-oxydianiline(ODA) in the solution of N, N-dimethylacetamide(DMAc) were synthesized by addition with different contents of nano-sized Al2O3via in situ polymerization. The microstructures of PI films were studied by scanning electronic microscope(SEM). The analysis result showed that nano-Al2O3particles were well dispersed in the PI matrix. However, it could also be found that the agglomeration of particles increased with the doped contents. The transmittance of UV-Vis, thermal stability, mechanical performance and corona-resistance of PI composite films have also be measured and analyzed. UV-Vis transmittance analysis revealed that the transmittance of composite films was lower than pure film, and with the increasing of nano-Al2O3particle content, the transparence will continue to fall down. Results from thermal gravimetric analysis on both pure film and composite films showed that the composite films have a better thermal stability than pure film. Results also indicated that the thermal stability of composite films would continually rise with the increasing of nano-Al2O3particle content. When the Al2O3content was16wt%, the decomposition temperature reached593.15℃, which was11℃higher than pure film.Among the composite films, the mechanical performance of PI/nano-Al2O3composite film was the best when the doping amount was8wt%. The tensile strength and elongation at break of composite film were119.9MPa and20.0%, respectively. The electric breakdown strength of composite films was better than pure film, and with increasing content of inorganic, it’s value have a trend of first increment and then reduction. The8wt%Al2O3/PI composite film had the highest electric breakdown strength, which was229.7kV/mm. Comparison with pure PI film, the corona-resistance time of composite film was longer. It proved that the nano-Al2O3particles can enhance the corona resistance as well.