Studies On Preparation And Properties Of Polyimide/Alumina Nanocomposite Films

Polyimides （PIs） have been extensively applied in the aero-space, microelectronic and electrical industries for insulation owing to their outstanding combination of thermal, mechanical, and electrical properties over an extremely wide temperature range. In recent years, the trend in AC motors has been towards higher operating voltages, an unavoidable consequence of which is increased attack on the insulation system due to corona discharge. Therefore, it is necessary to develop high-voltage insulation materials that perform well in corona-discharge environments. PI composite materials have received considerable attention due to the significant improvements compared with the pristine PI that can be achieved by introducing inorganic additives. By virtue of its high temperature resistance, good chemical stability, higher thermal conductivity, and its wider application in electrical devices, Al₂O₃ is considered to be one of the most important inorganic components for enhancing the corona resistance of dielectric materials. PI/Al₂O₃ nanocomposite materials can generally be prepared by mixing the polyamic acid （PAA） with the hydrolysate of aluminum isopropoxide by sol–gel method or be prepared by dispersing nano Al₂O₃ powder and the coupling agent in the solvent by means of an ultrasonic device and then be mixed with PAA. However, higher doping content usually leads to aggregation which impedes the further improvement of corona resistance of the composite films.In this dissertation, in order to improve the corona resistance of PI film, the sol ofγ-Al₂O₃ and AlOOH （boehmite） were synthesized from aluminum isopropoxide by hydrothermal process, and then PI/γ-Al₂O₃ and PI/AlOOH nanocomposite films were prepared using pyromellitic dianhydride （PMDA） and 4,4’-oxydianiline（ODA） as monomers by in situ process. The relationships among morphology of the alumina gel, structure of the nanocomposite films and the corona resistance time in the two systems were investigated.The results indicated thatγ-Al₂O₃ nanoparticles were well dispersed in the PI matrix even when the doping content was as high as 25wt%. A novel interpenetrated structure was clearly observed when the doping content exceeded 9wt%. Combing the preparation method and characterization results, we put forward the structure model and the formation mechanism of PI/γ-Al₂O₃nanocomposite films.Study on dielectric property of PI/γ-Al₂O₃ films revealed that dielectric coefficient decreased with increasing frequency and increased with increasing 2doping content. New phenomena were observed in the dielectric spectra of the films: dielectric loss decreased in the frequency region 10²-10⁴Hz, but increased in the frequency region 10⁴-10⁵Hz. With increasingγ-Al₂O₃ content, the magnitude of dielectric loss increased in the frequency region 10²-10⁴Hz, but decreased in the frequency region 10⁴-10⁵Hz.PI/γ-Al₂O₃ films were characterized by Electrostatic Force Microscope （EFM） and X-ray photoelectron spectroscopy （XPS）. The results revealed that the surface potential on the inorganic phase was higher than that on the PI matrix, and this was beneficial to shield and deflect the electrons arrived to the film surface in the corona process. Furthermore, theγ-Al₂O₃ content of the composite film in the inner part was higher than that on the surface.The corona resistance time of PI/γ-Al₂O₃ nanocomposite films was increased with increasingγ-Al₂O₃ content. It was found to be 25.4 times longer than that of a pure PI film whenγ-Al₂O₃ content was 25wt%. The reasons of enhanced corona resistance property were proposed combined with characterization results.The results revealed that 20–30nm sized spherical boehmite particles were homogenously dispersed in PI/AlOOH nanocomposite films without agglomeration. Thermal stabilities and corona resistance time of PI/AlOOH films were increased when the Al₂O₃ contents were less than 9wt%. The corona resistance time of PI/γ-Al₂O₃ nanocomposite films was increased with increasingγ-Al₂O₃ content, and it was found to be 13 times longer than that of the pure PI films when AlOOH content was 9wt%. However, obvious aggregation was observed in PI/AlOOH films containing higher content of AlOOH.