Corona Resistant Property And Mechanism Of Polyimide/Al₂O₃Composite Films

Polymer materials have been widely employed for various contemporaryapplications such as insulation material, frequency conversion motor, fuel cells, andaerospace for their outstanding electrical properties, thermal properties and mechanicalproperties. With the development of nano technology, design and development of newpolymer matrix materials have become a hot issue in the study of material science. Thepolyimide films doped with Al₂O₃have been prepared. Al₂O₃has higher specificsurface area and relatively lower electrical conductivity. Corona resistance and itsmechanism have been investigated.In this paper, the monolayer and trilayer composite films are prepared by in-situpolymerization method. The structure have been researched by X-ray diffraction,transmission electron microscopy, scanning electron microscopy. The data shows that,Al₂O₃decreases the intensity of polyimide peak and the orientation degree of PI. Thesize of Al₂O₃is10²0nm, Al₂O₃formes interface with polyimide, the thickness ofthe interface layer is3⁶nm. In the trilayer films, the thicknesses of the middle andhybrid layer are about10μm. Some Al₂O₃permeates into the hybrid layer.Corona resistance, surface resistance, threshold electric field, volume resistivity ofthe films have been tested by corona testing equipment, the surface resistance testequipment, electric conduction current test system, dielectric spectrometer, respectively.Aging rate and surface morphology of the films have been tested by atomic forcemicroscope and scanning electron microscopy. The results show that, the trilayer filmshave better corona resistance than monolayer film, the corona resistant life ofPI/Al₂O₃20wt.%has the best corona resistance, about27.8times of pure PI. Withthe increasing of Al₂O₃, surface resistance of the films decreases significantly andelectric field homogenization has been enhanced. In corona aging, surface aging rateof PI/Al₂O₃20wt.%is much lower than that of PI. Al₂O₃increases the conductancecurrent and threshold of electric field. The monolayer PI/Al₂O₃20wt.%film has thebest threshold electric field. Al₂O₃formes micelles with polyimide and formes theblocking corona layer. With the increasing of Al₂O₃, the volume resistivities of thefilms decreases. By contrast with monolayer, trilayer films had higher volume resistivity.Based on the experimental results, corona resistant mechanism of the thrilayerfilms is put forward. In hybrid layer, Al₂O₃reduces surface resistance of the films,uniforms the surface electric field, reduces the surface aging rate per unit area. Al₂O₃introduces more trap energy level, improves the electrical current, makes the carriertransports in Al₂O₃and interface layer, and reduces the probability of carriercolliding defects. Meanwhile, Al₂O₃reduces the producing probability of the hotcarrier. What is more, Al₂O₃has huge specific surface area and stable chemicalproperties, contacts with polyimide closely, forms the blocking corona layer. Themiddle layer with relatively higher volume resistivity, reduces the number of carriers,and forms carrier barrier with hybrid layer, make electric field stress concentration inthe hybrid layer.