Impact of nano-Al2O3 coating on the dielectric properties and High-Frequency surface electrical strength of the polyimide films

Polyimide (PI) films used for inter-turn insulation of power electronic transformer (PET) are prone to premature insulation failure. Using ion-exchange technology to modify the surface of PI films effectively improves its high-frequency (HF) insulation strength. In this paper, the HF discharge characteristics of PI films were studied coated a nano-Al2O3 layer. The experimental results illustrate that the introduction of nano-Al2O3 coating can effectively improve the HF surface insulation strength of modified PIs, in which the maximum discharge lifetime is increased to 5.6 times of the pristine PI, and the creeping flashover voltage raised from the initial 6.34 kV to 11.98 kV, an increase of 88.96%. The enhanced mechanism of HF insulation performance of modified PIs is as follows: On the one hand, formed the functional nano-Al2O3 coating improves the thermal management ability and the inorganic–organic phase structure of the modified PIs, which is effective in reducing the temperature rise of materials. On the other hand, the introduction of the nano-Al2O3 coating increases the density of the shallow trap, increasing the surface conductivity of modified PIs, and hindering the development of HF surface discharge. This paper offers a prospective strategy for polyimide dielectrics with superior HF insulation performance.