High frequency insulation performance of multi-dimensional modified polyimide composite films of aluminum nitride and boron nitride

Polyimide (PI) is widely used as interturn insulation in solid-state transformers (SSTs) for its corona resistance, but its low thermal conductivity and high dielectric loss cause performance degradation under long-term high-frequency electrothermal stress. Adopting multi-dimensional modification design is an effective solution. In this paper, single-dimensional aluminum nitride (AlN) and multi-dimensional aluminum nitride and boron nitride (AlN-BN) modified PI composites were prepared. The surface of the filler was modified by polydopamine (PDA) and the KH550 silane coupling agent. Through microstructural analysis, thermal and mechanical characterization and high-frequency insulation testing, the enhancement mechanism of high-frequency insulation by the multi-dimensional modification was revealed from the aspects of thermal conductivity, dielectric properties, electrical conductivity and trap characteristics. The results show that the AlN-BN/PI composites exhibits enhancements in thermal conductivity, mechanical properties and high-frequency insulation performance. At 10 wt%, the thermal conductivity of AlN-BN/PI composites increases to 0.311 W/(m·K) (85.12 % higher than pure PI and 26.42 % higher than AlN/PI). Under 3 kV/20 kHz, the 10 wt% AlN-BN/PI composites achieves a corona aging lifetime of 69.12 min, (372 % higher than pure PI and 42 % higher than AlN/PI). The enhanced insulation performance of the multi-dimensional composites is attributed to: 1) the AlN-BN "sphere–sheet wrapping" structure constructs a thermal conduction network and mitigates dielectric mismatch and loss; 2) multi-dimensional modification enhances charge dynamics through elevated conductivity and shallow trap density, effectively suppressing electric field distortion. These findings advance insulation system optimization for solid-state transformers.