Polyimide films coated by magnetron sputtered boron nitride for high-temperature capacitor dielectrics

Polymers are widely used as dielectric materials in dielectric capacitors, which are the key component of various circuits in power electronic devices. The sharply increased conduction loss at elevated temperatures limits their high-temperature applications. To address this issue, here in this study boron nitride (BN)/polyimide (PI)/BN sandwich-structured films were developed by depositing BN of optimal thickness onto both sides of PI films using magnetron sputtering. Dielectric, insulating and energy-storage performance at 150 °C of the resultant composite films were investigated. The experimental results showed that a dense layer of hexagonal boron nitride (h-BN) was formed on the surface of the polyimide and the coating layer could effectively suppress the charge injection at the electrode/dielectric interfaces by improving the barrier height. Consequently, the sandwiched film of optimal BN thickness exhibited a declined leakage current of about one order of magnitude lower and an improved breakdown strength by 12% in comparison to the neat PI. The sandwiched film could discharge an energy density of 0.493 J/cm ³ with charge-discharge efficiency of over 90% under an electric field of 200 MV/m at 150 °C, while the neat PI only possesses an efficiency of 72% and a much lower discharged energy density.