Effect of Thermal Cycle Ageing on the Breakdown Performance of Epoxy and Its Micro-composites

Epoxy-based materials are widely used in electronic devices as the main insulation due to their excellent dielectric and thermal properties. However, thermal cycles originating from the drastic changes in working temperature can greatly accelerate their degradation. In this paper, the effect of thermal cycle ageing (-55 ~ 150 °C) on the AC breakdown strength (EB) of neat epoxy (EP) and micron boron-nitride/epoxy (BN/EP) composites are investigated. The aging process can be understood by two stages. During the first stage, EB of EP experienced an evident rise from 64.73 to 76.17 kV/mm, and that of BN/EP displayed a similar growth from 73.01 to 77.41 kV/mm. The variation of their glass transition temperature (Tg) was consistent with EB. Results of Fourier transform infrared spectroscopy and the trap characteristics indicate that post-curing of epoxy matrix and the effect of low temperature were both responsible for the initial increase. In the later stage, owing to the effects of thermal-oxidative reactions of epoxy resin, EB of EP dropped to 63.11 kV/mm at 150 cycles. There was a more drastic decline of BN/EP to 61.38 kV/mm, and this severer deterioration might be attributed to the thermal expansion coefficients mismatch between BN fillers and epoxy matrix.