Effect of frequency of microsecond pulsed electric field on orientation of boron nitride nanosheets and thermal conductivity of epoxy resin-based composites

Abstract

Nanosecond pulsed electric fields over 200 kV/mm have been used for filler alignment in polymer-based thin film (120-250 μm) composites to improve their thermal conductivity, but little research has been performed on bulk composites because of the requirement of an extremely high nanosecond pulsed voltage. In this paper, a microsecond pulsed voltage with a pulse width of 1 μs and an electric field of only 11.76 kV/mm was adopted for the first time to prepare bulk composites of 1.7 mm. The effects of frequency on nanosheets' orientation and the thermal conductivity of epoxy composites were studied. The orientation degree of BN nanosheets was determined based on cross-sectional scanning electron microscopy images and X-ray diffraction peaks. The orientation degree and thermal conductivity of epoxy composites increased with the increase of the frequency, but the increase rate was lower under higher frequencies. The thermal conductivity of composites containing 10 wt. % BN nanosheets prepared under 100 Hz was 0.588 W/mK, which was more than twice that of composites without an applied electric field. Additionally, an effective medium theory model was adopted for thermal conductivity modeling. In addition, thermogravimetric and differential scanning calorimetry analyses were performed to evaluate the thermal properties of the composites. The results showed that after BN orientation in epoxy resin, the composites had higher glass transition temperature and better thermal stability. Microsecond pulsed electric fields are promising for preparing bulk insulation composites with higher thermal conductivity.