Development of Epoxy/BN Composites with High Thermal Conductivity and Sufficient Dielectric Breakdown Strength Part I -Sample Preparations and Thermal Conductivity

The aim of this research is to find a way to achieve the epoxy composites with both high thermal conductivity and acceptable dielectric breakdown (BD) strength. As high thermal conductivity, low permittivity and low thermal expansion coefficient of filler can endow composite with higher thermal conductivity, higher BD strength and lower thermal expansion coefficient respectively, BN (boron nitride) with high thermal conductivity, low permittivity and low thermal expansion coefficient was adopted as main filler in the research. Thermal conductivity was investigated in this part. The BD strength of samples will be discussed in Part II. Neat epoxy and other 25 kinds of epoxy/BN composites were prepared by a hot press method. Most of BN fillers were surface modified with silane coupling agent through ethanol/water reflux method to improve thermal conductivity. The values of 2.91 W/m.K, 3.95 W/m.K and 10.1 W/m.K as thermal conductivity were obtained for the composites that was single-loaded with h-BN(hexagonal boron nitride), c-BN (cubic boron nitride) or conglomerated h-BN, respectively. They were further improved to 5.26 W/m.K, 5.94 W/m.K and 12.3 W/m.K, respectively, by adding extra smaller AlN (aluminum nitride) to fill the voids in sample. Thermal conductivity of samples changes with the ratio of cBN and h-BN when c-BN and h-BN were co-loaded. A value of 5.74 W/m.K as maximum was obtained at their ratio of 1 to 1 when total filler content is 80 wt%. A much higher value of 7.69 W/m.K was obtained by adding extra AlN. From the experiment data, it is concluded that the filler orientation in vertical direction of sample surface and the decrease of voids in sample are very important to obtain high thermal conductivity, and that the filler surface modification is also necessary to improve thermal conductivity especially for epoxy/c-BN composites, and addition of nano silica in small amount can also increase thermal conductivity if sample is prepared appropriately.