Effect of micro and nano-size boron nitride and silicon carbide on thermal properties and partial discharge resistance of silicone elastomer composite

This study introduces silicone elastomers with micro and nano-sized boron nitride (BN) and silicon carbide (SiC) particles with various doping levels to improve thermal properties and partial discharge resistance. The effect of micro and nano-fillers on thermal conductivity, coefficient of thermal expansion (CTE), and thermal stability are investigated. Polarization and depolarization current and partial discharge are measured to investigate the non-linear conductivity, trap energy density distribution, and partial discharge resistance. Experimental results show higher thermal conductivity, lower CTE, and better thermal stability than the original silicone elastomer. Large size fillers dominate the thermal conductivity when the doping level is low, whereas the composite microstructure plays a significant role in the thermal conductivity when the doping level is high. The combination of different filler type and size has less effect on thermal stability and CTE compared to the effect of the doping level. According to the depolarization current, the composite's trap depth is generally shallower than that of the original silicone elastomer. With increased doping level, the shallow trap density increases, providing hopping sites for carriers, whereas the deep trap density decreases, reducing the number of trapped charges. The partial discharge inception voltage is higher than that of the original silicone elastomer, and it increases with the doping level, which may be because the electrostatic field generated by diffused charges reduces the local electric field near the high voltage electrode.

1 Introduction

With the size decrease and power density increase of power electronic devices, thermal accumulation and electrical discharge pose serious challenges to device reliability and raise much attention. The next-generation power electronic devices made of wide bandgap (WBG) semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN), will become increasingly popular due to their better electrical, mechanical, and thermal properties than silicon devices. However, most WBG manufacturers, such as the Cree, Infineon, and Semikron, only allow their WBG products the maximum working temperature of 175°C, which is the same as that of silicon-based counterparts [1]. The corresponding packaging technology cannot realize the full potential of WBG devices. Packaging materials, such as silicone gel, epoxy, and polyimide, on the one way, are required to improve thermal properties including thermal conductivity, stability, and coefficient of thermal expansion (CTE). On the other way, the packaging materials should withstand high electrical stress, since the power electronic devices are required to operate at high voltages to increase power density.