Effects of nano-filler addition on partial discharge resistance and dielectric breakdown strength of Micro-Al₂O₃/epoxy composite

It is often observed that the insulation structure for an insulated gate bipolar transistor (IGBT) suffers from dielectric failure, when the insulation is made of epoxy resin to which micro fillers with a high thermal conductivity were added. In order to reveal the above phenomena and to clarify the breakdown (BD) mechanism, we have carried out experiments using an MB-PWB (metal-base printed wiring board) insulation simulated structure. As a result, it was clarified that the IGBT insulation breaks down after successive partial discharges (PDs). It was also elucidated that BD strength becomes lower, when epoxy resin was loaded with high content of micro-fillers. A trial was made to raise the once-lowered BD strength by adding nano-Al₂O₃ fillers. Three kinds of experiments were carried out, i.e. an MB-PWB insulation simulated structure for dielectric failure, a rod-to-plane electrode for PD erosion resistance, and a sphere-tosphere electrode for BD strength for four kinds of insulation samples, i.e. neat epoxy, 5-wt% nano-Al₂O ₃/epoxy composite, 60-wt% micro-Al₂O₃/epoxy composite, and combined 2-wt% nano- and 60-wt% micro-Al₂O ₃/epoxy composite. It was clarified that the nano-micro-composite is higher in both BD strength and PD resistance than the micro-composite. It should be noted that the addition of nano-fillers would provide an excellent approach that can increase the dielectric BD strength and time of micro-filled epoxy composites.