Effect of Different Types of Fullerene Addition on AC Dielectric Breakdown Strength of Epoxy Resin

This research intends to investigate the applicability of fullerene as an electrically insulating filler. So far, the authors reported that the addition of a very small amount of fullerene C 60 to epoxy resin (EP) improves the dielectric breakdown strength $(E_{\mathrm{b}})$ by about 30%. This report presents the study on the effect of four different types of fullerenes as fillers added to EP on the improvement of $E_{\mathrm{b}}$ . Fullerenes used are mixtures of C 60 and higher order one $\mathrm{C}_{70}$ with a different mixture rate, and the addition effect of sublimation-purified fullerene on $E_{\mathrm{b}}$ is also investigated. Experimental results reveal that samples added with sublimated fullerenes exhibit larger $E_{\mathrm{b}}$ than that of samples without sublimation. The sublimation-purified sample is also found to give almost no difference in $E_{\mathrm{b}}$ by the type or proportion of the added fullerenes $\mathrm{C}_{60}$ and $\mathrm{C}_{70}$ . An attempt is also made to discuss the mechanism of the improvement effect on $E_{\mathrm{b}}$ of fullerene by evaluating the electronic state of epoxies with different fullerene fillers and curing agent using quantum chemical calculation. As a result, it is shown that the introduction of $\mathrm{C}_{60}$ and $\mathrm{C}_{70}$ into the EP considering the curing agent introduces electron traps of 0.64 and 0.72 eV, respectively. It is interpreted that the introduced electron traps suppress the electron avalanche, and consequently improves the breakdown strength.