In order to investigate a material transformation of alumina (Al2O3) and an influence on leakage current by application of high DC voltage at high temperatures, the leakage current of two alumina samples were measured for 100 hours and the elemental analyses were carried out. The leakage current tended to decrease, and the electrical conductivity decreased with increasing applied voltage. The activation energies of electrical conduction of two samples were nearly equal. Thus, the reproducibility of these results was confirmed. After the experiments, deposits of impurities contained in alumina were observed in the surrounding area of the positive electrode. Furthermore, Na, which was contained in Pt paste used for preparing the electrodes, can be a charge career of alumina because it was detected from only negative electrodes. Therefore, the reason why the leakage current decreased is the decrease in the amount of charge careers because of the deposits of impurities.
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