Fig. 4 shows the behavior of leakage current of the samples
during various DC-accelerated aging stresses in accordance with
the amount of Y2O3. The samples doped with 0.25 mol% in the
amount of Y2O3 exhibited the thermal runaway at the first stress
(0.85 EB/115 1C/24 h). It is assumed that the thermal runaway of
the samples is predominantly attributed to a low sintered density
and high leakage current. A low sintered density decreases the
number of conduction paths, and eventually leads to the concentration of current. Furthermore, a high leakage current leads to high
joule heat loss. Therefore, the thermal runaway phenomenon of the
samples doped with 0.25 mol% in the amount of Y2O3 is attributed
to a low sintered density and a high leakage current as indicated in
Table 1. The sample doped with 0.5 mol% in the amount of Y2O3
exhibited high stability until the third stress (0.95 EB/125 1C/24 h),