The critical voltage systematically increases from 0 vol% up to 10 vol%, and then collapses for 20 and 40 vol%,which may result from that hick polymer layers cannot undergo uniform breakdown, and there are large areas of segregated polymer that further limits uniformity of the current distribution in the composites. The nonlinearity coefficients, α, are in the range of 1.5 and 7.1 and the breakdown electric field at 0.1 mA are summarized in Table 1.