The effects of additives on electrical degradation for ZnO varistors

The correlation between electrical characteristics and valences state of additives in the ZnO varistors were investigated by a V-I, an ICTS and an XPS measurements. It is found that Cr as additives had different contribution to nonlinear index alpha to Bi-Co or Bi-Mn added ZnO. Addition of V to Bi-Co or Bi-Mn added ZnO decreased alpha. Bi-Mn-Cr and Bi-Co-Cr added ZnO samples showed small ratio of the decrement of alpha after the electrical degradation. After electrical degradation, XPS spectra of Cr-2p(3/2) orbit for all samples added Cr showed the same binding energy. XPS spectra of V-2P(1/2) orbit for Bi-Mn-V added sample was shifted to high energy side about 1eV by the electrical degradation. In this case the alpha drastically decreased compared with that for Bi-Co-V added ZnO. These results show that the valence states for additives correlate to the alpha for ZnO varistors.