The effects of Si addition on electrical degradation of ZnO varistors

The effects of Si addition on the electrical degradation of Bi-Mn-Si-added ZnO varistors and Bi-Co-Si-added ZnO varistors were investigated based on voltage-current (V-I), and capacitance-voltage (C-V) characteristics, and by isothermal capacitance transient spectroscopy (ICTS), and X-ray diffraction (XRD). The optimum Si content to prevent the electrical degradation is approximately 700molppm for Bi-Mn-Si-added ZnO varistors and approximately 500molppm for Bi-Co-Si-added ZnO varistors. The trap level E-C-E-T for Bi-Mn-Si-added ZnO varistors showed the local maximum at the optimum Si content similar to the nonlinearity index alpha but that for Bi-Co-Si-added ZnO varistors showed the local minimum at the optimum Si content contrary to alpha. The density N-is of the interface trap levels obtained from G V characteristics and ICTS for both kinds of ZnO varistor showed the local minimum at the optimum Si content similar to Al-added Bi-Mn-added and Bi-Co-added ZnO varistors. It is found that the crystal structure of alpha-Bi2O3 in the grain boundary changed to 6Bi(2)O(3)-SiO2, and the intensity of the diffraction peak for (002) of ZnO sintered at 1150degreesC markedly decreased with addition Of SiO2.