Effect of thermal annealing on electrical degradation characteristics of Sb–Bi–Mn–Co-added ZnO varistors

The effects of thermally annealing Bi–Mn–Co–Sb2O3-added ZnO varistors on their electrical degradation were investigated. For the samples added with 0.01 mol% Sb2O3 and without Sb2O3, no marked difference in the nonlinearity index α of the voltage–current (V–I) characteristics was observed upon electrical degradation for the annealed and nonannealed samples. Upon increasing the amount of Sb2O3 added, the values of α increased after electrical degradation for the annealed samples. Moreover, the value of α after electrical degradation was proportional to the width of gauss function (width) of the X-ray diffraction peak for Zn2.33Sb0.67O4-type spinel particles under various annealing conditions. The added Sb2O3 did not dissolve in the ZnO grains but became segregated at grain boundaries. Therefore, it is speculated that the increase in the width of the spinel particles is due to the increase in the numbers of fine spinel particles at grain boundaries and triple points. Furthermore, it is suggested that the improvement of the electrical degradation is due to the decrease in the mobility of oxide ions or Zn2+ ions owing to their being blocked by uniformly dispersed fine spinel particles at grain boundaries.