The Effect of Bi2O3/SiO2 Molar Ratio and Annealing on the dc Degradation of ZnO Varistors

The dc electrical stability of ZnO varistors was effectively improved by controlling the composition proportion of Bi2O3 and SiO2 as well as annealing condition. The microstructure, current–voltage (I–V) property, and dc degradation characteristics are significantly affected by the molar ratio of Bi2O3 to SiO2 in composition. It is found that a phase transition from β-Bi2O3 (with dissolved Si) to Bi12SiO20, with a volume contraction of 5.88%, occurred after annealing at 850°C. The formed Bi12SiO20 plays an important role in improving the electrical stability by inhibiting the oxygen-desorption at the grain boundary.