Influence of WO 3 ‐Doping on the Microstructure and Electrical Properties of ZnO–Bi 2 O 3 Varistor Ceramics Sintered at 950°C

The phase evolution, microstructure, and electrical properties of WO 3 ‐doped ZnO–Bi 2 O 3 ‐based varistors were investigated for different amounts x (0 ≤ x ≤ 1.60 mol%) of the dopant. When x was less than 0.40, the dissolved W 6+ in the β‐Bi 2 O 3 acted as a donor in the grain boundaries and reduced the electrical properties of the ZnO varistors. However, when x was 0.40 mol%, which meant an amount of WO 3 equal to that of Bi 2 O 3 , the electrical properties dramatically increased, which means the W 6+ donor effect is removed at the grain boundaries because a new Bi 2 WO 6 phase was formed in the grain‐boundary regions. The Bi 2 WO 6 phase has high oxygen conductivity at high temperatures; it transfers more oxygen to the grain boundaries in order to further enhance the electrical properties. For x values higher than 0.40 (i.e., an addition of WO 3 that is greater than the content of Bi 2 O 3 ), the electrical properties were steadily reduced in comparison to the composition with x = 0.40. This could be explained by the reduced amount of Co, Mn, and Al at the grain boundaries and in the ZnO grains as a result of their incorporation into the ZnWO 4 phase. The electrical properties of the ZnO grains and the grain boundaries were in agreement with the results of the impedance spectroscopy analysis.