The influence of CeO₂ on the microstructure and electrical behaviour of ZnO-Bi₂O₃ based varistors

The processing-microstructure-property relations have been studied in order to understand the role of the addition of CeO₂ (up to 0.9 mole%) in the ZnO-Bi₂O₃ based varistor recipe. The microstructural investigation suggests that CeO₂ is segregated at the corners of the ZnO grains in addition to the existence of the ZnySb ₂O₁₂ spinel phase. However, the α-spinel phase was observed instead of the β-spinel phase that is usually found in most commercial and laboratory ZnO-Bi₂O₃ based varistors. The α-spinel phase is more stable than the β-spinel phase and does not transform to the pyrochlore phase during the cooling process. The most significant effect of the CeO₂ particles is the ZnO grain refinement owing to the pinning effect of the grain growth. The average grain size decreases from 7.8 to 5.7 μm when compared to the 0.9 mole% CeO ₂-added sample against the CeO₂-free sample. This grain refinement results in a significantly enhanced breakdown field when compared to the CeO₂-free sample. The coefficient of nonlinearity of the current-voltage (I-V) characteristics is found to be nearly identical for the CeO₂ added varistor materials. However, when a slower cooling cycle (1°C min^-1 instead of 4°C min^-1) is used in the sintering process, these varistor materials exhibited a high nonlinear coefficient (α = 29 ± 5) as extracted from the I-V behaviour.