Soft core behavior in ZnO-Bi2O3-based varistors containing oxides of Ce and Gd

The effects of sample length d(0) on the average breakdown electric field E, of ZnO-Bi2O3-based varistor materials containing oxides of Cc and Gd sintered in the temperature range from 1150 to 1200 degrees C were investigated. It is observed that E, decreases with increasing do. The breakdown electric field E-B(') at each location along the axial direction was measured by gradually grinding and lapping both sides of the sample. The experimental results indicate that EB' in the core of the varistor material is as low as about 35% of that in the surface layers. This behavior of varistor materials is termed the soft core phenomenon. Further experimental results reveal that the Schottky barrier height (P,, in the core of the material is much lower than in the surface layers. This is indicative of non-uniform distribution of breakdown electric field. The reasons for the formation of the soft core were explored. The experimental results of X-ray photoelectron spectroscopy analysis indicate that the binding energy of Zn in the core is lower than that in the surface layers implying variation in the chemical environment of the Zn atoms along the axial direction. The analysis of the phases obtained by X-ray diffraction showed no evidence of Bi2O3 phase in the core while Bi2O3 phase appears in the surface layers. Consequently, it is proposed that the soft core behavior originates from the decrease of Schottky barrier height along the axial direction of the varistor material doped with rare earth oxides, which may be caused by the non-uniform distribution of sintering atmosphere and phase morphology at the grain boundaries. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Wemheim.