Phase Transformation and Electrical Properties of Bi2O3-Based ZnO Varistor Doped with WO3

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The effects of WO3 additive on the phase transformation, microstructure, varistor properties, and capacitance–voltage ($C$–$V$) characteristics of a Bi2O3-based ZnO varistor were investigated. A series of Bi2WyO3(1+y) ($0\leq y\leq 2/7$) interstitial solid solutions were formed at the ZnO grain boundary when WO3 content ($x$) was in the range of 0–0.3 mol %, while the plate-like Bi2WO6 phases presented for further addition. The phase transformation between Bi2WyO3(1+y) ($0\leq y\leq 2/7$) solid solutions and the Bi2WO6 phase at the ZnO grain boundary affects the microstructure and electrical properties of the varistor ceramic. WO3, involved in the formation of interfacial states or deep bulk traps at grain boundaries in the range of $0\leq x\leq 0.3$, provides large potential barriers to enhance the nonlinearity of a varistor ceramic. However, for $x\geq 0.7$, the present of the low resistive Bi2WO6 at the ZnO grain boundary results in a lowering of potential barriers and the deterioration of nonlinearity. The varistor ceramics for $x=0.3$ mol % exhibited the best performance: voltage gradient $E_{\text{1mA}}=54.2$ V/mm, nonlinear coefficient $\alpha=36$, and leakage current $I_{\text{L}}=0.8$ μA, but changed to ohmic behavior for $x=1.1$ mol %.
2010-06-25