Abstract
The effect of Nb2O5 doping on microstructure and the electrical properties of ZnO–V2O5–MnO2–Co3O4–Dy2O3–Nb2O5-based varistors was investigated. The microstructure of all the samples consisted of a primary phase of ZnO grains and minor secondary phases such as Zn3(VO4)2, ZnVO4, and DyVO4, which act as liquid-phase sintering aids. The Nb2O5 doping did not have a significant effect on the density reached after sintering. However it increased the average grain sizes (d = 5.62–7.49 μm) compared with the Nb2O5-undoped sample. With increasing Nb2O5 doping level up to 0.1 mol%, the breakdown field (E1 mA) decreased from 7015 V cm−1 to 3358 V cm−1, and the nonlinear coefficient (α) increased from 26 to 37. However, further increase in doping level caused α to decrease. The highest value of the donor concentration and barrier height was obtained at 0.1 mol% Nb2O5, 5.9 × 1017 cm−3 and 1.20 eV, respectively.
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