Sintering temperature dependence of varistor properties and impedance spectroscopy behavior in ZnO based varistor ceramics

ZnO based varistor ceramics were prepared by a solid state reaction route, and the phase composition, microstructure and electrical properties were also investigated by XRD, SEM, a V-I source/measure unit and impedance spectroscopy. Results show that all the samples consist of mainly ZnO grains and secondary phases including Bi2O3, Zn7Sb2O12, Y2O3, Nd-rich and Y-containing Bi-rich phases. With the increase of sintering temperature, the number of grain boundaries and varistor voltage per grain boundary decreases, and the breakdown field (E-1mA) decreases from 556.4 to 319.4 V/mm. The nonlinear coefficient value (alpha) is in the range of 60-70, the leakage current density values (J(L)) of all the varistor ceramics are < 3 mu A/cm(2). Typically, the varistor ceramic sintered at 1,050 A degrees C exhibits good electrical properties of E-1mA = 556.4 V/mm, alpha = 61 and J(L) = 1.55 mu A/cm(2). Meanwhile, a schematic model for the electrical microstructure of ZnO based varistor is proposed with an equivalent circuit of R-g (RgbCgb) by impedance spectroscopy.