Effect of Sb/Bi Ratio on Sintering and Grain Boundary Properties of ZnO-Bi2O3-Sb2O3-NiO-Cr2O3 Varistor

We have examined the co-doping effects of 1/2 mol% NiO and 1/4 mol% Cr2O3 (Ni:Cr = 1: 1) on the reaction, microstructure, and electrical properties, such as the bulk defects and the grain boundary properties, of ZnO-Bi2O3-Sb2O3 (ZBS; Sb/Bi = 0.5, 1.0, and 2.0) varistors. The sintering and electrical properties of Ni, Cr-doped ZBS, ZBS(NiCr) varistors were controlled using the Sb/Bi ratio. Pyrochlore (Zn2Bi3Sb3O14), a-spinel (Zn7Sb2O12), and delta-Bi2O3 were detected for all of compositions. For the sample with Sb/Bi = 1.0, the Pyrochlore was decomposed and promoted densification at lower temperature by Ni rather than by Cr. A homogeneous microstructure was obtained for all of the samples affected by a-spinel. The varistor characteristics were not dramatically improved (non-linear coefficient, a = 5 similar to 24), and seemed to form Zn-1.(0.17 eV) and Vo. (0.33 eV) as dominant defects. From impedance and modulus spectroscopy, the grain boundaries were found to have been divided into two types, i.e., one is tentatively assigned to ZnO/Bi2O3 (Ni, Cr)/ZnO (0.98 eV) and the other is assigned to a ZnO/ZnO (similar to 1.5 eV) homojunction.