In this study we aims to examine the co-doping effects of 1/3 mol% Mn3O4+Co3O4 (1:1) on the reaction, microstructure, and electrical properties such as the bulk defects and grain boundary properties of ZnO-Bi2O3-Sb2O3 (ZBS; Sb/Bi=0.5, 1.0, and 2.0) varistors. The sintering and electrical properties of Mn,Co-doped ZBS, ZBS(MCo) varistors were controlled by Sb/Bi ratio. Pyrochlore (Zn2Bi3Sb3O14) was decomposed and promoted densification at lower temperature on heating in Sb/Bi=1.0 by Mn rather than Co. Pyrochlore on cooling was reproduced in all systems however, spinel (α- or β-polymorph) did not formed in Sb/Bi=0.5. More homogeneous microstructure was obtained in Sb/Bi≥1.0. In ZBS(MCo), the varistor characteristics were improved drastically (non-linear coefficient, α=30∼49), and seemed to form (0.17 eV) and (0.33 eV) as dominant defects. From impedance and modulus spectroscopy (IS & MS), the grain boundaries have divided into two types, i.e. the one is tentatively assign to ZnO/Bi2O3(Mn,Co)/ZnO (0.47 eV) and the other ZnO/ZnO (0.80∼0.89 eV) homojunctions.
