Abstract
In this study we aims to examine the co-doping effects of 1/3 mol% <TEX>$Mn_3O_4+Co_3O_4$</TEX> (1:1) on the reaction, microstructure, and electrical properties such as the bulk defects and grain boundary properties of <TEX>$ZnO-Bi_2O_3-Sb_2O_3$</TEX> (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 (<TEX>$Zn_2Bi_3Sb_3O_{14}$</TEX>) 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 (<TEX>${\alpha}$</TEX>- or <TEX>${\beta}$</TEX>-polymorph) did not formed in Sb/Bi=0.5. More homogeneous microstructure was obtained in <TEX>$Sb/Bi{\geq}1.0$</TEX> In ZBS(MCo), the varistor characteristics were improved drastically (non-linear coefficient, <TEX>${\alpha}$</TEX>=30~49), and seemed to form <TEX>$Zn_i^{..}$</TEX>(0.17 eV) and <TEX>$V_o^{\bullet}$</TEX>(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 <TEX>$ZnO/Bi_2O_3(Mn,Co)/ZnO$</TEX> (0.47 eV) and the other ZnO/ZnO (0.80~0.89 eV) homojunctions.
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