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http://dx.doi.org/10.3740/MRSK.2012.22.12.689

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

Hong, Youn-Woo (Electronic Materials Convergence Division, KICET)
Lee, Young-Jin (Electronic Materials Convergence Division, KICET)
Kim, Sei-Ki (Electronic Materials Convergence Division, KICET)
Kim, Jin-Ho (School of Materials Science and Engineering, Kyungpook National University)
Publication Information
Korean Journal of Materials Research / v.22, no.12, 2012 , pp. 689-695 More about this Journal
Abstract
We have examined the co-doping effects of 1/2 mol% NiO and 1/4 mol% $Cr_2O_3$ (Ni:Cr = 1:1) on the reaction, microstructure, and electrical properties, such as the bulk defects and the grain boundary properties, of ZnO-$Bi_2O_3-Sb_2O_3$ (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 ($Zn_2Bi_3Sb_3O_{14}$), ${\alpha}$-spinel ($Zn_7Sb_2O_{12}$), and ${\delta}-Bi_2O_3$ 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 ${\alpha}$-spinel. The varistor characteristics were not dramatically improved (non-linear coefficient, ${\alpha}$ = 5~24), and seemed to form ${Zn_i}^{{\cdot}{\cdot}}$(0.17 eV) and ${V_o}^{\cdot}$(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/$Bi_2O_3$ (Ni,Cr)/ZnO (0.98 eV) and the other is assigned to a ZnO/ZnO (~1.5 eV) homojunction.
Keywords
ZnO varistor; electrical properties; NiO; $Cr_2O_3$; sintering;
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