| More attentions have been paid to SnO2 based varistors for their excellent stable varistor properties, large nonlinear coefficient and ultrahigh breakdown voltage. Since finding the nonlinear electrical characteristics of SnO2CoO3Nb2O5 varistor system, material investigators have been trying to get more excellent SnO2CoO3Nb2O5 varistor system with higher nonlinearity by doping a series of metallic oxides. The subsequent investigations mainly imitated the physical mechanism of ZnO varistors, which are investigated by adding various dopants, but didn't form a systematical and self-contained theory. Based on the different microstructure of SnO2 varistors from that of ZnO varistors, investigators have proposed some of theory models. But these models are not integrated and systematical. The aim of this thesis is to find a model suitable for the mechanism of SnO2 varistors by investigating SnO2CoO3Nb2O5 varistor system thoroughly and systematically.In this thesis, the contrastive experiments were separately conducted on pure SnO2, SnNb, SnCo and SnCoNb materials to investigate their varistor properties at first. We analyzed the microstructures, measured the nonlinear electrical characteristics and dielectric properties of these materials. The existing defects theory in the crystal lattice was introduced to explain the forming and distributing of defects in the SnO2CoO3Nb2O5 varistor system. A more suitable grain-boundary defects barriers model was proposed. Based on the defects theory and my model, the same way was used to investigate the effects of metallic elements Sr, K, Er and Yb, doping to this varistor system, on the electrical properties of the varistor systems. Some new phenomena were found and were explained.It was found that the pure SnO2 exhibits no electrical nonlinearity for lacking of carriers, Nb2O5-doped SnO2 varistor exhibits electrical nonlinearity but its grains cannot conglomerate. Therefore, the density of Nb2O5-doped SnO2 is low (65.6%), which lead to a small electrical nonlinearity (a =2.7). Co2O3-doped SnO2 varistor is very dense (98.6%), but exhibits no electrical nonlinearity for lacking of carriers. The SnO2 varistor doped with Nb2O5 and Co2O3 exhibits high electrical nonlinearity (a =11.1) and density (98.3%). These experiments discovered, for the first time, that donor dopants are the crucial ingredients for the electrical nonlinearity origin of thevaristors, and acceptor dopant CO2O3 contributes nothing but only densifying the SnO2 varistors.Breakdown voltage of SnCh varistors doped with only Nb2Os and CO2O3 is not more than 300V/mm. After doping little SrCO3 or K2CO3, the SnO2Co2O3Nb2O5 varistor system exhibits excellent varistor properties with large nonlinear coefficient (a>19.0) and high breakdown voltage (more than 1200V/mm). So does the Er2O3-doped or Yb2O3-doped SnO2Co2O3Nb2O5 varistor system (EB>1000V/mm, a >17.0). The breakdown voltage of the E^C^-doped varistor sintered at 1250°C was even as high as 2270V/mm. The reactance Vs resistance relation of these materials was analyzed, and the resistances of grain-boundary and grain were determined. The resistance of grain-boundary and grain of SnC>2 varistors doped with only Nb2Os and Co2O3 is 246k Q ? cm and 57.5 Q. ■ cm, respectively. The grain-boundary resistance of 0.20mol%-Yb2O3-doped SnO2Co2O3Nb2O5 varistor was increased to 428k Q, ■ cm and its grain resistance was decreased to 11.2 Q ? cm. The increase of grain-boundary resistance is of benefit to raising the varistor breakdown voltage. Meanwhile, the decrease of grain resistance is of benefit to raising the surge bearing capacity. All the above dopants can improve the breakdown voltage of the SnC^C^C^NbiOs varistor system. Furthermore, they all have a critical doping concentration, which is related to the ionic radius of dopants. The breakdown voltage will increase rapidly when the amounts of dopants are more than the critical doping point. |
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