SnO₂Co₂O₃Nb₂O₅系列压敏电阻电学性质的研究

【作者】 亓鹏；

【导师】 王矜奉；

【作者基本信息】 山东大学 ， 凝聚态物理， 2005， 硕士

【摘要】 SnO₂压敏电阻因其具有高度稳定的压敏性能、优良的非线性和超高的压敏电压而备受关注。自从SnO₂Co₂O₃Nb₂O₅系列压敏材料的电学非线性被发现以来,材料研究者们一直在尝试通过一系列的掺杂改性,以期得到更加优异的压敏特性。后继的研究也大都是仿照ZnO的压敏机制,通过尝试各种不同的掺杂进行研究,没有形成系统而完备的理论体系。由于SnO₂压敏电阻的微观结构不同于以往的ZnO压敏电阻的微观结构,研究者们也提出了相应的理论模型,但并不统一,也不完善。基于这种思想,作者希望通过对SnO₂Co₂O₃Nb₂O₅系列压敏电阻作深入而系统的研究,来探索适用于SnO₂压敏电阻的理论模型。 本论文首先对纯SnO₂、SnNb、SnCo和SnCoNb的压敏性质分别做了对比实验研究,分析了它们的微观结构、测定了它们的非线性电学性质和介电性质。通过借鉴前人的晶格缺陷理论,分析了该系列压敏材料内部的缺陷形成过程及分布情况,提出了更加合理的晶界缺陷势模型。在此基础上,用同样的方法研究了碱土金属Sr和碱金属K等大离子、稀土元素Er和Yb等的掺杂对该系列压敏电阻电学性质的影响,发现了一些新的规律并对此作了解释。 研究中发现,纯SnO₂因其晶粒内部缺少载流子而不具有压敏性,通过施主掺杂Nb₂O₅后的材料开始显示压敏性,但由于其晶粒不能融合生长而致使密度极低（65.6%）,非线性系数也极低（α=2.7）。只掺杂Co₂O₃的材料致密度很高（98.6%）,与纯SnO₂一样因缺少载流子而不具有压敏性。同时掺杂Nb₂O₅和Co₂O₃的材料具有很好的非线性（α=11.1）和很高的致密度（98.3%）。这些实验,首次揭示了施主掺杂是引起SnO₂压敏性质的关键因素,受主掺杂Co₂O₃对压敏性质并无贡献,仅起到SnO₂致密化的作用。 但只掺杂Nb₂O₅和Co₂O₃的SnO₂材料压敏电压不足300V/mm。只需掺杂少量的SrCO₃或K₂CO₃等大半径的金属离子,材料的压敏电压就急剧升高到1200V/mm以上,同时非线性也有很大提高（α>19.0）。掺杂Er₂O₃或Yb₂O₃的材料同样具有很高的压敏电压（E_B>1000V/mm）和非线性系数（α>17.0）。当把掺杂Er₂O₃的材料的烧结温度降至1250℃时,材料的压敏电压甚至达到了2270V/mm。更多还原

【Abstract】 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.更多还原