The High Pressure Of Sno <sub> 2 </ Sub> Varistor Ceramics Research And New Low - High Dielectric Constant, Pressure-sensitive Lead-free Piezoelectric Materials To Explore

Since the nonlinear electrical properties of ZnO were found in 1969, thorough and extensive investigations have been carried out on this material. Varistor applications in protecting electric equipments and electrionic devices from shock had been practiced to the end of 1980's. However, the thermal instability and degradation problem of ZnO varistors have not been resolved because of their multi-phase structure induced by doping more dopants. Therefore, as the research on the improvement of ZnO electrical properties going on, the efforts to search new varistor materials have not been interrupted, In 1995, S. A. Pianaro et al. found a new varistor material, (Co, Nb)-doped SnO₂, which has only single phase and high relative density. Moreover, their nonlinear electrical properties exhibit a high value, which can be improved further. In this thesis, we found that the varistor electrical field of SnO₂ varistor material can be enhanced greatly based on the improvement of nonlinear electrical properties. The enhancement of varistor electrical field makes it possible to miniaturize the varistor devices. Further more, the fabrication costs would be depressed by reducing the dosage of raw materials. Thorough studies indicate that the decrease of grain size is the main reason for the increase of the varistor electrical field. In this thesis, besides doped SnO₂ varistors, we found some other new materials, such as WO₃ and composite SnO —Zn₂SnO₄ ceramics exhibit good nonlinear electrical behavior, In this thesis, we also successfully prepared some lead-free piezoceramics and obtained the Li₂CO₃-Sb₂O₃-doped (KNa)_0.5NbO₃ (KNN) ceramics having good piezoelectric properties.In the first chapter, we generally introduced some parameters and theory evolution of varistors. The current-voltage curve of a varistor can be ,divided into three parts, prebreakdown region, nonlinear region and upturn region. The most important electrical property of varistor is its nonlinear current-voltage characteristic in the nonlinear region. The nonlinear coefficient is the most important parameter to describe the varistor nonlinearity and the higher the nonlinear coefficient, the better its nonlinearity. Breakdown voltage is another important parameter, which determine the application situation of varistors. Any other parameters such as leakage current, current capability, permittivity etc. are also very important for varistors. The nonlinear...