Investigation On The Electrical Properties Of SnO₂-Zn₂SnO₄Varistor System

Since Matsuoka announced the development of ZnO in1969，more thorough and extensive investigations were carried out on this novel varistor material. The applications and theoretical researches of ZnO varistors had been practiced to the end of1980’s. At the same time, the thermal instability and degradation problem of ZnO varistors have not been resolved because of their multi-phase structure induced by doping more dopants. With the research on the improvement of ZnO electrical properties going on, the thermal instability and degradation problem of ZnO varistors have not been resolved, so the efforts to search new varistor material have not been interrupted. For examples, Rhodes and Yan in Bell Lab announced a new TiO2varistor system in1982, and in1994, Trontelj and Makarov found the WO3has nonlinear electrical properties. In successive researches, it was found that TiO2varistors and WO3were suited for low voltage applications for their low breakdown electrical fields, and WO3has a serious thermal instability owing to its multi-phase structure at ambient temperature. In1995, 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 the first chapter in this thesis, we generally introduced theory evolution and some parameters of varistos. The nolinear current-voltage characteristic in the nonlinear region is its most important electrical property. Breakdown voltage is another important parameter, which determines the application situation of varistors. Any other parameters such as permittivity and leakage current etc. are also very important for varistors. The grain boundary defect barrier model proposed by Pike et al. can explained the nonlinear electrical behavior of ZnO, and it was widely accepted for its well agreement with amount of experiment results. In the second chapter in this thesis, the effects of Nb on the electrical properties of SnO2-Zn2SnO4varistors were investigated. The breakdown voltage of SnO2-Zn2SnO4based varstors increased from28V/mm to530V/mm with Nb2O5concentration increasing from0.05to0.8mol%. The analysis on the barrier height at grain boundary reveals that the SnO2grain size reduction is the mainly cause of breakdown voltage and resistivity rise. The origin of the SnO2grain size decreasing with the increasing of Nb2O5concentration is explained.In the third chapter in this thesis, the effects of Cr2O3on the electrical properties of SnO2-Zn2SnO4varistors were investigated. The breakdown voltage of SnO2-Zn2SnO4based varstors increased from298V/mm to335V/mm and decreased from335V/mm to278V/mm with Cr2O3concentration increasing from0.03mol%to0.06mol%. With the increase of temperature, a dielectric peak appeared on temperature spectra of the sample. For all samples, the barrier height about0.8eV reveals that the varistor peroperties of SnO2-Zn2SnO4cerimacs is mainly determined by the grain size and the form of Cr2O3in the samples.