Study On The Structural And Electrical Properties Of Doped Or Composite Oxide Varistor Ceramics

With the highly nonlinear electrical characteristics, varistors are widely used as the surge protection in relatively low voltage circuits, lightning arresters and the transient protection in electronic power systems. The study on new varistor materials whose main component is WO₃ or TiO₂ is at the moment of deep development and several problems are to be solved. Because of these problems, TiO₂ and WO₃ based varisors were comprehensively analyzed in this paper. Three different series of samples was prepared by the way of doping and composite. The effect of component of doping and composite on varistor material was investigated. Some theories and models was analysed in the base of experiment The main working and results are as follows: (1) The effect of Y₂O₃ doping (0.2-2mol%) on the nonlinear electrical behavior and dielectric properties of WO₃-based ceramics was investigated. The measurement show that the sample containing 0.8mol% Y₂O₃ has the highest nonlinear coefficient αof 3.61 . As the increase of the amount of Y₂O₃, the dielectric constant(measured at 1kHz)of samples first increase and then drop .The sample containing 1.2mol% Y₂O₃ has the highest dielectric constant of 1.16×10(4measured at 1kHz). To estimate the resistance of the grain, the frequency dependence of impedance of the Y₂O₃ doped WO₃ ceramic was measured. The effect of Y₂O₃ additive content on the relation between resistance of grain and permittivity of WO₃-based ceramic can be explained by Debye relaxation relation . (2) The electrical properties of TiO₂·Nb₂O₅-based ceramics with various SrCO₃ contents have been investigated. The measurements show that the sample containing 0.75 mol% SrCO₃ has the highest nonlinear coefficient αof 13.15. The sample containing 1.0mol% SrCO₃ has the highest dielectric constant of 1.33×104(measured at 1kHz).In view of these electrical characteristics, the SrCO₃·TiO₂·Nb₂O₅ ceramics is a viable candidate for capacitor-varistor functional devices. In order to illustrate the grain boundary barrier formation in SrCO3·TiO2·Nb2O5 varistors, an interface defect model was introduced. (3) The probability of Cu/TiO2 composite varistor material was explored by the means of composite. The electrical and dielectric properties of TiO2·CuO-based ceramics have been investigated. The measurements show that the samples reveal their clear nonlinear property with the content of composite Cu increase. The sample containing 30mol% Cu has the highest nonlinear coefficient of 3.33. The result and the dielectric graph show that with increasing of compound Cu, the grain boundary resistance dropped, and the dielectric constant increased. Though the nonlinear coefficient of the samples which prepared by these means are low, the clear nonlinear character would be likely to pave the new way to the study of varistor materials.