Preparation And Electrical Properties Of High Voltage-gradient ZnO Varistors

Zinc oxide varistor is a multifunctional semiconductor ceramic,which is widely used in electronic circuit protection and power systems because of its excellent nonlinear ohmic characteristics and large energy absorption capability.As electronic devices have become smaller and lighter,it has been required to develop ZnO varistors having more excellent electrical properties.In this thesis,the effects of conventional additives（SnO₂,Sb₂O₃）and rare earth oxide additives（Gd₂O₃,Y₂O₃,La₂O₃）on the electrical properties of ZnO varistors at different doping levels and different sintering temperatures were investigated.The details are as follows:（1）A series of ZnO varistor varistors doped with additives SnO₂ and Sb₂O₃ were prepared by high temperature solid phase method.The microstructure and electrical properties of ZnO varistor samples were tested and characterized by X-ray diffraction,scanning electron microscopy and electrical performance test.The effects of SnO₂ and Sb₂O₃ on the electrical properties of the samples and their mechanisms were analyzed.The results show that the breakdown voltage,nonlinear coefficient and flow capacity of ZnO varistor samples change significantly with the changes of SnO₂ and Sb₂O₃ content.Based on the analysis of electrical properties,when the doping amount of SnO₂ is 0.6wt%,the varistor voltage gradient,nonlinear coefficient and flow capacity are the best,respectively,376.7V/mm,30.6,1996A.When the doping amount of Sb₂O₃ is 1.64wt%,the electrical properties of the sample are the best,and the breakdown voltage,nonlinear coefficient and flow capacity are 343.8V/mm,36.5,1827A,respectively.The effects of SnO₂ and Sb₂O₃additives on the electrical properties of the samples and the mechanism of action are similar.The incorporation of SnO₂ and Sb₂O₃ additives forms a spinel phase and a pyrochlore phase with a"pinning"effect inside the sample,which can inhibit ZnO grain growth,in addition,SnO₂ and Sb₂O₃ additives also have a significant effect on the interface charge density and the carrier concentration in the ZnO grains,thereby affecting the electrical properties of the varistors.（2）A series of ZnO varistor samples doped with rare earth oxide additives（Gd₂O₃,Y₂O₃,La₂O₃）were prepared by high temperature solid phase method.The results show that:for Gd₂O₃ doped ZnO varistor samples,with the increase of Gd₂O₃ content,the breakdown voltage of the sample increases monotonically,and the nonlinear coefficient shows a relationship of increasing first and then decreasing.The leakage current is first decrease and then increase.According to the comprehensive performance of ZnO varistor,the varistor has the best comprehensive performance when the doping amount of Gd₂O₃ is0.5wt%at 1050°C.The breakdown voltage is 374.8V/mm,The linear coefficient is 47.6and the leakage current is 2.1μA.In addition,the study of Y₂O₃ and La₂O₃ doped ZnO varistor samples shows that the influence of Y₂O₃ and La₂O₃ on the electrical properties of ZnO varistor is similar to that of Gd₂O₃.This is mainly because Gd₂O₃,Y₂O₃,and La₂O₃belong to rare earth oxides and have similar physicochemical properties.When they are added to ZnO varistor ceramics,they can react with ZnO or other additives in the formulation to form a large number of second phases.It inhibits the growth of ZnO grains and increases the breakdown voltage.The addition of rare earth oxides can increase the double Schottky barrier between the ZnO grain boundaries and improve the microscopic uniformity of the varistor ceramics,thereby improving the nonlinear characteristics and leakage current characteristics of the varistor ceramics.