Study On ZnO Varistor With High Voltage- Gradient And High Surge Current Withstanding Capacity

Zinc Oxide varistors which are charactered by high nonlinear voltage-current specialty and excellent surge withstanding capability have been found great applications in the fields of electrical and electronics to protect electrical circuits. However, this field is still expecting to improve the surge current withstanding capability as well as voltage-gradient. This thesis mainly aims to produce high voltage-gradient varistor with high surge current withstanding capability while maintaining other electrical characteristics.This thesis adopts conventional technique and Sol-Gel process to fabricate ZnO varistors and discusses effects of component and manufactural technique on the micro structure and electrical properties of ZnO varistors.First of all, Three series of ZnO varistor ceramic samples doped with Bi2O3, SnO2 and Sb2O3 have been prepared by conventional electronic ceramics preparation technology. The microstructure of the samples have been studied by Scanning Electron Microscope. The varistor voltage, nonlinear coefficient and leakage current of ZnO varistor ceramics doped with different molar content of Bi2O3, SnO2 and SD2O3 have been measured. The clamping voltage ratio and the variation of breakdown voltage after surge current have been calculated. The effect of those three kinds of dopant on the microstructure and electrical properties of ZnO-based varistor ceramics have been discussed. The results show that:with the increase of the content of Bi2O3, the breakdown voltage-gradient VlmA/mm and nonlinear coefficient a first increases and then decreases. However, the leakage current IL first decreases and then rises. As Bi2O3-doping amount is 0.310 mol%, optimum properties of ZnO varistor can be obtained, breakdown voltage-gradient VlmA/ mm=435V/mm, nonlinear coefficient α=42, leakage current IL=3μA, clamping voltage ratio K=1.96, variation of breakdown voltage after surge current AVlmA/VlmA=4.89%. with the decrease of the content of SnO2, the breakdown voltage-gradient VlmA/mm and nonlinear coefficient a decrease, the leakage current Ⅱ rises. As SnO2-doping amount is 0.574 mol%, optimum properties of ZnO varistor can be obtained, breakdown voltage-gradient VlmA/mm=408V/mm, nonlinear coefficient α=34, leakage current IL=5μA, clamping voltage ratio K=2.02, variation of breakdown voltage after surge current △VlmA/VlmA=9.13%. with the increase of the content of Sb2O3, the breakdown voltage-gradient VlmA/mm and nonlinear coefficient a first decreases and then increases. However, the leakage current Ⅱ first rises and then decreases. As Sb2O3-doping amount is 0.572 mol%, optimum properties of ZnO varistor can be obtained, breakdown voltage-gradient Vlm//mm=420V/mm, nonlinear coefficient a=38, leakage current IL=3μA, residue voltage ratio K=2.01, variation of breakdown voltage after surge current △VlmA/VlmA=7.58%.Then, Mn2O3,NiO,Bi2O3,Co3O4 and Mn2O3,NiO,Bi2O3,Co3O4,SnO2 composite powder which were made by sol-gel method were introduced into the manufacture process of ZnO varistors. Firstly, Mn, Ni, Bi, Co (and Sn) was made into composite powder in the Sol-Gel method. Secondly, we use the composite powder with other additives and grinding of Zinc Oxide to manufacture varistors with the size of 8.8 mm in diameter and lmm in thickness under proper technique. The microstructure and performances of ZnO varistors made from the Sol-gel method were tested and analyzed. Compared to the ZnO varistors made from the conventional technique, The ones made from the Sol-Gel method have stabler internal structure and narrower range particle size distribution. These additives elements also present more uniform distribution. They also show great improvement in electrical properties:varistor voltage and the nonlinearity of the varistors manufactured by the Sol-Gel method are more than those of the conventional method, the leakage current is smaller. The surge absorption capability of 8/20μs of varistors manufactured by the Sol-Gel method is also improved. As Mn, Ni, Bi, Co, Sn was made into composite powder in the Sol-Gel process, optimum properties of ZnO varistor can be obtained, breakdown voltage-gradient VlmA/mm =516V/mm, nonlinear coefficient a=43, leakage current IL=2μA, clamping voltage ratio K=1.78, variation of breakdown voltage after surge current A△VlmA/VlmA=4.41%.