Al-doped ZnO Varistor Ceramics Prepared By The Reduction-reoxidation Method

With the rapid development of electronic information technology,multilayer chip varistors are widely used for their small size,high surge withstand capability,quick response and so on.Generally noble metal such as Pt or Ag/Pd are employed as inner electrodes material,which accouting for over half of the total cost.Using base metal as inner electrodes is an effective way to cut cost.As base metal electrodes are easy to oxidize at high temperature,ZnO varistors is prepared by the reduction-reoxidation method.In this paper,Al-doped ZnO varistor ceramics prepared by the reduction-reoxidation method were studied.Firstly,the effect of direct doped Al was studied of ZnO varistors prepared by the reduction-reoxidation method.It was found that the residual voltage ratio and surge withstand capability improved significantly for Al-doped ceramic samples.The solid solution of Al in ZnO grains can improve the conductivity of ZnO grains and broaden the current range of nonlinear regions.Then process optimization of sintering temperature and re-oxidized temperature were discussed.Results showed that samples doped with 0.006mol%Al,sintered at 1000?in N₂-H₂ atmosphere,oxidized at 800?in air exhibited a nonlinear coefficient of 51.67,leakage current of 0.28?A/cm²,after surge current?8/20?s?,the residual voltage ratio was 2.16 and variation of breakdown voltage was 2.21%.On this basis,in order to further improve the residual voltage ratio and surge withstand capability,a two-step doping method of Al was proposed.ZnO varistor ceramics prepared by the two-step doping method of Al in air directly exhibited improved electrical propertie with respect to directly doped ones.Then,the two-step doping method was applied to ZnO varistor ceramics prepared by the reduction-reoxidation method,and Al content and sintering temperature are optimized.Ceramic samples doped with 0.006 mol%Al and sintered at 975oC exhibited improved electrical properties with a nonlinear coefficient of56.28,leakage current of 0.18?A/cm²,after surge current?8/20?s?the residual voltage ratio was 1.50 and variation of breakdown voltage was 1.56%,with respect to Al directly doped ones.Microstructure characterization and electrical properties analysis showed that the two-step doping method contributed to the solid solution of Al in ZnO grains to improve the conductivity of ZnO grains,and improvement in the distribution of Al was beneficial to the enhancement of the acceptor state density and barrier height of the grain boundary.Moreover,the densification of ZnO varistor ceramic improved and the porosity decreased.