Effects Of Additives On Properties Of High-voltage ZnO Varistor Ceramics Prepared By Two-step Sintering Method

Due to their high breakdown field E_b??1kV/mm?and nonlinear current-voltage?I-V?characteristics,high-voltage zinc oxide?ZnO?varistors are widely applied in many fields such as military and consumer electronics for the protection of circuit systems by absorbing the surge voltage.At present,high-voltage ZnO varistors are usually fabricated by a two-step sintering method.However,the major ingredient and additives used in this method are all nano-sized powders,and some disadvantages still remain like the unavailability and high costs of nano-sized additives.To address these shortcomings,a new preparation method is put forward for ZnO varistors with high breakdown field,high nonlinearity coefficient,and low leakage current in this present thesis.Additives were added in the form of micro-sized powders and aqueous solutions to the nano-sized ZnO powders?30 nm?,and ceramics were two-step sintered.Firstly,effects were investigated of the addition amount of micro-sized Bi₂O₃ and Sb₂O₃ powders and the first sintering temperature T₁ on the microstructure,phase evolution,sintering behavior and varistor properties of ceramics.Results indicated that the addition of micro-sized Bi₂O₃ powders facilitated the densification of nano-sized ZnO ceramics during sintering,while the addition of micro-sized Sb₂O₃ powders inhibited ZnO grain growth.The recipe was optimized to be 97.45 mol%ZnO+0.80 mol%MnO₂+1.00 mol%Co₂O₃+0.50 mol%Bi₂O₃+0.40 mol%Sb₂O₃,where the starting ZnO was nano-sized and the other oxides were all micro-sized.Varistor ceramics two-step sintered at T₁/T₂=850°C/750°C for 5 min/6 h from the optimal recipe showed an average grain size of 1.77?m,a high relative density of 95%,and excellent properties:E_b=1047.6 V/mm,?=52.1,J_L=0.72?A/cm².Next,Mn?NO₃?₂ and Co?CH₃COO?₂ aqueous solutions were used as starting materials instead of micro-sized MnO₂ and Co₂O₃ powders.Effects were then studied of the addition of these solutions and T₁ on the phase evolution,sintering behavior,and varistor properties of ceramics.The addition of Mn?NO₃?₂ and Co?CH₃COO?₂ caused the formation of a new phase with CoMn₂O₄ spinel structure.The XRD main peak intensity of the CoMn₂O₄ phase increased with increasing Mn²⁺and Co²⁺concentrations.The recipe was optimized to be97.45 mol%ZnO+0.80 mol%Mn?NO₃?₂+2.00 mol%Co?CH₃COO?₂+0.50 mol%Bi₂O₃+0.25 mol%Sb₂O₃.Varistor ceramics two-step sintered at T₁/T₂=850°C/750°C for 5min/6 h from the optimal recipe exhibited a high relative density of 92%and excellent properties:E_b=1282 V/mm,?=57.2,J_L=1.2?A/cm².In addition,ceramics with Mn?NO₃?₂ and Co?CH₃COO?₂ additions had a higher E_b and a higher?than those with micro-sized MnO₂ and Co₂O₃ powder additions when the molar concentration of Mn and Co in ceramics was 0.80 mol%and 2.00 mol%respectively.