Low-Temperature Sintering Of Low-Voltage ZnO Varistor And Multilayer ZnO Varistor Fabricated By Aqueous Tape Casting

Based on the development of low-voltage, low-temperature sintering and "green" manufacturing of multilayer varistors, ZnO-Bi2O3-TiO2-Co2O3-MnCO3 (ZBTCM) varistor ceramics are chosen as the research objective. In this dissertation, the nonlinear electrical properties and low-temperature sintering of ZBTCM varistor ceramic, and the multilayer varistors fabricated by aqueous tape casting were investigated.The effects of sintering temperature and heating rate on the densification, phase transformation, microstructure and electrical properties of ZBTCM varistor ceramic were investigated. When ZBTCM varistor ceramic is sintered at 1100℃with a heating rate of 60℃/h, the optimal density and electrical properties are determined as follows:densityρ=5.42g/cm3, voltage gradient E1mA=35.6V/mm, nonlinear coefficientα=33.3, leakage current density JL=0.31μA/cm2, the variation of V1mAunder 8/20μs pulseΔV1mA=15.4%. The varistor ceramic has a low E1mA and largeαvalue, but the 8/20μs surge current withstand capacity is poor. And the high sintering temperature could not meet the demand for using Ag as inner electrode for MLV.Thereafter, the effects of WO3 additive on the densification, phase transformation, microstructure and electrical properties of ZBTCM varistor ceramic are investigated. Bi2WiO3(1+i) (i=1/7,2/7) are formed when WO3 content x≤0.3mol%, which reduce the sintering temperature to 1000℃, increase the density of interface states and the barrier height, and improve the nonlinearity and 8/20μs surge current withstand capacity of ZBTCM varistor ceramic. However for x≥0.3mol%, the present of plate-like low resistivity Bi2WO6 inhibits the grain growth of ZnO and impairs the nonlinearity of ZBTCM varistor ceramic. The varistor ceramics with 0.3mol% WO3 sintered at 1000℃exhibit the best performance:E1mA=54.2V/mm,α=36, JL=0.6μA/cm2,ΔV1mA=8.9%.The influences of Cr2O3 additive on the densification, phase transformation, microstructure and electrical properties of ZBTCM varistor ceramic were studied. The sintering temperature could be reduced to 1000℃and the potential barrier height is increased, which is attributed to the solid solution of Cr2O3 into ZnO and Bi2O3 crystal lattice. When Cr2O3 content y value is equal to 0.3mol%, the E1mA and a reach the maximum of 90.8V/mm and 46. WhileΔV1mA was decreased to 10% for y≥0.3mol%. The results show that the nonlinearity and 8/20μs surge current withstand capacity of ZBTCM varistor ceramic was improved as the increasing Cr2O3 content, but the E1mA value was also obviously increased.The effects of ZnO-B2O3 frit on the densification, phase transformation, microstructure and electrical properties of ZBTCM varistor ceramic were investigated. When ZnO-B2O3 frit is added to ZBTCM system, the sintering temperature only lowers 50℃. The grain growth kinetic exponent nZ and apparent activation energy Qz are calculated as 4.54 and 316.5 kJ/mol at the temperature below 1050℃due to the non-melting ZnO-B2O3 frit pining at ZnO grain boundaries, which inhibits the grain growth of ZBTCM varistor ceramic. However, nz and Qz are 2.92 and 187 kJ/mol at the temperature higher than 1050℃due to the melting ZnO-B2O3 frit wetting the ZnO grain boundaries, which accelerates the grain growth of ZBTCM varistor ceramic. The ZnB2O4 phase, crystallized from ZnO-B2O3 frit, plays an important role in increasing the density of interface states and the barrier height. When ZnO-B2O3 frit content is equal to 0.1 wt%, the varistor ceramics sintered at 1050℃exhibit the best performance:E1mA=36.7V/mm, a=35.4, JL=0.35μA/cm2,ΔV1mA=12.1%, Whereas theΔV1mA value is a little larger than 10%.The effects of Bi2O3-B2O3 frit on the densification, phase transformation, microstructure and electrical properties of ZBTCM varistor ceramic were investigated. When Bi2O3-B2O3 frit is added, the densification temperature could be reduces to 900℃. The grain growth kinetic exponent nB and apparent activation energy QB are 2.15 and 146.2 kJ/mol, which are much lower than that of ZnO-B2O3 frit doped varistor. It indicates that Bi2O3-B2O3 frit was more effective in improving the sintering properties of ZBTCM varistor ceramic than ZnO-B2O3 frit. The Bi4B2O9 phase, crystallized from Bi2O3-B2O3 frit, plays an important role in increasing the barrier height and nonlinearity of ZBTCM varistor ceramic. When Bi2O3-B2O3 frit content is equal to 2wt%, the varistor ceramics sintered at 900℃exhibit the best performance:E1mA=124.9V/mm,α=46.2, JL=0.2μA/cm2,ΔV1mA=8.3％.The aqueous tape casting for ZBTCM varistor with 2wt% Bi2O3-B2O3 frit was researched. By optimizing the slurry composition, including binder, dispersant, plasticizer, surfactant and defoamer, the excellent green tape with a thickness of 40μm are successfully developed. The optimal recipe for the aqueous slurry is 50wt% solid loading,41.4wt% deionized water,3wt% PVA,0.5wt% PAA-NH4,2.7wt% glycerin,1.5wt% Span-20, 0.9wt%。When the green tape is sintered at 900℃, the electrical properties are determined as follows:E1mA=112.4V/mm,α=34.6, JL=0.5μA/cm2.Finally, the effects of isostatic pressures on the microstructure of green MLV, and the cofiring properties of the varistor ceramic tape with Ag were also studied. The result shows that the tapes and Ag pastes contact with each other compactly under 60Mpa. There is no chemical reaction and interfacial diffusion happed at the interface of MLV when sintered at 900℃, and the excellent electrical properties are determined as follows:V1mA=6.1V,α=28.1, IL=0.15μA.