Fundamentals Of The Synthesis Of ZnVMnNbO-based Varistor Ceramics With LaCe Rare Earth Semi-finished Product

ZnVMnNbO-based varistor ceramics have excellent varistor properties and can be sintered at 900?.The nonlinearity of ZnVMnNbO-based varistor ceramics can be further improved by small additions of Rare Earth?RE?dopants.However,the pure rare earth dopants used in the most of peer researches also directly increases the cost of the production and application of this material.Therefore,in this paper,Lanthanum?La?and Cerium?Ce?containing RE semi-finished product obtained on a local RE production line were thermal decomposed into La and Ce composite RE oxide and used as the sole RE source instead of pure rare earth oxides to fabricate high-performance ZnVMnNbO-based varistor ceramics by the traditional mixing oxide process.Then,XRD,SEM,EDS and standard I-V measurements were used to study the effects of the variations of the above-mentioned thermal decomposition product addition?0-0.25mol%?,sintering temperature?850-925??and soaking time?2-5h?on the varistor ceramics and its mechanism.Finally,under the optimum doping amount and optimum preparation process conditions,the difference of the effect of the addition of thermal decomposed LaCe RE semi-finished product and the LaCe pure RE mixture doping with the same molar content on the ZnVMnNbO-based varistor ceramics was studied.The results show that the basic ZnVMnNbO-based varistor ceramics are composed of the main crystalline phase of ZnO and the secondary phases of ZnV₂O₄,ZnMn₂O₄ and Zn₃?VO₄?₂.With the addition of thermal decomposed LaCe RE semi-finished product,Ce?La?VO₄ forms as the additional secondary crystalline phase.When the ceramics were sintered at 875?for 3h,the relative density of ceramics increases from 92.67%to 93.40%and the average grain diameter decreases from 5.62?m to 3.20?m with the increase of thermal decomposed LaCe RE semi-finished product doping.The results of I-V test showed that ZnVMnNbO ceramic with 0.1mol%thermal decomposed LaCe RE semi-finished product exhibits the optimum electrical performance characterized by the non-linear coefficient of 48.4,the breakdown voltage 554 V/mm,and the leakage current density 120?A/cm².The variation of sintering temperature and soaking time did lead to variations in the relative density,average grain diameter and nonlinear properties.However,the nonlinear coefficients of the samples with those variations were all lower than 48.4.Secondly,phenomenological method was used to further study the grain growth mechanism of ZnVMnNbO-based varistor ceramics and its change with adding RE dopant.The grain growth exponent of ZnVMnNbO-based varistor ceramic increases from 2.39 to2.95 with the addition of thermal decomposed LaCe RE semi-finished product dopant,while the grain growth activation energy escalates from 186 KJ/mol to 263 KJ/mol.These results indicate the grain growth of the ZnVMnNbO varistor ceramics during sintering process is mainly controlled by the phase boundary reaction between ZnO grain and V-rich liquid phase and the pinning effect of ZnMn₂O₄ particles.The grain growth of the ceramics doped with the thermal decomposed LaCe RE semi-finished product is further hindered by the Ce?La?VO₄ particle phase and the reduction of liquid phase content of Zn₃?VO₄?₂caused by it.Finally,the ceramic containing the mixture of pure La/Ce oxides have the same crystalline phase constituents and density as their counterparts containing above-mentioned thermal decomposed LaCe RE semi-finished product.Meanwhile,their non-linear coefficient is 45.0,the breakdown voltage is 577 V/mm,and the leakage current density is 107?A/cm²,indicating the overall nonlinear properties of LaCe pure RE mixture doped sample is slightly lower.Therefore,it is not only proved that the LaCe RE semi-finished product can be used to prepare ZnVMnNbO-based varistor ceramics with high nonlinearity,but also provides reference for the application of such RE semi-finished product in the fabrication of other high performance functional ceramics.