Study On High-performance ZnO-Bi₂O₃-Sb₂O₃ Composite Ceramic Varistor Materials

Three series of ZnO-based varistor materials with different doping levels of Bi₂O₃ and Sb₂O₃ were prepared. Their sintering behavior, electrical properties, and mechanical properties were investigated, respectively. It was found that:(1) Both bismuth and antimony oxides, Bi₂O₃ and Sb₂O₃, evaporated at 1115°C, but their amount, in which bismuth is the majority, is not significant. Bi₂O₃ functioned mainly as liquid during sintering to promote the sintering of ZnO, but it doesn't mean the materials will be denser with more Bi₂O₃ added. The bismuth-rich phase retracted into small pores during cooling, leaving the big pores as voids at room temperature. More Bi₂O₃ added would result in less increase in material densities and dramatic decrease in relative densities, and a little bit increase in grain sizes of matrix ZnO. Sb₂O₃ would react with ZnO matrix into spinel, Zn₇Sb₂O₁₂, which will pin at the grain boundary of ZnO to control the ZnO grain growth. The more Sb₂O₃ was added, the smaller the grain sizes of ZnO is. Appropriate amount of Sb₂O₃ added will yield denser materials.(2) when the doping level of Sb was small, with increasing amount of Sb₂O₃ added, the change in leakage currents was also small, the nonlinear coefficients (both nonlinearity coefficientα_L and breakdown nonlinearity coefficientα_B) would decrease and the field strengths (both field strengths E_L and breakdown field strengthΕ_B) increase; but when the doping levels of Sb were high, with more Sb₂O₃ added, the leakage currents would increase sharply,α_L andα_B decreased further, and E_L andΕ_B would, suddenly, drop down. With increasing doping levels of Bi, the leakage currents would increase,α_L andα_B increase, and E_L andΕ_B decrease.(3) Under constant amount of bismuth of 2 at%, with increasing amount of antimony in an appropriate range, the decreasing grain size of ZnO, almost unchanged porosity, and increasing relative density resulted in increase in modulus, bending strength and fracture toughness of the obtained materials. Under constant amount of antimony of 3 at%, with increasing amount of bismuth, the grain size and porosity increased, thus the relative density would decrease although the apparent density might increase. The resultant modulus, bending strength and fracture toughness all decreased. The optimum ZnO-based varistor material in this work had mechanical properties of elastic modulus of 114 GPa, flexure modulus of 115 GPa, bending strength of more than 120 MPa, and fracture toughness of higher than 1.87 MPa.m^-1/2.Due to the high-performances of the obtained materials under optimum conditions, they will be widely applied in varistors.