Study On Additive-doping Effects On Mictructure And Electrical Properties Of ZnO-Pr₆O₁₁ Based Varistors

ZnO varistors are electronic ceramic devices whose primary function is to sense and limit transient voltage surges, and to do so repeatedly without being destroyed. It can be used in ac or dc fields and over a wide range of voltage. For its versatility, ZnO varistors have been extensively used to protect various semiconductor devices, electric power systems. In this paper, ZnO-Pr₆O₁₁ based varistors doped with several different additives were prepared by conventional ceramics process. The effects of doping additives and doping level on microstructure and electrical properties were investigated. It was found that:(1) In ZnO-Pr₆O₁₁-TiO₂ based varistors, TiO₂ acted as an inhibitor of ZnO grain growth. When the TiO₂ doping level was no more than 1.0 mol%, the increase of TiO₂ doping level could increase the varistor voltage and nonlinear exponents, and decrease the leakage currents of the varistors. With the Pr₆O₁₁ doping amounts no more than 1.5 mol%, the increase of Pr₆O₁₁ doping amounts could increase the varistor voltage and nonlinear exponents, and decrease the leakage currents of the varistors. The optimum electrical properties of ZnO-Pr₆O₁₁-TiO₂ based varistors were achieved with varistor voltage 381 V/mm, nonlinear exponents 91 and leakage currents 27μA/cm2.(2) In ZnO-Pr₆O₁₁-ZrO₂ based varistors, ZrO₂ acted as an inhibitor of ZnO grain growth. With the increase of ZrO₂ doping contents, the varistor voltage incresed, but the leakage currents increased.When the ZrO₂ doping level was no more than 0.5 mol%, the increase of ZrO₂ doping level could increase the nonlinear exponents. With the Pr₆O₁₁ doping amounts no more than 1.5 mol%, the increase of Pr₆O₁₁ doping amounts could increase the varistor voltage and nonlinear exponents, and decrease the leakage currents of the varistors. The optimum electrical properties of ZnO-Pr₆O₁₁-ZrO₂ based varistors were achieved with varistor voltage 544 V/mm, nonlinear exponents 32 and leakage currents 130μA/cm2.(3) In ZnO-Pr₆O₁₁-SnO₂ based varistors, SnO₂ acted as an inhibitor of ZnO grain growth. With the increase of SnO₂ doping contents up to 1.0mol%, the varistor voltage incresed.When the SnO₂ doping level was no more than 0.5 mol%, the increase of SnO2 doping level could increase the nonlinear exponents, and decrease the leakage currents of the vaistors. With the Pr₆O₁₁ doping amounts no more than 1.5 mol%, the increase of Pr₆O₁₁ doping amounts could increase the varistor voltage. With the increase of Pr₆O₁₁ amounts up to 1.0 mol%, the nonlinear exponents increased and the leakage currents decreased. The optimum electrical properties of ZnO-Pr₆O₁₁-SnO2 based varistors were achieved with varistor voltage 700 V/mm, nonlinear exponents 29 and leakage currents 16μA/cm².(4) In ZnO-Pr₆O₁₁ based varistors, the doping of Li⁺, Na⁺, K⁺ and Rb⁺ inhibited the ZnO grain growth. The doping of Li⁺ made the ZnO-Pr₆O₁₁ based varistors showing insulating and ohmic properties. The doping of Na⁺, K⁺ and Rb⁺ could increase the nonlinear exponents and decrease the leakage currents of ZnO-Pr₆O₁₁ based varistors.