Research And Develop Of High Voltage TiO₂Ring Varistors

In recent years, with the wide application and development of IC and LSI in allkinds of home applications, communications equipment and industrial electronicequipment, a variety of semiconductor components often malfunction or componentdamage because of the sensitivity to overvoltage, surges, noise, etc. resulting in thegeneration of many accidents. Varistors came into being in order to eliminate noiseand arc, and the interference of pulse signals and body static to all the electricalequipments. Since zinc oxide varistor was proposed, zinc oxide varistors withexcellent varistor properties occupied the varistor device market for a long term,especially in the field of high voltage devices, had been widely used in high-voltagemotors and transmission lines. But as a pure resistive element, zinc oxide varistorscan not meet the needs of multi-functional development direction, therefore, toresearch and develop capacitor-varistor multifunctional varistors and to seekalternative product of zinc oxide varistors has become the current study. In lowvoltage areas, SrTiO₃Varistor began to study from1980s can fully meet the demands,thus the development of TiO₂high-voltage varistors has a broad prospect.In this paper, we first select Nb₂O₅as a donor dopant, select BaCO₃or SrCO₃asan acceptor dopant, select Sb₂O₃as sintering aids, to do orthogonal experiment in awide range of factors and levels, to find the law of doping components of theelectrical properties of varistors. The results show that Sr-doped samples have lowvaristor voltage and nonlinear coefficient, although there is a higher capacitance, butthe electrical properties can not meet the high-voltage varistor performancerequirements; however, Ba-doped samples have relatively high varistor voltage,nonlinear coefficient and capacitance, which is possible to meet the requirements ofhigh voltage varistors after improved. Orthogonal experiment in a small range offactors and levels is performed, the results show that the best doping amount of Nb is0.2mol%and the varistor voltage increases with the decreases of the doping amount of Ba and Bi and the decreases of sintering temprature. The obtained samples werecharacterized by XRD, SEM and TEM, the results show that the varistor voltage ofthe same doping system samples increases with the decreases of the grain size.Further research of the doping amount of Ba and Bi is carried out which find theextreme points of the Ba and Bi doping amount. The highest varistor voltage ofsamples in （Nb, Ba, Bi）-doped system is only224.61V/mm, although the otherelectrical properties are all in line with the indicator, this system can not been used tomass production because of the relatively low varistor voltage.Secondly, we select Sb₂O₃instead of Sb₂O₃as sintering aids, to investigate theinfluence of different Sb doping on the electrical properties of varistors. The reaultsshow that the varistor voltage and nonlinear coefficient of the sample decreases firstand then increases with the reducing of Sb doping amount, in which the extreme pointof Sb doping amount is0.1mol%. The obtained samples were characterized by XRD,SEM and TEM and the results show that the varistor voltage of the Sb doping systemsamples increases with the decreases of the grain size. The Sb-doped formula obtainsa wide range varistor voltage of60³60V/mm and the nonlinear coefficient and otherperformance parameters of samples which varistor voltage is higher than200V/mmare in line with the indicator. The conclusion is that we can use the Sb-doped formulaas the basis for the amplification experiment and mass production. In addition, weprocess the raw material of titanium dioxide by hydrothermal method, samplesprepared by the processed raw TiO₂have a high varistor voltage up to10²under1μAcurrent and only about20pF capacitance. It is infered that this device does not havethe voltage-current nonlinear properties, and the raw TiO₂used to prepare TiO₂varistors must be anatase.Finally, stack sintering experiments of Bi-doped formula and Sb-doped formulais performed to investigate the impact of the different stack sintering method on theelectrical properties of the two formula samples. The study find that stack sinteringcan significantly improve the varistor voltage and nonlinear coefficient of the sample,while the capacitance and dielectric loss decreases. The improvement role of sidevertical stack sintering on Bi-doped formula samples is more obvious while it is plant vertical stack sintering on Sb-doped formula samples. Moreover, the side verticalstack sintering makes a large difference on the varistor voltage between the threeelectrodes. Overall, the plant vertical stack sintering Sb-doped samples are more inline with performance requirements and production needs. Small quantities ofsamples are sintered in this method, all aspects of performance of the obtainedsamples are in line with the target and the qualification rate reaches100%.