Research On TiO₂ Varistor-ceramic Dual Function For Micro Dc Motors

In this article,the powder of raw materials was prepared by the method of ball milling.Different samples were prepared by doping different content of oxides such as Nb₂O₅,SrCO₃,SiO₂,Bi₂O₃ and selecting different sintering temperatures.Testing the voltage,nonlinear coefficient,apparent dielectric constant and dielectric loss of the samples to explore the influence of different transition metal elements on the electrical properties of TiO₂ varistor-ceramics.At the same time,the forming density of the green body was changed,and the samples of different molding density were prepared.Then analysis of green body forming density on the electrical properties of TiO₂ varistor-ceramics,physical properties and micro structure of the relationship was done in order to obtain the optimum density of green body forming.Testing the microstructure of different samples to study the impact of various dopants and preparation process on the microstructure of samples,analysis of the relationship between different oxides doping on microstructure and electrical properties of the samples,revealing the mechanism of doping modification.（1）In the process of preparation of low voltage TiO₂ varistor-ceramics,Nb₂O₅as donor dopant and the optimal doping amount is 1.0 mol%,while SrCO₃ as the acceptor dopant the optimum doping amount is 0.5 mol%.As sintering agent,Bi₂O₃doping effect of SiO₂ is more obvious,mainly in the low breakdown voltage,basically meet the requirements voltage range,at the same time when Bi₂O₃ used as sintering additives,with high capacity,relative to low loss characteristics etc.（2）The doping range in this paper,the breakdown voltage of TiO₂varistor-ceramics increase with the content of Nb₂O₅ increased first and then decreased,with the increase of the concentration of SrCO₃,with the increase of the concentration of SiO₂ increased first and then decreased,with the increase of the concentration of Bi₂O₃ increased after the first drop,and Nb₂O₅ is the main effect the factors of TiO₂ varistor ceramic-varistor voltage.（3）The optimum technological conditions of TiO₂ varistor-ceramics with Nb,Sr,Bi oxides addition are:Nb₂O₅,SrCO_3,Bi₂O₃ doped respectively:1.1 mol%,0.5mol%,0.55 mol%,sintering temperature of 1390℃,the breakdown voltage obtained in the optimum process conditions for sample 9.88V/mm,nonlinear coefficient is 2.59,electrostatic capacity is 37 nF,the difference between 9.51%pole.（4）During the firing process of TiO₂ varistor-ceramic,the control of the sintering temperature has the greatest influence on the sintering,and the different formulations are corresponding to different sintering temperature.（5）The formation density of green body has a strong influence on the electrical properties and microstructure of TiO₂ varistor-ceramics.When the density of the green body is 2.55 g/cm³,the average grain size is 5.911μm,the grain size standard deviation is 0.853μm,the crystal structure is the most uniform,and the crystal structure is the best.The densification of the TiO₂ varistor-ceramics can be accelerated,and the grain growth can also be promoted.The optimum shape of the green body is related to the formula of the sample and the size of the raw materials.（6）With the increase of CaCO₃ doping amount,the breakdown voltage of the sample decreases first and then increases.The nonlinear coefficient increases first and then decreases.Therefore,a small amount of CaCO₃ can be added to improve the nonlinear properties of the samples.The relative dielectric constant showed a trend of first increase and then decreased.The dielectric loss showed a tendency to decrease first and then increase.Because the ionic radius of Ca²⁺is much larger than the ionic radius of Ti⁴⁺,it will lead to lattice distortion,grow up so trace CaCO₃ will lead to the grain size and the formation of grain boundary barrier,narrow and high in grain boundaries.When the doping amount of CaCO₃ is 0.5 mol%,the electrical properties of the varistor-ceramics are the best.