Studies of Secondary Phases in(La,Nb)-Codoped TiO_2 Varistors Ceramics

Transition of secondary phase in TiO2 varistor ceramics doped with Nb2O5 and La2O3 and effects of secondary phase on properties of samples was investigated.The effects of different dopants and sintering temperature on varistor voltage V1 mA and nonlinear coefficient α were discussed by experimental method.SEM,XRD,XPS and EDAX were carried out to measured the microstructure,chemical composition and crystal phase of TiO2 ceramics.The results show that there exist secondary phases in(La,Nb) doped TiO2 varistors ceramics.With sintering temperature increasing,secondary phases will transit from LaNbO4 to LaNbTiO6.Ternary phase diagram of La2O3-Nb2O5-TiO2 was plotted based on binary phase diagram of La2O3-Nb2O5,Nb2O5-TiO2 and La2O3-TiO2.According to the ternary phase diagram,the relative content of secondary phases in TiO2 ceramics sintered at different temperature were estimated.It is suggested that segregations of secondary phases cause an increasing concentration of titanium vacancy V ″″Ti,which result in a higher V1 mA and α.