Formation mechanism of secondary phase in (La,Nb) codoped TiO2 ceramics varistor

Formation mechanism of secondary phase in (La,Nb) Codoped TiO2 ceramics varistor were investigated. The (La,Nb) Codoped TiO2 ceramics samples were prepared from anatase TiO2, Nb2O5 and La2O3 oxide powders by a traditional solid-state sintering method. Microstructure, chemistry composition, crystal structure, thermal etched groove and micrograph of in (La,Nb) Codoped TiO2 ceramics were by SEM, EDS, AFM and TEM. Formation mechanism of secondary phase were discussed by point defect thermodynamical analysis, grain boundary energy and materials structure measurement.The results show that secondary phase are originated from segregation of point defects NbTi andTiLa at grain boundaries in (La,Nb) Codoped TiO2 ceramics. The driving force of segregation is the elastic strain energy. The segregated ion nucleate at grain surface or grain boundaries interface with higher energy. The nucleation gradually grow to form secondary phase at during high-temperature sintering. The growth of secondary phase is mainly on crystal planes with higher energy to minimize the materials system energy.