Grain boundary segregation and the formation mechanism of secondary-phase in (Ce,Nb)-codoped TiO2 ceramics

Abstract

Secondary phases are common phenomena in doped titanium dioxide ceramics. Effects of secondary phases on the structure and performance of the doped TiO2 ceramic are crucial. Both boundaries segregation and formation mechanism of secondary phases should be comprehensively explored to enhance properties and application. (Ce,Nb)-codoped TiO2 ceramic samples were prepared from TiO2, Nb2O5, and CeO2 oxide powders by a traditional solid-state sintering method. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and wavelength-dispersive X-ray spectroscopy (WDS) were conducted to determine the sample microstructures, crystal structures, ionic valences, and chemical composition. The point defect distributions at the grain boundaries were calculated via point defect thermodynamic analysis, and the effects of Ce4+ and Ce3+ on segregation at the grain boundaries were compared. The XPS analysis showed that both ions are present in the TiO2 ceramics, and that the Ce3+ content increases with increasing sintering temperature. Further, secondary phases originate from segregation of the NbTi· and Ce´Ti point defects at the grain boundaries. The driving force for segregation is primarily the elastic strain energy, and CeNbO4 and CeNbTiO6 secondary phases occur in the (Ce,Nb)-codoped TiO2 ceramics. With increased sintering temperature, the secondary phase transforms from CeNbO4 to CeNbTiO6.