Densification Behavior of Ti-Doped Polycrystalline Alumina in a Nitrogen-Hydrogen Atmosphere

Abstract

The densification behavior during sintering of 0.1 mol% TiO₂-doped Al₂O₃ was measured in either an air or N₂+5%H₂ gas atmosphere at the sintering temperature of 1573–1673 K. The grain boundary diffusivity was evaluated from the densification rate. High-resolution transmission electron microscopy (HRTEM) and nano-probe energy-dispersive X-ray spectroscopy (EDS) analyses revealed that the doped Ti cations segregate in the vicinity of the grain boundaries in the Al₂O₃. An electron energy loss spectroscopy (EELS) investigation indicated that the valence state of Ti in the Al₂O₃ sintered in the reducing atmosphere was close to +3. The grain boundary diffusivity in undoped Al₂O₃ was insensitive to the atmosphere, but was enhanced by the grain boundary segregation of Ti⁴⁺. The grain boundary diffusivity of alumina in the reducing atmosphere was, however, retarded by the Ti³⁺-doping. The retarded diffusivity by Ti³⁺-doping must be related to the lack of aluminum vacancies and the large ionicity of Ti-O compared to Al-O.