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

The densification behavior during sintering of 0.1 mol% TiO2-doped Al2O3 was measured in either an air on N-2 + 5%H-2 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. [doi: 10.2320/matertrans.MC200829]