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

Abstract

The densification behavior during sintering of 0.1 mol% TiO 2 -doped Al 2 O 3 was measured in either an air or 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 A1 2 0 3 . An electron energy loss spectroscopy (EELS) investigation indicated that the valence state of Ti in the Al 2 O 3 sintered in the reducing atmosphere was close to +3. The grain boundary diffusivity in undoped A1 2 0 3 was insensitive to the atmosphere, but was enhanced by the grain boundary segregation of Ti 4+ . The grain boundary diffusivity of alumina in the reducing atmosphere was, however, retarded by the Ti 3+ -doping. The retarded diffusivity by Ti 3+ -doping must be related to the lack of aluminum vacancies and the large ionicity of Ti-O compared to Al-O.