Kinetics and Microstructural Evolution of Heterogeneous Transformation of θ‐Alumina to α‐Alumina

Ultrafine (<0.1 μm) high‐purity θ‐Al 2 O 3 powder containing 3–17.5 mol%α‐Al 2 O 3 seeds was used to investigate the kinetics and microstructural evolution of the θ‐Al 2 O 3 to α‐Al 2 O 3 transformation. The transformation and densification of the powder that occurred in sequence from 960° to 1100°C were characterized by quantitative X‐ray diffractometry, dilatometry, mercury intrusion porosimetry, and transmission and scanning electron microscopy. The relative bulk density and the fraction of α phase increased with annealing temperature and holding time, but the crystal size of the α phase remained ∼50 nm in all cases at the transformation stage (≤1020°C). The activation energy and the time exponent of the θ to α transformation were 650 ± 50 kJ/mol and 1.5, respectively. The results implied the transformation occurred at the interface via structure rearrangement caused by the diffusion of oxygen ions in the Al 2 O 3 lattice. A completely transformed α matrix of uniform porosity was the result of appropriate annealing processes (1020°C for 10 h) that considerably enhanced densification and reduced grain growth in the sintering stage. The Al 2 O 3 sample sintered at 1490°C for 1 h had a density of 99.4% of the theoretical density and average grain size of 1.67 μm.