Effect of Seeding and Water Vapor on the Nucleation and Growth of α‐Al 2 O 3 from γ‐Al 2 O 3

Isothermal transformation kinetics and coarsening rates were studied in unseeded and alpha‐Al 2 O 3 ‐seeded γ‐Al 2 O 3 powders heated in dry air and water vapor. Unseeded samples heated in dry air transformed to alpha‐Al 2 O 3 with an activation energy of 567 kJ/mol. Seeding with alpha‐Al 2 O 3 increased the transformation rates and reduced incubation times by providing low‐energy sites for nucleation/growth of the alpha‐Al 2 O 3 transformation. The activation energy for the transformation was reduced to 350 kJ/mol in seeded samples heated in dry air. Seeded samples completely transformed to alpha‐Al 2 O 3 after 1 h at 1050°C when heated in dry air compared to 1 h at 925°C when heated in saturated water vapor. The combined effects of a lower nucleation barrier due to seeding and the increased diffusion due to water vapor reduced the activation energy for the transformation by 390 kJ/mol and the transformation temperature by ∼225°C compared to the unseeded samples heated in dry air. The accelerated kinetics is believed to be due to increased surface diffusion.