Effect of seeding and water vapor on the nucleation and growth of α-Al₂O₃ from γ-Al₂O₃

Isothermal transformation kinetics and coarsening rates were studied in unseeded and α-Al₂O₃-seeded γ-Al₂O₃ powders heated in dry air and water vapor. Unseeded samples heated in dry air transformed to α-Al₂O₃ with an activation energy of 567 kJ/mol. Seeding with α-Al₂O₃ increased the transformation rates and reduced incubation times by providing low-energy sites for nucleation/growth of the α-Al₂O₃ 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 α-Al₂O₃ 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 approximately 225 °C compared to the unseeded samples heated in dry air. The accelerated kinetics is believed to be due to increased surface diffusion.