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 alpha-Al2O3-seeded gamma-Al2O3 powders heated in dry air and water vapor. Unseeded samples heated in dry air transformed to alpha-Al2O3 with an activation energy of 567 kJ/mol. Seeding with alpha-Al2O3 increased the transformation rates and reduced incubation times by providing low-energy sites for nucleation/growth of the alpha-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 alpha-Al2O3 after 1 h at 1050 degrees C when heated in dry air compared to 1 h at 925 degrees 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 similar to 225 degrees C compared to the unseeded samples heated in dry air. The accelerated kinetics is believed to be due to increased surface diffusion.