Effect of Seeding and Water Vapor on the Nucleation and Growth of α-Al2O3 from γ-Al2O3 |
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| Authors: | Roger B Bagwell Gary L Messing |
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| Affiliation: | Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 |
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| Abstract: | Isothermal transformation kinetics and coarsening rates were studied in unseeded and alpha-Al2O3-seeded γ-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°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. |
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