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The growth thermodynamics of nanoscaled α-Al2O3crystallites after its formation from θ-Al2O3phase transition by thermal treatment was examined. The crystallite growth characterized with quantized size increments can be performed through coalescence processes. The α-Al2O3nuclei with sizes 17−20 nm (dcα) coalesced to form α-crystallites of sizes 45−50 nm (dp), then by coalescence of which the stable α-crystallites (ds) with minimum sizes 75−80 nm were obtained. Coalescence of the dscrystallites leads to formation of the crystallites with vermicular growth. A growth thermodynamic model describing the three-staged size growth was proposed. It provides well-coincided calculations with the data observed. The surface free energy of θ-Al2O3, which is being reported for the first time, is 2.16 J/m2. [ABSTRACT FROM AUTHOR]

The possibility of eliminating finger or vermicular growth of α-Al2O3 particles obtained by calcination of boehmite was examined. Heterogeneous precipitation of boehmite in a well-dispersed θ-Al2O3 suspension was first prepared, in which the mass ratio of boehmite to θ-crystallite was evaluated to form agglomerates of similar sizes that will form α-Al2O3 crystallites of <100 nm in diameter. θ- to α-phase transformation of alumina experiences a nucleation and growth mechanism, with the critical size of nucleation being ∼25 nm for θ-Al2O3 and the size for accomplishment of transformation followed by finger growth being ∼100 nm. Hence, fabricating agglomerates that would form α-Al2O3 crystallites with sizes <100 nm accompanied with appropriate thermal treatments can be a method for obtaining α-Al2O3 crystallites free of finger growth. It is found that proper preparation of the agglomerate with appropriate size may initiate a simultaneous and lower temperature θ- to α-Al2O3 phase transformation for such powder systems, substantially limiting the mass transfer among the newly formed α-Al2O3 particles. Moreover, α-Al2O3 crystallites free of finger growth can be obtained. [ABSTRACT FROM AUTHOR]