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Crystal-growth-related microstructures and the length-to-diameter ratio of a single-crystal-type α-Al 2 O 3 nanofiber were examined using HR-TEM techniques. The fibers exhibited diameters ranging from 50 to 100 nm and lengths of several tens of micrometers. During thermal treatments, the alumina fiber went through phase transformations similar to boehmite. Therefore, the phase evolution, especially the final θ- to α-Al 2 O 3 stage of the phase transformation, may be the determining factor in the microstructural evolution of the nanofibers. HR-TEM techniques were utilized to demonstrate that the single crystals were formed by the coalescence of well-elongated α-Al 2 O 3 colonies. The fibers grew in the [1 1 0] or [1 1 2] direction instead of [0 0 1]. A thermodynamic analysis revealed that if the α-Al 2 O 3 nanofiber that transformed from θ-Al 2 O 3 behaved in a stable manner, there could be a size ratio limit for the length and diameter of each α-Al 2 O 3 colony. The smallest potential diameter was calculated to be around 17 nm.

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

This study examines inducing an additional phase transformation subsystem using α-Al2O3 seeding techniques in a θ-Al2O3 powder system that undergoes θ- to α-phase transformation. It is found that seeding induced subsystem occurs independently at temperatures lower than that of the parent (original) θ-powder system. The whole system is composed of two parallel and subsequently occurring phase transformation systems during the heat treatment: the seeding-induced subsystem and the residual parent system. The fraction of the seeding induced subsystem in the whole system increases and eventually can become predominant with greater amounts of α-Al2O3 added. The subsystem achieves faster growth of θ-crystallites up to the critical size (dcθ) of phase transformation and can begin the transformation at a lower temperature. Thus as the seeding-induced subsystem becomes predominant, the phase transformation behavior of the parent system is gradually obscured and is finally replaced by that of the subsystem, with an apparent reduction in transformation temperature of the overall system. The seeding affected subsystem exhibited lower activation energy in nucleation stage of the phase transformation. However, the activation energy of the growth stage for both seeded and unseeded systems may have similar values.