θ- to α-phase transformation subsystem induced by α-Al_2O_3-seeding in boehmite-derived nano-sized alumina powders

摘要

This study examines inducing an additional phase transformation subsystem using α-Al_2O_3 seeding techniques in a θ-Al_2O_3 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-Al_2O_3 added. The subsystem achieves faster growth of θ-crystallites up to the critical size (d_(cθ)) 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.