because of the following
reasons: first, the a-Al2O3 is the most thermodynamically stable
phase after calcination at high temperature, the transformation of
a-Al2O3 from u-Al2O3 always relates to a significant change in the
oxygen sublattice from cubic peaking to hexagonal close packing
[17], the reconstructive transformation needs relatively high
activation energy compared with g-Al2O3!s-Al2O3!u-Al2O3
transformation. As a result, the phase transformation temperature
for almost salt-derived aluminum hydroxides and hydrated
alumina form to a-Al2O3 is always at 1100–1250 C. the high
temperature easily prompt the grain growth of powder, which
makes it difficult to get nanoscale particles; secondly, the phase
transformation of a-Al2O3 from u-Al2O3 is performed by a
nucleation and growth mechanism, the a-Al2O3 nuclei form
within the ultrafine u-Al2O3 matrix, and then rapidly grow to
produce a-Al2O3 colonies [18], which inevitably results in certain
degree of particle coarsening