While the nanoscale a-alumina powder is difficult to obtain, 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. For the above reasons, the a-alumina prepared by common methods shows serious agglomeration, so eliminating the agglomeration in the subsequent mechanical treatment is necessary, which inevitably introduced impurities.