The microstructures of transition aluminas which were prepared by the dehydration of boehmite were characterized by using transmission electron microscopy. The presence of γ-, δ- and θ-aluminas was identified by means of selected-area electron diffraction. Modifications that resulted from the re-ordering of Al vacancies on octahedral sites in a cubic close-packed O network were detected and were analyzed by using high-resolution transmission electron microscopy, combined with image simulation. A close correspondence between observed and calculated images confirmed the ordering of vacant octahedral sites, located on {011} and {01¯1) planes, that formed a zig-zag configuration along the <010> direction. Two more arrangements of empty octahedral sites, but concentrated on {001} planes, were detected in sintered powder-gel agglomerates. Structural analysis suggested that the modifications were all associated with the rearrangement of vacant sites during the phase transformation from γ-alumina to δ-alumina to θ-alumina, and were probably driven by configurational entropy minimization.

Y.G.Wang, P.M.Bronsveld, J.T.M.De Hosson, B.Djuricic, D.McGarry, S.Pickering: Journal of the American Ceramic Society, 1998, 81[6], 1655-60