Aluminum hydroxide transformations during thermal and vapor heat treatments

We have studied the mechanisms underlying the formation of corundum crystals from hydrargillite and boehmite during thermal and vapor heat treatments and have identified the major stages in the formation of corundum structure, thought of as the evolution of the system toward the most thermodynamically stable state. The results indicate that isothermal autoclaving at temperatures from 390 to 450C gives rise to structural changes in alumina, which involve the following stages: the formation of microcrystalline boehmite from coarser hydrargillite crystals, accompanied by disintegration of agglomerates, boehmite flocculation and dehydration, conversion of the dehydrated boehmite to alpha-alumina within each floccule, and formation of corundum single crystals. Unflocculated boehmite crystals are absorbed by the growing crystal faces. If presynthesized boehmite is used as the precursor, the process also involves flocculation, dehydration, conversion to alpha-alumina, and formation of corundum single crystals. The structural changes induced in hydrargillite and boehmite crystals by annealing in air and vacuum at temperatures from 1400 to 1500C have been analyzed. The formation of alpha-alumina from boehmite below < 1200C is accompanied by no changes in crystal habit, whereas annealing in the range 1400-1500C gives rise to sintering of fine alumina crystals and changes in crystal habit. The degree of alignment of the crystals is shown to influence their structural transformations.