Influence of the preparative conditions on the precursor phases formed during the synthesis of beta-alumina by the wet chemical gel to crystallite conversions

Hydrated alumina gel, Al2O3 . yH(2)O (80<y<120) reacts with sodium hydroxide in water-miscible organic solvents yielding crystallites of either NaxAlOx(OH)(3-x) (0.17<x<0.23), having the gibbsite structure with inserted sodium ions, or NaxAlO1+x(OH)(1-x) (0.17<x<0.23) of boehmite structure. On calcination at elevated temperatures these precursors yield phase-pure beta "-(Na) alumina. A detailed investigation has been undertaken regarding the influence of preparative parameters on the precursor phases formed. It is observed that the non-aqueous solvents destabilise the gel irreversibly in the presence of alkali hydroxide by decreasing the effective ionic pressure within the gel cavities, and at the same time prevent the back diffusion of alkali ions inserted within the precursor lattice. In the presence of strongly desiccative (water attracting) solvents, dehydroxylation of (Na) gibbsite takes place under wet chemical conditions to yield pseudoboehmite. On calcination, both these phases give rise to intermediate alumina which, in turn, converts to beta "-alumina through a continuous solid state process. (C) 1998 Elsevier Science S.A.