Surface Energy and Thermodynamic Stability of g -Alumina: Effect of Dopants and Water.

Retaining large surface areas in alumina powders during high-temp. annealing is a major challenge in applications as catalyst supports and ceramic precursors. This is because the alumina surface area drastically decreases with transformation from the g modification (defect spinel structure) into the a modification (corundum structure). The objective of this work is to show the thermodn. basis of using additives, such as Zr and Mg, to control the g-Al2O3 surface and bulk energetics and to manipulate the transformation temp. and surface area. These additives are obsd. to change the pattern of phase transformation and densification. Direct measurements of heats of soln. in a lead borate melt of pure and doped alumina as a function of surface area enabled us to exptl. derive trends in the surface energies of hydroxylated surfaces. Accounting for heats of water adsorption measured on pure and doped alumina surfaces allowed us to delineate the thermodn. effects of hydration on surface energies. Zr-doped g-alumina showed a higher energy of the hydroxylated surface than did pure g-alumina but showed a lower energy of the anhyd. surface. Mg addn. does not change surface energies significantly but decreases the energetic instability of the bulk g phase