ALPHA ALUMINA FORMATION IN ALUMINA SOL-GELS.

摘要

The formation of (alpha)-Al(,2)O(,3) was investigated in Al(,2)O(,3) sol-gels derived from pseudoboehmite (AlO(OH)(.)xH(,2)O) and bayerite (Al(OH)(,3)). The aluminum hydroxide sols were prepared by hydrolysis of aluminum sec-butoxide (Al(OC(,4)H(,9))(,3)). Alpha-Al(,2)O(,3) formation was characterized in Al(,2)O(,3) gels annealed at 1000(DEGREES)C to 1200(DEGREES)C in air containing different concentrations of water vapor. The influence of Cr(,2)O(,3), Fe(,2)O(,3), and MgO on (alpha)-Al(,2)O(,3) formation was characterized at 1150(DEGREES)C in pseudoboehmite gels. The rate of (alpha)-Al(,2)O(,3) formation was characterized by using quantitative x-ray analysis. Microstructural analysis was done by transmission electron microscopy (T.E.M.) and scanning electron microscopy (S.E.M.).; The kinetic and microstructural results show that (alpha)-Al(,2)O(,3) formation occurs by nucleation and growth which is preceded by a incubation period. A large temperature dependence was found for (alpha)-Al(,2)O(,3) formation in pseudoboehmite gels. The rate of (alpha)-Al(,2)O(,3) formation is greater in bayerite gels than pseudoboehmite gels annealed at 1150(DEGREES)C. The rate of (alpha)-Al(,2)O(,3) formation was not affected by minor additions of MgO, Cr(,2)O(,3) and Fe(,2)O(,3) nor by varying the concentration of water vapor.; Microstructural evolution in Al(,2)O(,3) gels is dominated by a series of phase transformations, between the hydrous oxide to transitional Al(,2)O(,3) phases, between transitional phases, and final transformation to (alpha)-Al(,2)O(,3). The phase transformation to (alpha)-Al(,2)O(,3) is accompanied by particle and pore coarsening which is pronounced in the presence of water vapor.; Water vapor affects the growth morphology of (alpha)-Al(,2)O(,3) in pseudoboehmite gels, but not in bayerite gels. The normal growth morphology for (alpha)-Al(,2)O(,3) is vermicular. Discrete, discrete-vermicular, and elongated grains of (alpha)-Al(,2)O(,3) are the observed growth morphologies when water vapor affects growth; large pores and faceting are associated with these growth morphologies.; Water vapor enhances the nucleation density of (alpha)-Al(,2)O(,3) at 1000(DEGREES)C and 1050(DEGREES)C. The density of discrete grains of (alpha)-Al(,2)O(,3) was 10('13)-10('14)/cm('3) and independent of water vapor concentration. The density of vermicular colonies of (alpha)-Al(,2)O(,3) was 10('10)/cm('3) and independent of temperature.