Thermal Stability of Coprecipitated Al2O3-SiO2 Xerogels Prepared from Aluminium Nitrate Nonahydrate and Tetraethylorthosilicate.

Abstract

Amorphous Al2O3-SiO2 xerogels were prepared by coprecipitation and their thermal stability was investigated by powder X-ray diffraction, N2 gas adsorption and transmission electron microscopy. Xerogels with chemical compositions of 4, 30, 60, 80 and 100mol% Al2O3 were prepared by adding conc. NH4OH to an ethanol solution of aluminium nitrate nonahydrate (ANN) and tetraethylorthosilicate (TEOS). Deionized water was added to the solution to adjust the molar ratio H2O/TEOS≥18. The xerogels obtained by calcining at 300°C were heat-treated at 800-1500°C for 2h at heating and cooling rates of 10°C/min. The specific surface area (SA) of the xerogels heated at high temperatures showed large composition-dependent variations, the samples containing 60 and 80mol% Al2O3 having much higher SA values than the other xerogels. This is attributed to retarding effect of the SiO2 component for transition into α-Al2O3 phase by avoiding direct contact of γ-Al2O3 particles. On the other hand, the SA values of the SiO2-rich and Al2O3 xerogels decreased markedly at about 1000-1100°C with increasing of pore radius (rP). This steep decrease in SA and increase of rP are attributed mainly to densification by a viscous flow mechanism accompanied by abrupt particle growth in the SiO2-rich xerogels and by a particle growth mechanism accompanied by the γ-to-α-Al2O3 phase transition in the Al2O3 xerogel. The thermal stability of the xerogels is compared with those of other previously reported porous ceramics.