Thermal stability of coprecipitated Al₂O₃-SiO₂ xerogels prepared from aluminium nitrate nonahydrate and tetraethylorthosilicate

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 100 mol%Al₂O₃ were prepared by adding cone. 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 2 h at heating and cooling rates of 10°C/min. The specific surface area (S_A) of the xerogels heated at high temperatures showed large composition-dependent variations, the samples containing 60 and 80 mol%Al₂O₃ having much higher S_A 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 S_A values of the SiO₂-rich and Al₂O₃ xerogels decreased markedly at about 1000-1100°C with increasing of pore radius (r_p). This steep decrease in S_A and increase of r_P 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-αAl₂O₃ phase transition in the Al₂O₃ xerogel. The thermal stability of the xerogels is compared with those of other previously reported porous ceramics.