硝酸アルミニウム九水和物とオルトケイ酸エチルから調製したアルミナ-シリカ共沈キセロゲルの熱安定性

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Amorphous Al_2O_3-SiO_2 xerogels were prepared by coprecipitation and their thermal stability was investigated by powder X-ray diffraction, N_2 gas adsorption and transmission electron microscopy. Xerogels with chemical compositions of 4, 30, 60, 80 and 100 mol%Al_2O_3 were prepared by adding conc. NH_4OH to an ethanol solution of aluminium nitrate nonahydrate(ANN)and tetraethylorthosilicate(TEOS). Deionized water was added to the solution to adjust the molar ratio H_2O/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%Al_2O_3 having much higher S_A values than the other xerogels. This is attributed to retarding effect of the SiO_2 component for transition into α-Al_2O_3 phase by avoiding direct contact of γ-Al_2O_3 particles. On the other hand, the S_A values of the SiO_2-rich and Al_2O_3 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 SiO_2-rich xerogels and by a particle growth mechanism accompanied by the γ-to-α-Al_2O_3 phase transition in the A_2O_3 xerogel. The thermal stability of the xerogels is compared with those of other previously reported porous ceramics.
2000-02-01

硝酸アルミニウム九水和物とオルトケイ酸エチルから調製したアルミナ-シリカ共沈キセロゲルの熱安定性

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