Novel Al2O3–SiO2 composite aerogels with high specific surface area at elevated temperatures with different alumina/silica molar ratios prepared by a non-alkoxide sol–gel method†
Abstract
We have developed a new sol–gel route to synthesise Al2O3–SiO2 composite aerogels with different alumina/silica (Al/Si) molar ratios using an inexpensive inorganic salt. The approach is straightforward, inexpensive, and it produces monolithic mesoporous material with high specific surface area heat-treated at elevated temperatures. The effects of different Al/Si molar ratios and calcination temperatures on the microstructures and properties of Al2O3–SiO2 composite aerogels are investigated in this study. Results show that SiO2 is essentially amorphous, while Al2O3 predominately exists as polycrystalline boehmite for the as-dried composite aerogels. With the increase of Al/Si molar ratios, the morphologies change from connected spheroidal particles to nanometer-sized fibrous particles and web-like microstructures with varying diameters. As the heat treatment temperature increases to 600 °C, structural transition from boehmite to γ-Al2O3 occurs within all the composite aerogels, and mullitization firstly occurs with the Al/Si molar ratio of 1 at around 1000 °C. The specific surface area undergoes an increase–decrease–increase process at 600 °C and 1200 °C for the composite aerogels with different Al/Si molar ratios. The specific surface area is as high as 166 m2 g−1 at 1200 °C for the sample with an Al/Si molar ratio of 8, which is higher than ever reported. The thermal conductivities of mullite fiber mat reinforced aerogel composites at room temperatures are 0.023 W m−1 K−1, 0.029 W m−1 K−1 and 0.025 W m−1 K−1 with the Al/Si molar ratios of 2, 3 and 8, respectively, suitable for efficient thermal insulations uses.