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

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.