Synthesis of superhydrophobic crack-free monolithic silica aerogels via a vacuum freeze-drying process

Freeze-drying is a promising method for drying gels, but it is considered incapable of preparing bulk inorganic oxide aerogels. Herein, using tert-butanol/water co-solvent as the freeze-drying solvent, large-size crack-free monolithic silica aerogels with different absolute ethyl alcohol to tetraethylorthosilicate molar ratios were successfully synthesized via a vacuum freeze-drying process. Superhydrophobicity was then obtained through an efficient chemical vapor deposition hydrophobic modification process. As the molar ratio increased from 8 to 16, the density, linear shrinkage, specific surface area (SSA), and mechanical properties decreased, while the thermal conductivity decreased first and then increased. The freeze-dried silica aerogels show the lowest density of 0.078 g/cm³, the lowest linear shrinkage of 4.6%, the highest SSA of 962 m²/g, the highest Young's modulus of 904.3 kPa, and a lowest thermal conductivity of 0.026 W/(m·K). Despite the formed fine ice crystals compressing the gel skeleton to some extent in the freeze-drying process, the developed mesoporous skeleton structure is basically preserved, which ensures excellent thermal insulation and mechanical performance. This study demonstrates that high-quality monolithic inorganic oxide aerogels can be effectively prepared by the freeze-drying method, which provides them with another efficient drying method independent of supercritical fluid drying and ambient pressure drying methods.