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

Abstract 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/cm 3 , the lowest linear shrinkage of 4.6%, the highest SSA of 962 m 2 /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.