Preparation of high-strength silica aerogels by two-step surface modification via ambient pressure drying

High-strength silica aerogels were prepared successfully by a new two-step surface modification (TSSM) method via ambient pressure drying (APD). Methyltrimethoxysilane (MTMS) and Trimethylchlorosilane (TMCS) were used as silylating agents in TSSM. First, MTMS is used to consume the residual pore water in the gel, and bridge the links between the secondary particles, further improving the structure of the Si–O–Si network. And then, TMCS is used in the second surface modification due to its smaller steric hindrance, higher reactivity, and more –CH 3 groups, so that the –OH in the gel can be completely replaced by –CH 3 , thereby reducing the shrinkage and improving the hydrophobicity of aerogels during APD. On the basis of keeping the low density (0.1616 g/cm 3 ), low thermal conductivity (0.0226 W m −1  K −1 )) and high hydrophobicity, the prepared samples also have high compressive strength (~ 0.6 MPa) which is far greater than other silica aerogels at present, making it possible to prepare high-strength aerogel blocks via APD. Possible mechanism of the improvement of mechanical properties by TSSM is discussed.