New insights into the resistance of hydrophobic silica aerogel composite to water, moisture, temperature and heat-stress coupling

To achieve high-performance thermal insulation under complex environments, a hydrophobic silica aerogel composite (HSAC) reinforced with ceramic fiber paper is developed via in situ co-condensation of tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES). The effect of the MTES/TEOS (M/T) ratio on hydrophobicity, microstructure and thermal conductivity is investigated. The water contact angle of HSAC depends on the microstructure and chemical structure. HSAC with an M/T molar ratio of 1.4 (AC1.4) exhibits the highest water contact angle of 152 degrees. Increasing the M/T molar ratio to 1.5 does not increase the water contact angle further owing to the existence of unhydrolyzed ethoxy groups and microcracks. The increase in specific surface area and mesopore volume reduces the thermal conductivity of HSAC. AC1.4 achieves a high specific surface area of 1039 m(2)/g and mesopore volume of 3.06 cm(3)/g, and a low thermal conductivity of 0.02014-0.12334 W/(m<middle dot>K) at 25-700 degrees C. The increase in the thermal conductivity of AC1.4 is as low as 0.00063 W/(m<middle dot>K) after hygrothermal aging at 85 degrees C and 85 % relative humidity for 42 d, demonstrating excellent long-term hydrophobicity. Benefiting from the ceramic fiber with high-temperature resistance, AC1.4 exhibits superior thermal stability, which retains its hydrophobicity at 450 degrees C, a high specific surface area of 393 m(2)/g and mesopore volume of 1.99 cm(3)/g at 900 degrees C, and low thermal conductivity of 0.01956-0.03031 W/(m<middle dot>K) after thermal treatment at 300-900 degrees C. At 900 degrees C, the coupling of 1 MPa stress does not degrade the thermal insulation performance remarkably. Compared to its state-of-the-art counterparts, AC1.4 offers significant advantages in term of hydrophobicity, thermal stability, thermal conductivity, and resistance to stress at high temperatures, and thus shows great promise for thermal insulation under harsh conditions.