Thermal and Mechanical Properties of Density-Gradient Aerogels for Outer-Space Hypervelocity Particle Capture

Aerogels with densities in the range 40-175 mg . cm(-3) were prepared using a tetraethyl orthosilicate (TEOS) ethanol-water solution as the precursor and hydrofluorid acid as the catalyst via a sol-gel process and CO2 supercritical-fluid drying. The density-gradient aerogels were prepared using layer-by-layer gelation, sol co-gelation, and gradient-sol co-gelation methods and their gradient properties were studied systematically. The results show that aerogels with different densities all have a three-dimensional skeleton consisting of spherical particles of diameter about 40-90 nm. The lower the density is, the looser the skeleton and pore-size distributions are, and the larger the peak value of the pore size is. Gradient aerogels prepared via different methods exhibited graded, approximately gradient, or gradient distributions. Dynamic mechanical analysis indicates that the Young's moduli of the aerogels at -100 and 25 degrees C (changed from 4.6x10(5) to 1.9x10(5) Pa and from 5.0x10(5) to 2.1x10(5) Pa, respectively) tend to decrease with decreasing density. Thermal constants analysis shows that as the densities of the aerogels decrease, the thermal diffusion coefficients increase and the specific heat capacities decrease, but the thermal conductivities do not change monotonically.