关键词: 梯度, 气凝胶, 热学, 力学, 热导率, 模量, 低温

Abstract: 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 hydrofluoric 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 threedimensional 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 °C (changed from 4.6×10⁵ to 1.9×10⁵ Pa and from 5.0×10⁵ to 2.1×10⁵ 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.

Key words: Gradient, Aerogel, Thermal, Mechanics, Thermal conductivity, Modulus, Low temperature