Abstract
Aerogels are generally described in terms of brittle and elastic materials, like glasses or ceramics with poor and not predictable mechanical properties. We propose the synthesis of a nanocomposite aerogels with improved mechanical properties by addition of silica particles (aerosil; 20–100 nm) in the monomer solution. The elastic modulus and rupture strength increase, stiffening and strengthening the structure by a factor 5–8. The toughness, the critical flaw size, and the fracture energy increase also versus the silica particle concentration. Moreover, the mechanical strength distribution and the Weibull modulus, m, characterizing the statistical nature of flaw size in brittle materials show a more homogeneous flaw size distribution. Pores are considered as integral part of flaws, and small angle X-ray scattering shows that the usual fractal structure observed in aerogel disappears with the silica particle addition. The fractal structure in aerogel is characteristic of a large pore size distribution, but also of a large flaw size distribution. If nanocomposites behave as brittle material under a tension stress, they exhibit a different response when the structure is submitted to a compression: densification and plastic hardening. These opposite behaviors (brittle and plastic) are surprisingly related to the same kinds of gel features: pore volume, silanol content, and the pore size distribution. Because of improved mechanical properties, these nanocomposites aerogels could be used as host matrices for the synthesis of glass ceramics, doped glasses, and composites.