Plasticity in Aerogels

When gently stressed, aerogels show an elastic response. However it was found that under isostatic pressure aerogels display an irreversible shrinkage which may be attributed to plastic behaviour. As a consequence of this plastic shrinkage it is possible to densify and modify the elastic properties of aerogels at room temperature.The structural evolution is followed by Small Angle X ray Scattering and the increase of the connectivity is revealed by the evolution of the elastic properties of the material.The SAXS data show that the densification mechanism is different from that obtained by sintering at high temperature. The densification mechanism induces a textural change at the periphery of the constitutive clusters but not inside, conversely to a sintering effect. We also show that the elasticity of the material is strongly influenced by this structural transformation. The power law evolution of the elastic modulus as a function of the density, usually observed on as-prepared and sintered aerogels, is not valid for compressed material.