Evolution of the mechanical properties during the gel-glass process

Abstract

Different kinds of structure in alcogels and aerogels (fractal or not fractal) can be synthesized by a control of the chemical parameters and also by different steps in the preparation such as sintering and plastic compaction. The porosity of the gels is affected either by the adjustment of the gelifying concentration, by a precise control of the viscous flow sintering process, or by an isostatic pressure deformation. The different kinds of gels cover the whole range of porosity between 99% and 0%, and their mechanical properties (elastic modulus, strength, toughness) are strongly dependent on the porosity but also on their structure. We follow the mechanical properties of the over the whole process alcogel - aerogel - glass. They vary by five orders of magnitude as a function of the density, and for the same relative density, the elastic modulus and strength can increase by one order of magnitude due to a change in connectivity. The influence of the sintering process compared to isostatic pressure on the mechanical properties is explained by the associated structural changes.