The mechanical properties of silica gels (alcogels and aerogels) are measured by three point bending. The elastic moduli (E,G) and the fracture strength (σ) are investigated as functions of the concentration of silicon precursor, the catalysing conditions, and the aging time.
Elastic moduli and strength follow scaling laws as a function of the bulk density of the solid. The scaling exponents are discussed in terms of percolation theory.
The evolution of the mechanical properties is also followed during the steps of the gel to glass transformation:
- hypercritical drying
- oxidation of the organic groups
-densification heat treatment
At each step of the process the increases of the elastic moduli and σ are related to the gel structure. During the hypercritical drying flexible “dead ends” in the network can condense and form new links. The connectivity is also enhanced by the removal of the organic groups creating siloxane bonds. Finally densification leads to a material having mechanical properties identical to those of conventional vitreous silica.