Abstract
Silica aerogels with densities <0.2 g/cm 3 have low mechanical strength and stiffness and improved mechanical properties are of interest to increase their commercial viability. Silica solubility in water is dependent on particle diameter, temperature and pH and, hence, these parameters affect aging. The manner in which the stiffness of a tetramethoxysilane-based gel can be increased by aging in water has been investigated. Aging at 40, 70 and 100°C as a function of time has been studied by measuring wet gel shear modulus and modulus of rupture. Surface area, density and small-angle X-ray scattering were measured on aerogels produced by aging, washing with acetone and liquid CO 2 , and supercritically drying. The observed increase in shear modulus, G, to 1.6 MPa from 0.7 MPa is caused by neck growth between primary particles, dissolution of smaller primary particles followed by deposition into regions of negative curvature and continued condensation reactions. The highest G modulus was obtained after 3 h of aging at 100°C compared with 132 h at 40° C. For longer times, the stiffness actually decreased, presumably a result of continued coarsening.
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