PLASTIC BEHAVIOR OF AEROGELS UNDER ISOSTATIC PRESSURE

The bulk modulus of a set of aerogels was measured as a function of applied pressure by means of a Hg porosimeter. With increasing pressure, the aerogels, which are known to be elastic, display irreversible shrinkage corresponding to plastic behaviour. The magnitude of the plastic shrinkage and the value of the bulk modulus depend strongly on the volume fraction of solid in the aerogel. The plastic behaviour is related to the condensation reaction between silanols. Both plastic shrinkage and bulk modulus are clearly increased when the aerogel has undergone an oxidation treatment which induces the formation of silanols at the expense of organic groups. As a consequence of this plastic shrinkage, it is possible to densify and stiffen the aerogel at room temperature. The connectivity of the aerogel is strongly increased by the densification under isostatic pressure and the power laws describing the elastic constant evolution as a function of the bulk density lead to values of the power exponent different to those found in the literature. Fatigue effect due to cycling runs of pressurization and also delayed elasticity have been observed for the stiffer aerogels.