PLASTIC BEHAVIOR OF AEROGELS UNDER ISOSTATIC PRESSURE

The bulk modulus of a set of aerogels was measured as a function of ap plied pressure by means of a Hg porosimeter. With increasing pressure, the aerogels, which are known to be elastic, display irreversible shr inkage corresponding to plastic behaviour. The magnitude of the plasti c shrinkage and the value of the bulk modulus depend strongly on the v olume fraction of solid in the aerogel. The plastic behaviour is relat ed to the condensation reaction between silanols. Both plastic shrinka ge and bulk modulus are clearly increased when the aerogel has undergo ne an oxidation treatment which induces the formation of silanols at t he expense of organic groups. As a consequence of this plastic shrinka ge, it is possible to densify and stiffen the aerogel at room temperat ure. The connectivity of the aerogel is strongly increased by the dens ification under isostatic pressure and the power laws describing the e lastic constant evolution as a function of the bulk density lead to va lues 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.