Large Scale Porous Structure in Gels and Relationship with Their Plastic Behavior

Abstract

Silica gels (classical aerogels and composite aerogels) have been prepared by classical gelation and addition of silica soot in the gelifying solution before gelation. Due to the aggregation mechanisms, these structures are characterized by a fractal organization. The fractal network previously described in the literature (1-100 nm) which results from the aggregation mechanism of the organosiloxane is affected by the addition of the silica soots. Ultra Small Angle X-ray Scattering (USAXS) experiments (done at ESRF) shows that besides the fractal network built by the organosiloxane, the silica soots are forming another porous structure at a higher scale. The mechanical properties seem to be dependent on this large pore structure. Under isostatic pressure, aerogels display an irreversible shrinkage caused by plastic deformation. As a consequence of this plastic shrinkage it is possible to densify, by the pore collapse tending towards the silica glass. The densification mechanism is different from the one obtained by a sintering at high temperature. The pore collapse mechanism is favored by the large pores structure of the composite aerogels, in contrast to viscous sintering.