Densification behaviour of silica aerogels upon isothermal sintering

Isothermal sintering has been studied for a homologous series of supercritically dried base-catalyzed silica aerogels with different initial densities. The change in density and macroscopic viscosity has been recorded in situ using a dilatometer and a beam-bending viscosimeter. Changes of the nanostructural features of the aerogel network have been monitored by means of small angle X-ray scattering. The cylinder model introduced by Scherer is generally accepted to describe the sintering behaviour of bodies with moderate porosity, such as xerogels. In case of highly porous aerogels, however, preferential densification of large pores and little loss of specific surface area is observed, at least during the initial stages. For the samples under investigtion the increase in density and the corresponding variation of specific inner surface and viscosity are compared to the analytical predictions by Scherer, the scaling approach introduced by Sempéré et al. and the results obtained from numerical simulation of viscous flow.