DENSIFICATION BEHAVIOR OF SILICA AEROGELS UPON ISOTHERMAL SINTERING

Isothermal sintering has been studied for a homologous series of super critically dried base-catalyzed silica aerogels with different initial densities. The change in density and macroscopic viscosity has been r ecorded in situ using a dilatometer and a beam-bending viscosimeter. C hanges of the nanostructural features of the aerogel network have been monitored by means of small angle X-ray scattering. The cylinder mode l introduced by Scherer is generally accepted to describe the sinterin g behaviour of bodies with moderate porosity, such as xerogels. In cas e of highly porous aerogels, however, preferential densification of la rge pores and little loss of specific surface area is observed, at lea st during the initial stages. For the samples under investigtion the i ncrease in density and the corresponding variation of specific inner s urface and viscosity are compared to the analytical predictions by Sch erer, the scaling approach introduced by Sempere et al. and the result s obtained from numerical simulation of viscous flow.