The densification of monolithic gels

Abstract

The densification of gels to glasses with composition SiO 2 and 8Al 2O 392SiO 2 has been studied as a function of heat treatment between 120 and 1200°C using Raman spectroscopy together with measurements of density and specific surface area. Monolithic dry gels are prepared at 120°C. In the case of SiO 2 these have a density of 1.39 g/cm 3, a specific surface area of 790 m 2/g and a mean pore radius of 10 Å. In the case of 8Al 2O 392SiO 2 gels these values are 0.76 g/cm 3, 535 m 2/g and 50 Å, respectively. The Raman spectra of monolithic dry SiO 2 gels, measured as a function of temperature, show that both surface and bulk SiOH groups are present. The relative concentration of these groups is temperature dependent. The total OH content decreases with increasing temperature. The combined results of Raman, density and specific surface area measurements show that during densification two, partly related, processes occur, viz. pore collapse and the condensation of surface SiOH groups to SiOSi bonds.