Structural modifications of highly porous silica aerogels upon densification

Structural changes during isothermal sintering were studied for two base-catalyzed SiO₂ aerogels with initial densities ρ₁ = 122 and 256 kg m⁻³ by means of small-angle X-ray scattering (SAXS). The relevant structural parameters are the specific surface area, which is determined by the size of the compact primary particles, the mean diameter of the secondary particles, which represent the building blocks of the gel network, and finally the size of the macropores within the tenuous tertiary structure. The major finding of our investigations is the observation of two sintering processes with different time scales: a fast reduction of the size of the macropores and a slow increase in the diameter of the secondary particles as well as a relatively small decrease in the specific surface area. For the two samples investigated the changes of these parameters can be unequivocally correlated with the achieved density upon sintering – independent of the initial density.