Structural modifications of highly porous silica aerogels upon densification

Abstract

Structural changes during isothermal sintering were studied for two base-catalyzed SiO2 aerogels with initial densities p~ = 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.

Densification behaviour of silica aerogels upon isothermal sintering

“…Using nitrogen sorption van Nordstrand et al [4] observed in the early fifties a dominant shrinking of macropores and only a small decrease in specific surface area during the beginning of the sintering process. This result has been verified by our group using small angle X-ray scattering to probe the nanostructure [5,6]. Hereby, the specific surface as a function of the pore volume has been found to be the decisive parameter, which easily allows to decide between different model scenarios.…”

“…Assuming arbitrary pore size distributions it is useful to plot the specific surface versus the ratio pore volume over solid volume (Ve/Vs = ~b -1 -1) [5]. In this representation, uniform shrinkage of pores of all size shows up in a linear relationship; a steeper decrease indicates preferential sintering of micropores and vice versa.…”

Effects upon Nitrogen Sorption Analysis in Aerogels

“…While the surface areas extracted from NAD generally are consistent with data derived via other methods (e.g., SAXS (8) and TEM (9)), the total pore volume significantly failed to be fully detected for samples with porosities above 85 to 90% (10). Several mechanisms have been proposed to account for the discrepancy between the total specific pore volume V pore,total as calculated from bulk (ρ B ) and skeletal (ρ S ) densities of the sample (V pore,total = 1/ρ B − 1/ρ S ) and the one determined via NAD:…”

Porous Glasses, Binary Glasses, and Composite Glasses from Aerogels

“…The isothermal densification kinetics of aerogels have been found to fit the Scherer' model (cylinder model) [6,17,18], with viscosity as the only fitting parameter. This model describes viscous flow sintering of amorphous material over a wide range of porosity (0-95%).…”