High-temperature and low-temperature supercritical drying of aerogels – structural investigations with SAXS

Abstract

Base-catalyzed silica aerogels are composed of particles with a mean size of about 5 nm, which form a chain-like porous network. Up to now it has been assumed that supercritical drying (SCD) of highly porous gels in autoclaves leaves the structure nearly unchanged. Small-angle X-ray scattering (SAXS) measurements provide evidence that this is only true for the low-temperature CO2 drying process with a critical temperature /',=304.2 K and a critical pressure p,--73.9 × 105 Pa. In the high-temperature methanol process (7", = 512.5 K and p, = 80.9 × 105 Pa) with remaining water and catalyst, however, structural changes are introduced. The SAXS measurements can be explained by a narrowing of the particle-size distribution during the heating period of the autoclave process. In a double-logarithmic representation of the scattered intensity I vs the scattering vector q, intermediate slopes smaller than -4 in the Porod region as well as oscillations in an lq 4 VS q plot appear. On the contrary, SCD hardly affects the particle structure of acid-catalyzed gels and, if the basic solvent is exchanged for pure methanol, of base-catalyzed gels.

Thermal aging of silica gels in water

“…2(b)) that can be interpreted in terms of a slight Ostwald ripening process in accordance with the conclusions of Haereid et al [3]. The effect is very similar to the one observed for base-catalyzed silica aerogels derived from supercritical drying with respect to methanol (at T > 240°C) [12,22]; here the heat treatment takes place while heating the gel to a temperature above the critical point. Unfortunately, the SAXS data at high q values are too sensitive to the correction of the scattering for the cuvette to allow for reliable conclusions.…”

“…(a) Scattering intensity I times q 2 (I · q 2 ) and (b) I · q 4 vs. scattering vector q. The curves are normalized by a factor (Porod constant [22]) that takes into account the different external surface areas and densities of the microporous primary particles ( Table 2). Table 2 Structural and elastic properties of the aerogels q (kg m À3 ) Sound velocity [13] Nitrogen sorption analysis SAXS E (MPa) C S ext (m 2 g À1 ) S micro (m 2 g À1 ) r BT (nm) [ micropore contribution, the C-parameters for the treated und the untreated aerogel would be 73 and 120, respectively.…”

Switching silica aerogels from transparent to opaque

“…1, insert) is very similar to the features found for silica aerogels derived from high-temperature supercritical drying (e.g., [18]), where Ostwald ripening is thought to result in a narrowing of the size distribution of primary particles and a smoothening of the necks between adjacent entities. According to the SAXS data, the primary particles with a diameter of 8-10 nm form small clusters (Table 1) with about three to four times their size.…”

Structural modifications of highly porous silica aerogels upon densification

“…The intermediate slope in the q region between 0.06 and 0.12 A-lis found to be steeper than -4; this could be a hint that the blurred first scattering minimum of equal-shaped gel particles with nearly uniform size is observed. This narrowing of the particle-size distribution might be due to dissolution and reprecipitation phenomena of silica during the hot alcohol autoclave process as described, for example, by Wang, Emmerling, Tappert, Spormann, Fricke & Haubold (1991). Between 0.02 and 0.1 A ~ the sample with the higher density shows a power-law exponent of -2.3 compared to -2-0 for the lower-density specimen.…”