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.

Structural properties of organic aerogels were studied by ultra-small-angle X-ray scattering (USAXS) at the Synchrotron Radiation Laboratory HASYLAB at DESY, Hamburg, Germany. The organic aerogels were synthesized from the base-catalyzed sol-gel polymerization of resorcinol with formaldehyde (RF) followed by a supercritical drying process. RF aerogels are low-density materials with a solid matrix composed of interconnected colloidal-like particles. Scattering experiments were carried out using a crystal camera optimized for ultra-small-angle X-ray scattering with synchrotron radiation. The measured SAXS profiles revealed a scattering power depending upon synthetic conditions of the gels. RF aerogels were found to be homogeneous at length scales greater than 20 nm. From Guinier plots, radii of gyration R_g of 3-20 nm were computed. R_g appears to be a measure of the pore (cell) size. Although fractal silica aerogels show similar characteristics, fractal behavior of the organic aerogels is uncertain. These materials are best described as random aggregates of smooth colloidal like particles with open-cell porosity.

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 T_c = 304.2 K and a critical pressure p_c = 73.9 × 10⁵ Pa. In the high-temperature methanol process (T_c = 512.5 K and p_c = 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 versus the scattering vector q, intermediate slopes smaller than -4 in the Porod region as well as oscillations in an Iq⁴ 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.