Ostwald ripening of cobalt precipitates in silica aerogels? An ultra-small-angle X-ray scattering study

Monolithic silica aerogels with radial symmetry were synthesized by supercritical drying, doped to 2% and 10% with cobalt, and reduced with hydrogen. All samples were investigated with ultra-small-angle X-ray scattering. The nondoped aerogels have three populations of scatterers with radii of gyration of about 10, 40 and 60 - 70 Angstrom. The doped aerogels show an additional structure with a radius of gyration ranging from 1050 to 3000 Angstrom. This structure causes intensity oscillations, thus revealing a relatively narrow size distribution. Scattering curves of the 10%-doped aerogels fitted well to a Lifshitz-Slyozov-Wagner particle size distribution, thus revealing that Ostwald ripening might have occurred during aerogel preparation. The same range also shows differences depending on whether the samples were reduced, or in their as-prepared condition. Scattering curves obtained from the cylinder-axis region were different from the scattering curves obtained from the sample boundary, indicating a process-dependent skin effect.