Granular structure and fractal domains of silica aerogels

Observation of silica aerogels by scanning electron microscopy and transmission electron microscopy are presented. They are supplemented by information on pore volume and pore size distribution determined by thermoporometry. Comparison with small-angle neutron scattering allows precise characterization of the fractal domains. The authors identify three classes of structure. Primary chains, 30-50 AA in diameter, form the substructures of grains whose size varies from 100 to 500 AA depending on density. The grains form large-scale aggregates, leading to pores on scales exceeding 1000 AA. Since the system is demonstrated to be fractal between the chain and grain dimensions, they re-examine the phonon-fracton crossover and the fracton dimensionality determined by Brillouin scattering and Raman scattering.