STRUCTURE AND DYNAMICS OF COLLOIDAL AEROGELS

Structural investigations of aerogels prepared by destabilization of colloidal particles of silica are used to compare determinations performed in direct space by electron microscopy with those in reciprocal space, obtained by small-angle neutron and X-ray scattering. The particle sizes measured by these different techniques are in excellent agreement. Several series of mutually self-similar samples, whose constitutive elements are spherical particles, have been prepared. The interconnected structure has a fractal geometry, with a fractal dimension D almost-equal-to 1.8. The vibrational modes of the particles, studied by low-frequency Raman scattering, indicate a velocity of sound in the particles close to that in vitreous silica. The elastic modulus of these materials, deduced from Brillouin scattering measurements, is found to be remarkably small.