Brillouin scattering study of sintered and compressed aerogels

Elastic properties of densified aerogels were investigated by Brillouin light scattering in silica aerogels densified by viscous sintering and isostatic compression. Sound velocity and acoustic attenuation characterize the evolution of the elastic properties vs. the bulk density for the two sets of samples. Elastic modulus increases strongly during sintering while the attenuation alpha decreases, which is coherent with a larger connectivity in the solid network. Viscous flow sintering creates new siloxane bonds, eliminates pores and as expected, the aerogel stiffens. Compressed aerogel has a completely different behavior. In the low pressure range, elastic modulus decreases and alpha rises. This is attributed to breakage of links between clusters during compression. Weakening of the aerogels is the consequence of large strain of the solid network. For higher pressure, the density increase is accompanied by stiffening, suggesting that condensation occurs more than link breakage. (C) 1998 Elsevier Science B.V. All rights reserved.