Numerical study of the dynamic properties of silica aerogels

We have developed a realistic model for silica aerogels based on a diffusio n-limited cluster-cluster aggregation model in three dimensions with a prim ary particle represented by a sample of vitreous silica given by molecular dynamics simulations. Using the spectral moments method, with more than 30 000 computed moments, we calculated the densities of states and the dynamic structure factors of a simulated sample of silica aerogels of different ma croscopic densities. We reproduce the vibration spectra experimentally obta ined on basic silica, with a fracton region composed of two contributions a ssociated with stretching and bond-bending modes respectively. The spectral dimensions describing these contributions are close to the measured values . The analysis of our numerical results in the fight of the scaling argumen t is reported.