Structure and mechanical properties of silica aerogels and xerogels modeled by molecular dynamics simulation

In this article a study of the structural and mechanical properties of porous silica is presented. The procedure to prepare the samples consists in expanding single crystals of beta-cristobalite to reach the desired density followed by a thermal treatment. The resulting porous structures have densities in the range from 0.23 to 2.2 g/cm(3). The structure of the samples is studied by obtaining the fractal dimension using two different methods, one based on the pair distribution function and the other one based on the simulation of a scattering experiment. The values of the fractal dimension were found to be in good agreement with previously published data from experiments and comparable computer simulations. The mechanical properties, namely elastic modulus and strength, are studied through the simulation of a tension test. The elastic modulus and the strength relate to density by a power law characterized by exponents of 3.11 +/- 0.21 and 2.53 +/- 0.15 respectively. A comparison of that data to previously published data is included. The results proved that the direct expansion, coupled with thermal processing of the sample, leads to systems suitable to investigate the structure and the mechanical properties of silica aerogels. (C) 2010 Elsevier B.V. All rights reserved.