Structural and Mechanical Properties of Silica and Hybrid Aerogels and Xerogels Studied Using Molecular Dynamics Simulations

Abstract

The structural and mechanical properties of silica and hybrid aerogels and xerogels are studied using Molecular Dynamics simulations. Hybrid samples are created by adding methyl groups (CH 3) to the silica structure. Silica samples are generated expanding a dense silica glass in a simulation box of volume chosen to produce a porous sample of a specified density. The expanded glass is thermally treated using a combination of heating and cooling processes. The heating process increases the kinetic energy of the atoms in the system, which facilitates their movement throughout the system. The cooling process reduces the kinetic energy of the atoms and helps locking them in place forming the desired porous structure. A similar procedure is used to create the hybrid samples, but CH 3 −SiO 2 molecules are randomly inserted in the expanded structure before the thermal treatment of the sample. The structures are characterized estimating the fractal dimension of the samples. It is calculated using its relationship with the decay of the Pair Distribution Function of the samples in the intermediate range of structural arrangement. The fractal dimension of the samples used for this study compares favorably to the values found reported in experimental studies, which validates the preparation processes used here. The mechanical properties of the samples are obtained by modeling a tension test performed by stretching the sample along one direction. The stress vs. strain relationship is estimated for all the samples. The results reproduce trends similar to those reported from physical experiments. The computational models show that the addition of methyl groups reduces the stiffness of the structure allowing it to stretch more easily, and increasing the toughness of the samples. From the stress-strain plots is concluded that as the amount of methyl groups in the structure is increased also does the compliance of the sample, effectively making the nature of the material similar to a rubber-like material. These results are in agreement with physical experimental findings which show that adding methyl groups to silica allows compressing the samples more than 50% without destroying them. To my family, my life support.