A computational study of the structure and physical properties of sol -gel derived porous silica.

Abstract

This dissertation endeavors to provide a comprehensive analysis of structure-property relationships in nano-porous materials, using silica aerogels as a model. Key to this study was the generation of realistic structures. Therefore, molecular dynamics simulations based on a reactive three-body potential with charge-transfer were used to reproduce the sol-gel condensation process that underlies the formation of these nano-porous gels. First we characterize the geometry of the simulated gel structure. The inherent self-similarity in these structures was established by evaluating the fractal dimension <italic>d_f</italic> from geometric correlations. Comparing the fractal dimensions of condensed gels with those of porous structures produced by rupturing dense silica glass, it was found that the fractal dimension is lower for the reacted gels than for the ruptured silica, indicating a wider pore size distribution for the former material. Supercritical drying of aerogels was modeled by a gradual extraction of water from the system with simultaneous relaxation of pressure, and found to have a negligible impact on <italic> d_f</italic>. The ruptured silica systems, on the other hand, undergo significant changes in density and fractal dimension upon pressure relaxation. The degree of branching in these disordered structures was measured in terms of the connectivity dimension: the ratio of the fractal dimension of the minimum path spanning the network to the fractal dimension of the entire structure. Unlike the fractal dimension <italic>d_f</italic>, the connectivity dimension is similar for the reacted and ruptured structures, suggesting that the degree of branching is independent of the generating process. Next, to study the effect of the chemical environment on gel structure, the sol-gel condensation process was simulated in an aqueous environment with varying water-to-silicon ratio <italic>r.</italic> A high value of <italic> r</italic> deters the condensation process, possibly by promoting the reverse (hydrolysis) reaction. Simulations reveal three distinct growth regimes in gel formation, depending on the system density and the water-to-silicon ratio. These regimes result in different structures, including (i) compact silica clusters with radius of gyration 5--17 A, (ii) well-percolated network structures, and (iii) branched clusters of widely varying size. Finally, a detailed investigation of the mechanical behavior of reacted gels and ruptured porous silica was undertaken. The structural stability of the condensed gels, as revealed by the change in their fractal dimension during supercritical drying, is strongest for the percolated network structures of regime (ii). The evolution of the bulk modulus and Young's modulus of the ruptured silica with density is well described by power-law scaling. (Abstract shortened by UMI.)