Investigation of the Bulk Modulus of Silica Aerogel Using Molecular Dynamics Simulations of a Coarse-Grained Model

• Published in: The journal of physical chemistry. B Vol. 117; no. 23; pp. 7095 - 7105
• Main Authors: Ferreiro-Rangel, Carlos A, Gelb, Lev D
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 13-06-2013
• Subjects: Applied sciences; Bulk modulus; Compressing; Computer simulation; Condensed matter: structure, mechanical and thermal properties; Density; Elasticity, elastic constants; Exact sciences and technology; Fluctuation; Mechanical and acoustical properties of condensed matter; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Mechanical properties of solids; Metals. Metallurgy; Physics; Power law; Silica aerogels; Simulation; Particle size; Fluctuations; Experimental data; Ageing; Molecular dynamics method; Theoretical study; Mechanical properties; Silica; Relaxation; Pore size; Experimental result; Fractal dimension; Numerical simulation; Metric; Spherical particle; Microstructure; Power law; Distributed parameter system; Bulk modulus
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp3128737

Structural and mechanical properties of silica aerogels are studied using a flexible coarse-grained model and a variety of simulation techniques. The model, introduced in a previous study ( J. Phys. Chem. C 2007, 111, 15792−15802 ), consists of spherical “primary” gel particles that interact through weak nonbonded forces and through microscopically motivated interparticle bonds that may break and form during the simulations. Aerogel models are prepared using a three-stage protocol consisting of separate simulations of gelation, aging, and a final relaxation during which no further bond formation is permitted. Models of varying particle size, density, and size dispersity are considered. These are characterized in terms of fractal dimensions and pore size distributions, and generally good agreement with experimental data is obtained for these metrics. The bulk moduli of these materials are studied in detail. Two different techniques for obtaining the bulk modulus are considered, fluctuation analysis and direct compression/expansion simulations. We find that the fluctuation result can be subject to systematic error due to coupling with the simulation barostat but, if performed carefully, yields results equivalent with those of compression/expansion experiments. The dependence of the bulk modulus on density follows a power law with an exponent between 3.00 and 3.15, in agreement with reported experimental results. The best correlate for the bulk modulus appears to be the volumetric bond density, on which there is also a power law dependence. Polydisperse models exhibit lower bulk moduli than comparable monodisperse models, which is due to lower bond densities in the polydisperse materials.