13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Characterization of Mechanical and Physical Properties of Silica Aerogels Using Molecular Dynamics Simulation Jingjie Yeo1, Jincheng Lei2, Zishun Liu 2,*, Teng Yong Ng1 1 School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 (Singapore) 2 International Centre for Applied Mechanics, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong University, Xi’an, 710049, China * Corresponding author: zishunliu@mail.xjtu.edu.cn Abstract Silica aerogels are nanoporous ultralight materials with extreme materials properties: the highest specific surface area of any material; slowest speed of sound through any solid material; excellent thermal insulators. Using molecular dynamics and the Tersoff potential, re-parameterized to simulate silicon dioxide, we have modeled the porous structures of silica aerogels. Our study shows that this potential is more suitable for modeling thermal properties in amorphous silica than the widely-used BKS potential. Increasing densities of aerogel samples are generated using an expanding and quenching process. Analysis of the fractal dimensions indicates a good fit with previous theoretical and experimental results. Each sample’s thermal conductivity is determined using reverse non-equilibrium molecular dynamics. Results indicate that the power-law fit of our data reflects the power-law exponent found in experimental studies. The results are also of the same order of magnitude as experimental bulk aerogel, but they are consistently higher. Analysis of the pore size distribution shows, firstly, that such a disparity is due to limited pore sizes represented in a finite nano-sized system, and secondly, that increasing system length scales leads to corresponding increases in the pore sizes that can be represented. Furthermore, we attempt to determine the mechanical properties, such as Young’s modulus of aerogel by using the same potential. We can conclude from all these results that our model is very suitable for modeling the mechanical and physical properties of bulk silica aerogel, and that an appropriate system length scale can be chosen to suit the pore size regime of interest. Keywords molecular dynamics, silica aerogel, nanoporous ultralight materials. 1. Introduction Silica aerogel is an exceptionally percolated material [1], made from various sol-gel processes and supercritical drying. Some silica aerogel’s properties include very low density and extremely high thermal resistance [2, 3]. Previous experimental studies characterized silica aerogel’s thermal conductivity and transport mechanisms, and found that the solid thermal conductivity scaled with density via a power law: λs = Cρ α (1) where α was approximately 1.6 for densities between 0.3 to 1.0g/cm3 [4]. Previous numerical studies characterizing the properties of silica aerogel [5, 6] include accurate reproductions of the porous and fractal nature of silica aerogels [7-9]. Murillo et al. [9] devised an expanding, heating and quenching method to model silica aerogels, obtaining good fits for the elastic moduli in comparison with experimental results. Ng et al. [6] employed negative pressure rupturing with the van Beest, Kramer and van Santen (BKS) potential [10, 11], and determined their thermal conductivities. It was found that the power-law fit of the data corresponds to experimental bulk sintered aerogel. In this study, new methods are used to attain a closer fit of the thermal conductivity, in comparison with experimental data. We have found that the Tersoff potential, re-parameterized to model silicon dioxide, is more suitable than the BKS potential in predicting the thermal conductivity of amorphous silica. The solid thermal conductivity of silica aerogels are determined using reverse non-equilibrium MD (RNEMD) simulations, by generating the porous samples using the method by
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