Computer Simulation of Fracture in Aerogels
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0978-GG05-03
Computer Simulation of Fracture in Aerogels Brian Good Materials and Structures Division, NASA Glenn Research Center, 21000 Brookpark Road, MS 106-5, Cleveland, OH, 44135 ABSTRACT Aerogels are of interest to the aerospace community primarily for their thermal properties, notably their low thermal conductivities. While the gels are typically fragile, recent advances in the application of conformal polymer layers to these gels has made them potentially useful as lightweight structural materials as well. In this work, we investigate the strength and fracture behavior of silica aerogels using a molecular staticsbased computer simulation technique. The gels' structure is simulated via a Diffusion Limited Cluster Aggregation (DLCA) algorithm, which produces fractal structures representing experimentally observed aggregates of so-called secondary particles, themselves composed of amorphous silica primary particles an order of magnitude smaller. We have performed multi-length-scale simulations of fracture in silica aerogels, in which the interaction between two secondary particles is assumed to be described by a Morse pair potential parameterized such that the potential range is much smaller than the secondary particle size. These Morse parameters are obtained by atomistic simulation of models of the experimentally-observed amorphous silica "bridges," with the fracture behavior of these bridges modeled via molecular statics using a Morse/Coulomb potential for silica. We consider the energetics of the fracture, and compare qualitative features of low-and high-density gel fracture. INTRODUCTION Aerogels are low-density materials possessing properties that have maintained interest for a wide variety of applications [1-3]. In particular, the low thermal conductivities characteristic of such gels have led to the aerospace community’s interest in these materials as lightweight thermal insulation. Aerogels are also notoriously fragile, and to develop thermal insulation that is mechanically robust, researchers in our laboratory have developed a method for applying conformal polymer coatings aerogels that greatly improve the gels' strength while minimally impacting their insulating properties [4]. In order to provide a microscopic understanding of the mechanical behavior of the gels, and to provide a predictive tool of use in their further development, we have constructed a multiscale model for fracture in silica aerogels. The model is built on computer simulations using a modified diffusionlimited cluster aggregation (DLCA) scheme [5] for gel cluster structure, and an interparticle interaction potential based on results of atomistic simulations. STRUCTURAL MODEL Silica aerogels are known to exhibit a low-density pearl-necklace structure that consists of tangled strands of roughly spherical “secondary” particles. These particles
themselves exhibit complex internal structure, and are usually considered to consist of smaller “primary” particles of amorphous silica of less than bulk density. For the current work, we ign
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