Computer Simulation of Compressive Failure in Silica Aerogels
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1060-LL06-03
Computer Simulation of Compressive Failure in Silica Aerogels Brian S. Good Materials and Structures Division, NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, OH, 44135 ABSTRACT Historically, the low thermal conductivity of silica aerogels has made them of interest to the aerospace community for applications such as cryotank insulation. However, recent advances in the application of conformal polymer coatings to these gels have made them significantly stronger, and potentially useful as lightweight structural materials. In this work, we perform multiscale computer simulations to investigate the compressive strength and failure behavior of silica aerogels. The gels' nanostructure is simulated via the diffusion limited cluster aggregation (DLCA) process, which produces fractal aggregates that are structurally similar to experimentally observed gels. The largest distinct feature of the clusters is the so-called secondary particle, typically tens of nm in diameter, which is composed of primary particles of amorphous silica an order of magnitude smaller. The secondary particles are connected by amorphous silica bridges that are typically smaller in diameter than the particles they connect. We investigate compressive failure via the application of a uniaxial compressive strain to the DLCA clusters. In computing the energetics of the compression, the detailed structure of the secondary particles is ignored, and the interaction among secondary particles is described by a Morse pair potential parameterized such that the potential range is much smaller than the secondary particle size; an angular potential contribution is included in some of the simulations as well. The Morse and angular parameters are obtained by atomistic simulation of models of the interparticle bridges, with the compressive and bending behavior of these bridges modeled via molecular statics. We consider the energetics of compression and compressive failure, and compare qualitative features of low-and high-density gel failure. INTRODUCTION The low thermal conductivity of aerogels has made them of interest for a variety of applications [1-3]. Recent developments, notably the application of conformal polymer coatings to the gels [4], and the inclusion of fibers [5] have resulted in gels that are significantly stronger than simple silica gels, offering promise as lightweight structural materials. We have previously developed multiscale models for gel thermal conductivity [6] and gel tensile failure [7]. In this paper, we describe a study of the compressive failure of silica aerogels, based on our multiscale failure model. The model is built on computer simulations using a modified diffusion-limited cluster aggregation (DLCA) scheme [8] to generate model gel structures, with failure energetics computed using a multiscale molecular statics approach.
STRUCTURAL MODEL Silica aerogels exhibit multiple ranges of structure. Small “primary” particles of amorphous silica, a few nm in diameter, form larger “secondary” particles, tens of nm
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