Elasticity and strength of silica aerogels: A molecular dynamics study on large volumes

The nanoporous structure and the mechanical properties of silica aerogels are studied by molecular dynamics simulations on large samples. For the first time, atomistic simulations are able to reproduce a pore size distribution, centered on 10 nm, comparable to experimental values. Using an unprecedented combination for silica aerogels of large volumes, large strains, and relatively small strain-rates, direct evidence of their peculiar mechanical behavior at the nanoscale is provided, from elasticity to fracture. The surface stress that silica ligaments experience produces a significant tension-compression asymmetry and an unusual discontinuity in the Poisson effect. The ductility of highly porous silica aerogels arises from an interplay between surface stress and a significant amount of reorganization in ligaments. Taking advantage of the large volumes accessible here, the very heterogeneous nature of low-density silica aerogels is reproduced, with an impact both on their elasticity and on their strength. In particular, a clear dependence between tensile strength and sample volume is uncovered, which opens perspectives for the elaboration of multi-scale models. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.