Mechanics of Nanostructured Porous Silica Aerogel Resulting from Molecular Dynamics Simulations

Abstract

Silica aerogels are nanostructured, highly porous solids which have, compared to other soft materials, special mechanical properties, such as extremely low densities. In the present work, the mechanical properties of silica aerogels have been studied with molecular dynamics (MD) simulations. The aerogel model of 192 000 atoms was created with different densities by direct expansion of β-cristobalite and subjected to series of thermal treatments. Because of the high number of atoms and improved modeling procedure, the proposed model was more stable and showed significant improvement in the smoothness of the resulting stress-strain curves in comparison to previous models. Resulting Poisson's ratio values for silica aerogels lie between 0.18 and 0.21. The elasticity moduli display a power law dependence on the density, with the exponent estimated to be 3.25 ± 0.1. These results are in excellent agreement with reported experimental as well as computational values. Two different deformation scenarios have been discussed. Under tension, the low-density aerogels were more ductile while the denser ones behaved rather brittle. In the compression simulations of low-density aerogels, deformation occurred without significant increase in stress. However, for high densities, atoms offer a higher resistance to the deformation, resulting in a more stiff response and an early densification. The relationship between different mechanical parameters has been found in the cyclic loading simulations of silica aerogels with different densities. The residual strain grows linearly with the applied strain (≥0.16) and can be approximated by a phenomenological relation ϵ = 1.09ϵ - 0.12. The dissipation energy also varies with the compressive strain according to a power law with an exponent of 2.31 ± 0.07. Moreover, the tangent modulus under cyclic loading varies exponentially with the compressive strain. The results of the study pave the way toward multiscale modeling of silica as well as reinforced silica aerogels.

Nanocompression of secondary particles of silica aerogel

“…This is consistent with typical stress-strain curves of highly porous materials that exhibit substantial hardening by densification [28]. This result is also consistent with the MD simulation results of Patil et al [9] on silica aerogels with 280 kg.m −3 , which exhibit similar strain hardening levels up to large compressive strains ( ≥ 0.7). Note that the proposed fit only matches satisfactorily the indentation curves for δ R ≥ 0.06 (see inset of Figure 2a).…”

“…Molecular dynamics (MD) simulations are also a powerful tool to investigate these properties. However, past simulations focusing on porous SiO 2 were only carried at the length-scale of a few tenths of nanometers [9,10,11], with too small pore sizes when compared to the 10 nm range observed in experiments. These simulations offer interesting insights in the inner structure and the mechanical properties of secondary particles but cannot fully reproduce their peculiar heterogeneous microstructure.…”

On the Molecular to Continuum Modeling of Fiber‐Reinforced Composites

“…This potential is well suited for modeling of silica because it takes into account the pairwise interactions, the energy associated with the bonding angle and the orientation of three atoms. In our previous work, the creation of the all‐atom silica aerogel model has been discussed in detail. Here, only a brief description is presented.…”

“…However, the model approach may be applicable to many such composites having an isotropic orientation of fibers. The silica aerogel matrix is modeled based on our previous work . Double‐wall carbon nanotubes (DWCNTs) are modeled as short fibers dispersed in the aerogel matrix.…”

“…The silica aerogel matrix is modeled based on our previous work. [10] Double-wall carbon nanotubes (DWCNTs) are modeled as short fibers dispersed in the aerogel matrix. To this end, first, molecular dynamics (MD) simulations are presented.…”