Mechanical Properties and Deformation Mechanisms of Graphene Foams with Bi-Modal Sheet Thickness by Coarse-Grained Molecular Dynamics Simulations.

Shenggui Liu,Mindong Lyu,Chao Wang

doi:10.3390/ma14195622

Abstract

Graphene foams (GrFs) have been widely used as structural and/or functional materials in many practical applications. They are always assembled by thin and thick graphene sheets with multiple thicknesses; however, the effect of this basic structural feature has been poorly understood by existing theoretical models. Here, we propose a coarse-grained bi-modal GrF model composed of a mixture of 1-layer flexible and 8-layer stiff sheets to study the mechanical properties and deformation mechanisms based on the mesoscopic model of graphene sheets (Model. Simul. Mater. Sci. Eng. 2011, 19, 54003). It is found that the modulus increases almost linearly with an increased proportion of 8-layer sheets, which is well explained by the mixture rule; the strength decreases first and reaches the minimum value at a critical proportion of stiff sheets ~30%, which is well explained by the analysis of structural connectivity and deformation energy of bi-modal GrFs. Furthermore, high-stress regions are mainly dispersed in thick sheets, while large-strain areas mainly locate in thin ones. Both of them have a highly uneven distribution in GrFs due to the intrinsic heterogeneity in both structures and the mechanical properties of sheets. Moreover, the elastic recovery ability of GrFs can be enhanced by adding more thick sheets. These results should be helpful for us to understand and further guide the design of advanced GrF-based materials.

Highlights

Choosing the system with v = 50% as an example, in a wide range of tensile strain, the tensile stress is higher than that of the system containing a small amount of stiff sheets with v = 30%; the snapshots of the system at the tensile strain of 0, 0.25, 0.5 and 0.75 in Figure 2b-ii show that the system shrinks very little perpendicular to the tensile direction compared to the graphene foams (GrFs) composed entirely of
In order to explain this critical phenomenon, we study the connectivity of stiff sheets in GrFs and find that the connectivity transforms from 0 to 1 as v > 30% as shown in Figure 2d, i.e., a complete bearing path by composed of entirely stiff sheets forms at this critical proportion, and the strength of the system would be instead determined by mechanical properties of stiff sheets
The modulus of GrFs increases almost linearly with the increased proportion of the 8-layer sheets, which can be well explained by the mixture rule

Summary

Introduction

Graphene is the thinnest-known sheet, composed of a single layer of carbon atoms, which has been used as nanoscale building blocks to fabricate a good deal of macroscale bulk materials, such as one-dimensional graphene fibers [1,2,3,4], two-dimensional graphene papers [5] and three-dimensional (3D) graphene foams (GrFs) [6,7,8,9,10,11,12,13,14,15,16]. This critical phenomenon is explained by the analysis of structural connectivity by stiff sheets and the deformation energy of bi-modal GrFs. we find that high-stress states always emerge in stiff sheets, while flexible sheets experience larger deformation.

Numerical Model of Bi-Modal GrFs and Methodology

Stress–Strain Curves in Uniaxial Tension

The Distribution and Evolution of Deformation Energy

Effect of the Proportion of

Conclusions