Laboratory for Atomistic and Molecular Mechanics (LAMM), Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge 02139, MA, USA.
Nanoscale. 2017 Sep 21;9(36):13477-13484. doi: 10.1039/c7nr04455k.
Graphene has excellent mechanical, thermal and electrical properties. However, there are limitations in utilizing monolayers of graphene for mechanical engineering applications due to its atomic thickness and lack of bending rigidity. Synthesizing graphene aerogels or foams is one approach to utilize graphene in three-dimensional bulk forms. Recently, graphene with a gyroidal geometry has been proposed. A gyroid is a triply periodic minimal surface that allows graphene sheets to form a three-dimensional structure. Its light weight and high mechanical strength suggests that the graphene that constitutes this geometry can synergistically contribute to the mechanics of the bulk material. However, it is not clear whether gyroid graphene can preserve the high thermal conductivity of pristine graphene sheets. Here, we investigate the thermal conductivities of gyroid graphene with different porosities by using full-atom molecular dynamics simulations. In contrast to its excellent mechanical properties, we find that the thermal conductivity of gyroid graphene is more than 300 times lower than that of pristine graphene, with a bulk density of only about one-third of that of graphene. We derive a scaling law showing that the thermal conductivity does not vary much with different bulk densities, which contrasts the behavior of conventional porous materials. Our analysis shows that the poor thermal conductivity of gyroid graphene can be attributed to defects and curvatures of graphene, which increase with the density, resulting in the reduction of a phonon mean free path by phonon scattering. Our study shows that three-dimensional porous graphene has potential that may be utilized in designing new lightweight structural materials with low and density-insensitive thermal properties and superior mechanical strength.
石墨烯具有优异的机械、热和电性能。然而，由于其原子厚度和缺乏弯曲刚性，单层石墨烯在机械工程应用中存在局限性。合成石墨烯气凝胶或泡沫是将石墨烯用于三维块状形式的一种方法。最近，提出了具有回转体几何形状的石墨烯。回转体是一种三重周期性的最小曲面，允许石墨烯片形成三维结构。其重量轻和机械强度高表明构成这种几何形状的石墨烯可以协同促进块状材料的力学性能。然而，目前尚不清楚回转体石墨烯是否可以保持原始石墨烯片的高热导率。在这里，我们通过使用全原子分子动力学模拟研究了具有不同孔隙率的回转体石墨烯的热导率。与优异的机械性能相反，我们发现回转体石墨烯的热导率比原始石墨烯低 300 多倍，其体密度仅为石墨烯的三分之一左右。我们得出了一个标度律，表明热导率与不同的体密度变化不大，这与传统多孔材料的行为形成对比。我们的分析表明，回转体石墨烯的低热导率可以归因于石墨烯的缺陷和曲率，这些缺陷和曲率随密度的增加而增加，导致声子平均自由程减小，从而导致声子散射。我们的研究表明，三维多孔石墨烯具有潜力，可以用于设计具有低密度和密度不敏感热性能以及卓越机械强度的新型轻质结构材料。
