褶皱与晶界耦合作用对石墨烯断裂行为的影响

由CVD方法制备的石墨烯含有大量的晶界,通常还带有许多褶皱,本文通过分子动力学方法研究了具有褶皱和晶界的石墨烯 平面拉伸断裂行为,结果显示,在垂直晶界方向,褶皱能够显著提高小角度双晶石墨烯的断裂应力,断裂应力增幅最 大约为50%,褶皱对断裂应力的影响随晶界角的增大减弱,导致双晶石墨烯断裂应力对晶界角不敏感,只略低于单晶石墨 烯,和实验结果完全吻合;在沿晶界方向,褶皱对双晶石墨烯断裂应力影响不明显. 另外,褶皱可以显著提高双晶石墨烯的断 裂应变,增幅最大约为100%. 增强机制归纳主要如下:通过面外变形,褶皱可以部分释放晶界5-7环中C---C键的预拉伸变形, 提高双晶石墨烯的断裂应力;褶皱可以降低相邻5-7环之间相互作用,导致断裂应力对晶界角不敏感;在拉伸作用下,褶皱被部分 拉平,这可以显著降低C---C键面内拉伸变形,导致断裂应变显著增大. 本研究为准确理解多晶石墨烯断裂行为提供重要帮助. 

COUPLING EFFECTS OF WRINKLES AND GRAIN BOUNDARY ON THE FRACTURE OF GRAPHENE1)

Graphene fabricated via chemical vapor deposition (CVD) is typically polycrystalline and also includes many wrinkles. The fracture of the polycrystalline graphene with wrinkles under an uniaxial tensile load is investigated via molecular dynamics simulations. With a tensile load perpendicular to the grain boundary, wrinkles can significantly increase the failure stress of bi-crystalline graphene with a small tilt angle, and the increase in the failure stress is up to around 50%. The wrinkle effect on the failure stress decreases with the increase of the tilt angle, resulting in that the failure stress of bi-crystalline graphene is insensitive to the tilt angle and slightly lower than that of pristine graphene, which agrees with the experimental results very well. With a tensile load along the grain boundary, the failure stress is insensitive to the wrinkle. In addition, wrinkles can significantly increase the failure strain, up to 100%. The influence mechanism can be described as follows: wrinkles will cause the out-of-plane deformation in graphene, resulting a partial release of the tensile pre-stress induced via the 5-7 rings of the grain boundary and consequently an increase in the failure stress of bi-crystalline graphene; the interaction between 5-7 rings of the grain boundary is eliminated, resulting that the failure stress is insensitive to the tilt angle; the flattening of wrinkles can significantly decrease the stretching ratio of C---C bonds, resulting an obvious increase in the failure strain. The present study provides a useful help to understand the fracture of graphene.