Giant Stretchability and Reversibility of Tightly Wound Helical Carbon Nanotubes

There is a surging interest in 3D graphitic nanostructures which possess outstanding properties enabling them to be prime candidates for a new generation of nanodevices and energy-absorbing materials. Here we study the stretching instability and reversibility of tightly wound helical carbon nanotubes (HCNTs) by atomistic simulations. The intercoil van der Waals (vdW) interaction-induced flattening of HCNT walls prior to loading is constrained by the defects coordinated for the curvature formation of helices. The HCNTs exhibit extensive stretchability in the range from 400% to 1000% as a result of two distinct deformation mechanisms depending on the HCNT size. For small HCNTs tremendous deformation is achieved by domino-type partial fracture events, whereas for large HCNTs this is accomplished by stepwise buckling of coils. The formation and fracture of edge-closed graphene ribbons occur at lower temperatures, while at elevated temperatures the highly distributed fracture realizes a phenomenal stretchability. The results of cyclic stretching-reversing simulations of large HCNTs display pronounced hysteresis loops, which produce large energy dissipation via full recovery of buckling and vdW bondings. This study provides physical insights into the origins of high ductility and superior reversibility of hybrid CNT structures.

三维石墨基纳米结构因具备优异性能，成为新一代纳米器件与吸能材料的核心候选者，相关研究热度持续攀升。本文通过原子模拟，研究了紧密缠绕螺旋碳纳米管（helical carbon nanotubes, HCNTs）的拉伸不稳定性与可逆性。加载前，螺旋间范德华（van der Waals, vdW）相互作用会诱导HCNT管壁发生扁平化，而调控螺旋曲率形成的缺陷会抑制这一现象。根据尺寸差异，HCNT可通过两种截然不同的变形机制实现400%至1000%的超大拉伸率：小尺寸HCNT通过多米诺型局部断裂实现大幅变形，而大尺寸HCNT则通过螺旋线圈的逐步屈曲完成拉伸。低温环境下会形成边缘闭合石墨烯带并伴随断裂，而在高温条件下，弥散分布的断裂模式可实现惊人的拉伸性能。大尺寸HCNT的循环拉伸-回复模拟结果显示出显著的滞后回线，通过屈曲与范德华键合的完全恢复实现大量能量耗散。本研究为碳纳米管复合结构的高延展性与优异可逆性的起源提供了物理层面的认知。