Optimizing Structural and Mechanical Properties of Coiled Carbon Nanotubes with NSGA-II and Reactive Molecular Dynamics Simulation

Coiled carbon nanotubes (CCNTs) have increasingly become a vital factor in the new generation of nanodevices and energy-absorbing materials due to their outstanding properties. Here, the multiobjective optimization of CCNTs is applied to assess their mechanical properties. The best trade-off between conflicting mechanical properties (e.g., yield stress and yield strain) is demonstrated and the optimization of the geometry enables us to find the astonishing CCNTs with a stretchability of 400%. These structures have been recognized for the first time in the field. We derived several highly accurate analytical equations for the yield stress and yield strain by the implementation of multiobjective optimization and fitting a theoretical model to the results of molecular dynamics (MD) simulations. The optimized structures are highly resilient because of two distinct deformation mechanisms depending on the dimensions of CCNTs. For small CCNTs, extraordinary extensibility is mainly contributed by buckling and nanohinge-like deformation with maintaining the inner coil diameter. On the other hand, for large CCNTs, this is accomplished by the creation of a straight CNT-like structure in the inner-edge of the CCNT with a helical graphene ribbon twisted around it. Our work represents an important advance in the design of CCNT based mechanical nanodevices.

卷曲碳纳米管（coiled carbon nanotubes，CCNTs）因其优异的性能，已日益成为新一代纳米器件与吸能材料的核心材料。本文通过对CCNTs开展多目标优化以表征其力学性能：实现了相互制约的力学性能（如屈服应力与屈服应变）之间的最优权衡，并通过几何优化成功获得了性能优异、伸长率达400%的CCNTs结构，这类结构在该领域尚属首次被报道。本文通过开展多目标优化，并将理论模型拟合分子动力学（molecular dynamics，MD）模拟结果，推导得到了若干高精度的屈服应力与屈服应变解析方程。优化后的结构具备极高的回弹性，这源于两种取决于CCNTs尺寸的独特变形机制：对于小尺寸CCNTs，其优异的延展性主要源于在保持内部线圈直径不变的情况下发生的屈曲与类纳米铰链变形；而对于大尺寸CCNTs，该优异性能则通过在CCNTs内边缘形成直碳纳米管（carbon nanotube，CNT）状结构，并将螺旋石墨烯带缠绕于其上实现。本研究为基于CCNTs的力学纳米器件设计领域带来了重要进展。