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.