Mesoscopic Distinct Element Method for Computational Design of Carbon Nanotube Materials

Abstract

Carbon Nanotubes (CNTs) are hollow molecular cylinders conceptually formed by rolling single or multiple layers of graphene into tubes. CNT materials have become an attractive research subject during the last decades owning to the superior mechanical and electronic properties of individual CNTs. Developing applications, such as structural materials, supercapacitors, batteries or nanomechanical devices, depend on our ability to understand, model, and design the structure and properties of realistic CNT assemblies. Toward this goal, here we have applied a recently developed mesoscale computational method, titled the mesoscopic distinct element method (MDEM) that makes it possible to simulate the formation, stability, and mechanics of CNT aggregates and ultrathin CNT films. We first combine experiments and distinct element method simulations to understand the stability of rings and rackets formed by single-walled carbon nanotubes assembled into ropes. The obtained agreement validates MDEM and indicates that the stability of the experimental aggregates can be largely explained by the competition between bending and van der Waals adhesion energies. Next, we have considered the geometry and internal packing in twisted CNT ropes. Compared to the state of the art, MDEM accounts in a computationally tractable manner for both the deformation of the fiber and the distributed van der Waals cohesive energy between fibers. These features enable us to investigate the torsional response in a new regime where the twisted rope develops packing rearrangements and aspect-ratio-dependent geometric nonlinearities, in agreement with phenomenological models. Finally, we have performed MDEM simulations and developed an atomic-scale picture of the CNT network stress relaxation. On this basis, we put forward the concept of mesoscale design by the addition of excluded-volume interactions. Silicon nanoparticles are integrated into the model and the nanoparticle-filled networks present superior stability and mechanical response relative to those of pure films. The approach opens new possibilities for tuning the network microstructure in a manner that is compatible with flexible electronics applications. As a distinct direction, MDEM was explored for modeling the mechanics of nanocrystalline particles. Simulations that rely on the fitting of the peak stress, strain, and failure mode on the experimental testing of Au and CdS hollow nanocrystalline particles illustrate the promising potential of MDEM for bridging the atomistic-scale simulations with experimental testing data.