Gas-Phase Dynamics\nof Bundle Formation from High-Aspect-Ratio\nCarbon Nanotubes

In\nfloating catalyst chemical vapor deposition (FCCVD),\nhigh-aspect-ratio\ncarbon nanotubes (CNTs) are produced in the gas phase at high number\nconcentrations and undergo collision and agglomeration, eventually\ngiving rise to a macroscale aerogel, enabling functional material\nforms such as fibers or mats to be obtained directly from the synthesis\nprocess. The self-assembly behavior between high-aspect-ratio CNTs\ndictates the resulting morphology at the nanoscale and subsequently\nthe bulk properties of the CNT product. Reorientation between CNTs\nafter collision is a critical step that results in bundle formation\nand precedes aerogel formation. However, it has been challenging to\nstudy the phenomenon with existing methods as it spans multiple time\nand length scales. In this study, a physics-based semi-analytical\nmodel was developed to study the gas-phase reorientation dynamics\nof high-aspect-ratio CNTs and their bundles, with ±10% accuracy\ncompared with mesoscale molecular dynamics simulations, but at <0.1%\nthe computational cost. It was revealed that the reorientation time\nscale is dictated by the interplay among the van der Waals potential,\ndrag, and the geometric configuration of CNTs upon collision. This\nthen allows the time scale of reorientation (i.e., bundle formation)\nto be compared with other gas-phase dynamics in a typical FCCVD reactor\nand offers new insights into the self-assembly behavior of 1D nanoparticles\nin the gas phase.