10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Aerosol Particle Restructuring: Molecular Dynamics of High Aspect Ratio Carbon Nanotubes
NIKOLAOS KATERIS, Adam M Boies, University of Cambridge
Abstract Number: 698 Working Group: Materials Synthesis
Abstract The Floating Catalyst Continuous Vapour Deposition (FCCVD) process for carbon nanotube (CNT) production is extensively used, but the kinetics within the process are poorly understood. The process involves injection of methane, ferrocene, and thiophene into a hydrogen filled reactor, downstream of which CNT aerogel is continuously drawn. Individual CNTs grow on catalyst nanoparticles, which form after the decomposition of thiophene and ferrocene. The CNTs then collide and form bundles. Further collisions of the bundles lead to the formation of the aerogel. This work aims to explain the mechanism of aerogel formation.
The collision frequency of CNTs has been investigated through stochastic simulations [1]. These simulations allow the calculation of the collision rates of straight and curved CNTs. The specific purpose of this work is the determination of the realignment rate of CNTs and CNT bundles, in order to examine the aerogel formation criteria. This is achieved with a spatially adaptive mesoscale molecular model.
The molecular dynamics after collision are governed by intermolecular van der Waals forces and the time evolution of the molecule arrangement is calculated on LAMMPS. The mesoscale model performs Langevin Dynamics calculations within a simulation box. A bead-spring model is employed to model the axial and bending stiffness of CNTs. Intermolecular forces are modelled using a Lennard-Jones potential, with its parameters scaled according to Markus Buehler's 2006 paper [2], in order to account for the discrete nature of the beads.
It is observed that the bead spacing affects bundling time and using the conventional value of bead spacing equal to CNT diameter is excessively computationally expensive. Therefore, in order to simulate bundling of long CNTs, a mesoscale model of spatially adaptive resolution is developed. This novel approach to bead-spring modelling involves maintaining fine beads in areas where intermolecular forces are dominant and modelling the rest of the molecule with coarse beads. The simulation is regularly paused and the CNT length is repopulated by beads of resolution that provides the necessary accuracy in critical regions.
These modelling techniques enable the calculation of bundling rates of individual CNTs and CNT bundles. For instance, re-alignment time of two individual CNTs is in the order of 1 ns (for lengths of order of 100 nm), whereas their collision time is of the order of up to 100 ns, for a wide range of CNT lengths. Long CNTs and CNT bundles have slower re-alignment rates, which approach collision rates. This re-alignment data is vital for the understanding of the aerogel formation process. Multiple CNTs can also be placed in a simulation box to collide and form bundles, and eventually an aerogel, thus giving information about the critical values that enable aerogelation to occur.
References [1] Thaseem Thajudeen, Ranganathan Gopalakrishnan, and Christopher J. Hogan. The collision rate of nonspherical particles and aggregates for all diffusive knudsen numbers. Aerosol Sci. Technol, 46(11):1174–1186, 2012. [2] Markus J. Buehler. Mesoscale modeling of mechanics of carbon nanotubes: Self-assembly, selffolding, and fracture. J. Mater. Res., 21(11):28552869, 2006.