10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Dynamics of Carbon Nanotube Aerogel Formation
Christian Hoecker, Bhalerao Ajinkya, Nikolaos Kateris, Jean de La Verpilliere, Brian Graves, ADAM M BOIES, University of Cambridge, University of Minnesota
Abstract Number: 1561 Working Group: Aerosol Physics
Abstract The underlying mechanisms behind the formation of gas-phase synthesized carbon nanotube (CNT) aerogels within a floating-catalyst chemical vapour deposition (FCCVD) reactor are still not well understood. The production of a macroscopic material from nano components, allows for the process to produce continuous CNT materials that are in great demand. The mass-produced self-assembled CNTs have superior thermal, electrical and mechanical properties making them valuable for many industrial applications.
While the material properties and applications of mass-produced CNT aerogels are well documented, the criteria required for gelation have not been discovered. Attempts have been made to describe CNT aerogel formation by relating the individual CNT growth to particle size distributions in the reactor. These, however, disregard fundamental features of 1-D material collisions, CNT restructuring and the dynamics within the FCCVD reactor.
In this presentation, we report on the dynamics of 1-D materials undergoing Brownian motion, collisions and re-orientation. By employing both molecular dynamic and Langevin dynamic modelling, we demonstrate that under typical conditions the time-scale for collision is much longer than the time-scale for re-orientation of the CNTs. Upon collision the CNTs re-orient to align with each other to form bundles (5-20 CNTs/bundle) due to Van der Waals forces. At a critical dimension, the CNT bundling ceases and long-range connections are maintained, thus allowing the formation of an aerogel.
Our work demonstrates the dynamics of both CNT collisions and bundling. Langevin dynamic results show that CNT collisions when accounting for both translation and rotation are similar to those found for non-dimensional aggregate collisions when only translation is accounted for. Molecular dynamics modelling demonstrates that re-orientation occurs at short timescales, but grows with CNT length and diameter. The methods for molecular dynamic modelling of re-orientation are discussed and approaches to extend the length of CNTs within a reasonable computational time are presented.
The resulting bundles and gelation theories are used to describe the aggregation in CNT aerosols which eventually leads to an aerogel formed of CNTs within synthesis reactors. These findings can then, for example, be related to the continuous gas phase process for the bulk production of CNTs, to identify the critical length and number concentration of CNTs required for gelation. Identifying the critical criteria for aerogelation of 1-D materials will potentially unlock the ability to produce novel self-assembled materials in high throughput reactors, unlocking the potential for industrial scale up.