Carbon Nanotube Alignment within Macrostructures

Abstract

The enhancement of alignment in carbon nanotube (CNT) assemblies is an important challenge in scaling up their exceptional properties from the nano- to the macroscopic level. In this work, theoretical and computational studies were performed to understand how CNT alignment is measured, how it impacts the electrical and mechanical properties of CNT materials and how it can be enhanced during manufacturing by application of electric fields. We derived an inverse projection method for the facile measurement of three-dimensional orientation distribution functions from image data of general rod-like objects, including CNTs. The method establishes an exact mathematical relationship between the Chebyshev and Hermans order parameters, two popular metrics for quantifying alignment in 3D and 2D. The mechanics of CNTs interacting via van der Waals forces were simulated using a novel high-performance implementation of a mesoscopic interaction potential. For systems of two CNTs, bundling was shown to occur in two distinct modes: central rotation (slow) and zipping (fast). Zipping was found to occur nearly at the speed of sound, explaining the experimentally observed existence of bundles in bulk CNT structures. Large-scale CNT network structures were subjected to simulated tensile testing with a new mesoscopic friction model, which showed that the elastic modulus and tensile strength of CNT nanofilms increase exponentially with Chebyshev order parameter. In order to investigate electrical properties, a large tight-binding study of pairwise CNT contacts of different diameters and chiral angles was performed and showed that small contact angles favour inter-tube conductance with minimal backscattering. Registry effects were shown to be important only when the registry angle was less than 15°, indicating that materials containing wellaligned and predominantly armchair and zig-zag CNTs should have the best electrical properties. To investigate the effect of electric fields on CNT alignment, a statistical field theory for worm-like chains with aligning terms was derived and validated using Monte Carlo simulations. The theoretical model showed that static electric fields fail to substantially align CNTs due to thermally induced bending fluctuations. Alternating fields, on the other hand, were shown to induce currents in the CNT, in turn creating a Lorentz pressure which stiffens the CNT. The stiffening effect was shown to be strong enough to overcome the limitations of static fields when the frequency of the applied field was at least in the MHz regime, matching experimental observations. In conclusion, we demonstrated that alignment is crucial in the upscale of CNT macromaterials and can be facilitated by application of alternating electric fields.