Chapter 6 - Multiscale Engineering of Carbon Nanotube Fibers

Abstract

Their unusual, complex hierarchical structure and wide range of applications continue to fuel the study of macroscopic fibers of carbon nanotubes (CNTs). Depending on the feature under study, CNT fibers resemble carbon fibers, staple yarns, high-performance polymer fibers, metals, or activated carbon. Their synthesis routes are intrinsically relatively simple and enable the production of continuous fibers with high-structural control. This chapter deals first with the direct chemical vapor deposition (CVD) spinning process, which comprises the growth of building blocks and assembly in one stage. It presents current strategies for molecular control in terms of CNT morphology, number of layers, and chiral angle. It shows the central role of group 16 element promoters to grow a CNT aerogel that can be directly spun from the gas phase and controlling CNT characteristics. A unique aspect of CNT fibers is the confluence of length scales in a single material, with around 106 individual CNTs per cross section and thus about 109 of them in a meter of sample. This complex structure requires many complementary characterization techniques to encompass different structural features depending on properties of interest. The second part of the chapter discusses methods to control the spatial arrangement of CNTs in fibers in the direct spinning method and associated techniques to probe the resulting hierarchical structure. Vast information about the identity of constituent CNTs is obtained with Raman spectroscopy and electron diffraction. From the bundle level upward, wide- and small-angle X-ray scattering (WAXS and SAXS, respectively) provides details of the orientation and coherence of elements in the fibers. Porosity can be studied by gas adsorption and SAXS, and the material can be treated as a surface fractal. Finally, emerging examples of structure-property relations for CNT fibers produced with high composition, and orientation control are reviewed. Oriented fibers can be treated as a continuum of network of crystallites, similar to a polymer fiber, with tensile properties dictated by the CNT orientation distribution function. The relation between structural features and electrochemical properties is also analyzed for samples with few-layer CNTs that thus enable access to their quantum capacitance, closely related to their low-dimensional joint density of electronic energy states.