We report on the synthesis of kilometers of continuous macroscopic fibers made up of carbon nanotubes (CNT) of controlled number of layers, ranging from single-walled to multiwalled, tailored by the addition of sulfur as a catalyst promoter during chemical vapor deposition in the direct fiber spinning process. The progressive transition from single-walled through collapsed double-walled to multiwalled is clearly seen by an upshift in the 2D (G′) band and by other Raman spectra features. The increase in number of CNT layers and inner diameter results in a higher fiber macroscopic linear density and greater reaction yield (up to 9%). Through a combination of multiscale characterization techniques (X-ray photoelectron spectroscopy, organic elemental analysis, high-resolution transmission electron microscopy, thermogravimetric analysis, and synchrotron XRD) we establish the composition of the catalyst particles and position in the isothermal section of the C–Fe–S ternary diagram at 1400 °C. This helps explain the unusually low proportion of active catalyst particles in the direct spinning process (<0.1%) and the role of S in limiting C diffusion and resulting in catalyst particles not being in thermodynamic equilibrium with solid carbon, therefore producing graphitic edge growth instead of encapsulation. The increase in CNT layers is a consequence of particle coarsening and the ability of larger catalyst particles to accommodate more layers for the same composition.

We present a method to spin highly oriented continuous fibers of adjustable carbon nanotube (CNT) type, with mechanical properties in the high-performance range. By lowering the concentration of nanotubes in the gas phase, through either reduction of the precursor feed rate or increase in carrier gas flow rate, the density of entanglements is reduced and the CNT aerogel can thus be drawn (up to 18 draw ratio) and wound at fast rates (>50 m/min). This is achieved without affecting the synthesis process, as demonstrated by Raman spectroscopy, and implies that the parameters controlling composition in terms of CNT diameter and number of layers are decoupled from those fixing CNT orientation. Applying polymer fiber wet-spinning principles then, strong CNT fibers (1 GPa/SG) are produced under dilute conditions and high draw ratios, corresponding to highly aligned fibers (from wide- and small-angle X-ray scattering). This is demonstrated for fibers either made up of predominantly single-wall CNTs (SWCNTs) or predominantly multiwall CNTs (MWCNTs), which surprisingly have very similar tensile properties. Finally, we show that postspin densification has no substantial effect on either alignment or properties (mechanical and electrical). These results demonstrate a route to control CNT assembly and reinforce their potential as a high-performance fiber.

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We present evidence that high temperature CVD growth of SWNTs under conditions of continuous spinning of macroscopic fibers leads to an inherent predominance of high chiral angle CNTs, peaking at the armchair end. Raman, UV-vis-NIR absorption, and photoluminescence spectroscopy measurements show the prevalence of metallic SWNTs. The complete chiral angle distribution is obtained by electron diffraction of over 390 CNTs. It is biased towards high chiral angles and peaks at the armchair end (30°), in good agreement with the established atomistic models for SWNT growth from a liquid catalyst. Based on the Fe-C-S constituent binary and ternary phase diagrams, thermodynamic calculations of phase compositions from fast cooling and experimental evidence of a post-synthesis catalyst, the proposed thermodynamic path of the catalyst is to form a solid FCC Fe core and a liquid Fe-S shell. S in the outer liquid shell first stabilizes the edge of the nascent CNT, but once a graphitic wall forms it is rejected due to the high interfacial energy of the Fe-C-S alloy.