The CVD process for the spinning of carbon nanotube (CNT) fibres combines the nucleation, growth and aggregation of CNTs in the form of an aerogel with fibre spinning into a single process step. The optimisation of the process requires agility in multi-dimensional parameter space, so one tends to find parameter ‘islands’ where spinning is possible, while exploration tends to follow ‘routes’ through this space. Here, we follow two such routes, one of which drastically improves fibre purity, the other changes the nature of the nanotubes comprising the fibres from multiwall to single wall. In the first case there is only a modest enhancement of the mechanical properties, but in the second a very considerable improvement is seen. In terms of the conditions required to make fibres comprising predominately single wall CNTs, the key factor appears to be the rigorous control of the sulphur addition, in trace quantities, coupled with the availability of carbon atoms at the earliest stage after injection, typically in the range 400–500 °C. A model is presented for the role of sulphur in floating catalysts CNT synthesis.

A protocol has been developed for the production of epoxy-based composites containing high-volume fractions of aligned carbon nanotubes. The nanotubes were fabricated as continuous fibres or aligned mats directly from the CVD reactor, in which they were synthesized. The block composites with highly aligned and tightly packed nanotube assemblies were prepared via epoxy resin infiltration, and their volume fraction, distribution, and internal porosity being analysed prior to mechanical testing. The samples were tested in both axial tension and three-point bending. The results show that the strength and stiffness enhancements were close to pro rata with the volume fraction of the carbon nanotubes added. The failure modes were distinctly different from those characteristic of the conventional aligned carbon fibre composites. The fracture surface showed considerable evidence of pull-out of bundles of (~50) nanotubes, but the pull-out appeared to involve the resin matrix which drew out along with the bundles. Subsidiary cracks were bridged by nanotube bundles giving structures reminiscent of crazes in glassy polymers, what constitutes the distinct toughness mechanism and higher resistance to the transverse cracks propagation.

The uniaxial orientational order in a macromolecular system is usually specified using the Hermans factor which is equivalent to the second moment of the system's orientation distribution function (ODF) expanded in terms of Legendre polynomials. In this work, we show that for aligned materials that are two‐dimensional (2D) or have a measurable 2D intensity distribution, such as carbon nanotube (CNT) textiles, the Hermans factor is not appropriate. The ODF must be expanded in terms of Chebyshev polynomials and therefore, its second moment is a better measure of orientation in 2D. We also demonstrate that both orientation parameters (Hermans in three dimensional (3D) and Chebyshev in 2D) depend not only on the respective full‐width‐at‐half‐maximum of the peaks in the ODF but also on the shape of the fitted functions. Most importantly, we demonstrate a method to rapidly estimate the Chebyshev orientation parameter from a sample's 2D Fourier power spectrum, using an analysis program written in Python which is available for open access. As validation examples, we use digital photographs of dry spaghetti as well as scanning electron microscopy images of direct‐spun carbon nanotube fibers, proving the technique's applicability to a wide variety of fibers and images.

Carbon nanotube (CNT) fibres, especially if perfect in terms of their purity and alignment,\ud are extremely anisotropic. With their high axial strength but ready slippage between the\ud CNTs, there is utmost difficulty in transferring uniformly any applied force. Finite element\ud analysis is used to predict the stress distribution in CNT fibres loaded by grips attached to\ud their surface, along with the resulting tensile stress–strain curves. This study demonstrates\ud that, in accordance with St Venant’s principle, very considerable length-to-diameter ratios\ud (103) are required before the stress becomes uniform across the fibre, even at low strains.\ud It is proposed that lack of perfect orientation and presence of carbonaceous material\ud between bundles greatly enhances the stress transfer, thus increasing the load the fibre\ud can carry before failing by shear. It is suggested that a very high strength batch of fibres previously\ud observed experimentally had an unusually high concentration of internal particles,\ud meaning that the pressure exerted by the grips would assist stress transfer between the\ud layers. We conclude that the strength of CNT fibres depends on the specific testing geometries\ud and that imperfections, whether by virtue of less-than-perfect orientation or of\ud embedded impurities, can act as major positive contributors to the observed strength