With their impressive individual properties, carbon nanotubes should form high-performance fibers. We explored the roles of nanotube length and structure, fiber density, and nanotube orientation in achieving optimum mechanical properties. We found that carbon nanotube fiber, spun directly and continuously from gas phase as an aerogel, combines high strength and high stiffness (axial elastic modulus), with an energy to breakage (toughness) considerably greater than that of any commercial high-strength fiber. Different levels of carbon nanotube orientation, fiber density, and mechanical properties can be achieved by drawing the aerogel at various winding rates. The mechanical data obtained demonstrate the considerable potential of carbon nanotube assemblies in the quest for maximal mechanical performance. The statistical aspects of the mechanical data reveal the deleterious effect of defects and indicate strategies for future work.

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.

A model for the strength of pure carbon nanotube (CNT) fibers is derived and parametrized using experimental data and computational simulations. The model points to the parameters of the subunits that must be optimized in order to produce improvements in the strength of the macroscopic CNT fiber, primarily nanotube length and shear strength between CNTs. Fractography analysis of the CNT fibers reveals a fibrous fracture surface and indicates that fiber strength originates from resistance to nanotube pull-out and is thus proportional to the nanotube-nanotube interface contact area and shear strength. The contact area between adjacent nanotubes is determined by their degree of polygonization or collapse, which in turn depends on their diameter and number of layers. We show that larger diameter tubes with fewer walls have a greater degree of contact, as determined by continuum elasticity theory, molecular mechanics, and image analysis of transmission electron micrographs. According to our model, the axial stress in the CNTs is built up by stress transfer between adjacent CNTs through shear and is thus proportional to CNT length, as supported by data in the literature for CNT fibers produced by different methods and research groups. Our CNT fibers have a yarn-like structure in that rather than being solid, they are made of a network of filament subunits. Indeed, the model is consistent with those developed for conventional yarn-like fibers.

A fiber made of carbon nanotubes is shown to resemble a conventional yarn in terms of its structure, composed of discrete fibrillar sub‐units, and its properties, such as high flexibility in bending, cutting resistance and 100% knot efficiency. Its combination of yarn‐like character and tensile properties in the high‐performance range make this CNT fiber an exceptional new material.

This review paper summarizes the current state-of-art and challenges for the future developments of fiber-reinforced composites for structural applications with multifunctional capabilities. After a brief analysis of the reasons of the successful incorporation of fiberreinforced composites in many different industrial sectors, the review analyzes three critical factors that will define the future of composites. The first one is the application of novel fiberdeposition and preforming techniques together with innovative liquid moulding strategies, which will be combined by optimization tools based on novel multiscale modelling approaches, so fiber-reinforced composites with optimized properties can be designed and manufactured for each application. In addition, composite applications will be enhanced by the incorporation of multifunctional capabilities. Among them, electrical conductivity, energy storage (structural supercapacitors and batteries) and energy harvesting (piezoelectric and solar energy) seem to be the most promising ones.

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.

Carbon nanotube (CNT) fibers consist of a network of highly oriented carbon nanotube bundles. This paper explores the ingress of liquids into the contiguous internal pores between the bundles using measurements of contact angles and changes in fiber dimensions. The resultant effects on the internal structure of the fiber have been examined by WAXS and SAXS. A series of time-resolved experiments measured the influence of the structural changes on the electrical resistivity of the fiber. All organic liquids tested rapidly wicked into the fiber to fill its internal void structure. The local regions in which the nanotube bundles are aggregated to give a bundle network were broken up by the liquid ingress. For the range of organic penetrants examined, the strength of the effects on structure and electrical resistivity was correlated, not only with the degree to which the liquid reduced the nanotube surface energy, but also with the Hansen affinity parameters. The fact that liquid environments influence the electrical performance of these fibers is of significance if they are to replace copper as power and signal conductors, with added implications regarding the possible ingress of external insulating materials, and possibly also sensing applications.

There is an ever-growing need to protect our environment by increasing energy efficiency and developing "clean" energy sources. These are global challenges, and their resolution is vital to our energy security. Although many conventional materials, such as metals, ceramics, and plastics, cannot fulfil all requirements for these new technologies, many material combinations can offer synergistic effects that create improved and even new properties. The implementation of nanocarbons, such as graphene and carbon nanotubes, into nanocomposites and, more recently, into the new class of hybrids, are very promising examples. In contrast to classical nanocomposites, where a low volume fraction of the carbon component is mixed into a polymer or ceramic matrix, hybrids are materials in which nanocarbon is coated with a thin layer of the functional compound, which introduces the interface as a powerful new parameter. Based on interfacial charge and energy transfer processes, nanocarbon hybrids have shown increased sensitivities in gas sensors, improved efficiencies in photovoltaics, superior activities in photocatalysts, and enhanced capacities in supercapacitors. This review compares the characteristics and potentials of both nanocarbon composites and hybrids, highlights recent developments in their synthesis and discusses key challenges for their use in various energy applications.