Fractal carbon nanotube fibers with mesoporous crystalline structure

Abstract

Macroscopic fibres of carbon nanotubes are hierarchical structures combining long building blocks preferentially oriented along the fibre axis and a large porosity arising from the imperfect packing of bundles. Synchrotron small-angle X-ray scattering SAXS measurements show that such structure is a surface fractal with fractal dimension (Ds) of 2.5 for MWCNT fibres and 2.8 for SWCNT fibres. N2 adsorption measurements give similar values of 2.54 and 2.50, respectively. The fractal dimension and deviation from Porod's law are related to density fluctuations associated with the wide distribution of separations between CNTs. These fluctuations are also evident as diffuse wide-angle X-ray scattering (WAXS) from CNTs at distances above (Juan J. Vilatela)2 turbostratic separation. The structure of CNT fibres produced at different draw ratios is compared in terms of degree of orientation and characteristic lengths parallel and perpendicular to the fibre.Drawing not only increases alignment but also the fraction of graphitic planes forming coherent domains capable of taking part in stress transfer by shear; thus increasing both tensile modulus and strength. The invariant-normalized intensity of the (002) equatorial reflection thus takes the form of a degree of crystallinity closely related to tensile properties.

Small-Angle Scattering from Fractals: Differentiating between Various Types of Structures

“…It is well-known that in fractal-based materials, the structural hierarchy and the geometric symmetry within a single structure play an important role in determining their bulk and surface properties. Common applications of such materials include enhancing: the lithium storage performance of CuO nanomaterials with surface fractal characteristics [21], the rheological behavior of fractal-like aggregates in polymer nanocomposites [22], the compatibility and interfacial reactivity of high-performance layered silicate/epoxy nanocomposites [23], the tensile modulus and strength in carbon nanotube fibers with mesoporous crystalline structure [24] or the electrocatalytic activity, durability and stability in Sierpinski gasket-like Pt-Ag octahedral alloy nanocrystals [25]. In addition, the fractal structure [26], also gives rise to quantum effects in various materials such as in plasmonic structures with sub-nanometer gaps [27] or in organometal halide perovskite nanoplatelets [28].…”

Tensile properties of carbon nanotube fibres described by the fibrillar crystallite model

“…In this respect, we note that CNT fibres subjected to higher draw ratios have a greater fracture energy (( Figure 7b), whereas the fibrillar breakage model assumes this to be constant through a constant number of failing elemets. Our recent WAXS measurements on multifilament samples suggest that samples produced at higher draw ratios have a larger "degree of crystallinity", that is, a large fraction of graphitic planes at turbostratic separation in coherent domains [27], which might be responsible for the this increase in fracture energy and the small deviation from the predicted fracture enevelope. In Table 2 we compare the parameters extracted from the fibrillar crystallite analysis for different fibres, including our two types of CNT fibres, CF, high-performance polymer fibres and ductile polymer fibres [22,30].…”

“…The mechanical behavior of fibrous materials depends critically on their morphology [26]. In this regard, despite the complex hierarchical structure found in CNT fibres [27], their microstructure can be defined as fibrillar by noting that CNT bundles are essentially long fibrils well-aligned along the fibre axis, as shown in the electron micrograph in Fig.1 (a). It is precisely these fibrils which act as load-carrying elements and, therefore, their mechanical properties control to a large extent the final properties of the macroscopic fibre.…”

Enhanced Electro-Fenton Mineralization of Acid Orange 7 Using a Carbon Nanotube Fiber-Based Cathode

“…The carbon nanotube fiber electrode has a large porosity arising from the imperfect packing of nanotube bundles (Yue et al, 2017), as shown in Figure 1A. Yet, the CNT are also associated in long bundles, forming a highly conducting network of crystalline domains.…”