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.
In the presence of macroscopic fibres of carbon nanotubes (CNT), various semicrystalline polymers are shown to present accelerated crystallisation through the formation of a transcrystalline (TC) layer perpendicular to the fibre axis. From differential scanning calorimetry, polarized optical microscopy and X-ray diffraction we establish this to be due to much faster nucleation rates at the fibre surface. The formation of a TC layers is demonstrated for polyvinyldene fluoride, isotactic polypropylene and poly(lactic acid) in spite of the large differences in their chemistry and structure unit cells, suggesting that epitaxy in terms of lattice type or size matching is not a prerequisite. For the three polymers as well as poly(ether ether ketone), the TC layer is identically oriented with the chain axis in the lamella parallel to the CNTs, as observed by wide and small angle X-ray scattering. These results point to polymer chain orientation at the point of adsorption and the formation of a mesomorphic layer as possible steps in the fast nucleation of oriented lamella, with wetting of the CNT fibre surface by the molten semi-crystalline polymer a key condition for heterogeneous nucleation to take place.
In this study, large‐area (10 cm2) samples of heterojunctions between zinc oxide and carbon nanotube (CNT) fibre arrays were synthesised. A two‐step hydrothermal treatment was used to grow Zn nanowires (NWs) in situ directly onto the CNT array. This resulted in a bulk heterojunction that consisted of a five‐micron layer of ZnO crystals in contact with the built‐in CNT fabric current collector, with preferential orientation of the ZnO c axis perpendicular to the CNT sheet. The electrical properties of the heterojunction corresponded to a Schottky junction with a barrier height of 0.26 eV and an ideality factor of 2.9. Photoconductivity measurements indicated charge transfer through the ZnO/CNT interface, thus leading to a gain factor of approximately 100 and a responsivity as high as 5000 mA w−1 under UV irradiation. The resulting ZnO/CNT fibre hybrid was both piezoresistive and piezoelectric under transverse compression with an attractive format for energy‐harvesting applications.
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