Page 115 - Carbon Nanotube Fibres and Yarns
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Post-spinning treatments to carbon nanotube fibers    107


              600 times higher than that of the pure CNT fibers. Similarly, the Pd- and
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              Pt-coated CNT fibers showed high electrical conductivity of 2 × 10  and
                   3
              5 × 10  S/cm, respectively.
                 Instead of coating nanoparticles on the surface of the CNT fibers, Zhao
              et al. [37] introduced iodine atoms into the structure of double-walled CNT
              fibers (DWNT); the diameters were in the range of 2–3 nm. They reported
              the fabrication and doping of carbon nanotube cables with resistivity one
              order of magnitude closer to the resistivity of Cu than the predecessors.
              The superior conductivity was achieved by a synergistic effect between the
              unique structure of the CNTs and the rational design of processing and
              doping. The nanotubes aligned in one direction and the nanotube bun-
              dles were interconnected and formed into a continuous network. The small
              bundles and the nanotubes by themselves were several micrometers long.
              After the nanotubes were fabricated into the cable, the natural alignment of
              the nanotube “stocking” was retained, which turned out to be beneficial for
              the conductivity of the cable. Interestingly, the electrical conductivity of the
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              CNT fibers could reach up to 6.7 × 10  S/cm. Owing to the low density of
                                3
              the fibers (0.33 g/cm ), their specific conductivity was higher than that of
              copper and aluminum.

              6.4  Acid treatment

              Impurities in the CNT fibers can be reduced by optimizing their syn-
              thesis process [6, 38, 39] and/or adopting purification treatments [29–31,
              40, 41]. Due to their simplicity, cost efficiency, and scalability, oxidative
              purification in liquid phase or gas phase has been widely employed as a
              powerful posttreatment for purifying CNTs [42]. While the liquid-phase
              oxidation uses a base or acid solution, oxidation in gas phase uses air, oxy-
              gen, or other gases, and might be followed by acid leaching [43]. Although
              successful purification of CNT fibers by liquid-phase oxidation methods
              has been widely reported [29–31], studies on purifying CNT fibers by
              gas-phase oxidation is very limited, especially for CNT fibers spun by the
              floating catalyst method.

              6.4.1  Purification of CNT fibers
              The dissolution of CNTs in chlorosulfonic acid (HSO 3 Cl), the true solvent
              for CNTs [44], has drawn great attention owing to their ability to obtain
              well-controlled CNT morphologies [44] for production of different mac-
              roscopic CNT assemblies [1, 45]. While many studies have been conducted
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