Page 116 - Carbon Nanotube Fibres and Yarns
P. 116

108   Carbon Nanotube Fibers and Yarns


          on the dissolution of CNT powders in superacid [1, 44, 45], few works on
          the dissolution of the direct-spun CNT fibers have been reported. An oxi-
          dative purification approach using air, followed by HCl washing, is applied
          to purifying CNT fibers spun by the floating catalyst method.
             Fig. 6.2 shows TEM, TGA, and Raman results of the as-spun and pu-
          rified CNT fibers produced by the floating catalyst method. Only CNT
          fibers spun from toluene source are used for the purification due to their
          high purity. As can be seen in Fig. 6.2A, the as-spun CNT fibers mainly
          consist of double-walled nanotubes (DWNTs) with an average diameter
          of 5.5 ± 0.4 nm. Iron impurities encapsulated by graphitic layers can be
          observed in the fiber structure (Fig. 6.2B). After applying the purification
          treatment, almost no iron impurities are observed in the CNT structure
          shown in Fig. 6.2C, suggesting that the CNT fibers are much cleaner.
             In TGA analysis of CNTs, the weight loss below 400°C usually corre-
          sponds to the removal of amorphous and disordered carbon from the sam-
          ple [31, 46]. Generally speaking, CNTs start to be oxidized at above 400°C.
          The TGA result shown in Fig. 6.2E indicates that carbonaceous impurities
          in the purified CNT fibers are below 1% and the purification treatment
          reduces their iron impurities from 11.9% to 4.2%. The results are supported
          by a remarkable temperature upshift (50°C) of the CNT oxidative decom-
          position after the treatment (Fig. 6.2E). Due to the presence of the iron
          impurities, the activation energy of the CNTs is lowered and their parasitic
          oxidation is catalyzed [46]. Therefore, the reduction of the iron impurities
          improves the thermal stability of the CNT fibers after the purification treat-
          ment. These findings are in good agreement with the enhanced thermal
          stability of the purified CNT films reported by Lin et al. [43].
             The findings suggest that the purification treatment using air and HCl
          leaching can effectively reduce the iron and carbonaceous impurities from
          the direct-spun CNT fibers. The iron impurities in the as-spun CNT
          fibers could not be removed by simple acidic washing since carbon shells
          encapsulate them. However, these impurities catalyze the oxidation of
          the protective carbon shells, resulting in their lower oxidation resistance
          [42, 43]. As the oxidation resistances between the CNTs, amorphous car-
          bon, and multishell carbon nanocapsules are different, the catalyst impuri-
          ties are exposed and can be dissolved by HCl. Therefore, the purification
          treatment reduces the impurity content in the CNT fibers. However, due
          to the highly packed structures of the CNT fibers, not all CNTs are ex-
          posed to air for oxidation during the purification process. Therefore, a
          small amount of catalyst impurities are still left in the fiber structure [46].
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