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].