Page 118 - Carbon Nanotube Fibres and Yarns

110 Carbon Nanotube Fibers and Yarns

although they have better oxidation resistance than the amorphous carbon
[46]. The purification treatment might also decrease the number of CNTs
and left surviving CNTs with damage (Fig. 6.2D), resulting in a higher
defect density [42]. These findings are consistent with the results of the
increasing defects on the structure of CNT films [43] and CNT powders
[46] purified by air oxidation treatment. Therefore, the oxidative treatment
should be carefully designed and optimized to balance the impurity removal
requirements, CNT structure preservation, and enhanced performance of
CNT fibers for desired applications.
The CNT length distribution was estimated by using measured iso-
tropic cloud point, CNT diameter measured by TEM and viscosity-
average aspect ratio under the assumption of a log-normal distribution
[47]. The average CNT length was estimated to be 2 μm and about 90%
of the CNTs were shorter than 6 μm. The results were in good agreement
with the aforementioned formation of the CNT tactoids and the required
length of the short F-Actin tactoids reported by Oakes et al. [48]. The
length of the CNTs constituting the fibers was still longer than that of
many commercial SWNTs and DWNTs such as HiPco 183.6 (1.51 μm),
HiPco 188.3 (0.29 μm), UniDym OE (1.92 μm), and SWeNT CG300
(0.71 μm) [47].
The CNTs were likely shortened during purification treatment by ox-
idation from their ends. Therefore, the average CNT length in the as-spun
fibers might be longer than 2 μm. Furthermore, the CNT length and CNT
aspect ratio (length/diameter ratio) of the fibers can be improved by op-
timizing the oxidative purification procedure [42, 43] or improving the
synthesis process by controlling the iron catalyst size to reduce the CNT
diameters or employing different carbon sources and synthesis temperatures
to enhance CNT growth [6, 7, 49].

6.4.2 Effects of acidization on mechanical properties
of CNT fibers

The tensile performance of CNT fibers can be improved by CNT surface
modification through acid treatment [50]. Meng et al. immersed CNT fibers
spun from CNT arrays in HNO 3 (16 M) for several hours to modify the
CNT surface by introducing various functional groups, including hydroxyl
(–OH), methyl (–CH 3 )/methylene (–CH 2 –), and carbonyl (–C=O) [50].
The surface modification improved the load transfer between the CNTs
by enhancing the inter-tube interaction and interfacial shear property. The
changes were reflected in the fiber’s mechanical property with 50% increase