Page 60 - Carbon Nanotube Fibres and Yarns

52 Carbon Nanotube Fibers and Yarns

(A) (B)

Fig. 3.12 CNT alignment in fibers spun at different winding rates: (A) winding rate of
8 m/min and (B) winding rate of 55 m/min. (Reproduced with permission from Alemán B,
Reguero V, Mas B, Vilatela JJ. Strong carbon nanotube fibers by drawing inspiration from
polymer fiber spinning. ACS Nano (2015).)

The CNT bundle alignment significantly influences the measured me-
chanical properties. Increasing the winding rate can lead to a higher degree
of alignment [22, 48, 52]. Compared to a winding rate of 8 m/min, the
higher winding rate of 55 m/min increased the fiber tensile strength from
0.3 to 1 N/tex, and modulus from 5 to 40 N/tex (Fig. 3.12). This increase is
more than what is expected of the effect caused just by reduced fiber linear
density. The increase in winding rate is accompanied by a reduced concen-
tration of CNTs in the gas phase which reduces CNT entanglements [48].
In another study, with a winding rate increase from 5 to 20 m/min, fiber
tensile strength and modulus increased from 0.5 to 2 N/tex and from 10 to
80 N/tex, respectively [52]. Both improved CNT alignment and finer fiber
contribute to the substantial increase in fiber strength. Gapann et al. [62]
modeled the alignment effect on the thread strength, and suggested that
perfect aligned bundles are inefficient for shear stress transfer compared to
CNT fiber with less perfect orientation and with the presence of amor-
phous carbonaceous coating and impurity clusters.
Densification could improve the properties of the CNT fiber and sheet.
Acetone spraying merges CNT bundles, improves the bundle alignment,
and enhances their packing efficacy [63]. Aleman et al. [48] reported that
although spraying acetone could reduce the fiber diameter by a factor of
about 11, the alignment, electrical resistance, and specific tensile strength of
the resulting fiber stayed almost unchanged. They concluded that the moder-
ate densification could not bring CNT bundles sufficiently close to improve
charge and load transfer. Post-processing methods, such as stretching and
rolling, could also be used to provide stronger densification effect. Stretching
helps alignment and enhances the anisotropy of the material [64, 65].