Page 154 - Carbon Nanotube Fibres and Yarns

Carbon nanotube yarn structures and properties 147

Fig. 7.9 Yarn diameter and density as a function of die diameter, based on the data from
Ref. [30]. (Source: K. Sugano, M. Kurata, H. Kawada, Evaluation of mechanical properties of
untwisted carbon nanotube yarn for application to composite materials, Carbon 78 (2014)
356–365.)

7.1.3.4 Liquid-densified fibers
Twistless liquid-densified fibers often demonstrate irregular cross-section
shape and the shape usually changes along the fiber length. This makes it
difficult to estimate the fiber porosity and the nanotube packing density.
Liquid-densified yarns show more uniform nanotube packing density in
the yarn cross section than twisted yarns.
Qiu et al. [31] characterized diameters of the nanotube bundles and the
inter-bundle pores using the longitudinal section SEM images of as-spun fibers
(Fig. 7.10A, B, and E). The distribution of the bundle diameters was found to
be similar to the diameter observed for inter-bundle pore diameters (Fig. 7.10D
and E). Fig. 7.10F shows that the distribution of bundle size peaked at about
20–30 nm, while that of the pores reached a maximum around 30–40 nm.
Cho et al. [32] measured nanotube distribution in the cross section
of acetone-densified CNT yarns drawn directly from a floating catalyst
CVD furnace (Fig. 7.11 and Table 7.1). The as-spun acetone-densified
3
yarn had a density of 0.24 g/cm and a porosity of 0.84, which indi-
cates a rather low densification in comparison with the twisted yarns
in Fig. 7.5. When the acetone-densified yarn was further treated with
solvent 1-methyl-2-pyrrolidinone (NMP) or chlorosulfonic acid (CSA),
the yarn density was more than doubled (Table 7.1) and the yarn porosity
decreased to the level comparable to the twisted yarns with a 40-degree
twist angle [14]. Nanotube flattening was found with the NMP and CSA