Page 150 - Carbon Nanotube Fibres and Yarns

Carbon nanotube yarn structures and properties 143

Unlike conventional textile fibers, van der Waals forces between nano-
tubes are of high importance to the strength of unbonded CNT yarn. The
magnitude of the van der Waals force depends strongly on the distance
between the nanotubes, which is a function of nanotube packing fraction
in the yarn.

7.1.3.1 Twisted yarns
Generally speaking, CNTs are not uniformly distributed in CNT yarns.
The local nanotube packing fraction in a twisted CNT yarn can vary
considerably. Sears et al. [25] showed that nanotube packing fraction de-
creases from the center to the peripheral in twisted CNT yarns, as shown
in the SEM images taken from focused ion beam (FIB) sectioned yarns in
Fig. 7.4A. When studying the porous structure of CNT yarns, two types
of voids may be distinguished, i.e., voids between nanotubes in the same
bundle and voids between the bundles, as shown in Fig. 7.4B and C. Due to

Fig. 7.4 Nanotube packing density distribution in twisted CNT yarns. (A) SEM image of
CNT yarn cross section, showing radial density distribution [25]. (B, C) SEM images show-
ing CNT bundles and pores between CNT bundles [26]. (Panel (A) reprinted with permis-
sion from K. Sears, C. Skourtis, K. Atkinson, N. Finn, W. Humphries, Focused ion beam milling
of carbon nanotube yarns to study the relationship between structure and strength, Carbon
48 (15) (2010) 4450–4456.; Panels (B and C) reprinted with permission from D. Zhang, M.
Miao, H. Niu, Z. Wei, Core-spun carbon nanotube yarnsupercapacitors for wearable elec-
tronic textiles, ACS Nano 8 (5) (2014) 4571–4579.)