Page 168 - Carbon Nanotube Fibres and Yarns

Carbon nanotube yarn structures and properties 159

including the hollow center of the CNT. Its engineering strength, i.e., when
the hollowness is excluded, is lower. Some nanotubes can collapse into
ribbons and their engineering strength can approach the ultimate (intrin-
sic) strength as the void in the center diminishes. So far there has not been
an experimentally established direct relationship between the strength of
continuously made CNT yarn and the strength of its constituent nanotubes.
Hill et al. [52] measured the strength of CNT bundles taken from
3-mm-high CNT forests. The gauge length for the bundle test was 1 mm
and the two ends of the bundle were secured using epoxy resin. It was
estimated that 50% of the nanotubes in the bundle was continuous across
the gauge length. The number of CNTs in the reported bundle test was in
the order of a million, which is the same order of magnitude for a typical
CNT yarn. The measured bundle strength had a high reliance on bundle
size and method of densification. The highest specific strength and modulus
were 1.8 and 88.7 N/tex, respectively, achieved on toluene-densified CNT
bundles [52]. The measured bundle strength was thus one or two orders of
magnitude lower than the values measured on single nanotubes reported by
other researchers. Unfortunately, the tensile properties of single nanotubes
used in Hill’s bundle test were not measured, making it impossible to estab-
lish a relationship between single nanotube strength and bundle strength.
The reported CNT bundle strength of 1.8 N/tex is about twice the tenac-
ity of typical CNT yarns reported by many researchers. This bundle-to-yarn
strength ratio is similar to that between cotton fiber bundle and spun yarn
from the textile industry [51].

7.2.3.2 Nanotube length
Several control experiments pointed to an increasing trend of yarn strength
when longer CNTs were used. For example, Zhang et al. [13] reported that
when CNT length was increased from 0.3 to 0.65 mm, the yarn strength
improved from 0.3 to 0.85 GPa. When the CNT length was further in-
creased to 1 mm, the average yarn strength increased to 1.9 GPa [53]. Fang
et al. [41] grew CNT forests to a series of heights between 0.15 and 0.4 mm
to spin yarns of similar diameter (5–7 μm) and surface twist angle (17–
21 degrees). The yarn strength was found to increase from 310 MPa for the
shortest CNTs (0.15 mm) to about 420 MPa for the longest (0.4 mm). A less
consistent increasing trend of yarn strength with increasing CNT length
from 0.8 to 2.1 mm was reported by Ghemes et al. [54]. A substantially
low yarn strength at 1.4 mm was attributed to a thicker yarn sample. This
generally positive influence of nanotube length on yarn strength shown