Page 183 - Carbon Nanotube Fibres and Yarns

174 Carbon Nanotube Fibers and Yarns

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(PAN)-based carbon fibers 7–60 W m K in the longitudinal direction
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and 0.5–1.2 W m K in the transverse direction, respectively [101].
The thermal conductivity of a MWNT forest-drawn web was reported
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to be 50 W m K in the parallel direction and the thermal conductivity
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of its yarn was 26 W m K [77]. The nearly two orders of magnitude
difference between the single tubes and the bulk materials suggests that
thermal transport is largely dominated by the numerous highly resistive
thermal junctions between tubes in the CNT bulk materials.
Behabtu et al. [92] reported that solution-spun CNT fibers had an
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average thermal conductivity of 380 ± 15 W m K , which increased to
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635 W m K after iodine doping. Gspann et al. [102] produced CNT
fibers from the floating catalyst CVD method, which were treated by a 5%
stretch during solvent evaporation to improve nanotube alignment. This
process resulted in a yarn with a room temperature thermal conductivity
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as high as 770 ± 10 W m K . Koziol et al. [103] used a Veeco explorer
AFM thermal probe setup to measure the thermal conductivity of as-spun
CNT fibers produced by the floating catalyst CVD method and found
that the thermal conductivity was the highest around room temperature
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(1255 ± 317 W m K ) and then decreased with increasing probe tempera-
ture. These values are about 1/3 of that of single CNTs and are much higher
than high thermal conductivity metals such as copper and gold.
7.6 Outlook for CNT fiber strength

An often asked question is whether the properties of the CNT fibers and
yarns can be substantially further improved. As discussed in Chapter 8, a
number of excellent essays [104–106] attempted to answer this question
using different models of organizational geometries, mechanical behaviors,
and load transfer mechanisms of the CNTs. Here, we take a survey on what
have already been achieved in the utilization of intrinsic properties of poly-
mers in textile fibers to cast a light on the prospects of future commercial
CNT fibers and yarns.
Table 7.4 compares the theoretical values of tensile properties for poly-
mers and the corresponding values for commercial fibers made from these
polymers. The theoretical specific strength (tenacity) and modulus, as summa-
rized by Hongu et al. [107], were derived from ideal polymers comprising of
fully orientated and infinitely long molecules. The theoretical values given in
traditional unit g/d (gram per denier) in the reference have been converted
to N/tex. Note that the theoretical values are converted using theoretical