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ing process. The challenges in making nanotube fibers/yarns with desirable prop-
erties, according to recent work, are in achieving the maximum possible align-
ment of the nanotubes or their bundles within the yarn, increasing the nanotube
packing density within the yarn and enhancing the internal bonding among the
nanotubes. Following this approach then produced MWCNT single-ply yarns and
5-ply yarns, which were made by over twisting five single yarns and subsequent-
ly allowing them to relax until reaching a torque-balanced state. First, the CVD
synthesized MWCNTs are 300 mL long and 10 nm in diameter and they formed
about 20 nm diameter bundles in a nanotube forest. Simultaneous draw and twist
of the bundles produced 10 mL diameter single yarn. Besides single-ply and 5-ply
yarns, Many Researchers have also demonstrated the fabrication of other multiply
yarns which have been used in 3-D braiding process as well as Z-yarns in 3-D
weaving processes. It is currently possible to produce tens of meters of continu-
ous MWCNT yarns. It is reported that no visible damage to the nanotube yarns
is imparted by the braiding process and the 3-D braids are very fine, extremely
flexible, hold sufficient load, and are well suited for the use in any other textile
formation process, or directly as reinforcement for composites. The reported elas-
tic and strength properties of carbon nanotube composites so far are rather low in
comparison with conventional continuous carbon fiber composites. It is believed
that the properties can be substantially improved if the processing methods and
structures are optimized. Others also studied the electrical conductivity of CNT
yarns, 3-D braids and their composites. It is noted that 3-D textile composites,
including 3-D woven and 3-D braided materials, combine high in-plane mechani-
cal properties with substantially improved transverse strength, damage tolerance
and impact resistance. However, even relatively small volume content of the out-
of-plane fibers results in considerable increase of interstitial resin pockets, which
contributes to lower in-plane properties. Utilizing fine CNT yarns could dramati-
cally reduce the through the-thickness yarn size while still sufficiently improving
the composite transverse properties. Furthermore, they found that the electrical
conductivity of 3-D hybrid composites are many times greater than that of com-
monly produced nano composites made from low volume fraction dispersion of
relatively short CNTs in epoxy.
1.1.6.7.3 CARBON NANOTUBE FIBERS
The superb mechanical and physical properties of individual CNTs provide
the impetus for researchers in developing high-performance continuous fibers
basedupon carbon nanotubes. Unlike in the case of carbon fibers, the processing
of CNT fibers does not require the cross-linking step of the precursor structures.
As summarized results, the leading approaches for the production of CNT fibers
are (1) spinning from a lyotropic liquid crystalline suspension of nanotubes, in a
wet-spinning process similar to that used for polymeric fibers such as aramids,
(2) spinning from MWCNTs previously grown on a substrate as “semi aligned”
carpets and (3) spinning directly from an aerogel of SWCNTs and MWCNTs as
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