Page 146 - Carbon Nanotube Fibres and Yarns

Carbon nanotube yarn structures and properties 139

inherent shortcomings. Diameter measurement taken from random points
on a short yarn specimen is often not representative of the yarn sample. It
is especially problematic if the diameter measurement was taken on the
fracture end of a specimen, outside of the tensile test specimen, or from
another part of the yarn sample. Many CNT yarns and fibers have irregular
cross-sectional shapes (e.g., yarns produced by liquid densification) and thus
it is difficult to determine their diameter.
Laser diffraction method is widely used for monitoring wire diameter
[12]. This method has been used for measuring CNT yarn diameter [13–
15]. A laser diffraction system with multiple laser beams may be mounted
on a tensile tester to measure the diameter of the tensile specimen at several
points during tensile testing [14, 15] to allow the calculation of instanta-
neous stress and Poisson’s ratio.
Fig. 7.2 shows how the diameter of twisted CNT yarns is affected by
the strain applied to the yarns. The low twist yarn had an initial diameter
of 33.1 μm. At 0.0536 axial strain, the yarn diameter contracted to 20.9 μm.
This gives a huge Poisson’s ratio of 6.8, which is more than 20 times higher
than common solid materials (Poisson's ratio is about 0.3). A very large
disparity can appear when yarn diameter is used for calculating yarn tensile
stress [14]. On the other hand, the diameter change of highly twisted yarns
is much less dramatic, giving a Poisson’s ratio less than 1.
Textile yarns, especially yarns spun from staple fibers, do not have
well-defined cross-section boundaries although they are often approximated
to a circular shape to simplify analysis. The variability of yarn cross section
is associated with the random number of fibers along the yarn length, the
irregularity of constituent fibers, and the imperfect condition in forming
the yarn. Because porosity is inevitable in yarns produced from staple fibers,
the application of a relatively small tensile load to a twisted yarn, which pro-
duces only a small change in the yarn length, can cause a significant decrease
in yarn diameter. Generally speaking, it is difficult to make a precise deter-
mination of yarn diameter that is applicable to all downstream processes
and specifications. For these reasons, textile technologists prefer to specify
yarn size in terms of count number (traditionally used, expressed in length/
mass, e.g., in metric count, 1 Nm = 1 m/g) or linear density (preferred unit,
expressed in tex, 1 tex = 1 g/km = 1 mg/m). Unlike diameter, linear density
is an average value from a long length of yarn.
CNT fibers and yarns are also porous and often have irregular and incon-
sistent cross-sectional shape within a sample. Measuring CNT yarn thick-
ness by linear density has now become increasingly common. The linear