Page 59 - Carbon Nanotube Fibres and Yarns

Carbon nanotube fibers spun directly from furnace 51

The axial stress in a CNT fiber is limited by the stress transfer between
adjacent CNT bundles through interfacial shear [34, 61]. The strength of
CNT fibers is usually limited by the coherence of the network. Thus the
CNT fiber strength should be affected by the bundle length, the coef-
ficient of static friction between bundles, and the surface contact area
[61]. Longer gauge length used in tensile testing has an adverse effect
on the measured yarn strength, with lower strength measured at longer
gauge length. A detailed discussion on fiber strength variability is given
in Chapter 7.
The fiber tensile strength decreased with the linear density of the fiber
samples [60] (Fig. 3.11). This is consistent with dry-spun fibers from CNT
forests (see Chapter 7). If we continue reducing the linear density and di-
ameter to the greatest extent, the ultimate scenario is an individual CNT
which has an extremely high strength. So linear density should be included
when reporting fiber strength for a fair comparison.
The carbonaceous coatings and impurity clusters on the bundle sur-
face could enhance the tensile strength and stiffness through inter-bundle
adhesion. Large impurity clusters have a similar strength-enhancing effect
especially at shorter testing gauge length, although it reduces the degree of
alignment [21, 62]. With less impurity cluster the stiffness of the CNT fiber
could be increased [11].

1400 0.7
1300
1200 0.6
1100
1000 0.5
900 0.4
UTS (MPa) 700 0.3 UTS (N/TEX) Large Yarn PAR
800
915,919
600
500
400 0.2
300
200 0.1
100
0 0
0 10 20 30 40
TEX (g/km)
Fig. 3.11 The strength of CNT fiber as a function of linear density. (Reproduced with per-
mission from Schauer MW, Lashmore D, White B. Synthesis and properties of carbon nano-
tube yarns and textiles. MRS Proceedings 2008; 1081:1081-P03-05.)