Page 176 - Carbon Nanotube Fibres and Yarns

Carbon nanotube yarn structures and properties 167

irradiation of single multiwalled CNTs, commonly performed in evacuated
SEM or TEM chambers, can create between-wall bonds that increase the
total breaking force [62, 63]. Single-walled CNTs in a “rope” can be linked
to each other by a low dose of electron irradiation [64]. Theoretical mod-
eling has shown that cross-links between CNTs can be formed with the
participation of interstitial carbon atoms generated by irradiation, or more
rarely by a radiation-induced chemical reaction between carboxyl groups
[64]. Electron-beam radiation attenuates rapidly and thus is not a suitable
large-scale treatment of CNT yarns. Gamma radiation, on the other hand,
penetrates a great depth into carbon materials and can be adopted to treat
large volumes of material uniformly.
Miao et al. [65] reported that the gamma irradiation of CNT yarns in air
significantly improved the tensile strength of the yarns. The improvement
was much greater for tightly structured yarns (e.g., high twist yarns) than
for loosely structured yarns. Sonic pulse tests also showed increased sound
velocity and thus increased dynamic modulus of the CNT yarns as a result
of gamma irradiation. X-ray photoelectron spectroscopic analyses on parent
CNT forests showed that gamma irradiation treatment in air dramatically
increased the concentration of oxygen in the CNT assemblies in propor-
tion to radiation dose. This indicates that CNTs were oxidized under the
ionizing effect of the gamma irradiation. The oxygen species are believed to
contribute to the interaction between CNTs and thus improvement of the
yarn mechanical properties.

7.3 Dynamic properties
Tensile properties of materials measured on a tensile testing machine are
dependent on the strain rate used in testing. For example, polymer fibers
could increase their Young’s moduli by up to threefold as the straining rate
was increased from 1% per minute to 100,000% per minute [66]. On the
other hand, the modulus of linear-elastic fibers, such as the glass fiber, was
not as affected by the straining rate used in testing [67].
Wang et al. [68] used a free-falling Hopkinson tension bar to measure
the dynamic tensile strength of CNT yarns. The dynamic strength of CNT
yarn could be 35% stronger than that obtained using a quasistatic test. This
strengthening behavior could be expressed as a function of the strain rate
using a simplified form of the Johnson-Cook model,

ε )
+
σ = σ (10 0186ln  (7.6)
.
0