Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

28j 1 Introduction
Table 1.6 Experimental Young s modulus and tensile strength of
carbon nanotubes synthesized from different techniques.
Investigator
Young s
modulus
(TPa)
Tensile
strength
(GPa)
Nanotube
type
Synthesis
process Test method
Treacy et al. [142] 1.8 — MWNT Electric arc TEM (thermal
vibration)
Krishnam et al. [143] 1.25 — SWNT Laser ablation TEM (thermal
vibration)
Wong et al. [144] 1.28 — MWNT Electric arc AFM
Salvetat et al. [145] 0.81 — MWNT Electric arc AFM
Salvetat et al. [145] 0.027 — MWNT CVD AFM
Yu et al. [146] 0.27–0.95 11–63 MWNT Electric Arc SEM (tension)
Yu et al. [147] 1 30 SWNT rope Laser ablation SEM (tension)
Xie et al. [148] 0.45 3.6 MWNT
bundle
CVD SEM (tension)
Demczyk et al. [149] 0.9 150 MWNT Electric Arc TEM (tension)
the manufacture of automotive parts that require high strength and toughness.
Xie et al. also determined the stiffness of aligned CVD-MWNT bundle employing a
technique similar to Yu et al. [148]. Recently, Demczyk et al. performed in situ tension
measurement in a TEM for arc-grown MWNT, producing a stiffness of 0.9 TPa and a
tensile strength of 0.15 TPa [149]. The results of these investigators are summarized
in Table 1.6. From the above discussion, both theoretical and experimental studies
confirm that nanotubes have exceptional stiffness and strength. However, a large
variation in experimentally measured stiffness and strength is observed, particularly
a noticeable reduction in stiffness for CVD-MWNTs when compared with arc-grown
MWNTs. This can be expected as these reported values are obtained from a variety of
CNTs synthesized from various techniques having different lengths, diameters,
disorder structures, level of metal impurities and carbonaceous species.
As mentioned above, the fracture tensile strain of CNTs predicted by the MD
simulations could reach up to 30% [136], or even higher depending on simulated
temperature and interatomic potential model adopted [150]. Recently, Huang et al.
reported that the SWNT fractured at room temperature with a strain 15% [151].
However, the fracture tensile strain of SWNTcould reach up to 280% at temperatures
2000 C. (Figure 1.15). In other words, CNT deformed in a superplastic mode at
elevated temperatures. In the measurement, a piezo manipulator was used to pull
the SWNT to increase the strain under a constant bias of 2.3 V inside a TEM. The
temperatures are estimated to be 2000 C under a bias of 2.3 V. As a result, kinks and
pointdefectsarefullyactivatedin thenanotube, resultingin superplasticdeformation.
For VGCFs, the modulus has a strong dependence on the graphite plane misorientation
[13]. This is because VGCFs are formed by stacking graphite layers
into cone and cup structures. The graphite layers are not parallel to the fiber axis.
Accordingly, a wide range of stiffness values have been reported in the literature.