Biomedical Engineering Reference
In-Depth Information
production at relatively low cost together with a control of CNTs growth by
adjusting reaction parameters such as temperature, catalyst, hydrocarbon
source and the flow rate of gases. However, CNTs tend to be highly
entangled or aligned depending upon the parameters chosen. The
application of plasma during growth is reported to enhance the alignment
and straightness of the nanotubes at low temperatures (Hofmann et al.,
2003).
7.2.3 Properties of carbon nanotubes
Modulus of elasticity
In early measurements, the moduli of elasticity of MWCNTs and SWCNTs
were found to be 1.8 TPa and 1.25 TPa, respectively, by measuring the
amplitude of their thermal vibrations in a transmission electron microscope
(Krishnan et al., 1998, Treacy et al., 1996). Highly crystalline CNTs have an
axial stiffness of 1.1 TPa, which is close to theoretical prediction (Belytschko
et al., 2002, Lu, 1997, Yao and Lordi, 1998), while the bending stiffness of
CNTs determined by atomic force microscopy was found also to be around
1 TPa (Salvetat et al., 1999, Wong et al., 1997). CNTs grown by CVD have
comparatively lower stiffness due to the presence of a higher concentration
of structural defects than those produced by high-temperature techniques.
CNTs tend to bend to high deformations without fracture due to buckling
mechanisms involving reversible collapse of their cross-sections (Shaffer and
Sandler, 2007). CNTs may, therefore, exhibit different values of stiffness in
tension, compression and bending. The modulus of elasticity of CNTs is
independent of their chirality, but depends on diameter and on deviation
from a parallel alignment of graphitic layers (Shaffer and Sandler, 2007).
Tensile strength
Theoretically, the predicted tensile strength of CNTs is relatively high, 75-
135GPa (Zhao and Zhu, 2011). However, a wide range of values has been
observed experimentally; for example, individual SWCNTs and high-
temperature grown MWCNT shells showed values of 3.6-100GPa (Li
et al., 2000, Wang et al., 2010, Yu et al., 2000a) and 10-110GPa (Ding et al.,
2007, Peng et al., 2008, Yu et al., 2000b), respectively. The ropes/strands of
SWCNTs and low-temperature grown MWCNTs showed very low values of
only 1-1.7GPa (Li et al., 2005, Pan et al., 1999, Zhu et al., 2002). CVD-
grown MWCNTs also demonstrated low-compression strength of 2GPa
(Zhao et al., 2011). Nanoloading devices attached with AFM tips are
generally operated inside a SEM or TEM to measure tensile strength. A
brittle fracture mode has always been observed during tensile loadings of
