Biomedical Engineering Reference
In-Depth Information
8.3.5
Carbon Nanotubes
CNT powders can be classified as single-walled nanotube (SWCNT) or multiwalled
nanotube (MWCNT) powders. SWCNT powder consists of a single graphene sheet
rolled up as a seam-free tube. They can be thought of as a linearly extended fuller-
ene [
1,
7,
40,
41
]. SWCNTs usually exist as agglomerations due to the van der
Waals forces between each tube, with diameters on average between 0.7 and 2 nm,
whilst their lengths are often 5-30 m m [
1,
16
]. MWCNT powders can be thought of
as an array of SWCNT that are concentrically arranged inside one another with an
internal diameter as small as 2.2 nm. The distance between the individual SWCNT
that constitute MWCNT (or the graphite inter-layer separation) is typically 0.34 nm
[
1,
40,
41
] (Fig.
8.6
). Iijima [
40
] used electron diffraction to establish that the crys-
tal axis of the graphene tubes consisted of carbon-atom hexagons arranged in a
helical manner about the tube axis (Fig.
8.7
). The CNT ends are closed off by the
presence of pentagonal carbon rings near the tip regions, whilst deformations and
imperfections of the cylinder occur as pentagons or heptagons within the main
structure of the tube [
1
] .
SWCNTs are known to be stiff and exceptionally strong (high Young's modulus and
tensile strength). Furthermore, SWCNTs can stretch beyond 20 % of their original
length and bend over double without kinking [
7,
16
] . However, the mechanical
properties of individual CNT, whether of SWCNT or MWCNT arrangement, are the
subject of much research with a significant variation in recorded properties existing.
While direct measurement of mechanical properties for individual CNT remains
challenging and high degrees of experimental uncertainty exist, it is clear that the
use of CNT in various matrices can greatly enhance mechanical properties.
3nm
Fig. 8.6
Transmission
electron micrograph (TEM)
of different MWCNT
arrangements [
40
]
0.34nm
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