Biomedical Engineering Reference
In-Depth Information
SWCNTs while MWCNTs fail in tension via a sword-in-sheath mechanism;
stepwise or brittle fractures are generally observed during the bending of
MWCNTs (Zhao and Zhu, 2011). The low experimental strength values and
brittle fracture have been related to defects present in the as-synthesized
CNTs (Zhao and Zhu, 2011), as the strength of CNTs is affected by the
presence of defects as well as the interactions of nanotubes in a MWCNT
and in the bundles of SWCNTs (Shaffer and Sandler, 2007). A recent in-situ
TEM test verified that CNTs were broken at the defective locations and
CNTs with no defects showed strength approaching the theoretically
determined value (Wang et al., 2010).
Electrical conductivity
The room-temperature electrical conductivity of metallic SWCNTs was
found to be 10
5
-10
6
S/m but only 10 S/m for semiconducting SWCNTs
(Tans et al., 1997). Generally, the electrical conductivity of SWCNT bundles
has been found to swing between 10
4
S/m and 3
10
6
S/m (Bozhko et al.,
1998, Fischer et al., 1997, Kim et al., 1998), approaching the in-plane
conductivity of graphite 2.5
6
10
6
S/m (Charlier and Issi, 1995). The
electrical conductivities of individual MWCNTs have been found to be in
the range of 20 to 2
6
10
7
S/m (Ebbesen et al., 1996). The helicity of the
outermost tubes (Lee et al., 2000), ballistic effects, and the presence of
defects (Dai et al., 1996) affect the measured electrical conductivity of CNTs
significantly.
6
Thermal conductivity
High thermal conductivity values (i.e. 6600W/m.K) have been theoretically
predicted for individual SWCNTs (Berber et al., 2000, Che et al., 1999,
Hone et al., 1998). However, bulk specimens of SWCNTs demonstrated
considerably lower values, i.e.
4
36W/m.K for disordered SWCNTs (Hone
et al., 1998) and
200W/m.K for aligned SWCNTs (Hone et al., 2000, Pan,
2011). Similarly, the experimental thermal conductivities of individual
MWCNTs are very high, e.g. 3000W/m.K (Kim et al., 2001), while
MWCNTs in bulk form showed a value of only 25W/m.K (Yi et al., 1999).
Moreover, values as high as 200W/m.K have been determined for
MWCNTs (Yang et al., 2002) but randomly oriented MWCNTs con-
solidated by spark plasma sintering (SPS) showed a very low value of only
4.2W/m.K (Zhang et al., 2005). As well as the effects of anisotropy, this
sharp decrease in thermal conductivity of bulk CNT specimens may be
associated with the interfacial resistance; bundles, ropes, nests and mats of
CNTs have shown considerably lower values, sometimes as low as 0.7W/
m.K (Hone et al., 1998).
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