Page 179 - Carbon Nanotube Fibres and Yarns

170 Carbon Nanotube Fibers and Yarns

7.4 Electrical conductivity
Single-walled carbon nanotubes (SWNTs) may be either metallic or semi-
conducting depending on their chirality, i.e., how a single-layer graphene is
wrapped into a cylinder (nanotube). The electrical resistivity through the same
graphene layer in a MWNT is much lower than that between the coaxial layers
(between walls) [3]. Resistivity in MWNTs can also be reduced by subjecting
the CNTs to annealing [72] and by controlled defect creation that promotes
cross-shell bridging. Contact resistance between nanotubes depends strongly
on the atomic structure in the contact area and can vary by more than an or-
der of magnitude. The optimal electronic transport between nanotubes occurs
when the tubes are in atomic scale registry where atoms from one tube are
placed on top of another [73]. Resistivity of CNT bundles formed in synthesis
can vary greatly due to large differences in the structures of nanotubes and the
electrical testing methods used. The electrical conductivity of MWNT-based
macrostructures is generally lower than that of defect-free individual CNTs
due to the presence of amorphous carbon and other impurities, which cause
scattering and increase contact resistances [74, 75].
Chen et al. [76] reported the dependence of electrical conductivity of
random and aligned CNT films on nanotube structure (wall number and
diameter). The random films were made by dispersing CNT forest (500–
600 μm in length) in an organic solvent while the aligned films were pro-
duced by laying a CNT forest using a roller-press method. The two types
of films showed similar conductivity and reached a peak when the average
wall number was 2.7, corresponding to an average nanotube diameter of
5.4 nm. The appearance of the peak was attributed to the off-setting effects
of increasing nanotube conductivity and decreasing CNT packing density
of the films with the increase of nanotube wall number.
Aliev et al. [77] measured the temperature dependence of resistivity
in different CNT assemblies including yarns (Fig. 7.22). Unlike the pos-
itive coefficient typically observed in metallic materials, all the MWNT
assemblies showed a negative temperature coefficient of resistance (dR/
dT < 0), indicating their semiconducting characteristics. They proposed that
although the longer overlap of aligned MWNTs could not greatly reduce
the number of barriers to electron hopping process, alignment substantially
reduced the electron pathway and the electrical resistivity.
Typical values of room temperature electrical conductivity for CNT
4
4
yarns spun from forests are between 1.5 × 10 and 4.1 × 10 S/m [5, 13, 27,
78–80]. A major reason for this wide spread of value is the difference in yarn