Page 240 - Carbon Nanotube Fibres and Yarns

230 Carbon Nanotube Fibers and Yarns

absolute zero temperature. As temperature increases, electrons are thermally
excited into higher energy bands due to increase in the mobility and avail-
ability or concentration of free charge carriers. Generally, the conductivity
of CNT fibers increases, or their resistivity decreases with a decrease in tem-
perature. Above 110 K [68]–200 K [9], R increases with increasing T, like a
metal as seen in the zero-field R/T plot of aligned CNT fiber (Fig. 9.9).
This was attributed to the intrinsic contribution of the CNTs over the fi-
ber’s morphological factors, such as junctions and entanglements.
The decrease in resistivity with decreasing temperature persists to
a crossover temperature below which it is reversed and will start to in-
crease like a semiconductor. Currently, a wide range of values have been
reported as the crossover temperature for CNT fibers, ranging from 40 K
to 110 K [3,9,68–70]. This transport behavior has been attributed to many
phenomena including hopping and localization [3,71–73]. Localization
state crossover of transport electrons depends on the relative contents of
the metallic and semiconducting CNTs while hopping barriers have been
reported to be absent in pure metallic SWCNT networks according to
Yanagi et al. [72]. While according to Bulmer et al. [68], the resistivity of
CNT fiber begins to level-off below 2 K to an apparently finite value ap-
proaching absolute zero due to the fiber residing on the metal side of the
insulator-to-metal transition. Thus, by using a magneto-transport model,
they eliminated the hopping mechanisms because hopping is on the insu-
lator side of the insulator-to-metal transition where charge carriers are lo-
calized. Considering this model, the length and alignment of the nanotubes
are more critical to obtaining metal-like conductivity over other factors
including the concentration of metallic CNTs due to fewer extrinsic junc-
tions in a network of long and aligned CNTs compared to an unaligned
network of CNTs of discrete length.
It was also reported that the morphology of the fibers affects the features
of the ρ–T curves. Metallic features of the resistance-temperature curves
are obtained when the fiber structure has better alignment, purity, packing
density of the CNTs, dopant, and higher share of metallic CNTs in the
assembly [3,69,70,72–74]. Considering these factors, the resistance of the
semiconducting-like behavior for these materials will not increase signifi-
cantly with decreasing temperature as seen in Fig. 9.9A except for a devia-
tion from such morphology [9].
Another deviation was also reported for CNT fiber integrated in an ep-
oxy resin. Badaire et al. observed a decrease of resistivity of annealed CNT
fiber with stretch applied that is different from that of composite fiber [75].