Page 74 - Carbon Nanotube Fibres and Yarns

66 Carbon Nanotube Fibers and Yarns

Ericson et al. [3] prepared an 8 wt% dispersion of purified SWNTs in
102% sulfuric acid (2 wt% excess SO 3 ) in a nitrogen-purged dry box. The
mixture was manually mixed and then transferred to a mixing apparatus via
a stainless steel syringe. Extensive mixing was accomplished by two alter-
nating pneumatic pistons, which pushed the SWNT dope back and forth
through an actively rotating shear cell within an evacuated housing. When
the viscosity reached a steady state, the SWNT material was extruded
through a small capillary tube (<125 μm in diameter) into a coagulation
bath (diethyl ether, 5% sulfuric acid, or water) to form continuous lengths
of macroscopic pure SWNT fibers. Water-coagulated fibers were dried in
a vacuum oven at 100°C, followed by annealing in a flow of H 2 /Ar (1:1)
at 1 atm and 850°C for 1 h. The fibers were further annealed in vacuum at
~1100°C. The resulting neat fiber had a diameter of about 50 μm. The neat
SWNT fibers possessed a Young’s modulus of 120±10 GPa and a tensile
strength of 116±10 MPa. As the pure SWCNT fibers contained no poly-
mers, their electrical and thermal conductivities were very high, at 500 S/cm
−1
−1
and 21 W m K , respectively.
A key parameter for improving SWNT alignment and thus fiber prop-
erties was the shear rate within the extrusion orifice. For a given extrusion
rate, the shear rate scales with orifice diameter cubed. As the diameter was
decreased from 500 to 125 μm, both thermal and electrical conductivity
increased because of the increased CNT alignment. The as-spun fiber had
a low resistivity due to acid doping and also a low modulus. Annealing the
fiber in an inert gas at 850°C improved the modulus but simultaneously
increased fiber resistivity by an order of magnitude [12].
Behabtu et al. [13] reported production of ultrahigh conductivity
CNT fibers from high-quality bulk-grown CNTs by a high-throughput
wet spinning using both single and 19-hole spinnerets. The wet-spinning
process was essentially the same as that used to produce high-performance
industrial polymer fibers. The CNTs were from single- to five-walls (diam-
eter 2–6 nm) and from 3 to 11 μm in length. The bulk-grown CNTs were
dissolved in chlorosulfonic acid (CSA) at a concentration of 2–6 wt% and
filtered to form a spinnable liquid crystal dope. The dope was extruded
through a spinneret (65- to 130-μm diameter) into a coagulant (acetone
or water) to remove the acid. The forming filament was collected on a
winding drum. The linear velocity of the drum was higher than the dope
speed at the spinneret exit to ensure high CNT alignment by continuous
stretching and tensioning of the fibers. The fibers were further washed
in water and dried in an oven at 115°C. The average tensile strength,