Spinning out fibers

NANOTECHNOLOGY

Macroscopic assemblies of carbon nanotubes
(CNTs) in the form of fibers could be useful
for composites, actuators, power cables,
electrodes, and catalyst supports. However,
the fabrication of such fibers has, to date,
relied on post-processing techniques.
University of Cambridge researchers have
devised a means of spinning fibers or ribbons
of CNTs directly from the chemical vapor
deposition synthesis zone [Li et al.,
Sciencexpress, published online 11 March
2004, DOI: 10.1126/science.1094982].
A liquid source of carbon, ethanol (with
added ferrocene and thiophene), is injected
at a rate of 0.08-0.25 ml/min from the top
of a furnace into a hydrogen carrier gas
flowing at 400-800 ml/min at a temperature
of 1050-1200°C. The nanotubes form an
aerogel in the hot zone of the furnace, which
is stretched into a ‘sock’ by the gas flow.
Fibers can be drawn continuously and
directly from the aerogel using a rotating
rod. Either single- or multi-walled nanotubes
(SWNTs or MWNTs) can be formed in this
way, depending upon the synthesis
conditions. MWNTs are formed with a
thiophene concentration of 1.5-4.0 wt.%,
hydrogen flow of 400-800 ml/min, and a
temperature of 1100-1180°C, while SWNTs
require a thiophene concentration of only 0.5
wt.% but a higher flow rate (1200 ml/min)
and temperature (1200°C). A high hydrogen
flow rate is necessary to produce a ‘clean’

product. The MWNT fibers are typically
30 nm in diameter, SWNTs are 1.6-3.5 nm.
The properties of the fibers show slightly
higher than usual conductivity
(8.3 x 105 Ω -1 m-1) and strengths in the
range 0.05-0.5 N/Tex (equivalent to
0.1-1.0 GPa). The researchers are confident,
however, that these properties can be
improved upon with refinements to the
process itself and post-process treatment.
This direct spinning process opens up a
novel, one-step production route for
nanotube fibers, ribbons, and even coatings.
The process could also be applied to other
fibrous materials. The researchers are now
working with specialty chemicals
manufacturer Thomas Swan Ltd. to scale up
the process using a 4 m x 4m x 4m rig.
Cordelia Sealy
(a) Photo of nanotubes being wound from spindle (left) onto a
second spindle (right). (b) Scanning electron micrograph of a
fiber of well-aligned MWNTs (c). (d) Permanent twist introduced
into a nanotube fiber after removal from furnace.
(© 2004 AAAS.)

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