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Liquid
carbon plays role in nanotube formation
10 Feburary 2005
Researchers have discovered that the formation of
multi-walled carbon nanotubes in a pure carbon arc involves liquid
carbon. They believe that the nanotubes formed by homogeneous
nucleation inside droplets of the liquid. Their results could
open up the whole question of nanotube formation again.
"We were doing research on the electrical transport properties of
carbon nanotubes when we noticed that the nanotubes had these little
beads that looked like liquid drops on them," said Walt de Heer of the
Georgia Institute of Technology. "We hope our results will open up the
whole question of nanotube formation again."
The scientists grew the nanotubes by creating a carbon arc between two
carbon electrodes in a helium atmosphere. At higher helium pressures,
the process produced carbon nanotubes. At low pressures of helium, the
arc emitted fullerenes. The scientists say that unlike catalytically
produced multiwalled nanotubes, pure carbon-arc-produced nanotubes are
essentially defect-free.
The reaction formed carbon columns about 1 mm long and 0.1 mm wide.
Inside the columns were randomly aligned multi-walled carbon
nanotubes, around 3-20 nm in diameter and several micrometres long,
and graphitic nanoparticles. On the surface of the columns was a layer
around 100 nm thick, with nanotubes protruding from the surface. These
nanotubes often had beads of amorphous carbon around them.
"We asked the question, if the beads were once liquid carbon and the
nanotubes they are attached to are also carbon, why didn't the liquid
carbon dissolve the nanotube?," said de Heer. "The answer is that the
liquid must have been a glass at a lower temperature than the
nanotube."
The scientists reckon that electron bombardment from the cathode
heated up the carbon anode, causing the surface to liquefy in places
and eject liquid-carbon globules. The surface of the globule then
cooled fast, causing it to glassify. The interior cooled more slowly,
however, so that the liquid carbon supercooled and carbon crystals
nucleated and grew into nanotubes and graphitic nanoparticles.
Capillary forces resulted in the glass coating beading on the
nanotubes.
The researchers reckon that the radial temperature gradient in the
drop during cooling encouraged the carbon crystals to grow into
nanotubes.
The researchers reported their work in
Science (v307, p907, 2005). |
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