Dear leonardodavinci,
The three principal approaches to single-walled carbon nanotube (SWCNT)
fabrication are chemical vapor deposition, laser ablation, and arc
welding. All currently known methods consist of some variant of one
of these approaches. Other fabrication mechanisms may be discovered
in future, but for the time being we must restrict our attention to
these three.
The most recently developed production method is chemical vapor deposition
(CVD), which grows nanotubes in an enclosed high-temperature environment
permeated with a carbon-bearing gas. The decomposition of the gas results
in the gradual deposition of carbon on a prepared surface. This type
of manufacturing is especially well suited to electronic manufacturing
applications in which nanotube structures must be grown in precise
quantities and locations. Recent developments in the use of plasma as a
CVD growth environent have made it possible to grow CNTs at temperatures
below 100 degrees Celsius. CVD methods can also be harnessed to the
production of long strands of carbon nanotube, typically of the
multi-walled rather than single-walled variety, for use in ultra-strong
rope and similar products.
A thermal CVD reactor is simple and inexpensive to construct,
and consists of a quartz tube enclosed in a furnace. Typical
laboratory reactors use a 1 or 2" quartz tube, capable of holding
small substrates. The substrate material may be Si, mica, quartz,
or alumina. The setup needs a few mass flow controllers to meter
the gases and a pressure transducer to measure the pressure.
The growth may be carried out at atmospheric pressure or slightly
reduced pressures using a hydrocarbon or CO feedstock. The growth
temperature is in the range of 700-900 C. A theoretical study
of CNT formation suggests that a high kinetic energy (and thus
a high temperature, ? 900 C) and limited, low supply of carbon
are necessary to form SWCNTs.
NASA Ames Research Center: M. Meyyappan and D. Srivastava: Carbon
Nanotubes: Section 4.2
http://www.nanoelectronics.engr.scu.edu/nanotechnology/NanoTech%20Downloads/crc_chapter_11_14_01(section3-supple1).doc
It has been shown that CVD is amenable for nanotube growth
on patterned surfaces, suitable for fabrication of electronic
devices, sensors, field emitters and other applications where
controlled growth over masked areas is needed for further
processing. More recently, plasma enhanced CVD (PECVD) has
been investigated for its ability to produce vertically aligned
nanotubes. A variety of plasma sources and widely varying results
have been reported in the literature. [...]
The plasma enhancement in CVD first emerged in microelectonics
because certain processes cannot tolerate the high wafer
temperatures of the thermal CVD operation. The plasma CVD allows
an alternative at substantially lower wafer temperatures (room
temperature to 100 C) for many processes and hence has become
a key step in IC manufacturing. The low temperature operation
is possible because the precursor dissociation (necessary for
the deposition of all common semiconductor, metal and insulator
films) is enabled by the high-energy electrons in an otherwise
cold plasma.
Plasma Source Science and Technology: M. Meyyappan, L. Delzeit,
A. Cassell, and D. Hash: Carbon Nanotube Growth by PECVD
http://www.iop.org/EJ/article/0963-0252/12/2/312/ps3212.pdf
Midway in SWCNT fabrication history between arc welding and CVD is the
technique of laser ablation. In this process, a graphite rod inside
a cylindrical furnace with a controlled gas flow is vaporized by a
high-powered laser, throwing off carbon nanoparticles that coalesce
downstream in a quartz tube to form SWCNT structures. Laser ablation
requires costly apparatus to produce small quantities of high-quality
SWCNT with purity ranging from 70% to 90%.
In laser ablation, a target consisting of graphite mixed with
a small amount of transition metal particles as catalyst is
placed at the end of a quartz tube enclosed in a furnace [60].
The target is exposed to an argon ion laser beam which vaporizes
graphite and nucleates carbon nanotubes in the shockwave just
in front of the target. Argon flow through the reactor heated
to about 1200 C by the furnace carries the vapor and nucleated
nanotubes which continue to grow. The nanotubes are deposited on
the cooler walls of the quartz tube downstream from the furnace.
This produces a high percentage of SWCNTs (~70%) with the rest
being catalyst particles and soot.
NASA Ames Research Center: M. Meyyappan and D. Srivastava: Carbon
Nanotubes: Section 4.1
http://www.nanoelectronics.engr.scu.edu/nanotechnology/NanoTech%20Downloads/crc_chapter_11_14_01(section3-supple1).doc
Laser ablation products from fullerene materials have been studied
by transmission electron microscopy and Raman spectroscopy. Using
nickel and cobalt as a catalyst, single-wall carbon nanotubes
were produced at an ambient temperature of 400 °C. The results
were compared with those using graphite as starting materials. It
is suggested that the formation of single-wall carbon nanotubes
is controlled by both the availability of proper precursors and
the activity of the metal catalyst.
Applied Physics Letters -- November 15, 1999 -- Volume 75, Issue 20, pp.
3087-3089: Y. Zhang and S. Iijima: Formation of single-wall carbon
nanotubes by laser ablation of fullerenes at low temperature
http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=APPLAB000075000020003087000001&idtype=cvips&gifs=yes
The earliest method of SWCNT production, indeed the one that resulted in
the serendipitous discovery of carbon nanotubes, is arc welding. Here,
graphite is vaporized by a strong electric current to produce structurally
sound but relatively impure SWCNT. Although arc welding is not amenable
to the application of precise quantities such as those required
in nanoelectronics, it is a relatively inexpensive and high-volume
production method. A recent refinement of this technique developed at
NASA Goddard has resulted in much higher yield rates than the usual 30%,
approaching the 70% range.
The arc process involves striking a dc arc discharge in an inert
gas (such as argon or helium) between a set of graphite electrodes
[1,59]. The electric arc vaporizes a hollow graphite anode packed
with a mixture of a transition metal (such as Fe, Co or Ni) and
graphite powder. The inert gas flow is maintained at 50-600 Torr.
Nominal conditions involve 2000-3000 C, 100 amps and 20 volts.
This produces SWCNTs in mixture of MWCNTs and soot. The gas
pressure, flow rate, and metal concentration can be varied to
change the yield of nanotubes, but these parameters do not seem
to change the diameter distribution. Typical diameter distribution
of SWCNTs by this process appears to be 0.7-2 nm.
NASA Ames Research Center: M. Meyyappan and D. Srivastava: Carbon
Nanotubes: Section 4.1
http://www.nanoelectronics.engr.scu.edu/nanotechnology/NanoTech%20Downloads/crc_chapter_11_14_01(section3-supple1).doc
High yields (70-90%) of SWNTs close-packed in bundles can
be produced by laser ablation of carbon targets. The method
(electric-arc) used here is cheaper and easier, but previously
had only low yields of NTs. They show that it can generate large
quantities of SWNTs with characteristics similar to those obtained
by laser ablation.
Northwestern University: F. Fisher and C. Brinson: Carbon Nanotubes Literature
Review: page 6
http://www.tam.northwestern.edu/~ftf234/nano/LitReview/LitTry3/NanotubeReview022101web.pdf
When considering which of the three SWCNT fabrication methods is best
suited to large-scale manufacturing, our criteria consist of volume and
quality. By definition, large-scale manufacturing requires that apparatus
procured at a reasonable price be capable of producing significant
quantities of SWCNT. This rules out laser ablation, which requires
significant expenditures to produce small quantities of SWCNT.
For similar outlay, both CVD and arc discharge methods have been shown to
be capable of producing tens or hundreds of grams of carbon nanostructures
daily in each enclosure. However, the criterion of product quality demands
that we strike CVD methods for the time being. Although CVD yields can
be very pure, meaning that the proportion of non-CNT contaminant is low
compared to the number of CNT particles, the nanostructures themselves
tend to be compromised by extensive defects. Since large-scale production
requires consistent structural properties, CVD does not at present
appear to be as suitable an approach as arc discharge. Furthermore,
CVD is best equipped to fabricating multi-walled CNT structures rather
than the single-walled variety.
Therefore, the best currently known fabrication technique for large-scale
SWCNT manufacturing is the arc discharge process. Thanks to recent
advancements by nanostructure fabrication researchers at NASA Goddard
Space Flight Center, it is now possible to take advantage of the
high-yield properties of arc discharge while enjoying yield purities
comparable to those of CVD and laser ablation. The new methods, which
do without a metal catalyst, also reduce considerably the cost and
complexity of SWCNT manufacturing.
The early processes used for CNT production were laser ablation
and an arc discharg approach. [...] Of the two, laser ablation
is not amenable for scaleup whereas the arc discharge process
has been used to produce large quantities of CNTs.
Plasma Source Science and Technology: M. Meyyappan, L. Delzeit,
A. Cassell, and D. Hash: Carbon Nanotube Growth by PECVD
http://www.iop.org/EJ/article/0963-0252/12/2/312/ps3212.pdf
Large-scale synthesis of SWCNT by the arc discharge method
yielded quantities of tens of grams a day under arc conditions
of 40~60 A d.c. and helium pressures of 500 to 700 torr. Results
show that helium atmosphere strongly affects the yield of SWNTs,
and that the diameter distribution of the SWNTs is affected by
the catalyst.
Northwestern University: F. Fisher and C. Brinson: Carbon Nanotubes Literature
Review: page 3
http://www.tam.northwestern.edu/~ftf234/nano/LitReview/LitTry3/NanotubeReview022101web.pdf
NASA scientists have developed an SWCNT manufacturing process
that does not use a metal catalyst, resulting in simpler, safer,
and much less expensive production. Researchers used a helium
arc welding process to vaporize an amorphous carbon rod and
then form nanotubes by depositing the vapor onto a watercooled
carbon cathode. Analysis showed that this process yields bundles,
or ?ropes,? of single-walled nanotubes at a rate of 2 grams
per hour using a single setup.
NASA?s process offers several advantages over metal catalyst
production methods. For example, traditional catalytic arc
discharge methods produce an ?as prepared? sample with
a 30% to 50% SWCNT yield at a cost of approximately $100 per
gram. NASA?s method increased the SWCNT yield to an average
of 70% while significantly reducing the per-gram production cost.
Fuentek: Available Technologies: Producing Lower-Cost Single-Walled
Carbon Nanotubes Without Metal Catalysts
http://www.fuentek.com/technologies/carbon-nantubes.htm
It has been an interesting challenge to address this question on your
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Regards,
leapinglizard |
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