Biomedical Engineering Reference
In-Depth Information
9.3.4
Other Methods of CNTs Synthesis
It has been demonstrated that the catalytic growth of the CNTs can
be realized by reacting hydrocarbons or carbon monoxide (CO) with
the catalyst particles generated in situ inside a reactor. Smalley and
coworkers [137] have developed a gas phase catalytic process to
grow bulk quantities of SWCNTs. The carbon feedstock is CO gas
and the growth temperature is in the range 800-1200°C. Catalytic
particles for SWCNTs growth are generated in situ by thermal
decomposition of the iron pentacarbonyl in a reactor heated to the
high growth temperatures. CO provides the carbon feedstock for the
growth of the CNTs of the iron catalyst particles. CO is a very stable
gaseous molecule and does not produce unwanted amorphous carbon
carbonaceous material at high temperatures. However, this also
indicates that CO is not an efficient carbon source for the nanotube
growth. To enhance the CO carbon feedstock, Smalley and coworkers
have used high pressures of CO (up to 10 atm) to significantly speed
up the disproportionation of the CO molecules into carbon, and thus
enhance the growth of the SWCNTs. The SWCNTs produced this way
are as small as 0.7 nm in diameter, the same as that of a C
molecule.
These authors have also found that the yield of SWCNTs can be
increased by introducing a small concentration of methane into their
CO high-pressure reactor at the 1000-1100°C growth temperatures.
Methane (CH
60
) provides a more efficient carbon source than CO and
does not undergo appreciable pyrolysis under these conditions.
The high pressure CO catalytic growth approach is promising for
bulk production of the SWCNTs. Smalley and his coworkers at Rice
University have refined this process to produce large quantities of
SWCNTs with remarkable purity, at industrial level. The so-called
HiPco nanotubes (high-pressure conversion of carbon monoxide)
have received considerable attention as the technology has been
commercialized by Carbon Nanotechnologies Inc. (Houston, TX,
USA) for large-scale production of high-purity SWCNTs.
Cheng and coworkers [138] reported a method that employs
benzene as the carbon feedstock, hydrogen as the carrier gas, and
ferrocene as the catalyst precursor for SWCNTs' growth. In this
method, ferrocene is vaporized and carried into a reaction tube by
benzene and hydrogen gases. The reaction tube is heated at 1100-
1200°C. The vaporized ferrocene decomposes in the reactor, which
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