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<p>Manufacturing of carbon nanotubes (CNTs) via chemical vapor deposition (CVD) calls
for thermal treatment associated with gas-phase rearrangement and catalyst deposition
to achieve high cost efficiency and limited influence on environmental impact. Taking
advantage of higher degree of structure control and economical efficiency, catalytic
chemical vapor deposition (CCVD) has currently become the most prevailing synthesis
approach for the synthesis of large-scale pure CNTs in past years. Because the synthesis
process of CNTs dominates the potential ecotoxic impacts, materials consumption, energy
consumption and greenhouse gas emissions should be further limited to efficiently
reduce life cycle ecotoxicity of carbon naotubes. However, efforts to reduce energy
and material requirements in synthesis of CNTs by CCVD are hindered by a lack of mechanistic
understanding. In this thesis, the effect of operating parameters, especially the
temperature, carbon source concentration, and residence time on the synthesis were
studied to improve the production efficiency in a different angle. Thus, implications
on the choice of operating parameters could be provided to help the synthesis of carbon
nanotubes.</p><p>Here, we investigated the typical operating parameters in conditions
that have yielded successful CNT production in the published academic literature of
over seventy articles. The data were filtered by quality of the resultant product
and deemed either "successful" or "unsuccessful" according to the authors. Furthermore,
growth rate data were tabulated and used as performance metric for the process whenever
possible. The data provided us an opportunity to prompt possible and common methods
for practioners in the synthesis of CNTs and motivate routes to achieve energy and
material minimization.</p><p>The statistical analysis revealed that methane and ethylene
often rely on thermal conversion process to form direct carbon precursor; further,
methane and ethylene could not be the direct CNT precursors by themselves. Acetylene
does not show an additional energy demand or thermal conversion in the synthesis,
and it could be the direct CNT precursors by itself; or at least, it would be most
easily to get access to carbon nanotube growth while minimizing synthesis temperature.
In detail, methane employs more energy demand (Tavg=883℃)than ethylene (Tavg=766℃),
which in turn demands more energy than acetylene (Tavg=710℃) to successfully
synthesize carbon nanotubes. The distinction in energy demand could be the result
of kinetic energy requirements by the thermal conversion process of methane and ethylene
to form direct CNT precursors, and methane employs the highest activation demand among
three hydrocarbons. Thus, these results support the hypothesis that methane and ethylene
could be thermally converted to form acetylene before CNT incorporation.</p><p>In
addition, methane and ethylene show the demand for hydrogen in thermal conversion
process before CNT incorporation; whereas, hydrogen does not contribute to the synthesis
via acetylene before CNT incorporation, except the reduction of catalyst. At relatively
low hydrogen concentration, this work suggests that hydrogen prompts growth of carbon
nanotubes via methane and ethylene, probably by reducing the catalysts or participating
thermal reactions. In addition, "polymerization-like formation mechanism" could be
supported by the higher growth rate of CNTs via ethylene than acetylene.</p><p>There
could be an optimum residence time to maintain a relatively higher growth rate. At
too low residence time, carbon source could not be accumulated, causing a waste of
material; while too high residence time may cause the limitation of carbon source
supplement and accumulation of byproducts.</p><p>At last, high concentration of carbon
source and hydrogen could cause more energy consumption, while it helps to achieve
a high growth rate, due to the more presence of direct carbon precursor.</p>
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