Figure 4: Schematic of Laser Ablation Method [Rajesh et al.,
2014]
Chemical Vapor Deposition
(CVD)
The best and the most used technique for mass production of CNTs is
chemical vapor deposition. This is mainly because of the lower
temperatures involved in CVD and greater ability to control the nanotube
size by changing catalyst particle size. The catalyst particles act as a
surface for carbon precursor decomposition and CNT formation. A carbon
source in gaseous phase is passed over an energy source like a resistive
heating coil to energize the gaseous atoms. By controlling the catalyst
particle size, catalyst concentration, pressure, growth time, growth
temperature and gas flow rate, CNT molecules with required properties
can be grown on catalyst surface. Generally, forFigure 5: Schematic of Chemical Vapor Deposition [Rajesh et
al., 2014]
Other
techniques
- Spray pyrolysis
In spray pyrolysis, the carbon precursor and
the synthesis of
o
MWCNTs, low
catalyst material are both pumped into the
temperatures (600-900 C) are preferred and temperatures around
900oC-1200oC yield SWCNTs. This high
temperature requirement for SWCNT fabrication is due to its higher heat
of formation which is attributed to its small diameter that results in a
higher strain energy and curvature. The diameter of nanotubes is
directly linked with the size of catalyst particles used. Varying the
catalyst composition and coating catalyst particles on various
substrates to know the characteristics of the resulting CNTs formed has
been an area of research in the past decade. Several CNT hybrid
materials are prepared through CVD technique to increase
electrochemically active surface area and the fast electron/ ion
transfer. CVD method provides large production yield at lowest
prices. However, CVD technique imparts more deficiencies in the
fabricated CNT material. The CNTs can be extracted from the final
product by a purification process, before being used for specific
applications.reactor at the same time. The injection CVD process is another name for
it. It has an advantage over the traditional CVD approach in that liquid
hydrocarbon and catalyst can be added continuously into the reaction
zone, resulting in MWCNT output that is both inexpensive and semi-
continuous [Annubhawi et al., 2017]. Liquid
additives are also monitored and regulated.
Flame
synthesis
Flame synthesis provides a method for direct growth of nanotubes over
large area substrates. Continuous and quick heating is involved.
Inherently, flames provide either oxidizing or carbon-rich conditions,
allowing metal-oxides or carbon materials to expand easily [Hua et
al., 2019]. It can also be used in conditions without any confinement,
allowing for scalability and high deposition rates.
Mechanical Ball
Milling
Fine particles with a homogeneous size distribution can be produced by
milling
balls, and both wet and dry powders can be grinded at a low cost.
Nanotubes are ground into extremely fine powders using the ball milling
process. The collision between the tiny rigid balls in a hidden
container generates localised high pressure during the ball milling
process. Usually, ceramic, flint pebbles and stainless steel are used.
Selected chemicals are used inside grinding container to include
functional groups onto CNTs.