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

  1. 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.