Figure 3: Schematic of Arc Discharge Method [Rajesh et al., 2014]

Laser Ablation

High intensity laser light act as energy source to excite carbon atoms in laser ablation technique. Laser beam like YAG or CO2 vaporizes a graphite target under high temperature in an inert atmosphere. Carbon species are produced by the laser. These species are swept from a high temperature zone to a water cooled copper collector by the flowing inert gases. The reaction temperature determines the quality & yield. Small amounts of transition metal such as Ni, Fe or Co when added to the carbon target, SWCNTs are produced. Various parameters like catalyst composition, growth temperature, nature of gases & gas pressure, average nanotube diameter and size distribution can be varied by varying the laser used. Carbon atoms and molecules condense from these vaporized species to form large carbon clusters. Catalysts which also start to condense, but rather slowly, attaches to these carbon clusters. The tubular structured carbon molecules from this form SWCNTs. This process of CNT growth continues until conditions have cooled sufficiently so that carbon atoms can no longer diffuse through or over catalyst surface particle. This growth also gets hindered as the size of catalyst particles become too large. Selective Laser Melting (SLM) is another technique used for fabricating CNT reinforced Al nanocomposites[Dongdong et al, 2019]. This material is used to prepare Aluminium metal matrix which are used in combination with phase changing materials for energy storage. This process uses laser based additive manufacturing technique. Both arc discharge & laser ablation techniques are very expensive due to the high power equipment so they are mainly used for SWCNTs production. These methods also produce small quantities of CNTs with a lot of impurities.