Producing carbon nanotubes using the technology of DC thermal plasma torch

Carbon nanotubes (CNT) are long hollow fibers made exclusively of carbon. They are generally synthesized from the dissociation of a carbon source and the co-injection of a metal catalyst. CNT interest the scientific community due to their impressive properties. They possess a high modulus of elasticity, high tensile strain, superconductor and semiconductor behaviours, high thermal conductivity and storage capabilities. These properties have been recently exploited to develop promising applications in the field of advanced materials, electronic components, and storage of gas and materials. However, the implementation process of these products is hindered due to the unavailability of large quantities of CNT and the current high cost of production. Presently, CNT are commercially produced via the arc discharge on graphite electrodes and the laser ablation of graphite. These methods yield limited production rates and represent low scale-up potential. In this work, this problem is solved by developing a new process in which CNT are produced via the dissociation of a liquid carbonaceous raw material in the water-cooled nozzle of a 100 kW DC non-transferred thermal plasma torch. The attractive aspect of this process is its scalability, since the technology of DC thermal plasma torch operating at the megawatt level is readily available.Here, tetrachloroethylene (TCE) is selected as the carbon raw material. It is preliminarily vaporized and injected into the plasma torch nozzle. The metal catalyst for the growth of CNT is provided from the erosion of the torch electrodes made of tungsten. The erosion of the electrodes generates tungsten vapours that are nucleated into catalyst nanoparticles of 10-20 nm in diameter in the plasma torch nozzle. Multi-walled nanotubes (MWNT) of 50 urn in length and 10-30 run in diameter are found in a graphitized coating condensed on the walls of the water-cooled torch nozzle. These MWNT are particularly grown on the surface of the catalyst nanoparticles at a temperature above the peritectic transformation of tungsten carbide (WC). This synthesis method also generates byproducts. Carbon nano-onions and nanoparticles are notably observed in the soot.CNT mass yield values are evaluated from thermogravimetry analysis and corroborated using electron microscopy (SEM, TEM) and Raman spectroscopy. This led us to conduct a parametric study in order to determine the influence of the different operating parameters on the yields of carbon nanotubes. Low pressure, high TCE feed rate and long time of operation all enhance the yield of CNT. This parametric study and CFD modeling reveal that the formation of a supersonic jet, high vapour pressures of carbon and a control of the temperature profile within the water-cooled torch nozzle generate a rapid growth of CNT.Optimization work with the view to push the formation of CNT downstream of the torch nozzle and to synthesize thinner CNT is also exposed. In summary, the injection of the carbonaceous raw material in a hot torch nozzle prevents the condensation of a graphitized coating and enables continuous operation over several hours. Nickel and iron are injected into the plasma jet as metal catalyst. The resulting metal vapours are nucleated into small sized nanoparticles (≤ 10 nm). These successfully catalyze the synthesis of thinner CNT of ≤ 10 run in the soot collected downstream of the hot torch nozzle, some of which are SWNT of 1 nm-2.5 nm in diameter. The thermal plasma dissociation of toluene as liquid carbonaceous raw material generates hydrogen gas (H2) in the plasma jet, which sensibly reduces the formation of amorphous carbon and yields cleaner CNT