Influence of the anode filling carbon granulometry on plasma features in the electric arc process applied to carbon nanotubes synthesis

This paper is a part of a comprehensive study aiming to understand the relationships between the conditions of the electric arc plasma process applied to the single walled carbon nanotubes (SWCNTs) synthesis and the features of the resulting carbon products. The parameters for this work were the type of hybridization (either sp2 (graphite) or sp3 (diamond)) and the grain size of the carbon source (the anode). The physical properties of the plasma were determined by in situ optical emission spectroscopy. Atomic neutral nickel Ni I and carbon C I lines at 349.3 nm and 909.4 nm, respectively, were selected to quantify the radial concentration distribution of these species in the plasma. The Swan band C2(0,0) at 516.5 nm was used to determine the radial temperature and the C2 column density distributions at the centre of the plasma. Carbon materials formed were analysed using high-resolution transmission electron microscopy. A significant improvement of both yield and purity of the SWCNTs formed was found when the way the anode was prepared corresponded to the highest apparent density of the packed powder. This improvement was correlated with reproducible plasma characteristics, typically an axial temperature in the range 5500-6500 K, a temperature gradient lower than 2800 K mm-1 and a high carbon concentration (∼1025 m-3) associated with a C I/Ni I concentration ratio higher than 108. However, anodes prepared in such a way that their apparent density was lower-which corresponds to standard conditions commonly used for the regular electric arc production of SWCNTs-consistently resulted in the formation of the usual heterogeneous carbon product, including tubular and non-tubular nano-morphologies. These results were correlated with fluctuating, non-reproducible plasma characteristics, typically a high axial temperature (>7000 K), a rapid cooling within the first 0.5 mm away from the discharge axis (>3000 K) and a low carbon concentration (<1021 m-3). The relationships between the synthesis conditions, the resulting plasma features, and the characteristics of the carbon products obtained are finally fully explained, the results from the literature are understood, and the conditions of the plasma for the optimization of the SWCNT synthesis are revealed.