Dissociation of the products of graphite anode ablation in an arc producing nanoparticles. (July 2022)

• Record Type: Journal Article
• Authors: Nemchinsky, Valerian
• Is Part Of: Carbon trends. Number 8(2022)
• Publication Date: 2022-07
• Subjects: Carbon molecules -- Dissociation -- Anode thermal regime -- Graphitel -- Arc -- Nanoparticles -- Ablation rate; Carbon -- Periodicals; Carbon composites -- Periodicals; Carbon; Carbon composites; Periodicals; 620.193
• DOI: 10.1016/j.cartre.2022.100187
• Languages: English
• ISSNs: 2667-0569

Abstract

Abstract: An arc with ablating graphite anode is used to produce nanoparticles. At the high anode temperatures taking place in the arc, graphite sublimates almost exclusively in the form of molecular trimers C3 . However, optical methods of observation do not find any C3 in the plasma: only C2 close to the anode and single atoms in the bulk of the plasma. To explain this paradox, the process of ablation product dissociation is considered. It is suggested that trimers dissociate in the reaction C3 +C→ C2 +C2 so quickly that it hampers their observation. Considering dissociation further, it is also suggested that C2 dissociates by virtue of electron impact. Formulae to calculate rates of these reactions were proposed. The composition of the arc plasma was calculated in the case of a low ablation rate when dissociation processes only slightly disturb the heat balance of the plasma. Calculations confirm the hypothesis of a fast C3 dissociation: C3 exists in a narrow layer of less than a few tenth of a millimeter thickness. Also, it is shown that the composition of the plasma flow at the anode changes with the plasma velocity in the anode's vicinity: at high velocity, flux at the anode consists almost exclusively of evaporating trimers whereas at low velocity dimers and atoms returning to the anode are also represented in the flow. As a result, the cooling effect of ablation expressed as J/kg (even at a constant ablation rate in kg/s!) depends on gas-dynamic velocity close to the anode. The energy cost of sublimation decreases as plasma velocity decreases. This is related to different heat of sublimation to mass ratios of different carbon species. Graphical abstract: Image, graphical abstract