Analytical model of low and high ablation regimes in carbon arcs

Abstract

Graphite ablation in a presence of inert background gas is widely used in different methods for the synthesis of carbon nanotubes, including electric arc and laser/solar ablation. The ablation rate is an important characteristic of the synthesis process. It is known from multiple arc experiments that there are two distinguishable ablation regimes, so-called "low ablation" and "high ablation" regimes in which the ablation rate behaves rather differently with variation of the arc parameters. We developed a model that explains low and high ablation regimes by taking into account the presence of a background gas and its effects on the ablation rate. We derive analytical relations for these regimes and verify them by comparing them with full numerical solutions in a wide arc parameter range. We comprehensively validate the model by comparing to multiple experimental data on the ablation rate in carbon arcs, where various arc parameters were varied. Good qualitative and quantitative agreement between full numerical solutions, analytical solutions, and experimental data was obtained.

Determination of positive anode sheath in anodic carbon arc for synthesis of nanomaterials

“…The anode ablation rate and cathode deposition rates showed dependence on gap distance. A possible explanation of the diminished anode ablation rate in the larger 4.5 mm gap case is provided by [15]; as the gap increases, less of the thermal radiation emitted from the cathode surface reaches the anode surface, resulting in a lower anode surface temperature and hence lower anode ablation rate. The lower cathode deposition rates in the 4.5 mm gap case are attributed to increased radial losses of ablated material.…”

“…During the arc operation, carbon material is introduced into the arc plasma by the ablation of the graphite anode [5,[7][8][9]. The anode ablation depends on the power balance at the anode, which among different factors, should be influenced by whether the anode sheath is electron-repelling (negative anode sheath) or electron-attracting (positive anode sheath) [7,[12][13][14][15][16]. Another important feature of the carbon arc is the transition between socalled low and high ablation regimes which occurs at a certain arc current threshold.…”

“…Another important feature of the carbon arc is the transition between socalled low and high ablation regimes which occurs at a certain arc current threshold. This current threshold can be affected by the anode diameter, gas pressure, and the gap between the anode and the cathode [7,15,17]. For low ablation, the arc current is below the current threshold.…”

“…Recent theoretical studies have modeled the carbon arc plasma parameters and have predicted that the transition from low to high ablation mode could be explained by the presence of the background working gas (e.g. helium) impeding ablated carbon flow [15,16]. Models predict that this transition is also influenced by the sheath between the plasma and the anode (anode sheath) [15].…”

“…helium) impeding ablated carbon flow [15,16]. Models predict that this transition is also influenced by the sheath between the plasma and the anode (anode sheath) [15]. However, there is an unresolved discrepancy between these models and experiments.…”