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

TL;DR

This study investigates the positive anode sheath in atmospheric pressure anodic carbon arcs used for nanomaterial synthesis, revealing sheath characteristics, ablation modes, and electron temperature insights through combined probe and optical emission spectroscopy measurements.

Contribution

It provides experimental evidence of a positive anode sheath in the arc and analyzes its relation to ablation modes and arc voltage, advancing understanding of arc physics in nanomaterial production.

Findings

Confirmation of a positive anode sheath in both ablation modes

Low electron temperature (~0.6 eV) from optical emission spectroscopy

Higher arc voltage (~20 V) needed for arc stability than predicted

Abstract

In the atmospheric pressure anodic carbon arc, ablation of the anode serves as a feedstock of carbon for production of nanomaterials. It is known that the ablation of the graphite anode in this arc can have two distinctive modes with low and high ablation rates. The transition between these modes is governed by the power deposition at the arc attachment to the anode and depends on the gap between the anode and the cathode electrodes. Probe measurements combined with optical emission spectroscopy (OES) are used to analyze the voltage drop between the arc electrodes. These measurements corroborated previous predictions of a positive anode sheath (i.e. electron attracting sheath) in this arc, which appears in both low and high ablation modes. Another key result is a relatively low electron temperature (~ 0.6 eV) obtained from OES using a collisional radiative model. This result partially explains a higher arc voltage (~ 20 V) required to sustain the arc current of 50-70 A than predicted by existing simulations of this discharge.