Role of the cathode deposit in the carbon arc for the synthesis of nanomaterials

The atmospheric pressure carbon arc in helium is an important method for the production of nanomaterials. Typical arcs operate in a dc mode between a graphite anode, which is consumed, and a cathode which may be a lower melting point material. During arc operation, a carbon deposit is formed on the cathode surface. This deposit may contain different forms of the synthesised fullerenes. It is shown that this deposit plays a crucial role in conducting the arc current. Temperature measurements demonstrate that a sufficiently large area of the cathode deposit is hot enough for thermionic emission to be the source of most of the arc current. Due to the deposit's low thermal conductivity, the cathode behind the deposit does not reach its melting point. The role of the deposit in emitting electrons can probably be generalized for other arc synthesis methods with consumed anodes.