H_2: The Critical Juncture between Polymerization and Dissociation of Hydrocarbons in a Low-temperature Plasma

摘要

The chemistry in an argon plasma jet, admixed with a small percentage of CH_4, C_2H_2, H_2 and mixtures thereof, is investigated by means of residual gas analysis. Polymerization of such hydrocarbon precursors is known to occur when their densities exceed the Ar+ ion density. This paper shows that polymerization also occurs for precursor gas flows far below the initial Ar~+ ion flow emanating from the plasma source. This is entirely due to the negative effect of H_2 on the Ar~+ ion density. Adding 1-2% of H_2 to the total argon and hydrocarbon gas flow suffices to initiate polymerization. Although, H_2 can be injected directly into the system, fragmentation of the hydrocarbon precursors themselves can likewise supply (part of) the required H_2. Polymerization is furthermore enhanced when both precursors are used together. The contribution of C_3H_y species to the plasma chemistry will likewise be substantiated.