Abstract
The rapid formation of large molecules and the subsequent production of solid-state dust particles in a low-pressure discharge is unlikely, because of the low rates of the polymerization reactions and short lifetimes of the species. Here, we suggest that C dust particles can form in atypically low (10− 3 mbar)-pressure hydrocarbon plasmas if the dust charging time is much shorter than the gas residence time in the device; we present supporting experimental evidence for this. Such a condition can be obtained by the production of high-density plasmas. The results show that dust formation from the gaseous phase can occur in a much wider parameter range than is commonly assumed.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. The explanation of the formation of macroscopic particle agglomerates in rarefied media is an intriguing problem that appears in the context of laboratory and space physics observations. The fast formation of large molecules in atypically low (10-3 mbar) pressure conditions should be unlikely because of the low rates of polymerization reactions and short lifetimes of the species, and it is even more puzzling to understand the production of solid-state dust particles in tenuous chemically active plasma.
Main results. In this work, we suggest that C dust particles can be formed if the charging time is much shorter than the gas residence time in the device supported by experimental evidence. To fulfil such a condition at an extremely low gas pressure, a high density magnetized Ar/CH4 plasma has been utilized. The formation of spherical nanosize particles (of diameter ~20nm, see figure) and further dendritic agglomeration was detected by electron microscopy measurements. The chemical nature of the particles was studied ex situ by infrared spectroscopy. The production of acetylenic compounds (C2Hx) appears to be a key mechanism for the powder formation in all of the investigated hydrocarbon plasmas. The gas phase chemistry of rather high density methane plasmas was found to be in agreement with chemical kinetics phenomena occurring on timescales shorter than the gas flow time.
Wider implications. The problem of dust formation in a low-pressure plasma environment is interesting both for technological applications and nuclear fusion research.
Figure. SEM image of a Si wafer exposed to a 25% methane mixture at 10-3 mbar. The dust cluster shown is formed by nanospheres of carbon based particles.