The effects of dilution gas on nanoparticle growth in atmospheric-pressure acetylene microdischarges

A two-dimensional multi-fluid model is developed to investigate the effects of dilution gas on microplasma properties and nanoparticle behavior in atmospheric-pressure radio-frequency acetylene discharges. The percentage of dilution gases (argon and helium) percentage varied from 0% to 90%, with the pressure kept constant. Simulation results show that the dilution gas percentage has a significant influence on the spatial distributions of the electron density and temperature, as well as on the formation of nanoparticles in acetylene microplasmas. With increasing dilution gas percentage, the electron density profile changes continuously from being high at the edge to high in the center. A mode transition from a mixed discharge mode with both α regime and drift-ambipolar regime into α regime occurs, which is associated with a sudden decrease in the electron density of the presheaths and an increase in the electron temperature of the bulk plasma. The mode transition point corresponds to the lowest number density ratio of hydrocarbon ions to acetylene. The highest number density ratio is observed at a dilution percentage of 90%, and causes more effective nucleation and coagulation of nanoparticles. Furthermore, owing to the high ionization potential of helium, the transition point moves to a larger dilution gas percentage in C₂H₂/He microplasmas. Finally, the growth of nanoparticles via coagulation is studied.