Effects of process conditions on the chemistry of an Ar/C₂H₂ dust‐forming plasma

Abstract

A volume‐averaged model and numerical simulations are used to clarify the effects of process conditions on the plasma chemistry and species initiating the formation of nanoparticles in an Ar/C2H2 plasma. It is shown that Ar/C2H2 plasmas with low electron density, moderate input flux of acetylene and an electron energy distribution function (EEDF) close to the Druyvesteyn EEDF are the most suitable for the production of carbonaceous nanoparticles. These results are verified by direct comparison with experimental data and enable to formulate recommendations for future experiments with a controlled growth of nanoparticles in chemically active plasmas.

Plasma properties as function of time in Ar/C2H2 dust-forming plasma

“…The density of C2H3 + is found to be smaller (not shown in figure 3) than that of other ions, which agrees well with results of our previous studies [15]. The C2Hanions are produced in collisions of electrons with acetylene molecules, while they are mainly lost in collisions with positive ions and hydrogen atoms [28]. As a result, the rapid increase of ne in the beginning of phase I (as nd is set to 0) is also accompanied by the rapid increase in C2Hdensity (figure 2(b)).…”

“…Thus, in this model the total rate for loss of Arm in collisions with C2H2 is nearly the same as the rate in [61]. More information on the different reactions and rates for various generation and loss processes, as well as elastic and inelastic collisions taken in to account in the power balance (equation ( 4)), can be found in [15,28] and in Supporting Information to [28].…”

“…Since the loss of C2H2 molecules is essentially affected by their collisions with excited argon atoms and C2H radicals [28], their time-dependency is nearly the inverse to those for Arm and C2H (figures 1(a), 1(b) and 2(a)). Therefore, the minimum in nC2H2 is nearly at the same time as the maximums for Arm and C2H.…”

“…Its decrease with increasing nd during phase II is mainly due to the decrease of the densities of electrons (figure 6(c)) and H2 molecules (figure 5(b)) and due to the decrease of nm and nC2H in the second part of phase II (figures 6(a) and 6(b)). Note that the production of atomic hydrogen in Ar/C2H2 plasmas is mainly due to collisions of Ar* with C2H2, C4H2 and H2, of Ar + with H2 and of C2H with C2H2 and C4H2 , as well as due to electronimpact dissociation and ionization of H2 molecules (Supporting information to [28]).…”

Molecular growth paths and dust‐particles nucleation precursors in Ar/C2H2 low pressure discharges

“…This rate shows a maximum of 17% acetylene in the feed gas, which means that increasing C 2 H 2 % above this value, does not enhance C 2 H 2 conversion. The high conversion yields of acetylene obtained at low C 2 H 2 % were also observed experimentally [16,43,44] and predicted numerically [45] for similar discharges conditions by other groups. The primary fragmentation of C 2 H 2 results in C 2 H as a major radicalar species.…”