Influence of N₂, O₂, and H₂ admixtures on the electron power balance and neutral gas heating in microwave Ar plasmas at atmospheric pressure

Abstract

A combination of optical emission and absorption spectroscopy of argon 2p –1s transitions (Paschen notation) combined with collisional-radiative (CR) modelling of argon 2p states was used to characterize microwave argon plasmas at atmospheric pressure in presence of N2, O2, and H2 admixtures. In particular, the neutral gas temperature (obtained from the broadening of argon 2p 2–1s2 and 2p 3–1s2 emission lines), the number density of argon 1s5 atoms (obtained from absorption spectroscopy of the argon 2p 9–1s5 transition using a tunable laser diode), the electron temperature (obtained from the comparison between measured and simulated argon 2p -to-1s relative line emission intensities), and the electron density (obtained from the Stark broadening of the Hβ line and argon relative line emission intensities) were recorded as a function of the axial distance along the microwave plasma column. The results show that, for a given position in the plasma and a higher amount of admixture in the nominally pure argon plasma, the neutral gas temperature increases and the electron number density decreases, while the electron temperature and the population of argon metastable atoms first decreases and then increases at higher concentrations. With such information, a detailed analysis of the electron power balance was performed. It is found that less than 1% of the admixture in the argon plasma already absorbed more than 80% of the microwave power. Part of this energy is used for neutral gas heating, mostly through electron-impact excitation of rotational levels.

Microwave Plasma Jet in Water: Effect of Water Electrical Conductivity on Plasma Characteristics

“…Figure 10c shows that, regardless of water conductivity, continuously increases from ~2700 K to ~3400 K as N2 content increases from 0 to 2.5%. Previous studies have shown that such increase in temperature is due to the excitation of N2 rotational levels, with contributions from other phenomena such as excitation of vibrational and electronic levels, ionization, and dissociation [25], [28], [29]. Although this explanation is very well accepted for gas-phase microwave plasmas, our results show that it is also valid for the submerged MWPJ investigated in this study.…”

“…Adding N2 to Ar significantly influences plasma properties, including electron density, electron temperature, and gas temperature [28], [29]. Recently, we have shown that the addition of 1-2% N2 to the Ar flow in MWPJ improves the efficiency of MB degradation [21].…”

On the transition of reaction pathway during microwave plasma gas‐phase synthesis of graphene nanosheets: From amorphous to highly crystalline structure

“…For a successful graphene synthesis, a delicate balance among all these intrinsically interconnected processes is needed. Such balance can be significantly altered by the presence of a reactive gas (e.g., O 2 or H 2 ), as it affects both plasma parameters [ 50 ] and chemical composition. [ 51 ] This can be either unintentional (impurities) or intentional (deliberate admixture).…”

Characterization of a microwave plasma jet (TIAGO) in‐contact with water: Application in degradation of methylene blue dye

“…The results clearly show a decrease in the width of Hα lines, thus, a decrease in electron density with increasing N 2 content (phenomena already reported) for both conditions of d . In fact, the addition of N 2 to Ar, even at very low percentages (<1%), is well known to induce a decrease in electron density due to power loss by molecular excitation/ionization phenomena that undermine the ionization of Ar and the production of electrons …”

“…The results clearly show a decrease in the width of Hα lines, thus, a decrease in electron density with increasing N 2 content (phenomena already reported [34,35] ) for both conditions of d. In fact, the addition of N 2 to Ar, even at very low percentages (<1%), is well known to induce a decrease in electron density due to power loss by molecular excitation/ ionization phenomena that undermine the ionization of Ar and the production of electrons. [36] Gas temperature measurements in plasmas are often obtained by emission spectroscopy from the population distribution in rotational levels of excited states of diatomic molecules (such as OH, NH, N 2 + , etc.). In plasma conditions very close to ours, OH (A-X) was found to be the best reliable probe to obtain a satisfactory estimation of the gas temperature, due to the highly favorable exchange energy of heavy particles and the internal rotational-vibrational states of this molecular species.…”