Inspection of contamination in nitrogen plasmas by monitoring the temporal evolution of the UV bands of NO-γ and of the fourth positive system of N2

Abstract

In this study, contamination by oxygen species in nitrogen plasmas produced by the active screen system used for plasma nitriding has been investigated by optical emission spectroscopy in the spectral range of 200–900 nm. Temporal evolution of emission intensity of different species (N2, N2+, NO, OH, H, NH, and Fe) was monitored, as well as electrical characteristics (current and voltage) of the discharge produced by a pulsed unipolar power supply. In nitrogen plasma, it was found that the emission of oxygen-containing species (NO and OH) decreases with time, while Fe emission intensity increases. Such behavior is observed only when the discharge is initiated immediately after venting the reactor. Starting from the hypothesis of contamination of the reactor walls by water vapor, we propose an explanation based on the synergistic effect between the temperature and the reactive nitrogen created by the plasma. Such a long decay of NO emission was not observed in N2–H2 gas mixtures. After decontamination, the fourth positive system of N2 could be observed. Such a not commonly observed system can be used as a tool to control reactor cleanliness in pure nitrogen discharge for nitriding applications.

Laser absorption spectroscopy for plasma-assisted thermochemical treatment. Part I.: Applicability of the Beer–Lambert law and interpretation of spectroscopic data

“…Monitoring the composition of the discharge is important for the process control and optimization as well as for understanding of the chemical kinetics. Three approaches are broadly reported: optical emission spectroscopy (OES) [2,10,11], mass spectrometry (MS) [12][13][14] and laser absorption spectroscopy (LAS) [1,3,10,[15][16][17][18][19][20]. OES monitors exclusively electronically excited species and quantitative interpretation of OES data requires complex collisional-radiative models.…”

Laser absorption spectroscopy for plasma-assisted thermochemical treatment. Part II.: Impact of the carbon and water contaminants on a low-pressure N2–H2 discharge

“…Investigations of those surface plasma-chemical processes require monitoring of the discharge during multiple hours and complex theoretical considerations and therefore are sparsely reported. Recently it was shown that water vapour adsorbed during venting of the reactor contributes to the AS plasma nitriding process as a source of oxygen-containing species over a long period of time comparable with the duration of the thermochemical treatment [11]. This can be critically, because the presence of oxygen, even at a relative amount of 1%, can significantly affect PNC processes via reduction of carboncontaining active species in the treatment environment [12].…”