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

Abstract

The work is devoted to the development of laser absorption spectroscopy (LAS) of plasma-assisted processes for application under industrial conditions. The interpretation of the LAS measurements was revised by taking into consideration the temperature gradient along the absorption path, which is unavoidable in a reactor for thermochemical treatment. The revision is based on the measurement of HCN, NH3, H2O and CO molecular lines in an industrial-scale, active screen plasma nitrocarburizing (ASPNC) reactor with a steel active screen (AS). It shows that an effective temperature determined from Doppler broadening could be assigned to each measured spectral line. The effective temperature does not only reflect the temperature gradients along the line-of-sight but also the line strength dependence on temperature for the specific spectroscopic transition. Lower limit estimates of the molecular densities are proposed based on the determined effective temperatures under the assumption of a Boltzmann distribution of the population density over the molecular levels at any local volume of the reactor. For a more accurate interpretation of LAS data of plasma-assisted processes, the spatial distribution of the temperature along line-of-sight has to be known and needs to be taken into account to obtain the molecular densities.

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

“…ASPNC reactors have two types of surfaces, one are the steel AS, the model probe, and workload table with a temperature near the process temperature of about 800 K and the other type of surface are the reactor walls with a significantly lower temperature. As described in detail in part I, see [19], the region between AS and reactor walls has the strongest temperature gradient along the line-of-sight.…”

“…The density evolution of HCN, NH 3 , H 2 O and CO molecules over time was monitored by laser absorption spectroscopy (LAS) with a single pass of the laser beam through the reactor. The LAS system and the method of the density determination are described in detail in the previous part [19].…”

“…Preliminary results of the work were reported in [18]. The first part of the manuscript [19] discusses peculiarities of the LAS diagnostic for application in the active zone (e.g. treatment zone) of a reactor for thermochemical treatments.…”

“…In the case, when the temperature distribution along line-of-sight is unknown, the method to estimate lower limits of the molecular densities was proposed and applied to the obtained experimental data. Time evolutions of those densities obtained in [19] are analysed further in the current manuscript.…”

The Interplay Effects between Feed-Gas Composition and Bias Plasma Condition during Active-Screen Plasma Nitrocarburizing with a Solid Carbon Source

“…This waiting-time (stabilization time) between each measurement was determined to be the minimum interval required to reach stability of the treatment temperature and AS plasma power. The measurement and data evaluation procedures for LAS have been described recently in detail in [32]. Briefly, a single absorption path of 125 cm in length through the reactor was used, considering a 100 cm path inside the inner reactor volume and a 25 cm path through the reactor walls and holders of the infrared transmissive windows.…”