[Paper Review] Validation of non-equilibrium kinetics in CO2-N2 plasmas

C. Fromentin, Tiago Silva+4 more|arXiv (Cornell University)|Jan 21, 2023

Plasma Applications and DiagnosticsMedicine3 citations

TL;DR

This study validates a self-consistent kinetic model for CO2-N2 plasmas under non-equilibrium conditions, demonstrating that N2 addition enhances CO2 dissociation (up to 70%) and increases vibrational temperatures via V-V energy transfer and reduced quenching. The model accurately reproduces experimental data from DC glow discharges at 0.6–4 Torr and 50 mA, attributing improvements to N2-mediated energy transfer and reduced back-reactions.

ABSTRACT

This work explores the effect of N2 addition on CO2 dissociation and on the vibrational kinetics of CO2 and CO under various non-equilibrium plasma conditions. A self-consistent kinetic model, previously validated for pure CO2 and CO2-O2 discharges, is further extended by adding the kinetics of N2. The vibrational kinetics considered include levels up to v = 10 for CO, v = 59 for N2 and up to v1 = 2 and v2 = v3 = 5, respectively for the symmetric stretch, bending and asymmetric stretch modes of CO2, and account for electron-impact excitation and de-excitation (e-V), vibration-to-translation (V-T) and vibration-to-vibration energy exchange (V-V) processes. The kinetic scheme is validated by comparing the model predictions with recent experimental data measured in a DC glow discharge operating in pure CO2 and in CO2-N2 mixtures, at pressures in the range 0.6 - 4 Torr (80.00 - 533.33 Pa) and a current of 50 mA. The experimental results show a higher vibrational temperature of the different modes of CO2 and CO and an increased dissociation fraction of CO2, that can reach values as high as 70%, when N2 is added to the plasma. On the one hand, the simulations suggest that the former effect is the result of the CO2-N2 and CO-N2 V-V transfers and the reduction of quenching due to the decrease of atomic oxygen concentration; on the other hand, the dilution of CO2 and dissociation products, CO and O2, reduces the importance of back reactions and contributes to the higher CO2 dissociation fraction with increased N2 content in the mixture, while the N2(B3Pg) electronically excited state further enhances the CO2 dissociation.

Motivation & Objective

• To extend a validated kinetic model for pure CO2 and CO2-O2 plasmas to include N2 kinetics for CO2-N2 mixtures.
• To investigate the impact of N2 addition on CO2 dissociation efficiency and vibrational excitation under non-equilibrium plasma conditions.
• To validate the extended kinetic model against recent experimental data from DC glow discharges in CO2-N2 mixtures.
• To identify the dominant mechanisms enhancing CO2 dissociation and vibrational excitation in N2-doped CO2 plasmas.

Proposed method

• Development of a self-consistent kinetic model incorporating vibrational levels up to v = 10 for CO, v = 59 for N2, and v1 = 2, v2 = v3 = 5 for CO2 modes.
• Incorporation of electron-impact excitation/de-excitation (e-V), vibration-to-translation (V-T), and vibration-to-vibration (V-V) energy transfer processes.
• Simulation of DC glow discharge conditions at 0.6–4 Torr and 50 mA, with varying N2 fractions in CO2.
• Comparison of model predictions with experimental measurements of vibrational temperatures and CO2 dissociation fractions.
• Analysis of contributions from CO2-N2 and CO-N2 V-V transfers, reduced atomic oxygen quenching, and N2(B³Πg) electronic excitation.
• Use of a consistent set of rate coefficients and cross-sections for all species, ensuring thermodynamic consistency.

Experimental results

Research questions

• RQ1 How does N2 addition affect the vibrational temperature of CO2 and CO in non-equilibrium CO2-N2 plasmas?
• RQ2 What are the dominant energy transfer mechanisms responsible for enhanced CO2 dissociation in N2-doped CO2 plasmas?
• RQ3 To what extent does reduced atomic oxygen concentration due to N2 dilution suppress quenching and influence vibrational excitation?
• RQ4 How does N2(B³Πg) electronic excitation contribute to CO2 dissociation in the plasma?
• RQ5 Can the extended kinetic model accurately reproduce experimental CO2 dissociation fractions and vibrational temperatures in CO2-N2 mixtures?

Key findings

• N2 addition increases the vibrational temperature of CO2 and CO modes, with the effect most pronounced in the asymmetric stretch and bending modes.
• CO2 dissociation fractions reach up to 70% in CO2-N2 mixtures, significantly higher than in pure CO2 plasmas.
• CO2-N2 and CO-N2 V-V energy transfer processes are primary contributors to elevated vibrational temperatures.
• Reduced atomic oxygen concentration due to N2 dilution decreases quenching of vibrationally excited states, enhancing energy retention.
• Dilution of CO2 and its dissociation products reduces back-reaction probabilities, favoring net dissociation.
• The N2(B³Πg) electronically excited state further enhances CO2 dissociation by providing additional energy pathways.