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Vibrational excitation in a nitrogen plasma, generated by a dielectric-barrier discharge

Authors
Publisher
Technische Universiteit Eindhoven
Publication Date
Disciplines
  • Chemistry
  • Medicine

Abstract

In the past decades, the use of plasmas in chemical processes has increased significantly. Especially in non thermal plasmas, chemical processes can be run with high reactivity (i.e. high reaction rate coefficients), without spending much energy on (translational) heating of the gas. For this reason, plasma chemistry can offer an energy efficient option for inducing or assisting chemical reactions. The dielectric-barrier discharge (DBD) is a plasma chemical reactor configuration which is widely used, since it offers a convenient and cost-effective way for creating a non thermal plasma. Vibrational excitation and gas temperature (translational excitation) have a large influence on rate coefficients and the energy efficiency of plasma chemical reactions. Therefore, vibrational excitation and gas temperature of a nitrogen plasma, generated in a DBD reactor, are investigated in a wide range of the available process variables, namely applied voltage amplitude, pressure and residence time. Also the specific energy input and the reduced electric field, two other relevant plasma parameters in a DBD reactor, are determined as a function of these process variables in order to find possible correlations with vibrational excitation.Optical emission spectroscopy (OES) is used to determine vibrational excitation and gas temperature (translational temperature) of nitrogen molecules in the plasma and to estimate the reduced electric field in the plasma, an important parameter for all gas discharges. Current-voltage (I-U) measurements are performed to determine the specific energy coupled into the plasma, an important parameter regarding the energy efficiency of a chemical process, and to obtain a second estimate for the reduced electric field. The latter diagnostic is also used for a thorough electrical characterization of the gas discharge. From this characterization, the capacitances of the dielectric barriers and the discharge gap are determined, together with the voltage across the discharge gap, both averaged over the discharge phase and as a function of time.The capacitances of the dielectric barriers and the discharge gap correspond to theoretically calculated values. Contrary to a common assumption, the voltage across the discharge gap is not found to be constant during the discharge phase. From the same analysis, the effect of residual charge in a nitrogen DBD is observed. By comparing two methods for determining the voltage across the discharge gap, the possible effect of charge accumulation in a discharge filament is observed. Both methods for determining the reduced electric field yield similar values, though the trends are not identical. Under all conditions, vibrational excitation is found to be in Boltzmann equilibrium, making it possible to assign a vibrational temperature to each operating condition, ranging 2100-3200 K. Gas temperatures are found in the range of 340-650 K. The vibrational temperature is found to be positively correlated with the specific energy input (ranging 0.24-64 kJ per standard liter) and negatively correlated with the reduced electric field (340-530 Td, OES based values) which suggests that a high specific energy input and a low reduced electric field are beneficial for rate coefficients and the energy efficiency of a plasma chemical process in a nitrogen DBD. No conclusive scaling parameter for vibrational temperature is found in this work. For a pure nitrogen plasma, generated in a DBD reactor, the highest rate coefficients may be expected for high values of the applied voltage amplitude and medium values for the residence time, while the highest energy efficiency may be expected at low pressures.

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