Study Of NO₂in Air Plasma By Cavity Ring-down Spectroscopy

The generation of high energy electron, radicals and reactive species in low temperature plasma enables physical processes and chemical reactions which are normally difficult to conduct. Thus, it is an effective method for NOx removal. With the wide utilization of plasma technology in large scale industrial production, the by-products of gas discharge arouse attention gradually, in which NOx suffers the most. In order to look into the reactions of NOx in plasma, one of the problems is fast in situ measurement of NOx in plasma environments. As an absorption spectroscopy with long absorption length and high sensitivity, cavity ring-down spectroscopy (CRDS) can suffer laser intensity fluctuations and obtain absolute concentration without calibration, which makes it a suitable technology for trace gas detection. In this thesis, pulse cavity ring-down spectroscopy is employed to do quantitative in situ measurement of nitrogen dioxide (NO2) in dielectric barrier discharge (DBD) and DC pulse discharge. The main results are summarized as follows:1. A pulsed-CRDS system is developed with detection limit of4.7×10111cm-3and17.5ppb in standard air pressure. In low pressure DBD, the number density of NO2increases almost linearly with voltage with the fixed frequency of10kHz and voltage between900v and1300v. The number density of NO2increases as frequency rises with the fixed voltage of lkv and frequency between8kHz and12kHz. The increase of voltage and frequency results in the enhancement of current and power and in turn the concentration of NO2.The removal and remove rate in DBD with unchanged voltage and frequency is examined under different initial concentration of NO2. We find that the removal increases almost linearly with initial concentration of NO2while remove rate increases at first, then reaches the peak and decreases slowly with the trend of becoming constant in the end.2. Combined with digital delay generator and time window technique, CRDS system realizes time resolved measurement with resolution of the order of microsecond. With NO2initial concentration of8.88×1013cm-3, time evolution of NO2in low pressure DC pulse discharge afterglow is measured by the system. It shows that the concentration of NO2drops rapidly after discharge and stabilizes in several tens of microseconds. The NO2removed directly by pulse discharge is measured. Under high initial concentration, remove rate of NO2increases with larger pulse duration. Under low initial concentration, remove rate of NO2increases at first and then decreases. In addition, under low initial concentration, remove rate of NO2increases sharply.