In this paper, dielectric columns with different dielectric constants are employed as dielectric materials in the packed bed reactor to investigate the dynamic behaviors of plasma interaction processes. The effects of the dielectric constants (zirconia: ε = 25 and PTFE: ε = 2.5) on the production of reactive species are studied for plasma catalysis applications. Comparison studies of discharge images, electrical characteristics, discharge dynamic evolution and spatial-temporal resolved optical emission spectroscopy are carried on when zirconia and PTFE columns are employed. The results show that there are four discharge processes existing in the packed bed reactor: surface streamer on the dielectric column, local discharge at the contact point, surface discharge on the grounded dielectric plate, and the volume discharge. The production of reactive species such as N 2 (C 3 Π u ), N + 2 (B 2 Σ + u ) and O(3p 5 P) depend on the discharge processes to a great extent. The production of the N + 2 (B 2 Σ + u ) always accompanies the formation of the streamer by electrons direct impact process to excite the ground state nitrogen molecules to N + 2 (B 2 Σ + u ). The O(3p 5 P) is generated in two different ways, which plays a major role during the voltage pulse raising and falling time, respectively. The rst way is the direct and fast one-step ionization and excitation by high energy electrons with O 2 . The second way is the energy transfer from the nitrogen metastable N 2 (A 3 Σ + u ) and energetic electrons, in which the O is rst ionized from O 2 and then excited to O(3p 5 P). Furthermore, compared with a zirconia column, a PTFE column is more conductive to the generation of reactive species.
The fixation of atmospheric nitrogen into valuable compounds through reactive plasma processes has attracted intense interests due to its easy operation and compatibility with distributed renewable energy sources. However, practical implementation of plasma-assisted nitrogen fixation is hampered because of its relatively low throughput, which is dominantly limited by the unclear underlying mechanisms. In this study, effort was focused on the in situ production of key species in a DC-driven warm air glow discharge at atmospheric pressure with the help of advanced laser spectroscopic diagnostics. Laser Rayleigh scattering was applied to determine the gas temperature distribution in the discharge column. And mid-infrared quantum cascade laser absorption spectroscopy and one/two-photon absorption laser-induced fluorescence were performed on molecular nitric oxide (NO), atomic oxygen and nitrogen (O, N) for their absolute densities in the discharge. It is found that the spatial distributions of gas temperature, O and N atoms show peaks in the hot discharge center. In contrast, a hollow ‘doughnut’ shape characterized by the NO molecule was observed, particularly under conditions of high discharge current but low airflow rate. The steady-state simulation shows that the hollow pattern of NO is dominantly induced by the radial diffusion of species due to the steep spatial gradient of gas temperature in the discharge cross-section. Moreover, the reverse conversion by atomic N leads to a negative effect on the NO synthesis, especially at the discharge center where the N density and gas temperature are high. From the steady-state modeling, a similar hollow distribution of NO2 was depicted in the air glow discharge. These results demonstrate the strong dependence on atomic O for the major formation process of NO, and the importance of suppressing the reverse paths dominated by atomic N for higher NO production in the studied warm air plasma.
In this study, spatial-temporal resolved optical emission spectroscopy and electrical characteristics are employed to study the dynamic evolution of molecules, vibrational distributions, reactive species, and streamer head speed in the generation and propagation of atmospheric pressure plasma jets (APPJs). The images of discharge, waveforms of pulse peak voltage and discharge current, and spatial-temporal emission spectra of N 2 (C 3 Π u → B 3 Π g , 380.5 nm), N + 2 (B 2 Σ + u → X 2 Σ + u , 391.4 nm) and He (3s 3 S → 2p 3 P, 706.5 nm) are recorded, the relative vibration population of N 2 (C 3 Π u ) and the key dynamic process of each discharge pulse are discussed. The effects of pulse peak voltage on emission intensity, vibration population of N 2 (C 3 Π u ) and APPJ speed are also investigated. The results show that the streamer head speed is about 10 5 m s −1 under the pulse peak voltage of 5-9 kV, and both the streamer head speed and emission intensities increase with pulse peak voltage. The emission intensities of N 2 (C 3 Π u → B 3 Π g , 380.5 nm) rise for about 10 ns but fall for several tens of nanoseconds. During the plasma generation process, the direct electron impact process is dominant in generating electronic, vibrationally and rotationally excited N 2 . Several tens of nanoseconds after the generation, spontaneous emission, quenched by N 2 and O 2 dominate the decay process. While the step excitation and Penning ionization extend the duration time of emission intensity. It is also found that the ratio of N 2 (C, υ = 1)/N 2 (C, υ = 0) is at high level within 1µs. The evolution of the ratio is dominated by the direct electron impact in the initial time, and is then possibly influenced by the vibrational relaxation process and downward vibrational-vibrational energy transmission process.
The plasma in contact with liquids has led to various novel applications such as plasma biomedicine, material synthesis, and so on. However, the phenomenon of evaporation under plasma treatment and its impact on plasma–liquid interactions has a limited understanding. In this study, the spatially and temporally resolved behavior of water vapor production and its induced influences on plasma properties and gaseous chemistry were studied in detail in an atmospheric pressure pin‐to‐water pulsed He discharge. Diagnostic methods such as laser‐induced fluorescence (LIF) and high‐resolution optical emission spectroscopy (OES) were applied to determine the water vapor and OH radical densities, as well as key plasma parameters such as the gas temperature and electron density. It shows that the physicochemical properties of plasma vary among different discharge regions due to evaporation behavior stimulated during the pulsed discharge‐on phase. In addition, using simulation based on the experimental data, the mechanisms of how water vapor affects the observed spatiotemporal behaviors of OH radicals in different discharge regions are understood. Compared to the pin‐anode and liquid‐cathode sheath regions, proper electron parameters such as density and temperature, as well as water vapor density in the plasma‐positive column, significantly enhance the production of reactive OH radical through the dominant path of electron‐stimulated H2O dissociation. However, higher levels of electron parameters in the intense discharge region near the positive‐pin boundary enhance OH dissociation and finally result in the hollow distribution of OH density. From the global kinetic plasma simulation, the production of reactive hydroxide species playing key roles in plasma medicine treatments, such as O, H, HO2, H2O2, and hydrated ions including H+(H2O)4 and H+(H2O)5, are promoted noticeably as a result of the enhanced water evaporation process.
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