| The atmospheric-pressure dielectric barrier discharges driven by the high-voltage pulses with repetitive frequency (hereafter, called the atmospheric-pressure pulsed dielectric barrier discharges) are of not only good stability, uniformity, chemical activity and thermal non-equilibrium, but also strong ability of generating oxygen atom, ozone, hydroxyl and vacuum ultraviolet, as well as high electron temperature, plasma density and energy transfer efficiency. Especially, features and advantages of the atmospheric-pressure pulsed dielectric barrier discharge can well correspond to actual requirement in the applications of non-thermal equilibrium discharge plasma, which make it both suitable for carrying out large-scale industrialized applications of non-thermal equilibrium discharge plasmas and provide new development opportunity and the research subjects in the fields of gas discharge and discharge plasma. Due to the above, the atmospheric-pressure pulsed dielectric barrier discharges are of important significance for scientific research and wide practical application prospects, and their mechanisms and characteristics have become the hotspot subjects concerned deeply in the fields of gas discharge, plasma physics, plasma chemistry and plasma biomedical engineering.In this dissertation, a one-dimensional fluid model has been developed and used to numerically investigate the mechanisms and characteristics of the pulsed dielectric barrier discharges in argon (Ar), nitrogen (N2) and argon/oxygen (Ar/O2) mixture at atmospheric pressure. Formation mechanisms, evolution processes, discharge modes and properties of the non-thermal equilibrium discharge plasmas in different discharge working gases have been systematically simulated and analyzed. This dissertation includes the following contents and results:1. The mechanisms and characteristics of the pulsed dielectric barrier discharges in Ar and in N2 at atmospheric pressure have been modeled numerically and compared. The state of the art for the investigation on the atmospheric-pressure pulsed dielectric barrier discharges in these two gases has been fully introduced. The one-dimensional fluid model used in this dissertation is described in detail, including modeling structure, modeling particles, basic reactions, governing equations, numerical method and experimental examination. The important characteristic quantities of the discharge plasmas, i.e. discharge current density, gap voltage, dielectric voltage, particle density and gap electric field, have been systematically calculated, and the evolution processes, discharge modes and ionization mechanisms of the discharges in Ar and in N2 under the identical discharge conditions have been studied and compared. The main conclusions are obtained as follows:(1) The discharge in Ar occurs when the gap voltage reaches its maximum and it is in the form of two bipolar pulses, in which one occurs at the rising edge of the applied voltage and the other at the falling edge. The discharge in N2 occurs when the gap voltage starts to increase, and presents relatively stable development in a long time nearly equal to the pulse width.(2) The discharge in Ar is in atmospheric pressure glow discharge mode, and that in N2 is in atmospheric pressure Townsend discharge mode and in the transition status from the Townsend discharge mode to the glow discharge one, respectively, at the different stages of the applied voltage pulses.(3) In the two gases, electron impact ionization is a main ionization mechanism. In Ar, multi-step ionization is the key ionization mechanism since it is able to obviously enhance the ionization rate. In N2, penning ionization can continuously produce charged particles in a relative long time, and thus is also an important ionization mechanism.2. The effects of the discharge conditions on the atmospheric-pressure pulsed dielectric barrier discharges in Ar and in N2 have been systematically studied using the one-dimensional fluid mode. The discharge conditions refer to the operating conditions and the parameters of the applied voltage pulses. The former includes the gap width, dielectric thickness and relative dielectric constant, and the latter includes the amplitude, pulse width, frequency, rising time and falling time. Firstly, the effects of the operating conditions on the mechanisms and characteristics of the discharges in the two gases are studied, and then the pulse parameter dependences of the discharge characteristics in the two gases are calculated and analyzed. The main conclusions are obtained as follows:(1) For the discharges in Ar and in N2, discharge current densities, averaged electron densities and averaged dissipated power densities decrease with the increase of discharge gap width and with increasing dielectric thickness or with decreasing relative dielectric constant.(2) Under the different discharge conditions, the discharges in Ar are in atmospheric pressure glow discharge mode, but those in N2 are in atmospheric pressure Townsend discharge mode or in weak atmospheric pressure glow discharge mode. In addition, the formation of the atmospheric pressure glow discharge in N2 needs to satisfy the requirement of small discharge gap width, thin dielectric thickness and high relative dielectric constant.(3) Discharge current densities and averaged electron densities of the discharges in Ar and in N2 increase monotonously with increasing amplitude of the applied voltage. Increasing the frequency of the applied voltage, the discharge current densities in Ar decrease, but those in N2 increase. In addition, the averaged electron densities in the two gases are in approximately linear increases. With the increase of the pulse width of applied voltage, discharge current densities and averaged electron densities remain unchanged in general in Ar, but increase obviously in N2.(4) To increase the amplitude of the applied voltage can cause the change of discharge mode from weak atmospheric pressure glow discharge to standard one in Ar and from atmospheric pressure Townsend discharge to atmospheric pressure glow discharge in N2.3. Ar/O2 plasmas contain abundant active species and these species play a considerable important role in non-thermal equilibrium discharge plasma material processing and biomedical applications. Besides, to change the oxygen concentration in Ar/O2 mixture can obviously affect the particle densities of the active species and the corresponding application efficiency. Using a one-dimensional fluid model describing non-thermal equilibrium argon/oxygen discharges, the effects of the oxygen concentration on the important characteristic quantities of the discharges in Ar/O2 mixture have been systematically investigated, and the main conclusions are obtained as follows:(1) The waveform of discharge current density of the atmospheric-pressure pulsed dielectric barrier discharge in Ar/O2 mixture is of two bipolar discharge pulses in one period of the applied voltage, which is similar to that in the discharge in Ar. To increase the oxygen concentration in Ar/02 mixture can induce the decrease of the peak value of the first discharge current density and the lag of the time of its peak value, but has a minor influence on the second discharge current density.(2) To increase the oxygen concentration can lead to the decrease of the electron densities in cathode sheath and in plasma positive column regions of the discharge gap, especially for the first discharge. The breakdown voltage for the second discharge and the averaged electron temperature in the pulse duration of the applied voltage and in the time interval between two adjacent voltage pulses increase with increasing oxygen concentration. Besides, the averaged dissipated power density reaches its maximum when the oxygen concentration is 3%.(3) To increase the oxygen concentration can enhance the averaged particle densities of O+, O2(1△g) and O3, but reduce those of Ar+,O and O(1D). Under the different oxygen concentrations, O2+and O3- are the dominated positive and negative oxygen species, respectively, and the averaged particle densities of O, O2(1△g) and O3 are significantly higher than those of other particles.4. Both the contributions of the reaction paths to productions and consumptions of the corresponding particles and the space-time density evolutions of the oxygen species of the discharges in Ar/02 mixture have been further studied on the basis of the fluid model of describing the Ar/02 discharge plasmas. The calculations and statistic analyses on the production and consumption of the oxygen species in the discharges in Ar/02 mixture are performed. Time evolutions of axial distributions of the densities of different oxygen species are also presented systematically. The main conclusions are obtained as follows:(1) The main pathways of producing O, O(1D),02(1△g) and O-are the impact interactions between electrons and O2. The reactions between Ar+and ground state oxygen atoms or ground state molecules play an important role in the productions of O+and O2+. The reaction between neutral particles, i.e. O2+O2+O→O3+O2, generates the vast majority of O3. Also, the two reactions concerning O3, i.e. e+03→O2-+O and O-+O3→O3-+O, produce the majority of O2- and O3-(2) In the discharges in Ar/02 mixture, neutralization reactions between O2+and O-, O2- and O3- are the key pathways of consuming these four kinds of oxygen ions, the respective reaction of three particles O+, O and O(1D) with O2 is the main pathway of consuming these three particles, and both the reaction of 02(1△g) with O3 and the de-excitation reactions of O2(1△g) with electrons, O and O2 are the main pathways of consuming 02(1△g). The reactions between O3 and various neutral particles play the essential role in the consumption of ozone molecules.(3) For the oxygen species of the discharges in Ar/02 mixture, particle densities of O+and O(1D) are relatively low and they are of two obvious wave peaks in the duration of the applied voltage pulse, particle densities of O2+, O, O2(1△g) and O3 are considerably large and almost not vary with the time. In addition, particle densities of O-, O2- and O3- gradually increase in the duration of the applied voltage pulse and slowly decrease at time interval between the two adjacent voltage pulses. The maximums of particle densities of O- locate nearby the dielectric surfaces, but particle densities of O2- and O3- have relatively uniform distributions in the middle region of the discharge gap. |
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