| As a feasible discharge scheme for producing low-temperature plasmas at atmospheric pressure, DBD based cold plasma jets have received vigorous developments in recent years, and become one hot research issue in low temperature plasma field. Compared with traditional DBD discharge, cold plasma jets have the advantage of spatially separating the breadown region from the plasma jet region, namely the high energetic species (e.g. electrons, ions, excited atoms, molecules, and free radicals) produced in breakdown region are transported to the plasma jet region. Besides, cold plasma jets can provide a novel chemically active climate with low gas temperature. Their desirable properties, including easy operation and low cost have made them unique and promising for a number of important industrial applications, such as material synthesis, material surface modification, biomedical engineering, environmental engineering, especially materials which are sensitive to temperature.To enable it to large-scale industrial application, it is of great importance to optimize the performance of these kinds of cold plasma jet sources, especially to enlarge the dimension and stability of cold plasma jets. For the purpose, it is principal to diagnose the basic parameters relating to plasma state and to clarify the basic discharge physics of the cold plasma jet. Further exploration for the amplification method for the cold plasma jets is also necessary for their potential applications. To meet the reqiuements, this dissertation focuses on a novel DBD based cold plasma jet with large diameter and gains the achievements as following:1. A triple electrode discharge configuration is presented for producing large-diameter plasma jet near atmospheric pressure. The3electrodes are refered as the driving electrode (D); the floating assistance electrode (F) and the grounding electrode (G). Different from various cold atmospheric pressure plasma jets via concentric DBD configurations, the novel kind of configuration can generate large plasma jets especially with much larger dimension in diameter in pure argon or Ar/N2mixture near atmospheric pressure (750torr). The voltage and current waveform (â… -â…¤) measurement indicates that the discharge composes of two consequent sections, one of which occurs as coaxial DBD mode while the other forms large-diameter plasma jets. The overall temperature of the plasma jet is determined to be about300K by fitting the optical emission bands of N2(B-C) and OH(A-X). Therefore the plasma is believed as cold as those atmospheric pressure plasma jets with concentric DBD configurations. In order to characterize the spatial homogeneity of the large plasma jet, optical photos of the jet region are analyzed for the optical emission distribution of the jet region. Pixal brightness of the photos can represent the emissive specie density generated in thejet and thus can reflect the spatial distribution of the plasma jet.2. The triple electrode discharge configuration can also work with radio frenquency as driving power. Stable radio frequency (13.56MHz) plasma jet can be produced via suitable power input. By adjusting radio frenquency power level, three different discharge modes (a,7and their coexistence) are achieved in the discharge configuration. The vibrational temperature and rotational temperature (gas temperature) are investigated by means of optical emission spectroscopy. Based on the temperatures, the three modes can be discriminated clearly from each other. In order to produce more kinds of active species using the radio frenquency plasma jet. oxygen is added into the argon working gas to study the tolerence to hetero-gas of the plasma jet. From the plasma jet photos with different oxygen concentration, the plasma jet is compared in homogeneity and stability and an optimal oxygen concentration of0.9%is determined.3. Similar to traditional parallel plata DBD configuration, self-organization phenomena are also found in the DBD region of the triple electrode configuration. Not only discharge pattern structure can occur as the results of nonlinear spatial ordering, but also double-period discharge phenomenon occurs as the temporal behavior of nonlinear ordering. In order to study the pattern discharge mode of the novel triple electrode configuration, the DBD region of the triple electrode configuration is idealized into a ring-to-plane configuration. A symmetrical1-D self-organized discharge pattern is formed in argon near atmospheric pressure using the ring-to-plane configuration. The ring electrode is a circular tube with a wedge-shaped brim.The circular sharp end of the tube forms a periodic boundary in azimuthal direction and can advances the symmetry of1-D self-organized discharge. The discharge photos exhibit symmetrical discharge patterns that can regularly develop with applied voltage. If expanding the range of applied voltage, it is noticed experimentally that the discharge experiences usually three modes:the self-organized pattern mode, the diffuse mode and the filament mode, depending on discharge conditions, such as the gas flow rate, the working pressure, the gap distance between the electrodes and the applied voltage. Due to the unique design of circular electrode, which can eliminate the different physical conditions of the boundary discharge channels in1-D self-organized pattern and those in the middle of pattern, originated from the geometric boundary condition, realizes the each discharge channel of the pattern is accompanied by the neighboring discharge channels on both sides identically in discharge patterns and forms a stable highly symmetrical discharge patterns. The influences of experimental conditions (such as applied voltage, frequency, and gas component) on pattern structures are investigated, confirmes the identity of each discharge channel in patterns.4. Multi-period discharge mode can take place occasionally in dielectric barrier discharges, which is featured with period multiplication of the discharge current waveform refering to the voltage waveform. In order to study the multi-period discharge mode of the novel triple electrode configuration, the DBD region of the triple electrode configuration is equivalent topologically to a pin-to-plane configuration. A novel double-period discharge mode is found in argon DBD near atmospheric pressure and at low pressure with the pin-to-plane electrode configuration. The double-period discharge mode can survive over a wide range of driving voltage and pressure from near atmosphere pressure (750Torr) to low pressure (several Torr). From the discharge V-I waveforms, the double-period discharge mode evolves gradually with discharge voltage, which is obvious different from the multi-period mode in traditional DBDs. The double-period mode occurrence domain has been determined in the phase space spanned by the discharge parameters of driving voltage, pressure and frequency. To show the physics of the period doubling, it is attributed to the effect of the counter electric field of the barrier layer surface charge accumulation on the gas gap.breakdown.5. To produce various energetic species meeting different application fields, cold plasma jet with high stability requires stable discharge in various mixed gas. As regard the newly-developed cold plasma jet, the novel discharge structure is extended to two well-selected gas mixtures in view of two application potentials.(1) In order to improve the cold plasma jet in density and its stability, in O2mixed gas, the triple electrode discharge is enhanced by applying a DC voltage at the central driving electrode. Consequently, the floating electrode is then driven with AC voltage to produce DBD to the grounding electrode. By the enhancement design, a large-diameter plasma jet ban form in O2mixed gas. Increasing the DC voltage is used to investigate the ozone formation. The ozone concentration increases with DC voltage till a peak value, then reduces.(2) In CH3I mixed gas, to achieve high yield of atomic iodine, the triple electrode discharge structure has been changed by substituting the floating electrode with a large dielectric container. The resultant unique discharge structure can not only achieve plasma jet discharge in CH3I mixed gas, but also guide the atomic iodine products to flow out of the gas breakdown region effectively. |
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