| An atmospheric-pressure microwave-induced plasma has many advantages:on one hand, its discharge volume is of the great diameter with high throughput of working gas and its energy and active particle densities are high due to its high energy coupling efficiency; on the other hand, it eliminates use of an expensive vacuum system so the equipment investment and maintenance costs are reduced greatly and at the same time the continuous production becomes possible. Therefore atmospheric-pressure microwave-induced plasma has been widely applied in practice. In this work, microwave plasma is ignited by seeds electrons which are provided by the kilohertz excited Ar plasma jet. A stabilized N2/Ar microwave plasma torch flame with high concentration of active particles is produced in a quartz tube reactor by injection of working gas in vortex flow.First, we investigated the discharge mechanism of the atmospheric pressure waveguide-based microwave plasma by considering the role of discharge tube. we have found that the discharge tube play an important role in generation of the waveguide-based atmospheric-pressure microwave-induced plasma. When diameter of the aperture holes on the wide waveguide walls equals to that of discharge tube, that is ro=R, the plasma will be induced in an equivalent resonator with a certain Q value and present as a torch-like plasma. However, if there is a launching gap between aperture holes and the discharge tube, that is ro<R, a surfaguide plasma will be developed at atmospheric pressure, its efficiency depends on the launching gap.Furthermore, we carried out the diagnosis of atmospheric-pressure waveguide-based microwave-induced N2-Ar plasma torch by means of optical emission spectroscopy. We have found that both input power and mixture ratio of argon to nitrogen have great influence on electron density and gas temperature but have little influence on electron temperature. The electron density and gas temperature increase as the absorbed power increases and when the gas mixture ratio of nitrogen to argon is raised the electron density increases while the gas temperature decreases. Simultaneously, in our study we investigated the variation of the intensity of the band head of the first negative positive system N2+(B2∑u+-X2∑g+) and the nitrogen atomic line, we also found that the large density of electron density benefit for the generation of the metastable state A3∑a+, a’1∑u-of N2which are necessary for the dominant generation path of N2+. And it is known that the higher the concentration of N2+generated in discharge volume, the greater amount of atomic nitrogen. Consequently, high production rate of atomic nitrogen in plasma torch could be obtained by increasing the absorbed power level and the concentration of nitrogen.To investigate the physical mechanisms of atmospheric-pressure plasma nitridation and further improve the atmospheric-pressure plasma nitridation technology, the atmospheric-pressure microwave N2/H2plasma torch is generated and diagnosed by optical emission spectroscopy. It is found that a large amount of N atoms and NH radicals are generated in the plasma torch. The increase of hydrogen to nitrogen ratio can raise the concentration of NH radicals in microwave plasma while the intensity of spectral line of atomic nitrogen shows invariant. The emission intensity of N2+(391.4nm) increases with the increase of hydrogen to nitrogen ratio. And more mixture ratio of hydrogen causes the morphology of the plasma discharge to change with appearance that the afterglow shrinks greatly, which is attributed to the quenching effect of the hydrogen on the metastable state of N2. Therefore, an optimum hydrogen to nitrogen ratio is acquired to be in the range from0.25%to0.5%, in which H2has little influence of the plasma morphology and its reducibility can eliminate the oxide layer on the surface of metal which makes nitridation process proceed in the reducing environment and the produced NH radical will benefit for the nitridation. |
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