Source Characteristics Of Plasmas Excited By RF And DC At High Pressures

In the plasma generated at atmospheric and sub-atmospheric pressure,the frequent collisions between particles can directly affect plasma power absorption and may lead to various instabilities.On the other hand,due to higher gas temperatures and active species concentration,high-pressure plasma has unique advantages and great potentials in rapid and thermal treatment of materials.The characteristic and technology studies of high-pressure plasma source have kept the focus of low temperature plasma research for many years.In this thesis,the source characteristics of high-pressure plasmas,atmospheric pressure 30kW ICP torch,atmospheric pressure inductively coupled microplasma jet and sub-atmospheric pressure DC glow discharge have been investigated.In addition,the deposition of Si-B-N non-metallic alloy materials using low pressure RF plasma is also carried out.Firstly,a 30kW ICP torch operated at atmospheric pressure is constructed,where the discharge is excited by 2MHz RF power supply.The diameter of thermal plasma torch is 60mm.A plasma E-H mode transition caused by the change of input power is observed.According to the electromagnetic field model of ICP discharge,the relationship between RF frequency and skin depth is discussed,and the distribution of magnetic field,electric field intensity and current density under different skin depths is also elaborated.The physical mechanism of E-H mode conversion is analyzed.Experimental investigation of an atmospheric-pressure ICP micro-jet driven by 150MHz with variable property is implemented.An irreversible mode transition is observed under external triggering.The plasmas near room temperatures eject out from the exit of quartz tube as the plasmas are generated through one-time triggering by a spark igniter,and its length varies with the input voltage.By applying second-triggering,the plasmas fill the entire glass tube,and only slight changes in jet shapes with the input voltage are observed.Meanwhile,the jet temperature rises suddenly to more than 500K.The coil current measurements show different dependent traces by increasing the input voltage for discharging with and without second-triggering.It is considered that the E-H mode transition happens in the discharge jump,which is caused by instantaneous increase of electron density during second-triggering.Pulse modulation technology is used to enhance the stability of sub-atmospheric uniform glow plasma discharge.The effects of pulse duty ratio during discharge are investigated.As the duty ratio is higher than 73%at 1.7kHz pulse,the rise time is about 10-20ms for the voltage to reach the stable value.The current rises rapidly at first,and then presents a slow increase with discharge voltage.The total rising time for the current is about 100～200ms,and the current lags behind the voltage obviously.The time constant τ increases almost linearly as the duty ratio is decreased.This is because the electron density decays to a lower level at the beginning of the ascending channel under smaller duty ratio.While the average current is kept constant,it is necessary to increase the discharge voltage for a lower duty ratio,so that the discharge power is increased.As the time constant increases to a certain value,it is difficult to continue to maintaining glow discharge due to the attenuation of electron density,and the discharge becomes unstable or even quenched.Finally,the research on the deposition of Si-B-N alloy on the surface of iron-based materials is carried out by RF plasma vapor deposition.Microhardness and nanoindentation tests show that the surface hardness of the stainless steel is significantly improved after plasma deposition.In the friction and wear test,the high hardness and self-lubricating properties of prepared surface alloy are found to significantly improve the wear resistance of the sample.Further XPS analysis shows that the surface of the film layer is mainly composed of four elements:Si,B,N and O.The high hardness of the surface results from the strong covalent bond between the non-metallic elements,which is consistent with the results of the infrared spectroscopy.Through the scanning electron microscope observation,EDS spectrum test and layer-by-layer elemental analysis by GDS,it is found that a"nonmetal-transition-metal" structure is formed during the growth of the alloy film.