Investigations On Discharge Physics And Technology Application Of Direct Current High Pressure Glow Plasma

Due to high collision frequency between particles, direct current (dc) glow discharge plasmas with high pressure (100Torr-1atm) are characterized by high gas temperature and close to thermal equilibrium states. Owing to and high dissociation efficiency of gas molecules, the plasmas are abundant in active radicals, which make them very suitable for the rapid deposition of functional films. Furthermore, the simple device structure and low gas consumption make the plasmas meet the requirement of industrial application in large scale. Nevertheless, the high gas temperature of the plasmas can lead to serious inhomogeneity of macro-parameters, such as electric field, temperature of plasma, which apt to induce the glow to arc transition (GAT). Essentially the GAT is the result of development of micro-instabilities. The research of discharge physics and technology of high pressure dc glow plasmas in large area is significant for the development and application of new plasma sources.In the dissertation a set of apparatus for producing pulsed dc glow discharge plasma under high pressure was built. The discharge technique, mechanism and application in functional materials synthesis were studied systematically.After massive reformations of electrodes and optimization of discharge parameters, the apparatus now can produce glow discharge plasma with the max diameters of 80mm and the power of 15 kW under the gas pressure between 100 Torr and 200 Torr, In addition, the plasma can remain stable more than 600 h.By virtue of OES (optical emission spectroscopy) and oscillograph, the feature evolutions of the plasma with multiple parameters were investigated. Through the spatially resolved OES diagnosis, three-dimensional distributions of electron temperature, gas temperature and the main reactive particles were obtained. The research indicates that, in the high gas pressure plasmas, the gas heating is remarkable, the control of electrode temperature is crucial for the discharge stability. Thermal effect becomes one of important mechanisms for particles excitation and dissociation. It influences not only the concentrations and spatial distributions of active particles in plasma but also the discharge stability, sometimes combined with, in other times competing with, even dominating over electron collision effects. Different pulse frequency and duty cycle can lead to different non-equilibrium degree, influencing the radical concentrations. The spatially resolved diagnosis shows the cathode region constricts into a thin layer sticking to the surface of cathode in which a large voltage drop and electric field consist. The cathode region consumes most of the total discharge power, is the most important region in charge of the stability of discharge plasma. Due to the relatively high electron temperature, abundant spectrum lines present in the OES of the region. Positive column region occupies the major bulk of plasma. Because of relative lower electron temperature and higher gas temperature, the region is characterized by small non-equilibrium degree and strong thermal effect. By virtue of oscillograph, the typical decay time of plasma is found to be 65μs, far lower than that at low gas pressure. The rise time of plasma density is about hundreds of microseconds depending on the discharge parameters. The rise and decay rates of plasma are associated with discharge current, interelectrode gap, gas pressure, pulse frequency and duty circle, but seem to be irrelevant to gas flow rate, methane and argon concentration.The plasmas also were used to deposit thick diamond wafers in the mixture of hydrogen and methane. The diamond wafer with diameter of 75 mm and thickness of 5 mm was successfully synthesized. The diamond wafer's uniformity in quality and thickness is influenced by the joint and interacting effects of gas pressure, discharge current, size of substrate and proportion of methane. The excessive carbon powders in hydrocarbon plasmas are detrimental to the growth of thick diamond wafers.A self-organization three dimensions growth of centimeter-scale treelike diamond structures was firstly found in the high pressure dc hydrocarbon plasma. Numerous powders with negative charges formed in the dc plasma play a crucial role in the growth process of diamond tree. The findings were featured on Nature China as one of outstanding research results from Mainland China and Hong Kong in 2009. The findings demonstrate the possibility of using CVD for the rapid growth of complex diamond structures. In addition, two other self-organization glow structures in the plasma were also observed and discussed. One is the glow striations in the positive column region, whose size is related with the gas temperature. The other is the pearllike glow spots around the anode edge.