Microwave plasma torch operating at a low pressure for material processing

Most operations involving the use of a microwave plasma torch are carried out at atmospheric pressure as an open system. A microwave plasma torch operated in a closed and isolated environment may provide an opportunity for the mass production of chemically active radicals for various chemical and biological processes. This study presents a microwave plasma torch operating at a low pressure in an isolated chamber. A microwave torch operating at a low pressure may be economical in terms of capital costs, maintenance costs, and operational costs compared to other plasma devices used in material processing operations. The properties of the torch plasma at a low pressure were investigated. It is found that the plasma profile at a low pressure is asymmetric with higher density on the incoming side of the microwaves. This behavior of the torch inhibits high-power operation of the microwave plasma torch at a low pressure. However, the asymmetry of the plasma profile disappears under a high gas flow rate. It was also found that a microwave plasma torch used at a low pressure can efficiently produce an abundance of chemical radicals.     

Heat and mass transfer during in-flight nitridation of molybdenum disilicide powder in an induction plasma reactor

The present study reveals some of the important parameters which control the in-flight nitridation of molybdenum disilicide (MoSi₂) powders when carried out in an induction thermal plasma reactor. Initially, gradients of temperature, velocity and concentration were evaluated, using an enthalpy probe system, for the plasma flow without injection of MoSi₂ powders. Radial profiles were then measured at the torch exit to examine the mass and energy transfer mechanisms occurring under different nitridation conditions. These measurements were performed using an induction plasma torch connected to a 50 kW radio-frequency (r.f.) power supply, the torch being attached to a water cooled cylindrical reactor. The process operating conditions studied were plasma plate power, chamber pressure, sheath gas composition, composition and flow rate of quench gas. The effect of last named parameter on the nitridation of the powders was found to be the most important parameter in the nitridation process. The results show that there is an optimum flow rate value for each type of quench gas and the temperature and concentration mapping demonstrates that the combination of high temperatures and high concentrations of N₂ are necessary to reach maximum nitridation levels in MoSi₂.     

Electron number density measurement on a DC argon plasma jet by stark broadening of Ar I spectral line

In thermal plasma processing, input power and gas flow rate play a major role in controlling the plasma jet temperature, velocity and density. Emission spectroscopy study is an important method for plasma diagnostics. A DC atmospheric plasma spray torch was operated at different power levels and flow rates of plasma gas (argon). Electron number density of the plasma jet, the corresponding temperature and the degree of ionization were determined using stark broadening of the Ar I (430.010 nm) line, the atomic Boltzmann plot method and the Saha equation, respectively. In the present work, we have investigated the effect of input power, axial position of the plasma jet and gas flow rate on the electron number density in the plasma jet. While an increase in input power considerably increased the electron number density, gas flow rate did not show any significant effect on the same.     

Numerical and experimental studies on DC plasma spray torch

One of the challenging problems in the plasma spray technique is reproducibility of the coating quality. This problem is mainly associated with arc fluctuations, which affect the plasma jet temperature and velocity, inside the plasma torch. In this study, 3D numerical models are developed to study the arc behavior inside the torch and effect of arc fluctuations on plasma jet temperature and velocity. Plasma arc is simulated for different operating parameters. Different arc sizes are predicted by using thermo-dynamical principle of minimum entropy production for given torch power. The influence of arc current and gas flow rate on the Ar-N₂ plasma arc characteristics, plasma jet and torch efficiency is presented. Predicted torch efficiencies and arc voltages are comparable with measurements. At the nozzle exit, velocity shows stronger three-dimensional effect than temperature. Plasma jets are simulated using different nozzle exit profiles obtained from the plasma arc model and their temperature and velocity distributions are clarified.     

Analysis of high-frequency processes in an electric-discharge chamber of a three-phase plasma torch at high working gas pressure

The paper deals with electrophysical processes in an electric-discharge chamber of a three-phase AC plasma torch with nitrogen as working gas. The frequency characteristics of voltage pulsations depending on current, gas mass flow rate, power, and pressure are obtained. Photos of arc burning processes in the plasma torch discharge chamber at different parameters are presented. It is shown that two arcs burn simultaneously in the plasma torch chamber and the third one either extinguishes or arises. These investigations might be useful when creating various technological processes by applying three-phase AC plasma torches operating on inert gases.     

Numerical simulation of argon twin torch plasma arc for high heating efficiency

Plasma arc heating technology has been applied for volume reduction through melting of bottom and fly ash, and for producing slag. Recently, a twin torch plasma arc, which has two torches at the cathode and anode, has been anticipated for application to disposal of medical waste because it can treat a wide area and can treat non-conductive materials. For this study, a numerical simulation model of a twin torch plasma arc at opposite electrodes was developed to elucidate high-efficiency heating using a twin torch plasma arc. It is defined as Local Thermal Equilibrium (LTE) and calculated in Magneto-Hydro-Dynamic (MHD) equations. Furthermore, the temperature distribution and conditions of high heating efficiency with the radiation loss were addressed and compared to those of a single torch. The heating efficiency decreases with increasing radiation efficiency because of the temperature increment caused by the current and input power. The radiation efficiency of a twin torch is about 5% higher than that of single torch.     

A probe investigation of the structure of electrode microwave discharge in nitrogen

A probe investigation is performed of the structure of electrode microwave discharge in nitrogen at a pressure from 0.5 to 2.5 torr, incident power of up to 500 W, a power of 10 to 50 W absorbed in the system, and a gas flow rate from zero to 1000 cm³/min under normal conditions. It is demonstrated that, if the discharge diameter exceeds considerably that of antenna, the discharge is a close-to-spherical formation with a clearly defined boundary. The antenna is surrounded by a bright luminous film of plasma. A considerable nonuniformity of the parameters of the electron component of plasma and of constant potentials in plasma is demonstrated. It is possible that the discharge structure is related to the existence of a region of self-maintained discharge (luminous film surrounding the electrode) and a region of semi-self-maintained discharge (spherical region). The existence of considerable constant fields in the plasma of an electrode microwave discharge is found, whose emergence may be due to a nonlinear transformation of the microwave power maintaining the discharge. The discharge is surrounded by an isotropic region with decreasing electron temperature and concentration.     

大面积平面表面波等离子体的研究

低温等离子体技术已被广泛应用于各高科技领域 ,并且应用范围仍然在迅速拓展 ,这对等离子体本身提出了更高的要求 ,平面大面积、高密度均匀等离子体源是目前最迫切的需求之一。作者主要介绍表面波激发等离子体的原理 ,并在自行研制的一套平面大面积表面波等离子体源上 ,利用静电双探针测量了其Ar气放电的角向、径向和轴向的电子密度和温度。发现角向电子密度和温度均匀性与耦合天线及气压密切相关而与入射功率无关 ;径向电子密度和温度均匀性则与入射微波功率及气压密切相关而与耦合天线无关。因此 ,通过优化耦合天线来获得径向参数的均匀性及微波耦合效率 ,并增大微波功率、选择适当的气压 ,可产生大面积平面高密度等离子体     

Application of microwave air plasma in the destruction of trichloroethylene and carbon tetrachloride at atmospheric pressure

In this study, the destruction rate of a volatile waste destruction system based on a microwave plasma torch operating at atmospheric pressure was investigated. Atmospheric air was used to maintain the plasma and was introduced by a compressor, which resulted in lower operating costs compared to other gases such as argon and helium. To isolate the output gases and control the plasma discharge atmosphere, the plasma was coupled to a reactor. The effect of the gas flow rate, microwave power and initial concentration of compound on the destruction efficiency of the system was evaluated. In this study, trichloroethylene and carbon tetrachloride were used as representative volatile organic compounds to determine the destruction rate of the system. Based on the experimental results, at an applied microwave power less than 1000 W, the proposed system can reduce input concentrations in the ppmv range to output concentrations at the ppbv level. High air flow rates and initial concentrations produced energy efficiency values greater than 1000 g/kW h. The output gases and species present in the plasma were analysed by gas chromatography and optical emission spectroscopy, respectively, and negligible amounts of halogenated compounds resulting from the cleavage of C(2)HCl(3) and CCl(4) were observed. The gaseous byproducts of decomposition consisted mainly of CO(2), NO and N(2)O, as well as trace amounts of Cl(2) and solid CuCl.     

Numerical Analysis of Powder Properties in Low Power Plasma Torch

A mathematical model was presented to describe the particle trajectory, velocity and temperature properties in the low power plasma spraying torch (3.6 kW)in which powder particles were directly injected into the region between the cathode and anode. The results show that the characteristics of the particles by low power plasma spraying are similar to that by traditional APS( Atmosphere plasma spraying) in 40 kW. The velocities of the particles increase with the increase of inlet gas flow rate, current and percentage of nitrogen and hydrogen, while the temperature of the powder increase with the decrease of the gas flow rate and with the increase of current and percentage of nitrogen and hydrogen.     

Plasma spheroidization of iron powders in a non-transferred DC thermal plasma jet

In this paper, the results of plasma spheroidization of iron powders using a DC non-transferred plasma spray torch are presented. The morphology of the processed powders was characterized through scanning electron microscopy (SEM) and optical microscopy (OM). The percentages of spheroidized powders were calculated by the shape factors such as the Irregularity Parameter (IP) and Roundness (RN). A maximum of 83% of spheroidization can be achieved. The spheroidization results are compared with the theoretical estimation and they are found to be in good agreement. The phase composition of the spheroidized powder was analyzed by XRD. The effect of plasma jet temperature and plasma gas flow rate on spheroidization is discussed. At low plasma gas flow rates and at high plasma jet temperatures, the percentage of spheroidization is high.     

Spectral measurements of the gas and electron temperatures in the flame of a single-phase ac plasma generator

High-power electric-arc ac plasma generators find extensive application which is primarily associated with the problem of processing of organic waste and production of synthesis gas. The structure of a high-power single-chamber multiphase ac plasma generator with rail-type electrodes includes an injector of primary charge carriers which is a single-phase high-voltage plasma generator with a power of 10 kW or lower. This latter plasma generator may be further employed for solving other problems. Results are given of spectroscopic measurements of gas temperature fields at the nozzle exit section of a high-voltage plasma generator with rod electrodes located in a cylindrical channel. The pattern of variation of the temperature fields as a function of working gas flow rate is investigated.     

In-flight behavior of lanthanum zirconate (La2Zr2O7) particles in gas tunnel type plasma jet and its coating properties

Numerical simulation of in-flight particles in thermal plasma jet is one of the most important research fields. It has been used to analyze the influence of spray parameters on jet characteristics and to improve the quality of coating during plasma spraying. In this study, in-flight behavior of a group of lanthanum zirconate (La₂Zr₂O₇) particles with different diameters (10–60 μm) in gas tunnel type plasma jet is investigated by numerical modeling. The influence of torch power on the plasma jet is investigated and its interaction with different sizes of La₂Zr₂O₇ particles is studied under optimized spraying conditions. The resultant coating properties are also investigated and correlated with simulation results. The simulation results showed that the plasma jet temperature and velocity increased while increasing the torch power. Consequently, the in-flight particle temperature and velocity profile also increased with respect to the torch power.     

Uniform coating of CVD diamond on metallic wire substrates

Diamond was coated onto wire substrates of various transition metals (Mo, W or Ti) of 0.5 mm diameter by the microwave plasma CVD method from a gas mixture of the CO–H₂ system. The CVD conditions for a uniform diamond coating were microwave power, 750–1100 W; total pressure, 2000 Pa; total flow rate, 200 ml min-1; CO concentration, 5 vol%; treatment time, 5 h. The wire substrates were mounted vertically or horizontally on a pyrophyllite susceptor, which was placed parallel to the irradiation direction of microwave power. Homogeneous and fine-grained diamond film was prepared on the whole surface of horizontal W wire substrate with a wire height of 2 mm from the susceptor. To obtain a dense diamond coating, the height has to be as low as possible in the plasma region, where the plasma density is higher at lower substrate temperature. Low pressure and high microwave power were suited for fine-grained coating. Diamond deposition rate was found to be more dependent on pressure than substrate temperature. As the pressure increased, a glassy carbon film was formed instead of diamond. This revised version was published online in November 2006 with corrections to the Cover Date.     

Nanokristalline Werkstoffe hergestellt mit dem Thermischen HF‐Plasma

Nano‐crystalline materials manufactured with the Thermal RF‐Plasma The Inductively Coupled Plasma (ICP) at atmospheric pressure is particularly suited for melting and evaporation of materials. The electrodeless ICP can be generated without limitation of the kind of plasma forming gases. Therefore, using an argon‐oxygen gas mixture as sheath gas of the ICP nanophase coatings can be processed by synthesis with metal‐organic liquid precursors injected in the hot plasma core. For depositions, the plasma jet has to be supersonic. For particles which impinge onto the substrate placed near the nozzle exit of the plasma torch thin and dense coatings are obtained with crystallite sizes of 30‐ 40 nm. The composition and the grain size of as‐deposited coatings are analyzed by XRD.     

Assessment of a new carbon tetrachloride destruction system based on a microwave plasma torch operating at atmospheric pressure

A new system for destroying volatile organic waste based on a microwave plasma torch that operates at atmospheric pressure and is coupled to a reactor affording isolation of output gases and adjustment of the plasma discharge atmosphere is proposed. The system was assessed by using carbon tetrachloride as the target volatile organic compound (VOC) and argon as the main gas in a helium atmosphere. Under optimal conditions, a microwave power of less than 1000 W was found to reduce the CCl(4) concentration at the reactor outlet to the parts-per-billion level and hence to virtually completely destroy the VOC. With high argon flow-rates and CCl(4) concentrations, the energy efficiency can reach levels in excess of 3000 g/kWh. Output gases and species in the plasma, which were identified by gas chromatography and light emission spectroscopy, respectively, were found to include no halogen-containing derivatives resulting from the potential cleavage of CCl(4). In fact, the main gaseous byproducts obtained were CO(2), NO and N(2)O, in addition to small traces of Cl(2), and the solid byproducts Cl(2)Cu and various derivatives depending on the particular reactor zone.     

大面积高光学质量金钢石自支撑膜的制备

介绍了一种新型的磁控/流体动力学控制的大口径长通道直流电弧等离子体炬，并据此设计建造了100千瓦级高功率DC Arc Plasma Jet CVD金刚石膜沉积系统，讨论了该系统采用的半封闭式气体循环系统的工作原理，以及在气体循环条件下制备大面积高光学质量金刚石自支撑膜的研究结果。     

Beeinflussung der Hartstoffverteilung beim Plasma‐Pulver‐Auftragschweißen

Influencing the distribution of reinforcing particles in plasma transfer arc welding A study was made to examine the possibilities of modification the distribution of reinforcing particles in plasma transfer arc welding by using pulsed gas flows. It is shown, how modulated gas flows can be created. The effects of different modulated plasma gas flows were analysed by measuring the pressure distribution in the arc. By using a two powder feeding system, a cobalt based hardfacing alloy with different amounts of tungsten carbides was deposited on plates of carbon steel. It was investigated, how variable pulsed gas flows affect the distribution of the reinforcing carbides. The metallographic analysis shows a uniform distribution of the tungsten carbide in contrast with specimens, welded with constant plasma gas flow.     

Growth and characterization of diamond films deposited at high-pressure using a low-power microwave plasma reactor

Diamond films were deposited on molybdenum substrates from mixtures of methane diluted in hydrogen using a high-pressure microwave plasma reactor. In this reactor, a compressed waveguide structure was used to increase the electric field strength, and accordingly the reactor was able to operate stably with low gas flow rate and microwave power. The films deposited on 12 mm diameter substrates were characterized by film morphology, Raman spectra, growth rate and crystalline quality. The morphology of diamond films deposited in this reactor depends mainly on the substrate temperature. When the deposition pressure was 48 kPa and microwave power was only 800 W, high quality diamond films could be uniformly deposited with a growth rate around 20 μm/h.