Modeling of an inductively coupled plasma at reduced pressures

Plasma sintering experiments in this laboratory at reduced pressures revealed efficient heating of the ceramic sample due to recombination of dissociated and/or ionized species on the surface. For establishing a model for this plasma sintering process, it is necessary to first consider the plasma itself. Therefore, a suitable model for an RF inductively coupled plasma has been developed considering reduced pressures. As the pressure decreases, the electron density also decreases at a fixed electron temperature, causing substantial deviations from chemical equilibrium. Due to the poor collisional coupling between electrons and heavy particles at reduced pressures, large deviations from kinetic equilibrium have also to be expected. The model is based on a rotationally symmetric plasma contained in a quartz tube. The power level ranges from 1.5 to 3 kW and the operating pressure is varied from 1 to 0.01 atm. Both deviations from chemical and kinetic equilibrium are included in this model. Thermodynamic and transport properties for two-temperature plasmas are used for this modeling work. The results indicate that for pressures below 0.1 atm, there is a strong ambipolar flux of charge carriers to the confining walls, leading to significant variations of the temperature across the tube. The electron temperature increases rapidly as the pressure decreases, whereas the heavy-particle temperature decreases.     

A Thermodynamic Analysis of Nonequilibrium Argon Plasma Torches

The nonequilibrium process of argon plasma torches is analyzed theoretically. Thermodynamic diagrams of different degrees of ionization are developed to aid in understanding and analyzing the transition from chemical equilibrium to frozen flow in dc plasma torch operations. A thermodynamic model is developed to describe the nonequilibrium processes in a dc argon plasma torch. In the model the ionization process is approximated as a constant-pressure heating process, with little deviation from the equilibrium state upon completion of heating. If the plasma flow is frozen shortly after heating, the entropy increase is small during the transition from equilibrium to frozen flow. In this case the frozen flow will have nearly the same composition and entropy as the flow at the heating section exit. For singly ionized argon plasmas in the entropy range relevant to dc torch operation, the frozen flow solutions on the affinity–pressure diagram are found to be insensitive to entropy change. Therefore the present model predicts that argon plasmas generated at different power levels will have almost identical affinity at the torch exit for the same operating pressure. This prediction agrees with experimental observations except for very low torch power levels.     

The vapour pressures of several metal-2,2,6,6-tetramethyl-3,5-heptanedione complexes measured by a Knudsen effusion method

The melting points, vapour pressures and heats of sublimation of the complexes of Ga, In, Y, La, Gd, Fe, Zn, Pb and Li with 2,2,6,6-tetramethyl-3,5-heptanedione are reported for the temperature range below the melting point of 40? to 250?C and pressures from 10 to 10^?2 torr. The measurements were made with an easily constructed Knudsen effusion cell in conjunction with a Mettler thermobalance.     

Characterization of the Converging Jet Region in a Triple Torch Plasma Reactor

Enthalpy probe measurements were taken of the converging plasma plume in a triple torch plasma reactor and related to substrate heat flux measurements. Results show excellent entrainment of process gases injected into the converging plasma plume by way of the central injection probe. At lower pressures (40 kPa), the plasma volume is equivalent to at least a 3 cm diameter, 4 cm long cylinder, with relatively uniform temperature, velocity, and substrate heat flux profiles when compared to a typical dc arc jet. Converging plasma plume size, substrate heat flux, and enthalpy profiles are also shown to be a strong function of applied system power. Substrate heat flux measurements show smaller radial gradients than enthalpy probe measurements, because of the high radial velocity component of gases above the substrate boundary layer. Enthalpy probe measurements were also conducted for diamond deposition conditions and approximate temperature and velocity profiles obtained. Problems with the uniform gas mixture assumption prohibited more accurate measurements. Reproducibility of enthalpy measurement results was shown with an average standard deviation of 11.8% for the velocity and 7.6% for the temperature measurements.     

Behavior of small particles in a thermal plasma flow

In this paper computational results are presented which reveal the effects of the Knudsen number on heat transfer and drag of small particles in a flowing thermal argon plasma. The Knudsen number is restricted to moderate values so that temperature jump and velocity slip conditions may be employed, and for the governing equations the continuum approach remains valid. It is shown that the ratio of the heat fluxes with and without the Knudsen effect is almost identical to the ratio obtained by the authors for the case of pure heat conduction. This fact is very important for modeling of the behavior of particles injected into an actual plasma reactor when the Knudsen effect has to be taken into account.On leave from the Department of Engineering Mechanics, Tsinghua University, Beijing 100084, P.R.C.     

The electromagnetic performance of a surfatron-based coaxial microwave plasma torch

Low power microwave plasma torches are of particular interest to analytical chemists. The torch design investigated herein, called TPS, is based on the known surfatron structure to which a coaxial section is added consisting of an impedance transformer followed by the metallic nozzle at the tip of which the discharge occurs. A series of experiments illustrate the main electromagnetic features and performance of this novel coaxial microwave plasma torch operating at 2.45 GHz and with input power in the range 10–180 W. A specially devised slotted coaxial line with a movable probe arrangement can be inserted into the torch in place of the transformer section to provide in situ measurements of the plasma impedance. Analyzing these results, we show that the shape of the torch tuning characteristics can be controlled to improve the power transfer to the plasma and stability of operation with respect to changes in discharge conditions; under these conditions, the design of the device can be simplified. The procedures presented have a general character and can be applied to various torch configurations.     

Modeling of the Subsonic–Supersonic Flow and Heat Transfer in a DC Arc Plasma Torch

Two-dimensional modeling results are presented concerning the subsonic–supersonic flow and heat transfer within a DC plasma torch used for low-pressure (or soft vacuum) plasma spraying. The so-called fictitious anode method is used in the modeling in order to avoid inclusion of the complex three-dimensional effects near the anode arc root and also to avoid the forced heating of all the incoming cold gas stream by the arc. A nonorthogonal boundary-conforming grid, nonstaggered variable arrangement and the all-speed SIMPLE algorithm are employed for the solution of the governing equations, including gas viscous effects, temperature-dependent properties, and compressible effects. Good agreement of the predictions with available results for a few benchmarked compressible flow problems shows that the new version of the FAST-2D program can be employed for the present plasma flow modeling. Temperature, axial velocity, Mach number, and static pressure contours, and streamlines within the DC arc plasma torch are presented to show the flow and heat transfer characteristics. The flow transits gradually from upstream subsonic regime into downstream supersonic regime with the subsonic–supersonic transition within the cylindrical segment of the torch nozzle. Additional numerical tests show that gas viscosity and Lorentz force have only a slight effect on the plasma flow.     

Nonequilibrium modeling of low-pressure argon plasma jets; Part I: Laminar flow

This paper presents a modeling attempt related to low-pressure plasma spraying processes which find increasing applications for materials processing. After a review of the various models for ionization and recombination processes, a two-temperature model for argon plasmas in chemical (ionization) nonequilibrium is established using finite rate chemistry. Results of sample calculations manifest departures from kinetic as well as chemical equilibrium, demonstrating that the conventional models based on the LTE (local thermodynamic equilibrium) assumption cannot provide proper prediction for low-pressure plasma jets.     

Multiscale Finite Element Modeling of Arc Dynamics in a DC Plasma Torch

The dynamics of the electric arc inside a direct current non-transferred arc plasma torch are simulated using a three-dimensional, transient, equilibrium model. The fluid and electromagnetic equations are solved numerically in a fully coupled approach by a multiscale finite element method. Simulations of a torch operating with argon and argon–hydrogen under different operating conditions are presented. The model is able to predict the operation of the torch in steady and takeover modes without any further assumption on the reattachment process except for the use of an artificially high electrical conductivity near the electrodes, needed because of the equilibrium assumption. The results obtained indicate that the reattachment process in these operating modes may be driven by the movement of the arc rather than by a breakdown-like process. It is also found that, for a torch operating in these modes and using straight gas injection, the arc will tend to re-attach to the opposite side of its original attachment. This phenomenon seems to be produced by a net angular momentum on the arc due to the imbalance between magnetic and fluid drag forces.     

Reduction of TiO2 with Hydrogen Plasma

Experiments were performed in a thermal plasma furnace to examine the reduction of TiO₂ by hydrogen plasma. The plasma gas composition was 50% hydrogen–50% argon, the plasma torch input power was 13 kW, the TiO₂ initial mass was 10 g and the processing time was varied from 10 to 90 min. The reaction product contained between 67% and 73% titanium by mass for all tests. This level of reduction was consistent with chemical equilibrium modeling. There was no detectable enhancement of reduction due to the presence of atomic hydrogen. The Ti-O microstructures produced were characterized using quantitative SEM analysis. The microstructures showed a number of similarities with structures described by previous researchers.     

Biomass conversion to hydrogen-rich synthesis fuels using water steam plasma

In this study, an experimental plasma-chemical reactor equipped with an arc discharge water steam plasma torch was used for biomass conversion to hydrogen-rich synthesis fuels. Glycerol and crushed wood were used as biomass sources. The effects of different conversion parameters including the water steam flow rate, treated material flow rate, and plasma torch power were studied. The experimentally obtained results were compared with the model based on the thermodynamic equilibrium. Additionally, the quantification of the plasma conversion system in terms of energy efficiency and specific energy requirement was performed. It has been found that the synthesis gas can be effectively produced from the biomass using water steam plasma.     

Mathematical Simulation of Plasma-Chemical Coal Conversion

A mathematical model that describes the conversion of a coal-dust flow in a cylindrical plasma reactor is presented. The model describes a two-phase (coal particles + air) chemically reacting flow, which propagates in a channel with or without an internal heat source (an electric arc, a plasmatron torch, or exothermic chemical reactions). The model is based on the assumption that the process is quasi-stationary and one-dimensional; coal particles are taken isothermal, and ash is assumed an inert component. The model represents the composition of coals by their organic and mineral constituents. The model was implemented as a program for personal computers; calculations performed with the use of this program are in satisfactory agreement with experimental data.     

Nozzle optimization for dissociated species transport in low pressure plasma chemical vapor deposition

Numerical simulation has been used to study the fluid dynamics and chemical kinetics in a supersonic nozzle situated downstream of a plasma reactor. To assist in system scale-up and optimization, effective nozzle design can help in maximizing the transport of chemically active species to the substrate. This paper examines the chemical non-equilibrium of the flow, the effect of different flow parameters, and the effect of different nozzle geometries. A three species hydrogen-argon gas mixture was modeled with finite dissociation and recombination. Non-equilibrium transport and thermodynamic mixture properties based on species pair collision cross sections were implemented. Running a plasma torch off design power can significantly alter power losses through the nozzle wall and subsequently change the species distribution at the nozzle exit. Decreasing the effective nozzle throat diameter can notably decrease atomic concentrations. Small changes in upstream or downstream pressures have a negligible effect on supersonic species transport through the nozzle. Flow separation can be avoided by correctly designing the divergent portion of the nozzle.     

A low-pressure beenakker-type microwave-induced helium plasma source as a simultaneous multi-element gas chromatographic detector

A low-pressure version of a Beenakker-type microwave-induced helium plasma optical emission spectroscopy detector for gas chromatographic effluents is described. The plasma is sustained in a 1.3-rum i.d. quartz tube and is viewed axially through a quartz window. Operating characteristics of the source were studied for power levels of 15–115 W, for carrier-gas flows of 20–1000 mi min^?1, and for pressures of 2–700 torr. A gas chromatographic system involving a fused-silica capillary column is used as the sample introduction system for compounds containing carbon, hydrogen, nitrogen, sulfur, and chlorine. Elemental response factors and the precision of elemental response ratios were studied. The use of this detector in evaluating empirical formulae is also discussed. Empirical formulae for a number of hydrocarbons and sulfur-containing aliphatic and heterocyclic compounds are presented, together with a discussion of the factors that affect accuracy and precision. It is concluded that this type of detector combines some of the best fearutes of the atmospheric-pressure Beenakker and the Evenson-type sources.     

Preliminary spectroscopic experiments with helium microwave induced plasma produced in air by use of a new structure: the axial injection torch

In this experimental study we have used spectroscopic methods to characterize helium plasma obtained by means of a novel waveguide-fed microwave plasma torch at atmospheric pressure, the axial injection torch. This device produces a “plasma flame” by coupling high frequency (HF) power at 2.45 GHz to the discharge. Various flame parameters (namely the electron density number and the electron and gas temperatures) have been determined by using spectroscopic diagnostic techniques that provided an estimate in terms of the helium flow rate, absorbed HF power and axial position in the experiments. These preliminary results suggest some departure from local thermodynamic equilibrium (LTE) and seem to indicate the utility of the discharge as an excitation source for emission spectroscopy. Comparison with other microwave torches already described in the literature is made in terms of the electron density and the electron and gas temperature.     

Simulation of Gas-Dynamic and Thermal Processes in Vortex-Stabilized,Inductively Coupled Argon–Hydrogen Plasma

The flow of vortex-stabilized argon–hydrogen plasma in a radiofrequency induction (RFI) plasma torch has been investigated using modern methods of computational fluid dynamics. Optimal values of the torch power and energy release in plasma have been found at various argon to hydrogen ratios in the plasma gas mixture. The heat and kinetic fields determined by calculation for a plasma-chemical reactor can be of use in designing an RFI plasma torch in part concerning the determination of the optimum zone for feeding the reactants to the reactor.     

Effects of Nozzle Length and Process Parameters on Highly Constricted Oxygen Plasma Cutting Arc

The influence of nozzle length and two process parameters (arc current, mass flow rate) on the plasma cutting arc is investigated. Modeling results show that nozzle length and these two process parameters have essential effects on plasma arc characteristics. Long nozzle torch can provide high velocity plasma jet with high heat flux. Both arc voltage and chamber pressure increase with the nozzle length. High arc current increases plasma velocity and temperature, enhances heat flux and augments chamber pressure and thus, the shock wave. Strong mass flow has pinch effect on plasma arc inside the torch, enhances the arc voltage and power, therefore increases plasma velocity, temperature and heat flux.     

High pressure fast-flow technique for gas phase kinetics studies

A fast-flow technique suitable for measuring elementary rate constants over a wide range of pressures has been developed. The method operates under turbulent flow conditions, in contrast to laminar flow which characterizes the conventional low pressure technique. Flow visualization, velocity profile measurements, and tracer pulse studies have been carried out in a flow tube reactor to investigate the dynamics of both laminar and turbulent flow for chemical kinetics purposes. Furthermore, the wall collision frequency for the reactants has been determined: at the higher pressures it is greatly reduced in comparison with the frequency characteristic of the conventional low pressure laminar flow technique. Also, to test and validate the technique the bimolecular rate constants for the reactions H+Cl₂ and H+O₃ have been measured at total pressures in the 3–300 torr range; at pressures below 5 torr, as well as above 50 torr in the turbulent flow regime the agreement with the recommended literature values is excellent. © 1993 John Wiley & Sons, Inc.     

Parametric study of the decomposition of NH3 for an induction plasma reactor design

The present paper reports a study of the gas mixing and chemical transformation in an induction plasma reactor under atmospheric pressure, and its dependence on the plasma operating conditions. For this purpose, the thermal dissociation of ammonia into nitrogen and hydrogen was chosen because of the relative simplicity of the reactions involved and its use in a number of studies on plasma synthesis of ultrafine nitride ceramic powders using ammonia as nitriding agent. A hot-wall reactor configuration is investigated in which ammonia is injected radially through multiple orifices into the gases at the exit nozzle of an induction plasma torch. Concentration mapping in the mixing zone was carried out, using a VG-Micromass-PC 300 D quadrupole mass spectrometer, for different plasma power levels, in the range 13–24 kW. A 3-point injection mode is used with the injection ports oriented upstream at 45° to the torch axis. This allows uniform mixing of the injected gas in the plasma jet. A systematic study of the effects of plate power and ammonia and plasma gas flow rates on the mixing and dissociation of NH₃ in the reactor is reported. The results are analyzed and discussed from the viewpoint of their use for optimizing the design of induction plasma reactors, to he applied to the vapor-phase synthesis of ultrafine silicon nitride powders.     

Determination of arsenic by continuous hydride generation with direct introduction into an O2-argon microwave plasma torch atomic emission spectrometer

The determination of arsenic was studied with a simple and economic method. A continuous hydride generation system is interfaced to a microwave plasma torch atomic emission spectrometer (MPT-AES). Arsenic hydride is transferred directly and continuously by the carrier gas into the plasma torch without separation of hydrogen. When oxygen is introduced into the outer tube of the plasma torch, the plasma is more stable and has a higher tolerance to hydrogen. The detection limit (3σ) is 5.2 μg/L when the forward power is 100 W with argon as support gas. Application to the standard sample coal fly ash showed a comparable result to the certified quantity.