Emission characteristics of mixed gas plasmas in low-pressure glow discharges

Glow discharge optical emission spectrometry (GD-OES) with mixed plasma gases is reviewed. The major topic is the effect of type and content of gases added to an argon plasma on the emission characteristics as well as the excitation processes. Emphasis is placed on argon–helium, argon–oxygen, and argon–nitrogen mixed gas plasmas. Results for non-argon-matrix plasmas, such as neon–helium and nitrogen–helium mixtures, are also presented. Apart from the GD-OES, glow discharge mass spectrometry and furnace atomization plasma emission spectrometry with mixed plasma gases are also discussed.     

CFD Simulation of a Hydrogen／Argon Plasma Jet Reactor for Coal Pyrolysis

A Computational Fluid Dynamics (CFD) model was formulated for DC arc hydrogen/argon plasma jet re-actors used in the process of the thermal H2/Ar plasma pyrolysis of coal to acetylene. In this model, fluid flow, convective heat transfer and conjugate heat conductivity are considered simultaneously. The error caused by estimating the inner-wall temperature of a reactor is avoided. The thermodynamic and transport properties of the hydrogen/argon mixture plasma system, which are usually expressed by a set of discrete da-ta, are fitted into expressions that can be easily implemented in the program. The effects of the turbulence are modeled by two standard k-ε equations. The temperature field and velocity field in the plasma jet reactor were calculated by employing SIMPLEST algorithm. The knowledge and insight obtained are useful for the design improvement and scale-up of plasma reactors.     

Plasma Characteristics and Plasma-Feedstock Interaction Under PS-PVD Process Conditions

Plasma spray-physical vapor deposition (PS-PVD) is a novel coating process based on plasma spraying. In contrast to conventional methods, deposition takes place not only from liquid splats but also from nano-sized clusters and from the vapor phase. This offers new opportunities to obtain advanced microstructures and thus to comply with growing demands on modern functional coatings. In this study, different process conditions were investigated with regard to the application of the PS-PVD process for ceramic thermal barrier coatings. Plasma characteristics were calculated under chemical equilibrium conditions by minimizing the Gibbs energy. The plasma-feedstock interaction was modeled taking into account the particular conditions at very low pressure. Since the plasma is highly rarefied, the small feedstock particles are in the free molecular flow regime. Hence, continuum methods commonly used in fluid mechanics and heat transfer approaches with continuous boundary conditions are not appropriate; alternative methods based on the kinetic theory of gases are required. The experimental results confirm the predictions about the degree of vaporization made by such calculations. In particular, they show that the feedstock treatment mainly takes place within the very first trajectory segment between injector and jet expansion.     

Comparison of Laminar and Turbulent Thermal Plasma Jet Characteristics—A Modeling Study

Modeling results are presented to compare the characteristics of laminar and turbulent argon thermal plasma jets issuing into ambient air. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of ambient air into the laminar and turbulent argon plasma jects, respectively. It is shown that since only the molecular diffusion mechanism is involved in the laminar plasma jet, the mass flow rate of ambient air entrained into the laminar plasma jet is comparatively small and less dependent on the jet inlet velocity. On the other hand, since turbulent transport mechanism is dominant in the turbulent plasma jet, the entrainment rate of ambient air into the turbulent plasma jet is about one order of magnitude larger and almost directly proportional to the jet inlet velocity. As a result, the characteristics of laminar plasma jets are quite different from those of turbulent plasma jets. The length of the high-temperature region of the laminar plasma jet is much longer and increases notably with increasing jet inlet velocity or inlet temperature, while the length of the high-temperature region of the turbulent plasma jet is short and less influenced by the jet inlet velocity or inlet temperature. The predicted results are reasonably consistent with available experimental observation by using a DC arc plasma torch at arc currents 80–250 A and argon flow rates (1.8–7.0)×10⁻⁴ kg/s.     

Synthesis of Hydrogenated Graphene during Acetylene Conversion in Helium Plasma Jet

Hydrogenated graphene has been synthesized in one step by acetylene conversion in a helium plasma jet. A dc plasma torch with a diverging anode channel and a power up to 45 kW has been used to generate plasma. The obtained graphene materials have been studied by scanning electron microscopy, Raman spectroscopy, and elemental analysis. Hydrogen desorption from the samples synthesized has been studied by thermal analysis as a function of temperature. It has been found that during annealing in vacuum, the synthesis products change their morphology because of hydrogen release.     

Probe measurements in thermal plasma jets

Measurements of composition, temperature, and velocity in atmospheric argon plasma jets are reported, using enthalpy probes. The plasma jets are generated by a commercial type plasma gun and the measurements are expected to be of particular interest for industrial applications such as plasma spraying. Emphasis has been on the central and downstream regions of the plasma flame. The entrainment of air into the jet was found to be very high, even close to the axis of the jet. Gas samples analyzed with a gas chromatograph showed demixing of the air, i.e., nitrogen is more abundant in the jet than at room temperature. The high air entrainment has a strong cooling effect on the plasma, resulting in a rapid temperature drop along the axis. The influence of the argon flow rate and of the arc current on the jet's conditions was parametrically studied. Matching of the quantities measured in the jet with the torch input confirmed the validity of the results, and the relevance of enthalpy probe diagnostics in thermal plasma jets.     

Generation of Long, Laminar Plasma Jets at Atmospheric Pressure and Effects of Flow Turbulence

Long, laminar plasma jets at atmospheric pressure of pure argon and a mixture of argon and nitrogen with jet length up to 45 times its diameter could be generated with a DC arc torch by restricting the movement of arc root in the torch channel. Effects of torch structure, gas feeding, and characteristics of power supply on the length of plasma jets were experimentally examined. Plasma jets of considerable length and excellent stability could be obtained by regulating the generating parameters, including arc channel geometry, gas flow rate, and feeding methods, etc. Influence of flow turbulence at the torch nozzle exit on the temperature distribution of plasma jets was numerically simulated. The analysis indicated that laminar flow plasma with very low initial turbulent kinetic energy will produce a long jet with low axial temperature gradient. This kind of long laminar plasma jet could greatly improve the controllability for materials processing, compared with a short turbulent arc jet.     

Particle densities and non-equilibrium in a low-pressure argon plasma jet

Plasma diagnostic techniques have been employed to determine particle densities and temperatures in a low-pressure argon plasma jet generated by a cascade arc. These measurements allow characterization of the extent to which the plasma jet deviates from thermodynamic equilibrium and provide a basis for predicting how reactive gases will interact with the excited and ionized species in the plasma jet. It was found that the distribution of atomic states in the plasma jet is not adequately described by either local thermodynamic equilibrium (LTE) or partial local thermodynamic equilibrium (pLTE), and the jet was optically thick for 3p4s transitions across the jet radius. Excited argon neutrals outnumber ions by a large ratio, and dominate subsequent dissociation/excitation phenomena. The rate of methane destruction in the plasma jet shows that estimates for particle densities, temperature, and jet velocity are self-consistent.     

An Optical Emission Study on Expanding Low-Temperature Cascade Arc Plasmas

The optical emission spectra from expanding low-temperature cascade arc plasmas were studied. The objective of this study was to examine the distinctive features of low-temperature cascade arc plasmas in comparison with a radio frequency (RF) plasma source. The principal results obtained in this study were: (1) in an expanding cascade arc plasma jet, active heavy particles (mainly excited argon or helium neutral species under our operating conditions), rather than electrons, are responsible for the excitation of reactive species when a reactive gas is injected into the plasma jet, (2) the excitation of reactive species was found to be controlled by the electronic energy levels of these excited argon or helium neutrals, (3) changing the operating parameters affected only the emission intensities of excited species, and no effect on the emission nature of plasmas was observed.     

Experimental Comparison of Methane Pyrolysis in Thermal Plasma

Methane pyrolysis via thermal plasma was investigated experimentally on a 2 kW DC arc plasma setup in argon atmosphere. Two widely applied methane pyrolysis profiles, i.e., pre-mixing methane and argon before fed into plasma torch, and injecting methane into pure argon plasma jet at torch outlet, were compared. Performances of methane pyrolysis regarding to methane conversion, acetylene selectivity, acetylene specific energy requirement (SER), and plasma stability were concluded. Results showed that pre-mixing methane and argon before fed into plasma torch would be efficient in converting methane and acetylene production, with higher conversion of methane and lower SER to acetylene at a given specific energy. Also, methane in arc zone would cause periodic fluctuations of plasma voltage and power, which could be reduced by controlling methane fraction in feed. On the other hand, when methane was injected into argon plasma jet at torch outlet, the energy efficiency in converting methane and producing acetylene would be lower. And the plasma would barely participate in the reaction other than providing heat, but the erosion of electrode was much slower and slighter. It was also validated that the SER of acetylene was limited by the thermal loss of the setup due to size-effect of reactor.     

Fundamental studies of mixed-gas inductively coupled plasmas

The effects of adding foreign gases to the central-gas flow or the intermediate-gas flow of an argon inductively coupled plasma are presented. In particular, the influence of up to 16.7% added helium, nitrogen or hydrogen on radially-resolved electron number density, electron temperature, gas-kinetic temperature and calcium ion emission profiles is examined. It is shown that these gases affect not only the fundamental parameters and bulk properties of the plasma, but also how energy is coupled and transported through the discharge and how that energy interacts with the sample. For example, added helium causes an increase in the gas-kinetic temperature, most likely due to the higher thermal conductivity of helium compared to argon but, in general, does not appear to affect significantly either the electron temperature or electron concentration. The shift in the calcium ion emission profile towards lower regions in the discharge with added helium may be attributable to higher droplet desolvation and particle vaporization rates. In contrast, the addition of nitrogen or hydrogen to an Inductively Coupled Argon Plasma (Ar ICP) results in dramatic changes in all three fundamental plasma parameters: electron number density, electron temperature, and gas-kinetic temperature. The net effect of these molecular gases (N₂ or H₂) on calcium ion emission and on the fundamental plasma parameters is shown to be dependent on the amount of gas added to the plasma and whether the gas is introduced as part of the central- or intermediate-gas flow. In general, nitrogen added to the central-gas flow causes a significant reduction in the number of electrons throughout most of the discharge (over an order of magnitude in certain regions), mainly in the central and upper zones of the ICP. A drop of 3000–5000 K in the central channel electron temperature and a smaller drop in the gas-kinetic temperature are also observed when N₂ is added to the central-gas flow. In contrast, the introduction of nitrogen in the intermediate flow causes about a 1 × 10¹⁵ electrons cm⁻³ increase in the electron concentration in the low, toroidal regions of the plasma and an increase in the gas-kinetic temperature of around 1000 K throughout most of the discharge. As seen with the addition of nitrogen to the central-gas flow, the electron temperature is found to increase in the toroidal zones of the plasma when N₂ is added to the intermediate flow. These combined effects cause a 20-fold depression in the calcium ion emission intensity only a 1.7-fold depression when N₂ is added to the central- or intermediate-gas flows, respectively. On the other hand, hydrogen causes a depression in the electron concentration in the upper areas of the plasma when this gas is added to the central flow but increases the number of electrons in the same region when added to the intermediate flow. Hydrogen also causes a dramatic effect on the electron and gas-kinetic temperatures, significantly increasing both of these parameters throughout the discharge. An increase in the calcium ion emission intensity, accompanied by a downward shift, elongation and broadening of the calcium ion emission profile is also observed with H₂ addition.     

Plasma Spray Process Operating Parameters Optimization Based on Artificial Intelligence

During plasma spray process, many intrinsic operating parameters allow tailoring in-flight particle characteristics (temperature and velocity) by controlling the plasma jet properties, thus affecting the final coating characteristics. Among them, plasma flow mass enthalpy, flow thermal conductivity, momentum density, etc. result from the selection of extrinsic operating parameters such as the plasma torch nozzle geometry, the composition and flow rate of plasma forming gases, the arc current intensity, beside the coupled relationships between those operating parameters make difficult in a full prediction of their effects on coating properties. Moreover, temporal fluctuations (anode wear for example) require “real time” corrections to maintain particle characteristic to targeted values. An expert system is built to optimize and control some of the main extrinsic operating parameters. This expert system includes two parts: (1) an artificial neural network (ANN) which predicts an extrinsic operating window and (2) a fuzzy logic controller (FLC) to control it. The paper details the general architecture of the system, discusses its limits and the typical characteristic times. The result shows that ANN can predict the characteristics of particles in-flight from coating porosity within maximal error 3 and 2 % in temperature and velocity respectively. And ANN also can predict the operating parameters from in-flight particle characteristics with maximal error 2.34, 4.80 and 8.66 % in current intensity, argon flow rate, and hydrogen flow rate respectively.     

Mechanism of Spectrum Excitation in a Solid Aerosol-Laden Plasma Jet of a Two-Jet Argon Arc Plasmatron

A possible reason for the high intensity of the ion emission in the spectrum excitation in a plasma jet generated by a two-jet argon arc plasmatron was considered. The injection of a test substance as an air–solid suspension between the plasma jets (i.e., mixing of a hot plasma with a cold directional carrier-gas flow) created a radial temperature gradient and induced an intense argon influx from the dense plasma jets to the cold axial plasma zone used for analytical purposes. Favorable conditions were thus created for the analyte Penning impact ionization with argon ions. This was confirmed by the existence of a correlation between an increase in the intensity of ion lines with the carrier-gas flow rate (cooling rate) and the total energy of ionization and excitation of an element. It was shown that charge transfer from the argon ion to the analyte occurred only in the case when the total energy of the element was lower than 16 eV, i.e., lower than the ionization energy of argon plus its kinetic energy.     

A TEM Investigation of Columnar-Structured Thermal Barrier Coatings Deposited by Plasma Spray-Physical Vapor Deposition (PS-PVD)

The plasma spray-physical vapor deposition technique (PS-PVD) is used to deposit various types of ceramic coatings. Due to the low operating pressure and high enthalpy transfer to the feedstock, deposition from the vapor phase is very effective. The particular process conditions allow for the deposition of columnar microstructures when applying thermal barrier coatings (TBCs). These coatings show a high strain tolerance similar to those obtained by electron beam-physical vapor deposition (EB-PVD). But compared to EB-PVD, PS-PVD allows significantly reducing process time and costs. The application-related properties of PS-PVD TBCs have been investigated in earlier work, where the high potential of the process was described and where the good resistance to thermo-mechanical loading conditions was reported. But until now, the elementary mechanisms which govern the material deposition have not been fully understood and it is not clear, how the columnar structure is built up. Shadowing effects and diffusion processes are assumed to contribute to the formation of columnar microstructures in classical PVD processing routes. For such structures, crystallographic textures are characteristic. For PS-PVD, however, no crystallographic textures could initially be found using X-ray diffraction. In this work a more detailed TEM investigations and further XRD measurements of the columnar PS-PVD microstructure were performed. The smallest build units of the columnar TBC structure are referred to as sub-columns. The observed semi-single crystal structure of individual sub-columns was analyzed by means of diffraction experiments. The absence of texture in PS-PVD coatings is confirmed and elementary nucleation and growth mechanisms are discussed.     

Experimental Study on the Thermal Argon Plasma Generation and Jet Length Change Characteristics at Atmospheric Pressure

The generation, jet length and flow-regime change characteristics of argon plasma issuing into ambient air have been experimentally examined. Different torch structures have been used in the tests. Laminar plasma jets can be generated within a rather wide range of working-gas flow rates, and an unsteady transitional flow state exists between the laminar and turbulent flow regimes. The high-temperature region length of the laminar plasma jet can be over an order longer than that of the turbulent plasma jet and increases with increasing argon flow rate or arc current, while the jet length of the turbulent plasma is less influenced by the generating parameters. The flow field of the plasma jet has very high radial gradients of plasma parameters, and a Reynolds number alone calculated in the ordinary manner may not adequately serve as a criterion for transition. The laminar plasma jet can have a higher velocity than that of an unsteady or turbulent jet. The long laminar plasma jet has good stiffness to withstand the impact of laterally injected cold gas and particulate matter. It could be used as a rather ideal object for fundamental studies and be applied to novel materials processing due to its attractive stable and adjustable properties.     

Modeling on the Momentum and Heat/Mass Transfer Characteristics of an Argon Plasma Jet Issuing into Air Surroundings and Interacting with a Counter-Injected Argon Jet

Modeling study is performed to reveal the momentum and heat/mass transfer characteristics of a turbulent or laminar plasma reactor consisting of an argon plasma jet issuing into ambient air and interacting with a co-axially counter-injected argon jet. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of argon in the argon–air mixture for the laminar and the turbulent regimes, respectively. Modeling results presented include the streamline, isotherm and argon mass fraction distributions for the cases with different jet-inlet parameters and different distances between the counter-injected jet exit and the plasma torch exit. It is shown that there exists a quench layer with steep temperature gradients inside the reactor; a great amount of ambient air is always entrained into the plasma reactor; and the flow direction of the entrained air, the location and shape of the quench layer are dependent on the momentum flux ratio of the plasma jet to the counter-injected cold gas. Two quite different flow patterns are obtained at higher and lower momentum flux ratios, and thus there exists a critical momentum flux ratio to separate the different flow patterns and to obtain the widest quench layer. There exists a high argon concentration or even ‘air-free’ channel along the reactor axis. No appreciable difference is found between the turbulent and laminar plasma reactors in their overall plasma parameter distributions and the quench layer locations, but the values of the critical momentum flux ratio are somewhat different.     

Two-dimensional observation of emission spectra excited from laser induced plasmas and the application to emission spectrometric analysis.

The spatial distribution analysis of emission signals from a laser-induced plasma can provide information on the excitation mechanism as well as on the optimization of the analytical conditions when it is employed as a sampling and excitation source in optical emission spectrometry. A two-dimensionally imaging spectrometer system was employed to measure spatial variations in the emission intensities of a copper sample and plasma gases when krypton, argon, or helium was employed under various pressure conditions. The emission image of the Cu I 324.75-nm line consists of a breakdown spot and a plasma plume, where the breakdown zone expands toward the surrounding gas. The shape and the intensities of the plasma plume are strongly dependent on the kind and pressure of the plasma gas, while those of the breakdown zone are less influenced by these experimental parameters. This effect can be explained by the difference in the cross-section of collisions between krypton, argon, and helium. The signal-to-background ratio of the Cu I 324.75-nm line was estimated over two-dimensional images to determine the optimum position for analytical applications.     

Atomic emission spectroscopy for the on-line monitoring of incineration processes

A diagnostic measurement system based on atomic emission spectroscopy has been developed for the purpose of on-line monitoring of hazardous elements in industrial combustion gases. The aim was to construct a setup with a high durability for rough and variable experimental conditions, e.g. a strongly fluctuating gas composition, a high gas temperature and the presence of fly ash and corrosive effluents. Since the setup is primarily intended for the analysis of combustion gases with extremely high concentrations of pollutants, not much effort has been made to achieve low detection limits. It was found that an inductively coupled argon plasma was too sensitive to molecular gas introduction. Therefore, a microwave induced plasma torch, compromising both the demands of a high durability and an effective evaporation and excitation of the analyte was used as excitation source. The analysis system has been installed at an industrial hazardous waste incinerator and successfully tested on combustion gases present above the incineration process. Abundant elements as zinc, lead and sodium could be easily monitored.     

Quenching Experiment Study on Thermal Plasma Pyrolysis Process of Coal Tar

Quenching is a key approach to obtain high acetylene yield in the process of coal tar pyrolysis to produce acetylene in a thermal plasma reactor due to the thermodynamic characteristics of acetylene. Experiments of coal tar pyrolysis were carried out in a lab-scale H₂/Ar plasma reactor under various quenching conditions. Meanwhile, thermodynamic analysis was performed to assist the optimization of quenching temperature and the maximization of acetylene yield. As quenching media in the experiments, hydrogen, argon, methane, and water were used separately to study the influence of quenching process on acetylene yield and specific energy requirement. The experimental results indicate that the acetylene concentration in quenched product gas was significantly affected by quenching operation, and the acetylene yield was significantly affected by quenching medium flow rate. The acetylene yields of 24.6, 17.8, 44.9 and 23.6 wt% can be reached by using hydrogen, argon, methane, and water as quenching media, respectively. The specific energy requirement analysis indicates that process energy efficiency can be improved by a suitable quench operation.     

Fluid boundary layer effects in atmospheric-pressure plasma diamond film deposition

Diamond films were deposited in an atmospheric-pressure radio frequency plasma reactor. Hydrogen and methane were injected coaxially into the plasma as a high-velocity jet which impinged on the molybdenum substrate. In some cases argon was added to the reactant jet to increase its momentum, thereby reducing the boundary layer thickness. In most cases argon addition substantially, improved diamond growth. A numerical model was developed, which calculated two-dimensional reactor temperature and velocity, distributions, and the chemical kinetics in the boundary layer. The calculations indicate that under the experimental conditions argon addition reduced the thickness of the hydrogen nonequilibrium boundary layer from 3.5 to 1.0 mm. In addition, the calculations suggest that monatomic carbon may be a key diamond growth species under thermal plasma conditions.