Plasma Chemistry and Plasma Processing (v.37, #3)

Foreword by Maher Boulos; Pierre Fauchais (511-512).

The widespread application of electric arcs is closely related to the continuous research interest over the course of many years. The present survey is concerned with chemical and excitation nonequilibrium in atmospheric pressure argon plasma generated between a sharpened tungsten cathode and a flat copper anode at current levels of 35–200 A. Advanced fully nonequilibrium modelling is applied to simulate the combination of the wall-confined arc plasma column and the open region in front of the anode in a self-consistent manner in order to pay tribute to the tremendous research work that E. Pfender has done. The new modelling results are presented along with experimental and modelling results of the studies of E. Pfender and his group and other works of relevance.
Keywords: Arc plasma; Thermal plasma; Ionization nonequilibrium; Chemical nonequilibrium; Modelling

Gas metal arc welding (GMAW) processes are characterized by a high number of simultaneously running physical processes. The process capability is mainly determined by the properties of a metal vapour influenced arc and the material transfer. In recent years, experimental as well as numerical methods are being used increasingly in order to understand the complex interactions between the arc and material transfer. In this paper, we discuss the influence of metal vapour on GMAW processes in spray as well as pulsed material transfer mode. With respect to the high complexity of the process, experimental and numerical methods are combined in a targeted manner in order to obtain a high level of expressive capability with moderate numerical and experimental effort. The results illustrate the high influence of the changing vaporization rate not only on the arc properties but on the arc attachment at the filler wire. It could be shown, that in many cases the metal vapour concentration in the arc region has a greater influence on the arc properties and the material transfer than different shielding gas components like oxygen, hydrogen or helium.
Keywords: Numerical simulation; Gas metal arc welding (GMAW); Metal vapour; Material transfer; Droplet detachment

Electric Arc Fluctuations in DC Plasma Spray Torch by V. Rat; F. Mavier; J. F. Coudert (549-580).
Direct current plasma torches for plasma spraying applications generate electric arc instabilities. The resulting fluctuations of input electrical power hamper a proper control of heat and momentum transfers to materials for coating deposition. This paper gives an overview of major issues about arc instabilities in conventional DC plasma torches. Evidences of arc fluctuations and their consequences on plasma properties and on material treatments are illustrated. Driving forces applied to the arc creating its motion are described and emphasis is put on the restrike mode that depends on the arc reattachment and the boundary layer properties around the arc column. Besides the arc root shown as a key region of instability, the Helmholtz oscillation is also described and accounts for the whole plasma torch domain that can generate pressure fluctuations coupled with voltage ones.
Keywords: Electric arc; DC plasma spray torch; Voltage fluctuations; Fluctuation modes

In metallurgic applications of thermal plasmas the presence of metal vapour, even in small proportion tends to increase the electron number density and to modify some basic properties such as the electrical conductivity and the radiation emission. In this paper we focus on the influence of these vapours on the radiation properties. After the definition of some necessary and basic functions and laws we briefly present the mechanisms responsible for emission and absorption of radiation in thermal plasmas. Then an important section is devoted to the role of metal vapours on the net emission coefficient which is the most popular parameter used to evaluate the radiation power losses in general models. It is shown that metal vapours increase the emission especially at low and intermediate temperatures (T < 12,000 K) and that their relative influence depends on the nature of the initial gas and of the metal itself. We list a rather important number of references presenting calculation of net emission in various gas–metal mixtures. Finally we show in a last section the influence of metal radiation on general plasma properties such as the energy transfer (other methods than the net emission coefficient), the cooling effect, the global energy balance and the heating of particulates injected in the plasma. The most spectacular effects are the increase of radiation losses in the energy balance and the complex role of the metal in the local cooling of the plasma.
Keywords: Radiation; Thermal plasmas; Metal vapour; Net emission coefficient

This paper presents, through examples, the evolutions of atmospheric plasma spraying since the sixties. The drastic improvement of the spray conditions and coatings reproducibility during more than 50 years was linked both to researches in laboratories and developments of spray equipment’s (plasma torches, computerized control panels, robots to spray coatings on complex parts, sensors working in the harsh environment of spray booths…). This evolution is illustrated through the following topics: (1) plasma forming gas thermodynamic and transport properties either at local thermodynamic equilibrium or more recently at two temperatures; (2) evolution of plasma spray torches since the nineties; (3) plasma jet and in-flight particle measurements with laboratory equipment’s and then sensors in spray booths; (4) plasma jets and torches modeling as well as heat and momentum transfer to particles; (5) splats formation and layering.
Keywords: Plasma spraying; History; Spray conditions improvement

Main Issues for a Fully Predictive Plasma Spray Torch Model and Numerical Considerations by Christophe Chazelas; Juan Pablo Trelles; Isabelle Choquet; Armelle Vardelle (627-651).
Plasma spray is one of the most versatile and established techniques for the deposition of thick coatings that provide functional surfaces to protect or improve the performance of the substrate material. However, a greater understanding of plasma spray torch operation will result in improved control of process and coating properties and in the development of novel plasma spray processes and applications. The operation of plasma torches is controlled by coupled dynamic, thermal, chemical, electromagnetic, and acoustic phenomena that take place at different time and space scales. Computational modeling makes it possible to gain important insight into torch characteristics that are not practically accessible to experimental observations, such as the dynamics of the arc inside the plasma torch. This article describes the current main issues in carrying out plasma spray torch numerical simulations at a high level of fidelity. These issues encompass the use of non-chemical and non-thermodynamic equilibrium models, incorporation of electrodes with sheath models in the computational domain, and resolution of rapid transient events, including the so-called arc reattachment process. Practical considerations regarding model implementation are also discussed, particularly the need for the model to naturally reproduce the observed torch operation modes in terms of voltage and pressure fluctuations.
Keywords: Plasma spray torch; Numerical model; Two-temperature; Chemical non-equilibrium; Electrode sheath

Steam Torch Plasma Modelling by Jiří Jeništa (653-687).
Numerical modelling of physical properties and processes in an electric arc stabilized by a water vortex (steam torch) has been summarized in this review paper. One-fluid MHD equations are numerically solved for an axisymmetric thermal plasma flow inside a discharge chamber of the steam plasma torch. The steady state solution results are discussed for the range of currents 300–600 A with relatively low steam flow rate of about 0.3 g s−1. The maximum obtained velocities and temperatures—8500 m s−1, 26,300 K, are reported at the centre of the nozzle exit for 600 A. The evaporation of water, i.e. mass flow rate of steam, was predicted from a comparison between the present simulation and experiments. The generated plasma is mildly compressible (M < 0.7) with the inertial forces overwhelming the magnetic, viscous, centrifugal and Coriolis forces with the factor of 103. Our calculations showed that the most significant processes determining properties of the arc are the balance of the Joule heat with radiation and radial conduction losses from the arc. Rotation of plasma column due to the tangential velocity component has a negligible effect on the overall arc performance, however, the rotation of water induces fluctuations in the arc and in the plasma jet with characteristic frequency which is related to the frequency of rotation of water. Reabsorption of radiation occurs at the radial position higher than 2.5 mm from the arc axis. The amount of reabsorbed radiation is between 17 and 28%. LTE conditions are satisfied in the arc column with the 2 mm radius. Comparison between the present simulations and experiments shows good agreement with the current–voltage characteristics, radial velocity and temperature profiles, as well as with the other related numerical simulation.
Keywords: Arc; Evaporation; Mass flow rate; Water-vortex stabilization; Net emission coefficients; Partial characteristics; Local thermodynamic equilibrium

Tomographic Measurements of Temperature Fluctuations in an Air Plasma Cutting Torch by Jan Hlína; Jiří Šonský; Jan Gruber (689-699).
A tomographic optical system and method based on evaluations of plasma radiation in a wavelength range of 559–601 nm were used to acquire temperature distributions in an air plasma cutting torch in planes perpendicular to the arc axis with a time resolution of 1 μs. The derived frequency spectra and distributions of temperature fluctuations represented by standard deviations have shown significant variations in distributions of instabilities, depending on the time scales which are taken into account. The results confirmed the decisive role of arc current ripple modulation in the arc temperature fluctuations.
Keywords: Cutting arc; Air plasma; Tomography; Temperature distribution; Fluctuation

The Existence of Non-negatively Charged Dust Particles in Nonthermal Plasmas by M. Mamunuru; R. Le Picard; Y. Sakiyama; S. L. Girshick (701-715).
Particles in nonthermal dusty plasmas tend to charge negatively. However several effects can result in a significant fraction of the particles being neutral or positively charged, in which case they can deposit on surfaces that bound the plasma. Monte Carlo charging simulations were conducted to explore the effects of several parameters on the non-negative particle fraction of the stationary particle charge distribution. These simulations accounted for two effects not considered by the orbital motion limited theory of particle charging: single-particle charge limits, which were implemented by calculating electron tunneling currents from particles; and the increase in ion current to particles caused by charge-exchange collisions that occur within a particle’s capture radius. The effects of several parameters were considered, including particle size, in the range 1–10 nm; pressure, ranging from 0.1 to 10 Torr; electron temperature, from 1 to 5 eV; positive ion temperature, from 300 to 700 K; plasma electronegativity, characterized in terms of n +/n e ranging from 1 to 1000; and particle material, either SiO2 or Si. Within this parameter space, higher non-negative particle fractions are associated with smaller particle size, higher pressure, lower electron temperature, lower positive ion temperature, and higher electronegativity. Additionally, materials with lower electron affinities, such as SiO2, have higher non-negative particle fractions than materials with lower electron affinities, such as Si.
Keywords: Dusty plasmas; Particle charging; Monte Carlo simulations; Particle charge limits; Electron tunneling; Non-negative particles

The aim of this paper is to compare the effects of different mechanisms underlying the synthesis of copper nanoparticles using an atmospheric pressure radio-frequency induction thermal plasma. A design oriented modelling approach was used to parametrically investigate trends and impact of different parameters on the synthesis process through a thermo-fluid dynamic model coupled with electromagnetic field equations for describing the plasma behaviour and a moment method for describing nanoparticles nucleation, growth and transport. The effect of radiative losses from Cu vapour on the precursor evaporation efficiency is highlighted, with occurrence of loading effect even with low precursor feed rate due to the decrease in plasma temperature. A method to model nanoparticle deposition on a porous wall is proposed, in which a sticking coefficient is employed to model particle sticking on the porous wall used to carry a quench gas flow into the chamber. Two different reaction chamber designs combined with different quench gas injection strategies (injection through a porous wall for “active” quenching; injection of a shroud gas for “passive” quenching) are analysed in terms of process yield and size distribution of the synthetized nanoparticles. Conclusion can be drawn on the characteristics of each quenching strategy in terms of throughput and mean diameter of the synthesized nanoparticles.
Keywords: Thermal plasmas; Plasma modelling; Copper nanoparticles; Vapour radiation; Reaction chamber design

Steam Plasma Treatment of Organic Substances for Hydrogen and Syngas Production by M. Hrabovsky; M. Hlina; V. Kopecky; A. Maslani; O. Zivny; P. Krenek; A. Serov; O. Hurba (739-762).
Gasification of several organic materials in steam plasma generated in a special plasma torch with a water-stabilized arc was investigated. Thermal plasma with very high enthalpy and low mass flow rate is produced in an arc discharge which is in direct contact with water. Biomass and several types of solid and liquid organic waste were gasified by plasma aided reactions of materials with water, carbon dioxide and oxygen. Composition of produced gas, energy balance of gasification process and gasification efficiency were determined from measured data. Synthesis gas with high content of hydrogen and carbon monoxide and very low content of carbon dioxide, light hydrocarbons and tar was obtained for all tested materials. Comparison of measured data with results of theoretical computations confirmed that steam plasma gasification produces syngas with composition which is close to the one obtained from thermodynamic equilibrium calculations.
Keywords: Plasma gasification; Thermal plasma; Steam plasma; Syngas; Organic waste

Plasma-Assisted Treatment of Municipal Solid Waste: A Scenario Analysis by I. J. van der Walt; A. A. Jansen; P. L. Crouse (763-782).
Small-scale thermal treatment of municipal solid waste (MSW) was investigated using mass and energy balances based on the assumption of thermodynamic equilibrium. A typical average MSW composition from the literature was used as basis for modelling of a one ton per day waste gasification facility (plant). Syngas production by pyrolysis, stoichiometric O2 addition and auto-thermal (gasification with oxygen and/or air where no external heat input is required) combustion were considered. These cases were evaluated for production of electricity only, and steam. From purely thermodynamic considerations, it was observed that auto-thermal oxygen gasification produces the most electricity (47.00 kWe) and oxygen plasma gasification produces a positive net amount (4.07 kWe) on a 1 ton per day scale. Although auto-thermal air gasification also produces a net positive amount, the calorific value of the syngas is too low to fuel an internal combustion engine. As expected, the amount of steam generated by the different scenarios is high due to higher process efficiencies. The close-coupled auto-thermal oxygen process proved to be the most efficient. The cost of additional oxygen generation was however not taken into account, and may change the picture significantly.
Keywords: Gasification; Energy balance; Plasma; Thermodynamic study; MSW

A method of synthesizing functional nanostructured powders through reactive thermal plasma processing has been developed. Nano-sized oxide powders, including titanium dioxide and some functional oxides, were synthesized by the oxidation of liquid precursors. Oxides with the prescribed cation ratio of the liquid precursor can be synthesized with this technique, and it is possible to precisely adjust the chemical composition, which is linked to the appropriate functions of ceramic materials. Quench gases, either injected from the shoulder of the reactor or injected counter to the plasma plume from the bottom of the reactor, were used to vary the quench rate; therefore, the particle size of the resultant powders. The experimental results are well supported by numerical analysis on the effects of quench gases on the flow pattern and temperature field of thermal plasma as well as on the trajectory and temperature history of particles. Plasma-synthesized TiO2 nanoparticles showed phase preferences different from those synthesized by conventional wet-chemical processes. Nano-sized particles of high crystallinity and nonequilibrium chemical composition were formed in one step via reactive thermal plasma processing. The plasma-synthesized nanoparticles were spherical and hardly agglomerated, and high dispersion properties were observed, i.e., the plasma-synthesized TiO2 nanoparticles were individually dispersed in water.
Keywords: Nano-sized particle; Thermal plasma processing; Non-equilibrium chemical composition; Metastable phase; Phase formation; Optical and photocatalytic properties; Dispersibility

A Coupled Chemical Kinetic and Nucleation Model of Fume Formation in Metal–Inert-Gas/Metal–Active-Gas Welding by Hunkwan Park; Maximilian Mudra; Marcus Trautmann; Anthony B. Murphy (805-823).
A computational model of the formation of welding fume in arc plasmas, under conditions occurring in metal–inert-gas (MIG) and metal–active-gas (MAG) welding, is presented. The model couples the chemical kinetics occurring in high-temperature mixtures of iron vapour, oxygen and argon with a moment model of the nucleation and growth by condensation of iron and iron oxide nanoparticles. Results are presented for different iron vapour concentrations, oxygen-to-argon ratios, and quench rates. It is found that the presence of oxygen has important effects on the gas-phase chemistry and the properties of the nanoparticles. FeO nanoparticles are preferentially nucleated, and have smaller diameter than the Fe nanoparticles that are produced in the absence of oxygen. The final composition of the nanoparticles depends on the relative concentrations of iron and oxygen in the plasma. A three-dimensional arc model that includes vaporization of the wire electrode is used to predict temperature, velocity and iron vapour mass fraction distributions in typical MIG and MAG welding conditions. Calculations of nanoparticle formation and growth along streamlines confirm the importance of oxygen in determining the fume particle properties.
Keywords: Welding fume; Gas metal arc welding; MIG welding; MAG welding; Nanoparticle formation; Nucleation; Condensation; Computational modelling

This paper is a further development of the collisional sheath model at the thermionic cathode for two temperature modeling of thermal arcs that was recently suggested by Pekker and Hussary. In the present work, the Schottky correction factor to the work function of the electrode material is calculated taking into account the friction of ions in the sheath, while in the model of Pekker and Hussary it was calculated neglecting this friction. The model is applied to the cathode spot at the tungsten cathode in argon. It is demonstrated that a virtual cathode can be formed in the atmospheric pressure argon plasma at the cathode surface if the cathode current density is sufficiently small. The heat flux to the thermionic cathode due to charged particles and the heat flux to the plasma due to thermionic electrons are calculated. The obtained results are compared with the model of Pekker and Hussary. The sheath potential drop and the heat fluxes calculated by this model can be used as boundary conditions at the wall for the electric potential and for the energy equations for the electrons and heavy particles (ions and neutrals) in two temperature modeling of thermal plasma.
Keywords: Sheath; Thermal plasma; High-pressure arcs; Thermionic electron emission; Plasma modeling

A batch process is developed to generate sulphur functionalized graphene nanoflakes (S-GNFs), corresponding to nanoparticles of stacked graphene. The growth and functionalization of the catalysts are done in a single thermal plasma reactor. The GNFs are first grown through the decomposition of methane in the thermal plasma volume followed by homogeneous nucleation of the nanoparticles in the well-controlled recombining plasma stream allowing the 2-dimensional evolution of the nanoparticle morphology. The precursor feeding conditions are then changed to liquid carbon disulphide in order to generate sulphur-based functional groups on the nanoparticles. The plasma conditions and carbon disulphide injection are varied, and samples with tuneable amount of sulphur between 4 and 28 at% are obtained. The functional groups generated include polythiophene polymer partly covering the GNFs, sulphur functionalities implemented directly on the graphitic structure, and traces of orthorhombic sulphur. The S-GNFs exhibit higher electrocatalytic activity toward the oxygen reduction reaction in alkaline medium for the samples containing the highest amounts of sulphur.
Keywords: Graphene; Graphene nanoflakes; Thermal plasma functionalization; Carbon disulphide; Sulphur based catalyst; Oxygen reduction reaction

Uniform Surface Oxidation of an Si Substrate by a Planar Modulated Inductively Coupled Thermal Plasma with Molecular Gas Feed by Mai Kai Suan Tial; Yasunori Tanaka; Yuji Maruyama; Takumi Tsuchiya; Yoshihiko Uesugi; Tatsuo Ishijima (857-876).
A planar type of inductively coupled thermal plasma (ICTP) with coil current modulation was adopted for large-area rapid surface oxidation of a substrate. The planar ICTP has been developed by the authors for large-area surface modification processing. In addition, coil current modulation was used to control the temperature and chemical reaction fields in the planar ICTP. Firstly, a fundamental study of the operation of the planar modulated ICTP with a substrate was carried out by measuring its electrical properties and the visible light emission in this work. Secondly, spectroscopic observations were carried out to investigate the effect of the coil current modulation on the radiation intensity distribution of spectral lines from the planar ICTP on the substrate. Thirdly, surface oxidation tests were made for Si substrates by irradiation of the planar modulated ICTP at different modulation frequencies and different duty factors. Oxide layer thickness distribution fabricated on the substrate was measured to study the lateral uniformity of the oxidation processing by the modulated planar ICTP. Finally, it was found that adoption of the coil modulation can improve the uniformity of the oxidation processing by the planar modulated ICTP.
Keywords: Inductively coupled thermal plasma; Planar ICTP torch; Coil current modulation; Atomic emission spectroscopy; Si oxidation processing

Numerical Simulation of Nonequilibrium Species Diffusion in a Low-Power Nitrogen–Hydrogen Arcjet Thruster by Hai-Xing Wang; Qing-Song He; A. B. Murphy; Tao Zhu; Fu-Zhi Wei (877-895).
Species distributions in a low-power arcjet thruster are investigated using a two-dimensional thermal and chemical nonequilibrium numerical model that incorporates the self-consistent effective binary diffusion coefficient approximation treatment of diffusion. Plasma flows in arcjet thruster with different input mole ratios of nitrogen to hydrogen are modelled. It is found that species separation due to nonequilibrium chemical kinetic processes occurs mainly in the regions where the dissociation and ionization of nitrogen and hydrogen species take place. The enrichment of nitrogen molecules at the fringes of the arc and hydrogen molecules near the anode wall of the thruster occurs mainly because the recombination processes of these two gases occur in different temperature ranges. In the expansion portion of the thruster nozzle, the gas residence times are of the same order as some chemical kinetic processes. Comparison between the nitrogen and hydrogen species profiles at the constrictor and thruster exit shows that the recombination of hydrogen ions and atoms are dominant kinetic processes near the thruster centreline, while the chemical reactions of nitrogen species are almost frozen in the high speed flow. The effects of temperature and pressure gradients on the species diffusion inside the arcjet thruster are also presented, with thermal diffusion found to have a much larger influence than pressure diffusion.
Keywords: Species diffusion; Thermal plasma flow; Arcjet thruster