Plasma-forming Gas Composition Research Articles

Study of the electric arc dynamics in a cascaded-anode plasma torch

In plasma spraying process, the plasma jet stability depends on the electric arc movement inside the plasma torch. For conventional plasma torches with rod cathode and a large anodic surface, arc roots are free to move along the anode wall, leading to high plasma jet fluctuations. To overcome these instabilities, cascaded-anode plasma torches have been developed, reducing considerably the electric arc movement and thus the plasma jet fluctuations. Unfortunately, a small number of studies reports on the behaviour of theses torches including the arc instabilities and on electrode erosion process as well. In this work, several diagnostics have been set up to further understand the cascaded-anode plasma torch behaviour under different sets of operating conditions. Influence of plasma forming gas composition on the electric arc dynamic has been studied through anodic imaging. The issue of the intense plasma radiation masking is discussed since the light emitted by the arc column interferes with anodic arc roots observations. Multiple constricted arc roots have been observed and a preferential site for anodic arc root has been identified for all experimental conditions. The addition of a secondary plasma forming gas promotes a higher number of reattachments. The influence of the electric arc movement on plasma jet stability is also studied by following the plasma jet brightness fluctuations. The low voltage fluctuation frequency is also observed on brightness spectra, which means that these fluctuations could have an impact on in-flight particle treatment.

Effect of Nitrogen on the Plasma Decarburization of Chromium-Containing Steel

Abstract—To study the kinetics of the processes occurring in the region where a plasma torch interacts with high-chromium Kh18N10-steel-type iron melts, we investigate the influence of the partial nitrogen pressure in a plasma-forming gas on the decarburization when the melts are processed by an oxidizing plasma in a laboratory setup at a variable oxidation potential. A computer program for processing experimental data is developed. It takes into account the process of inert gas dilution of evolved carbon monoxide and dioxide with allowance for the true character of gas evolution. The kinetic parameters of the plasma decarburization of the high-chromium melts are determined, and the influence of the partial pressures of oxygen and nitrogen in the plasma-forming gas on the process kinetics is detected. The mechanisms of limiting the interaction at various stages of the process are proposed. The influence of nitrogen on the oxidative decarburization in a plasma furnace is studied. The presence of nitrogen in the plasma-forming gas composition is shown to decrease the rate and degree of carbon removal from the melts.

A novel plasma-based method for synthesis of As-Se-Te films: Impact of plasma parameters on the structure, composition, and optical properties

A novel Plasma Enhanced Chemical Vapor Deposition method for synthesis of ternary As-Se-Te films with varied stoichiometries is demonstrated. The innovative process has been developed in a non-equilibrium low-temperature argon and helium plasma under reduced pressure, employing volatile As, Se and Te as precursors. Utilization of inorganic precursors, in contrast to the typically used in CVD metal-organic precursors, has given us the chance to achieve сhalcogenide As-Se-Te films of very high quality in terms of surface smoothness, chemical and structural uniformity and purity. The impact of plasma parameters (energy input, electron temperature, plasma-forming gas composition) on the mechanism of growth, structure, composition, and optical properties of the ternary compounds was studied in detail. It is shown that the dissociation of As4, As2, Se6, Se2 and Te2 clusters due to electron impact is responsible for the film growth process and the electron temperature is the determining parameter for the thin film deposition rate. The newly developed process is cost-effective and enables deposition of high-quality As-Se-Te films for a number of device applications in the infra-red range of spectrum.

Calculation of Heat Transfer Characteristics of an Arc in a Gas Stream of Plasma-Arc Furnaces

Within the framework of the heat exchange model of electric arc, the arc characteristics in the gas jet of a melting plasmotron are calculated in the approximation of the interaction of a semi-infinite isothermal jet with a wall. The temperature profile, structure, and values of the heat fluxes of a part of the arc column in the melting space of the plasma arc furnace are determined to a distance from the cathode. For the given power and composition of plasma-forming gas, the electric and thermal characteristics of the plasmotron and its geometry are established. Examples are given of calculation of arc characteristics in argon and argon–hydrogen mixture in the initial period of melting of solid charge with stable arc combustion taking into account the influence of electric circuit parameters. The results on the temperature profile of the arc in a gas jet are presented.

Plasma surface modification of polypropylene track-etched membrane to improve its performance properties

The surface and electrochemical properties of polypropylene track-etched membrane treated by plasma of nitrogen, air and oxygen are studied. The effect of the plasma-forming gas composition on the surface morphology is considered. It has been found that the micro-relief of the membrane surface formed under the gas-discharge etching, changes. Moreover, the effect of the non-polymerizing gas plasma leads to formation of oxygen-containing functional groups, mostly carbonyl and carboxyl. It is shown that due to the formation of polar groups on the surface and its higher roughness, the wettability of the plasma-modified membranes improves. In addition, the presence of polar groups on the membrane surface layer modifies its electrochemical properties so that conductivity of plasma-treated membranes increase.

Diamond-Like Carbon Film Deposition Using DC Ion Source with Cold Hollow Cathode

Carbon diamond-like thin films on a silicon substrate were deposited by direct reactive ion beam method with an ion source based on Penning direct-current discharge system with cold hollow cathode. Deposition was performed under various conditions. The pressure (12–200 mPa) and the plasma-forming gas composition consisting of different organic compounds and hydrogen (C3H8, CH4, Si(CH3)2Cl2, H2), the voltage of accelerating gap in the range 0.5–5 kV, and the substrate temperature in the range 20–850°C were varied. Synthesized films were researched using nanoindentation, Raman, and FTIR spectroscopy methods. Analysis of the experimental results was made in accordance with a developed model describing processes of growth of the amorphous and crystalline carbon materials.

Surface and electrochemical properties of polypropylene track membrane modified by plasma of non-polymerizing gases

The surface and electrochemical properties of polypropylene track membrane treated by plasma of nitrogen, air, and oxygen are studied. The effect of the plasma-forming gas composition on the surface morphology is considered. The membrane surface microrelief formed during the gas-discharge etching is found to change. Moreover, the non-polymerizing gas plasma treatment induces oxidation of the membrane surface layer and generates oxygen-containing functional groups, mostly carbonyl and carboxyl. The higher membrane roughness and its hydrophilization is shown to lead to its better wettability. In addition, the presence of polar groups in the membrane surface layer modifies its hydrodynamic and electrochemical properties so that water permeability and conductivity of modified membranes increase.

PLASMA SPRAYING OF BORON CARBIDE USING TERNARY GAS MIXTURES

Materials which exhibit high specific heat and melting enthalpy combined with a low density, like boron carbide B4C, are difficult to melt by plasma spraying under standard spray conditions. Moreover, the injection of this low-density powder in the plasma jet is critical, especially when fine particles are used. The optimization of the plasma-forming gas composition may make the penetration of the particles in the gas flow easier, and improve their melting state. This paper presents an investigation of the effect of the composition of ternary gas mixtures of argon, helium and hydrogen on B4C particles heating and acceleration. The particle injection and the resulting spray pattern in the jet were controlled by using a 1D imaging technique. The particle velocity distribution was measured with a laser Doppler anemometer and the particle overall temperature with a high-speed pyrometer.

INFLUENCE OF VOLTAGE FLUCTUATIONS RELATED TO PLASMA TORCH WORKING CONDITIONS ON ZIRCONIA PARTICLE THERMAL TREATMENT

The microstructure of plasma sprayed coatings is strongly linked to particle trajectories depending among other parameters on arc root fluctuations. The latters depend on the torch anode-nozzle shape and internal diameter, the plasma forming gas composition and flow rate, and the anode erosion. Experiments were performed with yttria partially stabilized zirconia (8 wt% yttria) particles with a size distribution between 5 to 25 μm and either a PTF4 or a 3MB GE Sulzer-Metco torch. The PTF4 torch was fed with an Ar-Mb mixture and an arc current of 600 A, while the 3MB torch was fed with N2-H2 mixture with an arc current of 500 A. Particle mean temperatures and mean velocities were measured to evaluate the influence of voltage fluctuations on the treatment. This study shows that the torch working parameters, except the arc current, have nearly the same effects on the arc voltage fluctuations for the both torches. The 3MB torch voltage fluctuations are more important than those for a PTF4 torch, but they are less affected by the variations of spray parameters. Furthermore, the fluctuations of particle temperatures and velocities versus the voltage fluctuations are not the same.

Experimental design of plasma spraying and laser treatment of hydroxyapatite coatings

Plasma spray process of hydroxyapatite (Ca 10(PO 4) 6(OH) 2, HA) followed by laser treatment of obtained coatings were optimized by an advanced statistical planning of experiments. The full factorial design of 2 4 experiments was used to find effects of four principal parameters, i.e. electric power, plasma forming gas composition, carrier gas flow rate and laser power density onto microstructure of hydroxyapatite (HA) coatings and powders and depth of laser melted zone. The SAS and Statgraphics commercial softwares have been applied to obtain the mathematical model of influence of process parameters onto experimental responses. The chosen responses were the fraction of HA crystal phase and two phases of its decomposition α-tricalcium phosphate (Ca 3(PO 4) 2, α-TCP), tetracalcium phosphate (Ca 4P 2O 9, TTCP) and, on the other hand, the depth of laser melted zone in the coating. The two most important factors influencing these responses are electric power supplied to torch, laser power density. Laser power density is very important for the depth of laser melted zone. The crystal phase content of powders and coatings was determined using X-ray diffraction (XRD) quantitative analysis. The morphologies of coatings surfaces, cross-sections were characterized using scanning electron microscope (SEM).

Relationship between plasma spray operational parameters and microstructure of hydroxyapatite coatings and powder particles sprayed into water

A full factorial design of 2 4 experiments was used to determine the effects of four principal plasma spray parameters (i.e., plasma forming gas composition, electric power, spraying distance, and carrier gas flow rate on the microstructure of hydroxyapatite (HA) coatings and powders). The granulometry of powders was tested after spraying into water and compared to the initial one. The crystal phase content of powders and coatings was determined using quantitative X-ray diffraction (XRD) analysis. The morphologies of coatings surfaces, cross-sections, and particles sprayed into water were characterized using scanning electron microscopy (SEM). The mapping of elements inside plasma-treated powders was carried out using electron microprobe analysis (EMPA). Infrared spectroscopy (FTIR) allowed determination of the dehydration of HA under different spray conditions.

Production of asymmetric track membranes with a high permeability and separation selectivity

An investigation of the structure and surface properties of poly(ethylene) terephthalate track membranes (PET TM) with a changed pore profile subjected to the plasma RF-discharge is performed. It was arranged that effect of the non-polymerizing gas plasma on the membranes of researched sample leads to formation of asymmetric membranes possessing a higher flow rate. Choice of optimal discharge parameters allows one to etch the membrane in its layer. In this case in the membrane layer not affected by plasma, the mean pore diameter remains at its initial level. Preserving the initial (nominal) pore diameter allows one to perform the filtration processes with no decrease of separation selectivity. It is shown that the introducing of the polymerizing gas, cyclohexane, into the composition of plasma-forming gas provides improved hydrodynamic properties of asymmetric membranes.

OES Use and Vaporization Modeling for Fly-Ash Plasma Vitrification

Results are presented of optical emission spectroscopy (OES) application asa control tool to improve fly-ash plasma vitrification. A twin-torch plasmasystem has been used for the fly-ash processing, and a new OES method hasexamined metallic vapors above the melt. The method allows the study ofnonhomogeneous optically thin plasmas exhibiting a symmetry plane withoutsophisticated tomographic systems. The dc arc torches are mounted above acold crucible filled with a synthetic glass. The arc intensity is from200 to 400 A. Argon is introduced into the torches along the cathodeand the anode, while argon, oxygen or hydrogen are injected through thelance between the torches. Local plasma temperatures above the melt havebeen evaluated using measured relative intensities of spectral lines ofthe plasma-forming gas. Metallic vapor concentration in the plasma isdeduced from the intensity ratio of the metal–gas spectral lines. Leadoxide has been used to study heavy-metal behavior at the fly-ash plasmavitrification. Distribution of the lead along the crucible surface,depending on the plasma-forming gas composition as well as the concentrationevolution with time, have been examined. The elemental analysis of theresultant glass has been measured by scanning electron microscopy (SEM)with energy-dispersive spectrometry (EDS). A predictive model has beenadapted to simulate the noncongruent vaporization of heavy metals from themelt. According to the data obtained, steep variations of the volatility ofthe elements depend strongly on reducing properties of gases controllingthe plasma composition near the melted surface. In addition, the melttemperature and the redox potential of the gas phase are found to be themost critical parameters.

Sound velocity in different reacting thermal plasma systems

In most cases the energy dissipated in plasma jets used either,for heating or spraying is varied by changing the are current, the total gas floc+rate, and composition. However, when doing so, conditions are reached where the plasma jet may become supersonic. To predict such conditions or to characterize supersonic plasma jets the knowledge of the sound velocitya is mandatory The goal of this paper is to calculatea versus plasma forming gas composition, temperature, and pressure. Rigorous calculation would imply the knowledge of the chemical reaction kinetics, sound velocity depending strongly on this. Unfortunately such kinetics are generally lolknown for plasma jet floras and the only possibility is to determine the equilibrium sound velocitya calculated through the isentropic coefficient T. This coefficient has been calculated taking into account the dissociation and ionization reactions at equilibrium for temperatures ranging from 300 to 25,000 K and pressures between 0.1 and 1 Mpa for Ar, H2, He, Ar-He, Ar-H2, O2, N2, air, .steam, and methane.aγ often called the “frozen” sound velocity, was also calculated using γ (ratio of specific heats) instead of Γ.