Argon Arc Plasma Research Articles

Calculation of radiative heat transfer in argon arc plasmas

Radiation emission and absorption in arc plasmas are important energy transfer processes. Exact calculations, though possible in principle, are usually impossible in practice because of the need to treat a large number of spectral lines and also the continuum radiation in the whole spectrum range. Recently, we have used an approximate method of partial characteristics to evaluate the radiation intensities, radiation fluxes and the divergence of radiation fluxes for SF6 arc plasma with cylindrical symmetry. In this paper, we have extended our calculations toargon arc plasmas for the plasma pressures of 0.1, 0.5 and 1.0 MPa. We have calculated the coefficients of absorption for Ar plasmas at temperatures from 300 to 35 000 K, and have used these coefficients to calculate the partial characteristics. Both the continuum and the line spectra have been included in calculations. We have taken into account the radiative photo-recombination and bremsstrahlung for the continuous spectrum, and over 500 spectral lines for the discrete spectra. The method of partial characteristics has been applied to three-dimensional calculations of radiative heat transfer — i.e. radiation intensity, radiation flux and its divergence — in simplified temperature profiles. Conclusions have been made concerning validity and utilization of the method of partial characteristics in general gas dynamics problems.

Influence of the Electron Emission on the Properties of the Cathode Sheath

A simple cathode sheath model has been used to discuss the influence of the electron emission on the properties of the cathode sheath (CS). The model is applied to an argon arc plasma at atmospheric pressure with a thoriated tungsten cathode and the thermionic emission is mainly taken into account in this paper. However, the influence of the electric field intensity at the point of the cathode surface on the thermionic emission is also discussed briefly in this paper. The calculated results indicate that the electron emission has remarkably influence on the properties of the cathode sheath.

Diagnostics and modeling of a hollow-cathode arc deposition plasma

Methane or acetylene added to an argon or helium arc plasma is dissociated and/or ionized effectively in order to deposit amorphous carbon (a-C:H) layers. The plasma can be confined or spread out by a homogeneous or cusp magnetic field. The issue of the present study is whether the magnetic confinement has an influence on the type of ions governing the deposition and their energy. To this end, an energy-resolved mass spectrometer (plasma monitor) was used, screened off by a μ-metal cage. It appears that, for strong magnetic fields of up to 10 mT, besides CH3+ and C2H2+ ions (from the methane and acetylene admixture, respectively), also relatively more dissociated species like CHx+ (x=2, 1, 0) and CyHz (y=2, 1, 0; z=1, 0) arise. Especially with the C2H2 admixture, atomic C+ ions appear to become more abundant with increasing magnetic field. The energy of the ions appears to decrease with increasing magnetic field. With Langmuir cylindrical electric-probe measurements it was confirmed that this is related to a decreasing plasma potential. By means of a numerical model, spatially resolved density profiles of the most important species were calculated. Depending on the reactor parameters, relevant ion densities are of the order of 109–1010 cm−3, resulting in deposition rates of up to some μm h−1.

Effect on transferred ac arc plasma stability of increasing ambient temperature and superimposing pulse at current zero point

The use of alternating current (ac) arc plasma involves a problem of instability of the arc plasma at current zero point. This paper describes the effects of ambient temperature and of superimposing a pulse current on stability of the ac argon arc plasma; the pulse current (10 A) is superimposed on an ac current only at each current zero point. The ac arc plasma is generated between a tungsten electrode of a plasma torch and a copper counter electrode in a chamber. The ac current is 100-500 Arms and the arc plasma length is 40 mm. As the ambient temperature increases from 300 K to 900 K, the stored energy in the arc plasma increases and the required supply voltage to stabilize the arc plasma decreases by about 20% (from 270 to 220 Vrms). Additionally, superimposing a pulse current on an ac current causes the required supply voltage to decrease by about further 20% (from 220 to 180 Vrms).

Deposition behavior in a low-temperature cascade arc torch (CAT) polymerization process

Two kinds of hydrocarbon type monomers and three kinds of organosilicons were polymerized by a low-temperature cascade arc argon plasma torch. Their deposition behaviors were studied as a function of experimental parameters and monomer elemental compositions. It was found that the normalized deposition rate (DR), expressed as deposition yield of DR/(FM)m, was determined by a composite operational parameter, W*(FM)c/(FM)m, where W is the power input, and (FM)c and (FM)m are the mass flow rates of carrier gases and monomers, respectively. Experimental results indicated that the deposition yield is highly dependent on the elementary compositions of monomers. Optical emission spectroscopy study on the argon plasma torch showed that the emission intensity of excited argon neutrals was proportional to the value of the parameter W*(FM)c. These results further certified that excited argon neutrals are the main energy carriers from the cascade arc column to activate monomers in the argon plasma torch. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 967–982, 1999

Behaviour of Cavity Formation and Convection in FeOx-SiO2 Melts under Plasma Arc.

The behaviour of oxides melted by plasma arc has been studied. Oxides of FeOx and FeOx-SiO 2 were melted by argon plasma arc. A direct-current-transferred-type plasma generator was employed. Observations of the sample surface revealed that a cavity formed in the region where the plasma arc was directed and that surface flow was operative in the direction from the centre toward the edge of the melt. The depth of the cavity was governed by the balance between the hydrostatic pressure and the sum of the pressures caused by the arc and the argon gas flow. The main driving force of convection on the sample surface was the aerodynamic drag force. The temperature distribution was dependent on both the argon gas flow rate and the viscosity of the sample, because the heat from the plasma arc was mostly transported by convection on the sample surface.

True detection limit of rare earth elements in atomic emission analysis with an arc two-jet plasmatron

The “true detection limit” CL,true was measured by a method proposed by Boumans et al. for 10 prominent lines of La, Ce, Pr, Nd and Sm, the most abundant rare earth elements (REEs) in geological samples. It is demonstrated how spectral interferences can increase of CL,true. The spectra were excited in an argon arc plasma jet with evaporation powders of five complex natural samples. The data show that there is a significant increase of CL,true in real samples containing large concentrations of such elements as Ti, Zr, Cr, Nb, Ta and REEs emitting complex spectra.

Evaporation from synthetic incinerator ashes melted by plasma arc

Synthetic incinerator ashes of 40 mass %SiO 2–40CaO–20Al 2O 3 (named Glass) and 36 mass %SiO 2–36CaO–18Al 2O 3–10Fe 2O 3 (named Fe-Glass) were melted by argon plasma arc, with iron rods used for arc ignition. A direct current transferred type plasma generator was used with an output power of 6 kW. During arc melting, the iron rod was melted and located in the centre of the molten ash. The temperature at the boundary between the iron and ash melts was 2200 K and that at the edge of the ash 1870 K. X-Ray diffraction showed that the deposit vaporised from Glass was composed of Fe, FeSi and amorphous SiO 2. The evaporation rates from Glass and Fe-Glass were 3.8×10 −4 g/s and 1.4×10 −4 g/s, respectively. It has been concluded that the rate of evaporation from the molten ash is affected by the vaporisation reaction which determines the oxygen partial pressure in the whole system.

Degenerate four-wave mixing in equilibrium argon arc plasma

The non-intrusive degenerate four-wave mixing (DFWM) method was used to study the local thermal equilibrium atmospheric-pressure argon arc plasma. The laser wavelength was in resonance with the - ArI transition, corresponding to the 696.5 nm emission line. The Abrams - Lind theory was verified and proved to be valid under the conditions of our plasma. In the high-laser-intensity limit, the DFWM signals were shown to be exclusively dependent on the population difference between the relevant argon states. Well resolved axial and radial profiles of the plasma temperature and the electron density were determined.

Prediction of Surface Temperature on Metal Beads Subjected to Argon-Hydrogen Transferred Arc Plasma Impingemen.

Surface temperatures of metals subjected to transferred arc plasma impingement in 100 % argon and Ar-H2 (90 : 10 and 70 : 30 by volume) mixed gas arc plasmas are predicted based on a combined mathematical model of the arc plasma and the metal bead. The numerical simulation involves solution to conservation equations of mass, momentum, energy, and current continuity of the arc plasma and the target metals of Nb and Ta. An increase in hydrogen content of the introduced gas resulted to higher surface temperatures as well as an increase in the size of the molten pool. Predicted threshold arc current that causes an onset of melting is in good agreement with experiment.

A self-consistent model for the cathode fall region of an electric arc

A one-dimensional model of the non-equilibrium plasma region adjacent to an electric arc cathode is developed. The collisionless space-charge zone (sheath) and the hydrodynamically described ionization zone (pre-sheath) are connected to a unified model. In the pre-sheath, ionization, diffusion and decoupling of electron and heavy-particle temperature are considered. The voltage drop in the space-charge zone is computed self-consistently including thermionic electron emission at the cathode surface. This unified model yields the overall cathode potential drop (cathode fall), the extent of the non-equilibrium region and the net energy flux towards the cathode surface. In this paper, the model is applied to an argon arc plasma at atmospheric pressure with a thoriated tungsten cathode, as is typically used for welding of stainless steel materials.

Modeling of the Flow, Temperature and Concentration Fields in an Arc Plasma Reactor with Argon-Nitrogen Atmosphere.

A mathematical formulation was developed for describing the flow behavior, temperature profile and concentration fields in pure or mixed argon and nitrogen arc plasmas impinging on a metal target inside a reactor. The simulation involves solution to conservation equations of mass, momentum, energy, k-e model, chemical species, and current continuity. Combined diffusion coefficients were used in chemical species conservation to account for all species present in the plasma. An increase in the proportion of nitrogen introduced in the reactor results in higher arc velocity and temperature, reduced arc column diameter and increased heat flux imparted to the metal target. Demixing resulted to an increase of mass fraction of nitrogen at the central portion of the arc.

Numerical simulation of flow and temperature distribution in a transferred argon arc plasma enclosed in a chamber.

A mathematical model was developed for predicting the flow characteristics and temperature distribution of a transferred arc plasma enclosed in a cylindrical chamber. The simulation incorporated the k–ε turbulence model to account for turbulent flow of the introduced gas. The model was adopted to a supplied current of 200 A, an arc length of 0.01 m, and Argon gas inflow rates of 3.33 × 10–4 and 6.67 × 10–4 m3/s. Goodagreement of the calculated isotherms in the arc with measured temperatures available from the literature was obtained, thus validating the model. The gas introduced through the nozzle was observed to pass along the edge of the arc region and creep close to the wall, result in a recirculating flow that covers most of the chamber. A high temperature region concentrated only at the vicinity of the electrodes, while substantially lower temperatures were predicted in most sections of the chamber away from the arc.

Elimination of Boron in Molten Silicon by Reactive Rotating Plasma Arc Melting

An argon arc plasma remelting furnace with a rotational torch device was used to remove impurities in metallurgical-grade silicon under 60 kPa argon atmosphere. The radius of the rotation can be varied from 0 to 20 mm and the rate was controlled from 0 to 100 rpm. In the button melting practice, impurities such as Fe, Ti and Al remained unchanged in molten silicon. The Ca content decreased with melting time. The plasma of pure Ar or Ar with 2.6 vol% oxygen did not decrease the boron content remarkably, but the boron content was decreased by the plasma of lower oxygen partial pressure 0.66 vol%O 2 . The rate of silicon loss in the present study was as low as 6%/h. The boron content was decreased from 12 mass ppm to below 1 mass ppm within 15 min by using Ar + 1.24 vol%H 2 O, which was the highest water vapor pressure in the present study. The apparent rate constant for this case was 3.8 x 10 -3 s -1 . By rotating the plasma flame, the loss of silicon was unchanged and the reaction rate of boron removal increased. A continuous casting practice was developed using a water-cooled copper mold for the continuous removal of boron. Silicon ingots 40 mm in diameter were successfully cast.

Influence of molecular processes on the hydrogen atomic system in an expanding argon–hydrogen plasma

An expanding thermal arc plasma in argon–hydrogen is investigated by means of emission spectroscopy. The hydrogen can be added to the argon flow before it enters the thermal arc plasma source, or it can be flushed directly into the vacuum expansion vessel (1–20 vol % H2). The atomic state distribution function for hydrogen, measured at a downstream distance of 20 mm, turns out to be very different in the two cases. For injection in the arc, three-particle recombination is a primary source of hydrogen excitation, whereas measurements with hydrogen injected into the vessel clearly point to a molecular channel (dissociative recombination of formed ArH+) populating atomic hydrogen levels.

Interconversion of chlorofluorocarbons in plasmas

Chlorofluorocarbons undergo interconversion during destruction in an argon arc plasma, so that, for example, in the exhaust gas from destruction of CCl2F2, CClF3 is found to be the major residual ozone depleting substance: as electron capture detectors are 104 times less sensitive for CClF3, compared with CCl2F2, although these gases have the same ozone depleting potential, analysis of exhaust from destruction of chlorofluorocarbons is therefore not a trivial matter of determining only the level of input chlorofluorocarbon remaining.

Fabry–Pérot line shape analysis on an expanding cascaded arc plasma in argon

Fabry–Pérot line profile measurements have been used to obtain heavy particle temperatures and electron densities for an expanding cascaded arc plasma in argon. This was done for the argon 415.9 and 696.5 nm neutral lines as a function of the distance from the onset of the expansion. Temperatures in the range of 2000–12 000 K were obtained. The electron density in the beginning of the expansion appeared to be 5.6×1021 m−3. The 696.5 nm line profiles appeared to be asymmetric because of self-absorption by cool metastables around the plasma. The density and temperature of these metastables could be determined by fitting the measurements to a theoretical model, and appeared to be around 1017 m−3 and around 3000 K, respectively.

Atomic and molecular emission spectroscopy on an expanding argon/methane plasma

A supersonically expanding cascaded arc plasma in argon is analyzed axperimentally by emission spectroscopy. The thermal cascaded arc plasma is allowed to expand through a conically shaped nozzle in the arc anode into the vacuum vessel. In the nozzle monomers (CnHv) are injected. The monomers are dissociated and ionized by the argon carrier plasma, and transported toward a substrate by means of the expansion. Emission spectroscopy is used to obtain temperatures and particle densities. By varying external parameters (e.g., arc power, gas flow rates) plasma parameters can be linked with (e.g. parameters (e.g., refractive index).

Microstructure of the Intermetallic Compounds Al3(Ti1-XMx)

AbstractAn attempt was made to change the crystal structure of the intermetallic compound Al3Ti from DO22 to Ll2, by replacing Ti with several alloying elements. The alloying elements chosen in this study were Sc, Er and Lu with which Al forms intermetallic compounds having LI, structure. Ingots weighing about 50g of Al3(Ti1-xMx) — M represents alloying element and the x value is 0, 0.25 or 0.5 — were melted in an argon plasma arc furnace. X-ray diffraction, energy dispersive spectroscopy and scanning electron and optical microscopy were carried out to examine the microstructurc in relation to the kind and amount of alloying elements. Microstructural observation revealed the occurrence of an Ll2 phase in all M bearing alloys. Compression tests were also carried out to assess the ductility of these alloys. The results showed that the Sc alloying is effective to increase the room temperature ductility of Al3Ti.

Emission spectroscopy on a supersonically expanding argon/silane plasma

Results from emission spectroscopy measurements on an Ar/SiH4 plasma jet which is used for fast deposition of amorphous hydrogenated silicon are presented. The jet is produced by allowing a thermal cascaded arc plasma in argon (I=60 A, V=80 V, Ar flow=60 scc/s and pressure 4 × 104 Pa) to expand to a low pressure (100 Pa) background. In the resulting plasma SiH4 is injected in front of the stationary shock front. Assuming a partial local thermal equilibrium situation for higher excited atomic levels, emission spectroscopy methods yield electron densities (∼ 1018 m−3), electron temperatures (∼5000 K) as well as concentrations of H+, Si+, and Ar+ particles. The emission spectrum of the SiH radical, the A 2Δ–X 2Π electronic transition, is observed. Numerical simulations of this spectrum are performed, resulting in upper limits for the rotational and vibrational temperatures of 4000 and 5600 K, respectively. The results can be understood assuming that, in the expansion, charge exchange and dissociative recombination are dominant processes in the formation of species in excited states, notably Si+.