Volt-ampere characteristics of a longitudinally blown self-adjusting ARC in plasma generators with a rod cathode

Using the methods of regression analysis, consideration is given to the influence of various processes on the volt-ampere characteristics (VAC) of an arc discharge in a plasma generator with a hot rod cathode and a cold cylindrical anode with an arc length that is self-adjusting due to the mechanism of shunting. It is shown that heat-transfer processes play a dominant role in VAC formation.     

CO2-shielded arc as a high-intensity heat source

The characteristics of a CO₂-shielded arc are studied to evaluate its potential as a novel heat source for material processing, with lower costs and higher productivity than that of the tungsten–inert gas (TIG) arc. A double-gas-shielded system, using both CO₂ and an inert gas, is employed for the arc torch; this minimizes consumption of the tungsten electrode and gives arc stability equivalent to an argon TIG arc for 1800 s operation. The arc voltage of the CO₂-shielded arc is about 19 V for an arc current of 150 A and an arc gap of 3 mm, which is much higher than the 12 V obtained for an argon TIG arc. The CO₂ constricts the arc, resulting in an increase in the maximum heat flux density at the anode surface by a factor of about 10 relative to the TIG arc. The penetration depth of stainless steel melted by the CO₂-shielded arc is much larger than that for the argon TIG arc. It is concluded that the greater heating power of the CO₂-shielded arc, which is due to the greater arc constriction, in turn a consequence of the greater specific heat of CO₂, should lead to a large increase in material processing productivity.     

Generalization of volt-ampere characteristics of the electric arc in a double-jet plasmatron with tubular single-chamber electrode units

On a double-jet plasmatron with cylindrical single-chamber electrode units, an experimental study of volt-ampere characteristics of the electric arc is conducted at current 105–550 A, voltage 400–1320 V, total flow rate of the plasma-forming gas (air) (0.76–9.83)∙10⁻³ kg ⁄ s, angle between the cathode and anode parts 45–62°, distance between the cathode and anode axes at the outlet from nozzles 0.07–0.2 m, and outlet pressure of ~0.1 MPa. Correlations for these characteristics are obtained.     

Simulating effects of gas flow on arc plasma to anode heat transfer during incinerator ash treatment

The conventional method for converting incineration ash to slag employs a plasma type furnace. An analysis of ash melting characteristics was undertaken by changing the inner shield N₂ gas flow rate, assuming a hollow cathode and the ash anode. Results show that heat input intensity characteristics on the ash depend strongly on the gas flow rate through the central hall of the tube cathode arc (TCA). The maximum temperature of the ash surface becomes about 3000-4000 K in a melting state. For a gas flow rate greater than 2 L/min, the shape of the heat input intensity on the ash is annular because of the gas flow. By adjustment of the inner shield gas flow rate, more practical melting of a wider area of the ash can be achieved with the intense annular heat input on the ash.     

Carbon Nanotubes Grown on the Surface of Cathode Deposit by Arc Discharge

Abstract

Carbon nanotubes prepared by de arc discharge of graphite electrodes in He and CH₄ gas took markedly different morphology. Thick nanotubes embellished with many carbon nanoparticles were obtained by evaporation under high CH₄ gas pressure and high arc current. Thin and long carbon nanotubes were obtained under a CH₄ gas pressure of 50Torr and an arc current of 20A for the anode with a diameter of 6mm.     

NO x emission from electric arc furnace in steel industry: contribution from electric arc and co-combustion reactions

Off-gas measurements were conducted at industrial electric arc furnaces (EAF) in Germany in order to investigate the interrelation of NO _x emission with installed plant equipment (e.g. gas burner) and process data (e.g. carbon input). Off-gas data monitor rapid changes in off-gas composition, temperature, and volume flow rates of air into the furnace indicating the transient state batch process of scrap melting. From the measurements two distinct sources of NO _x emission are clearly distinguished: (1) NO _x formation in the electric arc plasma during the start-up period of the melting process in an oxidizing furnace atmosphere after the charging of the furnace. (2) NO _x formation from post-combustion of CO/CO₂/H₂ gas with air inside and outside the furnace. Whereas the contribution from arc ignition is similar for different types of EAFs, other contributions depend on furnace equipment and operation, e.g. gas burners, use of air as carrier medium for carbon or dust injection, air-tightness of furnace, and parameters of off-gas extraction by EAF dedusting system. The positive effect of the minimum volume flow rate of air into the furnace by controlled off-gas extraction to total NO _x emission is shown.     

The Shunting of Current and the Resultant Variation of Voltage in Channels of Plasmatrons with Self-Adjusting Length of Electric Arc

The characteristics of pulsation processes in dc plasmatrons with the cathode located in the channel center and the anode in the form of a cylindrical or a conical channel wall are investigated in plasmas of argon and nitrogen at atmospheric pressure. The fast Fourier transform of signals and their subsequent computer processing are used to obtain the dependences of the frequency of fluctuations of the arcing voltage on the working parameters of the plasmatron, namely, the electric arc current, the flow rate of plasma-forming gas, and the channel diameter. For plasmatrons with the self-adjusting length of the electric arc, analysis is performed of the mechanism of reclosing of the anode region of the arc, i.e., of the electric arc shunting associated with the stretching of the current filament by a flow of gas and with the electrodynamic interaction of different filament regions. A formula is derived which defines the dependence of the characteristic frequency of fluctuations of the arcing voltage on the external parameters of the problem, namely, the arc current, the flow rate of the working gas, and the characteristic channel diameter. It is demonstrated that the pattern of the dependence of the frequency of voltage fluctuations on the gas flow rate may vary with the values of the parameter of magnetohydrodynamic interaction. The formula generalizes the experimental results of numerous researchers obtained in a wide range of variation of the external parameters.     

Thin film deposition using a plasma source with a hot refractory anode vacuum arc

Vacuum arc generated plasma was used to deposit metallic Al, Zn, and Sn coatings on glass substrates. An arc mode with a refractory anode and an expendable cathode (the “hot refractory anode vacuum arc”), overcomes macroparticle (MP) contamination experienced in other arc modes. I = 100–225 A arcs were sustained between a water-cooled coating source cathode and an anode, which was heated by the arc, separated from each other by a 10-mm gap, for times up to 150 s. The distance from the arc axis to the substrate (L) was 80–165 mm. Film thickness was measured with a profilometer. It was found that the deposition rate increased with time to a peak, and then decreased to a steady-state value. The peak occurred earlier when using short anode (9 mm long), e.g., with the Al cathode, L = 110 mm, and I = 200 A, the peak was at t _p = 15 s after arc ignition while with the long anode t _p = 45 s. t _p decreased with I, from 45 s with I = 100 A, to 10 s with I = 225 A with the short anode. The peak is believed to appear due to initial condensation of cathode material (including MPs) on the cold anode, and its subsequent evaporation as the anode heated. In the later HRAVA steady state, a balance between condensation and evaporation on the anode is established. The deposition rate peak was significant with low melting temperature Al and Zn cathodes, which produce many MPs, and negligible with Cu and Ti cathodes.     

Similarity of plasmatron characteristics for longitudinal vortex flow around the arc with variations in the gas flow rate

The volt-ampere charateristics (VAC) of an elecric arc are correlated with longitudinal vortex air and nitrogen flows in plasmatrons with a rod-like cathode and two tubular electrodes. Two types of exponent approximations are used: a simple empirical formula and a relationship between generalized variables. It is shown by methods of regression analysis that the use of generalized variables provides similarity of the VAC upon variations in the gas-flow rate.Translated from Inzhenerno Fizicheskii Zhurnal, Vol. 68, No. 6, pp. 943–948, 1995.     

The preparation of carbon-coated iron nanocrystals produced from Fe2O3-containmg composite anode in arc discharge

Using an Fe₂O₃-containing composite anode instead of an Fe-containing composite anode in the Kratschmer-Huffman carbon arc method, carbon-coated Fe (not Fe₂O₃) nanocrystals are produced both in the soot on the reactor walls and in the cathode deposits. The encapsulates also contain a little iron carbide, but do not contain iron oxides, as identified by transmission electron microscopy ( TEM ) and X-ray diffraction (XRD). As compared with Ni₂O₃- and Co₂O₃-containing composite anodes, the action of the Fe₂O₃-containing composite anode is unique. When the Fe₂O₃ contents in the composite graphite rod for the carbon arc nanocrystal production are increased in the range 5-33 wt.%, the effect on the structure and diameter distribution of the iron nanocrystals is investigated. The diameter distribution of iron nanocrystals is about 1–30 nm. The effect of helium pressure in the range 80–680 torr on the yield of C_60/70 in carbon soot produced from the composite graphite rod has also been examined. We present a novel result about the relationship of C_60/70 yields and helium gas pressure using a composite anode differing remarkably from the result using a homogeneous graphite anode in arc discharge.     

Experimental investigation of the spatial displacements of an arc-heated plasma

Oscillations of an electric arc in a plasmatron with the fixed arc in wide ranges of currents and air flow rates have been investigated. The distribution functions of arc oscillations have been obtained for various currents, gas flow rates, and distances from a cathode.     

Optimization of the heating of a metal by an electric arc

We investigate and generalize the results of experimental measurements of heat fluxes in a metal anode from a plasma jet of a plasmatron with an external arc (PEA) whose operating conditions vary over a wide range.Notation efficiency of heating of the metal - Q_el heat flux into the calorimeter - I current of the electric arc - U voltage drop across the electric arc - V linear velocity of rotation of the sensor - r_o thermal radius of arc spot - _o time until surface of calorimeter begins to melt - t_p melting point of copper - t_o initial temperature of calorimeter surface - q specific heat flux in the heating spot - a thermal diffusivity - D diameter of rotating calorimeter - t₁ temperature of surrounding medium - t₂ temperature of the surface of the rotating calorimeter - G argon flow rate - h_s specific enthalpy of argon in the region near the anode - h_a specific enthalpy of argon at the surface of the calorimeter - k Boltzmann constant - T_e electron temperature in the anode region - e charge of the electron - U_a potential drop at the anode - _a work function of electrons leaving copper - L length of open part of the arc—distance from the cut in the nozzle of the PEA to the sensor - h depth of cathode insertion into the nozzle—distance from the cut in the PEA nozzle to the cathode - d diameter of the PEA nozzle - U _a ^* effective voltage equivalent Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 4, pp. 687–691, October, 1980.     

Numerical simulation of the spatial structure of a moving arc discharge

Results are given of the calculation of the structure of an arc discharge moving on parallel electrodes in air under the effect of a transverse magnetic field (0.086 T) at a current strength of 320 A. The numerical simulation is performed within an unsteady-state three-dimensional mathematical model of radiation magnetogasdynamics. The calculations reveal that fluctuations of values of physical parameters and of spatial shape of arc arise in the arc, which are caused by the gas flow past the discharge column and by unsteady-state processes in the electrode regions. Comparison is made with the available experimental data.     

Optimization of metal heating in plasma-mechanical treatment

Results are presented of an experimental investigation of the process of energy transfer to the anode of an arc in application to plasma-mechanical treatment of components.Notation Q_y energy introduced per unit mass of the layer being cut, W/kg - Q integral energy flux in the anode, W - I arc current, A - L spacing between the plasmotron nozzle exit and the component, m - h spacing between the plasmotron nozzle exist and the cathode, m - d plasmotron nozzle diameter, m - G gas consumption, kg/sec - q energy flux density in the anode spot, W/cm² - r spacing from the center of the anode spot to the point at which the energy flux density is calculated, cm - q_m energy flux density at the center of the anode spot, W/cm² - k_q concentration coefficient, cm^–2 - S_o degree of heat propagation, m - thermal diffusivity factor, m²/sec - time, sec - k spacing between the plasmotron and the cutter, m - v cutting rate, m/sec - coefficient of energy transmission to the anode - N electrical power of the arc, W Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 1, pp. 138–143, July, 1984.     

Improvement of the characteristics of a plasma torch at low gas flow rate

It is shown that the gas dynamics of a plasma torch and its characteristics depend on the configuration of the discharge chamber. Tapered boring at the cylindrical anode inlet (10° over a length of 11 mm) makes it possible to increase the arc voltage, torch efficiency, and enthalpy of the plasma jet at low gas flow rates. At high gas flow rates, the above parameters are higher in a torch where the anode does not have tapered boring. This is explained by turbulization of the heated gas.Academic Scientific Complex A. V. Luikov Heat and Mass Transfer Institute, Academy of Sciences of Belarus, Minsk, Belarus. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 66, No. 3, pp. 338–339, March, 1994.     

Synthesis of graphene flakes using a non-thermal plasma based on magnetically stabilized gliding arc discharge

Abstract

The use of non-thermal plasma at atmospheric pressure has emerged as a technique for the substrate-free, gas-phase synthesis of graphene nanoflakes (GNFs). In this paper, a non-thermal plasma based on magnetically stabilized gliding arc discharge (MSGAD) was employed to prepare GNFs. The effects of the carbon-containing precursor, plasma gas, and arc current on the GNFs synthesis were investigated. The technique produced GNFs with sizes of 50–200?nm and 1–20 layers, spherical carbon nanoparticles with 10–40?nm diameters, and graphitic particles. The results showed that the formation of GNFs depended on the selection of proper process parameters, such as precursors with a high H/C ratio, an Ar-N₂ plasma gas, a low arc current, a low precursor flow rate, and a suitable plasma gas flow rate. Correlations between the process parameters and the product morphology indicated that abundant H atoms and fewer polycyclic aromatic hydrocarbons were favorable for the formation of GNFs.     

Controlling the Placement of Spherical Nanoparticles in Electrically Driven Polymer Jets and its Application to Li‐Ion Battery Anodes

Employing circumferentially uniform air flow through the sheath layer of the concentric coaxial nozzle, the gas‐assisted electrospinning (GAES) utilizes both high electric field and controlled air flow to produce nanofibers. The ability to tailor the distribution of various nanofillers (1.85–12.92 vol% of spherical SiO₂ and Si nanoparticles) in a polyvinyl alcohol jet is demonstrated by varying airflow rates in GAES. The distribution of nanofillers is measured from transmission electron microscopy and is analyzed using an image processing technique to perform the dispersion area analysis and obtain the most probable separation between nanoparticles using fast Fourier transform (FFT). The analysis in this study indicates an additional 350% improvement in dispersion area with the application of high but controlled airflow, and a 75 percent decrease in separation between nanoparticles from the FFT. The experiments in this study are in good agreement with a coarse‐grained MD simulation prediction for a polymer nanocomposite system subjected to extensional deformation. Lastly, utilizing the sheath layer air flow in production of Li‐battery anode material, a 680 mAh g^?1 improvement is observed in capacity for nanofibers spun via GAES compared to ES at the same Si NP loading, which is associated with better dispersion of the electrochemically active nanoparticles.     

A two-dimensional mathematical model of a short vacuum arc in external magnetic field: results of numerical calculations

The paper deals with the investigation of the impact made by two-dimensional effects on the process of passage of current in a short vacuum arc in an axial magnetic field. A two-fluid mathematical model is used, which is based on hydrodynamic and electrodynamic equations. The axial magnetic field B _z affects significantly the magnitude of two-dimensional effects: the two-dimensional effects increase with decreasing B _z . The simulation results demonstrate that the contraction of plasma density exceeds that of current density. The distribution of anode drop of potential on the anode surface is nonuniform; in the case of certain (critical) values of current, the anode drop goes to zero on the external boundary of plasma. The dependence of the critical current on B _z is determined. The distribution of current density on the starting plane is nonuniform with a maximum on the axis, and the ion trajectories are inclined to the discharge axis. The possibility is discussed of matching the solution in the plasma region of vacuum arc with that for cathode flames.     

Effect of component thickness on lifetime and oxidation rate of chromia forming ferritic steels in low and high pO2 environments

Abstract

Long term oxidation tests were carried out with a high-Cr ferritic steel at 800°C and 900°C in simulated cathode and anode gas of a solid oxide fuel cell (air and an Ar/H₂/H₂O mixture respectively). It was found that with decreasing sample thickness the life time of the steel decreases due to breakaway phenomena. This effect is caused by faster exhaustion of the chromium reservoir from the bulk alloy in the case of thinner components. During air exposure the oxidation rates increase with decreasing specimen thickness and this has to be taken into account in the calculation of the Cr-reservoir exhaustion. This thickness dependence is not found during the exposures in simulated anode gas. Hence, especially for thin walled components, the oxidation rates in anode gas are substantially smaller and thus the life times are longer than during air exposure. The differences in oxidation behaviour in the two environments are discussed on the basis of scale formation mechanisms involving microcrack formation in the surface oxide scale and depletion of major and minor alloying additions in the bulk alloy.     

Anode mode in cathodic arc deposition apparatus with various cathodes and ambient gases

The anode mode of a vacuum arc in a cathodic arc deposition apparatus was observed as a function of ambient gas pressure ranging from 0.01 to 300 Pa. The chamber (400 mm in diameter and 600 mm in length) made of stainless steel (SUS304) acted as the anode. The arc was operated at a relatively low constant current of 50 A. The cathode materials used were Al, Ti, Fe, Ni, and Cu, and ambient gases were He, Ne, Ar, H₂, N₂, O₂, and CH₄. The principal results are as follows. (1) As the pressure was increased, the anode mode changed from diffuse-arc to footpoint to plane luminous to anode-spot mode. (2) The anode mode and resultant arc voltage increase were strongly dependent on gas species, and weakly on the cathode material. (3) Comparing diatomic and polyatomic (H₂, N₂, O₂, and CH₄) with mono-atomic molecule gases (He, Ne, and Ar), the onset pressure of the anode mode transition in the former was lower, the arc voltage higher, and the footpoints more numerous, smaller, and clearer. Both the dependence of the ambient pressure and the influence of the cathode materials and gas species on the anode mode changes were explained by the ion deficiency theory.