Effect of Anode Arc Root Position on the Behavior of the DC Non-transferred Plasma Jet at Field Free Region

A special bi-anode plasma torch that can change the anode arc root position without changing working gas flow rate has been developed to investigate the effect of anode arc root position on the behavior of the plasma jet. It has two nozzle-shaped anodes at different axial distances from the cathode tip. The arc root can be formed at anodes either close to the cathode tip (anode I) or far away from it (anode II) to obtain different attachment positions and arc voltages. The characteristics of pure argon plasma jets operated in different anode modes were measured in the field free region by using an emalpy probe, and the thermal efficiency of the torch was determined by measuring the temperature differences between cooling water flowing in and out of the torch. The results show that compared with the normal arc operated in anode I mode, the elongated arc operated in anode II mode significantly reduced the plasma energy loss inside the torch, resulting in a higher temperature and a higher velocity of the plasma jet in the field free region.     

Advances in Plasma Arc Cutting Technology: The Experimental Part of an Integrated Approach

The experimental part of an integrated approach to design and optimization of plasma arc cutting devices will be presented; in particular results obtained through diagnostics based on high speed imaging and Schlieren photography and some evidences obtained through experimental procedures. High speed imaging enabled to investigate start-up transition phenomena in both pilot arc and transferred arc mode, anode attachment behaviour during piercing and cutting phases, cathode attachment behaviour during start-up transient in PAC torches with both retract and high frequency pulse pilot arc ignition. Schlieren photography has been used to better understand the interaction between the plasma discharge and the kerf front. The behaviour of hafnium cathodes at high current levels at the beginning of their service life was experimentally investigated, with the final aim of characterizing phenomena that take place during those initial piercing and cutting phases and optimizing the initial shape of the surface of the emissive insert.     

Heat Generation and Particle Injection in a Thermal Plasma Torch

The operation of plasma guns used for plasma spraying involves a continuous movement of the anode arc root. The resulting fluctuations of voltage and thermal energy input introduce an undesirable element in the spray process. This paper deals with the effects of these arc instabilities on the plasma jet, and the behavior of particles injected in the flow. The first part refers to the formation of the plasma jet. Measurements show that the static behavior of the arc depends strongly upon the plasma-forming gas mixture, especially the mass flow rate, of the heavy gas, injection mode, nozzle diameter, and arc current. These parameters control the electric field in the arc column, the arc length, its stability, and the gas velocity and temperature. The dynamic behavior of the arc is examined to determine how the tempeature and velocity of the plasma gas vary with voltage variations. Relationships between the gas velocity at the nozzle exit and the lifetime of the arc roots, and the independent operating parameters of the gun can be established from a dimensional analysis. The second part discusses the interaction between the plasma jet and the particles injected into the flow. The parameters controlling particle injection and trajectory are examined to determine how injection velocity must vary with particle size and density to achieve a given trajectory. The effect of the transverse injection of the powder carrier gas is investigated using a 3-D computational fluid dynamics code. Finally, the effect of the jet fluctuations on particle trajectory is studied under the assumption that the jet velocity follows the voltage variation. The result is a continuous variation of the particle spray jet position in the flow. Experimental observations confirm the model predictions.     

Synthesis of Al Nanopowders in an Anodic Arc

Nanopowders of metals and metal oxides have been produced using an arc operated between a refractory rod anode and a hollow cathode (J. Haidar in A method and apparatus for production of material vapour, Australian Patent No. 756273, 1999). the arc attachment to the anode is through a small region of molten metal located at the tip of the rod anode. Heat from the arc evaporates the molten metal and the vapour is passed through the arc plasma before condensing into sub-micron particles downstream of the cathode. A precursor metal is continuously fed onto the tip of the anode to maintain the molten metal region and compensate for losses of materials due to evaporation. The particle size of the produced powder depends on the pressure in the arc chamber and for production of nanoparticles in the range below 100 nm we use a pressure of 100 torr. Aluminium has been used as a precursor material, leading to production of aluminium metal nanopowders when the arc is operated in argon and to aluminium oxide nanopowders for operation in air. For operation in air, the products are made of γ-Al₂O₃.     

Experimental Observations of Constricted and Diffuse Anode Attachment in a Magnetically Rotating Arc at Atmospheric Pressure

Plasma Chemistry and Plasma Processing - At atmospheric pressure, the anode attachment can appear in two different modes: constricted and diffuse. In this paper, a magnetically rotating arc plasma...     

Numerical Study of a Free-Burning Argon Arc with Anode Melting

Numerical modeling of free burning arcs and their electrodes is useful for clarifying the heat transfer phenomena in the welding process and to elucidate those effects which determine the weld penetration. This paper presents predictions for a stationary welding process by the free-burning argon arc. The whole region of the welding process, namely, tungsten cathode, arc plasma and stainless steel anode is treated in a unified numerical model to take into account the close interaction between the arc plasma and the molten anode. The time dependent development of two-dimensional distributions of temperature and velocity, in the whole region of the welding process, are predicted at a current of 150 A. The weld penetration geometry as a function of time is thus predicted. It is shown also that different surface tension properties can change the direction of re-circulatory flow in the molten anode and dramatically vary the weld penetration geometry.     

Anode Magnetron Enhanced DC Glow Discharge Processes for Plasma Surface Cleaning and Polymerization

The objective of this study was to examine some fundamental factors involved in the design and construction of the anode magnetron dc glow discharge processes as well as its performance in plasma cleaning and polymerization. Those advantages of anode magnetron include the capability of the magnetron to operate at low pressure, as well as decreasing the thickness of cathode dark space, i.e., the negative glow which contains a higher concentration of ions and active species was more closely to the cathode surface, which makes the plasma surface cleaning and polymerization an effective and uniform processes. The deposition rate at a given discharge power is increased by the presence of anode magnetrons, and is also much higher relative to rf and af. The refractive index of dc plasma film at a given polymer thickness (such as TMS, 70 nm, RI: 2.4) is higher than rf, af, and cascade arc plasma (RI: 1.6–1.7).     

Radial particle distributions in a d.c. arc—maximum off-axis

It is shown that for some elements evaporated from the anode of a d.c. arc the position of the maximum concentration of atomic particles (i.e. atoms + ions) occurs not on the axis of the arc column but some distance away from the centre, this distance being largely determined by the inside diameter of the electrode crater. It is shown that the magnitude of the off-axis peak increases with increasing volatility of the element concerned, but decreases with increasing electrical power generated in the plasma as well as with decreasing ionisation potential of the buffer metal. The proposed mechanism for this phenomenon is based on the fact that, due to thermal repulsion, cool vapours do not readily mix with hot gases. Thus volatile sample components evaporating from regions not immediately beneath the anode spot would tend to diffuse laterally around the arc rather than vertically into the hot plasma.     

Numerical simulation of a free-burning argon arc with copper evaporation from the anode

A free-burning, high-intensity argon arc at atmospheric pressure was modeled during the evaporation of copper vapor from the anode to study the impact of the vapor to the entire plasma region. A uniform and a Gaussian radial velocity distribution are adopted for the copper vapor at the anode boundary with a net mass flow rate known from the experiment. The effect of both velocity distributions on the temperature, mass flow, current flow, and Cu concentration was studied for the entire plasma region. The cathode region is not affected by the evaporated copper, and the Cu vapor concentration in the arc core is negligible.     

Effects of Copper Vapor on Heat Transfer from Atmospheric Nitrogen Plasma to a Molten Metal Anode

As an object of nitrogen plasma operated with an arc current to 200 A, an arc length about 35 mm, we evaluated heating efficiency from arc plasma to the molten copper anode and the water-cooled solid copper anode. The heating efficiency to the molten anode is smaller than that to the solid anode by about 20%. We focused on copper vapor concentration in plasma as a possible cause for a decrease in heating efficiency, and estimated it by means of the Cu and the N spectral line measurement. Simple numerical analysis, taking into consideration measured copper vapor concentration, suggested that an increase in electrical conductivity due to copper vapor, made the plasma temperature change and consequently caused a decrease in thermal conductivity. We concluded that one of the reasons for a decrease in heating efficiency would be caused by copper vapor contamination.     

The effect of a buffer compound on the temperature of the anode spot in a d.c. arc

The investigation reported in this paper shows that the temperature of the anode spot of a d.c. arc is, under given conditions, determined largely by two factors: the temperature at which the metal of the buffer compound volatilises and enters the arc plasma and the energy required to effect this volatilisation i.e. the latent heat of volatilisation.     

Computer modeling of negative electrode operation in lithium-ion battery: Model of equal-sized grains,galvanostatic discharge mode,calculation of characteristic parameters

A computer simulation of the negative electrode (anode) operation in a lithium-ion battery is performed. A complete research program is carried out in accordance with the recommendations of the theory of porous electrodes: the “model of equal-sized grains of two types” was studied, percolation properties of the anode active layer were researched, values of effective coefficients were calculated for charge transfer and mass transport, a complete system of equations describing operation of the anode is presented. Two specific cases of galvanostatic mode of anode discharge are considered in detail: an “ideal” anode and anode with nanosize particles. Working anode parameters are calculated: optimum bulk concentration of graphite in the active layer, active layer thickness, time of complete anode discharge, its specific electric capacitance and final potential on the active/layer interelectrode space interface. Advisability of working with anodes with nanosize grains and electrolyte with enhanced specific conductivity is shown.     

Improvement of plasma spraying efficiency and coating quality

A commercial torch has been modified to introduce an additional anti-vortex and shroud gas flow to counter the detrimental effects brought about by the vortex plasma gas flow which is used to stabilize the cathode arc attachment and to increase the anode life. Deposition efficiency and coating quality are used as criteria to judge the modified versus the nonmodified torch. High-speed videography and computerized image analysis systems are used to determine the particle trajectories, velocities, and the plasma jet geometry. The results show that the additional anti-vortex and shroud gas flow to the torch can keep the particles closer to the torch axis and reduce the amount of entrainment of cold air into the plasma jet. The consequence is that deposition efficiency and coating quality are substantially improved.     

Synthesis of aluminum nitride in transferred arc plasma furnaces

Ultrafine particles of aluminum tnitride (AIN) arc produced by a transferred an plasma. Two devices are used: a transferred arc plasma on aluminum natal in nitrogen or nitrogenlammonia atmospheres, and a item concept of transferred arc plasma when, DC anode and cathode ares are coupled together above an alumintun melt. Equilibrium chemical compositions mere calculated. The temperature distributions in the plasma are measured hr emission spectroscopy Flit, powder, made from 99.8%, aluminum ingot, it as analyzed and confirmed to be 99.3%, of hexagonal phase aluminum nitride. In othertests, from 99.99% aluminum ingot, a translucent AIN vinter was obtained. The densification behavior was assessed by hot pressing and by pressureless sintering, with and without additives. The thermal conductivities are given.     

Mathematical Modeling of Silica Anode Decomposition

The volatilization of quartz in a transferred arc plasma followed byquench and recondensation is a promising route to the production offumed silica. In this work, an existing model of a transferred arcwas modified and combined with a newly developed model of a moltensilica anode to predict the behavior of a transferred arc evaporatoras a function of current and plasma gas flow rate. The model predictstemperature, current, and flow fields in both the plasma and anode aswell as evaporation rates. Although quantitative agreement withexperimental results was not possible because of insufficient propertydata for silica at high temperature, the results were within an orderof magnitude of those measured experimentally. The model developed isuseful for the design and scaleup of this type of reactor.     

Parameters controlling the generation and properties of plasma sprayed zirconia coatings

D.C. plasma jets temperature and velocity distributions as well as the arc root fluctuations at the anode were studied for Ar-H₂ (25 vol%) plasma forming gases. The parameters were the arc current up to 700 A, the total gas flow rate up to 100 slm, and the nozzle diameter which was varied from 6 to 10 mm. The trajectories of partially stabilized zirconia particles into the jet were studied by a 2D laser imaging technique and two fast (100 ns) two color pyrometers. The results have revealed the difficulty to inject small particles into the plasma flow since most were found to by-pass the jet rather than penetrate it. The results also show the broad trajectory distribution within the jet and the influence of the arc root fluctuations on the mean particle trajectory distribution within the jet. Beside the measurements of the particle surface temperature and velocity distributions in flight, the particle flattening and the cooling of the resulting splats were studied statistically for single particles all over the spray cone. Such studies have emphasized the drastic influence of the substrates or previously deposited layers temperature on the contact between them and the splats. At 200–300°C this contact is excellent (cooling rates of the order of 100 K/μs for 1 μm thick splats) and it results in a columnar growth within the splats and the layered splats of a bead (up to 500 layered splats). This growth can be observed through passes provided the bead surface temperature has not cooled too much (a few tens of K) before the next bead covers it. A/C values up to 60 MPa were achieved with PSZ coatings. The effect of impact velocity of the particles, of substrate preheating temperature, of relative movments torch to substrate, of substrate oxidation on A/C values and splat formation were also studied.     

Operating characteristics and energy distribution in transferred plasma arc systems

A specially designed plasma chamber was constructed to study the operating characteristics of a dc plasma-transferred arc of argon, struck between a fluid convective cathode and a water-cooled anode. The arc voltage increased markedly with arc length and with an increase in the inlet velocity of the argon flow past the cathode tip, and much less with an increase in current. Radiation from the plasma column to the chamber walls and transfer of energy to the anode were the two principal modes of transfer of the arc energy. The former was dominant in the case of long arcs and at high inlet argon velocities. At the anode, the major contribution was from electron transfer, which occurred on a very small area of the anode (～5 mm in diameter). Convective heat transfer from the plasma was somewhat less. In all cases, the arc energy contributions to cathode cooling and to the exit gas enthalpy were small. From total heat flux and radiative heat transfer measurements, it was estimated that the plasma temperature just above the anode was in the range 10,000–12,000 K. Preliminary experiments with an anode consisting of molten copper showed that the arc root was no longer fixed but moved around continuously. The arc was othwewise quite stable, and its operating characteristics differed little from those reported for solid anodes, in spite of the greater extent of metal vaporization.     

Characterization of a twin-torch transferred dc arc

The results of a twin-torch transferred de arc .study are presented. The arc system consists of two torches of opposite polarity, and a coupling zone of plasma jets located between them. The torch configuration increases the system reliability and efficiency during material plasma processing. The results of the study present data for the voltage-current characteristics, general behavior of the twin-torch arc, and spatial distribution of the plasma parameters. The plasma parameters have been measured using optical emission spectroscopy for a 200 A (20 k W) do arc, at atmospheric pressure, with argon and nitrogen introduced as plasma forming gases into the anode and the cathode units, respectively. The measurement technique used has allowed the determination of local electron density and temperature values in an inhomogeneous plasma volume having no axial sysmmetry. The data obtained illustrate the novel features of the twin-torch transfrred do arc for its applications in plasma processing.     

Effect of Cathode Nozzle Geometry and Process Parameters on the Energy Distribution for an Argon Transferred Arc

The influence of two nozzle geometries and three process parameters (arc current, arc length and plasma sheath gas flow rate) on the energy distribution for an argon transferred arc is investigated. Measurements are reported for a straight bore cylindrical and for a convergent nozzle, with arc currents of 100 A and 200 A and electrode gaps of 10 mm and 20 mm. These correspond to typical operating parameters generally used in plasma transferred arc cutting and welding operations. The experimental set up consisted of three principal components: the cathode-torch assembly, the external, water-cooled anode, and the reactor chamber. For each set of measurements the power delivered to each system component was measured through calorimetric means, as function of the arc’s operating conditions. The results obtained from this study show that the shape of the cathode torch nozzle has an important influence on arc behaviour and on the energy distribution between the different system components. A convergent nozzle results in higher arc voltages, and consequently, in higher powers being generated in the discharge for the same applied arc current, when compared to the case of a straight bore nozzle. This effect is attributed to the fluidynamic constriction of the arc root attachment, and the consequential increase in the arc voltage and thus, in the Joule heating. The experimental data so obtained is compared with the predictions of a numerical model for the electric arc, based on the solution of the Navier–Stokes and Maxwell equations, using the commercial code FLUENT©. The original code was enhanced with dedicated subroutines to account for the strong temperature dependence of the thermodynamic and transport properties under plasma conditions. The computational domain includes the heat conduction within the solid electrodes and the arc-electrode interactions, in order to be able to calculate the heat distribution in the overall system. The level of agreement achieved between the experimental data and the model predictions confirms the suitability of the proposed, “relatively simple” model as a tool to use for the design and optimization of transferred arc processes and related devices. This conclusion was further supported by spectroscopic measurements of the temperature profiles present in the arc column and image analysis of the intensity distribution within the arc, under the same operating conditions.     

Modeling of the Subsonic–Supersonic Flow and Heat Transfer in a DC Arc Plasma Torch

Two-dimensional modeling results are presented concerning the subsonic–supersonic flow and heat transfer within a DC plasma torch used for low-pressure (or soft vacuum) plasma spraying. The so-called fictitious anode method is used in the modeling in order to avoid inclusion of the complex three-dimensional effects near the anode arc root and also to avoid the forced heating of all the incoming cold gas stream by the arc. A nonorthogonal boundary-conforming grid, nonstaggered variable arrangement and the all-speed SIMPLE algorithm are employed for the solution of the governing equations, including gas viscous effects, temperature-dependent properties, and compressible effects. Good agreement of the predictions with available results for a few benchmarked compressible flow problems shows that the new version of the FAST-2D program can be employed for the present plasma flow modeling. Temperature, axial velocity, Mach number, and static pressure contours, and streamlines within the DC arc plasma torch are presented to show the flow and heat transfer characteristics. The flow transits gradually from upstream subsonic regime into downstream supersonic regime with the subsonic–supersonic transition within the cylindrical segment of the torch nozzle. Additional numerical tests show that gas viscosity and Lorentz force have only a slight effect on the plasma flow.