Non-equilibrium plasma property of ArGTA in low current range

Although the LTE assumption is effective to evaluate a high temperature region in the arc column in Gas Tungsten Arc (GTA) which is a kind of a transfer-type plasma torch, it is difficult to apply it to a low temperature region such as the fringe of the arc column or an electrical sheath due to the decrease of collision frequency. Especially, in order to consider the effect of chemical reaction between the arc plasma and the surface of the anode material, non-equilibrium property of the arc plasma should be considered without the LTE assumption, since it is required to understand the precise property of the arc plasma closed to the anode surface. Therefore, we have developed a non-equilibrium simulation model of GTA. In this paper, we will report simulation results of plasma property of argon GTA at atmospheric pressure. As a result it was found that the non-equilibrium property appears below the collision frequency of 3.8 ? 10¹¹/m³/s corresponding to the plasma temperature of 10,000 K in case of an arc current of 150 A.     

Numerical and experimental studies on DC plasma spray torch

One of the challenging problems in the plasma spray technique is reproducibility of the coating quality. This problem is mainly associated with arc fluctuations, which affect the plasma jet temperature and velocity, inside the plasma torch. In this study, 3D numerical models are developed to study the arc behavior inside the torch and effect of arc fluctuations on plasma jet temperature and velocity. Plasma arc is simulated for different operating parameters. Different arc sizes are predicted by using thermo-dynamical principle of minimum entropy production for given torch power. The influence of arc current and gas flow rate on the Ar-N₂ plasma arc characteristics, plasma jet and torch efficiency is presented. Predicted torch efficiencies and arc voltages are comparable with measurements. At the nozzle exit, velocity shows stronger three-dimensional effect than temperature. Plasma jets are simulated using different nozzle exit profiles obtained from the plasma arc model and their temperature and velocity distributions are clarified.     

等离子体喷枪中的射流脉动控制

电弧等离子体喷枪中的射流脉动是影响等离子体喷涂质量的一个非常重要的因素,射流脉动很大程度上是由于等离子体喷枪内电弧分流引起的.分析了电弧分流的机理及其对射流脉动的影响.在此基础上,设计了一种直流等离子体喷枪,采用在阳极和阴极中间设置隔离段的方法来限制电弧的大尺度分流.为了研究电弧分流对射流脉动的影响以及检验新喷枪对射流脉动的抑制作用,而建立了专门的试验装置并进行了相关的试验.试验结果表明,电弧分流和射流脉动的频谱特性是一致的,在使用新等离子体喷枪时,电弧分流明显得到了限制.     

Modeling of the supersonic argon plasma jet at low gas pressure environment

Numerical simulation results are presented concerning the heat transfer and fluid flow within the supersonic argon plasma jet encountered in low pressure (or soft vacuum) plasma spraying (LPPS). The plasma parameters at the inlet section of the plasma jet are taken from our modeling results of the subsonic-to-supersonic d.c. arc plasma torch. The mach number, temperature and static pressure at the center of the plasma jet on the torch exit section are 2.8, 13 200 K and 6000 Pa, respectively, whereas the environment (i.e. vacuum chamber) pressure is 0.1 atm. Those parameters are typical for LPPS. The plasma jet is assumed to be axi-symmetrical and in local thermodynamic equilibrium state. The All-Speed SIMPLE algorithm is coupled with the FAST-2D program to simulate the whole plasma jet containing both the supersonic and subsonic flow regions. Modeling results clearly show that there exist several successive temperature, velocity and static wave crests and troughs. The fluctuation magnitudes of those parameters reduce rapidly in the flow direction, along with the flow transformation from the supersonic flow regime into the subsonic flow regime. The existence of a series of compression and expansion waves in the region near the torch nozzle exit shows clearly the over-expanded characteristics of the supersonic plasma flow.     

Numerical analysis and experiments on transferred plasma torches for finding appropriate operating conditions and electrode configuration for a waste melting process

Thermal plasma characteristics of transferred plasma torches are numerically and experimentally investigated under atmospheric conditions to find the effects of operating variables and electrode arrangements on them. A control volume method and a modified SIMPLER algorithm are used for numerical analysis, and the temperature distributions of argon plasma are calculated in different torch operating conditions of a typical transferred arc torch. Transferred plasma torches are designed and fabricated, which have six different electrode arrangements, respectively, consisting of a conical rod cathode and a nozzle in the torch, and a distant anode material. The dynamic behaviors of arc voltage are measured to obtain stable arc conditions, and a similarity criterion is determined to analyze static behaviors of arc voltage. For predicting the heat transfer rate to melted material from arc column, measurements are made for the heat loss at the anode material and fractions of input power transferred to the anode. Furthermore, thermal plasma temperatures are measured by the optical emission spectroscopy of an Ar I line. As a result of the present work, an appropriate electrode configuration and operating conditions for waste melting process are presented for the optimization of transferred plasma torches.     

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.     

Electron number density measurement on a DC argon plasma jet by stark broadening of Ar I spectral line

In thermal plasma processing, input power and gas flow rate play a major role in controlling the plasma jet temperature, velocity and density. Emission spectroscopy study is an important method for plasma diagnostics. A DC atmospheric plasma spray torch was operated at different power levels and flow rates of plasma gas (argon). Electron number density of the plasma jet, the corresponding temperature and the degree of ionization were determined using stark broadening of the Ar I (430.010 nm) line, the atomic Boltzmann plot method and the Saha equation, respectively. In the present work, we have investigated the effect of input power, axial position of the plasma jet and gas flow rate on the electron number density in the plasma jet. While an increase in input power considerably increased the electron number density, gas flow rate did not show any significant effect on the same.     

Stability conditions of argon and helium gas mixtures in an atmospheric pressure plasma jet

Non-equilibrium plasmas can be generated by atmospheric pressure glow discharges, amongst others by atmospheric pressure plasma jets (APPJ), which feature a capacitive radio-frequency discharge between bare metallic electrodes.We investigated the stability conditions for discharges in an APPJ operated with helium-argon mixtures. Uniform α-discharges can be sustained in mixtures ranging from pure helium to pure argon. The ignition voltage increases drastically with argon content. There is also an upper limit for the existence of the α-mode, where α-sheath breakdown occurs. Critical plasma parameters for the α-mode were determined by equivalent circuit models and discussed in respect to dependences on the different models. A critical electron density of 2.4×10¹¹ cm⁻³ was obtained for pure helium. It increases steadily with argon content and reaches a value of 1.2×10¹² cm⁻³ for pure argon. Sheath thicknesses for α-sheath breakdown were calculated in the range of 0.17-0.29 mm for helium-argon mixtures.     

Numerical simulation of argon twin torch plasma arc for high heating efficiency

Plasma arc heating technology has been applied for volume reduction through melting of bottom and fly ash, and for producing slag. Recently, a twin torch plasma arc, which has two torches at the cathode and anode, has been anticipated for application to disposal of medical waste because it can treat a wide area and can treat non-conductive materials. For this study, a numerical simulation model of a twin torch plasma arc at opposite electrodes was developed to elucidate high-efficiency heating using a twin torch plasma arc. It is defined as Local Thermal Equilibrium (LTE) and calculated in Magneto-Hydro-Dynamic (MHD) equations. Furthermore, the temperature distribution and conditions of high heating efficiency with the radiation loss were addressed and compared to those of a single torch. The heating efficiency decreases with increasing radiation efficiency because of the temperature increment caused by the current and input power. The radiation efficiency of a twin torch is about 5% higher than that of single torch.     

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.     

Anode jet generation conditions affected by the radial current density distribution for maintaining current continuity

To perform high-quality welding, the heat input from the arc to the base metal is controlled. However, an anode jet sometimes occurs. In such cases, control is difficult and weld defects occur. This study elucidates how anode jet generation is affected by the radial current density distribution for maintaining the current continuity. The anode jet starts to occur at 15–20 mm of interelectrode distance. The arc temperature decreases because the arc radius increases. Then electrical conductivity decreases sharply depending on the temperature. If electrical conductivity decreases, then the current density decreases, and the arc cannot maintain the current continuity. When the mean current density is 8.7 × 10⁵ A/m², the flow velocity is zero at 15 mm. Therefore, this minimum value of the mean current density is the anode jet generation condition.     

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.     

Study of titanium nitride deposition by supersonic plasma spraying

In this study, titanium nitride (TiN) deposition by reactive spraying was carried out under a low-pressure environment using a DC arc plasma jet generator with a supersonic expansion nozzle. Titanium powders were injected using a hollow cathode with argon gas, and nitrogen or nitrogen and hydrogen mixture was used as the plasma gas. Microstructure and properties of the coatings were examined using scanning electron microscope and X-ray diffraction. A dense TiN coating with a Vickers hardness of 2000 Hv was formed at a substrate temperature of 700 °C with a low input power of 5.3 kW. The results showed that the supersonic plasma jet in thermodynamic and chemical nonequilibrium state exhibits high potentials for reactive spraying.     

Measurement of total energy flux density at a substrate during TiOx thin film deposition by using a plasma jet system

The total energy flux density delivered to an electrically isolated substrate in a low-pressure pulsed DC hollow cathode plasma jet sputtering system during TiO₂ thin film deposition has been quantified. The plasma source was operated in constant average current mode and in a mixture of argon and oxygen or only in pure argon working gas. A titanium nozzle served as the hollow cathode. The total energy flux density measurements were made using a planar calorimeter probe. The main results from the calorimeter probe showed clearly that the total energy flux density at the electrically isolated substrate decreases significantly with duty cycle from 100% (DC mode) to 10% at a given pulsing frequency 2.5 kHz. A local maximum at duty cycle 60% for only pure argon operation has been observed. In addition, the voltage waveforms on the hollow cathode and before the ballast resistor have been saved for pulsed DC measurements for both pure argon and argon + oxygen mixture. A similar transient phenomenon on the cathode voltage and discharge current as observed recently in mid-frequency pulsed DC magnetron discharge has been discovered in the hollow cathode plasma jet sputtering system. We can conclude from these preliminary measurements that the main asset of the pulsed DC hollow cathode plasma jet discharge as distinct from the DC driving of the same plasma system lies in the possibility to reduce or to increase energy influx on the floating substrate within the change of duty cycle.     

Atmospheric pressure plasma jet operated at narrow gap spacings

The atmospheric pressure plasma jet (APPJ) makes use of a dielectric-barrier-free radio-frequency (RF) glow discharge for the production of a non-equilibrium plasma. Usually the APPJ is operated in the alpha mode at gap spacings in the mm range, where the alpha sheath thickness is in the order of 200-300 μm. Narrow gap spacings are experimentally not yet investigated, but it is expected that the bulk region of the alpha discharge should disappear and the discharge should exhibit a sheath-only structure.In order to provide experimental evidence for such situations, APPJs with gap spacings down to 0.1 mm are investigated. The electric properties of the APPJ are studied by measuring the current and voltage characteristics. Time-averaged images of the front view of the discharge are taken with a digital camera. By using an image-intensified gateable video camera the time development of the discharge is studied with nanosecond resolution.It was possible to sustain alpha discharges at gap spacings down to 100 μm, whereby the voltage needed decreases down to an rms voltage of 70 V. A weak indication for a laterally oscillating sheath in the 100 μm gap was found.     

Excitation temperature and electron number density behavior of atmospheric pressure D.C. argon plasma jet during spheroidization of nickel

The excitation temperature and electron number density of the atmospheric pressure D.C. argon plasma jet during spheroidization of nickel have been measured at 5 mm from the nozzle exit by optical emission spectroscopic technique. Emission spectra of the argon plasma, argon plasma with carrier gas and carrier gas with nickel powder were recorded in 400-450 nm wavelength interval. The effect of carrier gas and powder loading on the excitation temperature and electron number density of the plasma jet were determined using atomic Boltzmann plot method and stark broadening of the Ar I (430.010 nm) line respectively. The experiment was done at 6.5, 7.9, 11.4 and 12.1 kW input power levels. Argon was used as plasma gas and also carrier gas. Nickel powders in the size ranging from 40 to 100 μm were processed. On introduction of carrier gas and nickel powder loading, the excitation temperature and electron density of the jet were found to decrease. From the results, the degree of ionization of the plasma jet was calculated by using the measured excitation temperature and electron density values. The spheroidzed nickel powder was characterized by SEM, optical photographs and XRD.     

Synthesis of carbon coatings employing a plasma torch from an argon-acetylene gas mixture at reduced pressure

Amorphous hydrogenated carbon films (a-C:H) were formed on Si (1 1 1) wafers from an argon-acetylene gas mixture at a reduced pressure of 1000 Pa using a direct current (DC) plasma torch discharge. The Ar/C₂H₂ gas volume ratio varied from 1:1 to 8:1, the distance between plasma torch exit and the samples 0.04-0.095 m. The DC plasma torch technique allows the production of thick (∼90 μm) coatings at 0.3 μm/s growth rates. Raman spectra shape, D and G peak positions and the intensity ratio (I_D/I_G) show an increase of sp³ bond fraction with decreasing acetylene flow in argon plasma. Reflectance of the coatings deposited at Ar/C₂H₂=8:1 is high (∼97%) and slightly increases with increasing distance between samples and plasma torch exit.     

Phenomenological representation of mechanical spectroscopy of high damping MnCuNiFe alloy

ABSTRACT

MnCuNiFe damping alloy was prepared to characterise the dynamic mechanical behaviour under varied frequency by employing dynamic mechanical analyzer, optical microscopy and X-ray diffraction. The relationship of characteristic temperatures is disentangled by antiferromagnetic transition (AFT), the strain glass transition and reverse martensitic transformation. It is reported that although only f.c.c. (γ) phase exists in the alloy under solution state, twins are induced by AFT. The aged alloy shows an elevated damping capacity while the maximum internal friction decreases from 0.1 to 10?Hz and then increases to 150?Hz, demonstrating the system resonant frequency of about 10?Hz under the vibration mode of double cantilever beam.     

Transferred arc plasma processing of mullite-zirconia composite from natural bauxite and zircon sand

Low cost mullite-zirconia composites were prepared from the mixtures of natural bauxite and zircon sand by using transferred arc plasma processing. In this paper, a mixture of natural bauxite and zircon in the ratio of 7:3 by weight (based on composition of 3:2 mullite) was ball milled for 4 h and melted in the transferred arc plasma for 2 and 4 min. Argon was used as plasma forming gas. The torch was operated at 5 kW input power. The phase and microstructure formation of melted samples were investigated by XRD and SEM images. The results show that the processing time is a key factor to get a single phase mullite-zirconia composites with required microstructure.     

Visualization of the process of metal heating and melting in the anode zone in an arc discharge with a tungsten electrode

Pictures of an electric arc burning in argon, obtained by means of a digital camera, within the different domains of wavelengths of the visible spectrum are presented. Maps of the thermal fields of the heating spot are plotted, and the plasma temperature in the anode arc zone is calculated. The application potential of the digital image technology in the visible wavelength domain for analysis of the processes directly in the anode arc zone and for estimation of the arc column parameters is shown.