Reactions of hydrocarbons in a supersonic vacuum plasma jet

The plasma plume of a hydrogen plasma jet used for diamond synthesis is analyzed by a Pitot tube and by mass spectrometry. In the investigated pressure range of 2–10 mbar, supersonic gas velocities with Mach numbers of up to 2 were observed, which decreased with increasing pressure and increasing distance from the nozzle. The injection of the carbon-containing species either at the exit of the jet nozzle or simply into the background gas of the reaction chamber confirmed the importance of recirculation of background gas into the plasma plume. In the case of background injection the rise of the total carbon content in the plume with increasing distance from the nozzle is much slower than in the case of nozzle injection. The results of a numerical model of the hydrocarbon gas-phase reactions in the jet are presented. The model considers the entrainment of background gas into the plasma plume. Two domains along the jet axis can be distinguished. The first one in the vicinity of the nozzle is dominated by methyl radicals, the second one by atomic carbon. Increase of the hydrogen dissociation level results in the broadening of the atomic carbon domain and the rise of C₂ far from the nozzle. Background injection of CH₄ leads to lower total carbon content in the plume but has little effect on the species distribution along the jet axis.     

Particle Injection in Direct Current Air Plasma Spray: Salient Observations and Optimization Strategies

External injection of high-melting point low thermal conductivity ceramics orthogonal to a typical direct current thermal plasma jet plays a vital role in determining the in-flight state of the particles and the process downstream. The interactions between low density ceramic particles and high temperature plasma jet is quite complex, which influences the spray process and associated deposition. Detailed in-flight particle diagnostics as well as spray stream visualization have significantly enhanced our capability to diagnose and control the process. In this paper we present some salient observations on the role of key variables on particle injection. A number of experiments were conducted using a 7MB torch (Sulzer Metco, Westbury, NY) with both Ar–H₂ and N₂–H₂ plasma gases, where the carrier gas flow to inject Yttria Stabilized Zirconia (YSZ) was varied systematically and the resulting in-flight particle state was captured using an array of particle and spray stream sensors arranged in a 3D set-up. A notable observation is the existence of a “sweet-spot” in the plasma jet where the particle temperatures and velocities achieved a maximum. This sweet-spot can be characterized by the plume position (location of centroid of the spray stream) rather than carrier gas flow rate and is independent of primary gas flows and other process/material conditions. This result suggests a possible approach to optimize particle injection independent of plasma-forming-torch-parameters. Controlling particle injection at this sweet-spot has shown to benefit the overall process efficiency (in terms of melting) and process reliability (both in-flight measurement and coating build-up) with concomitant application benefits.     

The Effects of Added Hydrogen on a Helium Atmospheric-Pressure Plasma Jet Ambient Desorption/Ionization Source

We present mass spectrometric data demonstrating the effect that hydrogen has on a helium-based dielectric-barrier discharge (DBD) atmospheric-pressure plasma jet used as an ambient desorption/ionization (ADI) source. The addition of 0.9 % hydrogen to the helium support gas in a 35-W plasma jet increased signals for a range of test analytes, with enhancement factors of up to 68, without proportional increases in background levels. The changes in signal levels result from a combination of changes in the desorption kinetics from the surface and increased ion production in the gas phase. The enhancement in ADI-MS performance despite the quenching of key plasma species reported in earlier studies suggests that ionization with a H₂/He plasma jet is the result of an alternate mechanism involving the direct generation of ionized hydrogen.      

Direct Measurement of the Gas Entrainment Into a Turbulent Thermal Plasma Jet

An experimental study is conducted to investigate the entrainment characteristics of a turbulent thermal plasma jet issuing from a DC arc plasma torch operating at atmospheric pressure. The mass flow rate of the ambient gas entrained into the turbulent plasma jet is directly measured by use of the so-called “porous-wall chamber” technique. It is shown that a large amount of ambient gas is entrained into the turbulent plasma jet. With the increase of the gas mass flow rate at the plasma jet inlet or the plasma torch exit, the mass flow rate of entrained ambient gas almost linearly increases but its ratio to the jet-inlet mass flow rate decreases. The mass flow rate of the entrained gas increases with the increase of the arc current or jet length. It is also found that using different ways to inject the plasma-forming gas into the plasma torch affects the entrainment rate of the turbulent plasma jet. The entrainment rate expression established previously by Ricou and Spalding (J. Fluid Mech. 11: 21, 1961) for the turbulent isothermal jets has been used to correlate the experimental data of the entrainment rates of the turbulent thermal plasma jet, and the entrainment coefficient in the entrainment rate expression is found to be in range from 0.40 to 0.47 for the turbulent thermal plasma jet under study.     

Experimental Testing of a Curvilinear Gas Shroud Nozzle for Improved Plasma Spraying

The performance of a new gas shroud nozzle attachment for plasma spraying was tested experimentally using particle diagnostics, flow visualization, and coating characterization techniques. A nozzle attachment with curvilinear inner walls was tested and compared with a commercially available conical nozzle. Particle temperatures were measured with a high-speed ratio pyrometer and particle velocities were measured with an intensified camera and a two-laser illumination system. Flow visualization was performed by seeding the surrounding air with smoke. Particle temperatures at the spraying distance were 300 K higher with the curvilinear insert. The plasma jet was narrower but the particle velocity distribution at the spraying distance was unchanged. Higher temperatures and improved particle melting with the curvilinear insert resulted in a reduction in coating porosity (from 7.0 to 7.2 to 4.5–5.1%) and an increase in coating adhesion strength (from 27.2 to 42 MPa). Shrouding as injected through a circular slot around the nozzle exit was also seen to provide better protection than gas injected with the standard sixteen-port configuration.     

Effects of the Length of a Cylindrical Solid Shield on the Entrainment of Ambient Air into Turbulent and Laminar Impinging Argon Plasma Jets

When materials processing is conducted in air surroundings by use of an impinging plasma jet, the ambient air will be entrained into the materials processing region, resulting in unfavorable oxidation of the feedstock metal particles injected into the plasma jet and of metallic substrate material. Using a cylindrical solid shield may avoid the air entrainment if the shield length is suitably selected and this approach has the merit that expensive vacuum chamber and its pumping system are not needed. Modeling study is thus conducted to reveal how the length of the cylindrical solid shield affects the ambient air entrainment when materials processing (spraying, remelting, hardening, etc.) is conducted by use of a turbulent or laminar argon plasma jet impinging normally upon a flat substrate in atmospheric air. It is shown that the mass flow rate of the ambient air entrained into the impinging plasma jet cannot be appreciably reduced unless the cylindrical shield is long enough. In order to completely avoid the air entrainment, the gap between the downstream-end section of the cylindrical solid shield and the substrate surface must be carefully selected, and the suitable size of the gap for the turbulent plasma jet is appreciably larger than that for the laminar one. The overheating of the solid shield or the substrate could become a problem for the turbulent case, and thus additional cooling measure may be needed when the entrainment of ambient air into the turbulent impinging plasma jet is to be completely avoided.     

Modeling Study on the Entrainment of Ambient Air into Subsonic Laminar and Turbulent Argon Plasma Jets

Modeling study is performed to reveal the special features of the entrainment of ambient air into subsonic laminar and turbulent argon plasma jets. Two different types of jet flows are considered, i.e., the argon plasma jet is impinging normally upon a flat substrate located in atmospheric air surroundings or is freely issuing into the ambient air. It is found that the existence of the substrate not only changes the plasma temperature, velocity and species concentration distributions in the near-substrate region, but also significantly enhances the mass flow rate of the ambient air entrained into the jet due to the additional contribution to the gas entrainment of the wall jet formed along the substrate surface. The fraction of the additional entrainment of the wall jet in the total entrained-air flow rate is especially high for the laminar impinging plasma jet and for the case with shorter substrate standoff distances. Similarly to the case of cold-gas free jets, the maximum mass flow-rate of ambient gas entrained into the turbulent impinging or free plasma jet is approximately directly proportional to the mass flow rate at the jet inlet. The maximum mass flow-rate of ambient gas entrained into the laminar impinging plasma jet slightly increases with increasing jet-inlet velocity but decreases with increasing jet-inlet temperature.     

Monitoring Hydrogen Plasma Reduction of Oxides by Na D Lines

Out of many applications of hydrogen plasma, reduction of metal oxides is an important one. The reduction can be carried out using carbon or hydrogen. While carrying out the reduction of hematite (Fe₂O₃) in hydrogen plasma, an attempt was made to characterize the hydrogen plasma by optical emission spectroscopy. The spectroscopic results provide some new and useful information. In addition to the hydrogen emission lines, two prominent lines at 589 and 589.6 nm were observed. These two lines are confirmed to be sodium D1 and D2 (Na D lines) by comparing with a low pressure sodium vapour lamp (LPSVL). The source of the trace amount of sodium is also confirmed to be from the metal oxide sample as an impurity. These lines are found to be very sensitive to various process parameters such as gas flow rate, microwave power, and reduction chamber pressure. The temporal variation of these two Na D lines also shows a characteristic trend during metal oxide reduction in hydrogen plasma. The weight loss and the X-ray diffraction analyses of reduced Fe₂O₃ sample for different time duration provides the evidence of correlation with Na D lines’ intensity trend. This trend can be used to monitor the state and completion of hydrogen plasma based reduction reaction. In processes where Na is not associated with metal oxide, trace amount of Na in its molecular form such as NaOH can be introduced for monitoring the plasma process parameters as well as the plasma based reduction process.     

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.     

Excitation Temperature and Constituent Concentration Profiles of the Plasma Jet Under Plasma Spray-PVD Conditions

Plasma spray-physical vapor deposition (PS-PVD) is a promising technology to produce columnar structured thermal barrier coatings with excellent cyclic lifetime. The characteristics of plasma jets generated by standard plasma gases in the PS-PVD process, argon and helium, have been studied by optical emission spectroscopy. Abel inversion was introduced to reconstruct the spatial characteristics. In the central area of the plasma jet, the ionization of argon was found to be one of the reasons for low emission of atomic argon. Another reason could be the demixing so that helium prevails around the central axis of the plasma jet. The excitation temperature of argon was calculated by the Boltzmann plot method. Its values decreased from the center to the edge of the plasma jet. Applying the same method, a spurious high excitation temperature of helium was obtained, which could be caused by the strong deviation from local thermal equilibrium of helium. The addition of hydrogen into plasma gases leads to a lower excitation temperature, however a higher substrate temperature due to the high thermal conductivity induced by the dissociation of hydrogen. A loading effect is exerted by the feedstock powder on the plasma jet, which was found to reduce the average excitation temperature considerably by more than 700 K in the Ar/He jet.     

Plasma conductivity as a probe for ambient air admixture in an atmospheric pressure plasma jet

By utilizing a fully floating double electrical probe system, the conductivity of a linear atmospheric pressure plasma jet, utilizing nitrogen as process gas, was measured. The floating probe makes it possible to measure currents in the nanoamp range, in an environment where capacitive coupling of the probes to the powered electrodes is on the order of several kilovolts. Using a chemical kinetic model, the production of reactive nitrogen oxide and hydrogen-containing species through admixture of ambient humid air is determined and compared to the measured gas conductivity. The chemical kinetic model predicts an enhanced diffusion coefficient for admixture of O₂ and H₂O from ambient air of 2.7 cm² s^?1, compared to a literature value of 0.21 cm² s^?1, which is attributed to rapid mixing between the plasma jets and the surrounding air. The dominant charge carriers contributing to the conductivity, aside from electrons, are NO⁺, NO₂ ^? and NO₃ ^?. Upon admixture of O₂ and H₂O, the dominant neutral products formed in the N₂ plasma jet are O, NO and N₂O, while O₂(¹Δ_g) singlet oxygen is the only dominant excited species.     

On the Interaction of Cold Atmospheric Pressure Plasma with Surfaces of Bio-molecules and Model Polymers

We review studies of surface-interaction mechanisms for a surface microdischarge (SMD) and an atmospheric pressure plasma jet (APPJ) with model polymers and biomolecules in our laboratory. We discuss the influence of plasma source type, operating parameters, and gaseous environments on surface modifications and biological deactivation. We focus on mild, remote conditions where the visible plasma plume does not contact the surface. For an APPJ fed with Ar, the interaction of the plasma plume with O₂ and/or N₂ gaseous environments leads to oxidation and surface-bound NO_x even on materials containing neither oxygen nor nitrogen. The APPJ also modifies photo-sensitive polymers. Using optical filters, these modifications were shown to result in part from irradiation with vacuum ultraviolet (VUV) photons in a spectral range corresponding to Ar excimer emission. No VUV-induced effects were seen for the SMD source operated with O₂/N₂. SMD treatments using O₂/N₂ mixtures result in surface oxidation and nitridation. A new surface-bound species, NO₃, has been measured on the polymers and biomolecules. Depending on the gas chemistry and film molecular structure, the NO₃ surface concentration can reach 10 %. Both surface NO₃ on plasma-treated films of lipopolysaccharide (LPS), an immune stimulating biomolecule found in bacteria such as E. c oli, and overall surface oxidation correlate with LPS biological deactivation as evaluated using an enzyme-linked immunosorbent assay. Ambient humidity was studied using the SMD and was found to decrease overall surface modifications including NO₃ and biodeactivation for O₂-rich conditions. Lastly, we discuss possible mechanisms and compare our results with published simulation studies.     

Hydrogen Production from Methane Through Pulsed DC Plasma

A non-equilibrium warm plasma reactor has been constructed for methane reforming and hydrogen production. The discharge reactor was derived with 20 kV pulsed DC power supply with pulse duration of 4 µs, pulse frequency of 33 kHz. Electrical and optical characterizations of the reactor have been investigated. The electrical characteristics of the discharge revealed that the discharge was ignited by streamer to glow transition. The optical characteristics of the discharge revealed that the discharge was found to be strongly non-equilibrium with rotational temperature (T_rot) of 2873 K and vibrational temperature (T_vib) of 12,130 K. The Stark broadening of the emitted H_α line profile was used to deduce the electron density, which was found to be in the order of 10¹⁶ cm^?3. Methane conversion was strongly dependent upon the applied voltage and the methane flow rate. In general, under the specified operating condition, a methane conversion percentage of about 92% and a maximum hydrogen selectivity of 44.6% have been achieved. Specific energy consumption of methane conversion (SEC) and specific energy requirements for hydrogen formation (SER) of 5 eV/molecule has been achieved simultaneously with a maximum hydrogen production energy cost of about 3.8 µg/J. Finally, the decomposition of methane gas resulted in the deposition of an important byproduct namely graphene oxide.     

Numerical Modeling of Thin Film Deposition in Expanding Thermal Plasma

Expanding thermal plasma (ETP) is a widely used technique for deposition of a thin layer of ceramic materials and metal oxide on a substrate for a wide range of applications including abrasion resistance, UV absorption, as well as conductive and optical coatings. The coating quality is found to be dependent on operating parameters as well as reactor designs. In this article, we have presented a CFD based model of the ETP process to simulate the deposition of silica-like coatings on a polycarbonate substrate. Along with the flow-thermal model of plasma jet expansion process, the study also reports the development of a simplified gas phase and surface reaction model to simulate the coating phenomena. The model has been used further to study the effect of various operating conditions on the coating thickness, viz. reactor pressure, reagent flow rate, distance of the substrate from the arc and substrate alignment.     

Electric Arc Fluctuations in DC Plasma Spray Torch

Direct current plasma torches for plasma spraying applications generate electric arc instabilities. The resulting fluctuations of input electrical power hamper a proper control of heat and momentum transfers to materials for coating deposition. This paper gives an overview of major issues about arc instabilities in conventional DC plasma torches. Evidences of arc fluctuations and their consequences on plasma properties and on material treatments are illustrated. Driving forces applied to the arc creating its motion are described and emphasis is put on the restrike mode that depends on the arc reattachment and the boundary layer properties around the arc column. Besides the arc root shown as a key region of instability, the Helmholtz oscillation is also described and accounts for the whole plasma torch domain that can generate pressure fluctuations coupled with voltage ones.     

Syngas Production from Propane Using Atmospheric Non-thermal Plasma

Propane steam reforming using a sliding discharge reactor was investigated under atmospheric pressure and low temperature (420 K). Non-thermal plasma steam reforming proceeded efficiently and hydrogen was formed as a main product (H₂ concentration up to 50%). By-products (C₂-hydrocarbons, methane, carbon dioxide) were measured with concentrations lower than 6%. The mean electrical power injected in the discharge is less than 2 kW. The process efficiency is described in terms of propane conversion rate, steam reforming and cracking selectivity, as well as by-products production. Chemical processes modelling based on classical thermodynamic equilibrium reactor is also proposed. Calculated data fit quiet well experimental results and indicate that the improvement of C₃H₈ conversion and then H₂ production can be achieved by increasing the gas fraction through the discharge. By improving the reactor design, the non-thermal plasma has a potential for being an effective way for supplying hydrogen or synthesis gas.     

Hydrogen Production from Ethanol Decomposition by Two Microwave Atmospheric Pressure Plasma Sources: Surfatron and TIAGO Torch

Molecular hydrogen production from ethanol decomposition by two microwave atmospheric pressure plasma sources (surfatron and Torche a Injection Axiale sur Guide d’Onde (TIAGO) torch) was studied by optical emission spectroscopy and mass spectrometry. In both cases ethanol was almost completely decomposed, thus giving place to molecular hydrogen. However, the atmosphere surrounding the discharge significantly influences the overall decomposition process. When the surfatron is used, C₂H₂ and CO are obtained as exhaust gases. Likewise, H₂O and HCN are also detected at plasma exit when sustained with the TIAGO torch.     

Comparison between Two Turbulence Models and Analysis of the Effect of the Substrate Movement on the Flow Field of a Plasma Jet

In the first part of the present study, an appropriate inflow turbulent boundary condition is chosen. Then, a comparison is made between two turbulence models for a plasma jet discharged into air atmosphere. The plasma jet gas phase flow is predicted with the standard k–ɛ model and the RNG model of turbulence. Particles behavior is modeled using stochastic particles trajectories. A validation of the plasma jet model is made by comparison with experimental data. This part of the study shows that the flow features are better predicted with the RNG model. The choice of appropriate boundary conditions seems to be crucial for a better simulation of plasma thermal spraying. Afterwards, computations are performed for projection of Ni particles. It is found that the computed particles velocities and temperatures are also better predicted with the RNG model compared with the k–ɛ model. The second part of this study is concerned with the effect of the substrate movement on the gas flow field. This is performed in order to simulate a realistic coatings process where a relative movement between the torch and the substrate always exists. Three substrate velocities have been used and it is found that the flow fields are affected only very near the substrate wall.     

Characterization of the Converging Jet Region in a Triple Torch Plasma Reactor

Enthalpy probe measurements were taken of the converging plasma plume in a triple torch plasma reactor and related to substrate heat flux measurements. Results show excellent entrainment of process gases injected into the converging plasma plume by way of the central injection probe. At lower pressures (40 kPa), the plasma volume is equivalent to at least a 3 cm diameter, 4 cm long cylinder, with relatively uniform temperature, velocity, and substrate heat flux profiles when compared to a typical dc arc jet. Converging plasma plume size, substrate heat flux, and enthalpy profiles are also shown to be a strong function of applied system power. Substrate heat flux measurements show smaller radial gradients than enthalpy probe measurements, because of the high radial velocity component of gases above the substrate boundary layer. Enthalpy probe measurements were also conducted for diamond deposition conditions and approximate temperature and velocity profiles obtained. Problems with the uniform gas mixture assumption prohibited more accurate measurements. Reproducibility of enthalpy measurement results was shown with an average standard deviation of 11.8% for the velocity and 7.6% for the temperature measurements.