Modeling the first instant of the interaction between a liquid and a plasma jet with a compressible approach

In this work, the numerical simulation of the interaction between a liquid jet and a plasma flow is investigated for understanding and predicting the physical parameters involved in the liquid plasma spraying process. This process is used for the production of high performance coatings to obtain thinner deposits (< 100 μm) than in conventional plasma spraying. The up-to-date goal is to describe the dispersion of liquid in order to understand the effect of injection conditions on the surface coating quality. This work proposes an original model for dealing with three-dimensional interactions between a plasma flow and a liquid phase with Volume Of Fluid (VOF) methods. A compressible model, capable of managing incompressible two-phase flows as well as compressible motions, is used. First comparisons with experimental photography show rather good agreement during the first moments of the injection.     

Contribution to the modeling of the interaction between a plasma flow and a liquid jet

A numerical simulation of the interaction between a plasma flow and a liquid jet was investigated, leading to the proposal of a compressible model, based on augmented Lagrangian, Large Eddy Simulation (LES) turbulence modeling and Volume of Fluid (VOF) approaches, capable of managing incompressible two-phase flows as well as turbulent compressible motions. The VOF method utilized volume markers to advect the local concentration of gas and liquid in a Eulerian manner. The numerical model was validated on single-phase plasma configurations as well as two-phase cross flow liquid jet interactions. Finally, an example of the first simulations of the interactions between a liquid jet and a plasma is presented. However improvements should be realized as to increase the speed of the VOF-SM algorithm or add specific subgrid models related to jet fragmentation and phase change.     

Numerical study of the relative importance of turbulence, particle size and density, and injection parameters on particle behavior during thermal plasma spraying

Numerical modeling is used to systematically examine the effects of turbulence, injection, and particle characteristics on particle behavior during thermal plasma spraying. Using the computer program LAVA (Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID), a steady-state plasma jet typical of a commercial torch at normal operating conditions is first developed. Then, assuming a single particle composition (ZrO₂) and injection location, real world complexity (e.g., turbulent dispersion, particle size and density, injection velocity, and direction) is introduced “one phenomenon at a time” to distinguish and characterize its effect and enable comparisons of separate effects. A final calculation then considers all phenomena simultaneously, to enable further comparisons. Investigating each phenomenon separately provides valuable insight into particle behavior. For the typical plasma jet and injection conditions considered, particle dispersion in the injection direction is most significantly affected by (in order of decreasing importance): particle size distribution, injection velocity distribution, turbulence, and injection direction distribution or particle density distribution. Only the distribution of injection directions and turbulence affect dispersion normal to the injection direction and are of similar magnitude in this study. With regards to particle velocity and temperature, particle size is clearly the dominant effect.     

Zirconium vacuum arc operation in a mixture of Ar and O2 gases: Ar effect on the arcing characteristics,deposition rate and coating properties

The effect of oxygen and argon partial pressures (P_O2, P_Ar) in a Zr vacuum arc on plasma ion current density J_p, arc voltage V_arc, deposition rate v_d, and selected coating properties was determined. A d.c. arc current of I_arc = 100 A was initiated between a Zr cathode and a grounded anode. Cathode spots produced a plasma jet, which entered a 1/8 torus macroparticle (MP) filter. The plasma was guided by a d.c. magnetic field through an aperture to a glass substrate or a flat disk probe, mounted on a rotatable holder. J_p was measured with the probe, negatively biased to V_b = − 60 V. Coating thickness was measured using a profilometer, and coating properties were investigated using optical microscopy, energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), nano-indentation and optical analysis.     

From DC Time-Dependent Thermal Plasma Generation to Suspension Plasma-Spraying Interactions

This paper proposes an original route for modeling the time-dependent behavior of a plasma jet issued from a DC plasma-spraying torch operating with various kinds of gas mixtures. The hydrodynamic interactions between this jet and a liquid jet for suspension plasma-spraying or a classical particle injection for the deposition of coatings are studied. In a first step, the classical plasma spraying process was explored using the FLUENT CFD code. Zirconia particles, defined as Lagrangian particles, were injected in an Ar/H₂ flow and their positions, kinetic and thermal states were compared with experimental results. The trend and intensity of the values demonstrated a rather good agreement. In a second step, the suspension plasma spraying was investigated with the AQUILON CFD to simulate interactions between the plasma and aqueous jets. An Ar/H₂ plasma flow was simulated with the Large Eddy Scale turbulence model assumption, in which a liquid jet had been introduced. The behavior observed during the first stage of the interactions between the two fluids corresponded to expectations.     

Influence of gas flow on argon microwave plasma jet at atmospheric pressure

Many kinds of an atmospheric-pressure plasma jet have been developed and used for widespread applications such as a surface treatment and modified. This study focused on the argon atmospheric-pressure microplasma jet generated by discharging of RF power of 2.45 GHz microwave. The plasma jet shows sensitivity to surrounding environment: pressure, temperature and gaseous species. It is therefore absolutely imperative that a nature of atmospheric-pressure plasma jet should be understood from a point of fluid dynamics. This study, therefore, focused on the interrelationship between the plasma jet and the working gas. Motion of the plasma jet and the working gas was evaluated by velocity measurement and fast photography. As a result, the unsteady sinusoidal waving motion in the radial direction of a torch was observed. Advection velocity of the plasma in just downstream region of the torch exit increases with the supplying flow rate, and the velocity ratio is in the range of 0.75-0.87.     

Liquid Precursor Plasma Spraying: Modeling the Interactions Between the Transient Plasma Jet and the Droplets

Plasma spraying using liquid feedstock makes it possible to produce thin coatings (<100 μm) with more refined microstructures than in conventional plasma spraying. However, the low density of the feedstock droplets makes them very sensitive to the instantaneous characteristics of the fluctuating plasma jet at the location where they are injected. In this study, the interactions between the fluctuating plasma jet and droplets are explored by using numerical simulations. The computations are based on a three-dimensional and time-dependent model of the plasma jet that couples the dynamic behaviour of the arc inside the torch and the plasma jet issuing from the plasma torch. The turbulence that develops in the jet flow issuing in air is modeled by a large Eddy simulation model that computes the largest structures of the flow which carry most of the energy and momentum. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Characteristics of atmospheric pressure plasma jet generated by compact and inexpensive high voltage modulator

An atmospheric pressure plasma jet has been successfully generated using a compact high voltage modulator driven by 12 V alkaline batteries. A jet nozzle was composed of a quartz tube with two cylindrical electrodes. The grounded electrode was rolled on the tube and the powered one was inserted in the tube for discharging at lower voltage. V-Q Lissajous analysis of the plasma jet indicated that energy and power consumed for the plasma generation were linear along the distance between the electrodes. Length of the plasma plume from the tip of the tube was 11 mm for the gap length of 5 mm and the input voltage DC of 12 V. At the input voltage, the energy and power consumed for the plasma generation were 4.1 μJ and 0.12 W, respectively. Optical emission spectroscopy analysis of the plasma showed that the plasma contained hydroxyl radicals and exited nitrogen molecules which are chemically active species. The plasma jet can be applied to plasma cleaning for material surface though was generated with the alkaline batteries.     

Fabrication of finely structured silicon-supported SOFC with anode deposited by multi-phase plasma spraying

The mechanically mixed NiO/YSZ powder was usually used as the anode material of atmospheric plasma sprayed (APS) solid oxide fuel cells (SOFC). Big particles and the non-uniform distribution of the pores were observed in the resultant anode layer. To overcome the limitations, a method of fabricating anode layer by multi-phase plasma spraying (MPS) was proposed in this paper. The NiO and YSZ powders were delivered into plasma jet by a separate injection, where nitrogen carrier was employed to feed micrometer-sized NiO powder and liquid carrier was to feed submicrometer-sized YSZ powder. Suspension plasma spraying (SPS) was applied to fabricate dense electrolyte layer. The microstructure and composition of coatings were characterized by SEM and EDS. The results showed that finely structured anode layer with small particle size (d ∼ 2 μm) was achieved by the MPS method. The MPS anode layer was porous with the porosity of 32.1% while the APS anode layer was 22.6%. Three kinds of elements (Ni, Y, Zr) were observed in the MPS anode layer and the NiO content was calculated to be 49.6 wt%. In the SPS process, the suspension flow rate was matched to the plasma gas flow rate to obtain proper injection condition.     

A comparative study on the flexural performance of plasma treated polypropylene fiber reinforced cementitious composites

The flexural performance of cementitious composites incorporating low frequency cold plasma treated polypropylene (PP) fibers has been investigated in this study. Polypropylene fibers were subjected to low frequency plasmas (40 kHz) of argon and oxygen gases at various power levels (60, 120, and 180 W) for different exposure periods (0.5, 1, 2, 5, 8 and 30 min). Possible physical changes on fiber surfaces were observed by scanning electron microscope (SEM) at micrometers scale. Meanwhile, the surface wettability of polypropylene plates was monitored by means of static contact angle measurements. Modified fibers subjected to 60 W–8 min, 120 W–2 min, 180 W–2 min and 180 W–30 min of argon and oxygen plasmas which were selected from the SEM and wettability observations. These modified fibers have been used in the production of cementitious composites. The flexural performance of 40 mm × 40 mm × 160 mm prismatic samples were characterised by means of first cracking strength, flexural strength, and toughness values. Test results showed that appropriate low frequency plasma treatment conditions lead to an improvement in the flexural strength and toughness of polypropylene fiber reinforced cementitious composites. From the SEM images and wettability measurements, it was postulated that main mechanism of performance improvement is the result of physical adhesion increase between polypropylene and matrix. Adhesion improvement is attributed to the relatively closer spacing of fiber and matrix which can be explained by modification of fibers’ surface from hydrophobic to hydrophilic by plasma exposure.     

Surface modification of polymer fibre by the new atmospheric pressure cold plasma jet

A new cold plasma jet has been developed for surface modification of materials at atmospheric pressure. This new cold plasma jet generator is composed of two concentric cylindrical all-metal tube electrodes. The argon is fed into the inner-grounded electrode, the outer electrode is connected to the high-voltage power supply and covered with a layer of dielectric, and then a stable cold plasma jet is formed and blown out into air. The plasma gas temperature is only 25–30 °C. Preliminary results are presented on the modification of polypropylene (PP) and polyethylene terephthalate (PET) fibres by this cold plasma jet. The water contact angle of these materials is found to decrease after plasma treatment and it will recover a little in two months. The chemical changes on the surface of polymers are studied by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM) is used to study the changes in surface feature of polymers due to plasma treatment. The hydrophilicity and surface structure of these materials after plasma treatment are discussed. The results show that such a plasma jet is effective.     

Mechanisms of films and coatings formation from gaseous and liquid precursors with low pressure plasma spray equipment

Plasma spraying is a well developed and widely used technology, successfully applied for ceramic and metal coatings in many fields of applications such as aeronautics, gas turbine, automotive or medical. The coatings obtained are usually intentionally porous and thick (more than 100 μm). Presently, thin films (< µm) are deposited using various physical vapour deposition (PVD) or chemical vapour deposition (CVD) processes with low deposition rates. In this paper, we make use of the high enthalpy and high ionisation degree of the plasma jet of conventional plasma spraying guns operated at low pressure (mbar) to obtain dense coatings by CVD from gaseous and/or liquid precursors. The advantages of such thermal plasma CVD processes are the high deposition rates to obtain dense and thin layers, and the possibility of combining these thin films with thermally sprayed coatings using the same equipment.An efficient injection and mixing of the liquid and gaseous precursors in the plasma jet, which is especially challenging for liquids in low-pressure processes, has been obtained by extensive developments and proper equipment design. Results of several different coatings based on liquid and gaseous precursors are presented. In particular, SiO_x thin films from HMDSO (Hexamethyldisiloxane, C₆H₁₈OSi₂) precursor and oxygen can be deposited over large areas (50 cm diameter) at typical deposition rates of 35 nm/s, with a precursor-to-film conversion efficiency exceeding 50%. For the case of amorphous carbon deposited from CH₄ or C₂H₄, deposition rates exceeding 25 nm/s are obtained. Results from mass spectrometry of the gas sampled in the plasma jet by an enthalpy probe show that the depletion of hydrocarbon precursors can reach 95% and that higher hydrocarbon species are formed by secondary reactions. In the case of carbon-containing precursors, results from mapping of the optical emission intensity throughout the plasma jet volume are presented. The formation and transport of excited precursor-based species, such as CH, C, C₂, and H are addressed. These results show, in particular, that the very high dissociation efficiency of the precursors takes place through (dissociative) charge exchange from Ar⁺ ions and subsequent dissociative recombination with low energy electrons. The peculiarities of plasma chemistry taking place in the low-pressure plasma jet compared to conventional low-density non-equilibrium plasmas are outlined.     

Influence of pore size on penetration of surface modification into woven fabric treated with atmospheric pressure plasma jet

One of the biggest difference between atmospheric pressure plasma jet (APPJ) and other plasma surface treatment is that only one side (top) of substrate is contacted with plasma jet at atmospheric pressure while in other plasmas two sides (top and bottom) of substrate are both contacted with plasma. The modification of the bottom side of woven fabric treated by APPJ is largely dependent on the penetration of active species in plasma jet, which is accordingly affected by plasma parameters and the structure of materials. In order to investigate the influence of pore size on penetration of surface modification into woven fabric treated with APPJ, four kinds of polyester woven fabrics with different pore sizes were used as the model porous medium. Two groups of parallel polyester fibers are respectively and tightly pasted on the top and bottom side of each fabric. Penetration of plasma effects through the pores was detected by changes in contact angle on the bottom side before and after APPJ treatment. It was further demonstrated by changes in surface morphology and chemical composition using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) analyses. The degree of penetration of APPJ surface modification was increased with the increasing pore size. Complete penetration was realized in fabric with pore size larger than 200 μm and nearly no penetration was found in fabric with the pore size smaller than 10 μm. This is attributed to more active species in plasma jet diffusing through the larger pores in fabric. Those species can reach the bottom side without losing their modifying ability during the movement process. Therefore the pore size might be a more important factor affecting penetration of APPJ surface modification into woven fabric.     

A new method for immunosensor preparation: Atmospheric plasma torch

This paper describes a new method for the modification of piezoelectric quartz crystals for immunosensor preparation. The surfaces of the piezoelectric quartz crystals were modified employing Atmospheric Pressure Plasma System using different monomers such as Ethylene diamine (EDA), 1,4-Diaminobutane (DAB) and n-Butylamine (BA). The atmospheric pressure plasma system was adjusted to use the temperature sensitive and volatile monomers. A tunnel from stainless steel was constructed and combined with the plasma torch to introduce the monomers into the plasma jet and preventing the mixing of atmospheric air with the plasma jet. Plasma parameters were; power: 1 kW, pressure: 1 atm, speed of plasma particles: 219 m/s, and exposure time 10 min for EDA and BA monomers, 15 min for DAB monomer. Crystals were characterized by frequency shift measurements, Atomic Force Microscopy (AFM) imaging, contact angle measurements and Infrared (IR) spectra. Frequency shifts for EDA, DAB and BA modified crystals after plasma modification were 56 ± 10, 143 ± 3 and 904 ± 3 Hz respectively. AFM images showed the presence of a film on the surfaces for all monomers. N-H, C-N, C-H and CO group bands were determined in the IR spectra of the crystals. Decrease in the contact angle values of the modified crystals indicated the increase in hydrophilicity. Those results showed that amine containing films on the crystal surfaces were successfully deposited using atmospheric pressure plasma torch. The chemical groups on quartz surface were further activated with gluteraldehyde (GA) and protein immobilisation properties were checked with test antibody (anti-aflatoxin B1) in liquid phase quartz crystal microbalance system. The dip coating method was chosen for immobilisation procedure. After the immobilisation step, chemical structure of surfaces was analyzed by X-ray Photoelectron Spectroscopy.     

Modeling of liquid ceramic precursor droplets in a high velocity oxy-fuel flame jet

Production of coatings by high velocity oxy-fuel (HVOF) flame jet processing of liquid precursor droplets can be an attractive alternative method to plasma processing. This article concerns modeling of the thermophysical processes in liquid ceramic precursor droplets injected into an HVOF flame jet. The model consists of several sub-models that include aerodynamic droplet break-up, heat and mass transfer within individual droplets exposed to the HVOF environment and precipitation of ceramic precursors. A parametric study is presented for the initial droplet size, concentration of the dissolved salts and the external temperature and velocity field of the HVOF jet to explore processing conditions and injection parameters that lead to different precipitate morphologies. It is found that the high velocity of the jet induces shear break-up into several μm diameter droplets. This leads to better entrainment and rapid heat-up in the HVOF jet. Upon processing, small droplets (<5 μm) are predicted to undergo volumetric precipitation and form solid particles prior to impact at the deposit location. Droplets larger than 5 μm are predicted to form hollow or precursor containing shells similar to those processed in a DC arc plasma. However, it is found that the lower temperature of the HVOF jet compared to plasma results in slower vaporization and solute mass diffusion time inside the droplet, leading to comparatively thicker shells. These shell-type morphologies may further experience internal pressurization, resulting in possibly shattering and secondary atomization of the trapped liquid. The consequences of these different particle states on the coating microstructure are also discussed in this article.     

Characterisation of fresh surface oxidation films formed on pure molten magnesium in different atmospheres

This paper examines the early stages of surface oxidation of liquid magnesium under argon, air, and air mixed with protective fluorine-bearing gases. Surface film characteristics such as morphology, thickness and composition are determined. In all cases except argon the film was locally uniform with no evidence of specific nuclei.In air, the film thickness was 15-150 nm. Under fluorine-bearing gas mixtures the surface film was a mixed fluoride and oxide and more even: 70-100 nm thick under SF₆ and 30-50 nm under 1,1,1,2-tetrafluoroethane. The latter had a substantially lower O:F ratio. Complete conversion of available fluorine into the film was indicated.     

Ar/O2 gas pressure dependences of a pulsed zirconium arc and extracted ion characteristics

A pulsed dc zirconium arc discharge is generated in an argon diluted oxygen gas by separating a pin electrode as an anode from the cathode. The arc is transiently generated, and its life time is approximately 3 ms for a series resistance of 1 Ω and a dc output of 33 V. The life is prolonged and the plasma becomes stable with increasing the arc current. A target with a diameter of 100 mm is set at 150 mm from the arc source, and is immersed in the plasma. A pulse voltage is applied to the target to extract ions from the plasma. The ion current is not detected after approximately 8 ms since the plasma initiation. When the plasma is generated in oxygen without argon, the plasma generation time is scattered, and the plasma is unstable. An ion density is estimated from the temporal behavior of the target voltage in the recovery region after the pulse voltage. The ion density at the target is approximately 2.5 × 10¹⁵ m^− 3 at a mixed gas pressure of 1.9 Pa, which corresponds to the plasma density of 1.1 × 10¹⁷ m^− 3 under an assumption of electron temperature of 1 eV.     

Influence of absorbed moisture on solubility of poly(vinyl alcohol) film during atmospheric pressure plasma jet treatment

Moisture in the hydrophilic material may potentially influence the plasma treatment effect. In order to understand how moisture absorbed into PVA affects the result of plasma treatment to the polymer, atactic poly(vinyl alcohol) (PVA) films with moisture regain (MR) of 2.5%, 9.3% and 78.3% corresponding to 10%, 65% and 98% relative humidity (RH), respectively were exposed to atmospheric pressure plasma jet. Another group was annealed at 140 °C for 20 min to discern the thermal effects from those due to plasma treatment. Scanning electron microscope (SEM) showed lamellae crystal structures were on the surface of the films with 65% and 98% RH, while some bubbles or salt grains appeared on the surface of film with 10% RH and the annealed film respectively. X-ray photoelectron spectroscopy (XPS) analysis indicated that oxygen concentration increased for plasma treated films with 65% and 98% RH and decreased for that with 10% RH. X-ray diffraction analysis (XRD) shows an increase in crystallinity in all the plasma treated films. It was found that the solubility of all the treated films was decreased, especially for the plasma treated film under 98% RH which is nearly insoluble in water at 50 °C for 20 min.     

Modulation of field emission properties of DLC films with the incorporation of nanocrystalline silver nanoparticles by CVD technique

Composite films containing Ag nanoparticles embedded in diamond-like carbon (DLC) matrix were deposited on glass substrates by using capacitively coupled plasma (CCP) r.f. chemical vapour deposition technique (CVD). Amount of silver incorporation was controlled by the amount of argon in the argon and methane mixture in the plasma. Field emission properties of these films are reported here. Films became sp² rich with increased incorporation of silver nanoparticles in the DLC matrix. Field enhancement factor increased significantly for films with higher silver content in the DLC matrix. Work function (?) values obtained from the Fowler-Nordheim model varied between 0.04 and 0.10 eV while the field factor (β) varied between 336 and 2759. The critical field was found to vary between 10 and 100 V/μm.     

Hydrogen-free carbon thin films prepared by new type surface-wave-sustained plasma (SWP)

In this paper is described the simple structure of a new type surface-wave-sustained plasma (SWP) source without a magnetic field surrounding the chamber wall. In the source, the plasma is excited and sustained by 2.45 GHz microwaves, and the plasma density is measured by a single Langmuir probe in the target direction. The results indicate that the electron density obtained in this system is as high as 9 × 10¹¹ cm^− 3 even at a low pressure of 2.8 Pa. A graphite target (99.998%) and argon (99.999%) are used for depositing hydrogen-free amorphous carbon films by the new SWP source. The Raman spectra of the carbon films were obtained, and the results denote that the structure of the carbon films prepared by SWP is typical of diamond-like carbon; the Raman intensity ratio I_D/I_G is 2.97. The surface morphology was investigated by using an atomic force microscope (AFM). The images demonstrate that the hydrogen-free carbon films deposited by SWP have a very smooth surface, with a grain size of about 20 nm and surface roughness R_a of about 0.778 nm.