Experimental Study on the Thermal Argon Plasma Generation and Jet Length Change Characteristics at Atmospheric Pressure

The generation, jet length and flow-regime change characteristics of argon plasma issuing into ambient air have been experimentally examined. Different torch structures have been used in the tests. Laminar plasma jets can be generated within a rather wide range of working-gas flow rates, and an unsteady transitional flow state exists between the laminar and turbulent flow regimes. The high-temperature region length of the laminar plasma jet can be over an order longer than that of the turbulent plasma jet and increases with increasing argon flow rate or arc current, while the jet length of the turbulent plasma is less influenced by the generating parameters. The flow field of the plasma jet has very high radial gradients of plasma parameters, and a Reynolds number alone calculated in the ordinary manner may not adequately serve as a criterion for transition. The laminar plasma jet can have a higher velocity than that of an unsteady or turbulent jet. The long laminar plasma jet has good stiffness to withstand the impact of laterally injected cold gas and particulate matter. It could be used as a rather ideal object for fundamental studies and be applied to novel materials processing due to its attractive stable and adjustable properties.     

Generation of Long, Laminar Plasma Jets at Atmospheric Pressure and Effects of Flow Turbulence

Long, laminar plasma jets at atmospheric pressure of pure argon and a mixture of argon and nitrogen with jet length up to 45 times its diameter could be generated with a DC arc torch by restricting the movement of arc root in the torch channel. Effects of torch structure, gas feeding, and characteristics of power supply on the length of plasma jets were experimentally examined. Plasma jets of considerable length and excellent stability could be obtained by regulating the generating parameters, including arc channel geometry, gas flow rate, and feeding methods, etc. Influence of flow turbulence at the torch nozzle exit on the temperature distribution of plasma jets was numerically simulated. The analysis indicated that laminar flow plasma with very low initial turbulent kinetic energy will produce a long jet with low axial temperature gradient. This kind of long laminar plasma jet could greatly improve the controllability for materials processing, compared with a short turbulent arc jet.     

大尺寸大气压空气等离子体射流设备研究

本文提出了一种特殊的大尺寸大气压空气等离子体射流设备。该设备采用一种高分子耐高温涂料作为介质进行放电,通过气体渐扩通道,产生大尺寸空气等离子体射流,射流直径最宽处可以达到28mm,长度可以达到数十mm。本文对射流中的活性物质进行光谱测量,同时对射流宏观温度进行测量,指出该射流温度非常接近室温,可以用于温度敏感材料的表面处理等方面。     

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.     

Ashing of Organic Compounds with Spray-Type Plasma Reactor at Atmospheric Pressure

We examined the ashing treatment at atmospheric pressure by means of three spray-type reactors fed with O₂/He or O₂/Ar mixture gases. These differed in the size or the shape of their nozzles. Such reactors were able to ash an organic compound (OFPR-800; a photoresist) even at atmospheric pressure. The results showed that the following procedures are important for increasing the ashing rate: to make the gas speed after blowing out fast; to decrease the O₂ content while increasing the gas speed; and to use a gas mixture which has a slow decay rate of the active species, such as the oxygen radicals. Especially, when we used O₂/Ar mixture gas for the ashing treatment, the ashing rate became quite fast and was as fast as that of a general low pressure glow plasma.     

Novel AC and DC Non-Thermal Plasma Sources for Cold Surface Treatment of Polymer Films and Fabrics at Atmospheric Pressure

Novel types of non-thermal plasma sources at atmospheric pressure based on multi-pin DC (direct current) diffusive glow discharge and AC (alternative current) streamer barrier corona have been elaborated and tested successfully for cold surface treatment of polymer films [polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET),] and polyester fabric. Results on physical properties ofdischarges mentioned and output energy characteristics of new plasma sources as well as data on after-treatment changes in wettability of films and fabrics are presented. The main goal of this study was to find out the experimental conditions for gas discharge and surface processing to achieve a remarkable wettability change for a short treatment time.     

Surface Functionalization at Atmospheric Pressure by DBD-Based Pulsed Plasma Polymerization

Dielectric barrier discharge (DBD)-based plasma deposition at atmospheric pressure, using glycidyl methacrylate (GMA, methacrylic acid-[2,3-epoxy-propyl ester]) as a prototype precursor was investigated in order to evaluate the applicability of dielectric barrier discharges to obtain plasma polymers with a high degree of structural retention of the starting precursor. Using pulsed excitation of the discharge, up to about 90% of the epoxy groups of GMA can be retained in plasma polymers obtainableat deposition rates in the order of 3–5nm/s. Preliminary investigations of the mechanism of film formation under pulsed plasma conditions indicate that the reaction of intact monomer molecules withsurface radicals generated during the pulses play a prominent role.     

Synthesis of Ozone at Atmospheric Pressure by a Quenched Induction-Coupled Plasma Torch

The technical feasibility of using an induction-coupled plasma (ICP) torch to synthesize ozone at atmospheric pressure is explored. Ozone concentrations up to ~250 ppm were achieved using a thermal plasma reactor system based on an ICP torch operating at 2.5 MHz and ~11 kVA with an argon/oxygen mixture as the plasma-forming gas. The corresponding production rate and yield were ~20 g ozone/hr and ~2g ozone/kWh, respectively. A gaseous oxygen quench formed ozone by rapid mixing of molecular oxygen with atomic oxygen produced by the torch. The ozone concentration in the reaction chamber was measured by Fourier Transform infrared (FTIR) spectroscopy over a wide range of experimental conditions and configurations. The geometry of the quench gas flow, the quench flow velocity, and the quench flow rate played important roles in determining the ozone concentration. The ozone concentration was sensitive to the torch RF power, but was insensitive to the torch gas flow rates. These observations are interpreted within the framework of a simple model of ozone synthesis.     

Comparison of Laminar and Turbulent Thermal Plasma Jet Characteristics—A Modeling Study

Modeling results are presented to compare the characteristics of laminar and turbulent argon thermal plasma jets issuing into ambient air. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of ambient air into the laminar and turbulent argon plasma jects, respectively. It is shown that since only the molecular diffusion mechanism is involved in the laminar plasma jet, the mass flow rate of ambient air entrained into the laminar plasma jet is comparatively small and less dependent on the jet inlet velocity. On the other hand, since turbulent transport mechanism is dominant in the turbulent plasma jet, the entrainment rate of ambient air into the turbulent plasma jet is about one order of magnitude larger and almost directly proportional to the jet inlet velocity. As a result, the characteristics of laminar plasma jets are quite different from those of turbulent plasma jets. The length of the high-temperature region of the laminar plasma jet is much longer and increases notably with increasing jet inlet velocity or inlet temperature, while the length of the high-temperature region of the turbulent plasma jet is short and less influenced by the jet inlet velocity or inlet temperature. The predicted results are reasonably consistent with available experimental observation by using a DC arc plasma torch at arc currents 80–250 A and argon flow rates (1.8–7.0)×10⁻⁴ kg/s.     

Excitation of Species in an Expanded Argon Microwave Plasma at Atmospheric Pressure

The excitation capability of an argon microwave plasma flame expanded at atmospheric pressure has been studied. For this purpose, argon with different proportions of nitrogen was introduced at the end of the expanded flame, where the population densities of the atomic argon levels were still high enough. Optical emission spectra allowed the identification of different excited species in the plasma. When argon containing nitrogen was added at the end of the plasma flame, all argon lines emitted in this region were highly quenched, emission due to species containing nitrogen (NH, CN) was enhanced and a noticeable increase in the emission of N₂ (C ³Π_u ? B ³Π_g) was observed. On the contrary, the weak emission of $ {\text{N}}_{2}^{ + } \left( {B^{ 2} \sum_{u}^{ + } - X^{ 2} \sum_{g}^{ + } } \right) $ was scarcely affected. According to these results it is possible to conclude that metastable argon atoms from the expanded flame are the main energy carriers when generating N₂ reactive species in this plasma zone.     

Development of Atmospheric Pressure Low Temperature Surface Discharge Plasma Torch and Application to Polypropylene Surface Treatment

We have finally succeeded in producing the plasma jet by use of the surface discharge plasma torch that can be expected to make larger the diameter of torch in the comparatively easy way. It can be checked that the active species in the jet obtained are different depending on the direction of connection, and also it was clearly found that much O and N₂ is included in them. Consequently, etching was confirmed at the position of 10 mm from the torch end in the surface treatment of polypropylene film, but etching was not confirmed at the position of 20 mm.     

Numerical Simulation of Downstream Kinetics of an Atmospheric Pressure Nitrogen Plasma Jet Using Laminar, Modified Laminar, and Turbulent Models

We present numerical simulation of the nitrogen atmospheric pressure plasma jet (APPJ) using three fluid models—namely, laminar model, modified laminar model, and turbulent (k-ε) model—coupled with gas-phase reaction kinetics. The spatial profiles of the light emission intensities, gas temperature, and NO density predicted by the turbulent model show a better agreement with the experimental observations, compared with laminar and modified laminar models. We have demonstrated that the turbulent model shows more oxygen entrainment, more mixing with the ambient air, and a lower axial velocity at the downstream. These allow the turbulent model to more precisely capture the APPJ characteristics than the laminar and modified laminar models do.     

A Thermodynamic Analysis of Nonequilibrium Argon Plasma Torches

The nonequilibrium process of argon plasma torches is analyzed theoretically. Thermodynamic diagrams of different degrees of ionization are developed to aid in understanding and analyzing the transition from chemical equilibrium to frozen flow in dc plasma torch operations. A thermodynamic model is developed to describe the nonequilibrium processes in a dc argon plasma torch. In the model the ionization process is approximated as a constant-pressure heating process, with little deviation from the equilibrium state upon completion of heating. If the plasma flow is frozen shortly after heating, the entropy increase is small during the transition from equilibrium to frozen flow. In this case the frozen flow will have nearly the same composition and entropy as the flow at the heating section exit. For singly ionized argon plasmas in the entropy range relevant to dc torch operation, the frozen flow solutions on the affinity–pressure diagram are found to be insensitive to entropy change. Therefore the present model predicts that argon plasmas generated at different power levels will have almost identical affinity at the torch exit for the same operating pressure. This prediction agrees with experimental observations except for very low torch power levels.     

Abnormal Optical Emissions in a Condensing Monosilane–Argon Gas Jet Activated by Electron Beam Plasma

Emissions from Si, SiH, H, Ar, and Ar⁺ are investigated for a free supersonic jet of a mixture of monosilane 5% (or 10%) with argon; the jet is activated by electron- beam plasma, and all the emissions are measured as a function of pressure, temperature in the gas source, the nozzle-to-electron beam distance, and the beam current. It is found that for certain parameters a process initiates which inhibits radiation of the spectral lines of Si, SiH, and H, and stimulates radiation of some spectral lines of argon atoms. It is shown that these anomalies are connected with a condensation process in the jet and, apparently, they are caused by electron-induced fluorescence of monosilane-containing clusters. The mechanisms for abnormal emissions of argon atoms and electron-excited fragments of monosilane molecule are suggested.     

Atmospheric Pressure Plasma Deposition of Polyfuran

The atmospheric pressure radiofrequency (RF) plasma polymerization of furan was carried out with the objective of synthesizing polyfuran thin film. The structure, compositions and morphology of the plasma deposited polyfuran film were investigated by Fourier transform infrared (FTIR), atomic force microscopy (AFM), ultraviolet‐visible absorption spectroscopy (UV‐vis) and thermogravimetric analysis (TGA). The formation of polyfuran was confirmed using FTIR and UV‐visible analysis. The properties of plasma‐deposited polyfuran were compared with those of chemically synthesized polyfuran. Although the plasma deposited thin film polyfuran shows lower thermal stability than that of chemically synthesized polyfuran. It has better solubility in CHCl₃, also. Thin uniform polyfuran films are obtained in plasma assisted polyfuran deposition, while particles are obtained in chemical polyfuran polymerization.     

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.     

Application of Spray-Type Atmospheric Pressure Glow Plasma Reactor: Ashing of Organic Compounds II

When plasma treatment is carried out in the after glow region of an electrical discharge, the decay rate and the density of the active species are very important factors for the treatment efficiency. They are known to depend on the linear gas flow rate (gas velocity) and on the residence time of the treatment gas in the discharge zone, respectively. In our previous study, we found that the spray-type atmospheric pressure glow plasma reactor with O₂/He or O₂/Ar mixture treatment gases had a satisfactory ashing rate of a solid organic compound (OFPR-800; a photoresist). However, the relationships between the gas velocity or the residence time and the ashing rate had not yet been examined. The present study showed clearly that the gas velocity influenced only the transit time, that is the time which the gas mixture took to progress from the slit nozzle to the sample surface, but it did not influence the generation of the active species. On the other hand, the generation rate of active species in the discharge zone was found to be strongly dependent on the residence time. The ashing rate was found to increase with increasing the residence time up to about 30 ms, beyond which it saturated. From optical emission spectroscopy measurements, the maximum ashing rate could be correlated with the emission intensities corresponding to He 3p³P-2s³S (388.8 nm) and O 3d⁵D-3p⁵P (926.5 nm) bands. These results are of practical interest.     

Atmospheric Pressure Plasma Surface Treatment of Polymers and Influence on Cell Cultivation

Atmospheric plasma treatment is an effective and economical surface treatment technique. The main advantage of this technique is that the bulk properties of the material remain unchanged while the surface properties and biocompatibility are enhanced. Polymers are used in many biomedical applications; such as implants, because of their variable bulk properties. On the other hand, their surface properties are inadequate which demands certain surface treatments including atmospheric pressure plasma treatment. In biomedical applications, surface treatment is important to promote good cell adhesion, proliferation, and growth. This article aim is to give an overview of different atmospheric pressure plasma treatments of polymer surface, and their influence on cell-material interaction with different cell lines.     

Determination of the Nitrogen Atom Density in the Afterglow of a Nitrogen and Helium,Nonequilibrium, Atmospheric Pressure Plasma

Three methods have been examined for evaluating the concentration of nitrogen atoms in the afterglow of a nonequilibrium, helium-stabilized, atmospheric pressure plasma. These are nitric oxide titration, absolute emission intensity of N₂(B ³_g) and temporal decay of the N₂(B ³_g) emission. To employ the second method, the rate constants for the recombination of N atoms into N₂(B ³_g), at different vibrational levels of the B state, were determined. The third newly developed method has three advantages over the other two techniques: (1) it can predict the N-atom density for the entire afterglow, (2) it does not require calibration of the N₂(B ³_g) emission intensity, and (3) it does not disturb the gas flow. According to these measurements, the atmospheric pressure plasma produced a high density of nitrogen atoms, exceeding 4.0×10¹⁵ cm^–3 at the edge of the discharge for 10 Torr N₂ in 745 Torr He at 375 K and 15.5 W/cm³.     

Characteristics of Atmospheric Pressure Air Glow Discharge with Aqueous Electrolyte Cathode

We investigated the characteristics of atmospheric pressure (AP) air glow discharge with aqueous electrolyte cathode. Distilled water or aqueous solutions of HNO₃, KCl, KBr and KI were utilized as a cathode. The cathode voltage drop, electric field strength, gas temperature as well as emission intensity of some lines of OI, H, K, Na and bands of N₂, OH, NO were measured at discharge current from 10 to 50 mA. The procedure of gas temperature measurement on the base of non-resolved structure of N₂(C³Π_u → B³Π_g) bands was described. The reduced electric field strength (E/N) was obtained, which showed that the discharge existed in the normal mode. On the base of these data, the regularities of optical emission were discussed.