Comparative study of analytical inductively-coupled argon-plasma discharges using different outer gases

The analytical capabilities of high power (2-4 kW) ICPs with argon as inner and intermediate gas and different outer gases (argon, nitrogen, oxygen and air) were studied under optimum and compromise operating conditions. Under the optimum conditions, the lowest detection limits for elements with sensitive atom lines (C, B, Zn) or ion lines (Mg, Mn, Fe, Cr, Ti, V) were achieved with argon as outer gas and an observation height of 13 mm. Under compromise conditions (3 kW, aerosol gas gauge-pressure 3 bar) the lowest detection limits for the atom lines were also found with a pure argon plasma at an observation height of 13 mm. For ion lines, however, the argon/oxygen and argon/nitrogen plasmas and an observation height of 8 mm were better. The detection limits were poorer in the presence of an aluminium matrix; under the optimum operating conditions, the relative increase in detection limit was smaller with the argon/oxygen and argon/air ICPs than with the pure argon or argon/nitrogen ICPs. It was found that the interferences arising from an easily ionizable matrix are lower with a diatomic gas than with argon as outer gas. The interferences when the argon/nitrogen, argon/oxygen and argon/air plasmas are used are similar and practically independent of the nebulizer-gas pressure applied.     

Emission characteristics of nickel ionic lines excited by reduced-pressure laser-induced plasmas using argon, krypton, nitrogen, and air as the plasma gas

The emission characteristics of nickel ionic lines in low-pressure laser-induced plasmas are investigated when argon, krypton, nitrogen, or air gas was employed as the plasma gas. The spectrum patterns and the relative intensities of the ionic lines are measured with and without a blind cylinder surrounding the sample surface to separate the detected emission area into two portions roughly: an initial breakdown zone and an expansion zone of the plasma. Their emission intensities are strongly dependent on both the kind and the pressure of the plasma gas. Different major ionic lines are observed in the argon and the krypton plasmas: for example, the Ni II 230.010-nm line (8.25 eV) for argon and the Ni II 231.604-nm line (6.39 eV) for krypton. The excitation mechanism of these ionic lines is considered to be a resonance charge-transfer collision with argon or krypton ion due to good energy matching to the corresponding energy levels of nickel ion. These ionic lines measured with the blind cylinder at reduced pressures of around 1300 Pa give the largest signal-to-background ratios; therefore, the analytical application under such optimum plasma conditions is recommended.     

Calorimetric and dimensional studies on inductively coupled plasmas

The validity of the use of metallic loads to obtain formulae for the power dissipated in plasmas is confirmed by direct calorimetry on plasmas. As a rule of thumb, when high powers are used, about 40% of the power supplied to the generator reaches the plasma. The power in the plasma, as a function of plate kVA seems to be independent of plasma gas flow coolant. Measurements of plasma volumes show that those obtained with a nitrogen coolant are smaller than with an argon coolant; because of this, and in spite of higher losses to the coolant, nitrogen-cooled plasmas have higher power densities.     

A microwave induced plasma system for the maintenance of moderate power plasmas of helium,argon, nitrogen and air

A microwave induced plasma system capable of maintaining stable plasmas of each of the gases helium, argon, nitrogen and air is presented. The system is capable of operation at powers of up to 500 W. The TM₀₁₀ cavity design is similar to that previously described in the literature with some modifications. A demountable torch facilitates centering of diffuse plasmas of helium, nitrogen and air by providing 6 flows directed tangentially within the quartz tube. This torch was not useful for argon plasmas. Toroidal argon plasmas were maintained with a threaded quartz tube arrangement. The heat generated by these plasmas was dissipated by an outer sheath of coolant air. Details of the design and preliminary characterization of each plasma system is presented.     

Transport coefficients of argon,nitrogen, oxygen,argon-nitrogen,and argon-oxygen plasmas

Calculated values of the viscosity, thermal conductivity, and electrical conductivity of argon, nitrogen, and oxygen plasmas, and mixtures of argon anti nitrogen and of argon anti oxygen, are presented. In addition, combined ordinary, pressure, and thermal diffusion coefficients are given for the gas mixtures. These three combined diffusion coefficients fully describe di fusion of the two gases, irrespective of their degree of dissociation or ionizati on. The calculations, which assume local thermodynamic equilibrium, are performed! for atmospheric-pressure plasmas in the temperature range /torn 300 to 30,000 K. A number of the collision integrals used in calculating the transport coefficients are significantly more accurate than values used in previous theoretical studies, resulting in more reliable values of the transport coefficients. The results are compared with those of published theoretical and experimental studies.     

Electrical Features of Radio-frequency,Atmospheric-pressure,Bare-metallic-electrode Glow Discharges

Radio-frequency (RF), atmospheric-pressure glow discharge (APGD) plasmas with bare metallic electrodes have promising prospects in the fields of plasma-aided etching, deposition, disinfection and sterilization, etc. In this paper, an induced gas discharge approach is proposed for obtaining the RF, atmospheric-pressure, γ-mode, glow discharges with pure nitrogen or air as the primary plasma-working gas using bare metallic electrodes. The discharge characteristics, including the discharge mode, the breakdown voltage and discharge voltage for sustaining α mode and/or γ mode discharges, of the RF APGD plasmas of helium, argon, nitrogen, air or their mixtures using a planar-type plasma generator are presented in this study. The uniformity (no filaments) of the discharges is confirmed by the images taken by an iCCD with a short exposure time (10 ns). The effects of different gap spacings and electrode materials on the discharge characteristics, the variations of the sheath thickness and the electron number density are also studied in this paper.     

Emission characteristics of mixed gas plasmas in low-pressure glow discharges

Glow discharge optical emission spectrometry (GD-OES) with mixed plasma gases is reviewed. The major topic is the effect of type and content of gases added to an argon plasma on the emission characteristics as well as the excitation processes. Emphasis is placed on argon–helium, argon–oxygen, and argon–nitrogen mixed gas plasmas. Results for non-argon-matrix plasmas, such as neon–helium and nitrogen–helium mixtures, are also presented. Apart from the GD-OES, glow discharge mass spectrometry and furnace atomization plasma emission spectrometry with mixed plasma gases are also discussed.     

The dehydrogenation of isopropanol on a nickel-manganese catalyst subjected to treatment in glow-discharge oxygen,argon, and hydrogen plasmas

The dependence of the activity of the (Ni 20 wt %-Mn 4 wt %)/SiO₂ catalyst on the treatment of its surface in glow-discharge oxygen, argon, and hydrogen plasmas was studied. Catalytic experiments were performed in a flow reactor and under static conditions in a vacuum. The highest activity was observed after catalyst treatment in an argon plasma. Glow-discharge plasma treatment changed the structure and number of active centers, which resulted in a change in the reaction mechanism. Ab initio quantum-chemical calculations were performed using the Hartree-Fock (UHF) method for the Ni₅ cluster. The results substantiated the suggested that the active center contained the hydrogen atom.     

Surface Modification of Poly-ε-Caprolactone with an Atmospheric Pressure Plasma Jet

In this work, poly-ε-caprolactone samples are modified by an atmospheric pressure plasma jet in pure argon and argon/water vapour mixtures. In a first part of the paper, the chemical species present in the plasma jet are identified by optical emission spectroscopy and it was found that plasmas generated in argon/0.05 % water vapour mixtures show the highest emission intensity of OH (A–X) at 308 nm. In a subsequent section, plasma jet surface treatments in argon and argon/water vapour mixtures have been investigated using contact angle measurements and X-ray photoelectron spectroscopy. The polymer samples modified with the plasma jet show a significant decrease in water contact angle due to the incorporation of oxygen-containing groups, such as C–O, C=O and O–C=O. The most efficient oxygen inclusion was however found when 0.05 % of water vapour is added to the argon feeding gas, which correlates with the highest intensity of OH (X) radicals. By optimizing the OH (X) radical yield in the plasma jet, the highest polymer modification efficiency can thus be obtained.     

An Optical Emission Study on Expanding Low-Temperature Cascade Arc Plasmas

The optical emission spectra from expanding low-temperature cascade arc plasmas were studied. The objective of this study was to examine the distinctive features of low-temperature cascade arc plasmas in comparison with a radio frequency (RF) plasma source. The principal results obtained in this study were: (1) in an expanding cascade arc plasma jet, active heavy particles (mainly excited argon or helium neutral species under our operating conditions), rather than electrons, are responsible for the excitation of reactive species when a reactive gas is injected into the plasma jet, (2) the excitation of reactive species was found to be controlled by the electronic energy levels of these excited argon or helium neutrals, (3) changing the operating parameters affected only the emission intensities of excited species, and no effect on the emission nature of plasmas was observed.     

Behavior and surface energies of polybenzoxazines formed by polymerization with argon,oxygen, and hydrogen plasmas

Polybenzoxazine (PBZZ) thin films can be fabricated by the plasma‐polymerization technique with, as the energy source, plasmas of argon, oxygen, or hydrogen atoms and ions. When benzoxazine (BZZ) films are polymerized through the use of high‐energy argon atoms, electronegative oxygen atoms, or excited hydrogen atoms, the PBZZ films that form possess different properties and morphologies in their surfaces. High‐energy argon atoms provide a thermodynamic factor to initiate the ring‐opening polymerization of BZZ and result in the polymer surface having a grid‐like structure. The ring‐opening polymerization of the BZZ film that is initiated by cationic species such as oxygen atoms in plasma, is propagated around nodule structures to form the PBZZ. The excited hydrogen atom plasma initiates both polymerization and decomposition reactions simultaneously in the BZZ film and results in the formation of a porous structure on the PBZZ surface. We evaluated the surface energies of the PBZZ films polymerized by the action of these three plasmas by measuring the contact angles of diiodomethane and water droplets. The surface roughness of the films range from 0.5 to 26 nm, depending on the type of carrier gas and the plasma‐polymerization time. By estimating changes in thickness, we found that the PBZZ film synthesized by the oxygen plasma‐polymerization process undergoes the slowest rate of etching in CF₄ plasma. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4063–4074, 2004     

Surface modification of poly(tetrafluoroethylene) by a low-temperature cascade arc torch and radio-frequency plasmas

Poly(tetrafluoroethylene) (PTFE) films were treated with a low-temperature cascade arc torch (LTCAT) and radio-frequency (RF) plasmas of argon and hydrogen. The plasma-treatment effect on the PTFE surface was studied with contact-angle measurement and scanning electron spectroscopy (SEM). LTCAT argon plasma, which is recognized as a beam of excited argon neutrals, was very efficient at improving the surface hydrophilicity of PTFE. For both the LTCAT and RF operation, argon plasma was more effective at modifying the surface wettability of PTFE films than hydrogen plasma was. Furthermore, the sample positions (inside or beyond the glow region) had a strong impact on the efficiency of the plasma treatment. SEM surface images indicated that no significant morphology change was induced on the PTFE films exposed to a LTCAT and RF argon plasmas. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4432–4441, 1999     

Steel surface processing by a continuous optical discharge plasma

The influence of a continuous optical discharge (COD) plasma generated in flowing argon and argon-nitrogen mixture on alloy and carbon steel surfaces has been investigated. Changes in the structure, element composition, and microhardness of steel surface layers under the action of a COD plasma and laser beam were observed. In the presence of a COD plasma the microhardness distribution in hardened zone reaches its maximum at the metal surface and is different from that measured for samples processed by laser beam only.     

Infrared spectra of formaldehyde complexes with hydrogen bromide,chlorine and iodine chloride

The infrared spectra of the formaldehyde complexes with hydrogen bromide, chlorine and iodine chloride have been studied. The CH-stretching fundamentals of formaldehyde undergo significant blue shifts in these complexes. The shifts are larger in nitrogen than in argon matrices. The hydrogen bromide complex has been studied in mixed argon-nitrogen matrices.     

Critical analysis of viscosity data of thermal argon plasmas at atmospheric pressure

Reliable values of the viscosity in thermal argon plasmas are most important for our understanding of the momentum transfer and for realistic modeling of various plasma applications. Despite numerous attempts to determine reliable viscosity values over the last three decades, discrepancies still exist among the data reported by different authors. In this paper, a critical analysis is undertaken of calculated and experimental data of the argon viscosity based on recent publications. Our recalculation of viscosities in thermal argon plasmas are performed by using Lennard-Jones, Morse, Aziz, and exponential repulsive potentials for Ar-Ar atom interactions in different temperature ranges from 300 to 20,000 K. The contributions of elastic collisions of e-Ar, e-Ar⁺, and Ar⁺-Ar, as well as charge exchange of Ar⁺-Ar, to the viscosity become important with increasing temperature and degree of ionization in argon plasmas. Based on a critical analysis and recalculations, improved values of the argon viscosity are recommended, covering temperatures from 300 to 20,000 K. Polynomial expressions have been developed for calculating argon viscosities, which will be useful for numerical work and other applications of thermal argon plasmas at atmospheric pressure.     

Effects of argon dilution on the translational and rotational temperatures of SiH in silane and disilane plasmas

The effects of argon dilution on the translational and rotational temperatures of SiH in both silane and disilane plasmas have been investigated using the imaging of radicals interacting with surfaces (IRIS) technique. The average rotational temperature of SiH determined from the SiH excitation spectra is approximately 500 K in both SiH(4)/Ar and Si(2)H(6)/Ar plasmas, with no obvious dependence on the fraction of argon dilution. Modeling of kinetic data yields average SiH translational temperatures of approximately 1000 K, with no dependence on the fraction of argon in the SiH(4)/Ar plasmas within the studied range. In the Si(2)H(6)/Ar plasmas, however, the translational temperature decreases from approximately 1000 to approximately 550 K as the Ar fraction in the plasma increases. Thus, at the highest Ar fractions, the translational and rotational temperatures are nearly identical, indicating that the SiH radicals are thermally equilibrated. The underlying chemistry and mechanisms of SiH energy equilibration in Ar-diluted plasmas are discussed.     

Nonequilibrium modeling of low-pressure argon plasma jets; Part I: Laminar flow

This paper presents a modeling attempt related to low-pressure plasma spraying processes which find increasing applications for materials processing. After a review of the various models for ionization and recombination processes, a two-temperature model for argon plasmas in chemical (ionization) nonequilibrium is established using finite rate chemistry. Results of sample calculations manifest departures from kinetic as well as chemical equilibrium, demonstrating that the conventional models based on the LTE (local thermodynamic equilibrium) assumption cannot provide proper prediction for low-pressure plasma jets.     

Some fundamental measurements of the atmospheric-pressure,microwave-induced discharge

Measurements of several fundamental parameters in a microwave-induced atmospheric-pressure flowing plasma are presented. Optical and electrical measurements were performed on argon and argon/nitrogen plasmas in the region 1–7 cm outside the cavity, as the applied microwave power, and plasma composition were varied.The stability of the plasma, atomic emission from the argon support gas, and emission from the analyte species, are proportional to the electron density. The observed electron density was varied when the power was changed,-when an electrophilic species was added, and as the observation zone was moved relative to the microwave field. In all cases, the change in the emission signal followed the change in electron density.The electron temperature, as measured by the double-probe method, is related to the kinetic energy of the fastest-moving electrons in the plasma. It is unchanged by variations in power, plasma gas composition, flow rate, and is independent of the location of the probes relative to the cavity. The spectroscopic and electrical data are consistent with excitation by ion—electron radiative recombination.     

Transport coefficients of air,argon-air,nitrogen-air,and oxygen-air plasmas

Calculated values of the viscosity, thermal conductivity and electrical conductivity of air and mixtures of air and argon, air and nitrogen, and air and oxygen at high temperatures are presented. In addition, combined ordinary, pressure, and thermal diffusion coefficients are given for the gas mixtures. The calculations, which assione local thermodynamic equilibrium, are performed for atmospheric pressure plasmas in the temperature range from 300 to 30,000 K. The results for air plasmas are compared with those of published theoretical and experimental studies. Significant discrepancies are found with the other theoretical studies; these are attributed to differences in the collision integrals used in calculating the transport coefficients. A number of the collision integrals used here are significantly more accurate than values used previously, resulting in more reliable values of the transport coefficients.     

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.