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.     

Radio-frequency induction plasmas at atmospheric pressure: Mixtures of hydrogen,nitrogen, and oxygen with argon

Numerical calculations are reported which simulate atmospheric-pressure radiofrequency induction plasmas consisting of either pure argon or mixtures of argon with hydrogen, nitrogen, or oxygen. These calculations are compared to observations of laboratory plasmas generated with the same geometry and run conditions. The major features of the laboratory plasmas are predicted well by the calculations: the pure argon plasma is the largest, with the argon-oxygen plasma slightly smaller. The argon-nitrogen plasma is considerably smaller and the argon-hydrogen plasma is the shortest, although somewhat fatter than the argon-nitrogen case. The calculations are not entirely successful in predicting the exact location of the plasmas relative to the coils. A likely explanation is that there is significant uncertainty regarding the actual power coupled to the laboratory plasmas.     

Carbon monoxide production in silent discharge plasmas of air and air-methane mixtures

CO production in high-voltage alternating current (HVAC) silent discharge plasmas of air and air-methane mixtures at atmospheric pressure has been investigated by matrix isolation FTIR spectroscopy. In pure air, CO is produced by decomposition of CO₂. A steady-state CO/CO₂ ratio was determined by varying the flow rate. CO production was considerably enhanced when methane was added to the plasmas. CO production was observed even at very low oxygen concentrations, and did not noticeably decrease due to secondary oxidation reactions, even when methane was discharged in a pure oxygen carrier. CO production in air-methane mixtures is shown to depend on input power. CO production from CO₂ and hydrocarbons in air appears to be a significant obstacle for development of a plasma-based air purification device.     

Correlation Between the Electrical and Optical Properties of an Atmospheric Pressure Plasma During Siloxane Coating Deposition

The effect of varying process parameters on atmospheric plasma characteristics and properties of nanometre thick siloxane coatings is investigated in a reel-to-reel deposition process. Varying plasma operation modes were observed with increasing applied power for helium and helium/oxygen plasmas. The electrical and optical behaviour of the dielectric barrier discharge were determined from current/voltage, emission spectroscopy and time resolved light emission measurements. As applied power increased, multiple discharge events occurred, producing a uniform multi-peak pseudoglow discharge, resulting in an increase in the discharge gas temperature. The effects of different operating modes on coating oxidation and growth rates were examined by injecting hexamethyldisiloxane liquid precursor into the chamber under varying operating conditions. A quenching effect on the plasma was observed, causing a decrease in plasma input power and emission intensity. Siloxane coatings deposited in helium plasmas had a higher organic component and higher growth rates than those deposited in helium/oxygen plasmas.     

Selective surface modification of polymers by means of hydrogen and oxygen plasmas

An ESCA investigation has been made of the changes in surface functionalization for a series of polymers effected by means of low-powered inductively coupled rf plasmas excited in hydrogen and oxygen. Reactions in each case are confined to the outermost surfaces of the polymer films and the use of oxygen plasmas leads to extensive oxidative functionalization. Bisphenol-A polycarbonate and polyethylene terephthalate exhibit similar overall reactivities to both oxygen and hydrogen plasmas, while polystyrene is shown to be substantially more reactive than high-density polyethylene to the plasma treatments of interest in this work. Comparison has been made of the effects of straight hydrogen and oxygen plasmas and of sequential hydrogen/oxygen and oxygen/hydrogen plasma treatments.     

Plasma reactions of self-assembled monolayers to model oxygen atom effects on polymers

The plasma treatment of self-assembled monolayers of octadecyl mercaptan on gold substrates has been investigated as a model for oxygen atom effects on polymers. Both O₂ and H₂O low pressure gas plasmas have been used. X-ray photoelectron spectroscopy has revealed that the two plasma treatments differ from each other in the extent of oxidation and etch rate with O₂ being the more aggressive plasma. The results have confirmed that the plasma modification of organic surfaces involves a balance between surface oxidation and surface etching. The well-defined structure of the monolayer enables quantitation of these atom-substrate reactions. © 1998 John Wiley & Sons, Ltd.     

Etching processes of polytetrafluoroethylene surfaces exposed to He and He-O2 atmospheric post-discharges

A comparative study of polytetrafluoroethylene (PTFE) surfaces treated by the post-discharge of He and He-O(2) plasmas at atmospheric pressure is presented. The characterization of treated PTFE surfaces and the species involved in the surface modification are related. In pure He plasmas, no significant change of the surface has been observed by X-ray photoelectron spectroscopy (XPS), dynamic water contact angles (dWCA) and atomic force microscopy (AFM), in spite of important mass losses recorded. According to these observations, a layer-by-layer physical etching without any preferential orientation is proposed, where the highly energetic helium metastables are the main species responsible for the scission of -(CF(2))(n)- chains. In He-O(2) plasmas, as the density of helium metastables decreases as a function of the oxygen flow rate, the treatment leads to fewer species ejected from the PTFE surfaces (in agreement with mass loss measurements and the detection of fluorinated species onto aluminum foil). However, the dWCA and AFM measurements show an increase in the hydrophobicity and the roughness of the surface. The observed alveolar structures are assumed to be caused by an anisotropic etching where the oxygen atoms etch mainly the amorphous phase.     

Vacuum-ultraviolet induced oxidation of the polymers polyethylene and polypropylene

The emission from low-pressure microwave plasmas in the vacuum-ultraviolet (VUV) region (λ < 200 nm) was investigated in order to use these plasmas as light sources for the study of the VUV photochemistry of polyethylene (PE) and polypropylene (PP) as part of the study of plasma-polymer interaction. These polymers, immersed in low-presure oxygen, were exposed to radiation with wavelengths down to 112 nm, the cut off of magnesium fluoride used as a window to separate the polymer specimen from the plasma light source. Total oxygen incorporation in the surface [O], and the formation of hydroxyl, carbonyl, and carboxyl groups were measured using XPS in combination with chemical derivatizations, particularly their dependence upon the radiation spectrum and the oxygen pressure around the sample. In most experiments the surface oxygen concentration [O] attained a constant value that appears to be related to the initial oxidation rate; this suggests a competition between oxygen incorporation and chain scission reactions, followed by the removal of volatile oxidation products. PE is usually oxidized to a higher level than PP, the latter appearing to be more susceptible to reaction with atomic oxygen than PE. A general initiation mechanism for the VUV experiments is proposed that allows us to explain the observed differences in behavior between PE and PP, and the results obtained under different irradiation conditions. The nature of oxidation products is in both cases very similar to what is observed after direct plasma treatment of the polymers. We conclude that short wavelength radiation contributes very appreciably to the observed surface modification effects during plasma treatment of PE and PP. © 1995 John Wiley & Sons, Inc.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 17. Effect of oxygen availability

The gas in contact with polyethylene has considerable impact on its oxidation. The rate of oxidation product formation is mostly larger with oxygen blanketing than in air. Similarly, the rate in air is larger than that under nitrogen blanketing. Moreover, the relative effect of the surrounding gas is depending heavily on the particular oxidation product considered. The effect on the alcohol concentration on passing from air to pure oxygen is the same as that on the hydroperoxide concentration. It is only under pure nitrogen that alcohol formation is relatively more affected than hydroperoxide formation. The overall carbonyl groups as well as the ketones show the expected ranking, i.e. faster rate in pure oxygen than in air and faster rate in air than under pure nitrogen. However, carboxylic acids are formed much faster in oxygen than in air. For the acids the results in air and under nitrogen are significantly closer in the initial stages of processing than the results obtained under pure oxygen. This is different for γ-lactones for which formation is faster in oxygen than in air where it is faster than under nitrogen. With trans-vinylene groups the situation is opposite to that observed for carboxylic acids: the rate of formation is close for the experiments performed under air and under oxygen and significantly faster than under nitrogen. The results for hydroperoxides, alcohols and ketones are easily interpreted taking into account the kinetics developed in previous work. Fitting the data to the heterogeneous kinetics shows the effect of the oxygen concentration on this kinetics. It is especially unexpected with respect to its impact on the initiation rate. It is discussed taking into account various possibilities. The only one that is compatible with all the data envisages chain initiation resulting from interaction of oxygen with strained polymer molecules.     

Singlet oxygen and photooxidation of polymers

The rates of accumulation of oxygen-containing species in photooxidation have been determined for a series of polymers such as polystyrene, polyisoprene, and polybutadiene in a wide range of singlet oxygen stationary concentrations in the polymers. An increase in singlet oxygen (¹O₂) concentration was achieved by introducing a dye which is a ¹O₂ donor into a polymer and by irradiating a sample with an additional source of light absorbed by the dye only. To decrease ¹O₂ concentration, 1,2,5-trimethyl-4-hydroxyphenylpiperidine, which is a quencher of ¹O₂, was introduced into a sample. The ¹O₂ stationary concentration in an oxidized polymer was measured via singlet oxygen oxidation of 2,2,6,6-tetramethyl-4-oxypiperidine which leads to the formation of the corresponding nitroxyl radical. The photooxidation rate has been found to vary only slightly with ¹O₂ concentration, even when the concentration changes 10–25-fold. Thus, ¹O₂ does not participate appreciably in the process of polymer photooxidation. It may be due to the lowering of ¹O₂ reactivity toward unsaturated groups in the polymer matrix as compared with that in solution.     

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.     

Thermodynamic and transport properties of argon/carbon and helium/carbon mixtures in fullerene synthesis

The equilibrium composition and thermodynamic and transport properties of argon; carbon and helium/carbon mixtures are calculated in the temperature range 300–20,000 K. The curves for the composition of mixtures of 50%, carbon in argon or helium are shown fir a pressure of 1.33 × 10⁴ Pa. The calculations for the heat capacity at constant pressure (C_p) and transport coefficients are validated with other studies, for the cases or pure argon and pure helium at a pressure of 10⁵ Pa. The properties of mixtures with various proportions of carbon in argon and helium are calculated. Results are presented at pressures of 105 and 1.33 × 10⁴ Pa, typical of reactors for the synthesis of fullerenes and nanotubes. It is observed that the properties of carbon and mixtures of carbon with a buffer gas (argon or helium) are very different from those of the buffer gas, thus the need to consider this effect in simulations. In general, the mixtures follow trends intermediate to those of the pure gases from which they are composed except for the thermal conductivity which shows a deviation from this tendency in the region between 11,500 and 19,000 K for argon/carbon mixtures and between 8,000 and 12,000 K for helium/carbon mixtures. Also, the electrical conductivity of mixtures of low carbon concentration is very close to that ofpure carbon. A datafile containing the transport properties of mixtures for pressures between 10⁴ and 10⁵ Pa is available free of charge from the authors.     

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.     

Emission characteristics of Grimm-style glow discharge plasmas with helium matrix plasma gas containing small amounts of nitrogen

Glow discharge plasmas with helium–(0–16%) nitrogen mixed gas were investigated as an excitation source in optical emission spectrometry. The addition increases the sputtering rate as well as the discharge current, because nitrogen molecular ions, which act as primary ions for the cathode sputtering, are produced through Penning-type ionization collisions between helium metastables and nitrogen molecules. The intensity of a silver atomic line, Ag I 338.29 nm, is monotonically elevated along with the nitrogen partial pressure added. However, the intensities of silver ionic lines, such as Ag II 243.78 nm and Ag II 224.36 nm, gave different dependence from the intensity of the atomic line: Their intensities had maximum values at a nitrogen pressure of 30 Pa when the helium pressure and the discharge voltage were kept at 2000 Pa and 1300 V. This effect is principally because the excitations of these ionic lines are caused by collisions of the second kind with helium excited species such as helium metastables and helium ion, which are quenched through collisions with nitrogen molecules added to the helium plasma. The sputtering rate could be controlled by adding small amounts of nitrogen to the helium plasma, whereas the cathode sputtering hardly occurs in the pure helium plasma.     

Thin film deposition from plasmas of tetramethylsilane–helium–argon mixtures with oxygen and with nitrogen

Films were produced by plasma enhanced chemical vapor deposition (PECVD) of tetramethylsilane (TMS)–helium–argon mixtures with either oxygen or nitrogen in a vacuum system fed with radiofrequency power. Actinometric optical emission spectroscopy was used to determine trends in the concentrations of plasma species of interest (H, CH, O, CO, and CN) as a function of the ratio of the inorganic reactive gas (oxygen or nitrogen) to the monomer (TMS) in the system feed. As the ratio of oxygen to TMS in the feed is increased, the degree of oxygenation of the deposited material, as revealed by transmission infrared spectroscopy, is also increased. Similarly, the degree of nitrogenation of the films increases with increasing nitrogen to monomer ratio in the feed. Strong correlations exist between the plasma concentrations of the above-mentioned plasma species and film structure and composition. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1873–1879, 1998     

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.     

Comparative studies of reactions of commercial polymers with molecular oxygen,singlet oxygen,atomic oxygen and ozone—II. Reactions with 1,2-polybutadiene

The oxidations of 1,2-polybutadiene by molecular oxygen, singlet oxygen, atomic oxygen and ozone have been studied using u.v. and i.r. spectroscopic methods. Some possible implications of the results of oxidation in the presence of singlet oxygen (parallel free radical oxidation) and atomic oxygen (formation of NO₂ and its reaction with polymer) are discussed. Crosslinking was observed during all types of oxidation. A new mechanism involving formation of free radicals has been considered in detail. During ozonization of 1,2-polybutadiene, formation of formaldehyde and formic acid were detected. An ozonization mechanism has been proposed.     

New power source from fractional quantum energy levels of atomic hydrogen that surpasses internal combustion

Extreme ultraviolet (EUV) spectroscopy was recorded on microwave discharges of helium with 2% hydrogen. Novel emission lines were observed with energies of q·13.6 eV where q=1,2,3,4,6,7,8,9, or 11 or these lines inelastically scattered by helium atoms wherein 21.2 eV was absorbed in the excitation of He (1s²) to He (1s¹2p¹). These lines were identified as hydrogen transitions to electronic energy levels below the ‘ground’ state corresponding to fractional quantum numbers. Significant line broadening corresponding to an average hydrogen atom temperature of 33–38 eV was observed for helium–hydrogen discharge plasmas; whereas pure hydrogen showed no excessive broadening corresponding to an average hydrogen atom temperature of ≈3 eV. Since a significant increase in H temperature was observed with helium–hydrogen discharge plasmas, and energetic hydrino lines were observed at short wavelengths in the corresponding microwave plasmas that required a very significant reaction rate due to low photon detection efficiency in this region, the power balance was measured on the helium–hydrogen microwave plasmas. With a microwave input power of 30 W, the thermal output power was measured to be at least 300 W corresponding to a reactor temperature rise from room temperature to 900 °C within 90 s, a power density of 30 MW/m³, and an energy balance of about −4×10⁵ kJ/mol H₂ compared to the enthalpy of combustion of hydrogen of −241.8 kJ/mol H₂.     

Studies on the photooxidation mechanism of polymers. V. Oxidation of polybutadienes by singlet oxygen from microwave discharge and in dye-photosensitized reactions

The singlet oxygen oxidation of cis- and trans- 1,4-polybutadienes was studied by using singlet oxygen generated in a microwave generator and in dye-photosensitized reactions of these polymers in the solid state and in solution. It was shown that the reactions of singlet oxygen result in formation of hydroperoxide groups, whereas ultraviolet oxidation by molecular oxygen in addition leads to formation of carbonyl groups. During dye-photosensitized oxidation of polydienes in benzene solution, a very rapid decrease in the molecular weight was observed.     

Comparative studies of reactions of commercial polymers with molecular oxygen,singlet oxygen,atomic oxygen and ozone—I. Reactions with cis-1,4-polybutadiene

The oxidation of cis-1,4-polybutadiene by molecular oxygen, singlet oxygen, atomic oxygen and ozone has been studied using u.v. and i.r. spectroscopic methods. Some possible implications of the results of oxidation in the presence of singlet oxygen (parallel free radical oxidation) and atomic oxygen (formation of NO₂ and its reaction with polymer) are discussed. Chain scission was observed during all types of oxidation. A new mechanism involving opening of double bonds and formation of biradicals has been considered in detail.