K. V. Kozlov

The barrier discharge: basic properties and applications to surface treatment

Abstract Barrier discharges (BDs) produce highly non-equilibrium plasmas in a controllable way at atmospheric pressure, and at moderate gas temperature. They provide the effective generation of atoms, radicals and excited species by energetic electrons. In the case of operation in noble gases (or noble gas/halogen gas mixtures), they are sources of an intensive UV and VUV excimer radiation. There are two different modes of BDs. Generally they are operated in the filamentary one. Under special conditions, a diffuse mode can be generated. Their physical properties are discussed, and the main electric parameters, necessary for the controlled BD operation, are listed. Recent results on spatially and temporally resolved spectroscopic investigations by cross-correlation technique are presented. BDs are applied for a long time in the wide field of plasma treatment and layer deposition. An overview on these applications is given. Selected representative examples are outlined in more detail. In particular, the surface treatment by filamentary and diffuse BDs, and the VUV catalyzed deposition of metallic layers are discussed. BDs have a great flexibility with respect to their geometrical shape, working gas mixture and operation parameters. Generally, the scaling-up to large dimensions is of no problem. The possibility to treat or coat surfaces at low gas temperature and pressures close to atmospheric once is an important advantage for their application.

Spatio-temporally resolved spectroscopic diagnostics of the barrier discharge in air at atmospheric pressure

The technique of spatially resolved cross-correlation spectroscopy (CCS) is used to carry out diagnostic measurements of the barrier discharge (BD) in air at atmospheric pressure. Quantitative estimates for electric field strength E(x,t) and for relative electron density ne(x,t)/nemax are derived from the experimentally determined spatio-temporal distributions of the luminosity for the spectral bands of the 0-0 transitions of the second positive system of N2 (λ = 337.1 nm) and the first negative system of N2+ (λ = 391.5 nm). These results are used to test the validity of some physical models of electrical breakdown in a BD. The influence of the spatio-temporal structure of the discharge on the chemical kinetics of ozone synthesis is studied by means of a semi-empirical method based on the results of spatially resolved CCS measurements.

Investigation of the filamentary and diffuse mode of barrier discharges in N2/O2 mixtures at atmospheric pressure by cross-correlation spectroscopy

The techniques of spatially resolved cross-correlation spectroscopy (CCS) and current pulse oscillography were used to carry out systematic investigations of the barrier discharge (BD) in the binary gas mixtures N2/O2 at atmospheric pressure. At very low oxygen concentrations (<500 ppm), the BD was observed in a so-called diffuse mode (also referred to as atmospheric pressure glow discharge, glow silent discharge or homogeneous BD). In the case of the BD filamentary mode, the spatio-temporal distributions of the BD radiation intensities were recorded for the spectral bands of the 0–0 transitions of the second positive (λ = 337 nm) and first negative system of molecular nitrogen (λ = 391 nm). In the case of the diffuse mode, the spectral bands λ = 337 nm, λ = 260 nm (0–3 transition of the γ -system of NO) and λ = 557 nm (radiation of ON2 excimer) were used for this purpose. The velocities of the cathode-directed ionizing waves as well as the effective lifetimes of the excited states N2(C 3 � u)υ� =0 and N + (B 2 � + u )υ � =0 were evaluated from the CCS data. Special attention was devoted to the investigation of the transition between the filamentary and diffuse modes of the BD, this transition being caused by the variation of oxygen content within the range 500–1000 ppm. (Some figures in this article are in colour only in the electronic version)

Intensity ratio of spectral bands of nitrogen as a measure of electric field strength in plasmas

The ratios of intensities of the spectral bands of molecular nitrogen corresponding to transitions N + (B 2 � + g ,v = 0) → N + (X 2 � + ,v = 0), N2(C 3 � u ,v = 0) → N2(B 3 � g ,v = 0) and N2(C 3 � u ,v = 2) → N2(B 3 � g ,v = 5) as a function of the applied electric field strength were measured for air in the pressure range of 300 to 10 5 Pa. The non-self-sustaining dc discharge in a parallel-plane gap was used for excitation of gas molecules. The reduced field strength was varied in the range of (150–5000) × 10 −21 Vm 2 . The measured ratio of intensities as a function of electric field strength is compared with the theoretical estimates made by other authors. The obtained intensity ratio versus field strength curves can be used for field strength estimation in plasmas if the nitrogen molecules are excited dominantly from the ground state directly by the electron impact. (Some figures in this article are in colour only in the electronic version)

Investigation of the coplanar barrier discharge in synthetic air at atmospheric pressure by cross-correlation spectroscopy

The barrier discharge in the coplanar arrangement operating in a single-filament mode was studied spectroscopically. The evolution of the discharge luminosity was measured by the technique of cross-correlation spectroscopy. The 1D-spatially and temporally resolved luminosities of the first negative (at 391.5 nm) and the second positive (at 337.1 nm) system of molecular nitrogen were recorded using the above-mentioned technique. A cathode-directed ionizing wave (IW) was clearly seen on the plot for radiation intensity at 337.1 nm. In addition to this, also observed was a wave of the enhanced electric field propagating over the anode. In this paper, the propagation of these waves is described and their velocities are determined. The discharge evolution is divided into three phases—the Townsend phase, the phase of the IWs propagation and the extinction phase. Since the above-mentioned luminosity distributions could be interpreted approximately as the electric field (for 391.5 nm) and the electron density (for 337.1 nm) distribution, the qualitative description of the discharge is made accordingly. All these parameters are compared with similar measurements of the volume discharge. Apart from this, an attempt to determine the reduced electric field is made according to the kinetic model. (Some figures in this article are in colour only in the electronic version)

Synthesis of organic compounds from mixtures of methane with carbon dioxide in dielectric-barrier discharges at atmospheric pressures

The entire range of gas phase reaction products, depending on the composition of initial binary mixtures of methane and carbon dioxide in dielectric barrier discharges, has been determined (saturated as well as unsaturated hydrocarbons and oxygenated organic compounds). The macro-kinetics of the basic chemical pathways of the system under consideration has been investigated. This system is found to display a strong feedback effect (positive or negative, depending on the initial state of the surfaces, as well as the chemical composition of the feed-gas mixture). It is demonstrated that these properties undergo significant changes during operation, due to surface modification processes (polymer film deposition, its oxidation or reduction). They are found to exert a considerable influence on the chemical efficiency of the discharge (for example, on the absolute and relative chemical yields of the reaction products), the Lissajous figures appear to be a sensitive tool to monitor the operation conditions of the discharge.

Reply to comments on ‘Intensity ratio of spectral bands of nitrogen as a measure of electric field strength in plasmas’

It is argued that the experimental results presented by Paris et al in J. Phys. D: Appl. Phys. (2005) 38 3894 are correct. It is shown that there is no evidence of filamentation or local electric field reinforcement in discharge. It is discussed that reaction scheme and rate constants for deactivation of excited nitrogen ion proposed by Pancheshnyi in his Comment must be specified for conditions corresponding to the experiment carried out by Paris et al.

Progress in the Visualization of Filamentary Gas Discharges. Part 1: Milestones and Diagnostics of Dielectric-barrier Discharges by Cross-correlation Spectroscopy

Abstract Filamentary gas discharges at atmospheric pressure, namely dielectric-barrier discharges and coronas, produce highly non-equilibrium plasmas in a controllable way at a moderate gas temperature. They provide the effective generation of atoms, radicals and excited species by energetic electrons. Consequently, they have found many applications, among others in environmental protection. The controlled application of these discharges requires a good understanding of their physical properties and the knowledge of the basic plasma parameters. Milestones on this way and the progress in the plasma diagnostics are listed of. Recently, the technique of spatially resolved cross-correlation spectroscopy (CCS) (with a resolution in the sub-ns and sub-mm range) has been used to contribute to a better understanding of these discharges. The application of the CCS method is described in more detail.

Properties of the dielectric barrier discharge in mixtures of methane with helium

The influence of the chemical composition of the feeding gas mixtures CH4-He on the physical properties of dielectric barrier discharge was investigated experimentally as well as theoretically. Certain changes in the shape of charge-voltage characteristics (Lissajous figures) were observed when the content of He was varied within the range 0–90%. An increase of the He concentration was found to cause monotonous but not linear decrease of the ignition voltage as well as of the burning voltage of the discharge. Measurements of the burning voltage were used to estimate the characteristic energy of electrons in the mixtures under consideration. These estimates were compared with experimentally determined values of the energy consumption for the synthesis of C2H2, C2H4 and C2H6, respectively. It was demonstrated that the observed dependencies of the energy consumption upon He content can be explained taking into consideration the influence of the chemical composition of feeding gas mixture upon the characteristic energy of electrons in the dielectric barrier discharge.

Cross-correlation spectroscopy in investigations of filamentary gas discharges at atmospheric pressure

The technique of spatially resolved cross-correlation spectroscopy is used to carry out diagnostic measurements of the barrier discharge in air at atmospheric pressure. Quantitative estimates for electric field strength E(x,t) and for relative electron density ne(x,t)/nemax are derived from the experimentally determined spatio-temporal distributions of the luminosity for the spectral bands of the 0-0 transitions of the 2nd positive system of N2 (? = 337.2 nm) and the 1st negative system of N2+ (? = 391.5 nm). Experimental results related to the investigations of coronas and pulsed spark discharges are briefly reviewed.