Scaling of Shielded Sliding Discharges for Environmental Applications

Shielded sliding discharges are nanosecond streamer discharges which develop along a dielectric between metal foil electrodes, with one of the foils extended over the entire rear of the dielectric layer. The electrode configuration not only allowed rearranging discharges in parallel due to the decoupling effect of the metal layer, but also to modify the electric field distribution in such a way that components normal to the surface are enhanced, leading to an increased energy density in the discharge plasma. By varying the electrode gap, the applied voltage, and the repetition rate, it is shown that by keeping the average electric field constant, the discharge voltage can be reduced from tens of kV to values on the order of a few kV, but only at the expense of a reduced energy density of the plasma. Varying the repetition rate from 20 to 500 Hz resulted in a slightly reduced energy per pulse, likely caused by residual charges on the dielectric surface. Measurements of the NO conversion to NO₂ and ozone synthesis in dry air showed that the conversion is only dependent on the energy density of the discharge plasma. Although reducing the pulse voltage from the tens of kV range to that of few kV, and possibly even lower, causes a reduction in energy density, this loss can be compensated for by increasing the electrode gap area. This and the possibility to form discharge arrays allows generating large volume discharge reactors for environmental applications, at modest pulsed voltages.     

Analysis and Review of Chemical Reactions and Transport Processes in Pulsed Electrical Discharge Plasma Formed Directly in Liquid Water

Electrical discharges formed directly in liquid water include three general cases where (a) streamer-like plasma channels form in, but do not span, the electrode gap, (b) spark discharges produce transient plasma channels that span the electrode gap, and (c) arc discharges form plasma channels with relatively longer life times. Other factors including the input energy (from <1?J/pulse to >1?kJ/pulse) as well as solution properties and the rates of energy delivery affect the nature of the discharge channels. An understanding of the formation of chemical species, including the highly reactive hydroxyl radical and more stable molecular species such as hydrogen and hydrogen peroxide, in such plasma requires determination of temporal and spatial variations of temperature, pressure, plasma volume, and electrical characteristics including current, voltage (electric field), and plasma conductivity. In spark and arc discharges analysis of the physical processes has focused on hydrodynamic and thermal characterization, while only a limited amount of work has connected these physical processes to chemical reactions. On the other hand, the most successful model of the chemical reactions in streamer-like discharges relies on simple assumptions concerning the temperature and pressure in the plasma channels, while analysis of the physical processes is more limited. This paper reviews the literature on the mathematical modeling of electrical discharges in liquid water spanning the range from streamer-like to spark and arc discharges, and compares the properties and processes in these electrical discharges to those in electron beam radiolysis and ultrasound.     

Parameters of an Electric Discharge in Electrolytes and Physicochemical Processes in an Electrolyte Plasma

The parameters of an atmospheric-pressure electric discharge across an active metal electrode immersed in an electrolyte and the electrolyte surface (the thickness of the plasma sheath produced around the active electrode, the distribution of electric potential in the sheath, electrolyte temperature near the sheath boundary and electrode surface) were measured, and the plasma parameters (electric field strength, mean energy and number density of electrons) were determined. It was shown that the plasma of these atmospheric-pressure discharges is nonequilibrium and the discharge in electrolytes is a new type of electric discharge.     

一种用于高场非对称波形离子迁移谱系统的新型敞开式化学离子源

根据高场非对称波形离子迁移谱(FAIMS)系统的要求, 本文提出了一种新型敞开式直流电晕放电化学离子源. 该离子源主要由内线电极、外筒电极和牵引电极组成, 内、外电极半径分别是0.08、2 mm. 筒壁电极上开有对称的4个槽, 用于通入样品和牵引离子. 质谱实验结果表明, 该离子源能够在敞开环境下很好地离子化丙酮、乙醇、苯胺、N,N-二甲基甲酰胺、甲基磷酸二甲酯(DMMP)、乙酸乙酯、甲酸、乙酸、苯酚等正、负电性物质. 静电计测试实验结果说明该离子源能够稳定地产生离子电流. 通过分析不同时刻的谱图发现, 在不同时间点上产生的主要离子相同, 具有很好的稳定性. 利用感应耦合等离子体(ICP)工艺在硅片上加工了该离子源, 从而验证了该结构可以由微机电系统(MEMS)加工技术实现. 该离子源具有体积小、结构简单、无辐射、工作稳定等特点, 不仅可以满足FAIMS系统的要求, 还可用于敞开式质谱、微型质谱仪、离子迁移谱(IMS)等仪器.     

Parametric studies of emission from a plasma gun source for atomic spectroscopy

A detailed parametric study of the effects of electrical discharge parameters on the spatially and temporally resolved emission from the expelled plume of a plasma gun atomic emission source was performed. Expulsion properties were found to depend on the dI/dt of the discharge current. Low-inductance discharges produced plasmas with propagation velocities (about 1.2 km s⁻¹) which were independent of discharge energy and probably controlled by the properties of the support gas in the discharge chamber. Added-inductance discharges produced plumes with significantly lower propagation velocities and less efficient atomization and expulsion of analyte. Continuum background emission was attributable to two processes, depending upon the spatial location in the plasma: decaying background from the rapidly cooling expelled plasma (lower regions) and collision of the initially-expelled plasma with some of the continuously-expelled plasma gun vapor (upper regions). The line-to-background ratio is optimized when using the highest-energy, low-inductance discharges coupled with integration of emission only from the upper regions of the plume (>10 mm) after the first emission pulse (> 75 μs).     

Characterization of a glow discharge plasma as a function of sampling orifice potential

Plasma diagnostic studies have been carried out on the discharge source of a commercial glow discharge quadrupole mass spectrometer. Plasma parameters were determined using an electrostatic probe with the objective of determining the dependence (if any) of these parameters on the voltage placed on an auxiliary electrode immersed in the plasma. The biased electrode utilized in this study was the sampling orifice element itself. Our results indicate that, for positive orifice voltages with respect to the grounded anode, variations in the plasma potential and ion energy can be correlated directly to the bias placed on the sampling orifice. The dependence of the electron temperature on this parameter is observed to be more complex in nature, and electron number densities show little significant variation with respect to sampling orifice bias. In addition, increased orifice voltages result in an increase in the ion signal intensity measured with the mass spectrometer. Based on the results obtained here, we feel that this increase is due primarily to an increase in ion transmission to the quadrupole arising from the increased ion energy.     

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.     

Nitric Oxide Conversion and Ozone Synthesis in a Shielded Sliding Discharge Reactor with Positive and Negative Streamers

Positive and negative streamer discharges in atmospheric pressure air were generated in a shielded sliding discharge reactor at operating voltages as low as 5 kV for a gap length of 1.6 cm. In this reactor, electrodes are placed on top of a dielectric layer and one of the electrodes, generally the one on ground potential, is connected to a conductive layer on the opposite side of the dielectric. The energy per pulse, at the same applied voltage, was more than a factor of seven higher than that of pulsed corona discharges, and more than a factor of two higher than that of sliding discharges without a shield. It is explained on the basis of enhanced electric fields, particularly at the plasma emitting electrode. Specific input energy required for 50 % removal from ~1,000 ppm initial NO could be reduced to ~18 eV/molecule when ozone in the exhaust of negative streamers was utilized. For sliding discharges and pulsed corona discharges this value was ~25 eV/molecule and it was 35 eV/molecule for positive shielded sliding discharges. Also, the ozone energy yield from dry air was up to ~130 g/kW h and highest for negative streamer discharges in shielded sliding discharge reactors. The high energy density in negative streamer discharges in the shielded discharge reactor at the relatively low applied voltages might not only allow expansion of basic studies on negative streamers, but also open the path to industrial applications, which have so far been focused on positive streamer discharges.     

Combined electrical discharges and their applications in atomic emission spectrometry of powdered samples

Certain features of ordinary combined discharges which represent the superposition on a burning d.c. arc of an impulse discharge; and double combined discharges when two impulse discharges with a controlled time interval between them are imposed on a burning d.c. arc were studied. Spectra of electrode material and of dried residue sample (dry residue of CuSO₄·5H₂O solution) excited using these discharges are compared. The utilisation of the combined discharge source for the determination of ordinary (Se, P, As, Zn) and “poorly excited” (Cl) elements and a special case—the determination of boron isotopic composition are also discussed.     

Influence of Plasma Regimes and Catalysts on Ethanethiol Oxidation

Plasma oxidation of ethanethiol in air was investigated using three plasma regimes: surface dielectric pulsed corona discharge, surface dielectric barrier discharge and pulsed corona discharge (PCD) in the plasma reactor. Catalytic plasma degradation of ethanethiol was also performed on the singular or binary metals doped ?èCAl₂O₃. The ethanethiol removal rate increased with increasing energy density but energy efficiency was first increased and then decreased with increasing energy density under three various types of discharges. PCD plasma required the lowest energy density at the similar ethanethiol removal performance compared with the other two plasma discharges. The main intermediate by-products of ethanethiol oxidation by plasma are CH₃CHO, HCHO, CO and CO₂. The sum of these intermediate products selectivities is 19?C43?%, implying that some other intermediates containing carbon were undetermined. When using PCD plasma combined with catalysts, ethanethiol removal rate and energy efficiency were all evidently improved. The maximum energy efficiency was achieved about 200?g kWh^?1 using Fe?CMn/?èCAl₂O₃ assisted PCD plasma, which was about 4.4 times when using PCD plasma alone. The mechanism of ethanethiol oxidation is also discussed.     

Application of infrared spectroscopy to monitoring gas insulated high-voltage equipment: electrode material-dependent SF(6) decomposition

Sulfur hexafluoride is a chemically inert gas which is used in gas insulated substations (GIS) and other high-voltage equipment, leading to a significant enhancement of apparatus lifetime and reductions in installation size and maintenance requirements compared to conventional air insulated substations. However, component failures due to aging of the gas through electrical discharges may occur, and on-site monitoring for risk assessment is needed. Infrared spectroscopy was used for the analysis of gaseous by-products generated from electrical discharges in sulfur hexafluoride gas. An infrared monitoring system was developed using a micro-cell coupled to an FTIR spectrometer by silver halide fibers. Partial least-squares calibration was applied by using a limited number of optimally selected spectral variables. Emphasis was placed on the determination of main decomposition products, such as SOF(2), SOF(4), and SO(2)F(2). Besides the different electrical conditions, the material of the plane counter electrode of the discharge chamber was also varied between silver, aluminum, copper, tungsten, or tungsten/copper alloy. For the spark experiments the point electrode was the same material as chosen for the plane electrode, whereas for partial discharges a stainless steel needle was employed. Complementary investigations on the chemical composition within the solid counter electrode material by secondary neutral mass spectrometry (SNMS) were also carried out. Under sparking conditions, the electrode material plays an important role in the decomposition rates of the gas-phase, but no relevant material dependence could be observed under partial discharge conditions.     

Synthesis of Ozone in a Surface Barrier Discharge with a Plasma Electrode

The synthesis of ozone from oxygen in a cylindrical ozonizer operating under surface discharge conditions with a plasma electrode was studied. The conditions of ozone synthesis were optimized. The dependence of ozone concentration and specific energy consumption on gas pressure in the plasma electrode and on distance between the coils of a corona electrode was determined. The results were compared with data obtained with the use of classical surface barrier discharge.__________Translated from Khimiya Vysokikh Energii, Vol. 39, No. 4, 2005, pp. 307–311.Original Russian Text Copyright © 2005 by Alemskaya, Lelevkin, Tokarev, Yudanov.     

电晕放电二氧化碳冷等离子体转化特性研究

在常压、室温条件下利用电晕放电使二氧化碳通过冷等离子体反应分解为一氧化碳和氧气,由四极质谱在线定量在分析产物组成。考察反应条件（电晕类型、能量密度、气体流量等）对反应转化率的影响,分析了该反应的能量效率。当放电功率为40W、CO2流量为30mL.min^-1时,正电晕等离子体CO2分解反应的转化率为15．2％;CO2,专座经率随体系能量密度的增加上升,随反应时间的增加而增大,当CO2流量为90mL.min^-1、正电晕放电功率为37．6W时,反应体系的能量效率为5．89％。实验发现,正电晕放电时CO2的转化率高负电晕的转化率。     

Argon Versus Helium Dielectric Barrier Discharge for Surface Modification of Polypropylene and Poly(methyl methacrylate) Films

A comparison is made for the helium and argon dielectric barrier discharge, in symmetrical electrode configuration, and on the plasma-induced effects, under He and Ar gaseous environment, on polypropylene and poly methyl methacrylate films, selected as polymers bearing distinct structure, functionality, polarity, degree of oxidation and crystallinity, thus allowing an analysis on the role of plasma active species created in inert gas environment in the surface mechanisms and assessment on the discharge efficiency in surface treatment. It is shown that He allows more fine tuning of the discharge parameters and control over the discharge energy, offering overall a larger range of variation both for plasma conditions and surface properties. The effects of the two discharges are similar at the material surface, whereas He-DBD induces a higher level of modification within the depth of the material. Although the amount of activated oxidizing species is lower in Ar-DBD, it conducts to comparable surface modification, under, indeed, much limited plasma parameters, which points to the role of oxygen due to residual air. The nature of the polymer, mainly the presence of intrinsic oxidized functionalities in its structure, limits only the overall level of modification, whereas the discharge controls the timescale and the mechanisms. Helium offers indeed some obvious advantages to produce and stabilize the discharge at atmospheric pressure, due to particular discharge physics. Nonetheless, the discharge parameters and the surface properties for Ar-DBD treated samples show a good balance, supporting the substitution of He with Ar for surface processing applications.     

Characterization of a circular thin film plasma source for atomic emission spectroscopy

A plasma source for analytical atomic emission spectroscopy is described based on the electrical vaporization by capacitive discharge of a thin Ag film deposited on a polycarbonate membrane filter. The source is designed for the rapid, direct analysis of solid powder samples collected by filtration from fluid media. A concentric electrode system consisting of a ring-shaped graphite electrode placed on the thin film surface and a pointed graphite rod located under the membrane substrate results in a plasma with cylindrical symmetry and a radial current path. Discharge current vs time and intensity vs time profiles are compared for the concentric electrode geometry and the linear geometry used in previous studies. Two values of tank circuit inductance also are compared. Both neutral-atom and ion line radiation from an Mn sample are more intense when the center electrode is initially cathodic. The inside diameter of the ring-shaped electrode and thus the surface area of the Ag film exposed to the plasma have relatively little effect on the intensity of continuum background and line radiation from a sample deposited near the center of the film. Particle size effects, while significant, are smaller than with the linear electrode geometry. Analytical curves are presented for several lines using both a low-inductance and a high-inductance discharge. Log-log slopes range from about 0.85 to 1.05 for ion lines. Detection limits are somewhat poorer than with the linear geometry.     

储氢合金电极的表面修饰研究

利用等离子体技术对AA型MH/Ni电池的储氢合金电极进行了镀覆导电膜层的表面修饰,用XRD及SEM对电极结构进行了表征.极片经过表面修饰的电池,其内阻降低了24％,放电容量有了明显提高,5C (7.5 A) 放电容量提高了200 mA•h,放电平台电压提高了约0.14 V,导电膜层还起到了电极保护层的作用,抑制了合金的粉化,提高了电池的循环稳定性.同时,电池内压显著降低,电池性能有了较大改善.     

Transformation of Organic Solvents into Carbon-Based Materials by Liquid-Phase Plasmas

The possibility of modifying chemical properties of organic liquids under the influence of strong electric fields created by pulsed electrical discharges with energies on the order of J/pulse is quite intriguing. Considering the majority of carbon-based materials today are almost exclusively synthesized from gaseous precursors, the realization of this process even seems necessary. The goal of this study was to examine the possibility of synthesizing carbon materials using streamer-like electrical discharges in three different organic liquids: methanol, acetone and pentane. The morphology of the deposited carbon was imaged using scanning electron microscopy whereas energy dispersive X-ray spectroscopy was used to analyze chemical composition of the resulting films and particles. The results have shown that electrical discharges in all three liquids result in the carbon deposition on the high-voltage electrode. Depending on the type of the organic liquid, the thickness of the carbon layer deposited on the electrode can rise 10?C70?% above baseline levels and the deposition is accompanied by a change in surface morphology of the electrode. Electrical discharges in acetone and pentane also result in the deposition of solid carbon particles in the bulk liquid. The mechanism for the formation of solid carbon byproducts was correlated with chemical reactions in plasma.     

Compensation voltage (CV) peak shapes using a domed FAIMS with the inner electrode translated to various longitudinal positions

High-field asymmetric waveform ion mobility spectrometry (FAIMS) separates ions at atmospheric pressure based on the difference in the mobility of an ion in a strong electric field and in a weak electric field. This field-dependent mobility of an ion is reflected in the compensation voltage (CV) at which the ion is transmitted through FAIMS at an applied asymmetric waveform dispersion voltage (DV). In this report, we show that experimental CV peak shapes using dome tipped inner electrode FAIMS prototypes with inner/outer electrode radii of: (1) 0.2/0.4 cm and (2) 0.4/0.6 cm are a function of the longitudinal position of the inner electrode. Varying the longitudinal position of the inner electrode modifies the electric fields between the surfaces of the hemispherical shaped inner electrode and the outer electrode in the vicinity of the ion outlet. In this region the position-dependent electric field strength (E/N) effectively forms a second tandem FAIMS analyzer region having differing ion separation properties. The final tandem FAIMS separation is the intersection of the CV windows of these two differing FAIMS separations and, therefore, the peak width in the CV scan is dependent on the longitudinal tip displacement (LTD) of the inner electrode. CV scans are shown for a LTD range of 0.14 to 0.4 cm. These scans illustrate that it is possible to control the FAIMS resolution (CV/peak width) from about 1 for the 0.2/0.4 cm electrode set at intermediate longitudinal position to over 10 at the narrowest distance between the inner electrode and the ion outlet.     

Study of high-voltage breakdown and material consumption in spark-source mass spectrometry and their significance in analytical applications

The breakdown voltage has been found to be dependent on the gap width between the electrodes and on the melting point of the sample elements in spark-source mass-spectrometry (SSMS). The number of discharges per pulse train and the time required to reach the first discharge depend only on the chosen breakdown voltage. The spark gap is proportional to the "radius" of the volume sampled (for a given element) and this radius is linearly related to the reciprocal of the melting point of the elements (23 different elements, metals or semiconductors), when fixed spark-parameters are used. The effect of electrode temperature on material consumption can be qualitatively explained by a fictive increase or decrease in melting point of the element. Knowledge of the relations between the different spark and instrumental parameters and the volume or weight of sample consumed can be applied to the study of the homogeneity of samples, to in-depth analysis by SSMS and to the analysis of microsamples.     

Deposition of polyacrylic acid films on PDMS substrate in dielectric barrier corona discharge at atmospheric pressure

This paper reports on deposition of acrylic acid films polymerized by an efficient and cost‐effective technique of dielectric barrier corona discharge at atmospheric pressure. The liquid acrylic acid was vaporized and carried by argon gas into plasma to deposit polyacrylic acid films on polydimethylsiloxane substrate. A nonthermal corona discharge was generated in a pyrex flask using a steel tube‐to‐plate asymmetric electrode configuration. The plasma was excited using an in‐house developed power supply operating with continuous wave signals of 10‐kHz frequency. The emission spectra of plasma species were recorded to know their contribution during deposition process. The deposited surfaces were characterized using contact angle measurements, atomic force microscopy, Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy and film thickness measurements. A maximum film growth rate of 363 nm/min was achieved under optimal condition of discharge. The results suggest that this plasma technique is capable of depositing organic coatings with a high concentration of carboxylic functional groups that could be potentially used for biomedical and microfluidic applications.