Particulates formed by a stabilized high voltage spark discharge

Light scattering was used to demonstrate the presence of small (?1 μm) particles in the immediate surroundings of a stable spark discharge. Several parameters, including electrode composition and surface condition, which are of importance in emission spectrochemical analysis, were found to be of importance in affecting the scattering signal. Electron micrographs of the heaviest particles revealed two distinct types of particles. Analytical implications are discussed.     

Effects of anodic spark oxidation by pulse power on titanium substrates

The characteristics of the oxide layer of titanium generated by anodic spark oxidation are affected significantly by the process variables. In this study, electrochemical treatments were performed while applying a direct current, a pulse current, and a reverse pulse current during anodic spark oxidation. A mixed solution of 0.015 M DL‐α‐GP (DL‐α‐glycerophosphate disodium salt) and 0.2 M CA (calcium acetate) was used as the electrolyte. The pore size generated after anodic spark oxidation was smallest in the group exposed to the reverse pulse current followed in order by the pulse current and direct current. Anatase was the major crystal phase of the TiO₂ produced on the surfaces subjected to 280 V, and the rutile phase was additionally detected in the group subjected to 320 V. The crystals precipitated on the surface after the hydrothermal treatment were hydroxyapatite (HA) crystals that had a polygonal bar‐shaped needle structure. Good activity was observed in the 320‐V pulse treated group, in which very thin needle‐shaped crystals were observed after immersing the samples in Hanks' solution for 4 weeks. The cell viability was increased greatly by anodic spark oxidation, and the surface roughness was also increased. It is believed that the surface treated using a pulse current has excellent characteristics, making it suitable for applications in biomaterials. Copyright © 2010 John Wiley & Sons, Ltd.     

Time-Resolved Spark-Source Mass Spectrometry; The Effect Of Spark Duration On Ion Intensity And Precision Of Analysis

Abstract

A radio-frequency probe and a synchronized electronic ion-beam chapper have been used to determine ion-intensity variation with spark duration. Large peaks in multiply charged ion intensities are detected in the early stages of the radio-frequency spark, while later in the spark period the singly charged species predominate. The effects of these variations on the precision of analysis have been determined. A gate was selected in the spark pulse, and a factor of two increase in precision was obtained.     

A Compact,High Speed Ion Beam Chopper

Abstract

A simple, compact pulse circuit was designed for use as an ion beam chopper with both synchronous and asynchronous triggering. The rise and fall times of the beam-on pulse are 60 ns and 320 ns, respectively, with a beam suppression potential of 180 V. The beam chopper was interfaced to a spark-source mass spectrometer for which the precision of analyses can be improved by reducing effects of sample or spark inhomogeneities. Additionally, the synchronous mode of chopping provides a means for temporal analysis of the spark.     

Effects of 50 Hz sinusoidal magnetic fields and spark discharges on human lymphocytes in vitro

Human peripheral blood lymphocytes were exposed in vitro to 50 Hz sinusoidal magnetic fields of 30 μT, 300 μT, and 1 mT, and were monitored for genotoxic effects. No effects on chromosomal aberrations, micronuclei or proliferation indices were noticed. A weak effect on sister chromatid exchanges, noted in one series of experiments, could not be verified when the series was repeated. Exposure of whole blood to spark discharges of up to 3.65 kVcm⁻¹ and 2.6 μs duration (10 pulses) did not result In chromosomal aberrations. Cell destruction from spark discharges was extensive. The conductivity of blood during a spark discharge pulse was examined and was shown to decrease by a factor of 2–3 at the beginning of the pulse owing to the β-relaxation of cells. After 0.5 μs the conductivity approached a constant level.     

A High-Efficiency Reactor for the Pulsed Plasma Conversion of Methane

The authors recently developed a high-frequency pulsed plasma process for methane conversion to acetylene and hydrogen using a co-axial cylindrical (CAC) type of reactor. The energy efficiency represented by methane conversion rate per unit input energy has been improved so that such a pulsed plasma has potential for commercial acetylene production. A pulsed plasma consists of a pulsed corona discharge and a pulsed spark discharge. Most of energy is injected over the duration of the pulsed spark discharge. Methane conversion using this kind of pulsed plasma is a kind of pyrolysis enhanced by the pulsed spark discharge. In this study, a point-to-point (PTP) type of reactor that can produce a discharge channel over the duration of a pulse discharge was used for the pulsed plasma conversion of methane. The energy efficiency and carbon formation on electrodes have been improved. The influences of pulse frequency and pulse voltage on methane conversion rate and product selectivity were investigated. The features of methane conversion using PTP and CAC reactors were discussed.     

Statistical evaluation of single sparks

Single spark evaluation (SSE) needs time resolved multispectral detection of individual emission intensities from the spark plasma. The classification and statistical evaluation of these data, typically 300 data per second and spectral channel have to be processed. Experiments with metal samples have shown that the single spark pulse height distributions can be satisfactorily approximated by a Gaussian function if the respective element is homogeneously distributed within the sample. If there are nonhomogeneities of the analyte concentration in the sample, such as inclusions, than significant deviations from the Gaussian distribution are observed. The integrals over the Gaussian and non-Gaussian parts can be used to quantify the respective components of the analyte. This procedure could be succesfully applied to calibrate the contents of soluble and insoluble Al in a set of steel samples. SSE allows also the calculation of correlations between pulse height distributions of different emission lines. One finds a range from anticorrelation to correlation for various line combinations. This information can help to select reference lines for improving the precision.     

Observations on the formation of spark discharge and its influence by an external magnetic field

Described are a series of experiments executed to characterize the physical and chemical nature of spark discharge in argon at atmospheric pressure. Results are reported concerning the expansion properties of the spark and the distribution and longevity of cathode material in and around the spark channel following cessation of the discharge current. The nature of a previously reported symmetrical post-discharge torus is studied at high spark repetition rates. The interaction of residual vapor on following sparks is investigated. Basic features of the spark process are generalized with a set of Schlieren photographs that show bulk motion of the spark plasma. Time-resolved and space-resolved emission and absorption experiments on magnesium electrodes suggest a charged nature to the post-discharge environment. The bulk plasma character is probed further with external magnetic fields to determine the existence of charged species within the post-discharge torus, and the effects of magnetic forces on its formation.     

Pulsed Discharge Regeneration of Diesel Particulate Filters

A novel method for the removal of soot from a diesel particulate filter using pulsed electric discharges is presented. High voltage pulses of between 18 and 25 kV of nano to microsecond duration and with pulse energies of typically 100–200 mJ were applied to the filter via a series spark gap. Initial slow erosion of the soot layer proceeds via the formation of microdischarges. Subsequent spark discharges removed the accumulated soot more effectively from a larger filter volume. Average soot removal rates of ～0.1–0.2 g/min were achieved at 50 Hz breakdown frequency by optimizing both electrode geometry and breakdown voltage. On-engine long term testing of the technology showed soot removal by pulsed discharge to be reliable, efficient and uniform; a total of 100 g of soot was deposited and removed over 18 filter regeneration cycles.     

Chemical effects of self-sustained spark discharge: Simulation of processes in a liquid

A numerical model has been proposed for the formation of active species in water by the action of pulsed UV radiation generated in self-sustained spark discharge with an energy per pulse of 5.9 × 10^?2 J, a pulse duration of 100 μs, and a pulse repetition frequency of 10 Hz. The primary products formed in water are $HO_2^ \bullet$ radicals, O^· atoms, and N₂O molecules. The yield of nitric acid and low (experimentally undetectable) yields of ozone and hydrogen peroxide are described in terms of the model. The concentrations of the active species in water have been calculated. The results can be used in biophysical research.     

Synthesis and Microstructure of Anodic Spark Deposited SrZrO3

 The advanced plasma electrochemical process of anodic spark deposition was used to prepare polycrystalline strontium zirconate layers on metal surfaces by an anodic conversion of the zirconium metal substrate and the metal ions in an aqueous electrolyte. On the metal substrate a thin passivating barrier film of zirconia first forms, which subsequently changes to the perovskite structure of SrZrO₃ during the anodic spark process. The typical surface morphology was characterized by scanning electron microscope. X-ray diffraction studies showed that the layers are composed of polycrystalline strontium zirconate and microcrystalline phases of zirconium oxides. The chemical state of zirconium was investigated by X-ray photoelectron spectroscopy.     

Preparation of nanosized nickel powder by direct-current electrolysis combined with high-voltage spark discharge

The possibility of obtaining nanosized nickel powder by simultaneous dc electrolysis and high-voltage spark discharge has been explored. The powder has been prepared from a nickel sulfate-containing electrolyte using soluble nickel anodes at a cathodic current density in the range of 11000–19000 A/m², a voltage on the spark gap of 12–16 kV, and a pulse repetition frequency of 0.5 Hz. The effect of various factors on the synthesis of the powder has been studied and the optimum conditions for its preparation have been found using mathematical experiment design. The size distribution of particles has been determined with a particle size analyzer. The BET specific surface area of the powder has been measured to be 11.7 ± 2.1 m²/g.     

The formation of ozone and UV radiation from high-power pulsed electric discharges

High-power electric discharges with pulse energies of from 0.15 J to 4 kJ were studied. The yields of UV photons and ozone were found to be approximately equal, which led us to conclude that discharge conditions under which UV radiation and ozone fully destroyed each other were possible. If ozone formation was suppressed, as when a negative volume charge was created in the spark gap region, the flux of UV photons reached 3 × 10²³ photons/(cm² s).     

Ignition of Quiescent Lean Propane–Air Mixtures at High Pressure by Nanosecond Repetitively Pulsed Discharges

We present an experimental study of lean mixture ignition by nanosecond repetitively pulsed (NRP) discharges. The plasma is created in a lean propane/air mixture at pressure up to 10 bar and equivalence ratio 0.7, premixed in a constant volume vessel. We characterize the initial spark radius, the ignition kernel development and the flame propagation as a function of pressure (up to 10 bar) and the pulse energy (1–6 mJ per pulse). Comparisons with a conventional igniter show that better results are obtained with NRP discharges in terms of flame propagation speed, in particular at high pressure, due to the increased wrinkling of the flame front that is induced by NRP discharges.     

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.     

High Voltage Spark Excitation of Some Organic Molecules in Nitrogen and Argon Atmospheres

Abstract

A simple high voltage ac spark produced between tungsten electrodes in both nitrogen and argon atmospheres is investigated as an emissive detector for certain organic molecules. Volatile vapours entering the electrode gap lower the resistance and an increased current flows in the spark. The energy available, particularly using nitrogen as a carrier gas, is sufficient to excite UV/visible spectra for various organic compounds; carbon species may be detected at 389 nm (CN), sulphur compounds at 257 nm (CS), halogenated compounds at various tungsten atomic lines and iodine at 206.2 nm (iodine atomic).     

The Pyrolysis Property of a Pulsed Plasma of Methane

This study examined the discharge property of methane in a pulsed plasma using single and sequential pulse modes. A pulsed corona discharge occurred, followed by a pulsed spark discharge. An equivalent single channel model (ESC model) for the pulsed plasma of methane was established. The maximum temperature of methane in the discharge channel was estimated. The pyrolysis property of the channel methane was estimated from Senkin simulation and single channel discharge experiment.     

Rapid dissolution of metal in aqueous media using a modified commercial spark source unit

In this preliminary study the feasibility of a modified spark source was investigated as a method of rapidly dissolving solid conductive samples in aqueous media prior to analysis by atomic spectroscopy. The spark source, originally designed for spark emission spectroscopy in air, was modified by the installation of a spark ablation vessel. This spark ablation vessel was designed for spark ablating samples in aqueous solution, such as deionised water. Samples such as mild steel and brass were ablated in 5–10 ml of deionised water for 2–30 s producing a colloidal suspension. The suspension was readily dissolved by adding 100 μl of concentrated HCl or HNO₃. In this paper the spark ablation vessel is described as well as some of the properties of spark ablation in aqueous solutions. Spark ablation rates on mild steel were measured with respect to spark ablation parameters such as applied current (power), polarity and spark time. Using mild steel as a test sample, spark ablation rates varied from 40 μg s⁻¹, with 2.5 A of applied current, to 70 μg s⁻¹ with 10 A of applied current to the electrodes. The feasibility of using this technique for analysing trace levels (μg g⁻¹) of elements in solid samples was also demonstrated for elements such as Ni in brass with inductively coupled plasma mass spectrometry (ICP-MS). Quantification of selected elements (Cr, Ni, Mn and Cu) in a certified alloy (SRM 663) and a non-certified stainless steel showed good agreement between the measured values using spark ablation and the accepted values.     

Applications of spark-source mass spectrometry for localized microanalysis,film analysis and depth analysis

Spark-source mass spectrometry, which is a typical bulk analysis technique, can also be used to study lateral profiles extending over millimeter distances, and for in-depth, layer and microsample analysis. Such analyses require knowledge of the influence of instrumental parameters such as breakdown voltage, pulse repetition frequency, pulse length and slit settings on the material consumed per spark pulse or per unit time, on the crater dimensions and on the ion yield. A study for over 20 matrices is reported. Applications are for lateral analysis (diffusion profile of beryllium in a Cu/CuBe diffusion couple), in-depth analysis (diffusion profile of nickel in copper), layer analysis (carbon in 3-μm gold layers, 3-μm GaAs epitaxial layer) and of microsample analysis (water residues). The effect of the amount of material sampled per analysis on the obtainable precision is demonstrated.     

Low Cost Compact Nanosecond Pulsed Plasma System for Environmental and Biomedical Applications

Nanosecond pulsed non-thermal atmospheric-pressure plasmas are promising for numerous applications including air and water purification, ozone synthesis, surface sterilization, material processing, and biomedical care. However, the high cost of the nanosecond pulsed power sources has hindered adaptation of the plasma-based technologies for clinical and industrial use. This paper presents a low cost (<100US$) nanosecond pulsed plasma system that consists of a Cockcroft–Walton high voltage charging circuit, a compact nanosecond pulse generator using a spark gap as switch, and a plasma reactor. The nanosecond pulse power source requires only a 12 V DC input, hence is battery operable. Through the optimization of the experimental parameters, pulses with a peak voltage >10 kV, a 3 ns rise time (10 to 90 %), and a 10 ns pulse duration (full width at half maximum) at a pulse repetition rate of up to 500 Hz were achieved in the present study. It has been successfully tested to power three different plasma reactors to form pulsed corona discharges, dielectric barrier discharges, and sliding discharges. The energy efficiency of such a nanosecond pulsed sliding discharge system was assessed in the context of ozone synthesis using air or oxygen as the feed gas, and was found comparable to a previously reported non-thermal plasma system that used commercial high voltage pulsed power sources. This study demonstrated that this low-cost nanosecond pulsed power source can prove to be an energy efficient and simple supply to drive various non-thermal atmospheric-pressure plasma reactors for environmental, medical and other applications.