Comparison of two cold atmospheric pressure plasma jet configurations in argon

The present study compares the operation of two cold atmospheric plasma jet (CAPJ) configurations: needle-to-cylinder electrode configuration (CAPJ I) and single high-voltage cylinder electrode around the quartz tube (CAPJ II). The CAPJs were operated in argon flowing through a quartz capillary with 0.5-mm inner diameter into the ambient air, and the plasma was generated by sinusoidal kHz frequency AC power supplies. The main emphasis of the study was on the mechanism of the initiation of ionization waves for these two configurations. For both CAPJs, there appeared several ionization waves during one half-period of the applied voltage waveform, and the number of ionization waves increased at higher voltage amplitudes. However, we discovered marked differences in the initiation of the ionization waves for two different CAPJ configuration. The applied voltage controlled the initiation of consecutive ionization waves, which propagated from the grounded electrode towards the tube orifice in CAPJ I. In the case of CAPJ II, certain time had to pass for the initiation of a new ionization wave, and subsequent ionization waves within the same half-period started at the tube orifice. In addition to the differences in the initiation of the ionization waves, we observed that the CAPJ I was ignited and sustained at lower voltages, while CAPJ II produced a longer plasma jet. The observed advantages and deficiencies of investigated CAPJ configurations point out their potential in different applications.     

Deposition of colloidal gold nanoparticles by fully pulsed-voltage-controlled electrohydrodynamic atomisation

Pulsed electrohydrodynamic atomisation (EHDA) of aqueous 10 nm gold colloid in a full voltage-controlled form was investigated. By using 4 µm and 20 μm nozzles, electrified fluid jet was emitted and Au nanoparticles in the jet were deposited onto a silicon substrate. Scanning electron microscopy (SEM) revealed that different morphology of the artifact was formed by using different voltages pulses. Particularly, island-liked artifact down to 10 μm can be produced regularly in the case of cone-jet mode by low voltage pulse. Our results demonstrate pulsed EHDA is a promising approach in creating micro-patterns of colloid-based nanomaterials.     

Enhancement of VUV Emission from a Coaxial Xenon Excimer Ultraviolet Lamp Driven by Distorted Bipolar Square Voltages

Enhancement of vacuum UV emission (172 nm VUV) from a coaxial xenon excimer UV lamp (EUV) driven by distorted 50 kHz bipolar square voltages, as compared to that by sinusoidal voltages, is investigated numerically in this paper. A self‐consistent radial one‐dimensional fluid model, taking into consideration non‐local electron energy balance, is employed to simulate the discharge physics and chemistry. The discharge is divided into two three‐period portions; these include: the pre‐discharge, the discharge (most intense at 172 nm VUV emission) and the post‐discharge periods. The results show that the efficiency of VUV emission using the distorted bipolar square voltages is much greater than when using sinusoidal voltages; this is attributed to two major mechanisms. The first is the much larger rate of change of the voltage in bipolar square voltages, in which only the electrons can efficiently absorb the power in a very short period of time. Energetic electrons then generate a higher concentration of metastable (and also excited dimer) xenon that is distributed more uniformly across the gap, for a longer period of time during the discharge process. The second is the comparably smaller amount of “wasted” power deposition by Xe⁺₂ in the post‐discharge period, as driven by distorted bipolar square voltages, because of the nearly vanishing gap voltage caused by the shielding effect resulting from accumulated charges on both dielectric surfaces (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A Particle‐in‐Cell Simulation of the Breakdown Mechanism in Microdischarges with an Improved Secondary Emission Model

In this paper, the failure of the breakdown voltage from the Paschen's law at extremely small electrode separations is studied. The electrical breakdown in microgaps occurs at the voltages far below the Paschen curve minimum breakdown limit and the modified Paschen curve should be used. Offered explanation for the departure from the Paschen's law at small gap spacings is based on the increasing of the yield of the secondary electrons. The high electric fields existing in small gaps may enhance the secondary electron yield and this would lead to a lowering of the breakdown voltage and to the departure from the Paschen's law. Particlein‐cell/Monte‐Carlo (PIC/MCC) simulations with a new secondary emission model have been performed to estimate the importance of this mechanism in the discharge breakdown. Obtained simulation results suggest that deviations from the Paschen curve across the micron and submicorn gap spacing can be attributed to the ion‐enhanced field emissions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An atmospheric-pressure nitrogen-plasma jet produced from microdischarges in a porous dielectric

This report presents an atmospheric-pressure nitrogen-plasma jet generated from microdischarges in a porous dielectric. A plasma jet with a length of 42 mm was produced by feeding nitrogen gas through a porous alumina installed between an outer electrode and a hollow inner electrode and by applying 60 Hz sinusoidal voltage wave to the electrodes. Microdischarges in the porous alumina are ejected as a plasma jet from the outer electrode through a 1 mm hole by increasing the applied voltage, showing that the temperature of the jet decreases to a value close to room temperature. Even at a frequency as low as 60 Hz, the plasma that evolves from a large amount of microdischarge inside a porous dielectric can have characteristics that are similar to those generated at several hundreds of kilohertz. From the electrical measurements, it is expected that not only the steady generation but also the frequency of the pulses resulting from the microdischarges in the porous dielectric play an important role in obtaining a stable plasma jet. We also identified the various excited plasma species produced from the plasma jet by an optical emission spectroscopy.     

Development of a new atmospheric pressure cold plasma jet generator and application in sterilization

This paper reports that a new plasma generator at atmospheric pressure, which is composed of two homocentric cylindrical all-metal tubes, successfully generates a cold plasma jet. The inside tube electrode is connected to ground, the outside tube electrode is connected to a high-voltage power supply, and a dielectric layer is covered on the outside tube electrode. When the reactor is operated by low-frequency (6 kHz--20 kHz) AC supply in atmospheric pressure and argon is steadily fed as a discharge gas through inside tube electrode, a cold plasma jet is blown out into air and the plasma gas temperature is only 25--30℃. The electric character of the discharge is studied by using digital real-time oscilloscope (TDS 200-Series), and the discharge is capacitive. Preliminary results are presented on the decontamination of E.colis bacteria and Bacillus subtilis bacteria by this plasma jet, and an optical emission analysis of the plasma jet is presented in this paper. The ozone concentration generated by the plasma jet is 1.0×10¹⁶cm^-3 which is acquired by using the ultraviolet absorption spectroscopy.     

Thorough small‐angle X‐ray scattering analysis of the instability of liquid micro‐jets in air

Liquid jets are of interest, both for their industrial relevance and for scientific applications (more important, in particular for X‐rays, after the advent of free‐electron lasers that require liquid jets as sample carrier). Instability mechanisms have been described theoretically and by numerical simulation, but confirmed by few experimental techniques. In fact, these are mainly based on cameras, which is limited by the imaging resolution, and on light scattering, which is hindered by absorption, reflection, Mie scattering and multiple scattering due to complex air/liquid interfaces during jet break‐up. In this communication it is demonstrated that synchrotron small‐angle X‐ray scattering (SAXS) can give quantitative information on liquid jet dynamics at the nanoscale, by detecting time‐dependent morphology and break‐up length. Jets ejected from circular tubes of different diameters (100–450 µm) and speeds (0.7–21 m s^?1) have been explored to cover the Rayleigh and first wind‐induced regimes. Various solvents (water, ethanol, 2‐propanol) and their mixtures have been examined. The determination of the liquid jet behaviour becomes essential, as it provides background data in subsequent studies of chemical and biological reactions using SAXS or X‐ray diffraction based on synchrotron radiation and free‐electron lasers.     

The Role of the Field Emission Effect in the Breakdown Mechanism of Direct‐Current Helium Discharges in Micrometer Gaps

This paper contains results of experimental studies of the direct current breakdown voltage curves and volt‐ampere characteristics of discharges generated in a system consisting of two plane‐parallel tungsten and molybdenum electrodes at separations from 100 µ m to 1 µ m. The measurements were performed in the pressure range from 22.5 Torr to 738 Torr. The results are presented in the form of Paschen curves. Based on the measured breakdown voltage curves, the effective yields have been estimated in the case of different cathode materials. Differences between them are attributed to the influence of the work function of the cathode material on the current‐voltage characteristics due to field emission effect in small gaps and high pressures. At low‐pressures, however, vaporation of impurities from the electrodes material becomes significant. The present paper delivers new data on DC breakdown under these experimental conditions and conditions on the validity of the Paschen law in helium and provides better insight into the role of the field emission and the electrode materials on the breakdown voltage. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Long‐Time Evolution of a Low Pressure Laboratory Plasma after Application of Transient high Voltage Positive Pulses

The long‐time evolution of weakly‐collisional plasma with application of high voltage positive pulses to an electrode immersed in plasma, with pulse widths less than as well as more than ion plasma periods, is studied. The plasma is produced by electron impact ionization of argon or helium gas, where electrons are coming out from dc biased hot thoriated tungsten filaments. It is observed that during the temporal evolution of argon plasma, a beam component exists along with temporal bulk electrons giving rise to a double hump profile of transient Electron Distribution Function (EDF). However, in the case of temporal evolution of helium plasma, only a bulk electron population is present. The obtained results are explained by understanding the role played by thermionically emitted electrons during the plasma evolution, the role of the difference of ionization rates of helium and argon, and the higher temporal plasma potential. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stochastic and Relaxation Processes in Argon by Measurements of Dynamic Breakdown Voltages

Statistically based measurements of breakdown voltages U _b and breakdown delay times td and their variations in transient regimes of establishment and relaxation of discharges are a convenient method to study stochastic processes of electrical breakdown of gases, as well as relaxation kinetics in afterglow. In this paper the measurements and statistical analysis of the dynamic breakdown voltages U _b for linearly rising (ramp) pulses in argon at 1.33 mbar and the rates of voltage rise k up to 800 V s ^–1 are presented. It was found that electrical breakdowns by linearly rising (ramp) pulses is an inhomogeneous Poisson process caused by primary and secondary ionization coefficients α , γ and electron yield Y variations on the voltage (time). The experimental breakdown voltage distributions were fitted by theoretical distributions by applying approximate analytical and numerical models. The afterglow kinetics in argon was studied based on the dependence of the initial electron yield on the relaxation time Y ₀ (τ ) derived from fitting of distributions. The space charge decay was explained by the surface recombination of nitrogen atoms present as impurities. The afterglow kinetics and the surface recombination coefficients on the gas tube and cathode were determined from a gas‐phase model. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Self‐Referenced Measurement of Light Waves

The wave nature of light has been known for more than 100 years. However, it is still very difficult to directly sample an optical field transient on a time scale below the oscillation period. It has been naturally believed that the field oscillation can be detected only by using a reference pulse that has a shorter duration than the period of the oscillation. In this Letter, an experimental demonstration of a self‐referenced light wave measurement is shown. Frequency‐resolved optical gating capable of carrier‐envelope phase determination was used to measure the complete electric field evolution of a 21‐fs few‐cycle pulse without any reference pulses. A second harmonic generation frequency‐resolved optical gating spectrogram and an interferogram between second harmonic and self‐diffraction signals were simultaneously recorded. The phase‐controlled light waves of few‐cycle pulses were clearly observed by using this method.     

Electrostatic cnoidal waves and soliton structures in multi‐ions plasmas

Using a reductive perturbation technique (RPT), the Korteweg‐de Vries (KdV) equation for nonlinear electrostatic waves in multi‐ion plasmas is derived with appropriate boundary conditions. Furthermore, compressive and rarefactive cnoidal wave and soliton solutions are discussed. In our model, the multi‐ion plasma consists of light dynamic warm ions, heavy cold ions, and inertialess electrons, which follows the Maxwell‐Boltzmann distribution. It is observed that in such an unmagnetized multi‐ion plasma, two characteristic electrostatic waves i.e., slow ion‐acoustic (SIA) waves and fast ion‐acoustic (FIA) waves, can propagate. The results are discussed by considering two types of multi‐ion plasmas i.e., H⁺–O⁺–e plasma and H^?–O⁺–e plasma that exist in space plasmas. It is found that for H⁺–O⁺–e plasma, the SIA cnoidal wave and soliton form both positive (compressive) and negative (rarefactive) potential pulses, which depend on the temperature and density of the light and warm ions. However, only electrostatic positive potential structures are obtained for FIA cnoidal wave and soliton in H⁺–O⁺–e plasma. In the case of H^?–O⁺–e plasma, the SIA cnoidal wave and soliton form only compressive structures, while the FIA cnoidal wave and soliton compose rarefactive structures. The effects of light ions' density and temperature on nonlinear potential structures are investigated in detail. The parametric results are also demonstrated, which are applicable to space and laboratory multi‐ion plasma situations.     

Ultra‐thin optical grade scCVD diamond as X‐ray beam position monitor

Results of measurements made at the SIRIUS beamline of the SOLEIL synchrotron for a new X‐ray beam position monitor based on a super‐thin single crystal of diamond grown by chemical vapor deposition (CVD) are presented. This detector is a quadrant electrode design processed on a 3 µm‐thick membrane obtained by argon–oxygen plasma etching the central area of a CVD‐grown diamond plate of 60 µm thickness. The membrane transmits more than 50% of the incident 1.3 keV energy X‐ray beam. The diamond plate was of moderate purity (～1 p.p.m. nitrogen), but the X‐ray beam induced current (XBIC) measurements nevertheless showed a photo‐charge collection efficiency approaching 100% for an electric field of 2 V µm^?1, corresponding to an applied bias voltage of only 6 V. XBIC mapping of the membrane showed an inhomogeneity of more than 10% across the membrane, corresponding to the measured variation in the thickness of the diamond plate before the plasma etching process. The measured XBIC signal‐to‐dark‐current ratio of the device was greater than 10⁵, and the X‐ray beam position resolution of the device was better than a micrometer for a 1 kHz sampling rate.     

Transient demonstration of exciton behaviours in solid state cathodoluminescence under different driving voltage

In the solid state cathodoluminescence (SSCL), organic materials were excited by hot electrons accelerated in silicon oxide (SiO₂) layer under alternating current (AC). In this paper exciton behaviours were analysed by using transient spectra under different driving voltages. The threshold voltages of SSCL and exciton ionization were obtained from the transient spectra. The recombination radiation occurred when the driving voltage went beyond the threshold voltage of exciton ionization. From the transient spectrum of two kinds of luminescence (exciton emission and recombination radiation), it was demonstrated that recombination radiation should benefit from the exciton ionization.     

Enhanced Device Performances of Blue‐Emitting PLEDs Coupled with Silver‐Nanoicosahedrons

Silver‐nanoicosahedron particles (AgNIPs) are produced by chemical reduction and photochemical methods and doped into the hole transport layer (HTL) or emissive layer (EML) of blue‐emitting polymer light‐emitting diodes (PLEDs) to improve their luminous efficiency. The optimal distributed‐densities of the AgNIPs are determined from current density–voltage–luminance measurements at different doping concentrations. The AgNIP dopant doses that maximize the average luminous efficiency of the proposed PLED are 6.71 µg cm^?2 in EML (achieving 3.48 cd A^?1) and 6.88 µg cm^?2 in HTL (achieving 3.35 cd A^?1). Although the luminous efficiencies of the blue‐emitting PLEDs fabricated by both doping methods are not significantly different, the maximum plasmonic enhancement (around 30‐fold) of the blue‐emitting PLED with AgNIPs in EML is red‐shifted to the green region (≈530 nm in the electroluminescence spectrum), seriously degrading the luminescent monochromaticity of the blue‐emitting PLED. The maximum plasmonic enhancement (around 33‐fold) of blue‐emitting PLED with AgNIPs in HTL occurred at 430 nm, overlapping the localized surface‐plasmon resonance extinctions of the AgNIPs in HTL (425 nm), thus favoring the enhancement of fluorescence emission. Therefore, to enhance the large‐area emission of blue‐emitting PLEDs, the AgNIPs should be doped in the HTL rather than the EML.     

Direct‐current nanogenerator based on ZnO nanotube arrays

Due to the special structure of a nanotube (NT), the different potential from piezoelectricity can be naturally divided by the hollow of the tube when the NT is bent or deformed. Furthermore, both the bent/deformed inner and outer wall can form a steady voltage/current, which might enhance the output voltage/current. We demonstrate a direct‐current nanogenerator (NG) based on zinc oxide (ZnO) NT arrays driven by an ultrasonic wave. The average output voltage is ca. 0.10 mV and the current density is ca. 0.069 μA/mm². Our study shows that the maximum power output of the ZnO NT array NG is 0.112 nW and the power density is 1.4 nW/cm². The success of energy harvesting from ZnO NTs reveals the potential of using nanogenerators for tubular piezoelectric materials. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of the pulse leading-edge time on the dielectric strength of a vacuum gap

Optimal regimes for electrode conditioning in a vacuum by applying voltage pulses with different waveforms are considered. For nanosecond pulses with a constant duration (t _p = const), the impulse dielectric strength for an oblique voltage wave is shown to be more than four times higher than for a rectangular pulse with an infinitely short leading-edge duration. The dependences of the dielectric strength on the conditioning pulse duration in the range 10^?10 < t _p < 10^?3 s for pulses with different rise rates are obtained. The dielectric strength increases from 2 × 10⁷ V/m for microsecond pulses to 10¹⁰ V/m for subnanosecond pulses.     

Plasma response to transient high voltage pulses

This review reports on plasma response to transient high voltage pulses in a low pressure unmagnetized plasma. Mainly, the experiments are reviewed, when a disc electrode (metallic and dielectric) is biased pulsed negative or positive. The main aim is to review the electron loss in plasmas and particle balance during the negative pulse electrode biasing, when the applied pulse width is less than the ion plasma period. Though the applied pulse width is less than the ion plasma period, ion rarefaction waves are excited. The solitary electron holes are reviewed for positive pulsed bias to the electrode. Also the excitation of waves (solitary electron and ion holes) is reviewed for a metallic electrode covered by a dielectric material. The wave excitation during and after the pulse withdrawal, excitation and propagation characteristics of various electrostatic plasma waves are reviewed here.     

Carbon nanotube–polybithiophene photovoltaic devices with high open‐circuit voltage

We report the preparation of photovoltaic devices using modified single wall carbon nanotubes, SWNTs. Devices are produced stacking on top of fluorine‐doped tin‐oxide, an electrochemically deposited polybithiophene layer, a layer of SWNT blended with poly(3‐octylthiophene) and an evaporated top metal contact, Ca/Al or Al. Ca/Al‐top‐electrode devices achieve open‐circuit voltages of 1.81 V and average power conversion efficiency of 1.48% at irradiance of 15.5 W m^–2, spectrally distributed following AM1.5. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)