Negative Corona Current Pulses in SF6 – Air Mixtures

Current waveforms of first negative corona pulses have been measured in dry air + SF₆ mixtures at a pressure of 50 kPa and various overvoltages. Effects of changing cathode secondary electron emission were studied using a copper cathode coated by CuI and graphite. It is concluded that in the mixtures containing less than 10 % of SF₆ the negative corona pulse is associated with the formation of a cathode‐directed streamer in the immediate vicinity of the cathode. In the mixtures containing more than 20 % of SF₆ the streamer is quenched and, consequently, the discharge is governed by the Townsend ionisation mechanism fed by cathode photoemission processes.     

Comparison of the first negative corona current pulses in N2 + SF6 and CO2 + SF6 mixtures

The current waveforms of the first negative corona pulses in a small point-to-plane gap have been measured with a nanosecond time resolution in N₂ + SF₆ and CO₂ + SF₆ mixtures at a pressure 50 kPa for various contents of SF₆ as a function of applied gap voltages. The physical mechanism for the pulses in these mixtures with low concentration of SF₆ has been described using the streamer-based theory. The influence of changing admixtures of SF₆ in N₂ and CO₂ has been compared. Differences in the pulse waveforms observed in N₂- and CO₂-based gas mixtures are explained by differences in the first and second Townsend ionization coefficients. This work was supported by the Grant Agency VEGA from the Ministry of Education of Slovak Republic under contracts 1/1011/04 and 1/2017/05.     

The first Trichel pulse of negative corona discharge in N2 with a small admixture of SF6

Current growth waveforms of transient negative point-to-plane discharge in N₂ and in N₂ with 0·025-0·1% admixture of SF₆ at a pressure of 40 kPa have been measured and compared. The transient glow discharge regime in pure N₂ was found to be preceded by a peaked current signal of conspicuous similarity to the first Trichel pulse rise and its initial decay in N₂+SF₆ mixture. The results are in a basic agreement with a hypothesis on streamer mechanism for Trichel pulse and indicate occurrence of field emission during the rise of Trichel pulse current.     

Stepped form of negative corona current pulses in hydrogen

Current wave forms of initial stages of discharge formation in a short negative point-to-plane gap have been measured with a nanosecond time resolution in hydrogen at pressures (12.5–76) kPa and for various overvoltages and cathode point radii. The measurements revealed the existence of a stepped form of negative corona current pulses in hydrogen. To test existing models for negative corona pulse formation, effects of changing cathode secondary electron emission were studied using copper and brass cathodes coated by CuI and graphite. It is concluded that a negative corona pulse is associated with the ignition of a cathode-directed streamer in the vicinity of the cathode. We report what we believe are the first experimental observations of non-Trichel oscillations of negative corona current with a frequency of (1–10) MHz. This work was supported by the Scientific Grant Agency of the Ministry of Education and Academy of Sciences of Slovak Republic (Project No. 1/5190/98).     

The impulse dielectric behavior of N2 gas in sphere-plane gap

The focus of this work has been on the pre-breakdown phenomena and the breakdown characteristics of N₂ gas in a sphere-plane gap under various impulse voltages. Both electrical and optical experimental investigation methods were used. Following parameters were considered: gas pressure range from 0.2 to 0.6 MPa, electric field utilization factor of the electrode configuration 71%, positive and negative impulse waveforms with the rise time of 500 ns, 1.2 μs and 180 μs. The observed discharge processes before the breakdown through the light emission images by the ICCD camera are in good agreement with the streamer mechanism. Under both polarity stresses, discharges are initially concentrated around the tip of the sphere and later pointing towards the earth electrode. However, negative streamers are thinner and more diffuse. As expected, the breakdown voltages for negative polarity are lower than those for positive polarity regardless of the gas pressure and shape of the applied impulse voltage. The breakdown voltage is increased with shortening the rise time of pulse waveforms. As a substitute for SF₆, N₂ gas under pressures above 0.3 MPa can reach the standard rated withstand voltage for 24 kV C-GIS.     

Current analysis of DC negative corona discharge in a wire-cylinder configuration at high ambient temperatures

To study the characteristics of DC negative corona discharge in a wire-cylinder configuration at an ambient temperature range of 350–850 °C, the I–V characteristics and the current composition are analyzed under different conditions. A simple method is proposed to determine the DC corona onset threshold voltage. At high ambient temperatures, in the DC negative corona discharge gap, some electrons are not attached to the electronegative gas molecules and move to the anode tube. Thus, these electrons form an electron current, which may account for most of the total discharging current. The ratio of the electron current to the total discharging current increases with increasing temperature. In a mixture of O₂ and N₂ and a mixture of CO₂ and N₂, the ratio of electron current increases with increasing N₂ content in the mixtures. The cathode material has little influence on the corona discharge characteristics at high ambient temperatures.     

Characteristics of DC discharge in a wire-cylinder configuration at high ambient temperatures

In the present work, the characteristics of direct-current (DC) discharge in a wire-cylinder configuration at an ambient temperature range of 350–850 °C were studied by analyzing photographs of the discharging process and the corresponding V–I characteristics, with the aim of facilitating the application of plasma technology in the fields of energy and the environment. The influences of the ambient temperature, the inter-electrode gap, the gas medium and the cathode material on the DC discharge were investigated. The corona-onset threshold voltage (COTV) and the spark-breakdown threshold voltage (SBTV) decrease as the ambient temperature increases, and the SBTV decreases more rapidly. Increasing the inter-electrode gap enlarges the difference between the SBTV and the COTV. After spark breakdown, in an air atmosphere, glow discharge is more likely to take place under conditions of high ambient temperatures or small inter-electrode gaps. The values of the SBTV in different atmospheres have the following relation: air ≈ O₂ > CO₂. At an ambient temperature range of 350–850 °C and in an atmosphere of N₂, glow discharge and arc discharge occur successively as the output voltage of the power supply is increased, while in an atmosphere of O₂ and CO₂, only corona and arc discharge are successively observed. In an air atmosphere, when the inter-electrode gap is 29 mm, corona, glow and arc discharge occur successively with increasing output voltage when the ambient temperature is 850 °C, while only corona and arc discharge appear when the temperature is 350–750 °C. When the inter-electrode gap is 5 mm in an air atmosphere, corona, glow and arc discharge occur successively in an ambient temperature range of 350–850 °C. The cathode material has a minor influence on the COTV and a more significant influence on the SBTV. In a device using a cathode with a low work function, the SBTV is low, and the power to maintain arc discharge is small.     

Positive point-to-plane corona discharge forms in O2-N2 mixtures

An extensive survey has been made of positive point-to-plane corona discharges with 0–100% O₂ in O₂-N₂ mixtures in the pressure range 0.5–100 kPa. Selected results are presented in the form of pressure-voltage diagrams. They show the regions of existence for the different point-plane discharge forms observed: burst pulses, glow corona with oscillations, streamers, sparks, and a pulse- and oscillationless glow-spike corona. The latter positive corona discharge form has formerly only been observed in very pure nitrogen. The main aim of the paper is to give a short account of these different forms of positive coronas and their stability or oscillations, and to guide experimenters in establishing experimental conditions for future studies of these phenomena. Dedicated to Prof. Jan Janča on the occasion of his 60th birthday.     

Cathode-sheath driven low-speed aerodynamic flow actuation using direct-current surface glow discharges

Flow actuation by a continuous/pulsed, direct-current (DC) surface glow discharge is explored. The discharge comprises an array of pin electrode pairs flush mounted on a dielectric actuator surface that lies adjacent to stagnant air. Strong electrostatic fields produced in the cathode sheath region of the discharge provides a motive force on the ions which in turn drag the background gas resulting in directed air flow from the anode to the cathode. The induced flow velocity is estimated by particle image velocimetry (PIV) at 10 Hz with TiO₂ seeding. For a pulsed DC discharge with peak power of 5 W per electrode pair, the induced flow velocity reaches peak values of about 1.7 m/s which is comparable to dielectric-barrier discharge (DBD) or corona discharge actuators. The actuation effect quantified by the magnitude of induced velocity increases as the pulse frequency increases from 0 to 1 kHz. The actuation effect decreases for further increase in frequency above 1 kHz. Decreased actuation effect at high frequency is accompanied by structural change in the discharge. At fixed frequency of 1 kHz, flow actuation effect is highest for a square wave pulse with a duty cycle of 50% indicating that pulsed DC discharges produces better actuation than continuous DC with a corresponding reduction in energy consumption.     

全固态激励小型TEA CO_2激光器的研制

基于国产非晶态合金磁芯,研制了采用可控硅开关、脉冲升压变压器、以及两级磁脉冲压缩网络的全固态激励电路系统,并且应用于放电体积为29 cm3,工作气压为100 kPa的电晕预电离小型TEA CO2激光器。讨论了提高系统能量传输效率和减小系统体积的设计方法,并且测量了系统的工作性能以及各部分的能量损失。实验结果表明:磁脉冲压缩网络的能量传输效率大于83%,全固态激励系统的总效率大于75%;连接激光器负载时,输出脉冲的电压峰值约为22 kV,电流上升时间约为100 ns;得到了脉冲能量109 mJ,宽度70ns的激光输出,激光器整体效率约为3.3%。在目前的封离体积与气体循环方式限制下,激光器最大重复频率约为100 Hz,而激励电路部分可以达到400 Hz的工作频率。     

EHD atomization characteristics of a point-plate system with a porous point electrode

An electrohydrodynamic (EHD) atomization from a point-to-plate system, with a wet porous point as a corona electrode, has been studied. And the atomized water droplets from the wet porous point, as well as the water droplet traces, the water droplet charge-to-mass ratios, and the water droplet number concentrations, were investigated. It was observed that the wet porous point can atomize abundant amounts of water droplet, 2.8, 2.6 and 2.2 mg/min for negative, AC and positive corona, respectively. The migrated water droplet traces were observed. The positive wet porous point atomized very fine water droplets than those obtained with the negative wet porous point. Moreover, the water droplets atomized from the AC corona showed granular-like larger traces. A weak corona discharge can atomize water droplets very effectively. On the other hand, an intensive corona discharge can eject more water droplets. As a result with the wet porous point, the maximum corona-current-based and corona-power-based water droplet atomization yields of Y_C = 3.34, 3.32 and 3.25 μg/μAs and Y_P = 0.35, 0.40 and 0.27 mg/Ws have been obtained for the negative, AC and positive corona discharges.     

Novel long-pulse TE CO2 laser excited by pulser–sustainer discharge

A novel long-pulse TE CO₂ laser with UV-preionization is presented. With an active volume of 1.17 l and gas pressure of 30 kPa, the laser can discharge stably with low pulser energy and high sustainer energy. Various long-pulse discharges such as 12, 20 or 25 μs are demonstrated. At discharge pulse width of 23.9 μs, maximum output laser energy of 6.8 J is obtained at an efficiency of 9.0%.     

Experimental study of minimum ignition energy of lean H2-N2O mixtures

Ignition energies for short duration (<50 ns) spark discharges were measured for undiluted and nitrogen-diluted H₂-N₂O mixtures of equivalence ratios ? = 0.15 and 0.2, dilution of 0% and 20% N₂, and initial pressures of 15–25 kPa. The ignition events were analyzed using statistical tools and the probability of ignition versus spark energy density (spark energy divided by the spark length) was obtained. The simple cylindrical ignition kernel model was compared against the results from the present study. Initial pressure has a significant effect on the width of the probability distribution, ranging from a broad (P = 15 kPa) to a narrow (P = 25 kPa) probability distribution indicating that the statistical variation of median spark energy density increases as initial pressure of the mixture decreases. A change in the equivalence ratio from 0.15 to 0.2 had a small effect on the median spark energy density. The addition of 20% N₂ dilution caused a significant increase in the median spark energy density when compared to no dilution. The extrapolation of the present results to atmospheric pressure, stoichiometric H₂-N₂O indicates that the electrostatic discharge ignition hazards are comparable to or greater than H₂-O₂ mixtures.     

Influence of grain size on luminescence properties of micro- and nanopowder Zn2V2O7 vanadate

Micro- and spongiform nanocrystalline Zn₂V₂O₇ compounds were synthesized by hydrothermal and solid-state reaction techniques, and their morphological features were investigated by scanning electron microscopy (SEM). The grain size ranges of the produced powders were 3–15, 0.5–2 μm, and 50–500 nm. The luminescence spectra of these compounds were measured under pulse cathode beam and photoexcitation (200–400 nm). The luminescence decay properties of Zn₂V₂O₇ were studied.It is found that the intensity, maximum position of luminescence spectra and luminescence decay time of Zn₂V₂O₇ samples depend considerably on the grain size of the synthesized powders. The processes of energy relaxation in Zn₂V₂O₇ and the observed size effect on the luminescence properties are also discussed.     

Dynamics of TiO2-based photoelectrochemical cell

The present work reports the effect of light on the open-circuit voltage of a photoelectrochemical cell (PEC) formed of TiO₂ photoanode, Pt cathode and Na₂SO₄ (0.35 M) aqueous solution as electrolyte. The studies included the measurements of the electromotive force (EMF) during the light-off and light-on cycles for the PEC involving photoanode that was made of both oxidised and reduced TiO₂ thin films. These specimens were formed by oxidation of the titanium metal at high and low oxygen activities. This was achieved by the imposition of the gas phase of two different compositions, including air, p(O₂) = 21 kPa, and the hydrogen–water vapour mixture, p(O₂) = 10^-10p({\rm O}_2) = 10^{-10} Pa, at 1,123 K and subsequent cooling to room temperature. The determined data indicate that the PEC formed of the oxidised specimen exhibits larger EMF and a substantially better stability in time. It is, therefore, concluded that the TiO₂ obtained in air exhibits superior performance-related properties compared to the reduced specimen. The obtained experimental EMF data are considered in terms of the effect of light on the reactivity of TiO₂ with oxygen and water and the related charge transfer.     

Characteristics of negative corona discharge in the working media of atmospheric-pressure nitrogen lasers

We present the results of investigations of the electrical and optical characteristics of a negative-polarity corona discharge excited in systems of “pins-mesh” and “pins-plane” electrodes in a He/N₂ mixture at atmospheric pressure. In order that such a corona discharge could be applied in systems of electric pumping of the working medium of atmospheric-pressure N₂-lasers, the optimum conditions should be: the total pressure of the mixture ≤150 kPa and the nitrogen pressure ≤5 kPa. Uzhgorod State University, 46, Pidgirna St., Uzhgorod, 290000, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 837–840, November–December, 1997.     

Cathode materials (NiO–LiFeO2–LiCoO2) for molten carbonate fuel cell: A diffraction,NMR and conductivity study

The search for a NiO-based cathode for molten carbonate fuel cell has been directed towards solid solutions containing LiFeO₂ and LiCoO₂ in addition to nickel oxide [A. Wijayasinghe, B. Bergman, C. Lagergren, Electrochim. Acta 49 (2004) 4709.]. NiO and LiFeO₂ are fully soluble in one another, while ternary solid solutions have been previously obtained only for small (< 18%) and high (> 90%) LiCoO₂ molar contents. The goal of this work is to identify and characterize the compositions with the most promising electrical conductivity, which may be used as cathode materials. We present data relative to compositions having a constant NiO/LiFeO₂ molar ratio of 3:1 and a LiCoO₂ content from 5 to 30 mol%.     

Cathodic performance of V2O5 nanowires and reduced graphene oxide composites for lithium ion batteries

In this study, vanadium pentoxide (V₂O₅) nanowires (NWs) with a diameter of 100–200 nm and a length of up to several micrometers as cathode for lithium ion batteries are synthesize using an electrospinning method. The reduced graphene oxide (rGO) and V₂O₅ NWs (GVO) composites are form by wet mixing the electrospun V₂O₅ NWs and rGO. Surface morphologies, microstructure and elemental mapping, and chemical bonding states of the composites are characterize. The initial and 60 cycles discharge capacities of GVO composite composed of 1 wt% rGO show up to 225 mAh g⁻¹ and 125 mAh g⁻¹, even higher than pure V₂O₅ NWs, when the lithium ion battery cycled between 2.0 and 4.0 V with a rate of 0.2 C, because of highly conductive rGO. The GVO composite could be promising as a high performance cathode for lithium ion batteries.     

LiPF6-EC-MPC electrolyte for LiMn2O4 cathode in lithium-ion battery

Methyl propyl carbonate (MPC) is a promising single solvent for lithium-ion battery without addition of ethylene carbonate (EC), but it is unstable upon cycling because of exposure to the spinel LiMn₂O₄ cathode. Thus, we attempted to add EC to MPC in order to form LiPF₆-EC-MPC electrolyte; the effects of solvent ratio and salt concentration on the cycling performance of LiMn₂O₄ cathode were also investigated. The experiments were characterized by conductivity measurements, charge-discharge at a constant current density and voltage–capacity curves at low temperature. To further enhance our understanding of the performance improvement of LiMn₂O₄/Li cells, the electrochemical characterization techniques (such as, LSV, EIS) were performed on these cells. The results show that the ionic conductivity of the electrolyte and the cycling performance of the spinel LiMn₂O₄ cathode have been dramatically enhanced. From the point of view of operation at low temperature (− 20 °C), 1 M LiPF₆ EC/MPC (1/3) electrolyte is highly recommended for spinel LiMn₂O₄ cathode in lithium-ion battery.     

Effect of chitosan on deposition of LiCoO2 thin film for Li-ion batteries

Deposition of LiCoO₂ thin film using chitosan-added precursor solution was found to be a cost-effective way to fabricate cathode for Li-ion thin film batteries. The structures and electrochemical performance of such LiCoO₂ cathode were characterized by using an X-ray diffracotmeter (XRD), FTIR and charge–discharge tests. After annealing at ca. 500 °C, the results of XRD showed that the LiCoO₂ gel started to crystallize and showed hexagonal phase with a space group of R3?m. The enhanced stability of the precursor solution by the addition of chitosan is attributed to the complexation between metal ions and the ?NH₂ groups of chitosan.The electrochemical behaviour for the deposited films calcined at 700 °C for 4 h was also characterized by charge–discharge test. The result revealed that the film deposited from chitosan-containing precursor solution possesses an initial discharge capacity of 129 mAh g^? 1.