Fluid-chemical modeling of the near-cathode sheath formation process in a high current broken in DC air circuit breaker

When the contacts of a medium-voltage DC air circuit breaker(DCCB) are separated, the energy distribution of the arc is determined by the formation process of the near-electrode sheath. Therefore, the voltage drop through the near-electrode sheath is an important means to build up the arc voltage, which directly determines the current-limiting performance of the DCCB. A numerical model to describe the near-electrode sheath formation process can provide insight into the physical mechanism of the arc formation, and thus provide a method for arc energy regulation. In this work, we establish a two-dimensional axisymmetric time-varying model of a medium-voltage DCCB arc when interrupted by high current based on a fluid-chemical model involving 16 kinds of species and 46 collision reactions. The transient distributions of electron number density, positive and negative ion number density, net space charge density, axial electric field, axial potential between electrodes, and near-cathode sheath are obtained from the numerical model. The computational results show that the electron density in the arc column increases, then decreases, and then stabilizes during the near-cathode sheath formation process, and the arc column's diameter gradually becomes wider. The 11.14 V–12.33 V drops along the17 μm space charge layer away from the cathode(65.5 k V/m–72.5 k V/m) when the current varies from 20 k A–80 k A.The homogeneous external magnetic field has little effect on the distribution of particles in the near-cathode sheath core,but the electron number density at the near-cathode sheath periphery can increase as the magnetic field increases and the homogeneous external magnetic field will lead to arc diffusion. The validity of the numerical model can be proven by comparison with the experiment.

A Hydrogen Iron Flow Battery with High Current Density and Long Cyclability Enabled Through Circular Water Management

The hydrogen-iron(HyFe)flow cell has great potential for long-duration energy storage by capitalizing on the advantages of both electrolyzers and flow batteries.However,its operation at high current density(high power)and over continuous cycling testing has yet to be demonstrated.In this paper,we discuss our design and demonstration of a water management strategy that supports high current and long cycling performance of a HyFe flow cell.Water molecules associated with the movement of protons from the iron electrode to the hydrogen electrode are sufficient to hydrate the membrane and electrode at a low current density of 100 mA cm^(-2)during the charge process.At higher charge current density,more aggressive measures must be taken to counter back-diffusion driven by the acid concentration gradient between the iron and hydrogen electrodes.Our water management approach is based on water vapor feeding in the hydrogen electrode,and water evaporation in the iron electrode,thus enabling the high current density operation of 300 mA cm^(-2).

Noble-Metal-Free Oxygen Evolution Reaction Electrocatalysts Working at High Current Densities over 1000 mA cm^(-2):From Fundamental Understanding to Design Principles

Alkaline water electrolysis provides a promising route for"green hydrogen"generation,where anodic oxygen evolution reaction(OER)plays a crucial role in coupling with cathodic hydrogen evolution reaction.To date,the development of highly active and durable OER catalysts based on earth-abundant elements has drawn wide attention;nevertheless,their performance under high current densities(HCDs≥1000 mA cm^(-2))has been less emphasized.This situation has seriously impeded large-scale electrolysis industrialization.In this review,in order to provide a guideline for designing high-performance OER electrocatalysts,the effects of HCD on catalytic performance involving electron transfer,mass transfer,and physical/chemical stability are summarized.Furthermore,the design principles were pointed out for obtaining efficient and robust OER electrocatalysts in light of recent progress of OER electrocatalysts working above 1000 mA cm^(-2).These include the aspects of developing self-supported catalytic electrodes,enhancing intrinsic activity,enhancing the catalyst-support interaction,engineering surface wettability,and introducing protective layer.Finally,summaries and outlooks in achieving OER at industrially relevant HCDs are proposed.

Adaptive controller design based on input-output signal selection for voltage source converter high voltage direct current systems to improve power system stability

An input-output signal selection based on Phillips-Heffron model of a parallel high voltage alternative current/high voltage direct current(HVAC/HVDC) power system is presented to study power system stability. It is well known that appropriate coupling of inputs-outputs signals in the multivariable HVDC-HVAC system can improve the performance of designed supplemetary controller. In this work, different analysis techniques are used to measure controllability and observability of electromechanical oscillation mode. Also inputs–outputs interactions are considered and suggestions are drawn to select the best signal pair through the system inputs-outputs. In addition, a supplementary online adaptive controller for nonlinear HVDC to damp low frequency oscillations in a weakly connected system is proposed. The results obtained using MATLAB software show that the best output-input for damping controller design is rotor speed deviation as out put and phase angle of rectifier as in put. Also response of system equipped with adaptive damping controller based on HVDC system has appropriate performance when it is faced with faults and disturbance.

Optical fiber-based power and data delivery system for high voltage electronic current transformer

This paper describes a prototype power delivery system developed for high voltage electronic current transformer （ECT） that uses laser light to transfer power to and communicates with the primary converter. The design is based on optical-to-electrical power converters, solid-state diode lasers and optical fibers. Command signals are transmitted via the same up-fiber used to send power from secondary power supply to primary converter. The upward data transmission is completed during the brief interruption of power delivery without affecting steady power-supply. A simple comparator added to the primary converter can take the command data. Experimental results show that the fibers can provide reliable up-link for data transmission at 200 kb/s from the secondary to the primary converter. Based on the delivery system, the secondary converter can control three auxiliary channels to provide additional information. These monitoring channels are used in a time-multiplexing mode to provide information about the operation temperature, voltage and current at the remote unit for monitoring the ECT. This preventive maintenance or built-in test can increase reliability by giving early warning for necessary maintenance request.

An Economical Method for Preparative Purification of Five Alkaloids from Coptis Chinensis Franch by High-Speed Counter-Current Chromatography Using Singled Prepared Solvent System by GC

Coptis chinensis Franch, a widely used Traditional Chinese Medicine, shows various kinds of bioactivity. The major active components of the herb are considered to be alkaloids. Thus, preparative separation of these alkaloids is critical important for further pharmacology and mechanism studies. In the paper, five alkaloids from C. chinensis were purified by HSCCC using the solvent system composed of chloro-form-metha- nol-water (2:1:1, v/v/v) single prepared. The content of each solvent in solvent system were determined by gas chromatography (GC), then according the ratios of solvents in each phase to prepare the mobile and stationary phase respectively. And a comparative study was carried out between together preparation and single preparation of the solvent system. The purities and recoveries of all the products were over 98.5% and 92%. However, 134 mL chloroform, 336 mL methanol and 452 mL water were saved when the two phase were singled by GC. Our research showed an economical method for separating alkaloids from C. chinensis by HSCCC using the solvent system single prepared by GC.

Surface modification of Cu-25Cr alloy induced by high current pulsed electron beam

A Cu-25Cr alloy prepared by vacuum induction melting method was treated by the high current pulsed electron beam （HCPEB） with pulse numbers ranging from 1 to 100. Surface morphologies and microstructures of the alloy before and after the treatment were investigated by scanning electron microscopy and X-ray diffraction. The results show that significant surface modification can be induced by HCPEB with the pulse number reaching 10. Craters with typical morphologies on the Cu-25Cr alloy surface are formed due to the dynamic thermal field induced by the HCPEB. Micro-cracks, as a unique feature, are well revealed in the irradiated Cu-25Cr specimens and attributed to quasi-static thermal stresses accumulated along the specimen surface. The amount of cracks is found to increase with the pulse number and a preference of these cracks to Cr phases rather than Cu phases is also noted. Another characteristic produced by the HCPEB is the fine Cr spheroids, which are determined to be due to occurrence of liquid phase separation in the Cu-25Cr alloy. In addition, an examination on surface roughness of all specimens reveals that more pulses will produce a roughened surface, as a result of compromising the above features.

N‑Doped Graphene‑Decorated NiCo Alloy Coupled with Mesoporous NiCoMoO Nano‑sheet Heterojunction for Enhanced Water Electrolysis Activity at High Current Density

Developing highly effective and stable non-noble metalbased bifunctional catalyst working at high current density is an urgent issue for water electrolysis(WE).Herein,we prepare the N-doped graphene-decorated NiCo alloy coupled with mesoporous NiCoMoO nano-sheet grown on 3D nickel foam(NiCo@C-NiCoMoO/NF)for water splitting.NiCo@C-NiCoMoO/NF exhibits outstanding activity with low overpotentials for hydrogen and oxygen evolution reaction(HER:39/266 mV;OER:260/390 mV)at±10 and±1000 mA cm^(−2).More importantly,in 6.0 M KOH solution at 60℃ for WE,it only requires 1.90 V to reach 1000 mA cm−2 and shows excellent stability for 43 h,exhibiting the potential for actual application.The good performance can be assigned to N-doped graphene-decorated NiCo alloy and mesoporous NiCoMoO nano-sheet,which not only increase the intrinsic activity and expose abundant catalytic activity sites,but also enhance its chemical and mechanical stability.This work thus could provide a promising material for industrial hydrogen production.

Research of growth mechanism of ceramic coatings fabricated by micro-arc oxidation on magnesium alloys at high current mode

Micro-arc oxidation(MAO)coatings of ZK60 magnesium alloys were formed in a self-developed dual electrolyte composed of sodium silicate and phosphate at the high constant current of 1.8 A(15 A/dm^(2)).The MAO process and growth mechanism were investigated by scanning electron microscopy(SEM)coupled with an energy dispersive spectrometer(EDS),confocal laser scanning microscopy and X-ray diffraction(XRD).The results indicate that the growth process of MAO coating mainly goes through“forming→puncturing→rapid growth of micro-arc oxidation→large arc discharge→self-repairing”.The coating grows inward and outward at the same time in the initial stage,but outward growth of the coating is dominant later.Mg,Mg_(2)SiO_(4) and MgO are the main phases of ceramic coating.

The Development of a Megawatt-Level High Current Ion Source

Neutral beam injection is one of the main plasma heating methods in nuclear fusion devices. In order to support the scientific study of the Experimental Advanced Superconducting Tokamak （EAST）, a megawatt-level high current ion source is designed and manufactured in the ASIPP, and the progress and preliminary test results will be presented in this paper.

A theoretical analysis of the electromigration-induced void morphological evolution under high current density

In this work, analysis of electromigration-induced void morphological evolution in solder interconnects is performed based on mass diffusion theory. The analysis is conducted for three typical experimentally observed void shapes: circular, ellipse, and cardioid. Void morphological evolution is governed by the competition between the electric field and surface capillary force. In the developed model, both the electric field and capillary force on the void's surface are solved analytically. Based on the mass conversation principle, the normal velocity on the void surface during diffusion is obtained. The void morphological evolution behavior is investigated, and a physical model is developed to predict void collapse to a crack or to split into sub-voids under electric current. It is noted that when the electric current is being applied from the horizontal direction, a circular void may either move stably along the electric current direction or collapse to a finger shape, depending on the relative magnitude of the electric current and surface capillary force. However, the elliptical-shaped void will elongate along the electric current direction and finally collapse to the finger shape. On the other hand, the cardioid-shaped void could bifurcate into two sub-voids when the electric current reaches a critical value. The theoretical predictions agree well with the experimental observations.

Compounded Surface Modification of ZK60 Mg Alloy by High Current Pulsed Electron Beam+Micro-plasma Oxidation

In this study, compounded surface modification technology-high current pulsed electron beam （HCPEB） ＋ micro-plasma oxidation （MPO） was applied to treat ZK60 Mg alloys. The characteristics of the microstructure of ZK60 Mg alloy after single MPO and HCPEB＋MPO compounded treatment were investigated by SEM. The results showed that the density of the ceramic layer of HCPEB＋MPO-treated ZK60 Mg alloy was improved and defects were reduced compared to that under MPO treatment alone. Surface modified layer of ZK60 Mg alloys treated by HCPEB＋MPO was divided into three zones, namely the top loose ceramic zone, middle compact zone and inside HCPEB-induced melted zone. Corrosion resistance of ZK60 Mg alloy before and after the compounded surface modification was measured in a solution of 3.5% NaCl by potentiodynamic polarization curves. It was found that the corrosion current density of ZK60 Mg alloys could be reduced by about three orders of magnitude, from 311μA/cm^2 of the original sample to 0.2μA/cm^2 of the HCPEB＋MPO-treated sample. This indicates the great application potential of the HCPEB＋MPO compounded surface modification technology in improving the corrosion resistance of ZK60 Mg alloys in the future.

Ultra Fast Shutter Driven by Pulsed High Current

Radiation simulation utilizing plasma radiation sources (PRS) generates a large number of undesirable debris, which may damage the expensive diagnosing detectors. An ultra fast shutter (UFS) driven by pulsed high current can erect a physical barrier to the slowly moving debris after allowing the passage of X-ray photons. The UFS consists of a pair of thin metal foils twisting the parallel axes in a Nylon cassette, compressed with an outer magnetic field, generated from a fast capacitor bank, discharging into a single turn loop. A typical capacitor bank is of 7.5μF charging voltages varying from 30 kV to 45 kV, with corresponding currents of approximately 90 kA to140 kA and discharging current periods of approximately 13.1μs. A shutter closing time as fast as 38 microseconds has been obtained with an aluminium foil thickness of 100 micrometers and a cross-sectional area of 15 mm by 20 mm. The design, construction and the expressions of the valve-closing time of the UFS are presented along with the measured results of valve-closing velocities.

Study of wavelet transform type high-current transformer

The wavelet transformation is applied to the high current transformer.The high current transformer elaborated in the paper is mainly applied to the measurement of AC/DC high current.The principle of the transformer is the Hall direct measurement principle.The transformer has the following three characteristics:firstly, the effect of the remnant field of the iron core on the measurement is decreased;secondly,because the temperature compensation is adopted,the transformer has good temperature charactreristic;thirdly,be cause the wavelet transfomation technology is adopted,the transformer has the capacity of good antijanming.

Regulation of hydrogen evolution performance of titanium oxide-carbon composites at high current density with a Ti-O hybrid orbital

Rational design and controllable synthesis of practical electrodes with high sta bility and activity at high current density for a hydrogen evolution reaction(HER)are critical for renewable and sustainable energy conversion.However,high-performance TiO_(2)-based electrocatalysts for HER are quite limited,and the cat alytic active centers still remain elusive.Herein,a simple strategy is demonstrated for the synthesis of TiO_(2)-carbon composite(TiO_(2)/C)with high HER performance and stability.The remarkable HER performance of TiO_(2)/C can be ascribed to the doping of carbon atoms,which leads to stronger hybridization of Ti 3d and O 2p orbitals,thus substantially improving the electrocatalytic efficiency.This study elucidates that the hydrogen evolution activity of oxide electrocatalysts can be largely improved by regulating their electronic structures by doping carbon atoms and also provides an effective strategy for designing heterostructured electro catalysts with high catalytic activity and stability at high current density for HER.

A MOF derived hierarchically porous 3D N‐CoP_(x)/Ni_(2)P electrode for accelerating hydrogen evolution at high current densities

Hydrogen evolution reaction is a critical reaction in water splitting for hydrogen production.However,developing effective and stable non‐noble‐metal electrocatalysts which work well at high current densities demanded by industry still remain great challenge.Herein,taking advantage of the highly tunable metal‐organic framework(MOF)templates,nitrogen doped binary transition metal phosphides electrocatalysts(N‐CoP_(x)/Ni_(2)P)with three‐dimensional(3D)conductive network structure were successfully synthesized.The 3D open porous channels could expose more catalytically active sites;nitrogen doping and the synergistic effect between CoP and Ni_(2)P can increase the electron density of Co atoms at active sites,further optimizing the Gibbs free energy of hydrogen(ΔGH*)and water(ΔG_(H_(2)O*)).As a result,the obtained N‐CoP_(x)/Ni_(2)P catalyst exhibits extraordinary electrocatalytic activity in a wide pH range.Especially,it requires an extremely low overpotential of 152 mV to deliver a high current density of 650 mA cm^(–2) in alkaline media.This work may shed some light on the rational design of cheap electrocatalysts and electrode materials that work well at high current densities.

Preparative isolation and purification of 12,13-dihydroxyeuparin from Radix Eupatorii Chinensis by high-speed counter-current chromatography

An efficient method for the isolation and purification of 12,13-dihydroxyeuparin from Radix Eupatorii Chinensis by high speed counter-current chromatography (HSCCC) was established in this paper. The ether extracts of Radix Eupatorii Chinensis were purified by HSCCC with a solvent system of hexyl hydride-ethyl acetate-methanol-water (1:2:1:2, v/v/v/v). The upper phase was used as the stationary phase and the lower phase as the mobile phase. About 8.4 mg of 12, 13-dihydroxyeuparin was obtained from 200 mg of ether extracts from Radix Eupatorii Chinensis in one-step HSCCC separation, with the purity of 96.71%, as determined by HPLC. After methanol- water recrystallization, the purity of 12,13-dihydroxyeuparin reached 99.83%. Such a simple and effective method was fairly useful to prepare pure compound as reference substances for related study on Radix Eupatorii Chinensis.

Characterizing modified surface layer of 316L stainless steel treated by high current pulsed electron beam

High current pulsed electron beam(HCPEB) is now developing as a useful tool for surface modification of materials.When concentrated electron flux transferring its energy into the surface layer of target material within a short pulse time,coupled thermal and stress processes would lead to the formation of metastalbe microstructure with improved properties.In the present work,HCPEB treatment of 316L stainless steel(SS) was carried out and the microstructural changes in modified surface layer were characterized with optical microscopy,X-ray diffractometry and electron backscatter diffractometry(EBSD) techniques.The corrosion resistance of modified surface was measured in a 5wt.% salt solution.The evolution regularity of surface craters and grain refinement effect,as well as the preferred orientation of(111) crystal plane occurring in the HCPEB treatment under different working parameters were discussed along with their influence on corrosion resistance.

High-Performance In0.23Ga0.77As Channel MOSFETs with High Current Ratio Ion/Ioff Grown on Semi-insulating GaAs Substrates by MOCVD

We demonstrate high-performance In0.23 Ga0.77 As channel metal-oxide-semiconductor field-effect transistors （ MOS- FETs） with high on-current to off-current （Ion/Ioff） ratio grown on semi-insulating GaAs wafers by metal-organic chemical vapor deposition （MOCVD）. The 2μm channel-length devices exhibit a peak extrinsic transeonductance of 150 mS/mm and a drain current up to 500 mA/mm. The maximum effective mobility is 1680 cm2/Vs extracted by the split C-V method. Furthermore, the Ion/Ioff ratio is significantly improved from approximately 4.5 × 10^3 up to approximately 4.32 × 10^4 by controlling the etch thickness of In0.49Ga0.51P, The high drain current and high Ion/Ioff ratio of the In0.23Ga0.77As channel MOSFETs are achieved due to the high effective mobility and the low gate leakage current density.

Microstructure and properties of Nb/Ta multilayer films irradiated by a high current pulse electron beam

Nb/Ta multilayer films deposited on Ti6A14V substrate with Nb and Ta monolayer thicknesses of 30 nm, 120 nm, and 240 nm were irradiated by a high current pulse electron beam （HCPEB） to prepare Nb-Ta alloyed layers. The mi- crostructure and the composition of the outmost surface of melted alloyed layers were investigated using a transmission electron microscope （TEM） equipped with an X-ray energy dispersive spectrometer （EDS） attachment. The Ta content of the alloyed surface layer prepared from the monolayer of thickness 30 nm, 120 nm, and 240 nm was- 27.7 at.%, 6.37 at.%, and 0 at.%, respectively. It was found that the Ta content in the alloyed layer plays a dominant role in the microstructure of the films. The hardness and the wear rate of the alloyed layers decrease with the increasing content of Ta in the surface laver.