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 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.

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.

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.

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.

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.

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.

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.

A novel 4H-SiC lateral bipolar junction transistor structure with high voltage and high current gain

In this paper, a novel structure of a 4H-SiC lateral bipolar junction transistor （LBJT） with a base tield plate and double RESURF in the drift region is presented. Collector-base junction depletion extension in the base region is restricted by the base field plate. Thin base as well as low base doping of the LBJT therefore can be achieved under the condition of avalanche breakdown. Simulation results show that thin base of 0.32 μm and base doping of 3 × 1017 cm 3 are obtained, and corresponding current gain is as high as 247 with avalanche breakdown voltage of 3309 V when the drift region length is 30 μm. Besides, an investigation of a 4H-SiC vertical BJT （VBJT） with comparable breakdown voltage （3357 V） shows that the minimum base width of 0.25 ~tm and base doping as high as 8 × 10^17 cm^-3 contribute to a maximum current gain of only 128.

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.

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.

High performance electrocoagulation process in treating palm oil mill effluent using high current intensity application

Electrocoagulation process using high current intensity to treat palm oil mill effluent(POME) was investigated in this study.Various operating parameters such as electrolysis time, inter-electrode distance and initial pH were carried out to determine the efficient process condition on the removal of chemical oxygen demand(COD),biological oxygen demand(BOD) and suspended solids(SS).The highest treatment was achieved at 50 min with the removal efficiencies for COD, BOD and SS obtained as 85%, 83%, and 84%, respectively.More than 50 min treatment showed the fluctuated trends of removal efficiencies which can be considered insignificant.The application of higher current resulted in higher removals of organics while the gas bubbles also assisted in removing the pollutant particles by floatation.In an inter-electrode distance study, the removal efficiency decreased when inter-electrode distance was either higher or lower than 10 mm due to the increase of solution resistance and the decrease of anode active surface area.In initial pH study, it was found that high removal efficiencies were achieved in slightly acidic POME sample rather than in neutral or basic condition.An electrocoagulation process by using the optimum operating parameters was able to remove COD, BOD and SS up to 95%, 94% and 96% respectively.The experimental results confirm that application of high current intensity in electrocoagulation provided high treatment efficiency at a reduced reaction time.

Improvement of Wear Resistance of Magnesium Alloy AZ91HP by High Current Pulsed Electron Beam Treatment

Surface modification of magnesium alloy AZ91HP (9wt%Al, 0.5wt%Zn, 0.5wt%Mn, Mg remaining percentage) by high current pulsed electron beam (HCPEB) treatment was studied in this paper. The secondary phase MgnAln is nearly completely dissolved and as a result, a super-saturated solid solution forms on the re-melted surface. The microhardness is increased both in and far beyond the heat-affected zone (HAZ), reaching about 250um. Measurements on sliding wear have shown that the wear resistance of the treated samples was improved by a factor of about 2.4 as compared to the as-received sample. It is also found that the sliding wear resistance can be further improved by surface alloying with TiN.

Improvement of wear resistance of AZ31 and AZ91HP by high current pulsed electron beam treatment

The surface modification of magnesium alloys (AZ31 and AZ91HP) was studied by a high current pulsed electron beam(HCPEB). The results show that the cross-sectional microhardness of treated samples increases not only in the heat affected zone(HAZ), but also beyond HAZ, reaching over 250μm. This is due to the action of quasi-static thermal stress and the shock thermal stress wave with materials, which result in its fast deformation on the surface layer and so increases microhardness. For the AZ91HP alloy, a nearly complete dissolution of the intermetallic phase Mg_ 17Al_ 12 is observed, and a super-saturated solid solution forms on the re-melted surface, which is due to the solute trapping effect during the fast solidification process. Measurements on sliding wear show that wear resistance is improved by approximately 5.6 and 2.4 times for the AZ31 and AZ91HP respectively, as compared with as-received samples.

Phase Evolution Study and Optimization of the Heat Treatment Process for High Current Capacity Bi-2223 Tapes

Powder in tube process（PIT） was adopted for the fabrication of single filament Bi-2223 tapes, and a heat treatment process including the first heat treatment（HT1）, intermediate rolling（IR）, and second heat treatment（HT2） was performed. The phase evolution mechanism and microstructure changes during these heat treatment processes were systematically discussed. The influences of HT1 parameters on the phase evolution process of Bi-2223 tapes were discussed. With the optimized HT1 process, a proper Bi-2223 content of about 90% was achieved. HT2 process was also optimized by adding a post annealing process. An obvious increase of current capacity was obtained due to the enhancement of intergrain connections. Single filament Bi-2223 tapes with the critical current of Ic-90 A were fabricated with the optimized sintering process.

Development of a high current short pulse electron gun

Dielectric wall accelerator(DWA), towards high gradient acceleration field(30 MeV/m–100 MeV/m), is under development at Institute of Modern Physics. A prototype was designed and constructed to prove the principle. This needs a short pulse high current electron source to match the acceleration field generated by the Blumlein-type pulse forming lines(PFLs). In this paper, we report the design and test of a new type short pulse high current electron gun based on principle of vacuum arc discharge. Electron beams of 100 mA with pulse width of 10 ns were obtained.

Numerical simulation of thermal-mechanical process of Al-Si-Pb alloy treated by high current pulsed electron beam

The modified microstructure of Al-Si-Pb alloys irradiated by high current electron beam (HCPEB) reveals three distinct regions: a molten zone, an overlapped zone of heat-affected and quasistatic thermal stress-affected zone, and a transition zone followed by the substrate. The hardness and wear properties of the alloys were significantly improved. To better understand these changes in microstructure and properties, the physical model for the simulation of temperature and quasistatic stress fields was established. Based on experimental investigation and physical models, the temperature field and stress field were simulated for Al-Si-Pb alloy. The starting melting position, largest crater depth, melting layer thickness, and quasistatic stress distribution were obtained. These results reveal the mechanism of crater formation on the surface and improvement of hardness and wear resistance.

Mitigating the Reconstruction of Metal Sulfides for Ultrastable Oxygen Evolution at High Current Density

Metal sulfides are emerging highly active electrocatalysts for the oxygen evolution reaction(OER),but still suffer from the instability caused by their inevitable reconstruction,especially at industrial-level current density.Here,it is discovered that Fe-incorporated Ni3S2 nanowires can deliver extraordinary durability with an ultralow potential degradation rate of 0.006 mV/h in alkaline electrolytes made with fresh water and seawater at a benchmark of 500 mA cm^(-2) while meeting the industrial activity requirement for overpotential less than 300 mV(290 mV).Systematic experiments and theoretical simulations suggest that after forming the S-doped NiFeOOH shell to boost intrinsic activity,Fe incorporation effectivelymitigates the reconstruction of the Ni_(3)S_(2) nanowire core by restraining Ni oxidation and S dissolution,justifying the performance.This work highlights the significance of circumventing reconstruction and provides a strategy to explore practical chalcogenides-based OER electrocatalysts.

High current limits in chemical vapor deposited graphene spintronic devices

Understanding the stability and current-carrying capacity of graphene spintronic devices is key to their applications in graphene channel-based spin current sensors,spin-torque oscillators,and potential spin-integrated circuits.However,despite the demonstrated high current densities in exfoliated graphene,the current-carrying capacity of large-scale chemical vapor deposited(CVD)graphene is not established.Particularly,the grainy nature of chemical vapor deposited graphene and the presence of a tunnel barrier in CVD graphene spin devices pose questions about the stability of high current electrical spin injection.In this work,we observe that despite structural imperfections,CVD graphene sustains remarkably highest currents of 5.2×10^(8)A/cm^(2),up to two orders higher than previously reported values in multilayer CVD graphene,with the capacity primarily dependent upon the sheet resistance of graphene.Furthermore,we notice a reversible regime,up to which CVD graphene can be operated without degradation with operating currents as high as 108 A/cm^(2),significantly high and durable over long time of operation with spin valve signals observed up to such high current densities.At the same time,the tunnel barrier resistance can be modified by the application of high currents.Our results demonstrate the robustness of large-scale CVD graphene and bring fresh insights for engineering and harnessing pure spin currents for innovative device applications.