Current densities in an illuminated perfectly-conducting sheet

When electromagnetic radiation illuminates a perfectly-conducting metal, it induces a current density on its surface. We consider a thin perfectly-conducting sheet, possibly finite in extent and possibly with apertures in it, so that incident radiation will induce a current density at both sides of the material. The sheet current density is the sum of the current densities at both sides, and this effective current density generates the scattered field. We show that when this sheet current density is known, its splitting in surface current densities at both sides of the sheet is unique, and determined by the incident field in a simple way. A set of two coupled equations for this sheet current density is derived, which holds for any spatial structure of the incident field. This approach to scattering by a sheet is illustrated by considering a plane wave incident on a mirror and on the Sommerfeld half-plane.     

Calculation of the critical current density of hard superconductors from the magnetization of cylindrical samples

A new approximation formula for the critical current density is presented which is more exact than those proposed previously and allows the evaluation of the current density in the range of the initial magnetization curve. The relative error is calculated for one practical case and is plotted for the different branches of the magnetization curve.     

Active current management for four-rail railguns

A system of auxiliary conductors designed to reduce current density peaks in railgun rails for four-rail, round-bore railguns is described. The effects on rail current density and projectile force are discussed. Railgun cross-sectional designs are presented for round-bore, four-rail railguns which operate at lower peak current densities and develop greater projectile forces than conventional two-rail, round-bore railguns     

Scintillation in anodic Ta2O5 films

The scintillation of anodic tantalum oxide was investigated by counting the number of breakdown events during anodization at a constant current density. A theory is developed which qualitatively explains the variation in the number of breakdown pulses with time and voltage for different current densities and different electrolyte resistivities. The theory also allows definition of a limiting anodization voltage which increases with the logarithm of the current density. Several experiments are presented which are in agreement with the theory.     

高效电镀电流密度与腐蚀裂纹研究

针对汽车工业急待解决的连杆腐蚀裂纹造成的减振器失效问题，对腐蚀裂纹建立了毛细管模型，试验研究了不同电流密度高效镀铬条件下减振器连杆镀层的微裂纹特征和腐蚀裂纹扩展规律。结果表明：电流密度为58A／dm^2时镀铬层微裂纹数量多且细密，在此电流密度下由于腐蚀电流的分散连杆基体的耐蚀性较好；镀铬层极化电流密度和裂纹长径比呈双曲线关系，电流密度为58A／dm^2时镀层腐蚀裂纹短而浅，耐腐蚀性能好。     

A path-independent integral for 2-dimensional cracks in homogeneous isotropic conductive plate

A path-independent integral which is denoted by j_e, is introduced for the 2-dimensional crack problems in the homogeneous isotropic conductor in which the steady current flows. By utilizing the j_e-integral the distributions of the electric potential, current density and the Joule heating rate near the crack tip are derived. It is shown that the j_e-integral provides a parameter which dominates the distributions of the electric potential and current density near the crack tip as the square of the amplitude.     

对靶非平衡磁控溅射系统平面探针诊断

对靶非平衡磁控溅射系统由两个相对磁控溅射靶和一个磁镜场约束系统构成，通过强调电磁线圈的激励电流可以控制磁镜场的空间分布状态，测量了合成磁场的空间分布状态。采用平面探针方法，在两靶中间位置测量了收集电流密度。结果表明，探针的收集电流密度随磁镜场的激励电流增加显著增大，在偏压150V、工作气体Ar、压力0.2Pa和磁控溅射靶工作电流3A时，收集的电流密度可以达到5.77mA/cm^2。     

考虑电子热初速分布计算磁聚焦强流电子注的新方法

电子的热初速对强流电子注磁聚焦系统的性能具有重大的影响.本文提出了一种新方法,系统地考虑电子热初速分布和空间电荷效应计算非层流强流电子注聚焦系统;导出了局部正交曲线坐标系中电流密度分布的演化方程;编写了强流电子注磁聚焦的软件,研究了热初速分布对周期磁聚焦电子注电流密度分布及其演化的影响.     

Spin transfer effect in magnetic tunnel junction with a nano-current-channel Layer in free layer

The current-induced magnetization switching (spin transfer effect) in a low resistance-area (RA) product magnetic tunnel junction (MTJ) device with critical current density of 1.4/spl times/10/sup 7/ A/cm/sup 2/ was demonstrated. The RA product of the MTJ is 4.2 /spl Omega//spl mu/m/sup 2/ and the magnetoresistance (MR) ratio induced by current is up to 16%. An MTJ structure with a novel nano-current-channel (NCC) layer inserted into the free layer for the current-induced magnetization switching by lower current density was proposed and prototyped. By using the current confined effect, the local current density in the integrated free layer was sufficiently high to switch the magnetization locally. Such local magnetization reversal helped to reverse the magnetic moments around together with the polarized current and spread out the switching of the entire free layer through the superparamagnetic nano-channels. The critical current density was reduced to 4.2/spl times/10/sup 6/ A/cm/sup 2/, which is only one quarter of that for a pure MTJ structure.     

PI衬底上电沉积Cu薄膜的晶面择优取向

采用硫酸盐电沉积法,利用X射线衍射仪(XRD)、扫描电镜(SEM)等手段研究了不同电沉积条件下在PI膜表面制备的Cu薄膜的品而择优取向、平均晶粒尺寸及表面形貌.结果表明,沉积层的晶面择优取向受Cu薄膜厚度和电流密度影响,电流密度较小(0.2 A/dm2)和较大(3.5～5.5 A/dm2)时,电沉积Cu膜分别容易得到(111)和(220)晶面择优取向,较大电流密度有利于晶核的形成,薄膜表面平均颗粒尺寸较小.     

Two-dimensional kinetic model of short high-current vacuum-arc discharge

A two-dimensional mathematical model of a short high-current vacuum-arc discharge is developed, according to which magnetized electrons move in a hydrodynamic regime and fast cathode ions propagate in a free flight regime in a two-dimensional electric field. The proposed model takes into account the distribution of ions with respect to their escape angles from the cathode plasma boundary. A method for calculation of the plasma density distribution in the interelectrode gap is proposed. Two-dimensional distributions of the plasma density, electric field, and discharge current density in an external magnetic field are calculated. It is shown that ion trajectories exhibit mutual intersections, partly return to the cathode, and partly rotate in the oppositely oriented electric field at the side boundary of plasma. A decrease in the applied magnetic field intensity leads to a decrease in the number of ion trajectories reaching the anode (ion starvation), which can result in the violation of a stationary current transfer.     

Current Density Distribution in Double-Sided GTAW Process

A three dimensional current density distribution in double-sided gas tungsten arc welding (GTAW) proces is performed. The current density distribution field is evaluated by numerically solving Maxwell equations in the domain of the workpiece. In the boundary condition, the current density on. the workpiece surface is assumed as Gaussian distribution. Results show that the current distribution in DSAW is different from single side arc welding, it is more concentrated and the relative location of the two torches influence the current distribution greatly.     

Design of solder joints for fundamental studies on the effects of electromigration

The continuous miniaturization of high performance electronic devices has reached a level at which current densities are large enough to make electromigration (EM) a significant issue affecting the electrical and mechanical reliability of solder joints. A new design of solder joints that controls the extent of regions experiencing relatively uniform current density, as well as regions with large current density gradient was developed. Current density distribution of this newly designed solder joint was calculated using finite element analysis (FEA), which was used to guide the characterization of EM of real solder joints. As a part of the effort in evaluating the suitability of the new joint configuration for evaluating the fundamental issues in EM, eutectic PbSn solder joints were fabricated using this design. EM effects due to applied current, current density distribution, and joint thickness of eutectic PbSn solder joints present in this joint configuration were investigated. Findings based on this new design can facilitate fundamental studies of EM issues that affect the reliability of solder joints.     

Design of half-million bit wire memory

A high volumetric density wire memory stack of 12 000 bits/in³was designed. The high bit density was obtained by the introduction of a novel and continuous digit line printed circuit which is also used to separate and support the plated wires. The printed circuit is also used for interconnecting the planes of the memory stack. Analysis of digit current and signal coupling is given. The advantages of the proposed memory stack configuration are apparent. These include low digit current and digit signal coupling along with good word and digit noise cancellations and high bit density.     

Measurement of Critical Current Density in Bulk Superconductors Based on Small Point Contact Characterization

A new approach to measure the critical current density of bulk superconductors is presented in this paper. In the proposed technique, the characterization uses a pulsed current which passes through a small point contact at the middle of the bulk and distributes uniformly, while the other contact is a copper ring around the bulk. By increasing the pulsed current above a threshold current, a normal zone appears and develops into a semi-oblate spheroid forming around the contact point. By analyzing its size and shape, and electromagnetic fundamentals, the voltage–current relationship in the normal zone is obtainable. In addition, experimental works have been performed and the results are in agreement with the theoretical analysis. In this characterization method, the obtained voltage–current relationship and the acquired experimental results lead to the critical current density measurement in the bulk superconductor pellet.     

三维石墨烯纳米球的制备及其在锂离子电池中的应用

采用直流电弧放电法制备出一种三维石墨烯纳米球材料。采用扫描电镜(SEM)、透射电镜(TEM)、拉曼光谱和X射线衍射光谱(XRD)等测试方法对三维石墨烯纳米球的形貌和结构进行了表征和研究。通过交流阻抗(EIS)、恒流充放电和循环稳定性测试等电化学测试手段来研究三维石墨烯纳米球作为锂离子电池负极材料的电化学性能。结果表明, 在电流密度为0.05 A/g下, 三维石墨烯纳米球作为锂离子电池负极材料的首次放电容量为485.9 mAh/g, 高于炭黑作负极的放电容量(401 mAh/g); 当电流密度为1 A/g时, 三维石墨烯纳米球负极材料仍然具有185.4 mAh/g的放电容量。在电流密度分别为0.5 A/g和2.5 A/g下, 充放电循环100次以后, 三维石墨烯纳米球的比容量几乎没有衰减, 这表明三维石墨烯纳米球作为锂离子电池的负极材料比炭黑具有更大的容量, 同时具有优异的循环稳定性。     

On work function and characteristics of anomalous electrodeposited nickel-cobalt films

The anomalous cobalt content in the electrodeposited nickel-cobalt (Ni-Co) alloy films significantly influenced by the current density was related to the variation of morphology and electron work function (EWF) of the films. The characteristics and EWF of Ni-Co films were investigated by scanning electron microscope with an attached energy dispersive X-ray spectrometer, X-ray diffraction, ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe technique, respectively. As the current density increased from 1 to 20 ampere per square decimeter (A/dm²), the Co atomic concentration (at.%) in Ni-Co greatly decreased from 22.5 at.% to 13.2 at.% correspondingly. The surface morphology of film obtained at low current density became smoother than that at high current density. Both UPS and Kelvin probe results showed the same trend of EWF variation which increased with increasing current density from 1 to 10 A/dm² and kept nearly unchanged at 10-20 A/dm². The smooth Ni-Co film with low EWF could be achieved at low current density. In comparison, Kelvin probe operated at atmosphere ambient could be a good candidate for EWF measurement because of the lower cost and easier operation than UPS at ultra high vacuum.     

Transient Electromagnetic and Heat Transfer Characteristics of the MgB2 Superconductor in Liquid Helium

A transient current density analysis is carried out for the type II superconductor (SC) material MgB₂ in liquid Helium. Variable magnetic fields are used as the unknown in solving the curl-curl relations of the electromagnetic fields applying the finite element method (FEM) for 2D axis symmetric cylindrical wires. Assuming an exponential current rise, the magnetic flux, current density and temperature distribution in the SC and He are calculated. This study gives the limiting current values to avoid the normal state, the temperature distribution profile with time as well as the changes in the magnitude of the current and time constant. Oscillations and non-uniform temperature distributions are observed in He and in the SC respectively if one varies the parameters which are related to the different heat transfer mechanisms. A slight instability in the current distribution is also observed which might transfer to a thermal instability, i.e. could signal He boiling.     

Optimization of superconducting solenoid

The maximum current of a superconducting solenoid is restricted by the maximum magnetic field. Therefore current density in low magnetic field regions becomes much smaller than the critical current density at those magnetic fields. This means that the superconductors are used inefficiently. A simple way to use superconductors more efficiently is to divide the solenoid into several sections according to their magnetic field strength. A method of minimizing the winding volume of the superconducting solenoid by such a division is described using certain approximations. This method is also very convenient for designing the solenoid with the highest current density or the most economical solenoid made with different superconducting materials.     

Observation of self-magnetic field relaxations in Bi2223 and Y123 HTS tapes after over-current pulse and DC current operation

The development of power transmission lines based on long-length HTS tapes requires the production of high quality tapes. Due to fault conditions, technical mistakes and human errors during the operation of a DC power transmission line, an over-current pulse, several times larger than the rated current, could occur. To study the effect of such over-current pulses on the transport current density distribution in the HTS tapes, we simulated two start-up scenarios for one BSCCO and two YBCO tapes. The first start-up scenario is an initial over-current pulse during which the transport current was turned on rapidly, rising to 900 A during the first milliseconds, then reduced to a 100 A DC current. The second start-up scenario is normal operation, and involved increasing the transport current slowly from 0 A to 100 A at a rate of 1 A/s. For both scenarios, we then measured the vertical component of the self-magnetic field by means of a Hall probe above the tape, and afterward, by solving a linear equation of the inverse problem we obtain the current density profiles. We observe a change of the self-magnetic field above the edge of the BSCCO and YBCO tapes during 30 min after the 5 ms of over-current pulse and during the normal operation. The current density profiles are peaked in the centre for over-current pulse, and more peaked around the edge of the HTS tape for normal operation, which means that the limited time over-current pulse changes the current density profiles of the HTS tapes. We observe also a loop of current for YBCO tapes and we show the role of the HTS tape stabilizer.