Anode jet generation conditions affected by the radial current density distribution for maintaining current continuity

To perform high-quality welding, the heat input from the arc to the base metal is controlled. However, an anode jet sometimes occurs. In such cases, control is difficult and weld defects occur. This study elucidates how anode jet generation is affected by the radial current density distribution for maintaining the current continuity. The anode jet starts to occur at 15–20 mm of interelectrode distance. The arc temperature decreases because the arc radius increases. Then electrical conductivity decreases sharply depending on the temperature. If electrical conductivity decreases, then the current density decreases, and the arc cannot maintain the current continuity. When the mean current density is 8.7 × 10⁵ A/m², the flow velocity is zero at 15 mm. Therefore, this minimum value of the mean current density is the anode jet generation condition.     

On the limiting factors of the critical current density in high-T c superconducting ceramics

The temperature dependence of the critical current density at high temperatures and in weak applied magnetic field for YBa₂Cu₃O_7?y ceramic samples with a pronounced granular character is analyzed. The experimental results can be explained in terms of thermally activated motion of the intergrain Josephson vortices at grain boundaries, which may be an indication that the actual limiting factor for the critical current density in ceramic samples results from a weak pinning force density for the intergrain vortices rather than from the weak-link quenching.     

On the limiting factors of the critical current density in high-Tc superconducting ceramics

The temperature dependence of the critical current density at high temperatures and in weak applied magnetic field for YBa₂Cu₃O_7–y ceramic samples with a pronounced granular character is analyzed. The experimental results can be explained in terms of thermally activated motion of the intergrain Josephson vortices at grain boundaries, which may be an indication that the actual limiting factor for the critical current density in ceramic samples results from a weak pinning force density for the intergrain vortices rather than from the weak-link quenching.     

An innovative method for the microstructural modification of TiAl alloy solidified via direct electric current application

Ti-48Al-2Cr-2Nb alloy solidified with the application of direct electric current has a refined and homogeneous microstructure without segregation. We observed an initial decrease followed by a subsequent increase in grain size and lamellar spacing, with the increase in current density. Similar trend can also be obtained by varying the amount of α2-phase(Ti_3Al). Using a directional solidification processing method,the columnar crystal microstructure transforms into an equiaxed crystal microstructure at a current density of 32–64 m A/mm~2. High dislocation density is also introduced with a minimum cross-sectional grain size of 460 μm at a current density of 64 mA/mm~2. The application of electric current alters the free energy of the critical nucleus and temperature via joule heating, causing a transformation from a columnar grain microstructure into an equiaxed grain microstructure. The increase in current density leads to a rise of the nucleation rate, and a resulting undercooling combined with temperature gradient contribute to growth of the primary phase, which finally results in grain coarsening at a critical current density of 96 mA/mm~2.The climb and cross-slip of dislocation and the migration of grain boundary ultimately create variable lamellae spacing of TiAl alloy.     

Direct Electrodeposition of Cu-Ni-W Alloys for the Liners for Shaped Charges

The Cu-Ni-W alloys for the liners for shaped charges were successfully prepared by direct current(DC) electrodeposition.The influence of cathode current density on morphology,microstructure and composition of the Cu-Ni-W alloys was investigated by means of scanning electron microscopy(SEM),X-ray diffraction(XRD) and energy dispersive spectrometry(EDS).With the increasing of current density,the copper content reduces gradually and the nickel and tungsten content increase;the crystallite size is decreased.Whe...     

Hydrogen permeation behaviour and associated phase transformations in annealed AISI304 stainless steels

The effective diffusion coefficient and subsurface concentration of hydrogen in annealed AISI304 austenitic stainless steels have been measured by the electrochemical permeation method. The effects of different cathodic current densities on the effective diffusion coefficient, hydrogen concentration beneath the cathodic surface and steady state permeation current density have been studied. The value of the effective diffusion coefficient for the permeated specimens increases with increasing charging current density. The hydrogen subsurface concentration and steady state permeation current density first increase with increasing charging current density, then decrease with increasing current density. X-ray diffraction analyses were used to investigate the phase transformation during hydrogen charging. The results revealed that cathodic charging resulted in the formation of a considerable amount of ε and α′ martensites, which will increase with charging time.     

Pulse-Width Dependence in Current-Driven Magnetization Reversal Using GaMnAs-Based Double-Barrier Magnetic Tunnel Junction

We have investigated the current pulse width dependence on current-driven magnetization reversal in double-barrier structures using GaMnAs-based magnetic tunneling junctions (MTJ) in order to clarify the origin of low threshold current density for current-driven magnetization reversal. Comparing with the case of single-barrier MTJ, the pulse-width dependence reveals that threshold current density is reduced by double-barrier MTJ. We confirmed that the threshold current density in the order of 10⁴ A/cm² is estimated considering the effect of current pulse width.     

The Influence of BaZrO3 on the Magnetic Response of (Bi,Pb) : 2223 High-Temperature Superconductors

DC magnetization and AC complex susceptibility measurements on (Bi,Pb) : 2223 high-temperature superconductors impurified with various amounts of BaZrO₃ are presented. The results are discussed in the frame of the critical state model, and the values of the inter- and intragranular critical current density as well as of the field for full penetration are estimated. The values of the intergranular critical current density are consistent with those obtained from transport measurements. The intragranular critical current density and the field for full penetration have similar values from both DC magnetization and AC susceptibility measurements. It was shown that, in the (Bi,Pb) : 2223 system, BaZrO₃ impurification changes only the properties of the intergrain matrix, while the superconducting properties of the grains are not modified.     

Flux lattice dislocation motion in the mixed state

It is shown that flux lattice dislocations (FLDs) may be investigated by observing the alternating voltage accompanying an alternating current. This method has the advantage that the FLDs are trapped within the sample and the observed ac voltages are constant. In the dc case FLDs may give rise only to transient voltages, because the current sweeps the FLDs out of the sample. Measurement on Nb_0.5 Ta_0.5 strongly suggest that electric fields may arise in this material as a result of FLD motion. The density of FLDs appears to depend on the magnetic history of the sample. It may be increased by applying a pulse of direct current; apparently, a sufficiently large current may operate FLD sources so that the density of FLDs is increased. The FLD sources are probably not all alike, for the results suggest that a large current operates more sources than a small current. The experimental results support the hypothesis that certain peculiarities of the V/I characteristics are due to FLD multiplication.     

Study of the effect of plasma current density on the formation of titanium nitride and titanium oxynitride thin films prepared by reactive DC magnetron sputtering

Titanium nitride and titanium oxynitride films were deposited by varying the plasma current density from 10 mA/cm² to 40 mA/cm² using DC magnetron sputtering at constant gas flow rate and deposition time. Samples were characterized by Grazing Incidence X-Ray Diffraction, XPS, Nanoindentation and colorimetric analysis. Different coloured films like golden, blue, pink and green were obtained at different current densities. At lower current density (10 mA/cm²), golden coloured stoichiometric titanium nitride film was formed. At higher current densities (20, 30 and 40 mA/cm²), non stoichiometric Titanium oxynitride films of colour blue, pink and green were formed respectively. The thickness of the films increased with plasma current density from 43 nm to 117 nm. It was found that the colour variation was not only due to thickness of the film but also due to oxygen atoms replacing the nitrogen positions in TiN lattice. Hardness and Young Modulus of the films were found to decrease from 17.49 GPa to 7.05 GPa and 319.58 GPa-246.77 GPa respectively with increasing plasma current density. This variation of hardness and Young Modulus of the films can be speculated due to change in crystal orientation caused by oxygen incorporation in the films. The film resistivity increased from 16.46 × 10⁻⁴ to 3.28 × 10⁻¹ Ω cm for increasing plasma current density caused due to oxygen incorporation in the crystal lattice.     

The Effect of the Flow of Atoms Sputtered from the Anode on the Flow of Current in a Vacuum Arc

The paper deals with the problem on the effect of atoms sputtered from the anode on the flow of fast cathode ions in a vacuum discharge at relatively low (100 A/cm²) values of current density. The problem is solved using a model of instantaneous ionization and a model allowing for the finite ionization length. The calculations result in determining the boundary of the region of stable passage of current depending on the density of arc current, the density of the flow of sputtered atoms, and on their velocity. It is demonstrated that, in the studied range of currents, it is impractical to include the fast ion pressure in the equations of motion and balance for energy, because these ions hardly collide with one another during the time of their transit through the discharge gap. The calculation results enable one to determine the range of validity of the model of instantaneous ionization beyond which one must allow for the dynamics of the process of ionization of atoms in the anode region.     

Initiation of mode I degradation in sodium-beta alumina electrolytes

The extension of initial surface cracks by the focusing of the ionic current in beta alumina electrolytes (Mode I degradation) is discussed in terms of existing models. Focusing for an ion current impinging on an elliptic-cylindrical flaw is calculated by solving for the electric potential with suitable boundary conditions. The current density distribution along the crack is used to calculate the sodium flow velocity and Poiseuille pressure inside the flaw. Calculated critical current densities using aK _lc criterion are several orders of magnitude higher than measured average critical current densities. This implies a lower effectiveK _lc for electrolytic degradation than for mechanical testing. Current density enhancement around insulating barriers, such as non-wetted surface areas, is also calculated using elliptic-cylindrical coordinates. Significant current density enhancements are found, but they are localized in very small regions. Crack growth would occur within these regions, but should be arrested once the flaw extends past the high current density zone. A plausible mechanism for decreasingK _lc in the electrolytic case is discussed.     

Angular Dependence of the Radiation Power of a Josephson STAR-emitter

We calculate the angular dependence of the power of stimulated terahertz amplified radiation (STAR) emitted from a dc voltage applied across a stack of intrinsic Josephson junctions. During coherent emission, we assume that a spatially uniform ac Josephson current density in the stack acts as a surface electric current density antenna source, and the cavity features of the stack are contained in a magnetic surface current density source. A superconducting substrate acts as a perfect magnetic conductor with H _||,ac=0 on its surface. The combined results agree very well with recent experimental observations. Existing Bi₂Sr₂CaCu₂O_8+δ crystals atop perfect electric conductors could have Josephson STAR-emitter power in excess of 5 mW, acceptable for many device applications.     

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.     

Visualization of the spatial distribution of current densities in a photographic system with a semiconductor photodetector

Abstract

This paper studies the visualization of the spatial distribution pattern of current density in a semiconductor photographic system with a gallium arsenide semiconductor photodetector. The spatial distribution of the current in the filaments was determined by photometric analysis of the gas discharge light emission when a current was passed through a photographic cell. This method ensured spatial resolution of ~ 10/mm and made it possible to describe quantitatively the distributions involving a drop in current density of ? 10². Transformation of the profile and amplitude of the current density of the filaments in the different regions of the current-voltage characteristic (CVC) has been studied. The filamentation (i.e. an inhomogencous distribution of the current density) was primarily due to the formation of a space charge of positive ions in the discharge gap between the photodetector and a transparent anode plate that changed the discharge from the Townsend type to the glow type.     

Effect of the ion beam current density on the formation of implanted metal nanoparticles in a dielectric matrix

The effect of the ion beam current density, varied within 4–15 μA/cm², on the formation of metal nanoparticles in a subsurface layer of SiO² substrates implanted with 30-keV Ag⁺ ions to a dose of 5×10 16 cm⁻² was studied by optical spectroscopy and atomic force microscopy techniques. An increase in the ion beam current density leads to the formation of nanoparticles of a greater size as a result of the glass substrate heating and due to an increase in the diffusion mobility of implanted silver atoms. These results suggest the possibility of controlling the dimensions of implanted nanoparticles in dielectrics by means of variation of the ion beam current density during the process.     

Destruction of the phase coherence by magnetic field in the fluctuation region of thin superconducting films

Reduction of the gradient of current density is observed near T _c in thin superconducting films. This effect is explained by the influence of fluctuation superconductivity on the current density distribution. The observed suppression of this effect by low magnetic field is explained by the destruction of phase coherence by magnetic field. The current density distribution in high magnetic field does not differ from that in the normal state down to low temperature (T << T _c2 ). This is interpreted as evidence that the superconducting state without pinning below H _c2 is not a vortex liquid but a state without phase coherence.     

Features of the recombination processes in InGaN/GaN based LEDs at high densities of injection current

It is established that the low-frequency noise density S in InGaN/GaN based light-emitting diodes (LEDs) operating a current densities j > 20 A/cm² depends on the current density as S ∼ j ³. This dependence is indicative of the formation of new nonradiative recombination centers, which may account for a drop in the external quantum efficiency of LEDs operating at high current densities.     

多孔硅的制备及其吸杂处理对电学性能的影响

多孔硅吸杂是减少晶体硅中杂质和缺陷,提高太阳能电池转换效率的有效方法。采用电化学腐蚀方法在单晶硅片上制备多孔硅,通过观察多孔硅的形貌、结构及单晶硅片的电阻率变化,研究不同电流密度制备的多孔硅对吸杂效果的影响,并从多孔硅的结构出发探究多孔硅吸杂的机理。结果表明,随电流密度增加,孔隙率明显增加,多孔硅在电流密度为100mA/cm2时,孔隙率最大;电流密度越大,多孔硅伴随所产生的弹性机械应力增加,晶格常数相应增加,这两个因素都有利于缺陷和金属杂质在多孔硅层-基底界面处迁移和富集,导致单晶硅吸杂后电阻率增大。