Minimum Current Density Research Articles

Repassivation of Pits in Aluminum Thin Films

The effect of metal film thickness on repassivation of pits in sputter‐deposited Al thin films was investigated in chloride solutions. The repassivation potential and the critical current density, which is the pit current density below which pits stop growing, were determined for pits in Al thin films ranging from 100 Å to 43 μm in thickness. The repassivation potential first decreased as thickness increased from 100 to 4350 Å, and then increased as the film thickness increased further. This behavior was found to be a consequence of the pit current‐density/potential relationship. The critical current density, a more informative parameter, decreased for increasing metal film thickness, even when the repassivation potential increased. The critical current density is the minimum current density needed to maintain the critical pit environment and prevent repassivation. The repassivation potential for a given metal film thickness is the potential at which the pit current density drops below the critical value. Mass‐transport and ohmic resistance both increase as the metal film thickness increases, but the former enhances pit stability and the latter destabilizes pitting in this system. Pit repassivation, and thus stability, are strongly influenced by mass‐transport considerations for pits in very thin pits, even though dissolution at low potentials is not under pure mass‐transport control. Ohmic effects become increasingly important as the film thickness increases.

A three-dimensional finite element model of human transthoracic defibrillation: paddle placement and size.

A detailed 3-D finite element model of the conductive anatomy of the human thorax has been constructed to quantitatively assess the current density distribution produced in the heart and thorax during transthoracic defibrillation. The model is based on a series of cross-sectional CT scans and incorporates isotropic conductivities for eight tissues and an approximation of the anisotropic conductivity of skeletal muscle. Current density distributions were determined and compared for four paddle pairs and two paddle sizes. Our results show that the myocardial current density distributions resulting from a defibrillation shock were fairly uniform for the paddle pairs and sizes examined in this study. Specific details of the spatial distribution of the current density magnitudes in the heart were found to depend on paddle placement and size. When the minimum current necessary to defibrillate was delivered, the maximum myocardial current density produced with any of the paddle sizes and positions examined was less than four times the minimum current density necessary to render a myocyte in a fibrillating heart inexcitable, and less than 40% of the damage threshold. These results suggest that common clinically used defibrillation paddle positions have a safety margin as large as 2.5 for current and approximately 6 for energy.

Characterization of sidewall defects in selective epitaxial growth of silicon

The sidewall defects in high quality selective epitaxial growth (SEG) of silicon were characterized. Three different SEG diode structures were fabricated and the bulk and perimeter defects were characterized through electrical measurements and transmission electron microscopy (TEM). The structures investigated were SEG grown in a 1.2 μm thick wet-etched field oxide, SEG grown in 1.2 μm thick reactive-ion etched field oxide, and SEG grown in a 1.2 μm high and 0.3 μm wide sidewall oxide cavity. The thin sidewall oxide cavity SEG diode showed the best ideality factors and minimum saturation current densities for diodes intersecting the sidewall, indicating the least thermal stress generated at the SEG/oxide sidewall interface during the cool-down period. Cross-sectional TEM micrographs showed no defects in the bulk SEG or at the sidewall, indicating that the thermal stress in all the processes was not high enough to cause plastic deformation, dislocations, or stacking faults.

Minimum critical current density of 10 5 A/cm 2 at 77 K in the thin layer of Bi 1.8Pb 0.4Sr 2.0Ca 2.2Cu 3.0O y superconductor near the Ag in Ag-sheathed tapes

We report for the first time a minimum critical current density in a BSCCO/Ag polycrystalline sample of 1.1. × 5 A/cm 2 at 77 K in a self-field. For practical applications of (Bi, Pb) 2Sr 2Ca 2Cu 3O y (BSCCO-2223) high-temperature superconducting material, the critical current density ( J c) is a limiting factor. A requirement for a J c of 10 5 A/cm 2 in long length wires and tapes, at 77 K and high magnetic field, has been established. The transport J c were measured on samples that retained a 10 μm thickness of the superconducting core after they were removed from rolled tapes. The thin superconducting layer region next to the silver sheath was shown to be the actual current carrying region in the tape. These results confirm the presence of a high critical current density region adjacent to the silver-sheathed tapes, and provide a basis for further improvements in J c. Regions of silver were also found to be present in the superconducting core.

Irreversibility line and thermal depinning in YBa 2 Cu 3 O 7-δ

By measuring magnetic torque moment in a field-sweeping process, the temperature and field dependence of the critical current density j (with a criterion of electric field) and the normalized relaxation rate Q = d ln j/d ln E of a YBa2Cu3O7-δ thin film were obtained. With a minimum current density (jmin = 10 Acm-2) the irreversibility lines at different sweeping rates were determined. It was found that these irreversibility lines cannot be fitted to either the melting line or the vortex-glass transition. All the data can be interpreted by the thermally-assisted-flux-flow model. Further investigation shows that, at irreversible temperature and field, Uc is much smaller than kBT, which indicates that the thermal depinning is the real origin of the irreversibility line.

The critical current density distribution in a TIBaCaCuO thin film ring resonator

Low temperature scanning electron microscopy (LTSEM) offers the possibility for spatially resolved investigation of superconducting properties of the integrated microwave devices made of high T c thin films. In this paper the LTSEM has been used to study the spatial distribution of the critical current density J c in a millimetre-wave microstripline ring resonator made of TlBaCaCuo thin film. The resonator was patterned by standard photolithography and wet etching on a TlBaCaCuo thin film fabricated by DC sputtering on a LaAlO 3 substrate with post-annealing. The experimental results shows that the inhomogeneity of the J c distribution in the device is not very serious. The maximum local critical current density is less than 1.5 times the minimum local critical current density. It seems to us that the J c distribution in the ring area is determined by the inhomogeneity of the superconducting properties of the thin film and is not influenced by the geometrical pattern of the device.

The critical current density distribution in a TlBaCaCuO thin film ring resonator

Abstract Low temperature scanning electron microscopy (LTSEM) offers the possibility for spatially resolved investigation of superconducting properties of the integrated microwave devices made of high T c thin films. In this paper the LTSEM has been used to study the spatial distribution of the critical current density J c in a millimetre-wave microstripline ring resonator made of TlBaCaCuO thin film. The resonator was patterned by standard photolithography and wet etching on a TlBaCaCuO thin film fabricated by DC sputtering on a LaAlO 3 substrate with post-annealing. The experimental result shows that the inhomogeneity of the J c distribution in the device is not very serious. The maximum local critical current density is less than 1.5 times the minimum local critical current density. It seems to us that the J c distribution in the ring area is determined by the inhomogeneity of the superconducting properties of the thin film and is not influenced by the geometrical pattern of the device.

Structure-dependent threshold current density for CdZnSe-based II-VI semiconductor lasers

The optical gain and threshold current density of CdZnSe-based semiconductor lasers are calculated theoretically with strain and quantum confinement effects taken into account. Both diffusion and drift components are incorporated into the calculation of leakage current. Optimized structures are obtained for CdZnSe-based lasers with different structures through minimizing the threshold current density. A minimum current density of about 200 A/cm/sup 2/ is obtained for a Cd/sub 0.2/Zn/sub 0.8/Se/ZnS/sub 0.06/Se/sub 0.94//Zn/sub 0.75/Mg/sub 0.25/S/sub 0.42/Se/sub 0.58/ modified MQW laser.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Gas diffusion electrode-membrane cells: effects of ion transport

Ion current limitations are explored for a gas diffusion electrode attached to a flooded membrane. Conservation expressions are solved for reversible electrochemical reactions (oxidation-reduction) in the presence of a stationary supporting electrolyte. Assuming that the gasous reactant and product diffusion times are much smaller than the ion transfer step, minimum current densities are presented corresponding to either a reaction or larger ion diffusion limit. It is demonstrated that the distinction between both cases disappears if the ratio of the membrane thickness to the reaction penetration depth based on the exchange current is large or K a K h 2 >1

Possibility of amplification of microwaves using the negative conduction of an electron beam (polytron)

The possibility to amplify waves in the waveguide or strip line penetrated by a transverse electron beam has been considered. Electrons can be emitted by FEAS or an extended thermionic cathode located along the line. The structure using FEAS is similar to the microtriode amplifier, but the input signal is supplied into the anode line instead of the cathode one. Wave amplification along the waveguide or line can be achieved due to the well-known effect of negative conduction of electron beam, used formerly in oscillating monotrons with cavities. If the transit angle of electrons drifting across the waveguide or line is 2π &amp;lt;θ &amp;lt;2.8π, the electrons at an average give the energy to the electromagnetic field, and if the current density is sufficient, it becomes possible not only to compensate losses, but to amplify the signal in the frequency band. The amplifier designed on the basis of this principle may be called a polytron. The analyses of polytron has shown that the amplification of millimetric waves is possible for the existing current densities being provided by thermionic cathodes or by FEAS. For example, minimum current densities Je,min providing the loss compensation in copper waveguide are Je,min =73 A/cm2 when λ=2 mm; Je,min =13 A/cm2 when λ=4 mm; Je,min =2.3 A/cm2 when λ=8 mm. The value of amplification depends on the difference between the working current density and the minimum one and the required distance between wide walls of the waveguide or between strip-line conductors makes up 10’s to 100’s of microns. The conducted analysis has shown the proposed principle of amplification in vacuum microelectronics to be promising.

Spatial variation of the critical current density of the high T c superconducting thin films

Low-temperature scanning electron microscopy (LTSEM) is a promising measuring technique for probing the spatial distribution of the superconducting properties of high Tc thin films. In the presented article we have given some experimental observations on the spatial variation of the critical current density of two kinds of high Tc thin films using a LTSEM. The results show that in the polycrystalline TlBaCaCuO film prepared by dc magnetron sputtering, the maximum local critical current density is about 3–4 times greater than the minimum local critical current density. The microstructure analysis implies that the strong inhomogeneous critical current density distribution in the TlBaCaCuO film is due to its granular structures and the existence of several structural phases. For c-axis oriented epitaxial YBaCuO thin film deposited in situ by laser ablation, the results indicate a less inhomogeneous critical current density distribution. The maximum local critical current density is less than 1.5 times the minimum local critical current density in the epitaxial thin film.

A minimum profile uniform current density electrode.

Present methods of determining the safe injected charge levels for disk-type electrodes are given in terms of an average charge density, although the charge density is higher near the periphery of the electrode. This paper describes an electrode that produces an injected charge density that is uniform over the surface of the electrode and thus permits maximum utilization of the surface. Charge density is the time integral of current density, and the alteration of the current density is obtained by adding curvature to the electrode and recessing it within a cylindrical insulating well. A novel numerical method is used to determine the recession and curvature, and this numerical method is also presented. The benefit of this technique is that it permits a reduction in the electrode size while maintaining the maximum safe injected charge level of a disk-type electrode. A minimum profile uniform current density electrode and the algorithms used in its design are presented in this paper. Finally, a flat electrode that is recessed by as little as 1/10 of its diameter is shown to have an injected current density on the electrode surface that is superior to that of a flat surface mounted electrode.

Microstructure and corrosion behaviour of chromium nitride grown by dynamical ion beam mixing

A chromium nitride coating on pure polycrystalline nickel was obtained by dynamical ion beam mixing (DIBM). The chemical composition and microstructure of the surface layer were examined by Auger electron spectroscopy and transmission electron microscopy, and their effects on corrosion behaviour were studied. It was found that the DIBM CrN specimen surface region consisted of amorphous Cr-N-C, Cr 2(CN), Cr, CrN, Cr 2N, Ni-Cr amorphous phases and a dislocation cell structure. Three-sweep potentiokinetic measurement of the specimens in de-aerated CH 3COOH-CH 3COONa (pH 6.7, 25°C) showed that the critical current density and minimum anodic passivating current density are lowered by two orders of magnitude for the DIBM CrN specimens. The reasons for the improvement in corrosion resistance are discussed with respect to morphology and the microstructure of DIBM CrN.

A planar InP/InGaAs avalanche photodiode with floating guard ring and double diffused junction

The authors discuss the fabrication, performance, and design of a novel, planar In/sub 0.53/Ga/sub 0.47/ As/InP separate absorption and multiplication region avalanche photodiode (SAM-APD) with floating guard rings and a double Zn diffused junction. The APD, grown by both vapor phase epitaxy and metalorganic vapor phase epitaxy, is observed to have a uniform gain of 85, a minimum primary dark current density of 5*10/sup -6/ A/cm/sup 2/ at 90% of breakdown, and a capacitance of 0.4 pF for a front-side illuminated device. Both experimental and analytical results show that the double-diffused floating guard ring structure prevents edge breakdown, and also greatly reduces the electric field along the semiconductor/insulator surface. The operation mechanisms and the optimum design of the planar APD based on a two-dimensional device model are discussed. >

A Kinetic Study of Carbon Electrode Reactions in Various Molten Fluorides

The electrochemical behavior of several kinds of carbon have been studied in various molten fluorides. The temperature range of the electrolyte employed in this experiment is related to the melting point of each melt, e.g., 50°–120°C for, 80°–150°C for , 260°C for , and 470°–600°C for . The values of the peak current density at ca. 6V vs. Pt and the minimum current density at the potentials over 6V vs. Pt were measured from the anodic polarization curves obtained by cyclic voltammetry with a sweep rate of 200 mV · s−1, and then they were interpreted kinetically. The value of was theoretically proved to be proportional to the rate of decomposition of film formed on the surface of carbon anode. From the plots of against the reciprocal of absolute temperature (1/T), the activation energy of the disproportionation of film obtained in the molten system was 22.0–25.9 kcal · mol−1. In contrast, that in each case of the molten , , and was only 5.5–9.4 kcal · mol−1. However, the carbon anode was often broken in the presence of . The disproportionation of the film is catalyzed by and the physical etching of the carbon anode could be explained in terms of the formation of an intercalation compound containing .

Electron field emission through a very thin oxide layer

Field emission of an emitter covered with a very thin oxide layer is modeled and calculated numerically. The additional barrier due to the oxide layer is included in the tunneling problem by using the standard WKB method. The following physical parameters are considered: the oxide barrier height, the oxide conduction band-edge electron effective mass, and the oxide dielectric constant. Compared with the Fowler-Nordheim equation, which was derived for a clean metallic emitter, the calculation shows a reduction of the emission current density from an emitter covered with an oxide layer a few monolayers thick. After reaching a minimum emission current density, the emission increases when the thickness of the oxide layer increases further. Finally, the emission current density is saturated and stabilized.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

X-ray laser by channeling radiation in the presence of a distributed feedback

The X-ray radiation from the beam of relativistic particles channeled in a crystal is analyzed in the presense of distributed feedback caused by the Bragg reflection from atomic planes. The theory developed considers the contribution of the beam to the system polarizability tensor. When the beam current density exceeds the so-called threshold value, laser generation starts. The influence of absorption and problem geometry on the threshold value has been studied. It is shown that non-one-dimensional geometry provides a considerably lower generation threshold than the conventional one-dimensional geometry. Estimates of the minimum threshold beam current density and of the number of induced X-ray quanta are given taking into account multiple scattering. The possibility of experimental observation of induced quanta is also discussed.

Nondestructive Evaluation of In-Service Toughness Degradation for Cr-Mo-V Steam Turbine Rotor Steel

Abstract Embrittlement was investigated for five retired and three laboratory-aged steam turbine rotors by sampling Charpy V-notch test coupons and testing them. A nondestructive evaluation method of toughness degradation was discussed. The investigation has revealed that Cr-Mo-V rotor steel becomes embrittled by phosphorus segregation at the grain boundaries over a long period of use at high temperatures. The phosphorus-enhanced toughness degradation can be nondestructively detected by the electrochemical polarization (ECP) test. An approximate formula to evaluate the degraded fracture appearance transition temperature (FATT) of a specific Cr-Mo-V rotor was established that included the nondestructive information of the ECP minimum current density, grain size, and data given from the fabrication record. The nondestructive estimation procedure for the ductile fibrosity percentage versus the temperature curve for the specific rotor is also presented.

Finite element analysis of cardiac defibrillation current distributions

We have developed a two-dimensional finite element model of the canine heart and thorax to examine different aspects of the distribution of current through cardiac tissue during defibrillation. This model allows us to compare various electrode configurations for the implantable cardioverter/defibrillator. Since we do not yet know the electrical criteria to apply for predicting defibrillation thresholds, such as the minimum current density required for defibrillation or the critical mass if indeed such quantities are applicable, we measured defibrillation energy in dogs to determine the voltages to apply to the model for calculating current distributions. By analyzing isopotential contours, current lines, power distributions, current density histograms, and cumulative current distributions, we estimated the critical fraction and threshold current density for defibrillation, compared various electrode configurations, and assessed the sensitivity of the defibrillation threshold to electrode position, patch size, and tissue conductivity. We found that blood can shunt defibrillation current away from the myocardium, particularly in configurations using a two-electrode catheter, that myocardial tissue conductivity strongly affects the current distributions, and that epicardial patch size is more important that subcutaneous patch size. Our results are consistent with successful defibrillation requiring that 80 +/- 5% of the heart must be rendered inexcitable by a current density of 35 +/- 5 mA/cm2 or greater. This two-dimensional, isotropic model has allowed us to analyze some of the determinants of defibrillation, but more detailed interpretation of experimental data may require the extension of the model to three dimensions.

HighT c of YBa2Cu3O x implanted N2

N 2 implantation increases T c under low acceleration energy for high T c YBa 2 Cu 3 O x . The maximum value of T coff is shown at 500 sec of the implantation time. The influence of the current density on T coff is obtained. T coff of the implanted sample above 5 mA mm −2 of the current density remarkably approaches that before the implantation. The maximum T coff value and its increase by N 2 implantation are 96.8 K and 4.1 K at 5×10 −3 mA mm −2 of the detectable minimum current density