Pulsed Current Technique Research Articles

Preparation of Silica Glass by Pulsed Electric Current Method.

A new fabrication method involving pulsed electric current heating was used to investigate shortening the time and lowering the cost of fabrication of silica glass. The advantages of this technique on the functionality of silica glass are also discussed. Silica powder was filled in a graphite mold having a diameter of 20mm. The temperature on the external mold surface was raised from 1773 to 1873K at heating rates of 50 and 100K/min under a constant uniaxial pressure of 20MPa. As a result, transparent silica glass free of bubbles was successfully fabricated at the heating rate of 50K/min. However, in the case of 100K/min, crystalline particles were formed on the bulk surface in addition to numerous bubbles. These crystals are attributed to a high temperature gradient, so that the heating at the bulk surface is insufficient and the temperature does not reach the melting point of silica. Investigation of the physical, mechanical and optical properties revealed that amorphous silica obtained by the pulsed electric current heating technique has the same density, refractive index, elastic modulus and absorbance as fused silica.

Dynamic electrical response of YBaCuO thin films as a function of microstructure in view of applications to agile electronics

The electrical characteristics of YBaCuO thin films sputtered on LaAlO 3 and MgO single-crystal substrates have been measured using a pulsed current technique, to avoid ohmic heating effects- The results are discussed in relation with deposition temperature (for films deposited on LaAlO 3 ) and substrate preparation (for films deposited on MgO). In the latter case, results are also discussed in the framework of a statistical model, which provides an empirical approach for the pinning phenomena in YBaCuO films, when a static magnetic field is applied.

Tellurium electrodeposition like a precursor to prepare the p-CdTe semiconductor

In this work, some aspects about the electrochemical behavior of tellurium, which is a precursor in the preparation of the p-CdTe semiconductor, are presented. Cadmium telluride has been used in the photoelectrochemical generation of hydrogen and such semiconductor material can be obtained by electrochemistry. By using the rotating disk electrode technique the kinetics of the tellurium deposition process on solid tellurium electrodes was analyzed and the kinetic parameters are presented. Electrochemical techniques in nonstationary diffusional regime were used and, in order to investigate the probable existence of adsorbed electroactive species on the electrode surface, the pulsed current electrolysis technique in a controlled hydrodynamic regime was applied.

Influence of welding techniques on microstructure and pitting corrosion behaviour of 1441 grade Al–Li alloy gas tungsten arc welds

AbstractThe effects of changing the welding technique on the weld bead solidification structure and resistance to pitting corrosion of an aeronautical grade Al-Li based alloy have been studied. Russian alloy 1441, with a nominal composition Al-1.9Li-1.8Cu-1.0Mg-0.1Zr (wt-%) and in the form of sheet, was welded using constant current, pulsed current and arc oscillation gas tungsten arc welding techniques. Under the optimum conditions of welding, the solidification structure was coarse columnar in constant current welds, fine equiaxed in pulsed current welds and fine equiaxed intermixed with fine cellular grains in arc oscillation welds. The distribution of interdendritic (and intergranular) non-equilibrium solidification products became increasing discrete on changing from constant current to pulsed current and arc oscillation welding. Resitance to pitting corrosion was found to be greatest in welds having a discrete second phase distribution, i.e. those produced using the pulsed current and arc oscillatio...

On-site determination of corrosion rate in reinforced concrete structures by use of galvanostatic pulses

Galvanostatically induced potential transients can be used to determine the corrosion rate and double layer capacitance ( C) of steel in reinforced concrete structures, provided that the steel surface area involved in the measurements is known. An approximate capacitance value, based on previous experience, can also be used to determine polarisation resistance even if the area of the working electrode (WE) is unknown, as in the case of measurements on life-size structures using small counter electrodes. In the case of small specimens, in which direct determination of C is possible, the reliability of the pulse technique is comparable to that of other widely accepted electrochemical procedures. In the case of life-size structures, its reliability is lower due to the need to use estimated values of C. The results indicate that neither the magnitude of the perturbing pulse nor the lack of uniformity of the polarisation on the WE are factors that significantly prejudice application of the current pulse technique in on-site measurements on large reinforced concrete structures.

Precision electrical trimming of very low TCR poly-SiGe resistors

Precision electrical trimming of stacked Si/SiGe polycrystalline resistors available from the extrinsic base structure of a SiGe BiCMOS technology has been demonstrated for the first time. It is shown that pulse current trimming techniques can be used to trim the poly-SiGe resistors by up to 50% from their original values with accuracy better than /spl plusmn/0.5%. The temperature coefficient of resistance (TCR) is shown to be linearly proportional to the percent change in electrically trimmed poly-SiGe resistance. Finally, we demonstrate resistance cycling using an electrical trim/recovery sequence, indicating that the technique is reversible and is governed by dopant segregation/diffusion mechanisms. The results are consistent with those obtained on conventional polysilicon resistors suggesting that the introduction of a strained SiGe layer does not adversely affect the electrical trim properties of these resistors.

Lead dioxide coating obtained by pulsed current technique

The electrocatalytic β-lead dioxide coating on a Ti/SnO 2 substrate was obtained with the use of the pulsed current technique. The effect of the pulsed current density on the heterogeneous nucleation, on the morphology of the coating as well as on its crystalline form was studied by means of scanning electron microscopy (SEM) and X-ray diffraction. The obtained results indicate that the increase in the pulsed current density induces a preferential heterogeneous nucleation on the substrate, reduces the dimensions of the crystals, effects their orientation and increases the internal stress of the lead dioxide coating.

Structure and Soft Magnetic Properties of Bulk Fe-Al-Ga-P-C-B-Si Glassy Alloys Prepared by Consolidating Glassy Powders

AbstractWith the aim of developing a bulk glassy Fe-based alloy with good soft magnetic properties by the powder metallurgy technique, we have applied the pulse current sintering technique to a Fe70Al5Ga2P9.65C5.75B4.6Si3 glassy alloy powder with a large supercooled liquid region of 60K before crystallization. The existence of the supercooled liquid region was found to enable us to form a bulk glassy alloy with a very high relative density of 99%. The resulting bulk glassy alloy exhibits good soft magnetic properties, i.e., 1.17T for flux density at a field of 800A/m, 12.0A/m for coercive force and 8000 for maximum permeability which are much superior to those for the bulk amorphous Fe-Si-B alloy prepared by the same sintering method. The much better soft magnetic properties for the multicomponent Fe-based bulk alloy are attributed to the combination of the high relative density and the unique amorphous structure with the features of high packing density and long-range homogeneous atomic configurations. The first success of forming the bulk amorphous alloy with good soft magnetic properties by the powder metallurgy technique is expected to enable us to use as practical soft magnetic materials.

Pulse electrodeposition of titanium on carbon steel in the LiF-NaF-KF eutectic melt

Electrodeposition of titanium was carried out in the K3TiF6–LiF–NaF–KF melt using both direct (DC) and unipolar pulse current (PC) techniques. Dense and smooth titanium coatings were obtained by PC plating at 750 °C whereas DC plating led to rough and dendritic deposits. The best results were obtained using a 100 μC cm−2 pulse charge and a cathodic current density of 50 and 75 mA cm−2. The cathodic current efficiency was in the range 60–65%. The titanium deposits obtained under such conditions behaved similarly to CP-titanium in NaCl and HNO3 solutions at room temperature.

Nonequilibrium superconductivity under spin-polarized quasiparticle currents in perovskite ferromagnet-insulator-superconductor heterostructures

We report phenomena manifesting nonequilibrium superconductivity induced by spin-polarized quasiparticles in perovskite ferromagnet-insulator-superconductor (F-I-S) heterostructures. Measurements of the critical current ${J}_{c},$ using a pulsed current technique to minimize Joule heating, reveal a monotonic increase with increasing insulator thickness; and for F-I-S with thin insulating barriers, a slight increase in ${J}_{c}$ is observed under small injection currents ${I}_{m}$ from the ferromagnet, followed by a strong suppression of ${J}_{c}$ under large ${I}_{m}.$ In contrast, no effect of injection on ${J}_{c}$ can be detected in the N-I-S control sample (N: nonmagnetic metal). The spin diffusion time and length in cuprate superconductors are also estimated and compared with our experimental results.

Magnetic field and current density dependence of the mixed-state Hall effect in Bi 2Sr 2CaCu 2O x

We report on measurements of the mixed-state Hall effect in Bi 2Sr 2CaCu 2O x thin films in magnetic fields ranging from B=60 mT to B=1 T and current densities j up to 3 MA cm −2. To reduce the problem of sample heating, we employed a pulsed current technique and developed a new heating correction procedure. We observe a double sign reversal of the Hall resistivity ρ yx in the mixed state, resulting in two peaks which both depend on B and j. The position of the sign changes only varies with B, but is found to be independent of j. The Hall conductivity σ xy turns positive again in the superconducting regime and dramatically increases towards lower temperatures and current densities, as the pinning is reduced by intense currents.

Piezoelectric effects in the optical properties of strained InGaN quantum wells

We investigated the carrier-injection effects in the emission spectrum of strained GaN/InGaN/AlGaN quantum well (QW) blue emitters using a pulsed current excitation technique. Spectral blueshift as large as 80 meV in the emission peak energy was observed as the injection current increases from 1 mA to 1 A. Based on a self-consistent calculation that couples the Poisson equation with a wurtzite-type Rashba–Sheka–Pikus Hamiltonian, four important interactions are evaluated in order to determine the optical properties of InGaN QW. It is shown that the spectral redshifting caused by a piezoelectricity induced quantum confined Stark effect and carrier-induced band gap renormalization is counteracted by a blueshift due to the band filling and charge screening effects. The increase of InGaN QW emission peak energy and intensity with injected carriers suggests a dominant contribution from the latter in a band-to-band recombination process.

Structure asymmetry effects in the optical gain of piezostrained InGaN quantum wells

We investigate the effects of structural asymmetry on the electronic and optical properties of indium gallium nitride (InGaN) quantum wells (QW's). Using a pulsed current excitation technique, spectral blue shift as large as 80 meV is observed in a strained 3.0-nm In/sub 0.2/Ga/sub 0.8/N QW as the pulsed current increases from 1 mA to 1 A. Based on a self-consistent calculation, we are able to quantify a gain competition process among the interactions of piezoelectricity, many-body, charge screening, and band filling effects. Such interactions are shown to provide a mechanism for shaping the QW confined potential such that superior carrier confinement and charge screening of the piezoelectric field can be obtained in the asymmetric InGaN QW. At high carrier injection of N/sub inj/>2/spl times/10/sup 19/ cm/sup -3/, a tenfold increase in the TE-polarized optical gain can be achieved by using the asymmetric GaN-InGaN-AlGaN QW instead of the symmetric InGaN-AlGaN QW. Due to the diminishing of piezoelectricity-induced quantum-confined Stark effect, the calculated optical gain spectra of the asymmetric InGaN QW exhibit a spectral blue shift with respect to those of the symmetric InGaN QW.

The practical realisation of zinc–iron CMA coatings

Abstract A detailed study of the production opportunities for composition modulated alloy electrodeposits by pulsed current techniques with Zinc-Iron alloys is reported. It is shown that by using a chloride solution, with the additional capability of variable agitation rates, a full range of alloy compositions is possible with nanometre layering attainable using single or double bath methods. Furthermore, by the use of a high concentration of ammonium chloride ostensibly as `conductivity' salt, the mechanism of deposition may be modified through control of a thin cathode oxide/hydroxide film.

Expansion of spark channels in air and possibilities of plasma crowbar in high pulsed current and magnetic field technique

The radial expansion velocities and diameters of spark channels in air are measured by the plasma-metal contact method. An LC discharge circuit with energy up to 50 kJ and with current frequencies ranging from ∼102 to ∼104 Hz served as the source of the channels. It is concluded on the basis of the results obtained that the previously proposed automatic plasma crowbar based on the expansion of spark channels in gases is a universal switch for an inductive load in LC discharge circuits that could find very wide applications in high pulsed current and magnetic field technology.

CDW dynamics in the quasi-one-dimensional molecular conductors (Per) 2M(mnt) 2, (M=Au and Pt)

A comparative study by pulsed current techniques of the Charge Density Wave (CDW) dynamics and the temperature dependence of the non-linear electrical transport threshold field, E t, for the quasi-one-dimensional molecular conductors (Per) 2M(mnt) 2, with M=Au and Pt, is presented. The voltage response to unipolar current pulse shows a similar regime in the two compounds with a quite rigid CDW. E t has a fast increase upon cooling in both compounds but has much larger values for M=Pt and presents a non universal temperature dependence.

Electrodeposition of copper and nickel on InBi alloy electrodes

Electrodeposition of copper and nickel was investigated on the 67–33 w/w InBi soft alloy, which has a low melting point (72°C). The electrochemical behaviour of this alloy was found to be similar to that of pure indium in a Watt's type bath. Direct and pulsed current techniques were compared. For copper, a current density ranging from 500 to 1000Am−2 was found to be suitable to obtain regular and shiny deposits at 25°C. For nickel, at 45°C, the current density range used was 500 to 1250Am−2 to obtain good deposits, that were not affected by hydrogen evolution. Pulsed and direct currents were found to be equally efficient. The main interest of electrodeposition on InBi alloy was the ease of removal of the alloy by simple melting, and consequently to make electrodes of unusual shapes which are normally difficult to produce industrially. Only a small amount of InBi alloy remained in the inner part of the deposit.

High current density measurements of giant magnetoresistive spin-valves for magnetic recording and sensor applications

High current density measurements on giant magnetoresistive NiFe-Cu-NiFe-FeMn spin-valve devices are presented. The spin-valve magnetoresistive response, /spl Delta/R/R, is highly temperature dependent; at temperatures near the blocking temperature there is a considerable reduction of pinning field and decrease in device response. It is desirable to measure certain high current density effects such as electromigration and self-field effects separately from the effects of ohmic heating. For this purpose, we introduce pulsed current techniques necessary to reduce ohmic heating in wafer-level device measurements and we deduce a required pulse width of 15 ns to minimize device heating.

An alternating pulse current technique for potentiometric stripping analysis (APC-PSA).

The response signal in PSA measurements - the transition time tau - can be considerably amplified by using a special pulse technique which we call "alternating pulse current (APC)". The sensitivity of the method is thereby considerably increased. This is achieved by modulating the current with a high frequency in a square wave mode during the stripping phase in such a way, that cathodic and anodic pulses alternate, so that the species to be determined is stripped and - at least partially - redeposited during the procedure. The transition time tau is not affected by the absolute values of the currents and of the times they flow. The amplification is determined by their respective ratios only. The equations holding are derived and proven experimentally by measurements on very dilute Pb solutions. The APC-PSA technique has also been applied for systems termed 'irreversible' such as U and Tc, where also a considerable amplification could be achieved. The probable reasons are discussed and future developments and investigations are depicted.

Exploring the dissipative regime of superconductors for practical current-lead applications

A pulsed-current technique was used to measure the extended I–V characteristics of a wide variety of prototype high-temperature-superconductor (HTS) leads. It was found that the average resistivity rises with J(≳Jc) more gradually than in conventional superconductors—often remaining very small compared to silver, for values of J(≫Jc) that are high enough to be practically useful. This observation, combined with the low thermal conductivity (∼50 times smaller than Ag), should extend the utility of HTS leads to the dissipative regime where J/Jc≫1.