Large DC Currents Research Articles

Domain Transformation and MI of Melt-extracted Co68.15Fe4.35Si12.25B13.25Nb1Cu1 Microwires by Cryogenic Joule Annealing

Giant magneto-impedance (GMI) effect and domain transformation for melt-extracted Co68.15Fe4.35Si12.25B13.25Nb1Cu1 amorphous wires have been induced by a cryogenic Joule annealing (CJA) treatment with different DC current amplitude (0~350 mA) for 240s. Experimental results indicate that the maximum GMI ratio ([ΔΖ∕Ζ0]max) achieves to 188.1% with exciting field increasing to 1.8 Oe monotonically for 300 mA annealing treated wires, which can attribute to the surface complex domain structure change formed by CJA treatment. The liquid nitrogen can protect the wire from crystallization when applied large DC currents. Moreover, the CJA treatment can improve the response sensitivities effectively. These remarkable characteristics make the melt-extracted microwires by CJA tailoring as the promising candidate material for small-sized magnetic sensors.

Experimental analysis of biomass pyrolysis using microwave-induced plasma

Energy-from-waste, a process that converts high calorific wastes to energy, has posed a possible renewable energy route, in addition to reducing waste volumes being sent to landfill. One technology that is effective at near complete organic–inorganic dissociation is plasma gasification. However, a precursor to generating this plasma is to create an electrical arc by a large DC current, which is highly energy intensive. This study, however, examines a novel method of producing plasma by microwaves, which is much more energy efficient. To test its suitability in waste and biomass treatment, three 10g triplicate waste wood (biomass) samples was pyrolysed using microwave-induced plasma in a lab-based reactor. The resultant gas was siphoned from the reactor exhaust pipe and characterised using a Gasmet DX400 FTIR analyser. To determine the proportion of gas flow between the siphon tube and exhaust, a mass balance model of the system was constructed. After applying the appropriate correction factor, the mean mass ratio of liquid, solid, and gas was found to be 66:20:13. The start and final masses were recorded and compared with literature values. Mean mass loss was determined to be 7.96g (79.6 wt.%), which is indicative of complete pyrolysis.

Chirality control of magnetic vortex in a square Py dot using current-induced Oersted field

We have proposed a method for controlling the vortex chirality in a squared permalloy dot by using the circular Oersted field locally induced by flowing a DC current across a small Py/Cu junctions. The reliability of the chirality control has been evaluated by measuring the nonlocal spin valve signal. The desired vortex chirality has been obtained when the injecting DC current has a moderate magnitude. However, the large DC current is found to reduce the control reliability. Another possibility for controlling the vortex structure using the large DC current injection was also discussed.

Switching behavior of La1−xCaxMnO3 thin films grown on SrTiO3 substrates

We report the switching behavior of resistive states induced by current flowing in La1−xCaxMnO3 films. The transport behavior upon treatments by applying a large dc current has been investigated in the absence of magnetic field. A switching from low to high resistive state was found upon applying a large dc current for the both compositions. The critical currents for the switching are 10.5mA (density of 2.3×103A∕cm2) and 6.7mA (density of 1.6×103A∕cm2) for compositions x=0.2 and 0.3, respectively. Our repeated measurements indicate that the high resistive state appeared in the film x=0.2 is much stable. Exposing the sample in air and room temperature does not cause any change of the transport properties. However, for the film with x=0.3, the induced high resistive state behaves with unstable characteristics. Keeping the sample at a low temperature of 20K, a relaxation to a higher resistive state takes place. The mechanism of phase separation was taken into account in the interpretation of the observed phenomena.

The abnormal electroresistance behavior observed in epitaxial La 0.8Ca 0.2MnO 3 thin films

La 0.8Ca 0.2MnO 3 epitaxial thin films were fabricated on SrTiO 3 (100) substrates by using pulsed-laser deposition (PLD) technique. The transport behavior upon treatments by applying a large dc current has been investigated. It is found that when the applied dc current exceeds a threshold value the electric resistivity in these films could be significantly enhanced in the whole measured temperature range from 10 to 300 K. Simultaneously, abnormal electroresistance behavior appears. The enhanced resistance turns out to be sensitive to a weak current. Even a very small dc current could remarkably depress the high resistance, showing an unusual colossal ER effect. ER reaches ∼1175% at temperatures lower than ∼50 K, and ∼705% at 300 K for a current changing from 0.7 to 11 μA. Structure measurements on micro-area by micron-XRD technique were carried out. No change of the structure and lattice parameter was found upon current treatments.

Thermal modulator array for analyte modulation and comprehensive two‐dimensional gas chromatography

AbstractSegmented thermal modulators were fabricated for the modulation of column effluent in one‐dimensional gas chromatography, comprehensive two‐dimensional gas chromatography, and as sample introduction devices in other analytical techniques. Thermal desorption of analytes from the modulator was achieved by computer‐controlled application of large DC currents at low voltages sequentially to consecutive segments or stages of the modulator heaters for periods between 100 and 400 ms per segment. This procedure produced sharply focussed analyte peaks. Unsegmented heaters and heaters having 2, 4, 6, 8, and 10 segments or stages were compared with respect to their focusing efficiency. The 10‐stage heater was found to give sharper focusing than heaters with a smaller number of segments. Analyte pulses with peak widths at half‐height of ca. 150 ms were generated by the 10‐stage heater. The application of a 10‐stage modulator in a comprehensive two‐dimensional analysis of a light petroleum condensate gave results of acceptable quality.

Synthetic analysis of periodically stimulated excitable and oscillatory membrane models

Many excitable neuronal membranes become oscillatory when stimulated by large enough dc currents. In this paper we investigate how the transition from excitable to oscillatory regimes affects the response of the membrane to periodic pulse trains. To this end, we examine how the dynamics of periodically stimulated FitzHugh-Nagumo neuron model changes as the system switches from excitability to oscillation. We show that, despite the important change in the asymptotic dynamics of the unperturbed model, $p:q$ phase-locking (i.e., the model membrane discharges q times in p interstimulus intervals and q input-output intervals repeat periodically) regions in the stimulus period-stimulus amplitude parameter plane (Arnold tongues) change continuously when the model changes from excitable to oscillatory. We provide further evidence for the continuous change of the Arnold tongues by using an analytically tractable one-dimensional map that approximates the Poincar\'e map of the forced system. We argue that the smooth change in the Arnold tongues results from the fact that, despite the qualitative difference between the asymptotic dynamics of unforced excitable and oscillatory regimes, other aspects of the dynamics such as the wave form of individual action potentials, are similar in the two regimes.

Effects of surface oxidation on magnetic properties of amorphous alloy for choke core use

In order to develop a non-gap type choke core, effects of annealing in oxygen atmosphere on magnetic properties for FeCrBSiC amorphous alloy were investigated. After annealing for 2 hrs at 480/spl deg/C, optimum magnetic properties for choke core were obtained, where the effective permeability was about 400 and was almost constant up to 1 MHz, and the decrease in permeability upon large bias dc current of 16 A was very little. Furthermore it was shown that the ac magnetic loss was very low as compared to the conventional choke cores. With the increase in annealing temperature the magnetic hysteresis loop was inclined without a substantial increase of coercive force and a microstructural change of amorphous phase up to 480/spl deg/C. It was considered that these choke properties were basically due to the interaction between oxidation-induced stress and magnetostriction which brought about the induced magnetic anisotropy perpendicular to the ribbon surface, and also the good soft magnetic properties of amorphous phase.

The low frequency conductance of bipolar membranes demonstrates the presence of a depletion layer

The electrical conductance of a 2-sheet bipolar membrane was measured as a function of frequency over the range 0.1 Hz–10 kHz in electrolyte solutions containing 1, 10 and 100 mM KCl. The 2-sheet bipolar membrane was also separated into its monopolar regions and their electrical properties were examined individually. The low frequency conductance of the bipolar membrane was always significantly less than the value which would have been obtained if the monopolar regions were too far apart to interact. The data suggest that the separation of the ion exchange regions was approximately ≤1 nm, even in the absence of a large DC current.

A false-balance-avoidance transductor for measuring large DC currents

A two-core feedback-controlled transductor for measuring large dc currents is presented. It is capable of measuring bidirectional dc currents with the accuracy of 100 ppm and of returning to its normal operating condition once it is stuck in the false balance state. It can be applied to the situation in which the abrupt jumps of the busbar current take place frequently.

Magnetic field dependence of threshold for chaotic acoustoelectric oscillations in ndashInSb at 77 K

We report on experimental evidences that identify more clearly the piezoelectric origin of chaotic acoustoelectric voltage oscillations observed in ndashInSb in a transverse magnetic field at 77 K. Measurements of the threshold current for chaotic behaviour showed that oscillations are reduced in amplitude when large DC currents driving the chaos are injected in a direction perpendicular to the [1 1 1] piezoelectric orientation as expected. The threshold current density for both nonlinear and chaotic behaviour is seen to decrease with the magnitude of the transverse magnetic field. Furthermore, we observed that the chaotic threshold current density saturates at high field towards a value consistent with the zero field threshold for a nonlinear acoustoelectric instability. This behaviour can be surprisingly interpreted with a linear model of the acoustic wave gain used to explain sound amplification and domain formation in Insb along a piezoelectric direction.

Characterization of Polymer Electrolyte Fuel Cells Using AC Impedance Spectroscopy

The ac impedance spectra of polymer electrolyte fuel cell (PEFC) cathodes measured under various experimental conditions are analyzed. The measurements were carried out in the presence of large dc currents. The impedance spectrum of the air cathode is shown to contain two features: a higher frequency loop or arc determined by interfacial charge‐transfer resistance and catalyst layer properties and a lower frequency loop determined by gas‐phase transport limitations in the backing. The lower frequency loop is absent from the spectrum of cathodes operating on pure oxygen. Properties of measured impedance spectra are analyzed by a PEFC model to probe the effect of ac perturbation. Comparison of model predictions to observed data is made by simultaneous least squares fitting of a set of spectra measured for several cathode potentials. The spectra reveal various charge and mass‐transfer effects in the cathode catalyst layer and in the hydrophobic cathode backing. Three different types of losses caused by insufficient cell hydration, having to do with interfacial kinetics, catalyst layer proton conductivity, and membrane conductivity, are clearly resolved in these impedance spectra. The data reveal that the effective tortuous path length for gas diffusion in the cathode backing is about 2.6 times the backing thickness.

Transition from steady to periodic liquid‐metal magnetohydrodynamic flow in a sliding electrical contact

In homopolar motors and generators, large dc electric currents pass through the sliding electrical contacts between rotating copper disks (rotors) and static copper surfaces shrouding the rotor tips (stators). A liquid metal in the small radial gap between the rotor tip and concentric stator surface can provide a low‐resistance, low‐drag electrical contact. Since there is a strong magnetic field in the region of the electrical contacts, there are large electromagnetic body forces on the liquid metal. The primary, azimuthal motion consists of simple Couette flow, plus an electromagnetically driven flow with large extremes of the azimuthal velocity near the rotor corners. The secondary flow involves the radial and axial velocity components, is driven by the centrifugal force associated with the primary flow, and is opposed by the electromagnetic body force, so that the circulation varies inversely as the square of the magnetic‐field strength. Three flow regimes are identified as the angular velocity Ω of the rotor is increased. For small Ω, the primary flow is decoupled from the secondary flow. As Ω increases, the secondary flow begins to convect the azimuthal‐velocity peaks radially outward, which in turn changes the centrifugal force driving the secondary flow. At some critical value of Ω, the flow becomes periodic through the coupling of the primary and secondary flows. The azimuthal‐velocity peaks begin to move radially in and out with an accompanying oscillation in the secondary‐flow strength.

Vacuum switchgear for fusion experiments

Fusion experiments frequently have demanding switching requirements that cannot be satisfied by conventional switchgear. Two frequent requirements and specialized vacuum switches designed to meet them are presented. Experimental results from testing of prototype devices are discussed. A frequently encountered switch application is the interruption of a high level, pulsed, dc current followed by a fast rising recovery voltage. A 7-in. vacuum interrupter with an axial magnetic field has proven to be an effective switch for currents up to 25 kA. Beyond 25 kA, a parallel configuration becomes necessary. The influence of the axial magnetic field on the behavior of parallel vacuum interrupters and test results up to 50 kA are discussed. Also, test results on experimental devices with novel electrode geometries such as the rod-array vacuum interrupter and the Amsler contact interrupter are presented. Interrupters which can carry large dc currents on a continuous basis are frequently required in fusion applications. This requirement occurs in superconducting coil protection circuits and generally involves recovery voltages of 5 kV or less. A specially designed vacuum interrupter with water-cooled electrodes has been used to extend the continuous current rating of a commercial interrupter from 1200 A to 13 kA. Two second generation devices with special electrode materials are currently being developed for use with a special actuator and are intended to extend the continuous rating to 25 kA.

A Pulsed Hall Device Gaussmeter

A crucial measurement in the Astron device of the controlled thermonuclear research program is to monitor a small (10-G) transient magnetic field caused by a moving group of electrons. The Astron device involves high currents, large magnetic fields, radiation problems, and gigantic radio frequency (RF) noise. The evolution of a Hall device gaussmeter to directly measure this phenomenon is explained. The result is a simple device for the measurement of a small signal buried in noise of the combined form of large magnetic field, RF power, gamma radiation, and high dc currents.

Design Fundamentals of Current Transductors for Large DC Currents

Design Fundamentals of Current Transductors for Large DC Currents

Untersuchungen über die statistischen Schwankungen der infraroten Strahlung einer Hochdruck-Hochstrom-Argonentladung

An argon discharge at high pressure (up to 20 atmosph.) and large dc current is described. This discharge is used in the investigation of spontaneous fluctuations of photon radiation. Electron and ion densities are calculated as functions of pressure and temperature from which the continuous emission and absorption coefficients are determined. By comparing the calculated emission with the measured distribution of the infrared spectral radiation intensity in a disk of plasma, which is 0,2 mm in front of the cathode, the radial temperature distribution in the disk is determined. The temperature in the arc axis was found to be independent of the gas pressure and must be higher than 28 000°K. Investigations on electrical conductivity confirm the temperature distributions. Finally, the statistical radiation fluctuations can be calculated by means of the equations given in Part I and are compared with the experimental results. The infrared detector used for all these measurements is also described in Part I. A satisfactory coincidence between calculated and measured values was found.

Theory of Simulated-Skin-Effect Filters a Thin Film Approach to EMI

The theory of a new, ceramic lowpass filter, highly effective at very high frequencies, is discussed. Its excellent performance is based on simulation of the skin effect whereby the VHF, UHF and microwave currents are forced along a thin, electrically permeable resistive film deposited inside a high-? dielectric tube while a large dc current may flow through the coaxial conductor. Theory and experiment are in excellent agreement. In several respects these filters are better than cascaded filters (ferrites and dielectrics combined) which were, thus far, considered the best available. The simulated-skin-effect filter shows great promise of becoming a significant means in combatting conducted electromagnetic interference.