Pulsed Current Measurements Research Articles

Analysis of room-temperature results on normally conducting and superconducting channels through polymer films

There are strong reasons from dc and pulsed-current measurements, and from thermal conductivity results, for thinking that narrow channels through films of oxidized atatic polypropylene are superconducting at room temperature. It is thought that the conducting channels, with diameters less than or of the order of a micrometre, are composed of smaller nanofilaments, with diameters of the order of a nanometre. In the present paper a possible explanation is given of measurements which show that the average resistance of non-superconducting channels through films increases with film thickness more slowly than linearly. This result is interpreted in terms of how the Bose condensation temperatures of bosons in arrays of nanofilaments depend on the length and numbers of filaments, and examples are given of parameters of the arrays which could explain the data. The dispersion for the bosons is assumed to consist of a sum of linear and quadratic terms, which is an approximate type of dispersion reported for Cooper pairs. In order to fit the data with the model used, it is necessary to suppose that values of superconducting T c for channels composed of large numbers of filaments are only slightly above room temperature. It is argued that the larger T cs reported in 1989 when currents of 0.5 A were passed through channels may arise because (a) currents concentrate in a subchannel of smaller width than the original channel, and (b) current–current interactions draw the filaments of the subchannel sufficiently close together to increase the transverse bandwidth and T c in the model by the required amount.

Continuous Fabrication of IBAD-MgO Based Coated Conductors

Reel-to-reel tape processing capable of fabricating tens of meters of coated conductor has been set up at Los Alamos National Laboratory. These reel-to-reel facilities include electropolishing, IBAD-MgO texturing, buffer layer deposition, REBaCuO pulsed laser deposition and position dependent critical current and ac-loss measurement. The systems enable development of long-length coated conductors as well as high-throughput experimentation. IBAD-MgO is an extremely fast process, demonstrated at 100 meters/hour, but requires smooth surfaces over long lengths, which is achieved in our laboratory by electropolishing of metal alloy tapes. Barrier and buffer layers are optimized to minimize interdiffusion from the substrate into the superconductor and to maximize crystalline alignment. YBCO is deposited in a multi-zone heater that allows for an engineered heating power profile as the YBCO film is deposited. Currently the best critical current result on a 1 meter length for a 3 /spl mu/m thick YBCO layer is 350 A (self field, 75.5 K).

Influence of the composition on the structure and electrochemical characteristics of the PEFC cathode

The influence the composition of the cathode has on its structure and electrochemical performance was investigated for a Nafion content spanning from 10 to 70 wt.%. The cathodes were formed on a Nafion membrane by the spray method and using 20 wt.% Pt on Vulcan (E-TEK). Materials characterisation (SEM, STEM, gas and mercury porosimetry, electron conductivity) and electrochemical characterisation (steady-state polarisation curve, impedance spectroscopy in O 2 and current-pulse measurements in N 2) were performed. The impedance spectra were analysed using our dynamic agglomerate model. The results indicate that the agglomerate model is valid until a Nafion content of about 45 wt.%. Pt/C and Nafion are homogeneously mixed for any composition and no Nafion film was observed. The cathodes containing 36–43 wt.% Nafion display a single or double Tafel slope behaviour ascribed to diffusion limitations in the agglomerates. At larger Nafion content, the agglomerate model can describe the curves only by assuming a diffusion coefficient 3–4 decades smaller than that of gases. At such compositions, the porosity was only 10%. These results were interpreted as a blocking of the pores and a non-percolating pore system for too large Nafion contents.

Study of single-event current pulses induced in SOI diodes by collimated swift heavy-ions micro-beams

We have succeeded in measuring single-event current pulses in silicon-on-insulator (SOI) structures using collimated heavy-ion beams in conjunction with a wide-bandwidth current–pulse measurement system. The total amount of collected charges for ions incident on the p +n junction indicate collection from below the top silicon layer in the SOI diode. For such high energies, SOI structures were found to be an ineffective means for ensuring the radiation hardness of electronic devices used in space.

Time-Resolved Switching Studies in a-Si:H and Related Films

AbstractSwitching in a-Si:H and a-Si:HNx layers is investigated by pulse current transient and Auger scanning microspectroscopy measurements. Switching in a-Si:H with Ag and Cr contacts exhibits 2 different regimes depending on the voltage pulse polarity. With a positive top Ag contact, switching occurs in nanoseconds after a certain latency time, which depends on voltage exponentially. For a negative Ag contact, there is no latency time provided the voltage exceeds a certain critical value. This might be related to interface effects on contact properties or field-assisted metal diffusion. Scanning Auger element micromaps reveal metallic filaments in the switched films. They contain both Ag and Cr throughout the film thickness. Two phases of the filament formation are suggested – a precursor phase and a post-switching phase characterized by local heating and atomic diffusion. Soft and hard switching are observed in a-Si:HNx films simultaneously and their rates depend strongly on the contact material and applied voltage. Soft switching might be related to the charge trapping in this wide bandgap material.

The Oxide/Nitride Interface: a study for gate dielectrics and field passivation

ABSTRACTThe study of the effects of substrate surface preparation of GaN, both in-situ and ex-situ and the subsequent deposition of dielectric materials is necessary to create a viable GaN FET technology. Surface preparation techniques have been explored using RHEED, AES, SIMS and C-V measurements to produce films of low interface trap density, 1–2E11 eV−1cm−2. A similar study of the as-fabricated HEMT surface was carried out to create a cleaning procedure prior to dielectric passivation. Dielectric films of Sc2O3 and MgO were deposited via gas-source MBE. Post-deposition materials characterization included AES, TEM, XRR and XPS, as well as gate pulse and isolation current measurements for the passivated HEMT devices. From this study, the relationship between the interface structure and chemistry and the quality of the oxide/nitride electrical interface has been determined. The resulting process has led to the near elimination of the current collapse phenomenon. In addition, the resulting oxide/nitride interface quality has allowed for the first demonstration of inversion in GaN.

Studies of single-event transient current induced in GaAs and Si diodes by energetic heavy ions

To investigate the tolerance of single-event upset (SEU) for GaAs devices, we have measured the transient currents induced in GaAs diodes by energetic heavy particles such as 15 MeV oxygen ions. The measurements were made by combining a focused ion microbeam and a wide-bandwidth current-pulse measurement system. The resulting transients were found to be markedly different to those measured in Si diodes. This result can be mostly attributed to a difference in the carrier drift velocities between GaAs and Si. The amount of collected charge in GaAs diodes was also found to be less than that in Si diodes with similar device properties. This fact strongly suggests that GaAs devices have a higher SEU tolerance than Si devices.

Phase slippage at the interface: normal metal/sliding charge-density wave

Phase slippage is required at the current electrodes of quasi-one-dimensional conductors with a charge density wave (CDW) ground state for the conversion from free to condensed carriers. We have performed at the ESRF high-resolution X-ray measurements of the spatially varying shift q( x) of the CDW satellite wave vector between current contacts on a thin NbSe 3 whisker in the sliding state. Applying direct currents, we observe at 90 K a steep exponential decrease of the shift within a few hundred microns from the contact. The CDW strain profile q( x) reflects the carrier conversion process, via nucleation and growth of phase-dislocation loops. Pulsed current measurements of the shift q show important differences between pulsed and dc current data, revealing a spatially dependent relaxational behaviour of the CDW strain. Using time-resolved high spatial resolution X-ray we observe at 300 μm from the electrode a stretched exponential-type decay of the shift q( t) upon switching off the current ( T=75 K) : q(t)=q 0[ exp(−t/τ) μ] with τ=23 ms and μ=0.36.

Frequency domain lifetime characterization

Time-based measurements are commonly used for lifetime characterization of semiconductors. We have developed the theory, verified by experiment, of frequency-based lifetime characterization as an alternative to time-based measurements for MOS devices biased in inversion. One consideration during lifetime/diffusion length measurements, is whether the near-surface space-charge region or the bulk or quasineutral region is characterized. To characterize the near-surface space-charge region of the device, one usually makes room temperature pulsed MOS capacitor or diode leakage current measurements. We show that room-temperature, frequency-domain capacitance, conductance, or resistance measurements characterize the quasineutral bulk, not the space charge region, in contrast to room-temperature pulsed MOS-C or diode leakage current measurements which characterize the space-charge region.

Current conversion in the sliding charge density wave state of NbSe 3

High spatial resolution (30–100 μm) X-ray measurements of the satellite positional shift q have been carried out in the upper charge-density-wave state of NbSe 3. We determined the spatial distribution of the CDW plastic deformations between electrodes under applied currents. With direct current we observe a steep exponential decrease of q with position x along the sample near the contacts followed by a linear dependence in the central part of the sample. This second regime is attributed to transverse pinning of the CDW dislocations while the first is dominated by phase slip processes. The observed differences between data from direct current and pulsed current measurements indicate a significant, strongly spatially dependent relaxational behaviour of the CDW deformations.

High pulsed current and voltage measurements on sintered BiPbSrCaCuO ceramics superconductors

High pulsed currents attaining 3300 A with a duration varying from 1.25 to 20 milliseconds have been applied at 77 K to sintered Bi1.8Pb0.4Sr2Ca2.2Cu3O10.3+x long bars. Measurements of voltage versus applied current determine the critical current Ic = 60 A, and the conditions for superconductivity recovery. Two types of behaviour are observed: (i) When the mean temperature T raises up to T < 95 K a critical current is recovered at the end of the pulse. (ii) When the temperature goes beyond 95 K, Ic goes to zero after the pulse. The temperature increases up to 43 K for a 20 ms pulse. No deterioration in the sample is observed and experiments can be reproducibly repeated. Voltages as high as 90 V have been measured, corresponding to an electrical field of 1100 V/m. The maximum instantaneous power applied to the material is 180 GW/m3 and the maximum energy per unit volume transmitted to the sample after a 20 ms pulse is 75 MJ/m3.

Non-invasive assessment of skin barrier integrity and skin irritation following iontophoretic current application in humans

Iontophoresis is believed to have great added value for the delivery of drugs to the systemic circulation via the transdermal route. Whether iontophoretic transdermal systems can be applied for a wide variety of drugs, will at the same time depend on the intrinsic safety of current application to the skin. We investigated the effect of electric current in vivo on human skin by measuring two different aspects of the irritant response: (a) the effect on the stratum corneum barrier function by measuring transepidermal water loss (TEWL); (b) effects at the level of the dermal vascular bed by measuring the erythematous response with Laser Doppler Flowmetry (LDF). Nine volunteers entered the study. DC (direct constant current) profiles were applied to the volar forearms of the subjects for 30 min. Following the application of DC and pulsed DC current profiles, TEWL and LDF measurements were carried out on these sites, on so-called passive control sites (occluded by the application chamber containing the buffer solution, but without applying current) and on untreated sites. TEWL values measured directly after patch removal were 1.5-2.0 times higher than the baseline values for TEWL regardless the presence or absence of electric current. The effect of the electric current application on the TEWL followed the same relaxation profile as the effect of buffer application without current. The erythematous response however did differ significantly from the controls. The response on the anodal sites was 1.5–2.7 times higher than at the cathodal sites. The erythema lasted from 25 to 69 min. The time span for relaxation of the erythema was independent of the current profile (DC vs. pulsed DC) but was longer at the anodal sites than at the cathodal sites. It was concluded from this study that after a single current application the effect on the barrier to water loss is negligible because the effect on TEWL cannot be distinguished from the occlusive effect resulting from the application of the aqueous buffer alone, without current. The erythematous response to current application was relatively moderate and dependent on current direction.

Electrochemical Behavior of LiMn2 O 4 and LiCoO2 Thin Films Produced with Pulsed Laser Deposition

Thin films of and have been prepared by pulsed laser deposition on heated stainless steel substrates. These films have thicknesses from 0.2 to 1.5 μm and are crystalline without postdeposition annealing. The films' electrochemical properties were studied with cyclic voltammetry, current pulse measurements, and galvanostatic charge/discharge techniques. Film capacity densities as high as 56 and 62 μAh/cm2‐μm were measured for and , respectively. Chemical diffusivities on the order of and were measured for and , respectively. Some of the films were cycled electrochemically for up to 300 cycles against lithium metal in 1 M /propylene carbonate electrolyte, demonstrating the promise of pulsed laser deposition for the production of cathode films for rechargeable lithium microbatteries.

Statistical Studies of the Anisotropy of the Pinning Mechanism in YBaCuO Films Deposited on Polycrystalline YSZ Substrates

We have studied the I-V characteristics and the behaviour of the critical current density of YBa 2 Cu 3 O 7-x films, prepared by sputtering, at different values of applied magnetic field and of temperature. The superconductor was deposited onto polycrystalline ZrO2 (8 mol % Y 2 O 3 ). The I-V characteristics were obtained by using a short current pulse measurement technique in order to avoid heating phenomena. We have used a theoretical model to describe the I-V characteristics, based on a statistical treatment of the motion of vortices in an arbitrary pinning potential. Besides the determination of the critical current density J c , this model allows to characterise the pinning sites by a function giving their energetic distribution. In this way we have been able to make a clear distinction between intrinsic and extrinsic pinning in sputtered YBa 2 Cu 3 O 7-x films.

Evaluating the effects of optical and carrier losses in etched-post vertical cavity lasers

We demonstrate the combined effects of optical scattering loss and surface recombination (or carrier diffusion) on the performance and scalability of etched-post vertical cavity lasers (VCLs). The size dependence of optical losses and threshold gain are determined from pulsed measurements of external quantum efficiency. Deeper etch depths result in a stronger radial dependence of the threshold gain, which quickly increases the threshold current density. With optical loss accounted for, pulsed threshold current density measurements give the extra information needed for evaluating carrier loss. Surface recombination or carrier diffusion also results in threshold current density increases, but scalability is ultimately limited by the ability of the active region to provide enough gain for smaller size, higher optical loss devices. Even with these losses, three-quantum-well VCLs with shallow etches have threshold currents as low as 420 μA.

Characterization of thin p-clad InGaAs single-quantum-well lasers

Low-ridge, thin p-clad InGaAs single-quantum-well (SQW) lasers with shiny Au contacts have been fabricated using a pulsed anodic oxidation technique. Although the ridge is very low (130 nm), the lateral refractive index step is sufficiently large to provide strong lateral waveguiding. Pulsed current measurements of threshold current, operating wavelength, and lateral far field as a function of ridge width are presented and compared with measured values reported for low-ridge and oxide-defined lasers fabricated from conventional thick p-clad material. The CW performance characteristics of five-micron-wide, low-ridge, thin p-clad lasers are shown to be comparable to those of conventional InGaAs SQW high-ridge waveguide lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Pulse measurements using LED in dielectric liquids under impulse voltages

Current pulses, in the form of partial discharges, are known to be precursors of electrical breakdown in dielectric liquids. The precise measurement of these pulses is of great value for the understanding of the breakdown mechanism. The measurement of current pulses under impulse voltage is very difficult when using conventional electrical measurement systems. To overcome this difficulty, a novel measuring system using fiber optics and a light emitting diode (LED) has been developed. In this paper, the improvements in time response and charge sensitivity of such a LED current-pulse measurement system are described. Using this system and a high speed shadow-graph assembly under impulse conditions, the current pulse and the streamer initiation in dielectric liquids are measured simultaneously.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Practical PCA-DA system for measurement of leakage current pulses on polymer insulators under wet contaminated conditions

A practical PC-DA system for automatically measuring and recording leakage currents and discharge pulses over long periods is described. A standard PC with an industry standard analog-to-digital converter plug-in board is used in the system. The software is designed to sample eight channels (insulators) up to 1000 Hz each. The test results from studies of insulators in a fog-chamber environment and in outdoor polluted conditions are reported. The performance of the system has been satisfactory.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Recent advances in partial discharge measurement capabilities at NIST

Three techniques for measuring the properties of partial discharges (PDs) are described. The first is concerned with an advanced, real-time PD measurement system that allows a complete characterization of the stochastic properties of PD. With this system it is possible to measure a set of conditional PD pulse-amplitude and pulse-time separation distributions from which memory effects characteristic of the discharge phenomena can be quantified and interpreted. Results for pulsating negative corona discharges in gases are shown. The second technique allows PD location in cables using time-domain reflectometry with appropriate statistical analysis. With the third technique, simultaneous measurements are made of the optical and electrical characteristics of PD in liquid dielectrics using fast photography combined with broadband. low-noise pulse current measurements. >

CDW depinning in the blue bronze: A study bby current pulse measurements, proton channeling, electron paramagnetic resonance

CDW depinning in the blue bronze has been investigated using three techniques: voltage response to current pulses, proton channeling and electron paramagnetic resonance. The voltage response involves four distinct regimes as the amplitude of the pulse increases. The backscattering yield of protons increases when the applied dc voltage exceeds the threshold value for depinning. An asymmetry of the EPR spectra at low temperatures is found as the threshold is approached from below. These results indicate that CDW structural defects play a predominant role in the depinning process.