Currents In Excess Research Articles

Asymmetrical electrohydrogenation of CO2 to ethanol with copper–gold heterojunctions

Copper is distinctive in electrocatalyzing reduction of CO2 into various energy-dense forms, but it often suffers from limited product selectivity including ethanol in competition with ethylene. Here, we describe systematically designed, bimetallic electrocatalysts based on copper/gold heterojunctions with a faradaic efficiency toward ethanol of 60% at currents in excess of 500 mA cm-2. In the modified catalyst, the ratio of ethanol to ethylene is enhanced by a factor of 200 compared to copper catalysts. Analysis by ATR-IR measurements under operating conditions, and by computational simulations, suggests that reduction of CO2 at the copper/gold heterojunction is dominated by generation of the intermediate OCCOH*. The latter is a key contributor in the overall, asymmetrical electrohydrogenation of CO2 giving ethanol rather than ethylene.

Large-Amplitude Internal Wave Transformation into Shallow Water

Abstract Large-amplitude internal solitary wave (ISW) shoaling, breaking, and run-up was tracked continuously by a dense and rapidly sampling array spanning depths from 500 m to shore near Dongsha Atoll in the South China Sea. Incident ISW amplitudes ranged between 78 and 146 m with propagation speeds between 1.40 and 2.38 m s−1. The ratio between wave amplitude and a critical amplitude A0 controlled breaking type and was related to wave speed cp and depth. Fissioning ISWs generated larger trailing elevation waves when the thermocline was deep and evolved into onshore propagating bores in depths near 100 m. Collapsing ISWs contained significant mixing and little upslope bore propagation. Bores contained significant onshore near-bottom kinetic and potential energy flux and significant offshore rundown and relaxation phases before and after the bore front passage, respectively. Bores on the shallow forereef drove bottom temperature variation in excess of 10°C and near-bottom cross-shore currents in excess of 0.4 m s−1. Bores decelerated upslope, consistent with upslope two-layer gravity current theory, though run-up extent Xr was offshore of the predicted gravity current location. Background stratification affected the bore run-up, with Xr farther offshore when the Korteweg–de Vries nonlinearity coefficient α was negative. Fronts associated with the shoaling local internal tide, but equal in magnitude to the soliton-generated bores, were observed onshore of 20-m depth.

The Mechanisms of Tsunami Amplification and the Earthquake Source of the 2021 M 7 Acapulco, Mexico, Earthquake

ABSTRACT We present a slip model for the 2021 M 7 Acapulco, Mexico, earthquake produced by inversion of strong motion, Global Navigation Satellite Systems, tide gauge, and Interferometric Synthetic Aperture Radar data. The earthquake occurs within the Guerrero gap, identified as a region of concern for its seismogenic potential and paucity of large events. We find that rupture was compact, constrained to depths between 10 and 20 km, and consistent of two main slip patches. The slip model leaves a broad swath of the megathrust unbroken, and, whether the event signals a reactivation of large earthquakes in the region remains unknown. We find that tide gauge recordings inside Acapulco Bay for the M 7 1962 earthquake and the 2021 event are strikingly similar, thus we interpret this as weak evidence that 2021 is a repeat of 1962. We also produce a high-resolution hydrodynamic model of the resulting tsunami using the slip model as initial condition and place special emphasis in understanding the long duration (∼17 hr) of waves inside the bay. We find that simple bay resonance alone does not account for the features of the event. Rather it is a complex interaction with shelf modes and edge waves that continuously re-excite the bay resonance that leads to the protracted tsunami disturbances. Furthermore, we find that significant currents in excess of 1 m/s occur in localized portions of the bay even when wave amplitudes remain small.

P-type electrical contacts for 2D transition-metal dichalcogenides.

Digital logic circuits are based on complementary pairs of n- and p-type field effect transistors (FETs) via complementary metal oxide semiconductor technology. In three-dimensional (3D) or bulk semiconductors, substitutional doping of acceptor or donor impurities is used to achieve p- and n-type FETs. However, the controllable p-type doping of low-dimensional semiconductors such as two-dimensional (2D) transition-metal dichalcogenides (TMDs) has proved to be challenging. Although it is possible to achieve high-quality, low-resistance n-type van der Waals (vdW) contacts on 2D TMDs1-5, obtaining p-type devices by evaporating high-work-function metals onto 2D TMDs has not been realized so far. Here we report high-performance p-type devices on single- and few-layered molybdenum disulfide and tungsten diselenide based on industry-compatible electron beam evaporation of high-work-function metals such as palladium and platinum. Using atomic resolution imaging and spectroscopy, we demonstrate near-ideal vdW interfaces without chemical interactions between the 2D TMDs and 3D metals. Electronic transport measurements reveal that the Fermi level is unpinned and p-type FETs based on vdW contacts exhibit low contact resistance of 3.3 kΩ µm, high mobility values of approximately 190 cm2 V-1 s-1 at room temperature, saturation currents in excess of 10-5 A μm-1 and an on/off ratio of 107. We also demonstrate an ultra-thin photovoltaic cell based on n- and p-type vdW contacts with an open circuit voltage of 0.6 V and a power conversion efficiency of 0.82%.

Silicon strip defects and their impact on electrical performance of readout electronics

In preparation for the High Luminosity LHC (HL-LHC) runs, the ATLAS inner detector will be completely replaced with an all silicon Inner Tracker (ITk). Hybrid silicon pixel modules will be used for the innermost tracking layers, and silicon micro-strip detectors will be used the outer layers of the tracker. During the production of the detector, the sensors, readout electronics, and other components will undergo a series of quality control (QC) and quality assurance tests. Defects in the fabrication of the sensors will be flagged early in the manufacturer's and ATLAS QC tests. A study of the influence of sensor defects was performed to assess the characteristics of these defects in completed modules, and whether any defect posed a risk to the operation of the front-end readout electronics. All defects were found to have no impact on the performance of the front-end readout electronics for healthy amplifier channels, and defects that short the coupling between the strip implant and the metal readout electrode were only noticeable for strip leakage currents in excess of 250 nA, beyond the end-of-life currents expected for most sensors at the HL-LHC.

Enhancement-mode atomic-layer thin In2O3 transistors with maximum current exceeding 2 A/mm at drain voltage of 0.7 V enabled by oxygen plasma treatment

In this Letter, enhancement-mode operation in devices with 1.5 nm atomic-layer thin In2O3 channels over a wide range of channel lengths down to 40 nm is demonstrated using an O2 plasma treatment at room temperature. Drain currents (ID) in excess of 2 A/mm at a drain-to-source bias (VDS) of 0.7 V are achieved in enhancement mode with significantly improved subthreshold swing down to near-ideal 65 mV/dec, suggesting that O2 plasma treatment is very effective at reducing bulk and interface defects. By using low-temperature O2 plasma, the fabrication process remains back-end-of-line compatible while enabling a clear route toward high-performance In2O3 transistors and circuitry.

High Precision Wide Bandwidth DC Current Transducer Based on the Platiše Flux Sensor

In the last decade, we observed a noticeable increase in direct-current systems (DC), particularly in solar power generation, grid storage systems, and electric mobility. Some of these systems may require high-voltage isolation and peak currents in excess of kA. The existing standard compact and lower cost current sensing solutions hardly ever achieve an overall measurement uncertainty below 1% mainly due to offsets and hysteresis; their typical bandwidth is about 250 kHz, and they may also be noisy. This article presents a new method of isolated DC and AC current measurement based on a single gapless core and the innovative Platiše Flux Sensor. After verification in a mixed-signal simulator, the method was implemented in a functional prototype of a DC current transducer (CT) and thoroughly tested in a reference setup. The performance tests showed a low offset and hysteresis, a bandwidth in the MHz range, low power consumption, and low noise operation. Furthermore, the low current transducer achieved a typical uncertainty of less than 0.2% and a linearity of less than 200 ppm, which indicates an overall superior performance compared to representative comparable CTs based on alternative technologies. In addition to the areas of application mentioned above, the new type of DC-CT can be used for general purpose metering, measurement instrumentation, and high power DC and AC systems.

Superconducting microwire detectors based on WSi with single-photon sensitivity in the near-infrared

We report on the fabrication and characterization of single-photon-sensitive WSi superconducting detectors with wire widths from 1 μm to 3 μm. The devices achieve the saturated internal detection efficiency at a wavelength of 1.55 μm and exhibit maximum count rates in excess of 105 s−1. We also investigate the material properties of the silicon-rich WSi films used for these devices. We find that many devices with active lengths of several hundred micrometers exhibit critical currents in excess of 50% of the depairing current. A meandered detector with a 2.0 μm wire width is demonstrated over a surface area of 362 × 362 μm2, showcasing the material and device quality achieved.

A digital feedback controller for stabilizing large electric currents to the ppm level for Feshbach resonance studies.

Magnetic Feshbach resonances are a key tool in the field of ultracold quantum gases, but their full exploitation requires the generation of large, stable magnetic fields up to 1000 G with fractional stabilities of better than 10-4. Design considerations for electromagnets producing these fields, such as optical access and fast dynamical response, mean that electric currents in excess of 100 A are often needed to obtain the requisite field strengths. We describe a simple digital proportional-integral-derivative current controller constructed using a field-programmable gate array and off-the-shelf evaluation boards that allows for gain scheduling, enabling optimal control of current sources with non-linear actuators. Our controller can stabilize an electric current of 337.5 A to the level of 7.5 × 10-7 in an averaging time of 10 min and with a control bandwidth of 2 kHz.

Use of tapered Pyrex capillary tubes to increase the mechanical stability of multiwall carbon nanotubes field emitters

In this study, NanocylTM NC 7000 Thin Multiwall Carbon Nanotubes (MWCNTs) were used with a high aspect ratio (>150) made by the process of catalytic chemical vapor deposition (CCVD). The field emitter tips were prepared by inserting these MWCT into fine glass capillary tubes that were pulled at high temperatures and then cut. Measurements were carried out under ultra-high vacuum (UHV) conditions with a base pressure of 10−9 mbar. The data show the effects of initial conditioning of MWCNT and hysteresis. Compression of the MWCNT by the capillary tubes appears to provide adequate mechanical support without requiring the use of a low-melting point electrically-conductive binder as has been used previously. Emission currents in excess of 1 μA were obtained so this technique shows promise as a reliable, stable, powerful electron source.

Carbon nanotube wires with continuous current rating exceeding 20 Amperes

A process to fabricate carbon nanotube (CNT) wires with diameters greater than 1 cm and continuous current carrying capability exceeding 20 A is demonstrated. Wires larger than 5 mm are formed using a multi-step radial densification process that begins with a densified CNT wire core followed by successive wrapping of additional CNT material to increase the wire size. This process allows for a wide range of wire diameters to be fabricated, with and without potassium tetrabromoaurate (KAuBr4) chemical doping, and the resulting electrical and thermal properties to be characterized. Electrical measurements are performed with on/off current steps to obtain the maximum current before reaching a peak CNT wire temperature of 100 °C and before failure, yielding values of instantaneous currents in excess of 45 A for KAuBr4 doped CNT wires with a diameter of 6 mm achieved prior to failure. The peak temperature of the wires at failure (∼530 °C) is correlated with the primary decomposition peak observed in thermal gravimetric analysis of a wire sample confirming that oxidation is the primary failure mode of CNT wires operated in air. The in operando stability of doped CNT wires is confirmed by monitoring the resistance and temperature, which remain largely unaltered over 40 days and 1 day for wires with 1.5 mm and 11.2 mm diameters, respectively. The 100 °C continuous current rating, or ampacity, is measured for a range of doped CNT wire diameters and corresponding linear mass densities ρL. To describe the results, a new form of the fuse-law, where the critical current is defined as I∝ρL3/4, is developed and shows good agreement with the experimental data. Ultimately, CNT wires are shown to be stable electrical conductors, with failure current densities in excess of 50 A in the case of a convectively cooled 11.2 mm doped CNT wire, and amenable for use in applications that have long-term, high-current demands.

Initial results in SST-1 after up-gradation

SST-1 Tokamak has recently completed the 1st phase of up-gradation with successful installation and integration of all its First Wall components. The First Wall of SST-1 comprises of ∼ 3800 high heat flux compatible graphite tiles being assembled and installed on 132 CuCrZr heat sink back plates engraved with ∼ 4 km of leak tight baking and cooling channels in five major sub groups equipped with ∼ 400 sensors and weighing ∼ 6000 kg in total in thirteen isolated galvanic and six isolated hydraulic circuits. The phase-1 up-gradation spectrum also includes addition of Supersonic Molecular Beam Injection (SMBI) both on the in-board and out-board side, installation of fast reciprocating probes, adding some edge plasma probe diagnostics in the SOL region, installation and integration of segmented and up-down symmetric radial coils aiding/controlling plasma rotations, introduction of plasma position feedback and density controls etc. Post phase-I up-gradation spanning from Nov 2014 till June 2016, initial plasma experiments in up-graded SST-1 have begun since Aug 2016 after a brief engineering validation period in SST-1. The first experiments in SST-1 have revealed interesting aspects on the ‘eddy currents in the First Wall support structures’ influencing the ‘magnetic Null evolution dynamics’ and the subsequent plasma start-up characteristics after the ECH pre-ionization, the influence of the first walls on the ‘field errors’ and the resulting locked modes observed, the magnetic index influencing the evolution of the equilibrium of the plasma column, low density supra-thermal electron induced discharges and normal ohmic discharges etc. Presently; repeatable ohmic discharges regimes in SST-1 having plasma currents in excess of 65 KA (qa ∼ 3.8, BT = 1.5 T) with a current ramp rates ∼ 1.2 MA/s over a duration of ∼ 300 ms with line averaged densities ∼ 0.8 × 1019 and temperatures ∼ 200 eV with copious MHD signatures have been experimentally established. Further elongation of the plasma duration up to one second or more with position and density feedback as well as coupling of Lower Hybrid waves are currently being persuaded in SST-1 apart from increasing the core plasma parameters with further optimizations and with wall conditioning.

Forced flow He vapor cooled critical current testing facility for measurements of superconductors in a wide temperature and magnetic field range

As superconducting materials find their way into applications, there is increasing need to verify their performance at operating conditions. Testing of critical current with respect to temperature and magnetic field is of particular importance. However, testing facilities covering a range of temperatures and magnetic fields can be costly, especially when considering the cooling power required in the cryogenic system in the temperature range below 65K (inaccessible for LN2). Critical currents in excess of 500A are common for commercial samples, making the testing of such samples difficult in setups cooled via a cryocooler, moreover it often does not represent the actual cooling conditions that the sample will experience in service. This work reports the design and operation of a low-cost critical current testing facility, capable of testing samples in a temperature range of 10–65K, with magnetic field up to 1.6T and measuring critical currents up to 900A with variable cooling power.

A Research Tool to Evaluate Safety Response of Lithium Batteries to Internal Short Ciurcuit

Safety of Li batteries is a major concern for many applications. One of major safety concerns is thermal runaway due internal short circuits created because of dormant internal defects. New processes, designs, and materials are being investigated to improve safety of batteries, particularly to prevent thermal runaway due to internal short circuits (ISC). The challenge is how to evaluate these improvements as existing methods using external triggers, such as nail or blunt rod penetration, crush, and heating, are widely known to be deficient. In response to this challenge, we have developed a very thin and small “thermal switch” to be implanted in a Li-ion cell for triggering ISC on demand to see if the cell goes to thermal runaway. This “thermal switch” or “ISC emulator device” consists of very thin layers of copper, wax, aluminum, and separator that are all compatible with the Li-Ion chemistries. The ISC device is inactive when is placed anywhere in a cell during its fabrication; the cell could be cycles many times without any performance issues. ISC device is activated by applying heat to raise the temperature of the thermal switch to tunable 45°C-60°C to melt the thin layer of wax and create a current path between positive and negative electrode and or current collectors. The ISC device can create any of 4 types of internal short circuit: +ve current collector to –ve current collector, anode active to +ve current collector, cathode active to –ve current collector, and anode active to cathode active. The ISC device can work in any li-ion cell format (cylindrical can and prismatic pouch/can). With support from DOE’s Vehicle Technologies Office, we have demonstrated that more than ISC devices are triggered in a cell creating ISC. In laboratory testing, the activated device can handle currents in excess of 300 A to simulate hard shorts (< 2 mohms). Phase change melting has been 100% successful during trigger tests to go from non-conducting to conducting. Experiments have shown that the creating ISC in cells can lead to thermal runaway depending on the type of internal short circuit is created, the safety measures in the cell, and the chemistry. We have provided more than 300 ISC devices to several cylindrical and pouch cell manufacture to implant them into their cell for evaluation of their safety supplying them to their clients for evaluation. The ISC device has been able to initiate thermal runaway in portion of the cells tested. In this paper, we provide details of the ISC device and the results of triggering the device. We believe that the ISC device a useful research tool for evaluating the safety of lithium ion batteries in response to inherent internal defects. Figure 1

Study of MHD activities in the plasma of SST-1

Steady State Superconducting Tokamak (SST-1) is a medium size Tokamak in operation at the Institute for Plasma Research, India. SST-1 has been consistently producing plasma currents in excess of 60kA, with plasma durations above 400ms and a central magnetic field of 1.5T over last few experimental campaigns of 2014. Investigation of these experimental data suggests the presence of MHD activity in the SST-1 plasma. Further analysis clearly explains the behavior of MHD instabilities observed (i.e. tearing modes with m=2, n=1), estimating the growth rate and the island width in the SST-1 plasma. Poloidal magnetic field and Toroidal magnetic field fluctuations in SST-1 are observed using Mirnov coils. Onsets of disruptions in connection with MHD activities have been correlated with other diagnostics such as ECE, Density and Hα etc. The observations have been cross compared with the theoretical calculations and are found to be in good agreement.

High-Mobility Stable 1200-V, 150-A 4H-SiC DMOSFET Long-Term Reliability Analysis Under High Current Density Transient Conditions

For SiC DMOSFETs to obtain widespread usage in power electronics their long-term operational ability to handle the stressful transient current and high temperatures common in power electronics needs to be further verified. To determine the long-term reliability of a single 4H-SiC DMOSFET, the effects of extreme high current density were evaluated. The 4H-SiC DMOSFET has an active conducting area of 40 mm2, and is rated for 1200 V and 150 A. The device was electrically stressed by hard-switching transient currents in excess of four times the given rating (>600 A) corresponding to a current density of 1500 A/cm2. Periodically throughout testing, several device characteristics including $R_{{\bf DS}({\bf on})}$ and $V_{{\bf GS}({\bf th})}$ were measured. After 500 000 switching cycles, the device showed a 6.77% decrease in $R_{{\bf DS}({\bf on})}$ , and only a 132-mV decreased in $V_{{\bf GS}({\bf th})}$ . Additionally, the dc characteristics of the device were analyzed from 25 to 150 °C and revealed a 200-mV increase in on-state voltage drop at 20 A and a 2-V reduction in $V_{{\bf GS}({\bf th})}$ at 150 °C. These results show this SiC DMOSFET has robust long-term reliability in high-power applications that are susceptible to pulse over currents, such as pulsed power modulators and hard-switched power electronics.

Correlating atomic structure and transport in suspended graphene nanoribbons.

Graphene nanoribbons (GNRs) are promising candidates for next generation integrated circuit (IC) components; this fact motivates exploration of the relationship between crystallographic structure and transport of graphene patterned at IC-relevant length scales (<10 nm). We report on the controlled fabrication of pristine, freestanding GNRs with widths as small as 0.7 nm, paired with simultaneous lattice-resolution imaging and electrical transport characterization, all conducted within an aberration-corrected transmission electron microscope. Few-layer GNRs very frequently formed bonded-bilayers and were remarkably robust, sustaining currents in excess of 1.5 μA per carbon bond across a 5 atom-wide ribbon. We found that the intrinsic conductance of a sub-10 nm bonded bilayer GNR scaled with width as GBL(w) ≈ 3/4(e2/h)w, where w is the width in nanometers, while a monolayer GNR was roughly five times less conductive. Nanosculpted, crystalline monolayer GNRs exhibited armchair-terminated edges after current annealing, presenting a pathway for the controlled fabrication of semiconducting GNRs with known edge geometry. Finally, we report on simulations of quantum transport in GNRs that are in qualitative agreement with the observations.

Low-Bremsstrahlung X-Ray Source Using a Low-Voltage High-Current-Density Nanostructured Field Emission Cathode and a Transmission Anode for Markerless Soft Tissue Imaging

We report the design, fabrication, and proof-of-concept characterization of an X-ray generator for improved X-ray absorption imaging that uses a nanostructured field emission cathode as the electron source and a microstructured transmission anode as the X-ray generating structure. Field emission cathodes consume less power, respond faster, and tolerate lower vacuum than the thermionic cathodes used in conventional X-ray generators. The use of a transmission anode, instead of a conventional reflection anode, allows filtering of the background radiation (bremsstrahlung) while allowing efficient generation of X-ray at lower voltages by exciting atomic shell transitions, resulting in emission of X-ray with narrow spectral linewidth for sharper imaging of biological tissue. The fabricated field emission cathode contains arrays of self-aligned and gated silicon field emitters. The field emission cathodes turn on at bias voltages as low as 25 V, and their gates transmit almost 100% of the electrons to the anode. The cathodes produce per-emitter electron currents in excess of $2~\mu $ A (current density >2 A/cm $^{2}$ ) at a bias voltage of 80 V. A desktop rig is built to generate X-ray with a field emission cathode and transmission anode. Using the facility, we obtained X-ray absorption images of several objects. The images clearly show details under $500~\mu $ m in size, as well as soft tissue and fine bone structures without using contrast agents. [2014-0087]

Resilient, Nanostructured, High-Current, and Low-Voltage Neutralizers for Electric Propulsion of Small Spacecraft in Low Earth Orbit

We report propellantless neutralizers resilient to oxygen and low-vacuum environments based on arrays of Pt-coated, self-aligned, and gated Si field emitters. These devices emit currents in excess of 1 mA at bias voltages of less than 120 V, adequate for neutralizing the plume of a small spacecraft's electric propulsion system. The reported devices produce similar currents at fivefold less voltage and emitting area than state-of-the-art CNT neutralizers. Long-term (3 hours) continuous emission in a 1 μTorr oxygen partial pressure environment was demonstrated, confirming the compatibility of these neutralizers with low Earth orbit (LEO) conditions. A robust processing sequence was developed that could be employed for high-yield fabrication of large-area field emission neutralizers with active areas larger than 10 cm2 for current emission higher than 100 mA.

TETRA observation of gamma‐rays at ground level associated with nearby thunderstorms

AbstractTerrestrial gamma‐ray flashes (TGFs)—very short, intense bursts of electrons, positrons, and energetic photons originating from terrestrial thunderstorms—have been detected with satellite instruments. TGF and Energetic Thunderstorm Rooftop Array (TETRA), an array of NaI(Tl) scintillators at Louisiana State University, has now been used to detect similar bursts of 50 keV to over 2 MeV gamma‐rays at ground level. After 2.6 years of observation, 24 events with durations 0.02–4.2 ms have been detected associated with nearby lightning, three of them coincident events observed by detectors separated by ~1000 m. Nine of the events occurred within 6 ms and 5 km of negative polarity cloud‐to‐ground lightning strokes with measured currents in excess of 20 kA. The events reported here constitute the first catalog of TGFs observed at ground level in close proximity to the acceleration site.