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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Submicron area NbN/MgO/NbN tunnel junctions for SIS mixer applications

The development of submicron area mixer elements for operation in the submillimeter wave range is discussed. High-current-density NbN/MgO/NbN tunnel junctions with areas down to 0.1 sq microns have been fabricated in both planar and edge geometries. The planar junctions were fabricated from in situ deposited trilayers using electron-beam lithography to pattern submicron area mesas. Modifications of fabrication techniques used in larger-area NbN tunnel junctions are required and are discussed. The NbN/MgO/NbN edge junction process using sapphire substrates has been transferred to technologically important quartz substrates using MgO buffer layers to minimize substrate interactions. The two junction geometries are compared and contrasted in the context of submillimeter wave mixer applications.

Leduc, H. G.↗
NASA NTRS1991

Inverse photoelectron spectrometer with magnetically focused electron gun

An inverse photoelectron spectrometer is described which is based on the design of a magnetically focused low energy electron gun. The magnetic lens extends its field over a relatively large segment of the electron trajectory, which could provide a better focusing effect on a high-current-density low-velocity electron beam, providing the magnetic field in the vicinity of the target is reduced sufficiently to preserve the collinearity of the beam. In order to prove the concept, ray tracing is conducted using the Herrmannsfeldt program for solving electron trajectories in electrostatic and magnetostatic focusing systems. The program allows the calculation of the angles of the electron trajectories with the z axis, at the target location. The results of the ray-tracing procedure conducted for this gun are discussed. Some of the advantages of the magnetic focusing are also discussed.

Krainsky, Isay L.↗
NASA NTRS1992

New approach to the design of Schottky barrier diodes for THz mixers

Near-ideal GaAs Schottky barrier diodes especially designed for mixing applications in the THz frequency range are presented. A diode fabrication process for submicron diodes with near-ideal electrical and noise characteristics is described. This process is based on the electrolytic pulse etching of GaAs in combination with an in-situ platinum plating for the formation of the Schottky contacts. Schottky barrier diodes with a diameter of 1 micron fabricated by the process have already shown excellent results in a 650 GHz waveguide mixer at room temperature. A conversion loss of 7.5 dB and a mixer noise temperature of less than 2000 K have been obtained at an intermediate frequency of 4 GHz. The optimization of the diode structure and the technology was possible due to the development of a generalized Schottky barrier diode model which is valid also at high current densities. The common diode design and optimization is discussed on the basis of the classical theory. However, the conventional fomulas are valid only in a limited forward bias range corresponding to currents much smaller than the operating currents under submillimeter mixing conditions. The generalized new model takes into account not only the phenomena occurring at the junction such as current dependent recombination and drift/diffusion velocities, but also mobility and electron temperature variations in the undepleted epi-layer. Calculated diode I/V and noise characteristics are in excellent agreement with the measured values. Thus, the model offers the possibility of optimizing the diode structure and predicting the diode performance under mixing conditions at THz frequencies.

Jelenski, A.↗
NASA NTRS1993

Fabrication of high-quality superconductor-insulator-superconductor junctions on thin SiN membranes

We have successfully fabricated high-quality and high-current density superconductor-insulator-superconductor (SIS) junctions on freestanding thin silicon nitride (SIN) membranes. These devices can be used in a novel millimeter-wave and THz receiver system which is made using micromachining. The SIS junctions with planar antennas were fabricated first on a silicon wafer covered with a SiN membrane, the Si wafer underneath was then etched away using an anisotropic KOH etchant. The current-voltage characteristics of the SIS junctions remained unchanged after the whole process, and the junctions and the membrane survived thermal cycling.

Garcia, Edouard↗
NASA NTRS1996

Submillimeter-Wave Receiver Containing An SIS Mixer

Submillimeter-wave heterodyne receiver designed to operate at input frequencies in range of 480 to 650 GHz. Intended for use in radio astronomy at frequency of 547 or 626 GHz. Heart of receiver is waveguide mixer that includes adjustable backshort and electric-field-plane tuner. Mixing element high-current-density superconductor/insulator/superconductor (SIS) tunnel junction integrated with superconductive microstrip radio-frequency circuit that tunes out capacitance of junction; matching complex impedance of junction to available tuning range of waveguide mount.

Febvre, Pascal↗
NASA NTRS2003

Differential Sputtering Behavior of Pyrolytic Graphite and Carbon-Carbon Composite Under Xenon Bombardment

A differential sputter yield measurement technique is described, which consists of a quartz crystal monitor that is swept at constant radial distance from a small target region where a high current density xenon ion beam is aimed. This apparatus has been used to characterize the sputtering behavior of various forms of carbon including polycrystalline graphite, pyrolytic graphite, and PVD-infiltrated and pyrolized carbon-carbon composites. Sputter yield data are presented for pyrolytic graphite and carbon-carbon composite over a range of xenon ion energies from 200 eV to 1 keV and angles of incidence from 0 deg (normal incidence) to 60 deg .

Williams, John D.↗
NASA NTRS1995

Chemical Vapor-Deposited (CVD) Diamond Films for Electronic Applications

Diamond films have a variety of useful applications as electron emitters in devices such as magnetrons, electron multipliers, displays, and sensors. Secondary electron emission is the effect in which electrons are emitted from the near surface of a material because of energetic incident electrons. The total secondary yield coefficient, which is the ratio of the number of secondary electrons to the number of incident electrons, generally ranges from 2 to 4 for most materials used in such applications. It was discovered recently at the NASA Lewis Research Center that chemical vapor-deposited (CVD) diamond films have very high secondary electron yields, particularly when they are coated with thin layers of CsI. For CsI-coated diamond films, the total secondary yield coefficient can exceed 60. In addition, diamond films exhibit field emission at fields orders of magnitude lower than for existing state-of-the-art emitters. Present state-of-the-art microfabricated field emitters generally require applied fields above 5x10^7 V/cm. Research on field emission from CVD diamond and high-pressure, high-temperature diamond has shown that field emission can be obtained at fields as low as 2x10^4 V/cm. It has also been shown that thin layers of metals, such as gold, and of alkali halides, such as CsI, can significantly increase field emission and stability. Emitters with nanometer-scale lithography will be able to obtain high-current densities with voltages on the order of only 10 to 15 V.

Source record↗
NASA NTRS2003

Quantum and semi-classical transport in RTDs using NEMO 1-D

NEMO 1-D has been developed primarily for the simulation of resonant tunneling diodes, and quantitative and predictive agreements with experimental high performance, high current density devices have been achieved in the past.

transport tunneling quantum↗
NASA NTRS2004

Recent progress in the development of antimonide-based mid-ir interband cascade lasers

Antimonide-based type-II interband cascade lasers are promising in becoming efficient and compact mid-infrared light sources for many applications such as gas sensing and environmental monitoring. Significant progress toward such a goal has recently been made in terms of achieving low threshold current densities, high temperature operation, and continuous wave operation of single-mode distributed feedback lasers. We review the recent progress of Sb-based mid-IR IC lasers and address the requirements for further improvements in their performance.

semiconductor superlattices↗
NASA NTRS2006

Carbon Nanotube Interconnect

Method and system for fabricating an electrical interconnect capable of supporting very high current densities ( 10(exp 6)-10(exp 10) Amps/sq cm), using an array of one or more carbon nanotubes (CNTs). The CNT array is grown in a selected spaced apart pattern, preferably with multi-wall CNTs, and a selected insulating material, such as SiOw, or SiuNv is deposited using CVD to encapsulate each CNT in the array. An exposed surface of the insulating material is planarized to provide one or more exposed electrical contacts for one or more CNTs.

Li, Jun↗
NASA NTRS2007

Arrays of Bundles of Carbon Nanotubes as Field Emitters

Experiments have shown that with suitable choices of critical dimensions, planar arrays of bundles of carbon nanotubes (see figure) can serve as high-current-density field emitter (cold-cathode) electron sources. Whereas some hot-cathode electron sources must be operated at supply potentials of thousands of volts, these cold-cathode sources generate comparable current densities when operated at tens of volts. Consequently, arrays of bundles of carbon nanotubes might prove useful as cold-cathode sources in miniature, lightweight electron-beam devices (e.g., nanoklystrons) soon to be developed. Prior to the experiments, all reported efforts to develop carbon-nanotube-based field-emission sources had yielded low current densities from a few hundred microamperes to a few hundred milliamperes per square centimeter. An electrostatic screening effect, in which taller nanotubes screen the shorter ones from participating in field emission, was conjectured to be what restricts the emission of electrons to such low levels. It was further conjectured that the screening effect could be reduced and thus emission levels increased by increasing the spacing between nanotubes to at least by a factor of one to two times the height of the nanotubes. While this change might increase the emission from individual nanotubes, it would decrease the number of nanotubes per unit area and thereby reduce the total possible emission current. Therefore, to maximize the area-averaged current density, it would be necessary to find an optimum combination of nanotube spacing and nanotube height. The present concept of using an array of bundles of nanotubes arises partly from the concept of optimizing the spacing and height of field emitters. It also arises partly from the idea that single nanotubes may have short lifetimes as field emitters, whereas bundles of nanotubes could afford redundancy so that the loss of a single nanotube would not significantly reduce the overall field emission.

Manohara, Harish↗
NASA NTRS2007

Process for High-Rate Fabrication of Alumina Nanotemplates

An anodizing process, at an early stage of development at the time of reporting the information for this article, has shown promise as a means of fabricating alumina nanotemplates integrated with silicon wafers. Alumina nanotemplates are basically layers of alumina, typically several microns thick, in which are formed approximately regular hexagonal arrays of holes having typical diameters of the order of 10 to 100 nm. Interest in alumina nanotemplates has grown in recent years because they have been found to be useful as templates in the fabrication of nanoscale magnetic, electronic, optoelectronic, and other devices. The present anodizing process is attractive for the fabrication of alumina nanotemplates integrated with silicon wafers in two respects: (1) the process involves self-ordering of the holes; that is, the holes as formed by the process are spontaneously arranged in approximately regular hexagonal arrays; and (2) the rates of growth (that is, elongation) of the holes are high enough to make the process compatible with other processes used in the mass production of integrated circuits. In preparation for fabrication of alumina nanotemplates in this process, one first uses electron-beam evaporation to deposit thin films of titanium, followed by thin films of aluminum, on silicon wafers. Then the alumina nanotemplates are formed by anodizing the aluminum layers, as described below. In experiments in which the process was partially developed, the titanium films were 200 A thick and the aluminum films were 5 m thick. The aluminum films were oxidized to alumina, and the arrays of holes were formed by anodizing the aluminum in aqueous solutions of sulfuric and/or oxalic acid at room temperature (see figure). The diameters, spacings, and rates of growth of the holes were found to depend, variously, on the composition of the anodizing solution, the applied current, or the applied potential, as follows: In galvanostatically controlled anodizing, regardless of the chemical composition of the solution, relatively high current densities (50 to 100 mA/cm2) resulted in arrays of holes that were more nearly regular than were those formed at lower current densities. . The rates of elongation of the holes were found to depend linearly on the applied current density: the observed factor of proportionality was 1.2 (m/h)/(mA/cm2). For a given fixed current density and room temperature, the hole diameters were found to depend mainly on the chemical compositions of the anodizing solutions. The holes produced in sulfuric acid solutions were smaller than those produced in oxalic acid solutions. The arrays of holes produced in sulfuric acid were more ordered than were those produced in oxalic acid. . The breakdown voltage was found to decrease logarithmically with increasing concentration of sulfuric acid. The breakdown voltage was also found to decrease with temperature and to be accompanied by a decrease in hole diameter. The hole diameter was found to vary linearly with applied potential, with a slope of 2.1 nm/V. This slope differs from slopes (2.2 and 2.77 nm/V) reported for similar prior measurements on nanotemplates made from bulk aluminum. The differences among these slopes may be attributable to differences among impurities and defects in bulk and electron-beam-evaporated aluminum specimens.

Myung, Nosang↗
NASA NTRS2010

Field Testing of High Current Electrokinetic Nanoparticle Treatment for Corrosion Mitigation in Reinforced Concrete

Electrokinetic Nanoparticle (EN) treatment was used as a rapid repair measure to mitigate chloride induced corrosion of reinforced concrete in the field. EN treatment uses an electric field to transport positively charged nanoparticles to the reinforcement through the concrete capillary pores. Cylindrical reinforced concrete specimens were batched with 4.5 wt % salt content (based on cement mass). Three distinct electrokinetic treatments were conducted using high current density (up to 5 A/m2) to form a chloride penetration barrier that was established in 5 days, as opposed to the traditional 6-8 weeks, generally required for electrochemical chloride extraction (ECE). These treatments included basic EN treatment, EN with additional calcium treatment, and basic ECE treatment. Field exposures were conducted at the NASA Beachside Corrosion Test Site, Kennedy Space Center, Florida, USA. The specimens were subjected to sea water immersion at the test site as a posttreatment exposure. Following a 30-day post-treatment exposure period, the specimens were subjected to indirect tensile testing to evaluate treatment impact. The EN treated specimens exhibited 60% and 30% increases in tensile strength as compared to the untreated controls and ECE treated specimens respectively. The surfaces of the reinforcement bars of the control specimens were 67% covered by corrosion products. In contrast, the EN treated specimens exhibited corrosion coverage of only 4%. Scanning electron microscopy (SEM) revealed a dense concrete microstructure adjacent to the bars of the treated specimens as compared to the control and ECE specimens. Energy dispersive spectroscopic (EDS) analysis of the polished EN treated specimens showed a reduction in chloride content by a factor of 20 adjacent to the bars. This study demonstrated that EN treatment was successful in forming a chloride penetration barrier rapidly. This work also showed that the chloride barrier was effective when samples were exposed to field conditions at one of the most severely corrosive environments in North America.

Cardenas, Henry↗
NASA NTRSDate not supplied

Predicting Thermal Response in a Li-ion Cell on a UAV Fight Profile

As the energy storage devices continue to "pack" more energy in a small space, any damage, battery component failure, manufacturing defect, or electrically abusing the battery can lead to catastrophic thermal runaway events. A catastrophic thermal event in a cell leads to high temperature, in some instances spewing of battery materials due to gas development from side reactions initiated due to high internal temperatures. Also, a thermal runaway event can propagate from a single "failed" cell to the pack in a battery pack, leading to a more significant event. Mitigating a thermal runaway event is important in the commercial and automotive sectors. However, preventing such events in an electric aircraft (or air taxis) is paramount due to the lack of alternatives in the event of a failure. Battery prognostics algorithms allow the prediction of state-of-charge (SOC) and end-of-life (EOL) of a Li-ion battery in a UAV (unmanned air vehicle) [1]. For this presentation, we will extend this two-level battery predictive algorithm to predict SOC, EOL, and estimated maximum temperature during a simulated flight. The model is extended by integrating a lumped physics-driven thermal model for high current densities [2]. The parameters used to control SOC and EOL are maximum storable charge, time constant for Li-ion diffusivity in the carbon particles, and internal cell resistance. Cycling leads to an increase in the heat generated by an aged Li-ion cell with a LiyCoO2 (LCO) cathode and a LixC6 (MCMB) anode. The aging of a cell leads to increase in SEI layer thickness, the diffusion time for the lithium ions inside the electrodes, and the local reaction rates, in addition to the thermodynamic abuse caused by fixed cycling voltages controlled by a Battery Management System. As the battery ages, the cell resistance increases, while the onset temperature of the thermal runaway decreases (depends on the cell chemistry and cell abuse history). Any large deviation of the cell temperature from the estimated (expected) value can identify a faulty cell. Since SEI decomposition has the lowest onset temperature in the series of reactions leading to thermal runaway, the model considers the self-heating rate of the SEI decomposition as onset temperature (similar to Ref. [3]). The parameters in the Arrhenius equation for the SEI heating rate depend on the number of cycles, the cell's operating temperature, and the cell's abuse history [4,5]. Coupling the electrochemical, thermal, and aging model allow the prognostic algorithm to estimate a typical cell voltage and temperature as a function of age (cycling and calendar), whose departure from measured values from the BMS is used to identify a safety event. In addition, we will present the results from two simulated flight scenarios for a UAV: typical and extreme, since the power requirements vary significantly during take-off, landing, and changing altitudes, while the power requirements remain low during the cruise. For this presentation, the power requirement for a battery pack in a UAV is scaled to a single cell. This cell is cycled through a simulated profile, and the data is collected and used to predict a safety event.

Li-ion↗
NASA NTRS1989

Unsteady density-current equations for highly curved terrain

New nonlinear partial differential equations containing terrain curvature and its rate of change are derived that describe the flow of an atmospheric density current. Unlike the classical hydraulic-type equations for density currents, the new equations are valid for two-dimensional, gradually varied flow over highly curved terrain, hence suitable for computing unsteady (or steady) flows over arbitrary mountain/valley profiles. The model assumes the atmosphere above the density current exerts a known arbitrary variable pressure upon the unknown interface. Later this is specialized to the varying hydrostatic pressure of the atmosphere above. The new equations yield the variable velocity distribution, the interface position, and the pressure distribution that contains a centrifugal component, often significantly larger than its hydrostatic component. These partial differential equations are hyperbolic, and the characteristic equations and characteristic directions are derived. Using these to form a characteristic mesh, a hypothetical unsteady curved-flow problem is calculated, not based upon observed data, merely as an example to illustrate the simplicity of their application to unsteady flows over mountains.

Sivakumaran, N. S.↗
NASA NTRS1995

Angular dependence of critical current density and magnetoresistance of sputtered high-T(sub c)-films

The angular dependence of the critical current density and the magnetoresistance of high-T(sub c)-films in high and low magnetic fields and for different temperatures were measured to investigate the flux pinning and the superconducting properties. A comparison of the results for the different superconductors shows their increasing dependence on the angle Theta between the magnetic field and the c-axis of the film due to the anisotropy of the chosen superconductor. Furthermore the influence of the current direction to the Theta-rotation plane is discussed.

Geerkens, A.↗
DOE OSTI2024

Quadrupling the depairing current density in the iron-based superconductor SmFeAsO1–xHx

Abstract Iron-based 1111-type superconductors display high critical temperatures and relatively high critical current densities J c . The typical approach to increasing J c is to introduce defects to control dissipative vortex motion. However, when optimized, this approach is theoretically predicted to be limited to achieving a maximum J c of only ∼30% of the depairing current density J d , which depends on the coherence length and the penetration depth. Here we dramatically boost J c in SmFeAsO 1– x H x films using a thermodynamic approach aimed at increasing J d and incorporating vortex pinning centres. Specifically, we reduce the penetration depth, coherence length and critical field anisotropy by increasing the carrier density through high electron doping using H substitution. Remarkably, the quadrupled J d reaches 415 MA cm –2 , a value comparable to cuprates. Finally, by introducing defects using proton irradiation, we obtain high J c values in fields up to 25 T. We apply this method to other iron-based superconductors and achieve a similar enhancement of current densities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗