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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Spindt cold cathode electron gun development program

A thin film field emission cathode array and an electron gun based on this emitter array are summarized. Fabricating state of the art cathodes for testing at NASA and NRL, advancing the fabrication technology, developing wedge shaped emitters, and performing emission tests are covered. An anistropic dry etching process (reactive ion beam etching) developed that leads to increasing the packing density of the emitter tips to about 5 x 10 to the 6th power/square cm. Tests with small arrays of emitter tips having about 10 tips has demonstrated current densities of over 100 A/sq cm. Several times using cathodes having a packing density of 1.25 x 10 to the 6th power tips/sq cm. Indications are that the higher packing density achievable with the dry etch process may extend this capability to the 500 A/sq cm range and beyond. The wedge emitter geometry was developed and shown to produce emission. This geometry can (in principle) extend the current density capability of the cathodes beyond the 500 A/sq cm level. An emission microscope was built and tested for use with the cathodes.

Spindt, C. A.↗
NASA NTRS1985

Kapton charging characteristics: Effects of material thickness and electron-energy distribution

Charging characteristics of polyimide (Kapton) of varying thicknesses under irradiation by a very-low-curent-density electron beam, with the back surface of the sample grounded are reported. These charging characteristics are in good agreement with a simple analytical model which predicts that in thin samples at low current density, sample surface potential is limited by conduction leakage through the bulk material. The charging of Kapton in a low-current-density electron beam in which the beam energy was modulated to simulate Maxwellian and biMaxwellian distribution functions is measured.

Williamson, W. S.↗
NASA NTRS1990

An Ohmic model for charge transport in a semiconductor

This analysis applies to a quasi-neutral region of uniformly doped semiconductor material. The objective is to solve for the current density in terms of the carrier density and the electric potential boundary values. It is shown that the combined effects of drift and diffusion can be calculated by assuming the current density to obey Ohm's law, but with modified electric potential boundary values.

Edmonds, Larry D.↗
NASA NTRS1978

Solid Polymer Electrolyte (SPE) fuel cell technology program

Many previously demonstrated improved fuel cell features were consolidated to (1) obtain a better understanding of the observed characteristics of the operating laboratory-sized cells; (2) evaluate appropriate improved fuel cell features in 0.7 sq ft cell hardware; and (3) study the resultant fuel cell capability and determine its impact on various potential fuel cell space missions. The observed performance characteristics of the fuel cell at high temperatures and high current densities were matched with a theoretical model based on the change in Gibbs free energy voltage with respect to temperature and internal resistance change with current density. Excellent agreement between the observed and model performance was obtained. The observed performance decay with operational time on cells with very low noble metal loadings (0.05 mg/sq cm) were shown to be related to loss in surface area. Cells with the baseline amount of noble catalyst electrode loading demonstrated over 40,000 hours of stable performance.

Source record↗
NASA NTRS1986

A numerical model of magnetosphere-ionosphere coupling Preliminary results

A three-dimensional simulation model was developed to study magnetosphere-ionosphere coupling in the auroral region. One-fluid MHD equations are adopted to model the magnetosphere, and current density continuity equations were solved consistently to model the ionosphere. In the preliminary simulation runs described here, the electrodynamics of region 1 field-aligned currents were modeled. Initially, the electric field is taken to be in the magnetospheric equatorial plane. Alfven waves then propagate down to the ionosphere, accompanying a field-aligned current and exciting the electrostatic potential in the ionosphere by electrodynamic coupling. The results of the preliminary runs directly correspond with the fundamental characteristics of global magnetosphere-ionosphere coupling. These characteristics include an ionospheric electrostatic potential which varies in its development in time, depending upon the ratio of ionospheric resistance to magnetospheric impedance, the flowing of field-aligned currents into the ionosphere on the dawnside and out on the duskside, and the distribution along the geomagnetic field of the amplitude of the field-aligned current density in proportion to the field intensity.

Watanabe, K.↗
NASA NTRS1993

Coupled structural, thermal, phase-change and electromagnetic analysis for superconductors, volume 2

Two families of parametrized mixed variational principles for linear electromagnetodynamics are constructed. The first family is applicable when the current density distribution is known a priori. Its six independent fields are magnetic intensity and flux density, magnetic potential, electric intensity and flux density and electric potential. Through appropriate specialization of parameters the first principle reduces to more conventional principles proposed in the literature. The second family is appropriate when the current density distribution and a conjugate Lagrange multiplier field are adjoined, giving a total of eight independently varied fields. In this case it is shown that a conventional variational principle exists only in the time-independent (static) case. Several static functionals with reduced number of varied fields are presented. The application of one of these principles to construct finite elements with current prediction capabilities is illustrated with a numerical example.

Felippa, Carlos A.↗
NASA NTRS2010

Fully Printed High-Frequency Phased-Array Antenna on Flexible Substrate

To address the issues of flexible electronics needed for surface-to-surface, surface-to-orbit, and back-to-Earth communications necessary for manned exploration of the Moon, Mars, and beyond, a room-temperature printing process has been developed to create active, phased-array antennas (PAAs) on a flexible Kapton substrate. Field effect transistors (FETs) based on carbon nanotubes (CNTs), with many unique physical properties, were successfully proven feasible for phased-array antenna systems. The carrier mobility of an individual CNT is estimated to be at least 100,000 sq cm/V(dot)s. The CNT network in solution has carrier mobility as high as 46,770 sq cm/V(dot)s, and has a large current-density carrying capacity of approx. 1,000 mA/sq cm , which corresponds to a high carrying power of over 2,000 mW/ sq cm. Such high carrier mobility, and large current carrying capacity, allows the achievement of high-speed (>100 GHz), high-power, flexible electronic circuits that can be monolithically integrated on NASA s active phasedarray antennas for various applications, such as pressurized rovers, pressurized habitats, and spacesuits, as well as for locating beacon towers for lunar surface navigation, which will likely be performed at S-band and attached to a mobile astronaut. A fully printed 2-bit 2-element phasedarray antenna (PAA) working at 5.6 GHz, incorporating the CNT FETs as phase shifters, is demonstrated. The PAA is printed out at room temperature on 100-mm thick Kapton substrate. Four CNT FETs are printed together with microstrip time delay lines to function as a 2-bit phase shifter. The FET switch exhibits a switching speed of 0.2 ns, and works well for a 5.6-GHz RF signal. The operating frequency is measured to be 5.6 GHz, versus the state-of-the-art flexible FET operating frequency of 52 MHz. The source-drain current density is measured to be over 1,000 mA/sq cm, while the conventional organic FETs, and single carbon nanotube-based FETs, are typically in the mA to mA/sq cm range. The switching voltage used is 1.8 V, while the state-of-the-art flexible FET has a gate voltage around 50 V. The gate voltage can effectively control the source-drain current with an ON-OFF ratio of over 1,000 obtained at a low Vds bias of 1.8 V. The azimuth steering angles of PAA are measured at 0deg, -14.5deg, -30deg, and 48.6deg. The measured far-field patterns agree well with simulation results. The efficiency of the 2-bit 2-element PAA is measured to be 39 percent, including the loss of transmission line, FET switch, and coupling loss of RF probes. With further optimization, the efficiency is expected to be around 50-60 percent.

Chen, Yihong↗
NASA NTRS1986

Modeling for CO poisoning of a fuel cell anode

Poisoning losses in a half-cell in the 110-190 C temperature range have been measured in 100 wt pct H3PO4 for various mixtures of H2, CO, and CO2 gases in order to investigate the polarization loss due to poisoning by CO of a porous fuel cell Pt anode. At a fixed current density, the poisoning loss was found to vary linearly with ln of the CO/H2 concentration ratio, although deviations from linearity were noted at lower temperatures and higher current densities for high CO/H2 concentration ratios. The surface coverages of CO were also found to vary linearly with ln of the CO/H2 concentration ratio. A general adsorption relationship is derived. Standard free energies for CO adsorption were found to vary from -14.5 to -12.1 kcal/mol in the 130-190 C temperature range. The standard entropy for CO adsorption was found to be -39 cal/mol per deg K.

Dhar, H. P.↗
NASA NTRS1995

Growth and Analysis of Highly Oriented (11n) BCSCO Films for Device Research

Films of BCSCO superconductor of the type Bi2CaSr2Cu2O(x), have been grown by liquid phase epitaxy method (LPE), using a partially closed growth chamber. The films were grown on (001) and (110) NdGaO3 substrates by slow cooling process in an optimized temperature range below the peritectic melting point (880 C) of Bi2CaSr2Cu2O8. Optimization of parameters, such as seed rotation, soak of initial growth temperature and growth period results in the formation of 2122 phase BCSCO films. The films grown at rotation rates of less than 30 and more than 70 rpm are observed to be associated with the second phase of Sr-Ca-Cu-O system. Higher growth temperatures (greater than 860 C) also encourage to the formation of this phase. XRD measurements show that the films grown on (110) NdGaO3 have a preferred (11n)-orientation. It is pertinent to mention here that in our earlier results published elsewhere we obtained c-axis oriented Bi2CaSr2Cu2O8 phase films on (001) NdGaO3 substrate. Critical current density is found to be higher for the films grown on (110) than (001) NdGaO3 substrate orientation. The best values, zero resistance (T(sab co)) and critical current density obtained are 87 K and 10(exp 5) A/sq cm respectively.

Raina, K. K.↗
NASA NTRS1982

Ion extraction capabilities of closely spaced grids

The ion extraction capabilities of accelerator systems with small screen hole diameters (less than 2.0 mm) are investigated at net-accelerating voltages of 100, 300, and 500 V. Results show that the impingement-limited perveance is not dramatically affected by reductions in screen hole diameter to 1.0 mm, but impingement-limited performance was found to be dependent on the grid separation distance, the discharge-to-total accelerating voltage ratio, and the net-to-total accelerating voltage ratio. Results obtained using small hole diameters and closely spaced grids indicate a new mode of grid operation where high current density operation can be achieved with a specified net acceleration voltage by operating the grids at a high rather than low net-to-total acceleration voltage. Beam current densities as high as 25 mA/sq cm were obtained using grids with 1.0 mm diameter holes operating at a net accelerating voltage of 500 V.

Rovang, D. C.↗
NASA NTRS1987

Spacecraft dielectric surface charging property determination

The charging properties of 127 micron thick polyimide, (a commonly used spacecraft dielectric material) was measured under conditions of irradiation by a low-current-density electron beam with energy between 2 and 14 keV. The observed charging characteristics were consistent with predictions of the NASCAP computer model. The use of low electron current density results in a nonlinearity in the sample-potential versus beam-energy characteristic which is attributed to conduction leakage through the sample. Microdischarges were present at relatively low beam energies.

Williamson, W. S.↗
NASA NTRS1990

Electrochemical incineration of wastes

The novel technology of waste removal in space vehicles by electrochemical methods is presented to convert wastes into chemicals that can be eventually recycled. The important consideration for waste oxidation is to select a right kind of electrode (anode) material that should be stable under anodic conditions and also a poor electrocatalyst for oxygen and chlorine evolution. On the basis of long term electrolysis experiments on seven different electrodes and on the basis of total organic carbon reduced, two best electrodes were identified. The effect of redox ions on the electrolyte was studied. Though most of the experiments were done in mixtures of urine and waste, the experiments with redox couples involved 2.5 M sulfuric acid in order to avoid the precipitation of redox ions by urea. Two methods for long term electrolysis of waste were investigated: (1) the oxidation on Pt and lead dioxide electrodes using the galvanostatic methods; and (2) potentiostatic method on other electrodes. The advantage of the first method is the faster rate of oxidation. The chlorine evolution in the second method is ten times less then in the first. The accomplished research has shown that urine/feces mixtures can be oxidized to carbon dioxide and water, but current densities are low and must be improved. The perovskite and Ti4O7 coated with RuO2 are the best electrode materials found. Recent experiment with the redox agent improves the current density, however, sulphuric acid is required to keep the redox agent in solution to enhance oxidation effectively. It is desirable to reduce the use of acid and/or find substitutes.

Bhardwaj, R. C.↗
NASA NTRS1995

Growth and analysis of highly oriented (11n) BCSCO films for device research

Films of BCSCO superconductor of the type Bi2CaSr2Cu2Ox have been grown by liquid phase epitaxy method (LPE), using a partially closed growth chamber. The films were grown on (001) and (110) NdGaO3 substrates by slow cooling process in an optimized temperature range below the peritectic melting point (880 C) of Bi2CaSr2Cu2O8. Optimization of parameters, such as seed rotation, soak of initial growth temperature and growth period results in the formation of 2122 phase BCSCO films. The films grown at rotation rates of less than 30 and more than 70 rpm are observed to be associated with the second phase of Sr-Ca-Cu-O system. Higher growth temperatures (is greater than 860 C) also encourage to the formation of this phase. X-Ray Diffraction (XRD) measurements show that the films grown on (110) NdGaO3 have a preferred (11 n)-orientation. It is pertinent to mention here that in our earlier results published elsewhere we obtained c-axis oriented Bi2CaSr2Cu2O8 phase films on (001) NdGaO3 substrate. Critical current density is found to be higher for the films grown on (110) than (001) NdGaO3 substrate orientation. The best values of zero resistance (T(sub co)) and critical current density obtained are 87 K and 105 A/sq cm, respectively.

Raina, K. K.↗
NASA NTRS2021

Effect of Reactant Purity on Proton Exchange Membrane Fuel Cell Performance

NASA has set a goal to return to the Moon and to establish a sustained lunar presence. Many applicable lander and upperstage vehicle concepts utilize cryogenic H2 and O2 propellants. This propellant selection enables appealing potential mission concepts wherein electricity is generated by operating a fuel cell on residual H2 and O2 propellants. This concept depends on the capability of the fuel cell to utilize dry, propellant-grade reactants with concentrations of up to 30 percent He present. This study consists of the evaluation of a 12-cell non-flow-through proton exchange membrane fuel cell stack of 50 sq. cm active area with passive water removal. This stack was supplied with three levels of reactant purity: >99.999, 99.1, and 70 mol%, with the remainder made up of He. The reactant humidity and flow-through rate were also assessed as performance factors, and the fuel stack repeatedly supported a load profile with current densities up to 500 mA/sq. cm (25 A). The fuel cell performed consistently over the course of testing, with cell voltages decreasing approximately 50 mV at the maximum current density when supplied with reactants with 30 percent He present.

electrochemistry↗
NASA NTRS1974

UCB current detector experiment on Swedish auroral payloads

A split Langmuir probe has been developed to make in situ measurements of ionospheric current density and plasma bulk flow. The probe consists of two conducting elements that are separated by a thin insulator that shield each other over a 2 pi solid angle, and that are simultaneously swept from negative to positive with respect to the plasma. By measuring the current to each plate and the difference current between plates, information is obtained on the plasma's current density, bulk flow, electron temperature, and density. The instrument was successfully flown twice on sounding rockets into auroral events. Measurement data indicate that the total auroral current configuration is composed of several alternating east and west electrojets associated with several alternating up and down Birkeland currents.

Mozer, F.↗
NASA NTRS1991

Reduction in bearing size due to superconductors in magnetic bearings

A design concept that reduces the size of magnetic bearings is assessed. The small size will enable magnetic bearings to fit into limited available bearing volume of cryogenic machinery. The design concept, called SUPERC, uses (high Tc) superconductors or high-purity aluminum conductors in windings instead of copper. The relatively high-current density of these conductors reduces the slot radial thickness for windings, which reduces the size of the bearings. MTI developed a sizing program called SUPERC that translates the high-current density of these conductors into smaller sized bearings. This program was used to size a superconducting bearing to carry a 500 lb. load. The sizes of magnetic bearings needed by various design concepts are as follows: SUPERC design concept = 3.75 in.; magnet-bias design concept = 5.25 in.; and all electromagnet design concept = 7.0 in. These results indicate that the SUPERC design concept can significantly reduce the size of the bearing. This reduction, in turn, reduces the weight and yields a lighter bearing. Since the superconductors have inherently near-zero resistance, they are also expected to save power needed for operation considerably.

Rao, Dantam K.↗
NASA NTRS1985

Influence of the electron source distribution on field-aligned currents

The field-aligned current density above a discrete auroral arc has been deduced from the downward electron flux and magnetic field measurements onboard the rocket Porcupine flight 4. Both measurements show that the field-aligned current density is, in spite of decreasing peak energies towards the edge of the arc, about 4 times higher there than in the center of the arc. This can be explained by using the single particle description for an anisotropic electron source distribution.

Bruening, K.↗