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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

InGaAsSb/GaSb thermophotovoltaic cells

AstroPower is developing InGaAsSb thermophotovoltaic (TPV) devices. This photovoltaic cell is a tvo-layer epitaxial InGaAsSb structure formed by liquid-phase epitaxy on a GaSb substrate The (direct) bandgap of the In(1-x)Ga(x)As(1-y)Sb(y) alloy is 0.50 to 0.55 eV, depending on its exact alloy composition (x,y); and is closely lattice-matched to the GaSb substrate. The use of the quaternary alloy as opposed to a ternary alloy -- such as, for example, InGaAs/InP -- permits low bandgap devices optimized for 1000 to 1500 C thermal sources with, at the same time, near-exact lattice matching to the GaSb substrate. Lattice-matching is important since even a small degree of lattice mismatch degrades device performance and reliability and increases processing complexity. Internal quantum efficiencies as high as 95% have been measured at a wavelength of 2 microns. At 1 micron wavelengths, internal quantum efficiencies of 55% have been observed. The open-circuit voltage at currents of 0.3 A/cm(exp 2) is 0.220 volts and 0.260 V for current densities of 2 A/cm(exp 2). Fill factors of 56% have been measured at 60 mA/cm(exp 2). However as current density increases there is some decrease in fill factor. Our results to date snow that the GaSb-based quaternary compounds provide a viable and high performance energy conversion solution for thermophotovoltaic systems operating with 1000 to 1500 C source temperatures.

Shellenbarger, Z. A.↗
NASA NTRS2017

High-Power Performance of a 100-kW Class Nested Hall Thruster

The performance of a three-channel, 100-kW class nested Hall thruster was evaluated on xenon propellant for total powers up to 102 kW at NASA Glenn Research Center. The thruster demonstrated stable operation in all seven available channel combinations at discharge voltages from 300 V to 500 V and three different current densities. The resulting test matrix contained forty-six unique conditions ranging from 5 to 102 kW total power and 16 to 247 A discharge current. At each operating condition, thruster performance was measured, and from these measurements specific impulse and efficiency were calculated. All seven channel combinations showed similar performance at a given discharge voltage and current density. The largest thrust recorded was 5.4 N ± 0.1 N at 99 kW, 400 V discharge voltage. Total efficiency and specific impulse ranged from 0.54 to 0.67 ± 0.03 and 1800 seconds to 2650 seconds ± 60 seconds, respectively. It was found that the thrust of the three channels firing together was not larger than the sum of each channel firing individually. Discharge current oscillations were also characterized with peak-to-peak and root-mean-square values and with power spectral density analysis. The implications of these results are discussed in the context of operation beyond 100 kW, as well as the general viability of NHT technology for future mission applications.

Hall, Scott J.↗
NASA NTRS1983

Ionospheric and Birkeland current distributions inferred from the MAGSAT magnetometer data

Ionospheric and field-aligned sheet current density distributions are presently inferred by means of MAGSAT vector magnetometer data, together with an accurate magnetic field model. By comparing Hall current densities inferred from the MAGSAT data and those inferred from simultaneously recorded ground based data acquired by the Scandinavian magnetometer array, it is determined that the former have previously been underestimated due to high damping of magnetic variations with high spatial wave numbers between the ionosphere and the MAGSAT orbit. Among important results of this study is noted the fact that the Birkeland and electrojet current systems are colocated. The analyses have shown a tendency for triangular rather than constant electrojet current distributions as a function of latitude, consistent with the statistical, uniform regions 1 and 2 Birkeland current patterns.

Zanetti, L. J.↗
NASA NTRS1995

Electromagnetic Scattering From a Rectangular Cavity Recessed in a 3-D Conducting Surface

The problem of electromagnetic (EM) scattering from an aperture backed by a rectangular cavity recessed in a three-dimensional conducting body is analyzed using the coupled field integral equation approach. Using the free-space Green's function, EM fields scattered outside the cavity are determined in terms of (1) an equivalent electric surface current density flowing on the three-dimensional conducting surface of the object including the cavity aperture and (2) an equivalent magnetic surface current density flowing over the aperture only. The EM fields inside the cavity are determined using the waveguide modal expansion functions. Making the total tangential electric and magnetic fields across the aperture continuous and subjecting the total tangential electric field on the outer conducting three-dimensional surface of the object to zero, a set of coupled integral equations is obtained. The equivalent electric and magnetic surface currents are then obtained by solving the coupled integral equation using the Method of Moments. The numerical results on scattering from rectangular cavities embedded in various three-dimensional objects are compared with the results obtained by other numerical techniques.

Deshpande, M. D.↗
NASA NTRS1987

Effect of inhibitors of auxin transport and of calmodulin on a gravisensing-dependent current in maize roots

Some characteristics of the gravity sensing mechanism in maize root caps were investigated using a bioelectric current as an indicator of gravity sensing. This technique involves the measurement of a change in the current density which arises at the columella region coincidently with the presentation time. Two inhibitors of auxin transport, triiodobenzoic acid and naphthylphthalamic acid, blocked gravitropic curvature but not the change in current density. Two inhibitors of calmodulin activity, compound 48/80 and calmidazolium, blocked both curvature and gravity-induced current. The results suggest that auxin transport is not a component of gravity sensing in the root cap. By contrast, the results suggest that calmodulin plays an intrinsic role in gravity sensing.

Non-NASA Center↗
NASA NTRS1969

Mass transport mechanism in porous fuel cell electrodes

Results of experiments on hydrogen-oxygen fuel cells show that higher current densities are obtained with cell anodes having a 100 micron thin active layer of porous nickel containing silver electrocatalyst. Increase in current density is attributed to a convective mass transport mechanism.

Jonsson, I.↗
NASA NTRS1990

Evaluation, development, and characterization of superconducting materials for space applications

The anisotropic electromagnetic features of a grain-aligned YBa2Cu3O(x) bulk sample derived from a process of long-time partial melt growth were investigated by the measurements of direct current magnetization (at 77 K) and alternating current susceptibility as a function of temperature, with the fields applied parallel and perpendicular to the c axis, respectively. The extended Bean model was further studied and applied to explain the experimental results. Upon comparison of the grain-aligned sample with pure single crystal materials, it is concluded that because of the existence of more effective pinning sites in the grain-aligned sample, not only its critical current density perpendicular to the c axis is improved, but the one parallel to the c axis is improved even more significantly. The anisotropy in the critical current densities in the grain-aligned sample at 77 K is at least one to two orders of magnitude smaller than in the pure single crystal. The measurement of anisotropy of alternating current susceptibility as a function of temperature, especially its imaginary part, shows that there are still some residues of interlayer weak links in the grain-aligned samples, but they are quite different from and far less serious than the weak links in the sintered sample.

Thorpe, Arthur N.↗
NASA NTRS1974

A theoretical analysis of the current-voltage characteristics of solar cells

The current-voltage characteristics and efficiencies of solar cells are discussed. For one solar cell structure detailed curves are presented which include carrier densities, current densities, potential, and quasi-Fermi levels at different voltage levels both with and without optically generated carriers (AMO conditions). In addition some results are presented concerning the influence of various parameter variations such as lifetime, cell thickness, and high-low junction width on solar cell performance.

Hauser, J. R.↗
NASA NTRS1981

Mechanism and models for zinc metal morphology in alkaline media

Based on experimental observations, a mechanism is presented to explain existence of the different morphologies of electrodeposited zinc in alkaline solution. The high current density dendrites appear to be due to more rapid growth on the nonbasal crystallographic planes than on the basal plane. The low current density moss apparently results from dissolution from the nonbasal planes at low cathodic voltages. Electrochemical models were sought which would produce such a phenomenon. The fundamental plating mechanism alone accounts only for different rates on different planes, not for zinc dissolution from a plane in the cathodic region. Fourteen models were explored; two models were in accord with the proposed mechanism. One involves rapid disproportionation of the zinc +1 species on the nonbasal planes. The other involves a redox reaction (corrosion) between the zinc-zincate and hydrogen-water systems.

May, C. E.↗
NASA NTRS1990

Electric current in a unipolar sunspot with an untwisted field

The return flux (RF) sunspot model is applied to a round, unipolar sunspot observed by H. Kawakami (1983). Solving the magnetohydrostatic problem using the gas pressure deficit between the umbral and quiet-sun atmospheres as a source function, a distribution of electric current density in an untwisted, unipolar sunspot as a function of height and radial distance from the sunspot center is observed. Maximum electric current density is about 32 mA/sq m at the bottom of the sunspot.

Osherovich, V. A.↗
NASA NTRS2006

Self-Passivating Lithium/Solid Electrolyte/Iodine Cells

Robust lithium/solid electrolyte/iodine electrochemical cells that offer significant advantages over commercial lithium/ iodine cells have been developed. At room temperature, these cells can be discharged at current densities 10 to 30 times those of commercial lithium/iodine cells. Moreover, from room temperature up to 80 C, the maximum discharge-current densities of these cells exceed those of all other solid-electrolyte-based cells. A cell of this type includes a metallic lithium anode in contact with a commercial flexible solid electrolyte film that, in turn, is in contact with an iodine/ graphite cathode. The solid electrolyte (the chemical composition of which has not been reported) offers the high ionic conductivity needed for high cell performance. However, the solid electrolyte exhibits an undesirable chemical reactivity to lithium that, if not mitigated, would render the solid electrolyte unsuitable for use in a lithium cell. In this cell, such mitigation is affected by the formation of a thin passivating layer of lithium iodide at the anode/electrolyte interface. Test cells of this type were fabricated from iodine/graphite cathode pellets, free-standing solid-electrolyte films, and lithium-foil anodes. The cathode mixtures were made by grinding together blends of nominally 10 weight percent graphite and 90 weight percent iodine. The cathode mixtures were then pressed into pellets at 36 kpsi (248 MPa) and inserted into coin-shaped stainless-steel cell cases that were coated with graphite paste to minimize corrosion. The solid-electrolyte film material was stamped to form circular pieces to fit in the coin cell cases, inserted in the cases, and pressed against the cathode pellets with polyethylene gaskets. Lithium-foil anodes were placed directly onto the electrolyte films. The layers described thus far were pressed and held together by stainless- steel shims, wave springs, and coin cell caps. The assembled cells were then crimped to form hermetic seals. It was found that the solid electrolyte films became discolored within seconds after they were placed in contact with the cathodes - a result of facile diffusion of iodine through the solid electrolyte material (see figure).

Bugga, Ratnakumar↗
NASA NTRS1995

An active homopolar magnetic bearing with high temperature superconductor (HTS) coils and ferromagnetic cores

A proof-of-feasibility demonstration showed that high temperature superconductor (HTS) coils can be used in a high-load, active magnetic bearing in liquid nitrogen. A homopolar radial bearing with commercially wound HTS (Bi 2223) bias and control coils produced over 200 lb (890 N) radial load capacity (measured non-rotating) and supported a shaft to 14000 rpm. The goal was to show that HTS coils can operate stably with ferromagnetic cores in a feedback controlled system at a current density similar to that in Cu in liquid nitrogen. Design compromises permitted use of circular coils with rectangular cross section. Conductor improvements will eventually permit coil shape optimization, higher current density and higher bearing load capacity. The bias coil, wound with non-twisted, multifilament HTS conductor, required negligible power to carry its direct current. The control coils were wound with monofilament HTS sheathed in Ag. These dissipated negligible power for direct current (i.e. for steady radial load components). When an alternating current (AC) was added, the AC component dissipated power which increased rapidly with frequency and quadratically with AC amplitude. In fact at frequencies above about 2 hz, the effective resistance of the control coil conductor actually exceeds that of the silver which is in electrical parallel with the oxide superconductor. This is at least qualitatively understandable in the context of a Bean-type model of flux and current penetration into a Type II superconductor. Fortunately the dynamic currents required for bearing stability are of small amplitude. These results show that while twisted multifilament conductor is not needed for stable levitation, twisted multifilaments will be required to reduce control power for sizable dynamic loads, such as those due to unbalance.

Brown, G. V.↗
NASA NTRS2021

Performance and High-Speed Characterization of a 100-kW Nested Hall Thruster

The performance of a three-channel, 100-kW class nested Hall thruster was evaluated on xenon propellant for total powers up to 102 kW. The thruster demonstrated stable operation in all seven available channel combinations at discharge voltages from 300 V to 500 V and three different current densities. The resulting test matrix contained forty-six unique conditions ranging from 5 to 102 kW total power and 16 to 247 A discharge current. At each operating condition, thrust and telemetry was measured, and from these measurements specific impulse and efficiency were calculated. All seven channel combinations showed similar performance at a given discharge voltage and current density. The largest thrust recorded was 5.4 N ± 0.1 N at 99 kW, 400 V discharge voltage. Total efficiency and specific impulse ranged from 0.54 to 0.67 ± 0.03 and 1800 seconds to 2650 seconds ± 60 seconds, respectively. It was found that the thrust of the three channels firing together was not larger than the sum of each channel firing individually. Discharge current oscillations were also characterized with peak-to-peak and root-mean-square values and with high-speed camera analysis, which provide insight into how the discharge channels oscillate, and how those oscillations are affected by the presence of other operating channels. The implications of these results are discussed in the context of operation beyond 100 kW, as well as the general viability of NHT technology for future mission applications.

High-power↗
NASA NTRS1989

Optimization of the lithium/thionyl chloride battery

A 1-D math model for the lithium/thionyl chloride primary cell is used in conjunction with a parameter estimation technique in order to estimate the electro-kinetic parameters of this electrochemical system. The electro-kinetic parameters include the anodic transfer coefficient and exchange current density of the lithium oxidation, alpha sub a,1 and i sub o,i,ref, the cathodic transfer coefficient and the effective exchange current density of the thionyl chloride reduction, alpha sub c,2 and a sup o i sub o,2,ref, and a morphology parameter, Xi. The parameter estimation is performed on simulated data first in order to gain confidence in the method. Data, reported in the literature, for a high rate discharge of an experimental lithium/thionyl chloride cell is used for an analysis.

White, Ralph E.↗
NASA NTRS1990

A model of the magnetospheric tail with current-free lobes

A model of the earth's tail field with no current density in its high latitude lobes is obtained as a modification of Tsyganenko's tail model. This is achieved by replacing Tsyganenko's current element with one in which the current density ends abruptly at a distance D from the central axis.

Stern, David P.↗
NASA NTRS1991

High-power generation in IMPATT devices in the 100-200-GHz range

A silicon double-drift IMPATT diode with high uniform doping levels was simulated. Simulation results show that it is possible for silicon IMPATT diodes to generate extremely high pulsed output power for frequencies above 100 GHz under high current-density operation. The highest output power matched to a 1-ohm load resistance obtained at 150 GHz is 37.7 W, with a dc current density of 200 kA/sq cm, although the calculated power conversion efficiency is low. It is also shown that the low-power conversion efficiency limits the diode's continuous wave power operation.

Chen, Chien-Chung↗
NASA NTRS2001

Performance Evaluation of Titanium Ion Optics for the NASA 30 cm Ion Thruster

The results of performance tests with titanium ion optics were presented and compared to those of molybdenum ion optics. Both titanium and molybdenum ion optics were initially operated until ion optics performance parameters achieved steady state values. Afterwards, performance characterizations were conducted. This permitted proper performance comparisons of titanium and molybdenum ion optics. Ion optics' performance A,as characterized over a broad thruster input power range of 0.5 to 3.0 kW. All performance parameters for titanium ion optics of achieved steady state values after processing 1200 gm of propellant. Molybdenum ion optics exhibited no burn-in. Impingement-limited total voltages for titanium ion optics where up to 55 V greater than those for molybdenum ion optics. Comparisons of electron backstreaming limits as a function of peak beam current density for molybdenum and titanium ion optics demonstrated that titanium ion optics operated with a higher electron backstreaming limit than molybdenum ion optics for a given peak beam current density. Screen grid ion transparencies for titanium ion optics were as much as 3.8 percent lower than those for molybdenum ion optics. Beam divergence half-angles that enclosed 95 percent of the total beam current for titanium ion optics were within 1 to 3 deg. of those for molybdenum ion optics. All beam divergence thrust correction factors for titanium ion optics were within 1 percent of those with molybdenum ion optics.

Soulas, George C.↗