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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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

High-voltage 6H-SiC p-n junction diodes

A chemical vapor deposition (CVD) process has been used to produce device structures of n- and p-type 6H-SiC epitaxial layers on commercially produced single-crystal 6H-SiC wafers. Mesa-style p-n junction diodes were successfully fabricated from these device structures using reactive ion etching, oxide passivation, and electrical contact metallization techniques. When tested in air, the 6H-SiC diodes displayed excellent rectification characteristics up to the highest temperature tested, 600 C. To observe avalanche breakdown of the p-n junction diodes, testing under a high-electrical-strength liquid was necessary. The avalanche breakdown voltage was 1000 V representing the highest reverse breakdown voltage to be reported for any CVD-grown SiC diode.

Matus, L. G.↗
NASA NTRS1990

A transient plasticity study and low cycle fatigue analysis of the Space Station Freedom photovoltaic solar array blanket

The Space Station Freedom photovoltaic solar array blanket assembly is comprised of several layers of materials having dissimilar elastic, thermal, and mechanical properties. The operating temperature of the solar array, which ranges from -75 to +60 C, along with the material incompatibility of the blanket assembly components combine to cause an elastic-plastic stress in the weld points of the assembly. The weld points are secondary structures in nature, merely serving as electrical junctions for gathering the current. The thermal mechanical loading of the blanket assembly operating in low earth orbit continually changes throughout each 90 min orbit, which raises the possibility of fatigue induced failure. A series of structural analyses were performed in an attempt to predict the fatigue life of the solar cell in the Space Station Freedom photovoltaic array blanket. A nonlinear elastic-plastic MSC/NASTRAN analysis followed by a fatigue calculation indicated a fatigue life of 92,000 to 160,000 cycles for the solar cell weld tabs. Additional analyses predict a permanent buckling phenomenon in the copper interconnect after the first loading cycle. This should reduce or eliminate the pulling of the copper interconnect on the joint where it is welded to the silicon solar cell. It is concluded that the actual fatigue life of the solar array blanket assembly should be significantly higher than the calculated 92,000 cycles, and thus the program requirement of 87,500 cycles (orbits) will be met. Another important conclusion that can be drawn from the overall analysis is that, the strain results obtained from the MSC/NASTRAN nonlinear module are accurate to use for low-cycle fatigue analysis, since both thermal cycle testing of solar cells and analysis have shown higher fatigue life than the minimum program requirement of 87,500 cycles.

Armand, Sasan C.↗
NASA NTRS1995

Effects of oxygen stoichiometry on the scaling behaviors of YBa2Cu3O(x) grain boundary weak-links

The effects of oxygen stoichiometry on the transport properties of the pulsed laser deposited YBa2Cu3O(x) bicrystalline grain boundary weak-link junctions were studied. It is found that not only the cross boundary resistive transition foot structure can be manipulated repeatedly with oxygen annealing processes but the junction behaviors are also altered in accordance. In the fully oxygenated state i.e with x = 7.0 in YBa2Cu3O(x) stoichiometry, the junction critical current exhibits a power of 2 scaling behavior with temperature. In contrast, when annealed in the conditions of oxygen-deficient state (e.g with x = 6.9 in YBa2Cu3O(x) stoichiometry) the junction critical current switches to a linear temperature dependence behavior. The results are tentatively attributed to the modification of the structure in the boundary area upon oxygen annealing, which, in turn, will affect the effective dimension of the geometrically constrained weak-link bridges. The detailed discussion on the responsible physical mechanisms as well as the implications of the present results on device applications will be given.

Wu, K. H.↗
DOE OSTI2025

Structures of RNA phosphotransferase Tpt1 reveal distinct binding modes for an RNA 2′-PO 4 splice junction versus a 5′-PO 4 mononucleotide

Tpt1 is a widely distributed enzyme that removes an internal RNA 2′-phosphate by transfer to NAD + , via a two-step reaction in which: (i) the RNA 2′-PO 4 attacks NAD + to form an RNA-2′-phospho-(ADP-ribose) intermediate and expel nicotinamide; and (ii) the ADP-ribose O2″ attacks the RNA 2′-phosphodiester to form 2′-OH RNA and ADP-ribose-1″,2″-cyclic phosphate products. Tpt1 can also execute a single-step ADP-ribosyltransferase reaction at a 5′-monophosphate nucleic acid terminus that installs a 5′-phospho-ADP-ribose cap structure. Here we present crystal structures of Tpt1 bound to an RNA containing an internal 2′-PO 4 mark (the substrate for the canonical Tpt1 pathway) and in a complex with 5′-AMP. We find that Tpt1 has distinct binding modes, whereby the RNA 2′-PO 4 and the AMP 5′-PO 4 are engaged by the same set of active site amino acids, but the 2′-PO 4 nucleoside and the 5′-nucleoside occupy different sites on the enzyme.

Biochemistry & Molecular Biology↗
NASA NTRS1972

Afar and ERTS-1 imagery

The author has identified the following significant results. The excellent ERTS-1 imagery of the Afar region of Ethiopia permits a preliminary revision to the analysis of the structures of this triple-rift junction, and also revisions to the outcrops of some lithological formations. The fault-belts of the Afar floor can now be mapped in fine detail. The Danakil horst is identified to be limited on its western side against Afar by a major fault-line, and it seems unlikely that the horst is the exposed, easterly portion of a west-dipping sialic block underlying all northern Afar. The Salt Plain appears to be a true graben. The Ethiopian plateau-Afar margin consists of a series of right-offset sectors, the offsets being marked by silicic volcanic centers. The nature of these offsets is related to the vexed question of cross-rift faulting. Such faulting is identifiable on the ERTS-1 imagery, both on the Afar floor, and in the monoclinally warped western margin. The significance of this faulting, though subordinate to the tensional faults of the fault-belts, awaits elucidation.

Mohr, P. A.↗
NASA NTRS1977

A capacitively shunted superconducting microbridge

A superconducting microbridge small enough to exhibit the Josephson effect has been shunted by a large external capacitance. Surprisingly little capacitive hysteresis is observed in its I-V characteristic. Prominent subgap structures are seen. The resistively shunted junction (RSJ) model fails to explain the observations.

Chiao, R. Y.↗
NASA NTRS1982

Towards a 700 mV silicon solar cell

The key to improved silicon solar cell performance lies in increasing cell open circuit voltage. Not only does improved voltage direclty increase cell efficiency, but it also increases the limiting value of fill factor and decreases the temperature sensitivity of the cell. Limits on attainable open circuit voltage are not well defined. A thermodynamic limit of 850 mV exists for black body silicon cells, with 700 mV long regarded as a practical limit. This paper describes experimental work which has resulted in experimental devices with open circuit voltages approaching 700 mV. Values up to 694 (AM0, 25 C) have been demonstrated. The cells are similar in structure to conventional p-n junction cells, but particular attention is paid to passivating the entire top surface of the cell, including regions under the top contact.

Green, M. A.↗
NASA NTRS1990

Geology of the Venus equatorial region from Pioneer Venus radar imaging

The data collected by Pioneer Venus have been used to map the broad radar characteristics of the equatorial region on the basis of radar brightness and texture. Seven units are defined and are used to assess the geologic character of the equatorial region. The correspondence between features in the 15 deg region of overlap between Pioneer Venus ans Venera 15/16 images is examined and used to extend units mapped in the northern high latitudes into the equatorial region. Using the properties of rms slope, reflectivity, the scattering behavior of the surface, and topographic signature, seven physiographic units are mapped on the basis of the distribution of the radar units in the equatorial region and are identified by increasing complexity as plains, dark halo plains, upland rises, upland plateaus, interhighland tectonic zones, tectonically segmented linear highlands, and tectonic junctions. The distribution of larger circular structures interpreted as coronae is also examined.

Senske, D. A.↗
NASA NTRS2004

Deep Structure of the Earth and Concentration of Metals in the Lithosphere: A Geodynamic Approach

A discussion of and introduction to satellite-altitude geopotential fields studies and their interpretation with emphasis on results from metalliferous regions will be given. The magnetic and gravimetric measurements from satellite altitudes show heterogeneity in deeper parts of the lithosphere. These patterns of magnetic anomalies do not only reveal the largest iron ore deposits such as Kiruna, Sweden and Kursk, Russia, but also linear features indicating structural discontinuities. Changes of magnetic amplitude of these patterns are caused by intersecting transverse fractures localizing magmatism and concentration of metals. The role of trans-regional mantle-rooted structural discontinuities in the concentration of metals will be discussed and a new type of mineral prognosis map will be presented. Deep-rooted structural discontinuities, defined by combination of geological and geophysical criteria, with spacing of several hundred kilometers, reveal a quite uniform pattern in the deeper parts of the lithosphere. As these structures provide favorable pathways for the ascent of heat, magmas and ore-forming fluids, their recognition is of crucial importance and can be used in the compilation of a new type of mineral prognosis map. Examples are shown from the United States, Canada, China, Burma, South America, Europe and Australia. The European example includes a pattern of east west trending structural discontinuities or belts and their junction with the NW-trending Tornqvist-Teisseyre Line. The Upper Silesian-Cracovian Zn-Pb district occurs along one of the latitudinal belts. Leslaw Teper of the University of Silesia has been invited to show the fractures in crystalline basement beneath the sediments hosting the Zn-Pb ores.

Taylor, Patrick T.↗
NASA NTRS2004

Contribution to the Study of Regional Magnetization of Satellite Magnetic Measurement: Magsat, Orsted, and Champ (with Gravity Field)

A discussion of and introduction to satellite-altitude geopotential field studies and their interpretation with emphasis on results from metalliferous regions will be given. The magnetic and gravimetric measurements from satellite altitudes show heterogeneity in deeper parts of the lithosphere. These patterns of magnetic anomalies do not only reveal the largest iron ore deposits such as Kiruna, Sweden, Kursk, Russia, and Banugi, Central African Republic but also linear features indicating structural discontinuities. Changes of magnetic amplitude of these patterns are caused by intersecting transverse fractures localizing magmatism and concentration of metals. In addition satellite altitude data are related to variations in crustal thickness and heat flow. Deep-rooted structural discontinuities, defined by combination of geological and geophysical criteria, with spacing of several hundred kilometers, reveal a quite uniform pattern in the deeper parts of the lithosphere. As these structures provide favorable pathways for the ascent of heat, magmas and ore-forming fluids, their recognition is of crucial importance and can be used in the compilation of a new type of mineral prognosis map. An example from Europe includes a pattern of east west trending structural discontinuities or belts and their junction with the NW-trending Tornqvist-Teisseyre Line. The Upper Silesian-Cracovian Zn-Pb district occurs along one of the latitudinal belts. Leslaw Teper of the University of Silesia has been invited to show the fractures in crystalline basement beneath the sediments hosting the Zn-Pb ores.

Taylor, Patrick T.↗
NASA NTRS2007

Four-Way Ka-Band Power Combiner

A waveguide structure for combining the outputs of four amplifiers operating at 35 GHz (Ka band) is based on a similar prior structure used in the X band. The structure is designed to function with low combining loss and low total reflected power at a center frequency of 35 GHz with a 160 MHz bandwidth. The structure (see figure) comprises mainly a junction of five rectangular waveguides in a radial waveguide. The outputs of the four amplifiers can be coupled in through any four of the five waveguide ports. Provided that these four signals are properly phased, they combine and come out through the fifth waveguide port.

Perez, Raul↗
DOE OSTI2025

Crystalline-symmetry-protected Majorana modes in coupled quantum dots

We propose a minimalist architecture for achieving various crystalline-symmetry-protected Majorana modes in an array of coupled quantum dots. Our framework is motivated by the recent experimental demonstrations of two-site and three-site artificial Kitaev chains in a similar setup. We find that introducing a π-phase domain wall in the Kitaev chain leads to a pair of mirror-protected Majorana zero modes located at or near the junction. Joining two π junctions into a closed loop, we can simulate two distinct classes of two-dimensional higher-order topological superconducting phases, both carrying symmetry-protected Majorana modes around the sample corners. As an extension of the π junction, we further consider a general vertex structure where n Kitaev chains meet, i.e., a Kitaev n vertex. We prove that such an n vertex, if respecting a dihedral symmetry group D n , necessarily carries n vertex-bound Majorana modes protected by the D n symmetry. Resilience of the junction and vertex Majorana bound states against disorder and correlation effects is also discussed. Our architecture paves the way for designing, constructing, and exploring a wide variety of artificial topological crystalline phases in quantum-dot experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1977

Nondestructive determination of the depth of planar p-n junctions by scanning electron microscopy

A method was developed for measuring nondestructively the depth of planar p-n junctions in simple devices as well as in integrated-circuit structures with the electron-beam induced current (EBIC) by scanning parallel to the junction in a scanning electron microscope (SEM). The results were found to be in good agreement with those obtained by the commonly used destructive method of lapping at an angle to the junction and staining to reveal the junction.

Chi, J.-Y.↗
NASA NTRS1996

A microstructural approach to cytoskeletal mechanics based on tensegrity

Mechanical properties of living cells are commonly described in terms of the laws of continuum mechanics. The purpose of this report is to consider the implications of an alternative approach that emphasizes the discrete nature of stress bearing elements in the cell and is based on the known structural properties of the cytoskeleton. We have noted previously that tensegrity architecture seems to capture essential qualitative features of cytoskeletal shape distortion in adherent cells (Ingber, 1993a; Wang et al., 1993). Here we extend those qualitative notions into a formal microstructural analysis. On the basis of that analysis we attempt to identify unifying principles that might underlie the shape stability of the cytoskeleton. For simplicity, we focus on a tensegrity structure containing six rigid struts interconnected by 24 linearly elastic cables. Cables carry initial tension ("prestress") counterbalanced by compression of struts. Two cases of interconnectedness between cables and struts are considered: one where they are connected by pin-joints, and the other where the cables run through frictionless loops at the junctions. At the molecular level, the pinned structure may represent the case in which different cytoskeletal filaments are cross-linked whereas the looped structure represents the case where they are free to slip past one another. The system is then subjected to uniaxial stretching. Using the principal of virtual work, stretching force vs. extension and structural stiffness vs. stretching force relationships are calculated for different prestresses. The stiffness is found to increase with increasing prestress and, at a given prestress, to increase approximately linearly with increasing stretching force. This behavior is consistent with observations in living endothelial cells exposed to shear stresses (Wang & Ingber, 1994). At a given prestress, the pinned structure is found to be stiffer than the looped one, a result consistent with data on mechanical behavior of isolated, cross-linked and uncross-linked actin networks (Wachsstock et al., 1993). On the basis of our analysis we concluded that architecture and the prestress of the cytoskeleton might be key features that underlie a cell's ability to regulate its shape.

NASA Discipline Cell Biology↗
NASA NTRS2002

Multi-junction Thin-film Solar Cells on Flexible Substrates for Space Power

The ultimate objective of the thin-film program at NASA GRC is development of a 20 percent AM0 thin-film device technology with high power/weight ratio. Several approaches are outlined to improve overall device efficiency and power/weight ratio. One approach involves the use of very lightweight flexible substrates such as polyimides (i.e., Kapton(Trademark)) or metal foil. Also, a compound semiconductor tandem device structure that can meet this objective is proposed and simulated using Analysis of Microelectronic and Photonic Structures (AMPS). AMPS modeling of current devices in tandem format indicate that AM0 efficiencies near 20 percent can be achieved. And with improvements in materials, efficiencies approaching 25 percent are achievable. Several important technical issues need to be resolved to realize these complex devices: development of a wide bandgap material with good electronic properties, development of transparent contacts, and targeting a 2-terminal device structure (with more complicated processing and tunnel junction) or 4-terminal device. Recent progress in the NASA GRC program is outlined.

Hepp, Aloysius F.↗