Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Ga”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Tectonic evolution of the Western Australian Shield

Geological and geochronological studies in the Western Australian Shield were updated. This terrane bears many similarities to the Indian Shield since they were neighboring parts of Gondwanaland. Western Australia consists of two cratons (Pilbara and Yilgarn) and four orogenic belts (Capricorn, Pingarra, Albany-Fraser, and Patterson), as well as some relatively young (1.6 to 0.75 Ga) sedimentary rocks. The two cratonic blocks are both older than about 2.5 Ga, and the orogenic belts range in age from 2.0 to 0.65 Ga.

Myers, John S.↗
NASA NTRS1988

Development of FIR arrays with integrating amplifiers

The development of optimized photoconductor arrays suitable for far infrared space astronomical applications are described. Although the primary impetus is the production of a 16 by 16 element Ge:Ga demonstration array for SIRTF, the extension of this technology to Large Deployable Reflector (LDR) is considered. The optimization of Ge:Ga and Ge:Be photoconductor materials is discussed. In collaboration with Lawrence Berkeley Laboratory, measurements of FIR photoconductors with quantum efficiencies greater than 20 percent at 100 micrometers, and dark currents below 300 electrons/s are presented. Integrating J-FET amplifier technology is discussed. The current generation of integrating amplifiers has a demonstrated read noise of less than 20 electrons for an integration time of 100 s. The design is shown for a stackable 16 x n Ge:Ga array that utilizes a 16-channel monolithic version of the J-FET integrator. A part of the design is the use of a thin, thermally insulating substrate that allows the electronics to operate at the optimum temperature of 50 K while maintaining thermal and optical isolation from the detectors at 2 K. The power dissipation for the array is less than 16 mW. The array design may particularly be applicable to high resolution imaging spectrometers for LDR.

Young, Erick T.↗
NASA NTRS1991

InP/Ga0.47In0.53As monolithic, two-junction, three-terminal tandem solar cells

The work presented has focussed on increasing the efficiency of InP-based solar cells through the development of a high-performance InP/Ga(0.47)In(0.53)As two-junction, three-terminal monolithic tandem cell. Such a tandem is particularly suited to space applications where a radiation-hard top cell (i.e., InP) is required. Furthermore, the InP/Ga(0.47)In(0.53)As materials system is lattice matched and offers a top cell/bottom cell bandgap differential (0.60 eV at 300 K) suitable for high tandem cell efficiencies under AMO illumination. A three-terminal configuration was chosen since it allows for independent power collection from each subcell in the monolithic stack, thus minimizing the adverse impact of radiation damage on the overall tandem efficiency. Realistic computer modeling calculations predict an efficiency boost of 7 to 11 percent from the Ga(0.47)In(0.53)As bottom cell under AMO illumination (25 C) for concentration ratios in the 1 to 1000 range. Thus, practical AMO efficiencies of 25 to 32 percent appear possible with the InP/Ga(0.47)In(0.53)As tandem cell. Prototype n/p/n InP/Ga(0.47)In(0.53)As monolithic tandem cells were fabricated and tested successfully. Using an aperture to define the illuminated areas, efficiency measurements performed on a non-optimized device under standard global illumination conditions (25 C) with no antireflection coating (ARC) give 12.2 percent for the InP top cell and 3.2 percent for the Ga(0.47)In(0.53)As bottom cell, yielding an overall tandem efficiency of 15.4 percent. With an ARC, the tandem efficiency could reach approximately 22 percent global and approximately 20 percent AMO. Additional details regarding the performance of individual InP and Ga(0.47)In(0.53)As component cells, fabrication and operation of complete tandem cells and methods for improving the tandem cell performance, are also discussed.

Wanlaas, M. W.↗
NASA NTRS1991

Calculation of Gallium-metal-Arsenic phase diagrams

Electrical contacts and metallization to GaAs solar cells must survive at high temperatures for several minutes under specific mission scenarios. The determination of which metallizations or alloy systems that are able to withstand extreme thermal excursions with minimum degradation to solar cell performance can be predicted by properly calculated temperature constitution phase diagrams. A method for calculating a ternary diagram and its three constituent binary phase diagrams is briefly outlined and ternary phase diagrams for three Ga-As-X alloy systems are presented. Free energy functions of the liquid and solid phase are approximated by the regular solution theory. Phase diagrams calculated using this method are presented for the Ga-As-Ge and Ga-As-Ag systems.

Scofield, J. D.↗
NASA NTRS1990

Potential, velocity, and density fields from redshift-distance samples: Application - Cosmography within 6000 kilometers per second

A potential flow reconstruction algorithm has been applied to the real universe to reconstruct the three-dimensional potential, velocity, and mass density fields smoothed on large scales. The results are shown as maps of these fields, revealing the three-dimensional structure within 6000 km/s distance from the Local Group. The dominant structure is an extended deep potential well in the Hydra-Centaurus region, stretching across the Galactic plane toward Pavo, broadly confirming the Great Attractor (GA) model of Lynden-Bell et al. (1988). The Local Supercluster appears to be an extended ridge on the near flank of the GA, proceeding through the Virgo Southern Extension to the Virgo and Ursa Major clusters. The Virgo cluster and the Local Group are both falling toward the bottom of the GA potential well with peculiar velocities of 658 + or - 121 km/s and 565 + or - 125 km/s, respectively.

Bertschinger, Edmund↗
NASA NTRS1991

Early mantle differentiation and its thermal consequences

This paper discusses early events (i.e., pre-3.8 Ga) leading to the mantle differentiation. Arguments are presented in favor of a pre-3.8 Ga basaltic crust covering the earth to balance the depletion of the mantle source of Archean continental crust. It is shown that isotopic and chemical considerations of surviving Archean rocks require the existence of a region enriched in Nd throughout much of the early and middle Archean, which was distinct from the continental crust. Data are presented on the essential mass balance for Nd between the depleted mantle and the enriched reservoir, showing that the enriched reservoir must have contained a large (about 30-70 percent) fraction of the total Nd inventory of the differentiated crust-mantle system at 3.8 Ga.

Galer, S. J. G.↗
NASA NTRS1991

Sm-Nd and Rb-Sr isotopic systematics of ureilites

The present evaluation of Sm-Nd and Rb-Sr isotopic data for seven ureilites establishes their divisions into three groups on the bases of Sm-Nd isotopic systematics. The first group of ureilites is made up of whole-rock samples whose highly depleted assemblages indicate Sm-Nd model ages consistent with 4.55 Ga. The second group of whole-rock samples of Kenna, Novo Urei, and ALHA77257 are heterogeneous mixtures of an unidentified light REE-enriched component and a light REE-depleted olivine-pyroxene assemblage. In the third group, LEW85440 neither has a model Sm-Nd age of 4.55 Ga nor plots of the 3.74 Ga isochron of the Kenna group; it might have had an evolution similar to the Kenna group, but involving different times and/or isotopic compositions.

Goodrich, Cyrena Anne↗
NASA NTRS1991

Searching for Crisium Basin ejecta - Chemistry and ages of Luna 20 impact melts

Chemical (INAA) and chronological (Ar-40 - Ar-39) analyses of six Luna 20 impact melts are performed, and these are compared to the results of Podosek et al. (1973), commonly taken to be representative of the Crisium Basin impact. At least two chemical groups of impact melts are identified. One is interpreted as melts derived from the local crust by craters over the last 3.9 Ga. Another group, which includes one of the samples dated by Podosek et al. (1973), is chemically and chronologically (3.85 + or - 0.02 Ga) indistinguishable from Apollo 17 samples interpreted as Serenitatis impact melts, and hence could be melt formed by that event and deposited at the Luna 20 site. One sample (22023,3,F) has a well-defined age of 3.895 + or - 0.017 Ga, and is chemically similar to, but distinct from, impact melts from other basins. This sample, and its chemistry and age are tentatively identified with the Crisium Basin impact. Whether the age of the Crisium impact is given by 22023,3,F, the samples analyzed by Podosek et al. (1973), or neither, it is clear that Crisium impact melt is not abundant in the Luna 20 collection.

Swindle, T. D.↗
NASA NTRS1991

The biogeochemistry of microbial mats, stromatolites and the ancient biosphere

Stromatolites offer an unparalleled geologic record of early life, because they constitute the oldest and most abundant recognizable remains of microbial ecosystems. Microbial mats are living homologs of stromatolites; thus, the physiology of the microbiota as well as the processes which create those features of mats (e.g., biomarker organic compounds, elemental and stable isotopic compositions) which are preserved in the ancient record. Observations of the carbon isotopic composition (delta C-13) of stromatolites and microbial mats were made and are consistent with the hypothesis that atmospheric CO2 concentrations have declined by at least one to two orders of magnitude during the past 2.5 Ga. Whereas delta C-13 values of carbonate carbon average about 0 permil during both the early and mid-Proterozoic, the delta C-13 values of stromatolitic organic matter increase from an average of -35 between 2.0 and 2.6 Ga ago to an average of about -28 about 1.0 Ga ago. Modern microbial mats in hypersaline environments have delta C-13 values typically in the range of -5 to -9, relative to an inorganic bicarbonate source at 0 permil. Both microbial mats and pur cultures of cyanobacteria grown in waters in near equilibrium with current atmospheric CO2 levels exhibit minimal discrimination against C-13. In contrast, hot spring cyanobacterial mats or cyanobacterial cultures grown under higher CO2 levels exhibit substantially greater discrimination. If care is taken to compare modern mats with stromatolites from comparable environments, it might be possible to estimate ancient levels of atmospheric CO2. In modern microbial mats, a tight coupling exists between photosynthetic organic carbon production and subsequent carbon oxidation, mostly by sulfate reduction. The rate of one process fuels a high rate of the other, with much of the sulfate reduction occurring within the same depth interval as oxygenic photosynthesis. Other aspects of this study are presented.

Desmarais, D. J.↗
NASA NTRS1991

Interstellar gas phase abundance of carbon, oxygen, nitrogen, copper, gallium, germanium, and krypton toward Zeta Ophiuchi

An analysis of weak (less than 10 mA) UV interstellar absorption line data obtained for the line of sight to the O9.5 IV star Zeta Oph is presented. Measurements of weak semiforbidden lines of N I, O I, Cu II, and a new UV detection of Na I are reported along with a small upper limit for C II. Interstellar detections of Ga II, Ge II, and Kr I are also presented. Ga, Ge, and Kr represent the heaviest elements detected in the ISM. A comparison of the derived column densities to cosmic abundances shows Ga to be depleted by about -1.2 dex while Ge is overabundant by +0.2 dex. Assuming Kr to be undepleted, a logarithmic cosmic abundance of Kr/H = 2.95 is obtained on the scale where H = 12.00.

Cardelli, Jason A.↗
NASA NTRS1992

Rhenium-osmium isotope systematics in meteorites. I - Magmatic iron meteorite groups IIAB and IIIAB

Resonance ionization mass spectrometry is used to determine the Re and Os abundances by isotope dilution (ID) and to measure Os-187/Os-186 ratios from 19 iron meteorites. Abundances range from 1.4 to 4800 ppb Re, and from 13 to 65,000 ppb Os, and generally agree well with previous ID and neutron activation results. The Re and Os data suggest that abundance trends in these iron groups may be entirely explained by fractional crystallization. Whole-rock isochrons for the IIAB and IIIAB groups are statistically indistinguishable. Pooled data yield an initial Os-187/Os-186 of 0.794 +/- 0.010 Ga. Given the errors in the slope and half life, this age does not differ significantly from the canonical chondrite age of 4.56 Ga, but could be as young as 4.46 Ga.

Morgan, John W.↗
NASA NTRS1992

The isotopic record of lunar volcanism

Some fundamental aspects of lunar history which have been established as a result of the Apollo program of lunary exploration and contemporaneous Soviet lunar missions are summarized, with emphasis on the isotopic data which record lunar global evolution most directly. It is shown that some lunar anorthosites formed about 4.45 Ga ago, consistent with the early formation of an anorthositic lunar crust. Some 'Mg-suite' norites, troctolites, and dunites also formed approximately contemporaneously with the oldest anorthosites, but others are up to 200-300 Ma younger. More evolved rocks (granites and quartz-monzodiorites) also formed between about 4.4 and 4.0 Ga ago. KREEP basalt volcanism began at least 4.1 Ga ago, approximately contemporaneously with that of the ancient, mare-like basalts, and continued during the cataclysm. The isotopic data record an early primary lunar differentiation during which mare basalt sources formed, probably by crystallization of cumulus minerals from a lunar magma ocean.

Nyquist, L. E.↗
NASA NTRS1991

High figure-of-merit n-type SiGe/GaP alloys

Improvements to the figure-of-merit of n-type SiGe have been achieved via a systematic anneal study aimed at determining the optimal P:Ga ratio. Figures-of-merit of 0.85 to 0.90 x 10-3K-1 have been routinely and reproducibly achieved starting with an initial P:Ga ratio of 3:1. These samples have carrier concentrations in excess of 4.0 x 1020/cu cm, the highest reported for SiGe. This value has been shown to be too high, resulting in unfavorably low Seebeck coefficients. The optimal carrier concentration has been shown to be 1.5-3.0 x 1020/cu cm. Thus, further improvements will be achieved by reductions in the P and Ga concentrations.

Scoville, A. N.↗
NASA NTRS1992

Structural review of the Vredefort dome

The structure of the older-than-3.2-Ga Archean basement and Archean-to-Precambrian sedimentary/volcanic rocks (3.07 to ca. 2.2 Ga) in the center of the Witwatersrand Basin to the southwest of Johannesburg (South Africa) is dominated by the ca. 2.0-Ga megascopic Vredefort 'Dome' structure. The effect of the 'Vredefort event' is demonstrably large and is evident within a northerly arc of about 100 km radius around the granitic core of the structure. Northerly asymmetric overturning of the strata is observed within the first 17 km (strata is horizontal in the south), followed by a 40-km-wide rim synclinorium. Fold and fault structures (normal, reverse, and strike-slip) are locally as well as regionally concentrically arranged with respect to the northern and western sides of the structure. The unusual category of brittle deformation, the so-called 'shock deformation', observed in the collar strata has attracted worldwide attention over the past two decades. These deformation phenomena include the presence of coesite and stishovite, mylonites, and pseudotachylites, cataclasis at a microscopic scale, and the ubiquitous development of multiply striated joint surfaces (which include shatter cones, orthogonal, curviplanar, and conjugate fractures). The macroscopic to microscopic deformation features have led to the formulation of various hypotheses to account for the origin of the Vredefort structure: (1) tectonic hypotheses--deep crustal shear model, doming and N-directed thrust fault model, fold interference model, and diapir model; (2) the exogenous bolide impact hypothesis; and (3) the endogenous cryptoexplosion model.

Colliston, W. P.↗
NASA NTRS1992

Thermobarometric studies on the Levack Gneisses: Footwall rocks to the Sudbury Igneous Complex

Granulite and amphibolite facies gneisses and migmatites of the Levack Gneiss Complex occupy a zone up to 8 km wide around the northern part of the Sudbury Igneous Complex (SIC). Orthopyroxene- and garnet-bearing tonalitic and semipelitic assemblages of granulite facies grade occur within 3 km of the SIC together with lenses of mafic and pyroxenitic rock compositions normally represented by an amphibole +/- cpx-rich assemblage; amphibolite facies assemblages dominate elsewhere in this terrain. These 2.711-Ga gneisses were introduced by (1) the Cartier Granite Batholith during late Archaean to early Proterozoic time and (2) the SIC, at 1.85 Ga, which produced a contact aureole 1-1.5 km wide in which pyroxene hornfelses are common within 200-300 m of the contact. A suite of 12 samples including both the opx-gt and amphibole-rich rock compositions have been studied. Garnets in the semipelitic gneisses are variably replaced by a plg-bio assemblage. Thermobarometric calculations using a variety of barometers and thermometers reported in the literature suggest that the granulite facies assemblages formed at depths in the 21-28 km range (6-8 kbar). Textures and mineral chemistry in the garnet-bearing semipelitic rocks indicate that this terrain underwent a second metamorphic event during uplift to depth in the 5-11 km range (2-3 kbar) and at temperatures as low as 500-550 C. This latter event is distinct from thermal recrystallization caused by the emplacement of the SIC; it probably represents metamorphism attributable to intrusion of the Cartier Granite Batholith. These data allow two interpretations for the crustal uplift of the Levack Gneisses: (1) The gneisses were tectonically uplifted prior to the Sudbury Event (due to intrusion of the Cartier Batholith); or (2) the gneisses were raised to epizonal levels as a result of meteorite impact at 1.85 Ga.

James, R. S.↗
NASA NTRS1992

Full design of fuzzy controllers using genetic algorithms

This paper examines the applicability of genetic algorithms (GA) in the complete design of fuzzy logic controllers. While GA has been used before in the development of rule sets or high performance membership functions, the interdependence between these two components dictates that they should be designed together simultaneously. GA is fully capable of creating complete fuzzy controllers given the equations of motion of the system, eliminating the need for human input in the design loop. We show the application of this new method to the development of a cart controller.

Homaifar, Abdollah↗
NASA NTRS1993

Adaptive process control using fuzzy logic and genetic algorithms

Researchers at the U.S. Bureau of Mines have developed adaptive process control systems in which genetic algorithms (GA's) are used to augment fuzzy logic controllers (FLC's). GA's are search algorithms that rapidly locate near-optimum solutions to a wide spectrum of problems by modeling the search procedures of natural genetics. FLC's are rule based systems that efficiently manipulate a problem environment by modeling the 'rule-of-thumb' strategy used in human decision making. Together, GA's and FLC's possess the capabilities necessary to produce powerful, efficient, and robust adaptive control systems. To perform efficiently, such control systems require a control element to manipulate the problem environment, and a learning element to adjust to the changes in the problem environment. Details of an overall adaptive control system are discussed. A specific laboratory acid-base pH system is used to demonstrate the ideas presented.

Karr, C. L.↗
NASA NTRS1993

Adaptive Process Control with Fuzzy Logic and Genetic Algorithms

Researchers at the U.S. Bureau of Mines have developed adaptive process control systems in which genetic algorithms (GA's) are used to augment fuzzy logic controllers (FLC's). GA's are search algorithms that rapidly locate near-optimum solutions to a wide spectrum of problems by modeling the search procedures of natural genetics. FLC's are rule based systems that efficiently manipulate a problem environment by modeling the 'rule-of-thumb' strategy used in human decision-making. Together, GA's and FLC's possess the capabilities necessary to produce powerful, efficient, and robust adaptive control systems. To perform efficiently, such control systems require a control element to manipulate the problem environment, an analysis element to recognize changes in the problem environment, and a learning element to adjust to the changes in the problem environment. Details of an overall adaptive control system are discussed. A specific laboratory acid-base pH system is used to demonstrate the ideas presented.

Karr, C. L.↗