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At least 1,693 records · Page 94

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NASA NTRS1988

Monolithic electro-optic modulator array

A PIN GaAlAs diode structure is provided with parameters for index guiding of light in a single mode. The index of refraction of the central layer I (which in practice may be lightly doped .pi. or .nu.) is greater than the p- and n-layers to create a slab waveguide in the transverse direction. Stripe contacts define separate waveguide channels that are separated electrically and optically by implanting protons or etching grooves between the stripe contacts in the upper layer. Separate reverse biasing voltages may be applied to the stripe contacts for modulation of the light in proportions to the voltage, either with absorption modulation, if the light wavelength is within about 500.ANG. of the bandgap of the .pi.-material, or phase-delay modulation, if the wavelength is separated from the bandgap of the .pi.-material by at least 900.ANG..

Robinson, Deborah L.↗

NASA NTRS2011

Measurement of Phased Array Point Spread Functions for Use with Beamforming

Microphone arrays can be used to localize and estimate the strengths of acoustic sources present in a region of interest. However, the array measurement of a region, or beam map, is not an accurate representation of the acoustic field in that region. The true acoustic field is convolved with the array s sampling response, or point spread function (PSF). Many techniques exist to remove the PSF's effect on the beam map via deconvolution. Currently these methods use a theoretical estimate of the array point spread function and perhaps account for installation offsets via determination of the microphone locations. This methodology fails to account for any reflections or scattering in the measurement setup and still requires both microphone magnitude and phase calibration, as well as a separate shear layer correction in an open-jet facility. The research presented seeks to investigate direct measurement of the array's PSF using a non-intrusive acoustic point source generated by a pulsed laser system. Experimental PSFs of the array are computed for different conditions to evaluate features such as shift-invariance, shear layers and model presence. Results show that experimental measurements trend with theory with regard to source offset. The source shows expected behavior due to shear layer refraction when observed in a flow, and application of a measured PSF to NACA 0012 aeroacoustic trailing-edge noise data shows a promising alternative to a classic shear layer correction method.

Bahr, Chris↗

NASA NTRS2010

New Neighbours: Modelling the Growing Population of gamma-ray Millisecond Pulsars

The Fermi Large Area Telescope, in collaboration with several groups from the radio community. have had marvelous success at uncovering new gamma-ray millisecond pulsars (MSPs). In fact, MSPs now make up a sizable fraction of the total number of known gamma-ray pulsars. The MSP population is characterized by a variety of pulse profile shapes, peak separations, and radio-to-gamma phase lags, with some members exhibiting nearly phase-aligned radio and gamma-ray light curves (LCs). The MSPs' short spin periods underline the importance of including special relativistic effects in LC calculations, even for emission originating from near the stellar surface. We present results on modelling and classification of MSP LCs using standard pulsar model geometries.

Venter, C.↗

NASA NTRS2014

Weather Avoidance Guidelines for NASA Global Hawk High-Altitude Unmanned Aircraft Systems (UAS)

The current Global Hawk flight rules would probably not have been effective in the single event of greatest concern (the Emily encounter). The cloud top had not reached 50,000 ft until minutes before the encounter. The TOT and lightning data would not have been available until near the overflight time since this was a rapidly growing cell. This case would have required a last‐minute diversion when lightning became frequent. Avoiding such a cell probably requires continual monitoring of the forward camera and storm scope, whether or not cloud tops have been exceeding specific limits. However, the current overflight rules as strictly interpreted would have prohibited significant fractions of the successful Global Hawk overpasses of Karl and Matthew that proved not to be hazardous. Many other high altitude aircraft (ER‐2 and Global Hawk) flights in NASA tropical cyclone field programs have successfully overflown deep convective clouds without incident.The convective cell that caused serious concern about the safety of the ER‐2 in Emily was especially strong for a tropical cyclone environment, probably as strong or stronger than any that was overflown by the ER‐2 in 20 previous flights over tropical cyclones. Specifically, what made that cell a safety concern was the magnitude of the vertical velocity of the updraft, at least 20 m/s (4000 ft/minute) at the time the ER‐2 overflew it. Such a strong updraft can generate strong gravity waves at and above the tropopause, posing a potential danger to aircraft far above the maximum altitude of the updraft itself or its associated cloud top. Indeed, the ER‐2 was probably at least 9000 ft above that cloud top. Cloud‐top height, by itself, is not an especially good indicator of the intensity of convection and the likelihood of turbulence. Nor is overflying high cloud tops (i.e. > 50,000 ft) of particular concern unless there is other evidence of very strong convective updrafts beneath those tops in the path of the aircraft. center dot Lightning, especially lightning with a high flash rate, is well correlated with convective intensity. Lightning with a minimal flash rate (say 1‐3 flashes per minute) is indicative of updraft speeds of about 10 m/s in the mixed phase region where charge is being separated, generally at altitudes about 20‐25 kft in a hurricane. That is still stronger than typical updrafts (more like 5 m/s). An unresolved issue is whether there is a high and instantaneous correlation between vertical velocity in the middle troposphere (necessary for lightning generation) and near cloud top (more direct concern for overflights).

Cecil, Daniel J.↗

NASA NTRS2014

Physics Based Model for Cryogenic Chilldown and Loading. Part I: Algorithm

We report the progress in the development of the physics based model for cryogenic chilldown and loading. The chilldown and loading is model as fully separated non-equilibrium two-phase flow of cryogenic fluid thermally coupled to the pipe walls. The solution follow closely nearly-implicit and semi-implicit algorithms developed for autonomous control of thermal-hydraulic systems developed by Idaho National Laboratory. A special attention is paid to the treatment of instabilities. The model is applied to the analysis of chilldown in rapid loading system developed at NASA-Kennedy Space Center. The nontrivial characteristic feature of the analyzed chilldown regime is its active control by dump valves. The numerical predictions are in reasonable agreement with the experimental time traces. The obtained results pave the way to the development of autonomous loading operation on the ground and space.

heat transfer↗

NASA NTRSDate not supplied

Evolution of Dendritic Extended 3D Patterns during Directional Solidification: Microgravity Experiments in DECLIC-DSI onboard ISS and Phase-field Modeling

To characterize the dynamical formation of three-dimensional (3D) arrays of dendrites under diffusive growth conditions, in situ monitoring of a series of experiments on a transparent succinonitrile – 0.46 wt% camphor model alloy was carried out under low gravity in the DECLIC Directional Solidification Insert onboard the International Space Station. These experiments offer continuous interface observation and enable the construction of space-time evolution maps of dendrite location and primary spacing during directional solidification. We present the evolution of dendritic extended 3D patterns for a large range of parameters displaying different levels of side branching in both experiments and phase-field simulations. The combined and separate effects of macroscopic interface curvature and crystal mis-orientation are evidenced and detailed. In addition, the aligned dendritic array caused by the invasion of dendrites formed at the edge of the crucible border is investigated.

Kaihua Ji↗

DOE OSTI2024

Speciation and Organic Phase Structure in Nitric Acid Extraction with Trioctylamine

Understanding chemical speciation and intermolecular interactions in multicomponent liquids is essential to understanding their phase and chemical equilibria, which underpin chemical separation processes, including solvent extraction. Here we report on the extraction of nitric acid from its aqueous solutions into organic solutions of trioctylamine (TOA) in toluene, investigated with spectroscopic, X-ray scattering, and computational tools to understand molecular speciation in the organic phase and its relationship with the nanoscale structure of the organic phase. Trends in acid and water extraction clearly show two and three regimes, respectively, indicating different stoichiometric relationships, but speciation of HNO 3 , water, and amine in these regimes is not apparent. 1 H NMR of the organic phase shows that there are at least two distinct acidic protons in the organic phase while ATR-FTIR results show that the organic phase with excess acid extraction is a mixture of trioctylammonium-nitrate ion pairs (TOAHNO 3 ), and undissociated HNO 3 molecules. Comparison with DFT-computed IR spectra show that the chain-like configurations of TOAHNO 3 HNO 3 H 2 O are favored over TOAHNO 3 H 2 OHNO 3 , i.e., direct interaction between the nitrate and HNO 3 molecules is more favored compared to a water-mediated interaction. SAXS of the organic phases were modeled as sums of Ornstein-Zernike (O-Z) scattering and a prepeak feature in the higher Q region that corresponds to extractant packing. The extraction of undissociated HNO 3 by the ion pairs leads to an increased X-ray scattering contrast in the organic phase without any significant change in the correlation length. These results show that the organic phase nanostructure is more sensitive to the concentration of TOAHNO 3 and is relatively unaffected by excess acid extraction. Finally, these findings will enable a molecular understanding of the mechanisms behind metal extraction from acidic media with basic extractants.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1992

Solidification in immiscible systems: The influence of processing conditions on resulting microstructures

Research carried out over the past two decades in which attempts have been made to produce desirable microstructures in immiscible alloys is reviewed. Factors affecting the formation of desirable microstructures in immiscible alloys are addressed and some of the techniques that have been successful in controlling these factors in order to produce useful structures are discussed. Some of the intended applications of these alloys require a dispersion of one phase in the other, while other applications favor the use of aligned, fibrous microstructures with continuity of both phases. The microstructures obtained in immiscible alloys are often strongly influenced by processing conditions. Perhaps the most well known effect is gravity induced sedimentation of the denser immiscible phase during solidification. This difficulty with sedimentation led to low gravity processing in attempts to produce dispersed microstructures. However, experimentation under low gravity conditions soon revealed that many other factors, including wetting of the crucible walls and surface tension driven flows, often resulted in complete separation and coalescence of the two immiscible phases. The production of aligned fibrous microstructures in hypermonotectic alloys appears to be even more difficult than the formation of a dispersion. Here, in addition to sedimentation and crucible wetting, convective and interfacial instabilities must be considered as well. Calculation of the limits of stability during directional solidification were carried out but require experimental verification.

Andrews, J. Barry↗

NASA NTRS2013

A Combined Solar Electric and Storable Chemical Propulsion Vehicle for Piloted Mars Missions

The Mars Design Reference Architecture (DRA) 5.0 explored a piloted Mars mission in the 2030 timeframe, focusing on architecture and technology choices. The DRA 5.0 focused on nuclear thermal and cryogenic chemical propulsion system options for the mission. Follow-on work explored both nuclear and solar electric options. One enticing option that was found in a NASA Collaborative Modeling for Parametric Assessment of Space Systems (COMPASS) design study used a combination of a 1-MW-class solar electric propulsion (SEP) system combined with storable chemical systems derived from the planned Orion crew vehicle. It was found that by using each propulsion system at the appropriate phase of the mission, the entire SEP stage and habitat could be placed into orbit with just two planned Space Launch System (SLS) heavy lift launch vehicles assuming the crew would meet up at the Earth-Moon (E-M) L2 point on a separate heavy-lift launch. These appropriate phases use high-thrust chemical propulsion only in gravity wells when the vehicle is piloted and solar electric propulsion for every other phase. Thus the SEP system performs the spiral of the unmanned vehicle from low Earth orbit (LEO) to E-M L2 where the vehicle meets up with the multi-purpose crew vehicle. From here SEP is used to place the vehicle on a trajectory to Mars. With SEP providing a large portion of the required capture and departure changes in velocity (delta V) at Mars, the delta V provided by the chemical propulsion is reduced by a factor of five from what would be needed with chemical propulsion alone at Mars. This trajectory also allows the SEP and habitat vehicle to arrive in the highly elliptic 1-sol parking orbit compatible with envisioned Mars landing concepts. This paper explores mission options using between SEP and chemical propulsion, the design of the SEP system including the solar array and electric propulsion systems, and packaging in the SLS shroud. Design trades of stay time, power level, specific impulse and propellant type are discussed.

Mercer, Carolyn R.↗

NASA NTRS2014

A Combined Solar Electric and Storable Chemical Propulsion Vehicle for Piloted Mars Missions

Mercer, Carolyn R.↗

NASA NTRS2016

40Ar/39Ar Ages of Carbonaceous Xenoliths in 2 HED Meteorites

The generally young K/Ar and 40Ar/39Ar ages of CM chondrites made us wonder whether carbonaceous xenoliths (CMX) entombed in Howardite–Eucrite–Diogenite (HED) meteorites might retain more radiogenic 40Ar than do ‘free-range’ CM-chondrites. To find out, we selected two HED breccias with carbonaceous inclusions in order to compare the 40Ar/39Ar release patterns and ages of the inclusions with those of nearby HED material. Carbonaceous inclusions (CMXs) in two HED meteorites lost a greater fraction of radiogenic 40Ar than did surrounding host material, but a smaller fraction of it than did free-range CM-chondrites such as Murchison or more heavily altered ones. Importantly, however, the siting of the CMXs in HED matrix did not prevent the 40Ar loss of about 40 percent of the radiogenic 40Ar, even from phases that degas at high laboratory temperatures. We infer that carbonaceous asteroids with perihelia of 1 astronomical unit probably experience losses of at least this size. The usefulness of 40Ar/39Ar dating for samples returned from C-type asteroids may hinge, therefore, on identifying and analyzing separately small quantities of the most retentive phases of carbonaceous chondrites.

Turrin, B.↗

DOE OSTI2021

Unexpected inverse correlations and cooperativity in ion-pair phase transfer

Liquid/liquid extraction is one of the most widely used separation and purification methods, where a forefront of research is the study of transport mechanisms for solute partitioning and the relationships that these have to solution structure at the phase boundary. To date, organized surface features that include protrusions, water-fingers, and molecular hinges have been reported. Many of these equilibrium studies have focused upon small-molecule transport – yet the extent to which the complexity of the solute, and the competition between different solutes, influence transport mechanisms have not been explored. Here we report molecular dynamics simulations that demonstrate that a metal salt (LiNO 3 ) can be transported via a protrusion mechanism that is remarkably similar to that reported for H 2 O by tri-butyl phosphate (TBP), a process that involves dimeric assemblies. Yet the LiNO 3 out-competes H 2 O for a bridging position between the extracting TBP dimer, which in-turn changes the preferred transport pathway of H 2 O. Examining the electrolyte concentration dependence on ion-pair transport unexpectedly reveals an inverse correlation with the extracting surfactant concentration. As [LiNO 3 ] increases, surface adsorbed TBP becomes a limiting reactant in correlation with an increased negative surface charge induced by excess interfacial NO 3 – , however the rate of transport is enhanced. Within the highly dynamic interfacial environment, we hypothesize that this unique cooperative effect may be due to perturbed surface organization that either decreases the energy of formation of transporting protrusion motifs or makes it easier for these self-assembled species to disengage from the surface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1999

Crystal Growth of II-VI Semiconducting Alloys by Directional Solidification

This research study is investigating the effects of a microgravity environment during the crystal growth of selected II-VI semiconducting alloys on their compositional, metallurgical, electrical and optical properties. The on-going work includes both Bridgman-Stockbarger and solvent growth methods, as well as growth in a magnetic field. The materials investigated are II-VI, Hg(1-x)Zn(x)Te, and Hg(1-x)Zn(x)Se, where x is between 0 and 1 inclusive, with particular emphasis on x-values appropriate for infrared detection and imaging in the 5 to 30 micron wavelength region. Wide separation between the liquidus and solidus of the phase diagrams with consequent segregation during solidification and problems associated with the high volatility of one of the components (Hg), make the preparation of homogeneous, high-quality, bulk crystals of the alloys an extremely difficult nearly an impossible task in a gravitational environment. The three-fold objectives of the on-going investigation are as follows: (1) To determine the relative contributions of gravitationally-driven fluid flows to the compositional redistribution observed during the unidirectional crystal growth of selected semiconducting solid solution alloys having large separation between the liquidus and solidus of the constitutional phase diagram; (2) To ascertain the potential role of irregular fluid flows and hydrostatic pressure effects in generation of extended crystal defects and second-phase inclusions in the crystals; and, (3) To obtain a limited amount of "high quality" materials needed for bulk crystal property characterizations and for the fabrication of various device structures needed to establish ultimate material performance limits. The flight portion of the study was to be accomplished by performing growth experiments using the Crystal Growth Furnace (CGF) manifested to fly on various Spacelab missions.

Lehoczky, Sandor L.↗

DOE OSTI2024

Metastable precipitation and ion–extractant transport in liquid–liquid separations of trivalent elements

The extractant-assisted transport of metal ions from aqueous to organic environments by liquid–liquid extraction has been widely used to separate and recover critical elements on an industrial scale. While current efforts focus on designing better extractants and optimizing process conditions, the mechanism that underlies ionic transport remains poorly understood. Here, we report a nonequilibrium process in the bulk aqueous phase that influences interfacial ion transport: the formation of metastable ion–extractant precipitates away from the liquid–liquid interface, separated from it by a depletion region without precipitates. Although the precipitate is soluble in the organic phase, the depletion region separates the two and ions are sequestered in a long-lived metastable state. Since precipitation removes extractants from the aqueous phase, even extractants that are sparingly soluble in water will continue to be withdrawn from the organic phase to feed the aqueous precipitation process. Solute concentrations in both phases and the aqueous pH influence the temporal evolution of the process and ionic partitioning between the precipitate and organic phase. Aqueous ion–extractant precipitation during liquid–liquid extraction provides a reaction path that can influence the extraction kinetics, which plays an important role in designing advanced processes to separate rare earths and other minerals.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

NASA NTRS1997

Phase dependencies of the human baroreceptor reflex

We studied the influence of respiratory and cardiac phase on responses of the cardiac pacemaker to brief (0.35-s) increases of carotid baroreceptor afferent traffic provoked by neck suction in seven healthy young adult subjects. Cardiac responses to neck suction were measured indirectly from electrocardiographic changes of heart period. Our results show that it is possible to separate the influences of respiratory and cardiac phases at the onset of a neck suction impulse by a product of two factors: one depending only on the respiratory phase and one depending only on the cardiac phase. This result is consistent with the hypothesis that efferent vagal activity is a function of afferent baroreceptor activity, whereas respiratory neurons modulate that medullary throughput independent of the cardiac phase. Furthermore, we have shown that stimulus broadening and stimulus cropping influence the outcome of neck suction experiments in a way that makes it virtually impossible to obtain information on the phase dependency of the cardiac pacemaker's sensitivity to vagal stimulation without accurate knowledge of the functional shape of stimulus broadening.

NASA Discipline Cardiopulmonary↗

DOE OSTI2021

CORRIGENDUM to Figures S3-S4 of "Quasi-stationary Planetary Scale Waves in Modern Climate Models"

In addition to fixing the color-coding legend, the new figures plot Fourier phase in a more easily recognizable format: polar coordinates rather than Cartesian coordinates. The same phase data is displayed—indicating the position of the first ridge to the east of the prime meridian—but the new plots avoid unphysical jumps, e.g. from +180° to -180° as longitude increases across the International Dateline. A slight disadvantage of the new format is that each hemisphere requires a separate plot. The wavenumber-1 Northern Hemisphere phase plots give, in effect, a view from above the North Pole, with Eurasia at the top and North America at the bottom. Wavenumber-1 Southern Hemisphere plots give an analogous view from the South Pole. Plots for higher wavenumbers, however, wrap around at fractions of 360° (180°, 120°, ...) and cannot be interpreted as a geographical view.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2020

Gas-Phase Complexes of Americium and Lanthanides with a Bis-triazinyl Pyridine: Reactivity and Bonding of Archetypes for F-Element Separations

Bis-triazinyl pyridines (BTPs) exhibit solution selectivity for trivalent americium over lanthanides (Ln), the origins of which remain uncertain. Here in this paper, electrospray ionization was used to generate gas-phase complexes [ML 3 ] 3+ , where M = La, Lu, or Am and L is EtBTP 2,6-bis(5,6-diethyl-1,2,4-triazin-3-yl)-pyridine. Collision-induced dissociation (CID) of [ML 3 ] 3+ in the presence of H 2 O yielded a protonated ligand [L(H)] + and hydroxide [ML 2 (OH)] 2+ or hydrate [ML(L–H)(H 2 O)] 2+ , where (L–H) - is a deprotonated ligand. Although solution affinities indicate stronger binding of BTPs toward Am 3+ versus Ln 3+ , the observed CID process is contrastingly more facile for M = Am versus Ln. To understand the disparity, density functional theory was employed to compute potential energy surfaces for two possible CID processes, for M = La and Am. In accordance with the CID results, both the rate determining transition state barrier and the net energy are lower for [AmL 3 ] 3+ versus [LaL 3 ] 3+ and for both product isomers, [ML 2 (OH)] 2+ and [ML(L–H)(H 2 O)] 2+ . More facile removal of a ligand from [AmL 3 ] 3+ by CID does not necessarily contradict stronger Am 3+ –L binding, as inferred from solution behavior. In particular, the formation of new bonds in the products can distort kinetics and thermodynamics expected for simple bond cleavage reactions. In addition to correctly predicting the seemingly anomalous CID behavior, the computational results indicate greater participation of Am 5f versus La 4f orbitals in metal–ligand bonding.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS2003

High Precision Laser Range Sensor

The present invention is an improved distance measuring interferometer that includes high speed phase modulators and additional phase meters to generate and analyze multiple heterodyne signal pairs with distinct frequencies. Modulation sidebands with large frequency separation are generated by the high speed electro-optic phase modulators, requiring only a single frequency stable laser source and eliminating the need for a fist laser to be tuned or stabilized relative to a second laser. The combination of signals produced by the modulated sidebands is separated and processed to give the target distance. The resulting metrology apparatus enables a sensor with submicron accuracy or better over a multi- kilometer ambiguity range.

Dubovitsky, Serge↗