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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 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Gravitational lensing limits on the cosmological constant in a flat universe

Inflationary cosmological theories predict, and some more general aesthetic criteria suggest, that the large-scale spatial curvature of the universe k should be accurately zero (i.e., flat), a condition which is satisfied when the universe's present mean density and the value of the cosmological constant Lambda have certain pairs of values. Available data on the frequency of multiple image-lensing of high-redshift quasars by galaxies suggest that the cosmological constant cannot make a dominant contribution to producing a flat universe. In particular, if the mean density of the universe is as small as the baryon density inferred from standard cosmic nucleosynthesis calculations or as determined from typical dynamical studies of galaxies and galaxy clusters, then a value of Lambda large enough to produce a k = 0 universe would result in a substantially higher frequency of multiple-image lensing of quasars than has been observed so far. Shortcomings of the available lens data and uncertainties concerning galaxy properties allow some possibility of escaping this conclusion, but systematic searches for a gravitational lenses and continuing investigations of galaxy mass distributions should soon provide decisive information. It is also noted that nonzero-curvature cosmological models can account for the observed frequency of galaxy-quasar lens systems and for a variety of other constraints.

Turner, Edwin L.↗
DOE OSTI2021

Hydrothermal Synthesis and Solid-State Laser Refrigeration of Ytterbium-Doped Potassium-Lutetium-Fluoride (KLF) Microcrystals

Hydrothermal methods are used for the first time to synthesize distinct crystallographic stoichiometries within the potassium-lutetium-fluoride phase diagram for applications in solid-state laser refrigeration. Four crystalline phases were synthesized hydrothermally and doped with 10% Yb(III) ions, namely, orthorhombic K 2 LuF 5 (space group Pnma), trigonal KLuF 4 (space group P3 1 21), orthorhombic KLu 2 F 7 (space group Pna2 1 ), and cubic KLu 3 F 10 (space group $Fm\bar{3m}$), with each phase exhibiting unique microcrystalline morphologies. Among the four phases, the most significant cooling was observed for the KLuF 4 phase, which showed an overall refrigeration of 8.6 ± 2.1 K below room temperature. Laser refrigeration for KLuF4 was measured by observing both the eigenfrequencies of optomechanical cantilevers in vacuum and also the Brownian dynamics of optically trapped microcrystals in water. Cooling was also observed for the first time for the K 2 LuF 5 phase in vacuum based on measurements of the mean luminescence wavelength of Yb(III) ions. Finally, cooling was not observed with the other two phases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Proximity to a critical point driven by electronic entropy in URu2Si2

Abstract The strongly correlated actinide metal URu 2 Si 2 exhibits a mean field-like second order phase transition at T o ≈ 17 K, yet lacks definitive signatures of a broken symmetry. Meanwhile, various experiments have also shown the electronic energy gap to closely resemble that resulting from hybridization between conduction electron and 5 f -electron states. We argue here, using thermodynamic measurements, that the above seemingly incompatible observations can be jointly understood by way of proximity to an entropy-driven critical point, in which the latent heat of a valence-type electronic instability is quenched by thermal excitations across a gap, driving the transition second order. Salient features of such a transition include a robust gap spanning highly degenerate features in the electronic density of states, that is weakly (if at all) suppressed by temperature on approaching T o , and an elliptical phase boundary in magnetic field and temperature that is Pauli paramagnetically limited at its critical magnetic field.

36 MATERIALS SCIENCE↗
DOE OSTI2021

The O K -2 V spectrum of CO: the influence of the second core-hole

Using synchrotron radiation in the tender X-ray regime, a photoelectron spectrum showing the formation of single site double-core-hole pre-edge states, involving the K shell of the O atom in CO, has been recorded by means of high-resolution electron spectroscopy. The experimentally observed structures have been simulated, interpreted and assigned, employing state-of-the-art ab initio quantum chemical calculations, on the basis of a theoretical model, accounting for their so-called direct or conjugate character. Features appearing above the double ionization threshold have been reproduced by taking into account the strong mixing between multi-excited and continuum states. The shift of the σ* resonance below the double ionization threshold, in combination with the non-negligible contributions of multi-excited configurations in the final states reached, gives rise to a series of avoided crossings between the different potential energy curves.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Phonon dynamics in the generalized Kitaev spin liquid

Candidate materials for the Kitaev spin liquid generically have residual interactions beyond the Kitaev coupling. It therefore becomes necessary to understand how signatures of the quantum spin liquid, e.g., fractionalization of the spin excitations, are affected by the presence of these interactions. Recently it was shown that phonon dynamics is an indirect but effective probe to study fractionalized excitations in the Kitaev spin liquid. Ultrasound experiments can measure sound attenuation, which should show characteristic temperature and angular dependence of the sound-attenuation coefficient if the scattering of phonons happens predominantly on Majorana fermions. So far the computation of the sound attenuation was only done in the pure spin-phonon coupled Kitaev model, without taking into account residual interactions. In order to understand experimental signatures, here we present a mean-field study of the sound attenuation in the generalized J-K_Γ model, which is relevant to candidate materials. Further, our findings show that as long as the system is in the spin liquid phase, characteristic features of the sound attenuation remain observable even in the presence of residual interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Rational Design of High-Performing Electrodes in Energy Storage Devices

Our approach in this project is to control conductivities of different ions by modulating the pore sizes in MOFs as sulfur hosts in cathodes. To validate our hypothesis that controlling the pore size of MOFs can effectively prevent LiPSs shuttling while allowing sufficient Li ion transport, which will ensure electrochemical cycling performance, we performed molecular dynamics (MD) simulations using a series of UiO MOFs with different linker sizes including LiPSs and electrolyte components (1M LiTFSI in DME/DOL 1:1 volume) in pores. Specifically, UiO-66, UiO-67, and UiO-68 are Zr-based MOFs with a linker length of 5.67 Å, 10.07 Å, and 14.35 Å, respectively, leading to different pore diameters of 8.4 Å, 12.8 Å, and 17.8 Å. We calculated the mean-squared displacements of Li and LiPS from MD simulations at 300 K for 50 ns or over to assess their mobility in MOFs.

25 ENERGY STORAGE↗
DOE OSTI2023

Searching in H I for Massive Low Surface Brightness Galaxies: Samples from HyperLeda and the UGC

A search has been made for 21 cm H I line emission in a total of 350 unique galaxies from two samples whose optical properties indicate they may be massive. The first consists of 241 low surface brightness (LSB) galaxies of morphological type Sb and later selected from the HyperLeda database and the second consists of 119 LSB galaxies from the UGC with morphological types Sd-m and later. Of the 350 unique galaxies, 239 were observed at the Nançay Radio Telescope, 161 at the Green Bank Telescope, and 66 at the Arecibo telescope. A total of 295 (84.3%) were detected, of which 253 (72.3%) appear to be uncontaminated by any other galaxies within the telescope beam. Finally, of the total detected, uncontaminated galaxies, at least 31 appear to be massive LSB galaxies, with a total H I mass ≥ 10 10 M ⊙ , for H 0 = 70 kms -1 Mpc -1 . If we expand the definition to also include galaxies with significant total (rather than just gas) mass, i.e., those with an inclination-corrected H I line width W 50,cor > 500 km s -1 , this brings the total number of massive LSB galaxies to 41. There are no obvious trends between the various measured global galaxy properties, particularly between mean surface brightness and galaxy mass.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1971

Partitioning of Eu and Sr between coexisting plagioclase and K-feldspar.

Minerals were separated by an EM approach and with the aid of liquids of great density. An analysis of K, Rb, Ca, Sr, Ha, and rare earth elements was conducted by means of a mass spectrometer isotope dilution technique. The behavior of the divalent europium ions during the partition process was found to be very similar to that of divalent strontium ions, taking into consideration data of the partition coefficients between coexisting feldspars in acidic rocks.

Nagasawa, H.↗
NASA NTRS1972

TiO2 film properties as a function of processing temperature, volume 3

Thin film TiO2 was produced at 150 C by chemical vapor deposition using hydrolysis of tetraisopropyl titanate. Films were amorphous as grown, but annealing in air caused crystallization, with anatase formed beginning at 350 C and rutile at 700 C. Density and index of refraction increased substantially with increasing anneal temperature, while etch susceptibility in HF and H2SO4 decreased. Comparison with literature data showed two groups of processes. One group yields films having properties that gradually approach those of rutile with increasing process temperature. The other group gives rutile directly at moderate temperatures. Deposition of amorphous film followed by etching and annealing is suggested as a means for pattern definition.

Fitzgibbons, E. T.↗
NASA NTRS1972

On the estimation and detection for optical turbulent channels.

Consideration of the problem of filtering and detecting signals received through a turbulent channel at optical frequencies. Adopting the semiclassical method of quantum mechanics, application is made to the direct detection of a single-path optical communication channel where the received field is assumed to be the coherent signal field degraded by the atmospheric turbulence with a signal-independent, additive, zero-mean Gaussian background radiation.

Harger, R. O.↗
NASA NTRS1978

Onset phenomena in self-field MPD arcjets

Voltage fluctuations in a multi-megawatt quasi-steady MPD accelerator, indicative of increased ablation and a consequent degradation in performance, are found to exist only over a narrow range of currents, above which the ablation continues. Within this range, the discharge is characterized by a thin cylindrical shell, 4.0 cm in diameter, concentric with the cathode surface, containing steep radial gradients in plasma luminosity, number density, current density, electric field and Hall parameter, and within which a significant fraction of the propellant is first ionized. Experiments have shown that this cylindrical shell can be totally eliminated by immersing a small object at critical locations within it, thereby increasing the local mass density by preferential ablation and suggesting further means of improving the accelerator design.

Rudolph, L. K.↗
NASA NTRS1978

Computerized symbolic manipulation in structural mechanics Progress and potential

Status and recent applications of computerized symbolic manipulation to structural mechanics problems are summarized. The applications discussed include; (1) generation of characteristic arrays of finite elements; (2) evaluation of effective stiffness and mass coefficients of continuum models for repetitive lattice structures; and (3) application of Rayleigh-Ritz technique to free vibration analysis of laminated composite elliptic plates. The major advantages of using computerized symbolic manipulation in each of these applications are outlined. A number of problem areas which limit the realization of the full potential of computerized symbolic manipulation in structural mechanics are examined and some of the means of alleviating them are discussed.

Noor, A. K.↗
NASA NTRS1979

Thermal structure and energy influx to the day- and nightside Venus ionosphere

The thermal structure of and energy influx to the dayside and nightside Venus ionosphere are discussed, based on data obtained by the Pioneer Venus spacecraft. Where the dayside ionosphere appears relatively constant in total ion concentration, ion and electron temperatures and ion composition below the ionopause, retarding potential analyzer measurements confirm the presence of strong spatial and temporal variations in nightside ion concentration. Ion temperatures on the nightside above 225 km are found to be larger by a factor of two or more than on the dayside, and comparable to electron temperatures on the day and night sides. Models of dayside and nightside ion temperatures consistent with observations require the heat input of 0.003 erg/sq cm sec and 0.005 erg/sq cm sec, respectively, to the ion gas to raise the ion temperature above that of the neutral gas, presumably by means of Joule heating.

Knudsen, W. C.↗
NASA NTRS1984

Three-dimensional laser Doppler anemometer measurements of a jet in a crossflow

A three-dimensional laser Doppler anemometer (3D-LDA) was used in a wind tunnel to measure a jet in a crossflow. Measurements were made in the vicinity of a 5-cm-diam jet which issued normally into a 10.65 m/sec wind tunnel crossflow; the velocity ratio Vjet/Vinf was 8. Detailed lateral surveys were made at two elevations (z = cm and 2 cm); both elevations were within the region affected by the boundary layer on the plate. The results are believed to provide reliable velocity field information in the boundary layer of the jet in a crossflow. Turbulence information also is available and believed to be roughly correct, although it may be subject to broadening effects for the lower values of turbulence. A weak vortex pair was observed in the wake at the plate surface. This structure existed in the boundary layer and built confidence because the 3D-LDA was, indeed, able to resolve fine detail in the wake. The capabilities of the 3D-LDA not only allow the making of the velocity surveys, but can be utilized to follow mean streamlines in the flow.

Snyder, P.↗
NASA NTRS1985

Results of the May 1982 ATMAP Campaign at the Meteor Wind Station Kuehlungsborn (54 Deg N, 12 Deg E), GDR

A sinusoisal least squares fit over the entire data of 1 to 6 May 1982 was computed. The results are shown. Wind results refere to an average height of 95 km (no height determination). The wind results correspond, in general, to the average behavior of early May. The zonal prevailing west wind, however, is stronger than the term mean value (-5 m/s). Also, the amplitude of the semidiurnal idal wind is larger (mean value 1977 to 1983: 8 m/s).

Greisiger, K. M.↗
NASA NTRS1985

Zonally averaged model of dynamics, chemistry and radiation for the atmosphere

A nongeostrophic theory of zonally averaged circulation is formulated using the nonlinear primitive equations on a sphere, taking advantage of the more direct relationship between the mean meridional circulation and diabatic heating rate which is available in isentropic coordinates. Possible differences between results of nongeostrophic theory and the commonly used geostrophic formulation are discussed concerning: (1) the role of eddy forcing of the diabatic circulation, and (2) the nonlinear nearly inviscid limit vs the geostrophic limit. Problems associated with the traditional Rossby number scaling in quasi-geostrophic formulations are pointed out and an alternate, more general scaling based on the smallness of mean meridional to zonal velocities for a rotating planet is suggested. Such a scaling recovers the geostrophic balanced wind relationship for the mean zonal flow but reveals that the mean meridional velocity is in general ageostrophic.

Tung, K. K.↗
NASA NTRS1986

A CS J = 2 1 survey of the galactic center region

A CS map of the galactic center region is presented consisting of 15,000 spectra covering -1 deg. less than 3. deg. 6 min., -0 deg.4 min. less than b less than 0 deg. 4 min., each having an rms noise of 0.15 K in 1 MHz filters. CS is a high-excitation molecule, meaning that it is excited into emission only when the ambient density is less than n much greater than or approx. 2 x 10 to the 4th power/cu cm CS emission in the inner 2 deg. of the galaxy is nearly as pervasive as CO emission, in stark contrast to the outer galaxy where CS emission is confined to cloud cores. Galactic center clouds are on average much more dense than outer Galaxy clouds. This can be understood as a necessary consequence of the strong tidal stresses in the inner galaxy.

Stark, A. A.↗