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At least 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

A hydrodynamic approach to cosmology: The mixed dark matter cosmological scenario

We compute the evolution of spatially flat, mixed cold and hot dark matter models containing both baryonic matter and two kinds of dark matter. Hydrodynamics is treated with a highly developed Eulerian hydrodynamic code (see Cen 1992). A standard particle-mesh (PM) code is also used in parallel to calculate the motion of the dark matter components. We adopt the following parameters: h equivalent to (sub 0)/100 km/s Mpc(exp -1) = 0.5, OMEGA(sub C) = 0.3, and OMEGA(sub B) = 0.06, with amplitude of the perturbation spectrum fixed by the Cosmic Background Explorer Satellite (COBE) Dark Matter Radiation (DMR) measurements (Smoot et al. 1992) being sigma (sub 8) = 0.67. Four different boxes are simulated with box sizes of L = (64, 16, 4, 1) h(exp -1) Mpc, respectively, the two small boxes providing good resolution but little valid information due to absence of large-scale power. We use 128(exp 3) approximate 10(exp 6.3) baryonic cells, 128(exp .3) cold dark matter particles, and 2 x 128(exp 3) hot dark matter particles. In addition to the dark matter we follow separately six baryonic species (H, H(+), He, He(+), He(++), e(-)) with allowance for both (nonequilibrium) collisional and radiative ionization in every cell. The background radiation field is also followed in detail with allowance made for both continuum and line processes, to allow nonequilibrium heating and cooling processes to be followed in detail. The mean final Zeldovich-Sunyaev y parameter is estimated to be y Bar = (5.4 + or - 2.7) x 10(exp -7) below currently attainable observations, with a rms fluctuation of approximately delta bar y = (0.6 + or - 3.0) x 10(exp -7) on arcminute scales. The rate of galaxy formation peaks at an even later epoch (z approximate 0.3) than in the standard (OMEGA = 1, sigma sub 8 = 0.67) cold dark matter (CDM) model (z approximate 0.5) and, at a redshift of z = 4, is nearly a factor of 100 lower than for the CDM model with the same value of sigma sub 8. With regard to mass function, the smallest objects are stabilized against collapse by thermal energy: the mass-weighted mass spectrum has a broad peak in the vicinity of M(sub B) = 10(exp 9.5) solar mass with a reasonable fit to the Schechter luminosity function if the ratio of baryon mass to blue light is approximately 4. In addition, one very large PM simulation was made in a box with size (320 h(exp - 1) Mpc) containing 3 x 200(exp 3) = 10(exp 7.4) particles. Utilizing this simulation we find that the model yields a cluster mass function which is about a factor of 4 higher than observed, but a cluster-cluster correlation length marginally lower than observed, but that both are closer to observations than in the (COBE) normalized CDM model. The one-dimensional pairwise velocity dispersion is 605 + or - 8 km/s at 1/h separation, lower than that of the DCM model normalized to COBE, but still significant higher than observations (Davis & Peebles 1983). A plausible velocity bias b(sub v) = 0.8 + or - 0.1 on this scale will reduce but not remove the discrepancy. The velocity auto-correlat ion function has a coherence length of 40/h Mpc, which is somewhat lower than the observed counterpart. In all these respects the model would be improved by decreasing the cold fraction of the dark OMEGA(sub CDM)/ (OMEGA(sub CDM) + OMEGA(sub HDB). But formation of galaxies and clusters of galaxies is much later in this model than in COBE-normalized CDM, perhaps too late. To improve on these constraints a larger ratio of OMEGA(sub CDM)/ (OMEGA(sub CDM) + OMEGA(sub HDM)) is required than the value of 0.67 adopted here. It does not seem possible to find a value for this ratio which would satisfy all tests. Overall, the model is similar both on large and intermediate scales to the standard CDM model normalized to the same value of sigma(sub B), but the problem with regard to late formation of galaxies is more severe in this model than in that CDM model. Adding hot dark matter, significantly improves the ability of the COBE-normalized CDM scenario to fit existing observations, but the model is in fact not as good as the CDM model with the same sigma(sub 8) and is still probably unsatisfactory with regard to several critical tests.

Cen, Renyue↗
NASA NTRS2008

Prospects for Studying Interstellar Magnetic Fields with a Far-Infrared Polarimeter for SAFIR

Polarimetry at mid-infrared through millimeter wavelengths using airborne and ground-based telescopes has revealed magnetic structures in dense molecular clouds in the interstellar medium, primarily in regions of star formation. Furthermore, spectropolarimetry has offered clues about the composition of the dust grains and the mechanism by which they are aligned with respect to the local magnetic field. The sensitivity of the observations to date has been limited by the emission from the atmosphere and warm telescopes. A factor of 1000 in sensitivity can be gained by using instead a cold space telescope. With 5 arcminute resolution, Planck will make the first submillimeter polarization survey of the full Galaxy early in the next decade. We discuss the science case for and basic design of a far-infrared polarimeter on the SAFIR space telescope, which offers resolution in the few arcsecond range and wavelength selection of cold and warm dust components. Key science themes include the formation and evolution of molecular clouds in nearby spiral galaxies, the magnetic structure of the Galactic center, and interstellar turbulence.

Dowell, C. Darren↗
NASA NTRSDate not supplied

37-Day Microgravity Exposure in 16-Week Female C57BL/6J Mice is Associated with Weight Bearing Site-Specific Bone Loss

Exposure to space environment factors, including microgravity unloading and space radiation, is well-known to be associated with rapid bone loss in mammals, but significant questions remain about mechanistic aspects of this process. Specifically, bone loss may vary depending on many interacting factors including age and skeletal maturity, physical activity, social versus individual housing, and habitat configuration. Likewise cellular tissue loss mechanisms in space may include changes to mineral homeostasis by activation of bone degradation by osteoclasts, or inhibition of bone formation by osteoblasts, and be caused by microgravity mechanical unloading or increased space radiation. In this study we used microcomputed tomography to quantify bone loss in multiple skeletal sites of group housed mice either exposed to 37 days in space on ISS during the Rodent Research 1, (RR1), experiments using the NASA Rodent Habitat, (RH), versus ground controls in the same habitats or standard vivarium cages. Our results in space versus matched RH ground controls show significant cancellous and cortical bone loss in the femur and tibia, but not in vertebrae, indicating that only weight-bearing bones experience significant bone loss during spaceflight. This finding suggests that space radiation at the low levels found in low Earth Orbit may not be a significant contributing factor to bone loss in space. Interestingly while mice maintained in the RH maintained or gained bone relative to Basal controls, mice kept in standard vivarium cages for 37 days showed significant loss in weight bearing bones, not as acute, but comparable in magnitude to spaceflight effects. These results suggest the RH configuration versus vivarium cages, may allow for greater mechanical loading of weight bearing bones as mice climb internal habitat surfaces, promoting maintenance of bone regenerative health.

mouse↗
DOE OSTI2021

Fermi-type particle acceleration from magnetic reconnection at the termination shock of a relativistic striped wind

An oblique-rotating pulsar generates a relativistic striped wind in a pulsar wind nebula (PWN). The termination shock of the PWN compresses the Poynting-flux-dominated flow and drives magnetic reconnection. By carrying out particle-in-cell simulations of the termination shock of the PWN, we study the shock structure as well as the energy conversion processes and particle acceleration mechanisms. With the recent advances in the numerical methods, we extend the simulations to the ultrarelativistic regime with a bulk Lorentz factor of up to γ 0 = 10 6 . Magnetic reconnection at the termination shock is highly efficient at converting magnetic energy to particle kinetic energy and accelerating particles to high energies. Similar to earlier studies, we find that the resulting energy spectra crucially depend on λ/d e (λ is the wavelength of the striped wind and de is the relativistic plasma skin depth). When λ/d e is large (λ ≳ 40d e ), the downstream particle spectra form a power-law distribution in the magnetically dominated relativistic wind regime. By analyzing particle trajectories and statistical quantities relevant to particle energization, we find that Fermi-type mechanism dominates the particle acceleration and power-law formation. We find that the results for particle acceleration are scalable as γ 0 and σ 0 increase to large values. The maximum energy for electrons and positrons can reach hundreds of TeV if the wind has a bulk Lorentz factor of γ0 ≈ 106 and a magnetization parameter of σ 0 = 10, which can explain the recent observations of high-energy gamma rays from PWNe

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1994

The CO/SiO radiative instability in cool star atmospheres revisited

We revisit the formation of radiative instabilities in cool star atmospheres and compare our results with those given by Muchmore, Nuth, & Stencel. We have considered the combined influence of CO and SiO molecules and have computed models for a grid of effective temperatures and geometrical dilution factors for the stellar radiation. Our results are based on the analysis of the energy balance of gas elements with prescribed thermodynamic properties. Our results show that radiative instabilities are most likely primarily caused by CO, whereas SiO is expected to play only a minor role, except when the CO density is reduced compared to Local Thermodynamic Equilibrium (LTE) values or the CO band can be assumed to be optically thick. The onset of radiative instabilities is expected to be strongly modified when dynamic phenomena such as stochastic shocks are present. Our results provide strong evidence that dust formation can most likely occur via a radiative instability alone. Therefore, we present a revised version of the Muchmore et al. dust formation paradigm, which also considers hydrodynamic cooling. The new paradigm is particularly relevant in cases where dust is formed relatively close to the stellar photosphere.

Cuntz, M.↗
DOE OSTI2024

Advancing Pore–Space–Partitioned Metal–Organic Frameworks with Isoreticular Cluster Concept

Trigonal planar M 3 (O/OH) trimers are among the most important clusters in inorganic chemistry and are the foundational features of multiple high-impact MOF platforms. Here we introduce a concept called isoreticular cluster series and demonstrate that M 3 (O/OH), as the first member of a supertrimer series, can be combined with a higher hierarchical member (double-deck trimer here) to advance isoreticular chemistry. We report here an isoreticular series of pore-space-partitioned MOFs called M 3 M 6 pacs made from co-assembly between M 3 single-deck trimer and M 3x2 double-deck trimer. Important factors were identified on this multi-modular MOF platform to guide optimization of each module, which enables the phase selection of M 3 M 6 pacs by overcoming the formation of previously-always-observed same-cluster phases. The new pacs materials exhibit high surface area and high uptake capacity for CO 2 and small hydrocarbons, as well as selective adsorption properties relevant to separation of industrially important mixtures such as C 2 H 2 /CO 2 and C 2 H 2 /C 2 H 4 . Furthermore, new M 3 M 6 pacs materials show electrocatalytic properties with high activity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Understanding the Importance of Labile Fe(III) during Fe(II)-Catalyzed Transformation of Metastable Iron Oxyhydroxides

Transformation of metastable Fe(III) oxyhydroxides is a prominent process in natural environments and can be significantly accelerated by the coexisting aqueous Fe(II) (Fe(II) aq ). Recent evidence points to the solution mass transfer of labile Fe(III) (Fe(III) labile ) as the primary intermediate species of general importance. However, a mechanistic aspect that remains unclear is the dependence of phase outcomes on the identity of the metastable Fe(III) oxyhydroxide precursor. Here, we compared the coupled evolution of Fe(II) species, solid phases, and Fe(III) labile throughout the Fe(II)-catalyzed transformation of lepidocrocite (Lp) versus ferrihydrite (Fh) at equal Fe(III) mass loadings with 0.2–1.0 mM Fe(II) aq at pH = 7.0. Similar to Fh, the conversion of Lp to product phases occurs by a dissolution–reprecipitation mechanism mediated by Fe(III) labile that seeds the nucleation of products. Though for Fh we observed a transformation to goethite (Gt), accompanied by the transient emergence and decline of Lp, for initial Lp we observed magnetite (Mt) as the main product. Further, a linear correlation between the formation rate of Mt and the effective supersaturation in terms of Fe(III) labile concentration shows that Fe(II)-induced transformation of Lp into Mt is governed by the classical nucleation theory. When Lp is replaced by equimolar Gt, Mt formation is suppressed by opening a lower barrier pathway to Gt by heterogeneous nucleation and growth on the added Gt seeds. The collective findings add to the mechanistic understanding of factors governing phase selections that impact iron bioavailability, system redox potential, and the fate and transport of coupled elements.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Probing Surface Plasmon Dynamics in Periodic Nanostructures through Ultrafast Electron Microscopy

Surface plasmon polaritons (SPPs) can be manipulated to localize and guide light in subwavelength distances, enabling them to find applications in a wide range of areas, from sensing to quantum computing. Among several methods of SPP excitation, periodic arrays of nano- and microstructures are of particular interest, as they enable engineering SPP properties through structural parameters. Here, in this study, using the photon-induced near-field electron microscopy (PINEM) technique, we investigated the mode formation, coupling, interference, and decay of SPPs in square and hexagonal arrays of circular nanoholes under both visible and near-infrared excitation. Polarization-resolved analysis revealed the key factors governing SPP localization and interference patterns, showing that the periodicity and symmetry of the array primarily determine the SPP interference patterns and their orientation, while pump polarization mainly modulates their intensity. Time-resolved PINEM measurements demonstrated the spatial dependence of the SPP temporal characteristics. In addition, cathodoluminescence (CL) spectroscopy was employed to examine the intrinsic plasmonic characteristics of the structure. Finite difference time domain (FDTD) simulations showed strong agreement with both PINEM and CL measurements on the spatial and spectral behavior of SPPs. Understanding the spatiotemporal dynamics of SPPs on nanostructures beyond the diffraction limit is crucial for optimizing plasmonic structures for advanced photonic and quantum technologies.

Plasmonics↗
DOE OSTI2025

Atomic-scale understanding of oxide growth and dissolution kinetics of Ni-Cr alloys

Aqueous corrosion of metals is governed by formation and dissolution of a passivating, multi-component surface oxide. Unfortunately, a detailed atomistic description is challenging due to the compositional complexity and the need to consider multiple kinetic factors simultaneously. To this end, we combine experiments with a first-principles-derived, multiscale computational framework that transcends thermodynamic descriptions to explicitly simulate the kinetic evolution of surface oxides of Ni-Cr alloys as a function of composition, temperature, pH, and applied voltage. In the absence of pitting, we identify three distinct voltage regimes, which are kinetically dominated by oxide growth, dissolution, and competitive dissolution and reprecipitation. Evolving compositional gradients and oxide thickness are revealed, including a transition between a metastable Ni-Cr mixed oxide and a thick, porous Ni-dominated oxide. Beyond elucidating the underlying physics, we highlight the need for competing kinetics in models to properly predict the transition from passivation to corrosion. Our results provide a key step towards co-design of alloy composition alongside environmental conditions for sustainable use across a variety of critical energy and infrastructure applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Crystallization of TiO 2 polymorphs from RF-sputtered, amorphous thin-film precursors

Crystalline TiO 2 films of anatase, brookite, and rutile are reproducibly made from amorphous precursors deposited by RF magnetron sputtering, producing large-area, single phase films of uniform thickness. Sputtered amorphous TiO 2 precursor thin films follow the general behavior observed for amorphous precursor thin films generated by pulsed laser deposition, namely, that oxygen deficiency is necessary for the formation of brookite and rutile. We quantify the oxygen deficiency and correlate it with the long wavelength optical absorption. We find that the precursor deposition rate is also a contributing factor to phase selection and that brookite and rutile form from films deposited more rapidly and anatase from films deposited more slowly. Sputtered and pulsed laser deposited amorphous precursor films prepared with similar oxygen deficiency and similar thickness result in the same final state after annealing, but the rate for sputtered precursors is slower.

14 SOLAR ENERGY↗
DOE OSTI2020

Experimental observation of magnetic dimers in diluted Yb:YAlO 3

In this paper, we present a comprehensive experimental investigation of Yb magnetic dimers in Yb 0.04 Y 0.96 AlO 3 , an Yb-doped yttrium aluminum perovskite YAlO3, by means of specific heat, magnetization, and high-resolution inelastic neutron scattering (INS) measurements. In our sample, the Yb ions are randomly distributed over the lattice and ~7% of Yb ions form quantum dimers due to nearest-neighbor antiferromagnetic coupling along the c axis. At zero field, the dimer formation manifests itself in an appearance of an inelastic peak at Δ ≈ 0.2 meV in the INS spectrum and a Schottky-like anomaly in the specific heat. The structure factor of the INS peak exhibits a cosine modulation along the $L$ direction, in agreement with the $c$-axis nearest-neighbor intradimer coupling. A careful fitting of the low-temperature specific heat shows that the excited state is a degenerate triplet, which indicates a surprisingly small anisotropy of the effective Yb-Yb exchange interaction despite the low crystal symmetry and anisotropic magnetic dipole contribution, in agreement with previous reports for the Yb parent compound, YbAlO 3 , and in contrast to Yb 2 Pt 2 Pb. The obtained results are precisely reproduced by analytical calculations for the Yb dimers.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Bench Testing Data and Report for an Early Prototype Pitch Resonator WEC

This dataset encompasses data and documentation from bench tests conducted on an early prototype of a "pitch resonator" wave energy converter (WEC). The testing aimed to validate numerical models and reduce risks associated with the pitch resonator concept, which is designed to convert the pitching and rolling motions of a buoy into electrical power. The project's goal is to provide supplementary power, in the range of 10-100 watts, to the National Science Foundation's Ocean Observatories Initiative Pioneer Array. Two distinct testing phases are documented: one using a single degree of freedom (1DOF) test rig, and another employing a six degree of freedom (6DOF) Stewart platform, known as the Large Amplitude Motion Platform (LAMP). These tests assessed various factors, such as system performance in different motion scenarios, the torque exerted by wave forces, and the impact of mounting configurations. The dataset includes raw test data in MATLAB (.mat) format, detailed metadata, and a report describing the experimental procedures and preliminary findings.

16 TIDAL AND WAVE POWER↗
NASA NTRS1980

Deflagration-to-detonation transition in granular HMX

Granular HMX of three degrees of fineness was packed into heavy-walled steel tubes closed at both ends. Ignition was obtained at one end using an intimate mixture of finely divided titanium and boron as an igniter that produced heat with little gas. The distance to detonation was determined by examination of the resulting tube fragments. By inserting tightly-fitted neoprene diaphragms periodically into the HMX column, it was shown that the role of convective combustion was limited to the initial stage of the deflagration to detonation (DDT) process. Experiments in which various combinations of two of the three types of HMX were loaded into the same tube showed that heating by adiabatic shear of explosive grains was an essential factor in the final buildup to detonation. A description of the DDT process is developed in which conductive burning is followed in turn by convective burning, bed collapse with plug formation, onset of accelerated burning at the front of the plug through heating by intercrystalline friction and adiabatic shear, and intense shock formation resulting in high-order detonation.

Campbell, A. W.↗
NASA NTRS1984

Life sciences and space research XXI(2); Proceedings of the Seventh and Eleventh Workshops, Graz, Austria, June 25-July 7, 1984

Space-based and space-related research in the life sciences is presented in reviews and reports. Topics examined include the long-term effects of weightlessness, cosmic chemistry and chemical and biological evolution, life-support systems for space travel, planetary protection, and the g-scale factor in gravitational biology. Consideration is given to the role of Ca ions in cytological effects of hypogravity, the organic aerosols of Titan, the role of meteorite impacts in the formation of organic molecules, prebiotic synthesis of purines and pyrimidines, atmosphere behavior of gas-closed mouse-algal systems, air and water regeneration in advanced regenerative environmental-control and life-support systems, and the influence of gravity on the development of animal systems.

Oser, H.↗
NASA NTRS1986

Corrosion pitting of SiC by molten salts

The corrosion of SiC by thin films of Na2CO3 and Na2SO4 at 1000 C is characterized by a severe pitting attack of the SiC substrate. A range of different Si and SiC substrates were examined to isolate the factors critical to pitting. Two types of pitting attack are identified: attack at structural discontinuities and a crater-like attack. The crater-like pits are correlated with bubble formation during oxidation of the SiC. It appears that bubbles create unprotected regions, which are susceptible to enhanced attack and, hence, pit formation.

Jacobson, N. S.↗
NASA NTRS1990

Managing Information On Costs

Cost Management Model, CMM, software tool for planning, tracking, and reporting costs and information related to costs. Capable of estimating costs, comparing estimated to actual costs, performing "what-if" analyses on estimates of costs, and providing mechanism to maintain data on costs in format oriented to management. Number of supportive cost methods built in: escalation rates, production-learning curves, activity/event schedules, unit production schedules, set of spread distributions, tables of rates and factors defined by user, and full arithmetic capability. Import/export capability possible with 20/20 Spreadsheet available on Data General equipment. Program requires AOS/VS operating system available on Data General MV series computers. Written mainly in FORTRAN 77 but uses SGU (Screen Generation Utility).

Taulbee, Zoe A.↗
NASA NTRS1992

Flux-limited diffusion in hydrodynamics

The radiation transport equation, correct to order v/c, is used to derive a flux limiter valid in high-velocity hydrodynamic flows. In the limit v = 0 the widely used result of Levermore and Pomraning (LP) (1982) is recovered. In the case of homologous flow (v varies as R) the LP result remains valid, but in the presence of shock waves and other nonhomologous flows differences may be large. We calculate the Eddington factors, which are significantly altered, and discuss the importance of having them possess the correct form in the presence of a shock wave, e.g., on the neutrino transport during the formation of a supernova.

Cooperstein, J.↗
NASA NTRS2002

Upper Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS

Current state-of-the-art environmental barrier coatings (EBCs) for Si-based ceramics consist of three layers: a silicon bond coat, an intermediate mullite (3Al2O3-2SiO2) or mullite + BSAS (1-xBaO-xSrO-Al2O3-2SiO2) layer, and a BSAS top coat. Areas of concern for long-term durability are environmental durability, chemical compatibility, silica volatility, phase stability, and thermal conductivity. Variants of this family of EBCs were applied to monolithic SiC and melt infiltrated SiC/SiC composites. Reaction between BSAS and silica results in low melting (approx. 1300 C) glasses at T > 1400 C, which can cause the spallation of the EBC. At temperatures greater than 1400 C, the BSAS top coat also degrades by formation of a porous structure, and it suffers significant recession via silica volatilization in water vapor-containing atmospheres. All of these degradation mechanisms can be EBC life-limiting factors. BSAS undergoes a very sluggish phase transformation (hexagonal celsian to monoclinic celsian), the implications of which are not fully understood at this point. There was evidence of rapid sintering at temperatures as low as 1300 C, as inferred from the sharp increase in thermal conductivity.

Lee, Kang N.↗