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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 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Diffusion in Immiscible Melts

The objective of this program is to measure the diffusion coefficients for molten Pb in Zn in the immiscible liquid-phase region. Diffusion couples of pure Pb and Zn were prepared using a shear cell. These have been placed in graphite crucibles and encapsulated in stainless steel cartridges and are awaiting the next Materials Experiment Assembly (MEA) flight opportunity. In flight, one couple will be soaked for 40 minutes at 440 deg C (just above the monotectic temperature) and the second couple will be soaked for 40 minutes 820 deg C (just above the consolute temperature). After the soak both samples will be rapidly quenched by flowing He to minimize redistribution of the immiscible phases. Post flight compositional analysis will be accomplished using X-ray fluorescence in the scanning electron microscopy.

Pond, R. B.↗
NASA NTRS1987

Simulations of the sun's polar magnetic fields during sunspot cycle 21

Regarding new bipolar magnetic regions as sources of flux, the evolution of the radial component of the solar photospheric magnetic field is simulated during 1976-1984, and the corresponding evolution of the line-of-sight polar fields as seen from earth is derived. The observed timing and strength of the polar-field reversal during cycle 21 can be accounted for by supergranular diffusion alone, for a diffusion coefficient of 800 sq km/sec. For an assumed 300 sq km/sec rate of diffusion, on the other hand, a poleward meridional flow with a moderately broad profile and a peak speed of 10 m/s reached at about 5 deg latitude is required to obtain agreement between the simulated and observed fields. Such a flow accelerates the transport of following-polarity flux to the polar caps, but also inhibits the diffusion of leading-polarity flux across the equator. For flows faster than about 10 m/s, the latter effect dominates, and the simulated polar fields reverse increasingly later and more weakly than the observed fields.

Devore, C. Richard↗
NASA NTRS1991

Single Doppler radar and lidar studies of the dynamics of mixed layers and overlying stable regions

Some examples of profiles of PBL quantities retrieved from Doppler radar and lidar observations are shown, and the extent to which the deduced quantities agree with the traditional eddy viscosity concept is examined. It is found in one case that the concept of 'top down bottom up' diffusion introduced by Wyngaard (1983) is more pertinent than that of the eddy viscosity. In another case it is found that, in stable layers with sufficiently large Richardson number, the momentum fluxes could be counter-gradient. In this latter case the use of a negative turbulence diffusion coefficient would result in an ill-posed differential equation, thus confusing the 'diffusion problem'.

Xu, Mei↗
NASA NTRS1996

Diffusion, Viscosity and Crystal Growth in Microgravity

The diffusivity of TriGlycine Sulfate (TGS), Potassium Dihydrogen Phosphate (KDP), Ammonium Dihydrogen Phosphate (ADF) and other compounds of interest to microgravity crystal growth, in supersaturated solutions as a function of solution concentration, 'age' and 'history was studied experimentally. The factors that affect the growth of crystals from water solutions in microgravity have been examined. Three non-linear optical materials have been studied, potassium dihydrogen phosphate (KDP), ammonium dihydrogen phosphate (ADP) and triglycine sulfate (TGC). The diffusion coefficient and viscosity of supersaturated water solutions were measured. Also theoretical model of diffusivity and viscosity in a metastable state, model of crystal growth from solution including non-linear time dependent diffusivity and viscosity effect and computer simulation of the crystal growth process which allows simulation of the microgravity crystal growth were developed.

Myerson, Allan S.↗
NASA NTRS2012

Plasma Diffusion in Self-Consistent Fluctuations

The problem of particle diffusion in position space, as a consequence ofeleclromagnetic fluctuations is addressed. Numerical results obtained with a self-consistent hybrid code are presented, and a method to calculate diffusion coefficient in the direction perpendicular to the mean magnetic field is proposed. The diffusion is estimated for two different types of fluctuations. The first type (resuiting from an agyrotropic in itiai setting)is stationary, wide band white noise, and associated to Gaussian probability distribution function for the magnetic fluctuations. The second type (result ing from a Kelvin-Helmholtz instability) is non-stationary, with a power-law spectrum, and a non-Gaussian probabi lity distribution function. The results of the study allow revisiting the question of loading particles of solar wind origin in the Earth magnetosphere.

Smets, R.↗
DOE OSTI2022

Temperature-Dependent Constrained Diffusion of Micro-Confined Alkylimidazolium Chloride Ionic Liquids

Alkylimidazolium chloride ionic liquids (ILs) have many uses in a variety of separation systems, including micro-confined separation systems. To understand the separation mechanism in these systems, the diffusion properties of analytes in ILs under relevant operating conditions, including micro-confinement dimension and temperature, should be known. For example, separation efficiencies for various IL-based microextraction techniques are dependent on the sample volume and temperature. Temperature-dependent (20–100 °C) fluorescence recovery after photobleaching (FRAP) was utilized to determine the diffusion properties of a zwitterionic, hydrophilic dye, ATTO 647, in alkylimidazolium chloride ILs in micro-confined geometries. These micro-confined geometries were generated by sandwiching the IL between glass substrates that were separated by ~1 to 100 μm. From the measured temperature-dependent FRAP data, we note alkyl chain length-, thickness-, and temperature-dependent diffusion coefficients, with values ranging from 0.021 to 46 μm 2 /s. Deviations from Brownian diffusion are observed at lower temperatures and increasingly less so at elevated temperatures; the differences are attributed to alterations in intermolecular interactions that reduce temperature-dependent nanoscale structural heterogeneities. Furthermore, the temperature- and thickness-dependent data provide a useful foundation for efficient design of micro-confined IL separation systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Chemical Mixing Induced by Internal Gravity Waves in Intermediate-mass Stars

Internal gravity waves can cause mixing in the radiative interiors of stars. We study this mixing by introducing tracer particles into 2D hydrodynamic simulations. Following the work of Rogers & McElwaine, we extend our study to different masses (3, 7, and 20 M ⊙ ) and ages (ZAMS, midMS, and TAMS). The diffusion profiles of these models are influenced by various parameters such as the Brunt–Väisälä frequency, density, thermal damping, the geometric effect, and the frequencies of waves contributing to these mixing profiles. We find that the mixing profile changes dramatically across age. In younger stars, we noted that the diffusion coefficient increases toward the surface, whereas in older stars the initial increase in the diffusion profile is followed by a decreasing trend. We also find that mixing is stronger in more massive stars. Hence, future stellar evolution models should include this variation. In order to aid the inclusion of this mixing in 1D stellar evolution models, we determine the dominant waves contributing to these mixing profiles and present a prescription that can be included in 1D models.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1973

The mobility and diffusion of ions in gases

Experimental and theoretical aspects of the mobility and diffusion of ions in gases are studied in detail. Some of the subjects discussed include ion-ion interaction, boundary condition and ion and electron behavior. Also discussed in separate chapters are the problems of the diffusion coefficients and the afterglow techniques. Finally, a special chapter studies the kinetic theory of diffusion and mobility, stressing the low-, medium- and high-field theory.

Mcdaniel, E. W.↗
NASA NTRS1985

Voyager observations of ion phase space densities in the Jovian magnetosphere

Data from the Voyager low-energy charged particle experiment (LECP) were used to calculate ion phase space densities in Jupiter's magnetosphere. The calculation of F at constant mu and J(2) requires the determination of particle fluxes at specific pitch angles and energies. It is shown that the greatest uncertainties in the determination of F from LECP data arise from the fits to the measured pitch angle distributions and differential energy spectra. An estimate is provided of this uncertainty and of others arising from model radial diffusion coefficients and magnetic fields. The general nature of the curves is consistent with inward diffusion of these energetic ions from the outer magnetosphere combined with losses near but starting beyond the orbit of Io. These losses are not consistent with simple satellite sweeping by Io alone and are probably due to an Io-torus wave-particle interaction. The lifetime against loss deduced from the data is approximately 20,000 s near Io and is a value consistent with strong diffusion losses. The measured ion loss rate declines much more rapidly than the strong diffusion loss rate as L increases from 7 to 9, suggesting that the loss rate is well below the strong-diffusion rate beyond 7 Jupiter radii for ions observed by the LECP.

Paonessa, M.↗
NASA NTRS1990

Injection and diffusive transport of suprathermal through energetic solar flare protons (35 keV to 20 MeV)

Consideration is given to the injection and interplanetary propagation of low-energy protons caused by the solar particle event of July 20, 1981, in which flare protons in the range from 35 keV to 20 MeV were observed by instruments on ISEE 3. The observed time-intensity and time-anisotropy profiles were fitted over the entire energy range using a model based on the spherically symmetric Fokker-Plank equation, including convection, diffusion, and adiabatic deceleration. The results are used to discuss the behavior of the radial interplanetary diffusion coefficient and the scattering mean free path for protons. Also, evidence is found for diffusive coronal shock acceleration of protons during the event.

Beeck, J.↗
NASA NTRS1995

Star formation activity in spiral galaxy disks and the properties of radio halos: Observational evidence for a direct dependence

In this article we address observationally the questions: how does star formation (SF) in the disks of galaxies lead to the creation of radio halos, and what minimum energy input into the interstellar medium (ISM) is needed to facilitate this? For the investigation we use a sample of five edge-on galaxies exhibiting radio continuum emmission in their halos and enhanced SF spread over large parts of their disks. In a detailed study of the two galaxies in our sample for which we have the best data, NGC 891 and NGC 4631, we show that the radio halos cut off abruptly at galactocentric radii smaller than those of the underlying thin radio disks. Our most important result is that the halo cutoffs are spatially coincident with the radii where the SF activity in the underlying disks drops sharply. The difference in radius of the emission distributions tracing ongoing SF in the disks (IRAS 50 micrometers, H alpha) versus that of the nonthermal radio continuum thin disks (tracing the distribution of cosmic-ray (CR) electrons) is typically a few kpc. This difference in extent is caused by CR diffusion. We have measured the CR diffusion coefficients in the thin disks of both NGC 891 and NGC 4631. For radial diffusion of CR electrons within the galactic disks the values are D(sub r) = 1.1-2.5 x 10 (exp 29) sq cm/s (NGC 4631) and D(sub r) = 1.2 x 10(exp 29) sq cm/s (NGC 891). For motions in the z-direction in areas within the thin disks where no outflows occur, we derive a firm upper limit of D(sub z) less than or equal to 0.2 x 10(exp 28) sq cm/s for NGC 891. The value for NGC 4631 is D(sub z = 1.4 x 10 (exp 28) sq cm/s. The other three galaxies in our sample, NGC 3044, NGC 4666, and NGC 5775 show (at the sensitivity of our data) less extended, more filamentary radio halos. Isolates spurs or filaments of nonthermal radio continuum emission in their halos are traced only above the most actively star-forming regions in the disks. This, in conjuction with the results obtained for NGC 891 and NGC 4631, suggests a direct connection between the shapes and radial extents of radio halos of normal galaxies and the level and the spatial distribution of the current SF in their disks. The existence of filamentary structures implies that expansion and convection of CR-heated plasma plays an important role even at low z-distances above the plane. Based on radio data we derive the disk-averaged mean energy input rates into the ISM due to supernova explosions per unit surface area in nine galaxies, including the ones in our sample. We find a clear trend in the sense that the galaxies with the highest mean energy input rates in the underlying disks have the most extended and pervasisve radio halos. Below a certain threshohld, outflows are inhibited. As a lower limit for this threshhold we derive an energy input rate of approximately 10 (exp -4) erg/s sq cm. The fact that radio halo emission is found only above the most luminous giant H 2 regions in late-type 'normal' galaxies (of type Sbc or later) indicates that only these are capable of initiating disk-halo interactions by locally exceeding the threshold. The distribution of the local outflows determines the overall shapes of the CR halos and leads to the observed filamentary structures.

Dahlem, Michael↗
DOE OSTI2023

Ionothermally Synthesized Nanoporous Ti 0.95 W 0.05 Nb 2 O 7 : a Novel Anode Material for High-Performance Lithium-Ion Batteries

Although TiNb 2 O 7 is regarded as a fast-rechargeable lithium-ion battery (LIB) anode material, the intrinsic poor electrochemical kinetics of TiNb 2 O 7 still dramatically impedes its development. Herein, an ionothermal synthesis-assisted doping strategy is proposed for the preparation of a new W 6+ -doped TiNb 2 O 7 material (Ti 0.95 W 0.05 Nb 2 O 7 ) with nanoporous structure (denoted as NPTWNO). The improved Li + diffusion coefficient of NPTWNO suggests that the ionic-liquid-templated nanoporous architecture improves the Li+ diffusion kinetics. The density functional theory computational study reveals that the doped W 6+ successfully boosts the electronic conductivity due to the narrowed conduction-valance bandgap resulted from charge redistribution, which is reflected by the electrochemical impedance spectroscopy data. With the simultaneously enhanced Li+ diffusivity and electronic conductivity, NPTWNO achieves fast-rechargeability in LIBs. So this work indicates the potential of ionothermal synthesis-assisted doping strategy on energy storage materials and offers NPTWNO material with promising electrochemical performance.

25 ENERGY STORAGE↗
DOE OSTI2020

Automated calculation and convergence of defect transport tensors

Defect diffusion is a key process in materials science and catalysis, but as migration mechanisms are often too complex to enumerate a priori, calculation of transport tensors typically have no measure of convergence and require significant end-user intervention. These two bottlenecks prevent high-throughput implementations essential to propagate model-form uncertainty from interatomic interactions to predictive simulations. In order to address these issues, we extend a massively parallel accelerated sampling scheme, autonomously controlled by Bayesian estimators of statewide sampling completeness, to build atomistic kinetic Monte Carlo models on a state-space irreducible under exchange and space group symmetries. Focusing on isolated defects, we derive analytic expressions for drift and diffusion coefficients, providing a convergence metric by calculating the Kullback–Leibler divergence across the ensemble of diffusion processes consistent with the sampling uncertainty. The autonomy and efficacy of the method is demonstrated on surface trimers in tungsten and Hexa-interstitials in magnesium oxide, both of which exhibit complex, correlated migration mechanisms.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Uncovering the hydride ion diffusion pathway in barium hydride via neutron spectroscopy

Solid state materials possessing the ability for fast ionic diffusion of hydrogen have immense appeal for a wide range of energy-related applications. Ionic hydrogen transport research is dominated by proton conductors, but recently a few examples of hydride ion conductors have been observed as well. Barium hydride, BaH 2 , undergoes a structural phase transition around 775 K that leads to an order of magnitude increase in the ionic conductivity. This material provides a prototypical system to understand hydride ion diffusion and how the altered structure produced by the phase transition can have an enormous impact on the diffusion. We employ quasielastic and inelastic neutron scattering to probe the atomic scale diffusion mechanism and vibrational dynamics of hydride ions in both the low- and high-temperature phases. Jump lengths, residence times, diffusion coefficients, and activation energies are extracted and compared to the crystal structure to uncover the diffusion pathways. We find that the hydrogen jump distances, residence times, and energy barriers become reduced following the phase transition, allowing for the efficient conduction of hydride ions through a series of hydrogen jumps of length L = 3.1 Å.

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

Diffusion characteristics of water molecules in a lamellar structure formed by triblock copolymers

The distribution and diffusion of water molecules are playing important roles in determining self-assembly and transport properties of polymeric systems. Small-angle neutron scattering (SANS) experiments and molecular dynamics (MD) simulation have been applied to understand the distribution of water molecules and their dynamics in the lamellar membrane formed by Pluronic L62 block copolymers. Penetration of water molecules into the polyethylene oxide (PEO) layers of the membranes has been estimated using scattering length density (SLD) profiles obtained from SANS measurements, which agree well with the molecular distribution observed from MD simulations. The water diffusion coefficient at different regions of the lamellar membrane was further investigated using MD simulation. The diffusion characteristic shows a transition from normal to anomalous diffusion as the position of the water molecule changes from the bulk to PEO and to the polypropylene oxide (PPO) layer. Here we find that water molecules within the PEO or PPO layers follow subdiffusive dynamics, which can be interpreted by the model of fractional Brownian motion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Ultrafast transient absorption measurements of photocarrier dynamics in PdSe 2

We investigate the photocarrier dynamics in bulk PdSe 2 , a layered transition metal dichalcogenide with a novel pentagonal structure and unique electronic and optical properties. Using femtosecond transient absorption microscopy, we study the behavior of photocarriers in mechanically exfoliated bulk PdSe 2 flakes at room temperature. By employing a 400 nm ultrafast laser pulse, electron–hole pairs are generated, and their dynamics are probed using an 800 nm detection pulse. Our findings reveal that the lifetime of photocarriers in bulk PdSe 2 is approximately 210 ps. Furthermore, by spatially resolving the differential reflection signal, we determine a photocarrier diffusion coefficient of about 7.3 cm 2 s –1 . Based on these results, we estimate a diffusion length of around 400 nm and a photocarrier mobility of approximately 300 cm 2 V –1 s –1 . Furthermore, these results shed light on the ultrafast optoelectronic properties of PdSe 2 , offer valuable insights into photocarriers in this emerging material, and enable design of high-performance optoelectronic devices based on PdSe 2 .

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1982

An electrochemical study of Ni/2+/, Co/2+/, and Zn/2+/ ions in melts of composition CaMgSi2O6

Results are reported for a study of the behavior of Ni(2+), Co(2+), and Zn(2+) in molten diopside, determined by cyclic voltametry, as a first step in adapting electrochemical methods for use in experimental geochemistry. The techniques used are described, the theory is briefly discussed, and it is demonstrated that Ni(2+), Co(2+), and Zn(2+) can be reduced, then reoxidized in an uncomplicated case of reversible charge transfer at the Pt electrode. This implies that the neutral metal atoms remain undissolved in the melt. The kinetics of the reduction process, the effects of concentration, the standard reduction potentials, and the diffusion coefficients are discussed in detail. The observations are consistent with the mechanism of Ni(2+) diffusion by transport from site to site. As the proportion of SiO2 in the melt increases, a higher activation energy is required for Ni(2+) to move, and the rate of diffusion is lowered.

Semkow, K. W.↗
NASA NTRS1983

Analytical and Numerical Solution for a Solidifying Liquid Alloy Slab

Numerical and analytical solutions are presented for the temperature and concentration distributions during the solidification of a binary liquid alloy slab. The slab is taken to be of a finite depth but infinite in the horizontal direction. The solidification process is started by withdrawing a fixed amount of heat from the lower surface of the slab. The upper surface of the slab is subjected to both radiation and convective conditions. The solution gives the concentration and temperature profiles and the interface position as a function of time. Due to the smallness of the mass diffusion coefficient in the solid, the numerical solution method breaks down whenever the ratio of the diffusivities in the solid and the liquid falls below a certain value. An analytical method is developed which gives accurate solution for any value of the diffusivity ratio.

Antar, B. N.↗