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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 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Nanoparticle scaffolds for syngas-fed solid oxide fuel cells

Incorporation of nanoparticles into devices such as solid oxide fuel cells (SOFCs) may provide benefits such as higher surface areas or finer control over microstructure. However, their use with traditional fabrication techniques such as screen-printing is problematic. Here, we show that mixing larger commercial particles with nanoparticles allows traditional ink formulation and screen-printing to be used while still providing benefits of nanoparticles such as increased porosity and lower sintering temperatures. SOFC anodes were produced by impregnating ceria–gadolinia (CGO) scaffolds with nickel nitrate solution. The scaffolds were produced from inks containing a mixture of hydrothermally-synthesised nanoparticle CGO, commercial CGO and polymeric pore formers. The scaffolds were heat-treated at either 1000 or 1300 °C, and were mechanically stable. In situ ultra-small X-ray scattering (USAXS) shows that the nanoparticles begin sintering around 900–1000 °C. Analysis by USAXS and scanning electron microscopy (SEM) revealed that the low temperature heat-treated scaffolds possessed higher porosity. Impregnated scaffolds were used to produce symmetrical cells, with the lower temperature heat-treated scaffolds showing improved gas diffusion, but poorer charge transfer. Using these scaffolds, lower temperature heat-treated cells of Ni–CGO/200 μm YSZ/CGO-LSCF performed better at 700 °C (and below) in hydrogen, and performed better at all temperatures using syngas, with power densities of up to 0.15 W cm -2 at 800 °C. This approach has the potential to allow the use of a wider range of materials and finer control over microstructure.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2019

Elucidating the nature of crystallization kinetics in Zr 46 Cu 46 Al 8 metallic glass through simultaneous WAXS/SAXS measurements

Simultaneous wide- and small-angle X-ray scattering (WAXS/SAXS) measurements were performed to investigate the low-temperature crystallization kinetics of Zr 46 Cu 46 Al 8 bulk metallic glass during isothermal annealing. Quantitative analysis of the WAXS intensity data indicated that the crystallization process was dominated by steady-state homogeneous nucleation and three-dimensional diffusion-controlled growth. This observation was corroborated by quantitative analysis of the SAXS data showing a saturation of the growth of nanoscale crystallites. Comparison of simultaneously measured WAXS and SAXS data showed evidence of composition fluctuation prior to crystallization, suggesting that the crystallization was facilitated by nanoscale phase separation which established regions of favorable compositions catalyzing crystallization. The temperature dependence of crystallization mechanisms and the resulting microstructures in metallic glass are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Nanosecond pulsed humid Ar plasma jet in air: shielding, discharge characteristics and atomic hydrogen production

Gas phase non-equilibrium plasmas jets containing water vapor are of growing interest for many applications. In this work, we report a detailed study of an atmospheric pressure nanosecond pulsed Ar + 0.26% H 2 O plasma jet. The plasma jet operates in an atmospheric pressure air surrounding but is shielded with a coaxial argon flow to limit the air diffusion into the jet effluent core. The jet impinges on a metal plate electrode and produces a stable plasma filament (transient spark) between the needle electrode in the jet and the metal plate. The stable plasma filament is characterized by spatially and time resolved electrical and optical diagnostics. This includes Rayleigh scattering, Stark broadening of the hydrogen Balmer lines and two-photon absorption laser induced fluorescence (TaLIF) to obtain the gas temperature, the electron density and the atomic hydrogen density respectively. Electron densities and atomic hydrogen densities up to $5 \times 10^{22}$ ${\rm m}^{-3}$ and $2 \times 10^{22}$ ${\rm m}^{-3}$ have been measured. This shows that atomic hydrogen is one of the main species in high density Ar-H 2 O plasmas. The gas temperature does not exceed 550 K in the core of the plasma. To enable in situ calibration of the H TaLIF at atmospheric pressure a previously published O density calibration scheme is extended to include a correction for the line profiles by including overlap integrals as required by H TaLIF. The line width of H TaLIF, due to collision broadening has the same trend as the neutral density obtained by Rayleigh scattering. This suggests the possibility to use this technique to in situ probe neutral gas densities.

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

Developing CdTe homojunctions applying high throughput deposition

In this project, we attempted to make CdTe homojunctions using single crystal p-type doped substrates and closed-space sublimation epitaxy (CSSE) n-type doped films. The project included modeling which determined the ideal thickness for the n-type layer as being <200 nm. Boules of high concentration indium-doped CdTe and CdSe 0.4 Te 0.6 (CST) were grown using modified vertical Bridgman (MVB) methods. Similarly, iodine-doped CdTe crystals were grown for the first time. Washington State University (WSU) stock phosphorus-doped CdTe was used as the p-type substrate layer. The crystals were characterized by Hall effect and time-resolved photoluminescence (TRPL, for electrical properties), photoluminescence microscopy (for uniformity), X-ray diffraction (for crystal structure), and glow discharge mass spectrometry (GDMS, for dopant and impurity concentration). CdTe:I crystals were also characterized by visible and infrared transmission measurements, and various Cd or Te heat treatments were performed to assess changes in optical and electrical properties. The grown n-type materials – CdTe:In, CdSe0.4Te0.6:In, and CdTe:I – were provided to the National Renewable Energy Laboratory (NREL) for growth of CSSE thick films for characterization by two photon TRPL (for carrier lifetime), electron back-scatter diffraction (EBSD, to assess epitaxy), and Hall effect. Several measurements of secondary ion mass spectroscopy (SIMS) were performed. Nearly 100% of the indium from the crystal was incorporated into the measured thick films, while only ~2-22% of the iodine from the crystal was incorporated. The net result of the diffusion issue is that homojunction devices created using CSSE have a buried homojunction, as indicated by the near infrared peak in the external quantum efficiency (EQE). Various parameterization of front and back contacts suggested that the poor device performance was primarily a result of this buried junction and not due to other effects. There may also be an issue with the CSSE film lifetime in addition to the dopant profile.

14 SOLAR ENERGY↗
DOE OSTI2021

Spatial correlations of entangled polymer dynamics

The spatial correlations of entangled polymer dynamics are examined by molecular dynamics simulations and neutron spin-echo spectroscopy. Due to the soft nature of topological constraints, the initial spatial decays of intermediate scattering functions of entangled chains are, to the first approximation, surprisingly similar to those of an unentangled system in the functional forms. However, entanglements reveal themselves as a long tail in the reciprocal-space correlations, implying a weak but persistent dynamic localization in real space. Furthermore, comparison with a number of existing theoretical models of entangled polymers suggests that they cannot fully describe the spatial correlations revealed by simulations and experiments. In particular, the strict one-dimensional diffusion idea of the original tube model is shown to be flawed. The dynamic spatial correlation analysis demonstrated in this work provides a useful tool for interrogating the dynamics of entangled polymers. Lastly, the failure of the investigated models to even qualitatively predict the spatial correlations of collective single-chain density fluctuations points to a possible critical role of incompressibility in polymer melt dynamics.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

Quantitative Determination of Metal Ion Adsorption on Cellulose Nanocrystals Surfaces

Nanocellulose is a bio-based material that holds significant potential in the field of water purification. Of particular interest is their potential use as a key sorbent material for the removal of metal ions from solution. However, the structure of metal ions adsorbed onto cellulose surfaces is not well understood. The focus of this work is to determine quantitatively the three-dimensional distribution of metal ions of different valencies surrounding negatively charged carboxylate functionalized cellulose nanocrystals (CNCs) using anomalous small-angle X-ray scattering (ASAXS). These distributions can affect the water and ionic permeability in these materials. The data show that increasing the carboxylate density on the surface of the CNCs from 740 to 1100 mmol/kg changed the nature of the structure of the adsorbed ions from a monolayer into a multilayer structure. The monolayer was modeled as a Stern layer around the CNC nanoparticles, whereas the multilayer structure was modeled as a diffuse layer on top of the Stern layer around the nanoparticles. Within the Stern layer, the maximum ion density increases from 1680 to 4350 mmol of Rb + /(kg of CNC) with the increase in the carboxylate density on the surface of the nanoparticles. Additionally, the data show that CNCs can leverage multiple mechanisms, such as electrostatic attraction and the chaotropic effect, to adsorb ions of different valencies. By understanding the spatial organization of the adsorbed metal ions, the design of cellulose-based sorbents can be further optimized to improve the uptake capacity and selectivity in separation applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Structure of a stratified CH 4 flame with H 2 addition

To explore the effect of H 2 addition (20 percent by volume) on stratified-premixed methane combustion in a turbulent flow, an experimental investigation on a new flame configuration of the Darmstadt stratified burner is conducted here. Major species concentrations and temperature are measured with high spatial resolution by 1D Raman-Rayleigh scattering. A conditioning on local equivalence ratio (range from φ = 0.45 to φ = 1.25) and local stratification is applied to the large dataset and allows to analyze the impact of H 2 addition on the flame structure. The local stratification level is determined as Δφ/ΔT at the location of maximum CO mass fraction for each instantaneous flame realization. Due to the H 2 addition, preferential diffusion of H 2 is different than in pure methane flames. In addition to diffusing out of the reaction zone where it is formed, particularly in rich conditions, H 2 also diffuses from the cold reactant mixture into the flame front. For rich conditions (φ = 1.05 to φ = 1.15) H 2 mass fractions are significantly elevated within the intermediate temperature range compared to fully-premixed laminar flame simulations. This elevation is attributed to preferential transport of H 2 into the rich flame front from adjacent even richer regions of the flow. Additionally, when the local stratification across the flame front is taken into account, it is revealed that the state-space relation of H 2 is not only a function of the local stoichiometry but also the local stratification level. In these flames H 2 is the only major species showing sensitivity of the state-space relation to an equivalence ratio gradient across the flame front.

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

Energetic Electron Precipitation Driven by Electromagnetic Ion Cyclotron Waves from ELFIN’s Low Altitude Perspective

We review comprehensive observations of electromagnetic ion cyclotron (EMIC) wave-driven energetic electron precipitation using data collected by the energetic electron detector on the Electron Losses and Fields InvestigatioN (ELFIN) mission, two polar-orbiting low-altitude spinning CubeSats, measuring 50-5000 keV electrons with good pitch-angle and energy resolution. EMIC wave-driven precipitation exhibits a distinct signature in energy-spectrograms of the precipitating-to-trapped flux ratio: peaks at >0.5 MeV which are abrupt (bursty) (lasting ~17 s, or ΔL ~ 0.56) with significant substructure (occasionally down to sub-second timescale). We attribute the bursty nature of the precipitation to the spatial extent and structuredness of the wave field at the equator. Multiple ELFIN passes over the same MLT sector allow us to study the spatial and temporal evolution of the EMIC wave - electron interaction region. Case studies employing conjugate ground-based or equatorial observations of the EMIC waves reveal that the energy of moderate and strong precipitation at ELFIN approximately agrees with theoretical expectations for cyclotron resonant interactions in a cold plasma. Using multiple years of ELFIN data uniformly distributed in local time, we assemble a statistical database of ~50 events of strong EMIC wave-driven precipitation. Most reside at L ~ 5 – 7 at dusk, while a smaller subset exists at L ~ 8 – 12 at post-midnight. The energies of the peak-precipitation ratio and of the half-peak precipitation ratio (our proxy for the minimum resonance energy) exhibit an L-shell dependence in good agreement with theoretical estimates based on prior statistical observations of EMIC wave power spectra. The precipitation ratio’s spectral shape for the most intense events has an exponential falloff away from the peak (i.e., on either side of ~ 1.45 MeV). It too agrees well with quasi-linear diffusion theory based on prior statistics of wave spectra. It should be noted though that this diffusive treatment likely includes effects from nonlinear resonant interactions (especially at high energies) and nonresonant effects from sharp wave packet edges (at low energies). Sub-MeV electron precipitation observed concurrently with strong EMIC wave-driven >1 MeV precipitation has a spectral shape that is consistent with efficient pitch-angle scattering down to ~ 200-300 keV by much less intense higher frequency EMIC waves at dusk (where such waves are most frequent). At ~100 keV, whistler-mode chorus may be implicated in concurrent precipitation. These results confirm the critical role of EMIC waves in driving relativistic electron losses. Nonlinear effects may abound and require further investigation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Understanding the ignition mechanism of high-pressure spray flames

A conceptual model for turbulent ignition in high-pressure spray flames is presented. The model is motivated by first-principles simulations and optical diagnostics applied to the Sandia n-dodecane experiment. The Lagrangian flamelet equations are combined with full LLNL kinetics (2755 species; 11,173 reactions) to resolve all time and length scales and chemical pathways of the ignition process at engine-relevant pressures and turbulence intensities unattainable using classic DNS. The first-principles value of the flamelet equations is established by a novel chemical explosive mode-diffusion time scale analysis of the fully-coupled chemical and turbulent time scales. Contrary to conventional wisdom, this analysis reveals that the high Damköhler number limit, a key requirement for the validity of the flamelet derivation from the reactive Navier–Stokes equations, applies during the entire ignition process. Corroborating Rayleigh-scattering and formaldehyde PLIF with simultaneous schlieren imaging of mixing and combustion are presented. Our combined analysis establishes a characteristic temporal evolution of the ignition process. First, a localized first-stage ignition event consistently occurs in highest temperature mixture regions. This initiates, owed to the intense scalar dissipation, a turbulent cool flame wave propagating from this ignition spot through the entire flow field. This wave significantly decreases the ignition delay of lower temperature mixture regions in comparison to their homogeneous reference. This explains the experimentally observed formaldehyde formation across the entire spray head prior to high-temperature ignition which consistently occurs first in a broad range of rich mixture regions. There, the combination of first-stage ignition delay, shortened by the cool flame wave, and the subsequent delay until second-stage ignition becomes minimal. A turbulent flame subsequently propagates rapidly through the entire mixture over time scales consistent with experimental observations. As a result, we demonstrate that the neglect of turbulence-chemistry-interactions fundamentally fails to capture the key features of this ignition process.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Temperature effects on the atomic structure and kinetics in single crystal electrochemistry

The influence of temperature on the atomic structure at the electrochemical interface has been studied using in-situ surface x-ray scattering (SXS) during the formation of metal monolayers on a Au(111) electrode. For the surface reconstruction of Au(111), higher temperatures increase the mobility of surface atoms in the unreconstructed phase which then determines the surface ordering during the formation of the reconstruction. For the underpotential deposition (UPD) systems, the surface diffusion of the depositing metal adatoms is significantly reduced at low temperatures which results in the frustration of ordered structures in the case of Cu UPD, occurring on a Br-modified surface, and in the formation of a disordered Ag monolayer during Ag UPD. The results indicate that temperature changes affect the mass transport and diffusion of metal adatoms on the electrode surface. This demonstrates the importance of including temperature as a variable in studying surface structure and reactions at the electrochemical interface.

Gründer, Yvonne↗
DOE OSTI2015

THE ACCRETION WIND MODEL OF FERMI BUBBLES. II. RADIATION

In a previous work, we have shown that the formation of Fermi bubbles can be due to the interaction between winds launched from the hot accretion flow in Sgr A* and the interstellar medium (ISM). In that work, we focus only on the morphology. In this paper we continue our study by calculating the gamma-ray radiation. Some cosmic-ray protons (CRp) and electrons (CRe) must be contained in the winds, which are likely formed by physical processes such as magnetic reconnection. We have performed MHD simulations to study the spatial distribution of CRp, considering the advection and diffusion of CRp in the presence of magnetic field. We find that a permeated zone is formed just outside of the contact discontinuity between winds and the ISM, where the collisions between CRp and thermal nuclei mainly occur. The decay of neutral pions generated in the collisions, combined with the inverse Compton scattering of background soft photons by the secondary leptons generated in the collisions and primary CRe, can well explain the observed gamma-ray spectral energy distribution. Other features such as the uniform surface brightness along the latitude and the boundary width of the bubbles are also explained. The advantage of this “accretion wind” model is that the adopted wind properties come from the detailed small-scale MHD numerical simulation of accretion flows and the value of mass accretion rate has independent observational evidences. The success of the model suggests that we may seriously consider the possibility that cavities and bubbles observed in other contexts such as galaxy clusters may be formed by winds rather than jets.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Shear-induced aggregation or disaggregation in edible oils: Models, computer simulation, and USAXS measurements

The effects of shear upon the aggregation of solid objects formed from solid triacylglycerols (TAGs) immersed in liquid TAG oils were modeled using Dissipative Particle Dynamics (DPD) and the predictions compared to experimental data using Ultra-Small Angle X-ray Scattering (USAXS). The solid components were represented by spheres interacting via attractive van der Waals forces and short range repulsive forces. A velocity was applied to the liquid particles nearest to the boundary, and Lees-Edwards boundary conditions were used to transmit this motion to non-boundary layers via dissipative interactions. The shear was created through the dissipative forces acting between liquid particles. Translational diffusion was simulated, and the Stokes-Einstein equation was used to relate DPD length and time scales to SI units for comparison with USAXS results. The SI values depended on how large the spherical particles were (250 nm vs. 25 nm). Aggregation was studied by (a) computing the Structure Function and (b) quantifying the number of pairs of solid spheres formed. Solid aggregation was found to be enhanced by low shear rates. As the shear rate was increased, a transition shear region was manifested in which aggregation was inhibited and shear banding was observed. Aggregation was inhibited, and eventually eliminated, by further increases in the shear rate. The magnitude of the transition region shear, $\dot{γ}_t$, depended on the size of the solid particles, which was confirmed experimentally.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Application of linear multifrequency-grey acceleration to preconditioned Krylov iterations for thermal radiation transport

The thermal radiative transfer (TRT) equations comprise a radiation equation coupled to the material internal energy equation. Linearization of these equations produces effective, thermally-redistributed scattering through absorption-reemission. In this paper, we investigate the effectiveness and efficiency of Linear-Multi-Frequency-Grey (LMFG) acceleration that has been reformulated for use as a preconditioner to Krylov iterative solution methods. We introduce two general frameworks, the scalar flux formulation (SFF) and the absorption rate formulation (ARF), and investigate their iterative properties in the absence and presence of true scattering. SFF has a group-dependent state size but may be formulated without inner iterations in the presence of scattering, while ARF has a group-independent state size but requires inner iterations when scattering is present. We compare and evaluate the computational cost and efficiency of LMFG applied to these two formulations using a direct solver for the preconditioners. Finally, this work is novel because the use of LMFG for the radiation transport equation, in conjunction with Krylov methods, involves special considerations not required for radiation diffusion.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Mixed Ionic Liquid Improves Electrolyte Dynamics in Supercapacitors

Well-tailored mixtures of distinct ionic liquids can act as optimal electrolytes that extend the operating electrochemical window and improve charge storage density in supercapacitors. Here, we explore two room-temperature ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimTFSI) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF 4 ). We study their electric double-layer behavior in the neat state and as binary mixtures on the external surfaces of onion-like carbon electrodes using quasielastic neutron scattering (QENS) and classical density functional theory techniques. Computational results reveal that a mixture with 4:1 EmimTFSI/EmimBF 4 volume ratio displaces the larger [TFSI – ] anions with smaller [BF 4 – ] ions, leading to an excess adsorption of [Emim + ] cations near the electrode surface. These findings are corroborated by the manifestation of nonuniform ion diffusivity change, complementing the description of structural modifications with changing composition, from QENS measurements. In conclusion, molecular-level understanding of ion packing near electrodes provides insight for design of ionic liquid formulations that enhance the performance of electrochemical energy storage devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Coherent neutron scattering and collective dynamics on mesoscale

By combining, and modestly extending, a variety of theoretical concepts for the dynamics of liquidsin the supercooled regime, we formulate a simple analytic model for the temperature and wavevectordependent collective density fluctuation relaxation time that is measurable using coherent dynamicneutron scattering. Comparison with experiments on the ionic glass-forming liquid Ca–K–NO3 in thelightly supercooled regime suggests the model captures the key physics in both the local cage andmesoscopic regimes, including the unusual wavevector dependence of the collective structural relaxationtime. The model is consistent with the idea that the decoupling between diffusion and viscosityis reflected in a different temperature dependence of the collective relaxation time at intermediatewavevectors and near the main (cage) peak of the static structure factor. More generally, our analysisprovides support for the ideas that decoupling information and growing dynamic length scales can beat least qualitatively deduced by analyzing the collective relaxation time as a function of temperatureand wavevector, and that there is a strong link between dynamic heterogeneity phenomena at thesingle and many particle level. Though very simple, the model can be applied to other systems, suchas molecular liquids.

Novikov, Vladimir↗