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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
DOE OSTI2014

TOF SIMS analysis and generation of white photoluminescence from strontium silicate codoped with europium and terbium

White light emitting terbium (Tb{sup 3+}) and europium (Eu{sup 3+}) codoped strontium silicate (Sr{sub 2}SiO{sub 4}) phosphors were prepared by a solid state reaction process. The structure, particle morphology, chemical composition, ion distribution, photoluminescence (PL), and decay characteristics of the phosphors were analyzed by x-ray diffraction (XRD), scanning electron microscopy (SEM), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and PL spectroscopy, respectively. The XRD data showed that our Sr{sub 2}SiO{sub 4} composed of two phases, namely, β-Sr{sub 2}SiO{sub 4} and α′-Sr{sub 2}SiO{sub 4}, and the α′-Sr{sub 2}SiO{sub 4} phase was more prominent than the β-Sr{sub 2}SiO{sub 4} phase. The SEM micrographs showed that the particles were agglomerated together and they did not have definite shapes. All ions (i.e., negative and positive) present in our materials were identified by TOF-SIMS. In addition, the chemical imaging performed with the TOF-SIMS demonstrated how the individual ions including the dopants (Eu{sup 3+} and Tb{sup 3+}) were distributed in the host lattice. White photoluminescence was observed when the Sr{sub 2}SiO{sub 4}:Tb{sup 3+}, Eu{sup 3+} phosphor was excited at 239 nm using a monochromatized xenon lamp as the excitation source. The phosphor exhibited fast decay lifetimes implying that it is not a good candidate for long afterglow applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Picosecond volume expansion drives a later-time insulator–metal transition in a nano-textured Mott insulator

There is significant technological interest in developing ever faster switching between different electronic and magnetic states of matter. Manipulating properties at terahertz rates requires accessing the intrinsic timescales of both electrons and associated phonons, which is possible with short-pulse photoexcitation. However, in many Mott insulators, the electronic transition is accompanied by the nucleation and growth of percolating domains of the changed lattice structure, leading to empirical timescales dominated by slowly coarsening dynamics. Here, in this study, we use time-resolved X-ray diffraction and reflectivity measurements to show that the photoinduced insulator-to-metal transition in an epitaxially strained Mott insulating thin film occurs without observable domain formation and coarsening effects, allowing the study of the intrinsic electronic and lattice dynamics. Above a fluence threshold, the initial electronic excitation drives a fast lattice rearrangement, which is followed by a slower electronic evolution into a metastable nonequilibrium state. Microscopic model calculations based on time-dependent dynamical mean-field theory and semiclassical lattice dynamics explain the threshold behaviour and elucidate the delayed onset of the electronic phase transition. This work highlights the importance of combined electronic and structural studies in unravelling the physics of dynamic transitions and the timescales of photoinduced processes. During a photoinduced phase transition, electronic rearrangements are usually faster than lattice ones. Time-resolved measurements now show that the insulator-to-metal transition in a thin-film Mott insulator is preceded by lattice reconfiguration.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Bidirectional Ultrafast Control of Charge Density Waves via Phase Competition

The intricate competition between coexisting charge density waves (CDWs) can lead to rich phenomena, offering unique opportunities for phase manipulation through electromagnetic stimuli. Here, leveraging time-resolved x-ray diffraction, we demonstrate ultrafast control of a CDW in EuTe 4 upon optical excitation. At low excitation intensities, the amplitude of one of the coexisting CDW orders increases at the expense of the competing CDW, whereas at high intensities, it exhibits a nonmonotonic temporal evolution characterized by both enhancement and reduction. This transient bidirectional controllability, tunable by adjusting photoexcitation intensity, arises from the interplay between optical quenching and phase-competition-induced enhancement. Our findings, supported by phenomenological time-dependent Ginzburg-Landau theory simulations, not only clarify the relationship between the two CDWs in EuTe 4 , but also highlight the versatility of optical control over order parameters enabled by phase competition.

charge density waves↗
DOE OSTI2019

Disentangling transient charge order from structural dynamics contributions during coherent atomic motion studied by ultrafast resonant x-ray diffraction [Disentangling charge and structural contributions during coherent atomic motions studied by ultrafast resonant x-ray diffraction]

We report on the ultrafast dynamics of charge order and structural response during the photoinduced suppression of charge and orbital order in a mixed-valence manganite. Employing femtosecond time-resolved resonant x-ray diffraction below and at the Mn K absorption edge, we present a method to disentangle the transient charge order and structural dynamics in thin films of Pr 0.5 Ca 0.5 MnO 3 . Based on the static resonant scattering spectra, we extract the dispersion correction of charge-ordered Mn 3+ and Mn 4+ ions, allowing us to separate the transient contributions of purely charge order from structural contributions to the scattering amplitude after optical excitation. Furthermore, our finding of a coherent structural mode at around 2.3THz, which primarily modulates the lattice but does not strongly affect the charge order, supports the picture of the charge order being the driving force of the combined charge, orbital, and structural transition.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Geographic variations in lowermost mantle structure from the ray parameters and decay constants of core-diffracted waves: CMBR Structure From P diff and S diff

We propose an array-based approach for constraining seismic velocity structure in the lowermost mantle by measuring the frequency dependence of the ray parameter p and decay constant γ for core-diffracted waves (P diff and SH diff ). The approach uses an iterative multichannel cross-correlation algorithm that solves for relative arrival times and amplitudes of core-diffracted waveforms from multiple peaks in normalized correlograms from pairs of coazimuthal stations. The approach is applied to 60 m b ≥ 5.8 earthquakes for P diff and 36 for SH diff during 2001–2002, with nearly 50,000 unique profile station pairs with epicentral-distance differences of Δd ≥ 10° and azimuth differences of Δφ ≤ 10°, sampling a significant portion of the lowermost mantle. Cap-averaging the resulting ray parameter estimates produces geographic variations that are largely consistent with the distribution of lowermost mantle large low-velocity provinces and seismically fast slab-accumulation regions seen in seismic tomography models, whose locations also greatly influence the geographic distribution of heat flow out of the core. Geographic variations in station-pair diffracted-wave decay constants differ from those of the ray parameters, suggesting that variations in the decay of core-diffracted waves are more linked to lowermost mantle seismic velocity gradients than absolute values of seismic velocity. The ray parameters and decay constants of core-diffracted waves are strongly frequency-dependent, and these frequency variations also vary significantly with geographic location. Combining lateral and vertical seismic-velocity variations with mineral-physics data on elasticity and conductivity of lowermost mantle species can provide constraints on D" composition and CMB heat flux.

58 GEOSCIENCES↗
DOE OSTI2017

Structural imaging of nanoscale phonon transport in ferroelectrics excited by metamaterial-enhanced terahertz fields

Nanoscale phonon transport is a key process that governs thermal conduction in a wide range of materials and devices. Creating controlled phonon populations by resonant excitation at terahertz (THz) frequencies can drastically change the characteristics of nanoscale thermal transport and allow a direct real-space characterization of phonon mean-free paths. Using metamaterial-enhanced terahertz excitation, we tailored a phononic excitation by selectively populating low-frequency phonons within a nanoscale volume in a ferroelectric BaTiO 3 thin film. Real-space time-resolved x-ray diffraction microscopy following THz excitation reveals ballistic phonon transport over a distance of hundreds of nm, two orders of magnitude longer than the averaged phonon mean-free path in BaTiO 3 . On longer length scales, diffusive phonon transport dominates the recovery of the transient strain response, largely due to heat conduction into the substrate. The measured real-space phonon transport can be directly compared with the phonon mean-free path as predicted by molecular dynamics modeling. In conclusion, this time-resolved real-space visualization of THz-matter interactions opens up opportunities to engineer and image nanoscale transient structural states with new functionalities.

36 MATERIALS SCIENCE↗
DOE OSTI2013

X-ray and neutron diffraction study of the defect crystal structure of the as-grown nonstoichiometric phase Y{sub 0.715}Ca{sub 0.285}F{sub 2.715}

This work begins a series of papers aimed at studying the defect structure of nonstoichiometric phases R{sub 1-y}Ca{sub y}F{sub 3-y} with a tysonite-type (LaF{sub 3}) structure. In the single-crystal structure of Y{sub 0.715}Ca{sub 0.285}F{sub 2.715} with a tysonite-type small unit cell (sp. gr. P6{sub 3}/mmc, a = 3.9095(2) A, c = 6.9829(2) A; Z = 2; R{sub w} = 2.16%), the displacements of Y{sup 3+} cations and F{sup 2-} anions from 6{sub 3} symmetry axes were observed for the first time. The X-ray diffraction pattern shows weak satellites insufficient for structural calculations. The LaF{sub 3} structure type is stabilized up and down on the temperature scale due to anion vacancies and the symmetrizing effect of Ca{sup 2+} cations lying on 6{sub 3} symmetry axes. At 120 Degree-Sign C the fluoride-ion conductivity in the nonstoichiometric phase Y{sub 0.715}Ca{sub 0.285}F{sub 2.715} is five orders of magnitude higher than that in the stoichiometric phase {beta}-YF{sub 3}. The transition to a superionic state is caused by a deviation from stoichiometry and is not associated with reconstructive phase transformation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Influence of local symmetry on lattice dynamics coupled to topological surface states

Herein we investigate coupled electron-lattice dynamics in the topological insulator Bi 2 Te 3 with time-resolved photoemission and time-resolved x-ray diffraction. It is well established that coherent phonons can be launched by optical excitation, but selection rules generally restrict these modes to zone-center wave vectors and Raman-active branches. We find that the topological surface state couples to additional modes, including a continuum of surface-projected bulk modes from both Raman and infrared branches, with possible contributions from surface-localized modes when they exist. Our calculations show that this surface vibrational spectrum occurs naturally as a consequence of the translational and inversion symmetries broken at the surface, without requiring the splitting-off of surface-localized phonon modes. The generality of this result suggests that coherent phonon spectra are useful by providing unique fingerprints for identifying surface states in more controversial materials. These effects may also expand the phase space for tailoring surface state wave functions via ultrafast optical excitation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Hydrogenation of CO 2 to Methanol on a Au δ+ –In 2 O 3–x Catalyst

CO 2 hydrogenation to methanol has attracted increasing attention with the development of renewable hydrogen. A big challenge is to identify catalysts able to achieve high conversion and selectivity. In this work, we report an In 2 O 3 -supported Au catalyst that exhibits excellent performance for hydrogenation of CO 2 selectively to methanol. In situ characterizations using time-resolved X-ray diffraction, ambient-pressure X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy confirm that a strong metal–support interaction leads to a reactive Au δ+ –In 2 O 3– x interface for activation and hydrogenation of CO 2 to methanol. An effective gold-indium oxide bonding favors the dispersion of the noble metal and prevents its sintering under reaction conditions. The methanol selectivity reaches 100% at temperatures below 225 °C and is more than 70% at 275 °C over the Au δ+ –In 2 O 3– x catalyst. It is even 67.8% with a space time yield of methanol of 0.47 g MeOH /(h·g cat ) at 300 °C, 5 MPa, and 21,000 cm 3 h –1 g cat –1 . The results obtained here represent the highest selectivity and activity ever reported for CO 2 hydrogenation over supported gold catalysts. Our study shows that the strong Au/In 2 O 3 interaction and the intrinsic chemical activity of In 2 O 3 can be used to significantly improve the catalytic performance of Au catalysts, providing promising routes for the rational design and application of Au catalysts beyond CO 2 hydrogenation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Magnetic Features of Boron Doping on Hf{sub 2}Co{sub 11} Thin Films

The effect of boron (B) doping on the structural and magnetic properties is studied systematically in the Hf{sub 2}Co{sub 11} thin films. The modified crystal microstructure and annealing process enhanced the coercive field up to 1754 Oe. In order to obtain superior magnetic properties, the optimum doping ratios should be properly controlled attentively. To identify the structure, B-doped Hf{sub 2}Co{sub 11} thin films were investigated by temperature-dependent X-ray diffraction analysis for the first time. The hard magnetic phase of Hf{sub 2}Co{sub 11}B{sub x} thin films was formed to orthorhombic phase with some Co impurities and had a crystallization temperature around 823 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Visualizing the chemistry and structure dynamics in lithium-ion batteries by in-situ neutron diffraction

We report an in-situ neutron diffraction study of a large format pouch battery cell. The succession of Li-Graphite intercalation phases was fully captured under an 1C charge-discharge condition (i.e., charge to full capacity in 1 hour). However, the lithiation and dilithiation pathways are distinctively different and, unlike in slowing charging experiments with which the Li-Graphite phase diagram was established, no LiC24 phase was found during charge at 1C rate. Approximately 75 mol. % of the graphite converts to LiC6 at full charge, and a lattice dilation as large as 4% was observed during a charge-discharge cycle. Our work demonstrates the potential of in-situ, time and spatially resolved neutron diffraction study of the dynamic chemical and structural changes in “real-world” batteries under realistic cycling conditions, which should provide microscopic insights on degradation and the important role of diffusion kinetics in energy storage materials.

25 ENERGY STORAGE↗
DOE OSTI2018

Microstructure and Tensile Properties of the Fe-32%Ni-4%Co Alloy During Cryorolling and Subsequent Annealing

The Fe-32%Ni-4%Co alloy was cryorolled to produce thickness reductions of 10, 40, 60 and 90%; then, the strips with a final thickness of 0.52 mm were annealed at 893, 923 and 943 K for different soaking times. The x-ray diffraction measurements indicated that both thermally induced α′-martensite (α′-M) and deformation-induced α′-M were present in the tested alloy. The thermally induced α′-M was mainly lath-shaped, while the morphology of the deformation-induced α′-M gradually changed from lath-shaped to irregular-shaped according to transmission electron microscopy observations. After the 90% reduction due to cryorolling, the total volume fraction of α′-M reached 75.6%, and the average ultimate tensile strength (UTS{sub av.}) was significantly improved from 430.3 to 1306.8 MPa. Upon subsequent annealing at 923 K for 5 min, the tested alloy mainly consisted of a large number of fine grains and a few coarse grains (CGs). In addition, several deformed structures were also observed. As the annealing time increased to 10 and 30 min, the fraction of CGs increased, and a bimodal grain size distribution was obtained, while the deformed structures were still present. The temperature of the reverse phase transformation of α′-M → γ was lower than the recrystallization temperature, which may be the main reason for the bimodal grain distribution. With this bimodal grain structure, the alloy exhibited a combination tensile strength of 543.3 MPa and an average elongation of 34.1%.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Two-step polarization reversal in biased ferroelectrics

Polarization reversal in polycrystalline ferroelectrics is shown to occur via two distinct and sequential domain reorientation steps. This reorientation sequence, which cannot be readily discriminated in the overall sample polarization, is made apparent using time-resolved high-energy x-ray diffraction. Upon application of electric fields opposite to the initial poling direction, two unique and significantly different time constants are observed. The first (faster time constant) is shown to be derived by the release of a residual stress due to initial electrical biasing and the second (slower time constant) due to the redevelopment of residual stress during further domain wall motion. A modified domain reorientation model is given that accurately describes the domain volume fraction evolution during the reversal process.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Time and frequency dependent mechanical properties of LaCoO 3 -based perovskites: Neutron diffraction and domain mobility

The study of domain wall movement and texture formation in ferroelastic LaCoO 3 perovskite under constant applied compressive stress has been performed using in situ neutron diffraction. Additionally, it was established that under constant applied compressive stress the domain walls show mobility that may lead both to the shrinkage (creep strain) and to the expansion (negative creep strain) of LaCoO 3 perovskite. The domain wall movement and texture formation can be explained by the availability, mobility, and interaction of twins, stacking faults, antiphase boundaries, dislocations, and point defects, such as oxygen vacancies and their complexes as well as impurity atoms. Lastly, the equation of motion was used to describe the possible mechanisms of domain wall movement under applied stress, and it was determined that the available solutions of this equation allow both for the shrinkage (creep strain) and for the expansion (negative creep strain) of LaCoO 3 perovskite to occur.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Development of a Neural Network Simulator for Studying the Constitutive Behavior of Structural Composite Materials

This paper introduces a recent development and application of a noncommercial artificial neural network (ANN) simulator with graphical user interface (GUI) to assist in rapid data modeling and analysis in the engineering diffraction field. The real-time network training/simulation monitoring tool has been customized for the study of constitutive behavior of engineering materials, and it has improved data mining and forecasting capabilities of neural networks. This software has been used to train and simulate the finite element modeling (FEM) data for a fiber composite system, both forward and inverse. The forward neural network simulation precisely reduplicates FEM results several orders of magnitude faster than the slow original FEM. The inverse simulation is more challenging; yet, material parameters can be meaningfully determined with the aid of parameter sensitivity information. The simulator GUI also reveals that output node size for materials parameter and input normalization method for strain data are critical train conditions in inverse network. The successful use of ANN modeling and simulator GUI has been validated through engineering neutron diffraction experimental data by determining constitutive laws of the real fiber composite materials via a mathematically rigorous and physically meaningful parameter search process, once the networks are successfully trained from the FEM database.

Na, Hyuntae↗
DOE OSTI2014

The Molecular Architecture for the Intermediate Filaments of Hard [alpha]-Keratin Based on the Superlattice Data Obtained from a Study ofMammals Using Synchrotron Fibre Diffraction

High- and low-angle X-ray diffraction studies of hard {alpha}-keratin have been studied, and various models have been proposed over the last 70 years. Most of these studies have been confined to one or two forms of alpha keratin. This high- and low-angle synchrotron fibre diffraction study extends the study to cover all available data for all known forms of hard {alpha}-keratin including hairs, fingernails, hooves, horn, and quills from mammals, marsupials, and a monotreme, and it confirms that the model proposed is universally acceptable for all mammals. A complete Bragg analysis of the meridional diffraction patterns, including multiple-time exposures to verify any weak reflections, verified the existence of a superlattice consisting of two infinite lattices and three finite lattices. An analysis of the equatorial patterns establishes the radii of the oligomeric levels of dimers, tetramers, and intermediate filaments (IFs) together with the centre to centre distance for the IFs, thus confirming the proposed helices within helices molecular architecture for hard {alpha}-keratin. The results verify that the structure proposed by Feughelman and James meets the criteria for a valid {alpha}-keratin structure.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2018

In Situ Elucidation of the Active State of Co–CeO x Catalysts in the Dry Reforming of Methane: The Important Role of the Reducible Oxide Support and Interactions with Cobalt

The activation of methane and its dry reforming with CO 2 was systematically studied over a series (2–30 wt %) of Co (~5 nm in size) loaded CeO 2 catalysts, with an effort to elucidate the interplay between Co and CeO 2 during the catalytic process using in situ methods. The results of in situ time-resolved X-ray diffraction (TR-XRD) show a strong interaction of methane with the CoOx–CeO 2 systems at temperatures between 200 and 350 °C. The hydrogen produced by the dissociation of C–H bonds in methane leads to a full reduction of Co oxide, Co 3 O 4 → CoO → Co, and a partial reduction of ceria with the formation of some Ce 3+ . Upon the addition of CO 2 , a catalytic cycle for dry reforming of methane (DRM) was achieved on the CoOx–CeO 2 powder catalysts at temperatures below 500 °C. A 10 wt % Co–CeO 2 catalyst was found to possess the best catalytic activity among various cobalt loading catalysts, and it exhibits a desirable stability for the DRM with a minimal effect of carbon accumulation. The phase transitions and the nature of active components in the catalyst were investigated under reaction conditions by in situ time-resolved XRD and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). These studies showed dynamic evolutions in the chemical composition of the catalysts under reaction conditions. CO 2 attenuated the reducing effects of methane. Under optimum CO- and H 2 -producing conditions, both XRD and AP-XPS indicated that the active phase involved a majority of metallic Co with a small amount of CoO, both supported on a partially reduced ceria (Ce 3+ /Ce 4+ ). Finally, we identified the importance of dispersing Co, anchoring it onto the ceria surface sites, and then utilizing the redox properties of CeO 2 for activating and then oxidatively converting methane while inhibiting coke formation. Furthermore, a synergistic effect between cobalt and ceria and likely the interfacial sitee are essential to successfully close the catalytic cycle.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗