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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 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Room-temperature mobility above 2200 cm{sup 2}/V·s of two-dimensional electron gas in a sharp-interface AlGaN/GaN heterostructure

A high mobility of 2250 cm{sup 2}/V·s of a two-dimensional electron gas (2DEG) in a metalorganic chemical vapor deposition-grown AlGaN/GaN heterostructure was demonstrated. The mobility enhancement was a result of better electron confinement due to a sharp AlGaN/GaN interface, as confirmed by scanning transmission electron microscopy analysis, not owing to the formation of a traditional thin AlN exclusion layer. Moreover, we found that the electron mobility in the sharp-interface heterostructures can sustain above 2000 cm{sup 2}/V·s for a wide range of 2DEG densities. Finally, it is promising that the sharp-interface AlGaN/GaN heterostructure would enable low contact resistance fabrication, less impurity-related scattering, and trapping than the AlGaN/AlN/GaN heterostructure, as the high-impurity-contained AlN is removed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Control of wetting and uniformity via ZrSi x formation in ceramic-to-metal joints fabricated using Ag-Zr brazes

The deposition of a 2.0 µm SiO 2 film on the alumina surface in Kovar TM /94% alumina joints enables the formation of a silicide reaction layer on the alumina during brazing with 97Ag2Zr1Cu. Additionally, the average and standard deviation of joint thickness decrease from 50 to 15 and 29 to 4 µm, respectively compared to joints without added SiO 2 . Finally, the average failure stress of these braze joints was 45 MPa, while that of similar joints without added SiO 2 was 90 MPa. Sessile drop experiments of 98Ag2Zr on SiO 2 and 99.6% Al 2 O 3 substrates show that the braze wets and spreads to 3x its original area on SiO 2 with a wetting angle near 0°, but remains the same area on 99.6% Al 2 O 3 with a wetting angle of 106.6°. Focused-ion-beam scanning electron microscopy analysis of a cross-section of the 98Ag2Zr sessile drop on the SiO 2 substrate has shown that Zr reacts with SiO 2 to form Zr oxide and silicide layers. Scanning transmission electron microscopy diffraction and energy dispersive X-ray spectroscopy analysis indicate this silicide layer contains tetragonal Zr 5 Si 4 . In conclusion, analysis shows the silicide layer enhances wetting and joint uniformity while unreacted SiO 2 embrittles the joint and degrades strength.

alumina↗
DOE OSTI2015

Surface determination through atomically resolved secondary-electron imaging

We report that unique determination of the atomic structure of technologically relevant surfaces is often limited by both a need for homogeneous crystals and ambiguity of registration between the surface and bulk. Atomically resolved secondary-electron imaging is extremely sensitive to this registration and is compatible with faceted nanomaterials, but has not been previously utilized for surface structure determination. Here we show a detailed experimental atomic-resolution secondary-electron microscopy analysis of the c(6 x 2) reconstruction on strontium titanate (001) coupled with careful simulation of secondary-electron images, density functional theory calculations and surface monolayer-sensitive aberration-corrected plan-view high-resolution transmission electron microscopy. Our work reveals several unexpected findings, including an amended registry of the surface on the bulk and strontium atoms with unusual seven-fold coordination within a typically high surface coverage of square pyramidal TiO 5 units. Lastly, dielectric screening is found to play a critical role in attenuating secondary-electron generation processes from valence orbitals.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

QTAIM studies of [Li(DMSO){sub n}]{sup +} and [Al(DMSO){sub n}]{sup 3+}

Geometry optimization of [Li(DMSO){sub n}]{sup +} and [Al(DMSO){sub n}]{sup 3+} complexes with n = 1 - 6 in DMSO (dimethyl sulfoxide, (CH{sub 3}){sub 2}SO) solutions is performed using DFT treatment with B3LYP hybrid functional and cc-pVDZ basis sets. Solvent effects are approximated within Integral Equation Formalism Polarisable Continuum Model. Electron structure of individual model systems is investigated in terms of Quantum Theory of Atoms-in-Molecule topological analysis of electron density. Metal-DMSO bonding through oxygen atom is preferred. The most probable coordination numbers n = 4 for Li{sup +} and n = 6 for Al{sup 3+} complexes with DMSO are concluded.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Observation of Seeded Mn K β Stimulated X-Ray Emission Using Two-Color X-Ray Free-Electron Laser Pulses

Kβ x-ray emission spectroscopy is a powerful probe for electronic structure analysis of 3d transition metal systems and their ultrafast dynamics. Selectively enhancing specific spectral regions would increase this sensitivity and provide fundamentally new insights. Recently we reported the observation and analysis of Kα amplified spontaneous x-ray emission from Mn solutions using an x-ray free-electron laser to create the 1s core-hole population inversion [Kroll et al., Phys. Rev. Lett. 120, 133203 (2018)]. To apply this new approach to the chemically more sensitive but much weaker Kβ x-ray emission lines requires a mechanism to outcompete the dominant amplification of the Kα emission. Here we report the observation of seeded amplified Kβ x-ray emission from a NaMnO 4 solution using two colors of x-ray free-electron laser pulses, one to create the 1s core-hole population inversion and the other to seed the amplified Kβ emission. Comparing the observed seeded amplified Kβ emission signal with that from conventional Kβ emission into the same solid angle, we obtain a signal enhancement of more than 10 5 . Finally, our findings are the first important step of enhancing and controlling the emission of selected final states of the Kβ spectrum with applications in chemical and materials science.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Structural and Electronic Effect Driven Distortions in Visible Light Absorbing Polar Materials A Ta 2 V 2 O 11 ( A = Sr, Pb)

Complex vanadates of tantalum(V), such as ATa 2 V 2 O 11 (A = Sr, Pb), are rare and underrated materials, which have potential application domains that could be substantially expanded, mitigating the existing controversy on their atomic and electronic organization. Herein, we present a thorough structural examination combining synchrotron powder X-ray diffraction-aided distortion mode analysis with computational methods to study hettotypes of SrTa 2 V 2 O 11 (STVO) and PbTa 2 V 2 O 11 (PTVO). Being distinct from the perovskite family due to the presence of [VO 4 ] groups, both compounds are polar dielectric materials with certain similarities to SBT and PBT Aurivillius phases. Applying the model of anions of metallic matrices to the analysis of electron localization functions calculated on top of as-established equilibrium structures helps retrace the effects in the Sr and Pb surroundings on the respective crystal packings of STVO and PTVO.

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

Towards Mitigating Electron Beam-Induced Damage in MOFs Using Low Dose Electron Microscopy for In-Situ Room-Temperature Measurements

Metal-Organic Frameworks (MOFs) have emerged as a versatile class of materials with applications in gas storage and separation, catalysis, and drug delivery [1]. Understanding their damage mechanisms in under various environmental conditions is crucial for optimizing performance and stability [2]. This study employs a combination of Length Structural Coherence (LSC) analysis in electron scattering and Electron Energy Loss Spectroscopy (EELS) to elucidate the damage mechanisms in two representative MOFs. MIL101(Fe), and MIL101(Cr). Here, the primary objective is to reveal structural and chemical changes that occur under electron beam irradiation in Scanning Transmission Electron Microscopy (STEM), both in the pristine state and during in-situ CO and CO 2 adsorption. By combining LSC in Radial Distribution Function (RDF) analysis with EELS, comprehensive insights into the nanoscale alterations are obtained and correlated to the electron dose rate.

dos Santos, Gabriel T. [Northwestern University, E↗
DOE OSTI2021

Statistical analysis of non-Maxwellian electron distribution functions measured with angularly resolved Thomson scattering

Angularly resolved Thomson scattering is a novel extension of Thomson scattering, enabling the measurement of the electron velocity distribution function over many orders of magnitude. Here, details of the theoretical basis of the technique and the instrument designed for this measurement are described. Angularly resolved Thomson-scattering data from several experiments are shown with descriptions of the corresponding distribution functions. A reduced model describing the distribution function is given and used to perform a Monte Carlo analysis of the uncertainty in the measurements. The electron density and temperature were determined to a precision of 12% and 21%, respectively, on average, while all other parameters defining the distribution function were generally determined to better than 20%. It was found that these uncertainties were primarily due to limited signal to noise and instrumental effects. Furthermore, measurements with this level of precision were sufficient to distinguish between Maxwellian and non-Maxwellian distribution functions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Sol-gel preparation and characterization of Er3+ doped TiO2 luminescent nanoparticles

Highlights: • Sol-gel derived Er{sup 3+} doped TiO{sub 2} nanoparticles for potential use in solar cells and bio imaging. • Influence of Er{sup 3+} doping into TiO{sub 2} with various Er{sup 3+} concentration on structural, optical, and luminescence properties. • Down-and up-conversion luminescence behaviour of Er{sup 3+} doped titanium dioxide. - Abstract: The present paper reports on down-and up-conversion luminescence behaviour of sol-gel derived erbium doped titanium dioxide with anatase structure. Through combined structural, optical and electron microscope analysis, effective and influence of Er{sup 3+} doping into TiO{sub 2} lattice has been demonstrated using X-Ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and optical reflectance spectra. XRD results showed only anatase diffraction peaks of TiO{sub 2}, indicating the formation of a pure anatase phase even after Er{sup 3+} incorporation into TiO{sub 2} lattice. Selected Area Electron Diffraction (SAED) confirmed that the synthesized TiO{sub 2} nanoparticles are polycrystalline in nature which correlated well with XRD findings. Upon excitation at 320 nm, two down-conversion contributions at 378 nm and 435 nm attributed to indirect band gap and defect-related emissions, respectively were observed from both pure and Er{sup 3+} doped TiO{sub 2} nanoparticles. On the other hand, strong green up-conversion emission centred at 544 nm ascribed to {sup 4}S{sub 3/2 }→ {sup 4}I{sub 15/2} transition of Er{sup 3+} was observed under 980 nm laser excitation for all Er{sup 3+} doped TiO{sub 2} samples. This result analysis brings insight on understanding of structural, optical and luminescence properties of Er{sup 3+} doped TiO{sub 2} nanoparticles for use in solar cells and bio imaging devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Demonstrating 1 nm-oxide-equivalent-thickness HfO{sub 2}/InSb structure with unpinning Fermi level and low gate leakage current density

In this work, the band alignment, interface, and electrical characteristics of HfO{sub 2}/InSb metal-oxide-semiconductor structure have been investigated. By using x-ray photoelectron spectroscopy analysis, the conduction band offset of 1.78 ± 0.1 eV and valence band offset of 3.35 ± 0.1 eV have been extracted. The transmission electron microscopy analysis has shown that HfO{sub 2} layer would be a good diffusion barrier for InSb. As a result, 1 nm equivalent-oxide-thickness in the 4 nm HfO{sub 2}/InSb structure has been demonstrated with unpinning Fermi level and low leakage current of 10{sup −4} A/cm{sup −2}. The D{sub it} value of smaller than 10{sup 12} eV{sup −1}cm{sup −2} has been obtained using conduction method.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Pt 8 cluster on alumina under a pressure of hydrogen: Support-dependent reconstruction from first-principles global optimization

Alumina supported Pt nano clusters under hydrogen environment play a crucial role for many heterogeneous catalysis applications. As such, we conducted grand canonical genetic algorithm (GCGA) simulations for supported Pt 8 clusters in hydrogen gas environment to study the intra-cluster, cluster-support, and cluster-adsorbate interactions. Two alumina surfaces, α-Al 2 O 3 (0001) and $γ$-Al 2 O 3 (100), and two conditions T=600°C, p H2 = 0.1bar and T=25°C, p H2 = 1.0 bar were considered corresponding to low and high hydrogen chemical potential $μ$ H , respectively. The low free energy ensemble of Pt 8 is decorated by a medium (2–12 H), resp. high (20–30 H), number of hydrogen atoms under equilibrium at low $μ$ H , respectively, high $μ$ H , and undergoes different morphology transformations on the two surfaces. On α-Al2O3(0001), Pt8 is mostly 3D but very fluxional in structure at low μH and converts to open one-layer 2D structures with minimal fluxionality at high μH, whereas on $γ$-Al 2 O 3 (100), the exact opposite occurs, and Pt 8 clusters present one-layer 2D shapes at low $μ$ H and switch into compact 3D shapes under high $μ$ H , during which the Pt 8 cluster preserves moderate fluxionality. Further analysis reveals similar Pt–Pt bond length increase when switching from low $μ$ H to high $μ$ H on both surfaces, though morphology transformations are different. Electronic structure analysis shows the existence of bonding interactions between Pt and Lewis acidic Al 3+ sites along with the Pt – O interaction, which implies the necessity to include Al neighbors to discuss the electronic structure of small Pt clusters.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

The mechanism of chemisorption of hydrogen atom on graphene: Insights from the reaction force and reaction electronic flux

At the PBE-D3/cc-pVDZ level of theory, the hydrogen chemisorption on graphene was analyzed using the reaction force and reaction electronic flux (REF) theories in combination with electron population analysis. It was found that chemisorption energy barrier is mainly dominated by structural work (∼73%) associated to the substrate reconstruction whereas the electronic work is the greatest contribution of the reverse energy barrier (∼67%) in the desorption process. Moreover, REF shows that hydrogen chemisorption is driven by charge transfer processes through four electronic events taking place as H approaches the adsorbent surface: (a) intramolecular charge transfer in the adsorbent surface; (b) surface reconstruction; (c) substrate magnetization and adsorbent carbon atom develops a sp{sup 3} hybridization to form the σC-H bond; and (d) spontaneous intermolecular charge transfer to reach the final chemisorbed state.

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

Axially Symmetric Energy Analyzer Based on the Electrostatic Decapole-Cylindrical Field

A new electron-optical scheme of an axially symmetric mirror energy analyzer based on the electrostatic decapole-cylindrical field is presented. A trajectory analysis of electron motion is performed. Design features of the proposed energy analyzer scheme are described. Second-order focusing regimes for two configurations are numerically calculated: (1) a ring source and its image located in the region of the inner cylinder and (2) a point source located at a great distance from the energy analyzer on the symmetry axis. It is shown that on the basis of the decapole-cylindrical field, a long-focal-length mirror energy analyzer with “axis–ring” type focusing can be built. Instrumental functions for two regimes of the energy analyzer are given; energy resolution and luminosity of the device are estimated.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Elucidating structural order and disorder phenomena in mullite-type Al{sub 4}B{sub 2}O{sub 9} by automated electron diffraction tomography

The crystal structure and disorder phenomena of Al{sub 4}B{sub 2}O{sub 9}, an aluminum borate from the mullite-type family, were studied using automated diffraction tomography (ADT), a recently established method for collection and analysis of electron diffraction data. Al{sub 4}B{sub 2}O{sub 9}, prepared by sol-gel approach, crystallizes in the monoclinic space group C2/m. The ab initio structure determination based on three-dimensional electron diffraction data from single ordered crystals reveals that edge-connected AlO{sub 6} octahedra expanding along the b axis constitute the backbone. The ordered structure (A) was confirmed by TEM and HAADF-STEM images. Furthermore, disordered crystals with diffuse scattering along the b axis are observed. Analysis of the modulation pattern implies a mean superstructure (AAB) with a threefold b axis, where B corresponds to an A layer shifted by ½a and ½c. Diffraction patterns simulated for the AAB sequence including additional stacking disorder are in good agreement with experimental electron diffraction patterns. - Graphical abstract: Crystal structure and disorder phenomena of B-rich Al{sub 4}B{sub 2}O{sub 9} studied by automated electron diffraction tomography (ADT) and described by diffraction simulation using DISCUS. - Highlights: • Ab-initio structure solution by electron diffraction from single nanocrystals. • Detected modulation corresponding mainly to three-fold superstructure. • Diffuse diffraction streaks caused by stacking faults in disordered crystals. • Observed streaks explained by simulated electron diffraction patterns.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

BAYESIAN CONFIDENCE LIMITS OF ELECTRON SPECTRA OBTAINED THROUGH REGULARIZED INVERSION OF SOLAR HARD X-RAY SPECTRA

Many astrophysical observations are characterized by a single, non-repeatable measurement of a source brightness or intensity, from which we are to construct estimates for the true intensity and its uncertainty. For example, the hard X-ray count spectrum from transient events such as solar flares can be observed only once, and from this single spectrum one must determine the best estimate of the underlying source spectrum I({epsilon}), and hence the form of the responsible electron spectrum F(E). Including statistical uncertainties on the measured count spectrum yields a 'confidence strip' that delineates the boundaries of electron spectra that are consistent with the observed photon spectrum. In this short article, we point out that the expectation values of the source brightness and its variance in a given photon energy bin are in general not (as has been assumed in prior works) equal to n, the number of counts observed in that energy bin. Rather, they depend both on n and on prior knowledge of the overall photon spectrum. Using Bayesian statistics, we provide an explicit procedure and formulas for determining the 'confidence strip' (Bayesian credible region) for F(E), thus providing rigorous bounds on the intensity and shape of the accelerated electron spectrum.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Influence of electron irradiation on the structural and thermal properties of silk fibroin films

Radiation-induced changes in Bombyx mori silk fibroin (SF) films under electron irradiation were investigated and correlated with dose. SF films were irradiated in air at room temperature using 8 MeV electron beam in the range 0-150 kGy. Various properties of the irradiated SF films were studied using X-ray diffraction (XRD), Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). Electron irradiation was found to induce changes in the physical and thermal properties, depending on the radiation dose.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Ferrous sulfate based low temperature synthesis and magnetic properties of nickel ferrite nanostructures

Highlights: • Novel low temperature synthesis of nickel ferrite nanoparticles. • Comparison with two conventional synthesis techniques including hydrothermal method. • XRD results confirm the formation of crystalline nickel ferrites at 110 °C. • Superparamagnetic particles with applications in drug delivery and hyperthermia. • Magnetic properties superior to conventional methods found in new process. - Abstract: We report a simple, low temperature and surfactant free co-precipitation method for the preparation of nickel ferrite nanostructures using ferrous sulfate as the iron precursor. The products obtained from this method were compared for their physical properties with nickel ferrites produced through conventional co-precipitation and hydrothermal methods which used ferric nitrate as the iron precursor. X-ray diffraction analysis confirmed the synthesis of single phase inverse spinel nanocrystalline nickel ferrites at temperature as low as 110 °C in the low temperature method. Electron microscopy analysis on the samples revealed the formation of nearly spherical nanostructures in the size range of 20–30 nm which are comparable to other conventional methods. Vibrating sample magnetometer measurements showed the formation of superparamagnetic particles with high magnetic saturation 41.3 emu/g which corresponds well with conventional synthesis methods. The spontaneous synthesis of the nickel ferrite nanoparticles by the low temperature synthesis method was attributed to the presence of 0.808 kJ mol{sup −1} of excess Gibbs free energy due to ferrous sulfate precursor.

36 MATERIALS SCIENCE↗