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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 1,567 records · Page 87
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Surface regulation enables high stability of single-crystal lithium-ion cathodes at high voltage

Abstract Single-crystal cathode materials for lithium-ion batteries have attracted increasing interest in providing greater capacity retention than their polycrystalline counterparts. However, after being cycled at high voltages, these single-crystal materials exhibit severe structural instability and capacity fade. Understanding how the surface structural changes determine the performance degradation over cycling is crucial, but remains elusive. Here, we investigate the correlation of the surface structure, internal strain, and capacity deterioration by using operando X-ray spectroscopy imaging and nano-tomography. We directly observe a close correlation between surface chemistry and phase distribution from homogeneity to heterogeneity, which induces heterogeneous internal strain within the particle and the resulting structural/performance degradation during cycling. We also discover that surface chemistry can significantly enhance the cyclic performance. Our modified process effectively regulates the performance fade issue of single-crystal cathode and provides new insights for improved design of high-capacity battery materials.

25 ENERGY STORAGE↗
DOE OSTI2020

Small Polarons and Surface Defects in Metal Oxide Photocatalysts Studied Using XUV Reflection–Absorption Spectroscopy

In photocatalytic transition metal oxides, the surface electronic structure and carrier kinetics are complicated by both charge trapping at defects and self-trapping by small polaron formation. These effects are specific to both the material and the type of defect or surface states involved. Here we review recent studies on ultrafast photoexcited charge carrier dynamics at the surfaces of alpha-Fe 2 O 3 (hematite) and NiO and at the interface of a alpha-Fe 2 O 3 /NiO heterojunction using extreme ultraviolet (XUV) reflection-absorption spectroscopy. We study dynamics of small polaron formation at hematite surfaces showing that the rate of carrier self-trapping is slower than in the bulk due to a greater lattice reorganization energy required for surface polaron formation. We also show that surface dynamics of hematite can be systematically tuned to control carrier transport properties through surface functionalization. To better understand the impact of defects on carrier events, we study charge carrier recombination of NiO. It is shown that O vacancies have no detrimental effect on carrier lifetime while grain boundaries cause fast recombination. Finally, we study a model heterojunction consisting of alpha-Fe 2 O 3 and NiO and find that interfacial charge transfer across these two materials occurs by a two-step mechanism where the interfacial electric field drives fast exciton dissociation followed by hole injection from alpha-Fe2O3 to NiO. Finally, these examples illustrate future opportunities to observe surface electron dynamics in photocatalytic materials with element and chemical state resolution using ultrafast XUV spectroscopy.

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

Crystalline to amorphous phase transition of tin oxide nanocrystals induced by SHI at low temperature

Tin oxide (SnO{sub 2}) thin films were deposited using pulsed laser deposition (PLD) technique on Si substrates. The as-deposited films were irradiated using 100 MeV Ag ions at different fluences ranging from 3x10{sup 13} to 3x10{sup 14} ions/cm{sup 2} at an incidence angle of 75 deg. with respect to surface normal at liquid nitrogen (LN2) temperature. The as-deposited and irradiated films have been characterized using X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques to study the modifications in structural and surface morphological properties. Nanocrystalline film become completely amorphous and nanograins of tin oxide disappeared from the surface as indicated by XRD spectra and AFM micrographs respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2014

“Conductive” yttria-stabilized zirconia as an epitaxial template for oxide heterostructures

We report an in situ thermochemical treatment that significantly increases the macroscopic electrical conductivity of insulating yttria-stabilized zirconia (YSZ) (001) single-crystalline substrates. We demonstrate the high-quality surface crystalline structure of the resulting “conductive” cYSZ (001) by low- and high-energy electron diffraction. Soft- and hard X-ray photoemission spectroscopy measurements reveal a sizable reduction of Zr cations to a metallic state and their homogeneous distribution within the cYSZ. We discuss the correlation between the microscopic chemical processes leading to the increased macroscopic metallicity. Finally, the heteroepitaxial growth of a functional magnetic oxide model system, ultrathin EuO on cYSZ (001), was demonstrated. cYSZ (001) thereby enables both high quality oxide heteroepitaxy and the advanced sample characterization by high electron-fluence characterization techniques.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Bird-Strike Analysis Using Eulerian Peridynamic Formulation

The purpose of this project between Sandia National Laboratories (SNL) and The Boeing Company (Boeing) was to develop an analysis capability for predicting the loads on aircraft structures due to the inflight impact of birds. The main technical accomplishment was the development and delivery to Boeing of a new software tool that predicts the pressure over a structure's surface area, as a function of time and impact conditions, during the impact. The benefit to Boeing, and to government agencies such as NASA and the FAA, is an accurate tool to help design structures to withstand potentially catastrophic damage due to bird strike.

42 ENGINEERING↗
DOE OSTI2014

Surface-induced charge at the Ge (001) surface and its interaction with self-interstitials

The Ge (001) surface with dimer structure, is negatively charged while into the bulk, positive charges are observed even deeper than the fifteenth layer from the surface. This is different from the Si case. This charge distribution can lead to the repulsion of positively charged self-interstitials by the positively charged near surface layer in an implantation or irradiation process. Self-interstitial reflection by Ge surfaces had been proposed to explain the results of diffusion experiments during irradiation whereby positively charged self-interstitials are generated by collisions of highly energetic particles with Ge atoms. We investigated different Ge (001) surface comparing an as-cleaved surface with dangling bonds to a surface with dimer structure, and to a surface terminated by hydrogen atoms. The effect of these different surface terminations on the surface-induced charges in the near surface bulk were calculated by ab initio techniques.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Electrical and optical properties of molybdenum doped zinc oxide films prepared by reactive RF magnetron sputtering

Molybdenum doped zinc oxide (MZO) films were deposited on to glass substrates held at temperatures in the range from 303 to 673 K by reactive RF magnetron sputtering method. The chemical composition, crystallographic structure and surface morphology, electrical and optical properties of the films were determined. The films contained the molybdenum of 2.7 at. % in ZnO. The films deposited at 303 K were of X-ray amorphous. The films formed at 473 K were of nanocrystalline in nature with wurtzite structure. The crystallite size of the films was increased with the increase of substrate temperature. The optical transmittance of the films was in the visible range was 80–85%. The molybdenum (2.7 at %) doped zinc oxide films deposited at substrate temperature of 573 K were of nanocrystalline with electrical resistivity of 7.2×10{sup −3} Ωcm, optical transmittance of 85 %, optical band gap of 3.35 eV and figure of merit 30.6 Ω{sup −1}cm{sup −1}.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Few-Layers Graphene Film and Copper Surface Morphology for Improved Corrosion Protection of Copper

Graphene has shown excellent corrosion protection of copper (Cu). The corrosion protection of Cu is governed by the characteristics of the deposited graphene and Cu surface morphology underneath. In this work, graphene films (1-2, 4-5 layers) were deposited on Cu using a chemical vapor deposition by changing hydrogen (H{sub 2}) concentration during the annealing stage. The chemical structure and surface microstructural features of graphene/Cu were inspected by Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. The electrochemical corrosion performance of graphene/Cu was studied in 0.5 M sodium chloride solution using potentiodynamic polarization and electrochemical impedance spectroscopic measurements. The 1-2 layers of graphene and rough Cu surface were obtained for low H{sub 2} concentrations (0 and 2.5%), whereas high H{sub 2} concentrations, 20 and 50%, resulted in a smooth Cu surface and 4-5 layers of graphene. Our results showed that Cu with a smooth surface and multilayer graphene film exhibited the best corrosion resistance against electrochemical degradation. Tafel analysis revealed that Cu with 4-5 layers of graphene coating corroded nearly three orders of magnitude slower than the annealed Cu, without graphene coating. The results of this study can be useful for several applications where Cu is in close contact with salt species.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Gas-phase surface modification to control catalyst structure and yields in methane dehydroaromatization

Methane dehydroaromatization (MDA) is a promising approach for direct methane transformation to aromatics and hydrogen. The benchmark catalyst Mo/H-ZSM-5 struggles to find commercial adoption because of thermodynamically-limited yields and rapid coking on Brønsted acid and molybdenum carbide species, especially on zeolite external surfaces. Here, gas-phase atomic layer deposition (ALD) overcoats H-ZSM-5 external surfaces with SiO 2 or Al 2 O 3 . NH 3 -TPD, HRTEM, and textural properties show that these overcoats exclusively passivate zeolite external surfaces. Under MDA conditions, SiO 2 gives softer coke and increases cumulative benzene yields by 25%, while Al 2 O 3 strongly decreases yields. H 2 -TPR and UV-visible and Raman spectroscopy show how the overcoats redisperse the MoO x precatalysts, especially over multiple deactivation and isothermal oxidative regeneration cycles. Combined with 27 Al-MAS NMR, MoO x redistribution and dealumination are seen as the causes of long-term deactivation over multiple regeneration cycles, and this process continues to occur regardless of the overcoat. Altogether, the deposition of a small amount of silica on the outer surface of Mo/H-ZSM-5 reduces the formation of hard coke, which could be regenerated by milder methods such as hydrogen treatment.

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

Metallic conductance at the interface of tri-color titanate superlattices

Ultra-thin tri-color (tri-layer) titanate superlattices ([3 u.c. LaTiO{sub 3}/2 u.c. SrTiO{sub 3}/3 u.c. YTiO{sub 3}], u.c. = unit cells) were grown in a layer-by-layer way on single crystal TbScO{sub 3} (110) substrates by pulsed laser deposition. High sample quality and electronic structure were characterized by the combination of in-situ photoelectron and ex-situ structure and surface morphology probes. Temperature-dependent sheet resistance indicates the presence of metallic interfaces in both [3 u.c. LaTiO{sub 3}/2 u.c. SrTiO{sub 3}] bi-layers and all the tri-color structures, whereas a [3 u.c. YTiO{sub 3}/2 u.c. SrTiO{sub 3}] bi-layer shows insulating behavior. Considering that in the bulk YTiO{sub 3} is ferromagnetic below 30 K, the tri-color titanate superlattices provide an opportunity to induce tunable spin-polarization into the two-dimensional electron gas with Mott carriers.

36 MATERIALS SCIENCE↗
DOE OSTI2016

First-principles study of electronic properties of FeSe{sub 1-x}S{sub x} alloys

We have studied the electronic and superconducting properties of FeSe{sub 1-x}S{sub x} (x = 0.0, 0.04) alloys by first-principles calculations using the Korringa-Kohn-Rostoker Atomic Sphere Approximation within the coherent potential approximation (KKR-ASA-CPA). The electronic structure calculations show the ground states of S-doped FeSe to be nonmagnetic. We present the results of our unpolarized calculations for these alloys in terms of density of states (DOS), band structures, Fermi surfaces and the superconducting transition temperature of FeSe and FeSe{sub 0.96}S{sub 0.04} alloys. We find that the substitution of S at Se site into FeSe exhibit the subtle changes in the electronic structure with respect to the parent FeSe. We have also estimated bare Sommerfeld constant (γ{sub b}), electron-phonon coupling constant (λ) and the superconducting transition temperature (T{sub c}) for these alloys, which were found to be in good agreement with experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

PEERING INTO THE GIANT-PLANET-FORMING REGION OF THE TW HYDRAE DISK WITH THE GEMINI PLANET IMAGER

We present Gemini Planet Imager (GPI) adaptive optics near-infrared images of the giant-planet-forming regions of the protoplanetary disk orbiting the nearby (D = 54 pc), pre-main-sequence (classical T Tauri) star TW Hydrae. The GPI images, which were obtained in coronagraphic/polarimetric mode, exploit starlight scattered off small dust grains to elucidate the surface density structure of the TW Hya disk from ∼80 AU to within ∼10 AU of the star at ∼1.5 AU resolution. The GPI polarized intensity images unambiguously confirm the presence of a gap in the radial surface brightness distribution of the inner disk. The gap is centered near ∼23 AU, with a width of ∼5 AU and a depth of ∼50%. In the context of recent simulations of giant-planet formation in gaseous, dusty disks orbiting pre-main-sequence stars, these results indicate that at least one young planet with a mass ∼0.2 M{sub J} could be present in the TW Hya disk at an orbital semimajor axis similar to that of Uranus. If this (proto)planet is actively accreting gas from the disk, it may be readily detectable by GPI or a similarly sensitive, high-resolution infrared imaging system.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Tunable electronic structure in dilute magnetic semiconductor Sr{sub 3}SnO/c-YSZ/Si (001) epitaxial heterostructures

We report a systematic study of the structural, physical, and chemical properties of epitaxial thin films of emerging dilute magnetic semiconductor (DMS) Sr{sub 3}SnO (SSO) integrated with Si (100) prepared by various post-growth annealing treatments. The transport properties of these films are primarily governed by oxygen vacancies and the results are explained with the variable-range hopping model. The increased oxygen vacancy concentration generated by post-growth vacuum annealing results in a shorter hopping distance and reduced hopping energy and Coulomb gap, leading to lower resistivity; oxygen annealing shows the opposite effects. The work function ranges from 4.54 to 4.02 eV and shows a negative linear relationship with oxygen vacancy concentration, accompanied by a 0.42 eV shift in the surface Fermi level. The transport and ultraviolet photoelectron spectroscopy probes agree quantitatively on measurement of the resistivity and surface electronic structure. The results provide a direct and consistent explanation that the property changes in the bulk and at the surface are primarily attributed to oxygen vacancies, which are believed to be the carriers in the SSO thin films. The ability to manipulate the work function and oxygen vacancy concentration in epitaxial DMS SSO thin films offers great potential for the development of spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Ultra-stable Molecule-Surface Architectures at Metal Oxides: Structure, Bonding, and Electron-transfer Processes

Research funded by this project focused on the development of improved strategies for functionalization of metal oxides to enhance charge-transfer processes relevant to solar energy conversion. Initial studies included Fe 2 O 3 , WO 3 , TiO 2 , SnO 2 , and ZnO as model oxide systems; these systems were chosen due to differences in metal oxidation state and chemical bonding types in these oxides. Later studies focused largely on SnO 2 and ZnO, as these materials show particularly promising surface chemistry, have high electron mobility, and can be readily grown in both spherical nanoparticles and as elongated nanorods. New molecules were synthesized that allowed the direct chemical assembly of novel nanoparticle “dyadic” structures in which two different oxide materials are chemically joined, leading to an interface that enhances the separation of of charge upon illumination. We demonstrated that such junctions enhance photocatalytic efficiency using model organic compounds. A separate effort focused on novel approaches to linking dye molecules to SnO 2 and ZnO as a way to enhance solar conversion efficiency. A novel type of surface binding through a dithione compound was discovered that leads to highly stable light-absorbing interfaces on ZnO.

14 SOLAR ENERGY↗
DOE OSTI2023

Molecular Dynamics Investigation of Nanoscale Hydrophobicity of Polymer Surfaces: What Makes Water Wet?

The wettability of a polymer surface–related to its hydrophobicity or tendency to repel water–can be crucial for determining its utility, such as for a coating or a purification membrane. While wettability is commonly associated with the macroscopic measurement of a contact angle between surface, water, and air, the molecular physics that underlie these macroscopic observations are not fully known, and anticipating relative behavior of different polymers is challenging. To address this gap in molecular-level understanding, we use molecular dynamics simulations to investigate and contrast interactions of water with six chemically distinct polymers: polytetrafluoroethylene, polyethylene, polyvinyl chloride, poly(methyl methacrylate), Nylon 66, and polyvinyl alcohol. We show that several prospective quantitative metrics for hydrophobicity agree well with experimental contact angles. Moreover, the behavior of water in proximity to these polymer surfaces can be distinguished with analysis of interfacial water dynamics, extent of hydrogen bonding, and molecular orientation–even when macroscopic measures of hydrophobicity are similar. The predominant factor dictating wettability is found to be the extent of hydrogen bonding between polymer and water, but the precise manifestation of hydrogen bonding and its impact on surface-water structure varies. In the absence of hydrogen bonding, other molecular interactions and polymer mechanics control hydrophobic ordering. Furthermore, these results provide new insights into how polymer chemistry specifically impacts water-polymer interactions and translates to surface hydrophobicity. Such factors may facilitate the design or processing of polymer surfaces to achieve targeted wetting behavior, and presented analyses can be useful in studying the interfacial physics of other systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Silver Nanoparticles with Surface-Bonded Oxygen for Highly Selective CO 2 Reduction

Here, the surface electronic structures of catalysts need to be carefully engineered in CO 2 reduction reaction (CO 2 RR), where the hydrogen evolution side reaction usually takes over under a significant overpotential, and thus dramatically lows the reaction selectivity. Surface oxides can play a critical role in tuning the surface oxidation state of metal catalysts for a proper binding with CO 2 RR reaction intermediates, which may significantly improve the catalyst activity and selectivity. Here, we demonstrate the importance of surface-bonded oxygen on silver nanoparticles in altering the reaction pathways and improving the CO 2 RR performances. A comparative investigation on air-annealed Ag (Air-Ag) catalyst with or without the post-treatment of H 2 thermal annealing (H 2 -Ag) was performed. In Air-Ag, the subsurface chemically bonded O species (O-Ag δ+ ) was identified by angle resolved X-ray photoelectron spectroscopy and X-ray absorption spectroscopy techniques, and contributed to the improved CO selectivity rather than H 2 in CO 2 RR electrolysis. As a result, while the maximal CO Faradaic efficiency of H 2 -Ag is at ~ 30 %, the Air-Ag catalyst presented a high CO selectivity of more than 90 % under a current density of ~ 21 mA/cm 2 .

36 MATERIALS SCIENCE↗