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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Electronic structure and surface properties of MgB 2 (0001) upon oxygen adsorption [Electronic structure and surface properties of MgB2: oxygen adsorption on MgB2(0001)]

Here, we use density-functional theory to investigate the bulk and surface properties of MgB 2 . The unique bonding structure of MgB 2 is investigated by Bader's atoms-in-molecules, charge density difference, and occupancy projected band structure analyses. Oxygen adsorption on the charge-depleted surfaces of MgB 2 is studied by a surface potential energy mapping method, reporting a complete map including low-symmetry binding sites. The B-terminated MgB 2 (0001) demonstrates reconstruction of the graphenelike B layer, and the reconstructed geometry exposes a threefold site of the subsurface Mg, making it accessible from the surface. Detailed reconstruction mechanisms are studied by simulated annealing method based on ab initio molecular dynamics and nudged elastic band calculations. The surface clustering of B atoms significantly modifies the B 2p states to occupy low energy valence states. The present paper emphasizes that a thorough understanding of the surface phase may explain an apparent inconsistency in the experimental surface characterization of MgB 2 . Furthermore, these results suggest that the surface passivation can be an important technical challenge when it comes to development of a superconducting device using MgB 2 .

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Laser-induced generation of surface periodic structures in media with nonlinear diffusion

A model of fast formation of high-contrast periodic structure appearing on a semiconductor surface under action of laser radiation is proposed. The process of growing a surface structure due to the interaction surface plasmon- polaritons excited on nonequilibrium electrons with incident laser radiation are considered in the framework of a medium with nonlinear diffusion of nonequilibrium carriers (defects). A resonance effect of superfast pico- and subpicosecond amplification of the plasmon-polariton structure generated on the surface, the realization of which can result in a high-contrast defect lattice.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Surface stability and the selection rules of substrate orientation for optimal growth of epitaxial II-VI semiconductors

The surface structures of ionic zinc-blende CdTe (001), (110), (111), and (211) surfaces are systematically studied by first-principles density functional calculations. Based on the surface structures and surface energies, we identify the detrimental twinning appearing in molecular beam epitaxy (MBE) growth of II-VI compounds as the (111) lamellar twin boundaries. To avoid the appearance of twinning in MBE growth, we propose the following selection rules for choosing optimal substrate orientations: (1) the surface should be nonpolar so that there is no large surface reconstructions that could act as a nucleation center and promote the formation of twins; (2) the surface structure should have low symmetry so that there are no multiple equivalent directions for growth. These straightforward rules, in consistent with experimental observations, provide guidelines for selecting proper substrates for high-quality MBE growth of II-VI compounds.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2006

The X-37 Hot Structure Control Surface Testing

Thermal-structural testing of three hot structure control surface subcomponent test articles (STA) designed for the X-37 (Boeing Phantom Works, Huntington Beach, California) Orbital Vehicle (OV) has been completed. The test articles were subcomponents of the X-37 OV bodyflap and flaperon control surfaces.

Hudson, Larry D.↗
DOE OSTI2021

Correlating 3D Surface Atomic Structure and Catalytic Activities of Pt Nanocrystals

Active sites and catalytic activity of heterogeneous catalysts is determined by their surface atomic structures. However, probing the surface structure at an atomic resolution is difficult, especially for solution ensembles of catalytic nanocrystals, which consist of heterogeneous particles with irregular shapes and surfaces. In this study, we constructed 3D maps of the coordination number (CN) and generalized CN ($\bar{C}\bar{N}$) for individual surface atoms of sub-3 nm Pt nanocrystals. Our results reveal that the synthesized Pt nanocrystals are enclosed by islands of atoms with nonuniform shapes that lead to complex surface structures, including a high ratio of low-coordination surface atoms, reduced domain size of low-index facets, and various types of exposed high-index facets. 3D maps of $\bar{C}\bar{N}$ are directly correlated to catalytic activities assigned to individual surface atoms with distinct local coordination structures, which explains the origin of high catalytic performance of small Pt nanocrystals in important reactions such as oxygen reduction reactions and CO electro-oxidation.

3D atomic structure↗
DOE OSTI2015

Surface and Structural Investigation of a MnO x Birnessite‐Type Water Oxidation Catalyst Formed under Photocatalytic Conditions

Abstract Catalytically active MnO x species have been reported to form in situ from various Mn‐complexes during electrocatalytic and solution‐based water oxidation when employing cerium(IV) ammonium ammonium nitrate (CAN) oxidant as a sacrificial reagent. The full structural characterization of these oxides may be complicated by the presence of support material and lack of a pure bulk phase. For the first time, we show that highly active MnO x catalysts form without supports in situ under photocatalytic conditions. Our most active 4 MnO x catalyst (∼0.84 mmol O 2 mol Mn −1 s −1 ) forms from a Mn 4 O 4 bearing a metal–organic framework. 4 MnO x is characterized by pair distribution function analysis (PDF), Raman spectroscopy, and HR‐TEM as a disordered, layered Mn‐oxide with high surface area (216 m 2 g −1 ) and small regions of crystallinity and layer flexibility. In contrast, the S MnO x formed from Mn 2+ salt gives an amorphous species of lower surface area (80 m 2 g −1 ) and lower activity (∼0.15 mmol O 2 mol Mn −1 s −1 ). We compare these catalysts to crystalline hexagonal birnessite, which activates under the same conditions. Full deconvolution of the XPS Mn2p 3/2 core levels detects enriched Mn 3+ and Mn 2+ content on the surfaces, which indicates possible disproportionation/comproportionation surface equilibria.

Deibert, Benjamin J.↗
DOE OSTI2017

Comparative analysis of image binarization methods for crack identification in concrete structures

Surface cracks in concrete structures are critical indicators of structural damage and durability. Manual visual inspection, the most commonly used method in practice, is inefficient from cost, time, accuracy, and safety perspectives. A promising alternative is computer vision-based methods that can automatically extract crack information from images. Image binarization, developed for text detection, is appropriate for crack identification, as texts and cracks are similar, consisting of distinguishable lines and curves. However, standardizing crack identification using image binarization is challenging, because binarization depends on the method and associated parameters. We investigate image binarization for crack identification, focusing on optimal parameter determination and comparative performance evaluation for five common binarization methods. Crack images are prepared to obtain optimal parameters by minimizing errors in estimated crack widths. Subsequently, comparative analysis is conducted using crack images with different conditions based on three performance evaluation criteria: crack width and length measurement accuracy and computation time.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Surface termination structure of α-Ga{sub 2}O{sub 3} film grown by mist chemical vapor deposition

The surface structure of α-Ga{sub 2}O{sub 3}(0001) grown on an α-Al{sub 2}O{sub 3}(0001) substrate by mist chemical vapor deposition was studied by coaxial impact-collision ion scattering spectroscopy (CAICISS) and atomic force microscopy (AFM). The minimum step height observed in the AFM image was 0.21 ± 0.01 nm, coinciding with the height of three atomic layers of α-Ga{sub 2}O{sub 3}(0001). It was revealed by CAICISS analysis that the surface of α-Ga{sub 2}O{sub 3}(0001) is terminated by a Ga layer followed by an O layer, which is consistent with the surface termination of α-Al{sub 2}O{sub 3}(0001). A structural model taking surface relaxation into account was also constructed by fitting the simulated curve for the azimuth angle dependence of the Ga intensity to the experimental dependence. The resultant structural model is similar to the model of an α-Al{sub 2}O{sub 3}(0001) surface, which indicates analogous behavior in corundum crystals.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Surface charge dynamics in photosynthetic membranes and the structural consequences

The strict stacking of plant photosynthetic membranes into granal structures plays a vital role in energy conversion. The molecular forces that lead to grana stacking, however, are poorly understood. Here we evaluate the interplay between repulsive electrostatic (F el ) and attractive van der Waals (F vdWaals ) forces in grana stacking. In contrast to previous reports, we find that the physicochemical balance between attractive and repulsive forces fully explains grana stacking. Extending the force balance analysis to lateral interactions within the oxygen-evolving photosystem II (PSII)–light harvesting complex II (LHCII) supercomplex reveals that supercomplex stability is very sensitive to F el changes. F el is highly dynamic, increasing up to 1.7-fold on addition of negative charges by phosphorylation of grana-hosted proteins. We show that this leads to specific destabilization of the supercomplex, and that changes in F el have contrasting effects on vertical stacking and lateral intramembrane organization. Lastly, this enables discrete biological control of these central structural features.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

Surface Atomic Structure and Functionality of Metallic Nanoparticles: A Case Study of Au–Pd Nanoalloy Catalysts

The surface atomic structure of metallic nanoparticles (NPs) plays a key role in shaping their physicochemical properties and response to external stimuli. Not surprisingly, current research increasingly focuses on exploiting its prime characteristics, including the amount, location, coordination, and electronic configuration of distinct surface atomic species, as tunable parameters for improving the functionality of metallic NPs in practical applications. The effort requires clear understanding of the extent to which changes in each of these characteristics would contribute to achieving the targeted functionality. This, in the first place, requires good knowledge of the actual surface of metallic NPs at atomic level. Through a case study on Au–Pd nanoalloy catalysts of industrial and environmental importance, we demonstrate that the surface atomic structure of metallic NPs can be determined in good detail by resonant high-energy X-ray diffraction (HE-XRD). Furthermore, using our experimental surface structure and CO oxidation activity data, we shed new light on the elusive origin of the remarkable catalytic synergy between surface Au and Pd atoms in the nanoalloys. In particular, we show that it arises from the formation of a specific “skin” on top of the nanoalloys that involves as many unlike, i.e., Au–Pd and Pd–Au, atomic pairs as possible given the overall chemical composition of the NPs. Moreover, unlike atoms from the “skin” interact strongly, including both changing their size and electronic structure in inverse proportions. That is, Au atoms shrink and acquire a partial positive charge of 5d-character whereas Pd atoms expand and become somewhat 4d-electron deficient. Accordingly, the reactivity of Au increases whereas Pd atoms become less reactive, as compared to atoms at the surface of pure Au and Pd NPs, respectively. Ultimately, this renders Au–Pd alloy NPs superb catalysts for CO oxidation reaction over a broad range of alloy compositions. Our findings are corroborated by DFT calculations based on a refined version of d-band center theory on the catalytic properties of late transition metals and alloys. Here, we discuss opportunities for improving the accuracy of current theory on surface-controlled properties of metallic NPs through augmenting the theory with surface structure data obtained by resonant XRD.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Structure of the Bastnäsite (001) Surface by Crystal Truncation Rod X-ray Diffraction and Ab Initio Molecular Dynamics: Implications for Separations of a Rare Earth Ore Mineral

Bastnäsite ((Ce,La)FCO 3 ) is the primary mineral source of light rare earth elements, but its surface structure is not well understood. This presents a major challenge in improving beneficiation strategies. In this work, a synergistic combination of X-ray scattering and ab initio molecular dynamics (AIMD) was used to gain atomistic insight into the interfacial structure of bastnäsite. Surface X-ray scattering was used to measure crystal truncation rods (CTRs) of the bastnäsite (001) surface, a significant crystal face with a previously unknown termination. The best-fit atomic-scale model of the CTR data features a carbonate layer at the surface, which is stabilized by the relaxation of carbonate groups from their bulk structural positions. AIMD simulations predict similar surface relaxations, which are shown to be influenced by the protonation of oxygen atoms at the surface. Evidence of ordered water at the interface is also observed in the best-fit model and AIMD simulations. Further, the presence of a carbonate layer at this dominant crystal surface is significant for improving separation technologies because most commonly used ligands utilize anionic functional groups to chelate metal cations at particle surfaces. Without modification, anionic ligands are expected to have poor affinity for the carbonate-terminated (001) surface.

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

Bulk and surface electronic structure of NiBi 3

Here, we present a high-resolution, angle-resolved photoemission spectroscopy study of the normal electronic state of the superconducting NiBi 3 . Our experimental results show a complex Fermi surface structure with many sheets along the Γ–$\textit{X}$ and Γ–$\textit{Y}$ directions of the Brillouin zone. The band structure presents a topological surface state (TSS) at the high symmetry Γ point with a surface Dirac point at the energy –0.185 eV. The Dirac-like cone presents a linear dispersion along $k_x$ while it presents saddle-like states along $k_y$ located in the vicinity of the surface Dirac point. Our results are in good agreement with results of density functional theory band structure calculations. We also discuss the topological band structure of the NiBi 3 compound.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Influence of Surface and Structural Variations in Donor–Acceptor–Donor Sensitizers on Photoelectrocatalytic Water Splitting

Conjugated organic chromophores composed of linked donor (D) and acceptor (A) moieties have attracted considerable attention for photoelectrochemical applications. In this work, we compare the optoelectronic properties and photoelectrochemical performance of two D–A–D structural isomers with thiophene-X-carboxylic acid (X denotes 3 and 2 positions) derivatives and 2,1,3-benzothiadiazole as the D and A moieties, respectively. 5,5′-(Benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(thiophene-3-carboxylic acid), BTD1, and 5,5′-(benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(thiophene-2-carboxylic acid), BTD2, were employed in the study to understand how structural isomers affect surface attachments within chromophore–catalyst assemblies and their influence on charge-transfer dynamics. Crystal structures revealed that varying the position of the −COOH anchoring group causes the molecules to either contort out of a plane (BTD1) or adopt a near-perfect planar conformation (BTD2). BTD1 and BTD2 were co-loaded with either a water oxidation catalyst, [Ru(2,6-bis(1-methylbenzimidazol-2-yl)pyridine)-(4,4′-((HO) 2 OPCH 2 )2-2,2′-bipyridine)(OH 2 )] 2 , RuCt 2+ , or proton reduction catalyst [Ni(P 2 Ph N 2 C 6 H 4 CH 2 PO 3 H 2 ) 2 ] 2+ , NiCt 2+ , on oxide electrodes to facilitate photodriven water splitting reactions. Emission quenching measurements indicate that both BTD1 and BTD2 inject electrons into n-type SnO 2 |TiO 2 electrodes and holes into p-type NiO semiconductors from their respective excited states at high efficiencies >60%. Photocurrent densities of chromophore–catalyst assemblies obtained using linear sweep voltammetry (LSV) show that BTD2-sensitized photoanodes generate significantly more photocurrent than BTD1-sensitized electrodes; however, both exhibit similar performances at the photocathode. Photoelectrocatyltic measurements demonstrate that both BTD1 and BTD2 performed similarly, generating Faradaic efficiencies of 39 and 38% at the anode or 61 and 79% at the cathode. Transient absorption measurements suggest that the differences between the LSV and photoelectrocatalytic measurements result from the differences in quantum yields of the photogenerated redox equivalents, which is also a reflection of the varying metal oxide surface conformation. Our findings suggest that BTD2 should be investigated further in photocathodic studies since it has the structural advantage of being incorporated into diverse types of chromophore–catalyst assemblies.

Chromophores↗
DOE OSTI2021

Enhancing positron production using front surface target structures

We report a target design which produced a substantial gain in relativistic electron-positron pair production using high-intensity lasers and targets with large-scale micro-structures on their surface. Comparing to an unstructured target, a selected Si microwire array target yielded a near 100% increase in the laser-to-positron conversion efficiency and produced a 10MeV increase in the average emitted positron energy under nominally the same experimental conditions. We had established a multi-scale particle-in-cell simulation scheme to simulate both the laser absorption and the subsequent pair productions in a thick metal target. Here, the experimental results are supported by the simulations demonstrating the performance increase is due to a higher conversion efficiency of laser energy into electrons with kinetic energies greater than 10 MeV due to enhanced direct laser acceleration of electrons enabled by the microwire array.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2026

Assessment of Near-Angle Scatter on Exo-Earth Coronagraphy

Near-Angle Scatter (NAS) of the host star’s light into the coronagraph dark hole may limit the ability of a potential Habitable Worlds Observatory (HWO) to detect and characterize an Earth-like planet around a Sun-like star. This paper summarizes a 5-year investigation of the impact of NAS on high-contrast coronagraphy. Science requirements were flowed down to a draft flux ratio noise ratio (FRN) error budget with allocations for NAS. The modeled sources of NAS include correlated (low and mid-spatial) surface structure, uncorrelated surface microroughness and coating structure, particulate contamination, micrometeoroid impacts and polarization leakage. The paper develops specifications for these sources of scattered light that meet their FRN error budget allocations, distinguishing between scatter that can and cannot be modulated. The development process utilizes an analytical expression that predicts scatter throughput into the dark hole based on BRDF/BSDF. Using this relationship, Photon Engineering performed a FRED® straylight analysis of the HWO EAC-1 (Exploratory Analytical Case #1) coronagraph to parametrically study predicted scattered light contrast in the dark hole. The model revealed that primary mirror microroughness or contamination is not a significant source of scattered light; the most critical surfaces are the smallest and those located closest to the coronagraph focal plane mask. The assessment found that uncorrelated microstructure, contamination, and micrometeoroid impacts require more investigation and potential significant sources of unmodulated scattered light, and highlights that slow thermal wavefront drift is an order of magnitude more impactful than fast mechanical jitter.

coronagraphy↗
DOE OSTI2018

Selective Electrocatalytic Reduction of CO 2 into CO at Small, Thiol-Capped Au/Cu Nanoparticles

The electrochemical CO 2 reduction reaction (CO 2 RR) is a promising approach for converting fossil fuel emissions into environmentally sustainable chemicals and fuels. The ability to control the surface structure of CO 2 RR nanocatalysts provides an opportunity to tune product selectivity. Bimetallic gold–copper catalysts have been identified as emerging electrocatalyst candidates, but Cu incorporation typically lowers product selectivity compared with pure Au. Here we show sustained CO selectivity and activity up to 49% Cu content in small (<2 nm), thiol-capped Au/Cu nanoparticles (NPs). Bimetallic NPs containing 49% Cu selectivity converted CO 2 into CO with 100 ± 6% CO Faradaic efficiency and average mass activity of ~500 mA/mg during a 12 h electrolysis experiment at –0.8 V vs RHE. Au/Cu NPs synthesized without thiol ligands selectively produced H 2 , whereas larger (>10 nm), thermally dethiolated Au/Cu NPs produced a wider product distribution including H 2 , CO, and C 2 H 4 . Density functional theory (DFT) modeling of CO 2 RR and H 2 evolution at realistic, thiol-capped Au/Cu NP structures indicated that copper–thiol surface structures sustained CO selectivity by stabilizing key *CO intermediates while making *H binding less favorable. Calculations also predicted that removing a significant fraction of the thiol ligands would increase *CO binding strength such that desorption of CO product molecules could become the most thermodynamically challenging step. This result, coupled with increased *H stability on dethiolated nanoclusters, points to decreased CO 2 RR selectivity for small, ligand-free catalysts, which is in line with experimental observations from our group and others. Here, our results demonstrate that thiol-ligand surface structures can sustain the CO selectivity of bimetallic Au/Cu NPs and reduce precious metal requirements for CO 2 RR.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗