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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 OSTI2016

CO adsorption on small Au{sub n} (n = 1–4) structures supported on hematite. I. Adsorption on iron terminated α-Fe{sub 2}O{sub 3} (0001) surface

This is the first of two papers dealing with the adsorption of Au and formation of Au{sub n} nanostructures (n = 1–4) on hematite (0001) surface and adsorption of CO thereon. The stoichiometric Fe-terminated (0001) surface of hematite was investigated using density functional theory in the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) form with Hubbard correction U, accounting for strong electron correlations (PBE+U). The structural, energetic, and electronic properties of the systems studied were examined for vertical and flattened configurations of Au{sub n} nanostructures adsorbed on the hematite surfaces. The flattened ones, which can be viewed as bilayer-like structures, were found energetically more favored than vertical ones. For both classes of structures the adsorption binding energy increases with the number of Au atoms in a structure. The adsorption of Au{sub n} induces charge rearrangement at the Au{sub n}/oxide contact which is reflected in work function changes. In most considered cases Au{sub n} adsorption increases the work function. A detailed analysis of the bonding electron charge is presented and the corresponding electron charge rearrangements at the contacts were quantified by a Bader charge analyses. The interaction of a CO molecule with the Au{sub n} nanostructures supported on α-Fe{sub 2}O{sub 3} (0001) and the oxide support was studied. It is found that the CO adsorption binding to the hematite supported Au{sub n} structures is more than twice as strong as to the bare hematite surface. Analysis of the Bader charges on the atoms showed that in each case CO binds to the most positively charged (cationic) atom of the Au{sub n} structure. Changes in the electronic structure of the Au{sub n} species and of the oxide support, and their consequences for the interactions with CO, are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2015

A Stochastic Scheduler for Integrated Arrival, Departure and Surface Operations in Los Angeles

In terminal airspace, integrating arrivals, departures, and surface operations with competing resources provides the potential of improving operational efficiency by removing barriers between different operations. This work develops a centralized stochastic scheduler for operations in a terminal area including airborne and surface operations using Non-dominated sorting genetic algorithm and Monte Carlo simulations. The scheduler handles completing resources between different flows, such as runway allocations, runway crossing, departure fixes, and other interaction way points between arrivals and departures. Meanwhile, the scheduler also takes time-varied uncertainties into account when optimizing schedules. The scheduler is run sequentially to identify the best and robust schedule for the next planning window. Resulting schedules decide the routes, speed or delays, and runway assignments with separation constraints at mergingdiverging waypoints in the air and crossing and separations on runways. The Los Angels terminal area was used as an example. The implementation of this stochastic scheduler for integrated arrival, departure and surface operations is completed. And several preliminary runs are finished for over 1,200 flights in LAX in a typical day. Sensitivity studies on various planning window sizes are presented, which shows that trade-off exits between planning window size and achievable minimum delay. Preliminary results on runway usage are also presented in this abstract. Because arrivals on the outer runways have to be followed by crossings on the inner runways, algorithmic runway allocation prefers inner runways for arrivals and outer runways for departures. More results will be presented in the final paper. And current terminal arrival and departure procedures based on first-come-first-serve procedure will also be set up and used as a baseline for comparison.

integrated scheduler↗
DOE OSTI2019

CO, CO 2 , and H 2 Interactions with (0001) and (001) Tungsten Carbide Surfaces: Importance of Carbon and Metal Sites

A systematic study on the adsorption of atomic and molecular hydrogen and carbon oxides on cubic (001) and hexagonal (0001) WC surfaces by periodical density functional theory is reported. Calculations have been performed by employing the Perdew–Burke–Ernzerhof exchange correlation functional with van der Waals corrections to account for the dispersive force term. In addition, dipole corrections were applied for W- and C-terminated hexagonal WC(0001) surfaces. Good agreement is found between calculated and reported data for representative bulk properties. Regarding surface properties, our results indicate that atomic hydrogen adsorbs quite strongly while H 2 does, in general, dissociatively on the studied surfaces, with very small energy barriers (<0.35 eV) for the cleavage of the H–H bonds. The C sites of the carbide play an essential role in the binding of H atoms and the cleavage of H–H bonds. Studies examining the interaction of tungsten carbide with CO and CO 2 also evidence the importance of C sites. The reactivity of C- and W-terminated (0001) hexagonal WC surfaces significantly differs. Atomic hydrogen, carbon monoxide, and CO 2 are more stable on a C- than on a W-terminated surface, and only this latter termination is able to cleave spontaneously a C–O bond of the CO 2 molecule. This difference in reactivity may open a number of possibilities for fine-tuning the selectivity of the resulting material or designing compounds catalytically active for specific reactions by carefully adjusting the proportion of C, W, and mixed terminations during the synthesis procedure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Direct conversion of hydride- to siloxane-terminated silicon quantum dots

Here, peripheral surface functionalization of hydride-terminated silicon quantum dots (SiQD) is necessary in order to minimize their oxidation/aggregation and allow for solution processability. Historically thermal hydrosilylation addition of alkenes and alkynes across the Si-H surface to form Si-C bonds has been the primary method to achieve this. Here we demonstrate a mild alternative approach to functionalize hydride-terminated SiQDs using bulky silanols in the presence of free-radical initiators to form stable siloxane (~Si-O-SiR 3 ) surfaces with hydrogen gas as a byproduct. This offers an alternative to existing methods of forming siloxane surfaces that require corrosive Si-Cl based chemistry with HCl byproducts. A 52 nm blue shift in the photoluminescent spectra of siloxane versus alkyl-functionalized SiQDs is observed that we explain using computational theory. Model compound synthesis of silane and silsesquioxane analogues is used to optimize surface chemistry and elucidate reaction mechanisms. Thorough characterization on the extent of siloxane surface coverage is provided using FTIR and XPS. As a result, TEM is used to demonstrate SiQD size and integrity after surface chemistry and product isolation.

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

Formic Acid Dissociative Adsorption on NiO(111): Energetics and Structure of Adsorbed Formate

Here, the dissociative adsorption of carboxylic acids on oxide surfaces is important for understanding adsorbed carboxylates, which are important as intermediates in catalytic reactions, for the organo-functionalization of oxide surfaces, and in many other aspects of oxide surface chemistry. We present here the first direct experimental measurement of the heat of dissociative adsorption of any carboxylic acid on any single-crystal oxide surface. The enthalpy of the dissociative adsorption of formic acid, the simplest carboxylic acid, to produce adsorbed formate and hydrogen (as a surface hydroxyl) on a (2 × 2)-NiO(111) surface is measured by single crystal adsorption calorimetry. The differential heat of adsorption decreases with formic acid coverage from 202 to 99 kJ/mol at saturation (0.25 ML). The structure of the adsorbed products is clarified by density functional theory (DFT) calculations, which provide energies in reasonable agreement with the calorimetry. These calculations show that formic acid readily dissociates on both the oxygen and Ni terminations of the octapolar NiO(111) surfaces, donating its acid H to a surface lattice oxygen, while HCOO adsorbs preferentially with bridging-type geometry near the M-O 3 /O-M 3 sites. The calculated energetics at low coverages agrees well with experimental data, while larger differences are observed at high coverage (0.25 ML). The large decrease in experimental heat of adsorption with coverage can be brought into agreement with the DFT energies if we assume that both types of octapolar surface terminations (O- and Ni-) are present on the starting surface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

A nitride-based epitaxial surface layer formed by ammonia treatment of silicene-terminated ZrB{sub 2}

We present a method for the formation of an epitaxial surface layer involving B, N, and Si atoms on a ZrB{sub 2}(0001) thin film on Si(111). It has the potential to be an insulating growth template for 2D semiconductors. The chemical reaction of NH{sub 3} molecules with the silicene-terminated ZrB{sub 2} surface was characterized by synchrotron-based, high-resolution core-level photoelectron spectroscopy and low-energy electron diffraction. In particular, the dissociative chemisorption of NH{sub 3} at 400 °C leads to surface nitridation, and subsequent annealing up to 830 °C results in a solid phase reaction with the ZrB{sub 2} subsurface layers. In this way, a new nitride-based epitaxial surface layer is formed with hexagonal symmetry and a single in-plane crystal orientation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2007

Neutral Solar Wind Generated by Lunar Exospheric Dust at the Terminator

We calculate the flux of neutral solar wind observed on the lunar surface at the terminator due to solar wind protons penetrating exospheric dust with: (1) grains larger that 0.1 microns and (2) grains larger than 0.01 microns. For grains larger than 0.1 microns, the ratio of the neutral solar wind to solar wind flux is estimated to be approx.10(exp -4)-10(exp -3) at solar wind speeds in excess of 800 km/s, but much lower (less than 10(exp -5) at average to low solar wind speeds. However, when the smaller grain sizes are considered, the ratio of the neutral solar wind flux to solar wind flux is estimated to be greater than or equal to 10(exp -5) at all speeds and at speeds in excess of 700 km/s reaches 10(exp -3)-10(exp -2). These neutral solar wind fluxes are easily measurable with current low energy neutral atom instrumentation. Observations of neutral solar wind from the surface of the Moon could provide a very sensitive determination of the distribution of very small dust grains in the lunar exosphere and would provide data complementary to optical measurements at ultraviolet and visible wavelengths. Furthermore, neutral solar wind, unlike its ionized counterpart, is .not held-off by magnetic anomalies, and may contribute to greater space weathering than expected in certain lunar locations.

Collier, Michael R.↗
DOE OSTI2016

One-dimensional nanoclustering of the Cu(100) surface under CO gas in the mbar pressure range

The bulk terminated Cu(100) surface becomes unstable in the presence of CO at room temperature when the pressure reaches the mbar range. Scanning tunneling microscopy images show that above 0.25 mbar the surface forms nanoclusters with CO attached to peripheral Cu atoms. At 20 mbar and above 3-atom wide one-dimensional nanoclusters parallel to < 001 > directions cover the surface, with CO on every Cu atom, increasing in density up to 115 mbar. Density functional theory explains the findings as a result of the detachment of Cu atoms from step edges caused by the stronger binding of CO relative to that on flat terraces.

Chemistry↗
DOE OSTI2022

Verification of stability and unraveling the electronic and physical properties of bulk and (001)-surfaces of newly synthesized Ti 2 ZnX (X = C, N) MAX phases

MAX phase family has been extended by the addition of late transition metals at the A-site with the expectation of diverse functional properties. Here, we present our systematic density functional investigation on the thermodynamic and phonon stabilities, elastic properties, including elastic constants, elastic moduli and elastic anisotropy of newly synthesized Ti 2 ZnX (X = C, N) phases in comparison with conventional Ti 2 AlX (X = C, N). Due to the smaller size of N as compared to C, the unit cell dimension is reduced when C atoms are replaced by N atoms at the X-site. Furthermore, the Ti 2 ZnC and Ti 2 ZnN are stable at the equilibrium volume of 110.84 Å 3 and 105.70 Å 3 . The thermodynamic, mechanical and dynamical stabilities are validated by estimating the formation energies, elastic constants and phonon dispersions, respectively. The elastic properties of Ti 2 ZnN are less anisotropic as compared to those of Ti 2 ZnC. To understand the thin-film characteristics in Ti 2 ZnX, the surface properties with (001)-terminated slabs are investigated. Both Ti 2 ZnX bulk and (001)-surfaces exhibit metal-like electronic structures. There is a strong covalent bonding between Ti-X and Ti-Zn atoms confirmed by the charge density map and Mulliken population analysis. Additional states are generated at the Fermi level (EF) due to the unusual d-p states hybridization between Ti and Zn atoms. The anisotropy in chemical bonding is confirmed by the cleavage energy difference between Ti-X and Ti-Zn. Here, Ti(X)-001 and Zn-001 terminations are stable surfaces; however, in terms of chemical potentials, Zn-001 termination is the most favourable in Ti 2 ZnX.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Low-index surface energies, cleavage energies, and surface relaxations for crystalline NiAl from first-principles calculations

NiAl surfaces frequently serve as a platform for studying a broad range of physical and chemical phenomena including chemisorption, catalysis, oxidation, alloy growth, and surface nanostructure formation. Knowledge of precise values for low-index surface energies of NiAl, the most fundamental quantities characterizing surface thermodynamics, is often invaluable for understanding of these phenomena. In all previous analyses for NiAl(100) and NiAl(111), Ni- and Al-terminations are not distinguished, and half of the cleavage energy (or equivalently, the average of the surface energies of two differently terminated surfaces) is always identified as the “surface energy”. No values are available for individual surface energies of Ni- or Al-terminated NiAl(100) or NiAl(111) surfaces, whereas knowledge of only cleavage energy is often insufficient for analyzing surface-associated behavior. As such, in this work we perform extensive first-principles density-functional-theory (DFT) calculations for surface energies and cleavage energies of NiAl(110), NiAl(100) and NiAl(111) by considering the chemical-potential-based formulations to clarify the ambiguity in their surface energies and cleavage energies. We obtain a surface-energy phase diagram for these three low-index surfaces versus the relevant chemical potential, as well as the chemical-potential-dependent Wulff plots for NiAl crystal equilibrium shapes. We also provide the surface-relaxation information from our DFT calculations for comparison with previous experimental data.

42 ENGINEERING↗
DOE OSTI2021

Engineering of Exciton Spatial Distribution in CdS Nanoplatelets

Zinc-blende CdS nanoplatelets with atomically flat and very large {100} basal planes terminated solely by one type of element (either Cd or S atoms) are synthesized. Optical spectroscopy, X-ray diffraction, X-ray absorption, and transmission electron microscopy confirm that the surface structures of newly developed S-terminated CdS nanoplatelets are at least as well-defined as the original Cd-terminated nanoplatelets. Band gaps of the nanoplatelets are found to depend on not only the quantum-confined dimension (thickness) but also the nature of the surface termination. The facet structure dictates the packing of the ligands (carboxylate for Cd-terminated nanoplatelets and alkyl for S-terminated nanoplatelets), which causes a difference in the lattice strain and significantly affects the optical spectral width. Experimental and theoretical results reveal that engineering the exciton spatial distribution by the tailored synthesis of semiconductor nanocrystals with a precisely controlled surface structure is fully possible, which should open a new door for delivering the long-promised potential of semiconductor nanocrystals.

Cadmium sulfide↗
NASA NTRS2010

Micro-LiDAR velocity, temperature, density, concentration sensor

A light scatter sensor includes a sensor body in which are positioned a plurality of optical fibers. The sensor body includes a surface, in one end of each of the optical fibers terminates at the surface of the sensor body. One of the optical fibers is an illumination fiber for emitting light. A plurality of second optical fibers are collection fibers for collecting scattered light signals. A light sensor processor is connected to the collection fibers to detect the scattered light signals.

Danehy, Paul M.↗
DOE OSTI2024

Direct evidence of low work function on SrVO 3 cathode using thermionic electron emission microscopy and high-field ultraviolet photoemission spectroscopy

Perovskite SrVO 3 has recently been proposed as a novel electron emission cathode material. Density functional theory (DFT) calculations suggest multiple low work function surfaces, and recent experimental efforts have consistently demonstrated effective work functions of ~2.7 eV for polycrystalline samples, both results suggesting, but not directly confirming, that some fraction of even lower work function surface is present. In this work, thermionic electron emission microscopy (ThEEM) and high-field ultraviolet photoemission spectroscopy (UPS) are used to study the local work function distribution and measure the work function of a partially oriented- (110)-SrVO 3 perovskite oxide cathode surface. Our results show direct evidence of low work function patches of about 2.0 eV on the cathode surface, with a corresponding onset of observable thermionic emission at 750 °C. We hypothesize that, in our ThEEM and UPS experiments, the high applied electric field suppresses the patch field effect, enabling the direct measurement of local work functions. This measured work function of 2.0 eV is comparable to the previous DFT-calculated work function values of the SrVO-terminated (110) SrVO 3 surface (2.3 eV) and SrO-terminated (100) surface (1.9 eV). The measured 2.0 eV value is also much lower than the work function for the (001) LaB 6 single crystal cathode (~2.7 eV) and comparable to the effective work function of B-type dispenser cathodes (~2.1 eV). If SrVO 3 thermionic emitters can be engineered to access domains of this low 2.0 eV work function, they have the potential to significantly improve thermionic emitter-based technologies.

36 MATERIALS SCIENCE↗
NASA NTRS2019

Measurement and Analysis of Terminal Shock Oscillation and Buffet Forcing Functions on a Launch Vehicle Payload Fairing

The buffet loads on a launch vehicle payload shroud can be impacted by the unsteadiness associated with a terminal shock at high subsonic speeds. At these conditions, flow accelerates to supersonic speeds on the nose of the payload fairing and is terminated by a normal shock on the cylindrical section downstream of the nose cone/cylinder shoulder. The location of the terminal shock and associated separated boundary layer is affected by the freestream Mach number, Reynolds number, and the pitch/yaw of the launch vehicle. Furthermore, even when the freestream conditions and vehicle attitude are constant, this terminal shock oscillates on the surface of the vehicle. The time-varying surface pressure associated with the terminal shock results in unsteady aerodynamic loads that may interact with vehicle structural dynamic modes and the guidance and control of the vehicle. Buffet testing of a 3-percent scale rigid buffet model of a launch vehicle cargo configuration with a tangent-ogive payload shroud was conducted in 2012 and in 2016. Initial buffet forcing functions (BFFs) utilized a coarse pressure sensor distribution on the vehicle surface in which a single longitudinal station with eight sensors observed the terminal shock environment at Mach 0.90. An examination of these circumferential pressures reveal large impulse-like pressure fluctuations and an asymmetry in pressure when the vehicle is at a nonzeroangle of attack that result in high BFFs. Revisions to the shock integration region were made based on computational fluid dynamics and shadowgraph video of shock motion to better represent the BFFs and reduce the high loads resulting from this environment. To more clearly understand this terminal shock environment, a second wind tunnel test was conducted with a dense distribution of 256 sensors at the terminal shock location. These sensor arrays presents a unique opportunity to observe the unsteady terminal shock environment and to characterize the impact of various integration schemes on the BFFs. This paper presents a summary of the development of BFFs for this terminal shock and a detailed analyses of shock region pressure coefficients, coherence, BFFs, shock location time histories, and power spectral density to help guide development of BFFs for other launch vehicle test and analysis programs.

Piatak, David J.↗
DOE OSTI2016

Adsorption and dissociation of molecular hydrogen on orthorhombic β-Mo 2 C and cubic δ-MoC (001) surfaces

Molybdenum carbides are increasingly used in heterogeneously catalyzed hydrogenation reactions, which imply the adsorption and dissociation of molecular hydrogen. In this paper, a systematic density functional theory based study, including or excluding dispersion terms, concerning the interaction and stability of H 2 with cubic δ-MoC(001) and orthorhombic β-Mo 2 C(001) surfaces, is presented. In the latter case the two possible C or Mo terminations are considered. In addition, different situations for the H covered surfaces are examined. Computational results including dispersive forces predict an essentially spontaneous dissociation of H 2 on β-Mo 2 C(001) independently of the surface termination, whereas on δ-MoC(001) molecular hydrogen dissociation implies a small but noticeable energy barrier. Furthermore, the ab initio thermodynamics formalism has been used to compare the stability of different H coverages. In conclusion, core level binding energies and vibrational frequencies are presented with the aim to assist the interpretation of yet unavailable data from X-ray photoelectron and infrared spectroscopies.

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

Atomic-Scale Understanding of Catalyst Activation: Carboxylic Acid Solutions, but Not the Acid Itself, Increase the Reactivity of Anatase (001) Faceted Nanocatalysts

Our ability to predict nanocatalyst reactivity has been hindered by our lack of atomic-scale understanding of nanocatalyst surface structure. Do nanocatalyst surfaces adopt a bulk-terminated structure or do they reconstruct to minimize their free energy, thereby lowering their reactivity as often observed in vacuum? Similarly, do nanocatalysts processed at high temperatures maintain their low reactivity, reconstructed surfaces when used at low temperatures? Using a new technique for the preparation of anatase nanocatalysts suitable for atomic-scale imaging and surface spectroscopy, we show that solution-prepared anatase is terminated by a monolayer of fluorine, which acts as an atomic-scale oleophobic coating, preventing the accumulation of adventitious carbon. We further show that the most common TiO 2 functionalization chemistry, a carboxylic acid solution, causes the spontaneous reorganization of a reconstructed anatase nanocatalyst, leading to a five-fold increase in reactive sites. This reorganization is not observed when carboxylic acids are deposited from the gas phase, suggesting that model experiments in vacuum environments can lead to a nonequilibrium, kinetically trapped state that may not be catalytically relevant. Aqueous carboxylic acid solutions produce densely packed carboxylate monolayers with richer adsorption geometries than previously predicted. Ab initio simulations show that although the carboxylate termination is somewhat less effective at removing surface stress than the reconstruction, it is more effective in lowering the surface energy. This observation suggests that bulk-terminated metal-oxide nanocrystals may be common in reactive environments, even if high temperatures are used to process the nanocatalyst or if the reactant is later rinsed off. As such, the assumption of a bulk-terminated surface may be a reasonable starting point for “materials-by-design” approaches to computationally engineered nanocatalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1976

A program of data synthesis from the ALSEP/CPLEE ALSEP/SIDE, and Explorer 35 magnetometer to investigate lunar terminator and nightside particle fluxes and surface interactions

Lunar nightside electron fluxes were studied with the aid of the ALSEP/CPLEE and other instruments. The flux events were shown to be due to (a) electrons propagating upstream from the earth's bow shock, (b) electrons thermalized and scattered to the lunar surface by disturbances along the boundary of the lunar solarwind cavity, and (c) solar wind electrons scattered to the lunar surface by lunar limb shocks and/or compressional disturbances. These electrons were identified as a cause of the high night surface negative potentials observed in tha ALSEP/SIDE ion data. A study was also made of the shadowing of magnetotail plasma sheet electrons by interactions between the lunar body and the ambient magnetic field and by interactions between charged particles and lunar remnant magnetic fields. These shadowing effects were shown to modify lunar surface and near-lunar potential distributions.

Reasoner, D. L.↗
NASA NTRS1997

Carbon Nanotubules: Building Blocks for Nanometer-Scale Engineering

Proximal probe technology has provided researchers with new ways to investigate and manipulate matter on the nanometer scale. We have studied, through molecular dynamics simulations, using a many-body empirical potential, the indentation of a hydrogen-terminated, diamond (111 ) surface, with a proximal probe tip that consists of an open, hydrogen-terminated, (10,10) carbon nanotubule. The simulations showed that upon indenting 1.8 A, the tubule deforms but returns to its original shape upon retraction. The Young's modulus of the tubule was determined using the predicted Euler buckling force and was found to be comparable to measured and calculated values. In a second series of simulations, an open (10, 10) nanotubule was heated to 4500 K and allowed to close. We find that at this temperature the resulting cap contains numerous imperfections, including some not mentioned previously in the literature.

Sinnott, Susan B.↗