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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 1,459 records · Page 81
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Effects of strain-dependent surface stress on the adhesive contact of a rigid sphere to a compliant substrate

Recent experiments have reported that the surface stress of soft elastic solids can increase rapidly with surface strain. For example, when a small hard sphere in adhesive contact with a soft silicone gel is slowly retracted from its rest position, it was found that the retraction force versus displacement relation cannot be explained either by the Johnson–Kendall–Roberts (JKR) theory or a recent indentation theory based on an isotropic surface stress that is independent of surface strain. In this paper, we address this problem using a finite element method to simulate the retraction process. Our numerical model does not have the restrictions of the aforementioned theories; that is, it can handle large nonlinear elastic deformation as well as a surface-strain-dependent surface stress. Our simulation is in good agreement with experimental force versus displacement data with no fitting parameters. Therefore, our results lend further support to the claim that significant strain-dependent surface stresses can occur in simple soft elastic gels. However, significant challenges remain in the reconciliation of theory and experiments, particularly regarding the geometry of the contact and substrate deformation.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Mesoscopic analyses of the impact of morphology and operating conditions on the transport resistances in a proton-exchange-membrane fuel-cell catalyst layer

Exploring the origins of local transport resistance and characterizing the oxygen transport resistances in the catalyst layer (RCL) are critical for cost reduction. In this paper, a comprehensive mesoscopic model for simulating coupled transport processes of oxygen and water vapor for different structural parameters under different operating conditions in reconstructed microstructures is proposed. The local transport resistance is calculated after achieving the limiting current density and the effective diffusivity of oxygen. The results demonstrate that RCL increases greatly with decreasing platinum loading (LPt) and the transport resistances in other components of the cell dominate for high-loadings. Both the reduced oxygen permeation coefficient in the ionomer thin-film and the adsorption resistance account for the origins of local transport resistance. The local transport resistance increases with the bare carbon ratio for a constant LPt and Pt/C ratio due to the decreased effective ionomer surface, and increases with the I/C ratio due to the increased ionomer thickness and decreased Knudsen diffusivity. Due to the presence of liquid water, a slight decrease followed by an increase of the local transport resistance versus relative humidity is obtained. The contribution of ionomer thin-films to RCL is more sensitive to liquid saturation compared with that of pores. This journal is

Mu, YT↗
DOE OSTI2023

Titanium dioxide and table sugar enhance the leaching of silver out of nanosilver packaging

We manufactured laboratory-scale food packages containing 2.57 ± 0.18 × 10 –3 wt% silver nanoparticles (AgNPs) and used them to show that table sugar (sucrose) and microcrystalline titanium dioxide (μTiO 2 ) enhance Ag migration from these packages and into aqueous food simulants. Ag migration into purified water was detected but was below the limit of ICP-MS quantitation, giving a range of potential Ag migration between 0.059 and 0.082 ng cm –2 packaging surface area. Ag migration into 9 wt% aqueous sucrose solution was 0.547 ± 0.084 ng cm –2 and migration into 9 wt% sucrose solution containing 0.01 wt% μTiO 2 was 0.724 ± 0.032 ng cm –2 . Total Ag migration into a 0.01 wt% μTiO 2 aqueous dispersion without sucrose was between 0.122 and 0.162 ng cm –2 , with upper and lower limits defined by the detectability of Ag in the supernatant phase of the simulant. If the midpoint of this range is taken as a baseline, these results imply that, compared to purified water, Ag migration was increased by approximately 10.3 times when the water simulant contained μTiO 2 and sucrose at commercially-relevant concentrations. Notably, the Ag migrated into water containing both ingredients exceeded the total Ag migrated into either of the single-ingredient simulants, pointing to a potential cooperative relationship between sucrose and μTiO 2 that possibly derives from binding and redox interactions between these two ingredients. Sucrose and μTiO 2 also both reduced a portion of migrated Ag + back into AgNPs, and μTiO 2 particles efficiently captured (>25% by mass) migrated Ag on their surfaces. Similar effects on migration were observed with nanocrystalline TiO 2 . Furthermore, these experiments are the first to show that TiO 2 particles exert a strong influence on the quantity and form of Ag that could migrate from AgNP-enabled packaging, suggesting that food formulations and interactions between individual food components may be important to consider when evaluating the fate of nanoparticles in these consumer applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Survey of Surveys: I. The largest compilation of radial velocities for the Galaxy

In the present-day panorama of large spectroscopic surveys, the amount, diversity, and complexity of the available data continuously increase. The overarching goal of studying the formation and evolution of our Galaxy is hampered by the heterogeneity of instruments, selection functions, analysis methods, and measured quantities. Here, we present a comprehensive catalogue, the Survey of Surveys (SoS), built by homogeneously merging the radial velocity (RV) determinations of the largest ground-based spectroscopic surveys to date, such as APOGEE, GALAH, Gaia-ESO, RAVE, and LAMOST, using Gaia as a reference. This pilot study serves to prove the concept and to test the methodology that we plan to apply in the future to the stellar parameters and abundance ratios as well. We have devised a multi-staged procedure that includes: (i) the cross match between Gaia and the spectroscopic surveys using the official Gaia cross-match algorithm, (ii) the normalisation of uncertainties using repeated measurements or the three-cornered hat method, (iii) the cross calibration of the RVs as a function of the main parameters on which depend (magnitude, effective temperature, surface gravity, metallicity, and signal-to-noise ratio) to remove trends and zero point offsets, and (iv) the comparison with external high-resolution samples, such as the Gaia RV standards and the Geneva-Copenhagen survey, to validate the homogenisation procedure and to calibrate the RV zero-point of the SoS catalogue. We provide the largest homogenised RV catalogue to date, containing almost 11 million stars, of which about half come exclusively from Gaia and half in combination with the ground-based surveys. We estimate the accuracy of the RV zero-point to be about 0.16–0.31 km s –1 and the RV precision to be in the range 0.05–1.50 km s –1 depending on the type of star and on its survey provenance. We validate the SoS RVs with open clusters from a high resolution homogeneous samples and provide the systemic velocity of 55 individual open clusters. Additionally, we provide median RVs for 532 clusters recently discovered by Gaia data. The SoS is publicly available and ready to be applied to various research projects, such as the study of star clusters, Galactic archaeology, stellar streams, or the characterisation of planet-hosting stars, to name a few. We also plan to include survey updates and more data sources in future versions of the SoS.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Equilibrium compositional distribution in freestanding ternary semiconductor quantum dots: The case of In x Ga 1-x As

Here, we report the findings of a systematic computational study that addresses the effects of surface segregation on the atomic distribution at equilibrium of constituent group-III atoms in freestanding ternary semiconductor In x Ga 1-x As nanocrystals. Our analysis is based on density functional theory calculations in conjunction with Monte Carlo simulations of the freestanding nanocrystals using a DFT-re-parameterized valence force field description of interatomic interactions. We have determined the equilibrium concentration profiles as a function of nanocrystal size (d), composition (x), and temperature (T). The ranges of d, x, and T are explored and demonstrate surface segregation and phase separation that leads to different extents of alloying in the nanocrystal core and in the near-surface regions. We find that formation of core/shell-like quantum dots characterized by an In-deficient core and an In-rich shell with a diffuse interface is favored at equilibrium. The analysis elucidates the relationship between the constituent species distribution in the nanocrystal and the parameters that can be tuned experimentally to design synthesis routes for tailoring the properties of ternary quantum dots.

solar (photovoltaic), charge transport, materials ↗
DOE OSTI2012

ZnO/anthracene based inorganic/organic nanowire heterostructure: Photoresponse and photoluminescence studies

The effects of surface modification of vertically aligned ZnO nanowires (NWs) with anthracene for the improved photocurrent, photoresponse, and UV photoluminescence have been investigated in this work. The formation of ZnO/anthracene based inorganic/organic NWs heterostructure by surface capping with anthracene solution was confirmed from the high resolution transmission electron microscopy and Fourier transport infrared spectroscopy analyses. After the anthracene capping of ZnO NWs, despite an increase in dark current, we obtained a significant improvement in the photocurrent and photoresponsivity. A sixfold improvement in the UV photocurrent-to-dark current ratio is obtained with capping. Compared to the uncapped NWs, the photoresponse is significantly faster for the ZnO/anthracene system with response and reset times of 1.5 and 1.6 s, respectively. The room temperature photoluminescence spectra show threefold enhanced UV emission with large enhancement in the ratio of UV to green emission intensities. The faster photoresponse and enhanced photocurrent from the ZnO/anthracene heterostructure are explained on the basis of modification of surface defects and interfacial charge transfer process.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Performance enhancement of organic light-emitting diodes by chlorine plasma treatment of indium tin oxide

The characteristics of green phosphorescent organic light-emitting diodes (OLEDs) fabricated on ITO/glass substrates pretreated with low-energy O{sub 2} and Cl{sub 2} plasma were compared. At 20 mA/cm{sup 2}, the OLEDs with O{sub 2} and Cl{sub 2} plasma-treated indium tin oxide (ITO) had voltages of 9.6 and 7.6 eV, and brightness of 9580 and 12380 cd/m{sup 2}, respectively. At {approx}10{sup 4} cd/m{sup 2}, the latter had a 30% higher external quantum efficiency and a 74% higher power efficiency. Photoelectron spectroscopies revealed that Cl{sub 2} plasma treatment created stable In-Cl bonds and raised the work function of ITO by up to 0.9 eV. These results suggest that the better energy level alignment at the chlorinated ITO/organic interface enhances hole injection, leading to more efficient and more reliable operation of the OLEDs. The developed plasma chlorination process is very effective for surface modification of ITO and compatible with the fabrication of various organic electronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Growth, steady-state, and time-resolved photoluminescence study of CdTe/MgCdTe double heterostructures on InSb substrates using molecular beam epitaxy

CdTe/MgCdTe double heterostructures (DHs) are grown on InSb substrates using molecular beam epitaxy and reveal strong photoluminescence with over double the intensity of a GaAs/AlGaAs DH with an identical layer structure design grown on GaAs. Time-resolved photoluminescence of the CdTe/MgCdTe DH gives a Shockley-Read-Hall recombination lifetime of 86 ns, which is more than one order of magnitude longer than that of typical polycrystalline CdTe films. These findings indicate that monocrystalline CdTe/MgCdTe DHs effectively reduce surface recombination, have limited nonradiative interface recombination, and are promising for solar cells that could reach power conversion efficiencies similar to that of GaAs.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Plasma emission spectroscopy for operating and developing the Spallation Neutron Source (SNS) H{sup −} ion sources

A RF-driven, Cs-enhanced H{sup −} ion source feeds the SNS accelerator with a high current (typically >50 mA), ∼1.0 ms pulsed beam at 60 Hz. To achieve the persistent high current beam for several weeks long service cycles, each newly installed ion source undergoes a rigorous conditioning and cesiation processes. Plasma conditioning outgases the system and sputter-cleans the ion conversion surfaces. A cesiation process immediately following the plasma conditioning releases Cs to provide coverage on the ion conversion surfaces. The effectiveness of the ion source conditioning and cesiation is monitored with plasma emission spectroscopy using a high-sensitivity optical spectrometer. Plasma emission spectroscopy is also used to provide a means for diagnosing and confirming a failure of the insulating coating of the ion source RF antenna which is immersed in the plasma. Emissions of composition elements of the antenna coating material, Na emission being the most significant, drastically elevate to signal a failure when it happens. Plasma spectra of the developmental ion source with an AlN (aluminum nitrite) chamber and an external RF antenna are also briefly discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Polymer escape from a confining potential

The rate of escape of polymers from a two-dimensionally confining potential well has been evaluated using self-avoiding as well as ideal chain representations of varying length, up to 80 beads. Long timescale Langevin trajectories were calculated using the path integral hyperdynamics method to evaluate the escape rate. A minimum is found in the rate for self-avoiding polymers of intermediate length while the escape rate decreases monotonically with polymer length for ideal polymers. The increase in the rate for long, self-avoiding polymers is ascribed to crowding in the potential well which reduces the free energy escape barrier. An effective potential curve obtained using the centroid as an independent variable was evaluated by thermodynamic averaging and Kramers rate theory then applied to estimate the escape rate. While the qualitative features are well reproduced by this approach, it significantly overestimates the rate, especially for the longer polymers. The reason for this is illustrated by constructing a two-dimensional effective energy surface using the radius of gyration as well as the centroid as controlled variables. This shows that the description of a transition state dividing surface using only the centroid fails to confine the system to the region corresponding to the free energy barrier and this problem becomes more pronounced the longer the polymer is. A proper definition of a transition state for polymer escape needs to take into account the shape as well as the location of the polymer.

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

Band alignment of HfO{sub 2}/Al{sub 0.25}Ga{sub 0.75}N determined by x-ray photoelectron spectroscopy: Effect of SiH{sub 4} surface treatment

The band-alignment of atomic layer deposited (ALD)-HfO{sub 2}/Al{sub 0.25}Ga{sub 0.75}N was studied by high resolution x-ray photoelectron spectroscopy measurements for both the non-passivated and SiH{sub 4} passivated AlGaN surfaces. The valence band offset and the conduction band offset for the ALD-HfO{sub 2}/Al{sub 0.25}Ga{sub 0.75}N interface were found to be 0.43 eV and 1.47 eV, respectively, for the non-passivated sample, and 0.59 eV and 1.31 eV, respectively, for the SiH{sub 4}-passivated sample. The difference in the band alignment is dominated by the band bending or band shift in the AlGaN substrate as a result of the different interlayers formed by the two surface preparations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Dissociation and recombination of D{sub 2} on Cu(111): Ab initio molecular dynamics calculations and improved analysis of desorption experiments

Obtaining quantitative agreement between theory and experiment for dissociative adsorption of hydrogen on and associative desorption of hydrogen from Cu(111) remains challenging. Particularly troubling is the fact that theory gives values for the high energy limit to the dissociative adsorption probability that is as much as two times larger than experiment. In the present work we approach this discrepancy in three ways. First, we carry out a new analysis of the raw experimental data for D{sub 2} associatively desorbing from Cu(111). We also perform new ab initio molecular dynamics (AIMD) calculations that include effects of surface atom motion. Finally, we simulate time-of-flight (TOF) spectra from the theoretical reaction probability curves and we directly compare them to the raw experimental data. The results show that the use of more flexible functional forms for fitting the raw TOF spectra gives fits that are in slightly better agreement with the raw data and in considerably better agreement with theory, even though the theoretical reaction probabilities still achieve higher values at high energies. The mean absolute error (MAE) for the energy E{sub 0} at which the reaction probability equals half the experimental saturation value is now lower than 1 kcal/mol, the limit that defines chemical accuracy, while a MAE of 1.5 kcal/mol was previously obtained. The new AIMD results are only slightly different from the previous static surface results and in slightly better agreement with experiment.

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

Formation of unique trimer of nitric oxide on Cu(111)

We report that NO molecules unexpectedly prefer a trimeric configuration on Cu(111). We used scanning tunneling microscopy (STM) at 6 K, and confirmed that the NO molecule is bonded to the face-centered-cubic hollow site in an upright configuration. The individual NO molecule is imaged as a ring protrusion, which is characteristic of the doubly degenerate 2π{sup *} orbital. A triangular trimer is thermodynamically more favorable than the monomer and dimer, and its bonding structure was characterized by STM manipulation. This unique behavior of NO on Cu(111) is ascribed to the threefold symmetry of the surface, facilitating effective mixing of the 2π{sup *} orbitals in a triangular configuration.

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

Nanoparticles and amyloid systems: A fatal encounter?

Nanoparticles (NPs) are used in many products of our daily life, however, there has been concern that they may also be harmful to human health. Recently NPs have been found to accelerate the fibrillation kinetics of amyloid systems. In the past this has been preliminarily attributed to a nucleation effect. Nanoparticle surfaces and interfaces appear to limit the degrees of freedom of amyloid systems (i.e., peptides and proteins) due to a phase space constraint such that rapid cross-beta structures are formed faster than without interface interactions and in turn fibril formation is enhanced significantly. Here we explore if lipid bilayers in the form of liposomes (140nm) also accelerate fibril formation for amyloid systems. We have investigated a fragment NNFGAIL of the Human islet amyloid polypeptide (hIAPP) in contact with 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) liposomes in aqueous solution. We found that the lipid bilayer vesicles do accelerate fibril formation in time-resolved off-line detected atomic force microscopy experiments. Characteristic Thioflavine-T fluorescence on the same structures verify that the structures consist of aggregated peptides in a typical cross-β-structure arrangement.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Transition Helmholtz free energy, entropy, and heat capacity of free-standing smectic films in water: A mean-field treatment

Using the extended McMillan's mean field approach with anisotropic forces a study of both the structural and thermodynamic properties of free-standing smectic film (FSSF) in water on heating to the isotropic temperature is carried out numerically. By solving the self-consistent nonlinear equations for the order parameters, we obtained that the smectic-A-isotropic (AI) transition occurs through the series of layer-thinning transitions causing the films to thin in the stepwise manner as the temperature is increased above the bulk smectic-A-isotropic temperature T{sub AI}(bulk). With enhanced pair interactions in the bounding layers, the smectic-isotropic transition corresponds to smectic melting of the central layers. The effects of surface “enhanced” pair interactions in the bounding layers and of film thickness on the orientational and translational order parameters, the Helmholtz free energy and entropy, as well as the temperature dependence of the heat capacity of FSSFs, have also been investigated. Reasonable agreement between the theoretically predicted and the experimentally obtained – by means of optical microscopy and ellipsometry techniques – data of the temperature when the thin decylcyanobiphenyl smectic film immersing in water ruptures has been obtained.

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

Si concentration dependence of structural inhomogeneities in Si-doped Al{sub x}Ga{sub 1−x}N/Al{sub y}Ga{sub 1−y}N multiple quantum well structures (x = 0.6) and its relationship with internal quantum efficiency

We investigated the distribution of luminescence in Si-doped Al{sub x}Ga{sub 1−x}N/Al{sub y}Ga{sub 1−y}N multiple quantum well (MQW) structures (x = 0.6) with different Si concentrations by cathodoluminescence (CL) mapping combined with scanning electron microscopy. The effects of surface morphology, dark spot density, and full width at half-maximum of spot CL spectra on internal quantum efficiency (IQE) were determined. A flat surface morphology and uniform CL map were observed for Si-doped AlGaN MQWs, in contrast to undoped AlGaN MQW and Si-doped AlGaN with relatively low Al content. The dark spot density in the Si-doped AlGaN MQWs increased exponentially as the Si concentration increased and did not explain the Si concentration dependence of IQE. In contrast, there was a clear correlation between the dark spot density and IQE of the AlGaN MQWs at a constant Si concentration. The emission energy distribution arising from the inhomogeneity of the relative Al content and the well layer thickness was estimated by monochromatic CL measurements, although there was almost no difference in the distribution for different Si concentrations. Therefore, the previously reported dependence of the defect complexes on Si concentration is reflected in the IQE of Si-doped AlGaN MQWs. Defect complexes composed of cation vacancies and impurities rather than dislocations and interfacial quality are the major contributor to the IQE of the Si-doped AlGaN MQWs with different Si concentrations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Effect of adatom deposition on surface magnetism and exchange coupling parameter in (0001) SmCo{sub 5} slabs

First principles calculations are performed on 3d-transition metal atom deposited (0001) surface of SmCo{sub 5} to understand the magnetic properties and the improvement of Curie temperature (T{sub c}). Various atomic sites are examined to identify the energetically feasible adsorption of adatom and it is found that the void site of Co-rich (0001) SmCo{sub 5} surface is the most favourable one to deposit. The surface magnetic moments of various adatom deposited SmCo{sub 5} surfaces are larger than the clean surface except for Cu and Zn. Eventually, the surface exchange coupling of clean and adatom deposited surface is found to increase for Mn, Fe, Co, Ni, and Cu deposited surfaces and this improvement results in the increase in T{sub c} of SmCo{sub 5} slab.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗