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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 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Enhancement of Eye Tracking for Highly Dynamic Displays

Eye-Tracking and Sandia have established a collaborative arrangement whereby the two entities worked in a no-funds arrangement to study and share information about interface, software and algorithmic capabilities for dynamic, user-driven workflows. Sandia researchers developed the GazeAppraise algorithm for analysis of so-called “smooth pursuit” eye tracking datasets and used Eye Tracking, Inc’s experimental tasks and data to evaluate algorithmic performance. Although the results were quite positive, the partners ultimately decided it did not fit Eye Tracking’s market niche, and Sandia is pursuing opensource alternatives for disseminating the Gaze Appraise algorithms.

McNamara, Laura A.↗
DOE OSTI2023

Solvent effects determine the sign of the charges of maximum entropy and capacitance at silver electrodes

Fully harnessing electrochemical interfaces for reactions requires a detailed understanding of solvent effects in the electrochemical double layer. Predicting the significant impact of solvents on entropic and electronic properties of electrochemical interfaces has remained an open challenge of computational electrochemistry. Using molecular dynamics simulations of silver–water and silver–acetonitrile interfaces, we show that switching the solvent changes the signs for both the charge of maximum capacitance (CMC) and charge of maximum entropy (CME). Contrasting the capacitance and CME behavior of these two interfaces, we demonstrate that the preferred orientation of the solvent molecule and the corresponding charge density determine the sign of the CMC and CME and, hence, the qualitatively different charge asymmetry of the electrochemical interface.

Chemistry↗
DOE OSTI2025

dynamics of organic-mineral interactions at the metal oxide-solution interface as studied via binding energetics (Final report)

This project focused on addressing longstanding fundamental and experimental uncertainties on how dissolved organic substances (DOS) interact with metal oxide surface under environmentally relevant conditions. By leveraging a custom-built real-time, in-tandem flow adsorption microcalorimetry-UV-Vis/fluorescence spectroscopy platform, we characterized the binding energetics, kinetics and mechanistic pathways driving DOS-metal oxide interactions at temporal resolution on the order of 1-5 seconds. We studied a diverse suite of model organic compounds/substances – including monocarboxylates (e.g. acetate and benzoate), di-carboxylates (oxalate and succinate), amino acids, amino-based nanparticles and natural organic matter – interacting at the mineral-water interface of structurally- and/or chemically distinct metal oxides (including SiO2, boehmite, ferrihydrite, and γ-Al2O3). Our results indicated that DOS-metal oxide interactions are governed by multi-step reaction pathways, often switching between distinct, resolvable enthalpy- and entropy-driven non-electrostatic or electrostatic configurations. To quantify these interactions, we developed and implemented an analytical workflow that integrates peak deconvolution and Monte-Carlo based error propagation to determine site-specific thermodynamic and kinetic parameters for individual binding/debinding events. In addition to resolving apparent first-order rate constants of each event, we were able to quantify associated apparent equilibrium constants as well as free energy, enthalpy and entropy contribution to the activation and subsequent progression of the binding/debinding process across compounds, compound class and metal oxide surfaces. The kinetic-thermodynamic data produced in this study captured how the interplay between oxide surface reactivity and DOS molecular structure jointly drives binding-debinding dynamics. Notably, that at pH below PZC of the oxide surface, neutral species were heavily involved in monocarboxylate binding, while anionic species drove dicarboxylate binding. Also, that among amino acids 1) positional isomers show distinctive binding characteristics to each other while enantiomers show no significant differences in binding characteristics, 2) molecules that bind via outer-sphere complexation show a larger entropic shift between binding and debinding with no impact on oxide surface while 3) inner-sphere interactions increased anion exchange capacity of the oxide surface. The new insights and data from this work has great potential for improving predictive modeling of carbon dynamics and specifically organic-mineral interactions in environmental and industrial systems.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2012

Simulation of film and droplet flow on wide aperture fractures using Smoothed Particle Hydrodynamics

Simulation of flow in fractured porous media represents a challenge due to the highly non-linear dynamics of fluid-air interfaces. Here we present small-scale flow simulations on wide aperture fractures using a modified three-dimensional multiphase SPH model \cite{Tartakovsky2005}. The model is modified to include the effects of random thermal noise. The model is able to reproduce a wide range of wetting conditions and Reynolds numbers encountered in laboratory experiments using pairwise fluid-fluid and solid-fluid interaction forces. Static and transient flow dynamics are compared to empirical and semianalytical solutions: (1) Droplets in a critical state are in agreement with laboratory experiments of \cite{ElSherbini2004}. (2) Well-defined random thermal noise is introduced via the fluctuation-dissipation theorem and its effect on dynamics of droplets in a critical state is investigated. (3) Transient flow dynamics on dry surfaces are validated using the dimensionless relationships established by \cite{Podgorski2001} and compared to (4) dynamics on prewetted surfaces where flow velocities are shown to be nearly tripled. Finally we establish flow regimes and occurrence of trailing films on initially dry fracture surfaces based on dimensionless scaling parameters and Reynolds numbers.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2023

$\mathrm{RADICAL}$-Pilot and $\mathrm{PMIx}$/$\mathrm{PRRTE}$: Executing Heterogeneous Workloads at Large Scale on Partitioned $\mathrm{HPC}$ Resources

Execution of heterogeneous workflows on high-performance computing (HPC) platforms present unprecedented resource management and execution coordination challenges for runtime systems. Task heterogeneity increases the complexity of resource and execution management, limiting the scalability and efficiency of workflow execution. Re-source partitioning and distribution of tasks execution over portioned re-sources promises to address those problems but we lack an experimental evaluation of its performance at scale. Here this paper provides a performance evaluation of the Process Management Interface for Exascale (PMIx) and its reference implementation PRRTE on the leadership-class HPC plat-form Summit, when integrated into a pilot-based runtime system called RADICAL-Pilot. We partition resources across multiple PRRTE Distributed Virtual Machine (DVM) environments, responsible for launching tasks via the PMIx interface. We experimentally measure the work-load execution performance in terms of task scheduling/launching rate and distribution of DVM task placement times, DVM startup and termination overheads on the Summit leadership-class HPC platform. Integrated solution with PMIx/PRRTE enables using an abstracted, standardized set of interfaces for orchestrating the launch process, dynamic process management and monitoring capabilities. It extends scaling capabilities allowing to overcome a limitation of other launching mechanisms (e.g., JSM/LSF). Explored different DVM setup configurations provide insights on DVM performance and a layout to leverage it. Our experimental results show that heterogeneous workload of 65,500 tasks on 2048 nodes, and partitioned across 32 DVMs, runs steady with resource utilization not lower than 52%. While having less concurrently executed tasks resource utilization is able to reach up to 85%, based on results of heterogeneous workload of 8200 tasks on 256 nodes and 2 DVMs.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2025

Chlorine gas and anion radical reactivity in molten salts and the link to chlorobasicity

Next-generation nuclear power plants may include exciting novel designs in which molten salts are the coolant or a combination of the coolant and fuel. Whereas it is straightforward to see why having a low volatility coolant can be advantageous for safety, much is not understood about the production of volatile halogen gases as a result of radiation and even less is known about the distribution of these species at and away from interfaces. Using first principles molecular dynamics simulations, we investigate the product of the disproportionation reaction between chlorine anion radicals (nominally Cl 2 ˙ − ) in the bulk and slab configurations. We find that the product depends on the chlorobasicity of the medium. For example, in ZnCl 2 , Cl 2 forms, but in a eutectic mixture of LiCl and KCl, Cl 3 − is formed as a product. We also find that Cl 3 − prefers to form at the vapor interface and this may have implications for corrosion and reactivity. Furthermore, the mechanisms of the mobility of Cl 2 and Cl 3 − are radically different, the first one being vehicular and the second Grotthus-like. Chlorobasicity is linked to the electronic structure of the host melt; ZnCl 2 forms extended networks along which metal ions and anionic counterions have significant electronic orbital overlap forming long, linear, molecular-like constructs; the opposite is true for the alkali metal eutectic salt.

Nguyen, Hung H. [Univ. of Iowa, Iowa City, IA (Uni↗
DOE OSTI2014

Atomistic modeling and HAADF investigations of misfit and threading dislocations in GaSb/GaAs hetero-structures for applications in high electron mobility transistors

A detailed investigation on the misfit and threading dislocations at GaSb/GaAs interface has been carried out using molecular dynamics simulation and quantitative electron microscopy techniques. The sources and propagation of misfit dislocations have been elucidated. The nature and formation mechanisms of the misfit dislocations as well as the role of Sb on the stability of the Lomer configuration have been explained.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Dynamics of Intact MexAB-OprM Efflux Pump: Focusing on the MexA-OprM Interface

Antibiotic efflux is one of the most critical mechanisms leading to bacterial multidrug resistance. Antibiotics are effluxed out of the bacterial cell by a tripartite efflux pump, a complex machinery comprised of outer membrane, periplasmic adaptor, and inner membrane protein components. Understanding the mechanism of efflux pump assembly and its dynamics could facilitate discovery of novel approaches to counteract antibiotic resistance in bacteria. We built here an intact atomistic model of the Pseudomonas aeruginosa MexAB-OprM pump in a Gram-negative membrane model that contained both inner and outer membranes separated by a periplasmic space. All-atom molecular dynamics (MD) simulations confirm that the fully assembled pump is stable in the microsecond timescale. Using a combination of all-atom and coarse-grained MD simulations and sequence covariation analysis, we characterized the interface between MexA and OprM in the context of the entire efflux pump. These analyses suggest a plausible mechanism by which OprM is activated via opening of its periplasmic aperture through a concerted interaction with MexA.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2019

Effect of Functional and Electron Correlation on the Structure and Spectroscopy of the Al 2 O 3 (001)–H 2 O Interface

Oxide–water interfaces are ubiquitous, with many applications in industry and the environment, yet there is a great deal of controversy over their properties and microscopic structure. This controversy stems, in part, from the unique H-bond networks formed at different surface terminations and mineral compositions. Density functional theory simulations of these interfaces require an accurate description of both the oxide mineral and water in diverse H-bond environments. Thus, herein we simulate the Al 2 O 3 (001)–H 2 O interface using the PBE, PBE-TS, RPBE, SCAN, and HSE06-TS functionals to determine how calculated interfacial properties depend on the choice of functional. We find that the structure of the first few layers of water at the surface is determined by electron correlation in a way that cannot be approximated using semiemipirical van der Waals corrections. Of the functionals investigated, we find that SCAN yields the most accurate interfacial structure, dynamics, and sum frequency generation spectrum. Furthermore, SCAN leads to a reduction in the order of the 2D H-bond network of water at the alumina surface predicted by GGA functionals, leading to a significant decrease in the anisotropy of the diffusion coefficient at the surface. We emphasize the importance of using a functional which accurately describes electron correlation for more complex oxides, such as transition-metal oxides, where electron correlation may play an even greater role in determining the structure and dynamics of the oxide–water interface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Emergence of complex-regular eutectic patterns in Al–Ge: Observations from correlative nano-imaging

Eutectic solidification exemplifies nonequilibrium pattern formation, making it a well-studied moving boundary problem. Yet the mechanisms behind the formation of complex-regular microstructures – particularly in highly anisotropic systems with a significant volume fraction of a faceted phase – remain poorly understood. Our understanding of such systems is made complicated by the nonlinear interface kinetics and unique growth dynamics characteristic of faceted phases. To address these challenges, we investigate a model Al–Ge eutectic system, where the faceted Ge phase constitutes a substantial volume fraction (~0.35) and where the two solid phases arrange into so-called “fishbone” or “feather” complex-regular patterns. Using synchrotron-based x-ray nano-imaging and nanotomography with high spatial resolution (22 nm per pixel), we capture in real-time the evolution of the solid–liquid interfaces and the resulting three-dimensional microstructures in this faceted/non-faceted eutectic system. By integrating these observations with electron backscattered diffraction, we elucidate the crystallographic biases on the solidification process and the mechanisms driving the formation of such complex-regular microstructures. These findings inform a new growth model for irregular eutectics in (near-)symmetrical phase diagrams, offering insight on advanced microstructural design and processing strategies. More broadly, we demonstrate how interfacial curvature is generated in irregular eutectic alloys and how it depends on the volume fraction of the faceted phase.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Transient Nanoscopy of Exciton Dynamics in 2D Transition Metal Dichalcogenides

Abstract The electronic and optical properties of 2D transition metal dichalcogenides are dominated by strong excitonic resonances. Exciton dynamics plays a critical role in the functionality and performance of many miniaturized 2D optoelectronic devices; however, the measurement of nanoscale excitonic behaviors remains challenging. Here, a near‐field transient nanoscopy is reported to probe exciton dynamics beyond the diffraction limit. Exciton recombination and exciton–exciton annihilation processes in monolayer and bilayer MoS 2 are studied as the proof‐of‐concept demonstration. Moreover, with the capability to access local sites, intriguing exciton dynamics near the monolayer‐bilayer interface and at the MoS 2 nano‐wrinkles are resolved. Such nanoscale resolution highlights the potential of this transient nanoscopy for fundamental investigation of exciton physics and further optimization of functional devices.

2D materials↗
DOE OSTI2021

Compositionally graded Ga 1-x In x P buffers grown by static and dynamic hydride vapor phase epitaxy at rates up to 1 μm/min

Here, we demonstrate Ga 1-x In x P compositionally graded buffers (CGBs) grown on GaAs with lattice constants between GaAs and InP by hydride vapor phase epitaxy (HVPE). Growth rates were up to ~1 μm/min and threading dislocation density (TDD) was as low as 1.0 x 10 6 cm -2 . We studied the effect of substrate offcut direction, growth rate, and strain grading rate on CGB defect structure. We compared the effect of a “dynamic grading” style, which creates compositional interfaces via mechanical transfer of a substrate between two growth chambers, vs. “static grading” where the CGB grows in a single chamber. Dynamic grading yielded smoother grades with higher relaxation, but TDD was not significantly different between the two styles. Substrate offcut direction was the most important factor for obtaining CGBs with low defect density. (001) substrates offcut towards (111)B yielded smoother CGBs with lower TDD compared to CGBs grown on substrates offcut towards (111)A. Transmission electron microscopy of static and dynamic CGBs grown on A and B-offcuts only found evidence of phase separation in a static A-offcut CGB, indicating that the B offcut limits phase separation, which in turn keeps TDD low. Reduced growth rate led to the appearance CuPt-type atomic ordering, which affected the distribution of dislocations on the active glide planes but did not alter TDD. Higher growth rates led to smoother CGBs and did not appreciably increase TDD as otherwise predicted by steady-state models of plastic relaxation. These results show HVPE’s promise for lattice-mismatched applications and low-c ost InP virtual substrates on GaAs

36 MATERIALS SCIENCE↗
DOE OSTI2018

Nanoscale view of assisted ion transport across the liquid–liquid interface

During solvent extraction, amphiphilic extractants assist the transport of metal ions across the liquid–liquid interface between an aqueous ionic solution and an organic solvent. Investigations of the role of the interface in ion transport challenge our ability to probe fast molecular processes at liquid–liquid interfaces on nanometer-length scales. Recent development of a thermal switch for solvent extraction has addressed this challenge, which has led to the characterization by X-ray surface scattering of interfacial intermediate states in the extraction process. Here, we review and extend these earlier results. We find that trivalent rare earth ions, Y(III) and Er(III), combine with bis(hexadecyl) phosphoric acid (DHDP) extractants to form inverted bilayer structures at the interface; these appear to be condensed phases of small ion–extractant complexes. The stability of this unconventional interfacial structure is verified by molecular dynamics simulations. The ion–extractant complexes at the interface are an intermediate state in the extraction process, characterizing the moment at which ions have been transported across the aqueous–organic interface, but have not yet been dispersed in the organic phase. In contrast, divalent Sr(II) forms an ion–extractant complex with DHDP that leaves it exposed to the water phase; this result implies that a second process that transports Sr(II) across the interface has yet to be observed. Calculations demonstrate that the budding of reverse micelles formed from interfacial Sr(II) ion–extractant complexes could transport Sr(II) across the interface. Our results suggest a connection between the observed interfacial structures and the extraction mechanism, which ultimately affects the extraction selectivity and kinetics.

Liang, Zhu↗
DOE OSTI2014

Length-scale dependent mechanical properties of Al-Cu eutectic alloy: Molecular dynamics based model and its experimental verification

This paper attempts to gain an understanding of the effect of lamellar length scale on the mechanical properties of two-phase metal-intermetallic eutectic structure. We first develop a molecular dynamics model for the in-situ grown eutectic interface followed by a model of deformation of Al-Al{sub 2}Cu lamellar eutectic. Leveraging the insights obtained from the simulation on the behaviour of dislocations at different length scales of the eutectic, we present and explain the experimental results on Al-Al{sub 2}Cu eutectic with various different lamellar spacing. The physics behind the mechanism is further quantified with help of atomic level energy model for different length scale as well as different strain. An atomic level energy partitioning of the lamellae and the interface regions reveals that the energy of the lamellae core are accumulated more due to dislocations irrespective of the length-scale. Whereas the energy of the interface is accumulated more due to dislocations when the length-scale is smaller, but the trend is reversed when the length-scale is large beyond a critical size of about 80 nm.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Laboratory-developed contact models controlling instability on frictional faults: FORESHOCKS ON FRICTIONAL FAULTS

Laboratory experiments were performed on a polymethyl methacrylate (PMMA)-PMMA frictional interface in a direct shear apparatus in order to gain understanding of fault dynamics leading to gross rupture. Actual asperity sizes and locations along the interface were characterized using a pressure-sensitive film. Slow aseismic slip accumulated nonuniformly along the fault and showed dependency on the applied normal force-increased normal force resulted in higher slip gradients. The slow slip front propagated from the trailing (pushed) edge into a region of more densely distributed asperities at rates between 1 and 9.5 mm/s. Foreshocks were detected and displayed impulsive signals with source radii ranging between 0.21 and 1.09 mm; measurements made using the pressure-sensitive film were between 0.05 and 1.2 mm. The spatiotemporal clustering of foreshocks and their relation to the elastodynamic energy released was dependent on the normal force. In the region where foreshocks occurred, qualitative optical measurements of the asperities along the interface were used to visualize dynamic changes occurring during the slow slip phase. To better understand the nucleation process, a quasi-static asperity finite element (FE) model was developed and focused in the region where foreshocks clustered. The FE model consisted of 172 asperities, located and sized based on pressure-sensitive film measurements. The numerical model provides a plausible explanation as to why foreshocks cluster in space and observed a normal force dependency and lend credence to Ohnaka’s nucleation model.

58 GEOSCIENCES↗
DOE OSTI2018

Amphiphile-Induced Phase Transition of Liquid Crystals at Aqueous Interfaces

We report that monolayer assemblies of amphiphiles at planar interfaces between thermotropic liquid crystals (LCs) and an aqueous phase can give rise to configurational transitions of the underlying LCs. A common assumption has been that a reconfiguration of the LC phase is caused by an interdigitation of the hydrophobic tails of amphiphiles with the molecules of the LC at the interface. A different mechanism is discovered here, whereby reorientation of the LC systems is shown to occur through lowering of the orientation-dependent surface energy of the LC due to formation of a thin isotropic layer at the aqueous interface. Using a combination of atomistic molecular dynamics simulations and experiments, we demonstrate that a monolayer of specific amphiphiles at an aqueous interface can cause a local nematic-to-isotropic phase transition of the LC by disturbing the antiparallel configuration of the LC molecules. Lastly, these results provide new insights into the interfacial, molecular-level organization of LCs that can be exploited for rational design of biological sensors and responsive systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Charge Recombination Dynamics in Sensitized SnO 2 /TiO 2 Core/Shell Photoanodes

Studies have been conducted to examine the mechanisms of charge recombination in dye-sensitized SnO 2 /TiO 2 core/shell films. Nanostructured SnO 2 /TiO 2 core/shell films varying in TiO 2 shell thicknesses were prepared via atomic layer deposition and sensitized with a phosphonate-derivatized ruthenium chromophore [Ru(bpy) 2 (4,4$'$-(PO 3 H 2 ) 2 bpy)] 2+ . Transient absorption spectroscopy was used to study the interfacial charge recombination dynamics for these core/shell materials. Charge recombination for sensitized, as-deposited SnO 2 /TiO 2 core/shell systems is dominated by a tunneling mechanism for shell thicknesses between 0 and 3.2 nm, with $β$ = 0.25 Å–1. For shell thicknesses greater than 3.2 nm, recombination primarily proceeds directly via electrons localized in the relatively thick TiO 2 shell. Annealing the SnO 2 /TiO 2 core/shell structure at 450°C affects the recombination dynamics substantially; charge recombination dynamics for the annealed films do not show a dependence on shell thickness and are comparable to ZrO 2 /TiO 2 control samples, suggesting the annealing process perturbs the core/shell interface. This analysis of charge recombination dynamics indicates that there is an optimum shell thickness to maximize charge separation lifetimes in dye-sensitized core/shell photoanodes and that the nature of the core/shell interface influences the efficacy of these materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗