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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 1,621 records · Page 90
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Gallium surface diffusion on GaAs (001) surfaces measured by crystallization dynamics of Ga droplets

We present accurate measurements of Ga cation surface diffusion on GaAs surfaces. The measurement method relies on atomic force microscopy measurement of the morphology of nano–disks that evolve, under group V supply, from nanoscale group III droplets, earlier deposited on the substrate surface. The dependence of the radius of such nano-droplets on crystallization conditions gives direct access to Ga diffusion length. We found an activation energy for Ga on GaAs(001) diffusion E{sub A}=1.31±0.15 eV, a diffusivity prefactor of D₀=0.53(×2.1±1) cm² s⁻¹ that we compare with the values present in literature. The obtained results permit to better understand the fundamental physics governing the motion of group III ad–atoms on III–V crystal surfaces and the fabrication of designable nanostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

General rules prospected for the liquid fragility in various material groups and different thermodynamic conditions

The fragility parameter has been acknowledged as one of the most important characteristics of glass-forming liquids. We show that the mystery of the dramatic change in molecular dynamics of systems approaching the glass transition can be better understood by the high pressure study of fragility parameters defined in different thermodynamic conditions. We formulate and experimentally confirm a few rules obeyed by the fragility parameters, which are also rationalized by the density scaling law and its modification suggested for associated liquids. In this way, we successfully explore and gain a new insight into the pressure effect on molecular dynamics of van der Waals liquids, polymer melts, ionic liquids, and hydrogen-bonded systems near the glass transition.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1991

Recursive renormalization group theory based subgrid modeling

Advancing the knowledge and understanding of turbulence theory is addressed. Specific problems to be addressed will include studies of subgrid models to understand the effects of unresolved small scale dynamics on the large scale motion which, if successful, might substantially reduce the number of degrees of freedom that need to be computed in turbulence simulation.

Zhou, YE↗
DOE OSTI2020

Atomic defects, functional groups and properties in MXenes

MXenes, a new family of functional two-dimensional (2D) materials, have shown great potential for an extensive variety of applications within the last decade. Atomic defects and functional groups in MXenes are known to have a tremendous influence on the functional properties. In this review, we focus on recent progress in the characterization of atomic defects and functional group chemistry in MXenes, and how to control them to directly influence various properties ( e.g ., electron transport, Li + adsorption, hydrogen evolution reaction (HER) activity, and magnetism) of 2D MXenes materials. Dynamic structural transformations such as oxidation and growth induced by atomic defects in MXenes are also discussed. The review thus provides perspectives on property optimization through atomic defect engineering, and bottom-up synthesis methods based on defect-assisted homoepitaxial growth of MXenes.

36 MATERIALS SCIENCE↗
NASA NTRS2005

Silica Aerogels Doped with Ru(II) Tris 1,l0-Phenanthro1ine)-Electron Acceptor Dyads: Improving the Dynamic Range, Sensitivity and Response Time of Sol-Gel Based Oxygen Sensors

Complexes 1 and 2 were characterized in fluid and frozen solution and as dopants of silica aerogels. The intramolecular quenching efficiency of pendant 4-benzoyl-N-methylpyridinium group (4BzPy) is solvent dependent: emission is quenched completely in acetonitrile but not in alcohols. On the other hand, N-benzyl-N'-methylviologen (BzMeV) quenches the emission in all solvents completely. The differences are traced electrochemically to a stronger solvation effect by the alcohol in the case of 1. In fiozen matrices or absorbed on the surfaces of silica aerogel, both 1 and 2 are photoluminescent. The lack of quenching has been traced to the environmental rigidity. When doped aerogels are cooled to 77K, the emission shifts to the blue and its intensity increases in analogy to what is observed with Ru(II) complexes in media undergoing fluid-to-rigid transition. The photoluminescence of 1 and 2 from the aerogel is quenched by oxygen diffusing through the pores. In the presence of oxygen, aerogels doped with 1 can modulate their emission over a wider dynamic range than aerogels doped with 2, and both are more sensitive than aerogels doped with Ru(II) tris(1,l0- phenanthroline). In contrast to frozen solutions, the luminescent moieties in the bulk of aerogels kept at 77K are still accessible, leading to more sensitive platforms for oxygen sensors than other ambient temperature configurations.

Kevebtusm Bucgikas↗
DOE OSTI2022

Generalized symmetries and Noether’s theorem in $\mathrm{QFT}$

We show that generalized symmetries cannot be charged under a continuous global symmetry having a Noether current. Further, only non-compact generalized symmetries can be charged under a continuous global symmetry. These results follow from a finer classification of twist operators, which naturally extends to finite group global symmetries. They unravel topological obstructions to the strong version of Noether’s theorem in QFT, even if under general conditions a global symmetry can be implemented locally by twist operators (weak version). We use these results to rederive Weinberg-Witten’s theorem within local QFT, generalizing it to massless particles in arbitrary dimensions and representations of the Lorentz group. Several examples with local twists but without Noether currents are described. We end up discussing the conditions for the strong version to hold, dynamical aspects of QFT’s with non-compact generalized symmetries, scale vs conformal invariance in QFT, connections with the Coleman-Mandula theorem and aspects of global symmetries in quantum gravity.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

134 billion intersecting brane models

The landscape of string vacua is very large, but generally expected to be finite in size. Enumerating the number and properties of the vacua is an important task for both the landscape and the swampland, in part to gain a deeper understanding of what is possible and “generic”. We obtain an exact counting of distinct intersecting brane vacua of type IIA string theory on the T 6 /Z 2 × Z 2 orientifold. Care is taken to only count gauge-inequivalent brane configurations. Leveraging the recursive nature by which branes may be added together one-by-one, we use dynamic programming to efficiently count the number of solutions of the tadpole, K-theory and supersymmetry consistency conditions. The distributions of 4D gauge group rank and complex structure moduli for the entire ensemble of intersecting brane vacua are presented. The methods we developed here may be useful in obtaining sharp upper and lower bounds on other corners of the landscape.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Effects of overstory composition and prescribed fire on fuel loading across a heterogeneous managed landscape in the southeastern USA.

In the southeastern USA, land use history, forest management and natural geomorphic features have created heterogeneous fuel loads. This apparent temporal and spatial variation in fuel loads make it difficult to reliably assess potential fire behavior from remotely sensed canopy variables to determine risk and to prescribe treatments. We examined this variation by exploring the relationships between overstory forest vegetation attributes, recent fire history, and selected surface fuel components across an 80,000 ha contiguous landscape. Measurements of dead and live vegetation components of surface fuels were obtained from 624 permanent plots, or about 1 plot per 100 ha of forest cover. Within forest vegetation groups, we modeled the relationship between individual surface fuel components and overstory stand age, basal area, site quality and recent fire history, then stochastically predicted fuel loads across the landscape using the same linkage variables. The fraction of the plot variation, i.e., R2, explained by predictive models for individual fuel components ranged from 0.05 to 0.66 for dead fuels and 0.03 to 0.97 for live fuels in pine dominated vegetation groups. Stand age and basal area were generally more important than recent fire history for predicting fuel loads. Mapped fuel loads using these regressor variables showed a very heterogeneous landscape even at the scale of a few square kilometers. The mapped patterns corresponded to stand based forest management disturbances that are reflected in age, basal area, and fire history. Recent fire history was significant in explaining variation in litter and duff biomass. Stand basal area was positively and consistently related to dead fuel biomass in most groups and was present in many predictive equations. Patterns in live fuel biomass were related to recent fire history, but the patterns were not consistent among forest vegetation groups. Age and basal area were related to live fuels in a complex manner that is likely confounded with periodic disturbances that disrupt stand dynamics. This study complements earlier hazardous fuels research in the southeastern USA, and indicates that succession, disturbance, site quality and decomposition interact with forest management practices to create variable spatial and temporal conditions. The inclusion of additional land use, disturbance history, and soil-topographic variables coupled to improved sampling methods may increase precision and subsequent fuel mapping.

09 BIOMASS FUELS↗
DOE OSTI2017

Computational Design of New Magnesium Electrolytes with Improved Properties

Here, we use computational design to examine 15 new electrolyte salt anions by performing chemical variations and mutations on the bis(trifluoromethane)sulfonamide (TFSI) anion. On the basis of our calculations, we propose two new anions as potential candidates for magnesium energy-storage systems, which are evolved from TFSI with the substitution of sulfur atoms in TFSI and the modification of functional groups. The applicability of these new anion salts is examined through comprehensive calculations using both first-principles as well as benchmarked classical molecular dynamics. We elucidate the important properties of these anions, including the electrochemical stability window, chemical decomposition, preferred solvation structure, diffusion coefficient, and other dynamical properties for 15 rationally designed molecules. Two of the designed anions are discovered to successfully avoid the vulnerability of TFSI during ion-pair charge-transfer reactions while retaining comparable or better performance of other properties. As such, our work provides, to our knowledge, the first theoretically designed electrolyte salt for contemporary multivalent batteries and provides guidance for the synthesis and testing of novel liquid electrochemical systems.

25 ENERGY STORAGE↗
DOE OSTI2018

Acidity Constants of the Hematite-Liquid Water Interface from Ab Initio Molecular Dynamics

The interface between transition metal oxides (TMO) and liquid water plays a crucial role in environmental chemistry, catalysis, and energy science. Yet, the mechanism and energetics of chemical transformations at solvated TMO surfaces is often unclear, largely because of the difficulty to characterize the active surface species experimentally. The hematite (alpha-Fe2O3)-liquid water interface is a case in point. Here we demonstrate that ab initio molecular dynamics is a viable tool for determining the protonation states of complex interfaces. The pK a values of the oxygen-terminated (001) surface group of hematite, OH,and half-layer terminated (012) surface groups, (OH)-O-2 and (OH2)-O-1, are predicted to be (18.5 +/- 0.3), (18.9 +/- 0.6), and (10.3 +/- 0.5) pK(a) units, respectively. These are in good agreement with recent bond-valence theory based estimates, and suggest that the deprotonation of these surfaces require significantly more free energy input than previously thought.

Gittus, Oliver↗
DOE OSTI2015

Energy Conversion in Polyelectrolyte Hydrogels

Using extensive molecular dynamics simulations of polyelectrolyte hydrogels we demonstrate that, on deformation, these hydrogels adjust their deformed state predominantly by altering electrostatic interactions between their charged groups rather than excluded-volume and bond energies. On deformation, due to the hydrogel’s inherent tendency to preserve electroneutrality in its interior, the translational entropy of counterions decreases and the total electrostatic energy becomes more attractive. This result is valid for a wide range of compression ratios and Bjerrum lengths. The change in the electrostatic energy is more marked in highly swollen gels at low ionic strengths. At high Bjerrum lengths, where most of the counterions are condensed on hydrogel chains and the gel resembles a neutral system, the electrostatic-energy change with deformation is weaker.

36 MATERIALS SCIENCE↗
DOE OSTI2023

A Nitrogen Out-of-Plane (NOOP) Mechanism for Imine-Based Light-Driven Molecular Motors

Light-driven molecular motors have generated considerable interest due to their potential applications in material and biological systems. Recently, Greb and Lehn reported a new class of molecular motors, chiral N-alkyl imines, which undergo unidirectional rotation induced by light and heat. The mechanism of unidirectional motion in molecular motors containing a C═N group has been assumed to consist of photoinduced torsion about the double bond. In this work, we present a computational study of the photoisomerization dynamics of a chiral N-alkyl imine motor. We find that the location and energetics of minimal energy conical intersections (MECIs) alone are insufficient to understand the mechanism of the motor. Furthermore, a key part of the mechanism consists of out-of-plane distortions of the N atom (followed by isomerization about the double bond). Dynamic effects and out-of-plane distortions are critical to understand the observed (rather low) quantum yield for photoisomerization. Finally, our results provide hints as to how the photoisomerization quantum yield might be increased, improving the efficiency of this class of molecular motors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Initial-site characterization of hydrogen migration following strong-field double-ionization of ethanol

Abstract An essential problem in photochemistry is understanding the coupling of electronic and nuclear dynamics in molecules, which manifests in processes such as hydrogen migration. Measurements of hydrogen migration in molecules that have more than two equivalent hydrogen sites, however, produce data that is difficult to compare with calculations because the initial hydrogen site is unknown. We demonstrate that coincidence ion-imaging measurements of a few deuterium-tagged isotopologues of ethanol can determine the contribution of each initial-site composition to hydrogen-rich fragments following strong-field double ionization. These site-specific probabilities produce benchmarks for calculations and answer outstanding questions about photofragmentation of ethanol dications; e.g., establishing that the central two hydrogen atoms are 15 times more likely to abstract the hydroxyl proton than a methyl-group proton to form H $${}_{3}^{+}$$ 3 + and that hydrogen scrambling, involving the exchange of hydrogen between different sites, is important in H 2 O + formation. The technique extends to dynamic variables and could, in principle, be applied to larger non-cyclic hydrocarbons.

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

Molecular engineering of fluoroether electrolytes for lithium metal batteries

Fluoroether solvents are promising electrolyte candidates for high-energy-density lithium metal batteries, where high ionic conductivity and oxidative stability are important metrics for design of new systems. Recent experiments have shown that these performance metrics, particularly stability, can be tuned by changing the fraction of ether and fluorine content. However, little is known about how different molecular architectures influence the underlying ion transport mechanisms and conductivity. Here, we use all-atom molecular dynamics simulations to elucidate the ion transport and solvation characteristics of fluoroether chains of varying length, and having different ether segment and fluorine terminal group contents. The design rules that emerge from this effort are that solvent size determines lithium-ion transport kinetics, solvation structure, and solvation energy. In particular, the mechanism for lithium-ion transport is found to shift from ion hopping between solvation sites located in different fluoroether chains in short-chain solvents, to ion–solvent co-diffusion in long-chain solvents, indicating that an optimum exists for molecules of intermediate length, where hopping is possible but solvent diffusion is fast. Consistent with these findings, our experimental measurements reveal a non-monotonic behavior of the effects of solvent size on lithium-ion conductivity, with a maximum occurring for medium-length solvent chains. Finally, a key design principle for achieving high ionic conductivity is that a trade-off is required between relying on shorter fluoroether chains having high self-diffusivity, and relying on longer chains that increase the stability of local solvation shells.

25 ENERGY STORAGE↗
DOE OSTI2014

Decay dynamics of nascent acetonitrile and nitromethane dipole-bound anions produced by intracluster charge-transfer

Decay dynamics of nascent dipole bound states of acetonitrile and nitromethane are examined using time-resolved photoelectron imaging of iodide-acetonitrile (I{sup −}·CH{sub 3}CN) and iodide-nitromethane (I{sup −}·CH{sub 3}NO{sub 2}) complexes. Dipole-bound anions are created by UV-initiated electron transfer to the molecule of interest from the associated iodide ion at energies just below the vertical detachment energy of the halide-molecule complex. The acetonitrile anion is observed to decay biexponentially with time constants in the range of 4–900 ps. In contrast, the dipole bound state of nitromethane decays rapidly over 400 fs to form the valence bound anion. The nitromethane valence anion species then decays biexponentially with time constants of 2 ps and 1200 ps. The biexponential decay dynamics in acetonitrile are interpreted as iodine atom loss and autodetachment from the excited dipole-bound anion, followed by slower autodetachment of the relaxed metastable ion, while the dynamics of the nitromethane system suggest that a dipole-bound anion to valence anion transition proceeds via intramolecular vibrational energy redistribution to nitro group modes in the vicinity of the iodine atom.

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

Ultrafast spectroscopy of coherent phonon in carbon nanotubes using sub-5-fs visible pulses

In the last two decades, nano materials are attracting many scientists’ interest for both basic and application viewpoints. In order to understand the properties of nano systems it is needed to understand the dynamic properties which control the specific properties of the systems. All the primary processes in nano systems are taking place in femtosecond regime. Our group has been able to stably generate visible to near-infrared sub-5-fs laser pulses using a noncollinear optical parametric amplifier (NOPA) by the combination of various novel techniques including non-collinear optical parametric amplifier, pulse compression by a prism pair and grating pair. We apply the sub-5-fs pulses to study real-time coherent phonon in a one-dimensional system of carbon nanotubes. We determine exciton-phonon coupling mechanisms by observing the breathing mode in semiconducting carbon nanotubes and show the effect of electronic transition affected by the vibrational mode.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Simulation of self-assembly of polyzwitterions into vesicles

Using the Langevin dynamics method and a coarse-grained model, we have researched the formation of vesicles by hydrophobic polymers consisting of periodically placed zwitterion side groups in dilute salt-free aqueous solutions. The zwitterions, being permanent charge dipoles, provide long-range electrostatic correlations which are interfered by the conformational entropy of the polymer. Our simulations are geared towards gaining conceptual understanding in these correlated dipolar systems, where theoretical calculations are at present formidable. A competition between hydrophobic interactions and dipole-dipole interactions leads to a series of self-assembled structures. As the spacing d between the successive zwitterion side groups decreases, single chains undergo globule → disk → worm-like structures. We have calculated the Flory-Huggins χ parameter for these systems in terms of d and monitored the radius of gyration, hydrodynamic radius, spatial correlations among hydrophobic and dipole monomers, and dipole-dipole orientational correlation functions. During the subsequent stages of self-assembly, these structures lead to larger globules and vesicles as d is decreased up to a threshold value, below which no large scale morphology forms. Finally the vesicles form via a polynucleation mechanism whereby disk-like structures form first, followed by their subsequent merger.

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

4 He+n+n continuum within an ab initio framework

In this study, the low-lying continuum spectrum of the 6 He nucleus is investigated for the first time within an ab initio framework that encompasses the 4 He+n+n three-cluster dynamics characterizing its lowest decay channel. This is achieved through an extension of the no-core shell model combined with the resonating-group method, in which energy-independent nonlocal interactions among three nuclear fragments can be calculated microscopically, starting from realistic nucleon-nucleon interactions and consistent ab initio many-body wave functions of the clusters. The three-cluster Schrödinger equation is solved with three-body scattering boundary conditions by means of the hyperspherical-harmonics method on a Lagrange mesh. Using a soft similarity-renormalization-group evolved chiral nucleon-nucleon potential, we find the known J π = 2 + resonance as well as a result consistent with a new low-lying second 2 + resonance recently observed at GANIL at ~2.6 MeV above the He6 ground state. We also find resonances in the 2 – , 1 + , and 0 – channels, while no low-lying resonances are present in the 0 + and 1 – channels.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗