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At least 1,675 records · Page 93
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Stopping Force Analysis of 235 U Elemental Fission Product Yields for $E_n$ = 0.11–92.4 MeV

We report most evaluated elemental fission product yield distributions are not experimentally measured. Instead, the majority of evaluated distributions are based on analytic expressions of the Zp-model for relevant cumulative yields. Here we report independent elemental fission product yield distributions of a 235 U target for incident neutron energies ranging from 0.11 MeV through 92.4 MeV. Atomic numbers are calculated by an approach that combines a 2E analysis with a stopping force analysis method, developed within this paper. These analyses are applied to more than 6.1 × 10 6 fission fragment ionization tracks captured within the NIFFTE (Neutron Induced Fission Fragment Tracking Experiment) collaboration fission time projection chamber (fissionTPC). A 3-Z resolution was obtained with the fissionTPC spatial and energy resolutions. Tabulated results are presented for the atomic yield and experimentally derived Zp values as a function of pre-neutron-emission fragment masses for the complete range of incident neutron energies. The stopping-force-derived Zp values tend to support the unchanged charge distribution theory within uncertainty.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Central vortex steady states and dynamics of Bose–Einstein condensates interacting with a microwave field

Here we study central vortex steady states and dynamics of a two-dimensional (2D), two-component, Gross–Pitaevskii equation (CGPE) system for two pseudo-spinor Bose Einstein condensates (BECs) interacting with an electromagnetic field (microwave) analytically and numerically. For the central vortex steady state at any given winding number S, we prove its existence in a reduced, single component detuning limit when contact-interaction strength β < β b , and nonexistence when β > β b , respectively, where β b is a threshold value, whose value is given in Theorem 3.1 in the paper. We extend the existence and nonexistence result to the general two pseudo-spinor case and prove that a central vortex steady state exists for any given S if β ≤ β b while it does not exist when β > 2β b . We then derive dynamical equations for some observables (expectations of the matter wave function) such as the center-of-mass, position of dispersion, the linear and angular momentum of the two pseudo-spinor CGPEs. Finally, numerical computations are brought in to validate and extend the existence of vortex steady state results to β b ≤ β < 2β b for the two pseudo-spinor case and to explore transient dynamics of the observables.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Impact of Chiral Symmetry Breaking on Spin-Texture and Lone Pair Expression in Chiral Crystals of Hybrid Antimony and Bismuth Halides

Chiral organic building blocks can be incorporated into hybrid organic-inorganic metal-halide crystalline semiconductors so as to control and impact the interconversion between light, charge, and spin. Here, we report a series of hybrid antimony and bismuth halide materials of the general formula MBA 4 B 2 X 10 (B = Sb 3+ , Bi 3+ ; X = Br - , I - ) and study how chiral symmetry breaking imposed by the templating organic chiral methylbenzylammonium (MBA) cations induces symmetry breaking within the inorganic sublattice and leads to a unique spin-texture. The chiral MBA cations introduce two structural modifications to the metal-halide sublattice that consists of isolated edge-sharing dimers of B 2 X 10 octahedra: (1) the dimers have a chiral spatial arrangement with respect to one another and (2) there is an asymmetric distortion of the two subunits within the individual dimers with a higher distortion caused by strong stereochemical activity of the Sb 5s 2 lone pair electrons. The structural distortions and chiral arrangement result in circular dichroism (CD) at the band edge of the inorganic framework with dissymmetry factors in the range of 10 -4 . Chiral spin-splitting of the inorganic states, caused by breaking of the inversion symmetry and large spin-orbit coupling, is studied via density functional theory (DFT), and a multiband effective mass theory was developed that links the DFT-derived spin-splitting of helical character (i.e., spin expectation value not perpendicular to the crystal momentum) to the observed CD. Furthermore, we also find broad red photoluminescence from the MBA 4 Sb 2 Br 10 compounds, which we attribute to self-trapped excitonic emission driven by the large distortion due to the lone pair expression.

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

Microemulsions as Emerging Electrolytes: The Correlation of Structure to Electrochemical Response

We describe the structural studies of microemulsions (μEs) prepared from water, toluene, butanol, and polysorbate 20 (PS20) that we recently used as electrolytes. Small-angle neutron scattering was used to monitor the development of the bicontinuous system as a function of the water-to-surfactant mass ratio on a constant oil-to-surfactant dilution line, revealing how the domain size, correlation length, amphiphilicity factor, and bending moduli change with composition. Kratky and Porod analyses are also employed, providing further structural detail of the scattering domains. Here, we demonstrate that controlling the water-to-surfactant ratio with a constant oil-to-surfactant dilution affects the bicontinuous phase, reveals a sizeable compositional region with structural similarities, and provides insight into the correlation of structure to physical properties. Voltammetric results are presented to examine how the evolution of the bicontinuous structure formed in a μE prepared from water, toluene, butanol, and PS20 contributes to the electrochemical response. These findings, therefore, provide essential information that will guide the formulation of μEs as electrolytes for energy storage.

25 ENERGY STORAGE↗
DOE OSTI2018

Ion Migration Studies in Exfoliated 2D Molybdenum Oxide via Ionic Liquid Gating for Neuromorphic Device Applications

The formation of an electric double layer in ionic liquid (IL) can electrostatically induce charge carriers and/or intercalate ions in and out of the lattice which can trigger a large change of the electronic, optical, and magnetic properties of materials and even modify the crystal structure. Here, we present a systematic study of ionic liquid gating of exfoliated 2D molybdenum trioxide (MoO 3 ) devices and correlate the resultant electrical properties to the electrochemical doping via ion migration during the IL biasing process. A nearly 9 orders of magnitude modulation of the MoO 3 conductivity is obtained for the two types of ionic liquids that are investigated. In addition, notably rapid on/off switching was realized through a lithium-containing ionic liquid whereas much slower modulation was induced via oxygen extraction/intercalation. Time of flight–secondary ion mass spectrometry confirms the Li intercalation. Density functional theory (DFT) calculations have been carried out to examine the underlying metallization mechanism. Results of short-pulse tests show the potential of these MoO 3 devices as neuromorphic computing elements due to their synaptic plasticity.

2D materials↗
DOE OSTI2024

Cooperative Effects Drive Water Oxidation Catalysis in Cobalt Electrocatalysts through the Destabilization of Intermediates

A barrier to understanding the factors driving catalysis in the oxygen evolution reaction (OER) is understanding multiple over- lapping redox transitions in the OER catalysts. The complexity of these transitions obscure the relationship between the coverage of adsorbates and OER kinetics, leading to an experimental challenge in measuring activity descriptors, such as binding energies, as well as adsorbate interactions, which may destabilize intermediates and modulate their binding energies. Herein, we utilize a newly designed optical spectroelectrochemistry system to measure these phenomena in order to contrast the behavior of two electrocatalysts, cobalt oxyhydroxide (CoOOH) and cobalt-iron hexacyanoferrate (cobalt-iron Prussian blue, CoFe-PB). Three distinct optical spectra are observed in each catalyst, corresponding to three separate redox transitions, the last of which we show to be active for the OER using time-resolved spectroscopy and electrochemical mass spectroscopy. By combining predictions from density functional theory with parameters obtained from electroadsorption isotherms, we demonstrate that a destabilization of catalytic intermediates occurs with increasing coverage. In CoOOH, a strong (~0.34 eV/ monolayer) destabilization of a strongly bound catalytic intermediate is observed, leading to a potential offset between the accumulation of the intermediate and measurable O 2 evolution. We contrast these data to CoFe-PB, where catalytic intermediate generation and O 2 evolution onset coincide due to weaker binding and destabilization (~0.19 eV/monolayer). By considering a correlation between activation energy and binding strength, we suggest that such adsorbate driven destabilization may account for a significant fraction of the observed OER catalytic activity in both materials. Finally, we disentangle the effects of adsorbate interactions on state coverages and kinetics to show how adsorbate interactions determine the observed Tafel slopes. Crucially, the case of CoFe-PB shows that, even where interactions are weaker, adsorption remains non-Nernstian, which strongly influences the observed Tafel slope.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Decoupling the catalytic and degradation mechanisms of cobalt active sites during acidic water oxidation

Advancement of iridium-free catalysts for the low-pH oxygen evolution reaction (OER) is required to enable multi-gigawatt-scale proton-exchange water electrolysis. Cobalt-based materials might address this requirement, but little is known about the mechanism of operation of these OER catalysts at low pH. Here we investigate the nature and evolution of the active cobalt sites along with charge- and mass-transfer processes that support their catalytic function within a cobalt–iron–lead oxide material using in situ spectroscopic, gravimetric and electrochemical techniques. We demonstrate that corrosion of the cobalt sites and their reformation through electrooxidation of dissolved Co 2+ do not affect the catalytic mechanism and are decoupled from the OER. The OER-coupled charge transfer is supported by Co (3+δ)+ -oxo-species, which are structurally different from those reported for alkaline/near-neutral conditions and are formed on a relatively slow timescale of minutes. These mechanistic insights might assist in developing genuinely practical catalysts for this vital technology.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Nano- to microscale three-dimensional morphology relevant to transport properties in reactive porous composite paint films

The quantitative evaluation of the three-dimensional (3D) morphology of porous composite materials is important for understanding mass transport phenomena, which further impact their functionalities and durability. Reactive porous paint materials are composites in nature and widely used in arts and technological applications. In artistic oil paintings, ambient moisture and water and organic solvents used in conservation treatments are known to trigger multiple physical and chemical degradation processes; however, there is no complete physical model that can quantitatively describe their transport in the paint films. In the present study, model oil paints with lead white (2PbCO 3 ·Pb(OH) 2 ) and zinc white (ZnO) pigments, which are frequently found in artistic oil paintings and are associated with the widespread heavy metal soap deterioration, were studied using synchrotron X-ray nano-tomography and unilateral nuclear magnetic resonance. This study aims to establish a relationship among the paints’ compositions, the 3D morphological properties and degradation. This connection is crucial for establishing reliable models that can predict transport properties of solvents used in conservation treatments and of species involved in deterioration reactions, such as soap formation.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Real time quantification of mixed ion and electron transfer associated with the doping of poly(3-hexylthiophene)

Conjugated polymers have promising applications in electronics and energy storage due to the polymer's tunable conductivity and redox activity. For example, the conductivity of poly(3-hexylthiophene) (P3HT) is heavily dependent upon the doping level and the dopant type. This feature becomes especially important when considering P3HT or similar conjugated polymers for devices that require switching between electronic states (conductive vs. insulating). In this study, the mechanism of mixed ion-electron transfer studied using electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) is discussed. During cyclic voltammetry and galvanostatic charge–discharge experiments, the mass change of a P3HT film is monitored in real time. Distinct mass transfer regions are quantified as a function of doping level and potential, which are then correlated to changes with in situ conductance and spectroelectrochemical response. To identify the time scale at which the doping reaction transitions from kinetic to diffusion control, electrochemical impedance spectroscopy is coupled with EQCM-D. Finally, this work gives valuable insight into the nature of mixed ion–electron transfer, including its time scale, as it relates to the electronic properties of P3HT.

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

Performance enhancement of aqueous ionic liquids with lower critical solution temperature (LCST) behavior through ternary mixtures

Thermally responsive ionic liquids (ILs) exhibit liquid–liquid phase separation when mixed with water and heated above a lower critical solution temperature (LCST), resulting in a water-rich (WR) and an IL-rich (ILR) phase. These binary IL–water mixtures can be employed in a variety of thermodynamic processes such as forward osmosis (FO) desalination, for which two solution properties are desirable: low phase separation temperature and high osmotic strength (osmolality). However, these two properties are interlinked, with ILs that exhibit higher osmotic strengths typically requiring higher phase separation temperatures. This behavior tends to arise from the hydrophilicity of the IL cations, which enhances osmotic strength while also elevating the phase separation temperature. In this work, we highlight a pathway to overcome this tradeoff by developing ternary IL mixtures (two ILs with varying cation hydrophilicity mixed with water), which lowers the phase separation temperature while maintaining and even enhancing the osmotic strength of the solution. We characterize the mixing behavior (osmolality, phase separation temperature, WR phase purity, and WR to ILR phase mass ratio) of four ILs as a function of their concentration in solution. We find that an enhancement of up to 81.6% in the osmolality with a concomitant reduction of up to 15.4% in the phase separation temperature can be achieved using this approach. The ternary mixture is also shown to improve the phase separation kinetics by nearly 95% compared to the binary mixture. Overall, this work highlights a new pathway to improve the performance of LCST ILs for water and energy applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Molecular beam scattering of neon from flat jets of cold salty water

Molecular beam scattering experiments are carried out to study collisions between Ne atoms (E i = 24.3 kJ mol-1) and the surface of a cold salty water (8 m LiBr(aq), 230 K) flat jet. Translational energy distributions are collected as a function of scattering angle using a rotatable mass spectrometer. Impulsive scattering and thermal desorption contribute to the overall scattering distributions, but impulsive scattering dominates at all three incidence angles explored. Highly super-specular scattering is observed in the impulsive scattering channel that is attributed to anisotropic momentum transfer to the liquid surface. The thermal desorption channel exhibits a cos θ angular distribution. Compared to Ne scattering from dodecane, fractional energy loss in the impulsive scattering channel is much larger across a wide range of deflection angles. A soft-sphere model is applied to investigate the kinematics of energy transfer between the scatterer and liquid surface. Fitting to this model yields an effective surface mass of 250-60 +100 amu and internal excitation of 11.8 ± 1.6 kJ mol-1, both of which are considerably larger than for Ne/dodecane. It thus appears that energy transfer to cold salty water is more efficient than to a dodecane liquid surface, a result attributed to the extensive hydrogen-bonded network of liquid water and roughness of the liquid surface.

Yang, Walt↗
DOE OSTI2014

Interactions of mobile helium clusters with surfaces and grain boundaries of plasma-exposed tungsten

We report results of atomistic computations for the interactions of small mobile helium clusters (He{sub n}) with free surfaces and grain boundaries (GBs) in tungsten toward development of continuum drift-diffusion-reaction models for the dynamics of mobile helium clusters in plasma-exposed tungsten. Molecular-statics (MS) simulations based on reliable many-body interatomic potentials are carried out for He{sub n} (1 ≤ n ≤ 7) clusters near sinks to obtain the potential energy profiles of the He{sub n} clusters as a function of the clusters' center-of-mass distance from a sink. Sinks investigated include surfaces, GBs, and regions in the vicinity of junctions where GBs intersect free surfaces. Elastic interaction potentials based on elastic inclusion theory provide an excellent description of the MS results for the cluster-sink interactions. The key parameter in the elastic models is the sink segregation strength, which is found to increase with increasing cluster size. Such cluster-sink interactions are responsible for the migration of small helium clusters by drift and for helium segregation on surfaces and grain boundaries in tungsten. Such helium segregation on sinks is observed in large-scale molecular-dynamics simulations of helium aggregation in model polycrystalline tungsten at 933 K upon helium implantation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Neutral and positively charged excitons in narrow quantum ring

We study theoretically quantized states of a neutral and a positively charged exciton (trion X{sup +}) confined in a heterostructure with the ring-like geometry. In order to assess the experimentally relevant domain of parameters, we adopt a simple model of a narrow ring when 3D wave equations for the neutral and positively charged excitons can be separated. By using the Fourier series method, we have calculated the energy spectra of excitons complexes in a quantum ring as a function of the electron-to-hole mass ratio, the ring radius, and the magnetic field strength. The quantum-size effect and the size-dependent magnetic oscillations of energy levels of excitons' complexes spectra have been revealed.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

The roles of the binodal curve and the spinodal curve in expansions from the supercritical state with flashing

Data extracted from time-of-flight (TOF) measurements made on steady-state He free jets at Göttingen already in 1986 and for pulsed Ne free jets investigated recently at Tel Aviv have been added to an earlier plot of terminal condensed-phase mass fraction x{sub 2∞} as a function of the dimensionless scaling parameter Γ. Γ characterizes the source (fluid species, temperature, pressure and throat diameter); values of x{sub 2∞} are extracted from TOF measurements using conservation of energy in the free-jet expansion. For nozzles consisting of an orifice in a thin plate; the extracted data yield 22 data points which are correlated satisfactorily by a single curve. The Ne free jets were expanded from a conical nozzle with a 20° half angle; the three extracted data points stand together but apart from the aforementioned curve, indicating that the presence of the conical wall influences significantly the expansion and hence the condensation. The 22 data points for the expansions via an orifice consist of 15 measurements with expansions from the gas-phase side of the binodal curve which crossed the binodal curve downstream from the sonic point and 7 measurements with expansions of the gas-phase product of the flashing which occurred after an expansion from the liquid-phase side of the binodal curve crossed the binodal curve upstream from the sonic point. The association of these 22 points with a single curve supports the alternating-phase model for flows with flashing upstream from the sonic point proposed earlier. In order to assess the role of the spinodal curve in such expansions, the spinodal curves for He and Ne were computed using general multi-parameter Helmholtz-free-energy equation-of-state formulations. Then, for the several sets of source-chamber conditions used in the free-jet measurements, thermodynamic states at key locations in the free-jet expansions (binodal curve, sonic point and spinodal curve) were evaluated, with the expansion presumed to be metastable from the binodal curve to the spinodal curve. TOF distributions with more than two peaks (interpreted earlier as superimposed alternating-state TOF distributions) indicated flashing of the metastable flow downstream from the binodal curve but upstream from the sonic point. This relatively early flashing is due apparently to destabilizing interactions with the walls of the source. If the expansion crosses the binodal curve downstream from the nozzle, the metastable fluid does not interact with surfaces and flashing might be delayed until the expansion reaches the spinodal curve. It is concluded that, if the expansion crosses the binodal curve before reaching the sonic point, the resulting metastable fluid downstream from the binodal curve interacts with the adjacent surfaces and flashes into liquid and vapor phases which expand alternately through the nozzle; the two associated alternating TOF distributions are superposed by the chopping process so that the result has the appearance of a single distribution with three peaks.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Intersubband energies in strain-compensated InGaN/AlInN quantum well structures

Intersubband transition energies in the conduction band for strain-compensated InGaN/AlInN quantum well (QW) structures were investigated as a function of strain based on an effective mass theory with the nonparabolicity taken into account. In the case of an InGaN/AlInN QW structure lattice-matched to GaN, the wavelength is shown to be longer than 1.55 μm. On the other hand, strain-compensated QW structures show that the wavelength of 1.55 μm can be reached even for the QW structure with a relatively small strain of 0.3 %. Hence, the strain-compensated QW structures can be used for telecommunication applications at 1.55 μm with a small strain, compared to conventional GaN/AlN QW structure.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Brownian relaxation of an inelastic sphere in air

The procedures that are used to calculate the forces and moments on an aerodynamic body in the rarefied gas of the upper atmosphere are applied to a small sphere of the size of an aerosol particle at sea level. While the gas-surface interaction model that provides accurate results for macroscopic bodies may not be appropriate for bodies that are comprised of only about a thousand atoms, it provides a limiting case that is more realistic than the elastic model. The paper concentrates on the transfer of energy from the air to an initially stationary sphere as it acquires Brownian motion. Individual particle trajectories vary wildly, but a clear relaxation process emerges from an ensemble average over tens of thousands of trajectories. The translational and rotational energies in equilibrium Brownian motion are determined. Empirical relationships are obtained for the mean translational and rotational relaxation times, the mean initial power input to the particle, the mean rates of energy transfer between the particle and air, and the diffusivity. These relationships are functions of the ratio of the particle mass to an average air molecule mass and the Knudsen number, which is the ratio of the mean free path in the air to the particle diameter. The ratio of the molecular radius to the particle radius also enters as a correction factor. The implications of Brownian relaxation for the second law of thermodynamics are discussed.

42 ENGINEERING↗
DOE OSTI2018

Impurity-derived p -type conductivity in cubic boron arsenide

Cubic boron arsenide (c-BAs) exhibits an ultrahigh thermal conductivity (κ) approaching 1300 Wm -1 K -1 at room temperature. However, c-BAs is believed to incorporate high concentrations of crystal imperfections that can both quench κ and act as sources of unintentional p-type conductivity. Although this behavior has been attributed to native defects, we show here, using optical and magnetic resonance spectroscopies together with first-principles calculations, that unintentional acceptor impurities such as silicon and/or carbon are more likely candidates for causing the observed conductivity. These results also clarify that the true low-temperature bandgap of c-BAs is 0.3 eV higher than the widely reported value of ~1.5 eV. Low-temperature photoluminescence measurements of c-BAs crystals reveal impurity-related recombination processes (including donor-acceptor pair recombination), and electron paramagnetic resonance experiments show evidence for effective mass-like shallow acceptors. Our hybrid density functional calculations indicate that native defects are incapable of giving rise to such signals. Instead, we find that group-IV impurities readily incorporate on the As site and act as shallow acceptors. Finally, such impurities can dominate the electrical properties of c-BAs, and their influence on phonon scattering must be considered when optimizing thermal conductivity.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗