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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Building and testing correlations for the estimation of one‐electron reduction potentials of a diverse set of organic molecules

We describe and evaluate a method for computationally predicting reduction potentials of a diverse group of organic molecules by linearly correlating calculated lowest unoccupied molecular orbital energies with ground state reduction potentials measured in acetonitrile. The approach is shown to provide a unique combination of extreme computational simplicity and excellent accuracy across a diverse range of organic structures and a wide window of reduction potentials. A disparate set of molecules (74 compounds belonging to six distinct structural families, comprised of molecules containing C, H, N, O, F, Cl, and Br, with functional groups including esters, ketones, halides, nitriles, quinones, alkenes, arenes, heteroarenes, and pyridinium and higher benzologs, all containing conjugated pi systems, spanning a 3.5‐V range of reduction potentials) was used to build the correlations. This methodology was found to be computationally inexpensive compared with other approaches and to permit the useful prediction of reduction potentials of additional molecules of diverse structural types not included in the families used to determine the correlation parameters. The effects of varying the basis set used in the B3LYP electronic structure calculations and including solvent (compared with calculations in gas phase) were also examined. It was found that the inclusion of a continuum solvent model in the calculations was required for accurate results, particularly when including cationic species in the correlations (although when only neutral molecules were examined, reasonable results could even be obtained in vacuo ). Several of the best correlations were used to predict the reduction potentials of seven much larger and structurally diverse chromophores that were not included in the correlation data set. Strong correlations ( r 2 values > 0.99) with very good predictive abilities (root mean square deviation < 60 mV) were found. This extremely simple and computationally efficient entirely closed‐shell methodology is proven robust and useful for the design of new molecules capable of participating in redox processes, including electron transfer reactions. Copyright © 2015 John Wiley & Sons, Ltd.

Méndez‐Hernández, Dalvin D.↗
DOE OSTI2017

Size dependent behavior of Fe 3 O 4 crystals during electrochemical (de)lithiation: an in situ X-ray diffraction, ex situ X-ray absorption spectroscopy, transmission electron microscopy and theoretical investigation

Here, the iron oxide magnetite, Fe 3 O 4 , is a promising conversion type lithium ion battery anode material due to its high natural abundance, low cost and high theoretical capacity. While the close packing of ions in the inverse spinel structure of Fe 3 O 4 enables high energy density, it also limits the kinetics of lithium ion diffusion in the material. Nanosizing of Fe 3 O 4 to reduce the diffusion path length is an effective strategy for overcoming this issue and results in improved rate capability. However, the impact of nanosizing on the multiple structural transformations that occur during the electrochemical (de)lithiation reaction in Fe 3 O 4 is poorly understood. In this study, the influence of crystallite size on the lithiation-conversion mechanisms in Fe 3 O 4 is investigated using complementary X-ray techniques along with transmission electron microscopy (TEM) and continuum level simulations on electrodes of two different Fe 3 O 4 crystallite sizes. In situ X-ray diffraction (XRD) measurements were utilized to track the changes to the crystalline phases during (de)lithiation. X-ray absorption spectroscopy (XAS) measurements at multiple points during the (de)lithiation processes provided local electronic and atomic structural information. Tracking the crystalline and nanocrystalline phases during the first (de)lithiation provides experimental evidence that (1) the lithiation mechanism is non-uniform and dependent on crystallite size, where increased Li + diffusion length in larger crystals results in conversion to Fe 0 metal while insertion of Li + into spinel-Fe 3 O 4 is still occurring, and (2) the disorder and size of the Fe metal domains formed when either material is fully lithiated impacts the homogeneity of the FeO phase formed during the subsequent delithiation.

25 ENERGY STORAGE↗
DOE OSTI2017

Temperature-modulated electronic structure of graphene on SiC: Possible roles of electron-electron interaction and strain

We have iobserved the electron band structure of graphene epitaxially grown on an SiC substrate using angle-resolved photoemission spectroscopy. The conical energy spectrum of graphene exhibits a minimum slope at ~50 K, which is accompanied by the minimum separation between its two branches. These observations provide a viable route towards the engineering of the electronic properties of graphene using temperature, while the latter suggests a possible evidence of gap engineering via strain induced by the substrate and modulated by temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Oscillating decay rate in electron capture and the neutrino mass difference

Reported oscillations in the rate of decay of certain ions by K-electron capture have raised questions about whether and how such oscillations can arise in quantum-mechanical theory and whether they can measure the neutrino mass difference. Here I show that simple principles of quantum mechanics answer some questions and clarify what must be performed theoretically or experimentally to answer some others. The principal result is that quantum mechanics does allow mass-difference-dependent oscillations in principle, but it imposes conditions not obeyed by the approximate dynamical models that have been put forth up to now. In particular, indirect coupling between two neutrino mass channels must be taken into account. What needs to be done experimentally and theoretically is discussed

Peshkin, Murray↗
DOE OSTI2014

A novel electron gun for inline MRI-linac configurations

Purpose: This work introduces a new electron gun geometry capable of robust functioning in the presence of a high strength external magnetic field for axisymmetric magnetic resonance imaging (MRI)-linac configurations. This allows an inline MRI-linac to operate without the need to isolate the linear accelerator (linac) using a magnetic shield. This MRI-linac integration approach not only leaves the magnet homogeneity unchanged but also provides the linac flexibility to move along the magnet axis of symmetry if the source to target distance needs to be adjusted. Methods: Simple electron gun geometry modifications of a Varian 600C electron gun are considered and solved in the presence of an external magnetic field in order to determine a set of design principles for the new geometry. Based on these results, a new gun geometry is proposed and optimized in the fringe field of a 0.5 T open bore MRI magnet (GE Signa SP). A computer model for the 6 MeV Varian 600C linac is used to determine the capture efficiency of the new electron gun-linac system in the presence of the fringe field of the same MRI scanner. The behavior of the new electron gun plus the linac system is also studied in the fringe fields of two other magnets, a 1.0 T prototype open bore magnet and a 1.5 T GE Conquest scanner. Results: Simple geometrical modifications of the original electron gun geometry do not provide feasible solutions. However, these tests show that a smaller transverse cathode diameter with a flat surface and a slightly larger anode diameter could alleviate the current loss due to beam interactions with the anode in the presence of magnetic fields. Based on these findings, an initial geometry resembling a parallel plate capacitor with a hole in the anode is proposed. The optimization procedure finds a cathode-anode distance of 5 mm, a focusing electrode angle of 5°, and an anode drift tube length of 17.1 mm. Also, the linac can be displaced with ±15 cm along the axis of the 0.5 T magnet without capture efficiency reduction below the experimental value in zero field. In this range of linac displacements, the electron beam generated by the new gun geometry is more effectively injected into the linac in the presence of an external magnetic field, resulting in approximately 20% increase of the target current compared to the original gun geometry behavior at zero field. The new gun geometry can generate and accelerate electron beams in external magnetic fields without current loss for fields higher than 0.11 T. The new electron-gun geometry is robust enough to function in the fringe fields of the other two magnets with a target current loss of no more than 16% with respect to the current obtained with no external magnetic fields. Conclusions: In this work, a specially designed electron gun was presented which can operate in the presence of axisymmetric strong magnetic fringe fields of MRI magnets. Computer simulations show that the electron gun can produce high quality beams which can be injected into a straight through linac such as Varian 600C and accelerated with more efficiency in the presence of the external magnetic fields. Also, the new configuration allows linac displacements along the magnet axis in a range equal to the diameter of the imaging spherical volume of the magnet under consideration. The new electron gun-linac system can function in the fringe field of a MRI magnet if the field strength at the cathode position is higher than 0.11 T. The capture efficiency of the linac depends on the magnetic field strength and the field gradient. The higher the gradient the better the capture efficiency. The capture efficiency does not degrade more than 16%.

43 PARTICLE ACCELERATORS↗
DOE OSTI2015

Experimental evidence of excited electron number density and temperature effects on electron-phonon coupling in gold films

The electronic transport properties of metals with weak electron-phonon coupling can be influenced by non-thermal electrons. Relaxation processes involving non-thermal electrons competing with the thermalized electron system have led to inconsistencies in the understanding of how electrons scatter and relax with the less energetic lattice. Recent theoretical and computational works have shown that the rate of energy relaxation with the metallic lattice will change depending on the thermalization state of the electrons. Even though 20 years of experimental works have focused on understanding and isolating these electronic relaxation mechanisms with short pulsed irradiation, discrepancies between these existing works have not clearly answered the fundamental question of the competing effects between non-thermal and thermal electrons losing energy to the lattice. In this work, we demonstrate the ability to measure the electron relaxation for varying degrees of both electron-electron and electron-phonon thermalization. This series of measurements of electronic relaxation over a predicted effective electron temperature range up to ∼3500 K and minimum lattice temperatures of 77 K validate recent computational and theoretical works that theorize how a nonequilibrium distribution of electrons transfers energy to the lattice. Utilizing this wide temperature range during pump-probe measurements of electron-phonon relaxation, we explain discrepancies in the past two decades of literature of electronic relaxation rates. We experimentally demonstrate that the electron-phonon coupling factor in gold increases with increasing lattice temperature and laser fluences. Specifically, we show that at low laser fluences corresponding to small electron perturbations, energy relaxation between electrons and phonons is mainly governed by non-thermal electrons, while at higher laser fluences, non-thermal electron scattering with the lattice is less influential on the energy relaxation mechanisms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Electron Mobility and Trapping in Ferrihydrite Nanoparticles

Iron is the most abundant transition metal in the Earth's crust, and naturally occurring iron oxide minerals play a commanding role in environmental redox reactions. Although iron oxide redox reactions are well studied, their precise mechanisms are not fully understood. Recent work has shown that these involve electron transfer pathways within the solid, suggesting that overall reaction rates could be dependent on electron mobility. Initial ultrafast spectroscopy studies of iron oxide nanoparticles sensitized by fluorescein derivatives supported a model for electron mobility based on polaronic hopping of electron charge carriers between iron sites, but the constitutive relationships between hopping mobilities and interfacial charge transfer processes has remained obscured. We developed a coarse-grained lattice Monte Carlo model to simulate the collective mobilities and lifetimes of these photoinjected electrons with respect to recombination with adsorbed dye molecules for the essential nanophase ferrihydrite, and tested predictions made by the simulations using pump-probe spectroscopy. We acquired optical transient absorption spectra as a function of particle size and under a variety of solution conditions, and used cryogenic transmission electron microscopy to determine the aggregation state of the nanoparticles. We observed biphasic electron recombination kinetics over timescales that spanned picoseconds to microseconds, the slower regime of which was fit with a stretched exponential decay function. The recombination rates were weakly affected by nanoparticle size and aggregation state, suspension pH, and the injection of multiple electrons per nanoparticle. We conclude that electron mobility indeed limits the rate of interfacial electron transfer in these systems with the slowest processes relating to escape from deep traps, the presence of which outweighs the influence of environmental factors such as pH-dependent surface charge.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Electronic coupling through natural amino acids

Myriad scientific domains concern themselves with biological electron transfer (ET) events that span across vast scales of rate and efficiency through a remarkably fine-tuned integration of amino acid (AA) sequences, electronic structure, dynamics, and environment interactions. Within this intricate scheme, many questions persist as to how proteins modulate electron-tunneling properties. To help elucidate these principles, we develop a model set of peptides representing the common α-helix and β-strand motifs including all natural AAs within implicit protein-environment solvation. Using an effective Hamiltonian strategy with density functional theory, we characterize the electronic coupling through these peptides, furthermore considering side-chain dynamics. For both motifs, predictions consistently show that backbone-mediated electronic coupling is distinctly sensitive to AA type (aliphatic, polar, aromatic, negatively charged and positively charged), and to side-chain orientation. The unique properties of these residues may be employed to design activated, deactivated, or switch-like superexchange pathways. Electronic structure calculations and Green’s function analyses indicate that localized shifts in the electron density along the peptide play a role in modulating these pathways, and further substantiate the experimentally observed behavior of proline residues as superbridges. The distinct sensitivities of tunneling pathways to sequence and conformation revealed in this electronic coupling database help improve our fundamental understanding of the broad diversity of ET reactivity and provide guiding principles for peptide design.

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

Kinetic electron and ion instability of the lunar wake simulated at physical mass ratio

The solar wind wake behind the moon is studied with 1D electrostatic particle-in-cell (PIC) simulations using a physical ion to electron mass ratio (unlike prior investigations); the simulations also apply more generally to supersonic flow of dense magnetized plasma past non-magnetic objects. In this work, a hybrid electrostatic Boltzmann electron treatment is first used to investigate the ion stability in the absence of kinetic electron effects, showing that the ions are two-stream unstable for downstream wake distances (in lunar radii) greater than about three times the solar wind Mach number. Simulations with PIC electrons are then used to show that kinetic electron effects can lead to disruption of the ion beams at least three times closer to the moon than in the hybrid simulations. This disruption occurs as the result of a novel wake phenomenon: the non-linear growth of electron holes spawned from a narrow dimple in the electron velocity distribution. Most of the holes arising from the dimple are small and quickly leave the wake, approximately following the unperturbed electron phase-space trajectories, but some holes originating near the center of the wake remain and grow large enough to trigger disruption of the ion beams. Lastly, non-linear kinetic-electron effects are therefore essential to a comprehensive understanding of the 1D electrostatic stability of such wakes, and possible observational signatures in ARTEMIS data from the lunar wake are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Nuclear-Electronic Orbital Ehrenfest Dynamics

The recently developed real-time nuclear-electronic orbital (RT-NEO) approach provides an elegant framework for treating electrons and selected nuclei, typically pro- tons, quantum mechanically in nonequilibrium dynamical processes. However, the RT-NEO approach neglects the motion of the other nuclei, preventing a complete de- scription of the coupled nuclear-electronic dynamics and spectroscopy. In this work, we describe the dynamical interaction between the other nuclei and the electron-proton subsystem with the mixed quantum-classical Ehrenfest dynamics method. The NEO- Ehrenfest approach propagates the electrons and quantum protons in a time-dependent variational framework, while the remaining nuclei move classically on the correspond- ing average electron-proton vibronic surface. This approach includes the non-Born- Oppenheimer eects between the electrons and quantum protons with RT-NEO and between the classical nuclei and the electron-proton subsystem with Ehrenfest dynam- ics. Spectral features for vibrational modes involving both quantum and classical nuclei are resolved from the time-dependent dipole moments. Moreover, nuclear quantum ef- fects are included directly in nonadiabatic dynamics simulations. The excited state intramolecular proton transfer in the o-hydroxybenzaldehyde molecule is shown to be faster and to exhibit a larger kinetic isotope eect with NEO-Ehrenfest compared to classical Ehrenfest dynamics. This work shows that the NEO-Ehrenfest method is a powerful tool to study dynamical processes with coupled electronic and nuclear degrees of freedom.

Zhao, Luning↗
DOE OSTI2014

Monte Carlo based beam model using a photon MLC for modulated electron radiotherapy

Purpose: Modulated electron radiotherapy (MERT) promises sparing of organs at risk for certain tumor sites. Any implementation of MERT treatment planning requires an accurate beam model. The aim of this work is the development of a beam model which reconstructs electron fields shaped using the Millennium photon multileaf collimator (MLC) (Varian Medical Systems, Inc., Palo Alto, CA) for a Varian linear accelerator (linac). Methods: This beam model is divided into an analytical part (two photon and two electron sources) and a Monte Carlo (MC) transport through the MLC. For dose calculation purposes the beam model has been coupled with a macro MC dose calculation algorithm. The commissioning process requires a set of measurements and precalculated MC input. The beam model has been commissioned at a source to surface distance of 70 cm for a Clinac 23EX (Varian Medical Systems, Inc., Palo Alto, CA) and a TrueBeam linac (Varian Medical Systems, Inc., Palo Alto, CA). For validation purposes, measured and calculated depth dose curves and dose profiles are compared for four different MLC shaped electron fields and all available energies. Furthermore, a measured two-dimensional dose distribution for patched segments consisting of three 18 MeV segments, three 12 MeV segments, and a 9 MeV segment is compared with corresponding dose calculations. Finally, measured and calculated two-dimensional dose distributions are compared for a circular segment encompassed with a C-shaped segment. Results: For 15 × 34, 5 × 5, and 2 × 2 cm{sup 2} fields differences between water phantom measurements and calculations using the beam model coupled with the macro MC dose calculation algorithm are generally within 2% of the maximal dose value or 2 mm distance to agreement (DTA) for all electron beam energies. For a more complex MLC pattern, differences between measurements and calculations are generally within 3% of the maximal dose value or 3 mm DTA for all electron beam energies. For the two-dimensional dose comparisons, the differences between calculations and measurements are generally within 2% of the maximal dose value or 2 mm DTA. Conclusions : The results of the dose comparisons suggest that the developed beam model is suitable to accurately reconstruct photon MLC shaped electron beams for a Clinac 23EX and a TrueBeam linac. Hence, in future work the beam model will be utilized to investigate the possibilities of MERT using the photon MLC to shape electron beams.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2016

Runaway electrons and magnetic island confinement

The breakup of magnetic surfaces is a central feature of ITER planning for the avoidance of damage due to runaway electrons. Rapid thermal quenches, which lead to large accelerating voltages, are thought to be due to magnetic surface breakup. Impurity injection to avoid and to mitigate both halo and runaway electron currents utilizes massive gas injection or shattered pellets. The actual deposition is away from the plasma center, and the breakup of magnetic surfaces is thought to spread the effects of the impurities across the plasma cross section. The breakup of magnetic surfaces would prevent runaway electrons from reaching relativistic energies were it not for the persistence of non-intercepting flux tubes. These are tubes of magnetic field lines that do not intercept the walls. In simulations and in magnetic field models, non-intercepting flux tubes are found to persist near the magnetic axis and in the cores of magnetic islands even when a large scale magnetic surface breakup occurs. As long as a few magnetic surfaces reform before all of the non-intercepting flux tubes dissipate, energetic electrons confined and accelerated in these flux tubes can serve as the seed electrons for a transfer of the overall plasma current from thermal to relativistic carriers. The acceleration of electrons is particularly strong because of the sudden changes in the poloidal flux that naturally occur in a rapid magnetic relaxation. Furthermore, the physics of magnetic islands as non-intercepting flux tubes is studied. Expressions are derived for (1) the size of islands required to confine energetic runaway electrons, (2) the accelerating electric field in an island, (3) the increase or reduction in the size of an island by the runaway electron current, (4) the approximate magnitude of the runaway current in an island, and (5) the time scale for the evolution of an island.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗