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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 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Analysis of lasers as a solution to efficiency droop in solid-state lighting

This letter analyzes the proposal to mitigate the efficiency droop in solid-state light emitters by replacing InGaN light-emitting diodes (LEDs) with lasers. The argument in favor of this approach is that carrier-population clamping after the onset of lasing limits carrier loss to that at threshold, while stimulated emission continues to grow with injection current. A fully quantized (carriers and light) theory that is applicable to LEDs and lasers (above and below threshold) is used to obtain a quantitative evaluation. The results confirm the potential advantage of higher laser output power and efficiency above lasing threshold, while also indicating disadvantages including low efficiency prior to lasing onset, sensitivity of lasing threshold to temperature, and the effects of catastrophic laser failure. As a result, a solution to some of these concerns is suggested that takes advantage of recent developments in nanolasers.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2015

Effects of the built-in electric field on polaron effects in a wurtzite ZnO/Mg{sub x}Zn{sub 1−x}O quantum well

The effects of the built-in electric field on the polarons in wurtzite ZnO/Mg{sub x}Zn{sub 1−x}O quantum wells are studied numerically via the improved Lee-Low-Pines intermediate coupling variational method. The contributions of the different branches of the optical phonons to the polaron energies of the ZnO/Mg{sub x}Zn{sub 1−x}O quantum wells are calculated as functions of well width d and composition x. The anisotropy effects of the electronic effective masses, the dielectric constants, and the frequencies of the different branches of the phonon modes (including both the longitudinal-like and transverse-like confined optical phonon modes, the interface optical phonon modes and the half-space phonon modes) on the polaron energies are considered in the calculations. Comparisons between the cases with and without the built-in electric field (F ≠ 0 and F = 0) are made for the optical phonon contributions to the polaron energies for the different branches of the phonon modes. The results show that the built-in electric field has marked effects on the contributions of the phonons with the different modes; in detail, it makes positive contributions to the interface and the half-space phonons, but negative contributions to the confined phonons, and thus its effect on the total phonon contribution is not obvious. Detailed comparisons of the contributions of the symmetric and antisymmetric phonon modes to the polaron energies as functions of d and x are also presented and a heuristic argument is provided to explain the numerical results.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Longitudinal relaxation in dipole-coupled homonuclear three-spin systems: Distinct correlations and odd spectral densities

A system of three dipole-coupled spins exhibits a surprisingly intricate relaxation behavior. Following Hubbard’s pioneering 1958 study, many authors have investigated different aspects of this problem. Nevertheless, on revisiting this classic relaxation problem, we obtain several new results, some of which are at variance with conventional wisdom. Most notably from a fundamental point of view, we find that the odd-valued spectral density function influences longitudinal relaxation. We also show that the effective longitudinal relaxation rate for a non-isochronous three-spin system can exhibit an unusual inverted dispersion step. To clarify these and other issues, we present a comprehensive theoretical treatment of longitudinal relaxation in a three-spin system of arbitrary geometry and with arbitrary rotational dynamics. By using the Liouville-space formulation of Bloch-Wangsness-Redfield theory and a basis of irreducible spherical tensor operators, we show that the number of relaxation components in the different cases can be deduced from symmetry arguments. For the isochronous case, we present the relaxation matrix in analytical form, whereas, for the non-isochronous case, we employ a computationally efficient approach based on the stochastic Liouville equation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Dynamics of the stochastic Lorenz chaotic system with long memory effects

Little seems to be known about the ergodic dynamics of stochastic systems with fractional noise. This paper is devoted to discern such long time dynamics through the stochastic Lorenz chaotic system (SLCS) with long memory effects. By a truncation technique, the SLCS is proved to generate a continuous stochastic dynamical system Λ. Based on the Krylov-Bogoliubov criterion, the required Lyapunov function is further established to ensure the existence of the invariant measure of Λ. Meanwhile, the uniqueness of the invariant measure of Λ is proved by examining the strong Feller property, together with an irreducibility argument. Therefore, the SLCS has exactly one adapted stationary solution.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Evaluating conducting network based transparent electrodes from geometrical considerations

Conducting nanowire networks have been developed as viable alternative to existing indium tin oxide based transparent electrode (TE). The nature of electrical conduction and process optimization for electrodes have gained much from the theoretical models based on percolation transport using Monte Carlo approach and applying Kirchhoff's law on individual junctions and loops. While most of the literature work pertaining to theoretical analysis is focussed on networks obtained from conducting rods (mostly considering only junction resistance), hardly any attention has been paid to those made using template based methods, wherein the structure of network is neither similar to network obtained from conducting rods nor similar to well periodic geometry. Here, we have attempted an analytical treatment based on geometrical arguments and applied image analysis on practical networks to gain deeper insight into conducting networked structure particularly in relation to sheet resistance and transmittance. Many literature examples reporting networks with straight or curvilinear wires with distributions in wire width and length have been analysed by treating the networks as two dimensional graphs and evaluating the sheet resistance based on wire density and wire width. The sheet resistance values from our analysis compare well with the experimental values. Our analysis on various examples has revealed that low sheet resistance is achieved with high wire density and compactness with straight rather than curvilinear wires and with narrower wire width distribution. Similarly, higher transmittance for given sheet resistance is possible with narrower wire width but of higher thickness, minimal curvilinearity, and maximum connectivity. For the purpose of evaluating active fraction of the network, the algorithm was made to distinguish and quantify current carrying backbone regions as against regions containing only dangling or isolated wires. The treatment can be helpful in predicting the properties of a network simply from image analysis and will be helpful in improvisation and comparison of various TEs and better understanding of electrical percolation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Rectified motion in an asymmetrically structured channel due to induced-charge electrokinetic and thermo-kinetic phenomena

It would be advantageous to move fluid by the gradient of random thermal noises that are omnipresent in the natural world. To achieve this motion, we propose a rectifier that uses a thermal noise along with induced-charge electroosmosis and electrophoresis (ICEO and ICEP) around a metal post cylinder in an asymmetrically structured channel and numerically examine its rectification performance. By the boundary element method combined with the thin double layer approximation, we find that rectified motion occurs in the asymmetrically structured channel due to ICEO and ICEP. Further, by thermodynamical and equivalent circuit methods, we discuss a thermal voltage that drives a rectifier consisting of a fluidic channel of an electrolyte and an impedance as a noise source. Our calculations show that fluid can be moved in the asymmetrically structured channel by the fluctuation of electric fields due to a thermal noise only when there is a temperature difference. In addition, our simple noise argument provides a different perspective for the thermo-kinetic phenomena (around a metal post) which was predicted based on the electrolyte Seebeck effect in our previous paper [H. Sugioka, “Nonlinear thermokinetic phenomena due to the Seebeck effect,” Langmuir 30, 8621 (2014)].

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Communication: Isotopic effects on tunneling motions in the water trimer

We present results of ring polymer molecular dynamics simulations that shed light on the effects of nuclear quantum fluctuations on tunneling motions in cyclic [H{sub 2}O]{sub 3} and [D{sub 2}O]{sub 3}, at the representative temperature of T = 75 K. In particular, we focus attention on free energies associated with two key isomerization processes: The first one corresponds to flipping transitions of dangling OH bonds, between up and down positions with respect to the O–O–O plane of the cluster; the second involves the interchange between connecting and dangling hydrogen bond character of the H-atoms in a tagged water molecule. Zero point energy and tunneling effects lead to sensible reductions of the free energy barriers. Due to the lighter nature of the H nuclei, these modifications are more marked in [H{sub 2}O]{sub 3} than in [D{sub 2}O]{sub 3}. Estimates of the characteristic time scales describing the flipping transitions are consistent with those predicted based on standard transition-state-approximation arguments.

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

Inter-phase heat transfer and energy coupling in turbulent dispersed multiphase flows

Here, the present paper addresses important fundamental issues of inter-phase heat transfer and energy coupling in turbulent dispersed multiphase flows through scaling analysis. In typical point-particle or two-fluid approaches, the fluid motion and convective heat transfer at the particle scale are not resolved and the momentum and energy coupling between fluid and particles are provided by proper closure models. By examining the kinetic energy transfer due to the coupling forces from the macroscale to microscale fluid motion, closure models are obtained for the contributions of the coupling forces to the energy coupling. Due to the inviscid origin of the added-mass force, its contribution to the microscale kinetic energy does not contribute to dissipative transfer to fluid internal energy as was done by the quasi-steady force. Time scale analysis shows that when the particle is larger than a critical diameter, the diffusive-unsteady kernel decays at a time scale that is smaller than the Kolmogorov time scale. As a result, the computationally costly Basset-like integral form of diffusive-unsteady heat transfer can be simplified to a non-integral form. Conventionally, the fluid-to-particle volumetric heat capacity ratio is used to evaluate the relative importance of the unsteady heat transfer to the energy balance of the particles. Therefore, for gas-particle flows, where the fluid-to-particle volumetric heat capacity ratio is small, unsteady heat transfer is usually ignored. However, the present scaling analysis shows that for small fluid-to-particle volumetric heat capacity ratio, the importance of the unsteady heat transfer actually depends on the ratio between the particle size and the Kolmogorov scale. Furthermore, the particle mass loading multiplied by the heat capacity ratio is usually used to estimate the importance of the thermal two-way coupling effect. Through scaling argument, improved estimates are established for the energy coupling parameters of each energy exchange mechanism between the phases.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Influence of electron-beam lithography exposure current level on the transport characteristics of graphene field effect transistors

Many factors have been identified to influence the electrical transport characteristics of graphene field-effect transistors. In this report, we examine the influence of the exposure current level used during electron beam lithography (EBL) for active region patterning. In the presence of a self-assembled hydrophobic residual layer generated by oxygen plasma etching covering the top surface of the graphene channel, we show that the use of low EBL current level results in higher mobility, lower residual carrier density, and charge neutrality point closer to 0 V, with reduced device-to-device variations. We show that this correlation originates from the resist heating dependent release of radicals from the resist material, near its interface with graphene, and its subsequent trapping by the hydrophobic polymer layer. Using a general model for resist heating, we calculate the difference in resist heating for different EBL current levels. We further corroborate our argument through control experiments, where radicals are either intentionally added or removed by other processes. We also utilize this finding to obtain mobilities in excess of 18 000 cm{sup 2}/V s on silicon dioxide substrates. We believe these results are applicable to other 2D materials such as transition metal dichalcogenides and nanoscale devices in general.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Quantum and classical contributions to linear magnetoresistance in topological insulator thin films

Three dimensional topological insulators possess backscattering immune relativistic Dirac fermions on their surface due to nontrivial topology of the bulk band structure. Both metallic and bulk insulating topological insulators exhibit weak-antilocalization in the low magnetic field and linear like magnetoresistance in higher fields. We explore the linear magnetoresistance in bulk insulating topological insulator Bi{sub 2-x}Sb{sub x}Te{sub 3-y}Se{sub y} thin films grown by pulsed laser deposition technique. Thin films of Bi{sub 2-x}Sb{sub x}Te{sub 3-y}Se{sub y} were found to be insulating in nature, which conclusively establishes the origin of linear magnetoresistance from surface Dirac states. The films were thoroughly characterized for their crystallinity and composition and then subjected to transport measurements. We present a careful analysis taking into considerations all the existing models of linear magnetoresistance. We comprehend that the competition between classical and quantum contributions to magnetoresistance results in linear magnetoresistance in high fields. We observe that the cross-over field decreases with increasing temperature and the physical argument for this behavior is explained.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Collective modes in two-dimensional one-component-plasma with logarithmic interaction

The collective modes of a familiar two-dimensional one-component-plasma with the repulsive logarithmic interaction between the particles are analysed using the quasi-crystalline approximation (QCA) combined with the molecular dynamic simulation of the equilibrium structural properties. It is found that the dispersion curves in the strongly coupled regime are virtually independent of the coupling strength. Arguments based on the excluded volume consideration for the radial distribution function allow us to derive very simple expressions for the dispersion relations, which show excellent agreement with the exact QCA dispersion over the entire domain of wavelengths. Comparison with the results of the conventional fluid analysis is performed, and the difference is explained.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Phenotype heterogeneity in cancer cell populations

Phenotype heterogeneity in cancer cell populations, be it of genetic, epigenetic or stochastic origin, has been identified as a main source of resistance to drug treatments and a major source of therapeutic failures in cancers. The molecular mechanisms of drug resistance are partly understood at the single cell level (e.g., overexpression of ABC transporters or of detoxication enzymes), but poorly predictable in tumours, where they are hypothesised to rely on heterogeneity at the cell population scale, which is thus the right level to describe cancer growth and optimise its control by therapeutic strategies in the clinic. We review a few results from the biological literature on the subject, and from mathematical models that have been published to predict and control evolution towards drug resistance in cancer cell populations. We propose, based on the latter, optimisation strategies of combined treatments to limit emergence of drug resistance to cytotoxic drugs in cancer cell populations, in the monoclonal situation, which limited as it is still retains consistent features of cell population heterogeneity. The polyclonal situation, that may be understood as “bet hedging” of the tumour, thus protecting itself from different sources of drug insults, may lie beyond such strategies and will need further developments. In the monoclonal situation, we have designed an optimised therapeutic strategy relying on a scheduled combination of cytotoxic and cytostatic treatments that can be adapted to different situations of cancer treatments. Finally, we review arguments for biological theoretical frameworks proposed at different time and development scales, the so-called atavistic model (diachronic view relying on Darwinian genotype selection in the coursof billions of years) and the Waddington-like epigenetic landscape endowed with evolutionary quasi-potential (synchronic view relying on Lamarckian phenotype instruction of a given genome by reversible mechanisms), to represent evolution towards heterogeneity, possibly polyclonal, in cancer cell populations and propose innovative directions for therapeutic strategies based on such frameworks.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Oscillation degeneracy in non-standard neutrino interactions

The standard theory describing neutrino oscillations only uses the interactions predicted by the Standard Model of particle physics. However, there is plenty of room for non-standard interactions (NSI) to exist. This is because extra interactions are allowed by experimental error bars and even expected at some level from effective theory arguments. This research is focused on examining the phenomenological consequences of the new physics of NSI at large atmospheric neutrino detectors like IceCube DeepCore. Of particular focus are the degeneracies between and within the standard neutrino oscillation parameters and the NSI parameters. These degeneracies will be explored both analytically and numerically, and strategies to lift them will also be discussed. This research is largely based on [1].

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Reactivity from excited state {sup 4}FeO{sup +} + CO sampled through reaction of ground state {sup 4}FeCO{sup +} + N{sub 2}O

The kinetics of the FeCO{sup +} + N{sub 2}O reaction have been studied at thermal energies (300–600 K) using a variable temperature selected ion flow tube apparatus. Rate constants and product branching fractions are reported. The reaction is modestly inefficient, proceeding with a rate constant of 6.2 × 10{sup −11} cm{sup 3} s{sup −1} at 300 K, with a small negative temperature dependence, declining to 4.4 × 10{sup −11} cm{sup 3} s{sup −1} at 600 K. Both Fe{sup +} and FeO{sup +} products are observed, with a constant branching ratio of approximately 40:60 at all temperatures. Calculation of the stationary points along the reaction coordinate shows that only the ground state quartet surface is initially sampled resulting in N{sub 2} elimination; a submerged barrier along this portion of the surface dictates the magnitude and temperature dependence of the total rate constant. The product branching fractions are determined by the behavior of the remaining {sup 4}OFeCO{sup +} fragment, and this behavior is compared to that found in the reaction of FeO{sup +} + CO, which initially forms {sup 6}OFeCO{sup +}. Thermodynamic and kinetic arguments are used to show that the spin-forbidden surface crossing in this region is efficient, proceeding with an average rate constant of greater than 10{sup 12} s{sup −1}.

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

Comparison of the telegraph and hyperdiffusion approximations in cosmic-ray transport

The telegraph equation and its generalizations have been repeatedly considered in the models of diffusive cosmic-ray transport. Yet the telegraph model has well-known limitations, and analytical arguments suggest that a hyperdiffusion model should serve as a more accurate alternative to the telegraph model, especially on the timescale of a few scattering times. We present a detailed side-by-side comparison of an evolving particle density profile, predicted by the telegraph and hyperdiffusion models in the context of a simple but physically meaningful initial-value problem, compare the predictions with the solution based on the Fokker–Planck equation, and discuss the applicability of the telegraph and hyperdiffusion approximations to the description of strongly anisotropic particle distributions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

First-principles study of crystal and electronic structure of rare-earth cobaltites

Using density functional theory plus self-consistent Hubbard U (DFT + U{sub sc}) calculations, we have investigated the structural and electronic properties of the rare-earth cobaltites RCoO{sub 3} (R = Pr – Lu). Our calculations show the evolution of crystal and electronic structure of the insulating low-spin RCoO{sub 3} with increasing rare-earth atomic number (decreasing ionic radius), including the invariance of the Co-O bond distance (d{sub Co–O}), the decrease of the Co-O-Co bond angle (Θ), and the increase of the crystal field splitting (Δ{sub CF}) and band gap energy (E{sub g}). Agreement with experiment for the latter improves considerably with the use of DFT + U{sub sc} and all trends are in good agreement with the experimental data. These trends enable a direct test of prior rationalizations of the trend in spin-gap associated with the spin crossover in this series, which is found to expose significant issues with simple band based arguments. We also examine the effect of placing the rare-earth f-electrons in the core region of the pseudopotential. The effect on lattice parameters and band structure is found to be small, but distinct for the special case of PrCoO{sub 3} where some f-states populate the middle of the gap, consistent with the recent reports of unique behavior in Pr-containing cobaltites. Overall, this study establishes a foundation for future predictive studies of thermally induced spin excitations in rare-earth cobaltites and similar systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Electrical properties of III-Nitride LEDs: Recombination-based injection model and theoretical limits to electrical efficiency and electroluminescent cooling

The current-voltage characteristic and ideality factor of III-Nitride quantum well light-emitting diodes (LEDs) grown on bulk GaN substrates are investigated. At operating temperature, these electrical properties exhibit a simple behavior. A model in which only active-region recombinations have a contribution to the LED current is found to account for experimental results. The limit of LED electrical efficiency is discussed based on the model and on thermodynamic arguments, and implications for electroluminescent cooling are examined.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Ferromagnetic bond of Li{sub 10} cluster: An alternative approach in terms of effective ferromagnetic sites

In this work, a model to explain the unusual stability of atomic lithium clusters in their highest spin multiplicity is presented and used to describe the ferromagnetic bonding of high-spin Li{sub 10} and Li{sub 8} clusters. The model associates the (lack of-)fitness of Heisenberg Hamiltonian with the degree of (de-)localization of the valence electrons in the cluster. It is shown that a regular Heisenberg Hamiltonian with four coupling constants cannot fully explain the energy of the different spin states. However, a more simple model in which electrons are located not at the position of the nuclei but at the position of the attractors of the electron localization function succeeds in explaining the energy spectrum and, at the same time, explains the ferromagnetic bond found by Shaik using arguments of valence bond theory. In this way, two different points of view, one more often used in physics, the Heisenberg model, and the other in chemistry, valence bond, come to the same answer to explain those atypical bonds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗