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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 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Monte-Carlo modeling of phonon thermal transport using DFT-based anisotropic dispersion relations over the full Brillouin zone

In this work, we present a Monte Carlo (MC) approach to solve the phonon Boltzmann transport equation (BTE) in which the anisotropic phonon dispersion relations over the full Brillouin zone (BZ) are used. In this approach, the discretization of the BZ used to compute the phonon relaxation time places constraints on the direction of scattered phonons in the real-space simulation domain. The phonon dispersion and phonon relaxation times are calculated using the density functional theory (DFT) approach. The modified MC approach is validated by a close examination of its ability to simulate phonon transport in both the ballistic and diffusive regimes for multiple materials including GaAs, InAs, ThO 2 , and α-U. In doing so, the phonon thermal conductivities from 100 K to 1000 K are calculated and compared with traditional non-transport solution of the phonon BTE. It is found that the phonon thermal conductivities of α-U and ThO 2 obtained from MC simulations using isotropic dispersion are larger than the values obtained using anisotropic phonon dispersion relations over the full BZ. The effect of phonon-defect scattering on the thermal conductivity of ThO 2 is also studied as an application of the current MC approach and found to agree with previously computed values in the literature. The MC solver developed here has been parallelized as a step to demonstrate its potential to solving computationally intensive phonon thermal transport problems at the mesoscale.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of preparation methods on dispersion stability and electrochemical performance of graphene sheets

Chemical exfoliation is one of the most important strategies for preparing graphene. The aggregation of graphene sheets severely prevents graphene from exhibiting excellent properties. However, there are no attempts to investigate the effect of preparation methods on the dispersity of graphene sheets. In this study, three chemical exfoliation methods, including Hummers method, modified Hummers method, and improved method, were used to prepare graphene sheets. The influence of preparation methods on the structure, dispersion stability in organic solvents, and electrochemical properties of graphene sheets were investigated. Fourier transform infrared microscopy, Raman spectra, transmission electron microscopy, and UV–vis spectrophotometry were employed to analyze the structure of the as-prepared graphene sheets. The results showed that graphene prepared by improved method exhibits excellent dispersity and stability in organic solvents without any additional stabilizer or modifier, which is attributed to the completely exfoliation and regular structure. Moreover, cyclic voltammetric and electrochemical impedance spectroscopy measurements showed that graphene prepared by improved method exhibits superior electrochemical properties than that prepared by the other two methods. - Graphical abstract: Graphene oxides with different oxidation degree were obtained via three methods, and then graphene with different crystal structures were created by chemical reduction of exfoliated graphene oxides. - Highlights: • Graphene oxides with different oxidation degree were obtained via three oxidation methods. • The influence of oxidation methods on microstructure of graphene was investigated. • The effect of oxidation methods on dispersion stability of graphene was investigated. • The effect of oxidation methods on electrochemical properties of graphene was discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Microstructural investigation of oxide dispersion strengthened alloy 617 after creep rupture at high temperature

Highlights: • The creep related microstructural evolution of ODS-617 at 950 °C under 25 MPa was investigated. • Dispersion of nanoscale Y-Al-O particles contributed to the superior creep properties of the ODS-617. • The creep mechanism of the ODS-617 at 950 °C under 25 MPa was Coble creep. • Cracks preferentially developed around the large rod-like AlN particles, resulting in the brittle creep fracture mode. - Abstract: Oxide dispersion strengthened (ODS) alloys have been widely considered as candidate structural materials for advanced nuclear reactors because of their good high temperature properties and irradiation resistance. In this study, the creep properties of oxide dispersion strengthened Alloy 617 (ODS-617) was examined at 950 °C under 25 MPa in the air. Conventional Alloy 617 tested under the same conditions was included for comparison. The ODS-617 exhibited superior creep resistance, including a much longer rupture life of 8533 h and a steady-state creep rate of 8.9 × 10{sup −8} (1/s), which was two orders of magnitude lower than that of the conventional alloy. The lower steady-state creep rate was related to the dispersion of nano-sized Y-Al-O particles, which may impede grain boundary diffusion. It seems that cracks preferentially developed near some large rod-like AlN particles, resulting in the brittle creep fracture mode.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Impact of boundary layer simulation on predicting radioactive pollutant dispersion: a case study for HANARO Research Reactor using the WRF-MMIF-CALPUFF modeling system

Wind plays an important role in cases of unexpected radioactive pollutant dispersion, deciding distribution and concentration of the leaked substance. The accurate prediction of wind has been challenging in numerical weather prediction models, especially near the surface because of the complex interaction between turbulent flow and topographic effect. As such, in this study, we investigated the characteristics of atmospheric dispersion of radioactive material (i.e. 137 Cs) according to the simulated boundary layer around the HANARO research nuclear reactor in Korea using the Weather Research and Forecasting (WRF)-Mesoscale Model Interface (MMIF)-California Puff (CALPUFF) model system. We examined the impacts of orographic drag on wind field, stability calculation methods, and planetary boundary layer parameterizations on the dispersion of radioactive material under a radioactive leaking scenario. We found that inclusion of the orographic drag effect in the WRF model improved the wind prediction most significantly over the complex terrain area, leading the model system to estimate the radioactive concentration near the reactor more conservatively. We also emphasized the importance of the stability calculation method and employing the skillful boundary layer parameterization to ensure more accurate low atmospheric conditions, in order to simulate more feasible spatial distribution of the radioactive dispersion in leaking scenarios.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2013

Derivation and generalization of the dispersion relation of rising-sun magnetron with sectorial and rectangular cavities

Field analysis method is used to derive the dispersion relation of rising-sun magnetron with sectorial and rectangular cavities. This dispersion relation is then extended to the general case in which the rising-sun magnetron can be with multi-group cavities of different shapes and sizes, and from which the dispersion relations of conventional magnetron, rising-sun magnetron, and magnetron-like device can be obtained directly. The results show that the relative errors between the theoretical and simulation values of the dispersion relation are less than 3%, the relative errors between the theoretical and simulation values of the cutoff frequencies of π mode are less than 2%. In addition, the influences of each structure parameter of the magnetron on the cutoff frequency of π mode and on the mode separation are investigated qualitatively and quantitatively, which may be of great interest to designing a frequency tuning magnetron.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

A dynamic model for the Lagrangian stochastic dispersion coefficient

A stochastic sub-grid model is often used to accurately represent particle dispersion in turbulent flows using large eddy simulations. Models of this type have a free parameter, the dispersion coefficient, which is not universal and is strongly grid-dependent. In the present paper, a dynamic model for the evaluation of the coefficient is proposed and validated in decaying homogeneous isotropic turbulence. The grid dependence of the static coefficient is investigated in a turbulent mixing layer and compared to the dynamic model. The dynamic model accurately predicts dispersion statistics and resolves the grid-dependence. Dispersion statistics of the dynamically calculated constant are more accurate than any static coefficient choice for a number of grid spacings. Furthermore, the dynamic model produces less numerical artefacts than a static model and exhibits smaller sensitivity in the results predicted for different particle relaxation times.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Metallophilic interactions from dispersion-corrected density-functional theory

In this article, we present the first comprehensive study of metallophilic (aurophilic) interactions using dispersion-corrected density-functional theory. Dispersion interactions (an essential component of metallophilicity) are treated using the exchange-hole dipole moment (XDM) model. By comparing against coupled-cluster benchmark calculations on simple dimers, we show that LC-ωPBE-XDM is a viable functional to study interactions between closed-shell transition metals and that it performs uniformly better than second-order Møller-Plesset theory, the basic computational technique used in previous works. We apply LC-ωPBE-XDM to address several open questions regarding metallophilicity, such as the interplay between dispersion and relativistic effects, the interaction strength along group 11, the additivity of homo- and hetero-metallophilic effects, the stability of [E(AuPH{sub 3}){sub 4}]{sup +} cations (E = N, P, As, Sb), and the role of metallophilic effects in crystal packing. We find that relativistic effects explain the prevalence of aurophilicity not by stabilizing metal-metal contacts, but by preventing gold from forming ionic structures involving bridge anions (which are otherwise common for Ag and Cu) as a result of the increased electron affinity of the metal. Dispersion effects are less important than previously assumed and their stabilization contribution is relatively independent of the metal.

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

Dispersion characteristics of three-dimensional dielectric-loaded grating for terahertz Smith-Purcell radiation

In this paper, a dielectric-loaded grating for Smith-Purcell device is proposed. The three-dimensional (3D) analytical theory for hot dispersion relation is obtained by using field matched method, which is solved by numerical simulations. The first and second order growth rates for the proposal model are analyzed, which is obtained by expanding hot dispersion equation at the operating point. The results show that the dispersion can be effectively weakened by introducing dielectric-loaded grating, in which the cutoff frequency is affected by the grating thickness. The dispersion curve becomes flatter and shifts towards lower frequency at the optimum grating parameters. The 3D particle-in-cell (PIC) simulation is also performed and the results are in good agreement with theoretical calculations. Comparing the first order growth rate with the second one, it reveals that the discrepancy is small when electron beam parameters are selected with small values. Otherwise, the discrepancy is large and cannot be ignored. To accurately describe the process of beam-wave interaction, the second order growth rate is necessary to apply.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Localized overlap algorithm for unexpanded dispersion energies

First-principles-based, linearly scaling algorithm has been developed for calculations of dispersion energies from frequency-dependent density susceptibility (FDDS) functions with account of charge-overlap effects. The transition densities in FDDSs are fitted by a set of auxiliary atom-centered functions. The terms in the dispersion energy expression involving products of such functions are computed using either the unexpanded (exact) formula or from inexpensive asymptotic expansions, depending on the location of these functions relative to the dimer configuration. This approach leads to significant savings of computational resources. In particular, for a dimer consisting of two elongated monomers with 81 atoms each in a head-to-head configuration, the most favorable case for our algorithm, a 43-fold speedup has been achieved while the approximate dispersion energy differs by less than 1% from that computed using the standard unexpanded approach. In contrast, the dispersion energy computed from the distributed asymptotic expansion differs by dozens of percent in the van der Waals minimum region. A further increase of the size of each monomer would result in only small increased costs since all the additional terms would be computed from the asymptotic expansion.

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

Nonreciprocal dispersion of spin waves in ferromagnetic thin films covered with a finite-conductivity metal

We study the effect of one-side metallization of a uniform ferromagnetic thin film on its spin-wave dispersion relation in the Damon–Eshbach geometry. Due to the finite conductivity of the metallic cover layer on the ferromagnetic film, the spin-wave dispersion relation may be nonreciprocal only in a limited wave-vector range. We provide an approximate analytical solution for the spin-wave frequency, discuss its validity, and compare it with numerical results. The dispersion is analyzed systematically by varying the parameters of the ferromagnetic film, the metal cover layer and the value of the external magnetic field. The conclusions drawn from this analysis allow us to define a structure based on a 30 nm thick CoFeB film with an experimentally accessible nonreciprocal dispersion relation in a relatively wide wave-vector range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Computation of generalized and exact dispersion relations for longitudinal plasma waves in nonextensive statistics and the effects of the nonextensivity on the oscillation modes and damps

We have derived generalized dispersion relations for longitudinal waves in collisionless thermal plasma using linear Vlasov-Poisson kinetic model and nonextensive distributions for electrons. The Maxwellian limit of the dispersion relations, where the q-nonextensive parameter tends to one, is calculated. The generalized dispersion relations are reduced to polynomials for some specific values of q. The well-known modes of oscillations such as the Langmuir and electron acoustic waves have been obtained by solving the dispersion relations. Some new modes of oscillation are also found. Finally, the dependence of the oscillation modes and damps on q is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Tunable effective nonlinear refractive index of graphene dispersions during the distortion of spatial self-phase modulation

Spatial self-phase modulation (SSPM) was observed directly when a focused He-Ne laser beam at 633 nm went through liquid-phase-exfoliated graphene dispersions. The diffraction pattern of SSPM was found to be distorted rapidly right after the incident beam horizontally passing through the dispersions, while no distortion for the vertically incident geometry. We show that the distortion is originated mainly from the non-axis-symmetrical thermal convections of the graphene nanosheets induced by laser heating, and the relative change of nonlinear refractive index can be determined by the ratio of the distortion angle to the half-cone angle. Therefore, the effective nonlinear refractive index of graphene dispersions can be tuned by changing the incident intensity and the temperature of the dispersions.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Dispersion interferometer using modulation amplitudes on LHD (invited)

Since a dispersion interferometer is insensitive to mechanical vibrations, a vibration compensation system is not necessary. The CO{sub 2} laser dispersion interferometer with phase modulations on the Large Helical Device utilizes the new phase extraction method which uses modulation amplitudes and can improve a disadvantage of the original dispersion interferometer: measurement errors caused by variations of detected intensities. The phase variation within ±2 × 10{sup 17} m{sup −3} is obtained without vibration compensation system. The measured line averaged electron density with the dispersion interferometer shows good agreement with that with the existing far infrared laser interferometer. Fringe jump errors in high density ranging up to 1.5 × 10{sup 20} m{sup −3} can be overcome by a sufficient sampling rate of about 100 kHz.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Extending the applicability of the Tkatchenko-Scheffler dispersion correction via iterative Hirshfeld partitioning

Recently we have demonstrated that the applicability of the Tkatchenko-Scheffler (TS) method for calculating dispersion corrections to density-functional theory can be extended to ionic systems if the Hirshfeld method for estimating effective volumes and charges of atoms in molecules or solids (AIM’s) is replaced by its iterative variant [T. Bučko, S. Lebègue, J. Hafner, and J. Ángyán, J. Chem. Theory Comput. 9, 4293 (2013)]. The standard Hirshfeld method uses neutral atoms as a reference, whereas in the iterative Hirshfeld (HI) scheme the fractionally charged atomic reference states are determined self-consistently. We show that the HI method predicts more realistic AIM charges and that the TS/HI approach leads to polarizabilities and C{sub 6} dispersion coefficients in ionic or partially ionic systems which are, as expected, larger for anions than for cations (in contrast to the conventional TS method). For crystalline materials, the new algorithm predicts polarizabilities per unit cell in better agreement with the values derived from the Clausius-Mosotti equation. The applicability of the TS/HI method has been tested for a wide variety of molecular and solid-state systems. It is demonstrated that for systems dominated by covalent interactions and/or dispersion forces the TS/HI method leads to the same results as the conventional TS approach. The difference between the TS/HI and TS approaches increases with increasing ionicity. A detailed comparison is presented for isoelectronic series of octet compounds, layered crystals, complex intermetallic compounds, and hydrides, and for crystals built of molecules or containing molecular anions. It is demonstrated that only the TS/HI method leads to accurate results for systems where both electrostatic and dispersion interactions are important, as illustrated for Li-intercalated graphite and for molecular adsorption on the surfaces in ionic solids and in the cavities of zeolites.

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

Kinetic transverse dispersion relation for relativistic magnetized electron-positron plasmas with Maxwell-Jüttner velocity distribution functions

We use a kinetic treatment to study the linear transverse dispersion relation for a magnetized isotropic relativistic electron-positron plasma with finite relativistic temperature. The explicit linear dispersion relation for electromagnetic waves propagating along a constant background magnetic field is presented, including an analytical continuation to the whole complex frequency plane for the case of Maxwell-Jüttner velocity distribution functions. This dispersion relation is studied numerically for various temperatures. For left-handed solutions, the system presents two branches, the electromagnetic ordinary mode and the Alfvén mode. In the low frequency regime, the Alfvén branch has two dispersive zones, the normal zone (where ∂ω/∂k > 0) and an anomalous zone (where ∂ω/∂k < 0). We find that in the anomalous zone of the Alfvén branch, the electromagnetic waves are damped, and there is a maximum wave number for which the Alfvén branch is suppressed. We also study the dependence of the Alfvén velocity and effective plasma frequency with the temperature. We complemented the analytical and numerical approaches with relativistic full particle simulations, which consistently agree with the analytical results.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Communication: A combined periodic density functional and incremental wave-function-based approach for the dispersion-accounting time-resolved dynamics of {sup 4}He nanodroplets on surfaces: {sup 4}He/graphene

In this work we propose a general strategy to calculate accurate He–surface interaction potentials. It extends the dispersionless density functional approach recently developed by Pernal et al. [Phys. Rev. Lett. 103, 263201 (2009)] to adsorbate-surface interactions by including periodic boundary conditions. We also introduce a scheme to parametrize the dispersion interaction by calculating two- and three-body dispersion terms at coupled cluster singles and doubles and perturbative triples (CCSD(T)) level via the method of increments [H. Stoll, J. Chem. Phys. 97, 8449 (1992)]. The performance of the composite approach is tested on {sup 4}He/graphene by determining the energies of the low-lying selective adsorption states, finding an excellent agreement with the best available theoretical data. Second, the capability of the approach to describe dispersionless correlation effects realistically is used to extract dispersion effects in time-dependent density functional simulations on the collision of {sup 4}He droplets with a single graphene sheet. It is found that dispersion effects play a key role in the fast spreading of the {sup 4}He nanodroplet, the evaporation-like process of helium atoms, and the formation of solid-like helium structures. These characteristics are expected to be quite general and highly relevant to explain experimental measurements with the newly developed helium droplet mediated deposition technique.

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

Gyro-viscosity and linear dispersion relations in pair-ion magnetized plasmas

A fluid theory has been developed by taking account of gyro-viscosity to study wave propagation characteristics in a homogeneous pair-ion magnetized plasma with a cylindrical symmetry. The exact dispersion relations derived by the Hankel-Fourier transformation are shown comparable with those observed in the experiment by Oohara and co-workers. The gyro-viscosity is responsible for the change in propagation characteristics of the ion cyclotron wave from forward to backward by suppressing the effect of the thermal pressure which normally causes the forward nature of dispersion. Although the experiment has been already explained by a kinetic theory by the present authors, the kinetic derivations are so involved because of exact particle orbits in phase space, finite Lamor radius effects, and higher order ion cyclotron resonances. The present fluid theory provides a simple and transparent structure to the dispersion relations since the gyro-viscosity is renormalized into the ion cyclotron frequency which itself indicates the backward nature of dispersion. The usual disadvantage of a fluid theory, which treats only fundamental modes of eigen-waves excited in a system and is not able to describe higher harmonics that a kinetic theory does, is compensated by simple derivations and clear picture based on the renormalization of the gyro-viscosity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Analysis of limited-diffractive and limited-dispersive X-waves generated by finite radial waveguides

In this work, we analyze the spatial and temporal features of electromagnetic X-waves propagating in free space and generated by planar radiating apertures. The performance of ideal X-waves is discussed and compared to practical cases where the important effects related to the finiteness of the radiating aperture and the wavenumber dispersion are taken into account. In particular, a practical device consisting of a radial waveguide loaded with radiating slots aligned along a spiral path is considered for the practical case in the millimeter-wave range. A common mathematical framework is defined for a precise comparison of the spatiotemporal properties and focusing capabilities of the generated X-wave. It is clearly shown that the fractional bandwidth of the radiating aperture has a key role in the longitudinal confinement of an X-wave in both ideal and practical cases. In addition, the finiteness of the radiating aperture as well as the wavenumber dispersion clearly affect both the transverse and the longitudinal profiles of the generated radiation as it travels beyond the depth-of-field of the generated X-wave. Nevertheless, the spatiotemporal properties of the X-wave are preserved even in this “dispersive-finite” case within a defined region and duration related to the nondiffractive range and fractional bandwidth of the spectral components of the generated X-wave. The proposed analysis may open new perspectives for the efficient generation of X-waves over finite radiating apertures at millimeter waves where the dispersive behavior of realistic devices is no longer negligible.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗