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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,441 records · Page 80
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Determination of oxygen diffusion kinetics during thin film ruthenium oxidation

In situ X-ray reflectivity was used to reveal oxygen diffusion kinetics for thermal oxidation of polycrystalline ruthenium thin films and accurate determination of activation energies for this process. Diffusion rates in nanometer thin RuO{sub 2} films were found to show Arrhenius behaviour. However, a gradual decrease in diffusion rates was observed with oxide growth, with the activation energy increasing from about 2.1 to 2.4 eV. Further exploration of the Arrhenius pre-exponential factor for diffusion process revealed that oxidation of polycrystalline ruthenium joins the class of materials that obey the Meyer-Neldel rule.

36 MATERIALS SCIENCE↗
DOE OSTI2015

A study on atomic diffusion behaviours in an Al-Mg compound casting process

Al and Mg alloys are main lightweight alloys of research interest and they both have superb material properties, i.e., low density and high specific strength, etc. Being different from Al alloys, the corrosion of Mg alloys is much more difficult to control. Therefore to combine merits of these two lightweight alloys as a composite-like structure is an ideal solution through using Al alloys as a protective layer for Mg alloys. Compound casting is a realistic technique to manufacture such a bi-metal structure. In this study, a compound casting technique is employed to fabricate bi-layered samples using Al and Mg and then the samples are analysed using electron probe micro-analyzer (EPMA) to determine diffusion behaviours between Al and Mg. The diffusion mechanism and behaviours between Al and Mg are studied numerically at atomic scale using molecular dynamics (MD) and parametric studies are conducted to find out influences of ambient temperature and pressure on the diffusion behaviours between Al and Mg. The results obtained clearly show the effectiveness of the compound casting process to increase the diffusion between Al and Mg and thus create the Al-base protection layer for Mg.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Comment on “Diffusion of n-type dopants in germanium” [Appl. Phys. Rev. 1, 011301 (2014)]

The authors of the above paper call into question recent evidence on the properties of self-interstitials, I, in Ge [Cowern et al., Phys. Rev. Lett. 110, 155501 (2013)]. We show that this judgment stems from invalid model assumptions during analysis of data on B marker-layer diffusion during proton irradiation, and that a corrected analysis fully supports the reported evidence. As previously stated, I-mediated self-diffusion in Ge exhibits two distinct regimes of temperature, T: high-T, dominated by amorphous-like mono-interstitial clusters—i-morphs—with self-diffusion entropy ≈30 k, and low-T, where transport is dominated by simple self-interstitials. In a transitional range centered on 475 °C both mechanisms contribute. The experimental I migration energy of 1.84 ± 0.26 eV reported by the Münster group based on measurements of self-diffusion during irradiation at 550 °C < T < 680 °C further establishes our proposed i-morph mechanism.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Communication: Probing anomalous diffusion in frequency space

Anomalous diffusion processes are usually detected by analyzing the time-dependent mean square displacement of the diffusing particles. The latter evolves asymptotically as W(t) ∼ 2D{sub α}t{sup α}, where D{sub α} is the fractional diffusion constant and 0 < α < 2. In this article we show that both D{sub α} and α can also be extracted from the low-frequency Fourier spectrum of the corresponding velocity autocorrelation function. This offers a simple method for the interpretation of quasielastic neutron scattering spectra from complex (bio)molecular systems, in which subdiffusive transport is frequently encountered. The approach is illustrated and validated by analyzing molecular dynamics simulations of molecular diffusion in a lipid POPC bilayer.

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

Influence of Dupree diffusivity on the occurrence scattering time advance in turbulent plasmas

The influence of Dupree diffusivity on the occurrence scattering time advance for the electron-ion collision is investigated in turbulent plasmas. The second-order eikonal method and the effective Dupree potential term associated with the plasma turbulence are employed to obtain the occurrence scattering time as a function of the diffusion coefficient, impact parameter, collision energy, thermal energy, and Debye length. The result shows that the occurrence scattering time advance decreases with an increase of the Dupree diffusivity. Hence, we have found that the influence of plasma turbulence diminishes the occurrence time advance in forward electron-ion collisions in thermal turbulent plasmas. The occurrence time advance shows that the propensity of the occurrence time advance increases with increasing scattering angle. It is also found that the effect of turbulence due to the Dupree diffusivity on the occurrence scattering time advance decreases with an increase of the thermal energy. In addition, the variation of the plasma turbulence on the occurrence scattering time advance due to the plasma parameters is also discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Insights into thermal diffusion of germanium and oxygen atoms in HfO{sub 2}/GeO{sub 2}/Ge gate stacks and their suppressed reaction with atomically thin AlO{sub x} interlayers

The thermal diffusion of germanium and oxygen atoms in HfO{sub 2}/GeO{sub 2}/Ge gate stacks was comprehensively evaluated by x-ray photoelectron spectroscopy and secondary ion mass spectrometry combined with an isotopic labeling technique. It was found that {sup 18}O-tracers composing the GeO{sub 2} underlayers diffuse within the HfO{sub 2} overlayers based on Fick's law with the low activation energy of about 0.5 eV. Although out-diffusion of the germanium atoms through HfO{sub 2} also proceeded at the low temperatures of around 200 °C, the diffusing germanium atoms preferentially segregated on the HfO{sub 2} surfaces, and the reaction was further enhanced at high temperatures with the assistance of GeO desorption. A technique to insert atomically thin AlO{sub x} interlayers between the HfO{sub 2} and GeO{sub 2} layers was proven to effectively suppress both of these independent germanium and oxygen intermixing reactions in the gate stacks.

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

Coarse-grained model of water diffusion and proton conductivity in hydrated polyelectrolyte membrane

Using dissipative particle dynamics (DPD), we simulate nanoscale segregation, water diffusion, and proton conductivity in hydrated sulfonated polystyrene (sPS). We employ a novel model [Lee et al. J. Chem. Theory Comput. 11(9), 4395-4403 (2015)] that incorporates protonation/deprotonation equilibria into DPD simulations. The polymer and water are modeled by coarse-grained beads interacting via short-range soft repulsion and smeared charge electrostatic potentials. The proton is introduced as a separate charged bead that forms dissociable Morse bonds with the base beads representing water and sulfonate anions. Morse bond formation and breakup artificially mimics the Grotthuss mechanism of proton hopping between the bases. The DPD model is parameterized by matching the proton mobility in bulk water, dissociation constant of benzenesulfonic acid, and liquid-liquid equilibrium of water-ethylbenzene solutions. The DPD simulations semi-quantitatively predict nanoscale segregation in the hydrated sPS into hydrophobic and hydrophilic subphases, water self-diffusion, and proton mobility. As the hydration level increases, the hydrophilic subphase exhibits a percolation transition from isolated water clusters to a 3D network. The analysis of hydrophilic subphase connectivity and water diffusion demonstrates the importance of the dynamic percolation effect of formation and breakup of temporary junctions between water clusters. The proposed DPD model qualitatively predicts the ratio of proton to water self-diffusion and its dependence on the hydration level that is in reasonable agreement with experiments.

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

Effect of Ni diffusion annealing temperature on crucial characterization of Bi-2223 superconducting system

In this current work, influence of diffusion annealing temperature (650-850°C) on the electrical, superconducting and structural characterizations of pure and Ni diffused Bi-2223 materials produced by conventional solid-state reaction route is surveyed by dc electrical resistivity (ρ-T), transport critical current density (J{sub c}) and powder X-ray diffraction (XRD) measurements. All the experimental findings indicate that the curial characteristics, being responsible for the novel and feasible applications, improve significantly with the enhancement of the diffusion annealing temperature up to the certain value of 700 °C after which they degrade dramatically, meaning that the properties initially get better and better as a consequence of the penetration of more and more Ni concentrations into the superconducting grains or over grain boundaries in the Bi-2223 crystal matrix. From the diffusion annealing temperature value of 700 °C onwards, the excess penetration of the Ni nanoparticles along with consecutively stacked layers in the Bi-2223 crystal structure is, however, unfavorable for the attractive and feasible applications due to new induced disorders, local structural distortions and artificial dislocations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

CMOS-compatible method for doping of buried vertical polysilicon structures by solid phase diffusion

Polysilicon receives attention nowadays as a means to incorporate 3D-structured photonic devices into silicon processes. However, doping of buried layers of a typical 3D structure has been a challenge. We present a method for doping of buried polysilicon layers by solid phase diffusion. Using an underlying silicon oxide layer as a dopant source facilitates diffusion of dopants into the bottom side of the polysilicon layer. The polysilicon is grown on top of the oxide layer, after the latter has been doped by ion implantation. Post-growth heat treatment drives in the dopant from the oxide into the polysilicon. To model the process, we studied the diffusion of the two most common silicon dopants, boron (B) and phosphorus (P), using secondary ion mass spectroscopy profiles. Our results show that shallow concentration profiles can be achieved in a buried polysilicon layer using the proposed technique. We present a quantitative 3D model for the diffusion of B and P in polysilicon, which turns the proposed method into an engineerable technique.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

A computational ab initio study of surface diffusion of sulfur on the CdTe (111) surface

In order to discern the formation of epitaxial growth of CdS shell over CdTe nanocrystals, kinetics related to the initial stages of the growth of CdS on CdTe is investigated using ab-initio methods. We report diffusion of sulfur adatom on the CdTe (111) A-type (Cd-terminated) and B-type (Te-terminated) surfaces within the density functional theory (DFT). The barriers are computed by applying the climbing Nudge Elastic Band (c-NEB) method. From the results surface hopping emerges as the major mode of diffusion. In addition, there is a distinct contribution from kick-out type diffusion in which a CdTe surface atom is kicked out from its position and is replaced by the diffusing sulfur atom. Also, surface vacancy substitution contributes to the concomitant dynamics. There are sites on the B- type surface that are competitively close in terms of the binding energy to the lowest energy site of epitaxy on the surface. The kick-out process is more likely for B-type surface where a Te atom of the surface is displaced by a sulfur adatom. Further, on the B-type surface, subsurface migration of sulfur is indicated. Furthermore, the binding energies of S on CdTe reveal that on the A-type surface, epitaxial sites provide relatively higher binding energies and barriers than on B-type.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Machine learning prediction of self-diffusion in Lennard-Jones fluids

In this work, different machine learning (ML) methods were explored for the prediction of self-diffusion in Lennard-Jones (LJ) fluids. Using a database of diffusion constants obtained from the molecular dynamics simulation literature, multiple Random Forest (RF) and Artificial Neural Net (ANN) regression models were developed and characterized. The role and improved performance of feature engineering coupled to the RF model development was also addressed. The performance of these different ML models was evaluated by comparing the prediction error to an existing empirical relationship used to describe LJ fluid diffusion. It was found that the ANN regression models provided superior prediction of diffusion in comparison to the existing empirical relationships.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Intrinsic anion diffusivity in lead halide perovskites is facilitated by a soft lattice

Significance Understanding and controlling the ion migration in a solid-state semiconductor is essential for device performance in modern photonics, electronics, and optoelectronics. Direct microstructural observation and atomic-scale mechanism of the intrinsic lattice diffusion are still highly challenging. Here, we report the direct visualization of halide anion interdiffusion in CsPbCl 3 –CsPbBr 3 single crystalline nanowire heterojunctions via nanoscale imaging, allowing for the direct measuring of intrinsic lattice diffusivities. The intrinsic halide diffusivities were found to be several orders of magnitudes lower than those reported in polycrystalline thin films. Together with multiscale modeling, we further demonstrate how anionic diffusion in lead halides is determined by low barriers to vacancy hopping and facilitated by the soft perovskite lattice.

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

Single-molecule diffusometry reveals no catalysis-induced diffusion enhancement of alkaline phosphatase as proposed by FCS experiments

Significance Recent experiments have suggested that the energy released by a chemical reaction can propel its enzyme catalyst (for example, alkaline phosphatase). However, this topic remains controversial, partially due to the indirect and ensemble nature of existing measurements. Here, we used recently developed single-molecule approaches to monitor directly the motions of individual proteins in aqueous solution and find that single alkaline phosphatase enzymes do not diffuse faster under catalysis. Instead, we demonstrate that interactions between the fluorescent dye and the enzyme’s substrate can produce the signature of apparent diffusion enhancement in fluorescence correlation spectroscopy, the standard ensemble assay currently used to study enzyme diffusion and indicate that single-molecule approaches provide a more robust means to investigate diffusion at the nanoscale.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Cross-correlation analysis of X-ray photon correlation spectroscopy to extract rotational diffusion coefficients

Coefficients for translational and rotational diffusion characterize the Brownian motion of particles. Emerging X-ray photon correlation spectroscopy (XPCS) experiments probe a broad range of length scales and time scales and are well-suited for investigation of Brownian motion. While methods for estimating the translational diffusion coefficients from XPCS are well-developed, there are no algorithms for measuring the rotational diffusion coefficients based on XPCS, even though the required raw data are accessible from such experiments. In this paper, we propose angular-temporal cross-correlation analysis of XPCS data and show that this information can be used to design a numerical algorithm (Multi-Tiered Estimation for Correlation Spectroscopy [MTECS]) for predicting the rotational diffusion coefficient utilizing the cross-correlation: This approach is applicable to other wavelengths beyond this regime. We verify the accuracy of this algorithmic approach across a range of simulated data.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2012

Binding Preferences, Surface Attachment, Diffusivity, and Orientation of a Family 1 Carbohydrate-Binding Module on Cellulose

Cellulase enzymes often contain carbohydrate-binding modules (CBMs) for binding to cellulose. The mechanisms by which CBMs recognize specific surfaces of cellulose and aid in deconstruction are essential to understand cellulase action. The Family 1 CBM from the Trichoderma reesei Family 7 cellobiohydrolase, Cel7A, is known to selectively bind to hydrophobic surfaces of native cellulose. It is most commonly suggested that three aromatic residues identify the planar binding face of this CBM, but several recent studies have challenged this hypothesis. Here, we use molecular simulation to study the CBM binding orientation and affinity on hydrophilic and hydrophobic cellulose surfaces. Roughly 43 {mu}s of molecular dynamics simulations were conducted, which enables statistically significant observations. We quantify the fractions of the CBMs that detach from crystal surfaces or diffuse to other surfaces, the diffusivity along the hydrophobic surface, and the overall orientation of the CBM on both hydrophobic and hydrophilic faces. The simulations demonstrate that there is a thermodynamic driving force for the Cel7A CBM to bind preferentially to the hydrophobic surface of cellulose relative to hydrophilic surfaces. In addition, the simulations demonstrate that the CBM can diffuse from hydrophilic surfaces to the hydrophobic surface, whereas the reverse transition is not observed. Lastly, our simulations suggest that the flat faces of Family 1 CBMs are the preferred binding surfaces. These results enhance our understanding of how Family 1 CBMs interact with and recognize specific cellulose surfaces and provide insights into the initial events of cellulase adsorption and diffusion on cellulose.

09 BIOMASS FUELS↗
DOE OSTI2011

THE FERMI GAMMA-RAY HAZE FROM DARK MATTER ANNIHILATIONS AND ANISOTROPIC DIFFUSION

Recent full-sky maps of the Galaxy from the Fermi Gamma-Ray Space Telescope have revealed a diffuse component of emission toward the Galactic center and extending up to roughly {+-}50{sup 0} in latitude. This Fermi 'haze' is the inverse Compton emission generated by the same electrons that generate the microwave synchrotron haze at Wilkinson Microwave Anisotropy Probe wavelengths. The gamma-ray haze has two distinct characteristics: the spectrum is significantly harder than emission elsewhere in the Galaxy and the morphology is elongated in latitude with respect to longitude with an axis ratio of {approx}2. If these electrons are generated through annihilations of dark matter (DM) particles in the Galactic halo, this morphology is difficult to realize with a standard spherical halo and isotropic cosmic-ray (CR) diffusion. However, we show that anisotropic diffusion along ordered magnetic field lines toward the center of the Galaxy coupled with a prolate DM halo can easily yield the required morphology without making unrealistic assumptions about diffusion parameters. Furthermore, a Sommerfeld enhancement to the self-annihilation cross-section of {approx}30 yields a good fit to the morphology, amplitude, and spectrum of both the gamma-ray and microwave haze. The model is also consistent with local CR measurements as well as cosmic microwave background constraints.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

EFFECTS OF PERPENDICULAR DIFFUSION ON ENERGETIC PARTICLES ACCELERATED BY THE INTERPLANETARY CORONAL MASS EJECTION SHOCK

In this work, based on a numerical solution of the focused transport equation, we obtained the intensity and anisotropy time profiles of solar energetic particles (SEPs) accelerated by an interplanetary shock in the three-dimensional Parker magnetic field. The shock is treated as a moving source of energetic particles with an assumed particle distribution function. We computed the time profiles of particle flux and anisotropy as measured by an observer at 1 AU, equatorial plane, and various longitudes with respect to the shock propagation direction. With perpendicular diffusion, energetic particles can cross magnetic field lines. Particles may be detected before the observer's field line is connected to the shock. After the observer's field line breaks from the shock front, the observer still can see more particles are injected into its field line. Our simulations show that the particle onset time, peak time, peak intensity, decay rate, and duration of SEP event could be significantly influenced by the effect of perpendicular diffusion. The anisotropy with perpendicular diffusion is almost the same as that without perpendicular diffusion, but there is an obvious difference at the moment when the observer's field line begins to be connected to the shock.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THEORETICAL EXPLANATION OF THE COSMIC-RAY PERPENDICULAR DIFFUSION COEFFICIENT IN THE NEARBY STARBURST GALAXY NGC 253

Diffusion coefficients are usually used to describe the propagation of cosmic rays through the universe. Whereas such transport parameters can be obtained from experiments in the solar system, it is difficult to determine diffusion coefficients in the Milky Way or in external galaxies. Recently, a value for the perpendicular diffusion coefficient in the nearby starburst galaxy NGC 253 has been proposed. In the present paper, we reproduce this value theoretically by using an advanced analytical theory for perpendicular diffusion.

79 ASTRONOMY AND ASTROPHYSICS↗