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Results for “DIFFUSION THEORY”

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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

First-principles multiple-barrier diffusion theory. The case study of interstitial diffusion in CdTe

The diffusion of particles in solid-state materials generally involves several sequential thermal-activation processes. However, presently, diffusion coefficient theory only deals with a single barrier, i.e., it lacks an accurate description to deal with multiple-barrier diffusion. Here, we develop a general diffusion coefficient theory for multiple-barrier diffusion. Using our diffusion theory and first-principles calculated hopping rates for each barrier, we calculate the diffusion coefficients of Cd, Cu, Te, and Cl interstitials in CdTe for their full multiple-barrier diffusion pathways. As a result, we found that the calculated diffusivity agrees well with the experimental measurement, thus justifying our theory, which is general for many other systems.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Peptide dynamics by molecular dynamics simulation and diffusion theory method with improved basis sets

Improved basis sets for the study of polymer dynamics by means of the diffusion theory, and tests on a melt of cis-1,4-polyisoprene decamers, and a toluene solution of a 71-mer syndiotactic trans-1,2-polypentadiene were presented recently [R. Gaspari and A. Rapallo, J. Chem. Phys. 128, 244109 (2008)]. The proposed hybrid basis approach (HBA) combined two techniques, the long time sorting procedure and the maximum correlation approximation. The HBA takes advantage of the strength of these two techniques, and its basis sets proved to be very effective and computationally convenient in describing both local and global dynamics in cases of flexible synthetic polymers where the repeating unit is a unique type of monomer. The question then arises if the same efficacy continues when the HBA is applied to polymers of different monomers, variable local stiffness along the chain and with longer persistence length, which have different local and global dynamical properties against the above-mentioned systems. Important examples of this kind of molecular chains are the proteins, so that a fragment of the protein transthyretin is chosen as the system of the present study. This peptide corresponds to a sequence that is structured in β-sheets of the protein and is located on the surface of the channel with thyroxin. The protein transthyretin forms amyloid fibrils in vivo, whereas the peptide fragment has been shown [C. P. Jaroniec, C. E. MacPhee, N. S. Astrof, C. M. Dobson, and R. G. Griffin, Proc. Natl. Acad. Sci. U.S.A. 99, 16748 (2002)] to form amyloid fibrils in vitro in extended β-sheet conformations. For these reasons the latter is given considerable attention in the literature and studied also as an isolated fragment in water solution where both experimental and theoretical efforts have indicated the propensity of the system to form β turns or α helices, but is otherwise predominantly unstructured. Differing from previous computational studies that employed implicit solvent, we performed in this work the classical molecular dynamics simulation on a realistic model solution with the peptide embedded in an explicit water environment, and calculated its dynamic properties both as an outcome of the simulations, and by the diffusion theory in reduced statistical-mechanical approach within HBA on the premise that the mode-coupling approach to the diffusion theory can give both the long-range and local dynamics starting from equilibrium averages which were obtained from detailed atomistic simulations.

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

Detailed numerical investigation of the Bohm limit in cosmic ray diffusion theory

A standard model in cosmic ray diffusion theory is the so-called Bohm limit in which the particle mean free path is assumed to be equal to the Larmor radius. This type of diffusion is often employed to model the propagation and acceleration of energetic particles. However, recent analytical and numerical work has shown that standard Bohm diffusion is not realistic. In the present paper, we perform test-particle simulations to explore particle diffusion in the strong turbulence limit in which the wave field is much stronger than the mean magnetic field. We show that there is indeed a lower limit of the particle mean free path along the mean field. In this limit, the mean free path is directly proportional to the unperturbed Larmor radius like in the traditional Bohm limit, but it is reduced by the factor δB/B {sub 0} where B {sub 0} is the mean field and δB the turbulent field. Although we focus on parallel diffusion, we also explore diffusion across the mean field in the strong turbulence limit.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Application of subgroup decomposition in diffusion theory to gas cooled thermal reactor problem

In this paper, the accuracy and computational efficiency of the subgroup decomposition (SGD) method in diffusion theory is assessed in a ID benchmark problem characteristic of gas cooled thermal systems. This method can be viewed as a significant improvement in accuracy of standard coarse-group calculations used for VHTR whole core analysis in which core environmental effect and energy angle coupling are pronounced. It is shown that a 2-group SGD calculation reproduces fine-group (47) results with 1.5 to 6 times faster computational speed depending on the stabilizing schemes while it is as efficient as single standard 6-group diffusion calculation. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2016

A Variational Nodal Approach to 2D/1D Pin Resolved Neutron Transport: I Diffusion Theory

The variational nodal method (VNM) as implemented in the VARIANT code has been widely employed to fast reactor cores, but much less so for thermal reactors. In what follows we report the initial phase of development of the PANX code (Purdue - Argonne - Northwestern - Xi'an) for three-dimensional pin resolved PWR transport calculations. In two dimensions, prototype modifications of VARIANT using finite elements to obtain heterogeneous nodes and quasi-reflected interface conditions to reduce the high angular-order interface conditions, have been successfully employed to the 2D C5G7 benchmark. In three dimensions, however, the polynomial interface conditions employed in VARIANT smear fuel and coolant at each axial interface, leading to lack of convergence as the axial mesh is refined. This problem is also encountered in the 2D/1D approximations in MOC based methods. The intent of this work is to eliminate these interface homogenization effects from VNM calculations. For clarity, in this work we examine these phenomena in diffusion theory. The next stage of development will incorporate spherical harmonic trial functions to achieve more accurate transport solutions. (authors)

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Generalized theory of diffusion based on kinetic theory

Here, we propose to use spin hydrodynamics, a two-fluid model of spin propagation, as a generalization of the diffusion equation. We show that in the dense limit spin hydrodynamics reduces to Fick's law and the diffusion equation. In the opposite limit spin hydrodynamics is equivalent to a collisionless Boltzmann treatment of spin propagation. Spin hydrodynamics avoids unphysical effects that arise when the diffusion equation is used to describe to a strongly interacting gas with a dilute corona. We apply spin hydrodynamics to the problem of spin diffusion in a trapped atomic gas. We find that the observed spin relaxation rate in the high-temperature limit [Sommer et al., Nature (London) 472, 201 (2011)] is consistent with the diffusion constant predicted by kinetic theory.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Statistical theory of diffusion in concentrated bcc and fcc alloys and concentration dependencies of diffusion coefficients in bcc alloys FeCu, FeMn, FeNi, and FeCr

The statistical theory of diffusion in concentrated bcc and fcc alloys with arbitrary pairwise interatomic interactions based on the master equation approach is developed. Vacancy–atom correlations are described using both the second-shell-jump and the nearest-neighbor-jump approximations which are shown to be usually sufficiently accurate. General expressions for Onsager coefficients in terms of microscopic interatomic interactions and some statistical averages are given. Both the analytical kinetic mean-field and the Monte Carlo methods for finding these averages are described. The theory developed is used to describe sharp concentration dependencies of diffusion coefficients in several iron-based alloy systems. For the bcc alloys FeCu, FeMn, and FeNi, we predict the notable increase of the iron self-diffusion coefficient with solute concentration c, up to several times, even though values of c possible for these alloys do not exceed some percent. For the bcc alloys FeCr at high temperatures T ≳ 1400 K, we show that the very strong and peculiar concentration dependencies of both tracer and chemical diffusion coefficients observed in these alloys can be naturally explained by the theory, without invoking exotic models discussed earlier.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Macromolecule Sorption and Diffusion in HEMA/MAA Hydrogels

Transient solute absorption and desorption concentration profiles were measured in a 70 wt % hydroxyethyl methacrylate (HEMA)/30 wt % methacrylic acid (MAA) anionic hydrogel using two-photon confocal microscopy. Dilute aqueous solutes included fluorescently labeled dextrans with molecular masses of 4, 10, and 20 kDa, and fluorescently labeled cationic avidin protein. Cross-linking densities with ethylene glycol dimethacrylate (EGDMA) varied from 0 to 1 wt % with polymer volume fractions increasing from 0.15 to 0.25. Average gel mesh sizes, determined from zero-frequency oscillatory shear storage moduli, ranged from about 3.6 to 8.4 nm over the cross-link ratios studied. All solutes exhibit Stokes-Einstein hydrodynamic radii obtained from measured free diffusion coefficients, D o , comparable to or larger than the average gel mesh size. In spite of considerable size exclusion, the studied solutes penetrate the gels indicating a range of mesh sizes available for transport. Transient uptake and release concentration profiles for FITC-dextrans follow simple diffusion theory with diffusion coefficients, D , essentially independent of loading or release characteristic of reversible absorption. Although strongly size-excluded, these solutes do not interact specifically with the polymer network. Diffusivities are accordingly predicted from a large-pore effective-medium (LPEM) model developed to account for solute size, hydrodynamic drag, and distribution of mesh sizes available for transport in the polymer network. For this class of solute, anrs of magnitude slower than that of dextran of similar hydrodynamic radius. Desorption of avidin is not cd using no adjustable parameters, diffusivities predicted from the new effective-medium model demonstrate good agreement with experiment. For the specific-interacting cationic protein, avidin, gel loading is 3 ordeomplete even after 2 weeks of extraction. On the basis of size alone, avidin is strongly size-excluded, yet it exhibits a partition coefficient of over 20. For the positively charged protein, we observed specific ion binding on the negatively charged carboxylate groups of MAA-decorated polymer strands in the larger mesh spaces. Finally, simple linear sorption kinetics gives an adsorption time constant of 5 min and a desorption time constant of about 20 days, suggesting nearly irreversible uptake of cationic avidin on the anionic gel matrix.

diffusion↗
DOE OSTI2022

The binding of atomic hydrogen on graphene from density functional theory and diffusion Monte Carlo calculations

In this research, density functional theory (DFT) and diffusion Monte Carlo (DMC) methods are used to calculate the binding energy of a H atom chemisorbed on the graphene surface. The DMC value of the binding energy is about 16% smaller in magnitude than the Perdew–Burke–Ernzerhof (PBE) result. The inclusion of exact exchange through the use of the Heyd–Scuseria–Ernzerhof functional brings the DFT value of the binding energy closer in line with the DMC result. It is also found that there are significant differences in the charge distributions determined using PBE and DMC approaches.

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

A Variational Nodal Approach to 2D/1D Pin Resolved Neutron Transport: II Spherical Harmonics

The variational nodal method (VNM) as implemented in the VARIANT code has found wide application to fast reactor problems where fuel assembly homogenization does not result in unacceptable errors. The objective of this research is to generalize VNM for application to PWR problems, where homogenization even at the pin-cell level often results in unacceptable errors. To accomplish this, we are developing the PANX code (Purdue - Argonne - Northwestern - Xi'an) to implement a 2D/1D VNM transport method without spatial homogenization employing spherical harmonics and diffusion theory, respectively, in the radial and axial directions. In the preceding paper, we employed diffusion theory in conjunction with finite element methods to implement heterogeneous VNM nodes, in which the problem of fuel-coolant smearing at axial interfaces, found in existing 2D/1D approximations, is eliminated. In this work, we generalize the resulting VNM nodes to couple spherical harmonics in the x-y plane with axial diffusion theory. We then apply the method to the three-dimensional C5G7 benchmark problem. (authors)

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

New theory of diffusive and coherent nature of optical wave via a quantum walk

We propose a new theory on a relation between diffusive and coherent nature in one dimensional wave mechanics based on a quantum walk. It is known that the quantum walk in homogeneous matrices provides the coherent property of wave mechanics. Using the recent result of a localization phenomenon in a one-dimensional quantum walk (Konno, 2010), we numerically show that the randomized localized matrices suppress the coherence and give diffusive nature. - Highlights: • A relation between diffusive and coherent nature of wave is obtained. • A rigorous decoherent process is observed in quantum walk. • Quantum walk for a point-process is numerically considered. • Transfer matrix theory with S-matrix in optics is connected with quantum walk.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Effects of interface electric field on the magnetoresistance in spin devices

An extension of the standard spin diffusion theory is presented by using a quantum diffusion theory via a density-gradient (DG) term that is suitable for describing interface quantum tunneling phenomena. The magnetoresistance (MR) ratio is greatly modified by the DG term through an interface electric field. We have also carried out spin injection and detection measurements using four-terminal Si devices. The local measurement shows that the MR ratio changes depending on the current direction. We show that the change of the MR ratio depending on the current direction comes from the DG term regarding the asymmetry of the two interface electronic structures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The modification of the nonlinear guiding center theory

We modify the NonLinear Guiding Center theory for perpendicular diffusion by replacing the spectral amplitude of the two-component model of magnetic turbulence with that of the two-dimensional model (following Shalchi), and replacing the constant a {sup 2}, which indicates the degree particles following the magnetic field line, with the variable a'{sup 2} as a function of the magnetic turbulence. We combine the modified model with the NonLinear PArallel diffusion theory to solve perpendicular and parallel diffusion coefficients simultaneously. It is shown that the new model agrees better with simulations. Furthermore, we fit the numerical results of the new model with polynomials, so that the parallel and perpendicular diffusion coefficients can be calculated directly without iteration of integrations, and many numerical calculations can be reduced.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2026

Nonlinear causality of Israel-Stewart theory with diffusion

We present the first fully nonlinear causality constraints in D = 3 + 1 dimensions for Israel-Stewart theory in the presence of energy and number diffusion in the Eckart and Landau hydrodynamic frames, respectively. These constraints are algebraic inequalities that make no assumption on the underlying geometry of the spacetime or the equation of state. In order to highlight the distinct physical and structural behavior of the two hydrodynamic frames, we discuss the special ultrarelativistic ideal gas equation of state considered in earlier literature in D = 1 + 1 dimensions, and show that our general D = 3 + 1 constraints reduce to their results upon an appropriate choice of angles. For this equation of state in both D = 1 + 1 and D = 3 + 1 dimensions one can show that: (i) there exists a region allowed by nonlinear causality in which the baryon current transitions into a spacelike vector in the Landau frame, and (ii) an analogous argument shows that the solutions of the Eckart frame equations of motion never violate the dominant energy condition, assuming nonlinear causality holds. Furthermore, we then compare our results with those from linearized Israel-Stewart theory and show that the linear causality bounds fail to capture the new physical constraints on energy and number diffusion that are successfully obtained through our nonlinear causality approach.

Quark-gluon plasma↗
DOE OSTI2020

Persistent compositions of non-stoichiometric compounds with low bulk diffusivity: A theory and application to Nb3Sn superconductors

Non-stoichiometric compounds may develop a composition gradient when they are formed by reactive diffusional processes. This paper reports an interesting phenomenon that in compounds with low bulk diffusivities, which rely mainly on grain boundary diffusion for their growth, the final bulk compositions may be far from equilibrium, with a very low bulk diffusivity leading to fixed (persistent) compositions – one such example is Nb 3 Sn, a superconductor. We investigated the microchemistry at the reactive interface using atom probe tomography to clarify the diffusion reaction mechanism for this low-bulk-diffusivity case and thus propose a theory for what determines the compound composition, using Nb 3 Sn as an example for concreteness. Using certain approximations, we derive an explicit analytical equation that illustrates what factors determine its composition profile. We compare our model with the known facts of Nb 3 Sn and see good agreement. In particular, this model predicts that internal oxidation may lead to higher Sn contents than conventional, non-oxidized Nb 3 Sn. Our measurements show that this is indeed true, and that the internally-oxidized Nb 3 Sn also has higher upper critical fields, achieving up to 28.2 T at 4.2 K. We discuss the general applicability of this model to non-stoichiometric compounds with low bulk diffusivity, and propose it as a tool to help in the design and processing of such materials for compositional control.

36 MATERIALS SCIENCE↗
DOE OSTI2021

First observation of increased DT yield over prediction due to addition of hydrogen

Here, in a number of reported instances, implosions utilizing fuel mixtures have resulted in anomalously low fusion yields below those predicted by radiation-hydrodynamics simulations. Inter-species ion diffusion has been suggested as a possible cause of the observed yield degradation in fuel mixture implosions. An experimental platform utilizing hydro-equivalent deuterium–tritium (DT), deuterium–tritium–hydrogen (DTH), and deuterium-tritium-helium3 (DT 3 He) capsule implosions was developed to determine whether the inter-species ion diffusion theory may describe the resulting fuel mixture implosion behavior. The implosion experiments were performed at the Omega laser facility. X-ray images and shell areal density diagnostics results show that the hydro-equivalent three capsules (DT, DTH, and DT 3 He) have similar compression behavior. However, nuclear yield deviation was observed from the scaling determined using a fusion yield formula. In the DT 3 He mixture, a reduced yield of a factor of 0.65 ± 0.13 was observed, which is similar to a yield reduction observed in D 3 He mixture by Rygg et al. (i.e., Rygg effect). In contrast, in the DTH mixture, a factor of 1.17 ±0.15 yield increase was observed, which we named the inverse Rygg effect. The yield increase observed in the DTH mixture is consistent with the inter-species ion diffusion theory where lighter H diffuses away from the core and concentrated DT in the core produces higher yield. An inter-species ion diffusion model, the Zimmerman–Paquette–Kagan–Zhdanov model, implemented in a Lagrangian radiation-hydrodynamics fluid code, was also used to analyze the present data, without the need to assume hydrodynamic equivalence of the capsules, but it does not completely explain the DTH or DT 3 He capsules although its effects are in the correct direction. Simulation-based Bayesian inference was used in the latter analysis to quantify the uncertainty in the numerical simulations. The simulation-based analysis resulted in an inferred Rygg-effect yield decrease factor of 0.91 ± 0.02 for the DT 3 He mixture, and an inferred inverse-Rygg yield increase factor of 1.21 ± 0.04 for the DTH mixture, based on simulations ignoring ion diffusion.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗