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All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Numerical Modeling and Dynamic Analysis of a Wave-Powered Reverse-Osmosis System

A wave energy converter (WEC) system has the potential to convert the wave energy resource directly into the high-pressure flow that is needed by the desalination system to pump saltwater to the reverse-osmosis membrane and provide the required pressure level to generate freshwater. In this study, a wave-to-water numerical model was developed to investigate the potential use of a wave-powered desalination system (WPDS) for water production. The model was developed by coupling a time-domain radiation-and-diffraction method-based numerical tool (WEC-Sim) for predicting the hydrodynamic performance of WECs with a solution-diffusion model that was used to simulate the reverse-osmosis (RO) process. The objective of this research is to evaluate the WPDS dynamics and the overall efficiency of the system. To evaluate the feasibility of the WPDS, the wave-to-water numerical model was applied to simulate a desalination system that used an oscillating surge WEC device to pump seawater through the system. The hydrodynamics WEC-Sim simulation results for the oscillating surge WEC device were validated against existing experimental data. The RO simulation was verified by comparing the results to those from the Dow Chemical Company’s reverse osmosis system analysis (ROSA) model, which has been widely used to design and simulate RO systems. The wave-to-water model was then used to analyze the WPDS under a range of wave conditions and for a two-WECs-coupled RO system to evaluate the influence of pressure and flow rate fluctuation on the WPDS performance. The results show that the instantaneous energy fluctuation from waves has a significant influence on the responding hydraulic pressure and flow rate, as well as the recovery ratio and, ultimately, the water-production quality. Nevertheless, it is possible to reduce the hydraulic fluctuation for different sea states while maintaining a certain level of freshwater production, and a WEC array that produces water can be a viable, near-term solution to the nation’s water supply. A discussion on the dynamic impact of hydraulic fluctuation on the WPDS performance and potential options to reduce the fluctuation and their trade-offs is also presented.

16 TIDAL AND WAVE POWER↗
DOE OSTI2016

THE ROLE OF THE DIFFUSIVE PROTONS IN THE GAMMA-RAY EMISSION OF SUPERNOVA REMNANT RX J1713.7–3946—A TWO-ZONE MODEL

RX J1713.7−3946 is a prototype in the γ-ray-bright supernova remnants (SNRs) and is in continuing debates on its hadronic versus leptonic origin of the γ-ray emission. We explore the role played by the diffusive relativistic protons that escape from the SNR shock wave in the γ-ray emission, apart from the high-energy particles’ emission from the inside of the SNR. In the scenario that the SNR shock propagates in a clumpy molecular cavity, we consider that the γ-ray emission from the inside of the SNR may arise either from the inverse Compton scattering or from the interaction between the trapped energetic protons and the shocked clumps. The dominant origin between them depends on the electron-to-proton number ratio. The diffusive protons that escaped from the shock wave during the expansion history can provide an outer hadronic γ-ray component by bombarding the surrounding dense matter. The broadband spectrum can be well explained by this two-zone model, in which the γ-ray emission from the inside governs the TeV band, while the outer emission component substantially contributes to the GeV γ-rays. The two-zone model can also explain the TeV γ-ray radial brightness profile that significantly stretches beyond the nonthermal X-ray-emitting region. In the calculation, we present a simplified algorithm for Li and Chen's “accumulative diffusion” model for escaping protons and apply the Markov Chain Monte Carlo method to constrain the physical parameters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Dynamical Threshold of Diluteness of Soft Colloids

The dynamics of soft colloids in solutions is characterized by internal collective motion as well as center-of-mass diffusion. Using neutron scattering we demonstrate that the competition between the relaxation processes associated with these two degrees of freedom results in strong dependence of dynamics and structure on colloid concentration, c, well below the overlap concentration c*. Triggered by the increasing inter-particle collisions, substantial structural dehydration and slowing-down of internal dynamics occurs before geometrically defined colloidal overlap develops. This observation is surprising since it is generally believed that the internal dynamics and conformation of soft colloidal particles essentially remain invariant below c*. The competition between these two relaxation processes gives rise to a new dynamically-defined dilute threshold concentration well below c*.

Chen, Wei-Ren↗
DOE OSTI2020

Stabilization/Solidification of Tritiated Water - 20135

Tritiated water was stabilized/solidified by using calcium oxide and a polymer. First, tritiated water was changed into the powder form of calcium hydroxide by addition of tritiated water into calcium oxide. Then the produced calcium hydroxide was solidified in waste form by using a polymer binder. The specimens of waste form prepared by surrogate calcium hydroxide were subjected to compressive strength testing and water immersion testing. In addition, the specimens of waste form prepared by tritiated calcium hydroxide were subjected to leach testing. The mean compressive strength of the specimens containing 40 wt % of the surrogate calcium hydroxide was about 98.5 MPa (14,300 psi). After water immersion testing, there was no distinct change in the appearance of the specimens. The mean compressive strength of the specimens after water immersion was 79.1 MPa (11,470 psi). The mean diffusion coefficient of tritium for the specimens containing 40 wt% of tritiated calcium hydroxide was 5.4 x 10-10 cm{sup 2}/s. These findings indicate that the waste form has enough mechanical integrity and durability for final disposal and releases tritium very slowly. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2015

Environmental cleanup of the East Tennessee Technology Park - Year three - A status report - 15522

On August 1, 2011, URS - CH2M Oak Ridge LLC (UCOR) began its five-year, $1.4 billion cleanup of the East Tennessee Technology Park (ETTP), the former Oak Ridge Gaseous Diffusion Plant, located on the U.S. Department of Energy's (DOE) Oak Ridge Reservation (Reservation) in Tennessee. Under the contract, the base period scope of work to be completed by 2016 includes demolition of both the K-25 and K-31 gaseous diffusion buildings and all demolition waste dispositioned, initiation of demolition on the remaining K-27 gaseous diffusion building, safe storage of transuranic (TRU) waste for eventual transfer to the Transuranic Waste Processing Center, previously designated -No-Path-To-Disposition Waste- dispositioned to the extent possible, and completion of design and engineering of a mercury treatment system to be installed at the Y-12 National Security Complex (Y-12). UCOR will have also managed DOE Oak Ridge Office of Environmental Management (OREM)-owned facilities at ETTP, Oak Ridge National Laboratory (ORNL), and Y-12 in a safe and cost-effective manner. Since assuming responsibilities as the ETTP and Oak Ridge Reservation cleanup contractor, UCOR has completed its life-cycle Performance Measurement Baseline; received its Earned Value Management System (EVMS) certification; completed the deactivation and demolition (D and D) of the 44-acre K-25 gaseous diffusion building; removed and disposed of a contaminated underground storage tank (Tank W-1A) at ORNL; dispositioned mercury-contaminated storage tanks at Y-12; completed closure of the Toxic Substances Control Act Incinerator; disposed of more than 158,000 cubic yards of cleanup waste while managing the on-site Environmental Management Waste Management Facility (EMWMF); and conducted operations, surveillance, and maintenance activities at 180 DOE EM facilities at ETTP, ORNL, and Y-12 in a safe and cost-effective manner. In 2014, UCOR completed the removal and disposal of all of the K-25 demolition debris; completed the removal of asbestos-laden transite siding on the K-31 gaseous diffusion building and began demolition of that 17-acre building; advanced the characterization and deactivation of the K-27 gaseous diffusion building, readying the building for the start of demolition in 2015; advanced the design and engineering of a Mercury Treatment Facility to be located at Y-12; continued the cost-effective and safe disposition of all cleanup wastes; and readied ETTP for Voluntary Protection Program Star Status Assessment in 2015. Project performance as of December 31, 2014, continued to be excellent: - Cost Performance Index - 1.12 - Schedule Performance Index - 1.02 At the same time, since safety is the foundation of all cleanup work, UCOR's safety record goes hand-in-hand with its excellent project performance. Through calendar year 2014, UCOR's recordable injury rate was 1.04 occurrences per 200,000 work hours, and the lost workday case rate was 0.52. This performance continues to be due, in large part, to the people and processes URS and CH2M HILL, the parent companies of UCOR, brought to the project. Key enablers included: - Selected and deployed experienced staff in key leadership positions throughout the organization, continually re-energizing the organization with new leaders - Implemented previously successful safety programs and initiatives from other DOE sites, such as DOE's Voluntary Protection Program - Brought fresh thinking and innovation to tackling tough project issues, such as the deactivation of the Technetium-99-contaminated areas of K-27, applying lessons learned from the D and D of K-25; and implementing a -waste factory- approach to cleanup - Maintained continuing alignment between UCOR's contract with DOE and its Performance Measurement Baseline - rapidly addressing contract changes when they arose - Maintained a readiness both from a process as well as experienced personnel perspective to support DOE OREM in performing other significant Oak Ridge Reservation cleanup activities. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2019

A higher order approximate static condensation method for multi-material diffusion problems

The paper studies an approximate static condensation method for the diffusion problem with discontinuous diffusion coefficients. The method allows for a general polygonal mesh which is unfitted to the material interfaces. Moreover, the interfaces can be discontinuous across the mesh edges as typical for numerical reconstructions using the volume or moment-of-fluid methods. We apply a mimetic finite difference method to solve local diffusion problems and use P 1 (mortar) edge elements to couple local problems into the global system. The condensation process and the properties of the resulting algebraic system are discussed. It is demonstrated that the method is second order accurate on smooth solutions and performs well for problems with high contrast in diffusion coefficients. Experiments also show the robustness with respect to position of the material interfaces against the underlying mesh.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2018

In Operando Analysis of Diffusion in Porous Metal‐Organic Framework Catalysts

Abstract The potential to exert atomistic control over the structure of site‐isolated catalyst sites, as well as the topology and chemical environment of interstitial pore spaces, has inspired efforts to apply porous metal‐organic frameworks (MOFs) as catalysts for fine chemical synthesis. In analogy to enzyme‐catalyzed reactions, MOF catalysts have been proposed as platforms in which substrate confinement could be used to achieve chemo‐ and stereoselectivities that are orthogonal to solution‐phase catalysts. In order to leverage the tunable pore topology of MOFs to impact catalyst selectivity, catalysis must proceed at interstitial catalyst sites, rather than at solvent‐exposed interfacial sites. This Minireview addresses challenges inherent to interstitial MOF catalysis by 1) describing the diffusional processes available to sorbates in porous materials, 2) discussing critical factors that impact the diffusion rate of substrates in porous materials, and 3) presenting in operando experimental strategies to assess the relative rates of substrate diffusion and catalyst turnover in MOF catalysis. It is anticipated that the continued development of in operando tools to evaluate substrate diffusion in porous catalysts will advance the application of these materials in fine chemical synthesis.

Gao, Wen‐Yang↗
DOE OSTI2024

Simulations of flaming combustion and flaming-to-smoldering transition in wildland fire spread at flame scale

Our objective in the present study is to provide basic insights into the coupling between external-gas and solid biomass vegetation processes that control the dynamics of flame spread in wildland fire problems. Here we focus on a modeling approach that resolves processes occurring at vegetation and flame scales, i.e., the formation of flammable vapors due to the thermal degradation of the solid biomass, the subsequent combustion in ambient air, the thermal feedback to the biomass through radiative and convective heat transfer, and the possible transition from flaming combustion (taking place outside of the solid biomass) to smoldering combustion (taking place inside the solid biomass). The capability uses a multiphase combustion framework and treats external-gas processes through a Large Eddy Simulation solver and solid biomass processes through a discrete particle model. The discrete particle model adopts a one-dimensional porous medium formulation, includes descriptions of drying, thermal pyrolysis, oxidative pyrolysis, and char oxidation, as well as a description of the external-gas-to-solid-biomass diffusion of oxygen mass; the discrete particle model thereby provides a treatment of in-depth oxidative processes and allows the simulation of smoldering combustion. The modeling capability is applied to the simulation of fire spread across a surrogate biomass vegetation bed corresponding to a discrete array of cylindrical-shaped, vertically-oriented, pine wood sticks, characterized by a monomodal size distribution, in horizontal flat terrain and under wind-aided conditions. The numerical results demonstrate that the model can simulate successful flaming-to-smoldering transition followed by complete biomass consumption.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2021

Measurement of Fission Product Concentration Profiles in AGR-3/4 TRISO Fuel Graphitic Matrix and Nuclear Graphites

The third Advanced Gas Reactor (AGR) irradiation experiment, AGR-3/4, was designed to investigate the migration of fission products in fuel compact graphitic matrix and reactor graphite components. Using destructive methods, radial fission product concentration profiles were measured for gamma-emitting fission products (e.g., Ag-110m, Cs-134, and Eu-154) and beta-emitting Sr-90 in irradiated graphitic and graphite components from six different AGR-3/4 irradiation capsules. These new measured concentration profiles can now be compared to non-destructive measurements and fission product transport simulations and will be used to derive new diffusivities and sorptivities to support refinement of fission product transport models and high-temperature gas-cooled reactor (HTGR) source-term analyses. Each capsule in the AGR-3/4 experiment had four fuel compacts in the middle of two concentric rings of graphitic matrix material, PCEA graphite, or IG-110 graphite. In addition to the approximately 1898 tristructural isotropic (TRISO) coated particles in each compact, there were 20 designed to fail (DTF) particles coated only in pyrocarbon so that they released fission products into the surrounding cylindrical rings of carbonaceous materials. Destructive sampling of the rings involved machining/milling material from around the circumference of the rings, collecting that material, and performing radiochemical analyses on it. Milling operations were performed in multiple steps or segments, and each segment was generally 0.508 mm (0.020 in) thick. Knowing the radial position at which each segment was milled, the volume of the milled material at each segment, and the fission product content in each segment, the radial fission product concentrations were constructed for select isotopes in each ring. Ag-110m profiles had the most variation. Some profiles were peaked at an inner or outer surface. Some were peaked at the middle of the ring wall thickness. Some increased radially outward, and some decreased radially outward. These types of variations and the fact that the measured profiles do not generally compare favorably with the transport model employed for AGR-3/4 may adversely impact the ability to extract reasonable transport parameters for this isotope. In many cases, the Cs-134 profiles decreased somewhat linearly in the outward radial direction, and in cursory comparisons, the shapes of these profiles appeared similar to those from model predictions. The step changes in concentration across the inner-outer ring gap were generally consistent with the model predictions as well. In some cases, there were local maxima in concentration at the outer surface of the rings. This suggests that fission products could have transported in the small gaps between the inner ring and the outer ring and between the outer ring and the sink ring such that some portion of a given fission product can bypass diffusion through the ring itself. The analysis of the small nubs on the outer surfaces of some of the outer rings revealed fission product concentrations in the nubs that were often higher than in the outermost segments of the rings. This further supports the hypothesis that short-circuit, gap transport occurred, causing relatively high surface concentrations on the outer surfaces of the rings. Eu-154 and Sr-90 profiles tend to have very similar shapes, suggesting that they transport via the same mechanisms. The observed profiles were indicative of a transport process where the isotopes are sorbed on the inner surface of the ring, but diffusion into the ring from that surface is quite slow. Some elevated concentrations of Sr-90 (relative to Eu-154) on the outer surface of a ring suggested that rapid, gas-gap transport of gaseous precursor Kr-90 and volatile Rb-90 could have occurred prior to their decaying to Sr-90. Overall, the Eu 154 and Sr-90 profiles were still very similar, which indicates that the transport of short-lived Sr-90 precursors is not a major effect. In some capsules, the qualitative Sr-90 behavior across the ring gaps was consistent with the model (using the available legacy Sr-90 transport parameters), but in other capsules the model was inconsistent with the measurements and seems to underestimate the amount of Sr-90 in the outer rings. The total ring Sr-90 inventories were estimated for all the rings that were subject to physical sampling. These results will be used to adjust the predicted particle and/or compact releases used in the AGR-3/4 fission product transport model. Given the different irradiation temperatures among the capsules and the rings, it was not possible to discern fundamental differences in the transport of isotopes within the different carbon materials, i.e., graphitic matrix, IG-110, or PCEA. It may be possible to do this in the course of determining transport from the concentration profiles in future work.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2018

Propagation and attenuation of sound in one-dimensional quantum liquids

At low temperatures, elementary excitations of a one-dimensional quantum liquid form a gas that can move as a whole with respect to the center of mass of the system. This internal motion attenuates at exponentially long time scales. As a result, in a broad range of frequencies, the liquid is described by two-fluid hydrodynamics, and the system supports two sound modes. The physical nature of the two sounds depends on whether the particles forming the quantum liquid have a spin degree of freedom. For particles with spin, the modes are analogous to the first and second sound modes in superfluid 4 He, which are the waves of density and entropy, respectively. When dissipative processes are taken into account, we find that at low frequencies, the second sound is transformed into heat diffusion, while the first sound mode remains weakly damped and becomes the ordinary sound. In a spinless liquid, the entropy and density oscillations are strongly coupled and the resulting sound modes are hybrids of the first and second sound. As the frequency is lowered and dissipation processes become important, the crossover to a single-fluid regime occurs in two steps. First the hybrid modes transform into predominantly density and entropy waves, similar to the first and second sound, and then the density wave transforms to the ordinary sound and the entropy wave becomes a heat-diffusion mode. Lastly, we account for the dissipation due to viscosity and intrinsic thermal conductivity of the gas of excitations, which controls attenuation of the sound modes at high frequencies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Graphene: a Barrier to Hydrogen Permeation

Small area freestanding monolayer graphene has been shown to be impermeable to standard gases including hydrogen and helium.1 Several recent reports have suggested that graphene can be used as a hydrogen permeation barrier on metals.2 Those studies have relied on electrochemical hydrogen permeation measurements, which do a good job reproducing bulk permeability compared to gaseous diffusion but do not always reflect the real surface effects due to the difference in driving forces. This problem is magnified when measuring the permeability of thin coatings, which are dominated by surface effects. For this reason, we performed gas driven hydrogen permeation measurements of graphene coated metal samples. The diffusion of hydrogen through a metal can be described using Fick's and Sievert's Laws. Graphene-coated copper foils were prepared using chemical vapor deposition (CVD) with a methane precursor. Photo-reduced graphene samples were prepared by first sputtering a layer (∼15 nm) of copper on top of stainless steel foils. Graphene oxide in ethanol solutions were then drop cast on the surface and photo reduced using a neon lamp. A custom-designed apparatus has been assembled for measuring the hydrogen permeability of various materials. The operating principle of the system is as follows: A sample is sealed between the fore-sample and post-sample chambers using standard vacuum gaskets. Hydrogen is introduced into the fore sample region at varying pressures while the post sample region is maintained under high vacuum. Due to the concentration gradient, hydrogen then diffuses through the material and is detected using a quadrupole mass spectrometer. This process is repeated at several temperatures. The mass spectrometer was calibrated using an orifice with known conductance in place of the sample. Data collection and reduction were automated using scripts written in LabVIEW and Octave, respectively. We saw a decrease in the hydrogen permeability of copper after adding graphene to the surface for small temperature steps. This result is also observed when considering temperature steps of 20 K. Future Work: Apply this methodology to graphene-coated 316L stainless steel. The hydrogen permeability of 316L steel should also decrease with the addition of graphene to the surface. Determine methods for graphene defect reduction.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Effects of anisotropies in turbulent magnetic diffusion in mean-field solar dynamo models

We study how anisotropies of turbulent diffusion affect the evolution of large-scale magnetic fields and the dynamo process on the Sun. The effect of anisotropy is calculated in a mean-field magnetohydrodynamics framework assuming that triple correlations provide relaxation to the turbulent electromotive force (so-called the 'minimal τ-approximation'). We examine two types of mean-field dynamo models: the well-known benchmark flux-transport model and a distributed-dynamo model with a subsurface rotational shear layer. For both models, we investigate effects of the double- and triple-cell meridional circulation, recently suggested by helioseismology and numerical simulations. To characterize the anisotropy effects, we introduce a parameter of anisotropy as a ratio of the radial and horizontal intensities of turbulent mixing. It is found that the anisotropy affects the distribution of magnetic fields inside the convection zone. The concentration of the magnetic flux near the bottom and top boundaries of the convection zone is greater when the anisotropy is stronger. It is shown that the critical dynamo number and the dynamo period approach to constant values for large values of the anisotropy parameter. The anisotropy reduces the overlap of toroidal magnetic fields generated in subsequent dynamo cycles, in the time-latitude 'butterfly' diagram. If we assume that sunspots are formed in the vicinity of the subsurface shear layer, then the distributed dynamo model with the anisotropic diffusivity satisfies the observational constraints from helioseismology and is consistent with the value of effective turbulent diffusion estimated from the dynamics of surface magnetic fields.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

An advanced hybrid reprocessing system based on UF{sub 6} volatilization and chromatographic separation

To recover U, Pu, MA (Np, Am, Cm) and some specific fission products FPs (Cs, Sr, Tc, etc.) from various spent nuclear fuels (LWR/FBR: Oxide, Metal Fuels), we are studying an advanced hybrid reprocessing system based on UF6 volatilization (Pyro) and chromatographic separation (Aqueous). Spent fuels are de-cladded by means of thermal and mechanical methods and then applied to the fluorination/volatilization process, which selectively recovers the most amount of U. Then, the remained fuel components are converted to oxides and dissolved by HNO{sub 3} solution. Compared to U, since Pu, MA and FPs are significantly less abundant in spent fuels, the scale of the aqueous separation process could become reasonably small and result in less waste. For the chromatographic separation processes, we have prepared different types of porous silica-based organic/inorganic adsorbents with fast diffusion kinetics, improved chemical stability and low pressure drop in a packed column. So they are advantageously applicable to efficient separation of the actinides and FP elements from the fuel dissolved solution. In this work, adsorption and separation behavior of representative actinides and FP elements was studied. Small scale separation tests using simulated and genuine fuel dissolved solutions were carried out to verify the feasibility of the proposed process. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2023

Solid-state reaction mediated microstructural evolution in a spark plasma sintered in situ Ti–B 4 C composite

For this study, a novel porosity-free in situ Ti–B 4 C composite was fabricated via spark plasma sintering (SPS). Detailed analysis of the phase evolution, coupling results from XRM, XRD and SEM-EBSD-EDS, indicate that primarily TiB 2 precipitates formed due to the in-situ reactions between the boron-carbide and titanium powders. The precipitation of TiB 2 resulted in the formation of a graphitic C-rich thin layer circumscribing the partially reacted B 4 C particles. Further progression of the in-situ reaction leads to the out-diffusion of the excess carbon from the B 4 C particles, across the graphite and TiB 2 phases, forming TiC upon reacting with the titanium matrix. Therefore, the final microstructure primarily consisted of TiB 2 , TiC, and partially reacted B 4 C phases, with small amounts of TiB, α-Ti, and graphite. Furthermore, the microstructure in these SPS processed in situ composites appeared to be the product of a solid-state Ti–B 4 C diffusion couple, substantially different from their fusion-based additively manufactured counterparts. Nano-indentation tests revealed a remarkably high average hardness of ~25 GPa for this SPS-processed Ti–B 4 C composite and comparable (with literature) phase-specific hardness and modulus values for the constituent TiB 2 , TiC, and B 4 C phases.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Ensemble variational Fokker-Planck methods for data assimilation

Particle flow filters solve Bayesian inference problems by smoothly transforming a set of particles into samples from the posterior distribution. Particles move in state space under the flow of an McKean-Vlasov-Itˆo process. This work introduces the Variational Fokker-Planck (VFP) framework for data assimilation, a general approach that includes previously known particle flow filters as special cases. The McKean-Vlasov-Itˆo process that transforms particles is defined via an optimal drift that depends on the selected diffusion term. It is established that the underlying probability density - sampled by the ensemble of particles - converges to the Bayesian posterior probability density. For a finite number of particles the optimal drift contains a regularization term that nudges particles toward becoming independent random variables. Based on this analysis, we derive computationally-feasible approximate regularization approaches that penalize the mutual information between pairs of particles, and avoid particle collapse. Moreover, the diffusion plays a role akin to a particle rejuvenation approach that aims to alleviate particle collapse. The VFP framework is very flexible. Different assumptions on prior and intermediate probability distributions can be used to implement the optimal drift, and localization and covariance shrinkage can be applied to alleviate the curse of dimensionality. A robust implicit-explicit method is discussed for the efficient integration of stiff McKean- Vlasov-Itˆo processes. Here, the effectiveness of the VFP framework is demonstrated on three progressively more challenging test problems, namely the Lorenz ’63, Lorenz ’96 and the quasi-geostrophic equations.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2021

Mechanism of Metal Intercalation under Graphene through Small Vacancy Defects

Metal intercalation under graphene has attracted extensive experimental and theoretical research because of its capability to manipulate the electronic structure and properties of graphene. However, the pathways and mechanisms of intercalation are still not well understood. Here, we systematically investigate the intercalation process of metal atoms through graphene vacancies using first-principles calculations. We show that the energy barrier for metal atom penetration through the vacancies in graphene is small as long as the size of the vacancy is larger than a mono-vacancy. However, metal atoms are strongly bonded to the vacancy so that the detachment energy of a metal atom from the vacancy is extremely high. This inhibits the diffusion of the metal atom into the gallery beneath the surface to complete the intercalation process. On the other hand, our calculation results show that the detachment energy of a metal atom from a metal dimer at small vacancy defects is significantly reduced, making intercalation much easier. Therefore, the key step limiting the intercalation process is the detachment of the metal atoms from vacancy defects. This finding from our study provides useful insight into the defect-assisted intercalation mechanism.

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

K-space algorithmic reconstruction (KAREN): a robust statistical methodology to separate Bragg and diffuse scattering

Diffuse scattering occurring in the Bragg diffraction pattern of a long-range-ordered structure represents local deviation from the governing regular lattice. However, interpreting the real-space structure from the diffraction pattern presents a significant challenge because of the dramatic difference in intensity between the Bragg and diffuse components of the total scattering function. In contrast to the sharp Bragg diffraction, the diffuse signal has generally been considered to be a weak expansive or continuous background signal. In this paper, using 1D and 2D models, it is demonstrated that diffuse scattering in fact consists of a complex array of high-frequency features that must not be averaged into a low-frequency background signal. To evaluate the actual diffuse scattering effectively, an algorithm has been developed that uses robust statistics and traditional signal processing techniques to identify Bragg peaks as signal outliers which can be removed from the overall scattering data and then replaced by statistically valid fill values. This method, described as a 'K-space algorithmic reconstruction' (KAREN), can identify Bragg reflections independent of prior knowledge of a system's unit cell. KAREN does not alter any data other than that in the immediate vicinity of the Bragg reflections, and reconstructs the diffuse component surrounding the Bragg peaks without introducing discontinuities which induce Fourier ripples or artifacts from underfilling `punched' voids. The KAREN algorithm for reconstructing diffuse scattering provides demonstrably better resolution than can be obtained from previously described punch-and-fill methods. The superior structural resolution obtained using the KAREN method is demonstrated by evaluating the complex ordered diffuse scattering observed from the neutron diffraction of a single plastic crystal of CBr 4 using pair distribution function analysis.

36 MATERIALS SCIENCE↗