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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Influence of cementation on the yield surface of rocks numerically determined from digital microstructures

Digital Rock Physics has reached a level of maturity on the characterisation of primary properties that depend on the microstructure – such as porosity, permeability or elastic moduli – by numerically solving field equations on μCT scan images of rock. After small deformations or at depth though, most rocks eventually reach their limit of elasticity and the complementary plastic properties are needed to describe the full mechanical behaviour. Currently, determination of a rock’s yield surface from its microstructure is often restricted to semi-analytical criteria derived by limit analysis or numerical simulations performed on idealised geometries. Such simplification lacks representativeness, particularly for processes that affect directly the pore-grain interface such as the cementation phenomenon, happening during diagenesis. Eventually, only direct numerical simulation of elasto-plasticity performed on digitalised microstructures can be used to assess the strength of different cemented materials and its evolution with the alteration of the microstructure. In this study, we provide a comprehensive parametric study on the impact of cementation on rock strength for real microstructures of cemented granular materials. Compared to most previous studies, the whole yield surface is determined numerically (using Finite Element Method) in order to assess the influence of cementation for different stress-paths. The previously known tendency of rock to strengthen with increasing cementation volume is verified. New results on the influence of cement property namely Young’s modulus, friction and cohesion on the rock’s yield surface are explored. The envelopes obtained are compared to the ones obtained by experimental data and existing models. Finally, the framework presented in this study showcases the wider possibility of determining any rock’s or porous material’s yield surface from its microstructure.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Off-Equilibrium Reactivity of Boron-Enriched Metal Diboride Surfaces in Electroreduction Conditions

Boron-based materials, featuring B-dependent reactivity and diverse phases, are emerging as promising catalyst systems. However, the catalytic mechanism on many borides remains poorly understood due to complex surface reconstructions under reaction conditions. Here, we investigate the MoB 2 surface in conditions of hydrogen evolution reaction in acidic media, using grand canonical global optimization, grand canonical density functional theory, ab initio molecular dynamics, free energy surface sampling, and an analytical model for electrochemical barrier evaluation. We propose a boron-enrichment strategy to tune the surface reactivity of the hexagonal face of MoB 2 . We reveal the dynamic nature of the B-enriched surface under H coverage and kinetic trapping of the system in the metastable regime with an extensive examination of the deactivation pathways. The metastable center B site on B-enriched surfaces, featuring buckled-up configuration and a usual relaxation effect, is found to be highly active toward HER via the Volmer–Heyrovsky mechanism. In conclusion, this work demonstrates how off-equilibrium behaviors can arise from the interplay between adsorbate coverage and surface reconstruction on a seemingly simple surface, and we present a theoretical framework and computational workflows to address these behaviors, along with other realistic complexities, in kinetics simulations.

Adsorption↗
DOE OSTI2026

Long‐term growth and persistence of granitic inselbergs in a semi‐arid cratonic landscape

Granitic inselbergs are enduring landforms that punctuate planation surfaces in tectonically stable settings, yet the mechanisms and timescales of their persistence remain debated. Here, in this study, we present a multinuclide cosmogenic dataset ( 10 Be, 26 Al and 21 Ne) from bedrock, fluvial sediments and planation surfaces in the semi-arid Central Ceará Domain, north-eastern Brazil, to quantify denudation rates and reconstruct the long-term dynamics of residual relief formation. This triple-nuclide approach enables independent validation of denudation contrasts and long-term exposure histories, providing quantitative constraints on inselberg evolution that a single-nuclide approach could not resolve. Bedrock denudation rates (1–4 m Myr −1 ) are systematically lower than basin-wide rates (8–14 m Myr −1 ), revealing a regional pattern of vertical differential denudation. When combined with summit elevations and surface exposure ages of up to 5 Myr, these contrasts suggest that cumulative relief growth has been operating over timescales of tens of millions of years (1–57 Myr). Morphological and structural observations further indicate localized mass wasting and heterogeneous erosion styles, modulated by rock fabric and joint density. Cosmogenic nuclide inventories in detrital sediments reveal partial reworking from older sediment stocks, including surface pebbles with exposure ages up to 5 Myr, which record multistage of burial and re-exposure histories and reflect episodic remobilization in low-connectivity, semi-arid catchments. Rather than static relicts of ancient surfaces, inselbergs in this cratonic setting emerge as dynamic landforms actively sustained by slow but persistent differential denudation. These results provide new constraints on long-term cratonic landscape evolution and underscore the interplay between lithological resistance, structural inheritance and sediment transfer processes in sustaining topographic relief. They also call for a critical reassessment of classical inselberg models, suggesting that hybrid mechanisms, rather than single-process paradigms, more accurately capture the complexity of residual landform development in cratonic interiors.

Cosmogenic nuclides↗
DOE OSTI2024

Fracture behavior of irradiation induced nanocrystalline UO 2 studied by in-situ mechanical testing in transmission electron microscopy

Uranium Dioxide (UO 2 ) is widely used as a fuel in current light water reactors (LWRs). Upon accumulation of radiation damage, LWR UO 2 fuel pellets start to develop a different microstructure at the pellet periphery when fuel burnup exceeds 45–50 GWd/tHM. The resulting porous, nanocrystalline microstructure is one of the most prominent microstructural changes occurring in such fuel. Its fracture mechanisms, which causes fuel fine fragmentation, could impact safety limits when the cladding breaches. Direct measurements of these properties are challenging, therefore a surrogate obtained via ion irradiation can be used. In this study, multiple microcantilevers were fabricated by focused ion beam from both fresh UO 2 and UO 2 irradiated with 84 MeV Xe 26+ ions to a peak dose of 1357 displacements per atom (dpa). Further, the irradiation produced a pseudo high burnup structure approximately 2 µm below the surface. In-situ nano-mechanical bending tests were conducted to investigate the fracture behavior and the effect of the surrogate UO 2 high burnup structure on local fracture properties. Fresh UO 2 fuel was observed to fracture in transgranular mode without nucleation or movement of dislocations. However, the Xe-irradiated nanocrystalline microcantilevers fractured along the grain boundaries, with no influence from the pre-existing micro-cracks in the microcantilever. Fracture toughness for this type of surrogate high burnup UO 2 structure is reported for the first time in literature. Both the fracture stress and toughness show degradation for UO 2 as a result of Xe-irradiation.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Physics-Informed and Data-Driven Prediction of Residual Stress in Three-Dimensional Machining

Efficient and reliable prediction of machining-induced residual stress (RS) is a key requirement for truly integrated computational materials engineering (ICME). Currently available process modeling approaches, including empirical, analytical, and numerical methodologies lack predictive power and require substantial calibration and validation data. Moreover, most model-based approaches consider only two-dimensional (2D) (i.e., orthogonal), cutting processes. Meanwhile, industrial processes such as milling, turning, and drilling are inherently three-dimensional (3D). The present work attempts to bridge the gap between 2D and 3D through careful consideration of the process physics, including geometric, kinematic, and size-effect constraints to realize robust prediction of how RS develops in 3D machining. Using a novel in-situ experimental technique and digital image correlation (DIC) to determine equivalent Hertzian contact widths, contact pressures, and friction coefficients, the proposed methodology leverages a discretized conversion algorithm that includes multi-pass shakedown effects. This paper presents a semi-analytical model to predict machining-induced RS in 3D turning operations, which are used representatively for 3D processes more generally. Rather than follow a ‘brute force’ 3D FEM approach or conduct countless experiments to train a purely data-driven machine learning algorithm, the proposed approach builds on previous 2D modeling work. Through careful consideration of the process physics, including complex geometry/kinematic considerations of 3D turning, the authors demonstrated an experimentally calibrated approach, as well as validation based on published RS data. Model predictions and previously published measurement data of RS depth profiles for turning of Inconel 718 were compared for a range of process parameters. Correlation between the proposed 3D model and validation data was found to be within the margin of experimental error for most conditions. The proposed model appears to capture the overall behavior of 3D RS depth profiles with acceptable accuracy, particularly the key metrics of near-surface stress, peak stress magnitude and location, as well as overall stress profile depth. This report presents a physics-informed, data-driven approach for efficient calibration of a 2D model for machining-induced RS through DIC analysis of in-situ characterized subsurface displacement fields.

42 ENGINEERING↗
DOE OSTI2022

MAGNETIC FIELD MAPPING OF 1.3 GHz SUPERCONDUCTING RADIO FREQUENCY NIOBIUM CAVITIES

Niobium is the material of choice to build superconducting radio frequency (SRF) cavities, which are fundamental building blocks of modern particle accelerators. These cavities require a cryogenic cool-down to ~2 - 4 K for optimum performance minimizing RF losses on the inner cavity surface. However, temperature-independent residual losses in SRF cavities cannot be prevented entirely. One of the significant contributor to residual losses is trapped magnetic flux. The flux trapping mechanism depends on different factors, such as surface preparations and cool-down conditions. We have developed a diagnostic magnetic field scanning system (MFSS) using Hall probes and anisotropic magneto-resistance sensors to study the spatial distribution of trapped flux in 1.3 GHz single-cell cavities. The first result from this newly commissioned system revealed that the trapped flux on the cavity surface might redistribute with increasing RF power. The MFSS was also able to capture significant magnetic field enhancement at specific cavity locations after a quench.

Parajuli, I. P.↗
DOE OSTI2022

Magnetic field mapping of superconducting radio frequency cavities

Superconducting radio frequency (SRF) cavities are the fundamental building blocks of modern particle accelerators. Niobium is the material of choice to build such cavities. These cavities require a cryogenic cool-down to ~2 ? 4 K for optimum performance minimizing RF losses on the inner cavity surface. However, temperature-independent residual losses in SRF cavities cannot be prevented entirely. One of the major sources of residual losses is trapped magnetic flux. The flux trapping mechanism depends on different factors, such as surface preparations and cool-down conditions. We have developed a diagnostic magnetic field scanning system (MFSS) using Hall probes and anisotropic magneto-resistance sensors to study the spatial distribution of trapped flux in 1.3 GHz single-cell cavities. The first result from this newly commissioned system revealed that the trapped flux on the cavity surface might redistribute with increasing RF power. The MFSS was also able to capture significant magnetic field enhancement at specific cavity locations after a quench.

Parajuli, Ishwari↗
DOE OSTI2020

Mechanism of Electrochemical Oxidation of Nitroxide Radicals in Ethaline Deep Eutectic Solvent

Deep eutectic solvents (DESs) are emerging as promising electrolytes for electrochemical energy storage applications. Electroactive nitroxide-radical-containing organics can be dissolved in DESs to facilitate redox reactions; however, mechanistic know-how of their charge transfer kinetics at the electrode surface is rather limited. Here, we investigate the mechanism underlying the electrochemical oxidation of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl (4-hydroxy-TEMPO). Using polarization measurements on a platinum rotating disk electrode and micro-electrode, we show that the anodic charge transfer coefficient (a) for one-electron transfer oxidations of TEMPO and4-hydroxy-TEMPO approaches 0.9 in DES as well as in aqueous electrolytes, i.e., a significant deviation fromα≈0.5 expected for symmetric redox behavior. To explain this observation, a two-step oxidation mechanism is proposed wherein the nitroxide-containing species undergo fast charge transfer at an electrode surface followed by slow rate-limiting desorption of the adsorbed oxidized species. Numerical simulations are reported to characterize how the proposed two-step mechanism manifests intransient cyclic voltammetry behavior of the 4-hydroxy-TEMPO oxidation reaction, and good agreement with experiments is noted.

25 ENERGY STORAGE↗
DOE OSTI2021

Proton irradiation-induced blistering in UO2

Abstract Proton (H + ) irradiation effects in polycrystalline UO 2 have been studied. The irradiation was carried out using three ion energies and two different ion fluxes at 600 °C. Scanning electron microscopy (SEM) investigations showed that significant surface flaking took place. Focused ion beam (FIB) milling in SEM was successfully applied for extracting lamellas from uneven blistered surfaces for transmission electron microscopy (TEM) investigations allowing detailed investigations for the degradation mechanisms. High-resolution TEM for the flaked UO 2 surfaces revealed that the implanted H + formed sharp two-dimensional cavities at the peak ion-stopping region instead of diffusing to the matrix. The resulting lateral stress likely caused UO 2 surface deterioration in good agreement with previous blistering and flaking studies on crystalline materials. Graphical abstract

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Elucidating the Initial Steps in α-Uranium Hydriding Using First-Principles Calculations

Hydrogen embrittlement of uranium, which arises due to the formation of a structurally weak pyrophoric hydride, poses a major safety risk in material applications. Previous experiments have shown that hydriding begins on the top or near the surface (i.e., subsurface) of α-uranium. However, the fundamental molecular-level mechanism of this process remains unknown. Here, in this work, starting from pristine α-U bulk and surfaces, we present a systematic investigation of possible mechanisms for the formation of metal hydride. Specifically, we address this problem by examining the individual steps of hydrogen embrittlement, including surface adsorption, subsurface absorption, and the interlayer diffusion of atomic hydrogen. Furthermore, by examining these processes across different facets, we highlight the importance of both (1) hydrogen monolayer coverage and (2) applied tensile strain on hydriding kinetics. Taken together, by studying previously overlooked phenomena, this study provides foundational insights into the initial steps of this overall complex process. We anticipate that this work will guide near-term future development of multiscale kinetic models for uranium hydriding and subsequently identify potential strategies to mitigate this undesired process.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Tritium Pressure Enhancement on the TPBAR Cladding by Physical Processes at the Getter

Tritium for the U.S. nuclear weapon stockpile is produced in tritium producing burnable absorber rods (TPBARs) inserted into Tennessee Valley Authoritys (TVA) light-water nuclear reactors. The rods are stainless steel tubes with a permeation barrier coating and internal components that generate and contain the tritium. The TPBAR incorporates a Ni-plated Zircoloy getter tube to capture tritium and prevent it from reaching the rod cladding and permeating into the environment. Under the conventional view of getter behavior, the tritium pressure outside the getter tube is expected to be limited to the equilibrium vapor pressure of Zr hydride at the temperature of the rod as long as the total hydrogen concentration remains below the capacity of the hydride. Since the tritium pressure is higher within the rod core, this behavior relies on the thin getters ability to hold off a differential tritium pressure. The effective tritium pressure on the cladding can also be enhanced by isotope exchange. Hydrogen ingress through the cladding from the reactor coolant creates a hydrogen pressure on the outer surface of the getter that can exchange with tritium, allowing the tritium partial pressure to increase toward this hydrogen gettering pressure. The goal of this work was to use laboratory-scale experiments to examine these mechanisms and create a model of getter behavior that describes tritium transport within the TPBAR. A third mechanism wherein the concentration at the outer surface of the getter is increased by the temperature gradient within the getter tube wall (the Soret effect) is not experimentally tested but is captured in the model. While not conclusively demonstrated by the experiments due to low pressure, high temperature, and small gap volume conditions, the model shows that when combined, the three mechanisms can explain both the magnitude and time dependence of the tritium release observed for reactor fuel assemblies with TPBARs. The model also shows how various modifications of the TPBAR design can reduce this tritium release into the environment.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2022

Tuning gold-based surface functionalization for streptavidin detection: A combined simulative and experimental study

A rationally designed gold-functionalized surface capable of capturing a target protein is presented using the biotin–streptavidin pair as a proof-of-concept. We carried out multiscale simulations to shed light on the binding mechanism of streptavidin on four differently biotinylated surfaces. Brownian Dynamics simulations were used to reveal the preferred initial orientation of streptavidin over the surfaces, whereas classical molecular dynamics was used to refine the binding poses and to investigate the fundamental forces involved in binding, and the binding kinetics. We assessed the binding events and the stability of the streptavidin attachment through a quartz crystal microbalance with dissipation monitoring (QCM-D). The sensing element comprises of biotinylated polyethylene glycol chains grafted on the sensor’s gold surface via thiol-Au chemistry. Finally, we compared the results from experiments and simulations. We found that the confined biotin moieties can specifically capture streptavidin from the liquid phase and provide guidelines on how to exploit the microscopic parameters obtained from simulations to guide the design of further biosensors with enhanced sensitivity.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Melt Pool and Heat Treatment Optimization for the Fabrication of High-Strength and High-Toughness Additively Manufactured 4340 Steel

Additively manufactured (AM) components offer superior design flexibility compared to their conventionally manufactured counterparts, and optimizing processing parameters is key to achieving high-quality depositions with desirable and predictable mechanical properties. This study was focused on 4340 steel fabricated using laser powder bed fusion (LPBF), and 42 laser power and scan speed combinations have been systematically investigated to determine an optimized melt pool geometry that would ensure fully-dense parts. The AM material was compared with a wrought 4340 equivalent and studied in two customized heat treated conditions, optimized for strength and toughness, respectively. The microstructures of the as-fabricated and heat treated AM and wrought materials were characterized to assess differences introduced by the layer-by-layer fabrication process and subsequent heat treatment. Tensile properties of both materials were also evaluated and demonstrate that the AM materials offer equal or superior properties compared to the wrought equivalents. Differences in fracture surface morphologies indicate the distinct failure mechanisms associated with the materials’ characteristic microstructures, and the role of inclusions in the failures was studied to elucidate these differences. Complementary to the experimental investigations, the dataset was leveraged to make recommendations for future design of experiments to optimize AM build parameters in other material systems. A statistical Monte Carlo analysis was used to predict the interpolation error produced using reduced datasets and to enable informed processing parameters selection. These findings are discussed to make recommendations for the use of AM materials for high-integrity structural applications.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Unraveling the Surface Termination and Evolution of Surface States for Electrocatalyst PtSn 4 in Alkaline HER

Semimetal PtSn 4 has been experimentally demonstrated as a promising topological electrocatalyst for the hydrogen evolution reaction (HER) under both acidic and alkaline conditions. While two possible mechanisms have been proposed to explain its activity, the role of its surface states in HER remains unclear. It is indeed in question how the surface states of this alloy evolve as HER proceeds. In this study, we investigate the surface termination that sustains conducting surface states on PtSn 4 , and we track their evolution during HER catalysis. We show that a reconstructed surface with a Sn-poor termination reproduces the scanning tunneling microscopy pattern observed in experiments and sustains a conducting surface. Through phase diagram and geometric structure analysis, we outline the HER profile following the Volmer–Heyrovsky mechanism. As hydrogen atoms adsorb onto the surface, the structure undergoes further reconstruction to an equilibrium phase with a coverage of two hydrides per unit cell. Meanwhile, the surface electronic bands evolve in response to interactions with the adsorbed hydrogen atoms. A hybridization diagram is further proposed for understanding the surface state evolution based on wave function and chemical bonding analyses. While the Pt atoms serve as conventional sites for hydrogen binding, the surface states of PtSn 4 are essential for stabilizing the hydrogen antibonding states via in-phase electronic interactions with the Sn components. This stabilization results in frontier surface bands that are responsible for driving the HER catalysis. Here, our findings provide a detailed description for the direct involvement of surface states on PtSn4 when employed as a catalyst for HER.

catalysts↗
DOE OSTI2023

Bimolecular Reaction Mechanism in the Amido Complex-Based Atomic Layer Deposition of HfO 2

The surface chemistry of the initial growth during the first or first few precursor cycles in atomic layer deposition is decisive for how the growth proceeds later on and thus for the quality of the thin films grown. Yet, although general schemes of the surface chemistry of atomic layer deposition have been developed for many processes and precursors, in many cases, knowledge of this surface chemistry remains far from complete. For the particular case of HfO 2 atomic layer deposition on a SiO 2 surface from an alkylamido-hafnium precursor and water, we address this lack by carrying out an operando atomic layer deposition experiment during the first cycle of atomic layer deposition. Ambient-pressure X-ray photoelectron spectroscopy and density functional theory together show that the decomposition of the metal precursor on the stoichiometric SiO 2 surface in the first half-cycle of atomic layer deposition proceeds via a bimolecular reaction mechanism. The reaction leads to the formation of Hf-bonded methyl methylene imine and free dimethylamine. In addition, ligand exchange takes place involving the surface hydroxyls adsorbed at defect sites of the SiO 2 surface.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Predicting Wind Loading and Instability in Solar Tracking PV Arrays

Wind loading and the fluctuating pressure loads it creates on PV panel surfaces are associated with multiple degradation mechanisms and failures. Modest wind speeds create reversing loads that can initiate cell cracks and weather cracked cells. Stronger wind speeds and extreme weather events can lead to larger scale forces and the aerodynamic instability known as torsional galloping. All these effects are dependent on the complex coupling between wind speed, panel orientation, and a myriad of other hardware and site-specific factors. In this work, we present the latest developments from our work to build an open-source, high-performance computing (HPC) fluid dynamics solver to predict and mitigate these effects. This simulation package allows users to easily specify different array layouts, solar-tracking angles, panel geometries, and weather conditions before automatically generating a refined computational mesh and solving for the unsteady loading on each panel surface. Small domains (e.g., a single panel row in isolation) can be solved on a modern laptop, while larger domains or very high-fidelity studies can be solved on distributed or HPC resources with minimal modifications to the underlying problem specification. We present preliminary case studies obtained using this simulation package and highlight how increased wind speeds combined with sub-optimal tracking angles can exacerbate degradation drivers.

aerodynamics↗
DOE OSTI2022

Surface Chemistry and Heterogeneous Processes in Solar-Driven Pyridine-Catalyzed CO 2 Reduction

The objective of our studies was to advance our fundamental understanding of the surface chemistry and heterogeneous processes that occur in solar-driven pyridine-catalyzed CO 2 reduction. Solar-driven CO 2 reduction to produce fuels offers tremendous promise but commercial deployment is impeded by the lack of suitable catalysts that simultaneously provide high efficiency and product selectivity. Pyridine (C 5 H 5 N) has been reported as an effective, selective co-catalyst in the conversion of CO 2 to methanol (CH 3 OH) in photoelectrochemical cells employing gallium phosphide (GaP) photocathodes. However, despite considerable electrochemical characterization and a large number of theoretical considerations, there are still many fundamental questions about the surface chemistry and heterogeneous processes that are involved in the conversion mechanism. Our research was aimed at investigating the role of the electrode surface and heterogeneous processes in this catalysis. Gaining this fundamental understanding of the origin of pyridine’s effectiveness is important to the continued optimization of solar-driven CO 2 reduction.

10 SYNTHETIC FUELS↗
DOE OSTI2023

Understanding of three different polyvinylpyrrolidone (PVP) based battery binders blends on graphene surfaces from first principles via DFT simulations

Binders play a crucial role in binding electrodes, current collectors, and conductive agents during battery manufacturing. The selection of the binders and their blends considerably affect various properties such as mechanical durability, adhesion performance, ionic/electronic conductivities, and solid electrolyte interface stability for lithium-ion batteries. In this work, polyvinylidene di-fluoride (PVDF), polyacrylic acid (PAA), polyvinylpyrrolidone (PVP), and lithiated polyacrylic acid (Li-PAA) were selected as the model binders, and their blends were studied on graphene surfaces. Further, among them, PVDF is hardly soluble in water unlike the other binders, so in practice, the blend of PVP:PVDF is not thought to be a proper combination. However, the PVP:PVDF binder has also been included because the purpose of this work is to calculate several electronic properties such as binding energies, intermolecular interaction energies, bond critical points, electron density, and the Laplacian of electron density at critical bonding regions of all these blends at the molecular level via density functional theory (DFT) simulations in order to evaluate and compare how is the interaction strength and bonding type of three different lithium-ion batteries (LIBs) binder blends on graphene surfaces. Most stable binder pairs and their binding mechanisms on graphene surfaces were studied. The delocalization of lithium in Li-PAA was studied with the presence of water.

25 ENERGY STORAGE↗