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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Atomic Diffusion, Segregation, and Grain Boundary Migration in Nickel-Based Alloys from Molecular Dynamics Simulations

Grain boundary diffusion and metal mobility in alloys control material performance in many applications and yet remain poorly understood at a mechanistic level. With advances in accessible time and length scales for computational molecular simulations, and recent force field developments, we now possess tools to help unravel those mechanisms. Using large-scale molecular dynamics simulations, here we examined vacancy-mediated diffusion processes in Ni-5Cr alloy with low and high-energy grain boundaries. We show that atomic diffusion inside the grain boundary plane is about four times higher than bulk diffusion, at any temperature, and exhibits a typical Arrhenius behavior with a very small energy barrier (0~.8 eV for Cr and 0.7 eV for Ni within 1300-1600 K). Additionally, the fastest diffusing species inverts; Cr diffusion was faster than Ni in the bulk but slower in the grain boundaries. This is attributed to the creation of high cohesive energy clusters of Cr at the grain boundary. Grain boundary migration was also observed to be temperature dependent and appears to be two times higher in the 5% Cr alloy than in pure Ni, highlighting the important role of the alloying element on grain boundary motion.

Simonnin, Pauline GN↗
DOE OSTI2021

Vacancy surface migration mechanisms in dilute nickel-chromium alloys

We investigate the unitary mechanisms related to the surface migration of vacancies in dilute Ni-Cr alloys via first-principle calculations. We survey a complete set of surface and sub-surface migration paths for vacancies near the (100) free surface and calculate the corresponding migration barriers. Our results show that a vacancy migrating towards the free surface will face lower energy barriers to migrate via atomic exchange with a neighboring Cr atom rather than with a Ni atom. Once a vacancy reaches the free surface, it will be trapped there. Our results also reveal that, when a Cr atom sits in the atomic plane just below the free surface, any in-plane vacancy hopping jump that would result in the vacancy sitting on top a subsurface Cr atom is energetically unfavorable. Taken together, these fundamental unitary surface migration mechanisms offer insights into the complex interactions between surface segregation and vacancy migration phenomena in Ni-Cr-based alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Sintering Mechanism of Pt/Al 2 O 3 in Complex Emission Gases Elucidated via In Situ Environmental STEM

Emission control catalysts are crucial for protecting human health by preventing the release of harmful gases and unburnt fuel into the atmosphere. These catalysts often face deactivation through sintering processes in high-temperature, chemically reactive environments containing multiple gas species. Here, we use in situ environmental scanning transmission electron microscopy to monitor the sintering behavior and transient morphologies of Pt/Al 2 O 3 in various relevant gas environments through controlled experiments. Our results reveal the particle migration and atomic ripening behavior of Pt/Al 2 O 3 at the atomic scale in the presence of water vapor and oxygen, which differs from behaviors observed in single gas environments. We identify an atomic ripening mechanism involving the dissociation and migration of Pt adatom chains from Pt nanoparticles, observed only in combinational gases. Finally, these findings provide valuable insights into catalyst degradation behavior in complex gas environments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Migration and aggregation of Pt atoms on metal oxide-supported ceria nanodomes control reverse water gas shift reaction activity

Abstract Single-atom catalysts (SACs) are particularly sensitive to external conditions, complicating the identification of catalytically active species and active sites under in situ or operando conditions. We developed a methodology for tracing the structural evolution of SACs to nanoparticles, identifying the active species and their link to the catalytic activity for the reverse water gas shift (RWGS) reaction. The new method is illustrated by studying structure-activity relationships in two materials containing Pt SACs on ceria nanodomes, supported on either ceria or titania. These materials exhibited distinctly different activities for CO production. Multimodal operando characterization attributed the enhanced activity of the titania-supported catalysts at temperatures below 320 ˚C to the formation of unique Pt sites at the ceria-titania interface capable of forming Pt nanoparticles, the active species for the RWGS reaction. Migration of Pt nanoparticles to titania support was found to be responsible for the deactivation of titania-supported catalysts at elevated temperatures. Tracking the migration of Pt atoms provides a new opportunity to investigate the activation and deactivation of Pt SACs for the RWGS reaction.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Atom-by-Atom Direct Writing

Direct-write processes enable the alteration or deposition of materials in a continuous, directable, sequential fashion. Here, in this work, we demonstrate an electron beam direct-write process in an aberration-corrected scanning transmission electron microscope. This process has several fundamental differences from conventional electron-beam-induced deposition techniques, where the electron beam dissociates precursor gases into chemically reactive products that bond to a substrate. Here, we use elemental tin (Sn) as a precursor and employ a different mechanism to facilitate deposition. The atomic-sized electron beam is used to generate chemically reactive point defects at desired locations in a graphene substrate. Temperature control of the sample is used to enable the precursor atoms to migrate across the surface and bond to the defect sites, thereby enabling atom-by-atom direct writing.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Superatoms as Superior Catalysts: ZrO versus Pd

Abstract Single‐atom catalysts are the focus of studies for over a decade due to their enhanced reactivity at smaller sizes. However, they have limitations as they offer only one active site, which may not be sufficient for reactions requiring the co‐adsorption of multiple reactants. Additionally, atoms can migrate on a substrate and coalesce, resulting in decreased reactivity. Here, an alternate path, a single‐superatom catalyst is provided. Superatoms are clusters of atoms that mimic the chemistry of atoms even if they do not contain a single atom whose chemistry they mimic. Motivated by an experimental paper on the photoelectron‐spectroscopy of negatively charged ions where ZrO is found to mimic properties of a Pd atom, first the reaction of Pd and ZrO with small molecules in the gas‐phase is studied and found that ZrO not only mimics the chemistry of Pd, but is able to activate these molecules more strongly than Pd. A detailed first‐principles study of CO 2 reduction (CO 2 ‐RR) and hydrogen evolution reactions (HER) on Pd and ZrO supported on graphene, Au(111), and Cu(111) surfaces shows that superatoms are indeed superior catalysts. The ability to design numerous superatoms by varying size and composition offers a promising new paradigm for catalyst design and synthesis.

Chemistry↗
DOE OSTI2023

Atomic Dynamics of Multi‐Interfacial Migration and Transformations

Redox-induced interconversions of metal oxidation states typically result in multiple phase boundaries that separate chemically and structurally distinct oxides and suboxides. Directly probing such multi-interfacial reactions is challenging because of the difficulty in simultaneously resolving the multiple reaction fronts at the atomic scale. Using the example of CuO reduction in H 2 gas, a reaction pathway of CuO → monoclinic m-Cu 4 O 3 → Cu 2 O is demonstrated and identifies interfacial reaction fronts at the atomic scale, where the Cu 2 O/m-Cu 4 O 3 interface shows a diffuse-type interfacial transformation; while the lateral flow of interfacial ledges appears to control the m-Cu 4 O 3 /CuO transformation. Together with atomistic modeling, it is shown that such a multi-interface transformation results from the surface-reaction-induced formation of oxygen vacancies that diffuse into deeper atomic layers, thereby resulting in the formation of the lower oxides of Cu 2 O and m-Cu 4 O 3 , and activate the interfacial transformations. In conclusion, these results demonstrate the lively dynamics at the reaction fronts of the multiple interfaces and have substantial implications for controlling the microstructure and interphase boundaries by coupling the interplay between the surface reaction dynamics and the resulting mass transport and phase evolution in the subsurface and bulk.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Chemical Passivation of Crystalline Si by Al 2 O 3 Deposited Using Atomic Layer Deposition: Implications for Solar Cells

The atomistic-level mechanism for the chemical passivation of the monocrystalline Si (c-Si) surface with thermally annealed Al 2 O 3 was studied using in situ infrared spectroscopy and photoconductance decay measurements. Al 2 O 3 was deposited on high-lifetime, float-zone c-Si substrates using atomic layer deposition (ALD) from trimethylaluminum (TMA), and H 2 O or O 3 . Surface-sensitive attenuated total reflection Fourier transform infrared spectroscopy was used to monitor the c-Si/Al 2 O 3 interface, as well as the bulk of the Al 2 O 3 film during the entire process. Our results show that some surface Si–H bonds are preserved after the ALD of Al 2 O 3 on H-terminated Si. During the annealing step at 400 °C, restructuring occurs at the c-Si/Al 2 O 3 interface to form interfacial SiO x . Isotope labeling was used to differentiate interfacial SiD bonds on the c-Si surface from H incorporated in Al 2 O 3 . Within the sensitivity of our infrared setup (~10 13 cm –2 ), we did not observe any net migration of atomic H or D from Al 2 O 3 to the c-Si/Al 2 O 3 interface. To isolate the effects of chemical and field-effect passivation of Al 2 O 3 thin films, we carried out surface passivation studies on c-Si/SiO 2 /Al 2 O 3 stacks. We also annealed these stacks in different atmospheres to test the influence of annealing atmospheres on the chemical passivation of c-Si by Al 2 O 3 and observed that O 2 -containing atmosphere led to the best surface chemical passivation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Zirconium Carbide Mediates Coke‐Resistant Methane Dry Reforming on Nickel‐Zirconium Catalysts

Abstract Graphitic deposits anti‐segregate into Ni 0 nanoparticles to provide restored CH 4 adsorption sites and near‐surface/dissolved C atoms, which migrate to the Ni 0 /ZrO 2 interface and induce local Zr x C y formation. The resulting oxygen‐deficient carbidic phase boundary sites assist in the kinetically enhanced CO 2 activation toward CO(g). This interface carbide mechanism allows for enhanced spillover of carbon to the ZrO 2 support, and represents an alternative catalyst regeneration pathway with respect to the reverse oxygen spillover on Ni‐CeZr x O y catalysts. It is therefore rather likely on supports with limited oxygen storage/exchange kinetics but significant carbothermal reducibility.

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

B 2 H 6 splitting on catalytic surfaces and role of BH 3 towards hydrogen spillover

A fundamental understanding of the spillover mechanism is an open and challenging problem and plays an important role in catalysis. In particular, bond-exchange spillover mechanism is considered to be effective for reversible storage and release of hydrogen at near ambient conditions. For this, three critical steps are needed: finding the right support (acceptor), the right catalyst to split H 2 , and ensuring that once H 2 is split, the H atoms can migrate on the surface with the help of secondary catalysts and eventually hydrogenate the entire material. In this paper we address these challenges using density functional theory. Here we show that BH 3 , a secondary catalyst, can be produced by symmetrically splitting its stable precursor, B 2 H 6 , on doped metal-free surfaces such as graphene and h-BN as well as on MOF5. In addition, to reduce computational cost, we develop structural descriptor and predictive model equation to effectively screen potential BH 3 binding sites. Symmetrical splitting of B 2 H 6 on different types of materials can address the hydrogen spillover challenge, making efficient storage of hydrogen possible.

08 HYDROGEN↗
DOE OSTI2021

Improving Thermal Stability of Perovskite Solar Cells by Suppressing Ion Migration Using Copolymer Grain Encapsulation

Thermal stability of organic–inorganic hybrid perovskites (OIHPs) remains as one of the critical challenges against the stable operation of perovskite solar cells (PSCs) in direct sunlight with elevated temperatures. Here, we show that the addition of a polystyrene-co-polyacrylonitrile (SAN) copolymer can significantly enhance thermal stability of OIHPs and improve the stability of the corresponding PSCs by suppressing the migration of organic cations in OHIP. The methylammonium lead iodide (MAPI) with SAN incorporated within the perovskite layer featured a superior thermal stability compared to pure MAPI without SAN, only displaying an average of 5–15% decrease in PCE even after continuous thermal aging for 24 h at 100 °C. The secondary ion mass spectrometry revealed that the thermal degradation of the pure MAPI was largely associated with MA+ out-migration. Conducting atomic force microscopy analysis further indicated that the incorporated SAN led to a suppression of ionic currents present at the grain boundaries of the perovskite film, which was understood by high immiscibility between SAN and MA+ components as confirmed by the experimentally estimated Flory–Huggins parameter between them. This study newly identifies a potential of using polymer grain encapsulation for enhancing thermal stability of OIHPs and their solar cell performance by suppressing the out-diffusion of cationic organic components.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Coordinatively and Spatially Coconfining High-Loading Atomic Sb in Sulfur-Rich 2D Carbon Matrix for Fast K + Diffusion and Storage

The atomically dispersed materials promise ultrafast redox kinetics for alkali-ion storage. However, their relatively low mass loading limits their application. In this work, well-dispersed Sb atoms with high-loading of 23.3 wt % anchored in sulfur-rich amorphous carbon-coated reduced graphene oxide matrix (SbSA/C) are prepared by a coordinative and spatial coconfinement methodology including freeze-casting the well-dissolved Sb-chelates (Sb-thioglycolate) within graphene oxide suspension and post heating-treatment. The reduced graphene oxide substrate features open two-dimensional spatial framework for loading atomically dispersive Sb species, and the pyrolyzed Sb-chelates would provide not only massive desirable heteroatoms (S, O) for coordinating the Sb atoms but also in situ pyrolytic carbon for further spatially separating those dispersive atoms. The coordinative and spatial coconfinement engineering also endows the SbSA/C composite with atomic-level Sb atoms against migration and agglomeration during electrochemical K-storage cycling. In situ TEM reveals the uniform potassiation behavior of the SbSA/C without obvious volume change; DFT calculation and electrochemical characterization suggest the significantly lower K-ion diffusion energy barrier. Therefore, on the basis of both the active Sb center and the coordinative heteroatom, the SbSA/C electrode delivers high fast-charging capacity, outstanding rate capability and long-lifespan performance in half/full K-ion batteries (e.g., a stable capacity of 331.3 mA h g –1 is maintained over 1100 cycles at 1.0 A g –1 for half-cell).

36 MATERIALS SCIENCE↗
DOE OSTI2021

Defect-driven selective metal oxidation at atomic scale

Nanoscale materials modified by crystal defects exhibit significantly different behaviours upon chemical reactions such as oxidation, catalysis, lithiation and epitaxial growth. However, unveiling the exact defect-controlled reaction dynamics (e.g. oxidation) at atomic scale remains a challenge for applications. Here, using in situ high-resolution transmission electron microscopy and first-principles calculations, we reveal the dynamics of a general site-selective oxidation behaviour in nanotwinned silver and palladium driven by individual stacking-faults and twin boundaries. The coherent planar defects crossing the surface exhibit the highest oxygen binding energies, leading to preferential nucleation of oxides at these intersections. Planar-fault mediated diffusion of oxygen atoms is shown to catalyse subsequent layer-by-layer inward oxide growth via atomic steps migrating on the oxide-metal interface. These findings provide an atomistic visualization of the complex reaction dynamics controlled by planar defects in metallic nanostructures, which could enable the modification of physiochemical performances in nanomaterials through defect engineering.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Phosphorus diffusion and deactivation during SiGe oxidation

Dopant profiles near the semiconductor–oxide interface are critical for microelectronic device performance. As the incorporation of Si 1−x Ge x into transistors continues to increase, it is necessary to understand the behavior of dopants in Si 1−x Ge x . In this paper, the diffusion and electrical activation of phosphorus within a strained, single-crystal Si 0.7 Ge 0.3 layer on Si during oxidation are reported. Both layers were uniformly doped, in situ, with an average phosphorus concentration of 4 × 10 19 atoms/cm 3 . After high-temperature oxidation, secondary ion mass spectrometry measurements revealed that the bulk of the phosphorus diffuses out of only the SiGe layer and segregates at the oxidizing SiGe–SiO 2 interface. Hall effect measurements corroborate the observed phosphorus loss and show that the phosphorus diffusing to the oxidizing interface is electrically inactive. Through density functional theory (DFT) calculations, it is shown that phosphorus interstitials prefer sites near the SiGe–SiO 2 interface. Finally, based on a combination of experimental data and DFT calculations, we propose that the phosphorus atoms are displaced from their lattice sites by Ge interstitials that are generated during SiGe oxidation. The phosphorus atoms then migrate toward the SiGe–SiO 2 interface through a novel mechanism of hopping between Ge sites as P–Ge split interstitials. Once they reach the interface, they are electrically inactive, potentially in the form of interstitial clusters or as part of the reconstructed interface or oxide.

Thornton, Chappel S.↗
DOE OSTI2024

Initial Development of Fusion Magnet Simulation Capabilities for Performance and Safety Evaluation Using the MOOSE Framework

Fusion energy holds the promise of being a transformative technology as a carbon-neutral, sustainable source of energy. Whole device modeling and the development of fusion digital twins will be increasingly important for emerging fusion device concepts at both national laboratories and within the commercial fusion industry. However, meeting the challenge of whole device modeling of fusion energy devices requires robust, multiphysics, multiscale modeling and simulation technologies capable of running on large-scale supercomputers. Detailed analysis of individual systems at-scale is also required to ensure safe and efficient operation as well as provide the safety basis for future device designs and licensing activities. In a tokamak, toroidal and poloidal magnets confine and shape the fusion plasma to promote the fusion reaction. High plasma temperatures and high magnetic field requirements in modern design concepts (leading to high amounts of energy stored within each magnet) impose electrical, thermal, and mechanical loads on the magnet components, which in turn impacts the safety considerations of the magnet and their supporting systems. Idaho National Laboratory (INL) has a history of working in this space, including development and benchmarking of the Magnetic System Circuitry Analysis Program (MSCAP) and Magnet Arcing (MAGARC) codes to study magnet quench events; notably, MAGARC was used to study quenching during the ITER Engineering Design Activity. However, these legacy codes and capabilities are not parallel and scalable, and new tools are required for future advances in this area, which leads to the INL-developed Multiphysics Object-Oriented Simulation Environment (MOOSE) framework. Developed originally for fission reactor systems under United States Department of Energy, Office of Nuclear Energy modeling and simulation programs, the MOOSE framework has been well-suited to multiscale, multiphysics modeling and simulation needs for nuclear systems. The framework is open-source, well-tested, under continuous development and deployment, and developed to a Nuclear Quality Assurance, Level 1 software quality standard. MOOSE has also been used in the fusion space previously in several projects: INL’s Tritium Migration Analysis Program, Version 8 (TMAP8) for tritium migration, UK Atomic Energy Authority’s A Unified Resource for OpenMC (fusion) Reactor Applications (AURORA) code for fusion thermo-mechanical and neutronics analysis, and Argonne National Laboratory’s Cardinal for high-fidelity computational fluid dynamics and neutronics. However, to model superconducting magnets, several MOOSE enhancements are required: additions to the current MOOSE electromagnetic capabilities, new material libraries for superconductors of interest (such as YBCO), as well as fusion-specific models for thermo-mechanics. This talk will discuss initial development activities to build these capabilities in MOOSE, focusing on initial validation and benchmarking activities. Proposed coupling workflows and future work to support the simulation of fusion magnets and magnet structural assemblies for performance and safety evaluation in MOOSE will also be discussed.

70 - PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Shear strain alters the structure and migration mechanism of self-interstitial atoms in copper

Here, we use atomistic modeling to show that externally applied shear strain causes the lowest energy self-interstitial atom (SIA) structure in copper (Cu) to change from a <100>-type dumbbell to a <110>-type dumbbell. Concurrently, SIA migration switches from the 3-D random walk characteristic of <100>-type dumbbells to a 1-D mechanism analogous to that of crowdion SIAs. Furthermore, the relative energies of these two dumbbell structures as a function of strain are well predicted using elastic dipole tensors computed at zero strain, indicating that examination of these tensors may be used to assess the likelihood of strain-induced SIA structure transitions in other materials. Changes in lowest energy SIA structures and associated migration mechanisms stand to impact predictions of SIA behavior in irradiated solids.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Photoelectron Spectroscopy and Theoretical Study on Monosolvated Cyanate Analogue Clusters ECX-· Sol (ECX- = NCSe-, AsCSe- and AsCS-; Sol = H2O, CH3CN)

Six monosolvated cyanate analogue clusters ECX-?Sol (ECX– = NCSe-, AsCSe-, and AsCS-; Sol = H2O and CH3CN) were investigated using negative ion photoelectron spectroscopy (NIPES). NIPES experiments show that these clusters possess overall similar spectra compared to their respective isolated ECX- anions but shift to higher electron binding energy with solvent CH3CN stabilizing the excess electrons slightly more than H2O. For the ECX-·H2O series, vertical detachment energies and their increments relative to the bare species are measured to be 3.700 / 0.370, 3.085 / 0.425, 3.085 / 0.430 eV for NCSe-, AsCSe- and AsCS-, respectively, while the corresponding values in ECX-·CH3CN series are 3.835 / 0.505, 3.145 / 0.475, and 3.135 / 0.480 eV. Ab initio electronic structure calculations indicate the excess charges located at the terminal N and Se atoms in NCSe- and migrated to the central C atom in AsCSe- and AsCS-. For NCSe-, the solvation is driven by the interactions with the two negatively charged terminal ends, while for AsCSe- and AsCS-, the solvation revolves around the interactions with the central C atom, where all the excess negative charge is concentrated. Two nearly degenerate isomers for NCSe-·H2O are identified, one forming a single strong N…H-O hydrogen bond (HB) and the other featuring a bidentate HB with two hydroxyl H atoms pointing to N and Se ends. In contrast, the negative central C atom in AsCSe-/AsCS- allows the formation of a bifurcated HB with H2O. Similar effects are observed for the acetonitrile case, in which the three H atoms of methyl group interact with the two negatively charged terminal ends in NCSe-, while prefer binding to the central negative carbon atom in AsCSe-/AsCS-. The different binding motifs derived in this work may suggest distinctly different solvation properties in NCSe- versus AsCSe-/AsCS-, with the former anion leading to asymmetric solvation at the N end of the solute, while the latter species creating more ‘isotropic’ solvation around the central C equatorial plane. This work was supported by U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, and performed using EMSL, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, which is operated by Battelle Memorial Institute for the DOE. The theoretical calculations were conducted on EMSL’s “Cascade” Supercomputer.

Yuan, Qinqin↗