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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Antithermal mobility in Σ7 and Σ9 grain boundaries caused by stick-slip stagnation of ordered atomic motions about Coincidence Site Lattice atoms

Antithermal grain boundary migration is an interesting phenomenon that has been recently discovered. These grain boundaries behave opposite to what is expected of thermally activated grain boundary migration; the antithermal grain boundaries exhibit mobility that decreases with increasing temperature. Two recent studies have provided insight into important behaviors correlated with antithermal migration. O'Brien and Foiles (J Mater. Sci. Vol. 51, p. 6607–6623, 2016) demonstrated that the high mobility exhibited by an antithermal Σ7 grain boundary is enabled by ordered atomic motions. Ulomek and Mohles (Acta Mater. Vol. 103, p. 424–432, 2016) demonstrated that the thermal migration of a different Σ7 grain boundary exhibited a two-step migration process. In this work, we demonstrate that the antithermal temperature dependence exhibited by these types of grain boundaries is caused by a combination of these two discoveries. Specifically, ordered atomic motions about Coincidence Site Lattice atoms (atoms common to the lattices on either side of the grain boundary), seen by O'Brien and Foiles, are interrupted by additional thermal energy at higher temperature. This is the source of the two-step process seen by Ulomek and Mohles. This two-step process is akin to stick-slip behaviors observed in shear coupled grain boundary migration. As temperature is increased, the duration of the stagnant portion increases and the mobility decreases. This behavior is demonstrated through molecular dynamics simulations in nickel bicrystals over temperatures ranging from 100 to 1400 K for Σ7 and Σ9 grain boundaries. For these Σ7 and Σ9 grain boundaries, those that have ordered atomic mechanisms are almost exclusively antithermal, while those that do not have ordered atomic mechanisms are thermally activated.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Equilibrium vortex lattices of a binary rotating atomic Bose–Einstein condensate with unequal atomic masses

We perform a detailed numerical study of the equilibrium ground-state structures of a binary rotating Bose–Einstein condensate with unequal atomic masses. Our results show that the ground-state distribution and its related vortex configurations are complex events that differ markedly depending strongly on the strength of rotation frequency, as well as on the ratio of atomic masses. We also discuss the structures and radii of the clouds, the number and the size of the core region of the vortices, as a function of the rotation frequency, and of the ratio of atomic masses, and the analytical results agree well with our numerical simulations. This work may open an alternate way in the quantum control of the binary rotating quantum gases with unequal atomic masses. - Highlights: • A binary quantum gases with unequal atomic masses is considered. • Effects of the ratio of atomic masses and rotation frequency are discussed in full parameter space. • The detailed information about both the cloud and vortices are also discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Synthesis and magnetic performance of gadolinium powder produced with rotating disk atomization

Spherical powder is key for achieving good heat transfer and maintaining low pressure drop for heat transfer fluid in magnetocaloric regenerator. Obtaining relatively large spherical powder of rare earth metals that are extremely sensitive to oxygen is challenging. Comparing to gas atomization and plasma rotating electrode methods, rotating disk atomization (RDA) method is more robust for obtaining a small amount of powder with a short turn-around time, thereby, more suitable for research and development of new magnetocaloric materials. In this work, commercially bulk gadolinium was successfully atomized to produce spherical powder in the targeted 150–250 μm range. The obtained powder exhibited the expected magnetocaloric performance with a maximum change of magnetic entropy of 12.1 J/kg K and a maximum adiabatic temperature change of 12.8 K with an applied magnetic field of 7 T.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Elucidating the Evolving Atomic Structure in Atomic Layer Deposition Reactions with in Situ XANES and Machine Learning

Precision synthesis of thin films requires an improved mechanistic understanding of the structural evolution of materials at the atomic scale. Atomic layer deposition (ALD) is a critical nanofabrication technique that enables fine-tuning of atomic structure and thickness as a result of its layer-by-layer growth behavior. In this study, in situ X-ray absorption spectra of the S K-edge during ALD growth of ZnS thin films on TiO 2 nanoparticles were collected and analyzed. The experimental results show that both sulfide and sulfate species form during the nucleation phase of ZnS on TiO 2 . As film growth proceeds, the S K-edge of the in situ ZnS film converges to that of a representative ex situ ALD ZnS film. By building representative atomistic models, a high-throughput screening method was developed to determine the most probable atomic configurations as the film structure evolves. Specifically, the screening method consisted of a supervised machine learning analysis of thousands of simulated X-ray absorption near edge structure (XANES) spectra. Atomic-level insight was gained into changes in the coordination environment of surface species as they transitioned from the nucleation phase toward the crystalline ZnS phase. The experimental and computational strategies presented herein provide an example of how in situ synchrotron-based characterization can be leveraged using robust modeling approaches to elucidate the ordering of atoms during thin-film growth.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Structural Evolution of Mixed-Addenda Keggin Polyoxometalate Anions with Atom-by-Atom Substitution

Polyoxometalates (POMs) are molecular metal oxides with distinctive electronic properties that make them promising materials for applications in energy, sensors, and memory devices. One of the most promising methods of tuning the stability, photochromic, redox, and electron-spin properties of POMs is through the substitution of the metal “addenda” atoms that, along with oxygen, constitute their cage-like structures. Because traditional synthesis methods typically produce a distribution of POMs, the isolation and characterization of multimetallic POMs with predetermined stoichiometry remains challenging. The presence of multiple energetically accessible isomers further complicates the experimental characterization and theoretical modeling of multimetallic POMs. Herein, we leverage the distinguishing mass-selection capabilities of ion soft landing to prepare stoichiometrically selected Keggin PMo x W 12-x O 40 3- (x = 0 – 6, 8, 10, and 12) POMs on self-assembled monolayer surfaces free of the solvent molecules and counterions that often confound characterization of complex species at interfaces. The structures of the supported POMs are characterized with atom-by-atom precision using in situ infrared (IR) reflection absorption spectroscopy complemented by detailed density functional theory calculations. Our joint experimental and theoretical results reveal an almost linear shift in the positions of the IR bands towards lower wavenumbers with an increase in the number of lighter molybdenum atoms compared to heavier W atoms in PMoxW 12-x O 40 3- . The theoretical calculations also indicate that numerous isomeric structures may be populated at the experimental conditions and, consequently, contribute to the overall IR spectra. In conclusion, our findings indicate that in addition to the number of substituted addenda atoms and the presence of multiple isomeric structures, interactions with the surface play an important role in determining the IR spectra and structure of supported bimetallic POMs.

Prabhakaran, Venkateshkumar [Pacific Northwest Nat↗
DOE OSTI2023

Influence of Light Atoms on Quantification of Atomic Column Positions in Distorted Perovskites with HAADF-STEM

Measurement of picometer-scale atomic displacements by aberration-corrected STEM has become invaluable in the study of crystalline materials, where it can elucidate ordering mechanisms and local heterogeneities. HAADF-STEM imaging, often used for such measurements due to its atomic number contrast, is generally considered insensitive to light atoms such as oxygen. Light atoms, however, still affect the propagation of the electron beam in the sample and, therefore, the collected signal. In this work, we demonstrate experimentally and through simulations that cation sites in distorted perovskites can appear to be displaced by several picometers from their true positions in shared cation-anion columns. The effect can be decreased through careful choice of sample thickness and beam voltage or can be entirely avoided if the experiment allows reorientation of the crystal along a more favorable zone axis. Therefore, it is crucial to consider the possible effects of light atoms and crystal symmetry and orientation when measuring atomic positions.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Stabilizing Single-Atom and Small-Domain Platinum via Combining Organometallic Chemisorption and Atomic Layer Deposition

Oxide-supported single-atom Pt materials are prepared by combining surface organometallic chemisorption with atomic layer deposition (ALD). Here Pt is supported as a discrete monatomic “pincer” complex, stabilized by an atomic layer deposition (ALD) derived oxide overcoat, and then calcined at 400 °C under O 2 . ALD-derived Al 2 O 3 , TiO 2 , and ZnO overlayers are effective in suppressing Pt sintering and significantly stabilizing single Pt atoms. Furthermore, this procedure decreases the overall Pt nuclearity (~1 nm average particle diameter) versus bare Pt (~3.8 nm average diameter), as assayed by aberration corrected HAADF-STEM. The TiO 2 and ZnO overcoats are significantly more effective at stabilizing single-atom Pt species and decreasing the overall Pt nuclearity than Al 2 O 3 overcoats. Vibrational spectroscopy of adsorbed CO also shows that oxidized Pt species commonly thought to be single Pt atoms are inactive for catalytic oxidation of adsorbed CO. CO chemisorption measurements show site blockage by the ALD overcoats.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗