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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Chiral Recognition: A Spin-Driven Process in Chiral Oligothiophene. A Chiral-Induced Spin Selectivity (CISS) Effect Manifestation

In this paper it is experimentally demonstrated that the electron-spin/molecular-handedness interaction plays a fundamental role in the chiral recognition process. This conclusion is inferred comparing current versus potential (I-V) curves recorded using chiral electrode surfaces, which are obtained via chemisorption of an enantiopure thiophene derivative: 3,3'-bibenzothiophene core functionalized with 2,2'-bithiophene wings (BT 2 T 4 ). The chiral recognition capability of these chiral-electrodes is probed via cyclic voltammetry measurements, where, Ag nanoparticles (AgNPs) capped with enantiopure (BT 2 T 4 ) (BT 2 T 4 )@AgNP) are used as the chiral redox probe. Then, the interface handedness is explored by recording spin-polarized (I-V) curves in spin-dependent electrochemistry (SDE) and magnetic-conductive atomic force microscopy (mc-AFM) experiments. The quality of the interfaces is thoroughly cross-checked using X-ray photoemission spectroscopy, Raman, electrodesorption measurements, which further substantiate the metal(electrode)-sulfur(thiophene) central role in the chemisorption process. Spin-polarization values of about 15% and 30% are obtained in the case of SDE and mc-AFM experiments, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Metal alloy microparticle tagging of UO 2 fuel pellets for nuclear forensics

A tagging strategy for UO 2 fuel pellets will be discussed which involves the creation of alloyed metal microparticles that can be tuned to create a unique identification (“barcode”) indicating provenance of the pellets. These specialized markers would contain high temperature resistant materials, such as Ti, Mo, and Cr, in small quantities such that they do not interfere with the performance of the pellets but are recoverable by standard analytical techniques. The Amazemet rePowder ultrasonic atomization system was used to manufacture the microparticles, and sample pellets were fabricated and sintered containing these dopants. Various techniques, such as Scanning Electron Microscopy (SEM) and Electron Probe Microanalysis (EPMA), were used to evaluate these sample pellets to determine if particles (and their alloy compositions) were recoverable. Details on how the particles were manufactured and characterized will be detailed, as well as results from their incorporation into test pellets. Next steps will involve the scale of this technique using more conventional mass production methods, as well as the implementation of other alloys.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

A Mechanism for Carbon Depletion at Bondline of High-Frequency Electric-Resistance-Welded X70 Pipeline Steel

The bondline of electric-resistance-welded (ERW) linepipe steel, often etched white (i.e., ferrite) in optical microscopy, is generally believed to be carbon depleted. The mechanism for the carbon depletion, however, is not fully understood by researchers. To this end, here we used atom probe tomography (APT) to measure elemental segregation of the as-welded and post-weld heat-treated bondline regions of X70 linepipe welds. The thin vertical features at the bondline in the as-welded condition were identified as carbon-rich martensite-austenite (M-A) constituents, and the majority ferrite phase in the bondline was identified as carbon-depleted ferrite. Following the post-weld normalization, all alloying elements, except Nb and Mo, are homogenized across the bondline and heat-affected zone. The carbon depletion in the ERW bondline was accurately measured. A new mechanism for carbon depletion has been proposed using Scheil calculations of elemental partitioning during weld formation. Segregation of elements in the heat-affected zone was shown to follow the negligible partitioning local equilibrium (NPLE) kinetics for bainite transformation.

36 MATERIALS SCIENCE↗
DOE OSTI2020

In situ nanoscale imaging of moiré superlattices in twisted van der Waals heterostructures

Abstract Direct visualization of nanometer-scale properties of moiré superlattices in van der Waals heterostructure devices is a critically needed diagnostic tool for study of the electronic and optical phenomena induced by the periodic variation of atomic structure in these complex systems. Conventional imaging methods are destructive and insensitive to the buried device geometries, preventing practical inspection. Here we report a versatile scanning probe microscopy employing infrared light for imaging moiré superlattices of twisted bilayers graphene encapsulated by hexagonal boron nitride. We map the pattern using the scattering dynamics of phonon polaritons launched in hexagonal boron nitride capping layers via its interaction with the buried moiré superlattices. We explore the origin of the double-line features imaged and show the mechanism of the underlying effective phase change of the phonon polariton reflectance at domain walls. The nano-imaging tool developed provides a non-destructive analytical approach to elucidate the complex physics of moiré engineered heterostructures.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Guided anisotropic oxygen transport in vacancy ordered oxides

Abstract Anisotropic and efficient transport of ions under external stimuli governs the operation and failure mechanisms of energy-conversion systems and microelectronics devices. However, fundamental understanding of ion hopping processes is impeded by the lack of atomically precise materials and probes that allow for the monitoring and control at the appropriate time- and length- scales. In this work, using in-situ transmission electron microscopy, we directly show that oxygen ion migration in vacancy ordered, semiconducting SrFeO 2.5 epitaxial thin films can be guided to proceed through two distinctly different diffusion pathways, each resulting in different polymorphs of SrFeO 2.75 with different ground electronic properties before reaching a fully oxidized, metallic SrFeO 3 phase. The diffusion steps and reaction intermediates are revealed by means of ab-initio calculations. The principles of controlling oxygen diffusion pathways and reaction intermediates demonstrated here may advance the rational design of structurally ordered oxides for tailored applications and provide insights for developing devices with multiple states of regulation.

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

Ultrafast charge transfer and carrier dynamics in a WS 2 /MoSe 2 few-layer van der Waals heterostructure

Photocarrier dynamics including interlayer charge transfer and intralayer valley scattering are studied in a heterostructure formed by trilayer WS 2 and MoSe 2 . The sample is fabricated by mechanical exfoliation and dry transfer and characterized by atomic force microscopy, Raman spectroscopy, and photoluminescence measurements. The conduction band minima of the two materials are nearly degenerate, representing a unique band alignment. Layer-selective pump–probe measurements are performed with three configurations to reveal a complete picture of the photocarrier dynamics. By comparing the heterostructure with the two individual trilayer materials in each experimental configuration, ultrafast interlayer charge transfer is unambiguously observed, which occurs on the same time scale as the intralayer valley scattering of the photocarriers. The quasi-equilibrium distribution of electrons in the two layers mediate the fast carrier recombination process. Furthermore, these results show that the band structures of the few-layer transition metal dichalcogenides can enable rich photocarrier dynamics with intermediate band alignments that are complementary to the previously studied monolayer–monolayer heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Imaging Nanoscale Energy Transport and Conversion with Ultrafast Electron Microscopy (Final Technical Report)

Light-matter interactions are ubiquitous in nature and reside at the heart of innumerable technologies. The cascade of processes that occur when a material absorbs a photon of light are exceedingly complex and are interwoven in both space and time, rendering precise determination of the atomic-scale and ultrafast mechanisms immensely challenging. The advent of methods for generating short pulses of light several decades ago led to major advances in understanding the initial moments of light absorption and the resultant effects, though directly interrogating the response of the atoms within the material continued to prove challenging. More recently, methods for generating ultrashort pulses of X-rays and fast electrons have opened the way to probing photoinduced structural dynamics of a wide range of matter in multiple phases. An especially promising laboratory-scale method is ultrafast electron microscopy (UEM), wherein the modalities of conventional transmission electron microscopes (imaging, diffraction, spectroscopy) are extended into the femtosecond temporal regime. Here, the ultrafast imaging and diffraction modalities of UEM were used to study the transient structural aspects of photoexcitation of semiconducting materials (e.g., spatially-resolved electron-phonon coupling, excitation and emission of acoustic phonons, and discrete nanoscale scattering processes). The project had three main objectives: (1) determination of the excitation mechanisms of dense, hypersonic charge-carrier waves and the spatially mediated means by which they couple to the lattice via coherent phonon emission, especially with UEM imaging, wherein effects of nanoscale structural and morphological features on the coupling and relaxation dynamics are expected to dictate nucleation sites and preferred wave vectors, (2) elucidation of photoinduced acoustic-phonon seeding, emergence, propagation, and decay over nanoscale crystal regions and especially with respect to local strain fields and atomic-scale disruptions in lattice order, and (3) interwoven with the first two were efforts aimed at realizing combined angstrom-femtosecond spatiotemporal imaging with UEM. The outcomes and impacts of this project were the generation of new knowledge with respect to fundamental light-matter interactions and, especially, the spatially-mediated excitation and evolution of the structural response of materials following coherent photoexcitation. Importantly, the spatial and temporal resolutions of the UEM imaging modalities used are well-suited for such studies and enable spatially-resolved mechanisms to be determined as a function of atomic order, structural features, and morphology. In addition, ultrafast crystallographic measurements were used to correlate real- and reciprocal-space dynamics in order to determine atomic-scale preferential wave vectors and ultrafast scattering mechanisms, especially as dictated by specimen boundary conditions. The obtained results, as detailed in peer-reviewed publications and presentations, illustrate the importance of ultrafast, angstrom-scale real-space imaging for developing a comprehensive understanding of energy transport and conversion in materials.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Metallic line defect in wide-bandgap transparent perovskite BaSnO 3

A line defect with metallic characteristics has been found in optically transparent BaSnO 3 perovskite thin films. The distinct atomic structure of the defect core, composed of Sn and O atoms, was visualized by atomic-resolution scanning transmission electron microscopy (STEM). When doped with La, dopants that replace Ba atoms preferentially segregate to specific crystallographic sites adjacent to the line defect. The electronic structure of the line defect probed in STEM with electron energy-loss spectroscopy was supported by ab initio theory, which indicates the presence of Fermi level–crossing electronic bands that originate from defect core atoms. These metallic line defects also act as electron sinks attracting additional negative charges in these wide-bandgap BaSnO 3 films.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Nano-compositional imaging of the lanthanum silicide system at THz wavelengths

Terahertz scattering-type scanning near-field optical microscopy (THz-sSNOM) provides a noninvasive way to probe the low frequency conductivity of materials and to characterize material compositions at the nanoscale. However, the potential capability of atomic compositional analysis with THz nanoscopy remains largely unexplored. Here, we perform THz near-field imaging and spectroscopy on a model rare-earth alloy of lanthanum silicide (La–Si) which is known to exhibit diverse compositional and structural phases. We identify subwavelength spatial variations in conductivity that is manifested as alloy microstructures down to much less than 1 μ m in size and is remarkably distinct from the surface topography of the material. Signal contrasts from the near-field scattering responses enable mapping the local silicon/lanthanum content differences. These observations demonstrate that THz-sSNOM offers a new avenue to investigate the compositional heterogeneity of material phases and their related nanoscale electrical as well as optical properties.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Probing topological quantum matter with scanning tunnelling microscopy

The search for topological phases of matter is evolving towards strongly interacting systems, including magnets and superconductors, where exotic effects emerge from the quantum-level interplay between geometry, correlation and topology. Over the past decade or so, scanning tunnelling microscopy has become a powerful tool to probe and discover emergent topological matter, because of its unprecedented spatial resolution, high-precision electronic detection and magnetic tunability. Scanning tunnelling microscopy can be used to probe various topological phenomena, as well as complement results from other techniques. We discuss some of these proof-of-principle methodologies applied to probe topology, with particular attention to studies performed under a tunable vector magnetic field, which is a relatively new direction of recent focus. Finally, we then project the future possibilities for atomic-resolution tunnelling methods in providing new insights into topological matter.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Minor Elements and Solidification Cracking During Laser Powder-Bed Fusion of a High $\gamma ^{\prime }$ CoNi-Base Superalloy

Here, the cracking behavior of a high γ' volume fraction CoNi-base superalloy fabricated via laser powder bed fusion (LPBF) is studied in relation to the content of carbon and boron. Severe cracking occurred with the increase in boron content from 0.08 to 0.16 at. pct (0.015 to 0.029 wt pct), while compositions with 0.1 to 0.36 at. pct C (0.02 to 0.076 wt pct) and 0.08 at. pct B exhibited minimal cracking. Assessment of cracks in the high-boron composition shows a variation in crack density with printing parameters, and alignment of the cracks with the build direction. Scanning electron microscopy (SEM) of the crack surfaces shows evidence of a solidification cracking mode. Differential thermal analysis (DTA) reveals a decreased incipient melting temperature for the high-boron composition, and atom probe tomography (APT) is used to measure the enrichment at grain boundaries, revealing distinct boron segregation. Scheil-Gulliver solidification simulations for the different C and B levels are consistent with the incipient melting behavior observed with DTA. Evaluation of the solidification cracking susceptibility from the simulations allow for comparison of the CoNi alloy behavior to Ni-base superalloys studied for LPBF fabrication and displays how such metrics may aid in the design of new precipitation-strengthened superalloys for additive manufacturing (AM).

36 MATERIALS SCIENCE↗
DOE OSTI2023

Nanoprecipitates to Enhance Radiation Tolerance in High-Entropy Alloys

The growth of advanced energy technologies for power generation is enabled by the design, development, and integration of structural materials that can withstand extreme environments, such as high temperatures, radiation damage, and corrosion. High-entropy alloys (HEAs) are a class of structural materials in which suitable chemical elements in four or more numbers are mixed to typically produce single-phase concentrated solid solution alloys (CSAs). Many of these alloys exhibit good radiation tolerance like limited void swelling and hardening up to relatively medium radiation doses (tens of displacements per atom (dpa)); however, at higher radiation damage levels (>50 dpa), some HEAs suffer from considerable void swelling limiting their near-term acceptance for advanced nuclear reactor concepts. In this study, we developed a HEA containing a high density of Cu-rich nanoprecipitates distributed in the HEA matrix. The Cu-added HEA, NiCoFeCrCu 0.12 , shows excellent void swelling resistance and negligible radiation-induced hardening upon irradiation up to high radiation doses (i.e., higher than 100 dpa). The void swelling resistance of the alloy is measured to be significantly better than NiCoFeCr CSA and austenitic stainless steels. Density functional theory simulations predict lower vacancy and interstitial formation energies at the coherent interfaces between Cu-rich nanoprecipitates and the HEA matrix. The alloy maintained a high sink strength achieved via nanoprecipitates and the coherent interface with the matrix at a high radiation dose (~50 dpa). From our experiments and simulations, the effective recombination of radiation-produced vacancies and interstitials at the coherent interfaces of the nanoprecipitates is suggested to be the critical mechanism responsible for the radiation tolerance of the alloy. Furthermore, the materials design strategy based on incorporating a high density of interfaces can be applied to high-entropy alloy systems to improve their radiation tolerance.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Core–Shell Gold Nanoparticle-Star Copolymer Composites with Gradient Transfer and Transport Properties: Toward Electro-Optical Sensors and Catalysis

A series of hybrid core–shell nanomaterials was prepared by solvent phase transfer methods with gold (Au) nanoparticles (NPs) as a core and star copolymers (h-PEI-b-PCL-Cbz) as a shell—consisting of hyperbranched polyethyleneimine (h-PEI) with different poly(ε-caprolactone) (PCL) chain lengths and a carbazole-contained terminating agent (Cbz). By varying the PCL chain length, these core–shell nanomaterials exhibited gradient energy transfer properties, surface hydrophobicity, electropolymerizability, and ionic probe transport and diffusion properties. The systematic copolymer formation was verified with quadruple-detector size exclusion chromatography (SEC) and 1H nuclear magnetic resonance (NMR). High-resolution atomic force microscopy-infrared spectroscopy (AFM-IR) chemical mapping verified nanoparticle-substrate adhesion and film formation. The design of these hybrid NPs and elucidation of gradient properties will be useful for future applications of the core–shell systems in electro-optical sensing and catalysis.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Pushing the limits: Resolving paleoseawater signatures in nanoscale fluid inclusions by atom probe tomography

New insight into the geochemistry of ancient environments can be gained through structural and chemical analyses of nanometer-scale features within minerals. Here, we present recent developments using atom probe tomography (APT) enabling direct visualization of nanoscale fluid inclusions trapped within pyrite (FeS 2 ) and thereby chemical characterization of remnant seawater. Pyrite framboids (spherical clusters of nanocrystals) were sampled from the Middle Devonian Leicester Pyrite Member (New York). Scanning transmission electron microscopy shows low density regions distributed within the pyrite consistent with nanoscale pores (<4 nm in size). APT 3D visualization and compositional mapping reveals that the nanopores are filled with water. The inclusions appear to preserve the elemental signature of the water column in which the framboids formed, specifically seawater components including Na, K, Mg, and Ca. Mg/Ca ratios within the pyrite were generally measured to be within 0.6±0.2 – consistent with calcite-dominated seawater conditions existing in the Middle Devonian. Furthermore, this study demonstrates the potential for a novel approach to reconstruct paleoenvironmental conditions from coupled elemental and structural analyses of nanoscale fluid inclusions.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2024

Topological Fermi-arc surface state covered by floating electrons on a two-dimensional electride

Two-dimensional electrides can acquire topologically non-trivial phases due to intriguing interplay between the cationic atomic layers and anionic electron layers. However, experimental evidence of topological surface states has yet to be verified. Here, via angle-resolved photoemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM), we probe the magnetic Weyl states of the ferromagnetic electride [Gd 2 C] 2+ ·2e - . In particular, the presence of Weyl cones and Fermi-arc states is demonstrated through photon energy-dependent ARPES measurements, agreeing with theoretical band structure calculations. Notably, the STM measurements reveal that the Fermi-arc states exist underneath a floating quantum electron liquid on the top Gd layer, forming double-stacked surface states in a heterostructure. Our work thus not only unveils the non-trivial topology of the [Gd 2 C] 2+ ·2e - electride but also realizes a surface heterostructure that can host phenomena distinct from the bulk.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Visualizing the Nanoscale Oxygen and Cation Transport Mechanisms during the Early Stages of Oxidation of Fe–Cr–Ni Alloy Using In Situ Atom Probe Tomography

Abstract Understanding the early stages of interactions between oxygen and material surfaces—especially at very high spatial resolutions—is highly beneficial for fields ranging from materials degradation, corrosion, geological sciences, forensics, and catalysis. The ability of in situ atom probe tomography (APT) is demonstrated to track the diffusion of oxygen and metal ions at nanoscale spatial resolution during the early stages of oxidation of a model Fe–Cr–Ni alloy. Using 18 O isotope tracers in these in situ APT experiments and complementary ex situ multimodal microscopy, spectroscopy, and computational simulations allows to precisely analyze the kinetics of oxidation and determine that outward cation diffusion to oxide/air interface is the primary mechanism for intragranular oxide growth in this alloy at 300 °C. This unique in situ isotopic tracer APT approach and the insights gained can be highly beneficial for studying early stages of gas–surface reactions in a broad array of materials.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Torsional force microscopy of van der Waals moirés and atomic lattices

In a stack of atomically thin van der Waals layers, introducing interlayer twist creates a moiré superlattice whose period is a function of twist angle. Changes in that twist angle of even hundredths of a degree can dramatically transform the system’s electronic properties. Setting a precise and uniform twist angle for a stack remains difficult; hence, determining that twist angle and mapping its spatial variation is very important. Techniques have emerged to do this by imaging the moiré, but most of these require sophisticated infrastructure, time-consuming sample preparation beyond stack synthesis, or both. In this work, we show that torsional force microscopy (TFM), a scanning probe technique sensitive to dynamic friction, can reveal surface and shallow subsurface structure of van der Waals stacks on multiple length scales: the moirés formed between bi-layers of graphene and between graphene and hexagonal boron nitride (hBN) and also the atomic crystal lattices of graphene and hBN. In TFM, torsional motion of an Atomic Force Microscope (AFM) cantilever is monitored as it is actively driven at a torsional resonance while a feedback loop maintains contact at a set force with the sample surface. TFM works at room temperature in air, with no need for an electrical bias between the tip and the sample, making it applicable to a wide array of samples. It should enable determination of precise structural information including twist angles and strain in moiré superlattices and crystallographic orientation of van der Waals flakes to support predictable moiré heterostructure fabrication.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2025

Methodology of Atomic Force Microscopy Visualization of Electrode–Electrolyte Interfaces

Electrochemical atomic force microscopy (EC-AFM) provides unprecedented insights into the microstructure of electrode–electrolyte interfaces during electrochemical reactions. However, performing EC-AFM measurements has many challenges, for example, drift, contamination, and probe degradation. We present solutions to these experimental issues through electrochemical cell design and carefully chosen experimental parameters. The possibility that the probes can react with the interface during scanning, generating false-positive electrochemical dynamics, is discussed as an example of the challenge of high-fidelity EC-AFM measurement. Here, we demonstrate this effect in highly ordered pyrolytic graphite and show that we could use electrochemical control of the AFM probe to enable high-fidelity in situ AFM visualization of solid–liquid interfaces during electrochemical reactions.

Electrochemical cells↗