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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Irradiation effects on Al 0.3 CoCrFeNi and CoCrMnFeNi high-entropy alloys, and 316H stainless steel at 500 °C

To evaluate the potential of high entropy alloys (HEAs) for nuclear applications, two HEAs, Al 0.3 CoCrFeNi and CoCrFeMnNi, and a conventional Type 316H stainless steel (SS) were irradiated with 1 MeV krypton ions at 500 degrees C up to 1 dpa, and examined in-situ with a transmission electron microscope (TEM). After irradiation, a high density of ordered L1 2 nanoparticles was observed in Al 0.3 CoCrFeNi. In contrast, no phase transformation was observed in CoCrFeMnNi and 316H SS. In the thin foil regions of TEM samples, stacking-fault tetrahedra were observed in the HEAs. In the thick foil regions, interstitial dislocation loops were observed for all alloys, and the areal loop density increased linearly with foil thickness. The Al 0.3 CoCrFeNi had the largest loop size (the lowest density), followed by the CoCrFeMnNi and then the 316H SS. The higher loop nucleation rate in the 316H SS was attributed to carbon content. Finally, the degree of irradiation hardening was slightly lower for the HEAs than for the 316H SS, which is a promising sign for the nuclear application of HEAs at high temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2021

In-situ ion irradiation study of alloy 709 stainless steels with different processing histories

Alloy 709 stainless steel (A709 SS) has gained an increased interest in nuclear applications due to its advanced properties at high temperatures over the widely used Type 316 SS. However, how this alloy behaves under irradiation with concurrent environmental factors, such as mechanical deformation and/or thermal annealing, has not been studied. In this work, we used in-situ ion irradiation inside a transmission electron microscope (TEM) to investigate the effects of irradiation on the microstructures of A709 SS samples with different prior processing histories, including as-received, annealed, and creep-deformed. It was found that, compared to room temperature irradiation, 600 degrees C irradiation led to larger dislocation loop sizes and lower loop densities. Furthermore, with 600 degrees C irradiation, the creep-deformed sample, which had the highest precipitate density and the highest dislocation density, had the smallest loop size and the lowest loop density compared to others. This observation has been rationalized with quantitative evaluation of the sink strength factors in each sample. This study shows that in-service deformation can significantly affect the irradiation performance in structural materials at reactor operating conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Phase stability, mechanical properties, and ion irradiation effects in face-centered cubic CrFeMnNi compositionally complex solid-solution alloys at high temperatures

We report two CrFeMnNi face-centered cubic complex concentrated solid-solution alloys (CSA) have been evaluated for phase stability, mechanical properties, and radiation damage effects from heavy ions. Cr 18 Fe 27 Mn 27 Ni 28 and Cr 15 Fe 35 Mn 15 Ni 35 were predicted by thermodynamic calculations to phase separate and maintain a single phase at 700 °C, respectively. Aging experiments at this temperature confirmed varying degrees of precipitation of a body-centered cubic phase in both Cr 18 Fe 27 Mn 27 Ni 28 and Cr 15 Fe 35 Mn 15 Ni 35 . The alloys showed promising strength in tensile deformation at room temperature, with yield strengths of 155 MPa and 151 MPa for Cr 18 Fe 27 Mn 27 Ni 28 and Cr 15 Fe3 5 Mn 15 Ni 35 , respectively. At 500 °C, the yield strength of Cr 18 Fe 27 Mn 27 Ni 28 fell to 93 MPa, and to 100 MPa in Cr 15 Fe3 5 Mn 15 Ni 35 . Unlike Cr 18 Fe 27 Mn 27 Ni 28 , Cr 15 Fe 35 Mn 15 Ni 35 gained some ductility at 500 °C compared to room temperature. The two CSAs were irradiated to 75 dpa at 500 °C in the plateau region of the displacement curve using 3.7 MeV Ni 2+ ions, alongside model alloy 709 as a reference. Irradiation results produced similar densities and sizes of dislocations loops in the two CSAs compared to the reference. However, while large voids form in the plateau region of Cr 18 Fe 27 Mn 27 Ni 28 , small voids form just beyond the displacement peak of Cr 15 Fe 35 Mn 15 Ni 35 . Atom probe tomography and energy dispersive X-ray spectroscopy-equipped scanning transmission electron microscopes were used to characterize the alloys for changes in chemical distribution.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2024

Mechanisms of ion irradiation induced ordering in amorphous TiO 2 nanotubes: Effects of ion mass and energy

Here, amorphous TiO 2 nanotubes were irradiated in-situ in a transmission electron microscope (TEM) with Kr + ions at energies of 46 keV, 150 keV, and 1 MeV and with 46 keV Xe + ions, to investigate the structural and morphological evolution of the nanotubes under irradiation. At all irradiation conditions, amorphous TiO 2 nanotubes exhibited significant morphological instability, and tended to undergo volumetric swelling with increasing ion counts, often until collapse of the original nanotube structure. Molecular dynamics (MD) simulations confirmed that irradiation-induced defects can explain the observed swelling. Structurally, nanotubes remain amorphous following all Kr + irradiation conditions, but irradiation with 46 keV Xe + leads to the formation of anatase nanocrystallites. Importantly, through systematically varying ion energy and ion species, we try to elucidate the influence of nuclear and electronic stopping power on ion irradiation induced changes. By contextualizing these results within the existing literature, we propose that the observed changes in TiO 2 nanotube morphology and structure could be due to a competition between two mechanisms: (1) disorder-induced swelling and (2) irradiation-induced amorphous-to-crystalline transformation.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Pulsed-Beam Transmission Electron Microscopy and Radiation Damage

Here, we review the use of pulsed electron-beams in transmission electron microscopes (TEMs) for the purpose of mitigating specimen damage. We begin by placing the importance of TEMs with respect to materials characterization into proper context, and we provide a brief overview of established methods for reducing or eliminating the deleterious effects of beam-induced damage. We then introduce the concept of pulsed-beam TEM, and we briefly describe the basic methods and instrument configurations used to create so-called temporally structured electron beams. Following a brief overview of the use of high-dose-rate pulsed-electron beams in cancer radiation therapy, we review historical speculations and more recent compelling but mostly anecdotal findings of a pulsed-beam TEM damage effect. This is followed by an in-depth technical review of recent works seeking to establish cause-and-effect relationships, to conclusively uncover the presence of an effect, and to explore the practicality of the approach. These studies, in particular, provide the most compelling evidence to date that using a pulsed electron beam in the TEM is indeed a viable way to mitigate damage. Throughout, we point out current gaps in understanding, and we conclude with a brief perspective of current needs and future directions.

36 MATERIALS SCIENCE↗
DOE OSTI2020

In situ indentation and high cycle tapping deformation responses in a nanolaminate crystalline/amorphous metal composite

The incorporation of nanostructured and amorphous metals into modern applications is reliant on the understanding of deformation and failure modes in constrained conditions. To study this, a 105 nm crystalline Cu/160 nm amorphous Cu 45 Zr 55 (at.%) multilayer structure was fabricated with the two crystalline layers sputter deposited between the top-middle-bottom amorphous layers and prepared to electron transparency. The multilayer was then in situ indented either under a single load to a depth of ~ 100 nm (max load of ~ 100 μN) or held at 20 μN and then repeatedly indented with an additional 5 μN up to 20,000 cycles in a transmission electron microscope to compare the deformation responses in the nanolaminate. For the single indentation test, the multilayer showed serrated load-displacement behavior upon initial indentation inductive of shear banding. Additionally, at an indentation depth of ~ 32 nm, the multilayer exhibited perfect plastic behavior and no strain hardening. Both indented and fatigue-indented films revealed diffraction contrast changes with deformation. Subsequent Automated Crystal Orientation Mapping (ACOM) measurements confirmed and quantified global texture changes in the crystalline layers with specifically identified grains revealing rotation. Using a finite element model, the in-plane displacement vectors under the indent mapped conditions where ACOM determined grain rotation was observed, indicating the stress flow induced grain rotation. Furthermore, the single indented Cu layers also exhibited evidence of deformation induced grain growth, which was not evident in the fatigue-indented Cu based multilayer. Finally, the single indented multilayer retained a significant plastic crater in the upper most amorphous layer that directly contacted the indenter; a negligible crater impression in the same region was observed in the fatigued tested multilayer. These differences are explained by the different loading methods, applied load, and deformation mechanisms experienced in the multilayers.

36 MATERIALS SCIENCE↗
DOE OSTI2024

A new paradigm in electron microscopy: Automated microstructure analysis utilizing a dynamic segmentation convolutional neutral network

Over the past half century, the transmission electron microscope enabled insight into the fundamental arrangements and structures of materials. State-of-the-art electron microscopes can acquire large image datasets across multiple imaging modalities. However, the manual annotation process for feature or defect quantification may not be feasible with the modern microscope. Convolutional neural networks emerged to characterize individual microstructural features from an image in a cost-effective, consistent manner. However, many of these neural network approaches rely on thousands to hundreds of thousands of manual annotations of each feature type across hundreds of images to train the network for adequate performance. This work focused on the development and application of a pixel-wise defect detection machine-learning dynamic segmentation convolutional neural network with associated automated acquisition and postprocessing to identify microstructural features rapidly and quantitatively from a small initial dataset incorporating multiple imaging modes. The approach was demonstrated for characterization of superalloy 718 from both single image acquisition on multiple detectors to in-situ evolution captured with a single detector on a standard desktop computer to demonstrate the low barrier to entry required for widespread adoption. Pixel-by-pixel class identification was excellent with strong identification of chemically distinct phases, structurally distinct phases, and defect structures, thus demonstrating the new paradigm of machine learning-assisted characterization.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Grain growth of nanocrystalline aluminum under tensile deformation: A combined in situ TEM and atomistic study

Nanocrystalline Al thin films have been strained in situ in a transmission electron microscope using two separate nanomechanical techniques involving a push-to-pull device and a microelectromechanical system (MEMS) device. Deformation-induced grain growth was observed to occur via stress-assisted grain boundary migration with extensive grain growth occurring in the necked region, indicating that the increase in local stress drives the boundary migration. Under applied tensile stresses close to the ultimate tensile strength of 450 MPa for a nanocrystalline Al specimen, measured boundary migration speeds are 0.2 – 0.7 nm s -1 for grains outside necked region and increases to 2.5 nm s -1 for grains within the necked region where the local estimated tensile stresses are elevated to around 630 MPa. By tracking grain boundary motion over time, molecular dynamics simulations showed qualitative agreement in terms of pronounced grain boundary migration with the experimental observations. The combined in situ observation and molecular dynamics simulation results underscore the important role of stress-driven grain growth in plastically deforming nanocrystalline metals, leading to intergranular fracture through predominant grain boundary sliding in regions with large localized deformation.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Size effect on the growth and pulverization behavior of Si nanodomains in SiO anode

Silicon monoxide (SiO) as a promising silicon-based anode electrode of lithium-ion batteries (LIBs) has been the subject of many recent investigations. However, the complex microstructural features of SiO hinder attempts to link the microstructure changes in SiO with its electrochemical performances upon electrochemical cycling. This study employs X-ray pair distribution function (PDF), with a high-resolution transmission electron microscope (HRTEM), to investigate the microstructure of Si nanodomains in SiO and its structural evolution over the electrochemical cycling process. The experiment results reveal the size evolution of Si nanodomains in SiO during the electrochemical cycling and find it highly dependent on the initial Si domain size in SiO. If the initial Si domain size is too large, the average size of Si nanodomains would increase at first and then decrease after a certain number of cycles, which indicates the pulverization of Si domains after the electrochemical-driven growth of Si nanodomains reaches a critical point. Furthermore, these results suggest that an optimal initial Si nanodomain size of 4-6 nm for SiO anode materials is essential to retard the growth and subsequent pulverization process of Si nanodomains and thus to improve the cyclability of SiO during long-term cycling.

25 ENERGY STORAGE↗
DOE OSTI2021

In situ study of the electronic structure of polar-to-polar SrTiO 3 /($000\bar1$)ZnO heterointerface

The SrTiO 3 (STO)/ZnO heterointerface, which is widely used in the fabrication of novel optoelectronic devices, is a classical system combining functional perovskite oxides and wurtzite-structure semiconductor materials. The electronic structure of the heterointerface often plays a significant role in controlling the functions of novel devices. In this study, the electronic structure was explored using in situ photoemission spectroscopy and X-ray absorption spectroscopy. X-ray diffraction results showed the coexistence of (111) STO and (011) STO orientations for the STO film deposited on the ZnO-($000\bar1$) substrate via pulsed laser deposition. High-resolution transmission electron microscopic results revealed two types of polar interfaces: [$11\bar2$][$10\bar1$](111) STO //[$1\bar210$][$10\bar10$]($000\bar1$) ZnO and [111][$2\bar1\bar1$](011) STO //[$10\bar21$][$10\bar10$]($000\bar1$)ZnO. In situ photoemission spectroscopic results revealed downward band bending and the transformation of the valence states of Ti from 4+ to 3+, with extra electrons transferring to the hybridization states between O 2p and Ti t 2g orbitals at the polar-to-polar STO/ZnO interface. We propose that the polar discontinuity drives the electron transfer to the STO/ZnO interface during the growth process. This study provides insight into the electronic structure of the STO/($000\bar1$)ZnO heterointerface.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2026

Irradiation-Induced Grain Growth of gamma-Fe2O3 and Fe3O4 at Cryogenic Temperatures

This work reports the first observations of grain growth under irradiation of Fe oxides, specifically maghemite gamma-Fe2O3 and magnetite Fe3O4. The Fe oxide thin films were grown by Pulsed Laser Deposition and irradiated in-situ in a Transmission Electron Microscope at - 223°C using 1 MeV Kr2+ ions up to 8.75 x 10^19 ions/m^2 to study grain growth kinetics under irradiation. Grain growth at such low temperatures appears to follow kinetics that can be accounted for by the thermal spike model developed for metals in the literature. The corresponding activation energies were calculated and compared with other oxides. The results demonstrate that gamma-Fe2O3, despite its larger initial grain size, exhibits surprisingly fast grain growth in comparison with Fe3O4, likely due to the presence of iron vacancies in the crystal structure that facilitate faster atomic diffusion under irradiation.

Kretov, Dmitrii↗
DOE OSTI2021

Probing molecular vibrations by monochromated electron microscopy

Chemical bonds fundamentally determine molecular properties and are prevalently characterized by various spectroscopic means such as infrared and Raman spectroscopies. However, the spatial resolution of these conventional approaches is insufficient to reveal nanoscale features. Recently, monochromated electron energy-loss spectroscopy (EELS) in the transmission electron microscope achieved a groundbreaking energy resolution of a few millielectronvolts and enabled direct observation of molecular vibrational spectrum with unmatched spatial resolution. Vibrational EELS is widely applicable to both organic and inorganic matter in the solid state or liquid phase. In this study, we introduce recent advancements and key concepts of this method, compare with other spectroscopic techniques, and discuss future developments for potential applications in research fields centered on catalysts, polymers, and live cells.

Vibrational Spectroscopy↗
DOE OSTI2020

Origin and Suppression of Beam Damage-Induced Oxygen-K Edge Artifact from γ-Al 2 O 3 using Cryo-EELS

Gamma-alumina (γ-Al 2 O 3 ), like other low-Z oxides, is readily damaged when exposed to an electron beam. This typically results in the formation of a characteristic pre-edge peak in the oxygen-K edge of electron energy-loss spectra (EELS) acquired during or after the damage process. This artifact can mask the presence of intrinsic O-K edge fine structure that would reveal chemical properties of the material; therefore, its suppression is key. In this work, we systematically investigate the conditions that give rise to the damage-induced O-K pre-edge peak and show that it can be effectively suppressed by performing EELS experiments at cryogenic (cryo) temperatures. Prolonged exposure of γ-Al 2 O 3 to a focused electron beam results in a hole bored through the sample; this was used as a reproducible beam damage condition. O-K edge EELS spectra were collected from a single-crystal γ-Al 2 O 3 sample both during and after focused 2 electron beam hole drilling, and at room and cryo temperatures, using a monochromated scanning transmission electron microscope (STEM). Furthermore, the characteristic 531 eV pre-edge peak visible in the room temperature EELS spectra was completely suppressed in the cryo-EELS spectra, even in the presence of a visible drilled hole. We then correlated these experimental observations with multiple-scattering EELS simulations to determine the likely atomistic origin of the damage-induced O-K pre-edge peak. The findings indicate that the pre-edge peak is caused primarily by the presence of surface O dimer (O-O) bonds formed during beam damage, and that operating at cryo temperature suppresses the formation of surface O-O bonds, thus preventing formation of the O-K pre-edge peak. Additionally, Al-L 2,3 edge EELS spectra revealed Al loss primarily from tetrahedral sites during hole drilling.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

$\mathrm{SIM}$-$\mathrm{STEM}$ $\mathrm{Lab}$: Incorporating Compressed Sensing Theory for Fast STEM Simulation

We report recently it has been shown that precise dose control and an increase in the overall acquisition speed of atomic resolution scanning transmission electron microscope (STEM) images can be achieved by acquiring only a small fraction of the pixels in the image experimentally and then reconstructing the full image using an inpainting algorithm. In this paper, we apply the same inpainting approach (a form of compressed sensing) to simulated, sub-sampled atomic resolution STEM images. We find that it is possible to significantly sub-sample the area that is simulated, the number of g-vectors contributing the image, and the number of frozen phonon configurations contributing to the final image while still producing an acceptable fit to a fully sampled simulation. Here we discuss the parameters that we use and how the resulting simulations can be quantifiably compared to the full simulations. As with any Compressed Sensing methodology, care must be taken to ensure that isolated events are not excluded from the process, but the observed increase in simulation speed provides significant opportunities for real time simulations, image classification and analytics to be performed as a supplement to experiments on a microscope to be developed in the future

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Secondary-electron imaging of bulk crystalline specimens in an aberration corrected $\text{STEM}$

Atomic-scale electron microscopy traditionally probes thin specimens, with thickness below 100 nm, and its feasibility for bulk samples has not been documented. Here in this study, we explore the practicality of scanning transmission electron microscope (STEM) imaging with secondary electrons (SE), using a silicon-wedge specimen having a maximum thickness of 18 μm. We find that the atomic structure is present in the entire thickness range of the SE images although the background intensity increases moderately with thickness. The consistent intensity of secondary electron (SE) images at atomic positions and the modest increase in intensity in between (background) observed in silicon suggest a limited contribution from SEs generated by backscattered electrons, a conclusion supported by our multislice calculations. We conclude that achieving atomic resolution in SE imaging for bulk specimens is indeed attainable using aberration-corrected STEM and that an aberration-corrected scanning electron microscope (SEM) may have the capacity for atomic-level resolution, holding great promise for future strides in materials research.

25 ENERGY STORAGE↗
DOE OSTI2025

Ultralow-temperature cryogenic transmission electron microscopy using a new helium flow cryostat stage

Advances in cryogenic electron microscopy have opened new avenues for probing quantum phenomena in correlated materials. This study reports the installation and performance of a new side-entry condenZero cryogenic cooling system for JEOL (Scanning) Transmission Electron Microscopes (S/TEM), utilizing compressed liquid helium (LHe) and designed for imaging and spectroscopy at ultra-low temperatures. The system includes an external dewar mounted on a vibration-damping stage and a pressurized, low-noise helium transfer line with a remotely controllable needle valve, ensuring stable and efficient LHe flow with minimal thermal and mechanical noise. Performance evaluation demonstrates a stable base temperature of 4.37 K measured using a Cernox bare chip sensor on the holder with temperature fluctuations within ±0.004 K. Complementary in-situ electron energy-loss spectroscopy (EELS) via aluminum bulk plasmon analysis was used to measure the local specimen temperature and validate cryogenic operation during experiments. The integration of cryogenic cooling with other microscopy techniques, including electron diffraction and Lorentz TEM, was demonstrated by resolving charge density wave (CDW) transitions in NbSe2 using electron diffraction, and imaging nanometric magnetic skyrmions in MnSi via Lorentz TEM. In conclusion, this platform provides reliable cryogenic operation below 7 K, establishing a low-drift route for direct visualization of electronic and magnetic phase transformations in quantum materials.

Charge density wave↗
DOE OSTI2025

Fabrication and characterization of boron-terminated tetravacancies in monolayer hBN using STEM, EELS and electron ptychography

Tetravacancies in monolayer hexagonal boron nitride (hBN) with consistent edge termination (boron or nitrogen) form triangular nanopores with electrostatic potentials that can be leveraged for applications such as selective ion transport and neuromorphic computing. In order to quantitatively predict the properties of these structures, an atomic-level understanding of their local electronic and chemical environments is required. Moreover, robust methods for their precision manufacture are needed. Here we use electron irradiation in a scanning transmission electron microscope (STEM) at a high dose rate to drive the formation of boron-terminated tetravacancies in monolayer hBN. Characterization of the defects is achieved using aberration-corrected STEM, monochromated electron energy-loss spectroscopy (EELS), and electron ptychography. Z-contrast in STEM and chemical fingerprinting by core-loss EELS enable identification of the edge terminations, while electron ptychography gives insight into structural relaxation of the tetravacancies and provides evidence of enhanced electron density around the defect perimeters indicative of bonding effects.

2D hBN↗