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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 955 records · Page 53
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Magnetic Phase Diagram of 2D Magnet Fe 5– x GeTe 2 Revealed by Cryogenic Lorentz 4D-STEM

A two-dimensional van der Waals ferromagnet Fe 5–x GeTe 2 (F5GT) exhibits room-temperature magnetic transition and magnetic anisotropy. Studies have identified diverse magnetic states, including stripe domains, skyrmionic type-I and topologically trivial type-II bubbles, depending on the magnetic and thermal history. Yet, the underlying micromagnetic energetics driving these states remains unclear. Here, we establish the magnetic phase diagram of bulk F5GT using cryogenic Lorentz four-dimensional scanning transmission electron microscopy. We reveal that stripe domains spontaneously form upon zero-field cooling, while metastable magnetic bubbles are stabilized by external fields. Transitions from type-I to type-II bubbles are driven by an oblique external field. Micromagnetic simulations confirm that these transitions arise from the interplay of uniaxial anisotropy, dipolar interactions, and external fields, without requiring significant Dzyaloshinskii–Moriya Interaction (DMI). In conclusion, these findings clarify the micromagnetic origin of spin textures of F5GT and establish it as a tunable platform for nanoscale topological magnetism.

2D Magnets↗
DOE OSTI2019

Deep Learning for Semantic Segmentation of Defects in Advanced STEM Images of Steels

Crystalline materials exhibit long-range ordered lattice unit, within which resides nonperiodic structural features called defects. These crystallographic defects play a vital role in determining the physical and mechanical properties of a wide range of material systems. While computer vision has demonstrated success in recognizing feature patterns in images with well defined contrast, automated identification of nanometer scale crystallographic defects in electron micrographs governed by complex contrast mechanism is still a challenging task. Here, building upon an advanced defect imaging mode that offers high feature clarity, we introduce DefectSegNet - a new convolutional neural network (CNN) architecture that performs semantic segmentation of three common crystallographic defects in structural alloys: dislocation lines, precipitates and voids. Results from supervised training of a small set of high-quality defect images of steels show high pixel-wise accuracy across all three types of defects: 91.60 ± 1.77% on dislocations, 93.39 ± 1.00% on precipitates, and 98.85 ± 0.56% on voids. We discuss the sources of uncertainties in CNN prediction, and the training data in terms of feature density, representation and homogeneity and their effects on deep learning performance. Further defect quantification using DefectSegNet prediction outperforms human expert average, presenting a promising new workflow for fast and statistically meaningful quantification of materials defects.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Direct visualization of the hydration layer on alumina nanoparticles with the fluid cell STEM in situ

Rheological behavior of aqueous suspensions containing nanometer-sized powders is of relevance to many branches of industry. Unusually high viscosities observed for suspensions of nanoparticles compared to those of micron size powders cannot be explained by current viscosity models. Formation of so-called hydration layer on alumina nanoparticles in water was hypothesized, but never observed experimentally. We report here on the direct visualization of aqueous suspensions of alumina with the fluid cell in situ . We observe the hydration layer formed over the particle aggregates and show that such hydrated aggregates constitute new particle assemblies and affect the flow behavior of the suspensions. We discuss how these hydrated nanoclusters alter the effective solid content and the viscosity of nanostructured suspensions. As a result, our findings elucidate the source of high viscosity observed for nanoparticle suspensions and are of direct relevance to many industrial sectors including materials, food, cosmetics, pharmaceutical among others employing colloidal slurries with nanometer-scale particles.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

STEM characterization of solar cells

Photovoltaic (PV) devices consisting of a thin-film CdTe absorber layer on glass support have long been considered one of the leading technologies for reaching the Department of Energy’s 2030 cost target of less than 0.03/kWh. Here, the currently used device architectures include a thin transparent conductive oxide (TCO) on top of the glass superstrate following by very thin (<100 nm) polycrystalline emitter layer, typically CdS or more recently MgZnO (MZO). The CdTe absorber layer is usually up to 4 µm thick and polycrystalline, followed by the back contact (Figure 1). Typical CdTe thin-film layers are formed from rapid deposition techniques, such as close-spaced sublimation (CSS) or high-rate vapor-transport deposition (HRVDT). Both processes result in a polycrystalline CdTe photo-absorbing film, and such devices can be manufactured cheaply and efficiently at large scale. Record cell efficiencies as high as 22.1% have been demonstrated. However, such cell performances are still far from the theoretical limits (~30% for CdTe), and improvements have mostly been due to better light collection, rather than improvements in absorber conversion efficiency. The remaining obstacles to further improving these devices toward their theoretical limits concern improvements in the fill factor (FF) and, more importantly, open-circuit voltage (V oc ), which is still far from its limit. In fact, it is widely accepted that V oc is the limiting factor since it reaches only about 60% of the CdTe bandgap. This is in contrast to GaAs solar cells, which have reached a V oc of 80% of the bandgap. The reasons for this discrepancy have been identified as the low p -type doping concentrations of CdTe due to the formation of compensating defect states, a poor p-n junction, and the need for better backside ohmic contacts. High interfacial defect density at grain boundaries (GBs) and intragrain dislocations is also one of the main concerns because they contribute to the dark current. This is supported by the study of single-crystal-based CdTe solar cells that were able to reach Voc in excess of 1 V.

14 SOLAR ENERGY↗
DOE OSTI2022

Dynamics of the gold–silicon eutectic reaction studied at limited length scales using in situ $\mathrm{TEM}$ and $\mathrm{STEM}$

We report the dynamics of the gold–silicon eutectic reaction in limited dimensions were studied using in situ transmission electron microscopy and scanning transmission electron microscopy heating experiments. The phase transformation, viewed in both plan-view and cross-section of the film, occurs through a complex combination of dislocation and grain boundary motion and diffusion of silicon along gold grain boundaries, which results in a dramatic change in the microstructure of the film. The conversion observed in cross-section shows that the eutectic mixture forms at the Au–Si interface and proceeds into the Au film at a discontinuous growth rate. This complex process can lead to a variety of microstructures depending on sample geometry, heating temperature, and the ratio of gold to silicon which was found to have the largest impact on the eutectic microstructure. The eutectic morphology varied from dendrites to hollow rectangular structures to Au–Si eutectic agglomerates with increasing silicon to gold ratio.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Insights on STEM Careers

This presentation will provide career advice for individuals seeking to go beyond just having a job to building a successful career in the areas of Science, Technology, Engineering, and Mathematics. Careful planning can be used to turn a job into a springboard for professional advancement and personal satisfaction. Topics to be addressed include setting priorities, understanding career ladders, making tough choices, overcoming stereotypes and assumptions by others, networking, developing a professional identify, and balancing a career with family and other personal responsibilities. Insights on the transition from individual technical work to leadership will also be provided. The author will draw upon experiences gained in academic, industrial, and government laboratory settings, as well as extensive professional service and community involvement.

99 GENERAL AND MISCELLANEOUS↗
DOE OSTI2017

Cyclic tensile strain enhances human mesenchymal stem cell Smad 2/3 activation and tenogenic differentiation in anisotropic collagen-glycosaminoglycan scaffolds

Orthopaedic injuries, particularly those involving ligaments and tendons, are some of the most commonly treated ailments in the United States and are associated with both high costs and poor outcomes. Regenerative medicine strategies for tendon injuries could be enhanced by three-dimensional biomaterials that can promote cell alignment and pro-tenogenic differentiation of patientderived MSCs. We have previously described a collagenglycosaminoglycan (CG) scaffold possessing aligned structural features able to promote bone marrow MSC differentiation towards a tenogenic lineage, in the absence of growth factor supplementation. We aimed to employ a bioreactor to enhance MSC tenogenic differentiation within the aligned CG scaffold via cyclic tensile strain (CTS), and further to evaluate the relative effects of strain cycle duration and extended application of repeated cycles of CTS on MSC response. Human MSCs were cultured in CG scaffolds for up to 6 d under static (unloaded) or cyclic tensile strain (1 Hz) for 10 min every 6 h. Time-dependent activation of ERK 1/2 and p38 mechanotransduction pathways was observed within each 6 h strain cycle. MSCs remained viable throughout the experiment and application of CTS robustly upregulated the expression of tendon-specific extracellular matrix proteins and phenotypic markers. Simultaneously, CTS promoted increased phosphorylation of Smad 2/3, suggesting a link between tensile stimulation and TGF-β family growth factor production. Together, we demonstrated the design, fabrication and validation of a high-throughput tensile stimulation bioreactor to increase MSC tenogenic differentiation in porous CG scaffolds.

59 BASIC BIOLOGICAL SCIENCES↗