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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 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Engineering magnetic topological insulators in Eu 5 M 2 X 6 Zintl compounds

Magnetic topological insulators provide a prominent material platform for quantum anomalous Hall physics and axion electrodynamics. However, the lack of material realizations with cleanly gapped surfaces hinders technological utilization of these exotic quantum phenomena. Here, using the Zintl concept and the properties of nonsymmorphic space groups, we computationally engineer magnetic topological insulators. Specifically, we explore Eu 5 M 2 X 6 (M=metal, X=pnictide) Zintl compounds and find that Eu 5 Ga 2 Sb 6 , Eu 5 Tl 2 Sb 6 , and Eu 5 In 2 Bi 6 form stable structures with nontrivial Z 2 indices. We also show that epitaxial and uniaxial strain can be used to control the Z 2 index and the bulk energy gap. Lastly, we discuss experimental progress towards the synthesis of the proposed candidates and provide insights that can be used in the search for robust magnetic topological insulators in Zintl compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Two-Dimensional Hexagonal Sheet of TiO 2

We report the ab initio discovery of a novel putative ground state for quasi two-dimensional TiO 2 through a structural search using the minima hopping method with an artificial neural network potential. Here, the structure is based on a honeycomb lattice and is energetically lower than the experimentally reported lepidocrocite sheet by 7 meV/atom and merely 13 meV/atom higher in energy than the rutile bulk structure. According to our calculations, the hexagonal sheet is stable against mechanical stress, chemically inert, and can be deposited on various substrates without disrupting the structure. Its properties differ significantly from all known TiO 2 bulk phases with a large gap of 5.05 eV that can be tuned through strain and defect engineering.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Dynamically tunable moiré exciton Rydberg states in a monolayer semiconductor on twisted bilayer graphene

Moiré excitons are emergent optical excitations in two-dimensional semiconductors with moiré superlattice potentials. Although these excitations have been observed on several platforms, a system with dynamically tunable moiré potential to tailor their properties is yet to be realized. Here we present a continuously tunable moiré potential in monolayer WSe 2 , enabled by its proximity to twisted bilayer graphene (TBG) near the magic angle. By tuning local charge density via gating, TBG provides a spatially varying and dynamically tunable dielectric superlattice for modulation of monolayer WSe 2 exciton wave functions. We observed emergent moiré exciton Rydberg branches with increased energy splitting following doping of TBG due to exciton wave function hybridization between bright and dark Rydberg states. In addition, emergent Rydberg states can probe strongly correlated states in TBG at the magic angle. Finally, our study provides a new platform for engineering moiré excitons and optical accessibility to electronic states with small correlation gaps in TBG.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Ultrafast band engineering and transient spin currents in antiferromagnetic oxides

Here, we report a dynamic structure and band engineering strategy with experimental protocols to induce indirect-to-direct band gap transitions and coherently oscillating pure spin-currents in three-dimensional antiferromagnets (AFM) using selective phononic excitations. In the Mott insulator LaTiO 3 , we show that a photo-induced nonequilibrium phonon mode amplitude destroys the spin and orbitally degenerate ground state, reduces the band gap by 160 meV and renormalizes the carrier masses. The time scale of this process is a few hundreds of femtoseconds. Then in the hole-doped correlated metallic titanate, we show how pure spin-currents can be achieved to yield spin-polarizations exceeding those observed in classic semiconductors. Last, we demonstrate the generality of the approach by applying it to the non-orbitally degenerate AFM CaMnO 3 . These results advance our understanding of electron-lattice interactions in structures out-of-equilibrium and establish a rational framework for designing dynamic phases that may be exploited in ultrafast optoelectronic and optospintronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Exploring layer thinning of exfoliated β-tellurene and room temperature photoluminescence with large exciton binding energy revealed in β-TeO 2

Due to its tunable bandgap, anisotropic behavior, and superior thermoelectric properties, device applications using layered tellurene (Te) are becoming more attractive. Here, we report a thinning technique for exfoliated tellurene nanosheets using thermal annealing in an oxygen environment. We characterize different thinning parameters, including temperature and annealing time. Based on our measurements, we show that controlled layer thinning occurs in the narrow temperature range of 325–350 °C. We also show a reliable method to form β-tellurene oxide (β-TeO 2 ), which is an emerging wide bandgap semiconductor with promising electronic and optoelectronic properties. This wide bandgap semiconductor exhibits a broad photoluminescence (PL) spectrum with multiple peaks covering the range of 1.76–2.08 eV. This PL emission, coupled with Raman spectra, is strong evidence of the formation of 2D β-TeO 2 . We discuss the results obtained and the mechanisms of Te thinning and β-TeO 2 formation at different temperature regimes. We also discuss the optical bandgap of β-TeO 2 and show the existence of pronounced excitonic effects evident by the large exciton binding energy in this 2D β-TeO 2 system that reach 1.54–1.62 eV for bulk and monolayer, respectively. Our work can be utilized to have better control over the Te nanosheet thickness. It also sheds light on the formation of well-controlled β-TeO 2 layered semiconductors for electronic and optoelectronic applications.

42 ENGINEERING↗
DOE OSTI2025

Charge transfer in transition metal dichalcogenide alloy heterostructures

Two-dimensional (2D) transition metal dichalcogenides and their alloys provide a unique platform for exploring interlayer charge transfer in van der Waals heterostructures. These structures are crucial for advancing the next-generation electronic, optoelectronic, and quantum devices. In this study, interlayer charge transfer in heterostructures composed of MoSe 2 , MoS 2 , and their alloy, MoSSe, is investigated using transient absorption, Raman, and photoluminescence spectroscopy. The experimental results reveal that electron transfer in the alloy heterostructures, MoSSe/MoS 2 and MoSe 2 /MoSSe, is faster than in the pure MoSe 2 /MoS 2 heterostructure, despite the smaller conduction band offsets of the alloy systems. Raman spectroscopy confirms that alloy layers support phonon modes matching those of the pure layers, aligning with theoretical models of phonon-assisted interlayer charge transfer. Additionally, efficient hole transfer is observed in both alloy heterostructures. The findings suggest transition metal dichalcogenides alloys can be used for engineering heterostructures with desired charge transfer properties. By leveraging compositionally tunable band gaps and optical properties, alloy-based heterostructures offer opportunities for designing tailored materials suitable for diverse applications such as photodetectors, light-emitting devices, and flexible electronics. Furthermore, the ultrafast charge transfer observed in these systems provides insights into the fundamental mechanisms governing interlayer interactions in 2D materials.

2D material↗
DOE OSTI2019

Localization and interactions in topological and nontopological bands in two dimensions

A two-dimensional electron gas in a high magnetic field displays macroscopically degenerate Landau levels, which can be split into Hofstadter subbands by means of a weak periodic potential. By carefully engineering such a potential, one can precisely tune the number, bandwidths, band gaps, and Chern character of these subbands. This allows a detailed study of the interplay of disorder, interaction, and topology in two-dimensional systems. We first explore the physics of disorder and single-particle localization in subbands derived from the lowest Landau level, that nevertheless may have a topological nature different from that of the entire lowest Landau level. By projecting the Hamiltonian onto subbands of interest, we systematically explore the localization properties of single-particle eigenstates in the presence of quenched disorder. Here, we then introduce electron-electron interactions and investigate the fate of many-body localization in subbands of varying topological character.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Quantum Spin-Valley Hall Kink States: From Concept to Materials Design

We propose a general and tunable platform to realize high-density arrays of quantum spin-valley Hall kink (QSVHK) states with spin-valley-momentum locking based on a two-dimensional hexagonal topological insulator. Through the analysis of Berry curvature and topological charge, the QSVHK states are found to be topologically protected by the valley-inversion and time-reversal symmetries. Remarkably, the conductance of QSVHK states remains quantized against both nonmagnetic short- and long-range and magnetic long-range disorder, verified by the Green-function calculations. Based on first-principles results and our fabricated samples, we show that QSVHK states, protected with a gap up to 287 meV, can be realized in bismuthene by alloy engineering, surface functionalization, or electric field, supporting nonvolatile applications of spin-valley filters, valves, and waveguides even at room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

A Four-Layer Cyber-Physical Security Model for Electric Machine Drives Considering Control Information Flow

Despite the IEEE Power Electronics Society (PELS) establishing Technical Committee 10 on Design Methodologies with a focus on the cyber-physical security of power electronics systems, a holistic design methodology for addressing security vulnerabilities remains underdeveloped. This gap largely stems from the limited integration of computer science and power/control engineering studies in this interdisciplinary field. Addressing the inadequacy of unilateral cyber or control perspectives, this article presents a novel four-layer cyber-physical security model specifically designed for electric machine drives. Central to this model is the innovative control information flow (CIF) model, residing within the control layer, which serves as a pivotal link between the cyber layer's vulnerable resources and the physical layer's state-space models. By mapping vulnerable resources to control variable space and tracing attack propagation, the CIF model facilitates accurate impact predictions based on tainted control laws. The effectiveness and validity of this proposed model are demonstrated through hardware experiments involving two typical cyber-attack scenarios, underscoring its potential as a comprehensive framework for multidisciplinary security strategies.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2022

Review of recent studies on nanoscale electrical junctions and contacts: Quantum tunneling, current crowding, and interface engineering

The study of charge carrier transport at nanoscale electrical contacts is crucial for the development of next-generation electronics. In this study, we review recent modeling efforts on quantum tunneling, current crowding, and contact resistance across electrical interfaces with nanometer scale dimensions. A generalized self-consistent model for quantum tunneling induced electron transport in metal–insulator–metal (MIM) junctions is summarized. Rectification of a dissimilar MIM junction is reviewed. A modified two-dimensional (2D) transmission line model is used to investigate the effects of spatially varying specific contact resistivity along the contact length. The model is applied to various types of electrical contacts, including ohmic contacts, MIM junction based tunneling contacts, and 2D-material-based Schottky contacts. Roughness engineering is recently proposed to offer a possible paradigm for reducing the contact resistance of 2D-material-based electrical contacts. Contact interface engineering, which can mitigate current crowding near electrical contacts by spatially designing the interface layer thickness or properties, without requiring an additional material or component, is briefly reviewed. Tunneling engineering is suggested to eliminate severe current crowding in highly conductive ohmic contacts by introducing a thin tunneling layer or gap between the contact members. Furthermore, unsolved problems and challenges are also discussed.

2D materials↗
DOE OSTI2017

Compact Models for Defect Diffusivity in Semiconductor Alloys.

Predicting transient effects caused by short - pulse neutron irradiation of electronic devices is an important part of Sandia's mission. For example , predicting the diffusion of radiation - induced point defects is needed with in Sandia's Qualification Alternative to the Sandia Pulsed Reactor (QASPR) pro gram since defect diffusion mediates transient gain recovery in QASPR electronic devices. Recently, the semiconductors used to fabricate radiation - hard electronic devices have begun to shift from silicon to III - V compounds such as GaAs, InAs , GaP and InP . An advantage of this shift is that it allows engineers to optimize the radiation hardness of electronic devices by using alloy s such as InGaAs and InGaP . However, the computer codes currently being used to simulate transient radiation effects in QASP R devices will need to be modified since they presume that defect properties (charge states, energy levels, and diffusivities) in these alloys do not change with time. This is not realistic since the energy and properties of a defect depend on the types of atoms near it and , therefore, on its location in the alloy. In particular, radiation - induced defects are created at nearly random locations in an alloy and the distribution of their local environments - and thus their energies and properties - evolves with time as the defects diffuse through the alloy . To incorporate these consequential effects into computer codes used to simulate transient radiation effects, we have developed procedures to accurately compute the time dependence of defect energies and properties and then formulate them within compact models that can be employed in these computer codes. In this document, we demonstrate these procedures for the case of the highly mobile P interstitial (I P ) in an InGaP alloy. Further dissemination only as authorized to U.S. Government agencies and their contractors; other requests shall be approved by the originating facility or higher DOE programmatic authority.

36 MATERIALS SCIENCE↗
DOE OSTI2020

First Look West (FLoW 2.0) (Final Technical Report for Closeout)

DOE’s Cleantech UP was an annual business plan competition, operating nationally between 2015 and 2018 through eight regional nodes that included Caltech’s FLOW. Cleantech UP focused on supporting university students at all levels creating cleantech startups. FLOW operated nationally with a focus on California. In support of the DOE’s objectives FLOW developed an immersive five point program that targeted major gaps in the educational support and practical resources available to scientists and engineers starting their first cleantech companies that prevented such young innovators from becoming entrepreneurs. Specifically, the FLOW program augmented the number of participants entering the energy entrepreneurship pipeline via the Cleantech UP competition; helped the US remain competitive by increasing the viability and financing potential of these cleantech companies started by young academics; and helped build the regional, state and national infrastructure that spurs and supports entrepreneurship. Each year the FLOW featured two major judging rounds for candidates drawn from as many as 80 applicants annually, providing essential experience and feedback that helped define the direction of these young entrepreneurs. The FLOW winners competed at the DOE’s national event, and FLOW companies took the top prize twice in the total history of the DOE program which was started in 2011.

96 KNOWLEDGE MANAGEMENT AND PRESERVATION↗
DOE OSTI2020

World Congress on High Entropy Alloys (HEA 2019) (Final Technical Report)

The World Congress on High Entropy Alloys (HEA 2019) was a new, cross-disciplinary technical forum designed to share the latest research advances in materials with high configurational entropy, including both single-phase and multiphase (compositionally complex) alloys. This inaugural World Congress on High Entropy Alloys (HEA 2019) was held in Seattle, Washington from November 17-20, 2019 with the goal of HEA 2019 to assemble the international community of HEA stakeholders – including researchers, educators, and engineers – to capture the current state-of-the-art, identify current gaps in HEA efforts, and to set the stage for the future growth and application of HEAs worldwide. HEA 2019 was the environment for leaders in this field to showcase their researc, and network with their peers to informally discuss the latest developments in high entropy alloys, while also making valuable professional connections. The goal of the organizers was to ensure a high-quality conference that held a strong international presence and aimed to highlight the important advances in HEA research on a global scale. Student participation was greatly encouraged in order to give students exposure to recent advances in this field. This meeting was open to all interested individuals and attempted to provide the most advanced technical meeting possible, with a mixture of attendees from research and development, industry, and education.

36 MATERIALS SCIENCE↗
NASA NTRS1994

Robotic Booking Of Coolant Tubes

Robotic tube-booking subsystem proposed for use in automated manufacturing cell described in "Robotic Processing of Rocket-Engine Nozzles" (MFS-29927). Includes electric or pneumatic end effector that inspects gaps under guidance of control processor connected to robotic vision subsystem. After inspecting each gap, end effector books tubes in vicinity, then reinspects to ensure attainment of desired gap. Makes entire tube-gap area brazeable, without damage to tubes, with consistent results. In addition, robotic booking takes less time and costs less than manual booking.

Wagner, Garret E.↗
NASA NTRS2007

Status of Superheated Spray and Post Combustor Particulate Modeling for NCC

At supersonic cruise conditions, high fuel temperatures, coupled with low pressures in the combustor, create potential for superheated fuel injection leading to shorter fuel jet break-up time and reduced spray penetration. Another issue particularly important to the supersonic cruise is the aircraft emissions contributing to the climate change in the atmosphere. Needless to say, aircraft emissions in general also contribute to the air pollution in the neighborhood of airports. The objectives of the present efforts are to establish baseline for prediction methods and experimental data for (a) liquid fuel atomization and vaporization at superheated conditions and (b) particle sampling systems and laboratory or engine testing environments, as well as to document current capabilities and identify gaps for future research.

Liu, Nan-Suey↗
NASA NTRS2016

Graphical Visualization of Human Exploration Capabilities

NASA's pioneering space strategy will require advanced capabilities to expand the boundaries of human exploration on the Journey to Mars (J2M). The Evolvable Mars Campaign (EMC) architecture serves as a framework to identify critical capabilities that need to be developed and tested in order to enable a range of human exploration destinations and missions. Agency-wide System Maturation Teams (SMT) are responsible for the maturation of these critical exploration capabilities and help formulate, guide and resolve performance gaps associated with the EMC-identified capabilities. Systems Capability Organization Reporting Engine boards (SCOREboards) were developed to integrate the SMT data sets into cohesive human exploration capability stories that can be used to promote dialog and communicate NASA's exploration investments. Each SCOREboard provides a graphical visualization of SMT capability development needs that enable exploration missions, and presents a comprehensive overview of data that outlines a roadmap of system maturation needs critical for the J2M. SCOREboards are generated by a computer program that extracts data from a main repository, sorts the data based on a tiered data reduction structure, and then plots the data according to specified user inputs. The ability to sort and plot varying data categories provides the flexibility to present specific SCOREboard capability roadmaps based on customer requests. This paper presents the development of the SCOREboard computer program and shows multiple complementary, yet different datasets through a unified format designed to facilitate comparison between datasets. Example SCOREboard capability roadmaps are presented followed by a discussion of how the roadmaps are used to: 1) communicate capability developments and readiness of systems for future missions, and 2) influence the definition of NASA's human exploration investment portfolio through capability-driven processes. The paper concludes with a description of planned future work to modify the computer program to include additional data and of alternate capability roadmap formats currently under consideration.

Rodgers, Erica M.↗
NASA NTRS2019

Feature Learning for Multispectral Satellite Imagery Classification Using Neural Architecture Search

Automated classification of remote sensing data is an integral tool for earth scientists, and deep learning has proven very successful at solving such problems. However, building deep learning models to process the data requires expert knowledge of machine learning. We introduce DELTA, a software toolkit to bridge this technical gap and make deep learning easily accessible to earth scientists. Visual feature engineering is a critical part of the machine learning lifecycle, and hence is a key area that will be automated by DELTA. Hand-engineered features can perform well, but require a cross functional team with expertise in both machine learning and the specific problem domain, which is costly in both researcher time and labor. The problem is more acute with multispectral satellite imagery, which requires considerable computational resources to process. In order to automate the feature learning process, a neural architecture search samples the space of asymmetric and symmetric autoencoders using evolutionary algorithms. Since denoising autoencoders have been shown to perform well for feature learning, the autoencoders are trained on various levels of noise and the features generated by the best performing autoencoders evaluated according to their performance on image classification tasks. The resulting features are demonstrated to be effective for Landsat-8 flood mapping, as well as benchmark datasets CIFAR10 and SVHN.

Robert Campbell↗
DOE OSTI2022

Resilient, Rural, and Revolutionary: Salisbury Square's Direct-Current Affordable Microgrid Community: Preprint

The technology to interconnect buildings with a dedicated direct-current (DC) power distribution network is in place today; what is missing is a turnkey approach to designing a DC microgrid - and the business models allowing such systems to be deployed, owned, and operated at scale. To close this gap, the Salisbury Square Development Team, comprising clean-energy experts, has engineered a resilient community DC microgrid for an affordable housing community in Randolph, Vermont. Ten single-family, occupant-owned residences and 12 multifamily rental units will share locally generated and stored solar energy via a DC power distribution bus capable of operating during extended grid outages. With a DC power distribution network in place, each home will be equipped with high-efficiency DC lighting and appliances, operating alongside alternating current (AC) appliances, even during an islanded mode of operation. To obtain a comprehensive understanding of what is possible and achievable, the Team collaborated with the local utility, regulatory agencies, a national laboratory, energy-as-service providers, and vendors. The collaborators evaluated microgrid typologies, business models, and energy modeling, and analyzed electrification and resilience. Further, the Team applied the URBANoptTM (Urban Renewable Building and Neighborhood optimization, NREL 2022) software development kit (SDK) to Salisbury Square's single-family and multifamily buildings to validate workflows and identify needs for advanced capability. This paper addresses the barriers to entry, scalability, and impact on residents and system ownership. It also examines the analysis that informed the design and engineering of the DC microgrid and the opportunities to streamline the process.

DER↗