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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 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Atomic scale investigation of aluminum incorporation, defects, and phase stability in β -(Al x Ga 1-x ) 2 O 3 films

The development of novel ultra-wide bandgap (UWBG) materials requires precise understanding of the atomic level structural origins that give rise to their important properties. We study the aluminum atom incorporation, defect formation, and their relationships with phase stability in β-(Al x Ga 1-x ) 2 O 3 films, a promising candidate for UWBG applications, to explain atomic scale structural characteristics and properties using a combination of quantitative scanning transmission electron microscopy (STEM) and density functional theory (DFT). Our STEM analysis indicates that ~54% of the incorporated Al substitutes on the octahedrally coordinated Ga 2 site in a series of films grown with different techniques and alloy concentrations. DFT calculations show that, while Al energetically prefers the octahedral site, surface reconstructions and kinetic limitations during the epitaxial growth are responsible for Al occupying both octahedral and tetrahedral sites in (Al x Ga 1-x ) 2 O 3 , ultimately limiting the stability of the β-phase at x < ~50%. Local heterogeneity of composition results in the formation of a planar defect, affecting the stability of the β-phase. The similarity of such inclusions to the metastable γ-phase is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effects of recoil spectra and electronic energy dissipation on defect survival in 3C-SiC

The coincidence of electronic and damage energy dissipation from energetic ions to an atomic lattice can significantly affect damage production along the ion trajectory due to spatial overlap of inelastic and elastic processes. Damage production and disordering in single crystal 3C-SiC from 5 MeV Si and 10 MeV Au ions is investigated using ion-channeling experiments. While defects are created by damage energy dissipation via elastic scattering, electronic energy dissipation via electron-phonon coupling decreases defect survival along the ion trajectory for Si ions. Finally, the more energetic recoil spectrum for 10 MeV Au ions leads to weaker spatial coupling of electronic and damage energy dissipation processes, and damage production is only weakly affected.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS

The ultrafast dynamics of photon-to-charge conversion in an organic light-harvesting system is studied by femtosecond time-resolved X-ray photoemission spectroscopy (TR-XPS) at the free-electron laser FLASH. This novel experimental technique provides site-specific information about charge separation and enables the monitoring of free charge carrier generation dynamics on their natural timescale, here applied to the model donor-acceptor system CuPc:C 60 . A previously unobserved channel for exciton dissociation into mobile charge carriers is identified, providing the first direct, real-time characterization of the timescale and efficiency of charge generation from low-energy charge-transfer states in an organic heterojunction. The findings give strong support to the emerging realization that charge separation even from energetically disfavored excitonic states is contributing significantly, indicating new options for light harvesting in organic heterojunctions.

14 SOLAR ENERGY↗
DOE OSTI2020

Catalytic properties of model supported nanoparticles

Nanoparticles of late transition metals are used as catalysts and electrocatalysts for industrial chemical reactions that produce fuels, convert chemical energy to electricity and clean up pollution associated with the generation and use of these fuels. For such applications, metals are usually bonded onto the surfaces of oxide or carbon support materials. To provide the energy needed for sustained economic development, we must develop new and improved solid catalysts and electrocatalysts for a variety of reactions that take better advantage of traditional and alternate energy sources (solar, wind, biomass, etc.) and avoid serious environmental problems. Additionally, since these metals are often expensive and in limited supply (e.g., Pt), we also need to develop catalysts that do not require precious metals. As a consequence, recent years have seen a dramatic increase in research aimed at providing the basic understanding needed to develop new and improved catalysts and electrocatalysts for a variety of reactions that involve nanoparticles of late transition metals supported on oxide or carbon materials. It is therefore timely to present a special issue that focusses on Catalytic Properties of Model Supported Nanoparticles. The collection highlights some of the most exciting new directions for research in this area, and provides in depth examples of some of the most cutting-edge methods, both experimental and theoretical, for studying this class of catalysts, which provide insights into structure, mechanisms, kinetics and energetics with unprecedented accuracy and detail. Finally, as Guest Editors, we present below a few of the many excellent papers published in the collection.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Treatment and aging studies of GaAs(111)B substrates for van der Waals chalcogenide film growth

GaAs(111)B are commercially available substrates widely used for the growth of van der Waals chalcogenide films. Wafer-scale, high-quality crystalline films can be deposited on GaAs(111)B substrates using molecular beam epitaxy. However, two obstacles persist in the use of GaAs(111)B: first, the surface dangling bonds make it challenging for the growth of van der Waals materials; second, the As-terminated surface is prone to aging in air. This study investigated a thermal treatment method for deoxidizing GaAs(111)B substrates while simultaneously passivating the surface dangling bonds with Se. By optimizing the treatment parameters, we obtained a flat and completely deoxidized platform for subsequent film growth, with highly reproducible operations. Furthermore, through first-principle calculations, we find that the most energetically favorable surface of GaAs(111)B after Se passivation consists of 25% As atoms and 75% Se atoms. Finally, we discovered that the common storage method using food-grade vacuum packaging cannot completely prevent substrate aging, and even after thermal treatment, aging still affects subsequent growth. Therefore, we recommend using N2-purged containers for better preservation.

Engineering↗
DOE OSTI2024

Controlled Dedoping and Redoping of N‐Doped Poly(benzodifurandione) (n‐PBDF)

Abstract The doping levels of conjugated polymers significantly influence their conductivity, energetics, and optical properties. Recently, a highly conductive n‐doped polymer called poly (3,7‐dihydrobenzo[1,2‐b:4,5‐b′]difuran‐2,6‐dione) (poly(benzodifurandione), n‐PBDF) is discovered, opening new possibilities for n‐type conducting polymers in printed electronics and other fields. Controlling the doping level of n‐PBDF is of great interest due to its wide range of potential applications. Here controlled dedoping and redoping of n‐PBDF is reported and a mechanistic understating of such a process is provided. Dedoping occurs through electron transfer and proton capture, wherein the ionic dopants, tris(4‐bromophenyl)ammoniumyl hexachloroantimonate (Magic Blue), exhibit efficient proton capture ability and stronger interaction with n‐PBDF, resulting in high dedoping efficiency. Moreover, chemically dedoped PBDF can be redoped using various proton‐coupled electron transfer agents. By manipulating the doping levels of n‐PBDF thin films, ranging from highly doped to dedoped states, the system demonstrates controllable conductivity in five orders of magnitude, adjustable optical properties, and energetics. As a result, these characteristics demonstrate the potential applications of n‐PBDF in organic electrochemical transistors and thermoelectrics.

Chemistry↗
DOE OSTI2020

The correlation between N deficiency and the mechanical properties of the Ti 2 AlNy MAX phase

The role of X deficiency on the mechanical properties of MAX phases was studied by synthesizing Ti 2 AlN through powder metallurgy in stoichiometric and sub/extra-stoichiometric nitrogen compositions. XRD analyses and ab initio calculations indicate that nitrogen vacancies result in a lattice contraction predominantly along the c-axis. The elastic moduli and intrinsic hardness of substoichiometric Ti 2 AlN 0.9 measured from nanoindentation tests are shown to be slightly smaller than that of Ti 2 AlN. The key mechanical indexes, bulk (B), shear (G) and Young’s (E) moduli as well as the hardness variation are calculated in density functional theory, and show different responses depending on the concentration of N vacancies. Overall, this joint experimental and theoretical study provides a full understanding of the energetics, chemical bonding, electronic structure, and mechanics of the N deficient MAX phases which would increase the application of nitride ceramics.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Competing electronic states emerging on polar surfaces

Excess charge on polar surfaces of ionic compounds is commonly described by the two-dimensional electron gas (2DEG) model, a homogeneous distribution of charge, spatially-confined in a few atomic layers. Here, by combining scanning probe microscopy with density functional theory calculations, we show that excess charge on the polar TaO 2 termination of KTaO 3 (001) forms more complex electronic states with different degrees of spatial and electronic localization: charge density waves (CDW) coexist with strongly-localized electron polarons and bipolarons. These surface electronic reconstructions, originating from the combined action of electron-lattice interaction and electronic correlation, are energetically more favorable than the 2DEG solution. They exhibit distinct spectroscopy signals and impact on the surface properties, as manifested by a local suppression of ferroelectric distortions.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Edge-Mediated Annihilation of Vacancy Clusters in Monolayer Molybdenum Diselenide (MoSe 2 ) under Electron Beam Irradiation

Here annihilation of vacancy clusters in monolayer molybdenum diselenide (MoSe2) under electron beam irradiation is reported. In situ high-resolution transmission electron microscopy observation reveals that the annihilation is achieved by diffusion of vacancies to the free edge near the vacancy clusters. Monte Carlo simulations confirm that it is energetically favorable for the vacancies to locate at the free edge. By computing the minimum energy path for the annihilation of one vacancy cluster as a case study, it is further shown that electron beam irradiation and pre-stress in the suspended MoSe2 monolayer are necessary for the vacancies to overcome the energy barriers for diffusion. The findings suggest a new mechanism of vacancy healing in 2D materials and broaden the capability of electron beam for defect engineering of 2D materials, a promising way of tuning their properties for engineering applications.

2D materials↗
DOE OSTI2025

Hydride Migration within RhH 2 Ag 19 Superatom: A Combined Neutron Diffraction and DFT Analysis

Abstract An investigation combining neutron diffraction and DFT allows determining the most likely hydride migration pathway within the icosahedral metal framework of [RhH 2 Ag 19 {S 2 P(O n Pr) 2 } 12 ] (RhH 2 Ag 19 ). Starting from the experimentally derived solid‐state structures, a computational analysis is able to reveal an energetically favorable migration pathway with a maximum energy barrier of 4.2 kcal mol −1 . The two hydrides migrate simultaneously within the Rh@Ag 12 icosahedral core, traversing several positional isomers. This study expands the understanding of hydride dynamics in nanoclusters and provides critical insights into the structural flexibility of the superatom framework. These findings have significant implications for hydrogen storage, catalysis, and the design of advanced hydride‐containing materials.

Chemistry↗
DOE OSTI2024

Effects of irradiation damage on the elastic properties and hardness of complex transition metal diborides

Multi-principal component transition metal (TM) diborides represent a class of high-entropy ceramics (HECs) that have received considerable interest in recent years owing to their promising properties for extreme environment applications that include thermal/ environmental barriers, hypersonic vehicles, turbine engines, and next-generation nuclear reactors. While the addition of chemical disorder through the random distribution of TM elements on the cation sublattice has offered opportunities to tailor elastic stiffness and hardness, the effects of irradiationinduced structural damage on the physical properties of these complex materials have remained largely unexplored. To this end, changes in the hardness and elastic moduli of a high-entropy TM diboride (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)B2 and three of its quaternary subsets following irradiation with 10 MeV gold (Au) ions to fluences of up to 6 × 10^15 Au cm-2 are investigated at the micrometer and sub-micrometer length-scales via the dispersion of laser-generated surface acoustic waves (SAW) and nanoindentation, respectively. The nanoindentation measurements show that the TM diborides exhibit an initial increase in hardness following irradiation with energetic Au ions, with a subsequent decrease in hardness following further irradiation. One quaternary composition, (Hf1/3Ta1/3Ti1/3)B2, exhibits a notable exception to the trend and continues to exhibit an increase in hardness with ion irradiation fluence. Although differences in the absolute values of the effective elastic moduli obtained from the measured SAW dispersion and nanoindentation are observed (and attributed to microstructural variations at the measurement length-scale), both techniques yield similar trends in the form of an initial reduction and subsequent saturation in the elastic modulus with increasing ion irradiation fluence. The quaternary TM diboride (Hf1/3Ta1/3Ti1/3)B2 again exhibits a departure from this trend. The high-entropy TM diboride (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)B2 exhibits the greatest recovery in hardness and modulus when irradiated to high ion fluences following initial changes at low fluence, indicating superior resistance to radiation-induced damage over its quaternary counterparts. Opportunities for designing HECs with superior hardness and modulus for enhanced radiation resistance (compared to their single constituent counterparts) by tailoring chemical disorder and bond character in the lattice are discussed.

36 - MATERIALS SCIENCE↗
DOE OSTI2025

Simultaneous hard x-ray Talbot phase and dark-field imaging in laser experiments at XFEL facilities

X-ray Free Electron Laser (XFEL) facilities offer unprecedented opportunities to advance instrumentation for studying matter under extreme conditions. In this study, we harnessed the enhanced x-ray capabilities of XFELs to demonstrate dark field imaging in laser-driven experiments at XFEL facilities. Utilizing a Talbot x-ray interferometer, we simultaneously captured transmission, dark-field, and differential phase contrast radiographs of laser-driven metallic foils. Our work showcases the feasibility of single-shot grating-based Talbot x-ray dark-field imaging in pump-probe experiments at XFEL facilities, opening doors to a wide range of hard x-ray imaging applications in material science and high-energy density physics.

Bouffetier, V. [Helmholtz-Zentrum Dresden-Rossendo↗
DOE OSTI2023

Delocalized polaron and Burstein-Moss shift induced by Li in α–V 2 O 5 : A DFT + DMFT study

For this work, we performed density functional theory (DFT)+U and dynamical mean field theory (DMFT) calculations with a continuous-time quantum Monte Carlo impurity solver to investigate the electronic properties of V 2 O 5 and Li x V 2 O 5 (x = 0.125 and 0.25). Pristine V 2 O 5 is a charge-transfer insulator with strong O p–V d hybridization, and it exhibits a large band gap (E gap ) as well as a nonzero conduction-band (CB) gap.We show that the band gap, the number of d electrons of vanadium, N d , and the CB gap for V 2 O 5 obtained from our DMFT calculations are in excellent agreement with the experimental values. While the DFT +U approach replicates the experimental band gap, it overestimates the value of N d and underestimates the CB gap. In the presence of low Li doping, the electronic properties of V 2 O 5 are mainly driven by a polaronic mechanism, and electron spin resonance and electron nuclear double resonance spectroscopies observed the coexistence of free and bound polarons. Notably, our DMFT results identify both polaron types, with the bound polaron being energetically preferred, while the DFT +U method only predicts the free polaron. Our DMFT analysis also reveals that increased Li doping leads to electron filling in the conduction band, shifting the Fermi level. This result is consistent with the observed Burstein-Moss shift upon enhanced Li doping, and we thus demonstrate that the DFT + DMFT approach can be used for an accurate and realistic description of strongly correlated materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Accelerating defect predictions in semiconductors using graph neural networks

First-principles computations reliably predict the energetics of point defects in semiconductors but are constrained by the expense of using large supercells and advanced levels of theory. Machine learning models trained on computational data, especially ones that sufficiently encode defect coordination environments, can be used to accelerate defect predictions. Here, we develop a framework for the prediction and screening of native defects and functional impurities in a chemical space of group IV, III–V, and II–VI zinc blende semiconductors, powered by crystal Graph-based Neural Networks (GNNs) trained on high-throughput density functional theory (DFT) data. Using an innovative approach of sampling partially optimized defect configurations from DFT calculations, we generate one of the largest computational defect datasets to date, containing many types of vacancies, self-interstitials, anti-site substitutions, impurity interstitials and substitutions, as well as some defect complexes. We applied three types of established GNN techniques, namely crystal graph convolutional neural network, materials graph network, and Atomistic Line Graph Neural Network (ALIGNN), to rigorously train models for predicting defect formation energy (DFE) in multiple charge states and chemical potential conditions. We find that ALIGNN yields the best DFE predictions with root mean square errors around 0.3 eV, which represents a prediction accuracy of 98% given the range of values within the dataset, improving significantly on the state-of-the-art. We further show that GNN-based defective structure optimization can take us close to DFT-optimized geometries at a fraction of the cost of full DFT. The current models are based on the semi-local generalized gradient approximation-Perdew–Burke–Ernzerhof (PBE) functional but are highly promising because of the correlation of computed energetics and defect levels with higher levels of theory and experimental data, the accuracy and necessity of discovering novel metastable and low energy defect structures at the PBE level of theory before advanced methods could be applied, and the ability to train multi-fidelity models in the future with new data from non-local functionals. The DFT-GNN models enable prediction and screening across thousands of hypothetical defects based on both unoptimized and partially optimized defective structures, helping identify electronically active defects in technologically important semiconductors.

Rahman, Md Habibur (ORCID:000000027705984X)↗
DOE OSTI2020

Failure investigation of nuclear grade POCO graphite target in high energy neutrino physics through numerical simulation

In many high energy physics neutrino beamlines, targets are made up of isotropic POCO graphite grade for production of neutrinos for high energy particle physics research and are bombarded with highly energetic pulsed proton beam. The pulsed proton beam with a small beam size creates thermal stress waves as well as radiation damage such as displacement damage, void formation, swelling, and gas formation amongst others. As a result, after a few years of operation one of the longest operated targets appears to have undergone bulk swelling and fractured along the beam centerline. A complex interaction of dynamic loading due to beam, material degradation due to irradiation are thought to have caused such a failure. Here, we present a numerical simulation to capture the combined effects of swelling and the dynamic effect of thermal stress waves due to beam heating, to explain the failure of target. An empirical formula has been developed to take into account swelling as a function of temperature and proton fluence and has been implemented in a commercial finite element code. Extensive X-ray diffraction (XRD) were performed on graphite samples from the failed target to understand the lattice parameter changes due to irradiation and build a valid empirical model. Simulation results show that swelling plays a major role in elevating the stress state in the material exceeding the failure strength while the pulsed beam heating also introduces fatigue loading that would explain brittle fracture emanating from inside of the material and propagating outward. Such empirical formulations will help in identifying the threshold values of critical parameters to extend the service life of future multi-megawatt neutrino targets and targets in other accelerator environments.

43 PARTICLE ACCELERATORS↗
DOE OSTI2025

Systematic softening in universal machine learning interatomic potentials

Machine learning interatomic potentials (MLIPs) have introduced a new paradigm for atomic simulations. Recent advancements have led to universal MLIPs (uMLIPs) that are pre-trained on diverse datasets, providing opportunities for universal force fields and foundational machine learning models. However, their performance in extrapolating to out-of-distribution complex atomic environments remains unclear. In this study, we highlight a consistent potential energy surface (PES) softening effect in three uMLIPs: M3GNet, CHGNet, and MACE-MP-0, which is characterized by energy and force underprediction in atomic-modeling benchmarks including surfaces, defects, solid-solution energetics, ion migration barriers, phonon vibration modes, and general high-energy states. The PES softening behavior originates primarily from the systematically underpredicted PES curvature, which derives from the biased sampling of near-equilibrium atomic arrangements in uMLIP pre-training datasets. Our findings suggest that a considerable fraction of uMLIP errors are highly systematic, and can therefore be efficiently corrected. We argue for the importance of a comprehensive materials dataset with improved PES sampling for next-generation foundational MLIPs.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Structural elucidation of polydopamine facilitated by ionic liquid solvation

Minimal understanding of the formation mechanism and structure of polydopamine (pDA) and its natural analogue, eumelanin, impedes the practical application of these versatile polymers and limits our knowledge of the origin of melanoma. The lack of conclusive structural evidence stems from the insolubility of these materials, which has spawned significantly diverse suggestions of pDA's structure in the literature. We discovered that pDA is soluble in certain ionic liquids. Using these ionic liquids (ILs) as solvents, we present an experimental methodology to solvate pDA, enabling us to identify pDA's chemical structure. The resolved pDA structure consists of self-assembled supramolecular aggregates that contribute to the increasing complexity of the polymer. The underlying molecular energetics of pDA solvation and a macroscopic picture of the disruption of the aggregates using IL solvents have been investigated, along with studies of the aggregation mechanism in water.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Influence of aluminum incorporation and aqueous conditions on metal ion release of high-Ni transition metal oxide nanomaterials

Developing a materials perspective of how to control the degradation and negative impact of complex metal oxides requires an integrated understanding of how these nanomaterials transform in the environment and interact with biological systems. Doping with aluminum is known to stabilize oxide materials, but has not been assessed cohesively from synthesis to environmental fate and biological impact. In the present study, the influence of aluminum doping on metal ion release from transition metal oxides was investigated by comparing aqueous transformations of lithium nickel cobalt aluminum oxide (LiNi 0.82 Co 0.15 Al 0.03 O 2 ; NCA) and lithium nickel cobalt oxide (LiNi 0.80 Co 0.20 O 2 ; NC) nanoparticles and by calculating the energetics of metal release using a density functional theory (DFT) and thermodynamics method. Two model environmental organisms were used to assess biological impact, and metal ion release was compared for NCA and NC nanoparticles incubated in their respective growth media: moderately hard reconstituted water (MHRW) for the freshwater invertebrate Daphnia magna (D. magna) and minimal growth medium for the Gram-negative bacterium Shewanella oneidensis MR-1 (S. oneidensis). Further, the amount of metal ions released was reduced for NCA compared to NC in MHRW, which correlated to changes in the modeled energetics of release upon Al substitution in the lattice. In minimal medium, metal ion release was approximately an order of magnitude higher compared to MHRW and was similar to the stoichiometry of the bulk nanoparticles for both NCA and NC. Interpretation of the release profiles and modeling indicated that the increase in total metal ion release and the reduced influence of Al doping arises from lactate complexation of metal ions in solution. The relative biological impacts of NC and NCA exposure for both S. oneidensis and D. magna were consistent with the metal release trends observed for minimal medium and MHRW, respectively. Together, these results demonstrate how a combined experimental and computational approach provides valuable insight into the aqueous transformations and biological impacts of complex metal oxide nanoparticles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗