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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 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Thermodynamic, structural and magnetic studies of phase transformations in MnAl nanocomposite alloys

Highlights: • Small changes in MnAl stoichiometry (5 at.%) produce different phase structure. • In Mn{sub 60}Al{sub 40} as-cast alloy only hcp ε phase can be found • In Mn{sub 55}Al{sub 45} as-cast alloy, there is a mixture of γ{sub 2} (Al{sub 8}Mn{sub 5}) and ε as well as ferromagnetic MnAl τ-phase. • Energies of phase transformation of about 165 kJ/mol for Mn{sub 60}Al{sub 40} and 290 kJ/mol for Mn{sub 55}Al{sub 45} alloys are found. • hcp ε phase decomposes through the migration of interphase interfaces. - Abstract: We have undertaken a temperature-dependent, investigation of the thermodynamics, structure, morphology and magnetism of two MnAl nanocomposite alloys (Mn{sub 60}Al{sub 40} and Mn{sub 55}Al{sub 45}). Differential scanning calorimetry (DSC) studies allowed the determination of the exo-effects occurring in the structural phase transformation of MnAl, while by using temperature-dependent X-ray diffraction of synchrotron radiation we were able to monitor the phase transformation effects and the evolution with temperature of various structural phases occurring in the samples. We have shown that slight changes in stoichiometry (5 at.%) give rise to different phase structure in the as-cast state. While in Mn{sub 60}Al{sub 40} only hcp ε phase can be found, in Mn{sub 55}Al{sub 45} as-cast alloy, there is a quite complex phase structure with a mixture of γ{sub 2} (Al{sub 8}Mn{sub 5}) and ε as well as ferromagnetic MnAl τ-phase. Activation energies of about 165 kJ/mol and 290 kJ/mol have been calculated from the Ozawa-Flynn-Wall analysis of DSC experimental data. These findings are in good agreement with the results obtained from XRD. For the Mn{sub 55}Al{sub 45} as-cast alloy, we have confirmed by temperature-dependent synchrotron XRD that the hcp ε phase decomposes through the migration of interphase interfaces with the transformation rate controlled by boundary diffusion processes. High-resolution transmission electron microscopy have confirmed the phase structure obtained by XRD, while magnetic properties, obtained for the as-cast alloys are consistent with the multiphase character of the samples and in good agreement with previously reported results. The magnetization does not saturate for the maximum applied field of about 4 × 10{sup 6} A/m and a marked coercivity of about 160 kA/m is obtained for the Mn{sub 55}Al{sub 45} as-cast alloy.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Interface potentials inside solid-state batteries: Origins and implications

Interface resistance has become a significant bottleneck for solid-state batteries (SSBs). Most studies of interface resistance have focused on extrinsic mechanisms such as interface reactions and imperfect contact between electrodes and solid electrolytes. Interface potentials are an important intrinsic mechanism that is often ignored. Here, we highlight Kelvin probe force microscopy (KPFM) as a tool to image the local potential at interfaces inside SSBs, examining the existing literature and discussing challenges in interpretation. Drawing analogies with electron transport in metal/semiconductor interfaces, we showcase a formalism that predicts intrinsic ionic resistance based on the properties of the contacting phases, and we emphasize that future battery designs should start from material pairs with low intrinsic resistance. Here, we conclude by outlining future directions in the study of interface potentials through both theory and experiment.

25 ENERGY STORAGE↗
NASA NTRS1975

Electron radiation effects on time-dependent dielectric breakdown in SiO2 films

An experiment testing the effect of ionizing radiation on breakdown characteristics of SiO2 films is presented. Silicon wafers were oxidized and metallized, and a capacitor array was etched into a control sample while the rest were first irradiated with 1 MeV electrons and then etched. Time-dependent dielectric tests were made on all the capacitors, and the average characteristics of 96 capacitors are illustrated graphically. The curves are consistent with the model of holes trapped in the SiO2 film during irradiation leading to a retarding field for positive ion emission and drift toward the interface. It is shown how an externally applied field is reduced by the trapped charge, and that changes in the dielectric breakdown properties of the SiO2 film after irradiation depend on the positive trapped charge near the metal interface.

Li, S. P.↗
DOE OSTI2021

Evolutionary inverse design of defects at graphene 2D lateral interfaces

Grain boundaries (GBs) in two-dimensional (2D) materials often have a profound impact on various material properties from mechanical to optical to electronic, yet predicting all possible GB formations is a challenge. Here, we introduce a workflow based on an evolutionary algorithm for exploring possible GBs formed at a lateral 2D interface. In a departure from conventional genetic algorithm based structure optimization methods, we perform genetic operations in the near interface region that allow us to be computationally efficient. We benchmark our method using graphene, which is a well-studied 2D material with a wide range of point defects. An empirical potential was used as the surrogate of the evolutionary search. More than 11.5 × 106 structures in total were evaluated for 128 GB orientations, and for each orientation, the ten best structures are recorded. A subset of low energy GBs predicted by empirical potential based search was relaxed by first-principles calculations and used to validate the energetic rank order. With the validated formation energy, we rank-ordered the best 128 GB structures and performed a detailed statistical analysis of primitive rings to find the correlation between the ring distribution and the formation energy. We found that for low energy GBs (below 0.5eV/Å), Stone–Wales defects will dominate, while structures with a higher energy (0.5–1.1eV/Å) show an increasing population of heptagons and nine-membered rings to form seven-nine defect pairs. For structures with energy higher than 1.1eV/Å, the percentage of octagons and nine-membered rings increases, which indicates that these two types of rings are not energetically favorable. Our proposed methodology is broadly applicable to explore defective low dimensional materials and represents a powerful tool that enables a systematic search of GBs of lateral interfaces for 2D materials.

Zhang, Jianan↗
DOE OSTI2019

Creating stable interfaces between reactive materials: titanium nitride protects photoabsorber–catalyst interface in water-splitting photocathodes

The development of a solar-driven water splitting device that replaces costly precious metals, while achieving stable high performance, is a major challenge. Transition metal phosphides are active and low-cost catalysts for the hydrogen evolution reaction (HER), although, none thus far have exhibited stable performance when interfaced with semiconductors. In this work, we report on a monolithic junction consisting of cubic-NiP 2 :TiN:Si, fabricated using both commercial and custom Si photovoltaics. Stable performance is achieved using an ultrathin film of crystalline TiN that effectively hinders atomic diffusion between interfaces during fabrication. Crystalline cubic-NiP 2 deposited on TiN/n + p-Si retains 97% of the bare Si photovoltage, comparable saturation current density to bare Si, and has a turnover frequency of 1.04 H 2 per site per s at -100 mV applied potential. In acid, it requires only -150 mV additional overpotential compared to the benchmark, Pt/TiN/n + p-Si, to reach a HER photocurrent density of -10 mA cm -2 . This photocathode maintains a stable H 2 photocurrent (±10%) for at least 125 hours, the duration of testing. When the same layers are fabricated on a commercial Si solar cell, this photocathode produced double the photocurrent density (36.3 mA cm -2 , under simulated 1.5 AM G illumination). Physical characterization gives detailed information on the properties responsible for the observed activity and durability of these interfaces. In general, the thin-film methodology presented here is widely applicable, demonstrates superior activity, and achieves long-term stability.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Integrated Nanomaterials

Nanomaterials offer exceptional properties for photonic, electronic, magnetic, structural, mechanical, chemical, nuclear, and biological functionality. Real access to enhanced functionality remains limited, however, without connecting the nanoscale across the mesoscale to macroscale assemblies and the ability to define and control nanomaterials organization, interactions, and interfaces. Integration is essential for accessing and controlling functionality to harness nanomaterials properties while also generating new behaviors and function. Integration thus provides a route to discovering, generating, and using intrinsic and emergent nanomaterials properties, which define the two focus areas of this leadership theme.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

A mass-conserving lattice Boltzmann method with dynamic grid refinement for immiscible two-phase flows

A mass-conserving lattice Boltzmann method (LBM) for multiphase flows is presented in this paper. The proposed LBM improves a previous model (Lee and Liu, 2010 [21]) in terms of mass conservation, speed-up, and efficiency, and also extends its capabilities for implementation on non-uniform grids. The presented model consists of a phase-field lattice Boltzmann equation (LBE) for tracking the interface between different fluids and a pressure-evolution LBM for recovering the hydrodynamic properties. In addition to the mass conservation property and the simplicity of the algorithm, the advantages of the current phase-field LBE are that it is an order of magnitude faster than the previous interface tracking LBE proposed by Lee and Liu (2010) [21] and it requires less memory resources for data storage. Meanwhile, the pressure-evolution LBM is equipped with a multi-relaxation-time (MRT) collision operator to facilitate attainability of small relaxation rates thereby allowing simulation of multiphase flows at higher Reynolds numbers. Additionally, we reformulate the presented MRT-LBM on nonuniform grids within an adaptive mesh refinement (AMR) framework. Various benchmark studies such as a rising bubble and a falling drop under buoyancy, droplet splashing on a wet surface, and droplet coalescence onto a fluid interface are conducted to examine the accuracy and versatility of the proposed AMR-LBM. The proposed model is further validated by comparing the results with other LB models on uniform grids. A factor of about 20 in savings of computational resources is achieved by using the proposed AMR-LBM. As a more demanding application, the Kelvin–Helmholtz instability (KHI) of a shear-layer flow is investigated for both density-matched and density-stratified binary fluids. The KHI results of the density-matched fluids are shown to be in good agreement with the benchmark AMR results based on the sharp-interface approach. When a density contrast between the two fluids exists, a typical chaotic structure in the flow field is observed at a Reynolds number of 10000, which indicates that the proposed model is a promising tool for direct numerical simulation of two-phase flows.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Integrated Nanotechnologies

Nanomaterials offer exceptional properties for photonic, electronic, quantum, magnetic, structural, mechanical, chemical, nuclear, and biological functionality. However, real access to enhanced functionality remains limited without connecting the nanoscale across the mesoscale to macroscale assemblies and the ability to define and control nanomaterials organization, interactions, and interfaces. Integration is essential for accessing and controlling functionality to harness nanomaterials properties while also generating new behaviors and properties. Integration thus provides a route to discovering, generating, and using intrinsic and emergent nanomaterials properties, which define the two focus areas of this leadership theme.

36 MATERIALS SCIENCE↗
DOE OSTI2016

First-principles calculations of thermoelectric properties of TiN/MgO superlattices: The route for an enhancement of thermoelectric effects in artificial nanostructures

We present the thermoelectric properties of TiN/MgO superlattices employing first-principles calculation techniques. The Seebeck coefficients, the electrical conductances, the thermal conductances, and the figure of merit are investigated employing electrical and thermal transport calculations based on density functional theory combined with the nonequilibrium Green's function and nonequilibrium molecular dynamics simulation methods. The TiN/MgO superlattices with a small lattice mismatch at the interfaces are ideal systems to study the way for an enhancement of thermoelectric properties in artificial nanostructures. We find that the interfacial scattering between the two materials in the metal/insulator superlattices causes the electrical conductance to change rapidly, which enhances the Seebeck coefficient significantly. We show that the figure of merit for the artificial superlattice nanostructures has a much larger value compared with that of the bulk material and changes drastically with the superlattice configurations at the atomistic level.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Accumulation-Type Ohmic van der Waals Contacts to Nearly Intrinsic WSe 2 Nanosheet-Based Channels: Implications for Field-Effect Transistors

We report the fabrication of ohmic van der Waals (vdW) contacts to nearly intrinsic WSe 2 nanosheet-based channels in field-effect transistors (FETs) using degenerately p-doped MoS2 (p + -MoS2) as a contact metal. We demonstrate that accumulation-type ohmic contacts and the high device performance are achievable without electrostatically gating the drain/source contact regions despite the nearly intrinsic nature of WSe 2 . Back-gated WSe 2 FETs with p + -MoS 2 bottom contacts (which screen the back-gate electric field in the drain/source regions) exhibit linear output characteristics, a high on/off ratio of 10 8 , and a high two-terminal field-effect mobility up to ~200 cm2 V –1 s –1 at room temperature. Our theoretical modeling reveals that the p + -MoS 2 /WSe 2 vdW junction behaves like a metal/semiconductor ohmic contact signified by a vanishingly thin space-charge region of ~1 nm on the p + -MoS 2 side and a substantial accumulation layer of free holes on the WSe 2 side, which is further verified by additional temperature-dependent and dual-gated measurements of WSe 2 FETs. We attribute the formation of accumulation-type ohmic contacts free of a Schottky barrier to the near absence of Fermi-level pinning at the vdW interface and the work function of p + -MoS 2 being larger than the ionization energy of WSe 2 . This study represents an important step toward achieving low-resistance ohmic contacts to two-dimensional (2D) semiconductors by eliminating the Fermi-level pinning effects, which is expected to have significant implications for next-generation 2D semiconductor-based nanoelectronics.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Battery charging goes quantum

Rechargeable lithium-ion batteries power consumer electronics and electric vehicles, making them an essential component of the modern economy. Although lithium-ion battery technology has improved continuously over the past decades, widespread adoption of electrified transportation requires charging in less than 15 min to be competitive with internal combustion engines. As a battery charges and discharges, lithium ions travel across the electrode-electrolyte interface. The rate at which lithium ions transfer is dictated by the structure and physical properties of electrolytes and lithium-storing electrodes. Yet, the exact chemical reaction mechanism underlying the insertion of lithium ions at the electrode-electrolyte interface remains elusive. On page 46 of this issue, Zhang et al. (1) report experimental evidence that shows that lithium-ion battery charge and discharge occur through a coupled ion-electron transfer mechanism. Furthermore, this could establish an experimental and theoretical platform to extract key parameters for optimizing charge transfer rates in lithium-ion batteries.

Warburton, Robert E. [Case Western Reserve Univers↗
DOE OSTI2025

What more can be done with XPS? Highly informative but underused approaches to XPS data collection and analysis

Because of the importance of surfaces and interfaces in many scientific and technological areas, the use of x-ray photoelectron spectroscopy (XPS) has been growing exponentially. Although XPS is being used to obtain useful information about the surface composition of samples, much more information about materials and their properties can be extracted from XPS data than commonly obtained. This paper describes some of the areas where alternative analysis methods or experimental design can obtain information about the near-surface region of a sample, often information not available in other ways. Experienced XPS analysts are familiar with many of these methods, but they may not be known to new or casual XPS users, and sometimes, they have not been used because of an inappropriately assumed complexity. The information available includes optical, electronic, and electrical properties; nanostructure; expanded chemical information; and enhanced analysis of biological materials and solid/liquid interfaces. Many of these analyses can be conducted on standard laboratory XPS systems, with either no or relatively minor system alterations. Topics discussed include (1) considerations beyond the “traditional” uniform surface layer composition calculation, (2) using the Auger parameter to determine a sample property, (3) use of the D parameter to identify sp 2 and sp 3 carbon information, (4) information from the XPS valence band, (5) using cryocooling to expand range of samples that can be analyzed and minimize damage, and (6) using electrical potential effects on XPS signals to extract chemically resolved electrical measurements including band alignment and electrical property information.

Baer, Donald R. [Pacific Northwest National Labora↗
DOE OSTI2024

Effect of H + Exchange and Surface Impurities on Bulk and Interfacial Electrochemistry of Garnet Solid Electrolytes

Contact loss and current constriction pose significant challenges at the Li metal interface of solid-state batteries. For garnet-structured Li 7 La 3 Zr 2 O 12 (LLZO), these effects are amplified by Li + /H + exchange and surface contamination reactions, which lead to conductivity losses and poor Li wetting. In this study, we utilize a variety of surface treatment processes across 37 cells to selectively induce proton exchange and contamination reactions in LLZO. The resulting bulk and surface chemistry is systematically characterized and correlated to changes in electrochemical properties. Additionally, we combine impedance analysis and finite element method modeling to deconvolute sources of impedance contributions at the Li metal interface. Specifically, we show that constriction impedance at the Li metal interface arises not solely from voids, but also from ionically-resistive surface contaminants. Further, these findings emphasize the connection between ionic conductivity and constriction, demonstrating that micron-scale ionically-resistive components increase constriction even with identical contact geometries. Finally, we leverage our comprehensive dataset to highlight unstable overpotential growth as a failure mechanism, additionally showing that the phase of a cell’s impedance is a sensitive indicator for the onset of interfacial instability. Overall, this study clarifies the impacts of proton exchange and surface contamination on electrochemical properties at the Li|solid electrolyte interface and elucidates insights that are generalizable to other solid-state battery systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Self-Assembly of MXene-Surfactants at Liquid–Liquid Interfaces: From Structured Liquids to 3D Aerogels

2D transition metal carbides and nitrides (MXenes), a class of emerging nanomaterials with intriguing properties, have attracted significant attention in recent years. However, owing to the highly hydrophilic nature of MXene nanosheets, assembly strategies of MXene at liquid-liquid interfaces have been very limited and challenging. Here in this paper, through the cooperative assembly of MXene and amine-functionalized polyhedral oligomeric silsesquioxane at the oil-water interface, we report the formation, assembly, and jamming of a new type MXene-based Janus-like nanoparticle surfactants, termed MXene-surfactants (MXSs), which can significantly enhance the interfacial activity of MXene nanosheets. More importantly, this simple assembly strategy opens a new platform for the fabrication of functional MXene assemblies from mesoscale (e.g., structured liquids) to macroscale (e.g., aerogels), that can be used for a range of applications, including nanocomposites, electronic devices, and all-liquid microfluidic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Review on modeling of the anode solid electrolyte interphase (SEI) for lithium-ion batteries

A passivation layer called the solid electrolyte interphase (SEI) is formed on electrode surfaces from decomposition products of electrolytes. The SEI allows Li + transport and blocks electrons in order to prevent further electrolyte decomposition and ensure continued electrochemical reactions. The formation and growth mechanism of the nanometer thick SEI films are yet to be completely understood owing to their complex structure and lack of reliable in situ experimental techniques. Significant advances in computational methods have made it possible to predictively model the fundamentals of SEI. This review aims to give an overview of state-of-the-art modeling progress in the investigation of SEI films on the anodes, ranging from electronic structure calculations to mesoscale modeling, covering the thermodynamics and kinetics of electrolyte reduction reactions, SEI formation, modification through electrolyte design, correlation of SEI properties with battery performance, and the artificial SEI design. Multi-scale simulations have been summarized and compared with each other as well as with experiments. Computational details of the fundamental properties of SEI, such as electron tunneling, Li-ion transport, chemical/mechanical stability of the bulk SEI and electrode/(SEI/) electrolyte interfaces have been discussed. This review shows the potential of computational approaches in the deconvolution of SEI properties and design of artificial SEI. We believe that computational modeling can be integrated with experiments to complement each other and lead to a better understanding of the complex SEI for the development of a highly efficient battery in the future.

25 ENERGY STORAGE↗
NASA NTRS1980

XPS study of the chemical structure of the nickel/silicon interface

The chemical nature of the Ni/Si, Ni/Ni2Si, and Si/Ni2Si interfaces is studied using X-ray photoelectron spectroscopy. Two approaches are employed: (1) the Ni/silicide and Si/silicide interfaces are examined by dynamically monitoring the advancing reaction front during in situ growth and (2) as-deposited Ni/Si interfacial properties are analyzed by sequential evaporating monolayer Ni films on Si. These experiments show that the Ni/Ni2Si interface consists of a Ni-rich silicide phase, while the Si/Ni2Si interface has a transitional region that is correspondingly Si-rich silicide.

Grunthaner, P. J.↗
DOE OSTI2020

Coupling CTH to Linear Acoustic Propagation across an Air-Earth Interface

The interface between the Earth and the atmosphere forms a strong contrast in material properties. As such, numerical issues can arise when simulating an elastic wavefield across such a boundary when using a numerical simulation scheme. This is exacerbated when two different simulation codes are coupled straddling that interface. In this report we document how we implement the coupling of CTH, a nonlinear shock physics code, to a linearized elastic/acoustic wave propagation algorithm, axiElasti, across the air-earth interface. We first qualitatively verify that this stable coupling between the two algorithms produces expected results with no visible effects of the coupling interface. We then verify the coupling interface quantitatively by checking consistency with results from previous work and with coupled acoustic-elastic seismo-acoustic source inversions in three earth materials.

58 GEOSCIENCES↗