Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “SCALE”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Measurement Error in Atomic-Scale Scanning Transmission Electron Microscopy—Energy-Dispersive X-Ray Spectroscopy (STEM-EDS) Mapping of a Model Oxide Interface

Abstract With the development of affordable aberration correctors, analytical scanning transmission electron microscopy (STEM) studies of complex interfaces can now be conducted at high spatial resolution at laboratories worldwide. Energy-dispersive X-ray spectroscopy (EDS) in particular has grown in popularity, as it enables elemental mapping over a wide range of ionization energies. However, the interpretation of atomically resolved data is greatly complicated by beam–sample interactions that are often overlooked by novice users. Here we describe the practical factors—namely, sample thickness and the choice of ionization edge—that affect the quantification of a model perovskite oxide interface. Our measurements of the same sample, in regions of different thickness, indicate that interface profiles can vary by as much as 2–5 unit cells, depending on the spectral feature. This finding is supported by multislice simulations, which reveal that on-axis maps of even perfectly abrupt interfaces exhibit significant delocalization. Quantification of thicker samples is further complicated by channeling to heavier sites across the interface, as well as an increased signal background. We show that extreme care must be taken to prepare samples to minimize channeling effects and argue that it may not be possible to extract atomically resolved information from many chemical maps.

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

Atomic Scale Account of the Surface Effect on Ionic Transport in Silver Hollandite

Here, nanosized electrodes for Li-ion batteries typically display improved electrochemical properties, which are generally attributed to the reduced dimensionality for lithiation. However, the intriguing roles of surface defects and disorder associated with the nanosized materials are often overlooked. Here, combining atomically resolved structural analysis with density functional theory calculations, we reveal that the formation of intrinsic oxygen vacancies near surface in silver hollandite nanorods modifies the local atomic structure and valence state. These surface reconstructions resulted from oxygen vacancies can significantly affect the diffusion pathways in what are otherwise one-dimensional (1D) tunneled structures. On the basis of energy barrier calculations, we demonstrate that the oxygen vacancies boost ionic transport through the edge sharing MnO 6 polyhedra in the a–b plane. Thus, within a single rod different from the inherent 1D tunnel diffusion in the interior, the ionic transport at oxygen vacancy decorated surfaces likely adopts a three-dimensional diffusion pathway including both tunnel and planar diffusion.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Large Scale GW Calculations

We present GW calculations of molecules, ordered and disordered solids and interfaces, which employ an efficient contour deformation technique for frequency integration and do not require the explicit evaluation of virtual electronic states nor the inversion of dielectric matrices. We also present a parallel implementation of the algorithm which takes advantage of separable expressions of both the single particle Green's function and the screened Coulomb interaction. The method can be used starting from density functional theory calculations performed with semilocal or hybrid functionals. We applied the newly developed technique to GW calculations of systems of unprecedented size, including water/semiconductor interfaces with thousands of electrons.

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

Molecular-Scale Structure of Electrode–Electrolyte Interfaces: The Case of Platinum in Aqueous Sulfuric Acid

Knowledge of the molecular composition and electronic structure of electrified solid-liquid interfaces is key to understanding elemental processes in heterogeneous reactions. Using X-ray absorption spectroscopy in the interface-sensitive electron yield mode (EY-XAS), first-principles electronic structure calculations, and multiscale simulations, we determined the chemical composition of the interfacial region of a polycrystalline platinum electrode in contact with aqueous sulfuric acid solution at potentials between the hydrogen and oxygen evolution reactions. We found that between 0.7 and 1.3 V vs Ag/AgCl the electrical double layer (EDL) region comprises adsorbed sulfate ions with hydrated hydronium ions in the next layer. No evidence was found for bisulfate or Pt-O/Pt-OH species, which have very distinctive spectral signatures. In addition to resolving the long-standing issue of the EDL structure, our work establishes interface- and element-sensitive EY-XAS as a powerful spectroscopic tool for studying condensed phase, buried solid-liquid interfaces relevant to various electrochemical processes and devices.

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

All-Scale Hierarchically Structured p-Type PbSe Alloys with High Thermoelectric Performance Enabled by Improved Band Degeneracy

We show an example of hierarchically designing electronic bands of PbSe toward excellent thermoelectric performance. We find that alloying 15 mol % PbTe into PbSe causes a negligible change in the light and heavy valence band energy offsets (ΔE V ) of PbSe around room temperature; however, with rising temperature it makes ΔE V decrease at a significantly higher rate than in PbSe. In other words, the temperature-induced valence band convergence of PbSe is accelerated by alloying with PbTe. On this basis, applying 3 mol % Cd substitution on the Pb sites of PbSe 0.85 Te 0.15 decreases ΔE V and enhances the Seebeck coefficient at all temperatures. Here, excess Cd precipitates out as CdSe 1-y Te y , whose valence band aligns with that of the p-type Na-doped PbSe 0.85 Te 0.15 matrix. This enables facile charge transport across the matrix/precipitate interfaces and retains the high carrier mobilities. Meanwhile, compared to PbSe the lattice thermal conductivity of PbSe 0.85 Te 0.15 is significantly decreased to its amorphous limit of 0.5 W m -1 K -1 . Consequently, a highest peak ZT of 1.7 at 900 K and a record high average ZT of ~1 (400-900 K) for a PbSe-based system are achieved in the composition Pb 0.95 Na 0.02 Cd 0.03 Se 0.85 Te 0.15 , which are ~70% and ~50% higher than those of Pb 0.98 Na 0.02 Se control sample, respectively.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗