SEARCH · Engineering Papers
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.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Abstract not provided
Abstract not provided
The design of distortions and rotations of the corner-connected BO6 octahedra across interfaces has emerged as an exciting platform to control electronic or ferroic behavior in ABO3 perovskite heterostructures. Here, we investigate isovalent manganite superlattices, [(La0.7Sr0.3MnO3)n/(Eu0.7Sr0.3MnO3)n]×m, as a route to spatial control over electronic bandwidth and ferromagnetism through the creation of octahedral superstructures. Electron energy loss spectroscopy confirms a uniform Mn valence state throughout the superlattices. In contrast, the presence of modulations of the MnO6 octahedral rotations along the growth direction commensurate with the superlattice period is revealed by scanning transmission electron microscopy and x-ray diffraction. We show that the Curie temperatures of the constituent materials can be systematically engineered via the octahedral superstructures leading to a modulated magnetization in samples where the superlattice period is larger than the interfacial octahedral coupling length scale, while a single magnetic transition is observed in the short period superlattices.
Explore the source record for details and available documents.
Abstract not provided
Explore the source record for details and available documents.
Organometallic Ir precatalysts have been found to yield homogeneous Ir-based water-oxidation catalysts (WOCs) with very high activity. The Cp*Ir catalyst series can operate under a variety of regimes: it can either act as a homogeneous or a heterogeneous catalyst; it can be driven by chemical, photochemical, or electrochemical methods; and the molecular catalyst can either act in solution or supported as a molecular unit on a variety of solid oxides. In addition to optimizing the various reaction conditions, work has continued to elucidate the catalyst activation mechanism and identify water-oxidation intermediates. Furthermore, this Perspective will describe the development of the Cp*Ir series, their many forms as WOCs, and their ongoing characterization.
Gallium-modified zeolites are known catalysts for the dehydrogenation of alkanes, reactivity that finds industrial application in the aromatization of light alkanes by Ga-ZSM5. While the role of gallium cations in alkane activation is well known, the oxidation state and coordination environment of gallium under reaction conditions has been the subject of debate. Edge shifts in Ga K-edge XANES spectra acquired under reaction conditions have long been interpreted as evidence for reduction of Ga(III) to Ga(I). However, a change in oxidation state is not the only factor that can give rise to a change in the XANES spectrum. In order to better understand the XANES spectra of working catalysts, we have synthesized a series of molecular model compounds and grafted surface organometallic Ga species and compared their XANES spectra to those of gallium-based catalysts acquired under reducing conditions. We demonstrate that changes in the identity and number of gallium nearest neighbors can give rise to changes in XANES spectra similar to those attributed in literature to changes in oxidation state. Specifically, spectral features previously attributed to Ga(I) may be equally well interpreted as evidence for low-coordinate Ga(III) alkyl or hydride species. Furthermore, these findings apply both to gallium-impregnated zeolite catalysts and to silica-supported single site gallium catalysts, the latter of which is found to be active and selective for dehydrogenation of propane and hydrogenation of propylene.
Explore the source record for details and available documents.
The Mn 4 CaO 5 cluster in photosystem II catalyzes the four-electron redox reaction of water oxidation in natural photosynthesis. This catalytic reaction cycles through four intermediate states (S i , i = 0 to 4), involving changes in the redox state of the four Mn atoms in the cluster. Recent studies suggest the presence and importance of isomorphous structures within the same redox/intermediate S-state. It is highly likely that geometric and electronic structural flexibility play a role in the catalytic mechanism. Among the catalytic intermediates that have been identified experimentally thus far, there is clear evidence of such isomorphism in the S2 state, with a high-spin (5/2) (HS) and a low spin (1/2) (LS) form, identified and characterized by their distinct electron paramagnetic resonance (EPR spectroscopy) signals. We studied these two S2 isomers with Mn extended X-ray absorption fine structure (EXAFS) and absorption and emission spectroscopy (XANES/XES) to characterize the structural and electronic structural properties. The geometric and electronic structure of the HS and LS S2 states are different as determined using Mn EXAFS and XANES/XES, respectively. The Mn K-edge XANES and XES for the HS form are different from the LS and indicate a slightly lower positive charge on the Mn atoms compared to the LS form. Based on the EXAFS results which are clearly different, we propose possible structural differences between the two spin states. As a result, such structural and magnetic redox-isomers if present at room temperature, will likely play a role in the mechanism for water-exchange/oxidation in photosynthesis.
Uranyl, ligand and solvent interactions lead to unique supramolecular assembly.
We demonstrate the coupling of liquid extraction surface analysis (LESA) to structures for lossless ion manipulations in conjunction with serpentine ultralong path with extending routing (SLIM SUPER) ion mobility-mass spectrometry (IM-MS) for the unambiguous annotation of important isomeric glycoforms in carbon-fixing communities. A major bottleneck of current mass spectrometry imaging (MSI) methods is their inability to accurately and confidently delineate amongst isomers within a single probing area. Our methodology provides gains in both sensitivity and IM resolution, while simultaneously gaining insight on the spatial distribution of these biological isomers in a mixed organism composed of moss, cyanobacteria and fungus. We envision this methodology will be broadly applicable to other mass spectrometry imaging modalities and invaluable in multiplexed, untargeted, analyses of a single probing location.