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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 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Trends and Descriptors of Metal-Modified Transition Metal Carbides for Hydrogen Evolution in Alkaline Electrolyte

Here in this study, the experimentally measured hydrogen evolution reaction exchange current densities for metal monolayer-modified transition metal carbides (TMCs) are correlated with density functional theory calculations of adsorbed hydrogen and hydroxyl binding energies. The correlation reveals a volcano relationship in alkaline electrolytes, while the hydroxyl binding energy does not appear to show a strong correlation. These results should provide guidance for further improving the electrocatalytic activity of metal-modified TMCs in an alkaline environment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Thermal Stabilization of Metal–Organic Framework-Derived Single-Site Catalytic Clusters through Nanocasting

Metal-organic frameworks (MOFs) provide convenient systems for organizing high concentrations of single catalytic sites derived from metallic or oxo-metallic nodes. However, high-temperature processes cause agglomeration of these nodes, so that the single-site character and catalytic activity are lost. In this work, we present a simple nanocasting approach to provide a thermally stable secondary scaffold for MOP-based catalytic single sites, preventing their aggregation even after exposure to air at 600 degrees C. We describe the nanocasting of NU-1000, a MOP with 3 nm channels and Lewis-acidic oxozirconium clusters, with silica. By condensing tetramethylorthosilicate within the NU-1000 pores via a vapor-phase HCl treatment, a silica layer is created on the inner walls of NU-1000. This silica layer provides anchoring sites for the oxozirconium clusters in NU-1000 after the organic linkers are removed at high temperatures. Differential pair distribution functions obtained from synchrotron X-ray scattering confirmed that isolated oxozirconium clusters are maintained in the heated nanocast materials. Pyridine adsorption experiments and a glucose isomerization reaction demonstrate that the clusters remain accessible to reagents and maintain their acidic character and catalytic activity even after the nanocast materials have been heated to 500-600 degrees C in air. Density functional theory calculations show a correlation between the Lewis acidity of the oxozirconium clusters and their catalytic activity. The ability to produce MOF-derived materials that retain their catalytic properties after exposure to high temperatures makes nanocasting a useful technique for obtaining single-site catalysts suitable for high-temperature reactions.

catalysis↗
DOE OSTI2017

Regulating the spatial distribution of metal nanoparticles within metal-organic frameworks to enhance catalytic efficiency

Composites incorporating metal nanoparticles (MNPs) within metal-organic frameworks (MOFs) have broad applications in many fields. However, the controlled spatial distribution of the MNPs within MOFs remains a challenge for addressing key issues in catalysis, for example, the efficiency of catalysts due to the limitation of molecular diffusion within MOF channels. We report a facile strategy that enables MNPs to be encapsulated into MOFs with controllable spatial localization by using metal oxide both as support to load MNPs and as a sacrificial template to grow MOFs. This strategy is versatile to a variety of MNPs and MOF crystals. By localizing the encapsulated MNPs closer to the surface of MOFs, the resultant MNPs@MOF composites not only exhibit effective selectivity derived from MOF cavities, but also enhanced catalytic activity due to the spatial regulation of MNPs as close as possible to the MOF surface.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Magnetic ordering through itinerant ferromagnetism in a metal-organic framework

In this report, the materials that combine magnetic order with other desirable physical attributes could find transformative applications in spintronics, quantum sensing, low-density magnets and gas separations. Among potential multifunctional magnetic materials, metal-organic frameworks, in particular, bear structures that offer intrinsic porosity, vast chemical and structural programmability, and the tunability of electronic properties. Nevertheless, magnetic order within metal-organic frameworks has generally been limited to low temperatures, owing largely to challenges in creating a strong magnetic exchange. Here we employ the phenomenon of itinerant ferromagnetism to realize magnetic ordering at T C = 225 K in a mixed-valence chromium(II/III) triazolate compound, which represents the highest ferromagnetic ordering temperature yet observed in a metal-organic framework. The itinerant ferromagnetism proceeds through a double-exchange mechanism, which results in a barrierless charge transport below the Curie temperature and a large negative magnetoresistance of 23% at 5 K. These observations suggest applications for double-exchange-based coordination solids in the emergent fields of magnetoelectrics and spintronics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Antiferromagnetic half-metals, gapless half-metals, and spin gapless semiconductors: The D0{sub 3}-type Heusler alloys

High-spin-polarization materials are desired for the realization of high-performance spintronic devices. We combine recent experimental and theoretical findings to theoretically design several high-spin-polarization materials in binary D0{sub 3}-type Heusler alloys: gapless (zero-gap) half-metallic ferrimagnets of V{sub 3}Si and V{sub 3}Ge, half-metallic antiferromagnets of Mn{sub 3}Al and Mn{sub 3}Ga, half-metallic ferrimagnets of Mn{sub 3}Si and Mn{sub 3}Ge, and a spin gapless semiconductor of Cr{sub 3}Al. The high spin polarization, zero net magnetic moment, zero energy gap, and slight disorder compared to the ternary and quaternary Heusler alloys make these binary materials promising candidates for spintronic applications. All results are obtained by the electronic structure calculations from first-principles.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Investigation of the impact of insulator material on the performance of dissimilar electrode metal-insulator-metal diodes

The performance of thin film metal-insulator-metal (MIM) diodes is investigated for a variety of large and small electron affinity insulators using ultrasmooth amorphous metal as the bottom electrode. Nb{sub 2}O{sub 5}, Ta{sub 2}O{sub 5}, ZrO{sub 2}, HfO{sub 2}, Al{sub 2}O{sub 3}, and SiO{sub 2} amorphous insulators are deposited via atomic layer deposition (ALD). Reflection electron energy loss spectroscopy (REELS) is utilized to measure the band-gap energy (E{sub G}) and energy position of intrinsic sub-gap defect states for each insulator. E{sub G} of as-deposited ALD insulators are found to be Nb{sub 2}O{sub 5} = 3.8 eV, Ta{sub 2}O{sub 5} = 4.4 eV, ZrO{sub 2} = 5.4 eV, HfO{sub 2} = 5.6 eV, Al{sub 2}O{sub 3} = 6.4 eV, and SiO{sub 2} = 8.8 eV with uncertainty of ±0.2 eV. Current vs. voltage asymmetry, non-linearity, turn-on voltage, and dominant conduction mechanisms are compared. Al{sub 2}O{sub 3} and SiO{sub 2} are found to operate based on Fowler-Nordheim tunneling. Al{sub 2}O{sub 3} shows the highest asymmetry. ZrO{sub 2}, Nb{sub 2}O{sub 5}, and Ta{sub 2}O{sub 5} based diodes are found to be dominated by Frenkel-Poole emission at large biases and exhibit lower asymmetry. The electrically estimated trap energy levels for defects that dominate Frenkel-Poole conduction are found to be consistent with the energy levels of surface oxygen vacancy defects observed in REELS measurements. For HfO{sub 2}, conduction is found to be a mix of trap assisted tunneling and Frenkel-Poole emission. Insulator selection criteria in regards to MIM diodes applications are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Ductility and work hardening in nano-sized metallic glasses

In-situ nano-tensile experiments on 70 nm-diameter free-standing electroplated NiP metallic glass nanostructures reveal tensile true strains of ∼18%, an amount comparable to compositionally identical 100 nm-diameter focused ion beam samples and ∼3 times greater than 100 nm-diameter electroplated samples. Simultaneous in-situ observations and stress-strain data during post-elastic deformation reveal necking and work hardening, features uncharacteristic for metallic glasses. The evolution of free volume within molecular dynamics-simulated samples suggests a free surface-mediated relaxation mechanism in nano-sized metallic glasses.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Helium atmospheric pressure plasma jets touching dielectric and metal surfaces

Atmospheric pressure plasma jets (APPJs) are being investigated in the context plasma medicine and biotechnology applications, and surface functionalization. The composition of the surface being treated ranges from plastics, liquids, and biological tissue, to metals. The dielectric constant of these materials ranges from as low as 1.5 for plastics to near 80 for liquids, and essentially infinite for metals. The electrical properties of the surface are not independent variables as the permittivity of the material being treated has an effect on the dynamics of the incident APPJ. In this paper, results are discussed from a computational investigation of the interaction of an APPJ incident onto materials of varying permittivity, and their impact on the discharge dynamics of the plasma jet. The computer model used in this investigation solves Poisson's equation, transport equations for charged and neutral species, the electron energy equation, and the Navier-Stokes equations for the neutral gas flow. The APPJ is sustained in He/O{sub 2} = 99.8/0.2 flowing into humid air, and is directed onto dielectric surfaces in contact with ground with dielectric constants ranging from 2 to 80, and a grounded metal surface. Low values of relative permittivity encourage propagation of the electric field into the treated material and formation and propagation of a surface ionization wave. High values of relative permittivity promote the restrike of the ionization wave and the formation of a conduction channel between the plasma discharge and the treated surface. The distribution of space charge surrounding the APPJ is discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Measurement and simulation of top- and bottom-illuminated solar-blind AlGaN metal-semiconductor-metal photodetectors with high external quantum efficiencies

A comprehensive study on top- and bottom-illuminated Al{sub 0.5}Ga{sub 0.5}N/AlN metal-semiconductor-metal (MSM) photodetectors having different AlGaN absorber layer thickness is presented. The measured external quantum efficiency (EQE) shows pronounced threshold and saturation behavior as a function of applied bias voltage up to 50 V reaching about 50% for 0.1 μm and 67% for 0.5 μm thick absorber layers under bottom illumination. All experimental findings are in very good accordance with two-dimensional drift-diffusion modeling results. By taking into account macroscopic polarization effects in the hexagonal metal-polar +c-plane AlGaN/AlN heterostructures, new insights into the general device functionality of AlGaN-based MSM photodetectors are obtained. The observed threshold/saturation behavior is caused by a bias-dependent extraction of photoexcited holes from the Al{sub 0.5}Ga{sub 0.5}N/AlN interface. While present under bottom illumination for any AlGaN layer thickness, under top illumination this mechanism influences the EQE-bias characteristics only for thin layers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Angular dependence of optical modes in metal-insulator-metal coupled quantum well infrared photodetector

We report the dependence of the near-field optical modes in metal-insulator-metal quantum well infrared photodetector (MIM-QWIP) on the incident angles. Three optical modes are observed and attributed to the 2nd- and the 3rd-order surface plasmon polariton (SPP) modes and the localized surface polariton (LSP) mode. In addition to the observation of a responsivity enhancement of 14 times by the LSP mode, the varying pattern of the three modes against the incident angle are revealed, in which the LSP mode is fixed while the 2nd SPP mode splits into two branches and the 3rd SPP mode red-shifts. The detailed mechanisms are analyzed and numerically simulated. The results fit the experiments very well, demonstrating the wavevector coupling effect between the incident light and the metal gratings on the SPP modes. Our work will pave the way to fully understanding the influence of incident angles on a detector’s response for applying the MIM-QWIP to focal plane arrays.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Jumping liquid metal droplet in electrolyte triggered by solid metal particles

We report the electron discharge effect due to point contact between liquid metal and solid metal particles in electrolyte. Adding nickel particles induces drastic hydrogen generating and intermittent jumping of a sub-millimeter EGaIn droplet in NaOH solution. Observations from different orientations disclose that such jumping behavior is triggered by pressurized bubbles under the assistance of interfacial interactions. Hydrogen evolution around particles provides clear evidence that such electric instability originates from the varied electric potential and morphology between the two metallic materials. The point-contact-induced charge concentration significantly enhances the near-surface electric field intensity at the particle tips and thus causes electric breakdown of the electrolyte.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Focusing of light at the nanoapex of a metal microtip located above the plane of a dielectric or metal

We study the focusing of the optical electromagnetic radiation energy into a nanoscale spatial region in the vicinity of the nanoapex of a metal microtip located near a dielectric or metal plane. Focusing arises from a symmetric convergence of a surface plasmon TM wave at the nanoapex. The metal boundary near the nanoapex is approximated by a paraboloid of revolution. A numerical model, based on the method of moments, is developed to find the electric field near the tip apex. Calculation results show that, compared to a single tip, the presence of a plane leads to an additional concentration of the electric field near the nanoapex. (nanofocusing)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

A multiple burst accretion model to describe the metallicity distributions and stellar mass–Metallicity relation for local dwarf galaxies

A one parameter model to describe the individual metallicity distributions and stellar mass–metallicity relation for dwarf galaxies is presented. This multiple-burst model is based on an accretion scenario, accomodates the observational constraint between z-bar and σ{sub z}{sup 2} recently established by Leaman, and predicts a slope consistent with the stellar mass–metallicity relation of Kirby et al. who showed that the local group dwarf spheroidal and dwarf irregular galaxies lie on the same relation. One interpretation of the model is that it describes star formation occuring either in gas rich mergers or at the intersection of colliding gas streams.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

METAL-POOR STARS OBSERVED WITH THE MAGELLAN TELESCOPE. I. CONSTRAINTS ON PROGENITOR MASS AND METALLICITY OF AGB STARS UNDERGOING s-PROCESS NUCLEOSYNTHESIS

We present a comprehensive abundance analysis of two newly discovered carbon-enhanced metal-poor (CEMP) stars. HE 2138-3336 is a s-process-rich star with [Fe/H] = -2.79, and has the highest [Pb/Fe] abundance ratio measured thus far, if non-local thermodynamic equilibrium corrections are included ([Pb/Fe] = +3.84). HE 2258-6358, with [Fe/H] = -2.67, exhibits enrichments in both s- and r-process elements. These stars were selected from a sample of candidate metal-poor stars from the Hamburg/ESO objective-prism survey, and followed up with medium-resolution (R {approx} 2000) spectroscopy with GEMINI/GMOS. We report here on derived abundances (or limits) for a total of 34 elements in each star, based on high-resolution (R {approx} 30, 000) spectroscopy obtained with Magellan-Clay/MIKE. Our results are compared to predictions from new theoretical asymptotic giant branch (AGB) nucleosynthesis models of 1.3 M{sub Sun} with [Fe/H] = -2.5 and -2.8, as well as to a set of AGB models of 1.0 to 6.0 M{sub Sun} at [Fe/H] = -2.3. The agreement with the model predictions suggests that the neutron-capture material in HE 2138-3336 originated from mass transfer from a binary companion star that previously went through the AGB phase, whereas for HE 2258-6358, an additional process has to be taken into account to explain its abundance pattern. We find that a narrow range of progenitor masses (1.0 {<=} M(M{sub Sun }) {<=} 1.3) and metallicities (-2.8 {<=} [Fe/H] {<=}-2.5) yield the best agreement with our observed elemental abundance patterns.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Insulator–metal transition induced by electric voltage in a ruthenate Mott insulator

We report the observation of electric-voltage induced insulator–metal phase transition in a ruthenate Mott insulator Ca 3 (Ru 0.9 Ti 0.1 ) 2 O 7 . We show that the electric field effect dominates and leads to a sharp phase transition at measurement temperatures far below the Mott transition, whereas the thermal effect becomes more significant and broadens the phase transition as the measurement temperature approaches the insulator–metal transition. Here, the electric field induced insulator–metal transition is presumably attributed to the avalanche breakdown of the correlated insulating state when driven out of equilibrium. This work highlights the strategy of using electric voltage to control the phase transition of this system in addition to other nonthermal parameters such as magnetic field and pressure reported previously.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗