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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 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Radiothermoluminescence of Oxide Nanostructures on Surfaces of Radiation-Oxidized Aluminum and Beryllium

The formation dynamics of oxide nanostructures on surfaces of oxidized aluminum and beryllium with water at room temperature and under radiation action has been studied by the radiothermoluminescence method. The role of intermediate surface active particles in the dynamics of changes of the oxidation process of aluminum and beryllium in an absorbed dose range of 0.5–120 kGy has been considered. It is established that, when nanostructure films are formed, molecular oxygen and other oxygen-containing ion-radical groups generated by γ irradiation play the main role. The radiation-oxidation kinetics of aluminum and beryllium is studied, and their chemisorption passivation is revealed. A possible mechanism of this process is discussed.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Multiple trapping on a comb structure as a model of electron transport in disordered nanostructured semiconductors

A model of dispersive transport in disordered nanostructured semiconductors has been proposed taking into account the percolation structure of a sample and joint action of several mechanisms. Topological and energy disorders have been simultaneously taken into account within the multiple trapping model on a comb structure modeling the percolation character of trajectories. The joint action of several mechanisms has been described within random walks with a mixture of waiting time distributions. Integral transport equations with fractional derivatives have been obtained for an arbitrary density of localized states. The kinetics of the transient current has been calculated within the proposed new model in order to analyze time-of-flight experiments for nanostructured semiconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Dramatic dwindling of the power spectrum of high order harmonics by shrinking of the gap size in bowtie nanostructures

Our work is based on high harmonic generation in a gaseous medium (helium ion), by exploiting gold bowtie nanostructures as laser field amplifiers. As the result of emission of a laser pulse, the wave function of the atom varies with time; so, it is necessary to solve 1D time-dependent Schrödinger equation by means of split operator method. By illumination of a short duration, long wavelength three color laser pulse inside the gap, the enhanced field not only changes with time, but also varies in space. In this work we considered this space inhomogeneity in linear and nonlinear schemes. We show that in nonlinear case, the plateau region is more extended. We also show that in larger gaps, cutoff occurs on higher frequencies. But limitation of electron motion in bowtie nanostructures leads to the choice of an optimum 16 nm gap size in our case. We predict that, by the superposition of supercontinuum harmonics, a 26 attosecond pulse can be generated.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Simulation and optical spectroscopy of a DC discharge in a CH{sub 4}/H{sub 2}/N{sub 2} mixture during deposition of nanostructured carbon films

Two-dimensional numerical simulations of a dc discharge in a CH{sub 4}/H{sub 2}/N{sub 2} mixture in the regime of deposition of nanostructured carbon films are carried out with account of the cathode electron beam effects. The distributions of the gas temperature and species number densities are calculated, and the main plasmachemical kinetic processes governing the distribution of methyl radicals above the substrate are analyzed. It is shown that the number density of methyl radicals above the substrate is several orders of magnitude higher than the number densities of other hydrocarbon radicals, which indicates that the former play a dominant role in the growth of nanostructured carbon films. The model is verified by comparing the measured optical emission profiles of the H(n ≡ 3), C{sub 2}{sup *}, CH*, and CN* species and the calculated number densities of excited species, as well as the measured and calculated values of the discharge voltage and heat fluxes onto the electrodes and reactor walls. The key role of ion–electron recombination and dissociative excitation of H{sub 2}, C{sub 2}H{sub 2}, CH{sub 4}, and HCN molecules in the generation of emitting species (first of all, in the cold regions adjacent to the electrodes) is revealed.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Fast optical detecting media based on semiconductor nanostructures for recording images obtained using charges of free photocarriers

Available published data on the properties of optical recording media based on semiconductor structures are reviewed. The principles of operation, structure, parameters, and the range of application for optical recording media based on MIS structures formed of photorefractive crystals with a thick layer of insulator and MIS structures with a liquid crystal as the insulator (the MIS LC modulators), as well as the effect of optical bistability in semiconductor structures (semiconductor MIS structures with nanodimensionally thin insulator (TI) layer, M(TI)S nanostructures). Special attention is paid to recording media based on the M(TI)S nanostructures promising for fast processing of highly informative images and to fabrication of optoelectronic correlators of images for noncoherent light.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Optical properties of nanostructured lead sulfide films with a D0{sub 3} cubic structure

The optical transmittance of nanostructured cubic PbS (lead sulfide) films with a D0{sub 3} structure is measured in the wavelength range 300 to 3200 nm. The film thicknesses are in the range {approx}120 to {approx}400 nm. Electron microscopy of the microstructure shows that about half of the films' particles are 60 nm or smaller in size. As the average particle size is decreased, the band gap E{sub g} increases from 0.85 to 1.5 eV, being noticeably larger than the band gap of coarse-grained PbS, 0.41 eV. This is indicative of a blue shift of the optical absorption band in nanostructured PbS films.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Prospects for the pulsed electrodeposition of zinc-oxide hierarchical nanostructures

Studies into the effect of the conditions of pulsed electrodeposition upon the structural and sub-structural parameters, morphology, and optical properties of ZnO-crystallite arrays make it possible to establish those parameters optimal for the formation of ZnO nanorods oriented normally to the substrate surface. These parameters are as follows: an electrolyte temperature of 70-85 Degree-Sign C, duty cycle of 40%, and a pulse-repetition frequency of 2 Hz. The nanorod dimensions can be varied by heating or cooling the electrolyte within the above-indicated limits; as a result, small-sized nanorods can be electrically deposited on the surface of larger nanorods to form hierarchical nanostructures. By varying the duty cycle, it is possible to modify the surface morphology of the arrays up to the formation of mesoporous ZnO networks. In combination with ZnO nanorods, such networks are capable of forming hierarchical nanostructures with large specific areas.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Electrochemical characteristics of nanostructured silicon anodes for lithium-ion batteries

High-aspect periodic structures with thin vertical walls are studied as regards their applicability as negative electrodes of lithium-ion batteries. The nanostructures are fabricated from single-crystal silicon using photolithography, electrochemical anodization, and subsequent anisotropic shaping. The capacity per unit of the visible surface area of the electrode and the specific internal surface area are compared for structures of varied architecture: 1D (wires), 2D (zigzag walls), and 3D structures (walls forming a grid). Main attention is given to testing the endurance of anodes based on zigzag and grid structures, performed by galvanostatic cycling in half-cells with a lithium counter electrode. The influence exerted by the geometric parameters of the structures and by the testing mode on the degradation rate is determined. It is shown that the limiting factor of the lithiation and delithiation processes is diffusion. The endurance of an electrode dramatically increases when the charging capacity is limited to ∼1000 mA h/g. In this case, nanostructures with 300-nm-thick walls, which underwent cyclic testing at a rate of 0.36C, retain a constant discharge capacity and a Coulomb efficiency close to 100% for more than 1000 cycles.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Calculation of the thermal conductivity of nanostructured Bi{sub 2}Te{sub 3} with the real phonon spectrum taken into account

The results of calculations of lattice thermal conductivity in bismuth telluride are presented and its decrease in a nanostructured material due to boundary scattering is estimated. Calculation is carried out using the lattice-dynamics method with the real phonon spectrum and the phonon–phonon interaction taken into account. Its results well agree with experimental data for the crystalline material. The estimates for the nanostructured material give a decrease in the thermal conductivity by 30% for grain sizes of 20 nm. Calculations by this method are compared with those obtained using approximate methods of description of the spectrum and scattering processes. It is shown that the least difference in the estimates (about 10%) can be obtained when using the approximation of a constant matrix element of phonon–phonon scattering with correction of the frequency dependence for acoustic modes.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Structure and Properties of Nanostructured YBa{sub 2}Cu{sub 3}O{sub 7–δ}, BiFeO{sub 3}, and Fe{sub 3}O{sub 4}

The development of new nanostructured materials based on YBa{sub 2}Cu{sub 3}O{sub 7–δ}, BiFeO{sub 3}, and Fe{sub 3}O{sub 4} compounds is considered. The structure, morphology, and properties of these materials are studied. The possibilities of fabricating YBa{sub 2}Cu{sub 3}O{sub 7–δ} ceramics with given densities from nanopowders in a single stage by an energy efficient method and growing superconducting films of the same composition on a silicon substrate (on a SiO{sub 2} layer) are demonstrated. The technique for fabricating BiFeO{sub 3} nanopowder, making it possible to obtain nanostructured ceramics without additional accompanied phases commonly forming during BiFeO{sub 3} synthesis is developed. Two methods of the single-stage synthesis of Fe{sub 3}O{sub 4} nanopowder are presented: burning of nitrate-organic precursors and the electrochemical three-electrode method in which one of the electrodes, i.e., an anode containing scrap iron and slurry, is used as an expendable material.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Study of the Effective Refractive Index Profile in Self-Assembling Nanostructured ITO Films

The optical characteristics and structural features of self-assembling nanostructured indium–tin oxide (ITO) films deposited onto substrates heated above 400°C are studied. Estimations of the distribution profiles of the material and effective refractive index in the films under study are obtained by computer simulation of a medium having spectral dependences of light transmission and reflection coefficients closest to those observed in the experiment. These results are in good agreement with scanning electron microscopy data for these films and make it possible to not only confirm the gradient character of such coatings but also restore some of its features. The further overgrowth of voids in self-assembling nanostructured films by means of the deposition of an additional material amount by magnetron sputtering at room temperature leads to a variation in the effective refractive index profile in them. Thus, it is possible to form transparent conductive films with an efficient refractive index profile adjusted for specific tasks. Adjustment of the refractive index profile in self-assembled ITO films acquires a special value for designing coatings with specified properties by virtue of a limited set of transparent conductive materials.

36 MATERIALS SCIENCE↗
DOE OSTI2019

On the Morphology and Optical Properties of Molybdenum Disulfide Nanostructures from a Monomolecular Layer to a Fractal-Like Substructure

The impact of layer thickness on the morphology and optical properties of MoS{sub 2} nanostructures, including monomolecular layers, formed by the carrier-gas-assisted transport of sulfur vapor to the hot zone of a reactor containing metallic molybdenum and subsequent deposition on mica (muscovite) substrates is investigated. Molybdenum disulfide nanostructures of different thicknesses grown at different temperatures of gas-transport synthesis are studied by atomic-force microscopy, optical absorption spectroscopy, and Raman spectroscopy. It is found that synthesis at temperatures of 525–600°C makes it possible to obtain monomolecular MoS{sub 2} layers containing trigonal domains and featuring direct-gap optical transitions at 1.84 eV with the formation of excitons at room temperature. Fractal-type MoS{sub 2} substructures are obtained for the first time. The frequencies of intralayer and interlayer vibrational modes E{sub 2g}{sup 1} and A{sub 1g}, respectively, in their Raman spectra (377.5 and 403.8 cm{sup –1}, respectively) differ both from the corresponding values for a monomolecular layer and the known frequencies for bulk samples. The frequency of the E{sub 2g}{sup 1} intralayer mode in these samples (377.5 cm{sup –1}) is the lowest of all previously reported.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Carbon Nanostructures on a Semiconductor Substrate

The analytical expressions for the density of states and the occupation numbers are obtained for simple models of carbon nanostructures (graphene–boron nitride lateral heterostructure, decorated zigzag edges of semi-infinity graphene and graphene nanoribbons, and decorated carbyne). The main attention is placed to the strong-coupling regime of the nanostructures with a semiconducting substrate. The numerical estimations are given for the SiC substrate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Fabrication of nanostructured silicon surface using selective chemical etching

A two-stage process based on selective chemical etching induced by metal nanoclusters is used to fabricate nanostructured surfaces of silicon plates with a relatively low reflectance. At silicon surfaces covered with silver nanoclusters, the SERS effect is observed for rhodamine concentrations of about 10{sup –12} M. At certain technological parameters, the depth of the nanostructured layer weakly depends on the conditions for the two-stage etching, in particular, etching time. Under otherwise equal conditions for etching, the rate of the formation of textured layer in the p-type silicon is two times greater than the formation rate in the n-type silicon.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

The Influence of Structural Factors on Strength and Electric Conductivity of Bulk Nanostructured Copper Alloys

The structural parameters ensuring high strength and low electrical resistance have been analyzed in Cu–Cr alloys exposed to severe plastic deformation (SPD) and aging. Specific resistance and strength characteristics have been modeled for a Cu–0.5 wt % Cr alloy with a nanostructure state caused by SPD. A comparison of modeling data with experimental values reveals the validity of the approach chosen for describing the above states. The formation of nanostructure states with high strength and low electrical resistance is shown to be due to simultaneous effects depending on the treatment conditions of materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Columnar Niobium Oxide Nanostructures: Mechanism of Formation, Microstructure, and Electrophysical Properties

The morphology and microstructure of columnar niobium oxide nanostructures are studied and the dependences of their morphological sizes on anodizing voltages (100–450 V) and anodic alumina pore diameters (40–150 nm) are established. The features of ion transport during local anodization of niobium are studied and the transport numbers of electrolyte anions and niobium cations are calculated; a mechanism of formation and growth is proposed and the phase composition and electrophysical properties of columnar nanostructures are studied.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

A New Type of Carbon Nanostructure on a Vicinal SiС(111)-8° Surface

The electronic properties of the Ba/SiC/Si(111)-8° nanointerface have been studied for the first time by photoelectron spectroscopy using synchrotron radiation. The experiments are conducted in situ in superhigh vacuum on submonolayer Ba coatings of SiC/Si(111)-8° samples grown by the method of substitution of atoms. It is found that the adsorption of Ba causes strong changes in the spectrum of the C 1s core level. It is shown that the effect is due to the formation of a new, previously unknown carbon nanostructure. It is found that the nanostructure is formed exclusively on the SiC vicinal surfaces in the presence of stabilizing adsorbed Ba metal atoms and consists of carbon rings, in which the chemical bonds are close in nature to the bonds characteristic of aromatic compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Stability of Y–Ti–O precipitates in friction stir welded nanostructured ferritic alloys

Nanostructured ferritic alloys, which have complex microstructures which consist of ultrafine ferritic grains with a dispersion of stable oxide particles and nanoclusters, are promising materials for fuel cladding and structural applications in the next generation nuclear reactor. This paper evaluates microstructure of friction stir welded nanostructured ferritic alloys using electron microscopy and atom probe tomography techniques. Atom probe tomography results revealed that nanoclusters are coarsened and inhomogeneously distributed in the stir zone and thermomechanically affected zone. Three hypotheses on coarsening of nanoclusters are presented. Finally, the hardness difference in different regions of friction stir weld has been explained.

36 MATERIALS SCIENCE↗