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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 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Schottky-contacted vertically self-aligned ZnO nanorods for hydrogen gas nanosensor applications

Vertically well aligned ZnO nanorods (NRs) were grown on Si(100) substrate using RF magnetron sputtering technique. Scanning electron microscopy images confirms uniform distribution of NRs on 2 in. wafer with average diameter, height and density being ∼75 nm, ∼850 nm, and ∼1.5 × 10{sup 10} cm{sup −2}, respectively. X-ray diffraction reveals that the ZnO NRs are grown along c-axis direction with wurtzite crystal structure. Cathodoluminescence spectroscopy, which shows a single strong peak around 3.24 eV with full width half maxima 130 meV, indicates the high crystalline and optical quality of ZnO and very low defect density. Vertically aligned nanosensors were fabricated by depositing gold circular Schottky contacts on ZnO NRs. Resistance responses of nanosensors were observed in the range from 50 to 150 °C in 1% and 5% hydrogen in argon environment, which is below and above the explosive limit (4%) of hydrogen in air. The nanosensor's sensitivity increases from 11% to 67% with temperature from 50 to 150 °C and also shows fast response time (9–16 s) and moderate recovery time (100–200 s). A sensing mechanism is proposed based on Schottky barrier changes at heterojunctions and change in depletion region of NRs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Band alignment of HfO{sub 2}/multilayer MoS{sub 2} interface determined by x-ray photoelectron spectroscopy: Effect of CHF{sub 3} treatment

The energy band alignment between HfO{sub 2}/multilayer (ML)-MoS{sub 2} was characterized using high-resolution x-ray photoelectron spectroscopy. The HfO{sub 2} was deposited using an atomic layer deposition tool, and ML-MoS{sub 2} was grown by chemical vapor deposition. A valence band offset (VBO) of 1.98 eV and a conduction band offset (CBO) of 2.72 eV were obtained for the HfO{sub 2}/ML-MoS{sub 2} interface without any treatment. With CHF{sub 3} plasma treatment, a VBO and a CBO across the HfO{sub 2}/ML-MoS{sub 2} interface were found to be 2.47 eV and 2.23 eV, respectively. The band alignment difference is believed to be dominated by the down-shift in the core level of Hf 4d and up-shift in the core level of Mo 3d, or the interface dipoles, which caused by the interfacial layer in rich of F.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Self-aligned grating couplers on template-stripped metal pyramids via nanostencil lithography

We combine nanostencil lithography and template stripping to create self-aligned patterns about the apex of ultrasmooth metal pyramids with high throughput. Three-dimensional patterns such as spiral and asymmetric linear gratings, which can couple incident light into a hot spot at the tip, are presented as examples of this fabrication method. Computer simulations demonstrate that spiral and linear diffraction grating patterns are both effective at coupling light to the tip. The self-aligned stencil lithography technique can be useful for integrating plasmonic couplers with sharp metallic tips for applications such as near-field optical spectroscopy, tip-based optical trapping, plasmonic sensing, and heat-assisted magnetic recording.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Behavior of oxidized platinum nanoparticles on an aligned carbon nanotube forest

We observed and analyzed the behavior of platinum nanoparticles (PtNPs) supported on aligned-carbon nanotubes (CNTs) at high temperatures by X-ray photoelectron spectroscopy and high-resolution transmission electron microscope observations. We found that the PtNPs moved toward the inner-side along each CNT on which they were deposited. The mechanism of this behavior is related to the redox reaction of Pt with the carbon atoms in the CNT. We also performed in-situ observation of this process at a high temperature using an environmental transmission electron microscope under an oxygen atmosphere. We found that the PtNPs penetrated down into a high-density aligned CNT forest along the tube axis and that the PtNPs changed their shape to fit the structure of the CNTs during their movement.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Alignment, orientation, and Coulomb explosion of difluoroiodobenzene studied with the pixel imaging mass spectrometry (PImMS) camera

Laser-induced adiabatic alignment and mixed-field orientation of 2,6-difluoroiodobenzene (C 6 H 3 F 2 I) molecules are probed by Coulomb explosion imaging following either near-infrared strong-field ionization or extreme-ultraviolet multi-photon inner-shell ionization using free-electron laser pulses. The resulting photoelectrons and fragment ions are captured by a double-sided velocity map imaging spectrometer and projected onto two position-sensitive detectors. The ion side of the spectrometer is equipped with a pixel imaging mass spectrometry camera, a time-stamping pixelated detector that can record the hit positions and arrival times of up to four ions per pixel per acquisition cycle. Thus, the time-of-flight trace and ion momentum distributions for all fragments can be recorded simultaneously. Finally, we show that we can obtain a high degree of one-and three-dimensional alignment and mixed-field orientation and compare the Coulomb explosion process induced at both wavelengths

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

Probing the band alignment in rectifying SrIrO 3 /Nb:SrTiO 3 heterostructures

Here, we have examined the band alignment in SrIrO 3 /Nb:SrTiO 3 (001) heterojunctions at room temperature using three independent techniques: current–voltage and capacitance–voltage measurements and internal photoemission spectroscopy. We find near-ideal rectifying behavior across the junction, which provides the opportunity to establish the band alignment via Schottky barrier height extractions in the metal-semiconductor junction approximation. The Schottky barrier height deduced from these measurements agrees well with each other within ~14%, with an average value of 1.44 ± 0.11 eV. These results provide a foundation for designing oxide heterostructures to harness the strong spin-orbit coupling and electrochemical properties of strontium iridate.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Angular alignment and fidelity of neutron phase-gratings for improved interferometer fringe visibility

The recent development of phase-grating moiré neutron interferometry promises a wide range of impactful experiments from dark-field imaging of material microstructure to precise measurements of fundamental constants. However, the contrast of 3% obtained using this moiré interferometer was well below the theoretical prediction of 30% using ideal gratings. It is suspected that non-ideal aspects of the phase-gratings was a leading contributor to this deficiency and that phase-gratings needed to be quantitatively assessed and optimized. Here we characterize neutron diffraction from phase-gratings using Bragg diffraction crystals to determine the optimal phase-grating orientations. We show well-defined diffraction peaks and explore perturbations to the diffraction peaks and the effects on interferometer contrast as a function of grating alignment. This technique promises to improve the contrast of the grating interferometers by providing in-situ aides to grating alignment.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

One-directional thermal transport in densely aligned single-wall carbon nanotube films

Individual carbon nanotubes (CNTs) possess extremely high thermal conductivities. However, the thermal conductivities and their anisotropy of macroscopic assemblies of CNTs have so far remained small. Here, we report the results of directional thermal transport measurements on a nearly perfectly aligned CNT film fabricated via controlled vacuum filtration. We found the thermal conductivity to be 43 ± 2.2 W m -1 K -1 with a record-high thermal anisotropy of 500. From the temperature dependence of the thermal conductivity and its agreement with the atomistic phonon transport calculation, we conclude that the effect of intertube thermal resistance on heat conduction in the alignment direction is negligible because of the large contact area between CNTs. These observations thus represent ideal unidirectional thermal transport, i.e., the thermal conductivity of the film is determined solely by that of individual CNTs.

42 ENGINEERING↗
DOE OSTI2020

Rotational self-alignment of graphene seeds for nanoribbon synthesis on Ge(001) via chemical vapor deposition

The chemical vapor deposition of CH 4 on Ge(001) results in the anisotropic synthesis of graphene nanoribbons that are aligned to Ge$\langle$110$\rangle$ and have faceted armchair edges, sub-10 nm widths, and lengths greater than 100 nm. The utilization of small graphene seeds to initiate nanoribbon synthesis provides control over the nanoribbon placement and orientation. However, in order to exclusively grow nanoribbons and suppress the concomitant growth of lower aspect ratio crystals, it is imperative to control the crystallographic orientation of the seeds with respect to the Ge lattice. Here, we demonstrate that when seeds are less than 18 nm in diameter, they are able to rotate upon annealing at 910 °C prior to nanoribbon synthesis. The effect of this rotation on the resulting nanoribbons’ orientation is characterized as a function of the diameter and initial crystallographic orientation of the seeds. The seeds preferentially rotate to an orientation in which an armchair direction of their lattice is parallel to Ge$\langle$110$\rangle$—subsequently maximizing the anisotropy in growth kinetics. By exploiting this seed rotation phenomenon, we demonstrate the fabrication of seamless nanoribbon meshes and gain understanding that will affect future efforts to create arrays of unidirectionally aligned nanoribbons.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Roles of bandgrading, lifetime, band alignment, and carrier concentration in high-efficiency CdSeTe solar cells

CdSeTe alloying has significantly increased the efficiency of CdTe-based solar technology. Here, computational modeling compares how different CdSeTe bandgrading, carrier lifetimes, band alignment, and carrier concentrations contribute to transport, recombination, and performance. We find that the gain in photocurrent caused by bandgap narrowing alone is insufficient to describe experimental efficiency gains. Performance can be increased by adjusting CdSeTe compositions and bandgrading depths. However, these performance gains are small relative to the contributions of enhanced lifetime by Se alloying, which can explain record cell efficiency gains with minimal open-circuit voltage loss despite significant bandgap narrowing. Similarly, CdSeTe band alignment shifts can significantly increase performance if front interface recombination is prevalent. For a wide range of CdSeTe grading profiles, the hole density is a critical component to achieve efficiencies exceeding 25%.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Pressure–strain interaction. II. Decomposition in magnetic field-aligned coordinates

In weakly collisional and collisionless magnetized plasmas, the pressure–strain interaction describes the rate of conversion between bulk flow and thermal energy density. In this study, we derive an analytical expression for the pressure–strain interaction in a coordinate system with an axis aligned with the local magnetic field. The result is eight groups of terms corresponding to different physical mechanisms that can contribute to the pressure–strain interaction. We provide a physical description of each term. Here, the results are immediately of interest to weakly collisional and collisionless magnetized plasmas and the fundamental processes that happen therein, including magnetic reconnection, magnetized plasma turbulence, and collisionless shocks. The terms in the field-aligned coordinate decomposition are likely accessible to measurement with satellite observations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Dangling bonds, the charge neutrality level, and band alignment in semiconductors

We present a systematic study of the electronic properties of dangling bonds (DBs) in a variety of semiconductors and examine the relationship between DBs and the charge neutrality level (CNL) in the context of band alignments of semiconductors. We use first-principles calculations based on density functional theory to assess the energetics of DBs in a set of diamond-structure group-IV and III–V or II–VI zinc-blende-structure semiconductors, considering both cation and anion-derived states. We examine the charge-state transition levels of DBs to assess whether they can serve as a CNL to align band structures, by comparing with offsets calculated from interface calculations. Our results show that this approach for evaluating the CNL yields quantitative results for band offsets and provides useful insights. We discuss the relation with alternative approaches for determination of CNLs based on branch-point energies or transition levels of interstitial hydrogen.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Electric fields imbue enzyme reactivity by aligning active site fragment orbitals

It is broadly recognized that intramolecular electric fields, produced by the protein scaffold and acting on the active site, facilitate enzymatic catalysis. This field effect can be described by several theoretical models, each of which is intuitive to varying degrees. In this contribution, we show that a fundamental effect of electric fields is to generate electrostatic potentials that facilitate the energetic alignment of reactant frontier orbitals. We apply this model to demystify the impact of electric fields on high-valent iron–oxo heme proteins: catalases, peroxidases, and peroxygenases/monooxygenases. Specifically, we show that this model easily accounts for the observed field-induced changes to the spin distribution within peroxidase active sites and explains the transition between epoxidation and hydroxylation pathways seen in Cytochrome P450 active site models. Thus, for the intuitive interpretation of the chemical effect of the field, the strategy involves analyzing the response of the orbitals of active site fragments, and their energetic alignment. We note that the energy difference between fragment orbitals involved in charge redistribution acts as a measure for the chemical hardness/softness of the reactive complex. This measure, and its sensitivity to electric fields, offers a single parameter model from which to quantitatively assess the effects of electric fields on reactivity and selectivity. Thus, the model provides an additional perspective to describe electrostatic preorganization and offers ways for its manipulation.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2012

MODELING THE JET KINEMATICS OF THE BLACK HOLE MICROQUASAR XTE J1550-564: A CONSTRAINT ON SPIN-ORBIT ALIGNMENT

Measurements of black hole spin made using the continuum-fitting method rely on the assumption that the inclination of the black hole's spin axis to our line of sight is the same as the orbital inclination angle i of the host binary system. The X-ray and radio jet data available for the microquasar XTE J1550-564 offer a rare opportunity to test this assumption. Following the work of others, we have modeled these data and thereby determined the inclination angle {theta} of the jet axis, which is presumed to be aligned with the black hole's spin axis. We find {theta} Almost-Equal-To 71 Degree-Sign and place an upper limit on the difference between the spin and orbital inclinations of |{theta} - i| < 12 Degree-Sign (90% confidence). Our measurement tests for misalignment along the line of sight while providing no constraint perpendicular to this plane. Our constraint on the misalignment angle supports the prediction that the spinning black hole in XTE J1550-564 has aligned itself with the orbital plane and provides support for the measurement of its spin via the continuum-fitting method. Our conclusions are based on a simple and reasonable model of a pair of symmetric jets propagating into a low-density cavity whose western wall is Almost-Equal-To 20% closer to XTE J1550-564 than its eastern wall.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

No evidence for Bardeen-Petterson alignment in GRMHD simulations and semi-analytic models of moderately thin, prograde, tilted accretion disks

In this paper, we introduce the first results that use data extracted directly from numerical simulations as inputs to the analytic twisted disk model of Zhuravlev and Ivanov. In both numerical and analytic approaches, fully relativistic models of tilted and twisted disks having a moderate effective viscosity around a slowly rotating Kerr black hole are considered. Qualitatively, the analytic model demonstrates the same dynamics as the simulations, although with some quantitative offset. Namely, the general relativistic magnetohydrodynamic simulations typically give smaller variations of tilt and twist across the disk. When the black hole and the disk rotate in the same direction, the simulated tilted disk and analytic model show no sign of Bardeen-Petterson alignment, even in the innermost parts of the disk where the characteristic time for relaxation to a quasi-stationary configuration is of the same order as the computation time. In the opposite case, when the direction of the disk's rotation is opposite to that of the black hole, a partial alignment is observed, in agreement with previous theoretical estimates. Thus, both fully numerical and analytic schemes demonstrate that the Bardeen-Petterson effect may not be possible for the case of prograde rotation provided that disk's effective viscosity is sufficiently small. This may have implications in modeling of different astrophysical phenomena such as disk spectra and jet orientation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

FORMATION AND ALIGNMENT OF ELONGATED, FRACTAL-LIKE WATER-ICE GRAINS IN EXTREMELY COLD, WEAKLY IONIZED PLASMA

Elongated, fractal-like water-ice grains are observed to form spontaneously when water vapor is injected into a weakly ionized laboratory plasma formed in a background gas cooled to an astrophysically relevant temperature. The water-ice grains form in 1–2 minutes, levitate with regular spacing, and are aligned parallel to the sheath electric field. Water-ice grains formed in plasma where the neutrals and ions have low mass, such as hydrogen and helium, are larger, more elongated, and more fractal-like than water-ice grains formed in plasmas where the neutrals and ions have high mass such as argon and krypton. Typical aspect ratios (length to width ratio) are as great as 5 while typical fractal dimensions are ∼1.7. Water-ice grain lengths in plasmas with low neutral and ion masses can be several hundred microns long. Infrared absorption spectroscopy reveals that the water-ice grains are crystalline and so are similar in constitution to the water-ice grains in protoplanetary disks, Saturn’s rings, and mesospheric clouds. The properties and behavior of these laboratory water-ice grains may provide insights into morphology and alignment behavior of water-ice grains in astrophysical dusty plasmas.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Intermittency and alignment in strong RMHD turbulence

We develop an analytic model of intermittent, three-dimensional, strong, reduced magnetohydrodynamic (RMHD) turbulence with zero cross helicity. We take the fluctuation amplitudes to have a log-Poisson distribution and incorporate into the model a new phenomenology of scale-dependent dynamic alignment between the Elsässer variables z{sup ±}. We find that the structure function 〈|Δz{sub λ}{sup ±}|{sup n}〉 scales as λ{sup 1−β{sup n}}, where Δz{sub λ}{sup ±} is the variation in z{sup ±} across a distance λ perpendicular to the magnetic field. We calculate the value of β to be ≃ 0.69 based on our assumption that the most intense coherent structures are two-dimensional with a volume filling factor ∝ λ. Two consequences of this structure-function scaling are that the total-energy power spectrum is ∝ k{sub ⊥}{sup −1.52} and the kurtosis of the fluctuations is ∝ λ{sup −0.27}. Our model resolves the problem that alignment angles defined in different ways exhibit different scalings. Specifically, we find that the energy-weighted average angle between the velocity and magnetic-field fluctuations is ∝ λ{sup 0.21}, the energy-weighted average angle between Δz{sup +} and Δz{sup −} is ∝ λ{sup 0.10}, and the average angle between Δz{sup +} and Δz{sup −} without energy weighting is ∝ [ln(L/λ)]{sup −1/2} when L/λ≫1, where L is the outer scale. We also carry out a direct numerical simulation of RMHD turbulence. The scalings in our model are similar to the scalings in this simulation as well as the structure-function scalings observed in the slow solar wind.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Chemical Solution Derived Planarization Layers for Highly Aligned IBAD MgO Templates

The main goal of this research is to develop a chemical solution derived planarization layer to fabricate highly aligned IBAD-MgO templates for the development of high temperature superconductor (HTS) based coated conductors. The standard IBAD-MgO template needs an additional electrochemical polishing step of the mechanically polished 50- m-thick Hastelloy C-276 substrates to ensure a flat and smooth surface for subsequent growth of multi-layer buffer architectures, which include: sputtered 80-nm Al2O3; sputtered 7-nm Y2O3; IBAD 10-nm MgO; sputtered 30-nm homo-epi MgO; and sputtered 30-nm LaMnO3 (LMO) layers. We have successfully developed a solution planarization layer that removes the electrochemical polishing step and also acts as a barrier layer. Crack-free, smooth Al2O3 layers were prepared on mechanically polished Hastelloy substrates using a chemical solution process. A nearly 10-15-nm thick Al2O3 layer was formed with each coating and the coating was repeated several times to achieve the desired film thickness with intermediate heat-treatments after each coating. The Al2O3 planarization layer significantly reduced the surface roughness of the substrate. The average surface roughness value, Ra for a starting substrate was 9-10 nm. After 8 coatings of Al2O3 layer, the Ra was reduced to 2 nm. Highly aligned IBAD-MgO layers with out-of-plane and in-plane textures comparable to the standard IBAD-MgO layers were successfully deposited on top of the solution planarization Al2O3 layers with an Y2O3 nucleation layer using a reel-to-reel ion-beam sputtering system. Both homo-epi MgO and LMO layers were subsequently deposited on the IBAD-MgO layers using rf sputtering to complete the buffer stack required for the growth of HTS films. YBa2Cu3O7- (YBCO) films with a thickness of 0.8 m deposited on these IBAD-MgO templates by pulsed laser deposition showed a high self-field critical current density, Jc of 3.04 MA/cm2 at 77 K and 6.05 MA/cm2 at 65 K. These results demonstrate that a low-cost chemical-solution-based, high-throughput Al2O3 planarization layer can remove the electro-polishing step and replace sputtered Al2O3 layers for the production of high Jc YBCO-coated conductors.

Paranthaman, Mariappan Parans↗