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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 1,711 records · Page 95
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Controlling compositional homogeneity and crystalline orientation in Bi 0.8 Sb 0.2 thermoelectric thin films [Control of composition and crystallinity in Bi 0.8 Sb 0.2 thermoelectric thin films].

Controlling alloy composition, crystalline quality, and crystal orientation is necessary to achieve high thermoelectric performance in Bi 1-x Sb x thin films. These microstructural attributes are demonstrated in this letter via co-sputter deposition of Bi and Sb metals on Si/SiO 2 substrates followed by ex-situ post anneals ranging from 200 – 300 °C in forming gas with rapid cooling to achieve orientation along the trigonal axis. We show with cross-sectional transmission electron microscopy and energy-dispersive X-ray spectrometry that 50 – 95% of the Sb segregates at the surface upon exposure to air during transfer. This then forms a nanocrystalline Sb 2 O 3 layer upon annealing, leaving the bulk of the film primarily Bi metal which is a poor thermoelectric material. We demonstrate a SiN capping technique to eliminate Sb segregation and preserve a uniform composition throughout the thickness of the film. Given that the Bi 1-x Sb x solid solution melting point depends on the Sb content, the SiN cap allows one to carefully approach but not exceed the melting point during annealing. This leads to the strong orientation along the trigonal axis and high crystalline quality desired for thermoelectric applications.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Ligand-surface interactions and surface oxidation of colloidal PbSe quantum dots revealed by thin-film positron annihilation methods

Positron Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) measurements reveal modifications of the electronic structure and composition at the surfaces of PbSe quantum dots (QDs), deposited as thin films, produced by various ligands containing either oxygen or nitrogen atoms. In particular, the 2D-ACAR measurements on thin films of colloidal PbSe QDs capped with oleic acid ligands yield an increased intensity in the electron momentum density (EMD) at high momenta compared to PbSe quantum dots capped with oleylamine. Moreover, the EMD of PbSe QDs is strongly affected by the small ethylenediamine ligands, since these molecules lead to small distances between QDs and favor neck formation between near neighbor QDs, inducing electronic coupling between neighboring QDs. The high sensitivity to the presence of oxygen atoms at the surface can be also exploited to monitor the surface oxidation of PbSe QDs upon exposure to air. Finally, our study clearly demonstrates that positron annihilation spectroscopy applied to thin films can probe surface transformations of colloidal semiconductor QDs embedded in functional layers.

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

Reversible control of magnetism in La 0.67 Sr 0.33 MnO 3 through chemically-induced oxygen migration

We demonstrate reversible control of magnetization and anisotropy in La 0.67 Sr 0.33 MnO 3 films through interfacial oxygen migration. Gd metal capping layers deposited onto La 0.67 Sr 0.33 MnO 3 leach oxygen from the film through a solid-state redox reaction to form porous Gd 2 O 3 . X-ray absorption and polarized neutron reflectometry measurements show Mn valence alterations consistent with high oxygen vacancy concentrations, resulting in suppressed magnetization and increased coercive fields. Effects of the oxygen migration are observed both at the interface and also throughout the majority of a 40 nm thick film, suggesting extensive diffusion of oxygen vacancies. After Gd-capped La 0.67 Sr 0.33 MnO 3 is exposed to atmospheric oxygen for a prolonged period of time, oxygen diffuses through the Gd 2 O 3 layer and the magnetization of the La 0.67 Sr 0.33 MnO 3 returns to the uncapped value. In conclusion, these findings showcase perovskite heterostructures as ideal candidates for developing functional interfaces through chemically-induced oxygen migration.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Improved performance in vertical GaN Schottky diode assisted by AlGaN tunneling barrier

In a vertical GaN Schottky barrier diode, the free electron concentration n in the 6-μm-thick drift layer was found to greatly impact the diode reverse leakage current, which increased from 2.1 × 10{sup −7} A to 3.9 × 10{sup −4} A as n increased from 7.5 × 10{sup 14 }cm{sup −3} to 6.3 × 10{sup 15 }cm{sup −3} at a reverse bias of 100 V. By capping the drift layer with an ultrathin 5-nm graded AlGaN layer, reverse leakage was reduced by more than three orders of magnitude with the same n in the drift layer. We attribute this to the increased Schottky barrier height with the AlGaN at the surface. Meanwhile, the polarization field within the graded AlGaN effectively shortened the depletion depth, which led to the formation of tunneling current at a relatively small forward bias. The turn-on voltage in the vertical Schottky diodes was reduced from 0.77 V to 0.67 V—an advantage in reducing conduction loss in power switching applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Fabrication and characterization of p{sup +}-i-p{sup +} type organic thin film transistors with electrodes of highly doped polymer

Organic thin film transistors (OTFTs) have been explored because of their advantageous features such as light-weight, flexible, and large-area. For more practical application of organic electronic devices, it is very important to realize OTFTs that are composed only of organic materials. In this paper, we have fabricated p{sup +}-i-p{sup +} type of OTFTs in which an intrinsic (i) regioregular poly (3-hexylthiophene) (P3HT) layer is used as the active layer and highly doped p-type (p{sup +}) P3HT is used as the source and drain electrodes. The 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F{sub 4}-TCNQ) was used as the p-type dopant. A fabricating method of p{sup +}-i-p{sup +} OTFTs has been developed by using SiO{sub 2} and aluminum films as capping layers for micro-scaled patterning of the p{sup +}-P3HT electrodes. The characteristics of the OTFTs were examined using the photoelectron spectroscopy and electrical measurements. We demonstrated that the fabricated p{sup +}-i-p{sup +} OTFTs work with carrier injection through a built-in potential at p{sup +}/i interfaces. We found that the p{sup +}-i-p{sup +} OTFTs exhibit better FET characteristics than the conventional P3HT-OTFT with metal (Au) electrodes, indicating that the influence of a carrier injection barrier at the interface between the electrode and the active layer was suppressed by replacing the metal electrodes with p{sup +}-P3HT layers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Crystal structure controlled synthesis and characterization of copper sulfide nanoparticles

Phase pure, controlled crystal structure of digenite (Cu{sub 9}S{sub 5}) copper sulfide nanoparticles were synthesized by hot injection method at the temperature of 180°C. The mixture of Oleylamine, 1-Octadecene and 1-Dodecanethiol were taken as solvent as well as capping agents. The effect of the mixture of solvents on the phase formation and morphology of the synthesized nanoparticles were analysed. The nanocrystals were characterized using X-Ray diffraction (XRD) which confirms the presence of single phase rhombohedral digenite Cu{sub 9}S{sub 5} NPs, Morphological analysis clearly depicts the formation of hexagonal faceted Cu{sub 9}S{sub 5} NPs, Energy dispersive X-ray absorption spectroscopy (EDS) reveals the stoichiometric ratio of 1.8:1 for synthesized NPs. From the UV-Vis absorption spectroscopy the bandgap value of Cu{sub 1.8}S is found to be 1.71 eV. The presence of capping agents along the surface of the Cu{sub 9}S{sub 5} NPs was confirmed from FTIR analysis.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Emission-wavelength tuning of InAs quantum dots grown on nitrogen-δ-doped GaAs(001)

We studied the structural and photoluminescence (PL) characteristics of InAs quantum dots (QDs) grown on nitrogen (N) δ-doped GaAs(001). The emission wavelength for low-density N-δ doping exhibited a blueshift with respect to that for undoped GaAs and was redshifted with increasing N-sheet density. This behavior corresponded to the variation in the In composition of the QDs. N-δ doping has two opposite and competing effects on the incorporation of Ga atoms from the underlying layer into the QDs during the QD growth. One is the enhancement of Ga incorporation induced by the lattice strain, which is due to the smaller radius of N atoms. The other is an effect blocking for Ga incorporation, which is due to the large bonding energy of Ga-N or In-N. At a low N-sheet density, the lattice-strain effect was dominant, while the blocking effect became larger with increasing N-sheet density. Therefore, the incorporation of Ga from the underlying layer depended on the N-sheet density. Since the In-Ga intermixing between the QDs and the GaAs cap layer during capping also depended on the size of the as-grown QDs, which was affected by the N-sheet density, the superposition of these three factors determined the composition of the QDs. In addition, the piezoelectric effect, which was induced with increased accumulation of lattice strain and the associated high In composition, also affected the PL properties of the QDs. As a result, tuning of the emission wavelength from 1.12 to 1.26 μm was achieved at room temperature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Lung scintigraphy in differential diagnosis of peripheral lung cancer and community-acquired pneumonia

Ventilation/perfusion lung scintigraphy was performed in 39 patients with verified diagnosis of community-acquired pneumonia (CAP) and in 14 patients with peripheral lung cancer. Ventilation/perfusion ratio, apical-basal gradients of ventilation (U/L(V)) and lung perfusion (U/L(P)), and alveolar capillary permeability of radionuclide aerosol were determined based on scintigraphy data. The study demonstrated that main signs of CAP were increases in ventilation/perfusion ratio, perfusion and ventilation gradient on a side of the diseased lung, and two-side increase in alveolar capillary permeability rate for radionuclide aerosol. Unlike this, scintigraphic signs of peripheral lung cancer comprise an increase in ventilation/perfusion ratio over 1.0 on a side of the diseased lung with its simultaneous decrease on a contralateral side, normal values of perfusion and ventilation gradients of both lungs, and delayed alveolar capillary clearance in the diseased lung compared with the intact lung.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2016

Optical Tamm states in one-dimensional superconducting photonic crystal

In this study, we investigate localized and resonant optical waves associated with a semi-infinite superlattice made out of superconductor-dielectric bilayers and terminated with a cap layer. Both transverse electric and transverse magnetic waves are considered. These surface modes are analogous to the so-called Tamm states associated with electronic states found at the surface of materials. The surface guided modes induced by the cap layer strongly depend on whether the superlattice ends with a superconductor or a dielectric layer, the thickness of the surface layer, the temperature of the superconductor layer as well as on the polarization of the waves. Different kinds of surface modes are found and their properties examined. These structures can be used to realize the highly sensitive photonic crystal sensors.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Cold atmospheric plasma jet in an axial DC electric field

Cold atmospheric plasma (CAP) jet is currently intensively investigated as a tool for new and potentially transformative cancer treatment modality. However, there are still many unknowns about the jet behavior that requires attention. In this paper, a helium CAP jet is tested in an electrostatic field generated by a copper ring. Using Rayleigh microwave scattering method, some delays of the electron density peaks for different ring potentials are observed. Meanwhile, a similar phenomenon associated with the bullet velocity is found. Chemical species distribution along the jet is analyzed based on the jet optical emission spectra. The spectra indicate that a lower ring potential, i.e., lower DC background electric field, can increase the amount of excited N{sub 2}, N{sub 2}{sup +}, He, and O in the region before the ring, but can decrease the amount of excited NO and HO almost along the entire jet. Combining all the results above, we discovered that an extra DC potential mainly affects the temporal plasma jet properties. Also, it is possible to manipulate the chemical compositions of the jet using a ring with certain electric potentials.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Characterization of hypervelocity metal fragments for explosive initiation

The fragment impact response of two plastic-bonded explosive (PBX) formulations was studied using explosively driven aluminum fragments. A generic aluminum-capped detonator generated sub-mm aluminum particles moving at hypersonic velocities. The ability of these fragments to initiate reaction or otherwise damage two PBX materials was assessed using go/no-go experiments at standoff distances of up to 160 mm. Lower density PBX 9407 (RDX-based) was initiable at up to 115 mm, while higher density PBX 9501 (HMX-based) was only initiable at up to 6 mm. Several techniques were used to characterize the size, distribution, and velocity of the particles. Witness plate materials, including copper and polycarbonate, and backlit high speed video were used to characterize the distribution of particles, finding that the aluminum cap did not fragment homogeneously but rather with larger particles in a ring surrounding finer particles. Finally, precise digital holography experiments were conducted to measure the three-dimensional shape and size of the fastest-moving fragments, which ranged between 100 and 700 μm and traveled between 2.2 and 3.2 km/s. Crucially, these experiments showed variability in the fragmentation in terms of the number of fragments at the leading edge of the fragment field, indicating that both single and multiple shock impacts could be imparted to the target material. As a result, these types of data are critical for safety experiments and hydrocode simulations to quantify shock-to-detonation transition mechanisms and the associated risk-margins for these materials.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Ultrafast-induced coherent acoustic phonons in the two-dimensional magnet CrSBr

Magnetism in two-dimensional (2D) van der Waals (vdW) crystals offers promising new directions for low-dimensional physics and devices. In this work, mega-electron volt (MeV) ultrafast electron diffraction was employed to investigate the ultrafast atomic dynamics of a novel, 2D vdW magnetic single-crystal CrSBr. Femtosecond (fs) optical pump pulses excited non-equilibrium atomic displacements shown to be coherent acoustic phonons (CAPs). Phonon frequencies were extracted by analyzing oscillations of different Bragg peak (BP) intensities and were determined to be GHz acoustic disturbances that propagated as strain waves. Phonon modes exhibit anisotropy with respect to the a and b crystal axes. Subharmonic phonon frequencies were also observed, and this provided a signature of nonlinear oscillatory coupling between the laser-induced pumping phonon frequency and secondary phonon frequencies. Thus, CrSBr was found to serve as a nonlinear phononic frequency converter. The ultrafast time dependence of the Bragg intensity was simulated by incorporating an oscillating deviation parameter ansatz into expressions for the dynamical scattering intensity yielded excellent modeling of the ultrafast structural dynamics of the photo-excited 2D crystal. Our work provides a foundation for exploring how fs light pulses can influence phonon dynamics in materials with strong spin-lattice coupling. These results suggest that CAPs can match the magnon frequencies and show the promise of CrSBr for use in optical-to-microwave transducers and phononic devices.

43 PARTICLE ACCELERATORS↗
DOE OSTI2022

Surface stability of SrNbO 3+δ grown by hybrid molecular beam epitaxy

4d transition metal oxides have emerged as promising materials for numerous applications including high mobility electronics. SrNbO 3 is one such candidate material, serving as a good donor material in interfacial oxide systems and exhibiting high electron mobility in ultrathin films. However, its synthesis is challenging due to the metastable nature of the d 1 Nb 4+ cation and the limitations in the delivery of refractory Nb. To date, films have been grown primarily by pulsed laser deposition (PLD), but development of a means to grow and stabilize the material via molecular beam epitaxy (MBE) would enable studies of interfacial phenomena and multilayer structures that may be challenging by PLD. To that end, SrNbO 3 thin films were grown using hybrid MBE for the first time using a tris(diethylamido)(tert-butylimido) niobium precursor for Nb and an elemental Sr source on GdScO 3 substrates. Varying thicknesses of insulating SrHfO 3 capping layers were deposited using a hafnium tert-butoxide precursor for Hf on top of SrNbO 3 films to preserve the metastable surface. Grown films were transferred in vacuo for x-ray photoelectron spectroscopy to quantify elemental composition, density of states at the Fermi energy, and Nb oxidation state. Ex situ studies by x-ray absorption near edge spectroscopy and scanning transmission electron microscopy illustrate that the SrHfO 3 capping plays an important role in preserving the crystalline quality of the material and the Nb 4d 1 metastable charge state under atmospheric conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Multi-mode analysis of surface losses in a superconducting microwave resonator in high magnetic fields

This paper reports on a surface impedance measurement of a bulk metal niobium–titanium superconducting radio frequency (SRF) cavity in a magnetic field (up to 10 T). Here, a novel method is employed to decompose the surface resistance contributions of the cylindrical cavity end caps and walls using measurements from multiple TM cavity modes. The results confirm that quality factor degradation of a NbTi SRF cavity in a high magnetic field is primarily from surfaces perpendicular to the field (the cavity end caps), while parallel surface resistances (the walls) remain relatively constant. This result is encouraging for applications needing high Q cavities in strong magnetic fields, such as the Axion Dark Matter eXperiment because it opens the possibility of hybrid SRF cavity construction to replace conventional copper cavities.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Structures of trehalose-6-phosphate phosphatase from pathogenic fungi reveal the mechanisms of substrate recognition and catalysis

Trehalose is a disaccharide essential for the survival and virulence of pathogenic fungi. The biosynthesis of trehalose requires trehalose-6-phosphate synthase, Tps1, and trehalose-6-phosphate phosphatase, Tps2. Here, we report the structures of the N-terminal domain of Tps2 (Tps2NTD) from Candida albicans , a transition-state complex of the Tps2 C-terminal trehalose-6-phosphate phosphatase domain (Tps2PD) bound to BeF 3 and trehalose, and catalytically dead Tps2PD(D24N) from Cryptococcus neoformans bound to trehalose-6-phosphate (T6P). The Tps2NTD closely resembles the structure of Tps1 but lacks any catalytic activity. The Tps2PD–BeF 3 –trehalose and Tps2PD(D24N)–T6P complex structures reveal a “closed” conformation that is effected by extensive interactions between each trehalose hydroxyl group and residues of the cap and core domains of the protein, thereby providing exquisite substrate specificity. Disruption of any of the direct substrate–protein residue interactions leads to significant or complete loss of phosphatase activity. Notably, the Tps2PD–BeF 3 –trehalose complex structure captures an aspartyl-BeF 3 covalent adduct, which closely mimics the proposed aspartyl-phosphate intermediate of the phosphatase catalytic cycle. Structures of substrate-free Tps2PD reveal an “open” conformation whereby the cap and core domains separate and visualize the striking conformational changes effected by substrate binding and product release and the role of two hinge regions centered at approximately residues 102–103 and 184–188. Significantly, tps2 Δ, tps2NTD Δ, and tps2D705N strains are unable to grow at elevated temperatures. Combined, these studies provide a deeper understanding of the substrate recognition and catalytic mechanism of Tps2 and provide a structural basis for the future design of novel antifungal compounds against a target found in three major fungal pathogens.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Tertiary folds of the SL5 RNA from the 5 ' proximal region of SARS-CoV-2 and related coronaviruses

Coronavirus genomes sequester their start codons within stem-loop 5 (SL5), a structured, 5' genomic RNA element. In most alpha- and betacoronaviruses, the secondary structure of SL5 is predicted to contain a four-way junction of helical stems, some of which are capped with UUYYGU hexaloops. Here, using cryogenic electron microscopy (cryo-EM) and computational modeling with biochemically determined secondary structures, we present three-dimensional structures of SL5 from six coronaviruses. The SL5 domain of betacoronavirus severe-acute-respiratory-syndrome-related coronavirus 2 (SARS-CoV-2), resolved at 4.7 Å resolution, exhibits a T-shaped structure, with its UUYYGU hexaloops at opposing ends of a coaxial stack, the T’s “arms.” Further analysis of SL5 domains from SARS-CoV-1 and MERS (7.1 and 6.4 to 6.9 Å resolution, respectively) indicate that the junction geometry and inter-hexaloop distances are conserved features across these human-infecting betacoronaviruses. The MERS SL5 domain displays an additional tertiary interaction, which is also observed in the non-human-infecting betacoronavirus BtCoV-HKU5 (5.9 to 8.0 Å resolution). SL5s from human-infecting alphacoronaviruses, HCoV-229E and HCoV-NL63 (6.5 and 8.4 to 9.0 Å resolution, respectively), exhibit the same coaxial stacks, including the UUYYGU-capped arms, but with a phylogenetically distinct crossing angle, an X-shape. As such, all SL5 domains studied herein fold into stable tertiary structures with cross-genus similarities and notable differences, with implications for potential protein-binding modes and therapeutic targets.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2015

The Crystal Structure of an Integral Membrane Fatty Acid α-Hydroxylase

Neuronal electrical impulse propagation is facilitated by the myelin sheath, a compact membrane surrounding the axon. The myelin sheath is highly enriched in galactosylceramide (GalCer) and its sulfated derivative sulfatide. Over 50% of GalCer and sulfatide in myelin is hydroxylated by the integral membrane enzyme fatty acid 2-hydroxylase (FA2H). GalCer hydroxylation contributes to the compact nature of the myelin membrane, and mutations in FA2H result in debilitating leukodystrophies and spastic paraparesis. We report here the 2.6 Å crystal structure of sphingolipid α-hydroxylase (Scs7p), a yeast homolog of FA2H. The Scs7p core is composed of a helical catalytic cap domain that sits atop four transmembrane helices that anchor the enzyme in the endoplasmic reticulum. The structure contains two zinc atoms coordinated by the side chains of 10 highly conserved histidines within a dimetal center located near the plane of the cytosolic membrane. We used a yeast genetic approach to confirm the important role of the dimetal-binding histidines in catalysis and identified Tyr-322 and Asp-323 as critical determinants involved in the hydroxylase reaction. Examination of the Scs7p structure, coupled with molecular dynamics simulations, allowed for the generation of a model of ceramide binding to Scs7p. Comparison of the Scs7p structure and substrate-binding model to the structure of steroyl-CoA desaturase revealed significant differences in the architecture of the catalytic cap domain and location of the dimetal centers with respect to the membrane. These observations provide insight into the different mechanisms of substrate binding and recognition of substrates by the hydroxylase and desaturase enzymes.

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

Air quality co-benefits of subnational carbon policies

To mitigate climate change, governments ranging from city to multi-national have adopted greenhouse gas (GHG) emissions reduction targets. While the location of GHG reductions does not affect their climate benefits, it can impact human health benefits associated with co-emitted pollutants. Here, an advanced modeling framework is used to explore how subnational level GHG targets influence air pollutant co-benefits from ground level ozone and fine particulate matter. Two carbon policy scenarios are analyzed, each reducing the same total amount of GHG emissions in the Northeast US: an economy-wide Cap and Trade (CAT) program reducing emissions from all sectors of the economy, and a Clean Energy Standard (CES) reducing emissions from the electricity sector only. Results suggest that a regional CES policy will cost about 10 times more than a CAT policy. Despite having the same regional targets in the Northeast, carbon leakage to non-capped regions varies between policies. Consequently, a regional CAT policy will result in national carbon reductions that are over six times greater than the carbon reduced by the CES in 2030. Monetized regional human health benefits of the CAT and CES policies are 844% and 185% of the costs of each policy, respectively. Benefits for both policies are thus estimated to exceed their costs in the Northeast US. The estimated value of human health co-benefits associated with air pollution reductions for the CES scenario is two times that of the CAT scenario. Implications: In this research, an advanced modeling framework is used to determine the potential impacts of regional carbon policies on air pollution co-benefits associated with ground level ozone and fine particulate matter. Study results show that spatially heterogeneous GHG policies have the potential to create areas of air pollution dis-benefit. It is also shown that monetized human health benefits within the area covered by policy may be larger than the model estimated cost of the policy. These findings are of particular interest both as U.S. states work to develop plans to meet state-level carbon emissions reduction targets set by the EPA through the Clean Power Plan, and in the absence of comprehensive national carbon policy.

54 ENVIRONMENTAL SCIENCES↗