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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 1,459 records · Page 81
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

A genetic algorithm for first principles global structure optimization of supported nano structures

We present a newly developed publicly available genetic algorithm (GA) for global structure optimisation within atomic scale modeling. The GA is focused on optimizations using first principles calculations, but it works equally well with empirical potentials. The implementation is described and benchmarked through a detailed statistical analysis employing averages across many independent runs of the GA. This analysis focuses on the practical use of GA’s with a description of optimal parameters to use. New results for the adsorption of M{sub 8} clusters (M = Ru, Rh, Pd, Ag, Pt, Au) on the stoichiometric rutile TiO{sub 2}(110) surface are presented showing the power of automated structure prediction and highlighting the diversity of metal cluster geometries at the atomic scale.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Evolution of structure in Na{sub 0.5}Bi{sub 0.5}TiO{sub 3} single crystals with BaTiO{sub 3}

The structural, dielectric, and piezoelectric properties of Na{sub 0.5}Bi{sub 0.5}TiO{sub 3}-x mol. %BaTiO{sub 3} (NBT-x%BT) crystals have been investigated. The dielectric and piezoelectric properties of NBT-x%BT were enhanced near x = 5–7. High resolution synchrotron x-ray powder diffraction studies revealed the presence of a phase boundary between monoclinic (Cc) and tetragonal (P4bm) phases near x = 5–7, where the dielectric and piezoelectric properties were enhanced.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Local atomic structure of K{sub x}Na{sub (1−x)}NbO{sub 3} by total x-ray diffraction

Local atomic structure of K{sub x}Na{sub (1−x)}NbO{sub 3} with 0.0 ≤ x ≤ 1.0 was studied using atomic Pair Distribution Function analysis based on x-ray diffraction. Powdered crystals were found to exhibit a re-entrant behavior by being orthorhombic (Amm2) for x < 0.42, monoclinic (Pm) for 0.42 ≤ x ≤ 0.63 and again orthorhombic (Amm2) for x > 0.63. Non-centrosymmetric structure of NaNbO{sub 3} (Amm2) was also evident in the piezoresponse force microscopy analysis revealing the presence of the ferroelectric domains and switching behavior. Lowering of the crystallographic symmetry for 0.42 ≤ x ≤ 0.63 is discussed in terms of differences in the sizes of Na{sup +} and K{sup +} ions and Na–O and K–O bond lengths. Besides being a bridging phase, as suggested by the previous studies on lead-based compositions, present study suggests that lower symmetry monoclinic phase of compositionally disordered perovskite solid solutions could also be a manifestation of the difference in the sizes of constituent ions and bond lengths.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Spin waves in micro-structured yttrium iron garnet nanometer-thick films

We investigated the spin-wave propagation in a micro-structured yttrium iron garnet waveguide of 40 nm thickness. Utilizing spatially-resolved Brillouin light scattering microscopy, an exponential decay of the spin-wave amplitude of (10.06 ± 0.83) μm was observed. This leads to an estimated Gilbert damping constant of α=(8.79±0.73)×10{sup −4}, which is larger than damping values obtained through ferromagnetic resonance measurements in unstructured films. The theoretically calculated spatial interference of waveguide modes was compared to the spin-wave pattern observed experimentally by means of Brillouin light scattering spectroscopy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Spectral and structural stability properties of charged particle dynamics in coupled lattices

It has been realized in recent years that coupled focusing lattices in accelerators and storage rings have significant advantages over conventional uncoupled focusing lattices, especially for high-intensity charged particle beams. A theoretical framework and associated tools for analyzing the spectral and structural stability properties of coupled lattices are formulated in this paper, based on the recently developed generalized Courant-Snyder theory for coupled lattices. It is shown that for periodic coupled lattices that are spectrally and structurally stable, the matrix envelope equation must admit matched solutions. Using the technique of normal form and pre-Iwasawa decomposition, a new method is developed to replace the (inefficient) shooting method for finding matched solutions for the matrix envelope equation. Stability properties of a continuously rotating quadrupole lattice are investigated. The Krein collision process for destabilization of the lattice is demonstrated.

43 PARTICLE ACCELERATORS↗
DOE OSTI2015

Computational extended magneto-hydrodynamical study of shock structure generated by flows past an obstacle

The magnetized shock problem is studied in the context where supersonic plasma flows past a solid obstacle. This problem exhibits interesting and important phenomena such as a bow shock, magnetotail formation, reconnection, and plasmoid formation. This study is carried out using a discontinuous Galerkin method to solve an extended magneto-hydrodynamic model (XMHD). The main goals of this paper are to present a reasonably complete picture of the properties of this interaction using the MHD model and then to compare the results to the XMHD model. The inflow parameters, such as the magnetosonic Mach number M{sub f} and the ratio of thermal pressure to magnetic pressure β, can significantly affect the physical structures of the flow-obstacle interaction. The Hall effect can also significantly influence the results in the regime in which the ion inertial length is numerically resolved. Most of the results presented are for the two-dimensional case; however, two three-dimensional simulations are presented to make a connection to the important case in which the solar wind interacts with a solid body and to explore the possibility of performing scaled laboratory experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

The high pressure structure and equation of state of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) up to 20 GPa: X-ray diffraction measurements and first principles molecular dynamics simulations

Recent theoretical studies of 2,6-diamino-3,5-dinitropyrazine-1-oxide (C{sub 4}H{sub 4}N{sub 6}O{sub 5} Lawrence Livermore Molecule No. 105, LLM-105) report unreacted high pressure equations of state that include several structural phase transitions, between 8 and 50 GPa, while one published experimental study reports equation of state (EOS) data up to a pressure of 6 GPa with no observed transition. Here we report the results of a synchrotron-based X-ray diffraction study and also ambient temperature isobaric-isothermal atomistic molecular dynamics simulations of LLM-105 up to 20 GPa. We find that the ambient pressure phase remains stable up to 20 GPa; there is no indication of a pressure induced phase transition. We do find a prominent decrease in b-axis compressibility starting at approximately 13 GPa and attribute the stiffening to a critical length where inter-sheet distance becomes similar to the intermolecular distance within individual sheets. The ambient temperature isothermal equation of state was determined through refinements of measured X-ray diffraction patterns. The pressure-volume data were fit using various EOS models to yield bulk moduli with corresponding pressure derivatives. We find very good agreement between the experimental and theoretically derived EOS.

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

Electrical and structural properties of In-implanted Si{sub 1−x}Ge{sub x} alloys

We report on the effects of dopant concentration and substrate stoichiometry on the electrical and structural properties of In-implanted Si{sub 1−x}Ge{sub x} alloys. Correlating the fraction of electrically active In atoms from Hall Effect measurements with the In atomic environment determined by X-ray absorption spectroscopy, we observed the transition from electrically active, substitutional In at low In concentration to electrically inactive metallic In at high In concentration. The In solid-solubility limit has been quantified and was dependent on the Si{sub 1−x}Ge{sub x} alloy stoichiometry; the solid-solubility limit increased as the Ge fraction increased. This result was consistent with density functional theory calculations of two In atoms in a Si{sub 1−x}Ge{sub x} supercell that demonstrated that In–In pairing was energetically favorable for x ≲ 0.7 and energetically unfavorable for x ≳ 0.7. Transmission electron microscopy imaging further complemented the results described earlier with the In concentration and Si{sub 1−x}Ge{sub x} alloy stoichiometry dependencies readily visible. We have demonstrated that low resistivity values can be achieved with In implantation in Si{sub 1−x}Ge{sub x} alloys, and this combination of dopant and substrate represents an effective doping protocol.

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

Imaging the inside of thick structures using cosmic rays

The authors present here a new method to image reinforcement elements inside thick structures and the results of a demonstration measurement performed on a mock-up wall built at Los Alamos National Laboratory. The method, referred to as “multiple scattering muon radiography”, relies on the use of cosmic-ray muons as probes. The work described in this article was performed to prove the viability of the technique as a means to image the interior of the dome of Florence Cathedral Santa Maria del Fiore, one of the UNESCO World Heritage sites and among the highest profile buildings in existence. Its result shows the effectiveness of the technique as a tool to radiograph thick structures and image denser object inside them.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Near infrared laser annealing of CdTe and in-situ measurement of the evolution of structural and optical properties

The high performance of polycrystalline CdTe thin film solar cells is enabled by annealing in the presence of Cl. This process is typically carried out for tens of minutes resulting in reduction of defect states within the bandgap among other beneficial effects. In this work, we investigate laser annealing as a means of rapidly annealing CdTe using a continuous wave sub-bandgap 1064 nm laser. The partial transmission of the beam allows us to monitor the annealing process in-situ and in real time. We find that optoelectronic and structural changes occur through two distinct kinetic processes resulting in the removal of deep defects and twinned regions, respectively. A multilayer optical model including surface roughness is used to interpret both the in-situ transmission as well as ex-situ reflectivity measurements. These experiments demonstrate beneficial material changes resulting from sub-bandgap laser-driven CdCl{sub 2} treatment of CdTe in minutes, which is an important step towards accelerating the processing of the CdTe absorber layer.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

ZnO nanoparticles obtained by ball milling technique: Structural, micro-structure, optical and photo-catalytic properties

The ZnO nanoparticles were obtained by ball milling of commercial grade ZnO powder at 250 rpm for 20 h and studied their structural, micro-structure, optical and photo-catalytic properties. Due to ball milling significant decrease in lattice parameters and average crystalline size is noticed for the as-milled ZnO nano powder. The HRSEM images of the as-milled powder consist of agglomerated fine spherical nanoparticles in the range of ~10-20 nm. The room temperature PL spectrum of as-milled ZnO nano powder excited under 320 nm reveals two emission bands at ~406 nm (violet emission) and ~639 nm (green emission). Interestingly about 98 % of photo degradation of methylene (MB) by the ZnO catalyst is achieved at 100 minutes of solar light irradiation.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Spin-orbit coupling, electron transport and pairing instabilities in two-dimensional square structures

Rashba spin-orbit effects and electron correlations in the two-dimensional cylindrical lattices of square geometries are assessed using mesoscopic two-, three- and four-leg ladder structures. Here the electron transport properties are systematically calculated by including the spin-orbit coupling in tight binding and Hubbard models threaded by a magnetic flux. These results highlight important aspects of possible symmetry breaking mechanisms in square ladder geometries driven by the combined effect of a magnetic gauge field spin-orbit interaction and temperature. The observed persistent current, spin and charge polarizations in the presence of spin-orbit coupling are driven by separation of electron and hole charges and opposite spins in real-space. The modeled spin-flip processes on the pairing mechanism induced by the spin-orbit coupling in assembled nanostructures (as arrays of clusters) engineered in various two-dimensional multi-leg structures provide an ideal playground for understanding spatial charge and spin density inhomogeneities leading to electron pairing and spontaneous phase separation instabilities in unconventional superconductors. Such studies also fall under the scope of current challenging problems in superconductivity and magnetism, topological insulators and spin dependent transport associated with numerous interfaces and heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

The effect of millisecond flash lamp annealing on electrical and structural properties of ZnO:Al/Si structures

The effect of millisecond flash lamp annealing (FLA) on aluminum doped ZnO (AZO) films and their interface with Si have been studied. The AZO films were deposited by magnetron sputtering on Si (100) substrates. The electrical and structural properties of the film and AZO/Si structures were characterized by current–voltage, capacitance–voltage, and deep level transient spectroscopy measurements, X-ray diffraction, and secondary ion mass spectrometry. The resistivity of the AZO film is reduced to a close to state-of-the-art value of 2 × 10{sup −4} Ω cm after FLA for 3 ms with an average energy density of 29 J/cm{sup 2}. In addition, most of the interfacial defects energy levels are simultaneously annealed out, except for one persisting shallow level, tentatively assigned to the vacancy-oxygen complex in Si, which was not affected by FLA. Subsequent to the FLA, the samples were treated in N{sub 2} or forming gas (FG) (N{sub 2}/H{sub 2}, 90/10%{sub mole}) ambient at 200–500 °C. The latter samples maintained the low resistivity achieved after the FLA, but not the former ones. The interfacial defect level persisting after the FLA is removed by the FG treatment, concurrently as another level emerges at ∼0.18 eV below the conduction band. The electrical data of the AZO films are discussed in term of point defects controlling the resistivity, and it is argued that the FLA promotes formation of electrically neutral clusters of Zink vacancies (V{sub Zn}'s) rather than passivating/compensating complexes between the Al donors and V{sub Zn}'s.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Aerodynamic levitator for in situ x-ray structure measurements on high temperature and molten nuclear fuel materials

An aerodynamic levitator with carbon dioxide laser beam heating was integrated with a hermetically sealed controlled atmosphere chamber and sample handling mechanism. The system enabled containment of radioactive samples and control of the process atmosphere chemistry. The chamber was typically operated at a pressure of approximately 0.9 bars to ensure containment of the materials being processed. Samples 2.5-3 mm in diameter were levitated in flowing gas to achieve containerless conditions. Levitated samples were heated to temperatures of up to 3500 °C with a partially focused carbon dioxide laser beam. Sample temperature was measured using an optical pyrometer. The sample environment was integrated with a high energy (100 keV) x-ray synchrotron beamline to enable in situ structure measurements to be made on levitated samples as they were heated, melted, and supercooled. The system was controlled from outside the x-ray beamline hutch by using a LabVIEW program. Measurements have been made on hot solid and molten uranium dioxide and binary uranium dioxide-zirconium dioxide compositions.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

Cooling rate dependence of structural order in Al{sub 90}Sm{sub 10} metallic glass

The atomic structure of Al{sub 90}Sm{sub 10} metallic glass is studied using molecular dynamics simulations. By performing a long sub-T{sub g} annealing, we developed a glass model closer to the experiments than the models prepared by continuous cooling. Using the cluster alignment method, we found that “3661” cluster is the dominating short-range order in the glass samples. The connection and arrangement of “3661” clusters, which define the medium-range order in the system, are enhanced significantly in the sub-T{sub g} annealed sample as compared with the fast cooled glass samples. Unlike some strong binary glass formers such as Cu{sub 64.5}Zr{sub 35.5}, the clusters representing the short-range order do not form an interconnected interpenetrating network in Al{sub 90}Sm{sub 10,} which has only marginal glass formability.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Room temperature negative differential resistance in terahertz quantum cascade laser structures

The mechanisms that limit the temperature performance of GaAs/Al 0.15 GaAs-based terahertz quantum cascade lasers (THz-QCLs) have been identified as thermally activated LO-phonon scattering and leakage of charge carriers into the continuum. Consequently, the combination of highly diagonal optical transition and higher barriers should significantly reduce the adverse effects of both mechanisms and lead to improved temperature performance. Here, we study the temperature performance of highly diagonal THz-QCLs with high barriers. Our analysis uncovers an additional leakage channel which is the thermal excitation of carriers into bounded higher energy levels, rather than the escape into the continuum. Based on this understanding, we have designed a structure with an increased intersubband spacing between the upper lasing level and excited states in a highly diagonal THz-QCL, which exhibits negative differential resistance even at room temperature. Furthermore, this result is a strong evidence for the effective suppression of the aforementioned leakage channel.

42 ENGINEERING↗
DOE OSTI2016

Comparative study of GeO{sub 2}/Ge and SiO{sub 2}/Si structures on anomalous charging of oxide films upon water adsorption revealed by ambient-pressure X-ray photoelectron spectroscopy

The energy difference between the oxide and bulk peaks in X-ray photoelectron spectroscopy (XPS) spectra was investigated for both GeO{sub 2}/Ge and SiO{sub 2}/Si structures with thickness-controlled water films. This was achieved by obtaining XPS spectra at various values of relative humidity (RH) of up to ∼15%. The increase in the energy shift is more significant for thermal GeO{sub 2} on Ge than for thermal SiO{sub 2} on Si above ∼10{sup −4}% RH, which is due to the larger amount of water molecules that infiltrate into the GeO{sub 2} film to form hydroxyls. Analyzing the origins of this energy shift, we propose that the positive charging of a partially hydroxylated GeO{sub 2} film, which is unrelated to X-ray irradiation, causes the larger energy shift for GeO{sub 2}/Ge than for SiO{sub 2}/Si. A possible microscopic mechanism of this intrinsic positive charging is the emission of electrons from adsorbed water species in the suboxide layer of the GeO{sub 2} film to the Ge bulk, leaving immobile cations or positively charged states in the oxide. This may be related to the reported negative shift of flat band voltages in metal-oxide-semiconductor diodes with an air-exposed GeO{sub 2} layer.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗