bluuebunny/crossref_metadata_2025_split huggingface dataset:
Defect Engineering and Atomic Relocation Processes in Impurity-Free Disordered GaAs and AlGaAs
Authors: Deenapanray P. N. K. , Krispin M. , Meyer W. E. , Tan H. H. , Jagadish C. , Auret F. D. 
Published: April 2011

ABSTRACTImpurity-free disordering (IFD) of GaAs and AlxGa1-xAs epitaxial layers using SiOx capping in conjunction with annealing was studied by deep level transient spectroscopy (DLTS) and capacitance-voltage (C-V) measurements. Three dominant electron traps S1 (EC– 0.23 eV), S2* (EC– 0.53 eV), and S4 (EC– 0.74 eV) are created in disordered n-type GaAs. The electron emission rate of S1 is enhanced in the presence of an externally applied electric field. We propose that S1 is a defect that may involve As-clustering or a complex of arsenic interstitials, Asi, and the arsenic-antisite, AsGa. S2* is shown to be the superposition of two defects, which may be VGa-related. S4 is identified as the defect EL2. Our preliminary results indicate that the same set of defects is created in disordered n-type AlxGa1-xAs, with S1 pinned to the conduction band edge, while S2* and S4 are pinned relative to the Fermi level. In contrast to disordering in n-type GaAs, IFD of p-type GaAs results in the pronounced increase in the free carrier concentration in the near-surface region of the disordered layer. Two electrically active defects HA (EV+ 0.39 eV) and HB2 (EV+ 0.54 eV), which we have attributed to Cu- and Asi/AsGa-related levels, respectively, are also observed in the disordered p-GaAs layers. IFD causes segregation of Zn dopant atoms and Cu towards the surface of IFD samples. This atomic relocation process poses serious limitations regarding the application of IFD to the band gap engineering of doped GaAs-based heterostructures.
Undoped gallium antimonide studied by positron annihilation spectroscopy
Authors: Ma S. K. , Ling C. C. , Weng H. M. , Hang D. S. 
Published: April 2011

ABSTRACTPositron lifetime spectroscopy has been used to study the vacancy type defects in undoped gallium antimonide. Temperature dependent positron trapping into the VGa-related defect having a characteristic lifetime of 310ps was observed in the as-grown sample. The lifetime data were well described by a model involving the thermal ionization (0/-) of the VGa-related defect and its ionization energy was found to be E(0/-)=83meV. For the electron irradiated sample, the VGa-related defect with lifetime of 310ps that was found in the non-irradiated samples was also identified. Moreover, another lifetime component (280ps) was only observed in the electron irradiated sample but not in the non-irradiated sample. It was also attributed to the VGa-related defect. The two identified VGa-related defects should have different microstructures because of their difference in characteristic lifetimes. The 280ps component remains thermally stable after the 500°C annealing while the 310ps component anneals at 300°C.
Characterization of zinc oxide single crystals for epitaxial wafer applications
Authors: Ohashi Naoki , Ohgaki Takeshi , Sugimura Shigeaki , Maeda Katsumi , Sakaguchi Isao , Ryoken Haruki , Niikura Ikuo , Sato Mitsuru , Haneda Hajime 
Published: April 2011

ABSTRACTZinc oxide (ZnO) single crystals were grown by the hydrothermal method using lithium and potassium hydroxide as mineralizer and properties of the grown crystals were characterized from the viewpoints of epitaxial wafer applications. The growth sector dependence of impurity and defect concentrations were characterized by secondary ion mass spectroscopy and photoluminescence. As a result, it was clearly shown that defect and impurity distribution in the obtained crystal was anisotropic, and this anisotropy is affected by the choice of the seed crystal shape and growth direction. Annealing effect on flatness of the wafer surface was also examined, and it was found that high temperature annealing with flat single crystalline cover is appropriate for removal of scratch and formation of atomically flat surface. Moreover, we show the possible miss-evaluation of Hall coefficient of ZnO due to anisotropy in defects and impurities distributions.
Energetic Materials Development at Technanogy Materials Development
Authors: Walter Kevin C. , Aumann Christopher E. , Carpenter R. Douglas , O'Neill Edward H. , Pesiri David R. 
Published: April 2011

ABSTRACTAn energetic material has been routinely manufactured from nano-metric powders of aluminum (Al) and molybdenum trioxide (MoO3). When optimized, the burn-rate of these materials (∼400 m/s) exceeds that of conventional thermites (based on micron-sized powders), but is less than that of conventional explosives. Similar burn-rates around 350 m/s are measured for these “super-thermites” using n-Al powder in the size range between 30 and 90 nm in diameter (20–60 m2/g, 60–80 wt%Al) and an oxygen to fuel (O/F) mass ratio of 1.4. The burn-rate decreases when the surface area of the MoO3 is decreased from 64 to 40 m2/g, or when O/F is changed from 1.2. Thus, for each average particle diameter, there is an optimum burn-rate at an O/F ratio that depends on the wt%Al present in the material and the particle size distribution of the powder. The burn-rate is dependent on several materials and processing factors such as the quality of the nano-metric ingredients, the processing method, and exposure to air and light, so the effect of aging and environmental exposure on the individual ingredients has been investigated. The results of this powder aging study suggests that the surface area of n-MoO3 can decrease two-fold within 10–12 days, and the Al-metal content in n-Al can decrease as much as 50% over two years. Adequate handling and storage procedures must therefore be followed for the effective use of nano-metric powders and their super-thermite mixes.
Characterization of Airborne Ultrafine and Nanometer Particles During Energetic Material Synthesis and Testing
Authors: Cheng Meng-Dawn , Jenkins Charles M. 
Published: April 2011

ABSTRACTSeveral experiments were conducted to improve our understanding of the properties of aerosol particles generated by detonation of conventional explosive and explosives prepared from nanophase materials. Initial number concentrations (∼ 106−107 cm−3) of particles produced by detonations of the nano-explosives were comparable to that produced by conventional explosive. In general, data taken by a time-of-flight aerodynamic sizer and a scanning differential mobility analyzer for the first sample indicate a multi-modal distribution that there were a peak between 0.7 and 0.9 μm, and one and/or two peaks smaller than 100 nm depending upon the explosive used. The material properties and formulation of the explosive appear to play a significant role in the enhanced particle growth and increased deposition velocity leading to a higher reduction rate of total particle concentrations. Furthermore, the high level of ultrafine particles and nanoparticles in addition to the enriched toxic metals, the biological properties (e.g., the cellular toxicity) of the detonation particles need to be investigated in the near future.
Synthesis and Characterization of Mixed Metal Oxide Nanocomposite Energetic Materials
Authors: Clapsaddle Brady J. , Zhao Lihua , Gash Alex E. , Satcher Joe H. , Shea Kenneth J. , Pantoya Michelle L. , Simpson Randall L. 
Published: April 2011

ABSTRACTIn the field of composite energetic materials, properties such as ingredient distribution, particle size, and morphology, affect both sensitivity and performance. Since the reaction kinetics of composite energetic materials are typically controlled by the mass transport rates between reactants, one would anticipate new and potentially exceptional performance from energetic nanocomposites. We have developed a new method of making nanostructured energetic materials, specifically explosives, propellants, and pyrotechnics, using sol-gel chemistry. A novel sol-gel approach has proven successful in preparing metal oxide/silicon oxide nanocomposites in which the metal oxide is the major component. Two of the metal oxides are tungsten trioxide and iron(III) oxide, both of which are of interest in the field of energetic materials. Furthermore, due to the large availability of organically functionalized silanes, the silicon oxide phase can be used as a unique way of introducing organic additives into the bulk metal oxide materials. As a result, the desired organic functionality is well dispersed throughout the composite material on the nanoscale. By introducing a fuel metal into the metal oxide/silicon oxide matrix, energetic materials based on thermite reactions can be fabricated. The resulting nanoscale distribution of all the ingredients displays energetic properties not seen in its microscale counterparts due to the expected increase of mass transport rates between the reactants. The synthesis and characterization of these metal oxide/silicon oxide nanocomposites and their performance as energetic materials will be discussed.
Oxidation Processes and Phase Changes in Metastable Al-Ti Mechanical Alloys
Authors: Zhu Xiaoying , Schoenitz Mirko , Dreizin Edward L. 
Published: April 2011

ABSTRACTOxidation of Al-Ti mechanical alloys with Ti concentrations from 5 to 25 at% was studied and compared to the oxidation of Al powders using thermal analysis in the temperature range of 300–1500 °C in oxygen. Differential scanning calorimetry (DSC) and differential thermal analysis (DTA) with simultaneous thermo-gravimetric analysis (TGA) were used to monitor phase changes and oxidation reactions. Intermediate reaction products were recovered at different temperatures and analyzed using x-ray diffraction (XRD) and scanning electron microscopy (SEM). Oxidation of all samples occurred stepwise. The temperature of the first oxidation step for Al-Ti mechanical alloys correlates with the exothermic formation of the Al3Ti intermetallic at T>700 °C and has a higher rate and greater degree of oxidation than the first oxidation step observed for pure aluminum. The second and third oxidation steps in mechanical alloys and aluminum occur at higher temperatures and both appear to be controlled by changes in the permeability of the Al2O3 films. The effect of the Al2O3 film becomes less noticeable at increased Ti concentrations.
Direct Evidence Of Chemical Reactions Induced By Shear-Strains
Authors: Gilman John J. 
Published: April 2011

ABSTRACTAfter a brief review of the history of mechanochemistry, and of the theoretical principles of chemical reactivity, four examples are described of reactions that demonstrate the importance of elastic shear strains compared with hydrostatic compressive strains (volume changes). Techniques for separating shear strains from volume changes, and for isolating elastic strains from plastic deformation are described. The latter (isolation) is achieved simply by localizing a strained region making it too small for dislocation nucleation. Shear strain acts by reducing the chemical hardness (activation energy) of a reactant. The four examples are: (1) “hammer chemistry” in which physisorped methane is struck by argon atoms with enough kinetic energy to cause chemisorption; (2) enhanced oxidation of silicon at stressed crack tips; (3) selective dissolution of crystals at screw dislocations; and (4) increased rates of catalyzed reactions when surface acoustic waves are passed through the catalyst.
Nanoscale Modeling of Shock-Induced Deformation of Diamond
Authors: Zybin S. V. , Oleinik I. I. , Elert M. L. , White C. T. 
Published: April 2011

ABSTRACTMolecular dynamics (MD) simulations of shock-induced deformations in diamond were performed using a reactive bond order (REBO) potential. A splitting of shock wave structure into elastic and crystal deformation fronts was observed in the [110] and [111] crystallographic directions above piston velocity thresholds of up ≈ 1.8 and 2.5 km/s, respectively. The crystal lattice response in a split two-wave regime consists of the relative movement of {111} planes in the diamond crystal and has different structural character for [110] and [111] shock waves. The strain produced by a [110] shock wave occurs only along one of the transverse crystalline directions, whereas in the [111] case crystal deformation involves the movement of the atoms in both transverse directions. To gain insight into the atomistic mechanisms of orientational dependence of shock compression of crystals, we have investigated in detail the constitutive stress-strain relationships under static uniaxial compression. The REBO potential gives a reasonably good description of stresses and energetics under moderate uniaxial compressions corresponding to an elastic shock wave regime. However, under compressions higher than 10% ([110] case) and 20% ([111] case) the REBO potential shows deficiencies in the quantitative description of stress response that might affect the MD picture of shock wave deformations in diamond.
Single-Wall Carbon Nanotubes Field Emission Properties: A Theoretical Study of the Effects of Cs
Authors: Akdim Brahim , Duan Xiaofeng , Shiffler Donald A. , Pachter Ruth 
Published: April 2011

ABSTRACTCarbon nanotubes-based materials appeal for explosive emission, in forming cathode plasma, of interest for high-power microwave tubes. Cs intercalation has demonstrated a significant reduction of the work function of carbon nanotubes, thus improving field emission properties. An understanding of the detailed adsorption effects is important because the current saturation is attributed, in part, to adsorption mechanisms. In this paper, we report a density functional theory study of the effects of Cs on field emission of single-wall carbon nanotubes (SWCNTs), as an example of an approach to be taken for a fundamental understanding of such properties.
Synthesis and Characterization of Gd(1−x)SrxCo(1−y)FeyO(3−Δ) as a Cathode Material for Intermediate Temperature Solid Oxide Fuel Cells
Authors: Dyck C. R. , Yu G. , Krstic V. D. 
Published: April 2011

ABSTRACTGd(1−x)SrxCoO(3−Δ) (GSC) is a promising cathode material system for use in IT-SOFCs due to its high catalytic activity for oxygen reduction and appreciable conductivity. However, it has a high thermal expansion coefficient that is unmatched to the common IT-SOFC electrolyte material, Ce0.8Gd0.2O(2−Δ) (CGO). Gd0.8Sr0.2CoO(3−Δ) (GS20C) was determined to provide the best balance of properties as a base composition in the GSC system for further study. GS20C exhibited electrical conductivity of 400 S cm−1 at 600°C and a linear thermal expansion coefficient of 23 ppm/°C. Manipulation of the Co-site in GS20C by Fe substitution to form Gd(0.8)Sr(0.2)Co(1−y)FeyO(3−Δ) (GS20CFY, Y= 0, 20, 40, 60, 80, 100 atomic%) resulted in a dramatic decrease in the thermal expansion coefficient to a level close to that of the electrolyte (∼13 ppm/°C). However, the decrease in thermal expansion was accompanied by a large decrease in the conductivity as the iron content was increased in the system (to ∼10 S cm−1). Alternatively, formation of GS20C/CGO composite cathodes resulted in thermal matching with the electrolyte material up to the IT-SOFC operating temperature of approximately 600°C with the maintenance of high electrical conductivity. Composite GS20C/CGO cathodes may reduce the problems associated with poor GSC thermal matching to the electrolyte without compromising other important cathodic properties.
Synthesis and Thermal Characteristics of Complex Metal Hydride: NaAlH4
Authors: Yebka Bouziane , Nazri Gholam-Abbas 
Published: April 2011

ABSTRACTComplex metal hydrides of general formula, ABH4 (A = alkali metals, B = third group elements such as B, Al, Ga) are potential candidates as hydrogen storage media for transportation. Thermal decomposition of complex hydrides generates hydrogen at elevated temperatures. The by -products of the dehydrogenation process can be regenerated using gaseous hydrogen at suitable temperature and pressure. The initial steps of thermal decomposition of NaAlH4 may be more complicated from the decomposition pathway reported in the literature. Close examination using thermal analysis by TGA, DSC and XRD measurements over the temperature range 30–500°C showed that the initial evolution of hydrogen occurred at a slow rate at ∼80°C, prior to fast decomposition at 190°C and at 260°C. Four regions of weight loss and five major endothermic peaks were measured during the thermal analysis. The effect of heating rate on the thermal analysis response showed that a high resolution of the thermal processes could be achieved at higher heating rates. Thermodynamic data was obtained for the various steps in the decomposition process including the formation of intermediate phases Na 3AlH6, and NaH. We also found that the decomposition of NaH is highly pressure dependent probably due to the high compressibility of the diffuse H− anion. The crystal-chemistry of NaAlH4 during decomposition has been established using X-ray diffraction analysis.
Ruthenium as Schottky metal for SiC-based high temperature hydrogen sensors
Authors: Basu S. , Roy S. , Jacob C. 
Published: April 2011

ABSTRACTSensor response characteristics of Ru/3C-SiC (epitaxial layer of SiC on Si substrates) Schottky junctions were studied at different temperatures (200 – 400 °C) in presence of varying concentrations of hydrogen from 5000 – 20000 ppm. The output signal of the sensor, the response time and the reversibility were investigated from the transient response characteristics of the sensors. The sensor parameters improved with higher operating temperature, up to 400 °C. The sensitivity of the sensors was found to be a function of applied bias across the Schottky junction. As compared with the Pd/3C-SiC junctions, the Ru/3C-SiC Schottky sensors showed higher resolution and better reversibility in the hydrogen concentration range 10000 to 20000 ppm. The SIMS (Secondary Ion Mass Spectrometry), RBS (Rutherford Backscattering Spectrometry) and GIXRD (Glancing Incidence X-ray Diffraction) studies indicated that up to 400 °C there was no formation of ruthenium silicide at the Ru/3C-SiC interface.
Transformation Crystallography and Plasticity of the δ→α′ Transformation in Plutonium Alloys
Authors: Krenn C. R. , Wall M. A. , Schwartz A. J. 
Published: April 2011

ABSTRACTIn delta phase Pu-Ga alloys, the transformation from the ductile face-centered cubic (fcc) δ phase that is retained at room temperature to the brittle low-temperature monoclinic alpha-prime phase is a thermally activated diffusionless transformation with double-c kinetics. Accurate modeling of the phase transformation requires detailed understanding of the role of plastic flow during the transformation and of the crystallographic transformation path. Using transmission electron microscopy (TEM), we find a significant increase in dislocation density in δ near the α′ plates, which suggests that plastic deformation contributes to the accommodation of the 20% reduction in volume during the transformation. Analysis of a series of optical micrographs of partially transformed alloys suggests that the α′ habit plane is usually nearly perpendicular to <111> δ. However, a small number of TEM observations support a habit plane near <112> or <123>, in agreement with earlier work.
Crystallographic Anisotropy in Compression of Uranium Metal to 100 GPa
Authors: Vohra Yogesh K. , Hope Kevin M. , Patterson J. Reed , Akella Jagannadham 
Published: April 2011

ABSTRACTNew high-pressure x-ray diffraction data on uranium metal (99.9 %) in a diamond anvil cell is presented to 100 GPa (Volume compression V/Vo = 0.700) at room temperature using a variety of pressure markers like ruby, copper, and platinum. The diffraction patterns are carefully indexed allowing for reversal of peak positions based on anisotropic compression. We report anisotropic compression of the orthorhombic unit cell with the axial ratio b/a increasing initially to 40 GPa followed by a rapid decrease at higher pressure. On the other hand, axial ratio c/a shows a rapid increase with increasing pressure followed by saturation at megabar pressures. The most recent full potential electronic structure calculations reproduce the increasing tend of axial ratio c/a to 100 GPa but do not explain the variation in the b/a ratio. Our detailed analysis of all available experimental data also indicates that the observed anisotropic effects are intrinsic to Uranium and are independent of the pressure medium used in the high-pressure experiments.
Advances in Understanding of the Crystal Chemistry of Hexavalent Uranium
Authors: Burns Peter C. 
Published: April 2011

ABSTRACTResearch concerning the crystal chemistry of hexavalent U by the Environmental Mineralogy and Crystal Structures research group at Notre Dame has resulted in the description of more than 110 new structures of uranyl compounds (including 36 minerals). New insights into the crystal chemistry of U6+ are presented, with emphasis on recently discovered novel structural connectivities. The structural hierarchy of uranyl minerals and compounds, which was first established for 180 structures in 1996, has been extended to include 145 new structures. The hierarchy is based upon polymerization of polyhedra containing higher-valence cations, and consists of five distinct classes: structures containing isolated polyhedra (7), finite clusters of polyhedra (41), chains of polyhedra (52), sheets of polyhedra (184), and frameworks of polyhedra (41). The dominance of sheets in uranyl compounds (57% of known structures) arises from the unequal distribution of bond-valences within the uranyl polyhedra. Topological relations of the sheets in uranyl compounds are best understood by analysis of the topological distribution of anions within sheets in which sharing of polyhedral edges dominates, and by graphical representation of the connectivity of polyhedra in cases where sharing of vertices of polyhedra dominates the sheet.
Pressure Dependence of Magnetic States of UGe2
Authors: Shick Alexander B. , Janiš Václav , Drchal Václav , Pickett Warren E. 
Published: April 2011

ABSTRACTThe correlated band theory picture (LSDA+U) has been applied to UGe2, in which superconductivity has been found to coexist with robust ferromagnetism. Over a range of volumes (i.e. pressures), two nearly degenerate states are obtained, which differ most strikingly in their orbital character (on uranium). The calculated moment, and its separation into spin and orbital parts, is consistent with recent polarized neutron scattering data. These two states are strong candidates for the two ferromagnetic phases, one low-temperature -- low-pressure, the other higher-temperature -- higher pressure. Magnetic waves built from fluctuations between these uranium configurations provide a possible new mechanism of pairing in UGe2.
Defect based spin mediation in δ-phase plutonium
Authors: Fluss Michael J. , Wirth Brian D. , Wall Mark , Felter Thomas E. , Caturla Maria J. , Kubota Alison , de la Rubia Tomas Diaz 
Published: April 2011

ABSTRACTWe earlier reported the measured decrease of electrical resistivity during isochronal-annealing of ion irradiation damage that was accumulated at low-temperature (10 or 20K), and the temperature dependence of the resistance of defect-populations produced by low-temperature damage-accumulation and annealing in a stabilized δ-phase plutonium alloy, Pu(3.3 at%Ga)[1]. We noted that the temperature dependence of the resistance of defects resulting from low-temperature damage accumulation and subsequent annealing exhibits a -ln(T) temperature dependence suggestive of a Kondo impurity. A discussion of a possible “structure-property” effect, as it might relate to the nature of the δ-phase of Pu, is presented.