bluuebunny/crossref_metadata_2025 huggingface dataset:
Design of composite channels for optimized transport in nitride devices
Authors: Singh Madhusudan , Singh Jasprit , Mishra Umesh K.
Published: April 2011

Heterostructure field effect transistors (HFETs) based on AlGaN / GaN structures have shown good performance as high power and high frequency devices. Theoretical simulations of transport in short channel HFETs show that there are several areas where considerable improvements in mobility, etc. can be made if thin composite structures (as considered in this work) can be utilized. We examine transport in a metal / AlGaN / InN / GaN composite structure. The InN region is very thin (∼ 15 Å) and is introduced to improve the low field transport without significantly impacting the device breakdown properties. Our model is capable of examining any other type of composite structure as well. Our simulation method consists of charge control solution of the heterostructure wave functions, followed by Monte Carlo simulation of scattering and free flight events. We present comparison of results on i) metal / AlGaN / GaN structure, and ii) a metal / AlGaN / GaN structure with a thin InN channel of the order of a few mono-layers. We find that mobility in the channel can improve considerably with very little effect on the mobility - charge product. Indeed, the charge density induced in the thin InN channel region is ∼ 1013cm∼2. While the peak velocities in a metal / AlGaN / InN / GaN structure exhibit an increase of nearly 30% over the values in metal / AlGaN / GaN structures, the low field mobilities are also increased. Low-field mobilities of ∼ 2500cm2(V ·s)∼ are predicted along with high sheet charges for low interface disorder for the structure with a thin InN layer. For higher degree of interface disorder (∼ 70Å), we have found good agreement with experimental Hall mobility data for similar structures. At higher electric fields, we find that most electron population transfers to higher valleys or other sub-bands that lie in AlGaN or GaN. This ensures that high field breakdown of low band gap InN layer is also suppressed.
Above and below Bandgap Excitation of Er-defect Complexes and Isolated Er in Er-implanted GaN
Authors: Braud A. , Abouzaid M. , Wojdak M. , Doualan J. L. , Moncorge R. , Pipeleers B. , Vantomme A.
Published: April 2011

Photoluminescence (PL) spectra, luminescence dynamics and luminescence saturation of the Er3+4I13/2 → 4I15/2 transition in Er-implanted GaN samples at 7K are investigated. Under below-gap excitation, different Er centers are identified. Several Er centers are clearly excited via local defects or impurities. Er-defect complexes excited by above or below bandgap light are compared. Luminescence dynamics study shows that the 4I13/2 manifold has a shorter lifetime when Er ions are part of Er-defect complexes than when Er ions are isolated from any defect. The saturation of Er luminescence is investigated for the different types of Er center corresponding to specific excitation wavelengths.
Magnetic properties of Mn-doped GaN, InGaN, and AlGaN
Authors: Reed M. L. , Berkman E. A. , Reed M. J. , Arkun F. E. , Chikyow T. , Bedair S. M. , Zavada J. M. , El-Masry N. A.
Published: April 2011

We report on the growth and magnetic properties of single crystal Mn-doped GaN, InGaN, and AlGaN films. The III-Nitride films were grown by MOCVD, while the Mn doping was performed by solid-state diffusion of a surface Mn layer deposited by pulsed laser ablation. Mn-doped InxGa1-xN films were grown with x < 0.15, where the easy axis of magnetization rotates from in-plane to out-of-plane by changing the InxGa1-xN thickness/strain-state of the film from compressively strained to relaxed. Mn-doped AlxGa1-xN films were grown with x < 0.40 showing ferromagnetic behavior above room temperature. SQUID measurements ruled out superparamagnetism within these films. By optimizing the growth and annealing conditions of Mn-doped III-Nitrides, we have achieved Curie temperatures in the range of 228 to 500K. These ferromagnetic Mn-doped III-Nitride films exhibit hysteresis with a coercivity of 100–500 Oe. TEM analysis showed no secondary phases within these films.
Combined MOCVD and MBE growth of GaN on porous SiC
Authors: Sagar Ashutosh , Feenstra R. M. , Inoki C. K. , Kuan T. S. , Koleske D. D.
Published: April 2011

GaN films have been grown homoepitaxially by MOCVD on MBE-grown GaN template layers, using both porous and nonporous SiC substrates. The effect of the porous SiC substrates on dislocations in the MBE and MOCVD GaN layers has been studied using TEM and x-ray characterization. A reduction in dislocation density from ≥1×1010 cm-2 in the MBE template to 2.5×109 cm-2 at the top of the MOCVD film is found, with similar final values in the MOCVD films for both porous and nonporous substrates. We discuss various mechanisms by which dislocation density is reduced in the MOCVD layers.
Atomic Structure of Defects in GaN:Mg grown with Ga polarity
Authors: Liliental-Weber Z. , Tomaszewicz T. , Zakharov D. , Jasinski J. , O'Keefe M. A. , Hautakangas S. , Laakso A. , Saarinen K.
Published: April 2011

Electron microscope phase images, produced by direct reconstruction of the scattered electron wave from a focal series of high-resolution images, were used to determine the nature of defects formed in GaN:Mg crystals. We studied bulk crystals grown from dilute solutions of atomic nitrogen in liquid gallium at high pressure and thin films grown by the MOCVD method. All the crystals were grown with Ga-polarity. In both types of samples the majority of defects were three dimensional Mg-rich hexagonal pyramids with bases on the (0001) plane and six walls on {1123} planes seen in cross-section as triangulars. Some other defects appear in cross-section as trapezoidal (rectangular) defects as a result of presence of truncated pyramids. Both type of defects have hollow centers. They are decorated by Mg on all six side walls and a base. The GaN which grows inside on the defect walls shows polarity inversion. It is shown that change of polarity starts from the defect tip and propagates to the base, and that the stacking sequence changes from ab in the matrix to bc inside the defect. Exchange of the Ga sublattice with the N sublattice within the defect leads to 0.6±0.2Å displacement between Ga sublattices outside and inside the defects. It is proposed that lateral overgrowth of the cavities formed within the defect takes place to restore matrix polarity on the defect base.
Influence of Growth Parameters on the Deep Level Spectrum in MBE-Grown n-GaN
Authors: Arehart A. R. , Poblenz C. , Heying B. , Speck J. S. , Mishra U. K. , DenBaars S. P. , Ringel S. A.
Published: April 2011

The impact of growth temperature and Ga/N flux ratio on deep levels in GaN grown by molecular beam epitaxy (MBE) is systematically investigated using both deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS) in a study designed to map out the presence and concentration of defects over a defined region of the MBE GaN growth phase diagram. A series of Si-doped GaN films were grown to cover a substrate temperature range and a Ga/N flux ratio range that spans from the N stable to the Ga droplet regimes along both variables. Identical growth templates were used to eliminate variations in dislocations between samples so that point defect variations could be tracked. For these samples, traps are detected at EC-Et=0.25, 0.60, 0.90, 1.35, 2.40, 3.04, and 3.28 eV. The near valence bands states at EC–3.04 and EC–3.28 eV are found to be strongly dependent on Ga/N flux with decreased concentrations as a function of increasing Ga flux toward the Ga droplet regime, but with little effect from growth temperature. The EC-1.35 eV level shows a strong dependence on growth temperature and only slight dependence on Ga/N flux ratio. In contrast, the concentration of the EC-Et=0.25, 0.90 eV levels increased with increasing Ga flux toward the Ga droplet regime, while the EC-Et=0.60 shows no dependence. The variation in concentration of the EC-2.40 eV level that has been related to VGa was difficult to quantify, but tends to increase towards nitrogen rich growth. The dependencies for the detected states with respect to growth temperature and Ga/N flux ratio suggest different physical point defect sources.
The Effects of Atmosphere, Temperature, and Bandgap on the Annealing of GaInNAs for Solar Cell Applications
Authors: Ptak A. J. , Kurtz Sarah , Young M. , Kramer C.
Published: April 2011

The effects of thermal annealing on the minority-carrier diffusion lengths and depletion widths of GaInNAs are studied. We find that diffusion lengths in as-grown, Be- and Si-doped GaInNAs are limited to less than about 0.2 μm for samples with low concentrations of nitrogen. For higher concentrations of nitrogen, the diffusion lengths are not measurable. Annealing under a variety of temperatures and atmospheres typically makes the diffusion lengths even shorter. These short diffusion lengths are not yet long enough for GaInNAs to be useful in a next-generation, four-junction structure. Using undoped GaInNAs in a p–i–n structure is a promising approach to increase device performance. Currently, however, the depletion widths are too small at the bandgaps necessary for solar cells, and annealing does not appear to improve them.
A study of anion exchange reactions at GaAs surfaces for heterojunction interface control
Authors: Losurdo Maria , Giuva Danilo , Capezzuto Pio , Bruno Giovanni , Brown Terence , Triplett Greg , May Gary , Brown April S.
Published: April 2011

GaPyAs1-y/GaAs, GaAsySb1-y/GaSb and GaSbyAs1-y/GaAs superlattices (SLs) grown by MBE, by exposure of GaAs to phosphorus and antimonide fluxes, and by exposure of GaSb to an arsenic flux, respectively, have been investigated. The focus is on the abruptness of interfaces and understanding the mechanisms associated with anion incorporation and exchange. In the case of the Sb flux interaction with the GaAs surface, the Sb segregation at the GaAs surface inhibits anion exchange. For the case of As over GaSb reactions, anion exchange results in the formation not only of the ternary alloy GaAsySb1-y, but also of isoelectronic compounds AsSbx that segregate at the GaSb/GaAs interface. In the case of the P flux interfaction with the GaAs surface, fast in-diffusion of P results in graded GaPyAs1-y layer formation.
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

Impurity-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

Positron 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

Zinc 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

An 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

Several 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

In 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

Oxidation 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

After 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

Molecular 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

Carbon 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.
Infrared Screening of Combinatorially Prepared Hydrogen Sorbing Metal Alloys
Authors: Olk C. H.
Published: April 2011

The investigation of many stoichiometric variations involving large numbers of combinations of elements offers a means to discover a hydride with optimal properties. We introduce the use of spatially resolved infrared imaging as a high throughput hydrogen storage candidate screening technique. Analysis is presented of a sample that consists of 16 separate Mg-Ni-Fe ternary pads and 32 Mg-Ni or Mg-Fe binary pads. Hydrogen sorption related emissivity changes observed indicate a substantial decrease in hydriding temperatures, which sensitively depends on composition.
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

Gd(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

Complex 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

Sensor 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.
Density Changes in Plutonium Observed from Accelerated Aging using Pu-238 Enrichment
Authors: Chung B. W. , Thompson S. R. , Woods C. H. , Hopkins D. J. , Gourdin W. H. , Ebbinghaus B. B.
Published: April 2011

Plutonium, because of its radioactive nature, ages from the “inside out” by means of self-irradiation damage and thus produces Frankel-type defects (vacancies and self-interstitial atoms) and defect clusters. The self-irradiation damage in Plutonium-239 occurs mainly by α-particle decay, where most of the damage comes from the U-235 recoil nucleus. The defects resulting from the residual lattice damage and helium in-growth could result in microstructural and physical property changes. Because these self-irradiation effects would normally require decades to measure, with a fraction (7.5 wt%) of Pu-238 is added to the reference plutonium alloy thus accelerating the aging process by approximately 18 times the normal rate. By monitoring the properties of the Pu-238 spiked alloy over a period of about 3.5 years, the properties of plutonium in storage can be projected for periods up to about 60 years. This paper presents density and volume changes observed from the immersion density and dilatometry measurements equivalent to aging the reference plutonium alloys to nine years.
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

In 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

New 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.
Nuclear Magnetic Resonance Studies of δ-Stabilized Plutonium
Authors: Curro N. J. , Morales L.
Published: April 2011

Nuclear Magnetic Resonance studies of Ga stabilized δ-Pu reveal detailed information about the local distortions surrounding the Ga impurities as well as provides information about the local spin fluctuations experienced by the Ga nuclei. The Ga NMR spectrum is inhomogeneously broadened by a distribution of local electric field gradients (EFGs), which indicates that the Ga experiences local distortions from cubic symmetry. The Knight shift and spin lattice relaxation rate indicate that the Ga is dominantly coupled to the Fermi surface via core polarization, and is inconsistent with magnetic order or low frequency spin correlations.
Advances in Understanding of the Crystal Chemistry of Hexavalent Uranium
Authors: Burns Peter C.
Published: April 2011

Research 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

The 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

We 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.
Development of Magneto-Optical Imaging for High-Throughput Characterization of Combinatorial Magnetic Thin Films
Authors: Zhao X. R. , Okazaki N. , Konishi Y. , Akahane K. , Kuang Z. , Ishibashi T. , Sato K. , Koinuma H. , Hasegawa T.
Published: April 2011

A new magneto-optical (MO) imaging system for high-throughput characterization of combinatorial magnetic thin films has been developed. The instrument allows us to measure both Faraday rotation and ellipticity maps at various wavelengths (400 nm∼1000 nm), different magnetic fields (0∼2000 G), and different temperatures (12 K∼300 K) for wide variety of materials. We used the magnetic circular dichroism (MCD) modulation technique to map MO properties, relatively free from substrate effects. The superiority of this system is that magnetic hysteresis curves of numerous specimens with different compositions prepared by the combinatorial technique can be simultaneously measured at one sweep of magnetic field, providing an efficient characterization method for combinatorial magnetic materials. We also confirmed that the system possesses enough spatial resolution and sensitivity for detecting MO signals of individual pixels contained in a combinatorial library.
Micro X-Ray Fluorescence As A High Throughput Screening Method For Combinatorial Chemistry
Authors: Havrilla George J. , Miller Thomasin , Warner Benjamin , Wells Cyndi
Published: April 2011

Micro X-ray fluorescence is used as a high-throughput screening method for combinatorial applications. These include chemical weapon degradation products, catalytic phosphate hydrolysis and radiologic dispersion device metals. In each application the intrinsic elemental signature was utilized to identify the lead hits for the combinatorial screening in locating peptide sequences exhibiting the desired binding characteristics. In addition to being nondestructive, rapid and non-perturbing to the binding event, this method can be used to quantify the amount of target material.
Machine-Learning Models for Combinatorial Catalyst Discovery
Authors: Landrum Gregory A. , Penzotti Julie , Putta Santosh
Published: April 2011

Standard machine-learning algorithms were used to build models capable of predicting the molecular weights of polymers generated by a homogeneous catalyst. Using descriptors calculated from only the two-dimensional structures of the ligands, the average accuracy of the models on an external validation data set was approximately 70%. Because the models show no bias and perform significantly better than equivalent models built using randomized data, we conclude that they learned useful rules and did not overfit the data.
Application of Combinatorial Chemistry Methods to The Development of Organic Coatings
Authors: Chisholm Bret , Potyrailo Radislav , Cawse James , Brennan Michael , Shaffer Ron
Published: April 2011

A combinatorial method for the development of organic clear coatings for plastic substrates has been developed. The combinatorial process or combinatorial factory has a throughput on the order of 100–200 coatings per day and possesses all of the aspects of a conventional combinatorial process such as automated sample preparation, miniaturized samples configured in an array format, high throughput screening of the properties of interest, and computer software developed for data management, storage, and analysis. The combinatorial factory greatly enhanced the rate of organic clear coat development.
Combinatorial Strategies for Thin Organic Films -Polythiophene via Surface Polymerization by Ion Assisted Deposition
Authors: Hanley Luke , Tepavcevic Sanja , Choi Yongsoo
Published: April 2011

Combinatorial strategies are applied to surface polymerization by ion assisted deposition (SPIAD) for the production of polythiophene films. Thiophene ion energy in the range of 25–200 eV and ion to α-terthiophene neutral ratio are varied to tune the extent of surface polymerization and film photoluminescence. SPIAD polythiophene films are grown by both mass-selected and non-mass-selected ions, while x-ray photoelectron spectroscopy and surface mass spectrometry are applied to determine film chemistry and the extent of polymerization. This work demonstrates that ion kinetic energy and ion to neutral ratio can be varied over a wide range in SPIAD to select films with useful optical or electronic properties. The essentially combinatorial approach of SPIAD can be dramatically extended by use of new ions and/or neutral species.
Combinatorial methods and high-throughput experimentation in synthetic polymer chemistry
Authors: Hoogenboom Richard , Meier Michael A. R. , Schubert Ulrich S.
Published: April 2011

Combinatorial chemistry has revolutionized the drug discovery as well as the catalyst discovery and optimization process during the last years. Nowadays, triggered by these developments, combinatorial methods and parallel chemistry also emerge in the field of material and polymer chemistry. Especially the field of polymer research seems to be perfectly suited for these approaches since many parameters can be varied during synthesis, processing, blending, formulation and compounding. Moreover, the screening of interesting parameters, such as molecular weight, polydispersity, polymerization kinetics, viscosity, hardness or stiffness, became feasible only recently.

Within this contribution we will describe our strategy to construct a most efficient workflow in the field of combinatorial polymer research. New developments made by our group as well as some general aspects will be discussed.
A Combinatorial Chemistry Study of YAG Nucleation
Authors: Hay Randall S.
Published: April 2011

A combinatorial method to screen large numbers of dopants that affect the nucleation of yttrium-aluminum garnet was developed. The method involves heat-treatment of electron-transparent sol-derived thin films on TEM grids, and counting the number and size of nuclei by TEM. Advantages and limitations of the technique are discussed. Preliminary results are presented.
The Development of Scanning Microwave Microscope for High-Throughput Characterization of Dielectric and Conducting Materials at Low Temperatures
Authors: Okazaki Sohei , Okazaki Noriaki , Sugaya Hidetaka , Zhao Xiaoru , Hasegawa Ken , Ahmet Parhat , Chikyow Toyohiro , Nishimura Jun , Fukumura Tomoteru , Kawasaki Masashi , Murakami Makoto , Mastumoto Yuji , Koinuma Hideomi , Hasegawa Tetsuya
Published: April 2011

We developed a scanning microwave microscope (SμM) designed for characterizing local electric properties at low temperatures. A high-Q λ/4coaxial cavity was used as a sensor probe, which can detect the change of quality factor due to the tip-sample interaction with enough accuracy. From the measurements of combinatorial samples, it was demonstrated that this SμM system has enough performance for high-throughput characterization of sample conductance under variable temperature conditions.
Data Mining Approach to Ab-Initio Prediction of Crystal Structure
Authors: Morgan Dane , Ceder Gerbrand , Curtarolo Stefano
Published: April 2011

Predicting crystal structure is one of the most fundamental problems in materials science and a key early step in computational materials design. Ab initio simulation methods are a powerful tool for predicting crystal structure, but are too slow to explore the extremely large space of possible structures for new alloys. Here we describe ongoing work on a novel method (Data Mining of Quantum Calculations, or DMQC) that applies data mining techniques to existing ab initio data in order to increase the efficiency of crystal structure prediction for new alloys. We find about a factor of three speedup in ab intio prediction of crystal structures using DMQC as compared to naïve random guessing. This study represents an extension of work done by Curtarolo, et al. [1] to a larger library of data.
Unique Indexing Scheme of Decagonal Phases Based on a Six Dimensional Model
Authors: Mandal R. K. , Pramanick A. K. , Sastry G. V. S. , Lele S.
Published: April 2011

Mandal and Lele (1989) have proposed a six dimensional model for the structural description of the decagonal phases. The integral linear combination of six basis vectors for indicating a physical vector in their model, however, leaves the problem of redundancy in indexing. While revisiting their model, we have noted that the condition of a null vector in physical space permits the formulation of unique indexing scheme both in physical reciprocal and direct spaces, we will demonstrate that our scheme, unlike all previously discussed ones, relies only on the information contained in the model. It will also be shown that diffracted spot having equivalent indices possesses identical intensity. This aspect, though equally important, has been totally ignored in the past. We shall substantiate our claim by taking examples from the known decagonal phases. We shall also present parity condition on indices that will be helpful in discussing subtle features of diffraction patterns. The notion of weak and strong diffracting conditions is explained based on the zone rule in physical space in terms of Cartesian components of direct and reciprocal vectors.
Electronic structure of Al-Cr-Fe intermetallics
Authors: Belin-Ferré Esther , Dankhazi Zoltan , Fontaine Marie-Françoise , Thirion Jean , de Weerd Marie-Cécile , Dubois Jean Marie
Published: April 2011

We report here the results obtained for the valence band of four compounds from the Al-Cr-Fe system, all being approximants of the decagonal quasicrystalline phase as probed by a combination of X-ray emission and X-ray photoemission spectroscopy techniques. Salient features ared-dandp-dhybridizations at the Fermi level and in its close vicinity as well as a repulsive interaction between thedstates of the two transition elements. Within about 4 eV below the Fermi level Al states are suggested to be of more localized-like character in the Al-Cr-Fe samples than in Al-Cu-Fe ones.
Plasticity of decagonal Al73Ni10Co17 quasicrystals
Authors: Schall Peter , Feuerbacher Michael , Urban Knut
Published: April 2011

We present a study of the deformation mechanism of decagonal Al73Ni10Co17 quasicrystals by means of transmission electron microscopy. We performed compression tests on single-quasicrystalline samples in three different orientations: with the compression axis parallel to, inclined by 45 ° and perpendicular to the tenfold axis of the decagonal quasicrystal. The deformed samples reveal characteristic orientation-dependent dislocation structures leading us to the conclusion that fundamentally different deformation mechanisms are involved in plastic deformation in the three deformation geometries. We explicitly identified the Burgers vectors of the dislocations as interatomic vectors in the structure of decagonal Al-Ni-Co.
Local Order in Single Grain Cd-Yb Icosahedral Phase
Authors: Wu Y. Q. , Kramer M. J. , Lograsso T. A.
Published: April 2011

Single grains of icosahedral Cd84Yb16 quasicrystalline phase were characterized using high resolution transmission electron microcopy (HRTEM) technique. Image reconstruction of the obtained multiple through-focal series for both the two and five-fold axes are consistent with the local cluster structure of concentric polyhedra consisting of Cd tetrahedron (1/3 occupied icosahedron), Cd dodecahedron, Yb icosahedron and a Cd icosidodecahedron. Edge-sharing of the clusters can result in complete tiling of the 2-D projections. Simple matching rules are being investigated to construct a 3-D model.