bluuebunny/crossref_metadata_2025_split huggingface dataset:
Recent Progress Toward an Integrated Multiscale-Multiphysics Model of Reactor Pressure Vessel Embrittlement
Authors: Wirth B. D. , Odette G. R. , Stoller R. E. 
Published: April 2011

ABSTRACTThe continued safe operation of nuclear reactors and their potential for lifetime extension depends on ensuring reactor pressure vessel integrity. Reactor pressure vessels and structural materials used in nuclear energy applications are exposed to intense neutron fields that create atomic displacements and ultimately change material properties. The physical processes involved in radiation damage are inherently multiscale, spanning more than 15 orders of magnitude in length and 24 orders of magnitude in time. This paper reports our progress in developing an integrated, multiscale-multiphysics (MSMP) model of radiation damage for the prediction of reactor pressure vessel embrittlement. Key features of the fully integrated MSMP model include: i) combined molecular dynamics (MD) and kinetic lattice Monte Carlo (KMC) simulations of cascade defect production and cascade aging to produce cross-sections for vacancy, self- interstitial and vacancy-solute cluster size classes for times on the order of seconds; ii) an integrated reaction rate theory and thermodynamic code to predict the evolution of nanostructural and nanochemical features for times on the order of decades; iii) a micromechanics model to calculate the resulting mechanical property changes. This paper will focus on the combined use of MD and KMC to simulate the long-term rearrangement (aging) of defects in displacement cascades and thus, produce late-time production cross-sections for vacancy and vacancy cluster features.
Combination of Continuum and Atomistic Approaches for the Study of Dislocation Nucleation from Atomic Size Surface Defects
Authors: Brochard Sandrine , Beauchamp Pierre , Grilhé Jean 
Published: April 2011

ABSTRACTAtomistic simulations realized on an f.c.c. crystal containing atomic size surface defects (step and groove) show that the defects are privileged sites for dislocation nucleation. Before nucleation, an elastic shear, precursor of the dislocation, appears in the plane in zone with the step where the dislocation will be nucleated. In order to explain the strong localization of the localized elastic precursor shear, we have analyzed the stress concentration near the surface defects using the continuum point force approach. For the step case, the origin of the localized shear is related to an increase in the interplanar separation due to the stress concentration.
Multi-Scale Simulations of Silicon Etching by Halides: Effects of Surface Reaction Rates.
Authors: Kratzer Matthias , Steinhögl Werner , Kersch Alfred , Sachse Tanja , Höink Volker 
Published: April 2011

ABSTRACTWe investigate the reactive ion etching of amorphous silicon by halides using a hierarchy of models on different time and length scales. The feature evolution is modeled using a two- dimensional cell based Monte-Carlo feature scale simulator. The fluxes, the energy distributions, and the angular distributions of the wafer-incident particles are provided by a hybrid plasma sheath simulator. The relevant surface reaction rates are calculated by a molecular dynamics simulator using a Stillinger-Weber representation of the interatomic potential. Our investigations show that the surface reaction rates are strongly determined by the particular surface morpho- logy, which, in turn, is strongly influenced by the kinetic properties of the impinging particles. Thus, we link the molecular dynamics simulator into the model as a whole.As results, we present calculations for the etching of amorphous silicon by fluorine, chlorine, and bromine. A Stillinger-Weber representation of the bromine and the silicon-bromine potential which was not yet available in literature is additionally developed. We discuss the different morphologies of halogenated silicon surfaces as a consequence of the energy distri- bution and the angular distribution of the impinging particles. Comparisons of the sputter yield functions of bare amorphous silicon surfaces and corresponding halogenated surfaces exhibit considerable differences, qualitative as well as quantitative.
Dielectric Polarization of Materials: A Modern View
Authors: Resta Raffaele 
Published: April 2011

ABSTRACTThe concept of macroscopic polarization is the basic one in the electrostatics of dielectric materials: but for many years this concept has evaded even a precise microscopic definition, and has severely challenged quantum-mechanical calculations. This concept has undergone a genuine revolution in recent years (1992 onwards). It is now pretty clear that—contrary to a widespread incorrect belief—macroscopic polarization has nothing to do with the periodic charge distribution of the polarized crystal: the former is essentially a property of the phase of the electronic wavefunction, while the latter is a property of its modulus. An outline of the modern viewpoint is presented. Experiments invariably address polarization derivatives (permittivity, piezoelectricity, pyroelectricity,…) or polarization differences (ferroelectricity), and these differences are measured as an integrated electrical current. The modern theory addresses this same current, which is dominated by the phase of the electronic wavefunctions. First-principle calculations based on this theory are in spectacular agreement with experiments, and provide thorough understanding of the behavior of dielectric materials.
AB-Initio Calculation of the optical Properties of BN(110) Surface.
Authors: Cappellini Giancarlo , Satta Guido , Palummo Maurizia , Onida Giovanni 
Published: April 2011

ABSTRACTWe study the optical properties of the nonpolar (110) surface of cubic Boron Nitride calculated within the first-pinciple DFT-LDA scheme. The reflectance anisotropy (RA) spectrum is analyzed in relation to the better known spectrum of the GaAs(110) surface. The influence of the ionicity on the surface relaxation, the surface states character and the surface optical spectra, is studied. Comparisons with existing data for the surface under study and results to the GaN(110) surface are also given.
Molecular-Dynamics Simulations of Magnetic Structures in Metallic Systems
Authors: Meyer Ralf , Lewis Laurent J. 
Published: April 2011

ABSTRACTRecently a method has been proposed which allows the calculation of complex magnetic structures from a simple d-band tight-binding Hamiltonian including Coulomb and exchange interactions with the help of a molecular-dynamics simulations. In this article an improved version of this ap- proach is suggested which retains the rotational symmetry of the simulated system. The improved algorithm is applied to systems with electron numbers in the range 7 ≤nd ≤ 8.
Efficient Reconstruction of Multi-Phase Morphologies from Correlation Functions
Authors: Rozman Michael G. , Utz Marcel 
Published: April 2011

ABSTRACTA highly efficient algorithm for the reconstruction of microstructures of heterogeneous media from spatial correlation functions is presented. Similar to previously proposed algorithms, the new method relies on Monte Carlo optimization, representing the microstructure on a discrete grid. An efficient way to update the correlation functions after local changes to the structure is introduced. In addition, the rate of convergence is substantially enhanced by selective Monte Carlo moves at interfaces. Speedups over prior methods of more than two orders of magnitude are thus achieved. The algorithm is ideally suited for parallel computers. The increase in efficiency brings new classes of problems within the realm of the tractable, notably those involving several different structural length scales and/or components.
Coupled atomistic-mesoscopic model of polycrystalline plasticity
Authors: Cleri Fabrizio , D'Agostino Gregorio , Satta Alessandra , Colombo Luciano 
Published: April 2011

ABSTRACTWe discuss a microstructure evolution framework which couples atomic-level information about extended-defect interactions into a mesoscopic model; the latter, in turn, describes the dy-namic evolution of a statistical population of grain boundaries and dislocations. Atomistic simulations are carried out by means of molecular dynamics simulations on both isolated and interacting dislocations, grain boundaries, triple junctions, microcracks; the reference material for such studies is, at present, Silicon with the Stillinger-Weber potential. The mesoscale model describes the motion of discrete triple junctions (and, consequently, of the continuous network of adjoining grain boundaries) embedded in a continuous medium containing a homogenous, evolving distribution of dislocations.
A Simulation Study of Tracer Diffusion Concentration Profiles Resulting from the Transition from Dislocation Pipes to a Grain Boundary Slab
Authors: Belova Irina V , Murch Graeme E 
Published: April 2011

ABSTRACTIn the present study we examine the well-known analysis in which the dislocation pipe diffusivity is determined by means of a grain boundary type analysis of the tail of a tracer concentration depth profile. We use a Monte Carlo grid method for testing the analysis. The results show that the analysis is really only satisfactory when the spacing between the dislocations is roughly twice the diffusion length (Dlt)½ where Dland t are the lattice diffusivity and time respectively.
A Kinetic Study of Hydrocarbons Reactivity on Palladium Catalysts through a DFT Approach
Authors: Bertani Valeria , Cavallotti Carlo , Masi Maurizio , Carrá Sergio 
Published: April 2011

ABSTRACTPalladium clusters have been chosen to represent a typical supported heterogeneous catalyst and their interaction with hydrocarbons has been investigated theoretically. The calculations were performed through density functional theory and the Becke-Lee-Yang-Parr hybrid (B3LYP) functional was adopted to calculate exchange and correlation energy. An effective core potential basis set (ECP on core electrons and Dunning/Huzinaga on outer electrons) was found sufficiently accurate to reproduce experimental data. Clusters containing up to seven Pd atoms were considered and their interaction with hydrogen, methane and ethane and their fragments was analyzed and a kinetic study of the system was performed. Transition states structures and energies were calculated through quantum mechanics and kinetic constants were derived from a statistic thermodynamic approach. On the basis of such information, a kinetic model that accounts for ethane transformations. Finally the kinetic scheme was embedded in a plug flow reactor model and simulations were performed to test the validity of the developed mechanism. In this way information obtained at the atomic scale were adopted to study phenomena occurring on the much higher reactor scale.
Residual Stresses in Silicon Nitride Based Composites Using Synchrotron Radiation
Authors: Lee Myungae , Xiao Yanan , Wittmer Dale E. , Mini Susan M. , Graber Timothy J. 
Published: April 2011

AbstractIn order to measure the residual strain (stress) in the silicon nitride based composites, synchrotron based x-ray diffraction was employed on the BESSRC beamline at the APS. The residual stress for both the baseline Si3N4 and the Si3N4-TiN composites were determined from the (441) and (531) reflections, obtained by applying the three-dimensional 2θ-sin2ψ method. In both the baseline Si3N4 and the Si3N4-TiN composites, after thermal shocking, compressive residual stresses were developed in both directions parallel and perpendicular to the specimen's surface. The average residual stresses in the direction parallel to the sample surface were much higher than in the perpendicular direction.The measured residual stresses were compared with the flexural strength and fracture toughness results to determine the effects of residual stress and thermal shocking on the strength and toughness of each composite. The results suggested that there should be a maximum thermal shock temperature, within the range of 1000 °C to 1100 °C, for improving the fracture toughness for both the baseline Si3N4 and the Si3N4-TiN composites. Also, the addition of the TiN appeared to improve both the strength and toughness of the baseline composition.
Microcracks in Carbon/Carbon Composites: A Microtomography Investigation using Synchrotron Radiation
Authors: Paris Oskar , Peterlik Herwig , Loidl Dieter , Rau Christoph , Weitkamp Timm 
Published: April 2011

AbstractThe mechanical properties of brittle matrix composites such as carbon/carbon (C/C) are closely related to the generation and propagation of microcracks. A better understanding of the role of microcracking requires a quantification of the three-dimensional morphology of the crack network. In this study we demonstrate that phase contrast microtomography using synchrotron radiation is a unique tool to get 3D information about cracks in C/C. This is shown for three different C/C specimens subjected to different final heat treatment temperatures (HTT). The results are discussed qualitatively with respect to the influence of HTT on the distribution of microcracks and their relevance for the mechanical properties of C/C.
Structure and Bonding of Metallic Nanowires Prepared in Nanoporous Alumina Membranes Studied by EXAFS, XANES and HED
Authors: Kröll M. , Benfield R. E. , Dore J. C. , Grandjean D. , Schmid G. 
Published: April 2011

AbstractThe structural properties of metallic nanowires prepared within the pores of nanoporous alumina membranes are investigated using EXAFS, XANES and High Energy X-Ray Diffraction (HED). Gold, silver, copper and iron nanowires show the same structure and atom-atom distances as the corresponding bulk metals. The structural properties of these metals do not change if the wire diameter is varied. Cobalt nanowires, however, show a size dependent mixture of fcc and hcp phases. The amount of fcc-Co, which in the bulk is only known as a high temperature modification, increases with decreasing wire diameter. This leads to interesting changes in the magnetic properties of Co nanowires.
X-RAY Magneto-Optics in Lanthanide Materials
Authors: Starke K. , Heigl F. , Vollmer A. , Kaindl G. 
Published: April 2011

AbstractWe demonstrate the feasibility of element-specific magnetization reversal studies applying x-ray magneto-optical Kerr effect (XMOKE) to lanthanide systems. In particular, we will first describe XMOKE spectra at 4d-4f excitation thresholds of Gd and Tb in some detail, before showing element specificity at the example of a Gd/Y/Tb-trilayer metal film. On the basis of experimental Tb M4,5absorption spectra, we may anticipate substantial MO signals in specular reectivity at M4;5edges of all lanthanide elements, provided they carry a non-vanishing net 4f-magnetization.
Society Photoemission and Photoabsorption Study of YNi2−xCoxB2C Superconductors
Authors: Hsu L.-S. , Guo G. Y. , Chen C.-J. , Lan M.-D. , Lee J.-F. 
Published: April 2011

AbstractThe electronic structures of five polycrystalline YNi2−xCoxB2C (x=0, 0.05, 0.1, 0.15, and 0.2) borocarbide superconductors were studied by photoemission and photoabsorption spectroscopies and theoretical calculations. The valence-band (VB) photoemission spectrum is compared with the theoretical total and partial density-of-states (DOS) curves. The VB satellite is peaked at a binding energy (EB) of 6 eV. The Ni K-edge x-ray absorption near edge spectra (XANES) are compared with the calculated XANES spectra for these intermetallic compounds. Extended x-ray absorption fine structure (EXAFS) spectra at the Ni and Co K edges are analyzed to yield the structural parameters. The decrease of the superconducting transition temperatures (Tc) with addition of Co dopant in these compounds is due to a decrease of the total DOS at the Fermi level (EF).
Proton-gated Molecular Devices Based on Rod-shaped Metal Complexes Immobilized on Solid Surface
Authors: Haga Masa-aki , Shiozawa Youichi , Suzuki Shinichiro , Inoue Miyao 
Published: April 2011

ABSTRACTRod-shaped dinuclear Ru/Os complexes, [Os(bimpy-X)(BL)M(bimpyH2)]n+( M = Os or Ru; bimpyH2 = 2,6-bis(benzimidazol-2-yl)pyridine, X = anchoring group such as disulfide and phosphonic acid, BL =bis tridentate ligand), as a potential candidate for molecular devices have been successfully self-assembled on gold or indium-tin oxide (ITO) electrode by using anchoring ligands. The Ru/Os assembled complexes on the solid surface were well characterized by means of MALDI-TOF mass spectrometry, XPS, and electrochemical measurements. Novel proton-coupled electron transfer reaction of the self-assembled dinuclear complex on M(bimpyH2) moiety (M = Ru or Os) was confirmed by analyzing the plots of oxidation potential, E1/2, vs pH. In the case of M = Os and BL = 1,4-bis(terpyridyl)benzene (btpb), two stepwise one-electron oxidation waves were observed at 0.67 V for the Os(III/II) process and 0.88 V vs Ag/AgCl for the Ru(III/II) process in CH3CN/H2O at pH 1.0. The E1/2-pH diagram reveals the intercrossing of Ru(II/III) and Os(II/III) processes at around pH 5.5. Therefore, the intramolecular electron transfer can be controlled by pH. Proton-gated electron rectifying effect was observed in this Ru-Os dinuclear complex.
Components for AlGaN/GaN Power Amplifiers
Authors: Jacobs B. , Wingelaar P. , Kramer M. , Falcone S. , Karouta F. , Hek P. De , Suijker E. , Dijk R. Van 
Published: April 2011

ABSTRACTOhmic contacts and Schottky contacts were made on an undoped AlGaN/GaN FET structure. Despite the high Al content (33%), we were still able to obtain a contact resistance of 0.3 ωmm. Pulsed measurements showed the large effect of self-heating even for circular contacts with a radius of 50 μm. The behavior ofthe Ni/Au Schottky contacts is according to the charge control model; the reverse current and capacitanceonly scale with the area of the diode. Tests with polygon type diodes showed no dependence of the reverse current on the number of polygon corners. The reverse current decreased when the devices were aged at 400°C for 30 hrs. Coplanar Waveguide discontinuities were realized on AlN substrates. A scalable lumped element model was derived from measurements for T-junctions, transmission lines, bends and crosses.
Analysis of Electron Transport in a High-Mobility Freestanding GaN Substrate Grown by Hydride Vapor-Phase Epitaxy
Authors: Yun F. , Morkoc H. , Rode D. L. , Tsen K. T. , Farina L. , Kurdak C. , Park S. S. , Lee K. Y. 
Published: April 2011

ABSTRACTSemiconductor nitrides grown on substrates with a large lattice mismatch typically contain extended and point defects that prevent the full potential of this material system from being attained. Among allthe substrate options explored so far, freestanding GaN templates appear ideal for homoepitaxial growth of GaN films. To this end, hydride vapor-phase epitaxial (HVPE) grown GaN templates with a thickness of more than 200 μm were thermally lifted off from the sapphire substrate and mechanically polished. The defect densityof such a template is expected to be non-uniform in the growth direction, especially near the back surface which was in close vicinity of the sapphire substrate. We, therefore, studied the transport properties of this template before and after the removal of a 30 μm region from the back-side. For as-prepared GaN, Hall mobilities of 1100 cm2/V-s and 6800 cm2/V-s were obtained at 295 K and 50 K, respectively. A simultaneous fitting of mobility and carrier concentration was used to quantify the contribution ofdifferent scattering mechanisms. When the backside was etched by ∼30 μm, Hall mobilities improved to 1200 cm2/V-s at 295 K and 7385 cmsup2/V-s at 48 K, respectively. A numerical solution of the Boltzmann transport equation (BTE) that deals with the inelastic nature of electron scattering by polar optical mode was employed to determine the acceptor concentration. Raman spectroscopy was employed to obtain LO and TO phonon energies, which were then used in the above-mentioned calculations. The best fittings of the mobility and carrier concentration data yield an average acceptor concentration of 4.9×1015 cm-3 and a donor concentration of 2.1×1016 cm-3 for the as-prepared GaN. The average acceptor concentration decreased to 2.4×1015 cm-3 after etching of the backside, which confirms that the etched-away region contained higher density of defects. The donor activation energy is derived to be 25.2 meV. Our analysis demonstrated high quality of the freestanding GaN substrate with the highest reported electron mobility for wurtzite GaN.
Growth of 3C-SiC Layers on Silicon Substrates with a Novel Stress Relaxation Structure
Authors: Irokawa Yoshihiro , Yamada Noboru , Kodama Masahito , Kachi Tetsu 
Published: April 2011

ABSTRACTSilicon (Si) substrates having cavities just beneath the surface layer (multi-cavity Si substrates) were examined whether they worked as the stress relaxation structure in 3C-SiC heteroepitaxial growth on Si. Single crystalline 3C-SiC layers were grown on the multi-cavity Si substrates by means of low pressure chemical vapor deposition (LPCVD). The layers' quality was characterized by the cross-sectional TEM observations and the Micro-Raman spectroscopy. The TEM results showed that this structure reduced the defect density in the 3C-SiC layers. The averaged full width at half-maximum (FWHM) of LO Raman mode in the 3C-SiC layerson the multi-cavity Si substrates became narrower than that on the conventional Si substrates. Furthermore, Schottky barrier structures showed that the reverse leakage current of the diodes using the multi-cavity Si substrates is smaller than that using the conventional Si substrates. These results indicate that the multi-cavity Si substrates are effective for stress relaxation in the 3C-SiC layers.
Electrical properties of MBE grown Si-doped AlxGa1−xN as a function of nominal Al mole fraction up to 0.5
Authors: Ahoujja M. , Yeo Y. K. , Hengehold R. L. , Nostrand J. E. Van 
Published: April 2011

ABSTRACTHall-effect measurements were conducted on Si-doped AlxGa1−xN films grown on sapphire substrate by gas source molecular beam epitaxy. The Al mole fraction in the 1 [.proportional]m thick AlxGa1−xN was 0.0, 0.3, and 0.5, and the Si doping concentration was kept at a nominal value of 1018 cm−3. Variable temperature Hall-effect measurements reveal a presence of a highly degenerate n-type region at the AlxGa1−xN /sapphire interface. This degenerate interfacial layer dominates the electrical properties below 30 K and significantly affects the properties of the AlxGa1−xN layer. Thus, by using a two-layer conducting model, the carrier concentration and mobility of the AlxGa1−xN layer alone are obtained.
Structural, optical and electrical properties of GaN films grown by metalorganic chemical vapor deposition on sapphire.
Authors: Visconti P. , Reshchikov M. A. , Yun F. , Jones K. M. , Morkoç H. , Passaseo A. , Piscopiello E. , Pomarico A. , Cingolani R. , Lomascolo M. , Catalano M. 
Published: April 2011

ABSTRACTProperties of GaN layers grown by metalorganic chemical vapor deposition (MOCVD) on c-plane of sapphire have been investigated using atomic force microscopy (AFM), wet etching for defect investigation, transmission electron microscopy (TEM), high-resolution X-ray diffraction, Hall effect measurements and low-temperature photoluminescence (PL). Tapping-mode AFM images of the as-grown samples showed atomically smooth surfaces (rms roughness ≍ 0.2 nm) consisting of terraces separated by about 3Å bi-layer steps. Hot H3PO4 chemical etching was used to produce hexagonal-shaped etch pits at the surface defect sites as revealed by AFM imaging. The obtained etch pit densities (9×108 - 2 ×109 cm−2) were in agreement with the dislocation density found by plan-view and cross-sectional TEM observations. The full-width at half-maximum (FWHM) of the X-ray diffraction rocking curve was about 4.8 and 3.9 arcmin for the symmetric (002) and asymmetric (104) directions, respectively. PL spectrum at 15 K demonstrated sharp peaks (FWHM ≍ 4 meV) in the excitonic region, which were attributed to free and bound excitons. The spectrum contained also weak PL bands with maxima at about 2.2, 2.9 and 3.27 eV, which have been attributed to three different acceptors.