bluuebunny/crossref_metadata_2025_split huggingface dataset:
An ab initio two-center tight-binding approach to simulations of complex materials properties
Authors: Frauenheim Th. , Porezag D. , Elstner M. , Jungnickel G. , Elsner J. , Haugk M. , Sieck A. , Seifert G. 
Published: April 2011

ABSTRACTWe describe theab initioconstruction of two-center tight-binding (TB) hamiltonians, which at a properly chosen input density upon non-selfconsistent solution of the related Kohn-Sham equations transform the energy within density-functional theory (DFT) into a tight-bindinglike expression. In cases, where the electron density of the interacting many-atom structure in good approximation may be represented as a sum of atomic-like densities, the method has been shown to operate highly transferable, being particularly successful in determining the properties of low-energy silicon clusters, in predicting the structure and vibrational signatures of fullerene oligomers, amorphous carbons and carbon nitrides and in simulating elementary growth reactions on diamond surfaces. The uncertainties within the standard non-SCF DF-TB-variant, however, increase if the chemical bonding is controlled by a delicate charge balance between different atomic constituents, as e.g. in organic molecules and in polar semiconductors. Therefore, we extend the standard TB-approach to the operation in a selfconsistent-charge mode(SCC-DFTB)in order to improve total energies, forces, and transferability in the presence of considerable long-range Coulomb interactions. By using a variational technique, we derive a transparent and readily calculable expression for the iterative modification of Hamiltonian matrix elements and show, that the final energy is a second order approximation to the total energy in density-functional theory,see M. Elstner et al., this Symposium.First successful applications to surface studies of GaAs and dislocation modeling in GaN will be presented.
Structure, Energy, and Electronic Properties of the Σ = 13 {510} Tilt Grain Boundary Structure In Si
Authors: Morris J. R. , Lu Z.-Y. , Ring D. M. , Xiang J.-B. , Ho K.-M. , Wang C. Z. , Fu C.-L. 
Published: April 2011

ABSTRACTWe have examined a variety of structures for the {510} symmetric tilt boundary in Si, using first-principles calculations. These calculations show that the observed structure in Si is the lowest energy structure. This structure is more complicated than what is necessary to preserve four-fold coordination. We compare the results to classical and tight-binding models, in order to test these empirical approaches.
Molecular Dynamics Simulations of Porous Silica
Authors: Beckers J.V.L. , De Leeuw S. W. 
Published: April 2011

ABSTRACTWe describe a new simulation method for the preparation of porous silica and present results from molecular dynamics simulations of the structures obtained. We start from a homogeneous liquid phase with reduced atomic charges. The charges are then slowly rescaled and the atoms start clustering to finally form a porous network. We observe that local ordering precedes formation of long range correlations. We investigate physical properties of porous silica such as porosity, internal surface and fractality. They are in reasonable agreement with experimental data, although internal surface and porosity seem to be systematically larger than those found in adsorption experiments. The vibrational and dielectric power spectra show an enhanced intensity in the low frequency region. These modes can be associated with slow dynamics of clusters.
An Atomistic Study of Ti Segregation to Lamellar Interfaces in Ti-RICH TiAl
Authors: Ito K. , Vitek V. 
Published: April 2011

ABSTRACTIn this paper we analyze the effect of the surplus of titanium in the bulk on γ/γ interfaces. Monte Carlo calculations using a central force many-body potential suggest that in Ti rich alloys titanium segregates to the 120° rotational fault and the pseudotwin. This leads to the formation of a thin region of the DO19 Ti3Al at these interfaces. While titanium does not segregate to the ordered twin, it does to the ordered twin with the APB. But in this case the interface dissociates into the 120° rotational fault and the pseudotwin. The calculations further show that there are two types of atomic sites at the interfaces. One is the same as in the ideal L10 the other, to which the segregation takes place, is specific for interfaces parallel to {111} planes. The specific distribution of the sites favored for segregation is the reason why segregation leads to the formation of a narrow region of the DO19 Ti3Al in the 120° rotational fault and the pseudotwin.
Evolution of Microstructure in the BCC-HCP Martensitic Phase Transition in Zirconium
Authors: Ackland G. J. , Pinsook U. 
Published: April 2011

ABSTRACTWe report molecular dynamics simulations of the martensitic phase transition from bcc to hep in zirconium. We show the evolution of a laminated twinned microstructure, in which some plastic deformation has occurred to rotate the twins manifested as basal stacking faults. This rotation is such as to alter the twinning angle from the 60° between the hep variants to the 61.5° angle of the low energy (1011) twins. These are thus identified as a cause of microscopic irreversibility in the transition.
Structural and Mechanical Properties of Nanophase Ni: A Molecular-Dynamics Study of the Influence of Grain-Boundary Structure on Elastic and Plastic Behavior
Authors: Van Swygenhoven H. , Spaczér M. , Caro A. 
Published: April 2011

ABSTRACTMolecular dynamics computer simulations of high load plastic deformation at temperatures up to 500K of Ni nanophase samples with mean grain size of 5 nm are reported. Two types of samples are considered: a polycrystal nucleated from different seeds, each having random location and random orientation, representing a sample with mainly high angle grain boundaries, and polycrystals with seeds located at the same places as before, but with a limited missorientation representing samples with mainly low angle grain boundaries. The structure of the grain boundaries is studied by means of pair distribution functions, coordination number, atom energetics, and common neighbour analysis. Plastic behaviour is interpreted in terms of grain-boundary viscosity, controlled by a self diffusion mechanism at the disordered interface activated by thermal energy and stress.
The Role of Preparation Conditions on the SIC(001) Surface Reconstructions: A First Principles Study
Authors: Galli Giulia , Catellani Alessandra , Gygi Francois 
Published: April 2011

ABSTRACTUsing first principles molecular dynamics, we have studied the reconstructions and thermal properties of the (001) surfaces of cubic SiC. Our calculations show that C-terminated surfaces can have different reconstructions, depending on preparation conditions and thermal treatment, and that the Si-terminated surface geometry can be substantially affected by the presence of stress. Our findings allow us to interpret recent experiments about (001) SiC surfaces, in particular STM images.
Stoichiometry and Structure of Polar Group-III Nitride Semiconductor Surfaces
Authors: Fritsch J. , Sankey O. F. , Schmidt K. E. , Page J. B. 
Published: April 2011

ABSTRACTWe use a local-orbital formalism based on density-functional theory to investigate the stoichiometry and structure of the cation- and anion-terminated (0001) surfaces of wurtzite-phase GaN and A1N. We compare the total energies computed for various p(2×2) reconstructions. First-layer atom deficient structures such as vacancies are found to be the most stable configurations for the anion- and cation-terminated surfaces. For metal rich growth conditions our calculations favor the adsorption of metal atoms. Surface chemical reactions relevant for the growth of thin nitride films, such as the adsorption of hydrogen and nitrogen from decomposed ammonia, are discussed. We determine the total energy differences for the co-adsorption of NH2 and H on different surface structures.
Ab Initio Study of Electronic and Geometric Structures of Metal/Ceramic Heterophase Boundaries
Authors: Köstlmeier S. , Elsässer C. , Meyer B. , Finnis M. W. 
Published: April 2011

ABSTRACTThe adhesion geometries of coherent cube-on-cube interfaces between spinel (MgAl2O4) and the two metals Al and Ag were determined by density functional band structure calculations in the local density approximation (LDA). For Al/MgAl2O4, for which experimental data are available, the calculated optimum interface geometry is in excellent agreement with HRTEM measurements (distance dint: 1.90 Å calc., 1.90±0.04 Å exp.).The work of adhesion Wad is calculated for three different high-symmetry translation states between an Al-0 terminated (001) surface of the spinel and the (001) surface of each of the metals. The binding energy curves display a distinct optimum for the adhesion of aluminum atoms on top of the spinel oxygen ions at a Wad value of 2.4 J/m2. For silver several adsorption sites are isoenergetic at 1.1 J/m2 and the intersections of the Wad (dint) curves indicate a low-energy dissociation path. A further analysis of the electronic structure in the Al/MgAl2O4 system reveals the charge redistribution in the metal towards the oxygen ions as the main contribution to bonding. On the contrary, polarization of the metal film is the major effect observed on the adhesion of Ag to the spinel substrate.
The Local Atomic Structure of Lal-xSrxCoO3: Effects Induced by The Spin-State and Non-Metal to Metal Transitions
Authors: Louca Despina , Sarrao J. L. , Kwei G. H. 
Published: April 2011

ABSTRACTThe pair density function (PDF) used in the analysis of pulsed neutron diffraction data of La1-xSrxCoO3 revealed new structural effects which are correlated to the susceptibility and transport transitions. The transition in the spin configuration of the Co ions from the low-spin (LS) to the high-spin (HS) state in the Co perovskite oxides can potentially induce structural distortions due to the coupling of the spin to the lattice and charge. The ground state of the pure compound, LaCoO3, is in the LS state and is non-magnetic. A transition occurs to the HS state at ∼ 50 K as indicated from the susceptibility measurements due to the thermal excitation of electrons to the eg level. The CoLSO6 octahedra associated with the Co ions in the LS configuration are distinguished from the CoHSO6 octahedra with the Co in the HS configuration because the CoLS-O bond length is shorter than the CoHS-O distance due to the different size of the corresponding Co ions. Such bond lengths are clearly identified in the local structure between 15 – 300 K. This finding is in contrast to the average structure which shows only one type of bond length in this temperature range but two types of bond lengths are suggested at considerably higher temperatures. This suggests that whereas the LS and HS CoO6-octahedra coexist, they are randomly distributed in the crystal lattice at lower temperatures and become ordered at higher temperatures. The introduction of charge carriers in the structure does not eliminate the coexistence of both the LS and HS states, indicating that with the transition to the ferromagnetic metallic state, the spin configuration is not entirely of the HS character and structural inhomogeneities are present.
Observation of Large Low Field Magnetoresistance in Ramp-Edge Tunneling Junctions Based on Doped Manganite Ferromagnetic Electrodes and A SrTiO3 Insulator
Authors: Kwon C. , Jia Q. X. , Fan Y. , Hundley M. F. , Reagor D. W. , Hawley M. E. , Peterson D. E. 
Published: April 2011

ABSTRACTWe report the fabrication of ferromagnet-insulator-ferromagnet junction devices using a ramp-edge geometry based on (La0.7Sr0.3)MnO3 ferromagnetic electrodes and a SrTiO3 insulator. The multilayer thin films were deposited using pulsed laser deposition and the devices were patterned using photolithography and ion milling. As expected from the spin-dependent tunneling, the junction magnetoresistance depends on the relative orientation of the magnetization in the electrodes. The maximum junction magnetoresistance (JMR) of 30 % is observed below 300 Oe at low temperatures (T < 100 K).
Microstructural Aspects of Nanocrystalline LiZn Ferrites Densified with Chemically Derived Additives
Authors: Cho Yong S. , Burdick Vemon L. , Amarakoon Vasantha R. W. , Underhill Elijah , Brissette Leo 
Published: April 2011

ABSTRACTDensification behavior and microstructural characteristics of nanocrystalline LiZn ferrites with chemically derived additives were investigated. Nanocrystalline Li0.3Zn0.4Fe2.3O4 powders having a ≈ 15 nm size were prepared at a low temperature of 450°C by a chemical synthesis using a combustible polyacrylic acid (PAA). Small amounts of Si, Ca and Mn were incorporated into the nanocrystalline ferrites via sol-gel reactions utilizing tetraethyl orthosilicate, calcium isopropoxide and manganese acetate. This process was believed to give a homogeneous distribution of the additives over the nanocrystalline ferrites. A uniform microstructure was obtained without any evidence of exaggerated grain growth after sintering at 1100°C. Saturation magnetization and coercive force were found to increase with the chemical additives. The results were compared with those of the same composition, but processed by the conventional batch-mixing of corresponding oxide additives.
Pressure and Isotope Effects in the Manganese-Oxide Perovskites
Authors: Goodenough J. B. , Zhou J.-S. 
Published: April 2011

ABSTRACTMeasurements of the temperature dependence of the resistivity ρ(T) and thermoelectric power α(T) under several hydrostatic pressures on l8O/l6O isotope-exchanged (La1-xNdxCa0.3MnO3 polycrystalline samples spanning the structural O'-O orthorhombic transition have demonstrated that the perovskite tolerance factor t increases with pressure, signaling an unusually compressible Mn-0 bond. They have also indicated a change at the O'-O transition from static to dynamic cooperative Jahn-Teller deformations, from a second-order to a first-order magnetic transition, from Mn(IV) to two-Mn polarons in the paramagnetic region, from Mn(IV) polarons to a vibronic state below Tc, and a phase segregation in the O phase above Tc that traps out mobile polarons into ferromagnetic Mn(IV)-rich clusters within a Mn(IV)-poor matrix. Specific heat data show a transfer of spin entropy to configurational entropy on cooling through Tcin the O phase, and a single-crystal study of La1-xSrxMnO3, x = 0.12 and 0.15, has demonstrated a transition from polaronic to itinerant e electrons below Tc within the O phase. Magnetic-susceptibility measurements in low fields confirm the phase segregation above Tc and the existence of a ferromagnetic glass below Tc in the O phase. The intrinsic “colossal” magnetoresistance (CMR) is attributed to the growth to their percolation threshold of the ferromagnetic clusters existing above Tc, and the dramatic rise in Tc with increasing tolerance factor t in the O-orthorhombic phase to an increase in the density, and also the mobility, of the untrapped polarons above Tc.
Magnetic Anisotropy and Lattice Distortions in The Doped Perovskite Manganites
Authors: Suzuki Y. , Hwang H. Y. , Cheong S-W. , Van Dover R. B. , Asamitsu A. , Tokura Y. 
Published: April 2011

ABSTRACTWe have investigated the magnetic anisotropies of doped manganite materials in epitaxial thin film and single crystal form. Structural characterization, including x-ray diffraction, Rudierford backscattering spectroscopy and atomic force misocroscopy, indicate that our epitaxial films arc single crystalline and have excellent crystallinity. Since lattice distortions greatly affect the magnetic and transport properties of this family of materials, it is not surprising to find the profound effect of strain in films due to the lattice mismatch between the substrate and film. Magnetic anisotropy results of single crystals, subject to no external stress, is compared to those of epitaxial films.
Evidence for a Jahn-Teller Distortion in the CMR Layered Manganite La1.4Sr1.6Mn2O7
Authors: Louca Despina , Kwei G. H. , Mitchell J. F. 
Published: April 2011

ABSTRACTThe changes with temperature in the crystallographic structure of the two-layered La1.4Sr1.6Mn2O7, are on average quite small but the atomic pair density function analysis of pulsed neutron diffraction data shows that the lattice is locally distorted in accordance with the change in the transport properties. In particular, while no Jahn-Teller (JT) distortion is expected in the layered compounds because the octahedral bilayers are almost cubic, lattice distortions attributed to a large JT effect are present and are of comparable magnitude as in the cubic perovskite system. This could in turn explain the similarity in their properties. The number of the JT distorted sites is reduced with temperature concomitantly with the decrease in resistivity of the ab-plane.
Mixed Ionic-Electronic Conduction in N1 Doped Lanthanum Gallate Perovskites
Authors: Long N. J. , Tuller H. L. , Hurt Lower , Zealand New 
Published: April 2011

ABSTRACTLanthanum gallate is a promising material for “monolithic” fuel cells or oxygen pumps, i.e. one in which the electrolyte and electrodes are formed from a common phase. We have investigated La1−xSrxGa1–yNiyO3 (LSGNx-y) with x=0.1 and y=0.2 and 0.5 as a potential cathode material for such an electrochemical device. The σ(PO2,T) for LSGN10–20 points to a p-type electronic conductivity at high PO2 and predominantly ionic conductivity at low PO2. LSGN10–50 has an electronic conductivity suitable for SOFC applications of approximately 50 S/cm in air at high temperature. AC impedance spectroscopy on an electron blocking cell of the form M/LSG/LSGN/LSG/M was used to isolate the ionic conductivity in the LSGN10–20 material. The ionic conductivity was found to have a similar magnitude and activation energy to that of undoped LSG material with σi= 0.12 S/cm at 800°C and EA= 1.0 ± 0.1 eV. Thermal expansion measurements on the LSGN materials were characterized as a function of temperature and dopant level and were found to match that of the electrolyte under opeating conditions.
Carbon and Fluorinated Carbon Materials for Fuel Cells
Authors: Wheeler D. , Luczak F. , Fredley R. , Cipollini N. 
Published: April 2011

ABSTRACTCarbon and fluorinated carbon materials are major constituents of phosphoric acid fuel cells and PEM fuel cells and the stability of these materials is critical for long life operation. Laboratory corrosion studies of separator plate materials were correlated with separator plate changes in commercial PAFC fuel cells. The addition of thin films of Teflon™ to the separator plates extends the life of the separator plates to 60,000+ hours through the formation of a temporary hydrophobic barrier. ESCA studies show the loss of hydrophobicity with time of PAFC electrodes to be a result of delamination of the Teflon from the carbon and not corrosion of the Teflon by phosphoric acid. The projected life of PAFC power plants has been confirmed by commercial operation of power plants for over 40,000 hours.
Accelerated Corrosion of Stainless Steels in the Presence of Molten Carbonate below 923 K
Authors: Ota Ken-ichiro , Toda Katsuya , Motohira Naobumi , Kamiya Nobuyuki 
Published: April 2011

ABSTRACTThe high temperature corrosion of stainless steels (SUS316L and SUS310S) in the presence of molten carbonate [(Li0.62K0.8)2CO3 and (Li0.52Na0.48)2CO3] has been studied in a CO2-O2 atmosphere by measuring the weight gain of the specimens.The corrosion of SUS316L significantly depended on the reaction conditions. With the carbonate coating (both Li/Na and Li/K carbonates), severe corrosion occurred during the initial period of the corrosion test below 923 K, especially around 823 K. The initial severe corrosion was a local corrosion which produced through holes in the metal specimens and occurred more clearly at low Pco2 with the Li/Na coating than with the Li/K coating. The corrosion became more severe at higher CO2 pressures and lower O2 pressures. In a pure CO2 atmosphere (without O2), the corrosion rate significantly increased at 823 K. The steel was corroded uniformly at that time.
LiCoO2 Thin Films Grown by Pulsed Laser Deposition on Low Cost Substrates
Authors: Ginley D. S. , Perkins J. D. , McGraw J. M. , Parilla P. A. , Fu M.L. , Rogers C. T. 
Published: April 2011

AbstractWe report on the use of pulsed laser depositon (PLD) to grow thin films of LiCoO2 on a number of low cost substrates including SnO2 coated Upilex, stainless steel and SnO2 coated glass. Highly textured (001) films grown on CVD deposited SnO2 films on 7059 glass, were obtained at 200 to 500 mTorr O2 and a temperature of 500 C. Similar texture was not obtained on the stainless or Upilex however dense films from crystalline to amorphous were obtained. The films were characterized by x-ray diffraction and Raman spectroscopy.
Advances in Battery Technologies and Markets: Material Science Aspects
Authors: Salkind A. J. 
Published: April 2011

AbstractAdvances in materials innovation, availability, processing, and volumetric and gravimetric utilization, have lead to dramatic changes in the technologies and markets for batteries, and other electrochemical storage devices, in recent years. The increased ability to cycle of the new materials and preparations has also resulted in a more dominant proportion of secondary batteries in the market. The growth of the various technology sectors of the battery and energy storage device industry will be compared to the relevant improvements in materials science. The developments needed in materials, composites, and processing techniques is reviewed, in order to match the ongoing changes in electronic circuits, power devices, transportation, and the growth of portable appliances. An analysis of the technical/market segments of the battery industry is presented.
A New Polymer Cathode-Conducting Polymer Interconnected With Sulfur-Sulfur Bonds: Poly(2,2'-Dithiodianiline)
Authors: Naoi Katsuhiko , Kawase Ken-ichi , Mori Mitsubiro , Komiyama Michiko 
Published: April 2011

AbstractPoly(2, 2'-Dithiodianiline)(Poly(DTDA)), a conducting polymer having disulfide bond in it, has been proposed as a new class of high energy storage material. DTDA has one S-S bond interconnected between two moieties of anilines. The structure of the poly(DTDA) was similar to that of polyaniline in addition that the S-S bond is preserved after electropolymerization of DTDA. The poly(DTDA) has some advantages because of its high theoretical energy density, faster kinetics and higher electrical conductivity than other organosulfur cathodes
Raman Scattering in LiCoO2 Single Crystals and Thin Films
Authors: Perkins J. D. , Fu M. L. , Trickett D. M. , McGraw J. M. , Ciszek T. F. , Pariila P. A. , Rogers C. T. , Ginley D. S. 
Published: April 2011

ABSTRACTWe report Raman scattering measurements for uniaxially textured and randomly oriented polycrystalline LiCoO2 min films as well as for LiCoO2 and LiCo0.4Al0.6O2 single crystals. For both the crystalline LiCoO2 thin film samples and the single crystal LiCoO2 samples, well defined phonon modes are observed at Raman shifts of 486 cm−1 and 596 cm−1corresponding to the expected Eg and Alg modes of the layered LiCoO2 crystal structure with R3m symmetry. Upon Al substitution for Co in LiCoO2, the two phonon modes appear to shift to higher energy, but further work is needed to clarify this point.