bluuebunny/crossref_metadata_2025 huggingface dataset:
Superlattice Calculation in an Empirical spds* Tight-Binding Model
Authors: Scholz R. , Jancu J.-M. , Bassani F.
Published: April 2011

We propose an empirical tight-binding method for tetrahedrally coordinated cubic materials and apply it to group IV and III-V semiconductors, extending existing calculations by the inclusion of all five d-orbitals per atom in the basis set. The symmetry character of the conduction states at the surface of the Brillouin zone is considerably improved compared to calculations in smaller bases, and the corresponding band positions can be obtained within the experimental uncertainties. Because the distance dependence of the tight-binding parameters is derived from deformation potentials, the model is particularly suited for an investigation of strained superlattices where the states at direct or pseudo-direct conduction band minima are composed of wavefunctions of all the minima at Γ, X, and L of the constituents. Investigations of GaAs/AlAs and short-period superlattices indicate a strong mixing between the conduction band valleys in the miniband structure, and the results are in better agreement with experiments than state-of-the-art empirical pseudopotential calculations.
Self-Consistent Tight-Binding Methods Applied to Semiconductor Nanostructures
Authors: Carlo Aldo Di
Published: April 2011

A self-consistent tight-binding approach applied to semiconductor nanostructure is presented. This allows us to describe electronic and optical properties of nanostructured devices beyond the usual envelope function approximation. Example of applications are given for High Electron Mobility Transistors (HEMTs) and non-linear optical devices.
A Novel Scheme for Accurate Md Simulations of Large Systems
Authors: De Vita Alessandro , Car Roberto
Published: April 2011

We present a simple and informationally efficient approach to electronic-structure-based simulations of large material science systems. The algorithm is based on a flexible embedding scheme, in which the parameters of a model potential are fitted at run time to some precise information relevant to localised portions of the system. Such information is computed separately on small subsystems by electronic-structure “black box” subprograms, e.g. based on tight-binding and/or ab initio models. The scheme allows to enforce electronic structure precision only when and where needed, and to minimise the computed information within a desired accuracy, which can be systematically controlled. Moreover, it is inherently linear scaling, and highly suitable for modern parallel platforms, including those based on non-uniform processing. The method is demonstrated by performing computations of tight-binding accuracy on solid state systems in the ten thousand atoms size scale.
Prediction Of Structure Candidates For Simple Ionic Compounds Using Global Optimisation
Authors: Schön J. C. , Jansen M.
Published: April 2011

A method is presented that allows, in principle, the prediction of the existence and structure of (metastable) compounds. We show the results of this approach for two examples of binary and ternary ionic compounds that have not been synthesized yet, but should stand a fair chance of being kinetically stable.
The Effects of the Electron-Phonon Interaction on the Vibrational Anomalies and Polymorphism in Titanium
Authors: Gavartin J. L. , Bacon D. J.
Published: April 2011

We apply the frozen phonon and molecular dynamics methods within the semiempirical orthogonal tight-binding framework to study the anomalous behaviour of the (0001) optical longitudinal (LO) and transverse (TO) phonons in the low temperature hep phase of Ti, and the ⅔[111]L and ½[110]T1 phonons in the high temperature bec phase. We demonstrate that, in agreement with previous findings in Zr, the anomalous thermal frequency shifts in hep Ti are related to the strong coupling of the electron density of states (DOS) to the particular lattice distortions. The distortions along the bec ⅔[111]L and ½[110]T1 phonons also significantly affect the DOS, resulting in the instability of these modes at low temperatures and triggering the bcc-hep and bcc-ω phase transformations.
Electronic Structure of Amorphous Silicon
Authors: Allan G. , Delerue C. , Lannoo M.
Published: April 2011

The electronic structure of a continuous network model of tetrahedrally bonded amorphous silicon (a-Si) and of a model hydrogenated amorphous silicon (a-Si:H) that we have built from the a-Si model are calculated in the tight binding approximation. The band edges near the gap are characterized by exponential tails of localized states induced mainly by the variations in bond angles. The spatial localization of the states is compared between a-Si and a-Si:H. Valence band offset between the amorphous and the crystalline phases is calculated.
Electronic Structure and Atomic Configuration of Extended Defects in Metals by First-Principles and Semiempirical TB-LMTO Methods
Authors: Šob M. , Turek I. , Vitek V.
Published: April 2011

We present two tight-binding linear muffin-tin orbitals (TB-LMTO) techniques for electronic structure calculations of extended defects (such as grain boundaries, interphase interfaces, surface layers etc.) in metals. The first is based on the first-principles self-consistent surface Green's function approach within the atomic-sphere approximation (ASA) utilizing two-dimensional periodicity in the layers parallel to the interface. In the second approach the Hamiltonian is constructed within the TB-LMTO-ASA as well, but semiempirical terms are employed to characterize the repulsive part of the interaction and the effect of electrons in interstitial space. While the adjustable parameters have only been fitted to the properties of ideal ground state structure, the semiempirical approach describes correctly the structural energy differences, phonon frequencies etc. Two examples are presented: the electronic structure of the Σ = 5(210)/[001] tilt grain boundary in tungsten is determined and the sensitivity of 4d magnetic moments in thin films to local environment is discussed. A comparison of the semiempirical TB-LMTO-ASA with the first-principles full-potential LMTO results is performed along the trigonal deformation path connecting the bcc, simple cubic and fee structures and the applicability of the semiempirical approach for simulating atomic structure of extended defects is assessed.
Defects in Aluminum: A Density Functional Study
Authors: Ramprasad R. , Atlas S. R.
Published: April 2011

We describe a new method of examining defects and their interactions with each other at a microscopic level of detail. This involves efficiently calculating local properties at arbitrary spatial points, applicable in periodic supercell calculations. In particular, we have identified two quantities, the local energy density, ε(r) and the local stress density, σαβ(r), which are constrained to yield the total energy and the average macroscopic stress tensor, respectively, when integrated over the entire volume of the supercell. In systems with defects, these microscopic quantities provide important insights about the local nature and environment of defects. For the test case of bulk Al with a point defect, we demonstrate that these concepts result in meaningful local quantities characteristic to the point defect. We propose to use these methods to study the microscopic nature of vacancies at a grain boundary and their interaction with each other.
Potential Energy Calculation of two Structures of the Σ=11 <011> Tilt Grain Boundary in Silicon and Germanium
Authors: Chen J. , Hairie A. , Lebouvffir B. , Nouet G. , Paumter E.
Published: April 2011

Available experimental data concerning two possible atomic structures (A and B) of the Σ=11 <011> tilt grain boundary in silicon and germanium are analyzed. Previous empirical calculations concerning low and high temperature stability of these structures are summarized and criticized. New calculations using the semi-empirical tight-binding method (SETBM) are presented for low temperature stability. The tight-binding parameters of Mercer and Chou give results in agreement with experimental observations by high resolution electron microscopy.
H2S and HS-Adsorption on a Charged Cu Surface in an Electrolyte: Effect of Ionic Strength
Authors: Wilson W. D. , Schaldach C. M.
Published: April 2011

We present a method for the calculation of the binding and rotational energies of neutral (H2S) and charged (HS-) molecules impinging upon a charged (Cu <100>) surface in the presence of an electrolyte. A molecular surface is constructed surrounding the H2S and HS-molecules forming boundary elements. A coupled Schrödinger-Poisson-Boltzmann iterative procedure treats the electronic structure of the molecules at the 6–31G**/MP2 level of theory and includes solvation effects through the single and double layers of charge induced by the electronic distribution. The molecule, together with its charged layers, forms a Molecular Single and Double Layer (MSDL), an object which then interacts with a Gouy-Chapman plane within the electrolyte. The additional induced charge at the molecular surface resulting from this electric field is obtained by solving a second set of boundary element equations. Repulsive interactions between the atoms of the molecule and those of the surface are obtained using a rigid-ion Hartree-Fock method. Binding energies of the molecule to the surface are determined as a function of the real surface charge imposed and also the ionic strength of the solution. It is found that surface charges can completely (180°) reorient these molecules and that the counterions in the solution can completely screen binding effects of even large surface charges.

Work supported by the United States Department of Energy under contract #DE-AC04–94AL85000.
Effects of Alloying Elements on Iron Grain Boundary Cohesion
Authors: Chen X. , Ellis D. E. , Olson G. B.
Published: April 2011

For a long time, understanding the mechanisms of impurity-promoted embrittlement in iron and the consequent cohesion(decohesion) effects has been a challenge for materials scientists. The role alloying elements play in impurity-promoted embrittlement is important due to either their direct intergranular cohesion(decohesion) effects or effects upon embrittling potency of other impurities. Some alloying elements like Pd and Mo are known to be helpful for intergranular cohesion in iron and some other alloying elements like Mn are known to segregate to and weaken iron grain boundaries dramatically[1]. There have been intensive investigations on these mechanisms for a long time and especially, with the progress in computing techniques in recent years, calculations on more realistic models have become possible[2–4]. In this paper we briefly present our studies on some selected alloying-element/iron grain boundaries(GB) and free surface(FS) systems. The effects of Pd, Mo, Mn and Cr on the Fe Σ5 (031) grain boundary and its corresponding (031) free surface are examined, using a combination of molecular dynamics(MD) and first-principles electronic structure calculations. Section 2 gives a brief introduction to the methods used and Section 3 gives the main results.
Influence of Aluminum Grain Boundary Misorientation on Penetration by Gallium
Authors: Hugo R. C. , Hoagland R. G.
Published: April 2011

Materials that undergo intergranular degradation processes, such as Stress Corrosioi Cracking (SCC) or certain types of Liquid Metal Embrittlement (LME), show variations in sus ceptibility with grain boundary character Samples with high fractions of “special” boundaries i.e. highly periodic boundaries with low-S Coincidence Site Lattices (CSL), exhibit higher resis tance to these types of attack[l,2]. Very useful comparisons have been drawn between low periodicity boundaries and high-periodicity boundaries; however, few attempts have been mad to date to explain variations in behavior between various “general” boundaries. The presen study employs the LME of aluminum by liquid gallium in an attempt to elucidate the variation in structure and properties of general grain boundaries.
Diffusion Processes and Pre-Exponential Factors in Homo-Epitaxial Growth on Ag(100)
Authors: Kürpick Ulrike , Rahman Talat S.
Published: April 2011

We provide a recipe for the evaluation of pre–factors for various diffusion processes relevant to crystal growth on Ag(100), based on explicit calculations of appropriate vibrational density of states using realistic many-body interaction potentials. Our calculated diffusion coefficients show that while hopping processes are favored for terrace self-diffusion, exchange processes dominate step-edge diffusion. We also find the characteristics of adatom diffusion along the < 100 >-step-edge to be strikingly different from the < 110 >-step-edge with interesting consequences for interlayer transport on this surface.
Monte Carlo Simulation of Magnetization Reversal Via Domain-Wall Motion in Fe Sesquilayers on W(110)
Authors: Kolesik M. , Novotny M. A. , Rikvold Per Arne
Published: April 2011

Iron sesquilayers are ultrathin films with coverages between one and two atomic mono-layers. They consist of an almost defect-free monolayer with compact islands of a second atomic layer on top. This variation of the film thickness results in a strong interaction between domain walls and the island structure. It makes these systems an ideal laboratory to study the dynamics of domain walls driven by weak external fields. We present computer simulations which provide insight into the role of the thermally activated nucleation processes by which a driven domain wall overcomes the obstacles created by the islands.
Molecular Dynamics Study of the Effect of Magnetic Field on the Static and Dynamical Properties of Two-Dimensional Coulomb Systems
Authors: Gumbs Godfrey , Dubey Girija S.
Published: April 2011

Molecular dynamics simulations are used to examine the effect of a uniform perpendicular magnetic field on a two-dimensional (2D) interacting electron system and we analyze how the magnetic field affects the single-particle properties of the system. In this simulation, we include the effect of the magnetic field classically through the Lorentz force. Both the Coulomb interaction and the magnetic field are included directly in the electron dynamics to study their combined effect on the transport properties of the 2D system. Results are presented for the pair correlation function, the mean square displacement and the density correlation function, in the presence and absence of an external magnetic field. Our simulation results, obtained from a first-principles calculation, clearly show that the external magnetic field has no effect on the static properties, but it affects the dynamics.
Contribution to the Dielectric Behavior of Relaxers from the Surface of Nano-Order Cluster in the Materials
Authors: Cheng Z. -Y. , Guo Aqiang , Katiyar R. S.
Published: April 2011

Based on the analysis of the dielectric behavior of relaxor ferroelectrics, it is assumed that the material has two polarization processes. Thus, a formula, which can fit the experimental results very well, is proposed to describe the temperature and frequency dependence of the dielectric constant. The fitted results show that there is a resonance process in the material and the peak of the dielectric constant is determined with both the relaxation and resonance processes. The relaxation time analysis shows that the peak of the dielectric constant indeed consists of two parts.
Monte Carlo Simulations of Fluid Phase Transitions in Micropores: Hysteresis and Van Der Waals-Type Loops
Authors: Gusev V. Yu.
Published: April 2011

We report the results of the Monte Carlo simulation study of the fluid phase transitions in micropores. The Gibbs, Canonical, and Grand Canonical ensembles were employed to obtain the adsorption isotherms of the Lennard-Jones nitrogen in carbon slit micropores at 77.4 K. It is shown for the first time that depending on the choice of the parameters of the Monte Carlo simulations, such as the number of particles in the simulation cells, the sampling from the Gibbs ensemble may supply either van der Waals-type or hysteresis loops in the phase transition zones. The existence of these loops is confirmed by independent sampling from the Canonical ensemble using Widom method of test particles. The Grand Canonical ensemble simulations always produce hysteresis loops within the phase transition zone. Outside the phase transition zones, sampling from all three ensembles yield essentially identical isotherms.
A Method to Predict the Adhesion Energy of Nonreactive Ceramic-Metal Wetting Based on Chemical Thermodynamics Considerations
Authors: Wu Jianxin , Wang Xiaodong
Published: April 2011

A previous model is refined to predict the adhesion energy of nonreactive ceramic-liquid metal wetting using the radii of metal atoms and ceramic ions and the enthalpies of mixing at infinite dilution. This method is convenient in that no fit to experiment data is required.
Interfacial Strain-Field in Thin-Film Microstructures
Authors: Shen Qun , Kycia Stefan
Published: April 2011

We present x-ray diffraction analysis of interfacial strain fields in crystalline semiconductor microstructures. In particular we report the observations of diffuse interference fringes, asymmetric grating diffraction patterns, and asymmetric crystal-truncation-rod profiles due to the straw-varying regions in a microstructure. From these diffraction features, both the longitudinal and the transverse strain gradients are determined, in addition to the average stram. These experimental results can be used to interpret other physical properties in microstructures such as band-gap energies and surface morphologies.
Atomistic Simulations of the Work of Adhesion at Metal Oxide Interfaces
Authors: Ohira T. , Inoue Y.
Published: April 2011

In this work we have studied magnetite scale adhesion on a tube made from some different type materials, and treated metal oxide interfaces between magnetite scales (Fe3O4) and stainless steel tube surface oxides (NiFe2O4) or chromium coating tube surface oxides (Cr2O3, FeCr2O4). In this paper we have defined new MEAM (Modified Embedded Atom Method) parameters for Fe-O, Cr-O, Ni-O, Fe-Ni, and Fe-Cr pair interactions based on experimental information such as lattice constants, cohesive energies, bulk moduli of those metal oxides, and have calculated the work of adhesion at Fe3O4 (110) / Fe3O4 (110), NiFe2O4(110), Cr2O3(110), Cr2O3(100), FeCr2O4(110) interfaces where a cluster-surface interfacial model is adopted. As the results of the calculation, it was found that there is an energy barrier, which prevents scales from approaching a tube surface, in a potential energy curve vs. a separation between scales and chromium coating surfaces. We further discuss the effect of surface directions at interfaces on the work of adhesion.
First Principles Calculations to Describe Zirconia Pseudopolymorphs
Authors: Jomard G. , Petit T. , Magaud L. , Pasturel A.
Published: April 2011

The structural and electronic properties of four different structures of zirconia (ZrO2) are studied using ab initio total energy calculations. The calculations are made in the framework of density functional (DFT) and pseudopotential theory. We compare results given within the LDA (Local Density Approximation) and including Generalized Gradient Corrections (GGCs) in the Perdew Wang and Perdew Becke formalisms. We present results for pure and defective zirconia (oxygen vacancies and Zr substitution by Fe) showing the effects of such point defects on tne relative structural stabilities of the different pseudopolymorphs.
Analysis of Cation Valences and Oxygen Vacancies in Magnetoresistive Oxides by Electron Energy-Loss Spectroscopy
Authors: Wang Z. L. , Yin J. S. , Berta Y. , Zhang J.
Published: April 2011

Magnetic oxides of (La,A)MnO3and (La,A)CoO3have two typical structural characteristics: cations with mixed valences and oxygen vacancies, which are required to balance the charge introduced by cation doping. The consequences introduced by each can be different, resulting in different properties. It is important to quantitatively determine the percentage of charges balanced by each, but this analysis is rather difficult particularly for thin films. This paper has demonstrated that electron energy-loss spectroscopy (EELS) can be an effective technique for analyzing Mn and Co magnetic oxides with the use of intensity ratio of white lines, leading to a new technique for quantifying oxygen vacancies in functional and smart materials.
Thin Film Growth and Magnetotransport Study of (La, Sr)MnO3
Authors: Manako Takashi , Obata Takeshi , Shimakawa Yuichi , Kubo Yoshimi
Published: April 2011

Thin-film samples of La1-xSrxMnO3 (x = 0.20 – 0.30) were grown by pulsed-laser deposition using various target compositions, substrate materials, growth temperatures, oxygen partial pressures, and laser-pulse repetition rates. The crystal structure and the transport and magnetic properties of these films were then examined. Of the growth conditions, the oxygen partial pressure (Po2) had the greatest influence on the electrical and magnetic properties. Films grown under a low Po2 had a low ferromagnetic transition temperature (Tc) and a wide resistive transition width. None of the heat treatments done after growth improved these films’ quality. The film morphology was significantly affected by the substrate material. Our x-ray diffraction analysis and AFM measurements revealed that the films deposited on both MgO (100) and LaAlO3 (100) were epitaxially grown but contained defect structures. In contrast, grain-free thin films were epitaxially grown on the SrTiO3 (100) substrates. The surface roughness of films grown on SrTiO3 was less than 0.3 nm, even for films up to 150 nm thick. Under optimized growth conditions, as-deposited films for x ≥ 0.2 showed a sharp transition in resistivity at Tc. Magnetoresistance at far below Tc was as low as that reported for single-crystal sample. Since large magnetoresistance was often observed in polycrystalline samples and believed to be a grain boundary effect, these results indicate the high quality of the films grown on the SrTiO3 substrates.
Calculated Transport and Magnetic Properties of some Perovskite Metallic Oxides AMO3
Authors: Santi G. , Jarlborg T.
Published: April 2011

We study some compounds of the perovskite (or pseudo-cubic perovskite) series AMO3, where M is a transition metal and A is Ca, Sr, or Nd, by LSDA self-consistent electronic structure calculations with the LMTO method. Transport and magnetic properties, as well as Fermi surfaces are calculated. These materials exhibit sharp density of states features in the vicinity of the Fermi level that strongly affect their transport and magnetic properties and make them very sensitive to structural deformation and stoichiometry. Calculated total energies are very close for anti-ferromagnetic and ferromagnetic solutions. This explains qualitatively the magnetoresistive anomalies shown by this family of compounds.
Experimental Determination of the key Energy Scales in the Colossal Magnetoresistive Manganites
Authors: Dessau D. S. , Saitoh T. , Park C.-H. , Shen Z.-X. , Moritomo Y. , Tokura Y.
Published: April 2011

We have performed X-ray absorption (XAS) and angle-resolved photoemission (ARPES) on single crystals of both the layered and cubic colossal magnetoresistive manganites to determine the electronic structure and the relevant energy scales in the problem: the intra-atomic exchange energy J (∼ 2.7 eV), the Jahn-Teller energy gain EJ-T (<.25 eV), the one-electron bandwidth W (>3 eV for layered compounds) and the lattice relaxation or polaron binding energy EB (.65 eV ferromagnetic phase and.8 eV paramagnetic phase). Lattice polarons are deemed important especially in the paramagnetic but also to a degree in the ferromagnetic phase. Due to the energy scale mismatch, the Jahn-Teller effect is unlikely to be the cause for these lattice polarons, at least for the layered samples.
Dry and Wet Etch Processes for NiMnSb, LaCaMnO3 and Related Materials
Authors: Hong J. , Wang J. J. , Lambers E. S. , Caballero J. A. , Childress J. R. , Pearton S. J. , Dahmen K. H. , Von Molnar S. , Cadieu F. J. , Sharifi F.
Published: April 2011

A variety of plasma etching chemistries were examined for patterning NiMnSb Heusler thin films and associated A12O3 barrier layers. Chemistries based on SF6 and Cl2 were all found to provide faster etch rates than pure Ar sputtering. In all cases the etch rates were strongly dependent on both the ion flux and ion energy. Selectivities of ≥20 for NiMnSb over A12O3 were obtained in SF6-based discharges, while selectivities ≤5 were typical in Cl2 and CH4/H2 plasma chemistries. Wet etch solutions of HF/H2O and HNO3/H2SO4/H2O were found to provide reaction-limited etching of NiMnSb that was either non-selective or selective, respectively, to A12O3. In addition we have developed dry etch processes based on Cl2/Ar at high ion densities for patterning of LaCaMnO3 (and SmCo permanent magnet biasing films) for magnetic sensor devices. Highly anisotropie features are produced in both materials, with smooth surface morphologies. In all cases, SiO2 or other dielectric materials must be used for masking since photoresist does not retain its geometrical integrity upon exposure to the high ion density plasma.
High Frequency Magneto-Electrodynamics of La1-xSrxMnO3 Single Crystals
Authors: Srikanth H. , Revcolevschi B. , Sridhar S. , Pinsard L. , Revcolevschi A.
Published: April 2011

The radio frequency (RF) response of La1-xSrxMnO3 single crystals reveal a variety of features associated with the structural, electronic and magnetic properties of the system. The resonance technique operating at ∼ 4 MHz employed in this study is sensitive to small changes in both the magnetic susceptibility and resistivity of the samples. Very sharp changes in frequency are observed at the ferromagnetic (FM) and structural phase transitions in both the metallic (x = 0.175) and insulating (0.125) crystals studied.

In addition to the known transitions identified as FM and orthorhombic distortions, our experiments show rich structures which are not observed in conventional DC magnetization and transport experiments. Our results demonstrate that RF experiments are ideally suited to investigate the complex phase diagram in the manganites.

The colossal frequency change that we observe at the FM transition in the La1-xSrxMnO3 crystals is indicative of the enormous potential for using these materials in high frequency switching applications.
Magnetic and Electronic Transport Properties of Single Crystal La0.64Pb0.36MnO3
Authors: Yang Jihui , Hu Siqing , Uher Ctirad , Han P. D. , Payne D. A.
Published: April 2011

We studied the magnetic and electronic transport properties of a single crystal sample of La0.64Pb0.36MnO3 in the temperature range 5 K to 350 K and magnetic field up to 5.5 T. A magnetic transition is found at 210 K. The single crystal sample is ferromagnetic below the transition temperature (Tc) and becomes paramagnetic at temperatures T > Tc. Magnetization measurements along three different orthorhombic crystal axes show no significant difference. The magnetoresistance approaches a maximum value of about -60% at Tc in 5 T magnetic field strength and has qualitatively different field dependence below and above Tc. The scaling behavior between resistivity and magnetic moment is examined for temperatures both below and above the transition. A low temperature (T<15 K) dp / dT < 0 effect is attributed to possible quantum tunneling of carriers between neighbouring distortions.
Epitaxial Growth Mechanism and Physical Properties of Ultra thin Films of La0.6Sr0.4MnO3
Authors: Konishi Yoshinori , Kasai Masahiro , Kawasaki Masashi , Tokura Yoshinori
Published: April 2011

Thin films of La1-xSrxMnO3 (x=0.4) were fabricated using pulsed laser deposition (PLD) methods. The surface morphology of the films was sensitively affected by oxygen pressure during deposition. At high oxygen pressure (∑150 mTorr), randomly aligned grains were nucleated on the epitaxial film. When the pressure was reduced to 100 mTorr, the epitaxial film had very smooth surface. Under this condition, the thickness dependence of resistivity and magnetization were analyzed. Even 6 nm thick film showed ferromagnetic metallic behavior. The AFM images of ulta thin films deposited on wet-etched SrTiO3 showed atomically flat terraces and 0.4 nm steps. The film growth mode can be tuned to either layer by layer or step flow by the deposition temperature.
Analysis of Mixtures of Gamma Lithium Aluminate, Lithium Aluminum Carbonate Hydroxide Hydrate, and Lithium Carbonate
Authors: Nemeth M. T. , Ford R. B. , Taylor T. A.
Published: April 2011

Lithium aluminate, LiA1O2is a ceramic powder which is used as the porous solid support for the electrolyte in molten carbonate fuel cells (MCFCs). It has previously been reported that gamma LiAlO2will convert to lithium aluminum carbonate hydroxide hydrate, Li2Al4(CO3)(OH)123H2O and Li2CO3when exposed to water vapor and carbon dioxide. We compare three techniques, weight gain, carbonate content and x-ray diffraction to measure the amount of conversion. The reaction may involve amorphous intermediates and no one technique by itself is satisfactory to study the conversion.
Crystal Structures of Mixed-Conducting Oxides Present in The Sr-Fe-Co-O System
Authors: Hodges J. P. , Jorgensen J. D. , Miller D. J. , Ma B. , Balachandran U. , Richardson J. W.
Published: April 2011

The potential applications of mixed-conducting ceramic oxides include solid-oxide fuel cells, rechargeable batteries, gas sensors and oxygen-permeable membranes. Several perovskite-derived mixed Sr-Fe-Co oxides show not only high electrical-conductivity but also appreciable oxygen-permeability at elevated temperatures. For example, dense ceramic membranes of SrFeCo0.5O3-δ can be used to separate oxygen from air without the need for external electrical circuitry. The separated oxygen can be directly used for the partial oxidation of methane to produce syngas. Quantitative phase analysis of the SrFeCo0.5O3-δ material has revealed that it is predominantly composed of two Sr-Fe-Co-O systems, Sr4Fe6-xCoxO13 and SrFe1−xCoxO3-δ. Here we report preliminary structural findings on the SrFe1−xCoxO3-δ (0 ≤ x ≥ 0.3) system.
Application of Eutectic Ceramic Mixtures for The Functional Components of High Temperature Sofc's
Authors: Gerk Ch. , Willert-Porada M.
Published: April 2011

A novel design for a high temperature SOFC, based on lamellar electrode-electrolyte segments obtained by solidification of an oxidic eutectic melt on an electrolyte substrate is presented. Such “composite” electrodes contain NiO or MnO - 8Y-ZrO2 lamellae, which after reduction / oxidation yield electrode-electrolyte lamellae with 1–2 μm width and a vertical dimension of> 100 μm, depending upon the amount of eutectic melt solidified on a polycrystalline substrate. The nucleation of the eutectic on a polycrystalline substrate followed by a semi-directional crystallization of the two phases yields a gradient of 3-phase boundaries over the height of such an electrode, with the number of 3-phase boundaries increasing towards the substrate.
Dynamics of the Perovskite-Based High-Temperature Proton-Conducting Oxides: Theory and Experiment
Authors: Karmonik C. , Yildirim T. , Udovic T. J. , Rush J. J. , Hempelmann R.
Published: April 2011

The dynamics of the doped perovskite-based high-temperature protonie conductors (HTPC) were studied by means of neutron vibrational spectroscopy (NVS) and first-principles pseudopotential supercell calculations. Vibrational spectra from hydrogen-charged samples with different rare-earth dopants revealed three well-defined vibrational bands in the energy ranges 20–60, 60–90, and 100–140 meV. The two lowest-energy bands were insensitive to the dopants. First-principles phonon calculations indicate that they are mainly associated with oxygen modes. In contrast, the high-energy band was very sensitive to the dopant, and in this case, calculations indicate that it is associated with OH bending modes.
Carbon Composite for A Pem Fuel Cell Bipolar Plate
Authors: Besmann T. M. , Klett J. W. , Burchell T. D.
Published: April 2011

The current major cost component for proton exchange membrane fuel cells is the bipolar plate. An option being explored for replacing the current, nominal machined graphite component is a molded carbon fiber material. One face and the volume of the component will be left porous, while the opposite surface and sides are hermetically sealed via chemical vapor infiltration of carbon. This paper will address initial work on the concept.
Novel Synthesis Process and Characterization of Li-Mn-O Spinels for Rechargeable Lithium Batteries
Authors: Takada Toshimi , Enoki Hirotoshi , Akiba Etsuo , Ishizu Takenori , Horiba Tatsuo
Published: April 2011

A new process has been developed for the synthesis of well-crystallized Li-Mn-O spinels with a homogeneous composition Li[LixMn2-x]O4(0.0≤x≤0.333) using the stoichiometry mixtures of lithium acetate and manganese nitrate as starting materials. The crystal structure of these compounds was studied with Rietveld refinements of the X-ray diffraction profiles. The lattice parameter of the spinels shows a strong dependence on the composition and manganese oxidation state. SEM micrographs indicate that the crystallites appear as single crystals. The size of the crystallites are in the range of 0.1 – 2μm, depending on the synthesis conditions. Samples with x≤0.125 show good electrode performance for the cell Li/Li1+xMn2-xO4in the 4V region, whereas Li4Mn5O2(x=0.333) spinels show good cyclability with a rechargeable capacity of over 100mAh/g in the 3V region.
LiMn2-xCuxO4 Spineis - 5 V Cathode Materials
Authors: Ein-Eli Yair , Howard W. F. , Lu Sharon H. , Mukerjee Sanjeev , McBreen James , Vaughey John T. , Thackeray Michael M.
Published: April 2011

A series of electroactive spinel compounds, LiMn2-xCuxO4 (0.1 ≤ x ≤ 0.5) has been studied by crystallographic, spectroscopie and electrochemical methods and by electron-microscopy. These LiMn2-xCuxO4 spinels are nearly identical in structure to cubic LiMn2O4 and successfully undergo reversible Li intercalation. The electrochemical data show slight shifts to higher voltage for the delithiation reaction that normally occurs at 4.1 V in standard Li1−xMn2O4 electrodes (1 ≥ x ≥ 0) corresponding to the oxidation of Mn3+ to Mn4+. The data also show a remarkable reversible electrochemical process at 4.9 V which is attributed to the oxidation of Cu2+ to Cu3+. The inclusion of Cu in the spinel structure enhances the electrochemical stability of these materials upon cycling. The initial capacity of LiMn2-xCuxO4 spinels decreases with increasing x from 130 mAh/g in LiMn2O4 (x=0) to 70 mAh/g in “LiMn1.5Cu0.5O4”(x=0.5). Although the powder X-ray diffraction pattern of “LiMn1.5Cu0.5 O4” shows a single-phase spinel product, neutron diffraction data show a small, but significant quantity of an impurity phase, the composition and structure of which could not be identified. X-ray absorption spectroscopy was used to gather information about the oxidation states of the manganese and copper ions. The composition of the spinel component in the LiMn1.5Cu0.5O4 was determined from X-ray diffraction and XANES data to be Li1.01Mn1.67Cu0.32O4 suggesting, to a best approximation, that the impurity in the sample was a lithium-copper-oxide phase.
Silver-Doped Vanadium Oxides as Host Materials for Lithium Intercalation
Authors: Coustier F. , Passerini S. , Hill J. , Smyrl W. H.
Published: April 2011

An improved cathodic material has been obtained by doping vanadium oxide hydrogel with silver. Silver-doped vanadium pentoxides with a silver molar fraction ranging from 0.01 to 1 were synthesized. With the successful doping, the electronic conductivity of V2O5 was increased by 2 to 3 orders of magnitude. The electrochemical performance of the silver doped materials is very high, up to 4 moles of lithium per mole of silver-doped V2O5 were found to be reversibly intercalated. In addition, the lithium diffusion coefficient is found to be high in the silver-doped material and with a smaller dependence on the lithium intercalation level. These enhancements resulted in high rates of insertion and delivered capacities.
Electric Double- Layer Capacitor using Organic Electrolyte
Authors: Morimoto T. , Tsushima M. , Suhara M. , Hiratsuka K. , Sanada Y. , Kawasato T.
Published: April 2011

Electric double-layer capacitors based on charge storage at the interface between a high surface area activated carbon electrode and a propylene carbonate solution are widely used as maintenance-free power sources for IC memories and microcomputers. New applications for electric double-layer capacitors have been proposed in recent years. The popularity of these devices is derived from their high energy density compared with conventional capacitors and their long cycle life and high power density relative to batteries.

The performance of the capacitor depends not only on the materials used in the cells but also on the construction of the cells. The performance of power capacitors for large power sources as well as materials and construction of these capacitors are described.
Modification of a Carbon Electrode Surface by Cold Plasma Treatment for Electric Double Layer Capacitors
Authors: Ishikawa Masashi , Sakamoto Atsushi , Morita Masayuki , Matsuda Yoshiharu , Ishida Koichi
Published: April 2011

Activated carbon fiber cloth (ACFC) electrodes whose surface was modified by “pulsed cold plasma”, i.e., low-temperature plasma generated by a pulsed electric power in argon-oxygen mixed gas at a reduced pressure, were applied to electric double layer (EDL) capacitors with an organic electrolyte composed of propylene carbonate and tetraethylammonium tetrafluoroborate (TEABF4). The treatment of the ACFC electrodes with the pulsed cold plasma increased total capacitance in the EDL capacitors. The observed increase in the total capacitance was ascribed mainly to an ascertained increase in capacitance of a negative ACFC electrode involving TEA+cation adsorption/desorption with the cold plasma treatment. No obvious increase in capacitance of a positive ACFC electrode involving BF4- anion adsorption/desorption was observed with the plasma treatment. The chemical and electrochemical characteristics of a treated ACFC interface were found to be favorable for TEA+cation adsorption/desorption.
First-Principles Theory of Cation and Intercalation Ordering in LixCoO2
Authors: Wolverton C. , Zunger Alex
Published: April 2011

Several types of cation- and vacancy-ordering exist in the LixCoO2 battery material. The ordering patterns are of interest due to the fact that they can control the voltage in rechargeable Li batteries. We present a first-principles total energy theory which can predict both cation-and vacancy-ordering patterns at both zero and finite temperatures. Also, by calculating the energetics of the Li intercalation reaction, this theory can provide first-principles predictions of battery voltages of Lix;CoO2/Li cells. Our calculations allow us to search the entire configurational space to predict the lowest-energy ground state structures, search for large voltage cathodes, explore metastable low-energy states, and extend our calculations to finite temperatures, thereby searching for order-disorder transitions and states of partial disorder.
Quantitative Icts Measurement of Interface States at Grain Boundaries in ZnO Varistors
Authors: Mukae K. , Tanaka A.
Published: April 2011

Isothermal capacitance transient spectroscopy(ICTS) measurement is applied to ZnO:Pr varistors. A simple peak corresponding to the interface states at grain boundaries was obtained and the energy level of the interface states revealed to be monoenergetic and located around 0.9eV below the conduction band. The cross section was calculated as around 10−14 cm2. Quantitative treatment of the ICTS intensity in relation to the density of interface states at grain boundaries was established. The density of interface states was obtained from the linear relation between ICTS intensity and reciprocal carrier density(ND). According to experiments on series of rare-earth doped ZnO varistors, the interface states of Pr, Tb or Nd doped ZnO varistors had higher density of states than La or Er doped varistors. Moreover, application of ICTS measurement to single grain boundary using microelectrodes revealed that higher density of interface states gave higher nonlinearity in I-V characteristics.
Current Localization, Non-Uniform Heating, and Failures of ZnO Varistors
Authors: Bartkowiak M.
Published: April 2011

Non-uniform heating of ZnO varistors by electrical pulses occurs on three different spatial scales: (1) microscopic (sub-micron), (2) intermediate (sub-millimiter), and (3) macroscopic (of order of millimeters or centimeters). Heating on these scales has different origins and different consequences for device failure in large and small varistors. On the microscopic scale, the heating localizes in strings of tiny hot spots. They occur at the grain boundaries in a conducting path where the potential is dropped across Schottky-type barriers. These observations are interpreted by applying transport theory and using computer simulations. It is shown that the heat transfer on a scale of the grain size is too fast to permit temperature differences that could cause a varistor failure. On an intermediate size scale, the heating is most intense along localized electrical paths. The high electrical conductivity of these paths has microstructural origin, i.e., it derives from the statistical fluctuations of grain sizes and grain boundary properties. Current localization on the intermediate size scale appears to be significant only in small varistors. On the macroscopic scale, current localization in large blocks can be attributed to inhomogeneities in the electrical properties which originate during ceramic processing. The resulting non-uniform heating is shown to cause destructive failures of large varistor blocks.
Current Flow and Structural Inhomogeneities in Nonlinear Materials
Authors: Greuter F. , Christen T. , Glatz-Reichenbach J.
Published: April 2011

Nonlinear current flow in real 3-d composite materials is far more complex than suggested by usual brickstone models. We present experimental results on controlled seeding of structural inhomogeneities in varistor ceramics. In particular, we discuss the influence of mesoscopic imperfections on the characteristic electrical parameters. A main effect is a change in the statistical distribution of the grain size. As a consequence, the breakdown voltage drops very sensitively due to the disorder, while the shape of the I-V curve is hardly affected. We discuss also the extremely nonuniform filamentary current pattern, being illustrated by electroluminescence studies.
Characterizing the Dispersion of Constituents in Concrete by Electrical Resistivity
Authors: Chen Pu-Woei , Fu Xuli , Chung D.D.L.
Published: April 2011

Characterization of the degree of dispersion of constituents in cement paste or mortar by DC electrical resistivity measurement was demonstrated for the constituent being much more conducting than cement (i.e., short carbon fibers) and much less conducting than cement (i.e., latex particles). The fiber dispersion is described by the ratio of the measured conductivity to the calculated value. The latex particle dispersion is described by a factor in an electrical conduction model.
Real Time Observation of Material Deformation Processes by Synchrotron White Beam X-ray Topography
Authors: Zhao Z. B. , Hershberger J. , Rek Z. U. , Bilello J. C.
Published: April 2011

An experimental apparatus which exploits synchrotron white beam X-ray diffraction topography has been developed to perform real time in situ observation of deformation processes of single crystals. The apparatus consists of a fully automated and remotely controlled mini-tensile device and a CCD based X-ray imaging detector with associated video system. Both tensile data and X-ray topographic images are recorded simultaneously. The materials under study include single crystals of Si, and single crystals or bicrystals of the refractory metals W and Mo. The deformation processes of these crystals have been studied by monitoring a selected transmission Laue spot while the specimen is mechanically cycled by loading and unloading. The X-ray images have been captured by a frame grabber and recorded via a VCR. Several phenomena have been observed, including bending contours, strain concentrations, reversible anelastic effect and microyielding. The observed stress relaxations are correlated to microstructural variations shown by X-ray topography. The stability of low angle grain boundaries in bicrystals; and the effect of high strength and toughness coatings on the deformation of single crystal substrates was investigated.
In Situ and Ex Situ Ellipsometric Characterization of Oxygen Plasma and UV Radiation Effects on Spacecraft Materials
Authors: Bungay C. L. , Tiwald T. E. , DeVries M. J. , Dworak B. J. , Woollam John A.
Published: April 2011

Atomic Oxygen (AO) and ultraviolet (UV) radiation contribute (including synergistically) to degradation of spacecraft materials in Low Earth Orbit (LEO). NASA is, therefore, interested in determining what effects the harsh LEO environment has on materials exposed to it, as well as develop materials that are more AO and UV resistant. The present work involves the study of AO and UV effects on polyarylene ether benzimidazole (PAEBI) with in situ and ex situ spectroscopic ellipsometry. PAEBI is a polymer proposed for space applications due to its reported ability to form a protective phosphorous oxide on the surface when exposed to AO. In our experiments PAEBI was exposed to UV radiation from a xenon lamp while in situ ellipsometry data were acquired. The effects of UV radiation were modeled as an exponentially graded layer on the surface of bulk PAEBI. The change in UV absorption spectra, depth profile of the index of refraction, and growth trends of the UV irradiated PAEBI were all studied in these experiments. In addition, PAEBI was exposed to an oxygen plasma to simulate the synergistic effects of AO and UV. Ellipsometry data were acquired in-line with both a UV-Visible ellipsometer and an infrared ellipsometer. The change in UV absorption bands and index of refraction due to synergistic AO/UV, as well as the growth trends of the oxide layer were studied.
Spectral-Component Monitoring of the Plumes Generated during the Deposition of RE(Y, Nd) Ba2Cu3O7−x Films by Pulsed Laser Ablation
Authors: Biggers R. , Varanasi C. , Maartense I. , Dempsey D. , Mastc D. , Liptak D. , Jones J. , Peterson T. L. , Murray T. , Busbee J.
Published: April 2011

Yba2Cu3OT7−x (Y123) and NdBa2Cu3O7−x (Nd123) thin films were deposited by PLD using a 248 nm wavelength KrF excimer laser. The emission spectra PLD process control techniques developed to grow reproducibly high quality Y123 thin films were also applied to the Nd123 depositions. The time-resolved spectral components of plumes generated from Y123 and Nd123 targets at the same deposition conditions were compared. When the two targets were mounted at the same distance from the substrate heater, the spectral emissions at 327 ± 5 nm (Cu*) were essentially identical for similar laser pulse energies and other deposition conditions (150 mTorr oxygen pressure, 760°C substrate heater temperatures, etc.). High quality Nd 123 films (Tc ˜93 K, Jc > 2x 106 A/cm2) are considerably more difficult to produce than those of Y123. Deposition conditions associated with PLD of high quality Y123 films produce very poor Nd 123 films. This observation is consistent with the belief that the nucleation and growth of Nd 123 are significantly different than that of Y 123.