bluuebunny/crossref_metadata_2025_split huggingface dataset:
Cluster Calculation of B and A1 Impurities in Amorphous Silicon
Authors: Cogordan J. A. , Sansores L. E. , Valladares A. A. 
Published: March 2011

ABSTRACTWe report our results of molecular structure calculations for B and A1 impurities. From our unoptimized ab initio calculations, we found the molecular electronic wave function to be unstable for both impurities. This instability was removed through a geometry optimization process. Local densities of states (LDOS) were computed for the optimized geometries. They show a rise of a peak at the tail of the valence LDOS; this feature is due to p orbitals of B and Al. The contribution is slightly higher for B than for A1 impurities. Charge contour plots are presented.
Diffusion Characteristics of Vacancies in Aluminum Interconnects.
Authors: Nandedkar A. S. 
Published: March 2011

ABSTRACTComputer simulations were performed to investigate the migration energy of vacancies in A1 in the presence of grain boundary, dislocation and various imposed forces (such as stress gradients and electro-migration). The energy values were significantly smaller when the vacancy moved into the grain boundary or a dislocation core than in opposite direction. This makes these defects sinks for vacancies from which they are unlikely to migrate out. The major time required for the movement of vacancies is thus limited to within a single grain. Calculations of migration time from the center of the grain to the grain boundary gives time needed for void formation. Calculations of this time assuming an electrical potential gradient along the metal line gave estimates consistent with experimental observations. The increased values of migration energy under compressive stresses suggest that void formation could be deterred by applying such stresses.
Extension of a New Semiempirical Method (BFS) and the Study of Ground State Properties of Binary Alloys
Authors: Bozzolo Guillermo , Ferrante John 
Published: March 2011

ABSTRACTWe extend the method of Bozzolo, Ferrante and Smith (BFS) for the study of alloy energetics to include a description of the local environment in specific ordered structures. The concept of bond-diagrams is introduced and applied to fcc binary compounds. A simple example of the parameterization of the bond-diagrams is done with reference to available first-principles calculations of Ni-Pt ordered alloys.
Calculation of the Thermopower of Ni-P Amorphous Alloys
Authors: Swihart James C. , Yang Hyungjin , Nicholson Donald M.C. 
Published: March 2011

ABSTRACTWe have modeled the Ni-P amorphous alloy system by a set of supercells on which we have carried out calculations of electronic and transport properties. The thermopower is determined by calculating the energy dependence of the dc electrical conductivity from the Kubo formalism. At 15% P we find -0.015 μV/K2 for the thermopower divided by the temperature, while for 25% P the calculated value is +0.008 μV/K2, in reasonable agreement with experiment, particularly with regard to the change of sign with concentration.
Electronic Structure of Ordered and Disordered Ternary Intermetallics
Authors: Wolverton C. , De Fontaine D. 
Published: March 2011

ABSTRACTA cluster expansion for energetics is combined with a direct, real-space method of studying the electronic structure of ordered and disordered ternary intermetallics. The electronic structure calculations are based on an explicit averaging of local quantities over a small number of randomly chosen configurations. Quantities such as densities of states, one-electron energies, etc., are computed within the framework of the first-principles tight-binding linear muffin-tin orbital method (TB-LMTO). Effective pair interactions, which describe the ordering tendencies of the alloy, are computed for the full ternary alloy. With this technique, then, the effects on ordering trends of ternary additions to a binary alloy may be obtained. Results for Ag-Pd-Rh and Ni-Al-Cu are shown. The self-consistency of these calculations is checked against the fully self-consistent ordered LMTO calculations.
Electronic Structure and Phase Stability Properties in Ternary Substitutional Alloys
Authors: Traiber Ariel J.S. , Sluiter Marcel , Turchi Patrice E.A. , Allen Samuel M. 
Published: March 2011

ABSTRACTElectronic structure and stability properties of ternary-metal alloys are examined using an extension of the Coherent Potential Approximation -Generalized Perturbation Method approach within the Tight-Binding description of the chemically random alloy. In particular, we report on calculations of density of states, mixing energies and effective cluster interactions which build up the ordering energy. The study focuses on the Ti-V-Fe system and its binary components.
Monte Carlo and Molecular Dynamics Validation of an N-Body Potential for Cu3Au
Authors: Rey-losada Carlos , Hayoun Marc , Pontikis Vassilis 
Published: March 2011

ABSTRACTWe present the results of Monte Carlo and Molecular Dynamics simulations of some thermodynamical properties of Cu3Au. The calculations rely on an empirical n-body potential that reproduces satisfactorily the critical temperature, Tc, the temperature dependence of the lattice constant and atomic vibrational amplitudes as well as Cowley's short-range order parameters above Tc. Our results show that relaxation effects decrease considerably the formation energy of antisite defects and therefore should explicitly be considered for a realistic description of the transition.
The Effects of Aromaticity and Reduced Bond Length Alternation on Linear and Third-Order Polarizabilities of Conjugated Organic Chromophores
Authors: Gorman Christopher B. , Marder Seth R. 
Published: March 2011

ABSTRACTThe geometries, the linear (α) and third-order (γ) polarizabilities of several bipolaron model compounds and cyanines are computed using the semi-empirical AM1 parameterization. The effects of aromaticity, charge localization and bond length alternation are considered.
Quantum Simulation of Amorphous Silicon: Preparation, Structure and Properties
Authors: Colombo L. , Servalli G. 
Published: March 2011

ABSTRACTWithin a tight-binding molecular dynamics scheme we investigate pure amorphous silicon (a-Si) obtained by direct quenching from the melt. Using different rates for the cooling process, we demonstrate that both structural and electronic properties of a-Si depend on the sample preparation. Possible size-effects are also investigated using 64- and 216-atom supercells. Finally, we discuss the reliability and transferability of the present scheme for large scale simulations of covalent materials.
Model of the Semiconductor-to-Semimetal Transition Modulation at Teraherz Frequencies
Authors: Cheng T. K. , Acioli L. , Vidal J. , Zeiger H. J. , Ippen E. P. , Dresselhaus G. , Dresselhaus M. S. 
Published: March 2011

ABSTRACTWe present experimental data which show that large amplitude coherent lattice vibrations can be impulsively excited using short optical pulses of light. Furthermore, using a quasi-equilibrium model, we show how the these coherent lattice vibrations suggest a ~ 7 THz modulation of the semiconductor-to-metal transition in Ti2O3.
Dynamical Ising Model Simulations of Nucleation and Growth in Copper-Cobalt Alloys
Authors: Cerezo Alfred , Hyde Jonathan M. , Miller Michael K. , Setna Rohan P. , Smith George D. W. 
Published: March 2011

ABSTRACTA simple dynamical Ising model on a fixed lattice with a single bond energy parameter has been used to simulate the kinetics of diffusion during solid-state phase transformations in binary metallic alloys. The results of these simulations are compared with direct real-space measurements of the atomic distributions of elements in alloys, obtained with the position-sensitive atom probe. Despite the simplicity of the model, there is good quantitative agreement between the development of micro-structure in the simulation and the nucleation and growth of cobalt-rich precipitates in copper-cobalt alloys.
Elastic Instabilities and Amorphization of Crystalline Silica Under Pressure
Authors: Chelikowsky James R. , Binggeli Nadia 
Published: March 2011

ABSTRACTSolid state amorphization can occur when a crystalline phase is compressed at a sufficiently low temperature to inhibit kinetically the transformation to a stable high pressure crystalline phase. An example of such a vitrification transformation occurs in α-quartz, the most stable phase of SiO2 at standard temperature and pressure conditions. Under pressure at room temperature α-quartz gradually transforms to an amorphous form in the range of 25-30 GPa. The driving force for this amorphization is not clear, and speculation has centered on mechanical instabilities of the quartz crystal under pressure. The elastic properties of α-quartz are studied as a function of pressure using both classical interatomic potentials, and ab initio pseudopotentials. In both cases, we find that the α-quartz structure becomes mechanically unstable at about 30GPa. This finding supports a picture in which the amorphization of quartz is triggered by the onset of a lattice shear-instability. The microscopic origin of this elastic softening is intimately related to the presence of an oxygen close-packed cubic arrangement in the quartz high pressure structure.
First-Principles Calculation of Force Constants and Full Phonon Dispersions
Authors: Wei Siqing , Chou M. Y. 
Published: March 2011

ABSTRACTWe calculated the real-space force constants and full phonon dispersion curves for elemental semiconductors (silicon and germanium) under the local-density approximation with the Hellmann-Feynman forces. The force constants are obtained through super- cell calculations for planar displacements in three different symmetry directions. From these real-space force constants the dynamical matrix for an arbitrary wave vector in the Brillouin zone can be constructed. The procedure is simple in concept and requires no complicated computer programing. It is also possible in principle to handle the anharmonic effects.
Quantum Dynamical Simulation of Many Electron-Phonon Coupled Systems on Parallel Computers
Authors: Nakano Aiichiro , Kalia Rajiv K. , Vashishta Priya 
Published: March 2011

ABSTRACTA quantum dynamical simulation method is developed to investigate coupled many electron-phonon systems. Both electron and phonon wave functions are numerically propagated in time. The method is applied to the study of resonant tunneling of an electron through double quantum dots. Phonon-induced electron localization is observed. The space- splitting Schrödinger solver and dynamical-simulated-annealing Poisson solver are implemented on an 8,192-node MP-1 computer from MasPar.
Simple Flexible Boundary Conditions for the Atomistic Simulation of Dislocation Core Structure and Motion
Authors: Pasianot Roberto , Savino Eduardo J. , Xie Zhao-Yang , Farkas Diana 
Published: March 2011

ABSTRACTFlexible boundary codes for the atomistic simulation of dislocations and other defects have been developed in the past mainly by Sinclair [1], Gehlen et al.[2], and Sinclair et al.[3]. These codes permitted the use of smaller atomic arrays than rigid boundary codes, gave descriptions of core non-linear effects and allowed fair assessments of the Peierls stress for dislocation motion. Green functions (continuum or discrete) or surface traction forces were used to relax the boundary atoms.A much simpler approach is followed here. Core and mobility effects at the boundary are accounted for by a dipole tensor centered at the dislocation line, whose components constitute six more parameters of the minimization process. Results are presented for [100] dislocations in NiAl. It is shown that, within the limitations of the technique, reliable values of the Peierls stress are obtained.
Analogy Between the Groundstate of an Antiferromagnet and the Mode of Quasi-Static Deformation of a Packing of Spheres.
Authors: Evesque P. 
Published: March 2011

ABSTRACTWe report on local and global processes during the mechanical deformation of packings of spheres. We demonstrate that solid friction imposes a counter-rotation interaction to two adjacent grains during the deformation in order to minimize losses. This is equivalent to introduce an anti-ferromagnetic spin-like interaction in the structure; the search for the deformation process is quite similar to that one of finding the groundstate of an anti- ferromagnetic disordered structure with special boundary conditions. We demonstrate experimentally that boundaries may produce leading frustration processes; in this case, they govern most of the deformation mode and cancel mostly the influence of bulk mechanical properties (at least in the case when global “antiferromagnetic” order of the bulk is allowed by the absence of frustration). Frustration due to triangular local structure is described and existence of “anti-ferromagnetic-like” domains is proved; these domains are induced by the adequation between the boundary conditions and the deformation process and the domain wall is a stress-localization band as named in classical soil mechanics.
Catalytic Effect of Some Oxides on Calcination of Sodium Nitrate
Authors: Sembira A. N. , Perez M. M. 
Published: March 2011

ABSTRACTThe calcination process used for sodium nitrate could be the most important part of the liquid-waste calcination process used in high-level-waste (HLW) disposal. A French process that involves conversion of the solution of nitrate fission products into a solid mixture of oxides and undecomposed nitrates. These products are then mixed with a glass-making mixture (called a frit) in an induction melting pot.In 1984, Utake et.al., (1) suggested the use of the Sol-Gel technique to decrease the calcination temperature of NaNO3.In this work, we used the thermal-analysis technique to study the catalytic effect of some glass network formers like SiO2, B2O3 and of some network modifiers like A12O3, Fe2O3, MgO, TiO2 ZnO. The best effect was achieved by adding 20% diatomaceous earth (Kieselguhr) and 20% B2O3 In that case, the calcination temperature onset decreased from 780°C to 565°C.The advantages of this process are as follows:1. The proposed additives (Kieselguhr and B2O3) are glass network formers that have to be added anyway in the next piocess step.2. In the proposed process, no sodium is added, and the only process change suggested is that the glass formers (or part of them) should added at the calcination step, rather than at the glass-melting step.3. The proposed modification is easily achieved and is expected to improve the homogenization of the final product.
Plutonium Leaching from a Reference Nuclear Waste Glass in Synthetic Interstitial Claywater
Authors: Wang Lian , Van Iseghem P. , Maes A. 
Published: March 2011

ABSTRACTThe steady-state concentration of Pu was measured upon leaching Pu doped glass in synthetic interstitial claywater (SIC) and humic acid free interstitial solution (IS). The solubility of PuO2·xH2O(am) was determined in the same chemical condition. The results then were compared with the Pu solubility calculated by using a published solubility product and complex stability constants. The humic acid (HA) influence on the Pu leaching from the glass and the colloid formation were investigated.
Effects of MgO on the Short- and Long-Term Stability of R7T7 and M7 Nuclear Waste Glass in Aqueous Media
Authors: Advocat T. , Crovisier J.L. , Dussossoy J.L. , Vernaz E. 
Published: March 2011

ABSTRACTExperiments were conducted in initially pure water at 90°C and 100°C with glass specimens enriched in MgO and in alkali metals relative to the standard light water R7T7 reference glass. Three experimental protocols were implemented: Soxhlet testing with leachant renewal, static leaching according to a modified MCC-1 method with an SA/V ratio of 50 m−1, and static leaching with glass powder for an SA/V ratio of 8000 m−1 The results clearly show that the short and, especially, the long term dissolution rates depend on the initial glass composition. Higher MgO, Na2O, Li2O and B2O3 concentrations reduce the glass resistance to aqueous corrosion, as already indicated by the calculated hydration energy values. These experiments illustrate the importance of the glass composition in insuring long-term material integrity.
Effects of α-Decay on Mechanical Properties of Simulated Nuclear Waste Glass
Authors: Inagaki Y. , Furuya H. , Ono Y. , Idemitsu K. , Banba T. , Matsumoto S. , Muraoka S. 
Published: March 2011

ABSTRACTA simulated nuclear waste glass was self-irradiated by doping with short-lived actinides of 238Pu and 244Cm. Changes in the hardness, the Young's modulus and the fracture toughness, as a function of irradiation dose, were measured by use of indentation techniques. The irradiated glass was annealed at temperatures from 573K to 723K for periods of up to 48hours, and the recovery of these changes were measured as a function of annealing temperature and time.It was observed that the hardness and the Young's modulus decreased, while the fracture toughness increased exponentially with the cumulative dose. The maximum values of the relative changes in the hardness, the Young's modulus and the fracture toughness were about −25%, −30% and +45%, respectively. The results of the annealing show that the hardness and the Young's modulus were almost recovered to the original values at temperatures above 673K within 10 hours, while the recovery of the fracture toughness was minimal in this region of temperature and time. The changes in the hardness and the Young's modulus can be well explained by the model, in which the changes is proportional to the volume fraction of damaged zones, F, and the recovery of F is first order. On the other hand, the changes in the fracture toughness cannot be explained by the model, which suggests that the mechanism of the change in the fracture toughness is different from that in the hardness and the Young's modulus.
Coupled Process Modeling and Waste-Package Performance
Authors: Mcgrail B. P. , Engel D. W. 
Published: March 2011

ABSTRACTThe interaction of borosilicate waste glasses with water has been studied extensively, and reasonably good models are available that describe the reaction kinetics and solution chemical effects. Unfortunately, these models have not been utilized in performance assessment analyses, except in estimating radionuclide solubilities at the waste form surface. A geochemical model has been incorporated in the AREST code to examine the coupled processes of glass dissolution and transport within the engineered barrier system. Our calculations show that the typical performance assessment analyses using fixed solubilities or constant reaction rate at the waste form surface are not always conservative or realistic predictions of radionuclide release. Varying the transport properties of the waste package materials is shown to produce counterintuitive effects on the release rates of some radionuclides. The use of noncoupled performance assessment models could lead a repository designer to an erroneous conclusion about the advantages of one waste package design or host rock setting over another.
Kinematics and Thermodynamics of Non-Stoichiometric Oxidation Phase Transitions in Spent Fuel
Authors: Stout R. B. , Kansa E.J. , Wijesinghe A.M. 
Published: March 2011

ABSTRACTAt low temperatures (<200°C), spent fuel from power reactors oxidizes from its UO2 lattice to a U4O9 lattice but with an oxygen-to-uranium (O/U) ratio of ∼2.4. Also, the weight gain time response has a plateau as the O/U approaches 2.4. Part of this response results from a geometrical dependency as a U4O9 oxidation front propagates into grain volumes of UO2. It may also be indicative of a metastable, non-stoichiometric U4O9 phase whose existence may inhibit the transition kinetics to the next expected phase of U3O8. To gain a mechanistic understanding and to plan future oxidation tests, lattice kinematic and thermodynamic models are developed for lattice deformations and energetics of lattice phase changes (UO2 → U4O9 → U3O7 → U3O8) that include zeroth order influences on oxidation kinetics due to interstitial oxygen atoms and vacancies plus interstitial and substitutional actinides and fission decay products in spent fuel.
The Possibility for Microbially Influenced Degradation of Cement Solidified Low-Level Radioactive Waste Forms
Authors: Rogers Robert D. , Hamilton Melinda A. , Mcconnell John W. 
Published: March 2011

ABSTRACTThe Nuclear Regulatory Commission (NRC) regulations 10 CFR Part 61, “Licensing Requirements for Land Disposal of Radioactive Waste,” regulate the disposal of radioactive waste and provides, among other stipulations, that class B and C low-level radioactive waste (LLW) be stabilized. This is intended to ensure that solidified waste does not structurally degrade and cause subsidence in the disposal unit's cover system. It is reasoned that deterioration of the waste form could adversely effect the stability of the burial site and lead to the release of radionuclides to the environment. Because of its apparent structural integrity, cement has been widely used as a binder to solidify LLW. However, the resulting preparations called pozzolanic cements are susceptible to failure due to the actions of stress and environment.This paper presents data from the literature that document the significance of biologically mediated chemical attack on concrete, in general. Concrete is susceptible to aggressive reaction with acids (both mineral and organic) of natural and anthropogenic origin. If persistent, such reactions ultimately lead to structural failure. Groups of microorganisms have been identified that are capable of metabolically converting organic and inorganic substrates into organic and mineral acids.The literature supports the conclusions that acid-producing bacteria of one type or another could be prevalent in all soils, even at depths expected for burial of LLW. Given the appropriate conditions of micro-environment and suitable substrate for growth, these bacteria will create conditions conducive to concrete deterioration. Growth substrates used by acid-producing bacteria (i.e., ammonia compounds, other reduced nitrogen compounds, sulfur and reduced sulfur compounds, reduced iron compounds, as well as organic carbon) could be naturally present in the disposal environment or be provided by the contents of the waste form.Sufficient evidence is presented which demonstrates the potential for microbially-influenced deterioration of cement-solidified LLW. These data are the basis for the NRC initiating the development of appropriate tests to determine the resistance of cement-solidified LLW to microbiologically induced degradation.
Effects of Microorganisms Growth on the Long-Term Stability of Cement and Bitumen
Authors: Libert Marie F. , Sellier R. , Jouquet G. , Trescinski M. , Spor H. 
Published: March 2011

ABSTRACTCement is used as a coating matrix for nuclear waste or as an engineered barrier of waste repositories situated in geological formations.The effect of mineral acids excreted by bacteria (Thiobacillus) or organic acids produced by fungi, on the biodegradation of cement is discussed. Organic acids are quantitatively and qualitatively determined during growth of fungi over a two-year period. Even with high pH conditions, pH of the cement ≈ 11, growth of microorganisms occurs.Biodeterioration of cement is expressed in terms of bioleaching velocity of calcium and is observed by electron microscopy.