bluuebunny/crossref_metadata_2025 huggingface dataset:
Preparation of Large Area Cross-Sectional Tem Specimen of Semiconducting Heteroepitaxial Materials
Authors: Tamura M. , Aoki S.
Published: March 2011

The sample preparation procedures which enable us to observe large areas over a few tens of microns in one-dimension of semiconducting heteroepitaxial materials are described. The main principle involves the careful grinding and polishing of samples. In these procedures, another side thinning of the specimen after finishing initial side polishing is carried out using a sample platform by hand throughout all of the following steps. It is shown that for some typical examples of heteroepitaxial films general information concerning the film growth modes and structures, as well as the defect morphologies and natures introduced during growth can be effectively obtained by using the present technique.
The Syntheses of Nanoscale Metal Particles Produced by the Beam Induced Decomposition of Metal Hydrides and Azides in A Transmission Electron Microscope
Authors: Herley P. J. , Fitzsimons N. P. , Jones W.
Published: March 2011

A novel and efficient technique for the generation of nanoscale metal particles by the electron beam decomposition of metal hydrides and azides is presented. Nanoscale metal particles down to ∼ 25 Å in diameter can be generated from metal hydrides e.g. copper from CuH and from metal azides e.g. silver from AgN3. The technique has been successfully applied to Be, Mg, Al, Cu, Pd, Ti, ‘Mn, Ni, Ag, Sn, Fe, Bi, Cd, Sb, Ln and Tl.
Fibxtem— Focussed Ion Beam Milling for TEM Sample Preparation
Authors: Basile David P. , Boylan Ron , Baker Brian , Hayes Kathy , Soza David
Published: March 2011

In the semiconductor industry, shrinking geometries and increasing process complexity have greatly increased the demand for TEM analysis of specific submicron regions. Until recently, samples of this nature have been difficult if not impossible to prepare. We have combined cross-sectional TEM sample preparation (XTEM) and the precise material sputtering of focussed ion beam milling (FIB) to thin samples to electron transparency. We call this sample preparation technique FIBXTEM.

Three advantages of this technique are:1) The area of interest can be analyzed in the scanning electron microscope before final thinning;2) Any specific defect area becomes a candidate for TEM analysis, including failed sub-micron structures; and3) Samples are generally artifact-free and of uniform thickness.

Key elements of the FIBXTEM technique include precision planar polishing, unique holders for mounting and transferring samples between systems, and the FIB-induced deposition of a sacrificial protective layer over the area of interest during ion thinning.

This technique extends the use of TEM analysis into new areas of semiconductor process development and failure analysis. Recent applications for materials problem solving and failure analysis are discussed.
Rapid Plan View Fabrication of Semiconductor Tem Samples for Interface Strain and Grain Size Analysis
Authors: Cole M. W. , Flemish J. R.
Published: March 2011

This study presents novel methods for chemically preparing back-side and double-side etched, plan-view TEM samples without use of jet thinning instrumentation in order to examine both surfaces and interfaces. Examples are drawn from the semiconductor industry, but this technique can be generalized for use in most materials systems. The methodology for single-sided etching consists of flat lapping and mechanical dimpling followed by material selective chemical etching to electron transparency. Interface defect analysis of heterostructures is achieved via double-sided etching to electron transparency using the proper selective chemical etchants. These techniques are presented not as replacements for conventional cross-sectional preparatory techniques, but instead, as a means for rapid sample preparation for situations where a large number of samples must be analyzed quickly, or in cases where the necessary equipment is lacking for crosssection preparation.
Thickness Variations and Surface Layers in Ultramicrotomed Sections and Their Effects on Elemental Mapping
Authors: Malls T. , Steele D.
Published: March 2011

While the thickness of ultramicrotomed sections is much more uniform than that of wedge-shaped foils, its variation is seen to be dependent on such factors as the type of material, use of embedding media, water bath interaction (anodic dissolution) and quality of the diamond knife edge. The latter also affects the thickness of surface oxides formed during sectioning. More deleterious surface layers are produced by partial breakdown and redeposition of embedding media under the electron beam. These artefacts indicate that ultramicrotomy is not the panacea for elemental X-ray mapping that might, at first thought, appear to be the case. For the example of a metal matrix composite, useable maps are obtainable, but even the best microtomed sections are limited in terms of mapping by factors such as the lengthy times for map acquisition caused by the low X-ray count rates resulting from thin specimens, particularly where low mass fractions are involved.
Application of Ultramicrotomy to Tem Specimen Preparation of Particulate Inclusion and Composite Materials.
Authors: Popoola O. O. , Cooper J. J. , Jakstys B. P. , Kriven W. M.
Published: March 2011

Ultramicrotomy has been used to prepare TEM specimens from nickel sulfide inclusions in plate glass and from calcium aluminate / polyvinyl alcohol composite materials. Microstructural studies showed that the inclusion probably contained at least two different phases, namely Ni7S6 and Ni3S2. Although microtomy-induced cracking and chatter were observed, important microstructural features such as twins and Moire patterns were preserved. The microchemistry of the polymer phase in the ceramic / polymer composite was precisely determined.
Silica/Silicone Nanocomposite Films: A New Concept in Corrosion Protection
Authors: Gentle Theresa E. , Baney Ronald H.
Published: March 2011

Thin films of silsesquioxane, (HSiO3/2)n, were applied to aluminum panels and to CMOS microelectronic circuit surfaces by spin or dip coating organic solutions of the silsesquioxane. Nanoporous silica was obtained by oxidation of the silsesquioxane. These nanoporous silica films were then vacuum infiltrated with various viscosities of polydimethylsiloxanes (PDMS) to form hydrophobic nanocomposites. The nanocomposite films were shown to provide superior hermetic protection against salt fog exposure when compared to PDMS and silica films alone.

The composite films were characterized by FTIR and optical microscopy. FTIR spectra showed that the silica served as a skeletal framework holding the hydrophobic PDMS in place and preventing loss of adhesion. This is in contrast to PDMS films alone in which blistering of the film from the substrate can occur, thus, allowing ions and moisture to reach the surface and corrosion to take place.
Microcrystalline Ceramic Composites by Active Filler Controlled Reaction Pyrolysis of Polymers
Authors: Greil Peter , Seibold Michael , Erny Tobias
Published: March 2011

Pyrolytic conversion of preceramic polymers such as polysilanes, -silazanes, or -siloxanes to ceramics may be significantly influenced by the resence of active filler dispersoids. Based on thermodynamic and microstructural considerations a variety of suitable polymer-filler systems can be found which allow the fabrication of microcrystalline composite materials with low dimensional change upon polymer- ceramic conversion. As an example the active filler controlled reaction pyrolysis of polysiloxane with addition of titanium powder was investigated. A composite material with microcrystalline titanium carbide inclusions embedded in an amorphous (< 1000 °C) or nanocrystalline (>1000 °C) silicon oxycarbide matrix was formed. Property changes with increasing pyrolysis temperature can be attributed to various microstructural transformations. Thus, a variety of potential fillers may be used to tailor the microstructure of polymer-derived ceramic composite materials in order to fabricate bulk materials and components with a broad range of compositions and properties.
Structure-Property Relationships in the Toughening of Poly(Methyl Methacrylate) by Sub-Micron Size, Multiple-Layer Particles
Authors: Archer A. C. , Lovell P. A. , McDonald J. , Sherratt M. N. , Young R. J.
Published: March 2011

Rubber-toughened poly(methyl methacrylate) materials have been prepared by blending poly(methyl methacrylate) with specially-synthesised, refractive index matched particles containing two, three and four radially-alternating rubbery and glassy layers. The paper describes the effects upon mechanical properties of (i) two-, three- and four-layer particle structure and (ii) particle diameter and glassy core size for three-layer particles.
A Study on the Blends of Epoxy/Polybutadiene and the Application to the Encapsulation of a Capacitor
Authors: Ren Zhongyuan , Qui Liying
Published: March 2011

This paper describes the blends of epoxy/polybutadiene and the application of the blends to the encapsulation of capacitors. Experiments showed that the hydroxy-carboxyl terminated polybutadiene (HCTPB) had a good toughening effect on epoxy resins, and the blends of epoxy/HTPB or epoxy/HCTPB had good craze resistance at low temperatures. The phase separation and dynamic mechanical analysis of these blends are discussed below.
Fracture Toughness and Fracture Mechanisms of PBT/PC/IM Blend
Authors: Wu Jingshen , Mai Yiu-Wing , Cotterell Brian
Published: March 2011

Static and impact fracture toughness of a Polybutylene terephthalate (PBT)/Polycarbonate (PC)/Impact modifier (IM) blend was studied at different temperatures. The experimental results were interpreted by the specific fracture work concept and J-integral analysis. It is found that the specific fracture work concept characterizes the impact behavior of the blend very well. In the static fracture tests the specific fracture work gives the crack initiation resistance of the blend which is consistent with the JIC value obtained. The effect of temperature was also examined and the fracture mechanisms were investigated via TEM and SEM. Extensive cavitation of the impact modifiers and plastic flow of matrix in the vicinity of the crack tip is believed to be the major toughening process of the enhanced fracture toughness.
High Glass Content Non-Shrinking Sol-Gel Composites via Silicic Acid Esters
Authors: Ellsworth Mark W. , Novak Bruce M.
Published: March 2011

The application of the sol-gel process for the formation. of inorganic-organic composites has received a great deal of attention in recent years. We have focused our efforts in this area toward the development of simultaneous polymerization/sol-gel reactions for the developement of inorganic-organic composites with a wide range of polymers. Further research in this area has led to the design and synthesis of tetraalkoxysilanes, and more recently, poly(silicic acid esters) possessing polymerizable alkoxides for the synthesis of non-shrinking sol-gel composites.
Effect of Pressure on Order and Stability in Alloys: The Case of Al-Ge
Authors: Turchi P. E. A. , Sluiter M. , Stocks G. M.
Published: March 2011

A parameter-free approach to phase stability in substitutional alloys is applied to the influence of pressure on order-disorder phenomena in Al-Ge. The methodology is based upon an application of the Generalized Perturbation Method to the Korringa-Kohn-Rostoker scattering formulation of the Coherent Potential Approximation. For fcc-based Al-Ge alloys, it is shown that the tendency towards phase separation at normal pressure originates from a structural difference between the pure species. By applying pressure, the structural energy difference is reduced, and a significant increase in the tendency towards order, especially for Al-rich alloys, is theoretically observed, leading, as an example, to the possible observation of a DO22 ordered state around 25 at. pct. Ge. The electronic origin of the ordering tendencies induced by pressure is discussed and the theoretical predictions are related to experimental facts.
A Tight-Binding-Bond Approach to Interatomic Forces in Disordered Transition Metal Alloys
Authors: Pasturel A.
Published: March 2011

A tight-binding-bond approach to interatomic forces in disordered transition metal-Aluminium alloys is presented. The bond-order is calculated on a Bethe lattice reference system, well adapted to topologically disordered alloy. It is shown that the bond-order depends strongly on the strength of the pd hybridization in the AB alloy, leading to non additive potentials with a strong preference for the formation of pair of unlike atoms and short bond-distances in the A-B pairs. This is illustrated by studying the structural properties of liquid Al80Ni20 and Al80Mn20 alloys using molecular dynamics simulations and by comparing our results with the available experimental ones.
Kinetics of Ordering in Cu3Au : An Atomistic Approach
Authors: Xi Zhigang , Chakraborty Bulbul
Published: March 2011

We study the kinetics of ordering in Cu3Au using a model Hamiltonian derived from the effective medium theory of chemical bonding. Monte Carlo simulations are used to investigate universal and non-universal features of the growth kinetics. Anisotropic scaling of the structure factor is observed in late-stage growth of ordered domains. The anisotropy is a non-universal feature determined by the details of the microscopic model, and we find that the anisotropy observed in the simulations is in excellent agreement with experiments on Cu3Au. The simulations are discussed in the context of theories of unstable growth. To our knowledge, this is the first study of kinetics in a realistic model Hamiltonian describing the material-specific properties of Cu3Au.
Real-Space Descriptions of Structural Energies in Metals
Authors: Carlsson A. E. , Zou J.
Published: March 2011

Two real-space methods for treating structural energetics of transition metals and compounds are described. The first uses a local description of the electronic density of states (DOS) in a tight-binding model to obtain an angular-force method containing up to four-body interaction terms. It is shown that this method yields bond-strengthening effects at surfaces which exceed those obtained by previous many-body potentials. Structural-energy calculations show that for W, several Frank-Kasper phases are only slightly higher in energy than the ground-state bcc structure; this near-degeneracy is driven by the energetic favorability of icosahedral sites. The second method uses a free-electron type approach to generate pair potentials for transition-metal solutes in Al. The calculated potentials have an oscillating form, with a much larger magnitude than those in Al. The potential is applied to complex Al-Mn phases, including the icosahedral quasicrystal. The results indicate that the oscillating pair potentials make a major contribution to stabilizing the complex phases.
Self Diffusion Parameters from Non-empirical Pair Potentials
Authors: Dorfman S. , Fuks D. , Pelleg J. , Rashkeev S.
Published: March 2011

A scheme for construction of the pair potential from non-empirical calculations of electronic structure of solids is suggested. As an example, parameters of Lennard-Jones potential are obtained for fccCs, based on LMTO calculations of energy parameters. Vacancy formation and migration energies for fccCs are calculated from this first-principles pair potential. In addition, the frequency of vibration and the jump probability of an atom are calculated and it is shown that they are direction dependent.
Molecular Mechanics and Dynamics Studies of Chemisorbed Monolayers of Alkanethiolates
Authors: Shnidman Yitzhak , Eilers James E. , Ulman Abraham , Sellers Harrell
Published: March 2011

We study ordering within closely packed monolayers of thiol derivatives self-assembled on gold substrates by chemisorption from solution. We discuss constant stress molecular mechanics and dynamics simulations of alkanethiolate monolayers on Au(111) and Au(l00) surfaces. In the first case, we use the MM2 force field augmented with classical chemisorption parameters obtained from fitting to results of ab initioquantum mechanical calculations. In the second, the chemisorption to the Au(l00) surface was accomplished by using harmonic constraints to restrict the thiolate groups to the square lattice. It is shown that two chemisorption modes of alkanethiolate on Au(l11) exist, and that both give closely packed ordered monolayers. In the system on Au(100), rearrangement towards close-packing is achieved at the expense of distortions that increase the intrachain elastic energy.
Exact Diagonalization Study of Real Materials
Authors: Chen Changfeng
Published: March 2011

We report some recent developments in theoretical modeling of real 3d magnetic materials, particularly interface and ultrathin-film structures. We focus on many-body correlation effects and describe the method of modeling realistic strongly correlated electron systems using and going beyond die local-density-approximation single-particle electronic structures. Group-theoretical techniques and high-performance computing facilities are systematically used in this work. Fundamental spectroscopic properties of several prototypical 3d transition-metal systems have been obtained and new features induced by many-body correlation have been discovered. Results will be presented and discussed.
Ground State Analysis on the Fcc Lattice With Four Pair Interactions
Authors: Garbulsky Gerardo D. , Tepesch Patrick D. , Ceder Gerbrand
Published: March 2011

We have partially solved the ground state problem of binary alloys on the fcc lattice with pair interactions up to the fourth nearest neighbor distance. Our results extend the study presented by Kanamori and Kakehashi [1], releasing the constraint they imposed on the nearest neighbor correlation. The solution we present increases the number of possible ground state structures by an order of magnitude with respect to previous results. We have applied both the polyhedron and the enumeration method. The latter proved more powerful when including interactions beyond the second nearest neighbor distance.
Parallel Algorithms for Molecular-Dynamics Simulations of Coulombic Systems
Authors: Li Wei , Kalia Rajiv K. , Leeuw Simon De , Nakano Aiichiro , Greenwell Donald , Vashishta Priya
Published: March 2011

In molecular-dynamics simulations for the long-range Coulomb interaction, a great deal of effort is devoted to reducing the computational complexity of the usual N2operations in the direct calculation. For bulk systems, we have designed a parallel algorithm based on the domain-decomposition strategy for the Ewald summation. The performance of the algorithm is evaluated on the in-house iPSC/860 system. We find that this algorithm reduces the computational complexity to O(N). For a 64,000-particle plasma in three dimension, the execution time on an 8-node system is 27.4 sec per MD time step. The interprocessor communication is a small fraction of the total execution time. We find linear speedups and a parallel efficiency of 0.85. For comparison, parallel algorithms are also designed for the Fast Multipole Method (FMM) - a divide and conquer scheme in which the system is divided into cubic subdomains and interactions between distant charged regions are calculated with a truncated multipole expansion. The performance of the FMM on Touchstone Delta machine is discussed.
Statistical Modelling in Materials Research
Authors: Pouskouleli G. , Ahmad A. , Wheat T.A. , Varma S. , Prasad S.E.
Published: March 2011

The role of statistical modelling in materials research is examined. It is shown that considerable benefits can be derived by following an appropriate full factorial design, in which a number of factors (process parameters) are varied simultaneously while the responses (materials properties) are monitored. It is also shown that by using a fractional factorial matrix, essentially the same conclusions can be drawn with a significantly smaller number of experimental trials. Examples drawn from recent materials research on glasses and ceramics are presented to demonstrate the effectiveness of this approach.
Interatomic Potentials From First-Principles Calculations
Authors: Ercolessi Furio , Adams James B.
Published: March 2011

We propose a new scheme to extract “optimal” interatomic potentials starting from a large number of atomic configurations (and their forces) obtained from first-principles calculations. The method appears to be able to overcome the difficulties encountered by traditional fitting approaches when using rich and complex analytical forms, and constitute a step forward towards large-scale simulations of condensed matter systems with a degree of accuracy comparable to that obtained by ab initio methods. A first exploratory application to aluminum is presented.
Long Time Dynamics of the Transverse Ising Model - Comparison With Data onLiTbF4
Authors: Sen Surajit , Florencio João , Cai Zhi-Xiong
Published: March 2011

We use the continued fraction formalism (CFF) to obtain the zz- component of the dynamical spin pair correlations of a transverse Ising model (TIM) in 2D at T = ∞ for the first time. Our calculations suggest that the correlation function decays algebraically at long times as t−α, 1.5 < α < 2.2, which is slower than gaussian decay found for the corresponding dynamical correlation in 1D. The calculations suggest that the corresponding dynamical correlation in 3 D is also likely to exhibit a power law decay. Our results compare favorably with the neutron scattering data on LiTbF4, for T > Tc, an induced moment ferromagnet.
Parallel Molecular Dynamics With the Embedded Atom Method
Authors: Plimpton Steven J. , Hendrickson Bruce A.
Published: March 2011

Parallel computing offers new capabilities for using molecular dynamics (MD) to simulate larger numbers of atoms and longer time scales. In this paper we discuss two methods we have used to implement the embedded atom method (EAM) formalism for molecular dynamics on multiple-instruction/multiple-data (MIMD) parallel computers. The first method (atom-decomposition) is simple and suitable for small numbers of atoms. The second method (force-decomposition) is new and is particularly appropriate for the EAM because all the computations are between pairs of atoms. Both methods have the advantage of not requiring any geometric information about the physical domain being simulated. We present timing results for the two parallel methods on a benchmark EAM problem and briefly indicate how the methods can be used in other kinds of materials MD simulations.
Diffusion of Metallic Impurities in Semiconductor Thin Film
Authors: Vaysleyb Alexander V.
Published: March 2011

The diffusion transport of metal impurities in semiconductors is considered. Critical magnitude of dislocation density and critical sample size are obtained at which the diffusion kinetics change from the interstitial impurity regime of diffusion transport to the self-interstitial regime or the vacancy regime. It is predicted that the critical density of dislocations decreases with increasing temperature.
A Method for the Determination of Real-Space Interatomic Force-Constants.
Authors: Quong Andrew A. , Liu Amy Y. , Klein Barry M.
Published: March 2011

We present a method for the self-consistent determination of inter-atomic force-constants. Using non-local ab-initio pseudopotentials to represent the ion-electron interaction and linear response theory to calculate the self-consistent change in the electron density, we are able to calculate the dynamical matrices at arbitrary points in the Brillouin zone. Diagonalization of the dynamical matrix yields phonon eigenvectors and eigenvalues, and fourier inversion yields the real-space interatomic force-constants. We present numerical results for the phonon-dispersion of a variety of metals.
Reconstruction of Twist Grain Boundaries in Gold
Authors: Phillpot S. R.
Published: March 2011

The reconstruction of high-angle twist grain boundaries on the four densest atomic planes in gold are investigated using the recently developed method of grand-canonical simulated quenching. It is found that the grain boundaries on the two densest planes, (111) and (100), do not reconstruct, while those on the (110) and (113) planes do.
Theoretical Prediction of the Donor/Acceptor Site-Dependence on First Hyperpolarizabilities in Conjugated Systems Containing Azomethine Bonds
Authors: Matsuura Azuma , Hayano Tomoaki
Published: March 2011

We calculated frequency-dependent first hyperpolarizabilities for electro-optic Pockels effect, β(ω; 0, ω), in π-conjugated molecules containing azomethine bonds using the CNDO/S-CI sum-over-states method. First, we examined the dependence of static β on formulas for the two-center Coulomb integral, γAB. The choice of the γAB formula has little effect on the relative β value, but a considerable effect on the absolute value. Then we investigated the relationship between the magnitude of β(ω; 0, ω) and the donor/acceptor substitution sites. We found that the molecule with no donors or acceptors can be divided into donor and acceptor regions whose existence causes the donor/acceptor site-dependence of β(ω; 0, ω).
Angular Dependence of the Magnetic Properties of Steels Under the Action of Uniaxial Stress
Authors: Kaminski D. A. , Jiles D.C. , Biner S.B. , Sablik M.J.
Published: March 2011

Angular dependence of structure sensitive magnetic properties such as coercivity and permeability has never been adequately explained by a quantitative physical model or theory. The present study investigates the angular dependence of magnetic properties of materials with different microstructures. Measurements were taken under compressive and tensile uniaxial stress. The materials investigated were plain carbon steels with compositions of 0.1 % -0.8% carbon. Alloy steels with compositions of 2.25 % Cr and 1.0 % Mo were also investigated. The effects of uniaxial stress on permeability along the direction of the stress can be described by including an effective field Hσ given by:

Experimental results are compared to model using Hσ.
Anharmonic Effects at the (100) and (110) Surfaces of Ni
Authors: Beaudet Yvon , Lewis Laurent J. , Persson Mats
Published: March 2011

We present the results of a detailed molecular-dynamics investigation of anharmonic effects at the (100) and (110) surfaces of Ni, with the interactions between atoms described using embedded-atom potentials. We find both surfaces to suffer an anomalously large thermal expansion, with a concomitant rapid increase of the mean-square amplitudes of vibration. For the (100) surface, our results for the surface expansion can be directly compared to the low-energy electron diffraction (LEED) measurements of Cao and Conrad;1 the agreement is excellent. The anomalies appear at a temperature of about 900 K for the (100) surface, well below the onset of disordering, thereby confirming that the observed anomalies indeed arise from anharmonicities. Our simulated LEED intensities are found to reproduce well the observed intensities. The phonon spectra in the anharmonic regime will be discussed, and a detailed comparison between the two surfaces will be established.
A Tight-Binding Molecular Dynamics Simulation of the Melting and Solidification of Silicon
Authors: Horsfield Andrew , Clancy Paulette
Published: March 2011

The melting of crystalline silicon and the cooling of liquid silicon are investigated using Molecular Dynamics. Both the Stillinger-Weber (SW) potential and the Tight-Binding Bond Model are used to calculate the forces. The electrical properties are investigated using an empirical pseudopotential method with a plane wave basis. The melting point of the solid is found to be about 2300K. The dependency of this temperature with cell size is investigated. On cooling, there are changes in some of the properties of the liquid: the energy per particle decreases, the diffusion constant decreases, and the low frequency electrical conductivity decreases slightly as the temperature decreases. Between 1180K and 980K the liquid undergoes a transition to a glassy phase. There are large changes in the pair correlation function, the SW three-body energy distribution, the diffusion constant, the density of electron single particle states and the electrical conductivity. All of these changes are consistent with increased tetrahedral bonding.
Molecular Dynamics Simulations and Theory of Interfaces of Oligomers
Authors: Harris Jonathan G. , Wang Yantse
Published: March 2011

These proceedings summarize recent work in our group studying the structure of interfaces involving molecular fluids. Two types of systems are discussed. First, we summarize simulations of the structure and surface tension of liquid-vapor interfaces of the alkanes eicosane and decane. Then, we describe the results of simulations of the confined films studied in surface force apparatus and atomic force microscopy experiments. Our simulations show that in both films of normal and branched alkanes, the formation of a layered structure is observed. The branching inhibits this layering, especially in the narrowest pores. In addition an examination of the molecular motions indicates that a transition to a solid or glassy state is not a prerequisite for layering or oscillating solvation forces.
Am 1 Semiempirical Mo Calculations of Torsional Barriers and Structural Reorganization in Radical Ions of Pbo and Pbzt Model Structures
Authors: Connolly John W. , Dudis Douglas S.
Published: March 2011

AM 1 semiempirical calculations on model structures of neutral, anionic and cationic poly(p-phenylenebenzobisthiazole), PBZT, offer an explanation why the PBZT anion has been shown to be an electrical conductor while the PBZT cation is not. To wit, the highest occupied molecular orbital (HOMO) in the PBZT model structure is centered primarily on the sulfur atoms, resulting in concentration of the positive charge on the sulfur atom when the cation is formed. The lowest occupied molecular orbital (LUMO) is far more delocalized and consequently anion formation produces a species which has electronic charge delocalized over the entire model structure. Similar calculations on model structures of neutral, anionic and cationic poly(p-phenyienebenzobisoxazole), PBO, indicate that both frontier orbitals, i. e., the HOMO and the LUMO are delocalized.
Nonlinear Molecular Dynamics and Monte Carlo Simulations of Crystals at Constant Temperature and Tensorial Pressure
Authors: Lill J.V.
Published: March 2011

Complimentary molecular dynamics and Metropolis Monte Carlo algorithms for the atomistic simulation of crystals at constant temperature and homogeneous tensorial pressure are summarized. The novel aspect of computational physics which unites these methods is the extension of the virial theorem to nonlinear elastic media. This guarantees the dynamical balance between the internal pressure, as determined by the interatomic potential, and effective external pressure, as determined by the applied laboratory pressure, and includes the elastic response of the material. Numerical examples are presented.
Computational Diagnostics for Detecting Phase Transitions During Nanoindentation
Authors: Lee Susanne M. , Hoover Carol G. , Kallman Jeffrey S. , Hoover William G. , De Groot Anthony J. , Wooten Frederick
Published: March 2011

We study nanoindentation of silicon using nonequilibrium molecular dynamics simulations with up to a million particles. Both crystalline and amorphous silicon samples are considered. We use computational diffraction patterns as a diagnostic tool for detecting phase transitions resulting from structural changes. Simulations of crystalline samples show a transition to the amorphous phase in a region a few atomic layers thick surrounding the lateral faces of the indentor, as has been suggested by experimental results. Our simulation results provide estimates for the yield strength (nanohardness) of silicon for a range of temperatures.
Generalized Melting Criterion for Amorphization
Authors: Devanathan R. , Lam N. Q. , Okamoto P. R. , Meshii M.
Published: March 2011

We present a thermodynamic model of solid-state amorphization based on a generalization of the well-known Lindemann criterion. The original Lindemann criterion proposes that melting occurs when the root-mean-square amplitude of thermal displacement exceeds a critical value. This criterion can be generalized to include solid-state amorphization by taking into account the static displacements. In an effort to verify the generalized melting criterion, we have performed molecular dynamics simulations of radiation-induced amorphization in NiZr, NiZr2, NiTi and FeTi using embedded-atom potentials. The average shear elastic constant G was calculated as a function of the total mean-square atomic displacement following random atom-exchanges and introduction of Frenkel pairs. Our results provide strong support for the generalized melting criterion.
The Si Vacancy: An Example of a Pressure-Sensitive Jahn-Teller System
Authors: Tarnow Eugen
Published: March 2011

We calculate the trends in the Jahn-Teller (JT) parameters of the Si vacancy as a function of the breathing mode coordinate. Using ab-initio local-density-functional theory and Hellman--Feynman forces we find the couplings between the JT modes and the breathing mode to be significant. Since the breathing mode coordinate is very sensitive to pressure we predict a pressure-sensitive JT contribution to the effective electronic correlation energy U. Furthermore we find that the tetragonal JT energy increasingly dominates over the trigonal energy as the breathing coordinate decreases due to a softening of the tetragonal spring constant.
Incorporation of Radionuclides into Mineral Phases via a Thermally Unstable Complexant Ligand
Authors: Apblett Allen W. , Georgieva Galina D. , Mague Joel T.
Published: March 2011

The complexation and thermal behavior of pyruvic acid oxime (PAO) with representative cations associated with nuclear reprocessing waste has been investigated. This ligand has the dual advantage that it precipitates most of the cations from solution and these complexes decompose at ca. 150°C to CO2, acetonitrile, and, initially, the metal hydroxide. Introduction of the appropriate elements required for Synroc phases to a simulated waste stream followed by NaPAO and disodium fumarate leads to precipitation of a metalorganic Synroc precursor in which the components have been intimately mixed at the molecular level. This approach circumvents incorporation of alkali metals in the final ceramic.
The Leaching of Pu, Am, Np and Tc from High-Level Waste Glasses in Clay Media
Authors: Lemmens K. , Van iseghem P. , Wang L.
Published: March 2011

The leaching behaviour of Pu, Am, Np and Tc from the high-level waste glasses of interest to Belgium has been studied. Corrosion tests were carried out on inactive glasses (the Cogéma R7T7 glass SON68 and the DWK Pamela glasses SM513 and SM527), doped with the radionuclides. Nuclide leaching was studied in contact with a slurry of Boom clay, diluted with claywater, at 90°C and 40°C, for durations of up to 4 years. The retention of Pu and Np in the glass surface layers was considerable for SON68 and SM513, but small for SM527. The retention of these elements decreased at lower temperature. Tc was not retained. The largest part of the leached activity is adsorbed onto the clay in an immobile form. Nuclide concentration in mobile form (smaller than 105 MWU) was found to be independent on duration for Pu and Am. Increasing concentrations were found for Np and Tc. The mobile concentrations were smaller in the presence of corrosion products.
Analytical Electron Microscopystudy of Colloids from Nuclear Waste Glass Reaction
Authors: Buck E.C. , Bates J.K. , Cunnane J.C. , Ebert W.L. , Feng X. , Wronkiewicz D.J.
Published: March 2011

An Analytical Electron Microscopy study of colloidal particles formed during reaction of waste glass has been performed. The effect of waste glass test parameters on colloid formation is examined. Characterization of phases present in the leachate of these tests has shown that layers spalled from the glass and precipitated phases are both sources of colloids in the leachate. Elements, such as uranium, have been found to concentrate within colloidal particles in the leachate.
Effects of Relative Surface Area and Leachant Composition on the 137Cs Leach Rate from Cement Waste Forms
Authors: Peric Aleksandar D. , Plecas Ilija B. , Glodic Snezana D.
Published: March 2011

Cement matrices, doped with 137Cs, were investigated to establish their physicochemical properties, mainly 137Cs leach-rate. Experiments were carried out in leachant media with different degrees of aggressiveness on the cement matrix, e.g., distilled water, Na2SO4 and NaCl solutions. A series of six cement samples, which were orthocylindrically-shaped, were treated in each of three leachant media. For each sample, the surface area exposed to leaching relative to the total surface area was increased from 1:6 to 6:6, at increments of 1/6 of the total surface area. Expected higher 137Cs leach-rates of the samples immersed in the more aggressive environment show very specific behavior and functional relations with respect to the exposed surface area. Radionuclide leach-rate coefficients did not exhibit linear behavior. Instead, leach-rate values were best fit by exponential equations to describe cement matrix behavior in the leachants. Different empirical equations were established to describe 137Cs leach-rates in the various leachants as a function of the relative surface area of the cement matrix exposed to the leachant.
The Role of Oxygen Diffusion in the Release of Technetium from Reducing Cementitious Waste Forms
Authors: Smith Robert W. , Walton John C.
Published: March 2011

Cementitious materials provide an ideal geochemical environment (e.g., high pH pore fluids and large surface areas for sorption) for immobilizing nuclear waste. The inclusion of reducing agents, such as blast furnace slag (BFS) can immobilize radionuclides by forming of solid sulfide phases. Thermodynamic calculations using the MINTEQ geochemical computer code indicate that elemental sulfur present in BFS reacts with the highly mobile pertechnetate anion (TcO4−) anion to form an insoluble technetium sulfide phase (Tc2S7(8)). Initially, the waste form very effectively immobilizes technetium. However, as oxygen diffuses into the waste form, an outer zone of oxidized concrete and a shrinking core of reduced intact concrete develops. Oxidation of sulfur in the outer zone results in increased technetium concentrations in the pore fluid because Tc2S7(8) oxidizes to the mobile TcO4− anion. The TcO4− anion can then diffuse from the waste form into the environment.

A mathematical model that accounts for diffusion of oxygen into concrete coupled with oxidation of sulfur and sulfide to sulfate has been developed. This model assumes the existence of an oxidized outer layer of concrete surrounding a shrinking core of reducing intact concrete. A sharp boundary between the two zones moves slowly inward resulting in oxidation of Tc2S7(8) and subsequent release of TcO4− via aqueous diffusion in the concrete pore fluids. The model indicates that this mechanism results in a linear dependance of release with the square root of time similar to pure diffusion. In addition, the release of technetium is related to the inverse of the square root of the concentration of BFS, indicating that performance will significantly increases with the addition of approximately 20 percent BFS to the cement mix.
Alkali-Activated Fly Ash-Slag Cement Based Nuclear Waste Forms
Authors: Jiang W. , Wu X. , Roy D.M.
Published: March 2011

This paper is based on the results of an in-progress research project on Alkali-Activated Cement System at MRL. The objective of this research is to establish the potential for large volume use of fly ash and slag as main components of the cement system. Alkali-activated Fly ash-slag Cement (AFC) was studied as a matrix for immobilization of nuclear waste. AFC is characterized by high early strength, high ultimate strength, low porosity, lower solubilities of the hydrates, and high resistance to chemical corrosion as well as to freezing and thawing. All these advanced properties are particularly favorable to the immobilization the nuclear wastes.
Critical Conditions for Initiation of Localized Corrosion of Mild Steels in Contact with Bentonite Used in Geological Disposal Packages of Nuclear Waste
Authors: Nakayama Guen , Akashi Masatsune
Published: March 2011

In the current design of geological disposal of high-level nuclear waste, the use of bentonite to stand as an artificial barrier-cum-buffer between the host rock and the packages made of mild steel is being investigated. Although mild steels commomly have been considered to be passivity in alkaline environments, under certain circumstances, they become liable to localized corrosion, e.g., pitting corrosion and crevice corrosion. Since bentonite can turn the environment alkaline to a pH of approximately 10 when it is mixed with groundwater, critical conditions for the initiation of localized corrosion of mild steel must be known to evaluate the extremely long time integrity of disposal packages serving in such an environment. This paper presents and discusses the observations and results acquired in a series of critical conditions for the initiation of localized corrosion of mild steels in various groundwater-bentonite environments at 20C, with a deaerated aqueous solution of 1 mMol/L [HCO3−] +10 ppm [CI−], simulating the natural groundwater and varying the bentonite content.
Aqueous Diffusion in Repository and Backfill Environments
Authors: Conca James L. , Apted Mick , Arthur Randy
Published: March 2011

Aqueous diffusion coefficients have been experimentally determined in a variety of porous/fractured geologic and engineered media. For performance assessment applications, the purely diffusive flux must be separated from retardation effects. The simple diffusion coefficient, D, does not include any transient chemical effects, e.g., sorption, which lower the diffusion coefficient for some finite time period until equilibrium is reached. D is primarily a function of volumetric water content, θ, and not material characteristics. At high water contents, D gradually declines as water content decreases, from 10−5 cm2/sec at θ ∼ 50% to 10−7 cm2/sec at θ ∼ 5%, followed by a sharp decline to 10−10 cm2/sec at θ ∼ 0.5%. Although surface diffusion has a strong experimental basis in the transport of gases along metal surfaces, experimental evidence for aqueous geologic/backfill/engineered systems strongly indicates that surface diffusion is not important, even in bentonite, because of the extremely poor connectivity among electric double-layers and the extremely low diffusivities and high ∂C/∂x at small area/point contacts which more than negate the increased flux along intragrain surfaces.
Study on Retardation Mechanism of 3H, 99Tc, 137Cs, 237Np and 241AM in Compacted Sodium Bentonite
Authors: Sato H. , Ashida T. , Kohara Y. , Yui M.
Published: March 2011

The apparent diffusion coefficients were measured at room temperature (about 23°C) under atmospheric condition by the one-dimensional non-steady state diffusion method for 3H, 99Tc, 137Cs, 237Np and 241Am in compacted sodium-bentonite saturated with water. Sodium-bentonite, which is commercially available as KunigelVl®, was used in this study. Experiments were carried out in the density range of 0.4–2.0 (×103kg/m3). Bentonite in the cell was prepared to be saturated with distilled water. The measured apparent diffusion coefficient decreases with increasing dry density of bentonite. That the apparent diffusion coefficient of 3H decreased as a function of dry density of bentonite appears to be the effect of the change of porous structure with dry density of bentonite. 99Tc may be retarded by anion-exclusion because dominant diffusion specie of 99Tc is pertechnetate ion under atmospheric condition. Retardation for 137Cs may be caused by ion-exchange on bentonite. The sorption, anion-exclusion and molecular filtration are considered as a retardation mechanism for 237Np and 241Am because those dominant species are negatively charged and of large ionic size.
Measurements of Grain-Boundary Inventories of 137Cs, 90Sr and 99Tc in Used Candu Fuel
Authors: Stroes-Gascoyne S. , Tait J.C. , Porth R.J. , Mcconnell J.L. , Barnsdale T.R. , Watson S.
Published: March 2011

Two methods were used to measure grain-boundary inventories of 137Cs, 90Sr and 99Tc in used CANDU fuel, to corroborate source term estimates based on a fission gas release code. Used fuels were partially oxidized at 200°C in air to overall compositions of UO2+x (0.15≤ × ≤0.25) to expose UO2 grain boundaries, followed by leaching in aqueous solution. Only a fraction (2 to 18%) of the calculated gap + grain-boundary inventories for 37Cs was released. This suggests that the calculations overestimate Cs release or that oxidation does not expose all grain boundaries, or that Cs release from grain boundaries is slow. Release of 90Sr (0.01 to 0.7%) agreed reasonably well with the source term estimates (0.001 to 0.3%). Release of 99Tc (0.3 to 1.5%) suggests that the source term estimate for the upper boundary of 99Tc release (25%) may be too high. A second technique involved leaching of crushed and size-fractionated used fuel in either a static or dynamic system. A direct one-to-one correlation between calculated and measured gap + grain-boundary inventories for 137Cs was found for low- and medium-power fuels.
Temperature and Moisture Distributions in a Clay Buffer Material Due to Thermal Gradients
Authors: Mohamed A.M.O. , Yong R.N. , Kjartanson B.
Published: March 2011

Several series of one-dimensional tests were used to investigate the nature of transient heat and moisture movements in a clay buffer under different imposed temperature gradients. The measured temperature and moisture profiles were used to calculate the diffusion parameters governing heat and moisture movement in the buffer material. The diffusion parameters are shown to depend on the moisture content, temperature and moisture equilibrium time.
Radiation Damage in NaCI: Calculations with an Extended Jain-Lidiard Model
Authors: Soppe Wim J. , Prij J.
Published: March 2011

The colloid growth due to irradiation in a rock salt formation is calculated with an extended version of the Jain-Lidiard model. The extensions of the model comprise a description of the nucleation stage of the colloids and the role of impurities on the formation of defect centers. Results of model calculations are shown for a representative design for a high-level radioactive waste repository in a rock salt formation. It is concluded that it is unlikely that, near the waste containers, the fraction of NaCl that will be converted to metallic Na and molecular Cl centers will exceed a few mole percent.