bluuebunny/crossref_metadata_2025 huggingface dataset:
Segregation at a Single Trap in the Presence of Fields
Authors: Taitelbaum Haim , Weiss George H.
Published: March 2011

There have been a number of recent investigations of segregation properties of diffusing particles in the presence of a single static trap in low dimensions. We study these properties when the diffusing particles are subject to different forms of external fields: global constant bias, random bias fields (Sinai model) and random transition rates. We discuss two measures of segregation, the distances from the trap either to the point at which the concentration profile reaches a specified fraction of its bulk value, or to the nearest unreacted particle. For the cases of global bias (both away from, and towards the trap) and random fields, we found that both measures of segregation have the same asymptotic temporal behavior, while for random transition rates they differ. We explain this difference by relating the nearest-neighbor distance measure to properties of hard-core diffusion in these systems. We also found anomalous spatial shapes for the profile in the vicinity of the trap in the random systems, as well as anomalous reaction rates.
Catalysis on a Fractal Lattice: A Model for Poisoning
Authors: Clement Eric , Leroux-Hugon Patrick , Argyrakis Panos
Published: March 2011

We present theoretical calculations and Monte-Carlo simulations of a surface reaction model proposed by Fichthorn, Gulari and Ziff (1), on a geometrically disordered lattice. Here we look at this model on a percolation cluster in euclidean dimensions d=2 and d=3. We address the problem of poisoning by a foreign inert species exactly at the percolation concentration.
Consolute Critical Phenomena in Dilute Silica Gel
Authors: Frisken B. J. , Ferri Fabio , Cannell David S.
Published: March 2011

The effect of even dilute silica networks on the critical phenomena of binary liquid mixtures is profound. The network preferentially adsorbs one component, preventing a portion of the mixture from participating in critical fluctuations. Fluctuations in the remaining mixture are found to decay with a non-exponential correlation function near the consolute point. A correlation function consisting of the sum of an exponential decay and a non-exponential term of either an activated or stretched exponential form fits the data well. In the presence of the silica network, the mixtures are observed to phase separate near the critical temperature of the pure system, but while still in the one-phase region of the pure system.
High Pressure NMR Studies of Confined Liquids
Authors: Jonas J. , Zhang Jing , Xu Shu
Published: March 2011

The main goal of our novel NMR experiments on confined liquids was to determine the effects of pressure on the dynamics of liquids in the surface layer by using the twostate, fast exchange model and compare them to the pressure effects observed for bulk liquids. With this goal in mind, the deuteron NMR spin-lattice relaxation times, T1, in liquid pyridine-d5, nitrobenzene-d5, and methylcyclohexape-d confined to sol-gel porous silica glasses with pore radii in the range from 18Å to 49˚A were measured as a function of pressure up to 5 kbar at 300 K. In another set of high resolution natural abundance 13C NMR experiments, the 13C relaxation behavior of each carbon in 2- ethylhexyl benzoate model lubricant was measured as a function of pressure in porous silica glasses. In fact, the described experimental approach which allows investigation of the effects of pressure on the dynamic behavior of surface-layer liquids, may provide a new tool in studies of model liquid lubricants at extreme conditions of pressure and temperature. In addition selected results of a recent study of acetonitrile-d3 are reviewed. The 2H and 14N spin-lattice relaxation times of acetonitrile-d3 in porous silica glasses were measured in order to study the confinement effects on the anisotropic reorientation characterized by rotational diffusion constants, D⊥ and D‖, of this symmetric-top molecule.
Simulation Study of Strain-Related Morphology in YBa2Cu3−x(Al,Fe)xO7
Authors: Cai Zhi-Xiong , Zhu Yimei , Welch David O.
Published: March 2011

A new simulation method which combines the merits of Monte Carlo simulation of a lattice gas model and the continuum elasticity theory is described. This method treats the elastic strain energy due to concentration fluctuation of interstitial as a perturbation of a lattice gas model Hamiltonian. We illustrate this method by calculating the diffuse scattering intensity of YBa2Cu3O7 systems doped with trivalent impurity atoms M such as Fe or Al. The oxygen concentration wave amplitudes cq were obtained from Monte Carlo simulations of an anisotropic lattice gas model which represents well the interaction between oxygen atoms in this system. These results are in turn used to calculate the diffuse X-ray scattering intensity caused by the displacement field using concentration wave/displacement wave approach. The results suggest that the small orthorhombic domains associated with the oxygen “cross-link” around impurity atoms M causes the diffuse scattering intensity to fall off with oxygen concentration wave vector q as 1/q2for small qand as 1/q4 for larger q.We also show that the size of such domain can be obtained from diffuse scattering data.
An Exact Approach to the Diluted Hubbard Model
Authors: Wang Chumin , Navarro O. , Oviedo-Roa R.
Published: March 2011

A new method to solve the extended Hubbard Hamiltonian for systems with few electrons is reported. This method is based on mapping the original many-body problem onto a tight-binding one in a higher dimensional space, which can be solved exactly. For one-and two-dimensional periodic lattices, the real-space pairing problem of two electrons with parallel and anti-parallel spins is analyzed by looking at the binding energy, the coherence length and the mobility of electron pairs. Likewise, some results of the three-body correlation are also reported.
Pseudopotential Portability in the Qmc Framework
Authors: Menchi M. , Bosin A. , Meloni F. , Bachelet G. B.
Published: March 2011

We have calculated the binding energies of several two-electron pseudoions using the Diffusion Quantum Monte Carlo method. The comparison between our results and the experiment suggests that HSC pseudopotentials are portable from the original single particle theory (DFT-LDA) to the “exact” many body one. Moreover we are able to evaluate the degree of portability of each effective field.
Ionic Overlayers on Corrugated Surfaces III : Dynamics in the Fluid Phase
Authors: Seong Hyangsuk , Mahanti S.D.
Published: March 2011

Using molecular dynamics simulation, we have studied the dynamics in the fluid phase of ionic overlayers on corrugated substrates. This is a good model to probe the dynamics of intercalants inside the galleries of intercalated systems. We have calculated the diffusion constant from the incoherent dynamic structure factor, de Genne oscillations in the q-dependence of the Rayleigh peak width and dispersion of Brillouin peaks have been obtained from the coherent dynamic structure factor. These results are compared with neutron scattering measurements in stage-2 RbC24 and earlier simulation results.
Constant-Temperature Molecular-Dynamics Simulation of the 2D Melting Transition of Rb and K
Authors: Fan J. D. , Cai Zhi-Xiong
Published: March 2011

The energy histogram method, introduced by Ferrenberg and Swendsen [Phys. Rev. Lett., 61, 2635, (1988) and 63, 1195, (1989)], was applied for the first time to the constant temperature molecular dynamics (MD) simulation of a two-dimensional (2D) system with incommensurate structures. We performed MD simulations for the stage-2 graphite intercalation compounds (GIC's) with Rb or K being the intercalants (Rb-GIC's and K- GIC's). The temperature dependence of the specific heat, Cv, is calculated for various sizes up to 864 atoms. The melting temperature was found to be 158 K for Rb-GIC's and 119 K for K-GIC's, respectively, which are in agreement with the experimental observations.
Methods for Determining Vacancy Formation Thermodynamic
Authors: Zhao L. , Najafabadl R. , Srolovitz D. J.
Published: March 2011

The vacancy formation thermodynamics in six fcc metals Ag, Au, Cu, Ni, Pd and Pt are determined from atomistic simulations as a function of temperature. This investigation is performed using the Embedded Atom Method interatomic potentials and the finite temperature properties are determined within the local harmonic and the quasiharmonic frameworks. We find that the temperature dependence of the vacancy formation energy can make a significant contribution to the vacancy concentration at high temperatures. An additional goal of the present study is to evaluate the accuracy of the local harmonic method under circumstances in which the excess entropy associated with the formation of a defect is very small. Our data demonstrate that while the errors associated with determining the vacancy formation entropy in the local harmonic model are large, a simple extension to the local harmonic method yields thermodynamic properties comparable to that obtained in the quasiharmonic model, but with much higher computational efficiency.
Equilibrium Unstable Point Defects in Solids
Authors: Koshkin V.M.
Published: March 2011

It is shown that in addition to Frenkel’s and Schottky’s defects there is another type of equilibrium defects, such as an unstable pairs (UP) : vacancy and interstitial atom. UP has the life time of the order of 10-1010-12s, independent of temperature. The review of the unstable pairs physics is presented. Numerous experimental evidences of equilibrium UP in semiconductors, insulators, especially in superionic crystals, and metals are discussed.
A Comparative Atomistic Study of the Structure of Grain Boundaries in Tungsten
Authors: Marinopoulos A. , Sob M. , Vitek V. , Carlsson A. E.
Published: March 2011

Most atomistic studies of grain boundaries have been carried out using central forces to describe atomic interactions. However, in transition metals with unfilled d-bands the angular dependence of interatomic forces may be important. The purpose of this paper is to investigate the significance of angular forces in the case of Tungsten. The calculations have been performed for the Σ5(210) symmetrical tilt grain boundary using two alternate approaches. First are the central-force many-body potentials of the Finnis-Sinclair type. The second are the angular dependent potentials obtained via a moment analysis of the electronic density of states. The results of these two approaches are compared by analyzing the boundary structures, the relative displacements of the adjoining grains and the expansion. Differences in structural characteristics are discussed in terms of the effect of angular forces.
A New Approach for Calculating Phonon Dispersions and Elastic Constants of Simple Metals
Authors: Li M. , Liu S.J. , Chen N.X.
Published: March 2011

A new method to calculate the phonon dispersions for simple metals is first introduced, based on the Mobius transform. The cohesive energy can be obtained from experiment or theory, then the pair and three—body potentials between atoms in metals can be obtained by use of the modified Mobius theorem. Based on only the pair potentials the phonon dispersions for Cu, A1 and Ni have been obtained without any adjustable parameters. The calculation of elastic constants has involved both the 2—and 3—body interactions, and the restriction of the Cauchy relation has been erased. The above result represents a significant improvement over the Carlsson — Gelatt — Ehrenreich method.
Hydrogen Complexes and their Vibrations in Proton and Deuteron Implanted Inp: Theory and Experiment
Authors: Talwar D. N. , Fischer D. W. , Manasreh M. O. , Matous G.
Published: March 2011

By using a Van de Graaff accelerator we have implanted 2 MeV protons and deuterons into InP samples. These implanted samples were annealed in 50 °C increments in the 200 to 600 °C temperature range for 30 minutes each and then measured by the infrared absorption spectroscopy. A group of four different local vibrational mode absorption lines is observed, each of which arises from the hydrogen-phosphorus stretching vibration with different defects or impurities at nearest-neighbor sites. Each mode exhibits an annealing behavior which is different than any of the other absorption lines. The annealing results are shown and discussed in terms of Green's function calculations of the defects responsible for the two absorption lines.
Structure and Growth of Small Palladium Clusters
Authors: Li Y. S. , Cai Y. , Newsam J. M.
Published: March 2011

We study the structure and growth sequence of small palladium clusters Pdn using the Many-Body Alloy (MBA) potential and simulated annealing techniques. Our results show the preference of compact polyhedral structures. These equilibrium structures are compared with the bulk Pd crystal in terms of cohesive energies and nearest neighbor distances. Both the cohesive energy and the nearest neighbor distance show a slow convergence to bulk behaviors. By analyzing the detailed structures and cohesive energies, we find that Pd4, Pd7 and Pd13 are magic number structures, which are the consequence of their high symmetry and large coordination number.
First-Principles Theory of Polarization in Ferroelectrics
Authors: Resta R. , Posternak M. , Baldereschi A.
Published: March 2011

We outline a modern theory of the spontaneous polarization P in pyroelectric and ferroelectric materials. Although P itself isnot an observable, the difference ΔP between two crystal states can indeed be measured and calculated. We define P as the difference between the polar structure and a suitably chosen nonpolar prototype structure. We previously proposed and implemented a supercell scheme in order to evaluate P in pyroelectric BeO; here we adopt an approach recently developed by King-Smith and Vanderbilt, where ΔP is obtained from the computation of Berry's phases, with no use of supercells. We apply this novel approach, which is numerically very convenient, in order to revisit our previous work on BeO. We then perform a first-principles investigation of the spontaneous polarization P of KNbO3 in its tetragonal phase, which is a well studied perovskite ferroelectric. Our calculated P value confirms the most recent experimental data. The polarization is linear in the ferroelectric distortion; the Born effective charges show strong variations from nominal ionic values, and a large inequivalence of the 0 ions. Only the highest nine valence-band states (O 2p) contribute to P, while all the other states behave as rigid core states.
Synthesis and Characterization of Periodic Polypeptides Containing Repeating —(AlaGly)xGluGly— Sequences
Authors: Deguchi Yoshikuni , Krejchi Mark T. , Borbely Janos , Fournier Maurille J. , Mason Thomas L. , Tirrell David A.
Published: March 2011

We have expressed in E. coli a series of periodic polypeptides represented by sequence 1. Our objective has been an understanding of the role of chemical sequence in determining the chain folding behavior of periodic macromolecules. Molecular organization has been examined by infrared spectroscopy and 1H and 13C NMR methods and a preliminary model of the folded structure has been developed.
Processing Natural and Reconstituted Silk Solutions Under Equilibrium and Non-Equilibrium Conditions
Authors: Viney Christopher , Huber Anne E. , Dunaway Dwayne L. , Case Steven T. , Kaplan David L.
Published: March 2011

A variety of natural silk secretions (from spiders, silkworms and aquatic insect larvae), and also reconstituted silk solutions, are able to form a nematic liquid crystalline phase. The anisotropic structures that self-assemble in this phase are formed from the isotropic phase by aggregation of molecules, rather than by individual molecules undergoing a conformational change to a rod-like form. This enables the molecules to retain their solubility in water while, simultaneously, the viscosity of the solution is reduced. The liquid crystalline phase is stable under a wide range of equilibrium conditions, but its ability to form is sensitive to the rate at which the initially isotropic solution is allowed to dry. The kinetics of phase transitions exhibited by solutions of silk proteins must be taken into account if solutions of silk fibroin are to be successfully processed in vitro.
Chiral Symmetry Breaking and Pattern Formation in Two-Dimensional Films
Authors: Selinger Jonathan V. , Wang Zhen-Gang , Bruinsma Robijn F.
Published: March 2011

Thin films of organic molecules, such as Langmuir monolayers and freely suspended smectic films, can exhibit a spontaneous breaking of chiral symmetry. This chiral symmetry breaking can occur through at least three possible mechanisms: (1) the relation between tilt order and bond-orientational order in a tilted hexatic phase, (2) a special packing of non-chiral molecules on a two-dimensional surface, and (3) phase separation of a racemic mixture. Because the chiral order parameter is coupled to variations in the direction of molecular tilt, chiral symmetry breaking leads to the formation of patterns in the tilt direction with one-dimensional or two-dimensional order. Using a Landau theory, we investigate these patterns and predict the critical behavior near the chiral symmetry breaking transition.
Genetic Coding in Biomineralization of Microlaminate Composites
Authors: Morse Daniel E. , Cariolou Marios A. , Stucky Galen D. , Zaremba Charlotite M. , Hansma Paul K.
Published: March 2011

Biomineralization is precisely controlled by complex templating relationships ultimately encoded in the genes. In the formation of the molluscan shell, polyanionic pleated sheet proteins serve as templates for the nucleation and epitaxial growth of calcium carbonate crystalline domains to yield microlaminate composites of exceptional strength and crystal ordering. The strength and fracture-resistance of these composites far exceed those of the minerals themselves, as a result of both the capacity for flexible deformation of the organic matrix layers and the retardation of crack propagation at each mineral-organic interface. The basic principles controlling low temperature biosynthesis of these materials thus are of both fundamental and applied importance. The abalone shell consists of microlaminates with a remarkable regularity of lamina thickness (ca. 0.5 micron), the formation of which defies present understanding. We have found that shells of abalone larvae formed prior to metamorphosis contain only aragonite, whereas the adult shell made after metamorphosis contains both aragonite and calcite. This transition is accompanied by a switch in genetic expression of the template proteins, suggesting that the premetamorphic protein may serve as a template for aragonite nucleation and growth, while template proteins synthesized after metamorphosis may direct crystallization of calcite. These analyses are based on improvements we recently reported for the detection and purification of proteins from the demineralized shell matrix. Genetic cloning experiments now in progress are aimed at discovering additional protein sequences responsible for the programmed control of crystal phase termination, since it is the termination and reinitiation of mineralization that is responsible for the regularity of highly ordered microlaminates produced in nature.
Characterization of New Ionically Conducting Peo-Based Networks by Diffusion, Elasticity Modulus and Ionic Conductivity Measurements
Authors: Cheradame Herve , Desbat F. , Mercier-Niddam P. , Boileau S.
Published: March 2011

Ionically conducting materials containing PEO were prepared from telechelic di(methyl-diethoxy-silane) PEO, synthesized by the hydrosilylation of telechelic diallyl-PEO with methyldiethoxysilane. The network is obtained by the usual sol-gel chemistry. Then, it is filled with LiClO4 by diffusion of the salt and further drying. A comparison is made with the same kind of materials crosslinked using urethane chemistry. Diffusion studies show that the diffusion coefficient of solvent is similar for both types of materials, whilst the ionic conductivity is higher for the networks crosslinked with siloxane bonds. An experiment of diffusion of LiClO4 without solvent showed that this salt has a diffusion coefficient of the order of 2.10-8 cm2.sec-1 at 34°C. The conductivity calculated from this determination is compatible with the mechanism of lithium cation transport by the diffusion of salt molecules. Elasticity modulus measurements show that the salt aggregates are essentially located within the crosslinks at low concentration, but also in the PEO chains for salt concentrations higher than 1 mol/l.
Ion Insertion in Highly Conjugated Polypyrrole
Authors: Skaarup S. , West K. , Zachau-Christiansen B. , Careem M.A.
Published: March 2011

Polypyrrole has been synthesized electrochemically at current densities from 6.4 μA/cm2 to 3.84 mA/cm2. Low current density is crucial to the formation of a highly conjugated modification as shown by the low value of the π-π* transition energy (2.83 eV). The low current form displays a cyclic voltammogram with a detailed structure with narrow peaks. Following the presence and evolution of the peaks is a powerful diagnostic method of monitoring the state of the polymer. The solvent (water-free propylene carbonate or acetonitrile) can be reversibly exchanged during oxidation/reduction cycling and has little effect on the nature of the polymer formed, but influences the properties during the doping process greatly. In contrast, the ion present during polymerization (ClO4 or CF3SO3) seems to irreversibly influence the morphology of the film. The highly conjugated, low current form is to be preferred as a well-defined point of reference close to the intrinsic properties of pure polypyrrole.
Variable Microwave Transmission Attenuation by Conducting Polymers
Authors: Rupich Martin W. , Liu Ying Ping , Kon Alan B.
Published: March 2011

The microwave properties of polypyrrole, polyaniline and poly(3-hydroquinonylpyrrole) films were examined as a function of doping level, conductivity, counter anion and thickness. The microwave transmittance of the polymer films was varied from less than 10% to greater than 90%; by oxidizing and reducing the polymers. The potential use of conductive polymers in the design of an electromagnetic modulator for the tunable attenuation of microwave radiation is described.
Structure and Dynamics of Network Glasses at Large Positive and Negative Pressures - a Molecular Dynamics Study
Authors: Vashishta Priya , Kalia Rajiv K.
Published: March 2011

Using the molecular dynamics (MD) method, the nature of crystalline fragments in SiSe2 glass (a-SiSe2) at normal pressure, structural transformation and the loss of intermediate range order in SiO2 glass at very large positive pressures, and the modification of SiO2 glass network at very large negative pressures have been investigated. Implementations of the MD algorithm on the Connection Machine and Intel iPSC/860 machine are discussed.
Structural and Dynamical Correlations in Stishovite and High Density Silica Glass
Authors: Jin Wei , Kalia Rajiv K. , Vashishta Priya
Published: March 2011

Pressure-induced structural transformation from tetrahedral to octahedral coordination and the destruction of intermediate-range order (IRO) are studied in silica glass (a-SiO 2) using the molecular-dynamics (MD) method. Changes in the position and height of the first sharp diffraction peak (FSDP) in the static structure factor, bond lengths, coordination numbers, bond-angle distributions, and statistics of rings are investigated as a function of density. Modifications of the vibrational density of states and participation ratio are also discussed.
Cation Ordering in Brownmillerite-Perovskite Intergrowth Structures
Authors: Thomas Julie K. , Krause Wendy E. , Loye Hans-Conrad Zur
Published: March 2011

Brownmillerite-perovskite intergrowths of the type m(A2B2O5)*n(ABO3), where m = 1 and n = 1, were investigated with respect to cation ordering. It was found that Sr31n2ZrO8, Sr31n2HfO8, Ba3Sc2TiO8 and Ba31n2TiO8 form in a disordered cubic structure, while Sr3Sc2ZrO8 and Ba3Sc2ZrO8 form an ordered cation arrangement. The Arrhenius plots of the oxygen ion conductivity of Ba3Sc2ZrO8, Sr31n2HfO8, and Ba31n2TiO8 contain discontinuities that appear to be caused by an oxygen vacancy order-disorder transition. High temperature powder-X ray diffraction did not show any evidence for cation ordering that might otherwise have created the changes in activation energy in the conductivity plots.
Electrical Behavior of Ca(UO2PO4)2.10.5H2O Films in Contact with the Generating Electrolytes
Authors: J.R. Ramos Barrado , J Benavente , S. Bruque , M. Martinez
Published: March 2011

The electrical response of a self -supported film (CaUP) in contact with different electrolyte media was studied by impedance spectroscopy for electrolyte concentrations ranging between 10−4 M and 10−2 M.The film was placed separating two aqueous solutions of the generating electrolytes, at the same concentrations, and two systems: i) UO2(NO3)2/CaUP/H3PO4; ii)Ca(NO3)2/CaUP/H3PO4, were considered.

The film equivalent circuit and characteristic parameters (Rf and Wf) for both systems were determined. Result show an important dependence of Rf values with concentration, whlckis related to the electrolyte invasion into the film matrix, but no significant changes in Wf values were found. Due to the film nature, the ionic diffusion process associated to this circuit element is mainly attributed to the protons.
Dielectric Properties of the Solid Solution Formed in the System LiTao3 - “Li2MnO4 ”
Authors: Villafuerte-Castrejón M.E. , Pineda J.L. , Huanosta A.
Published: March 2011

Impedance spectroscopy methods were used to characterize the electrical properties of the solid solution formed in the system LiTaO3 -“Li2Mn4O9”.

Involved parameters were calculated using equialent circuits. Activation energy values were decreasing from around 1.3 eV for lowest concentration of Mn4+ until around 0.8 eV for higher Mn4+ contents. Results will be discused in terms of Z' vs Z" and Arrhenius plots. The ferroelectric character was stablished using the real part of the permittivity against temperature. Curie temperature values Tc were found decreasing toward higher concentration of Mn4+ in the range 630-580°C.
Femtosecond Spectroscopy of Ferroelectric Perovskites: Explanation of Anomalous Polariton Dynamics in Lithium Tantalate
Authors: Wiederrecht Gary P. , Dougherty T. P. , Dhar L. , Nelson K. A.
Published: March 2011

Time resolved impulsive stimulated Raman scattering (ISRS) is used to characterize the lowest frequency A1 phonon-polariton mode in lithium tantalate. The anomalously high and wavevector-dependent damping rates observed are explained in terms of coupling of the polariton to a weakly Raman-active relaxational mode and to two heavily damped vibrational modes. The dynamics of the relaxational mode are explored further through temperature dependent ISRS studies. Femtosecond optical pulse shaping is used for multiple pulse ISRS at temperatures lower than 120K where single pulse excitation led to photorefractive damage.
In Situ X-Ray Investigation of Novel Protonic Conductors
Authors: Shekhtman Veniamin Sh. , Dilanyan Ruben A.
Published: March 2011

Systematic X-ray diffraction studies of thermal expansion in CsHSO4, CsDSO4, RbHSO4 and Rb3H(SeO4)2 crystals have been carried out. An analysis of the results showed that the fast protonic conductivity is closely connected with the characteristic features of the structure, symmetry and orientation of hydrogens bonds in the crystals of MeXAO4 (Me=NI4, Rb, Cs; X= D,H; A= S,Se) family.
Fundamental Study on the Solidification of Cs+ and Sr2+ with Hydrous TiIV Oxide Modified with Si and Zr
Authors: Inoue Y. , Yamazaki H. , Erza R.
Published: March 2011

Fundamental data are provided for the potential use of hydrous TiIV-si oxide (TS) and hydrous TiIV-Zr oxide (TZ) as the solidification media of radioactive Cs+ and Sr2+. The sorption capacity for Cs+ and Sr2+, at various conditions, and the structure and the composition of the exchangers in these ionic forms, at high temperature, were investigated, followed by measuring the leaching rate from the calcined exchangers into water at room temperature. TS uptakes more Cs+ and Sr2+ than TZ does. The leaching rate from calcined TS is much lower than that from calcined TZ, due to the formation of titanium silicates of Cs+ or Sr2+. Based on these results, TS is considered to be superior to TZ, in sorption capacity, and the fixation capability of Cs+ and Sr2+, on the exchanger matrix.
Dissolution of R7T7 Glass in Static and Flowing Conditions: Influence of Si Diffusion Mechanism in the Leached Layer
Authors: Delage F. , Larche F. , Vernaz E.
Published: March 2011

Leach tests with R7T7 nuclear waste glass in distilled water were conducted at 50 and 90°C under static and slow flowing conditions, with an SA/V ratio of 50 m−1. A computer model for glass dissolution (LIXIVER) Was used to interpret the experimental data. This mechanistic model is based on a combination of the first-order law governing surface reactions, the silicon mass transport equation for the interstitial solution in the alteration film, and an empirical law for partial silicon retention in the alteration layer. The LIXIVER model satisfactorily accounts for most of the experimental results. The importance of the silicon diffusion mechanism in the diffusion layer is stressed. Values are indicated for the apparent silicon diffusion coefficient, Dsi, which ranged from 10−16 to 10−14 m2·s−1 at 50°C, and from 10−15 to 10−13 m2·s−1 at 90°C.
High-Level Nuclear-Waste Borosilicate Glass: A Compendium of Characteristics
Authors: Cunnane J. C. , Bates J. K. , Ebert W. L. , Feng X. , Mazer J. J. , Wronkiewicz D. J. , Sproull J. , Bourcier W. L. , Mcgrail B. P.
Published: March 2011

With the imminent startup, in the United States, of facilities for vitrification of high-level nuclear waste, a document has been prepared that compiles the scientific basis for understanding the alteration of the waste glass products under the range of service conditions to which they may be exposed during storage, transportation, and eventual geologic disposal. A summary of selected parts of the content of this document is provided.

Waste glass alterations in a geologic repository may include corrosion of the glass network due to groundwater and/or water vapor contact. Experimental testing results are described and interpreted in terms of the underlying chemical reactions and physical processes involved. The status of mechanistic modeling, which can be used for long-term predictions, is described and the remaining uncertainties associated with long-term simulations are summarized.
Borate Substituted Ettringites
Authors: Csetenyi Laszlo J. , Glasser F. P.
Published: March 2011

The setting of cement is adversely affected by soluble borates. To reduce interference, the extent to which borate can be insolubilized has been investigated. One specific mechanism of insolubilization is by inclusion into ettringite. Ettringite, Ca6Al2(SO4)3(OH)12·26H2O, is a normal and stable constituent of Portland cement. It has an open but non-zeolitic framework. Borate can substitute partially or fully for sulfate.

Formation conditions, solubility and stability of borate ettringites, Ca6Al2(BO4)2-4(OH,O)12·26H2O, are characterised using XRD, IR, DTA, and SEM.

The potential durability of borate ettringites in a repository environment have been assessed by exposing it to Na-sulfate and Na-carbonate attack at different concentrations. Ion exchange occurs; back substitution of borate by sulfate is incomplete; high carbonate concentrations can, however, decompose borate ettringite.

On heat treatment up to 85°C the crystalline morphology and the OH arrangement of the structure are altered, but the X-ray powder pattern, and hence its structural framework are largely unaffected.

It is concluded that ettringite has potential to reduce the solubility of borate.
Mobility of Chelated Radionuclides in Engineered Concrete Barriers
Authors: Dicke Craig A. , Smith Robert W.
Published: March 2011

Concrete is a major component in many low-level radioactive waste (LLW) disposal facilities. The use of concrete is widespread because of its physical and structural properties and because it provides geochemical control on metal and radionuclide releases. Organic compounds are often disposed with radionuclides in LLW disposal facilities. Interactions between radionuclides and chelating agents must be evaluated to estimate mobility of radionuclides in concrete vaults. This paper quantifies the effects of two common organic components [citric acid and ethylenediaminetetraacetic acid (EDTA)] on radionuclide mobility in concrete barriers by using equilibrium geochemical calculations.

Equilibrium speciation calculations indicate that some radionuclides are chelated in groundwater (pH 7) but are destabilized in the highly alkaline (pH 13) concrete pore fluids. Radionuclides complexed by EDTA and citrate are replaced by calcium in the concrete pore fluids. In addition, the citrate nuclide complex reacts to form uncomplexed citrate in concrete pore fluids. The chemical performance of concrete LLW disposal facilities should not be compromised by small amounts of chelating agents disposed with some radionuclides. However, EDTA may form significant nickel and cobalt complexes above the pH important in the long-term service life of concrete barriers.
The Radiation Stability of Ground Granulated Blast Furnace Slag/ordinary Portland Cement Grouts Containing Organic Admixtures
Authors: Palmer John D , Fairhall Graham A
Published: March 2011

At the British Nuclear Fuels (BNFL) Sellafield reprocessing plant in the United Kingdom, cement grouts based on ground granulated blast-furnace slag (BFS) and ordinary Portland cement (OPC) are used extensively for immobilising radioactive wastes. These grouts have excluded organic admixtures in order to reduce process complexity and uncertainties, regarding the performance of organic admixtures with BFS/OPC grouts, particularly under irradiation.

This study has investigated the effects of sulfonated melamine formaldehyde and naphthalene condensates on grout properties. The results show grout settlement and strengths increase on addition of additives, with the additives remaining largely in the pore solution. Under irradiation the additives breakdown liberating hydrogen and carbon dioxide. Strength and product dimensions are unaffected by irradiation.
Ancient Metallurgy and Nuclear Waste Containment
Authors: Goodway Martha
Published: March 2011

Archaeological artifacts of glass, ceramic, and metal provide examples of long term durability and as such have been surveyed by the nuclear agencies of several countries as a possible guide to choices of materials for the containment of nuclear waste. In the case of metals evaluation is difficult because of the loss of many artifacts to recycling and corrosion processes, as well as by uncertainty as to the environmental history under which the remainder survived. More recently the study of ancient metallurgy has expanded to included other materials associated with metals processing.

It is suggested that an impermeable ceramic composite used in ancient metals processing installations should be reproduced and tested for its resistance to radiation damage. This material was synthesized more than two millennia ago and has a proven record of durability. These installations have had no maintenance but are intact, some still holding water.
Research Needs in HLW Disposal Programmes
Authors: Hadermann Joerg , Mccombie Charles
Published: March 2011

A repository for high-level radioactive waste (HLW) will not be in operation in Switzerland (or elsewhere) before the turn of the century. However, extensive investigations for disposal in specific regions or sites are ongoing and formal safety analyses have been performed in many countries. Broadly speaking, these analyses show the feasibility of the chosen options for deep geological disposal. At the present stage, before a licensing application, performance assessments have another important application: to identify further needs to improve system understanding, and to guide the necessary research activities. Performance assessments are thus indispensable tools for focussing in on research requirements and discriminating between necessary, and merely desirable or interesting, research projects.

Based on experience from assessments of HLW disposal in the crystalline of Northern Switzerland (Project Gewähr, KRISTALLIN-I) we consider in detail the chain of models resulting from a scenario analysis. For each model block (e.g., engineered barrier performance, hydrology, radionuclide transport), the adequacy of understanding is addressed and the necessary research needs pointed out. These needs cover a wide span, from a requirement for more reliable input numbers (example: long-term corrosion rate of glass) to a better understanding of important features (example: excavation-damaged-zone) and key mechanisms (example: sorption).
Experimental Study of Uranium(6+) Sorption on the Zeolite Mineral Clinoptilolite
Authors: Pabalan Roberto T. , Prikryl J. D. , Muller P. M. , Dietrich T. B.
Published: March 2011

Experiments on the sorption of uranium(6+) on clinoptilolite from solutions in equilibrium with atmospheric CO2(g) were conducted to understand the fundamental controls on uranium sorption on zeolite minerals, including the effects of pH, aqueous uranium speciation, and uranium concentration in solution. The results indicate that uranium(6+) species are strongly sorbed on the zeolite mineral clinoptilolite at near-neutral pH. The amount of uranium sorbed is strongly dependent on pH and, to some extent, on the total concentration of uranium. Uranium sorption on clinoptilolite is important in the pH range where UO2(OH)2°(aq) is the predominant uranium aqueous species, whereas sorption is inhibited at pH's where carbonateand hydroxy-carbonate-complexes are the primary uranyl species. Surface adsorption appearsto be the main sorption mechanism, although at pH<4 the results suggest ion exchange may occur between the UO2+2 ions in solution and the cations in the intracrystalline cation exchange sites of clinoptilolite.

The effectiveness of zeolite-rich horizons underneath Yucca Mountain, Nevada, as barriers to actinide transport through sorption processes will depend strongly on groundwater chemistry. Reliable predictions of radionuclide transport through these horizons will need to properly account for changes in solution chemistry.
The Influence of Non-Linear Sorption on Colloid Facilitated Radionuclide Transport Through Fractured Media
Authors: Smith Paul A.
Published: March 2011

In the safety assessment of nuclear waste repositories, sorption of radionuclides on the surfaces of colloids may significantly modify transport behaviour where colloid concentration is sufficiently high. In the case of fractured geological media, colloids may be excluded from matrix pores, in which case radionuclides bound to them are not subject to the retarding effects of matrix diffusion and sorption onto matrix pore surfaces.

A model is presented describing colloid facilitated transport through fractured media with non-linear sorption. A simple criterion is developed to predict when the presence of colloids will have a significant influence on transport and effects resulting from non-linearity of sorption are described. However, lack of comprehensive sorption data, as well as computational efficiency, mean that the use of a simplified transport model, with linear sorption both on pore surfaces and colloids, is desirable if it can be demonstrated to be conservative. A further criterion is developed to predict where such a model, with linear sorption calculated for the highest concentration encountered along the flow path, would be expected to yield conservative results.
Diffusivity and Porosity in Rock Matrix—Laboratory Methods Using Artificial and Natural Tracers
Authors: Valkiainen M. , Olin M. , Uusheimo K. , Kumpulainen H. , Lehikoinen J. , Muurinen A.
Published: March 2011

The nature of diffusivity and porosity in crystalline rock was studied by electrical conductivity measurements, steady-state diffusion experiments, saturation-leaching of tracers with cylindrical rock samples and analysis of the concentrations of different elements from core samples or pore water near fractures. The phenomena of main interest were dead-end porosity, ion-exclusion, sorption, and the continuity of pore networks. The modelling of experimental results was based on a modified Fick's second law for diffusion, which was solved either by analytical or numerical methods. The measured De and ε were found to statistically follow an exponential presentation: Archie's law. The existence of ion-exclusion for anions was confirmed. The connectivity of the pore network extended in the laboratory experiments at least six centimetres, in coarse-grained granite in nature several metres but in fine-grained rock samples of a uranium deposit the element mobilization effects could be seen only to the depth of 2–3 centimetres.
Model Validation and Decision Making: An Example Using the Twin Lakes Tracer Test
Authors: Olague Natalie E. , Davis Paul A. , Smith Darrla , Feeney Tom
Published: March 2011

An approach to the validation of ground-water flow and transport models that are used in support of licensing a radioactive waste site is proposed and demonstrated. The approach attempts to interpret modeling results in light of their intended purpose. Instead of trying to find the ‘best’ fit model to the experimental data, we attempt to determine the conservativeness or bias invoked by applying different models to simulate the same experiment. In this way we are able to provide guidance to the regulatory community on where they should expect model results to fall relative to actual data and provide some evidence that certain model approaches lead to conservative results while other modeling approaches lead to non-conservative results. Applying this approach to the Twin Lakes Tracer Test demonstrated that a simple one-dimensional flow and one-dimensional dispersion model consistently over-predicts the maximum concentration. A one-dimensional flow with three-dimensional dispersion and two dimensional flow with two dimensional advection-dispersion are conservative only if they employ laboratory scale dispersivities. Therefore, if a decision-maker were to accept a similar site based on results from these types of models, there would be a relatively low chance that he has accepted an unsafe site. However, if decision-makers were to reject a similar site based on these types of results, there would be a relatively large chance that they rejected a safe site.
Approaches to Long-Term Performance Assessment of Deep Underground Disposal of Radioactive Wastes: A European Perspective
Authors: Sumerling Trevor J. , Read David
Published: March 2011

The aim of long-term performance assessment (PA) is generally to develop confidence that the disposal system will perform within certain bounds and give rise to acceptably low impacts or risks. The PA procedure must be able to deal with:

- uncertainty due to incomplete understanding of the relevant processes;

- uncertainty due to limited sampling of the structures and characteristics of the system and especially the geological media;

- uncertainty about future conditions and processes leading to changes in the engineered barriers and natural environment that may affect radionuclide release, transport and exposure pathways.

A number of approaches have been taken to solving the problem of how to deal with such uncertainties. The differences in approach may be due to the stage of project development, differences in national regulatory guidance and the different r6les of the implementor and the regulator. This paper examines the rôles of the implementor and regulator and discusses possible approaches appropriate to both points of view.
Scanning Tunneling Microscopy on Charge Density Waves in Layered Compounds
Authors: De Hosson J. Th. M. , Van Bakel G. P. E. M.
Published: March 2011

Different layered transition metal dichalcogenides were subjected to scanning tunneling microscopy to reveal the electronic charge distribution associated with the charge density wave (CDW) part of the superstructure, in addition to the atomic corrugation. The observations presented display three regimes ranging from localized CDW centred around defects/impurities in the case of lT-TiS2, via an intermediate regime governed by overlapping envelope functions in 2H-NbSe2, to a fully developed CDW system in 1T-TaSe2 (as well in a large number of other compounds). The fact that these observations have been made in solids ranging from (dirty) semiconductor (1T-TiS2) to semimetal (1T-TaSe2) to metallic (2H-NbSe2) points at the general applicability of the phenomenological Ginzburg-Landau theory, employed to describe the various regimes in which the formation of charge density waves and the accompanying periodic lattice distortions appear to act.
Al Induced Reconstructions on the Si(111) Surfaces Studied by Scanning Tunneling Microscopy
Authors: Yoshimura Masamichi , Takaoka Katsuya , Yao Takafumi , Sato Tomoshige , Sueyoshi Takashi , Iwatsuki Masashi
Published: March 2011

The atomic arrangements of the α-7×7 phase and incommensurate phases of Al adsorbed Si(111) surface are investigated respectively by scanning tunneling microscopy (STM). STM images of α-7×7 surface reveal that characteristic triangular structures consisting of three Al adatoms situated on the center part of each half unit cell. The three Al adatoms form a triangular cluster and are bonded to the center adatoms. In this situation, the dangling bond of the center adatom is saturated, which would modify its electronic structure into non-metallic. The incommensurate phase is consists of approximately 9×9 structures, which are separated each other with misfit dislocations. In the “9×9” structure, individual Al atom is visible arranged in threefold symmetry.
Structure of the ∑=3 (111) Grain Boundary in Cu-1.5%Sb
Authors: Fonda Richard W. , Luzzi David E.
Published: March 2011

Grain boundaries in quenched and aged Cu-i.5%Sb were examined with Auger electron microscopy, transmission electron microscopy, and high resolution electron microscopy. The ∑=3 grain boundaries are strongly faceted, with the facets lying primarily along the coincident (111) planes of the two grains. The grain boundaries are enriched in antimony, as demonstrated by both AES and HREM. HREM images of the ∑=3 (111) ║ (111) grain boundary differ from those of the Cu-Bi ∑ =3 (111) ║ (111) grain boundary in the lack of a significant grain boundary expansion to accommodate the excess solute at the boundary. A preliminary investigation of the atomic structure of the ∑=3 (111) ║ (111) facet by HREM and multislice calculations is presented.
Compositional Analysis and High Resolution Imaging of Grain Boundaries in Pr-DOPED ZnO Ceramics
Authors: Solorzano I. G. , Sande J. B. Vander , Baek K. K. , Tuller H. L.
Published: March 2011

An electron microscopy investigation has been conducted in ZnO:Pr:Co varistor structures prepared by a new in-diffusion method. It has been shown that some boundaries host intergranular precipitates of a Prx Oy-based phase. All observations lead to the conclusion that grain boundaries exhibit Pr segregation, while Co is homogeneously distributed in the ZnO matrix. No continuous intermediate layer is present at the ZnO/ZnO boundary nor at the Pr2O3/ZnO interfaces. This is consistent with a barrier model based solely on a space charge depletion region(s).