bluuebunny/crossref_metadata_2025 huggingface dataset:
A new Physical Picture for Surface Diffusion at High Temperatures
Authors: Suni I. I. , Seebauer E. G.
Published: March 2011

Surface diffusion of In and Sb on Ge (111) has been measured by optical second harmonic Microscopy. This technique employs surface second harmonic generation to directly image submonolayer surface concentration profiles. At low coverage, the activation energy Ediff and pre-exponential factor D0 are extraordinarily large for both systems and cannot be explained by theories based on simple site-to-site hopping.

Molecular dynamics simulations of a Lennard-Jones (111) surface has revealed a new mechanism for high-temperature diffusion which is probably operative on Ge (111). At low coverages and high temperatures, adatoms sink into the top layer of Ge, and diffusion is dominated by thermal formation of adatom-vacancy pairs. This process is characterized by a high enthalpy of pair formation and a high entropy of activation due to lattice relaxation near the vacancy.

At high coverages, the behaviour of Sb and In are quite different. As the In coverage increases, a larger fraction of In resides in the adlayer, so the diffusion parameters slowly approach those for site-to-site hopping. For Sb there is evidence for immobile islands at higher coverages. This does not affect Ediff, but decreases D0 due to the smaller number of diffusion paths around the islands. The gradual change of In diffusion parameters with coverage is interrupted by an apparent phase transition at θ=0.16, where both Ediff and D0 peak sharply.
The Effect of Tungsten on the Nucleation and Growth of thin Aluminum Alloy Films
Authors: CAO Bea , David Theodore N. , Shin Hank , Fejes Peter , Hendrickson Les
Published: March 2011

A variety of alloying elements are currently being investigated for their effects on the mechanical properties and reliability of thin aluminum films. In the present study, scanning electron microscopy and transmission electron microscopy are used to study the nucleation and growth of Al-1.5wt%Cu and Al-1.5wt% Cu-0.2wt% W films. Differences in Microstructure, nucleation and growth behavior are observed and are explained in terms of changes in surface energies and atomic Mobilities.
In Situ Studies of Nitridation of Cu/Ti Thin Films Using Environmental Cell in Transmission Electron Microscopy
Authors: Sharma R. , Atzmon Z. , Mayer J. , Hong S. Q.
Published: March 2011

Co-deposited Cu/Ti thin films were heated at various temperatures in an ammonia ambient in an environmental cell fitted in to the column of transmission electron Microscope (TEM). The reaction dynamics was observed in situ and recorded on video using a TV camera with 1/30 sec. time resolution. The nitridation of titanium accompanied by nucleation and growth of copper particles started at 370°C. Ti2N formed at lower temperatures while TiN was formed above 400°C. The nucleation of crystals occurred simultaneously (within a Minute) throughout the film indicating no effect of electron beam on reaction process. The growth rate of copper particles was observed to vary slightly from one particle to another indicating varying growth rate for different facets.
The Commensurate Phase of Al Overlayers on the Si(111) Surfaces: STM Study of the γ-Phase Surface
Authors: Yoshimura M. , Takaoka K. , Yao T.
Published: March 2011

We have elucidated the atomic structure of the γ-phase, which appears at 1ML coverage of Al on the Si (111) surface, using scanning tunneling Microscopy (STM). The structure is based on the 9×9 dimer-adatom-stacking fault (DAS) structure. Al atoms replace Si atoms in the second layer of the DAS Model. As a result, excess silicon atoms are produced during the formation of the γ-phase and migrate on the surface to form Monatomic-height islands. The area ratio of the upper and lower terraces is in good agreement with the proposed formation Mechanism.
The Evolution of Strain Relaxation Close to The Critical Thickness
Authors: Kidd P. , Fewster P.F.
Published: March 2011

High resolution X-ray diffraction space mapping has been used to follow the change in the distribution of residual strain and localised relaxation in low mismatched epitaxial layers. Using this new technique, we have obtained a series of diffraction space maps of partially relaxed epitaxial layers of In.1Ga.9As on GaAs. The layers have different thicknesses and hence different degrees of strain relaxation. The diffuse scatter close to the Bragg peaks provides information about the imperfect and distorted regions in the structure and this has allowed us to examine the extent and distribution of residual strain close to the dislocations. We have followed the evolution of local relaxation, which is confined initially to regions around isolated dislocations, through to the case of overlapping dislocation strain fields, leading to a more homogeneous strain field distribution and microscopic and macroscopic tilting of the layers.
A Study of the Defect Structure in GaAS1−xPx/GaAs AS x<0.25
Authors: Aragon G. , De Castro M.J. , Molina S.I. , Gonzalez Y. , Gonzalez L. , Briones F. , Garcia R.
Published: March 2011

The defect structure of GaAsP layer grown by Atomic Layer Molecular Beam Epitaxy on (001) GaAs substrate has been studied by Transmission Electron Microscopy. The phosphorous content and the epilayer thickness have been changed below 25% and 1μm respectively. Three kinds of defect structure have been found: a) α-δ fringes at the interface for coherent epilayer, b) Misfit dislocation for thin epilayers and c) Multiple cracks normal to the interface and parallel to one <110> direction for thick epilayers.
Defect and Island Formation in Stranski-Krastanov Growth of Ge on Si (001)
Authors: Sakai Akira , Tatsumi Toru
Published: March 2011

We have examined macroscopic island (Macro-island) formation in Stranski-Krastanov (SK) growth of Ge on Si (001) surfaces at various growth temperatures in molecular beam epitaxy (MBE). The nucleation mode of Macro-island formation was observed to be predominantly heterogeneous at medium growth temperatures less than 450°C, but homogeneous at high temperatures. In the heterogeneous Mode, Macro-island formation is mediated by particular V-shaped defects running preferentially along the <110> direction, which form as a result of coalescence of the well-faceted islands.
MBE Doping Kinetics - A Rate Equation Study
Authors: Bendi Shridhar , Venkatasubramanian R. , Dorsey Donald L.
Published: March 2011

A rate equation model based on the master equation is developed for the study of MBE doping kinetics. The Model includes elementary surface processes such as adsorption, evaporation and migration of atoms. The Model is applied to the study of the surface segregation phenomenon during the In doping of Si. The doping studies were performed for the following growth conditions: T in the range 500 — 750°C; a growth rate of 1 μm/hr.; and a flux ratio, JIn/JSi equal to 2.0 × 10-4. The predicted sticking coefficient of In versus obtained shows excellent agreement with experiments. The sticking coefficient decreases with T due to surface segregation aided evaporation of In at higher temperature. The results of dopant depth profile also show excellent qualitative agreement with experiments. The surface segregation of In occurs due to a strong repulsive interaction between In and the host lattice. The results of this study show that there is a dopant depleted zone (DDZ) where the In concentration is lower than both the bulk and the top surface layer. The observed DDZ matches with that observed in experiments.
Computer Simulation of Surface Diffusion of Silver
Authors: Kumagai A. , Ogawa K. , Doyama Masao
Published: March 2011

The binding energies to silver (111) surface of a silver ad-atom and its cluster have been calculated. The activation energies of motion of these ad-atom clusters, vacancies and divacancies on silver (111) surface have been calculated by use of n-body embedded atom potentials and molecular dynamics.
Zone Descriptions of Film Structure-A Rationale
Authors: Smith David A. , Ibrahim A.
Published: March 2011

The Microstructure of films is a result of the interplay of nucleation and growth processes which occur during deposition. A columnar structure is characteristic of films irrespective of the material or the deposition process. The length scale of the microstructure depends systematically on the atomic mobility during growth.
White-Beam Transmission Characterization of Texture in Mo Thin Films and Mo/W Multilayers
Authors: Vill Marc , MALHOTRA S. G. , Rek Z. , Yalisove S. M. , Bilello J. C.
Published: March 2011

Synchrotron white-beam (polychromatic) x-ray radiation was used in a transmission Laue study of texture in monolithic mo films and Mo/W Multilayers. High intensity synchrotron radiation permits x-ray diffraction study of mo specimens >100μm in thickness, while 6μm is the practical thickness limit for a transmission study using a rotating anode x-ray source. The transmission Laue technique permits the collection of preferred orientation data from several families of diffracting planes on one photographic exposure. Polychromatic x-rays facilitate collection of diffraction information from a range of planar orientations without altering the orientation of the sample with respect to the incident x-ray beam. The texture information from a 35μm thick monolithic mo film and a 31μm thick multilayer of mo and W is presented. Two-fold symmetry seen in the {211} plane diffraction ring of the multilayer demonstrated both an in-plane and out-of-plane texture with a <110> growth direction. Three fold symmetry seen in the {310} and {222} plane diffraction rings of the monolithic mo film demonstrated both in-plane and out-of-plane texture which suggests a <111> growth direction.
Effect of Buried Surface Structure on Solid Phase Epitaxy of GE on SI (111)-7×7
Authors: Hellman Olof C.
Published: March 2011

We study the crystallization of a thin film of amorphous Ge deposited at room temperature on Si (111). Features of the silicon surface buried beneath the Ge film are shown to affect the rate of crystallization. In particular, solid phase epitaxy is observed to be enhanced at surface steps and defects in the surface reconstruction. It is further shown that one-dimensional crystallization patterns can be caused by impurity-Mediated crystallization. Precipitates of an impurity rich phase migrate in the plane of the film, leaving behind a crystalline trail. The Migration path of these precipitates is also dependent on the buried surface structure.
Growth, Structure, and Thin Film Stress in TiAl AND Ti3Al FILMS
Authors: Chinmulgund M. , Inturi R. B. , Barnard J. A.
Published: March 2011

Thin films of Ti, Al, TiAl and Ti3Al were deposited by dc magnetron sputtering onto 2” dia. oxidized Si<111> wafers and their mechanical properties were studied by measuring the internal stress in the films. Ti and Ti3Al films show hexagonal structure with preferred orientation in the (0002) direction. TiAl is tetragonal, nanocrystalline, and (111) oriented; Al is random fee polycrystalline in nature. Young's Modulii of thin films of these materials were calculated from the stress temperature plots. The E values of TiAI and Ti3Al thin films were found to be significantly higher than those of the bulk Materials.
Using Zeolites as Substrates for Diamond Thin Film Deposition
Authors: Kwan Michael C. , Gleason Karen K.
Published: March 2011

Zeolites have been used as a potential substrate for CVD diamond deposition. By saturating the pore channels of a silicalite crystal, which are spaced approximately 10 Å apart, with hydrocarbon seeds to induce and control nucleation, films have been grown in a hot filament chemical vapor deposition (HFCVD) system under standard deposition conditions, using acetone as the carbon source. The hydrocarbons used were adamantane, naphthalene, anthracene, 2,3-benzanthracene, and l,2:5,6-dibenzanthracene. To enhance nucleation, a high pressure pre-deposition period was used initially.

Characterization of these films through electron diffraction and Auger electron spectroscopy indicates that polycrystalline β-SiC imbedded in an amorphous matrix is initially formed and large, well faceted diamond crystals are subsequently nucleated.
X-ray Photoelectron Spectroscopy of the Interface Between Diamond Films and Tantalum Substrates
Authors: Waitew M.M. , Ismat Shah S.
Published: March 2011

Diamond films were deposited in a microwave plasma chemical vapor deposition (MPCVD) system on Ta substrates using a mixture of hydrogen and methane gases. The films were grown for varying lengths of time to provide samples with no diamond growth to a continuous diamond film. These films were analyzed using X-ray photoelectron spectroscopy (XPS) in order to understand the time dependent interactions between the substrate and the incoming carbon flux. Photoelectron peaks in the Ta 4f, C Is and Ols regions have been analyzed. In the initial stages of growth, a layer of carbide forms on the substrate. As the substrate becomes supersaturated with carbon, graphite starts to form on the surface. A diamond peak begins to appear after about 30 Minutes of deposition.
Development of Anisotropic GaAs (001) Surface Morphology During Molecular Beam Epitaxial Growth
Authors: Pidduck Allan J. , Smith G.W. , Keir A.M. , Whitehouse C.R.
Published: March 2011

We have studied the development of a microscopically ridged [110] Morphology during (001) GaAs Molecular beam epitaxy, as a function of layer thickness and growth temperature. The ridge slopes are consistent with the [110] separation required to incorporate a majority of adatoms by step-flow growth. Thus step-flow can be a dominant growth mode even on nominally on-axis (singular) substrates. With increasing epilayer thickness, the ridge slopes, and surface step density, remain approximately constant, while the ridge spacings, and therefore roughness amplitude, increase steadily.
Structure and morphology of Clean and Magnesium - Dosed Sapphire Surfaces
Authors: Yu Yan , Lad R.J.
Published: March 2011

The structure and morphology of r-axis (1012) and c-axis (0001) sapphire (a-Al2O3) surfaces were studied with Low Energy and Reflection High Energy Electron Diffraction (LEED and RHEED) and Atomic Force Microscopy (AFM). Following heat treatments at 1000°C in 5×10-7 torr O2, both surfaces exhibited (1×1) diffraction patterns. Regular step-terrace structure was observed on the (1012) surface with the AFM, but no step structure was visible on the (0001) surface. The initial growth of magnesium overlayers dosed onto the (1012) surface at 25°C in ultra-high vacuum was monitored with RHEED and the chemical interactions at the Mg / Al2O3 interface were characterized with X-ray Photoelectron Spectroscopy (XPS). During the initial deposition, very little interaction between mg and the Al2O3 substrate occurs and a very abrupt interface is formed. At larger mg doses, a textured polycrystalline mg overlayer is produced. Annealing the mg film above 300°C causes some oxidation of mg at the Mg / Al2O3 interface, but most of the mg desorbs from the surface.
Deposition and Properties of Epitaxial Ferrite Thin Films
Authors: Phillips Julia M. , Van Dover R. B. , Gyorgy E. M. , Marshall J. H.
Published: March 2011

We have grown thin films and multilayers of several ferrite materials with the spinel structure by pulsed laser deposition of stoichiometric targets. Epitaxial films can be grown on a variety of substrates including MgO, Al2O3, MgAl2O4, Y-stabilized ZrO2 (YSZ) and SrTiO3. Films on Mg- and Al-containing substrates have a low saturation Magnetization, Ms, while films on YSZ and SrTiO3 exhibit bulk values of Ms. The anisotropy can be lowered by a post growth anneal, resulting in a film with a permeability of 28.
Growth and Characterization of Silicon-Germanium Films on Oxide by VLPCVD/PE-VLPCVD
Authors: Tsai Julie A. , Reif Rafael
Published: March 2011

Results of Si1-xGex deposition on oxide-coated Si substrates using a PE-VLPCVD (Plasma-Enhanced Very-Low-Pressure Chemical Vapor Deposition) reactor are presented. Thin layers of poly-Si1-xGex deposited with SiH4 and GeH4 at ≤ 600°C had low C and O levels. Two growth modes were examined at 500°C: plasma-initiated thermal growth (VLPCVD) and full plasma-enhanced deposition (PE-VLPCVD). In both cases, Ge incorporation increases sublinearly with gas ratio, growth rates increase with Ge content, and the transition temperature between polycrystalline and amorphous deposition is lower for Si1-xGex than Si. On the other hand, compared to thermal growth, plasma-enhanced deposition promotes not only higher growth rates but also improved structural properties such as smoother surface Morphology, More columnar and oriented grains, and the unique feature of direct deposition onto oxide.
The As-Grown Microtopologies of Solution Grown GaAs (001) Vicinal Surfaces
Authors: Marek T. , Strunk H.P. , Bauser E. , Lu Y.C.
Published: March 2011

The observations of the as-solution-grown GaAs (001) vicinal surfaces yield three kinds of Microtopologies. Each of them develops in a certain range of growth temperature. In a low temperature range (492° C…600° C) we observe parallel growth steps with heights of one Monolayer. In a medium range, a network consisting of two sets of growth steps with larger heights is observed. At high growth temperatures (780° C…850° C) we find growth surfaces that are atomically rough. IMplications of these findings for modelling the crystal growth will be discussed.
Atiomiic Scale Interface Structure of In0.2Ga0.8As/GaAs Strained Layers Studied by Crosssectional Scanning Tunneijng Microscopy
Authors: Zheng J. F. , Salmeron M. B. , Weber E. R.
Published: March 2011

A molecular beam epitaxy-grown In0.2Ga0.8As/GaAs strained layer structure has been studied by scanning tunneling microscopy in cross-section on the (110) cleavage plane perpendicular to [001] the growth direction. Individual Indium atoms were differentially imaged in the group III sublattice, allowing a direct observation of the interface roughness due to the indium compositional fluctuation. In the In0.2Ga0.8As layers, Indium atoms are found in clusters preferentially along the growth direction with each cluster containing 2-3 indium atoms. Indium segregation induced asymmetrical interface broadening is studied on an atomic scale. The interface of In0.2Ga0.8As grown on GaAs is sharp within 2-4 atomic layers. The interface of GaAs grown on In0.2Ga0.8As is found to be broadened to about 5-10 atomic layers. The atomic scale fluctuation due to indium distribution is about 20 Å along the interface in this case. We conclude that clustering and segregation are the main reason for the In0.2Ga0.8As/GaAs interface roughness.
Dissociation of Misfit Dislocations in GeSi/{111}Si Interfaces
Authors: Ernst Frank
Published: March 2011

The accommodation of lattice mismatch is studied in Ge0.15Si0.85 layers grown epitaxially on {111}-oriented Si substrates by chemical vapor deposition (CVD) at 1100°C. Weak beam dark field microscopy reveals a regular misfit dislocation network, which resembles the honeycomb network of edge-type dislocations anticipated by the O-lattice theory. In contrast to the latter, however, the real network exhibits extended nodes where the misfit dislocations dissociate into misfit partial dislocations. Between the partials, high resolution transmission electron microscopy (HRTEM) reveals intrinsic and extrinsic stacking faults. Owing to the presence of these stacking faults, three different atomistic structures of the GeSi/Si interface coexist and compete for the interfacial area according to their energy. The observed configuration is shown to minimize the total energy of the interface.
Observation of Open-Ended Stacking Fault Tetrahedra in Si0.85Ge0.15 Grown on V-Grooved (001) Si and Planar (111) Si Substrates
Authors: Howard David J. , Paine David C.
Published: March 2011

A new strain relief mechanism was observed in thin films of Si0.85Ge0.15 grown by RTCVD (rapid thermal chemical vapor deposition) on patterned (001) and planar (111) Si substrates. The (001) Si substrates were lithographically patterned and anisotropically etched to produce a line pattern of V-shaped grooves running in the [110] direction where the walls of the grooves were the {111} crystal planes lying in the [110] zone. Cross-section and plan-view TEM (transmission electron microscopy) studies revealed the presence of open ended stacking fault tetrahedra in strained-layer Si0.85Ge0.15 grown both on (111) Si wafers and the {111} sidewalls of the patterned (001) Si wafers. No defects were observed in the (001) portions of the films grown on non-planar substrates.
In-Situ 1Mev Tem and Hrem Study of the Deformation and the Transformation of Symmetrical Tilt Grain Boundaries in Ge and Si.
Authors: Thibault Jany. , Baillin X. , Pelissier J. , Putaux J.L. , Michaud H.M.
Published: March 2011

The evolution under deformation of symmetrical GBs in Si and Ge has been studied both by in-situ TEM and HREM. The influence of the strain conditions (tension or compression) on the final structure through the entrance of different types of dislocations will be discussed. The influence of the deformation temperature on the multiplicity of the structures has been observed in a particular range of misorientation angles.
Atomistic Calculations and Hrtem Observations of an [001] Tilt Boundary in Rutile
Authors: Lee W.-Y. , Bristowe P.D. , Solorzano I.G. , Vandersande J.B.
Published: March 2011

The equilibrium atomic structure of the ∑=15 36.9° (210)[001] tilt boundary in rutile (TiO2) has been computed using an ionic shell model and compared to a high-resolution electron microscope image of the same boundary using image simulation. The lowest energy structure of the boundary is characterized by an in-plane translation of a/6[120] relative to the mirrorsymmetric CSL configuration in agreement with the electron microscope observations.
Computer Simulation of Tensile Deformation of Titanium Small Single Crystals with Stacking Faults
Authors: Aoshima M. , Kusube T. , Ida J. , Doyama Masao
Published: March 2011

Small single crystals of titanium with and without stacking faults have been pulled by use of the molecular dynamics method. The tensile axis was [0001] and the stacking fault was introduced on (0001) pianes. The yield stress was higher in the crystal with stacking faults. The deformation was complicated in the crystal with stacking fault. Dislocations were created near the tip of a crack and moved on (1122).
Sans Measurements of Deuterium Trapped at Grain Boundaries in Palladium
Authors: Heuser Brent J. , King John S. , Summerfield George C.
Published: March 2011

Small-angle neutron scattering from grain boundary interfaces has been investigated in polycrystal palladium with and without deuterium dissolved in the solution phase. Polycrystalline samples were prepared using two different procedures: (1) single crystal material was recrystallized at 517°C after 78% cold working, and (2) an extruded polycrystal rod was investigated as received, or recrystallized at 600°C and 680°C. The expected 1/Q2 scattering profile (where Q is the neutron wavevector transfer) from grain boundaries has been observed, and the absolute cross sections permit a determination of the grain boundary surface concentration of missing palladium atoms and of trapped deuterium. The vacancy and trapped deuterium surface concentration determined in these measurements was 0.2-0.3 vacancies/Å2 and 0.4-0.6 deuterons/Å2, respectively. The measurements were extended to the low Q region (Q ≥0.006 1/Å) where the scattering response was dominated by Porod 1/Q4 behavior typical of internal voids. This behavior was especially strong for the samples fabricated from the extruded polycrystal, but was also present in the as-received single crystal. The degradation of the single crystal Porod response after compressional cold working is in reasonable agreement with the calculated effect of flattening a spherical void.
Growth of Grain Boundary Precipitates as a Function of Misorientation in an Al-5 WT% Cu Alloy
Authors: Cantrell M. A. , Shiflet G. J.
Published: March 2011

The size evolution of θ phase (CuAl2) precipitates as a function of time was used to study the growth of grain boundary precipitates in an Al-5 wt % Cu alloy. The kinetics were: modeled using the Brailsford and Aaron treatment of the collector plate mechanism. It was found, for a given time, that the size varied in a reproducible manner as a function of misorientation between the grains. Precipitate size was found to vary from 5 x 10-6 cm to 5 x 10-5 cm, while misorientation varied from 20 to 50 degrees for a given heat treatment period. Grain boundary misorientation was determined to be the most important factor influencing precipitate size for a given grain boundary. The grain boundary plane orientation plays a secondary role in the growth of precipitates. From these data, the relationship between grain boundary misorientation and grain boundary diffusion has been determined.
Diffusion-Induced Grain Boundary Migration and Role of Defects
Authors: Chaki T.K.
Published: March 2011

A model is presented to explain various aspects of diffusion-induced grain boundary migration (DIGM). The driving energies of DIGM are identified as the free energy of mixing and the interface free energy, the former being predominant in most cases of DIGM. The grain boundary migrates due to thermally activated motion of atoms across the interface under the influence of the driving energies. An expression for the velocity of migration is derived. It is shown that this depends parabolically on the solute concentration, in agreement with experimental observations in the case of liquid film migration (LFM), which is analogous to DIGM. Furthermore, the velocity is proportional to lattice diffusivity ahead of the boundary. Recent results of enhancement of DIGM by ion bombardment is explained by radiation-enhanced lattice diffusivity due to introduction of point defects by the ions. The model also explains that diffusion-induced recrystallization (DIR) is due to a free energy decrease associated with the transformation from the amorphous phase in the grain boundary layer to the crystalline phase.
Interface Diffusion in Solids
Authors: Grinfeld Michael A.
Published: March 2011

Surface diffusion within intergrain space is studied theoretically with the help of the Onsageristic approach of irreversible thermodynamics. The grains are treated as isotropic elastic half solids. It is assumed that species of one of the grains are capable to leave their equilibrium positions in the parent solid body and then they migrate within the inter-grain space along the surface of their parent body. Decrease of the total accumulated energy is postulated to be a driving force of the diffusion processes.
Solid-Phase Epitaxy - Role of Point Defects in Amorphous Solids
Authors: Chaki T.K.
Published: March 2011

A model of solid-phase epitaxial growth (SPEG), explaining enhancing effects of ion-irradiation and dopants, is presented. The crystallization is by the adjustment of atomic positions in the amorphous side of the crystalline/amorphous (c-a) interface due to self-diffusion in the amorphous solid, assisted by a freeenergy decrease associated with the transformation from the amorphous (a) to crystalline (c) phase. Irradiation and electrically active dopants increase the selfdiffusivity of a-phase by generating point defects in the amorphous layer and thus enhance crystallization. An expression for the velocity of epitaxial growth is derived. The low activation energy of ion-induced SPEG is due to recombination of point defects in the a-phase.
Laboratory Testing of the Corrosion of Waste Glasses in Aqueous Environments - Effects of Experimental Parameters
Authors: Barkatt AA. , Sang Jing C. , Xing S.-B. , Guo Yan , Pegg I. L. , Barkatt AL.
Published: March 2011

Many types of procedures have been developed for testing of the chemical durability of nuclear waste forms. These procedures differ from each other in critical aspects, such as duration, replenishment or non-replenishment of the leachates, and S/V ratio. As a result, different answers to basic questions, such as how waste-form leachability depends on its chemical composition, are obtained when different test methods are used. Furthermore, the possibility that some glasses may exhibit a leach rate excursion within the test period causes the composition dependence to be an even more sensitive function of test duration and of leachant replenishment. These factors also complicate the use of test data for the prediction of long-term waste form behavior.
Applicatons of Hydration Thermodynamics to In-Situ Test Results
Authors: Plodinec M. J. , Wicks G. G.
Published: March 2011

An extremely important question for the eventual disposal of glass in natural environments is the relevance of laboratory testing of glass durability to the long-term performance of glass in geologic environments. The purpose of this study was to attempt to provide an empirical answer to that question, by applying the hydration thermodynamics approach (which has successfully been applied to laboratory tests of glass durability) to the results of longer-term testing in natural environments.

The results show that hydration thermodynamics is a useful tool for explaining the effects of glass composition observed in in-situ tests, in several environments. Thus, it appears to provide a link between laboratory tests of glass durability and the results of in-situ tests in natural environments. Perhaps the most important conclusion of this effort is that the in-situ test results emphasize the importance of control of chemical composition during glass production as a means of achieving a durable glass.
High Level Radioactive Waste Glass Production and Product Description
Authors: Sproull James F. , Marra Sharon L. , Jantzen Carol M.
Published: March 2011

A summary of the current state of defense high-level radioactive waste in the U.S. is presented. Reasons for the selection of borosilicate glass as the matrix for the waste glass product over other glasses is enunciated. The product and processing requirements which the waste glass must meet are summarized. Finally the effects of crystallization and glass-in-glass phase separation on waste glass durability are discussed. Crystallization, under typical cooling conditions, has minimal effect on durability but glass-in-glass phase separation should be avoided.
Generation of Volatile Organic Compounds by Alpha Particle Degradation of Wipp Plastic and Rubber Material
Authors: Reed Donald T. , Molecke Martin A.
Published: March 2011

The generation of volatile organic compounds (VOCs), hydrogen, and carbon oxides due to alpha particle irradiation of polyethylene, polyvinylchloride, hypalon, and neoprene, is being investigated. A wide diversity of VOCs was found including alkenes, alkanes, alcohols, ketones, benzene derivatives, and nitro compounds. Their yields however, were quite low. The relative amounts of these compounds depended on the material, atmosphere present, and the absorbed dose. This investigation will help evaluate the effect of ionizing radiation on the long-term performance assessment and regulatory compliance issues related to the Waste Isolation Pilot Plant (WIPP).
Development of a Gas-Generation Model for the Waste Isolation Pilot Plant
Authors: Brush L. H. , Garner J. W. , Storz L. J.
Published: March 2011

Design-basis transuranic (TRU) waste to be emplaced in the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico may generate significant quantities of gas, which may affect the performance of the WIPP with respect to regulations for radioactive and/or chemically hazardous waste constituents. We are developing a model to predict gas generation in WIPP disposal rooms during and after filling and sealing. Currently, the model includes: (1) oxic and anoxic corrosion of steels and other Fe-base alloys, including passivation and depassivation; (2) microbial degradation of cellulosics with O2, NO3-, FeO(OH) , SO42-, or CO2as the electron acceptor; (3) α radiolysis of brine; (4) consumption of CO2and, perhaps, H2S by Ca(OH)2(in cementitious materials) and CaO (a potential backfill additive). The code simulates these processes and interactions among them by converting reactants (steels, cellulosics, etc.) to gases and other products at experimentally observed or estimated rates and plotting temporal reaction paths in three-dimensional phase diagrams for solids in the Fe-H2O-CO2-H2-H2S system.
Laboratory Testing of Waste Glass Aqueous Corrosion; Effects of Experimental Parameters
Authors: Ebert W.L. , Mazer J.J.
Published: March 2011

A literature survey has been performed to assess the effects of the temperature, glass surface area/leachate volume ratio, leachant composition, leachant flow rate, and glass composition (actual radioactive vs. simulated glass) used in laboratory tests on the measured glass reaction rate. The effects of these parameters must be accounted for in mechanistic models used to project glass durability over long times. Test parameters can also be used to highlight particular processes in laboratory tests. Waste glass corrosion results as water diffusion, ion exchange, and hydrolysis reactions occur simultaneously to devitrify the glass and release soluble glass components into solution. The rates of these processes are interrelated by the effects of the solution chemistry and glass alteration phases on each process, and the dominant (fastest) process may change as the reaction progresses. Transport of components from the release sites into solution may also affect the observed corrosion rate. The reaction temperature will affect the rate of each process, while other parameters will affect the solution chemistry and the particular processes that are observed during the test. The early stages of corrosion will be observed under test conditions which maintain dilute leachates and the later stages will be observed under conditions that generate more concentrated leachate solutions. Typically, water diffusion and ion exchange reactions dominate the observed glass corrosion in dilute solutions, while hydrolysis reactions are dominant in more concentrated solutions. Which process controls the long-term glass corrosion is not fully understood, and the long-term corrosion rate may be either transport- or reaction-limited.
Densification of Salt-Occluded Zeolite a Powders to a Leach-Resistant Monolith
Authors: Lewis Michele A. , Fischer Donald F. , Murphy Christopher D.
Published: March 2011

Pyrochemical processing of spent fuel from the Integral Fast Reactor (IFR) yields a salt waste of LiCI-KCI that contains approximately 6 wt% fission products, primarily as CsCI and SrCl2. Past work has shown that zeolite A will preferentially sorb cesium and strontium and will encapsulate the salt waste in a leach-resistant, radiation-resistant aluminosilicate matrix. However, a method is still needed to convert the salt-occluded zeolite powders into a monolith suitable for geologic disposal. We are thus investigating a method that forms bonded zeolite by hot pressing a mixture of glass frit and sait-occluded zeolite powders at 990 K (717°C) and 28 MPa. The leach resistance of the bonded zeolite was measured in static leach tests run for 28 days in 363 K (90°C) deionized water. Normalized release rates of all elements in the bonded zeolite were low, <1 g/m2d. Thus, the bonded zeolite may be a suitable waste form for IFR salt waste.
Expansion of Cementitious Materials Exposed to Sulfate Solutions
Authors: Pommersheim James M. , Clifton James R.
Published: March 2011

Disposal of low-level radioactive waste in concrete vaults buried underground is being considered in the United States and a potentially significant degradation process is sulfate attack. The results of experimental studies and of modeling of the kinetics of sulfate attack of cement-based materials are given. It was concluded that the expansion process involves a two-stage mechanism. The first stage is likely to be diffusion controlled and the second stage appears to be acceleratory due to the development and spread of cracks.
Elements Present in Leach Solutions from Unsaturated Spent Fuel Tests
Authors: Finn P. A. , Bates J. K. , Hoh J. C. , Emery J. W. , Hafenrichter L D. , Buck E. C. , Gong M.
Published: March 2011

Preliminary results for the composition of the leachate from unsaturated tests at 90°C with spent fuel for two successive periods of ~60 days each with pretreated J-13 groundwater are reported. The pH of the leachate solutions ranged from 4 to 7. The americium concentration was 104 to 105 greater than that reported for saturated spent fuel tests in which the leachate pH was 8. The major fraction of material in the leachate was present as colloids containing both americium and curium. The presence of actinides in a form not currently directly included in repository radionuclide transport models provides information that can be used in spent fuel reaction modeling, the performance assessment of the repository and the design of the engineering barrier system.
Comparison of the Dissolution Behaviour of Various UO2Samples in Saline Solution at 100°C Under Reducing Conditions
Authors: Stroes-Gascoyne S. , Tait J.C. , Porth R.J. , McConnell J.L. , Duclos A.M.
Published: March 2011

The separate effects of alpha- and gamma-radiolysis on UO2dissolution can be studied with unirradiated UO2, whereas studies vith used nuclear fuel necessarily always include both alpha- and gamma-radiolysis effects. This paper attempts to separate these effects by comparing the leaching behaviour in saline solution of a number of UO2samples (each vith a particular radiation characteristic or chemical property inherent to used fuel) vith the leaching behaviour of used fuel. Data from leaching experiments vith lov- and high-burnup CANDU (CANada Deuterium Uranium) fuels are also compared. The results indicate that the presence of an alpha field at 100°C under reducing conditions does not increase UO2dissolution but suggest that the combined effects of the beta and gamma fields in used CANDU fuel may enhance UO2dissolution.
A Comparative Study of the Natural and Experimental Corrosion of Poorly Durable Potash-Lime-Silica Glasses
Authors: Cooper G.I. , Cox G.A.
Published: March 2011

Naturally corroded, poorly durable glasses are compared with results from MCC-4 leach tests on similar glasses. It is suggested that different corrosion products are formed at different temperatures; therefore, care should be taken when interpreting experimental leach rates. Solution composition, pH and flow around the sample can also have a profound effect on corrosion rates.
Effect of Surface Layers on the Dissolution of Nuclear Waste Glasses
Authors: Xing Shi-Ben , Buechele Andrew C. , Pegg Ian L.
Published: March 2011

Explanation of the striking non-linear effect of glass composition on the aqueous dissolution represents an important challenge to existing dissolution mechanisms. Surface layers that are formed during glass dissolution may play an important role in this effect. One chemically reactive and one less-reactive nuclear waste glass (leachate concentrations differ by about a factor of 10) were reacted in deionized water. Two types of glass powders were used: Type A powders were pristine glass powders; Type B powders were the glass powders which had been reacted for 120 days to develop the surface layers. Both the solution concentrations and the surface layers were investigated. The experimental observations indicate that: (i) There is a range of glass compositions over which small differences in composition lead to large changes in both reaction rates and surface layer thickness; and (ii) The reaction rate is strongly affected by the formation of the surface layer (the layer appears to be protective) and cannot be explained in terms of saturation effects alone. The findings are contrary to the conclusion of a previous study and serve to highlight the inadequacy of existing dissolution models predicated on an overly simplistic mechanism, especially with regard to glass composition effects.
Non-Linearity in Glass Composition Dependence of Dissolution Rates: Effect of Solution pH
Authors: Xing Shi-Ben , Muller Isabelle S. , Pegg Ian L.
Published: March 2011

In our previous studies on the optimization of glass compositions for high-level nuclear waste vitrification it was found that, over certain composition ranges, PCT leachate concentrations increased dramatically with very small changes in glass composition. The large differences that are observed between the leachate pH values for the “durable” and the “less-durable” glasses is one possible cause for this strongly non-linear glass composition effect; conversely, the pH difference may be merely another symptom. In this study, four simulated nuclear waste glasses (two of the less-durable and two of the durable types), were leached in both zwitterionic and inorganic buffer solutions, at fixed pH-values in the ranges of 7 to 12. The very different leaching behaviors of the two types of glasses persisted and, furthermore, different pH-dependence was found despite their very similar glass composition. This study suggests that the leachate pH difference observed between the less-durable and the durable glasses under uncontrolled pH conditions is not the major cause of the large difference of leaching behavior between those glasses. The normalized release ratios of soluble components (B, Li, Na) to Si show significant differences for the two types of glasses.
RBS and PIXE Analysis of Alteration Layers on Waste Glass in a Leaching Test Under Reducing Condition
Authors: Sasakawa Kaoru , Imakita Tsuyoshi , Wada Ryutaro , Matsuda Fumio
Published: March 2011

A corrosion test of model waste glass was carried out with and without a carbon steel under reducing condition in a glove box purged with nitrogen gas, and then the characteristics of alteration layers of the glass were examined.

On the glass surfaces corroded under reducing condition with carbon steel, a thick precipitated layer was observed by Scanning Electron Microscopy (SEM). In this layer concentrated iron was observed by Particle Induced X-ray Emission Spectrometry (PIXE) and Rutherford Backseattering Spectrometry (RBS). Such a thick precipitated layer was not observed on the corroded glass under oxidizing conditions with and without carbon steel and reducing condition without carbon steel.
XPS and XRD Studies of Samples from the Natural Fission Reactors in the Oklo Uranium Deposits
Authors: Sunder S. , Miller N.H. , Duclos A.M.
Published: March 2011

Mineral samples from the natural fission reactors 10 and 13 in the Oklo uranium deposits were studied using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) to gain information about the long-term behaviour of UO2 fuel in a geological disposal vault. Two samples from reactor zone 10 (samples # D81N-190292 and D73-88) and one sample from reactor zone 13 (sample # SD37-S2/CD) were analysed. Low-resolution XPS spectra were recorded to determine the major elements present in the ore. High-resolution spectra were recorded to gain information about the chemical state of the elements present in the mineral samples. The samples show low values for the U6+/U4+ ratio. The oxidation state of uranium in these samples is even lower than that in U409. The binding energies of the Pb 4f bands indicate most of the Pb is in the +2 oxidation state in these samples. The C Is band indicates the presence of organic carbon. XRD analysis shows that the main uranium-bearing phase is uraninite and lead is present mainly as galena. The significance of the results for nuclear fuel waste management is discussed.
Significance of the Effect of Mineral Alteration on Nuclide Migration
Authors: Murakami Takashi , Ohnuki Toshihiko , Isobe Hiroshi , Sato Tsutomu , Yanase Nobuyuki , Kimura Hideo
Published: March 2011

In order to clarify the effect of mineral alteration on nuclide migration, we examined the processes, mechanisms, and kinetics of chlorite weathering, and the uranium concentrations in minerals and rocks at Koongarra, Australia. The observed concentrations of uranium in rocks were compared to those calculated. The sequence of chlorite weathering may be simply expressed as a chlorite → vermiculite → kaolinite conversion. These minerals occur as a function of depth, which corresponds well to uranium concentrations on the meter scale. Iron minerals, closely related to the uranium redistribution, are released during the weathering. The first-order kinetic model of the weathering process suggests that the weathering rate is not constant but time-dependent. The uranium concentrations are qualitatively proportional to the extent of the weathering; the weathered part having higher uranium concentration. Uranium mainly occurs with iron minerals, and sub micron sized saléeite, a uranyl phosphate, is one of the most probable uranyl phases associated with the iron minerals. The uranium fixation mechanisms are probably saléeite microcrystal coprecipitation and sorption to the iron minerals. Our model, which describes uranium concentrations in rocks as a function of time, shows that the transition zone (a vermiculite dominant area) plays an important role in the uranium migration. We have established that weathering of chlorite has affected the redistribution of uranium for more than one million years. The present study demonstrates the significance of mineral alteration when we estimate nuclide migration for geologic time.
The Effects of Bacteria on Crystalline Rock
Authors: Ann Brown D.
Published: March 2011

Many reactions involving inorganic minerals at water-rock interfaces have now been recognized to be bacterially mediated; these reactions could have a significant effect in the excavation of vaults for toxic and radioactive waste disposal. To investigate the role that bacteria play in the natural aqueous environment of crystalline rock the microbial growth factors of nutrition, energy and environment are described. Microbial activity has been investigated in Atomic Energy of Canada's Underground Research Laboratory (URL), situated in the Archean granitic Lac du Bonnet Batholith, Winnipeg, Manitoba. Faults, initiated in the Early Proterozoic, and later-formed fractures, provide ground-water pathways. Planktonic bacteria, free-swimming in the groundwater, have been observed in over 100 underground borehole samples. The number of bacteria varied from 103to 105mL-1, and appeared to decrease with depth and with increased salinity of the water. However, in the natural environment of deep (100–500 m) crystalline rocks, where nutrition is limited, formation of biofilms by sessile bacteria is a successful survival strategy. Natural biofilms at the URL and biofilms grown in bioreactors have been studied. The biofilms can accumulate different elements, depending upon the local environment. Precipitates of iron have been found in all the biofilms studied, where they are either passively accumulated or utilized as an energy source. Within the biofilm active and extensive biogeochemical immobilization of dissolved elements is controlled by distinct bacterial activities which are sufficiently discrete for hematite and siderite to be precipitated in close proximity.
Sorption Mechanism of Pu, Am and Se on Sodium-Bentonite
Authors: Shibutani Tomoki , Yui Mikazu , Yoshikawa Hideki
Published: March 2011

The distribution coefficients (Kd) were measured for Pu, Am and Se on sodium-bentonite in bentonite-equilibrated distilled water in a pH range 4~ 10. Sodium-bentonite includes sodium-montmorillonite and ferrous minerals such as pyrite. The experiments were carried out by using the batch method under aerobic conditions for Pu, Am and under anaerobic conditions for Se. The distribution coefficients for Pu and Am were in the range 100~ 102m3/kg on sodium-bentonite in the above pH range. The sorption of Pu and Am was modeled by ion exchange of these cationic aqueous species with interlayer cations of sodium-bentonite. The distribution coefficients of Se on sodium-bentonite were interpreted by using a surface complexation model. The model was based on the assumption that the sorption behavior was dominated by ferric-oxyhydroxide, goethite, coating on the surface of pyrite. Sorption behavior of Am in compacted bentonite were also discussed by using cation exchange model.