bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling the Stress Evolution of Ion Beam Synthesized Nanocrystals
Authors: Yi D.O. , Sharp I. D. , Xu Q. , Liao C. Y. , Ager J. W. , Beeman J. W. , Liliental-Weber Z. , Yu K. M. , Zakharov D. , Haller E. E. , Chrzan D. C.
Published: April 2011

Under certain conditions, nucleation and growth can lead to substantial stresses in nanocrystals embedded in a host matrix. These stresses may be relaxed through subsequent annealing treatments. A model is presented for the relaxation of these stresses via diffusive processes within the matrix. The model reflects the effects of surface tension, potential phase transformations at or near the processing temperature, and differential thermal expansion. It is demonstrated that the model describes well the stress relaxation of ion beam synthesized Ge nanocrystals embedded in a silica matrix.
Atomistic Simulation of Dislocation Interactions with a σ = 5 (210) Grain Boundary during Nanoindentation of Ni
Authors: Jang Ho , Farkas Diana
Published: April 2011

Molecular dynamics simulations of nanoindentation were performed using embedded atom potentials. The indentation was simulated on a Ni substrate using a diamond-like spherical indenter. In this study, we focused on the interaction of a σ=5 (210) grain boundary with the dislocations that were nucleated and expanded as loops under the indenter during nano- indentation. The results showed that dislocation loops were nucleated beneath the surface and propagated on multiple {111} slip planes. These dislocations impinged into the grain boundary in the course of nanoindentation. The lattice dislocations changed the atom configuration at the boundary region as they merged into the grain boundary and at later stages they transmitted across the grain boundary. The results also showed that the presence of a grain boundary affected the indenting speed and dislocation motion during nanoindentation, with the grain boundary retarding the indentation process.
Synthesis and Characterization of Fe-Co Nanocrystalline Alloys
Authors: Mujahid Mohammad , Zhu Jie
Published: April 2011

Nanostructured alloys have great potential as soft magnetic materials. In particular, nanocrystalline Fe-Co based alloys are believed to be good candidates for imparting improved magnetic behavior in terms of higher permeability, lower coercivity, reduced hysteresis losses and higher Curie temperatures. In the present work, Fe-50at.%Co alloy powders have been prepared using mechanical alloying (MA) in a planetary ball mill under controlled environment. The particle size and morphology of MA powders was investigated using scanning electron microscopy. The crystal size and internal strain was measured using X-ray diffraction. It has been shown that the crystal size could be reduced down to less than 15 nm in these alloys. Finally, the influence of grain size and internal strain on the magnetic properties has been discussed.
Microstructural Refinement in Single-Phase Copper Solid Solutions by Machining
Authors: Swaminathan S. , Chandrasekar S. , Compton W. D. , Trumble K. P. , King A. H.
Published: April 2011

A study has been made of the effect of solute (Mn, Al, Ni) additions on microstructure refinement due to large strain deformation in single phase, copper solid solutions. The solutes were specifically selected for their influence on stacking fault energy (SFE) of copper, and the large strain deformation was imposed by chip formation in machining. The microstructure of Cu- 0.7at%Ni chip consists of elongated, sub-micrometer sized grains while Cu-7at%Al chip is made up of long, thin microbands with twins. The microstructure of the chip changes as the SFE of the material varies. With all of the solid solutions studied, the hardness of the chips is found to be significantly greater than that of the bulk material. Recrystallization temperature of solid solution chips is found to be higher than those of pure copper chips.
Effect of Oxygen and Nitrogen Doping on Mechanical Properties of Silicon Using Nanoindentation
Authors: Karoui A. , Rozgonyi G. , Ciszek T.
Published: April 2011

The effects of oxygen and nitrogen on the mechanical properties of Czochralski (CZ) and float zone silicon have been studied using nano-indentation. Nitrogen free FZ Si exhibited low hardness of 6.49 GPa and elastic modulus of 104 GPa. When doped with 2×1015cm−3 nitrogen, FZ Si hardness and elastic modulus increased to 8.2 and 182 GPa, respectively. In the near-surface denuded zone of N-doped CZ Si (N-CZ) the hardness correlates well with the O and N profiles. Distinct high hardness points, found in the O- and N- rich subsurface region, were attributed to precipitates. Nano-scratch tests of N-CZ Si confirmed the existence of hard phases, mostly small precipitates, whose density, estimated to be 2×1013cm−3, is in the range of previously suggested nuclei density in as-grown N-CZ silicon.
Critical Conditions for Dislocation Nucleation from Surface Steps
Authors: Li Chengzhi , Xu Guanshui
Published: April 2011

The critical conditions for dislocation nucleation from the surface steps of various geometries are analyzed based on the Peierls-Nabarro dislocation model. By modeling a surface step as part of a three- dimensional crack surface, the half space problem is transferred into an equivalent three dimensional crack problem in an infinite medium. The profiles of embryonic dislocations, corresponding to the relative displacements between the two adjacent atomic layers along slip planes, are then rigorously solved through the variational boundary integral method. The critical conditions for dislocation nucleation are determined by solving the stress dependent activation energies required to activate embryonic dislocations from their stable to unstable saddle point configurations. For a given slip plane, the effects of step geometry such as the step height and inclined angle on dislocation nucleation are analyzed in detail. The results show that the atomic scale steps may reduce the critical stress required for dislocation nucleation from the surface by several factors. Compared to previous analyses of this type of problem based on continuum elastic dislocation theory, the presented analysis eliminates the uncertain core cutoff parameter by allowing for the existence of an extended dislocation core as the embryonic dislocation evolves. Because of the serious limitation of direct atomic simulation for this type of problem, the presented methodology of incorporating atomic information into continuum approach appears to be particularly noteworthy for providing insights of energetics of the atomic processes involved in dislocation nucleation.
Investigation of Strain Accommodation upon Phase Transformation of Small Inclusions in Aluminum
Authors: Zhang L. H. , Johnson E. , Dahmen U.
Published: April 2011

The evolution of elastic strain caused by melting and solidification of small inclusions in aluminum was investigated by in-situ transmission electron microscopy. The appearance and subsequent decay of elastic strain during phase transformation of inclusions around 100nm in size were observed directly, and the decay rate was determined as a function of temperature. The mechanism of strain accommodation was studied by determining the activation energy of the process using alloy composition and inclusion size to control the transformation temperature.
Structures and Mechanical Properties of ECAP Processed 7075 Al Alloy upon Natural Aging and T651 Treatment
Authors: Zhao Yonghao , Liao Xiaozhou , Valiev Ruslan Z. , Zhu Yuntian T.
Published: April 2011

Equal-channel angular pressing (ECAP) processed ultrafine grained (UFG) and coarse grained (CG) 7075 Al alloys were treated by natural aging and T651 temper (annealed at 120 °C for 48 h in Ar atmosphere), respectively. Mechanical tests showed that for the UFG sample, the natural aging resulted in the highest strength (the ultimate tensile strength is 720 MPa). In contrast, for the CG sample, the T651 treatment resulted in a higher strength (the ultimate strength is 590 MPa) than the natural aging (530 MPa). Microstructural analyses indicated that the enhanced strength of the T651 treated CG sample was mainly caused by high densities of G- P zones and metastable η' precipitates. The enhanced strength of the naturally aged UFG sample was mainly caused by the high densities of G-P zones and dislocations. Upon T651 treatment, the dislocation density of the UFG sample deceased significantly, overcompensating the precipitation strengthening.
A Formalism to Describe Demixing of Mixed Oxides of Large and Small Grain Size in an Electric Field
Authors: Belova Irina V. , Brown Mandy J. , Murch Graeme E.
Published: April 2011

In this paper, we address the theory of electric field demixing of the cations in semi-conducting and initially homogeneous mixed ceramic oxides. We include the off-diagonal phenomenological transport coefficients by assuming a random distribution of cations and making use of an exact sum-rule expression. The steady state atomic composition profiles are shown to be quite different when different assumptions are made about vacancy equilibration. We show that in the case of large grained material (where vacancies in the interior do not equilibrate easily to the external oxygen partial pressure), a maximum develops in the steady state vacancy composition profiles. This behaviour is verified by independent Monte Carlo simulations.
Surface structure and composition of nanocrystalline SnO2 thin films obtained by Chemical Vapor Deposition
Authors: Barreca Davide , Bontempi Elza , Depero Laura E. , Maragno Cinzia , Tondello Eugenio
Published: April 2011

Nanocrystalline SnO2 thin films were synthesized by Chemical Vapor Deposition on Si(100) and Al2O3 substrates using bis(diethylamino)dimethylstannane(IV) [(CH3)2Sn(N(C2H5)2)2] as precursor. Film growth was performed at 400-500°C in an O2(H2O)+N2 atmosphere, with the aim of studying the effects of the synthesis conditions on the coating properties. The sample chemical composition and surface morphology were analyzed by X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM), while their structural features were investigated by Glancing Incidence X-ray Diffraction (GIXRD) and X-ray Reflectivity (XRR). In this paper, the attention is focused on the interplay between film nanostructure and morphology, with particular regard to the influence of the growth surface.
Shape, Size and Morphology Control of Inorganic Crystals With Self-Assembled Monolayers
Authors: Han Yong-Jin , Aizenberg Joanna
Published: April 2011

Self-assembled monolayers (SAMs) provide simple, yet sophisticated surfaces to mimic the effect of proteins associated with the process of biomineralization. A careful selection of organic molecules with an appropriate surface chemistry (i.e. HS-(CH2)n-X−supported on a metal surface) allows the nucleation and growth of oriented calcite crystals and provides opportunities to study the formation of inorganic crystals assisted by organic molecules. We have successfully crystallized calcite crystals on different SAMs in the presence of additives such as proteins and/or ions in solution, and found correlations between the orientations of crystals to their final shapes, sizes and morphologies. We report here our experimental results demonstrating how underlying organic molecules along with inorganic additives can control and mold the final shape, size and morphology of calcium carbonate crystals
Interaction Between Titanium Implant Surfaces and Hydrogen Peroxide in Biologically Relevant Environments
Authors: Muyco Julie , Ratto Timothy , Orme Christine , McKittrick Joanna , Frangos John
Published: April 2011

Titanium was exposed to dilute solutions of hydrogen peroxide (H2O2) to better characterize the interaction at the interface between the solution and metal. The intensity of light passing through films of known thickness of titanium on quartz was measured as a function of time in contact with H2O2in concentrations of 0.3% and 1.0%. An atomic force microscope (AFM) was used to record deflection-distance (force) curves as a probe approached the interface of titanium in contact with solution containing 0.3% of H2O2. The interaction layer measured using AFM techniques was much greater than the thickness of the titanium films used in this study. Raman spectroscopy taken during interaction shows the emergence of a Ti-peroxy gel and titania after 2 hours in contact with 0.3% H2O2solution.
Adhesion Kinetics of MC3T3-E1 Pre-Osteoblasts to Osteoconductive Porous Titanium Scaffolds
Authors: St-Pierre Jean-Philippe , Gauthier Maxime , Lefebvre Louis-Philippe , Tabrizian Maryam
Published: April 2011

Porous metallic scaffolds have recently gained recognition as a promising avenue toward the regeneration of damaged bone structures. Interest in these materials resides in their ability to guide bone growth by presenting a favorable structure for cellular adhesion and three-dimensional proliferation. A powder metallurgy process to fabricate titanium foams with favorable microstructural parameters for applications in bone engineering has recently been developed. This study assesses the potential of this novel material for applications as an osteoconductive scaffold throughin vitrocharacterization of early cellular interactions with titanium foams having pore sizes ranging from 167 to 500 µm. The foams exhibit no cytotoxic effects on J774 mouse macrophages while favoring adhesion and proliferation of MC3T3-E1 pre-osteoblasts. Three-dimensional morphology assumed by these cells on porous titanium suggests that the microstructure of the foams is biomimetic.
Fabrication of Nanoporous Silica Nanospheres and Nanotubes by Inorganic and Organic Double Templates
Authors: Chen Jian-Feng , Liu Runjing , Cao Dapeng , Shen Zhigang , Yun Jimmy , Wang Jiexin , Shao Lei , Zhou Weilie L.
Published: April 2011

Inorganic and organic double templates were used to fabricate silica nanospheres and nanotubes with nanochannels perpendicular to the shells. Sphere and needle like CaCO3 nanoparticles, synthesized by a high gravity reactive precipitation method, were used as inorganic templates and C16H33N(CH3)3Br (C16-CTAB) was used as an organic surfactant template. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were employed to characterize the nanoporous structure. The nanochannels were found perpendicular to the surface of nanospheres and nanotubes. The size of nanochannels is about 3~5 nm. The size of hollow nanosphers and nanotubes can be controlled by the inorganic CaCO3 nanoparticle templates and the nanochannels in the shells can be tuned by different surfactant micelles. The nanospheres and nanotubes with nanochannels perpendicular to the shells have a potential application in chemical bio-catalyst, bio-separation, and drug delivery.
Bio-inspired Crystal Growth Induced by Novel Organic Compounds
Authors: Dinsdale Nicholas B. , Heywood Brigid R.
Published: April 2011

The growth of biogenic inorganic crystals demands much attention given the exceptional control of nucleation and growth that is evidenced in biological systems. In many cases the biological mineralization products are crystals engineered to express unique forms and habits; this being linked to the bio-function of minerals. It is acknowledged that the formation of these biogenic minerals is controlled by a complex directory of organic macromolecules which are chemically tailored to mediate the crystallization sequelae. In this study, an homologous series of sulphonated calix[4]arenes (further modified by the addition of an alkyl substituent (R = (CH2)nCH3; n = 0-7)) were selected as chemical and structural mimics for some of the active components in the acidic macromolecules known to be associated with many biogenic mineral phases. Here, these calix[4]arenes were assayed for their ability to control the growth of barium sulphate. Their presence in the crystallization liquor induced the twinning of the barium sulphate crystals; two forms of twinning were observed. Interpenetrant (2-31) twins were noted for substituted calix[4]arenes when R = (CH2)2CH3; and interpenetrant {610} twins when R = (CH2)nCH3; n = 0-7. When R = (CH2)nCH3; n = 0 or 1, the crystals also evidenced a novel acicular growth form. These effects were linked to the chemical properties of the calixarenes and structural recognition motifs created at the calix[4]arene/barium sulphate interface following the adsorption of these sulphonated moieties onto the nascent crystals. The results offer an insight into the mechanisms which may mediate such effects in vivo.
Mechanistic Aspects of Fracture of Human Cortical Bone
Authors: Nalla Ravi K. , Kruzic Jamie J. , Kinney John H. , Ritchie R. O.
Published: April 2011

There has been growing interest of late in the fracture properties of human bone. As understanding such properties in the context of the hierarchical microstructure of bone is of obvious importance, this study addresses the evolution of the in vitro fracture toughness with crack extension (Resistance-curve behavior) in terms of the salient mechanisms involved. Fracture-mechanics based measurements were performed on compact-tension specimens hydrated in Hanks' Balanced Salt Solution using cortical bone from mid-diaphyses of 34-41 year-old human humeri. Post-test observations of the crack path were made by optical microscopy and three-dimensional X-ray computed tomography. The fracture toughness was found to rise linearly with crack extension with a mean crack-initiation toughness of Ko ∼ 2.0 MPa√m for crack growth in the proximal-distal direction. The increasing cracking resistance had its origins in several toughening mechanisms, most notably crack bridging by uncracked ligaments. Uncracked-ligament bridging, which was observed by tomography in the wake of the crack, was identified as the dominant toughening mechanism responsible for the observed Rcurve behavior through compliance-based experiments. The extent and nature of the bridging zone was examined quantitatively using multi-cutting compliance experiments in order to assess the bridging stress distribution. The results obtained in this study provide an improved understanding of the mechanisms associated with the failure of cortical bone, and as such are of importance from the perspective of developing a realistic framework for fracture risk assessment, and for determining how the increasing propensity for fracture with age can be prevented.
Do Natural Silks Make Good Engineering Materials?
Authors: Morrison Natalie A. , Bell Fraser I. , Beautrait Alexandre , Ritchie Joanne , Smith Christopher , McEwen Iain J. , Viney Christopher
Published: April 2011

Fast relaxation of stresses lower than the yield stress is demonstrated in Bombyx mori(silkworm) cocoon silk and Nephila clavipes (spider) major ampullate silk (MAS; dragline). Stress relaxation and creep make natural silk unsuitable as a long-term load-bearing material. Instead, silk-like materials are better suited to applications in which energy dissipation is important, and in which high loads need to be withstood on a once-off basis for only very short periods of time. Examples might include use as a ballistic material that arrests the penetration of fragments from the explosion of a pressure vessel, an aircraft luggage container, or a tyre. Treatment in a domestic microwave oven is shown to significantly reduce the rate of stress relaxation in both silkworm cocoon and spider MAS. Except for ductility, the tensile properties of cocoon silk measured in constant strain rate experiments are enhanced by this treatment. Initial experiments on MAS suggest that the tensile properties of this material also are enhanced by exposure to microwaves, in this case with the exception of initial modulus.
Use of Soft Lithography for Multi-layer MicroMolding (MMM) of 3-D PCL Scaffolds for Tissue Engineering
Authors: Sun Yang , Ferrell Nicholas , Hansford Derek J.
Published: April 2011

Tissue engineering scaffolds with precisely controlled geometries, particularly with surface features smaller than typical cell dimensions (1-10μm), can improve cellular adhesion and functionality. In this paper, soft lithography was used to fabricate polydimethylsiloxane (PDMS) stamps of arrays of parallel 5μm wide, 5μm deep grooves separated by 45 μm ridges, and an orthogonal grid of lines with the same geometry. Several methods were compared for the fabrication of 3-D multi-layer polycaprolactone (PCL) scaffolds with precise features. First, micromolding in capillaries (MIMIC) was used to deliver the polymer into the small grooves by capillarity; however the resultant lines were discontinuous and not able to form complete lines. Second, spin coating and oxygen plasma were combined to build 3-D scaffolds with the line pattern. The resultant scaffolds had good alignment and adhesion between layers; however, the upper layer collapsed due to the poor mechanical rigidity. Finally, a new multi-layer micromolding (MMM) method was developed and successfully applied with the grid pattern to fabricate 3-D scaffolds. Scanning electron microscopy (SEM) characterization showed that the multi-layered scaffolds had high porosity and precisely controlled 3-D structures.
Creep testing of thick-wall copper electron beam and friction stir welds
Authors: Andersson Henrik C. M. , Seitisleam Facredin , Sandstr Rolf
Published: April 2011

Abstract:Thick section copper canisters are planned to be used as a corrosion protection of nuclear waste disposal containers for long term underground deposal in Sweden. The copper canisters will have the top and possibly the bottom lid welded to the canister walls using electron beam or friction stir welding. Due to the high external hydrostatic pressure and the relatively high temperature of the waste during the first one hundred years the copper will creep. The creep process will close the manufacturing gap between the cast iron container and the copper canister. The creep ductility must be sufficient to avoid cracking of the weld.

Specimens cut from the friction stir welds and the electron beam welds have been creep tested at temperatures ranging from 75 to 175 °C. Cross-weld specimens were used for both friction stir and electron beam welds. Weld metal, heat affected zone and base metal were also studied for friction stir welds. The results for the electron beam welds show that the main creep deformation is concentrated to the weld metal where the failure takes place. Weld metal and most cross-weld tests of friction stir weld material show similar creep lives and ductility as base metal tests. Ductility at rupture was found to exceed 30% for friction stir weld specimens, and the Norton power law exponent was determined to be between 30 and 50.
Finite Element Modelling of Temperature Distribution in Friction Stir Welding Process and Its Influence on Distortion of Copper Canisters
Authors: Källgren Therese , Jin Lai-Zhe , Sandström Rolf
Published: April 2011

In an effort to enhance safety for long time disposal of waste nuclear fuel, friction stir welding has been developed as one alternative to seal copper canisters. To avoid the formation of voids and cracks during the welding process, an understanding of the heat and material flow andthereby the evolution of the microstructure, is of great importance. Finite element modelling has been used to simulate the heat and material flow as well as thermal expansion during the friction stir welding process. A model involving heat transfer, material flow, and continuum mechanics has been developed. The steady state solutions have been compared with experimental temperature observations as well as analytical solutions, showing good agreement. Temperature distribution is affected by the welding speed. For a given reference pointperpendicular to the welding direction, a lower welding speed corresponds to a higher peak temperature. The plunging position of welding tool influences the temperature distribution and therefore the displacement distribution of the weldment.
The Effect of Fabrication Processes on the Governing Mechanical Failure Criteria for Alloy 22, Ti-Grade 2/7, and Ti-Grade 5/24 Alloys
Authors: Csontos Aladar A. , Dunn Darrell S. , Pan Yi-Ming , Cragnolino Gustavo A.
Published: April 2011

The effect of fabrication processes on the governing mechanical failure criteria for Alloy 22, Ti-Grade 2/7, and Ti-Grade 5/24 has been evaluated through the development of material specific failure assessment diagrams (FAD). The Barsom and Rolfe Charpy V-Notch (CVN) energy to KICempirical relationship was shown to be conservative when compared to experimental KJC data for Alloy 600 and 690, which indicates that the KIC derivation is most likely conservative for Alloy 22 as well. In contrast, the CVN to KICH2 relationship was found to be non-conservative for Ti-Grade 2/7 and Ti-Grade 5/24. The derived KIC value for Alloy 22 was then used to construct a K-based FAD, which indicated that the governing mechanical failure theory was the Tresca criterion for plastic collapse. Both the derived KIC and experimental KQ values were used in the development of the Ti-Grade 2/7 and Ti-Grade 5/24 FADs. The Ti-Grade 2/7 FAD indicated that the governing mechanical failure theory is also plastic collapse while the Ti-Grade 5/24 FADsuggested mixed mode failure processes where neither linear-elastic fracture mechanics (LEFM) nor Tresca can accurately predict.
Technetium and Molybdenum in Oxide Spent Nuclear Fuel: Impact on Release Estimates
Authors: Fortner Jeffrey A. , Kropf A. Jeremy , Finch Robert J. , Cunnane James C.
Published: April 2011

Technetium-99 99Tc) is an important radionuclide in repository models owing to its relatively long half-life and the high aqueous solubility of compounds where it is inthe heptavalent state. The vast majority of the 99Tc inventory presently slatedfor disposal is contained in oxide commercial spent nuclear fuel (CSNF), where it is divided (along with its cohort, molybdenum) betweenexsolved, intermetallic “epsilon” particles and isolated, likely oxidized, atoms distributed in the uranium dioxide matrix. We present recent evidence from synchrotron x-ray fluorescence spectroscopy, electron microscopy, and fuel dissolution testing on the likely oxidation state, coordination environment, and physical disposition of technetium and molybdenum in CSNF. Effects of the relative proportioningof technetium and molybdenum among the metallic and oxidized states in CSNF, andtheir distribution in or near grain boundaries and gaps on release during CSNF corrosion testing are discussed.
An Experimental Basis for a Mixed Potential Model for Nuclear Fuel Corrosion within a Failed Waste Container
Authors: Shoesmith D.W. , Noël J.J. , Garisto F.
Published: April 2011

A mixed-potential model to predict the corrosion behaviour of nuclear fuel inside a failed carbon steel-lined copper waste container in a granitic repository is briefly described. A number of experiments underway to improve the mechanistic form of the model and to provide the necessary input data are discussed. A primary emphasis is placed on the consequences of the accumulation of corrosion product deposits on the fuel surface on the development of aggressive local chemistries, the cathodic reduction of H2O2 and potential for scavenging of H2O2 by the products of carbon steel corrosion (in particular H2).
Uncertainties in geochemical models of natural systems: Implications for performance assessments
Authors: Palenik Christopher S. , Jensen Keld A. , Ewing Rodney C.
Published: April 2011

Licensing a geologic repository for nuclear waste will be based primarily upon the results of a performance assessment (PA), which evaluates compliance of the repository with radiation exposure limits by modeling the engineered and natural barrier systems over extended periods (104 to 106 years). The properly completed PA must include an analysis of the sources of uncertainty, as well as the range of input parameter values, in order to determine whether the analysis is “robust”, “realistic” or “conservative”. This paper examines the sources of uncertainty that may be encountered in source-term and near-field models in a performance assessment of a repository for spent nuclear fuel by examining the uncertainties in modeling the geochemical behavior of the Bangombé natural reactor in Gabon.
Preferential Radionuclide Release Due to Alpha Decay: Effects on Repository Performance
Authors: Pickett David A. , Murphy William M.
Published: April 2011

We model preferential release of 237Np, 234U, 230Th, 226Ra, and 210Pb from disposed commercial spent nuclear fuel as a result of alpha recoil damage, using the U.S. Nuclear Regulatory Commission (NRC) Total-system Performance Assessment (TPA) model for the potential repository at Yucca Mountain. Time-dependent augmentation of the ingrown component is simulated by increasing the initial parent inventory; we have used a factor of five increase, based on natural system observations. For 237Np, the magnitude of preferential release is subject to solubility limits. Stochastic TPA runs show a significant effect on modeled dose of preferential 237Np release, but low impact from the other four radionuclides. The mechanism could be ineffective if 237Np is incorporated into secondary phases. While our results are exploratory in nature, this approach to modeling decay-related enhancement of release can be applied in other nuclear waste disposal settings.
Application of the TSPA Glass Degradation Model to Non-Conforming Waste Forms
Authors: Ebert W. L.
Published: April 2011

An approach is presented for determining if the models used to calculate the release of radionuclides from defense high-level radioactive waste (HLW) glass for total system performance assessment (TSPA) calculations can be used to account for the release of radionuclides from waste forms other than standard borosilicate glasses. The fractional release rates of radionuclides due to waste form degradation, the available surface area, and the radionuclide inventory in an alternative waste form can be compared with the corresponding models used in TSPA for HLW glasses to determine if those models adequately represent the waste form. This approach is demonstrated for the ceramic and metallic waste forms developed for electrometallurgically treated spent sodium-bonded nuclear fuel. Depending on the waste form, comparisons made with aspects of the HLW glass model may be based on similarities in degradation mechanisms or purely empirical.
Modelling of the Dounreay Recovered Spent Nuclear Fuel Database
Authors: Baikie I.D. , Dennis Frank , Scirea Massimo , Crawford Ron
Published: April 2011

We report the initial results of modelling studies performed on the Dounreay Database of Spent Nuclear fuel particles recovered from the marine environment and coastal foreshore adjacent to the Dounreay Fast Reactor Complex located in the North East coast of Scotland. We demonstrate how interrogation of the fuel-particle frequency versus 137Cs isotopic distribution allows us to generate particle behaviour models and residence lifetimes of various particle subgroups. Similarly we find the recovered fuel particle's spatial distribution and shape characteristics are determined by environmental selection rules governed by properties of adjacent sediment and sediment transport mechanisms.

Analysis of the Spent Fuel particle database allows us to speculate on the original Site discharge pathways and fuel composition at the time of discharge. Further such studies correlate well with other sample morphology and compositional data and allow us to speculate upon the ultimate fate of different spent fuel types.
Influence of the Near-Field Conditions on the Mobile Concentrations of Np and Tc Leached from Vitrified HLW
Authors: Pirlet Véra , Lemmens Karel , Iseghem Pierre Van
Published: April 2011

The solubility values usually used in Belgium for Performance Assessment of the disposal of HLW in deep geological formations are based on migration experiments in the far-field and, thus, can be considered as "far-field solubilities". The particular chemistry in the near-field of the repository will also influence the solubility of the released radionuclides. Therefore, the determination of maximum concentrations of the radionuclides in the near-field is also useful. This was approached by leach tests with doped glass in near-field conditions. The experiment consisted of static leach tests with the french R7T7 reference glass doped with 237Np (0.34 wt%) and 99Tc (0.12 wt%), two key radionuclides in the performance assessment of the disposal system. The glass was put in contact with FoCa7-clay together with pyrite, metallic iron and Boom clay water to simulate the near-field conditions under reducing conditions. The interpretation of the results is based on the measurement of the steady-state Np and Tc concentrations in the leachates after ultrafiltration through 100,000 MWCO membranes and on the activity measured on the clay after different test durations. Most of the Np and Tc activity (99.5%) was found associated to the clay and increasing as a function of time while the Np and Tc concentration in the leachates were stable and extremely low with a mean value of 1.5×10−10M for Np and 5.6×10−10M for Tc. This is due at least partially to sorption on the clay, but there are indications for precipitation as well.
Field-Scale Migration of99Tc and129I at the Nevada Test Site
Authors: Hu Qinhong , Smith David K.
Published: April 2011

The groundwater at the Nevada Test Site (NTS) contains many long-lived radionuclides, including99Tc (technetium) and129I (iodine), as a result of 828 underground nuclear weapons tests conducted between 1951 and 1992. We synthesized a body of data collected on the distribution of99Tc and129I in groundwater to assess their migration at NTS, at field scales over distances of hundreds of meters and for durations up to forty years and under hydrogeologic conditions very similar to the proposed geological repository at Yucca Mountain. The results of our study show that Tc does not necessarily exist as a mobile and conservative species TcO4−, as has been commonly assumed. This conclusion is corroborated by recent in situ redox potential measurements, which show that groundwaters at multiple locations of the NTS are not oxidizing, and mobility of reduced Tc species (TcO2·nH2O) is greatly decreased. Speciation of iodine and its associated reactivity is also complex in the groundwater at the NTS, and its effect on the mobility of iodine should be the subject of future studies.
Distributions of Concentration Limits for the Source Term in Performance Assessments
Authors: Murphy William M. , Soldavini Steve , Gallagher Colin M.
Published: April 2011

Radionuclide concentration limits are important parameters in performance assessments for geologic disposal of nuclear waste, but their statistical distributions are challenging to define. Thermodynamic solubilities provide an attractive theoretical constraint, but solubilities do not provide a basis for distribution functions, and concentration limits can exceed solubilities in irreversible systems. Distributions of natural concentrations are broad and do not correspond to distributions of concentration limits. Interpretations of natural nickel and lead concentrations suggest that they do not represent concentration limit distributions. Interpretation of groundwater data for dissolved calcium indicates a bimodal distribution, which hypothetically corresponds to a solubility-limited population and a population of values below the solubility limits.
HTO Diffusion in Oxfordian Limestone and Callovo-Oxfordian Argillite Formations
Authors: Descostes M. , Blin V. , Grenut B. , Meier P. , Tevissen E.
Published: April 2011

Diffusion coefficients and accessible porosities for HTO were measured on 30 samples from the Andra Underground Rock Laboratory in Meuse/Haute-Marne (France) using the through diffusion technique. Two distinct geological formations were studied: Oxfordian limestone and Callovo-oxfordian argillite between 166 and 477 m depth. The experiences were carried out with synthetic pore-water from each formation. The measured values of the effective diffusion coefficients (De) are ranging from 2.6× 10-12 to 12.4 × 10-11 m2 s-1 while accessible porosities (ε) are between 2.4 and 24%. Good correlations are found between these two parameters.
Experimental study of colloid interactions with rock surfaces
Authors: Alonso U. , Missana T. , Patelli A. , Ravagnan J. , Rigato V.
Published: April 2011

In subsurface environments, colloids may play a role in the transport of contaminants to the biosphere because of their high capacity to adsorb them. The electrostatic interactions between colloids and the heterogeneous rock surfaces will determine the fate and transport of both colloids and contaminants adsorbed on the particles. Thus, the aim of the present work was to study the electrostatic interactions between colloidal particles and a crystalline rock surface. Since different minerals are expected to act as preferential reactive areas, the spatial distribution of colloids on the mineral surfaces was studied, accounting several conditions, by means of the nuclear ion beam technique μ-Particle Induced X-ray Emission (μPIXE).
Behavior of UO2 in the RBMK-1000 Spent Fuel under Oxidizing Conditions
Authors: Kolyadin A. B. , Mishin V. Ya. , Mishin K. Ya. , Aloy A. S. , Koltsova T. I.
Published: April 2011

The oxidation of UO2–type spent nuclear fuel (SNF) in gaseousmedia was studied at different temperatures and oxygen contents using gravimetric and powder X-ray diffraction (XRD) techniques. The aim of the study was to determine the mechanism(s) of thermal-oxidation alteration of SNF during long-term dry storage. The samples used in the experiments were chips of RBMK-1000 fuel rods.Oxidation of UO2with a mean burn-up of 10.7 and 19.73 MW d/kg in humid air was observed at a temperature as low as 150°C. At 200°C nearly all of the UO2was transformed into U3O8 between 3500-4000 hours. In a humid nitrogen environment containing of 0.05-1.3 vol. % oxygen at 300°C, the UO2 completely transformed to U3O8 between 2500-3000 hours. Oxidation of UO2in samples with small amounts of jacket damage (e.g., <0.04 MM2)ll progresses more slowly and after â3000 hours the oxygen-to-uranium ratio was 2.56.Stabilization of the oxidation process was not observed in the fuel samples upto an O/U ratio of 2.4, which may be attributed to the smallburn-up of the fuel under investigation.
Leaching Behaviour of Low -Activity Alpha-Doped UO2.
Authors: Rondinella Vincenzo V. , Cobos Joaquin , Wiss Thierry
Published: April 2011

So-called alpha-doped UO2(i.e. UO2containing short-lived alpha-emitters) can reproduce the activity levels of spent fuel at different ages. Previous experiments showed significant effects of alpha-radiolysis at relatively high alpha-activity levels (e.g., in the range 108– 1010Bq/g). This study shows the results of leaching tests done on low alpha-activity material (106– 107Bq/g). UO2containing ∼10 and ∼1 wt. %233U was leached at room temperature in deionized and carbonated water under deaerated conditions. Higher release in carbonated water was observed. A clear radiolytic enhancement of the concentration of uranium in solution was observed for the material containing 10 wt. %233U. The dissolved U concentration was very close to the values previously observed for higher activity alpha-doped UO2and confirmed the finding that under relatively high surface/volume condition the measured amount of U in solution is essentially independent of the alpha-activity level. No significant radiolysis effect was detected for the material doped with 1%233U compared to undoped UO2after ∼4 months of leaching. The post- leaching SEM examination revealed no pronounced etching of the surfaces.
Spin Effects of Low-dimensional Electron Gases Studied by Far-infrared Photoconductivity Experiments
Authors: Hu C. -M.
Published: April 2011

We review our recent work on spin effects in low-dimensional electron gases studied using far-infrared photoconductivity technique. We measure the spin-orbit coupling parameter α via spectroscopy by detecting the combined resonance. Detailed filling-factor dependent study shows the collective nature of this excitation, in accordance to theoretical predictions that both Kohn and Larmor theorem are broken for long-wavelength excitations that changes both the Landau and spin quantum numbers. We find that the long spin-relaxation time of a two-dimensional electron gas results in a novel bolometric spin effect, which gives rise to a substantial photo resistance change by reversing the spin polarization of electrons at the Fermi-level.
Theory of Electrically Controlled Resonant Tunneling Spin Devices
Authors: Ting David Z.-Y. , Cartoixà Xavier
Published: April 2011

We report device concepts that exploit spin-orbit coupling for creating spin polarized current sources using nonmagnetic semiconductor resonant tunneling heterostructures, without external magnetic fields. The resonant interband tunneling spin filter exploits large valence band spin-orbit interaction to provide strong spin selectivity. The bi-directional spin pump induces the simultaneous flow of oppositely spin-polarized current components in opposite directions through spin-dependent resonant tunneling. The efficiency of resonant tunneling spin devices can be improved when the effects of structural inversion asymmetry (SIA) and bulk inversion asymmetry (BIA) are combined properly, and incorporated into device design. The current spin polarizations of the proposed devices are electrically controllable, and potentially amenable to high-speed modulation. In principle, the electrically modulated spin-polarized current source could be integrated in optoelectronic devices for added functionality.
Rashba spin-orbit coupling in InGaAs/InP quantum wires
Authors: Knobbe Jens , Guzenko Vitaliy A. , Schäpers Thomas
Published: April 2011

The effect of Rashba spin-orbit coupling on the transport properties of InGaAs/InP quantum wire structures is investigated. The geometry of the wire structures was defined by selective wet chemical etching. For wires without a gate a clear beating pattern, due to the presence of the Rashba spin-orbit coupling, is observed for wires with a width down to 600 nm. For narrower wires no beating pattern is found. The experimental observations are explained by contribution of the Rashba spin-orbit coupling to the one-dimensional magnetosubbands. By depleting the one-dimensional conductor by means of a gate electrode the Rashba coupling strength could be controlled.
Biomimetic Surfaces via Dextran Immobilization: Grafting Density and Surface Properties
Authors: Miksa Davide , Irish Elizabeth R. , Chen Dwayne , Composto Russell J. , Eckmann David M.
Published: April 2011

Biomimetic surfaces were prepared by chemisorption of oxidized dextran (Mw = 110 kDa) onto SiO2 substrates that were previously modified with aminopropyl-tri-ethoxy silane (APTES). The kinetics of dextran oxidation by sodium metaperiodate (NaIO4) were quantified by 1H NMR and pH measurements. The extent of oxidation was then used to control the morphology of the biomimetic surface. Oxidation times of 0.5, 1, 2, 4, and 24 hours resulted in <20, ∼30, ∼40, ∼50 and 100% oxidation, respectively. The surfaces were characterized by contact angle analysis and atomic force microscopy (AFM). Surfaces prepared with low oxidation times revealed a more densely packed “brushy” layer when imaged by AFM than those prepared at low oxidation times. Finally, the contact angle data revealed, quite unexpectedly, that the surface with the greatest entropic freedom (0.5 h) wetted the fastest and to the greatest extent (θAPTES>θ1h>θ2,4h>θ0.5h).
Fluorophore Conformation in Green Fluorescent Protein: A Quantum Mechanics/Molecular Mechanics Study
Authors: Trohalaki Steven , Patnaik Soumya S. , Pachter Ruth
Published: April 2011

Green Fluorescent Protein (GFP) is a widely used fluorescent marker exhibiting two excitation peaks – a strong peak at 398 nm and a second at 475 nm, with the fluorescence at ca. 510 nm. Its molecular structure consists of a β-barrel composed of 11 β-strands and a central helix containing the fluorophore. Two different forms of the fluorophore – a protonated/neutral fluorophore and a de-protonated/anionic fluorophore – are thought to be responsible for the two distinct spectroscopic states. Notably, the isolated fluorophore in solution is efficiently quenched. Conformational flexibility within the protein cavity is an implicitly important factor that governs the photochemistry of GFP. However, the literature contains accounts of studies that reach conflicting conclusions, claiming that either the fluorophore's barrier to internal rotation is negligibly small or that the protein cavity is not complementary to a planar fluorophore. In this work, we calculate the torsional potential of one of the two exocyclic bonds that connect the two rings in the fluorophore, taking into account its immediate environment by applying a quantum mechanics/molecular mechanics method, with the ultimate aim of evaluating the protein-environment effects on the fluorescence.
Teaching Undergraduates Nanotechnology
Authors: Shapter J.G. , Hale P. , Maddox L.M. , Ford M. J. , Waclawik E. R.
Published: April 2011

One of the first nanotechnology undergraduate degrees in the world was established at Flinders University in 2000. In this paper we present our experience of developing and delivering this degree in a climate where “traditional” physical sciences are under considerable strain. We will discuss the motivation for this initiative, structure of the established course and educational issues relating to its development.
Teaching Computational Materials Science for Nanoscale Science and Engineering
Authors: Glotzer S.C. , Horsch M.A. , Lamm M.H. , Iacovella C.R. , Zhang Z.L.
Published: April 2011

We describe the development of a graduate level course designed to teach computational materials science and its application to nanoscale science and engineering. We discuss the use of MatDL, a web-based digital library and materials science resource, as a collaborative learning tool within the context of the course.
AtomLab: A tool for teaching materials science and simulation on the atomic scale
Authors: Falk Michael L.
Published: April 2011

AtomLab is a publicly available software package created to help students and faculty design and execute simulations for instructional purposes. AtomLab has been utilized as a learning environment within which to teach simulation techniques to graduate students. It has also been used to provide conceptual tools for undergraduate education. This paper presents a brief description of the AtomLab environment and of an undergraduate educational module designed to teach the atomic mechanisms of transport. Data is presented regarding the educational outcomes that were met using this module and the student response to the exercise.
Goniometry of Direct Lattice Vectors Supporting Students' Comprehension of Crystallographic Core Concepts and Demonstrating Image-Based Nanocrystallography
Authors: Moeck P. , Padmanabhan K. , Qin W. , Fraundorf P.
Published: April 2011

We are of the opinion that students of an introductory materials science and engineering course should gain a thorough understanding of crystallographic core concepts by applying them quasi-experimentally in computer simulation sessions that run parallel to the lectures. Software simulations of goniometry of direct lattice vectors in a transmission electron microscope (TEM) will serve two purposes at once: to introduce students to practical aspects of electron microscopy and support their comprehension of crystallographic core concepts. We use the programming software Matlab and Java (Jmol applets) on a PC platform for the creation of software simulations that demonstrate this methodology and complement already existing software simulations. The newly created software is used in classroom demonstrations of an introductory materials science and engineering course at Portland State University and will become freely accessible over the internet. This software will also support and promote image-based nanocrystallography in TEM.