bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth of (110) GaAs/GaAs by Molecular Beam Epitaxy
Authors: Parechanian L.T. , Weber E.R. , Hierl T.L.
Published: March 2011

The simultaneous molecular beam epitaxy (MBE) growth of (100) and (110) GaAs/GaAsintentionally doped with Si(∼lE16/cm^3) was studied as a function of substrate temperature, arsenic overpressure, and epitaxial growth rate. The films wereanalyzed by scanning electron and optical microscopy, liquid helium photoluminescence (PL), and electronic characterization.

For the (110) epitaxal layers, an increase in morphological defect density and degradation of PL signal was observed with a lowering of the substrate temperature from 570C. Capacitance-voltage (CV) and Hall Effect measurements yield room temperature donor concentrations for the (100) films of n∼l5/cm^3 while the (110) layers exhibit electron concentrations of n∼2El7/cm^3. Hall measurements at 77K on the (100) films show the expected mobility enhancement of Si donors, whereas the (110) epi layers become insulating or greatly compensated. This behavior suggests that room temperature conduction in the (110) films is due to a deeper donor partially compensated by an acceptor level whose concentration is of the same order of magnitude as that of any electrically active Si. Temperature dependent Hall effect indicates that the activation energy of the deeper donor level lies ∼290 meV from the conduction band. PL and Hall effect indicate that the better quality (110) material is grown by increasingthe arsenic flux during MBE growth. The nature of the defects involved with the growth process will be discussed.
Optically Detected Magnetic Resonance of Copper Doped Gallium Phosphide
Authors: Gislason Haflidi P. , Watkins George D.
Published: March 2011

A deep photoluminescence (PL) band peaking at 1.08 eV in Cu-doped GaP has been studied by means of optically detected magnetic resonance (ODMR). We report an intense S=l ODMR spectrum arising from this PL band. The signal is strongly anisotropic and consistent with a center of triclinic symmetry. Some of the ODMR transitions show a well resolved hyperfine structure in agreement with the nuclear spin I=3/2 of Cu, thus suggesting the presence of Cu in the defect.
Defect Identification in High-Purity Semiconductors
Authors: Haller Eugene E.
Published: March 2011

The elemental semiconductors silicon and germanium can be purified to electrically active impurity concentrations as low as ∼1010cm−3. Highly sensitive, energy dispersive analytical techniques have been developed to identify and measure the concentration of the residual elemental impurities. The application of these techniques to very pure materials has also resulted in the discovery of a large number of new levels which are due to impurity/defect complexes. Photothermal ionization spectroscopy using uniaxial stress or a magnetic field, electron paramagnetic resonance, and doping experiments using stable and radioactive elements have been used in combination to identify the composition and the structure of some of the new centers.
Microscopic Identification of Optical Defects in Silicon by Photoluminescence
Authors: Sauer R.
Published: March 2011

The paper reviews isotope effects observed in the photoluminescence (or absorption) of deep defects in silicon introduced by irradiation damage, heating, quenching or during crystal growth. Three examples are discussed in more detail where photoluminescence methods, partially in combination with other analytical methods, have led to microscopic defect models.
Dangling Bonds and Sub-Gap Optical Absorption in Silicon
Authors: Seager C. H. , Lenahan P. M. , Brower K. L. , Mikawa R. E.
Published: March 2011

The silicon “dangling bond” defect plays a large part in controlling the electronic properties of a-Si, polycrystalline silicon, and the Si/Si0 2 interface. Jackson et al. have suggested that transitions of electrons occupying this defect produce the Urbach-like sub-gap absorption tail seen in two of these materials. We have performed optical and electron spin resonance measurements on polycrystalline silicon, plastically deformed silicon, and Si/Si02 interfaces to further examine this contention. In addition to seeing no measurable absorptance due to dangling bond interface states in the latter system, we conclude from the poor correlation of ESR signals with optical data that the Urbach tail in polycrystalline and deformed silicon is not due to transitions of dangling bond electrons.
A Relationship Between Interatomic Bonding and Electron Transport in Plasma–Deposited, Amorphous Metal Alloys
Authors: Pike G. E. , Hays A. K. , Nelson G. C. , Fritz I. J.
Published: March 2011

Amorphous metal alloy films have been produced using a new technique. Nominally NiP alloys are made by dissociating mixtures of nickel carbonyl and phosphine using a radio-frequency discharge in either an argon or a hydrogen carrier gas. This process incorporates significant amounts of carbon and oxygen impurities into the film. A previously unreported relationship between the interatomic bonding involving these impurities and the electronic transport properties of the films is discussed in this paper. The bonding studies were carried out principally using Auger lineshape analysis. Transport properties measured included 4-terminal resistivities, Hall mobilities and carrier concentrations, and Seebeck coefficients. Covalent bonding of Ni with C or P with O is strongly correlated with high resistivity, mainly through a decrease in carrier density.
Thermodynamic Considerations in Thin-Film Metallizations
Authors: Beyers Robert
Published: March 2011

The drive to produce smaller devices in integrated circuits is wellknown. Concurrent with this drive is the need to incorporate new materials, such as silicides, as interconnects, diffusion barriers, and contacts. During circuit fabrication, these materials are in contact with other solids and gases at elevated temperatures. Consequently, reactions may occur which degrade the materials present. Since the reactions of interest typically involve three elements distributed in several phases, ternary phase diagrams are required to predict the occurrence of a reaction and the subsequent reaction products, or, conversely, the stability of the phases present. Hence, ternary diagrams can provide important guidance in the selection of optimal materials and the design of reliable processing schedules.
Film Stress in Pd2Si Layers of Varying Thickness
Authors: Coulman Betty , Chen Haydn , Ritz Kenneth
Published: March 2011

Little is known about the development of film stresses in very thin films (thicknesses of the order of a few hundred Angstroms or less). As interest intensifies in the technological application of ever smaller quantities of materials, the behavior of very thin. films will undergo increasing scrutiny. In this study, the film stresses in evaporated Pd films and Pd2Si films formed by reaction with the Si substrates at 250°C were measured for thicknesses ranging from 7 to 107 nm. Stresses were calculated from the substrate radii of curvature determined by X-ray diffraction topography techniques (Lang and double crystal) and Stoney's equation. Because film continuity cannot be taken for granted at low coverage, the films were examined by electron microscopy in an attempt to correlate their morphology with the observed stresses.
Amorphous Semiconductor Multilayer Structures: Interface and Layer Thickness Effects in Photoluminescence
Authors: Tiedje T.
Published: March 2011

A number of new developments have occurred recently in research on the synthesis and properties of amorphous semiconductor multilayer structures (“amorphous superlattices”) since the discovery of this class of materials in 1983.1 This and more recentwork have shown that tetrahedrally bonded amorphous semiconductors can be fabricated in the form of multilayer structures, with highly uniform layers and atomically abrupt interfaces. The remarkably high degree of structural perfection in these materials on the length scale of the superlattice period (> 5A) has been demonstrated by transmission electron microscopy.
The Influence of Plasma Excitation Frequency on the Properties of a-SiC:H Produced in a Glow Discharge Plasma
Authors: Partlow W. D. , Herzig H.
Published: March 2011

We have deposited a-SiC:H thin films from a silane-methane glow discharge plasma in a planar reactor using high (13Mhz and low (12kHz) frequency plasma excitation. Films produced at these two frequencies have significantly different properties as measured by vacuum UV reflectivity measurements, XPS, and infrared spectroscopy. Samples produced at the low excitation frequencies, where higher ion fluxes and energies are expected, have higher VUV reflectivities, higher bulk plasmon energies, and fewer IR bands due to organic fragments. In order to relate the VUV reflectivity data to the material properties, it was fitted to a simple Lorentz model using parameters taken from XPS data. We will discussour interpretation of these data and also the dependence of film properties on the reactant gas composition.
Photoconductivity of Boron Doped A—SiN X:H
Authors: Nojima H. , Hayakawa T. , Imada E. , Kojima Y. , Narikawa S. , Matsuyama T. , Ehara S.
Published: March 2011

The photoconductivity of boron doped a—SiNx:H(x=O.07)has been studied by steady state secondary photocurrent, xerographic photodischarge, the time of flight measurement, and ESR. The dominant carrier of the photocurrent in a—SiNx:H is found to be electron. The μτ product of the electron decreases with increasirg doping amount of boron, while that of the hole increases and becomes largerthan 10-8cm2/V. From these results it is found that the life time of the hole becomes longer with increasing the amount of doping boron. The intensity of the ESR signal due to the neutral dangling bonds decreases monotonically with doping boron, and this decrease of the neutral dangling bond density is considered to be due to not only change of the occupation statistics, but mainly to the chemical or electrical interaction between boron and nitrogen.
Enhanced Stability of Amorphous Silicon Pin Solar Cells by Doping Profiles
Authors: Moeller M. , Kausche H. , Guenzel E. , Juergens W. , Stetter W.
Published: March 2011

pin cells with the light entering the n-layer (pinITO) or the p-layer (SnO2p(C)in) were prepared taking into account the ‘basic’ boron profile in the i-layer. Their efficiencies (up to 7.4% and 10.1%) show a light-induced degradation. This can be removed by doping the i-layer of pinITO cells with a decreasing boron profile. The initial efficiency is only slightly affected. Cells containing fluorine due to prior plasma-etching require higher amounts of boron for stability. For the cell type SnO2p(C)in, phosphorus profiles, however increasing towards the n-layer, lead to reduced cell degradation without affecting the efficiency. The results of profiling were interpreted in terms of a recombination model.
Amorphous Silicon Thin Film Transistors and Memory Devices
Authors: Lecomber P.G.
Published: March 2011

The preparation of amorphous silicon with a low density of defect states by the glow discharge decomposition of silane and the ability to control its electrical conductivity over many orders of magnitude by the addition of phosphine or diborane to the silane, stimulated a worldwide interest in this material and in its possible applications. This paper begins with a description of the preparation technique and a brief review of some of the important properties of the material. The fabrication and characteristics of a-Si thin-film field effect transistors will be described and followed by a discussion of the applications of these devices in large area liquid crystal displays, in simple logic circuits and in addressable image sensors. Finally, the use of a-Si in memory devices will be briefly described.
Ambipolar Field Effect Transistor
Authors: Pfleiderer H. , Kusian W.
Published: March 2011

The characteristics of a thin-film transistor using an amorphous-silicon film are presented. The appearance of electron and hole channels is made possible by ohmic source and drain contacts. A theoretical model explains the phenomena.
An Active Matrix Liquid Crystal Display Using a Novel Amorphous Silicon Switching Device
Authors: Boer W. Den , Payson J.S. , Skebe G. , Swartz L. , Yaniv Z.
Published: March 2011

A 32×32 pixel liquid crystal display using amorphous silicon n-i-n diodes as switching elements has been developed. The display can be operated at duty ratios up to 1:256.
High Deposition Rate Amorphous Silicon Alloy Xerographic Photoreceptor
Authors: Hudgens S.J. , Johncock A.G.
Published: March 2011

A new multilayer amorphous silicon alloy photoreceptor has been deposited at rates exceeding 36 µm/hr. using 2.45 GHz microwave glow discharge. The device whose structure is Al/a-Si:H:F (B-300)/a-Si:H:F (B-10)/a-Si:H:F:C is deposited in a powderless plasma deposition process which exhibits gas utilization efficiency approaching 100%. The xerographic performance of a 28µm device is: Vsat∼1100 V for a +7 KV corona; dark half decay time ≃5 sec; and photosensitivity ∼0.3 µJ/cm2 at λ = 650 nm. Stable, high quality xerographic images are obtained with these photoreceptors.
Transient Photoconductivity Studies of a-Si:H Interfaces
Authors: Street R. A.
Published: March 2011

The application of transient photoconductivity to the study of contacts and interfaces with a-Si:H is reviewed. The photocurrent is shown to contain three terms - one from the drift of photogenerated carriers, and two from contact and bulk effects due to the electric field induced by the drifting carriers. For different sample configurations, each of these terms can dominate, and each gives different information about a-Si:H bulk or surface electronic properties. The effects are illustrated with data from metal contacts, dielectric interfaces, doped layers and gap cell measurements.
Electronic States Associated with Grain Boundaries in Silicon
Authors: Shyu C. M. , Cheng L. J.
Published: March 2011

The density of states at the grain boundary is found to be a complicated function of both the defect state property and the physical location of the defect at the boundary. A modified double-depletion-layer model concerning the nonuniform distribution of defects along the boundary is presented. A method is developed which can estimate the level location from the complex DLTS spectra.
Electronic Structure of Grain Boundaries and Interfaces in Polycrystalline Zinc Oxide
Authors: Sukkar M. H. , Tuller H. L. , Johnson K. H.
Published: March 2011

Preliminary theoretical models for the electronic structure of grain boundaries and interfaces in polycrystalline ZnO have been constructed on the basis of self-consistent-field X-alpha scattered-wave (SCF-Xα-SW) cluster molecular-orbital calculations. The disposition and character of the interface states, relative to the valence and conduction bands of the otherwise perfect crystalline material, have been studied for clusters representing coordinatively unsaturated Zn surface sites and molecular O2 chemisorption thereon. The possible effects of the resuiting interface states on electron transport at grain boundaries in ZnO varistors have been addressed.
Charge Collection Characterization of Polycrystalline n-GaAs Layers for Solar Cells
Authors: Paz O. , Bhat K. N. , Borrego J. M.
Published: March 2011

N–type Ga-As polycrystalline layers, grown on Mo substrates by the metal-organic process were investigated using the SEM. Micrographs of charge collection contrast indicate a fairly random distribution of high collection (bright) grains. In a typical cell about 1/3 of its area is bright, with nonuniformities in collection current within and between grains. These results correlate well with Isc measurements, and some of the variations betweencells is explained in terms of insufficient doping of the grain boundaries.

Reducing the penetrating depth of the carriers' excitation volume results in lowering of the collection current. The measurements were normalized for changes in beam and EBIC current by a comparison with a single crystal cell of the same geometry. This degradation is explained in terms of contamination or damage of the layer close to the surface during growth.
Grain Boundaries in Crystallized Silicon Thin Films
Authors: Johnson N. M. , Biegelsen D. K. , Moyer M. D.
Published: March 2011

The presence of residual grain boundaries in laser-crystallized thin films of silicon necessitates an understanding of their properties and effects on device operation. In CW laser crystallized silicon islands, lateral p-n junction diodes have been used to evaluate the following: (1) enhanced arsenic diffusion along grain boundaries, (2) current-voltage characteristics, and (3) effects of hydrogenation on diode operation. To study the process of hydrogen passivation, deuterium has been used as a readily identifiable isotope which duplicates hydrogen chemistry. From secondary-ion mass spectrometry, diffusion of deuterium in single-crystal and polycrystalline silicon at low temperatures (e.g., 350 C) clearly demonstrates that grain-boundary diffusion dominates bulk diffusion.
Characterization of Grain Boundaries in Electronic Ceramics by Transmission Electron Microscopy
Authors: Clarke David R.
Published: March 2011

The principal high resolution transmission electron microscopy techniques used in characterizing grain boundaries in electronic ceramics are described, including those recently developed for detecting the presence of extremely thin (∼10Å) intergranular phases. The capabilities of the techniques are illustrated with examples drawn from studies of ZnO varistors, PTC BaTiO3 devices and boundary layer capacitors.
Diffusion of Arsenic and Phosphorus in Laserprocessed-Polycrystalline-Silicon-Thin-Films
Authors: Baumgart H. , Leamy H. J. , Trimble L. E. , Doherty C. J. , Celler G. K.
Published: March 2011

Grain boundary diffusion of arsenic and phosphorus in laser processed polycrystalline Si films has been investigated. Mesa diodes were fabricated in LPCVD Si thin films on SiO2 and subsequently recrystallized by cw Ar ion laser processing to form large grain material. The diffusion length of enhanced As and P diffusion along grain boundaries intersecting the p-n junction has been measured by the EBIC technique. Quantitative experimentation allowed determination of the grain boundary diffusion coefficients of As and P in the temperature range from 900°C to 1250°C.
Furnace Annealing of Ion Implanted Polycrystalline Silicon
Authors: Tandon J.L. , Harrison H.B. , Neoh C.L. , Short K.T. , Williams J.S.
Published: March 2011

Rutherford backscattering, Van der Pauw and TEM measurements were used to characterise the annealing behaviour of antimony implanted LPCVD polysilicon. High electrical activity without dopant redistribution was obtained for 600°C annealing of 1 × 1015 cm−2 and 3×1015 cm−2 antimony implanted samples. Subsequent annealing at temperatures ≥900° C resulted in considerable grain-boundary-assisted redistribution of antimony within the polycrystalline layers and associated changes in sheet resistance. Our results suggest that the sheet resistance of the films is controlled by dopant segregation at grain boundaries and the fraction of antimony distributed on active sites within individual grains.
Laser Recrystallized Polysilicon on Sio2 For High Performance Resistors
Authors: Shah Rajiv R. , Randy Hollingsworth D. , Lloyd Crosthwait D.
Published: March 2011

We demonstrate, for the first time, a technique for the fabrication of polysilicon resistors with excellent control and temperature characteristics: a standard deviation of 2.5 Ω/□ and temperature variation of -5.6% from 25° to 125°C for 1.96kΩ/□, 4.5 × 1014 cm˗2 boron implanted polysilicon resistors. The technique is based on fabricating resistors in laser recrystallized polysilicon on SiO2. Resistor performance is compared to that of similar resistors fabricated in standard LPCVD polysilicon and in single crystal silicon.
Grain Boundary Phenomena in ZnO Varistors
Authors: Einzinger Richard
Published: March 2011

The paper resumes essential grain boundary phenomena which were found experimentally by microcontact measurements on adjacent grains. The results suggest a ZnO homojunction model. Calculations of defect equilibria in the Schottky defect model show that homojunctions can be formed during the cooling period under the influence of an extrinsic donor species by a shift of the bulk defect equilibrium and intersecting diffusion profiles of oxygen- and zinc vacancies near the grain boundary.

Recent experiments indicate that heterogeneous Bi2O3-segregation layers with a dissolved zinc-antimony-oxide compound are transformed to ZnO epitaxial layers plus a fine grained pyrochlore phase of zinc-bismuth-antimony oxide during the cooling period. Thus epitaxial ZnO layers eliminate thin Bi2O3-films and form homogeneous connections of adjacent grains.
Secondary Ion Images of Impurities at Grain Boundaries in Polycrystalline Silicon
Authors: Pollock Gary A. , Deline V. R. , Furman B. K.
Published: March 2011

A CAMECA IMS-3F secondary ion microscope has been used to investigate the presence of impurities in cast polycrystalline silicon wafers and to determine their lateral distribution. Cesium and oxygen ion bombardment were used to selectively detect potential impurities and extract their lateral distribution with submicron lateral resolution.Images of H, C, O, P, and Cu segregated in grain boundaries have been obtained from polycrystalline silicon.
Validation of Hydrogeologic Models to Describe Ground-Water Flow in the Crystalline Basement of Northern Switzerland
Authors: Andrews Robert W. , Kimmeier Francesco , Perrochet Pierre , Kiraly Laszlo
Published: March 2011

Two hydrogeologic conceptual models have been developed to describe the ground-water flow regime in the crystalline rocks of northern Switzerland. The purpose of these models has been to characterize the regional and local ground-water flow systems and to define the groundwater travel paths and velocities from potential waste disposal areas to the biosphere. Several parameters have been used as validation tools in the modeling process. These include potentiometric data, volumetric discharge rates, infiltration rates, bulk and isotopic geochemistry, and temperature distributions. The results of utilizing these validation tools are described.
Confidence in Solutions of Flow Through Stochastically Generated Hard Rock Formations
Authors: Braester Carol , Thunvik Roger
Published: March 2011

Confidence in solutions of flow through stochastically generated hard rock formations was studied with the aid of a simplified synthetic model. The formation is conceptualized as a fracture network intersecting an impervious rock mass. The geometrical properties of the fracture network were assumed to be known while fracture transmissivities were considered a stochastic process.

First, network fracture transmissivities were generated using a given probability distribution function. This a priori model was considered the “true formation”. In a second step, only a limited amount of information, similar to that obtained in reality from boreholes, was used to construct a conditioned-by-measurement model.

Identical flow tests were performed on a formation constructed with limited data and on the “true formation”. The ratio of the rates of flow resulting from these tests was considered a measure of confidence in the stochastically generated formation. The results, with this model and a particular data set, show uncertainty values between 47% to 63%, corresponding to fracture sample sizes of 11% and 2% respectively, from the total number in the network.
The Assessment of Reducing Conditions at Depth in Granitic Rock
Authors: Wikberg Peter
Published: March 2011

Deep groundwaters from sites potentially suitable for the disposal of spent nuclear fuel have been chemically characterized. The waters are classified according to their contents of main constituents and pH as shallow or deep groundwaters. New equipment consisting of a mobile field laboratory and downhole pH and Eh probes has been successfully used in the investigations.

The deep groundwaters contain appreciable amounts of iron and sulphide and Eh values monitored downhole are more negative than those obtained from calculations based on the ferrous iron ferric hydroxide system. Laboratory simulations of the water rock interaction indicate that the dissolved oxygen is quickly removed when the water is in contact with the rock.
Investigations on the Role of Surface Layers in HLW Glass Leaching
Authors: Conradt R. , Roggendorf H. , Scholze H.
Published: March 2011

A corrosion test series was performed to clarify the role of reaction product layers on the corrosion of a simulated HLW borosilicate glass in a salt brine under hydrothermal conditions. The layers were unprotective at 200°C. At 120°C, slight protective effects ocurred when the leachant contained dissolved reaction products. The consequence for the long term behaviour between 120 and 200°C is a constant glass dissolution rate.
U.S. Nuclear Defense Waste Program Current Development and Key Research Areas
Authors: Walton Ray D. , Chacey Kenneth A.
Published: March 2011

The United States has been engaged in defense nuclear activities for over 40 years. The primary goal of the Defense Waste and Byproducts Management Program is to develop technology which ensures the safe, permanent disposal of all defense nuclear waste. Programs are in place at each U.S. Department of Energy site which address long-term strategy for permanent disposal of waste generated as a result of defense operations. Technology is developed for assessing the hazards, environmental impact, and cost of each long-term disposition alternative for selection and implementation. This paper addresses the key research areas and major facilities associated with the long-term management of defense nuclear waste.
Crystallization Behavior of a Ferri-Silicate α-Waste Glass
Authors: Stalios A. D. , De Batist R.
Published: March 2011

The crystallization behavior of a ferri-silicate α-waste glass was studied by means of several experimental techniques. The main crystal phase showed a plate-like morphology and was identified as a monoclinic pyroxene. The Johnson-Mehl-Avrami equation was used for the determination of the kinetic parameters of the process. Following both isothermal and non-isothermal techniques, the Avrami exponent, n, was found to be nearly one. The activation energy for crystal growth is Eg = 356 kJ mole−1. The crystallization process is governed by instantaneous nucleation and diffusion controlled two dimensional growth.
Redox Processes and UO2(s)Solubility: The Determination of the UO22+U4+Redox Potential at 25°C in HClO4Media of Different Strength
Authors: Bruno Jordi , Grenthe Ingmar , Lagerman B. O
Published: March 2011

The redox potential of the couple UO22+/U4+has been determined at 25°C in various HC1O4media by direct potentiometric determinations in 0.25, 0.5, 1, 2 and 3 M HC1O4media. Some spectrophotometric measurements of the equilibrium: Cu(s) + UO2+= U4++ Cu2++ H2O.
French High Level Nuclear Waste Program — Key Research Areas
Authors: Sombret Claude G.
Published: March 2011

The most important aspects of this research program concern disposal safety: the long-term behavior and sensitivity of the materials to the variability inherent in industrial processes, and the characterization of the final product. This research required different investigations involving various scientific fields, and implements radioactive and non-radioactive glass samples as well as industrial scale glass blocks. Certain studies have now been completed; others are still in progress.
Alpha-Recoil Effect on the Dissolution of Betafite: Rapid Natural Annealing of Radiation Damage Within A Metamict Phase
Authors: Eyal Y. , Lumpkin G. R. , Ewing R. C.
Published: March 2011

Betafite, a radioactive mineral, is the Ti-rich member of the pyrochlore group, A1–2B2X6Y0–1, (Fd3m, Z = 8), where A = Ca, Na, U, Th, REE (rare earth elements), Y, Ba, Sr, Bi, Pb; B = Nb, Ti, Ta, Zr, Sn, Fe: X = oxygen; Y = 0, OH and F. The pyrochlore structure (or structural derivatives, e.g. zirconolite in SYNROC) is a common constituent of polyphase, crystalline radioactive waste forms. Naturally occurring minerals with this structure often occur in the metamict state. Therefore, a principal concern is the effect of alpha-recoil damage on the structure and dissolution behavior of synthetic, analogue, waste-form phases which contain actinides.

We describe the dissolution behavior of 238U, 234U, 232Th, 230Th and 228Th in betafite during leaching in a bicarbonate-carbonate solution. Results indicate enhanced leaching of recently produced alpha-recoil damage as evidenced by the more rapid dissolution rate of 228Th relative to 232Th and 230Th, and only slightly enhanced leaching of older damage as evidenced by a small increase in the dissolution rate of 234U relative to 238U. These experiments demonstrate that individual alpha-recoil tracks are preserved for some time as disordered regions of higher chemical reactivity in already fully-damaged, aperiodic structures. We estimate an annealing time for the alpha-recoil tracks of 2,000 ± 1,300 yr in the metastable, aperiodic structure. Similar alpha-recoil tracks should be formed and annealed in other aperiodic nuclear waste forms, such as borosilicate glass.
An Investigation of Uranium L-Edges of Metamict and Annealed Betafite
Authors: Greegor R. B. , Lytle F. W. , Chakoumakos B. C. , Lumpkin G. R. , Ewing R. C.
Published: March 2011

The uranium site in naturally occurring metamict minerals of the pyrochlore group (A1–2B2O6Y0–1) has been investigated using x-ray absorption spectroscopy (XAS). Pyrochlore structures are common phases in proposed polycrystalline waste forms. Betafite, a member of the pyrochlore group (B = 2Ti ≥ Nb+Ta), exhibits U-O bond lengths of 1.94 and 2.37 A for the metamict state, and 2.03 and 2.51 A for the crystalline (annealed) state. The U-O bond lengths decrease (∼0.1 A) and there is a disruption in the second nearest neighbor periodicidy as material is converted from the crystalline to the metamict state.
German Nuclear High-Level Waste Program - Key Research Areas
Authors: Geipel Helmut
Published: March 2011

Based on more than 10 years of research and development, vitrification and high level waste disposal have reached the status of demonstration projects in the Federal Republic of Germany: hot operation of the PAMELA vitrification plant is scheduled for october 1985, and a disposal test with 30 canisters of high active glass is being prepared in the ASSE salt mine. Safety studies for a model repository led to a good understanding of the relevant phenomena; they will continue using sitespezific parameters. Modelling and computer codes will be further developed in international cooperation. In addition to reprocessing, the technology for direct disposal of spent fuel will be developed and demonstrated in the next years.
Determination of Near Field Excavation Disturbance in Crystalline Rock
Authors: Koopmans Robert , Hughes Robin W.
Published: March 2011

A procedure has been established to assess the effectiveness of various excavation techniques using a computerized borehole dilatometer system developed by the authors[l]. This system, which incorporates data acquisition and computer capabilities, allows data to be collected and analyzed quickly in the field. The zone of disturbance is determined by conducting borehole tests at relatively close intervals and profiling the results with depth from the excavation wall. A similar concept of using deformation moduli measurements from plate bearing tests to detect changes in bedrock conditions is discussed by Koopmans[2]. In general, lower moduli are obtained in disturbed zones which have become stress relieved due to rock relaxation and/or the presence of rock discontinuities while higher values are obtained in relatively undisturbed zones which have not become stress relieved due to an absence of rock relaxation and/or rock discontinuities.
Hard Clay as Host Medium Thermo-Mechanical Experiments and Model
Authors: Baldi G. , Borsetto M. , Hueckel T. , Peano A. , Tassoni E.
Published: March 2011

Clay's impermeability is an essential feature for the quality of a possible repository. However it results in a substantial pore water pressure rise induced by heating whose magnitude and dissipation rate depend on the clay's deformational response to heating. Experiments on thermal strains and effective stress are reported and the mathematical model implemented in a computer code is described. It allows study of a repository performance due to waste heating in both continental and subseabed clay formation.
Flow and Tracer Experiments in Crystalline Rocks: Results from Several Swedish in Situ Experiments
Authors: Abelin H. , Birgersson L. , Gidlund J. , Moreno L. , Neretnieks I. , Tunbrant S.
Published: March 2011

In a final repository for radioactive waste in crystalline rock, water flowing in the fractures may transport the radionuclides eventually leached from the waste. To be able to predict the migration of the radionuclides the processes involved must be understood. To quantify the processes, data from flow and transport in real fractures under realistic conditions are needed. Models used for prediction must include descriptions of the important processes and mechanisms.
The Effects of Simultaneous Heat and Mass Transport on Radionuclide Migration
Authors: Walton Frank B. , Ross J. P. M. , Juhnke D. G.
Published: March 2011

Five thermal-convection loops, constructed of granite, were operated for 350 d with a hot-side temperature of 60°C and a cold-side temperature of 20°C, to examine the effects of a thermal gradient on the convective transport of seven radionuclides. Radionuclide concentrations on the surface of the loop flow channels indicated that 125Sb, 60Co and 54Mn and precipitated iron oxyhydroxides were concentrated in the hot side of the loops. Concentrations of 144Ce and 99Tc were higher in the cold side of the loops. Both of these patterns were exhibited by 75Se. The concentration of 137Cs was not significantly affected by the thermal gradient. Radionuclide solubility and sorption were affected by variations in pH, Eh and ionic strength. Incorporation of the radionuclide into mineral lattices was found to play a key role in retarding the transport of all radionuclides tested.
Investigations Into the Mobilization of Actinides From ILW Conditioned in Cement and Bitumen and Studies of their Sorptive and Migration Behaviour in Both the Near- and Far-Fields for a Repository Contained in a Deep Salt Environment
Authors: Flambard A. R. , Fusban H.-U. , Keiling Ch. , Marx G.
Published: March 2011

Experiments have been carried out into the leaching of ILW conditioned in cement and bitumen in concentrated salt solutions. Although difficult to observe with real waste leachates, investigations into the leaching of a simulated waste in cement have indicated leached transuranic levels of ca. 10−9 M ; the amount of activity leached by a quinary salt solution being 102 - 103 times higher . This has been interpreted in terms of a pH effect. For the real waste, spectroscopy has indicated a significantly larger release of Cs from cement than from bitumen. For all waste samples a notable absence of colloidal material was observed ; an observation which can be explained in terms of the high solution ionic strengths and the corresponding influence upon radionuclide solvation.

Transuranic mobility studies through salt and sand from a salt dome in Northern Germany have shown the presence of at least two types of species of wildly differing mobility ; one migrating with approximately the same velocity as that of the solvent front and the other strongly retarded. Actinide recoveries (i.e. that passing through the columns) could be strongly influenced by either changing the system pH or by the addition of a competitor such as Ce ; the latter effect pointing to a competitive sorption.
Diffusivities in Crystalline Rock Materials
Authors: Skagius Kristina , Neretnieks Ivars
Published: March 2011

The diffusion of non-sorbing species in different crystalline rocks and fissure coating materials has been studied. The results show that the effective diffusivity of iodide, Uranine and Cr-EDTA in rock materials with fissure coating material is of the same magnitude or higher than in granites and gneisses. The results also show that it is not possible to assign one value to the diffusivity of a rock from a given area. The variations in properties are too large. The estimated effective diffusivity of iodide in rocks without fissure coating material was found to be in the range 1.10-14 m2/s to 70.10-14 m2/s.

To simulate the stress that exists in the bedrock at large depths, diffusion experiments with iodide and electrical resistivity measurements in rock materials under mechanical stress have been performed. It was found that the diffusivity in rock samples at 300–350 bars stress was reduced to 20–70% of the value in samples under atmospheric pressure.
Sorption and Diffusion of Radionuclides in Rock Matrix and Natural Fracture Surfaces Studied by Autoradiography
Authors: Muuronen S. , Kämäräinen E-L. , Jaakkola T. , Pinnioja S. , Lindberg A.
Published: March 2011

A method based on autoradiography was developed to determine the sorption and diffusion of cesium, strontium, cobalt, nickel, iodine and americium into rock matrix. Samples chosen for this study were filled and unfilled natural fracture surfaces and drill cores having a central drilled hole (”). Rock types were mica gneiss, tonalite and rapakivi granite, which were selected to represent the common rocks and minerals in Finnish bedrock. Distribution coefficients (Ka-values) of cesium and strontium determined for fissure surfaces and drill core cups were of the same order of magnitude. After three months contact time the greatest penetration depth for cesium was 2.5 mm, for a natural fissure surface sample of rapakivi granite. For strontium the penetration depths of 11 mm in three months and 35 mm in twelve months were found for filled natural fissure surface samples of rapakivi granite. The range of Da-values was 1.5 × 10−15 - 3.2 × 10−14 m2/s and 1.4 × 10−14 - 2.1 × 10−13 m2/s for cesium and strontium, resp. For cobalt the Da-values of 5 × 1016 m2/s in tonalite was obtained. In six months the penetration depths of nickel, iodine and americium were too low (< 0.5 mm) to allow calculation of Da.
Migration Behavior of U(VI) in Tuff
Authors: Sato Seichi , Furuya Hirotaka , Araya Sadao , Matsuo Kazumori
Published: March 2011

Adsorption coefficient and penetration behavior of U(VI) in Itado tuff were measured. The migration behavior of U(VI) in the tuff was discussed.

Adsorption coefficient of U(VI) was measured by a batch method using a wafer specimen and a powder specimen in pH region from 2 to 7. Adsorption coefficient was 17000 m1/g at the pH value of 5.5.

Penetration behavior of U2− (VI) in Itado tuff was studied as a function of pH, concentration of CO32− and temperature by contacting the rock specimen with U(VI) solution. The concentration profile and fractional amount of U(VI) penetrating into the rock specimen Mt/M were measured. The penetration behavior of U(VI) was seen to be diffusion accompanied by adsorption/desorption process. In the pH region from 3 to 4, it is likely that the penetration process of U(VI) is not sufficiently reversible but is competitive process consisting of diffusion and adsorption. In the pH region above 6, penetration depth was very thin because of both large adsorption coefficient and large adsorption rate constant.

The sectioning method was found to have an advantage to see the penetration behavior directly. But in the pH region from 6 to 7, the method has a shortage that diffusion coefficient, and adsorption and desorption rate constant could not be determined, because of small penetration depth of U(VI). From this point of view, the measurement of the fractional amount of penetration as a function of time should be carried out as well as that of concentration profile.
Ocean Floor Sediment as a Repository Barrier: Comparative Diffusion Data for Selected Radionuclides in Sediments from the Atlantic and Pacific Oceans
Authors: Schreiner F. , Sabau C. , Friedman A. , Fried S.
Published: March 2011

Effective diffusion coefficients for selected radionuclides have been measured in ocean floor sediments to provide data for the assessment of barrier effectiveness in subseabed repositories for nuclear waste. The sediments tested include illite-rich and smectite-rich red clays from the mid plate gyre region of the Pacific Ocean, reducing sediment from the continental shelf of the northwest coast of North America, and Atlantic Ocean sediments from the Southern Nares Abyssal Plain and the Great Meteor East region. Results show extremely small effective diffusion coefficients with values less than 10−14 m2s−1 for plutonium, americium, curium, thorium, and tin. Radionuclides with high diffusion coefficients of approximately 10−10 m2s− include the anionic species pertechnetate, TcO4−, iodide, I−, and selenite, SO3−2. Uranyl(VI) and neptunyl(V) ions which are stable in solution, have diffusion coefficients around 10−12m2s−1. The diffusion behavior of most radionuclides is similar in the oxygenated Pacific sediments and in the anoxic sediments from the Atlantic. An exception is neptunium, which is immobilized by Great Meteor East sedi-ment, but has high mobility in Southern Nares Abyssal Plain sediment. Under stagnant conditions a 30 m thick sediment layer forms an effective geologic barrier isolating radionuclides in a subseabed repository from the biosphere.
Heat Induced Fracturing of Rock in an Existing Uniaxial Stress Field
Authors: Mathis J. I. , Stehpansson O. , Bjarnason B. , Hakami H. , Herdocia A. , Mattila U. , Singh U.
Published: March 2011

The thermal fracturing of rock has been the object of several research projects, notably for initial rock breakage in mining [4] as well as crushing [6] In addition, the process has been studied carefully in regards to the storage of radioactive waste underground where rock fracturing could lead to a loss of radioactivity confinement. The Stripa Project, a project concerning large scale testing of procedures for underground storage of nuclear waste, probably has dealt most thoroughly with this subject by theoretical studies and in-situ heater testing in an attempt to describe the thermal failure process in rock [13]

This project was designed to test the agreement between theoretical and actual rock fracture times of a rock block, loaded with a physical as well as a thermal load. Laboratory testing consisted of physically loading center-drilled cubes of rock, 0.3 m on a side, uniaxially from 0 to 25 MPa. These were then thermally loaded with a nominal 3.7 kW (factory rating) cylindrical heater until failure occurred. This time to failure was recorded for comparison with a direct mathematical and a finite element solution. For both cases, calculations were performed at specific time-steps and an estimated failure time calculated from the compiled results.
Time-Resolved X-Ray Absorption of An Amorphous Si Foil During Pulsed Laser Irradiation
Authors: Murakami Kouichi , Gerritsen Hans C. , Brug Hedser Van , Bijkerk Fred , Saris Frans W. , Wiel Marnix J. Van Der
Published: March 2011

We report time-resolved X-ray absorption and extended X-ray absorption fine structure (EXAFS) measurements on amorphous silicon under nanosecond pulsed-laser irradiation. Each measurement was performed with one laser shot in the X-ray energy range from 90 to 300 eV. An X-ray absorption spectrum for induced liquid Si (liq*Si) was first observed above an energy density of 0.17 J/cm2. It differs significantly from the spectrum for amorphous Si and characteristically shows the disappearance of the Si-L(II,III) edge structure at around 100 eV. This phenomenon is interpreted in terms of a significant reduction in the 3s-like character of the unfilled part of the conduction band of liq*Si compared to that of amorphous Si. This is the first direct evidence that liq*Si has a metallic-like electronic structure. Timeresolved EXAFS results are also discussed briefly.
Direct Imaging of “Explosively” Propagating Buried Molten Layers In Amorphous Silicon Using Optical, Tem And Ion Backscattering Measurements
Authors: Lowndes D. H. , Jellison G. E. , Pennycook S. J. , Withrow S. P. , Mashburn D. N. , Wood R. F.
Published: March 2011

The behavior of pulsed laser-induced “explosively” propagating buried molten layers (BL) in ion implantation-amorphized silicon has been studied in a time- and spatially-resolved way, using nanosecond time-resolved reflectivity measurements, “Z-contrast” scanning transmission electron microscope (STEM) imaging of implanted Cu ions transported by the BL, and helium ion backscattering measurements. Infrared (1152 nm) reflectivity measurements allow the initial formation and subsequent motion of the BL to be followed continuously in time. The BL velocity is found to be a function of both its depth below the surface and of the incident KrF laser energy density (El); a maximum velocity of about 14 m/s is observed, implying an undercoolingvelocity relationship of about 14 K/(m/s). Z-contrast STEM measurements show that the final BL thickness is less than 15 nm. Time-resolved optical, TEM and ion backscattering measurements of the final BL depth, as a function of E1, are also found to be in excellent agreement with one another.
Raman Studies of Phase Transformations In Pulse Laser Irradiated Dielectric Films
Authors: Exarhos Gregory J.
Published: March 2011

Sputter deposited TiO2 and ZrO2 films on silica have been characterized with regard to thickness, phase purity, and residual film stress using nondestructive Raman spectroscopic techniques. Irradiated anatase coatings exhibit vibrational features that irreversibly shift to higher frequencies, suggesting an increase in localized stress. Catastrophic damage is manifested by the appearance of a higher density rutile phase. Rutile coatings examined after irradiation show only intensity changes, while amorphous coatings were found to rapidly crystallize under high-pulse energy irradiation. Results of these studies are compared with equilibrium Raman measurements of thermally-induced transformations in these materials.
Observation of Superheating During Picosecond Laser Melting ?
Authors: Fabricius N. , Hermes P. , Linde D. Von Der , Pospieszczyk A. , Stritzker B.
Published: March 2011

The surface temperature of GaAs during pulsed laser heating is obtained from the measured velocity distributions of evaporated atoms. For nanosecond pulses the observed solid-liquid transition temperature agrees with the equilibrium melting point TM. With picosecond pulses the temperature rises above TM by several hundred degrees before melting can be detected. This result is interpreted as evidence of superheating.
Picosecond Transient Reflectance Measurements of Crystallization In Pure Metals
Authors: MacDonald C.A. , Malvezzi A.M. , Spaepen F.
Published: March 2011

The kinetics of rapid crystallization in metals is investigated by measuring transient optical property changes in laser irradiated gold and copper films in the picosecond and nanosecond regimes. Melt thresholds are determined by examining concentration profiles of multi-layered films before and after irradiation.
Time-Resolved Laser Induced Transformations In Crystalline Te Thin Films
Authors: Marinero E. E. , Pamler W. , Chen M.
Published: March 2011

We report on time-resolved measurements of laser-induced transformations in Te thin films. Rapid thermal transformations are effected utilizing a KrF excimer laser (12 ns duration, 249 nm). The kinetics of the transformation is investigated utilizing transient conductance and infrared absorption measurements as well as reflectivity and transmissivity changes at 633 nm. The threshold for melting is accurately determined as well as melt velocities pertinent to the recrystallization kinetics of the molten films. Criteria for glass formation of Te are also discussed.