bluuebunny/crossref_metadata_2025 huggingface dataset:
Luminescence and Tem-Investigation of Laser Induced Defects in (AI,Ga)As Heterostructures
Authors: Zysset B. , Salathe R.P. , Martin J.L. , Gotthardt R. , Reinhart F.K.
Published: March 2011

Laser induced defects in (AI,Ga)As heterostructures have been investigated. Luminescencetopography reveals three different defects, a luminescent B, a nonradiative D as well asdark line defects (DLD). Luminescence and excitation spectra together with TEM measurements indicate a point defect or point defect complex for B and D. Defect B is described by a configuration coordinate (CC-) model with low vibrational energies. Defect DLD consists of extended dislocations.
Defect Scattering of Phonons in Phoiopcited GaAs
Authors: Culbertson J.C. , Klein P.B. , Wolf S.A. , Strom U.
Published: March 2011

The time resolved heat pulse generated by above band gap light is studied at 2 K in undoped, semi—insulating GaAs. The phonon lineshape changes with irradiation by 1.1 micron below gap light. The changes in phonon lineshape are reversed by raising the sanple teaperature to 130 K and recooling to 2M. The results provide strong evidence for the change in acoustic phonon scattering when the defect EL2 is transferred to its metastable state. The phonon scattering anisotropies observed for the [100] and [111] directions is indicative of an anisotropic phonon—defect interaction.
Defects in CZ—Silicon Investigated by the Positron Annihilation Techniques
Authors: Dannefaer S. , Kerr D.
Published: March 2011

Positron measurements on boron doped CZ—silicon show that clusters of oxygen interstitials trap positrons. Depending on whether a vacancy is associated to this cluster or not, the positron lifetimes exhibit changes from 240 to 100 ps. The positron data indicate the nature of the initial —or precursor — stages for thermal donor formation.
Surface Roughness of Optical Oxide Coatings Deposited From Solutions and the Morphological Effect of Different Deposition Methods
Authors: Yoldas Bulent E. , Partlow Debbie P.
Published: March 2011

Optical coatings may be deposited from clear polymeric solutions derived from metal-organic compounds. These coatings are converted to their oxide counterparts upon heating at temperatures of 400–500°C. In this work, the surface roughness of solution deposited SiO2 films were investigated in order to assess their suitability for such applications as waveguides, mirrors, etc.

SiO2 films deposited from polymeric siloxane solutions show rms surface roughness as low as 2.2Å in profile scans extending over 600 μm. The surface roughness of these films, as well as other properties, are found to be modified by the deposition method, e.g., spin or dip coating, as well as by the reaction parameters involved in the solution preparation.
Identification of Constituent Phases Within Thin Metal-Silicide Structures Using Surface X-Ray Diffraction
Authors: Jordan Harold , Seeharsha K. S. , Chunu D. W.
Published: March 2011

A relatively simple technique of surface x-ray diffraction (S-XRD) has been developed for the detection and identification of thin crystalline materials. The technique provides an abundance of diffraction lines with enhanced ability to detect and identify thin phases and to follow the sequence of phase formation within multiple-phase metal silicide structures.
Optical Properties of Hydrogenated Amorphous Carbon Films Grown From Methane Plasma
Authors: Pouch J. J. , Alterovitz S. A. , Warner J. D. , Liu D. C. , Lanford W. A.
Published: March 2011

We have used a 30 kHz ac glow discharge formed from methane gas to grow carbon films on InP substrates. Both the growth rate, and the relative Ar ion sputtering rate at 3 keV varied monotonically with deposition power. The Ar ion etchigg rate of the films decreased with deposition power. Results from the 15N nuclear reaction profile experiments indicated a slight drop in the hyorogen concentration as more energy was dissipated in the ac discharge. Values for the inoex of refraction and extinction coefficient ranged from 1.721 to 1.910 and 0 to -0.188, respectively. Optical bandgaps as high as 2.34 eV were determined.
Current Status of thin film HgCdTe: Relationship of Structure and Chemistry to Device Application
Authors: Krikorian E.
Published: March 2011

This paper addresses the current status of thin film HgCdTe and limitations in the currently achievable thin film properties. The emphasis is placed on Hg1−xCdxTe since this material is presently most seriously considered for infrared devices and particularly resistant to high quality film preparation due to large elemental vapor pressure differences. The current status of thin film HgCdTe is reviewed and the various thin film deposition techniques utilized to prepare HgCdTe thin films are compared. Finally, results are reviewed which still support the claim.that the less investigated technique of bias-sputtering, modified to accommodate deposition in a high partial pressure background of Hg vapor, can yield structurally high quality Hg1−xCdxTe films with device-worthy properties. Significant is the fact that the deposition process still provides the simple and systematic control over all relevant film properties without post-deposition treatment. Such control is required to achieve the desired material for device application. As is also reviewed here, this work was one of the first to demonstrate the ability to use low cost foreign substrates for HgCdTe thin film device fabrication.
Growth Behavior of Au on Semiconductor and Insulating Substrates
Authors: Alessandrini E. I. , Aboelfotoh M. O. , Laibowltz R. B. , Lacey J. A.
Published: March 2011

There is a great deal of interest in forming Au grains on semiconductor and insulating substratcs for device application. Reduction in density of grain boundaries is known to cause improvement of clectromigration in thin films. Transmission electron microscopy and diffraction have been used to study the growth behavior and morphology of vapor deposited Au on these substrates. Electron beam evaporation techniques were used to evaporate Au at high rates (50Å/sec or higher) on substrates held at low temperature (Rm. Temp. to 250°C). Large Au grains laave been found to form on both amorphous (Si3N4 and SiO2) and single crystal Si substrates. The grain size of Au was also found to be dependent on substrate surface roughness; rough surfaces tended to retard the formation of large grains. Grains as large as 1/2 μ in a matrix of smaller ones were observed in films as thin as 350Å Annealing up to 350°C, subsequent to deposition, did not appreciably increase the size of the larger grains, although the smaller ones grew by about 10%. The TEM analysis together with RBS and AES data showed no significant interaction between Au and Si (that is to say no atomic intermixing or silicide formation) under all deposition conditions as well as annealing heat treatments. The significance of this in relation to the growth behavior of Au on the insulating and single crystal Si substrates will be shown.
The Influence of Sprayed (Cd, Zn)S on Dipped CuxS Composition
Authors: Berote G. , De Puydt Y. , Minet J -P. , Pierson E. , Streydio J -M.
Published: March 2011

It is shown that the properties of sprayed (Cd, Zn)S layers obtained by means of a pneumatic nozzle depend dramatically on the type of flow gas used (air or nitrogen). The role of chemisorbed oxygen is demonstrated. Independently of the various heat treatments applied the resulting CuxS obtained has always a coefficient x closer to 2 when the flow gas is nitrogen. A subsequent annealing of the (Cd, Zn)S layers is found to increase the CuxS growth speed.

It is also shown that the quality of (Cd, Zn)S layers determines to a great extent that of the cuprous sulfide in (Cd, Zn)S-CuxS heterojunctions. The data are interpreted in terms of the modification of the layers structure, composition and charge carrier concentration.
Secondary Grain Growth and Graphoepitaxy in thin Au films on Submicrometer-Period Gratings
Authors: Wong Chee C. , Smith Henry I. , Thompson C. V.
Published: March 2011

Secondary grain growth in thin Au films on SiO2 substrates with periodic surface relief structures was studied as a model for the application of graphoepitaxy (the growth of orientated crystalline films through the use of artificial surface patterning). Secondary grain growth driven by sur-face energy anisotropy produces grains many times larger than the film thickness with uniform texture. In thin films of Au on SiO2, surface-energy- driven secondary grain growth was found to occur at room temperature as soon as the film becomes continuous, and was shown to be responsible for the {111} deposition texture. A square-wave-profile grating of 0.2 μm period, etched into the surface of the substrate, resulted in preferred growth of {111}-textured grains with <112> directions oriented parallel to the grating axis. It is proposed that surface energy minimization is responsible for this phenomenon.
The Significance of the Impact Velocity of Vacuum-Deposited Atoms for the Structure of thin Films
Authors: Fuchs H. , Gleiter H.
Published: March 2011

The goal in this study was to investigate the effect of the impact velocity of the atoms on the structure of thin films generated by vacuum deposition. For that purpose all atoms faster or slower than a pre-selected velocity, u, were removed with a velocity filter frm a beam of gold atoms obtained by thermal evaporation in vacuum (10−5 Pa). The residual atoms (with a velocity u)were deposited onto cold (80 K) glass substrates. The annealing behaviour of the films studied by means of d.c. resistance measurements indicates a correlation between film structure and impact velocity in the sense that films generated by means of low velocity atoms have a non-equilibrium structure which does not exist in films prepared by conventional vacuum deposition.
Epitaxial Relations in Fluoride Films Grown on GaAs{lll} and Ge(lll) Substrates
Authors: Tsutsui K. , Ishiwarya H. , Asano T. , Fijrukawa S.
Published: March 2011

Epitaxial relations in Cax Sr1−xF2(0⫶×⫶l) and BaF2 films grown on GaAs(111), (111) and Ge(111) substrates were investigated by He ion channeling analysis and transmission electron microscopy. Though the lattice constants of GaAs and Ge are nearly the same, the relations were found to be completely different. That is, the fluoride films on GaAs prefered to have an orientation identical to that of the substrate (type A orientation), whereas the orientation of the films on Ge was mainly rotated by 180° about the surface normal 〈111〉 axis (type B orientation). A hypothesis is proposed that the epitaxial relations in these systems are affected by the ionicity of the substrate as well as the lattice matching condition.
Defect States in Silicon Nitride
Authors: Robertson John , Powell Martin J.
Published: March 2011

The energy levels of defect centers in amorphous silicon nitride have been calculated. The results are related to recent photoemission and light-induced electron spin resonancedata. The Si dangling bond is argued to be the memory trap in MNOS devices and to be responsible for the electron accumulation at interfaces with amorphous silicon and for the n-type chargetransfer doping of amorphous silicon-silicon nitride superlattices.
On the Electronic Properties of Silicon-Tellurium Films
Authors: Faris F.A. , Al-Jassar A. , Wakim F.G. , Botros K.Z.
Published: March 2011

Silicon and Silicon-Tellurium films were prepared in an ultrahigh vacuum system. The d.c. electrical conductivity of these films were measured under vacuum before exposing them to the air. The thermal activation energies of Silicon-Tellurium films were always found to be greater than that of Silicon films. Furthermore the optical measurements on Silicon-Tellurium films indicated that the optical gap of these films decreases as the concentration of tellurium in the films increases. Electron microscope examinations revealed that the films were amorphous in structure and homogeneous in composition.
A Carrier Lifetime Model for the Optical Degradation of Amorphous Silicon Solar Cells
Authors: Smith Z E. , Wagner S.
Published: March 2011

The light-induced performance degradation of amorphous silicon solar cells is described well by a model in which the carrier lifetimes are determined by the dangling bond density. The kinetics of the defect generation follow the model in which band-to-band recombination provides the energy for the creation of dangling bonds, which in turn introduce gap states that reduce carrier lifetime. Degradation will be slower in solar cells operating at lower excess carrier concentrations. This is documented with a comparison of degradation data for cells of different i-layer thickness, cells operating at open circuit vs. load, and for single vs. cascade cells. The model also correctly predicts the relation between short circuit current and fill factor degradation. At sufficiently long times, the efficiency will decrease at approximately the same rate for all cell structures and dimensions, with an offset in time between different device types which can be calculated.
Conduction and Threshold Switching in Amorphous Silicon
Authors: Sa-Neto A.
Published: March 2011

We report on measurements of conduction, threshold switching and high frecuency oscillations (∼100 MHZ) on amorphous silicon prgduced by RF sputtering in lateral devices with a very small (4µm2 to 40µm2 ) metalic contact area. The results show that the mechanism of threshold switching in this material is not due to the formation of a metalic filament through diffusion of the contacts. A small amount of Hydrogen (PH2/PAr=0.025) is sufficient to inhibit threshold switching. The “Zero voltage” resistance is reproducible and seems to be independent of the switching events.
Problems in a-Si Photoelectric Devices; Photoreceptor and Vidicon
Authors: Shimizu Isamu
Published: March 2011

The history in developing a-Si image devices, photoreceptor and vidicon is briefly reviewed. Some problems to be solved are discussed for the device structures and preparation techniques of Si-base alloys and Si itself from technical point of view.
Fabrication of Amorphous Silicon Devices on Plastic Substrates
Authors: Jeffrey F. R. , Vernstrom G. D. , Aspen F. E. , Jacobson R. L.
Published: March 2011

As Amorphous silicon devices spread into more products, the advantages inherant in continuous processing utilizing a plastic roll substrate will assume greater importance. Advantages include cost, ease of handling, shipping weight, steady state processing conditions and feedback control available in continuous processing. A new set of problems and constraints also comes with plastic substrates. These problems include mechanical strength with flexing, substrate surface defects, differential expalsion coefficients, shrinkage, and contaminant control. These problems along with efforts to evaluate and overcome them are discussed.
Defects in Tetrahedrally Coordinated Amorphous Semiconductors
Authors: Taylor P.C.
Published: March 2011

In the tetrahedrally coordinated amorphous semiconductors the dominant defects deep in the gap are attributed to dangling bonds on the group IV atoms. These defects are commonly thought to have effective electronelectron correlation energies Ueff which are positive, although some tightbinding estimates suggest negative Ueff. Defect states near the band gap edges are invoked to account for many experimental results including the usual appearance of an Urbach absorption edge. These shallow defect states are usually attributed to strained bonds but two-fold-coordinated group IV atoms have also been suggested. The application of light of near-band-gap energies alters the deniity of paramagnetic dangling bonds. For large spin densities (ns ≥ 1017 cm−3) this increase is probably due to the creation of new defects, bui it is possible that at lower densities (ns ≤ cm−3) the rearrangement of electronic charge in existing defects is important. Impurities also contribute to the defects observed in tetrahedral amorphous semiconductors. Particular species include trapped atomic and molecular hydrogen, trapped N0−2 molecules, singly-coordinated oxygen atoms and E' centers.
Grain Boundary Structures and Properties in Polycrystalline Silicon
Authors: Tsuo Y.S. , Milstein J.B. , Surek T.
Published: March 2011

The method of preparation of polycrystalline silicon can have a strong influence on the types and distributions of grain boundaries, and thereby influence the electrical properties of devices made from such materials. Examples of methods employed in the preparation of polycrystalline silicon for solar cell applications include directional solidification (Czochralski pulling and various casting techniques), ribbon growth techniques (ribbon-to-ribbon, edgedefined film-fed growth, low-angle silicon sheet growth, edge supported pulling, silicon-on-ceramic), chemical and physical vapor deposition (CVD and PVD) on silicon and foreign substrates, recrystallization techniques (laser, electron beam), and others such as graphoepitaxy and electrodeposition. This paper reviews the important morphological features such as grain size and defect structures of the various polycrystalline silicon materials and the influence of growth parameters on these features. The effects of grain boundaries on the electrical and photovoltaic properties of various polycrystalline silicon materials will also be discussed.
Passive and Enhanced Grain Boundaries in Solar Cells by Gas Immersion Laser Diffusion
Authors: Turner G. B. , Tarrant D. , Aldrich D. , Pressley R. , Press R.
Published: March 2011

A new method for making p-n junctions based on immersion in a transparent dopant gas followed by irradiation with a pulsed laser is presented. The surface of the wafer melts, dopant from the gas dissolves in it, and is distributed by liquid diffusion before epitaxial regrowth. This technique, termed GILD for Gas Immersion Laser Diffusion, was used to make solar cells in both monocrystalline and polycrystalline silicon.

Laser photocurrent scanning shows that grain boundary recombination is reduced or eliminated in cells made by the GILD technique and current collection is enhanced at some grain boundaries. Heat treatment at 850 ° C prior to GILD causes the same grain boundaries to be recombination active as they are in ordinary diffused cells.
Surface Mobilities in Laser-Processed Polysilicon Films
Authors: Frye R. C. , Ng K. K.
Published: March 2011

Surface mobilities in laser-processed polysilicon films were measured using silicon-gate n-channel thin-film transistors of varying dimensions. The apparent surface mobility inferred from transconductance measurements was found to be a decreasing function of channel length. For very short (~0.3/µm) channels, this mobility approaches the surface mobility of identical devices fabricated on bulk silicon. Furthermore, the temperature dependence of the surface mobilities in polycrystalline and bulk films was found to be identical.

A novel EBIC technique was employed to examine the surface potential of transistors in operation. These measurements indicate a high degree of spatial nonuniformity in the inversion layer of polycrystalline films arising from the grain boundary region. A simple model of the transistors is presented which explains the geometry dependent surface mobility and its temperature dependence.
Metastable Barrier Voltage in ZnO Varistors
Authors: Gupta Tapan K. , Carlson William G. , Hall Barbara O.
Published: March 2011

A reversible change in barrier voltage is observed in assintered ZnO varistors upon application and removal of the applied voltage. This is attributed to a metastable component in the grain boundary Schottky barriers. Varistors show improved stability when the metastable component is removed or fixed by heat treatment.
A Method For The Tem Characterisation Of Grain Boundary Films In Ceramics
Authors: Jepps N.W. , Page T. F. , Stobbs W. M.
Published: March 2011

The behaviour of Fresnel fringes with focus has been investigated as a means of characterising grain boundaries in engineering ceramics. This technique, together with dark-field imaging, has been applied to the study of grain boundaries in two hot-pressed silicon carbides. It is demonstrated that these two techniques, applied together, provide the best means of obtaining a complete characterisation of grain boundary films, in terms of width, structure, and composition.
Transport Across Silicon Grain Boundaries
Authors: Werner J. , Jantsch W. , Froehner K.H. , Queisser H.J.
Published: March 2011

By comparison of the capacitance and the conductivity of p–type Si bicrystals, we show quantitatively that current transport occurs through thermionic emission of holes across the potential barrier, which is caused by charged donors in the grain boundary. Starting from this finding, we propose a simple model which allows for the first time a spectroscopic determination of the grain boundary density of states from photocapacitance data. Results indicate the presence of band tails and additional mid-gap states.
The Origin of Saline Groundwaters in Granitic Rocks: Evidence From Hydrothermal Experiments
Authors: Savage David , Cave Mark R. , Milodowski Antoni E
Published: March 2011

Hydrothermal rock-water experiments at 80°, 150° and 250° C have been carried out to investigate the origin of saline groundwaters in the Carnmenellis Granite, Cornwall, UK which have previously been attributed to be derived via dissolution of biotite and plagioclase feldspar by dilute meteoric groundwaters. The nature of product fluid and solid phases in the experiments was dominated by the dissolution of plagioclase and potassium feldspars, coupled with the precipitation of laumontite, calcite and smectite. However, the release of chloride from the rock was minimal, suggesting minor dissolution of the major chloride bearing mineral, biotite. The relative inertness and stability of biotite during the experiments was borne out by direct physical observation of reacted biotites and by consideration of the fluid phase composition in terms of thermodynamic mineral stability diagrams. It is concluded that the experimental and natural systems are buffered under different chemical conditions, which may be due to acidity generated via oxidation of localised sulphide zones within the granite and absent in the experiments. Dissolution of biotite may be an important mechanism for enhancing the salinity of groundwaters in granitic rocks if groundwaters are buffered to be outside the stability of biotite and convective circulation is significant.
Evaluation of Single-Hole Hydraulic Tests in Fractured Crystalline Rock by Steady-State and Transient Methods: A Comparison
Authors: Andersson J-E. , Persson O.
Published: March 2011

The results from a large number of single-hole packer tests in crystalline rock from three test sites in Sweden have been analysed statistically. Average hydraulic conductivity values for 25 m long test intervals along boreholes with a maximal length of about 700 m are used in this study. A comparison between steady state and transient analysis of the same test data has been performed.

The mean vaule of the hydraulic conductivity determined from steady state analysis was found to be about two to three times higher compared to transient analysis. However, in some cases the steady state analysis resulted in 10 to 20 times higher values compared to the transient analysis. Such divergence between the two analysis methods may be caused by deviations from the assumed flow pattern, borehole skin effects and influence of hydraulic boundaries.
United States Crystalline Repository Project - Key Research Areas
Authors: Patera Edward S.
Published: March 2011

The Crystalline Repository Project is responsible for siting the second high-level nuclear waste repository in crystalline rock for the United States Department of Energy. A methodology is being developed to define data and information needs and a way to evaluate that information. The areas of research the Crystalline Repository Project is involved in include fluid flow in a fractured network, coupled thermal, chemical and flow processes and cooperation in other nations and OECD research programs.
Static Leaching of Fully Radioactive Waste Glass at 90°C in the Presence of Bentonite, Granite, and Stainless Steel Corrosion Products
Authors: Hermansson Hans-Peter , Björner Inga-Kari , Christensen Hilbert , Ohe Toshiaki , Werme Lars
Published: March 2011

The results from the JSS phase II static glass leaching experiments are presented. The experiments have been performed in the presence of bentonite, granite and stainless steel corrosion products at 90°C for 28 days. The leaching behaviour of Cs has been analysed using gamma-spectrometry. The surface of the leached glass samples has been analysed by infrared reflection spectrometry.

The presence of uncompacted bentonite seems to have a very small, increasing, effect on the leach rate of the glass compared to when no bentonite is present. In the presence of compacted bentonite, a lower leach rate has beep founy than in the prgsencelof uncompacted bentonite. (For Cs 0.3 g × m−2 × d−1 and 0.7 g × m−2 × d−1 respectively.) The addition of stainless steel corrosion products seems to have a slightly increasing effect on the leach rate in the presence of compacted bentonite. The beneficial effect of granite on the leach rate seems to be overshadowed by the presence of uncompacted bentonite.
Leaching of Actinides from the French LWR Reference Glass
Authors: Fillet S. , Nogues J L. , Vernaz E. , Jacquet-Francillon N.
Published: March 2011

The French LWR reference glass was spiked successively with 237Np, 239Pu, 238Pu and 241Am to study the specific leaching behavior of each actinide, as part of a larqer program investigating the effects of a decay in this glass. This paper describes the preparation of the α doped glasses and presents the initial leaching results for neptunium, plutonium and americium.

Glass samples were leached for up to six months at 90°C under static conditions. Weight loss measurements were performed, and the leachate was analyzed by ICP for the inactive elements, and by a spectrometry for the actinides. The results provide a basis for comparing the retention capability of the glass for each actinide, and the alterability of the bulk glass.

Finally, a comparison between glass specimens doped with 239Pu and with 238Pu, having the same chemical composition but very different specific activities, allows the effects of radiolysis on leaching to be estimated.
Effects of Rocks and Backfill Materials on Waste Glass Leaching
Authors: Ishiguro K. , Sasaki N. , Kashihara H. , Yamamoto M.
Published: March 2011

Extensive studies have been made on the interactions between a waste glass and repository materials under static conditions. One of the PNC reference glasses was leached in the solution prepared from water in contact with crushed granite, tuff, diabase and backfill materials such as bentonite and zeolite. The leachant solutions except for some bentonite solutions reduced the glass leach rate compared with that measured in distilled water. The extent of the reduction was a function of silicon concentration in solution. The bentonite solutions enhanced the glass dissolution rate by a factor of 2 to 3 at low bentonite/water ratios but the effect was found to be less important at high bentonite/water ratios and in the long-term experiment. Addition of granite and zeolite to the bentonite solutions decreased the leach rate below the value measured in distilled water.
Influence of Higher Waste Loading on Crystallization of Titanosilicate Glass-Ceramics
Authors: Hayward P. J. , Vance E. R. , Cann C.D. , George I. M.
Published: March 2011

Sphene (CaTiSio5 ) is obtained as the main crystalline phase in Na2O-A12O3-CaO-TiO2-SiO2 glass-ceramics containing up to ∼ 15 wt% of simulated nuclear fuel recycle waste, by maintaining the precursor glass at ∼ 1050°C for sufficient time to allow nucleation and crystal growth. The implications for production of an engineering-scale waste form are discussed.
Source Term for the Activity Release from a Repository for Spent LWR Fuel
Authors: Wuertz Ruediger , Ellinger M.
Published: March 2011

In order to provide basic data for the calculation of the dose burden from a repository for spent LWR fuel in a salt dome, a formula is given for determining activity release into the surrounding salt brine under accident conditions. The main characteristic of this source term is a time-independent, instantaneous release of part of the activity and mobilization of another part at a continuous and temperature dependant rate, using nuclide-specific leach rates. Changes in leach rates resulting from reduction of the waste surface during mobilization and precipitation are taken into account. The model also takes into account axial and radial activity distribution in the fuel rods.
Evaluation of the Localised Corrosion of Carbon Steel Overpacks for Nuclear Waste Disposal in Granite Environments
Authors: Marsh G. P. , Taylor K. J. , Bland I. D. , Westcott C. , Tasker P. W. , Sharland S. M.
Published: March 2011

The application of carbon steel corrosion allowance containers for the long term encapsulation of HLW or spent fuel requires data on the likely rates of corrosion attack so that the metal thickness needed to prevent penetration can be estimated. This paper describes a joint mathematical modelling and experimental approach to the evaluation of the rate of localised corrosion.
Swiss HLw Research Programme: Key Research Areas
Authors: McCombie C.
Published: March 2011

The increased emphasis put upon disposal of HLW in recent years led to the growth of large numbers of research programmes in many countries. Many results of this research have flowed into projects developing and analysing geologic repository concepts; the basic feasibility of implementing safe disposal facilities is now generally accepted by the technical community (though not always by the public!)
Modelling Radionuclide Speciation and Solubility Limits in The Near-Field of a Deep Repository
Authors: Wanner Hans
Published: March 2011

In the safety analysis recently reported for a potential Swiss high-level waste repository, radionuclide speciation and solubility limits are calculated for expected granitic groundwater conditions. With the objective of deriving a more realistic description of radionuclide release from the near-field, an investigation has been initiated to quantitatively specify the chemistry of the near-field. In the Swiss case, the main components of the near-field are the glass waste-matrix, a thick cast steel canister horizontally stored in a drift, and a backfill of highly compacted bentonite.

Based on available experimental data, an ion-exchange model for sodium, potassium, magnesium, and calcium has been developed, in order to simulate the reaction of sodium bentonite backfill with groundwater. The model assumes equilibrium with calcite as long as sufficient carbonates remain in the bentonite, as well as quartz saturation. The application of this model to the reference groundwater used in ‘Project Gewaehr 85’ results in a significant rise in pH (by up to 3 units) as well as a marked increase in the carbonate concentration.

Neptunium and plutonium speciation and solubility limits are calculated for the reference groundwater chemistry gradually altered to that of saturated bentonite water and back again by a water exchange cycle model. The solubility limits estimated in this way generally turn out to be higher for the bentonite water than for the reference groundwater, mainly due to carbonate complexation of the actinide components AnO2+ and AnO22+. Uncertainties are particularly large for neptunium solubility due to its strong Eh dependence in bentonite water.
The Chemistry of the Near Field Environment.
Authors: Ewart F. T. , Sharland S. M. , Tasker P. W.
Published: March 2011

The chemistry of near field, i.e. the pH, Eh and potential complexing ions, determines the solubility of radionuclides from a waste source. It is often assumed that outside the repository this chemistry is that of the appropriate groundwater. However, the chemical environment around the repository itself is perturbed both by the near field components, e.g. wasteform, backfill etc., and by the act of building the repository.
The Chemical Environment in Cement Matrices
Authors: Angus M. J. , Glasser F. P.
Published: March 2011

General progress in modelling the behaviour of cement matrices and their interactions with wastes is reviewed in terms of appropriate model-ling concepts and their formulation in terms of parameters Which are susceptible to evaluation. The redox behaviour, particularly of slag-cement blends is shown to differ sharply from that of Portland cement matrices. Experimental methods of measuring Eh and poising capacities are described and the composition of pore fluids determined. Slag cements contain appreciable soluble Sn− species as well as S2O32− and SO42−
Near-Field Thermal Analysis of the Nuclear Waste Disposal Vault
Authors: Radhakrishna H. S. , Lau K-C. , Crawford A. M.
Published: March 2011

This paper describes an experimental and theoretical analysis of the near-field thermal performance of a backfilled disposal vault in which the waste containers are emplaced in boreholes and surrounded by a bentonite-sand buffer. Effects of heat and moisture diffusion on the thermal performance of clay-based buffer and backfills were studied by means of laboratory scale experiments. The bentonite-sand buffer exhibited cracking due to desiccation. The impact of such cracking on the perforamnce of buffer as an engineered barrier is being investigated.

An integrated finite difference computer code ‘TRUCHAM’ was written to analyze the transient heat and moisture flow regime in a disposal vault and was validated by a scale model heater experiment. It is hoped that with proper characterization of the material properties ‘TRUCHAM’ can be employed as a useful tool in assessing the thermal response of the prototype disposal vault. Further work is needed to integrate the effects of heat and moisture on the shrinkage and expansion of clay-based buffer and backfills.
NMR-Studies of Pore Water in Bentonite/Water/Electrolyte
Authors: Carlsson Torbjörn
Published: March 2011

The properties of water saturated bentonite systems are highly dependent on the pore water chemistry, water content and temperature. In this study relative changes in the physical state of pore water are studied by means of the nuclear magnetic resonance technique (NMR). It is found that the relative water mobility increases with increasing water content and/or increasing temperature, while an increase in the electrolyte content lowers the water mobility.
Transport of Radionuclides in Inhomogeneous Crystalline Rocks
Authors: Hadermann Joerg , Roesel Fritz , Mccombie Charles
Published: March 2011

In Switzerland high-level radioactive wastes are planned to be disposed of in deep-lying crystalline formations. The modelling approach for geospheric transport in the framework of a HLW safety assessment is described. The processes taken into account are advection and dispersion in water conducting veins or fractures. Out of these, nuclides may diffuse into a spatially limited zone of altered rock. Sorption of nuclides on the rock is taken into account. Diffusion out of fractures and sorption in the adjacent altered rock may often be represented by an effective surface sorption on the fracture walls. For transport of the 237-Np nuclide chain we present a sensitivity analysis. The performance of the geologic barrier depends most critically on the extent of the altered zones.
Sorption of Some Fission Products and Actinides in Concrete Systems
Authors: Höglund S. , Eliasson L. , Allard B. , Andersson K. , Torstenfelt B.
Published: March 2011

The sorption of some actinides (Th, U, Np, Pu and Am) and fission products (I, Cs) was measured on two types of Standard Portland cements as well as on samples from old (70 years) hydro power dam constructions using a batch technique. Pore water compositions were analysed, and artificial pore water solutions were used as aqueous phases in the experiments. Measurements were also performed on five other concrete types (not reported in detail in this paper) to illustrate the effects of the cement matrix composition on the sorption behaviour of the radionuclides.

The sorption of actinides in the trivalent (americium), tetravalent (thorium) pentavalent (neptunium) and hexavalent (uranium) states was high in all the studied concrete systems.

Generally, the sorption of cesium was low due to the low exchange capacity of the cement and the high concentration of competing cations in the pore waters.

The sorption of iodine was much higher than in most silicate minerals of geologic origin.

The differences between the various concrete systems were generally minor in terms of their sorbing capacities.
Formation and Transport of Americium Pseudocolloids in Aqueous Systems
Authors: Olofsson U. , Bengtsson M. , Allard B.
Published: March 2011

The sorption of americium on colloidal quartz and montmorillonite has been studied using a batch technique. Americium appeared to be distributed between all available surfaces in the system (particles and vessel walls). Distribution coefficients, defined as the ratio of the amount of americium per mass of colloidal matter and the concentration in the solution phase, were estimated to be of the same order of magnitude as obtained in measurements on crushed material of much larger particle sizes.

In the presence of sorbents (alumina or granite) the removal of americium from the solution was enhanced, either due to the desorption of the americium from the particle phase and resorption on all available surfaces (sorbent plus vessel wall) or the sorption of the whole colloidal aggregates on the sorbent.

No break-through of americium was observed in column experiments (with alumina or granite packing) after the injection of americium pseudocolloids (quartz or montmorillonite) followed by elution with large volumes of aqueous phase (up to 880 column volumes). However, a minor fraction of the americium passed through the column without significant retention, especially under conditions with high flow rate (short hold-up time in the column).
Transient Diffusional Release from Waste Packages in a Repository in Basalt
Authors: Bensky M. S. , Oliver D. L.
Published: March 2011

Analyses of diffusional release of several typical radionuclides in spent fuel from waste packages emplaced in a repository in basalt were conducted to assess the effects of system characteristics and boundary conditions on computed release rates. Radionuclide releases, including spatial and temporal variations that may be present, represent the source term for transport in the geohydrologic setting and are therefore critical to the assessment of repository acceptability.

Two mathematical approaches were utilized to determine radionuclide release rate versus time characteristics; (1) an analytical solution for one-deimensional diffusion based upon a Dirichlet (constant-concentration) boundary at the waste form surface; and (2) a finite-element numerical solution based upon a Neumann (zero-flux boundary at the waste form surface. The latter method is suitable for radionuclides such as 129I, whose total inventory in spent fuel could be quickly depleted from the waste form and dissolved in the pore spaces of the packing material surrounding the waste form and which, therefore, cannot be adequately represented by a constant concentration at the waste form (i.e., container) surface.

The analysis revealed several system characteristics that are not intuitively obvious. For example, strong sorption in the near-field host rock behaves like a strong mass sink and can yield calculated transient release rates exceeding allowable limits. Similarly, a short half-life effectively removes the radionuclide from the host rock, which induces a steep concentration gradient at the host rock/packing interface and thereby increases the diffusional release rate at that boundary.

Typical results for 79Se and 129I are presented to illustrate these effects. The effects of perturbations to key assumptions are shown to indicate the importance of (1) formulating models that accurately represent the physical system and (2) interpreting analytical results carefully to ensure understanding of the capability of the system.
Phase Diagram of Laser Induced Melt Morphologies On Silicon
Authors: Preston John S. , Sipe John E. , Driel Henry M. Van
Published: March 2011

During cw laser induced melting, over a large range of intensities, silicon films phase separate into patterns of coexisting solid and molten regions. We have identified several distinct and reproduceable morphologies of this inhomogeneous coexistence region ranging from random lamellae (“amorphous”) structures to periodically ordered (“crystalline”) strips. The type of morphology formed is a function of laser intensity and spot size and these parameters can be viewed as constituting the two axes for a steady state “phase diagram” of the structures. “Phase transitions” between these structures occur for small changes of the experimental parameters with the fraction of liquid being an order parameter.
Oscillatory Morphological Instabilities During Rapid Solidification A The Role of Diffusion In The Solid
Authors: Bansal Atul , Bose Arijit
Published: March 2011

Recent results by Coriell and Sekerka [J. Crystal Growth, 61, 499(1983)] on the oscillatory instability of a planar rapidly solidifying binary melt are extended to include diffusion in the solid phase. Under assumptions equivalent to those made by Coriell and Sekerka, it is shown that no matter how small the diffusion coefficient is in the solid, the system is stable to all oscillatory and non-oscillatory disturbance modes if the modified constitutional supercooling criterion is satisfied and if the nonriequilibrium segregation coefficient is zero. Thus, a range of the non-equilibrium segregation parameter exists where these results allow the possibility of instability, whereas Coriell and Sekerka predict that the system will be stable.

System stability is increased for both oscillatory and non-oscillatory modes. It is necessary for the diffusivity ratio Ds/D1 to be nearly 0.1 before oscillatory modes are affected. Both the critical wavelength of the disturbance as well as the oscillation frequency are reduced slightly from the case where diffusion in the solid is ignored.
Pulsed Laser Melting of Graphite
Authors: Braunstein G. , Steinbeck J. , Dresselhaus M. S. , Dresselhaus G. , Elman B. S. , Venkatesan T. , Wilkens B. , Jacobson D. C.
Published: March 2011

Experimental evidence for laser melting of graphite, by irradiation with 30ns pulses from a ruby laser, is presented. RBS-channeling analysis, Raman scattering and TEM measurements reveal that the surface of graphite melts at a threshold energy density of about 0.6 J/cm2. For laser pulse energy densities above 0.6 J/cm2, the melt front penetration depth increases nearly linearly with increasing energy density. An intense emission of carbon particles during and after irradiation is observed. The thickness of the carbon layer removed in this process also increases nearly linearly with increasing pulse fluence. A dramatic redistribution of ion implanted impurities is also observed. Furthermore, the crystalline structure of the resolidified material is shown to depend on the energy density of the laser pulse. In order to explain these phenomena, a model for laser melting of graphite at high temperatures to form liquid carbon has been developed in which a free electron gas approximation is used to describe the properties of liquid carbon. The model is solved numerically to give the time and depth dependences of the temperature as a function of the laser pulse energy density. Very good agreement is found between the observed melt depth dependence on laser pulse energy density, as determined by RBS-channeling, and the model calculations. The redistribution of ion implanted impurities and the modification of the crystalline structure, caused by the pulsed laser irradiation, are also consistent with the model and permit the determination, for the first time, of interfacial segregation coefficients for impurities in liquid carbon. The model also predicts that liquid carbon at low pressure (p < 1 kbar) has metallic properties.
Nanosecond Time Resolved Reflectivity Measurements At The Surface of Pulsed Laser Irradiated Graphite
Authors: Venkatesan T. , Steinbeck J. , Braunstein G. , Dresselhaus M. S. , Dresselhaus G. , Jacobson D. C. , Elman B. S.
Published: March 2011

Time resolved reflectivity measurements at the surface of pulsed laser irradiated graphite are found to be complicated by the evolution of carbon atoms from the surface which attenuates the probe beam. The evolution of the species occurs concomitant with the observation of the melt on a time scale of <2 nsec. Further, the emitted species have velocities of:≤ 2 x 105 cm/s. The experimental results suggest the formation of large clusters at the molten surface though more experiments are required to clearly distinguish the effect of clusters from those due to the formation of a randomly reflecting plasma.
Nucleation of Allotropic Phases During Pulsed Laser Annealing of Manganese
Authors: Perepezko J. H. , Follstaedt D. M. , Peercy P. S.
Published: March 2011

Manganese has four allotropes with an equilibrium melting point of the high temperature δ-phase at 1517 K and calculated metastable melting points for the ϒ, β and α phases at 1501 K, 1481 K and 1395 K, respectively. Our observations for Mn irradiated with a pulsed laser and supporting estimates of maximum allotropic transition rates indicate that transformations between allotropes are suppressed during heating with ~ 25 ns laser pulses, as well as during subsequent cooling. Upon pulsed heating of β-Mn to the melt threshold, the melt is undercooled 122 K below the δ-Mn melting point. For incident laser pulse energy densities near the melting threshold, resolidification involves regrowth of β-Mn from the substrate. At energy densities well above threshold, the ϒ-Mn phase forms by separate nucleation and growth from the undercooled melt, and is retained upon rapid solidification. From these results and analyses, we conclude that significant melt undercooling, which may exceed 100 K, can occur during pulsed laser melting of metallic crystals and that the resulting crystalline structure is determined by both thermodynamics and nucleation kinetics.
Laser-Material Interactions In Phase Change Optical Recording
Authors: Barton Roger , Rubin Kurt A.
Published: March 2011

We have studied laser induced crystallization and amorphization reactions in tellurium and tellurium alloy thin films, which are of interest for use in optical data storage. The writing characteristics of these films are determined by plotting changes in film reflectivity against the parameters of laser power and pulse width. The laser pulse conditions necessary for film melting, crystallization, and amorphization are identified and then checked against the results of a numerical heat diffusion model. In this way we determine the critical quenching rate for formation of the amorphous phase in pure tellurium (2 × 109 C/sec). Minimum crystallization times are determined and found to increase by orders of magnitude with addition of alloying elements such as germanium or selenium. TEM analysis reveals significant differences in the crystallization mechanisms of different alloy systems.
Au-Cluster Redistribution During Nanosecond Laser-Annealing of Metal/Insulator Matrices
Authors: Pamler W. , Marinero E. E. , Chen M. , Jipson V. B.
Published: March 2011

We report on the growth and redistribution of Au clusters caused by nanosecond laser interaction of Aux(TeO2 )1−x thin films with intense excimer laser radiation. This laser-induced phenomenon is studied in a time-resolved manner using transient reflectivity and transmissivity techniques. Structural and compositional changes are investigated using Rutherford Backscattering, XPS depth profiling, x-ray diffraction and conductivity measurements. Our studies indicate that melting of the binary structure initializes segregation, growth and coalescence of Au crystallites in the amorphous TeO2 matrix.