bluuebunny/crossref_metadata_2025_split huggingface dataset:
Coffinitization - A Mechanism for the Alteration of UO2 under Reducing Conditions
Authors: Janeczek J. , Ewing R.C. 
Published: March 2011

ABSTRACTA structural and chemical analogue of UO2 in spent fuel, uraninite, UO2+x, is unstable in the presence of dissolved silica (>10-3.6 mg/L) under reducing conditions and transforms into coffinite, USiO4 nH20. Coffinite may incorporate numerous elements into its structure including actinides and fission products. Conditions favorable for coffinitization of UO2 in spent fuel may occur in repositories in granitic, basaltic, and tuffaceous rocks.
Source-Term Constraints for the Proposed Repository at Yucca Mountain, Nevaida, Derived from the Natural Analog at PeÑa Blanca, Mexico
Authors: Murphy William M. , Pearcy English C. 
Published: March 2011

ABSTRACTThe source term for nuclear waste repository performance assessments can be constrained by the solubilities of radioelement-bearing solids and/or the rates of release of radioelements from nuclear waste forms. Both solubility and rate limits for the proposed repository at Yucca Mountain, Nevada, can be assessed using information from the natural analog at PeÑa Blanca, Mexico. Petrographic and field relations indicate that uraninite oxidation and transformation to secondary uranyl silicate minerals have been rapid relative to mass transport of uranium out of the PeÑa Blanca system. The rate limiting process for uranium removal is likely to be advective transport in groundwaters with uranium contents controlled by interactions with uranyl silicate minerals such as uranophane. A maximum limit on the rate of uraninite oxidation at PeÑa Blanca is calculated to be 0.032 tons of UO2per year using geologic constraints on the amount of oxidation and the available time.
Interaction of Water and Compacted Sodium-Bentonite in Simulated Nuclear Waste Disposal Conditions
Authors: Melamed A. , Pitkänen P. , Olin M. , Muurinen A. , Snellman M. 
Published: March 2011

ABSTRACTAlteration of compacted sodium-bentonite (Volclay MX-80) caused by groundwater in simulated repository conditions for high level radioactive waste, was studied in an experiment where bentonite (wrapped by a copper cylinder) was let to react with two types (A,B) of synthetic granitic groundwater that are distinguished by their initial concentration of potassium and chloride. The reaction took place in ambient conditions at a temperature of 75 °C and proceeded during several time intervals up to 36 months.At the end of each time interval the water was chemically analysed for determination of possible changes in composition. Chemical and mineralogical changes in the bentonite were investigated by using NH4Cl extractions, XRD and microprobe (SEM, EDS) analyses and were studied as a function of the reaction time (months) as well as of the distance (mm) from the contact front with water.Sodium ions were found to migrate out from the bentonite while being replaced by other cations such as calcium, magnesium and to some extent, particularly during the reaction of the bentonite with water B, by potassium. No clear evidence was found for the fixation of potassium ions in the interlayer position of montmorillonite clay and the transformation to illite. The main mineralogical change in the bentonite was from Na- to Ca-rich montmorillonite. Secondary processes were the dissolution-precipitation of sulphur compounds, dissolution of gypsum and carbonates and the dissolution-precipitation of copper compounds.
Characterization of Lanthanoid Phases Formed Upon Glass Dissolution in Salt Solutions
Authors: Rother Anette , Lutze Werner , Schubert-Bischoff Peter 
Published: March 2011

ABSTRACTThis communication gives a detailed characterization of some molybdate solid solutions and cerianite-type material which formed on the French borosilicate nuclear waste glass R7T7 upon corrosion in various saturated salt solutions at 110°C, 150 °C and 190 °C. The glass contained lanthanoid elements, such as neodymium, lanthanum, praseodymium, cerium and yttrium, but did not contain actinoid elements, except uranium and thorium. Various solid solutions containing lanthanoids (Ln) were found on the glass surface after corrosion, including powellite solid solutions and cerianite-type material. The secondary solid phases are characterized based on quantitative microchemical and structural analyses. These phases are expected to incorporate actinoids such as americium and curium in acid magnesiumcontaining salt solutions. The phases then constitute an additional barrier against migration of these radionuclides, which would otherwise be in the aqueous phase.
The Current Status of the Radionuclide Migration Experiment at the Grimsel Underground Rock Laboratory
Authors: Alexander W.R. , Bradbury M.H. , Mckinley I.G. , Heer W. , Eikenberg J. , Frick U. 
Published: March 2011

ABSTRACTThis paper reviews the current status of the Grimsel radionuclide migration experiment, with especial reference to the advances of the last four years. Details are provided on the unusually extensive support programme and the general philosophy of undertaking such a large and costly field experiment.
The Structure of Alteration Layers on Cast Glass Surfaces
Authors: Oversby V. M. , Phinney D. L. 
Published: March 2011

ABSTRACTAlteration layers developed on SRL-165 simulated waste glasses in dilute sodium silicate/bicarbonate leaching solutions have been examined by Secondary Ionization Mass Spectrometry (SIMS) using fine-scale, multiple-element depth profiling. Selected samples were examined with an imaging detector system, which demonstrated the horizontal homogeneity of the layer development at all depths within the layer. After 1 day of reaction at 90°C the reaction layer shows depletion of glass elements to a depth of 0.2 μm. The surface of the layer in contact with the solution shows enrichment of Si, Al, and alkali elements even at this short reaction time, suggesting the early stages of development of secondary aluminosilicate phases. With increased reaction time, the layer thickens to about 1.3 μm at 91 days, while the evidence for aluminosilicate development at the surface of the layer becomes more prominent. Penetration of hydrogen into the “unreacted” glass proceeds to a depth of about 0.5 μm deeper than the alkali depletion zone. This suggests the mechanism of initial reaction of the glass is by attack of the silicate structure by molecular water or hydroxide ion rather than by alkali-hydrogen ion exchange. The simple structure of the layers developed in the silicate solution is in contrast to the complexity of layer structure found when glasses are reacted in deionized water. Since the conditions for geologic disposal will be closer to those used in the silicate leaching experiments, these results hold promise for the ability to model the system to predict long-term performance after disposal in a repository.
Instability in a-Si:H Thin-Film Transistor: A New Method to Discriminate Between Charge Injection and Defect Creation
Authors: Foglietti P. , Fortunato G. , Mariucci L. , Parisi V. 
Published: March 2011

ABSTRACTIn the present work, in order to discriminate the main source of instability in a-Si:H TFTs, the determination of both threshold voltage and flat-band voltage has been performed after bias-stressing the devices with different gate voltages and at different temperatures. Flat-band voltage was determined by the space-charge photomodulation technique. From the close correlation observed between the two quantities, we conclude that the predominant instability mechanism is represented by change in the gate insulator charge at and near the semiconductor/insulator interface.
Electron Range and Electron Generation Function in a-SLH
Authors: Najar S. , Equer B. , Chévrier J-B. 
Published: March 2011

ABSTRACTThe characterization of a-Si:H structure, using an electron beam in the keV range, is an interesting alternative to the more common photon beam techniques. a-Si:H device characterization by the determination of collection efficiency and other electronic parameters can be performed by variable energy EBIC method. But this requires an accurate knowledge of electron generation function and electron range in a-Si:H. In this paper we present an experimental determination of electron generation function and electron range in a-Si:H. To do this, a series of identical n-i-p a-Si:H diodes with a thin aluminium top electrodes were fabricated and varying thicknesses of a-Si:H layer were deposited on it. In EBIC measurements, the n-i-p diode was reverse biased at maximum potential. The electron range of a-Si:H was determined directly by measuring the energy at which electron beam is completely stopped in the top layer and no carrier generation is possible in the n-i-p diode. The generation function is then deduced from EBIC contrast measurements between the aluminium electrode and the top a-Si:H layer.
Kinetics of a-Si:H Crystallization Induced by Gold at Low Temperatures
Authors: Pasa A.A. , Schubert M.B. , Abel C.-D. , Beyer W. , Losch W. , Bauer G.H. 
Published: March 2011

ABSTRACTThe Au-induced crystallization of a-Si:H has been studied by evaporating Au films of different thicknesses onto intrinsic glow discharge deposited a-Si:H layers. The presence of a sharp peak in the Raman spectra (FWHM≈9 cm-1, ω516 cm-1) of samples with a Au thickness larger than 2 nm, which have been annealed in vacuum at 400K≤T≤600K, indicate that the crystallites have approximately the same size (6nm) regardless of the annealing conditions. An investigation of crystallization versus Au-film thickness revealed, that the total crystallized volume is increasing with Au thickness, and furthermore a saturation of the crystallized volume takes place, most probably due to an exhaustion of the Au reservoir. The increase of crystallization rate with temperature follows an Arrhenius-like dependence with an activation energy of 1.1 eV. Changes in hydrogen content as a consequence of the crystallization have been monitored by H-effusion measurements: Au-coated a-Si:H samples show a strong H2 evolution at temperatures substantially lower than uncoated ones.
Microstructure, Optoelectronic Properties and Saturated Defect Density of A-SL:H Prepared in VHF-Glow Discharge Using AR and XE Dilution
Authors: Kroll U. , Finger F. , Dutta J. , Keppner H. , Shah A. , Howling A. , Dorier J.-L- , Hollenstein Ch. 
Published: March 2011

ABSTRACTFilms were prepared under argon and xenon dilution with silane concentrations ranging from 3 to 100% using the very high frequency glow discharge (VHF-GD) technique. The H-content and the microstructure were determined by IR-spectroscopy. A surface profiler was used to measure the stress and thickness of the films. The surface roughness of the films was evaluated by the UV-light reflectance loss. The samples were further characterized by dark- and photoconductivity, by CPM and by PDS, both in annealed and light-soaked state.Down to silane concentrations of about 10–20% film properties change only little; however, for both dilution series the microstructure parameter shows a minimum and the internal stress a maximum near 20% silane concentration. At still higher rare gas dilution the film properties change drastically. Surprisingly the photoconductivity remains almost constant for all gas dilutions. It is shown how these changes in the film properties are linked with the light induced degradation.
VHF Plasma Deposition: A Comparative Overview
Authors: Shah A. , Dutta J. , Wyrsch N. , Prasad K. , Curtins H. , Finger F. , Howling A. , Hollenstein Ch. 
Published: March 2011

ABSTRACTThe use of plasma excitation frequencies f in the VHF band (30–300 MHz), and particularly of f=70 MHz, for the high-rate deposition of amorphous hydrogenated silicon (a-Si:H) is described. Deposition rates, using monosilane (SiH4) as source gas, are thereby increased roughly five fold to over 10 Å/s as compared with the conventional case of RF plasma enhanced chemical vapour deposition with f=13.56 MHz. This may possibly be attributed to an enhancement in the high-energy tail of the electron energy distribution function (EEDF) of the plasma. Thereby, no noticeable deterioration in film properties is found.Characteristics of VHF-deposited a-Si:H films are extensively reported, including properties like microstructure, hydrogen effusion behaviour and its low internal mechanical stress. High quality hydrogenated microcrystalline silicon (μc-Si:H) can be deposited at low substrate temperature and low plasma power densities thanks to VHF glow discharge. This can be linked to a reduction in sheath potential and to the energy of the ions arriving at the growing surface. Thereafter, use of VHF plasma in applications such as 100 μm thick a-Si:H layer for particle detectors and powder-free deposition of solar cells with efficiencies over 8% are reported.
Growth and Structure of Microcrystalline Silicon by Reactive DC Magnetron Sputtering
Authors: Feng G. F. , Katiyar M. , Yang Y. H. , Abelson J. R. , Maley N. 
Published: March 2011

ABSTRACTWe have deposited microcrystalline silicon (μc-Si) at low temperature by reactive dc magnetron sputtering. The structure and crystallinity of the films are analyzed by in situ spectroscopie ellipsometry. μc-Si growth occurs at deposition conditions with substrate temperature between 150 – 300 °C and hydrogen partial pressure above 4 mtorr. We have also observed that the deposition rate strongly affects the interface structure. At a rate of 21 À/min, a 230 Å thick amorphous layer appears at the interface of film and glass substrate. The interface layer thickness decreases with deposition rate, and becomes undiscernible for growth rates ∼ 2 Å/min. The thickness of the interface layer is also found to depend on substrates.
Structural Studies of Amorphous Semiconductor Alloys Using X-Ray Absorption and Related Techniques
Authors: Boscherini F. 
Published: March 2011

ABSTRACTThe use of X-ray absorption spectroscopy and related techniques in the investigation of the structure of amorphous group IV based semiconductor alloys is briefly reviewed, with emphasis on recent results.In these materials the single scattering contribution to the x-ray absorption signal contains information mainly on first shell coordination; this is illustrated by showing trends in the local structure of amorphous semiconductor alloys. This type of analysis can be complemented by using other methods. For example, X-ray scattering yields information on second shell structure which reflects the influence of bond angle disorder.Recent results on a-Si-1-xCx:H are then reported. Presence of both carbidic and graphitic carbon, interatomic distances which are constant with composition and a clear tendency towards chemical order are found. These results are compared to some recent structural models.
Contributions to Silicon-Hydrogen Bond-Stretching Frequency in Amorphous Si Alloys
Authors: Jing Z. , Whitten J. L. , Lucovsky G. 
Published: March 2011

ABSTRACTWe have performed ab initio calculations and determined the bond-energies and vibrational frequencies of Si-H groups that are: i) attached to Si-atoms as their immediate, and also more distant neighbors; and ii) attached to three O-atoms as their immediate neighbors, but are connected to an all Si-atom matrix. These arrangements simulate bonding geometries on Si surfaces, and the calculated frequency for i) is in good agreement with that of an Si-H group on an Si surface. To compare these results with a-Si:H alloys it is necessary to take into account an additional factor: the effective dielectric constant of the host. We show how to do this, demonstrating the way results of the ab initio calculations should then be compared with experimental data.
Interpretation of Stretched-Exponential Defect Kinetics in a-Si:H
Authors: Redfield David 
Published: March 2011

ABSTRACTA physical interpretation is given for the widespread observation of stretched-exponential time dependences for both light-induced generation and thermal anneal behavior of a-Si:H. This interpretation is based on a distribution of simultaneous rate processes, each of which represents a subset of normal metastable defects having a simple exponential behavior; it is thus an adaptation of one used for relaxation in glasses. The distribution can be evaluated from an observed stretched exponential, and for the case of thermal anneal with activated time constants, the stretch parameter should increase with temperature quite generally. The shape and width of the distributions of time constants and activation energies are inferred from these relations and properties of observed stretched exponentials.
Potential Role for Charged Dangling Bonds in Transient-Lesr of Light-Soaked a-Si:H
Authors: Saleh Z. M. , Tarui H. , Tsuda S. , Nakano S. , Kuwano Y. 
Published: March 2011

ABSTRACTTransient light-induced electron spin resonance (LESR) at 120 K, has been used to investigate deep defects in a-Si:H through changes in the lineshape. When the lineshape is deconvoluted into narrow and broad components, the narrow component is found to decrease, relative to the broad component, with increasing light-soaking time. Similar changes are not observed, however, in as-deposited or annealed films regardless of deposition and annealing conditions. An important role for charged dangling bonds is proposed to explain these changes and we suggest that intrinsic (stable) and light-induced (metastable) defects play different roles in transient-LESR and may occupy different energy distributions in the gap.
Anomalous Diffusion of Hydrogen and the Dependence of the Diffusion Constants on Hydrogen Content in Hydrogenated Amorphous Silicon
Authors: Shinar R. , Jia H. , Wu X.-L. , Shinar J. 
Published: March 2011

ABSTRACTThe diffusion constant of hydrogen DH(t) in hydrogenated amorphous silicon (a-Si:H) is strongly dependent on the Si-bonded H content CH of the films. It increases by over four orders of magnitude for CH ranging from 1 to 19 at. %. In an rf sputter-deposited film of CH ∼5 at. % it increases with time at 300 ≤ T ≤ 362°C. The dispersion parameter α in DH(t) = D∞ (ωt)-αis thus negative. This observation and the increase of α with T above a sample-dependent temperature Tτ are discussed in relation to low temperature structural relaxation processes in the amorphous network.
Comparison of Electron Beam-Induced and Light-Induced Defect Saturation in a-Si:H
Authors: Grimbergen M. , Lopez-Otero A. , Fahrenbruch A. , Benatar L. , Redfield D. , Bube R. , McConville R. 
Published: March 2011

ABSTRACTGeneration, saturation, and annealing characteristics of metastable defects formed by electron beam irradiation at 20 keV and photon irradiation at 1.9 eV have been compared. Saturation density reached by electron irradiation is temperature independent over the range 225 K to 300 K, although a small activation energy of the generation rate may be present. This differs from observed temperature dependent light-induced saturation from 330 K to 470 K, although differences are expected because of the separate temperature ranges and dissimilar carrier excitation rates. The electron beam-induced saturated defect density is about 5 times larger than for light-induced saturation at 350 K and high light intensity (generation rate ≈ 1022cm-3s-1). Defects formed by electron irradiation anneal at 300 K with a stretched exponential time constant three orders of magnitude smaller than for light-induced defects. After electron irradiation, dark conductivity relaxes faster than photoconductivity. Once the dark Fermi level becomes constant during defect density relaxation, photoconductivity is inversely proportional to the defect density.
Photodegradataion and Stability of a-Si Prepared at High Deposition Rates
Authors: Vepřek Stan , Ambacher Oliver , Vaněček Milan 
Published: March 2011

ABSTRACTA-Si films with a constant optical band gap were prepared by plasma CVD from silane at deposition rates up to 17 Å/s under controllably varied conditons and characterized by a number of techniques. Photodegradation under AM1 simulator has been done for 50 hours and for 4–17 hours under 2 Watt/cm2 dye laser illumination at a wavelength corresponding to an absorption coefficient of about 104 cm-1 up to saturation. Samples with different initial value of σ(d) and σ(ph) show different rates of photodegra-dation, but they reach very similar saturation values. The fast photode-gradation with the dye laser is equivalent to a long term (several thousand hours) photodegradation under AM1. The AMI photodegradation for 50 to 100 hours does not yield any reliable measure of the stability of the films. The best films, which were prepared at a high deposition temperature show a low saturation defect density of about 5·1016 cm-3 (CPM-data).
Defect Kinetics and Saturation in Amorphous Silicon-Germanium Alloys
Authors: Schumm G. , Abel C.-D. , Bauer G.H. 
Published: March 2011

ABSTRACTA quantitative comparison of defect kinetics under high-intensity light-soaking was carried out on a-Si:H and a-SiGe:H. A simple technique that ensures identical experimental conditions (generation rates and generation profiles) in samples of different bandgaps is demonstrated. It is shown that the apparent dependence on bandgap of the susceptibility to light-induced degradation is mainly due to changes in intrinsic carrier concentrations without the need to adhere to any structural changes.
Subgap Absorption and Electrical Properties of Compositional Multilayers of a-Si:H and Alloys
Authors: Bernhard Norbert , Bauer Gottfried H. 
Published: March 2011

ABSTRACTUrbach tails and midgap absorption, as determined by PDS, and temperature dependent coplanar and sandwich conductivity were systematically investigated on compositional multilayers of a-Si:H and its alloys with Ge and C, under the main aspect of a critical evaluation with respect to a possible existence of quantum size effects. Both electronic well and barrier widths were varied, with and without keeping the mean composition of the multilayers constant. Neither the overall dependences of the Urbach slopes, nor of the conductivity activation energies and room temperature αdark values are consistent with a quantization picture. Other effects, as imposing on each other the own sublayer's degree of structural disorder, and adjustment of the Fermi level by charge transfer, play a dominant role and may feign quantum size effects, if in a multilayer series only variation of the well widths is considered.
Optimization of the Deposition Conditions for High-Gap a-Si,Ge:H,F Alloys
Authors: Morin P.A. , Wang N.W. , Wagner S. 
Published: March 2011

ABSTRACTWe report the deposition parameters for optimized a-Si,Ge:H,F alloys in the range of optical (Taue) gap 1.5 leV to 1.62eV. These deposition parameters were optimized using the saturated defect density as a figure of merit. We report initial defect densities at or below 2.5×1016 cm-3, saturated defect densities below 9×1016 cm-3, photoconductivities (at G = 1021 cm-3s-1) between 8.7×10-6 Scm-1 and 7×10-5 Scm-land photosensitivities between 104 and 105 for alloys in this range of optical gap.
Optoelectronic Properties of Amorphous SiGec Alloys
Authors: Kolodzey J. , Schwarz R. , Wang F. , Muschik T. , Krajewski J. , Shekhar R. , Barteau M. , Plättner R. , Günzel E. 
Published: March 2011

ABSTRACTWe describe the optoelectronic characteristics of hydrogenated amorphous silicon germanium carbon (a.Si1-x-yGexCy:H) alloys prepared by plasma deposition from SiH4/GeH4/CH4/H2 gas mixtures. a-Si1-x-yGexCy:H is a homogeneous random alloy having a variable optical gap depending on composition, with properties similar to those of amorphous Si-Ge alloys of the same optical gap but with improved thermal stability. Calculations show that if the ratio of Ge/C atomic fractions is 8.2, the average bond length matches that of unalloyed amorphous a-Si:H with the possibility of reduced defect densities at heterointerfaces. After light-soaking with high intensity white light, a sample having a 1.3 eV optical gap exhibited no Staebler-Wronski change in its properties.