bluuebunny/crossref_metadata_2025_split huggingface dataset:
Obsidians and Tektites: Natural Analogues for Water Diffusion in Nuclear Waste Glasses
Authors: Mazer James J. , Bates John K. , Stevenson Christopher M. , Bradley C. R. 
Published: March 2011

ABSTRACTMolecular water diffusion in natural obsidians and tektite was investigated in vapor hydration tests performed between 75 and 230°C for up to 400 days. Reaction progress was monitored using measurements of the birefringent hydration layer, an alteration feature associated with strain caused by molecular water diffusion in obsidians. The hydration rate constants and temperature dependence of the reaction are strongly correlated with the logarithm of the initial total water content of the glass. These values have been quantified for conditions relevant to the potential Yucca Mountain repository. The low initial total water concentrations of Savannah River Lab nuclear waste glasses produced at the bench-top scale help to minimze the effects of molecular water diffusion in waste glasses. The results of this study indicate that molecular water diffusion does not dominate waste glass reactions under conditions considered in this study. However, it is unknown whether molecular water diffusion will be important under other reaction conditions, especially longer time periods.
Experimental Determination of Parameters Governing Heat-Induced Moisture Movement in A Clay Buffer
Authors: Selvadurai A.P.S. 
Published: March 2011

ABSTRACTThis paper discusses the experimental evaluation of the hygro-thermal parameters governing coupled heat and moisture movement within a moist clay buffer, which is developed as an engineered barrier for the isolation of heat emitting nuclear waste containers in either a borehole environment or in an in-room emplacement scheme.
Hydrothermal Field Experiment Simulating Steel Canister Embedded in Expansive Clay - Physical Behavior of the Clay
Authors: Pusch Roland , Karnland Ola , Lajudie Alain , Atabek Rosemarie 
Published: March 2011

ABSTRACTField heat experiments with kaolinite/smectite clay surrounding heaters in boreholes were conducted for 0.7 and 4 years with temperatures up to 170-180°C. The short test gave a high degree of water saturation even in the hottest part (> 75 %) and almost no change in physical properties and mineral composition. The long test gave a dry inner zone of claystone, indicating gas formation, and rich precipitation of silica/aluminum compounds and sulphate minerals. Brittleness characterized the hot parts and stiffening occurred also in the colder parts due to precipitation of silica and aluminum.
Development of A Model for Handling the Movement of Redox Fronts and Other Sharp Reaction Fronts
Authors: C. Nyman , V. Ozolins , L. Moreno , I. Neretnieks 
Published: March 2011

ABSTRACTWe have developed a coupled transport and reaction computer program which is adapted to handle sharp reaction fronts. Such fronts develop in the ground and typical examples are redox fronts and dissolution/precipitation fronts. The program is developed to describe the chemical evolution around radioactive waste repositories but it is applicable to other situations as well. To validate the program we compare our results with data from other geochemical equilibrium programs, such as EQ3/6 [1], PHREEQE [2] and transport programs such as CHEQMATE [3]. Natural analogues like the Osamu Utsumi mine at Poços de Caldas in Brazil and the uranium ore body at Cigar Lake in Saskatoon, Canada, are studied using the program.
Modelling the Chemical Environment Near A Waste Disposal Site in A Granitic Host-Rock Influence of Factors Controlling the Solubility of Radionuclides
Authors: Barbier Jean , Fabriol Robert , Ouzounian Gerald 
Published: March 2011

ABSTRACTMaximum aqueous concentrations of six radionuclides (Sr, Np, U, Th, Pu and Tc) near a disposal site with a quartz sand or clay engineered barrier in a granitic environment, were calculated in a previous study by computer modelling. A sensitivity study has been carried out in order to assess the importance of three factors in determining the maximum concentrations likely to be found in solution: the maximum temperature reached, water salinity, and the dissolution constants used. In the circulating fluid, final concentrations for Pu, Tc and Th depend mostly on the final temperature. The solubility of Sr is influenced by the fluid salinity. The final concentration of Np is affected by the temperature of the waste disposal site. Uncertainty in the dissolution constants causes the greatest relative variation in calculated U concentrations.
Numerical Simulations of Heterogeneous Chemical Reactions Coupled to Fluid Flow in Varying Thermal Fields
Authors: Carnahan C. L. 
Published: March 2011

ABSTRACTA numerical simulator of reactive chemical transport with coupling from precipitation-dissolution reactions to fluid flow, via changes of porosity and permeability, is applied to precipitation-dissolution of quartz and calcite in spatially and temporally variable fields of temperature. Significant effects on fluid flow are found in the quartz-silicic acid system in the presence of a persistent, strong gradient of temperature. Transient heat flow in the quartz-silicic acid system and in a calcite-calcium ion-carbonato species system produces vanishingly small effects on fluid flow.
Non-Linear Effects of Glass Composition on Chemical Durability: Physical Stability of Surface Layers
Authors: Muller I. S. , Buechele A. C. , Pegg I. L. , Macedo P. B. 
Published: March 2011

ABSTRACTPresented here are results relating to the possible causes for the very strong nonlinearity in the relationship between glass composition and glass durability, as determined by MCC 3 or Product Consistency (PCT) tests. We have previously found a rapid increase in leachate concentrations with small changes in composition (as little as 1 weight percent SiO2 or Al2O3) over some composition ranges near those of interest for nuclear waste disposal. This rapid rise occurs over a narrow transition region in composition space that provides a natural division between more durable and less durable glasses. Our new data on glasses of interest to the West Valley Demonstration Project (WVDP) suggest that this effect is not due to phase separation. Results from leach tests conducted with and without constant agitation are compared in an attempt to determine whether the physical integrity of alteration layers plays an essential role, as might be the case if formation-spallation cycles were important in this process. Microstructural data are also presented in order to compare the alteration layers on the more durable and less durable glasses leached with and without agitation.
An Investigation of the Behaviour of a-Si:H Thin Film Transistors Fabricated With Different Proximity Recovery Layer Doping Levels
Authors: Willums M.F. , Lecomber P.G. , Hack M. 
Published: March 2011

ABSTRACTExperiments have been carried out on inverted staggered amorphous silicon (a-Si:H) thin film transistors (TFTs). Depending on the voltage stressing conditions the electronic characteristics of these devices tend to shift with time [1]. To slow these changes down, and to speed up the recovery of the TFT, a lightly doped compensation or proximity recovery layer (PRL) has been placed in close proximity to the active layer of the undoped a-Si:H [2]. The effects of varying the dopant concentrations in the PRL have been studied by comparing the PRL TFTs with conventional control TFTs containing no PRL.
High Electric Field Amorphous Silicon Structures : Application to Particle Detection
Authors: Equer B. , Chevrier J.B. 
Published: March 2011

ABSTRACTThin-film ionizing radiation detectors prepared from amorphous semiconductors are receiving a growing interest. To reach ultimate sensitivity, single particle detection is needed. Amorphous silicon detectors operated in this mode require a high polarization to overcome the limitations due to the low electron and hole effective mobilities and to charge trapping. In this review, we summarize results obtained on charged particle detection (electrons, protons and alphas) with special emphasis on the electron-hole pair generation and collection mechanisms. We report a study on the role of p-doped layers in obtaining high breakdown voltages (more than 7 × 105V/cm). Physical mechanisms underlying leakage current injection and generation are discussed.
Crystallization Kinetics of Hydrogenated Amorphous Silicon During Pulsed Excimer Laser Annealing
Authors: Ready S. E. , Roh J. H. , Boyce J. B. , Anderson G. B. 
Published: March 2011

ABSTRACTExplosive crystallization of amorphous silicon (a-Si) during pulsed laser annealing occurs at an intermediate laser energy fluence above the threshold for surface melting. Mediated by a molten silicon layer which is undercooled with respect to crystalline silicon and above the melting point of a-Si, the crystallization interface drives down into the sample, sustaining itself due to the difference in the latent heats of the crystalline and amorphous silicon. Explosive crystallization has been the subject of numerous studies which have for the most part been restricted to ion implanted amorphized layers in silicon bulk samples. In this study we examine the crystallization kinetics of vapor deposited thin films of hydrogenated a-Si for films of differing hydrogen content and substrate temperature. We reevaluate current models of interface and nucleation kinetics qualitatively in light of these results. The fundamental physical mechanisms in these non-equilibrium phase transitions during pulsed laser annealing are discussed.
Thin Film on Asic - A Novel Concept for Intelligent Image Sensors
Authors: Fischer H. , Schulte J. , Giehl J. , Böhm M. , Schmitt J. P. M. 
Published: March 2011

ABSTRACTTwo different pixel detectors have been fabricated, each of them consisting of an a-Si:H based photodiode layer on top of a crystalline silicon integrated circuit. Both sensors are arranged in a matrix and addressed columnwise, providing parallel readout of the matrix rows or optional random access. One sensor is an a-Si:H detector / x-Si switching transistor combination with 256 × 256 pixels and a pixel size of 100μm × 100μm. The signal transport in the array is examined, demonstrating its capability of very fast information readout. The second sensor, which consists of an array of 32 × 32 pixels on a 5μm PMOS ASIC, performs digital contour extraction. Experimental data on the performance of the intelligent array are also reported.
Comparison of Computer Simulations with Measurements From a-Si:H Imaging Arrays.
Authors: Yorkston J. , Antonuk L.E. , Seraji N. , Boudry J. , Huanga W. , Morton E.J. , Street R.A. 
Published: March 2011

ABSTRACTThe design and characterization of flat panel a-Si:H arrays for imaging applications in radiotherapy and diagnostic radiology has already begun. Future improvements in design and performance will be heavily reliant on the results of computer simulations capable of accurately predicting the behavior of the arrays in a variety of operating conditions. These results will allow the design of new arrays to be optimized for the particular application before they are committed to fabrication. This paper outlines the results of an investigation into the effects of varying a number of array parameters and operating conditions using a simulation code which models the operation of a-Si:H devices. The results are compared with actual measurements from arrays fabricated at Xerox PARC.
Charge-Sensitive Poly-Silicon TFT Amplifiers for a-Si:H Pixel Particle Detectors
Authors: Cho G. , Perez-Mendez V. , Hack M. , Lewis A. 
Published: March 2011

ABSTRACTPrototype charge-sensitive poly-Si TFT amplifiers have been made for the amplification of signals (from an a-Si:H pixel diode used as an ionizing particle detector). They consist of a charge-sensitive gain stage, a voltage gain stage and a source follower output stage. The gain-bandwidth product of the amplifier is ∼ 300 MHz. When the amplifier is connected to a pixel detector of 0.2 pF, it gives a charge-to-voltage gain of ∼ 0.02 mV/electrons with a pulse rise time less than 100 nsec. An equivalent noise charge of the front-end TFT is ∼ 1000 electrons for a shaping time of 1 μsec.
Effect of Annealing on Undoped a-Si:H Films Prepared by Mercury-Sensitized Photochemical Vapor Deposition
Authors: Niiyama T. , Nozaki H. 
Published: March 2011

ABSTRACTThe effects of annealing on undoped a-Si:H films were studied. Films of a-Si:H were prepared by mercury-sensitized photochemical vapor deposition, and were annealed immediately. The hydrogen configuration was altered in the annealing process below 200°C, although films were prepared at a substrate temperature of 230°C. At the same time, electron drift mobility greatly increased. These observations indicate a reduction in the number of tail states in the a-Si:H band-gap. Thus the annealing below the substrate temperature clearly influences not only the bonded-hydrogen atoms but also the Si-Si network in the films to some extent.
A Particular Photo-CVD Apparatus for Hydrogenated Amorphous Silicon Deposition
Authors: Manfredotti C. , Fizzotti F. , Osenga C. , Boero M. , Rigato V. , Quaranta A. 
Published: March 2011

ABSTRACTA new PHOTO-CVD apparatus has been built in order to deposit a – Si : H films and other kinds of amorphous thin films by a technique which is both simple and versatile. This apparatus is composed of three chambers connected together: a load-lock chamber, a process chamber and a third chamber for in-situ analysis of deposited films. A peculiarity of the lamp, a dielectric discharge lamp which can work with noble gases like Xe or Kr, is that it can be completely dismounted without breaking the vacuum in order to clean the optical MgF2 window. By this method, the deposition chamber can be kept in very clean conditions. In this apparatus, we started to deposit a – SixC1−x: H of very good quality, taking their thickness into account. These films have been completely characterized by chemical (RBS, ERDA) and optical (PDS) methods. Their quality can be compared with quality of a – Si : H samples of the same thickness obtained by PECVD.
Effects of Hydrogen Plasma Treatment on Hydrogenated Amorphous Silicon
Authors: Lee J.-K. , Schiff E. A. 
Published: March 2011

ABSTRACTWe have studied the effects of hydrogen plasma treatments of a-Si:H at elevated temperatures using conductivity, photoconductivity, and spin density measurements. Effects were measured as a function of temperature and of treatment time; control specimens were subjected to comparable treatments without the plasma. In no case did plasma treatment improve the specimen photoconductivity over the as-deposited state. At treatment temperatures (230 °C < T < 460 °C) plasma treatment reduced the photoconductivity below that of the control specimen. This effect may be evidence for plasma-assisted dehydro-genation of the specimen. The defect structural equilibration determined by hydrogen chemical potential depending on temperature and hydrogen plasma is also addressed.
Configurational Relaxation of the D Defect in Hydrogenated Amorphous Silicon as a Function of Fermi Energy
Authors: Leen Thomas M. , Rasmussen Randall J. , Cohen J. David 
Published: March 2011

ABSTRACTBy using light soaking and partial dark annealing to vary the Fermi level in n-type a-Si:H, we have examined the thermal emission of electrons from the dangling bond (D) defect. We find optical evidence for a change in the configuration of the D defect when EF = Ec-0.55±0.08eV. We find that the relaxation rate increases with temperature and increases as EF is brought closer to Ec. Voltage-pulse photocapacitance and depletion-width-modulated ESR show emission is predominantly from D° defects for short emission times and short filling pulse widths. With longer emission times and longer filling pulse widths, emission from D-dominates. We also find that the charge emission transient fits a universal scaling law under a variety of pulsing conditions, temperatures, and anneal states.
Optical Properties and Structure of Microcrystalline Silicon
Authors: Nguyen Hien V. , An Ilsin , Li Youming , Wronski C.R. , Collins R.W. 
Published: March 2011

ABSTRACTThe optical properties of thin film microcrystalline silicon (μc-Si:H) prepared by plasma-enhanced chemical vapor deposition (PECVD) have been studied by real time spectroscopie ellipsometry in the nucleation regime as isolated crystalline particles increase in size. A simple geometric model of nucleation allows us to remove the dominant effect of voids and extract the dielectric functions of the crystallites themselves. We find that the results can be understood in terms of a classical size effect whereby limitations on the electron mean free path by scattering at crystallite surfaces control the absorption onset from 2.0 to 3.0 eV. Finally, we describe how well-ordered, continuous 15 Å c-Si films can be prepared on metal substrates.
Origin of Surface Defects in a-Si:H Films
Authors: Yan H. , Morimoto A. , Kumeda M. , Shimizu T. , Yonezawa Y. 
Published: March 2011

ABSTRACTSurface oxidation and surface defect creation processes in a-Si:H films have been studied in detail by means of electron spin resonance(ESR) and X-ray photoelectron spectroscopy(XPS). It is found that Si dangling bonds created by the surface oxidation distribute far wider than the thickness of the SiO2 layer. These defects are also found to be removed out by annealing at around 100°C. These defects are proposed to be created by a stress in a-Si:H induced by the surface oxidation. Moreover, the presence of the surface defects unrelated to oxidation is shown for the first time by the present experiment. The origin of these defects, however, are not clear at present.
A Multiple Quantum NMR (MQNMR) Study of Hydrogen Microstructure in Boron Doped a-Si:H
Authors: Mitra S. , Levy D. H. , Gleason K. K. , Jia H. , Shinar J. 
Published: March 2011

ABSTRACTAn IR and multiple quantum NMR (MQNMR) study of hydrogen microstructure in three boron doped a-Si:H is discussed. The total Si-bonded H content of all films was 6.5 ± 1.0 at.% as determined by the 640 cm-1 IR wagging mode, but their boron content, which was determined by secondary ion mass spectrometry, ranged from 0.02 to 0.3 at. %. The number of correlated hydrogen, as measured at a preparation time of 600 μSwas found to be more weakly dependent on the boron content than previously observed in phosphorous-doped glow-discharge films. Upon annealing at 220 °C the MQNMR spectrum show a moderate increase in the number of correlated hydrogen in all three samples.
Small Angle X-Ray Scattering (Saxs) and IR Study of Microvoid Dynamics in Annealed RF Sputter-Deposited A-SI:H
Authors: Jia H. , Shinar J. , Chen Y. , Williamson D. L. 
Published: March 2011

ABSTRACTA SAXS and IR study of microvoid distribution and dynamics in a-Si:H deposited by rf sputtering at 200 – 600 W on nominally unheated substrates is described and discussed. The 640 cm-1 band of the 200 W film yielded a total Si-bonded H content CH=21 at. %; the 840 – 890 cm-1 band yielded a dihydride content CH2 3.4 at. %. The SAXS measurements yielded a microvoid volume fraction vf=8.2 vol.%, and tilting SAXS data indicated elongated voids consistent with a columnar microstructure. In the other films, 9<CH<12 at. % and CH2 was negligible, vf was -2 vol. %. Annealing from 250°C to 310°C for 6 hrs resulted basically in no changes of CH and vf. However CH decreased and vf increased significantly after annealing at 350°C and above. The results showed a strong correlation between the IR determined CH and CH2 and the SAXS determined vf.
Dipolar Measurements of Hydrogen in Amorphous Silicon
Authors: Hari P. , Taylor P.C. , Street R.A. 
Published: March 2011

ABSTRACTThe dipolar interaction of hydrogen (∼ 10 at. %) in boron-doped amorphous silicon has been studied using the Jeener-Broekaert pulse sequence. The sample was prepared on an Al foil substrate at a temperature of 230°C using a standard glow discharge system (operating at 1 W rf power). The diborane/silane ration was 10-4. The same sample was removed from the Al substrate using dilute hydrochloric acid. The Jeener-Broekaert pulse sequence consists of three pulses: π/2|x′ – τ1 – π/4|y′ – τ2 – π/4|y′ – echo. We measured T1D, the dipolar spin lattice relaxation time, for τ1 = 82 μs, τ1 = 100 μs and τ1 = 50 μs at 299°K. The value of τ1D was found to be independent of τ1. At 335°K we found τ1D to be much longer than at room temperature. The values of τ1D at 299 K, 314 K and 335 K are, respectively, 0.7 ms, 1.2 ms and 1.8 ms. From the data we estimate an activation energy for microscopic motion to be ∼ 0.2 eV.
Surface and Bulk GAP States Distributions in Amorphous Silicon Films as Obtained by Optical Methods
Authors: Amato G. , Benedetto G. , Boarino L. , Giorgis F. , Spagnolo R. 
Published: March 2011

ABSTRACTThe aim of this work is to achieve detailed information about surface and bulk density of states in device quality a-Si:H films by means of deconvolution of PDS spectra. Unambiguous deconvolution of the spectra has been performed by calculating the first derivative vs. energy of the absorption coefficient of the samples. The results are compared with those obtained by means of the integration rule. Calculations of the density of states carried out on samples having different thickness suggest that the defect distribution differs from surface to bulk. The surface defect peaks are found to lie closer to the corresponding mobility edges, with respect to the bulk defect peaks: these results agree well with those obtained by means of other techniques, like total yield spectroscopy and can be explained in terms of a defect pool model.The present deconvolution procedure when applied to samples having different thickness provides new results about the film's homogeneity in terms of deep defect and valence tail states.
Enhanced Hydrogen Diffusion Under Illumination in Hydrogenated Amorphous Silicon
Authors: Santos P.V. , Johnson N.M. , Street R.A. 
Published: March 2011

ABSTRACTWe provide experimental evidence for the fact that hydrogen diffusion in hydroge-nated amorphous silicon is controlled by the concentration of electronic carriers. It is experimentally demonstrated that the hydrogen diffusion coefficient (a) is enhanced if the carrier population is increased by illumination and (b) is strongly suppressed if carriers are extracted from the diffusion region by the application of an electric field.
Study of Short-Range Motion of Atomic Hydrogen in Amorphous Silicon by Neutron Reflectometry
Authors: Dozier W. D. , Herwig K. W. , Shinar R. , Jia H. , Shinar J. 
Published: March 2011

ABSTRACTPreliminary results of neutron reflectometry (NR) measurements on if sputter-deposited a-Si:H/a-Si:D bilayers indicate that this technique may be used to monitor H and D motions over distances of ≈ 10 to 200 Å with a nominal resolution of 5 – 10 Å. In studying rf sputter-deposited thin films containing a high density of microvoids annealed at 270 C, we found that the hydrogen diffused a distance of only ≈ 100 Å. Further annealing at 270 and 280 C produced no additional motion. This result is consistent with a model of this system in which the hydrogen is trapped in microvoids after moving a relatively short distance.
Concentration-Dependent Hydrogen Diffusivity in a-Si:H
Authors: Santos Paulo V. , Jackson Warren B. 
Published: March 2011

ABSTRACTWe present experimental evidence for an enhancement in hydrogen diffusivity in a-Si:H with hydrogen concentration. The concentration dependence is attributed to the saturation of the density of hydrogen traps consisting of Si-H bonds with increasing hydrogen concentration. As a consequence, hydrogen lifetime in mobile interstitial and/or bond-center sites is enhanced. The saturation occurs when the rate of trap filling by hydrogen injection from the plasma exceeds the rate of trap creation.