bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling of Geochemical Compatibility of Near Field Materials in Terms of Radionuclide Retention Properties
Authors: Lucille P. L. , Burnol A , Ollar Ph.
Published: April 2011

The containment of radionuclides over very long periods of time is based on the interposition of a multi-barrier system between the waste and the biosphere. A performance allocation study is usually conducted for each barrier. Each barrier is then designed, in terms of geometry and composition, to cope with the performance it has been allocated. However geochemical interactions will occur between the different barriers. An alkaline plume will be generated by cement materials, a redox front will be generated by container corrosion and geochemical gradients will be generated by the dissolution of artificial barriers by natural groundwater. Radionuclide retention mechanisms are strongly pH and Eh dependent, therefore the impact of these geochemical transients on retention must be quantitatively evaluated to check the performance of each barrier for realistic in-situ situations. To assess this impact, two types of engineered barriers (clay and cement) for a spent fuel repository are simulated with a coupled hydrogeochemical model. Comparisons between hydraulic heterogeneous (fractured) and simple homogeneous systems are also carried out in terms of waste dissolution.
Amorphisation of Perovskite: the Effectofcomposition and Pre-Existing Cation Vacancies
Authors: Smith Katherine L. , Lumpkin Gregory R. , Blackford Mark G. , Vance Eric R.
Published: April 2011

Eight perovskites of different compositions and pre-existing vacancy contents were irradiated with 1.5 MeV Kr+ ions using the HVEM-Tandem User Facility at Argonne National Laboratory. The critical dose of 1.5 MeV Kr+ ions for amorphisation (Dc) of NaNbO3, SrTiO3 and two natural perovskites varies with composition and may decrease with Na content. The D,c values of 5 members of the solid solution series from SrTiO3 to La0.67TiO3 do not exhibit a simple relationship with vacancy content: In particular, Dc(Sr0 7La0.2TiO3) is extraordinarily high (>170 × 1014 ions cm−2) compared to those of the other (Sr, La)-perovskites in the series, which have Dc values between 2.4 and 11 × 1014 ions cm−2.
A Model for Dissolution of Cao-Sio2-H2O Gel at Ca/Si<1 by Considering Disordered Structure
Authors: Rahman Md. Mazibur , Nagasaki Shinya , Tanaka Satoru
Published: April 2011

A model for dissolution of CaO-SiO2-H2O (C-S-H) gel is proposed by considering a Margules type of non-ideal mixture of binary solid solutions at Ca/Si (C/S)<1. The Guggenheim and Prausnitz equations are used to represent the activities of model solids as a function of mole fractions. The Gibbs-Duhem equation, together with the activities of model solids, is applied to express the conditional solubility products of model solids as a function of C/S ratio. The determination of Guggenheim’s empirical parameters is performed with the geochemical code PHREEQE on experimental data. The dissolution model is developed by considering the effect of disorder in the structure of C-S-H gel at C/S<1. The solubility results predicted by the proposed model are comparable with experimental data. The study predicts some structural information of C-S-H gel at C/S<1.
Gallium Removal from Weapons-Grade Plutonium and Cerium Oxide Surrogate by a Thermal Technique
Authors: Park YoungSoo , Kolman David G. , Ziraffe Hisham , Haertling Carol , Butt Darryl P.
Published: April 2011

This paper describes the process of gallium removal from Ga2O3 -doped CeO2−x a surrogate for weapons-grade PuO2−x. Gallium is removed from the surrogate feedstock material using thermal techniques. An Ar-6% H, gas was used in order to reduce the oxide to gaseous Ga2O. Experimental results were shown in the temperature range of 600°C to 1200°C as a function of time and sample geometry. The results to date have shown that CeO2−x is a very good surrogate for PuO2−x.
Colloidal Agglomerates in Tank Sludge and Their Impact on Waste Processing
Authors: Tingey J. M. , Bunker B. C. , Graff G. L. , Keeper K. D. , Lea A. S. , Rector D. R.
Published: April 2011

Disposal of millions of gallons of existing radioactive wastes in underground storage tanks is a major remediation activity for the United States Department of Energy. These wastes include a substantial volume of insoluble sludges consisting of submicron colloidal particles. Processing these sludges under the proposed processing conditions presents unique challenges in retrieval transport, separation, and solidification of these waste streams. Depending on processing conditions, these colloidal particles can form agglomerated networks having high viscosities that could clog transfer lines or produce high volumes of low-density sediments that interfere with solid-liquid separations. Under different conditions, these particles can be dispersed to form very fine suspended particles that do not settle. Given the wide range of waste chemistries present at Department of Energy sites, it is impractical to measure the properties of all treatment procedures. Under the current research activities, the underlying principles of colloid chemistry and physics are being studied to predict and eventually control the physical properties of sludge suspensions and sediment layers in tank wastes and other waste processing streams. Proposed tank processing strategies include retrieval transport, and solid-liquid separations in basic (pH 10 to 14), high ionic strength (0.1 to 1.0 M) salt solutions. The effect of salt concentration, ionic strength, and salt composition on the physical properties such as viscosity, agglomerate size, and sedimentation of model suspensions containing mixtures of one or two of the major components found in actual wastes have been measured to understand how agglomeration influences processing. Property models developed from theory and experiment on these simple suspensions are then applied to explain the results obtained on actual wastes.
Alpha-Spectroscopy and Leach Testing of Synroc Doped With Actinide Elements
Authors: Hart K. P. , Vance E. R. , Stanojevic R. , Day R. A.
Published: April 2011

Synroc specimens doped individually with ˜ 1 wt% of Np or Pu have been studied, after leaching for >2,500 days, using α-spectroscopy and SEM. In the last leaching period, the leachant was replaced on either a daily or monthly basis. When the leachant was replaced on a daily basis the release rate of Np was similar to that measured when the samples were first leached, whereas for the monthly replacement the release rates of Np or Pu were about a factor of 20 to 30 lower than that for daily replacement. These findings agree generally with the results obtained from surface examination of the samples which showed that the thickness of actinidedepleted surface layer of anatase was 0.3 μm for monthly replacement but only 0.1 μm when the leachant was replaced frequently. Overall, the results suggest that release of actinides from Synroc are controlled by solubility limiting effects at the surface area to volume ratios employed in MCC-1 tests.
Demonstration of the defense waste processing facility vitrification process for tank 42 radioactive sludge – melter feed preparation
Authors: Fellinger T. L. , Bibler N. E. , Marshall K. M. , Crawford C. L. , Hay M. S.
Published: April 2011

The Defense Waste Processing Facility (DWPF), at the Savannah River Site (SRS), is processing and immobilizing the radioactive high level waste sludge at SRS into a durable borosilicate glass for final geological disposal. The DWPF is currently processing the second, million gallon batch of radioactive sludge. This second batch is primarily from Tank 42. Each time a new batch of radioactive sludge is to be processed by the DWPF, the process flowsheet is to be tested and demonstrated to ensure an acceptable melter feed and glass can be made. This demonstration was completed in the Shielded Cells Facility in the Savannah River Technology Center at SRS.

This paper presents the processing and offgas data, and compositional analyses obtained during the preparation of a melter feed for this demonstration. A second paper in this conference describes the properties of the glass produced from this feed. The demonstration used Tank 42 sludge slurry and the DWPF process control strategy for blending the sludge slurry with Frit 200 to make an acceptable melter feed. To prepare feed for the melter, the flowsheet requires that the radioactive sludge slurry be treated with nitric and formic acid to adjust rheology and remove mercury. During this step, hydrogen is formed from the decomposition of the formic acid. The acidified sludge slurry is then mixed with the prescribed amount of glass forming frit and evaporated to the proper weight percent solids to prepare feed to the melter. During this step hydrogen is also formed. Results indicate that the H2 generation rate is below the DWPF safety limits and an acceptable melter feed was produced.
Quantification of Water-Content of Simulated Nuclear Waste Glasses Using Nuclear Reaction Measurements
Authors: Sundaram S. K. , Thevuthasan S. , McCready D. E.
Published: April 2011

Water content of glasses is an important factor in glass manufacture. In the case of slurryfed processing of waste glass melters, the water content of waste glasses may change the waste form properties significantly. A reliable, non-contact and quantitative measurement technique, with a spatial resolution of about 2-mm was used to determine the concentration of dissolved water molecules in simulated nuclear waste glasses. The 19F resonant reaction was used. The 1H (19F, αγ)16O reaction requires a resonance energy that is available by use of a 6.471 MeV19F beam. A series of ten segments of slurry-fed, joule-heated, research-scale melter tests were conducted to determine the redox effects on processing of various feeds. Representative glass samples were collected at the middle of each segment. Their hydrogen contents were measured. The hydrogen content was correlated to the feed processing and redox of the glasses.
Low Temperature Vitrification of Radioiodine Using AgI-Ag2O-P2O5 Glass System
Authors: Fujihara H. , Murase T. , Nisli T. , Noshita K. , Yoshida T. , Matsuda M.
Published: April 2011

A new vitrification process for radioiodine has been developed for safe disposal of the spent iodine adsorbent generated from reprocessing off-gas systems. The proposed process consists of the following two steps: (1) separation of radioiodine as AgI from spent iodine adsorbent and (2) vitrification of the separated AgI with silver phosphate. An AgI-Ag2O-P2O5 glass system can homogeneously contain up to 60mol%AgI, which leads to high volume reduction efficiency (approximately 1/25). It also can vitrify the AgI without volatilization of iodine because of its low melting temperature (below 400°C). The leachabilities of iodine from simulated vitrified waste forms were evaluated by the MCC-1 static leach test in an anaerobically controlled glove box, which was purged by nitrogen gas with 3% hydrogen. The leaching behavior of the Agl-Ag2O-P2O5 glass system was influenced by the composition of the actual glass, type of leachant, and redox conditions. When the leach test was carried out using simulated ground wateroriginating from rainfall, the leach rate of iodine from 3AgI-Ag4P2O7 glass was 6×10−8g/cm2/d in the early period of the leach test at 35°C under the reducing condition, after which it decreased to 3×10−9g/cm2/d. This glass showed a lower leach rate in the simulated ground water originating from sea water or cement saturated water, since a precipitation layer was formed on the surface.
Thermochemical Models for Nuclear Waste Glass Subsystems – MgO-CaO and MgO-Al2O3
Authors: Besmann T. M. , Spear K. E. , Beahm E. C.
Published: April 2011

A relatively simple model, the associate species model, is being applied to nuclear waste glass compositions in order to accurately predict behavior and thermodynamic activities in the material. In the model, the glass is treated as a supercooled liquid, with the liquid species allowed to exist below their melting point. The approach requires an initial assembly of binary and ternary oxide liquid solution data that sufficiently reproduce the equilibrium phase diagrams. Two binary oxide subsystems, MgO-CaO and MgO-Al2O3, have been modeled and results compared to published phase diagrams. Computed activities of the glass constituent species are plotted as a function of composition at 1200°C.
Leaching of Spent Fuel in Cladding Segments and After Removal From Cladding
Authors: Oversby V. M.
Published: April 2011

In the early 1980s, tests of the leaching behavior of spent light water reactor fuel were conducted in Sweden by SKB and in the USA by the NNWSI Project. Both organizations used fuels with similar burnup, leaching solutions with similar chemical compositions, and conducted the tests at ambient hot cell temperature. Most of the test results were closely similar. The exception was in the recovery of actinide elements at the end of leaching cycles. In the NNWSI tests, the test vessels were stripped with nitric acid at the end of the leaching cycle. When the actinide inventories recovered in the original leaching cycle plus vessel rinse solutions were added to the amount recovered from the acid stripping, the relative abundances of uranium, plutonium, and other actinides were approximately the same as their inventories in the fuel samples. In the SKB tests, the materials recovered from stripping the leaching vessels were low in amount and interpreted to be fine fuel fragments. Only a few percent of the plutonium inventory that would correspond to the uranium recovered in solution was accounted for in the solution samples. To investigate the reasons for this difference in recovery of the actinides, a new test was undertaken using a fuel sample that had been leached for several years using the SKB methods. The fuel was removed from the cladding after a total of a bit more than two years of leaching inside the cladding. The bare fuel was then leached for several cycles using a geometry that simulated the NNWSI tests and a test procedure that was similar to that used in the NNWSI tests. The cladding from which the fuel was removed was also leached in a separate vessel in parallel with the bare fuel leaching. This paper presents the results of the test series and discusses the effects of specimen geometry on the mobility of actinides during leaching. During leaching of spent fuel inside the original cladding, a secondary phase containing Pu seems to form. This phase appears to be less easily dissolved than spent fuel itself.
Microstructure Evolution and Weathering Reactions of Synroc Samples Crystallized from CaZrTi2O7 melts: TEM/AEM investigation and geochemical modeling
Authors: Xu Huifang , Wang Yifeng
Published: April 2011

Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and analytical electron microscopy (AEM) studies have been conducted on samples crystallized from a melt with a composition of zirconolite {(Ca0.9Gd0.1)Zr(Ti1.9Al0.1)2, O7}. The formationof a whole suite of Synroc minerals (zirconia, ZrTiO4, zirconolite, perovskite, and rutile) has been observed. The formation of these minerals follows the crystallization sequence of Ti-bearing zirconia → ZrTiO4 phase → Zr-rich zirconolite → Zr-poor zirconolite →
rutile/perovskite. This sequence is induced by a fractional crystallization process, in which Zr-rich mineral phases tend to crystallize first, resulting in continuous depletion of Zr in melt. Consistent with this melt compositional evolution, Zr content in the zirconolite decreases from the area next to ZrTiO4 phase to the area next to rutile or perovskite. High-resolution TEM images show that there are no glassy phases at the grain boundary between zirconolite and perovskite. The fractional crystallization-induced textural heterogeneity may have a significant impact on the incorporation of radionuclides into crystalline phases and the resistance of radioniclides to leaching processes. Exsolution lamellae and multiple twinning result from the phase transition from tetragonal zirconia to monoclinic zirconia may decrease durability of the Synroc. Fast cooling of melt may produce more zirconolite phase and
relatively uniform texture. In general, however, a Synroc prepared by a through-melt method is less uniform in texture than that prepared by a through-sol-gel method. The reaction path calculation for the alteration of U-bearing zirconolite in an oxidizing fluid shows that zirconolite is first altered into a perovskite-like phase (CaZrO3), followed by rutile (TiO2 ), and U6+ -bearing phases of soddyite [(UO2)2SiO42H20] and haiweeite [Ca(U02)2Si6O15·5H2O].
The Effect f Waste Package and Engineered Barrier System Design:Options on Long-Term Waste Package Degradation Characteristics
Authors: Lee J. H. , Mon K. G. , Bullard B. E. , Longsine D. E.
Published: April 2011

Sensitivity analyses were conducted to evaluate the impact of a number of waste package and engineered barrier system (EBS) design options on long-term waste package degradation in the potential repository at Yucca Mountain, Nevada. The evaluations were performed by analyzing the waste package degradation in terms of the first breach and first pit-breach profiles with time. Results for each design option were assessed by comparing to results for the reference-case design. An important finding from the analyses is that the waste package lifetime could be extended substantially by limiting and delaying the contact of dripping water with the Alloy 22 corrosion resistant material (CRM) barrier. In the current model, this would keep the CRM barrier outside the conditions that makes the alloy susceptible to localized corrosion. Uncertainty in waste package degradation analysis would be reduced by improved understanding and characterization of the processes that create the local exposure condition on waste package.
Principal Component Analyses of Potential Repository Groundwaters
Authors: Sasamoto H. , Salter P. , Apted M. , Yuiv M.
Published: April 2011

The chemical composition of ambient groundwater for a geological, high level radioactive waste repository is of crucial significance to issues such as radioelement solubility limits, sorption, corrosion of the overpack, behavior of compacted clay buffers, and many other factors involved in repository safety assessment. At this time, there are no candidate repository sites established in Japan for the geological disposal of high-level radioactive waste, and only generic rock formations are under consideration. It is important that a small, but representative set of groundwater types be identified so that defensible models and data for generic repository performance assessment can be established. Over 15,000 separate analyses of Japanese groundwaters have been compiled into a data set for the purpose of evaluating the range of geochemical conditions for waste repositories in Japan. This paper demonstrates the use of a multivariate statistical analysis technique, principal component analysis (PCA), to derive a set of statistically based, representative groundwater categories from the multiple chemical components and temperature that characterize the deep Japanese groundwater analyses. PCA also can be used to guide the selection of groundwaters that could be used in scenario analyses of future geological events in Japan.
Radionuclide Sorption at Yucca Mountain, Nevada - A Demonstration of an Alternative Approach for Performance Assessment
Authors: Turner David R. , Pabalan Roberto T. , Prikryl James D. , Paul Bertetti F.
Published: April 2011

An approach is developed for including aspects of mechanistic models of radionuclide sorption into performance assessment (PA) calculations. Water chemistry data from the vicinity of Yucca Mountain (YM), Nevada are screened and used to calculate the ranges in key parameters that could exert control on radionuclide sorption behavior. Using a diffuse-layer surface complexation model, sorption parameters for Np(V) and U(VI) are calculated based on the chemistry of each water sample. Model results suggest that lognormal probability distribution functions (PDFs) of sorption parameters are appropriate for most of the samples; the total calculated range is almost five orders of magnitude for Np(V) sorption and nine orders of magnitude for U(VI) sorption, but most samples fall in a narrower range. Finally, statistical correlation between the calculated Np(V) and U(VI) sorption parameters can be included as input into PA sampling routines, so that the value selected for one radionuclide sorption parameter is conditioned by its statistical relationship to the others. The approaches outlined here can be adapted readily to current PA efforts, using site-specific information to provide geochemical constraints on PDFs for radionuclide transport parameters.
Kinetics of MgO Dissolution and Buffering of Fluids in the Waste Isolation Pilot Plant (Wipp) Repository
Authors: Monastra Valerie , Grandstaff D. E.
Published: April 2011

DOE has proposed to add MgO to backfill in the WIPP repository to stabilize pH and reduce gas pressures and spallation. Experiments show that MgO dissolution rates are proportional to surface area, nearly independent of pH, and decrease with increasing dissolved NaCl. The probable repository MgO dissolution rate will be ≤ 10 to 20 μmol m2 min−1. Simulations indicate that MgO dissolution will buffer pH and provide stabilizing cement; however, under highly reducing conditions MgO will not control total gas pressures. Adding oxidizing agents or materials containing oxidized species, for example anhydrite, to the backfill, might control gas pressures.
Modeling the Durability of Silica Additives Proposed to Protect Nuclear Waste Glass Packages in A Geological Repository
Authors: Minet Yves , Vernaz Étienne
Published: April 2011

Silica additives have been proposed to protect nuclear waste glass packages in deep geological repositories by presaturating silicon adsorption sites* in the media surrounding the package (engineered barrier, canister and overpack corrosion products). The durability of such additives was estimated using a one-dimensional dissolution-transport model developed at the CEA's Rhône Valley Research Center for the French reference glass in typical disposal sites. The silica additive would be placed between the wasteform and the overpack and thus dissolved before the waste glass. Its lifetime is proportional to Madd2(where Madd is the added silica mass) up to a critical value Mcrit at which it increases considerably (from 103 or 104 to 108 years) due to saturation of the adsorption sites. Excess silica consumption is then limited by the typically very low (3 × 10−3 l·m−2y−1 ) groundwater flow in the geological site. After consumption of the added silica, the evolution of the altered waste glass fraction is comparable to the evolution without added silica if Madd « Mcrit, but much lower if Madd ≈ Mcrit or Madd > Mcrit. These findings were confirmed by some experimental evidence, and establish that a few kilograms of added silica for a 400 kg package can considerably enhance the waste glass durability.
Modeled Near-Field Environment Porosity Modification due to Coupled Thermohydrologic and Geochemical Processes
Authors: Nitao John J. , Glassley William E.
Published: April 2011

Heat generated by waste packages in nuclear waste repositories can modify rock properties by instigating mineral dissolution and precipitation along hydrothermal flow pathways. Modeling this reactive transport requires coupling fluid flow to permeability changes resulting from dissolution and precipitation. Modification of the NUFT thermohydrologic (TH) code package to account for this coupling in a simplified geochemical system has been used to model the timedependent change in porosity, permeability, matrix and fracture saturation, and temperature in the vicinity of waste-emplacement drifts, using conditions anticipated for the potential Yucca Mountain repository. The results show dramatic porosity reduction approximately 10 m above emplacement drifts within a few hundred years. Most of this reduction is attributed to deposition of solute load at the boiling front, although some of it also results from decreasing temperature along the flow path. The actual distribution of the nearly sealed region is sensitive to the time-dependent characteristics of the thermal load imposed on the environment and suggests that the geometry of the sealed region can be engineered by managing the waste-emplacement strategy and schedule.
Determination of the Porosity, Permeability and Diffusivity of Rock in the Excavation-Disturbed Zone Around Full-Scale Deposition Holes Using the He-Gas Method
Authors: Autio J. , Timonen J. , Aaltonen T. , Laajalahti M. , Kuoppamäki K. , Maaranen J.
Published: April 2011

In a nuclear waste repository, rock in the excavation-disturbed zone adjacent to the walls of deposition holes for waste canisters is a potential pathway for the transport of corrosive agents and radionuclides. Three experimental holes the size of deposition holes in a KBS-3 type repository (depth 7.5 m and diameter 1.5 m) were bored in hard granitic rock in the Research Tunnel at Olkiluoto and the porosities, effective diffusivities and permeabilities of rock in the excavation-disturbed zone were determined in a direction parallel to the disturbed surface using He-gas methods. Permeability and diffusivity in a direction parallel to the rock schistosity was found to be clearly larger than in a direction perpendicular to it.
Corrosion Protection of Metallic Waste Packages Using Thermal Sprayed Ceramic Coatings
Authors: Wilfinger K. R. , Farmer J. C. , Hopper R. W. , Shell T. E.
Published: April 2011

Ceramic coated carbon steel coupons were corrosion tested in water with dissolved salts to simulate exposure to evaporation concentrated groundwater in an underground nuclear repository. Metallography revealed no corrosion at the ceramic metal interface of dense coatings, even though electrical measurements demonstrated that the coatings were slightly porous. Experimental results and a model to predict corrosion rates influenced by a porous ceramic coating and coating lifetimes are presented.
Zircaloy Corrosion in a Repository Environment
Authors: Christian Jerry D.
Published: April 2011

Assessments are made of the corrosion characteristics of spent nuclear fuel Zircaloy cladding in a Yucca mountain repository environment and the potential for the cladding to provide protection against radionuclide release following waste package failure. Considerations and assumptions includes a waste package life near 10,000 years and air-saturated water contacted with waste package corrosion product goethite, based on the near-field geochemical environment evaluated in the Yucca Mountain Viability Assessment [3]. Literature corrosion data (general, pitting, and localized crevice attack) are evaluated on the basis of these conditions and the expected chemical environments that can result on the surface of the fuel. General corrosion of Zircaloy is expected to be negligible and result in a lifetime of the SNF cladding of several hundred thousand years, approaching a million years. General surface pitting is not expected. Effects of crevice localized corrosion for periods beyond 10,000 years are uncertain and require modeling development and experimental characterization. Details of the evaluations that provide the basis for the conclusions are presented.
Sectrophotometric Investigation of U(VI) Chloride Complexation in the Naci/NaClO4 system
Authors: Paviet-Hartmann Patricia , Lin Mavis R.
Published: April 2011

Post closure radioactive release scenarios from geologic salt formation, such as the WIPP (Waste Isolation Pilot Plant) (USA) include hydrologic transport of radionuclides through a chloride saturated aquifer. Consequently, the understanding of actinide solution chemistry in brines is essential for modeling requiring accurate knowledge of the interaction between AnO22+ and chloride ions. Complexation constants of two U(VI) chloride species UO2Cl+ and UO2Cl20 have been intensively studied for about 40 years using different methods. However, large uncertainties reflect the general difficulty in determining accurate stability constants of weak complexes. In order to model the behavior of U(VI) in brines, we studied the formation of its chloride complexes by UV-Vis spectroscopy as a function of the NaCI concentration at 25 °C. The experiments were performed at constant ionic strength by varying the concentration ratio of NaCI and NaClO4. Deconvolution resulted in single component absorption sepectrum for UO2Cl+ and UO2Cl20. The apparent stability constants of UO2Cl+ and UO2Cl20 are determined at different ionic strengths. The experimental data are used to parameterize using the SIT approach.
He-Dilution to Increase Deposition Rate and Feedstock Utilization During the Growth of a-Si:H and a-SiGe:H Alloys
Authors: Middya A. R. , Wood G. , Lin G. H. , Carlson D. E.
Published: April 2011

We report on the development of helium diluted a-Si:H and a-SiGe:H solar cells with higher deposition rates and better feedstock utilization than devices made with hydrogen dilution. Both the initial and the stabilized efficiencies of the He-diluted single-junction aSi:H and a-SiGe:H cells are similar to those of hydrogen-diluted cells with state-of-the-art intrinsic materials. The total fabrication time for tandem cells has been reduced by 17% by using helium dilution without loss in initial and stabilized efficiency.
ESR Measurements of a-Si:H and a-Si0.5Ge0.5:H Films Under Solid-Phasecrystallization
Authors: Yun I. H. , Roh O. H. , Lee J.-K.
Published: April 2011

We have investigated the solid-phase crystallization of a-Si1-xGex:H (x=0 and 0.5) films by using electron spin resonance and x-ray diffraction. The films were deposited on Coming 1737 glass in a plasma-enhanced chemical vapor deposition system using SiH4 and GeH4 gases. The films were then annealed to be crystallized at 600°C. It was observed that, for the a-Si:H film, both the spin density and the g-value first increased with annealing time, and then rapidly decreased as the film was crystallized. For the a-Si0.5Ge0.5:H film, the Ge dangling bond spin density increased from 3 × 1018 cm-3 to 2 × 1019 cm3 for the first stage of annealing and then decreased to 3 × 1017 cm-3 after being crystallized; the Si dangling bond spin density just increased to about 2 × 1017 cm-3 and remained nearly constant for further annealing. It is thought that exodiffusion of hydrogen resulted in the increase of spin density in the beginning, and then some portions of amorphous components were converted into the crystalline phase by further annealing. And the keen correlation between the dependence of x-ray peak intensity and the dependence of Ge dangling bond spin density on the annealing time suggests that the Ge dangling bonds rather than Si dangling bonds play an important role in the crystallization of the Si0.5Ge0.5 film.
Hydrogen Transport in Phosphorus and Boron Dopedpolycrystalline Silicon
Authors: Nickel N. H. , Kaiser I.
Published: April 2011

Hydrogen diffusion in phosphorus and boron doped polycrystalline silicon was investigated by deuterium diffusion experiments. The presence of dopants enhances hydrogen diffusion. The effective diffusion coefficient Deff is thermally activated and the activation energy varies between 0.1 and 0.4 eV. This is accompanied by a variation of the diffusion prefactor by 12 orders of magnitude. Using the theoretical diffusion prefactor the actual energy EA was calculated from Deff.EA also depends strongly on the Fermi energy and exhibits a similar dependence as the formation energies of H+ and H- in single crystal silicon.
Characterization of Hydrogen in Hydrogenated Nano-Crystalline Silicon
Authors: Yamamoto K. , Itoh T. , Ushikoshi K. , Nonomura S. , Nitta S.
Published: April 2011

The characterization of hydrogen in hydrogenated nano-crystalline silicon (nc-Si:H) films with various crystallinity has been studied using FTIR absorption spectroscopy and gas effusion spectroscopy. In gas effusion spectrum of a nc-Si:H, three evolution peaks of H2 are found near 400, 500 and 600°C. From the deconvolution of the gas effusion spectra, the hydrogen concentration in amorphous region dose not depend on the volume fraction of crystalline. The surface density of hydrogen at the interface and in the grain boundaries of nano-crystalline grains decreases with the volume fraction of crystalline. The properties of hydrogen in nc-Si:H films are discussed with the results of FTIR absorption spectroscopy.
Structural Changes in a-Si:H Studied by X-Ray Photoemission Spectroscopy
Authors: Sheng Shuran , Sacher Edward , Yelon Arthur , Branz Howard M. , Masson Denis P.
Published: April 2011

X-ray irradiation-induced structural changes in undoped a-Si:H have been investigated in detail by X-ray photoemission spectroscopy (XPS). The Si2s and the Si2p peaks were found to shift simultaneously to lower bonding energies, by the same amount, with X-ray irradiation. The shifts are near saturation, at about 0.1 eV, after one hour of irradiation at the intensity used; they can be reversed almost completely, seemingly with an activation energy lower than that for the metastable changes in electronic properties (Staebler-Wronski effect). The present results suggest that essentially the whole Si network structure is affected by the X-ray irradiation.
Infrared Electroabsorption Spectra in Amorphous Silicon Solar Cells
Authors: Lyou J. H. , Schiff Eric A. , Hegedus Steven S. , Guha S. , Yang J.
Published: April 2011

We report measurements of the infrared spectrum detected by modulating the reverse-bias voltage across amorphous silicon pin solar cells and Schottky barrier diodes. We find a band with a peak energy of 0.8 eV. The existence of this band has not, to our knowledge, been reported previously. The strength of the infrared band depends linearly upon applied bias, as opposed to the quadratic dependence for interband electroabsorption in amorphous silicon.

The band's peak energy agrees fairly well with the known optical transition energies for dangling bond defects, but the linear dependence on bias and the magnitude of the signal are surprising if interpreted using an analogy to interband electroabsorption. A model based on absorption by defects near the n/i interface of the diodes accounts well for the infrared spectrum.
Stable Amorphous Silicon and Improved Microcrystalline Silicon by Photon-Assisted Electron Cyclotron Resonance Chemical Vapor Deposition
Authors: Song Young J. , Anderson Wayne A.
Published: April 2011

Amorphous (a-Si) and microcrystalline silicon (μc-Si) are widely used in photovoltaics and thin film transistors. These products suffer from instability and less than desired electrical properties. We have utilized photon-assisted electron cyclotron resonance chemical vapor deposition (PA-ECRCVD) resulting in great improvement in both areas. For example, PA-ECRCVD compared with conventional ECR-CVD gives carrier lifetime of 1.35 4ts compared to 0.17 μs, and photovoltaic solar cell efficiency of 10 % compared to 5.9 %. Moreover, the PAECRCVD cell only degraded to 9.8 % compared to the ECR-CVD cell degradation to 5.5 %, under long term exposure to tungsten lamp illumination. In addition, PA-ECRCVD gives much enhanced crystallinity in μc-Si as revealed by atomic force microscopy and Raman spectroscopy.
A Novel P-Type Nanocrystalline Si Buffer at the P/I Interface of A-SI Solar Cells for High Stabilized Efficiency
Authors: Lee Chang Hyun , Lim Koeng Su
Published: April 2011

We investigated the properties of a novel p-type nanocrystalline Si (p-nc-Si) prepared onto p-a-SiC and the effect of using the buffer with an energy bandgap over 1.9 eV at the p/i interface on the performance of p/i/n type amorphous silicon based solar cells. At the initial growth stage of the p-nc-Si onto p-a-SiC, Si nanocrystallites are proved to be formed in amorphous matrix within the thickness of less than 100Å. The open circuit voltage and the blue response of the cell were improved significantly by inserting the film at the p/i interface as a buffer as compared with the bufferless cell. We found from a numerical simulation using the Gummel-Sharfetter method that the buffering effect of the p-nc-Si is originated from the reduction of highly defective region with a short life-time in the vicinity of the p/i interface.
High Rates and Very Low Temperature Fabrication of Polycrystalline Silicon From Fluorinated Source GAS and Their Transport Properties
Authors: Kamiya T. , Nakahata K. , Ro K. , Fortmann C. M. , Shimizu I.
Published: April 2011

Low temperature (50-300°C) growth of polycrystalline silicon (poly-Si) by very high frequency (100MHz) glow-discharge plasma enhanced CVD using SiF4 and H2mixtures was studied. The poly-Si microstructure was strongly affected by the SiF4/H2 gas flow ratio. For example, either (220) or (400) preferentially oriented films were prepared by appropriate SiF4/H2 ratio selection. The addition of small SiH4 flows to the SiF4/H2 mixtures could be used to increase the growth rate while the SiF4/H2 continued to control the film structures such as preferential orientation. Highly crystalline films were grown at a growth rate of 0.52nm/s using SiF4/H2/SiH4 flow rates of 30/90/2.Osccm (respectively). However, at higher SiH4 flows amorphous films were deposited. Under the small SiF4/H2 ratio condition, highly crystallized poly-Si was grown at temperatures as low as 50°C. N/i/Pt Schottky diode solar cells were prepared using these poly-Si for both the n- and the i-layers. These solar cells exhibited good performance; for example, open circuit voltages over 0.32V. N-i-p solar cell results are very promising with 6.2% of conversion efficiency being achieved in the initial trials.
Structure and Optoelectronic Properties as a Function of Hydrogen Dilution of Micro-Crystalline Silicon Films Prepared by Hot Wire Chemical Vapor Deposition
Authors: Yue Guozhen , Lin Jing , Wang Qi , Han Daxing
Published: April 2011

Films prepared by hot wire CVD using H dilution ratio, R=H 2/SiH4, from 1 to 20 were studied by X-ray, Raman, PL, and conductivity measurements. We found that (a) when the dilution ratio reached R=3, the structure transition from amorphous to microcrystalline growth occured; meanwhile, PL spectrum showed a dual-peak at 1.3 and 1.0 eV; (b) the total intensity, band width, and peak position of the low energy PL band decreased with increasing H dilution; (c) both the Raman and PL measured from the transparent substrate side showed that initial growth tends to be amorphous and a portion of μc-Si was formed when R ≥ 5; and (d) the conductivity activation energy first decreased from 0.68 to 0.15 eV when the film transition from a- to μc-Si; then increased slightly with increasing μc-Si fraction. The results demonstrate that the variation of the H-dilution ratio has significant effects on both the film structures and the optoelectric properties.
High Quality Microcrystalline Silicon-Carbide Films Prepared by Photo-CVD Method Using Ethylene Gas as a Carbon Source
Authors: Myong Seung Yeop , Lee Hyung Kew , Yoon Euisik , Lim Koeng Su
Published: April 2011

Hydrogenated boron-doped microcrystalline silicon-carbide (p-μc-SiC:H) films were grown by a photo chemical vapor deposition (photo-CVD) method from silane (SiH4), hydrogen (H2), diborane (B2H6), and ethylene (C2H4) gases. Since the photo-CVD is a mild process (~10mW/cm2), we can avoid the ion damage of the film, which is inevitable during the deposition of μc-SiC:H employing conventional PECVD technique. A dark conductivity as high as 5 × 10-1 S/cm, together with an optical bandgap of 2 eV, was obtained by the C2H4 addition, which is the first approach in photo-CVD systems. From the Raman and FTIR spectra, it is clear that our p-μc-SiC:H films are made up of crystalline silicon grains embedded in amorphous silicon-carbide tissue. We investigate the role of the hydrogen dilution and ethylene addition on the electrical, optical, and structural properties of p-μc-SiC:H films.
Structured Polysilicon for Photonic Applications
Authors: Fleming J.G. , Lin Shawn-Yu
Published: April 2011

Three-dimensional photonic lattices have been fabricated in the infrared using a combination of advanced silicon processes. The structures display bandgaps centered at 12μ. and 1.55μ.
Quasi Drift and Quasi Diffusion: A Grain size Dependent Polysilicon Tft Model
Authors: Eccleston W.
Published: April 2011

Most of the models for Thin Film Transistors take no account of the way that electrons move across grain boundaries. The purpose of this work is to produce an analytical model, physically based, that takes account of both the change in height of the potential barriers that separate the grains, and the change of carrier density in the grains with gate voltage. The high current and subthreshold regions are treated. The model enables the change of field effect mobility with grain size and temperature to be determined. Two closed form expressions are provided which should be of value to both device and circuit designers, as well as providing insight to the physical processes occurring in such devices. They form a close analogy with the electrical characteristics of MOSFETs on crystalline silicon, diffusion and drift being replaced by quasi-diffusion and primary quasi-drift.
Application of Self-Aligned Amorphous SI Thin-Film Transistors
Authors: Lu J.P. , Mei P. , Chua C. , Ho J. , Wang Y. , Boyce J. B. , Lujan R.
Published: April 2011

We have successfully used self-aligned Amorphous Si Thin-Film Transistors, fabricated by a laser doping/annealing process, to construct dynamic shift register circuits, which can be used as gate-line drivers or in other peripheral circuits for flat-panel displays and imagers. Taking advantage of easily scaling down the TFT channel length in a self-aligned process, much higher circuit speeds can be achieved compared to that of circuits using conventional TFTs. We have successfully demonstrated a four-phase dynamic shift register, operating at a clock speed higher that 250 kHz (1 μs for each clock phase) built on 3 μm channel length TFTs. This new technology opens up possibilities for integrating peripheral circuits in flat-panel displays and imagers based on a-Si TFTs.
Signal Amplification and Leakage Current Suppression in Amorphous Silicon P-I-N Diodes by Field Profile Tailoring
Authors: Hong W.S. , Zhong F. , Mireshghi A. , Perez-Mendez V.
Published: April 2011

The performance of amorphous silicon p-i-n diodes as radiation detectors in terms of signal amplitude can be greatly improved when there is a built-in signal gain mechanism. We describe an avalanche gain mechanism which is achieved by introducing stacked intrinsic, p-type, and n-type layers into the diode structure. We replaced the intrinsic layer of the conventional p-i-n diode with i1-p-i2-n-i3 multilayers. The i2 layer (typically 1~3 μm) achieves an electric field > 106 V/cm, while maintaining the p-i interfaces to the metallic contact at electric fields < 7 × 104 V/cm, when the diode is fully depleted. For use in photo-diode applications the whole structure is less than 10 μm thick. Avalanche gains of 10~50 can be obtained when the diode is biased to ~500 V. Also, dividing the electrodes to strips of 2 μm width and 20 μm pitch reduced the leakage current up to an order of magnitude, and increased light transmission without creating inactive regions.
Noise of a-Si:H Pin Diode Pixels in Imagers at Different Operating Conditions
Authors: Blecher F. , Schneider B. , Sterzel J. , Böhm M.
Published: April 2011

The noise current spectral density of an a-Si:H pin diode can be calculated with experimentally determined flicker noise coefficients by superposition of the shot and flicker noise spectra of photocurrent and dark current. The dependence of the flicker noise current spectral density in pin diodes on the pixel area is calculated with our expansion of Hooge's law for flicker noise. We propose a new method for the calculation of dynamic range (DR) and signal-to-noise ratio (SNR) in pin diode pixels as a function of pixel area, dark current, photocurrent and the integration time of the imager. DR and SNR of the pin diodes are calculated for Thin Film on ASIC (TFA) image sensors.
Oriented Carbon Nanotube Growth for Field Emission Applications
Authors: Obraztsov A.N. , Pavlovsky I. , Volkov A.P. , Kuznetsov V.L. , Chuvilin A.L.
Published: April 2011

Oriented carbon nanotube films were grown using a method of chemical vapor deposition in hydrogen/methane plasma activated by glow discharge. The film phase composition and structural features were studied by Raman, SEM, TEM, and HRTEM techniques. Field emission properties of the films were examined to obtain I-V characteristics and the field emission site distribution. The I-V curves in Fowler-Nordheim coordinates were linear, that is typical for the field emission, with the threshold average field about 1.5 V/μm and the emission current density up to 50 mA/cm2 at the field of 5 V/μm. The emission site density reached 107 cm2 at the same value of electrtic field.
Field Emission Properties of Thin Molybdenum Carbide and Diamond Films Deposited by Dielectrophoresis
Authors: Rouse Ambrosio A. , Bernhard John B. , Sosa Edward D. , Golden David E.
Published: April 2011

Molybdenum carbide and diamond films were deposited at room temperature by dielectrophoresis on molybdenum foil and field emission tips. Films deposited on flat surfaces were characterized with XPS, UPS and SEM. Field emission Fowler-Nordheim current-voltage characteristics and electron energy distribution measurements were obtained using films deposited on field emission tips as part of a single aperture gated diode in a VG ESCALAB II system. Molybdenum trioxide was present after molybdenum carbide deposition at room temperature on all samples. At a temperature of about 650 °C, the dominant oxide changes to molybdenum dioxide. At about 800 °C, the oxygen content of the films significantly decreases and molybdenum carbide with significant graphite content remains. The UPS data for these samples were obtained at different annealing temperatures and a number of photon energies to determine photoelectric work functions for molybdenum trioxide, molybdenum dioxide and molybdenum carbide. The same kind of data was obtained for diamond deposited on molybdenum and molybdenum carbide coated foils and field emission tips.
High-Dielectric Constant Insulators and the AMLCD Process: Is Amorphous BST a Candidate?
Authors: Andy P. , Neumayer D. , Duncombe P. , Dimitrakopoulos C. , Libsch F. , Laibowitz R. , Grill A. , Wisnieff R.
Published: April 2011

As the active matrix flat panel market continues to grow, manufacturers are constantly looking for ways of gaining an advantage in the marketplace typically through increased performance, expanded functionality and/or reduced cost. The continuing trend of larger panel sizes and higher resolution poses interesting challenges for manufacturers in the areas of image quality and brightness. In particular, it has been clearly shown that as pixel dimensions shrink, relative crosstalk coupling coefficients increase while the area available for pixel electrodes and storage capacitors (Cδ,t) decreases, so that charge compensation drive schemes must be employed to eliminate visual artifacts. One approach to minimizing crosstalk and flicker is to increase Cδ,t, however this requires increased pixel area, usually at the expense of brightness due to smaller aperture ratio. Depending on design and process flow, it may be possible to replace the typical Cδ,t, insulator, SiNx:H, with a material of higher dielectric constant thereby vastly improving electrical performance without sacrificing brightness.