bluuebunny/crossref_metadata_2025 huggingface dataset:
The Solubility Limited Source Term for Cement - Conditioned Wastes: A Status Report
Authors: Glasser F. P.
Published: April 2011

Abstact

An important function of the cement backfill in a nuclear waste repository is to react with aqueous waste species and reduce their solubility. However, to quantify backfill performance it is first necessary to prove the existence and establish the nature of the chemical solubility controls. This can be done by characterising the solubility-limiting phases, determining their solubility and thermodynamic functions, and assessing their stability and persistence and solubility during backfill degradation. Much of the necessary data must be acquired experimentally. The title paper describes briefly the progresss of experimental work on selected species including nickel, chromium(III,VI), tin(IV) molybdenum(VI), uranium(VI), Ce(III,IV), thorium, actinide simulants (III, IV) and chloride. Data needs are assessed and although much experimental work remains to be done, methodologies have been developed which will expedite progress. The expectation of a more quantitative performance assessment of cement barriers is, therefore, attainable.
A model for Redox Control in A Cementitious Repository
Authors: Heath T. G. , Tweed C. J.
Published: April 2011

Calculations have been performed to investigate long timescale redox control in a cementitious deep waste repository, once there has been significant corrosion of the steel containers in the near field. The methodology considers reaction of the porewater with redoxsensitive solids. The solids were selected based on considerations of the repository concept, the waste inventory and the processing of the waste into a form suitable for disposal. A number of factors that may affect the redox potential have been considered. Amongst these are the form of the redox-controlling solids, the effect of changes in pH, the effect of sorption onto the cementitious backfill, and the effect of the association of dissolved radionuclides with complexants arising from the degradation of organic wastes. The calculations suggest that for the United Kingdom Nirex Limited (Nirex) disposal concept and inventory, the redox system in the near field of the repository would be controlled by the iron(II)/iron(III) redox couple. This would maintain the redox potential in the repository near field at a low value for timescales in excess of 1 million years.
Ion Selective Resins: Development and Applications for Nuclear Waste Management
Authors: Czerwinski K. R. , Draye M. , Favre-Réguillon A. , Foos J. , Guy A. , Lemaire M.
Published: April 2011

Organic based ion selective resins have some similar attributes: case of synthesis, high metal ion complexation ability, and flexibility for different nuclear waste management applications. For most chelating polymers, the ligand is deemed to be of primary importance for the interaction with the targeted metal ion. The role of the polymer matrix is usually ignored. For ion specific resins, the polymer structure is formed to a specific metal ion. Using the molecular imprinting technique, resins can be formed with functional groups and cavities for a target metal ion. Ion selective resins have been developed for the separation of Cs. The methods and concepts used for the development of the Cs specific resins have been applied to the development of selective resins for Eu (a trivalent actinide model). The resulting resins are characterized by FTIR spectroscopy, moisture regain, and ion exchange capacity. The incorporation of 8-hydroxyquinoline into the resin increases selectivity for Eu over La. The results for the Eu study indicate ion specific resins can be developed for the separation of trivalent actinides from nuclear waste.
Development of Metal Recycling Process from Decommissioned Nuclear Power Plants
Authors: Ishikura T. , Abe S. , Sakai H. , Takahashi J.
Published: April 2011

Dismantling and demolition of a nuclear power plant generates a large amount of metal waste. Recycling of such waste for resource is important because of the limited capacity of the disposal site. Authors have developed metal recycling processes. It is found that pyrometallurgical separation techniques can effectively separate nickel and cobalt, which contain main contributors of radioactivity, from iron and chromium.
Optimization of immobilization of strontium and uranium by the solid matrix
Authors: Raicevic S. , Plecas I. , Lalovic D. I. , Veljkovic V.
Published: April 2011

One of the basic physical parameters which defines: (1) the capacity of the solid matrix for the incorporation (sorption) of the impurity and (2) the stability of the solid matrix -impurity system, is the ion-ion interaction potential, representing the main term of the cohesive energy. Using this parameter determined in the frame of the pseudopotential theory and pseudoatomic approximation we have investigated the systems HAP-Sr and HAP-UO2. In analysis, the hydroxyapatite (HAP, Ca10(PO4)6(OH)2) was selected as a model solid matrix since insoluble phosphates, phosphate ceramic and apatite-like materials are candidates for the immobilization of radionuclides. The substitution of calcium atoms by different impurities in HAP can be presented by the formula: Ca10-xMx(PO4)6(OH)2, where 0 ≤ x ≤ 10 and M = Sr2+, UO22+. It has been shown that (1) Sr can be safely immobilized by HAP, natural apatites and phosphate rocks in the whole range of its concentrations, and (2) that HAP is not an appropriate solid-matrix for immobilization of UO22+ because its incorporation in the low concentration results in a decrease of the matrix stability, stimulating formation of the inhomogeneous structure containing isolated clusters of the UO2- apatite solid phase.
Optimization of heat-liberating batches for ash residue stabilization
Authors: Karlina O. K. , Varlackova G. A. , Ojovan M. I. , Tivansky V. M. , Dmitriev S. A.
Published: April 2011

Our study is concerned on development of a new stabilisation method of ash residue obtained after incineration of solid radioactive waste by means of heat liberating batches - compositions with redox properties. The results of optimisation of heat-liberating batches with the purpose of increasing process safety and obtained product quality of are discussed.
Zirconoiate corrosion in dilute acidic and basic fluids at 180-700° C and 50 MPa
Authors: Malmström J. , Reusser E. , Giere R. , Lumpkin G. R. , Düggelin M. , Mathys D. , Guggenheim R.
Published: April 2011

AbstractI

Corrosion of zirconolite in a closed system at elevated temperature and pressure for various fluid compositions has been investigated by several analytical techniques. Together with previous studies, the results indicate a weak corrosion below 250° C at 5 MPa, Above 400° C, secondary TiO2 phases crystallize in acidic and neutral fluids on the corroded surfaces and in near-surface pores, At 550° C and above, zirconolite starts to be replaced by perovskite and “calzirtite” in basic fluids. Our results indicate that zirconolite is reasonably durable in acidic. neutral and basic fluids up to 500° C and 50 MPa which corresponds to deep borehole conditions.
Corrosion Behavior of Pyrochlore-Rich Titanate Ceramics For Plutonium Disposition; Impurity Effects
Authors: Bakel A. J. , Zyryanov V. N. , Mertz C. J. , Buck E. C. , Chamberlain D. B.
Published: April 2011

Four titanate ceramics were characterized and tested. These ceramics were similar to those proposed for Pu disposition, with Ce as a surrogate for Pu. The baseline ceramic contained Ti, U, Ca, Hf, Gd, and Ce, and was made up of only four crystalline phases: pyrochlore, zirconolite, rutile, and brannerite. The three other ceramics contained different amounts of impurities that are expected in the feed. Impurities are defined as any element other than Ti, U, Ca, Hf, Gd, and Ce. The ceramics that contained impurities contained different phases than the baseline. The addition of impurities led to the absence of brannerite and the presence of amorphous silicate, Ca-Al-Ti, and perovskite phases. The results from 3 day, 90°C MCC-I tests with impurity ceramics were significantly different than the results from tests with the baseline ceramic. Overall, the addition of impurities to these titanate ceramics altered the phase assemblages, which in turn, affected the corrosion behavior.
High-temperature melter tests for vitrificationof bnfl high-level nuclear wastes
Authors: Matlack Keith S. , Muller Isabelle S. , Hojaji Hamid , Pegg Ian L. , Ahearn Catherine , Scales Charles R.
Published: April 2011

Vitrification has become the international method of choice for the immobilization of highlevel nuclear wastes (HLW) from both commercial and defense fuel reprocessing activities. However, there are clear economic incentives for "next-generation" melters that might achieve increased waste loadings and higher throughput rates. In addition, BNFL is investigating potential immobilization processes for projected high-zirconium waste streams that would be generated from advanced nuclear fuel reprocessing processes. One approach towards addressing both of these challenges is waste vitrification at temperatures above the limits that are imposed on all currently operating HLW melters (around 1100 -1200°C). A unique high-temperature joule-heated melter that has been operating at the Vitreous State Laboratory for several years was used to conduct a matrix of tests designed to address these issues. The tests used simulants of two reprocessing waste streams: (i) A blend of "Magnox" and "Oxide" wastes, similar to those currently being vitrified at BNFL's Sellafield site; and (ii) The projected high-zirconium wastes. Each type of waste was investigated both as a wet slurry feed using chemical additives and as a dry calcined waste feed mixed with glass frit. Three target glass compositions were designed for these tests in order to permit processing over a wide range of temperatures. Data were collected on processing rates, off-gas composition, glass product composition and product quality, as well as on general processing behavior. In all cases, glass production rates increased rapidly with processing temperature, typically doubling for every 100°C, and decreased with increasing water content of the feed. The glass production rates per unit melt surface area were significantly higher than those typically observed for hot-wall induction melters operating under comparable conditions. Interestingly, off-gas emissions were much greater for the slurry feeds than for the comparable solid feeds. The resulting glass products were characterized for melt viscosity and electrical conductivity, leachability by both soxhlet and PCT procedures, glass transition temperature, and the amounts and types of any secondary phases that were present.
Study of crystallization mechanisms in the french nuclear waste glass
Authors: Orlhac X. , Fillet C. , Phalippou J.
Published: April 2011

The development of glass materials for long-term storage of high-level waste implies determining the glass thermal stability and notably assessing the risk of devitrification. Previous studies of the French nuclear waste glass have identified the crystalline phases and the domains in which they form, and have shown that devitrification is minimal. Modeling the long-term crystallization behavior requires an investigation of the thermodynamic and kinetic mechanisms liable to induce crystallization during cooling. The first step in this approach is to determine the nucleation and growth curves for each of the component phases. Crystals including CaMoO4, CeO2 and ZnCr2O4 were identified in glasses heat-treated at temperatures between 630°C and 1110°C, and the time and thermal dependence of the CaMoO4 morphology were evaluated. The nucleation and growth kinetics of these phases were determined by optical microscopy and SEM, and the impact of impurities was addressed by studying two glasses, with and without platinoid elements. The results indicated enhanced nucleation kinetics in glass containing platinoid elements. No induction time was observed before permanent nucleation in either of the glasses, and rapid saturation of nucleation kinetics—synonymous with the depletion of active centers of nucleation— was detected after a few hours. Furthermore, similar growth kinetics were observed in both glasses. The nucleation and growth curves coincided for all the phases. Peak values were much higher for nucleation than for growth kinetics, confirming the need for a thorough investigation of the mechanisms occurring in and below the glass transition range, i.e. in the non-equilibrium state.
A Mechanistic Model of Spent Fuel Dissolution, Secondary Mineral Precipitation, and Np Release
Authors: Chen Y. , Siegmann E. , Mattie P. , McNeish J. , Sevougian S. D. , Andrews R.
Published: April 2011

A mechanistic spent fuel dissolution model has been developed, based on the generalreaction-transport code of AREST-CT. It considers the dissolution of spent fuel under flow conditions. The kinetic reactions of spent fuel dissolution and precipitation of schoepite, uranophane, soddyite, and Na-boltwoodite are included in the model. The results of model prediction are compared against the results of drip-tests that simulate the conditions that may occur in the Yucca Mountain Repository. Comparison shows that the modeling results match the laboratory observations very well and no contradiction has been found. It indicates that the model is a reasonably good representation of the real system.

After validation, the model was used to investigate the release rate of Np from the dissolution of secondary uranyl minerals by examining various degrees of Np incorporation into secondary uranyl minerals. The predicted Np concentration in the aqueous phase is 3 orders of magnitude lower than the upper-bound of the Np solubility range currently used in DOE performance assessment analyses. It suggests that the Np solubility range currently used is too conservative and could be replaced with more realistic values.
Investigations of the Nizhnekanskiy Granitoid Massif (Middle Siberia, Russia) as a Promising Territory for Deep Geological Disposal of HLW: Results of Pre-Exploration Stages of the Work
Authors: Anderson E. B. , Shabalev S. I. , Savonenkov V. G , Lyubtseva E. F. , Rogozin Yu. M.
Published: April 2011

To decide the final fate of high-level radioactive wastes that are in Krasnoyarsk Land, Russia, and, most importantly the wastes from a weapons complex at the Mining-Chemical Combine (MCC, Krasnoyarsk-26), suitable sites for geological disposal of the wastes are being sought in Middle Siberia. The ultimate goal is to select one site for development of a repository for these wastes. As a result of complex geological-geophysical work completed to date, the two most promising sites are in the northern part of the Nizhnekanskiy granitoid massif (the Yenisei Ridge). These sites are approximately 25 km from the MCC. Based on geotectonic analysis and electrical surveying data, these sites are composed of low-permeability biotite granite that extends to relatively great depth. At present, both sites are undergoing more detailed studies to define the most suitable one for detailed geological exploration. Radiogeochemical studies showed that granitoid rock samples from the massif exhibited high retention for major long-lived radionuclides in the wastes. The general results of the investigations are presented.
Synthesis of Garnet/Perovskite-Based Ceramic for the Immobilization of Pu-Residue Wastes
Authors: Burakov B. E. , Anderson E. B. , Knecht D. A. , Zamoryanskaya M. A. , Strykanova E. E. , Yagovkina M. A.
Published: April 2011

The use of garnet/perovskite-based ceramic, with formula type (Y, Gd,..)3(AI, Ga,..)5O1212/(Y, Gd,..)(A1, Ga,..)O3, was tested for immobilizing plutonium residue wastes. Pu residue wastes originate from nuclear weapons production and can contain more than 50% of impurities including such elements as Am, Al, Mg, Ga, Fe, K, La, Na, Mo, Nd, Si, Ta, Ce, Ba, B, W, Zn, Zr, C and Cl. While for some of these residues, direct conversion to typical glass or ceramic forms may be difficult, ceramic forms based on durable actinide host-phases are preferred for Pu, Am and other actinides immobilization. Garnet/perovskite crystalline host-phases are chemically and mechanically durable and desirable for the incorporation of Pu and most of the impurity elements in the Pu residue wastes in the lattices of host-phases in the form of solid solutions. Experiments on the synthesis of garnet/perovskite ceramic samples were carried out using melting in air at temperatures from 1300°C (for samples doped with 10 wt.% Pu residue waste simulant) to 2000°C (for samples doped with 10 wt.% Ce or U). Samples were studied by XRD, SEM and cathodoluminescence techniques. It was found that the garnet phase can incorporate upto 6 wt.% Ce and up to 4.0-5.5 wt.% U, which is correlated with the increase of Ga content and decrease of Al content in the melt. In one of the features of the melt, the perovskite phase formation substitutes for the formation of garnet. The capacity of the perovskite lattice to accommodate Ce and U is higher than the capacity of garnet, reaching about 8 and 7 wt.%, respectively. It was shown that cathodoluminescence can be effectively used to determine the valence state of Ce and U, an important step to optimize the starting precursor preparation. Incase of U4+in the melt, the charge-compensating elements (Sn2+, Ca2+...) are needed to successfully incorporate U in the garnet lattice.
Numerical Simulation of the Interaction Between Granitic Groundwater, Engineered Clay Barrier and Iron Canister
Authors: Trotignon L. , Faure M.-H. , Cranga M. , Peycelon H.
Published: April 2011

The geochemical evolution of a clay engineered barrier subjected simultaneously to granitic groundwater on its external boundary and to a steady flux of iron released by canister corrosion on the other boundary is simulated for time periods up to 1000 y. A multisite ion-exchange model combined with dissolution/precipitation of accessory minerals (calcite, gypsum, quartz, goethite, pyrite, pyrrhotite, iron silicate, magnetite) is used to describe the geochemical evolution of the clay barrier. Diffusive transport of aqueous species and dissolved gases is simulated in a one dimension space. Retroaction of chemical reactions on the corrosion process of the canister is not included in the model. The temperature is assumed to be constant (25°C). Calculations are performed with the IMPACT geochemical coupled code. Two main scenarios are considered: (i) the reduction of sulfates (gypsum) is kinetically hindered, (ii) the reduction of sulfates is possible and leads to the precipitation of sulfides. Main results obtained are dealing with H2(gas) generation and control, pH transient and buffering effect of the clay barrier system. These simulations show that the hydrogen partial pressure in the clay barrier could possibly stay for long periods of time at a very low level due to interactions with mineral buffers or sinks. In the sulfate reduction scenario, a transient pH, xcursion is predicted at the vicinity of the canister. Future developpements of this work include the taking into account of a temperature gradient. Improvement of the geochemical model and reaction pathway is also necessary and will be done with the guidance of on going experimental work.
Modelling of the Interaction Between an Engineered Clay Barrier and Concrete Structures in a Deep Storage Vault
Authors: Trotignon L. , Peycelon H. , Cranga M. , Adenot F.
Published: April 2011

The degradation of concrete structures in a storage vault will release an alkaline plume influencing the geochemical evolution of clay engineered barriers or plugs. The studied configuration has a representative scale and is composed of a rectangular clay barrier interfaced on one side with a cement block and on the other side with an external granitic field. A clay model including major and accessory minerals is used; cement material is supposed to be made of portlandite and CSH minerals. The clay barrier and the cement block are assumed to be initially saturated and in equilibrium with their respective interstitial water. Diffusive transport of aqueous species coupled to chemistry is simulated in a one dimension space up to 10000 y. The feed-back of the porosity evolution on the transport properties is not taken into account. The temperature is supposed to be constant (25°C). Calculations are performed with the TRIO-EF coupled transport-chemistry code.

The main results deal with the evolution of the pH profile across the clay barrier, the dissolution of clay minerals and the precipitation of new solid phases in the clay such as albite, gibbsite and CSH. These simulations show that the pH excursion includes only a minor part of the clay barrier at 10 000 y and that the CSH precipitation retards the alkaline plume progression. The appearance of local accumulations of newly precipitated minerals might reduce strongly the porosity. Future developpements of this work include: (i) the completion of the geochemical model by introducing other minerals in concrete (other CSH phases and sulfo-aluminates) and ion exchange sites within the clay, (ii) the implementation of a precipitation/dissolution model in order to describe on a realistic way the porosity profile evolution and possible clogging phenomena.
Reaction of Montmorillonite in Alkaline Solution at 60°C, 90°C, 120°C and 180°C
Authors: Amaya Takayuki , Shimojo Mikio , Fujihara Hiroshi , Yokoyama Katsuhiko
Published: April 2011

The reaction of montmorillonite was investigated. Three kinds of bentonites with different montmorillonite composition were mixed with 0.3M NaOH solution and 0.3M Ca(OH)2 slurry. They were immersed at 60°C, 90°C, 120°C and 180°C for one month, three months and six months. The concentrations of the soluble ions were measured and the bentonites were analyzed quantitatively after the immersion. 50% of the montmorillonite was reacted within two weeks at greater than 90°C. Montmorillonite reacts less when mixed with Si-minerals. It extensively reacted in 0.3M Ca(OH)2 slurry. These results suggest that the reaction mechanism of the montmorillonite in alkaline solution was dominantly Si dissolution, and would decrease by controlling the concentration of Si ion. The cement / bentonite system under Si saturated conditions is discussed.
Time Scales for Dissolution of Calcite Fracture Fillings and Implications for Saturated Zone Radionuclide Transport at Yucca Mountain, Nevada
Authors: Winterle James R. , Murphy William M.
Published: April 2011

An analysis was performed to estimate time scales for dissolution of calcite fracture fillings in the fractured tuff aquifer that underlies Yucca Mountain (YM), Nevada, where groundwater is chemically undersaturated with respect to calcite. The impetus for this analysis originates from speculation that undissolved calcite in the saturated zone is evidence for limited diffusive exchange between fracture and matrix waters. Assuming that matrix diffusion is the rate-limiting process, the time scale for dissolution of calcite fracture fillings depends on the amount of calcite initially deposited, the distance between flowing fractures, the degree of chemical disequilibrium, and the rate of diffusion. Assuming geochemistry of J-13 well water in free-flowing fractures, estimated time scales for complete dissolution of matrix-entrapped calcite range from about 104yr for a 2 mm-thick deposit located I m from a flowing fracture, to over 107 yr for a 2 cm-thick deposit located 100 m from a flowing fracture. We conclude that, given the geochemical and hydrologic characteristics observed at YM, the persistence of calcite minerals over geologic time scales in aquifers where flowing water is under-saturated with calcite does not necessarily preclude matrix diffusion as a dilution mechanism. However, our model suggests that the effective spacing between flowing fractures may be large enough to diminish the overall benefit of matrix diffusion to proposed high-level waste repository performance.
Passivation Behavior of Mild Steel Used for Nuclear:Waste Disposal Package
Authors: Fukaya Yuichi , Akashi Masatsune
Published: April 2011

Mild steel has been investigated as one of the overpack materials for geological disposal of high-level nuclear waste. The present paper discussed the passivation behavior of mild steel using an electrochemical approach to evaluate the critical condition for passivation. The critical current density for passivation, icris, of mild steel in 0.1 mol /L [HCO3 + CO32−] solution at room temperature was found to decrease with increasing pH of the solution. The depassivation pH, pHd, at room temperature was determined to be about 7.5 by comparisons between icris and the diffusion-limiting current density, iL02, for dissolved oxygen of 8.1 ppm in equilibrium with atmospheric oxygen. The anodic current density of mild steel in the solution of pH = 8 at 90°C was also measured and was found to decrease rapidly with time, leading to the absence of a critical current density for passivation. Thus, mild steel can be self-passivated in the system. An empirically determined corrosion phase diagram is provided for mild steel in 0.1 mol/L [HCO3− + CO32−] solution at 90°C, based on these findings and some published data.
A Modeling Study of General Corrosion of Copper Overpack for Geological Isolation of High-Level Radioactive Waste
Authors: Honda A. , Taniguchi N , Ishikawa H. , Kawasaki M.
Published: April 2011

This paper describes a modeling study for general corrosion of copper which is a candidate material for high-level radioactive waste overpacks. The model is a mixed-potential model combined with diffusive transport of reactants and reaction products. The rest potential and corrosion rate of copper in aerated solution were measured while controlling the thickness of a diffusive solution layer on the copper surface using a rotating-disk electrode. Experimental data were used for validation of the model.
Synthesis and Characterization of CS-Bearing Apatites
Authors: Senamaud N. , Bernache-Assollant D. , Carpena J. , Pin C.
Published: April 2011

The French Act, dated 30 December 1991, related to “research concerning the elimination of radioactive wastes”, defines the research programs including the study of new conditioning processes and long-term surface storage of these wastes. The research of new conditioning processes mainly concerns separated long-lived radionuclides.
Corrosion Testing of Stainless Steel-Zirconium:Metal Waste Forms
Authors: Abraham D. P. , Simpson L. J. , Devries M. J. , Mcdeavitt S. M.
Published: April 2011

Stainless steel-zirconium (SS-Zr) alloys have been developed as waste forms for the disposal of metallic waste generated during the electrometallurgical treatment of spent nuclear fuel. The waste forms incorporate irradiated cladding hulls, components of the alloy fuel, noble metal fission products, and actinide elements. The baseline waste form is a stainless steel- 15 wt% zirconium (SS-15Zr) alloy. This article presents microstructures and some of the corrosion studies being conducted on the waste form alloys. Electrochemical corrosio n, immersion corrosion, and vapor hydration tests have been performed on various alloy compositions to evaluate corrosion behavior and resistance to selective leaching of simulated fission products. The SS-Zr waste forms immobilize and retain fission products very effectively and show potential for acceptance as high-level nuclear waste forms.
High-Rate Growth of Stable a-Si:H
Authors: Takagi T. , Hayashi R. , Payne A. , Futako W. , Nishimoto T. , Takai M. , Kondo M. , Matsuda A.
Published: April 2011

Correlation between the gas phase species in silane plasma measured by mass spectrometry and the properties of hydrogenated amorphous silicon (a-Si:H) films deposited by plasma enhanced chemical vapour deposition (PECVD) has been investigated. We have especially been interested in the higher-order silane related species in the plasma, whose contribution to the film growth is considered to be the cause of light-induced degradation in the film quality, especially at high growth rate. In this study, we varied excitation frequency, gas pressure and power density to vary the growth rates of a-Si:H films ranging from 2 Å/s to 20 Å/s.

Molecular density ratio of trisilane, representative of higher silane related radicals, to monosilane has shown a clear correspondence to the fill factor after light soaking of Schottky cells fabricated on the resulting films.
Analysis of Plasma Properties and Deposition of Amorphous Silicon Alloy Solar Cells Using Very High Frequency Glow Discharge
Authors: Yan B. , Yang J. , Guha S. , Gallagher A.
Published: April 2011

Positive ionic energy distributions in modified very-high-frequency (MVHF) and radio frequency (RF) glow discharges were measured using a retarding field analyzer. The ionic energy distribution for H2 plasma with 75 MHz excitation at a pressure of 0.1 torr has a peak at 22 eV with a half-width of about 6 eV. However, with 13.56 MHz excitation, the peak appears at 37 eV with a much broader half-width of 18 eV. The introduction of SiH4 to the plasma shifts the distribution to lower energy. Increasing the pressure not only shifts the distribution to lower energy but also broadens the distribution. In addition, the ionic current intensity to the substrate is about five times higher for MVHF plasma than for RF plasma. In order to study the effect of ion bombardment, the deposition of a-Si alloy solar cells using MVHF was investigated in detail at different pressures and external biases. Lowering the pressure and negatively biasing the substrate increases ion bombardment energy and results in a deterioration of cell performance. It indicates that ion bombardment is not beneficial for making solar cells using MVHF. By optimizing the deposition conditions, a 10.8% initial efficiency of a-Si/a-SiGe/SiGe triple-junction solar cell was achieved at a deposition rate of 0.6 nm/sec.
Performance of a-Si-H Solar Cells at Higher Growth Rates
Authors: Ganguly G. , Lin G. , Chen L.F. , He M. , Wood G. , Carlson D. , Arya R.
Published: April 2011

We have studied the effects of external growth parameters during the deposition of the i-layers of a-Si p-i-n solar cells using dc plasma decomposition of silane-hydrogen mixtures at growth rates of up to 3A/s. The loss of initial performance with increasing growth rate is mainly due to a loss of short-circuit current. The relative degradation of efficiency upon extended light soaking also increases with growth rate, and is mainly due to a decrease in the fill factor. Systematic comparisons of the performance and its degradation with changes in growth conditions reveal that these two components of the total degradation have distinct origins.
Preparation of Triple-Junction A-Si:H NIP Based Solar Cells at Deposition Rates of 10 Å/s using a Very High Frequency Technique
Authors: Jones S.J. , Deng X. , Liu T. , Izu M.
Published: April 2011

In an effort to find an alternative deposition method to the standard low deposition rate 13.56 M-z PECVD technique, the feasibility of using a 70 MiHz rf plasma frequency to prepare a-Si:H based i-layer materials at high rates for nip based triple-junction solar cells has been tested. As a prelude to multi-junction cell fabrication, the deposition conditions used to make single-junction a-Si:H and a-SiGe:H cells using this Very High Frequency (VHF) method have been varied to optimize the material quality and the cell efficiencies. It was found that the efficiencies and the light stability for both a-Si:H and a-SiGe:H single-junction cells remain relatively constant as the i-layer deposition rate is varied from 1 to 10 Å/s. Also these stable efficiencies are similar to those for cells made at low deposition rates (1 Å/s) using the standard 13.56 MHz PECVD technique and the same deposition equipment. Using the knowledge obtained in the fabrication of the single-junction devices, a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cells have been fabricated with all of the i-layers prepared using the VHF technique and deposition rates near 10 Å/s. Thin doped layers for these devices were prepared using the standard 13.56 MIHz rf frequency and deposition rates near 1 Å/s. Pre-light soaked efficiencies of greater than 10% have been obtained for these cells prepared at the high rates. In addition, after 600 hrs. of light soaking under white light conditions, the cell efficiencies degraded by only 10-13%, values similar to the degree of degradation for high efficiency triple-junction cells made by the standard 13.56 MiHz method using i-layer deposition rates near 1 Å/s. Thus, use of this VHF method in the production of large area a-Si:H based multi-junction solar modules will allow for higher i-layer deposition rates, higher module throughput and reduced module cost.
Low-Temperature Preparation of Poly-Si Thin-Films Having Giant Grains
Authors: Yeh Wen-Chang , Matsumura Masakiyo
Published: April 2011

A low heat-capacitance substrate has been introduced to lateral grain-growth for Si thin-films. Grains as large as 13μm could be fabricated by irradiation of a single shot of an excimer-laser light pulse with intensity gradient. The film has an orientation in the (110) direction, different from (111) for the conventional excimer-laser-annealed film, and is applicable to a seed layer for solid-phase epitaxial-growth aiming at solar cell application.
Development of Ultra-Clean Plasma Deposition Process
Authors: Kamei Toshihiro , Matsuda Akihisa
Published: April 2011

We have developed a new type of ultra-high vacuum plasma-enhanced chemical vapor deposition (UHV/PECVD) system. According to high sensitivity secondary ion mass spectrometry, device quality hydrogenated amorphous silicon (a-Si:H) films deposited at 250°C at a deposition rate of 1 Å/s contains 1015 cm-3 of O, 1015 cm-3 of C, and 1014 cm-3 of N impurities, while low defect hydrogenated microcrystalline silicon (μc-Si:H) films deposited at 200°C at a very low rate of 0.1 Å/s include 1016 cm-3 of O, 1015 cm-3 of C and 1016 cm-3 of N. These are the lowest concentrations of atmospheric contaminants for these kinds of materials observed so far. The essential features of the present UHV/PECVD system are an extremely low outgassing rate of 8×10-9 Torr·s, extremely low partial pressure of contaminant gas species <10-12 Torn, and purification of feed gas SiH4 at “point of use”. These efforts are quite important not only for clarifying the microscopic mechanism of photo-induced degradation in a-Si:H, but also for enlarging the crystalline grain size in μc-Si:H. μc-Si:H with a grain size of ≍1000 Å as determined by Scherrer's formula can be obtained at the higher rate of 1.5 Å/s by utilizing a VHF (Very High Frequency) plasma. The specific origins of impurities in the films are also discussed.
Lateral Solid Phase Crystallization of Amorphous Silicon Under High Pressure
Authors: Lee Seung-Mahn , Singh Rajiv K.
Published: April 2011

We have investigated a novel surface-seeded crystallization technique at low processing temperatures (≤ 550°C) and high pressures (10MPa~25MPa) using polished polycrystalline diamond seeds. By controlling the high pressure, the nucleation and growth of silicon can be controlled to obtain improved quality silicon films on amorphous substrates at low temperatures. Depending on the annealing temperature and applied pressure, the orientation of crystallized silicon thin films varies as seen by x-ray diffraction and transmission electron microscopy results. In addition, crystallization of amorphous silicon thin films has effect on their roughness.
Spectroscopic Investigations of Crystallinity and Electronic-Structural Transitions Due to Solid Phase Crystallization of Amorphous Si1-xGex
Authors: Yamaguchi S. , Sugh N. , Nakagawa K. , Miyao M.
Published: April 2011

Solid phase crystallization of Si1-xGex, have been investigated by means of change in crystallinity and crystallization temperature using the ellipsometric spectroscopy (2-5 eV). Dispersion analysis on the spectra revealed that the Si1-xGex alloys show both abrupt increase and rapid saturation of the crystallinities according to the structural transitions. The saturated values of crystallinities did not reach those of single crystals in all samples and rapidly decreased with increase of Ge concentration. Ion-implanted amorphous SilxGe, exhibited much lower activation energies of re-crystallization than those of the deposited amorphous samples. Furthermore, their crystallization temperature were strongly depend on the species of implanted ions such as As+ and BF2+. It was found that the crystallization velocity of the As+-implanted Si1-xGex was estimated as 4 times faster than that of the BF2+-implanted Si1-xGex. Itinerant property of Ge in Si may give rise to the decrease of the crystallinities as well as the decrease of the activation energy of solid phase crystallization which may be strongly affected by the catalytic effects of the implanted impurities.
Comparative Study of Hydrogen Diffusion in Hot-Wire and Glow-Discharge-Deposited a-Si:H
Authors: Shinar J. , Shinar R. , Junge K. E. , Iwaniczko E. , Mahan A. H. , Crandall R. S. , Branz H. M.
Published: April 2011

Long-range atomic H motion in hot-wire deposited (HW) a-Si:H is compared directly to that in glow-discharge deposited (GD) a-Si:H by monitoring the deuterium secondary ion mass spectrometry (DSLMS) profiles in [GD a-Si:H]/[GD a-Si:(H,D)]/[HW a-Si:H] multilayers vs annealing temperature and time. While the profiles in the GD layer are in excellent agreement with complementary error-function behavior and previous studies, the profiles in the HW layer suggest that the multiple-trapping motion of the H and D atoms is much slower, possibly due to an interface layer of defects. However, an exponential “tail” of D atoms extends deep into the HW layer, probably due to a long diffusion length of mobile D atoms, consistent with the established release times of H and D from the GD layer and H loss typical during growth of HW films. The results are also discussed in terms of the H exchange model and compared to previous NMIR studies of HW a-Si:H, which suggest that most of the hydrogen in the HW layer is concentrated in H-rich clusters dispersed in a network of very low H content.
The Formation and Behavior of Particles in Silane Discharges
Authors: Gallagher Alan
Published: April 2011

Particle growth in silane RF discharges, and the incorporation of particles into hydrogenated-amorphous-silicon (a-Si:H) devices is described. These particles have a structure similar to a-Si:H, but their incorporation into the device is believed to yield harmful voids and interfaces. Measurements of particle density and growth in a silane RF plasma, for particle diameters of 8-50 nm, are described. This particle growth rate is very rapid, and decreases in density during the growth indicate a major flux of these size particles to the substrate. Particle densities are a very strong function of pressure, film growth rate and electrode gap, increasing orders of magnitude for small increases in each parameter. A full plasma- chemistry model for particle growth from SiHm radicals and ions has been developed, and is outlined. It yields particle densities and growth rates, as a function of plasma parameters, which are in qualitative agreement with the data. It also indicates that, in addition to the diameter >2 nm particles that have been observed in films, a very large flux of SixH,, molecular radicals with × >1 also incorporate into the film. It appears that these large radicals yield more than 1% of the film for typical device-deposition conditions, so this may have a serious effect on device properties.
Structural Origin of Bulk Molecular Hydrogen in Hydrogenated Amorphous Silicon
Authors: Liu Xiao , Pohl R.O. , Crandall R.S.
Published: April 2011

The elastic anomaly observed previously at the triple point of bulk molecular hydrogen in hydrogenated amorphous silicon films prepared by hot-wire chemical-vapor deposition has also been observed in deuterated films at the triple point of D2. The origin of this anomaly has now been traced to bubbles formed at the crystalline-amorphous interface. An upper limit of the pressure in these bubbles at their formation temperature, 440°C, has been estimated to be 11 MPa, and is suggested to be a measure of the bonding strength between film and substrate at that temperature. Bubble formation after heat treatment at 400°C has also been observed in films prepared by plasma-enhanced chemical-vapor deposition. The internal friction anomalies resemble those observed previously in cold-worked hydrogenated iron where they have been interpreted through plastic deformation of solid hydrogen in voids.
Effects of Chlorine on Dopant Activation in a-Si:H
Authors: Payne Adam M. , Wagner Sigurd
Published: April 2011

The incorporation of chlorine has a significant effect on the dark conductivity of doped and undoped hydrogenated amorphous silicon (a-Si:H). The dark conductivity of a-Si:H films deposited from dichlorosilane (SiCl2H2) and SiH4, and doped with diborane, increases by as much as a factor of 100 over the usual a-Si:H,B films deposited without SiCl2H2. The effect is observed at gas phase concentrations of diborane ranging from 0.006% to 0.5%, and for both DC and RF plasma depositions, although it is more noticeable for the DC discharge. An increase in dark conductivity is also observed in B doped a-Si,C:H films deposited with dichlorosilane, albeit coupled with a change in the Tauc gap. Chlorine reduces the conductivity of undoped and phosphorus doped a-Si:H films. Undoped a-Si:H films deposited from SiCl2H2 and SiH4 have a dark conductivity of ~1. 10-12 S.cm-1, which is an order of magnitude lower than films deposited from pure SiH4. We discuss several alternatives for the mechanism of chlorine enhanced or reduced dopant activation. We have made solar cells using chlorinated p-type a-SiC:H films as the p-layers.
Microcrystalline Silicon - Relation of Transport Properties and Microstructure
Authors: Kočka J. , Fejfar A. , Vorlíček V. , Stuchlíková H. , Stuchlík J.
Published: April 2011

Understanding of transport in hydrogenated microcrystalline silicon (μc-Si:H) is difficult due to its complicated microstructure (grains, grain boundaries, amorphous tissue). μc-Si:H layers often exhibit preferential orientation leading to transport anisotropy. Furthermore, specific μc-Si:H growth features lead to the thickness dependence of the structure and properties.

μc-Si:H incubation layer was studied by AFM with conductive cantilever measuring simultaneously morphology and local conductivity maps with submicron resolution. Clear identification of Si crystallites (with size of few tens of nanometers) is demonstrated. The crystalline fraction at the surface may be easily evaluated.

For the charge collection in solar cells we need to study transport perpendicular to the substrate. Measurement of frequency spectra of A.C. conductivity is introduced as a new tool which can exclude the influence of contact barriers in sandwich geometry and can be used for finding the “true” conductivity perpendicular to the substrate. Using this technique transport anisotropy in some μc-Si:H samples was clearly demonstrated.

Finally, it is shown how the transport properties change with growing μc-Si:H thickness and how these changes correlate with the structure observed by AFM.
Stress in Hydrogenated Microcrystalline Silicon Thin Films
Authors: Peiró D. , Voz C. , Bertomeu J. , Andreu J. , Martínez E. , Esteve J.
Published: April 2011

Hydrogenated microcrystalline silicon films have been obtained by hot-wire chemical vapor deposition (HWCVD) in a silane and hydrogen mixture at low pressure (<5 × 10-2 mbar). The structure of the samples and the residual stress were characterised by X- ray diffraction (XRD). Raman spectroscopy was used to estimate the volume fraction of the crystalline phase, which is in the range of 86 % to 98%. The stress values range between 150 and -140 MPa. The mechanical properties were studied by nanoindentation. Unlike monocrystalline wafers, there is no evidence of abrupt changes in the force-penetration plot, which have been attributed to a pressure-induced phase transition. The hardness was 12.5 GPa for the best samples, which is close to that obtained for silicon wafers.
Long-Term Stability of Microcrystalline Silicon P-I-N Solar Cells Exposed to Sun Light
Authors: Sanguino P. , Koynov S. , Schwarzl R. , Fernandes M. , Vieira M. , Manso R. , Joyce A. , Collares-Pereira M.
Published: April 2011

The performance of an entirely microcrystalline p-i-n solar cell was monitored during a long-term outdoor test in Lisbon starting in September 1998. A small decrease of the short circuit current was observed after 5 months of operation. The open-circuit voltage remained stable around 400 mV. From the analysis of the I-V characteristic in dark and under illumination we could identify the weak points of the test structure, like large series resistance, high recombination rate, and intensity-dependent collection efficiency.
Structure Sensitive Hydrogenation Effects in Polysilicon High Voltage thin Film Transistors
Authors: Clough F.J. , Xu Y.Z. , Narayanan E.M. Sankara , Cross R.
Published: April 2011

Hydrogen passivation of grain boundary and in-grain defects is a key process step in the fabrication of high quality poly-Si TFTs. The sensitivity of the hydrogenation process to device geometry is therefore an important consideration. The effects of rf-plasma hydrogenation on the operating performance of a range of self-aligned and offset drain (Loff = 5 to 40 μm) poly-Si TFT configurations is reported. The hydrogenation of offset drain structures results in a predictable increase in the pre-threshold slope and a reduction in the device threshold voltage. However, extended hydrogenation (up to 12 h) can result in a significant reduction in the device drive current (by up to 2 orders). A similar effect is observed in metal field plate HVTFTs in which some portion of the offset region is un-modulated by the additional electrode. The on state conduction in the offset region is examined as a function of hydrogenation time, temperature and planar electric field. The increase in the on resistance is attributed to a reduction in the poly-Si defect density, which moderates carrier transport through the offset region.
Rugged a-Si:H TFTs on Plastic Substrates
Authors: Gleskovas H. , Wagner S. , Suo Z.
Published: April 2011

Much of the mechanical strain in semiconductor devices can be relieved when they are made on compliant substrates. We demonstrate this strain relief with amorphous silicon thin-film transistors (a-Si:H TFTs) made on 25-μm thick polyimide foil, which can be bent to radii of curvature R down to 0.5 mm without substantial change in electrical characteristics. At R= 0.5 mm the channel area of the TFTs is strained by ~ 1%. The reduction in bending radius, from R ≠ 2 mm on steel foil of the same thickness, agrees with the theoretical prediction that changing from a stiff to a compliant substrate can reduce the bending strain in the device plane by a factor of up to 5.
CAT-CVD Process and its Application to Preparation of Si-Based Thin Films
Authors: Matsumura Hideki , Masuda Atsushi , Izumi Akira
Published: April 2011

This is to review the present understanding on Cat-CVD (catalytic chemical vapor deposition) or hot wire CVD. Firstly, the deposition mechanism in Cat-CVD process is briefly mentioned along with key issues such as the effect of heat radiation and a method to avoid contamination from the catalyzer. Secondly, the properties of Cat-CVD Si-based thin films such as amorphous silicon (a-Si), polycrystalline silicon (poly-Si) and silicon nitride (SiNx) films are demonstrated, and finally, the feasibility of such films for industrial application is discussed.