bluuebunny/crossref_metadata_2025 huggingface dataset:
Intergrowth Structures in Synthetic Pyrochlores: Implications for Radiation Damage Effects and Waste Form Formulation
Authors: Buck E. C. , Chamberlain D. B. , Gieré R.
Published: April 2011

Titanate-based ceramic waste forms are currently under development for the immobilization of excess weapons plutonium. Both Hf and Gd are added to the ceramic formulation as neutron absorbers in order to satisfy a defense-in-depth concept for the waste form. The introduction of significant amounts of hafnium may be responsible for the presence of zirconolite-2M crystals in pyrochlore-based ceramics and the formation of zirconolite lamellae within pyrochlore. The zirconolite grows epitaxially on { 111 }planes of pyrochlore. Although the zirconolite lamellae within pyrochlore are non-cubic, any volume expansion due to radiation damage in the pyrochlore should still be isotropic; in addition, the presence of these intergrowths may allow some stress relief in the ceramic.
Behavior of simulated hanford slurries during conversion to glass
Authors: Darab John G. , Meiers Eve M. , Smith Peter A.
Published: April 2011

Mixtures of simulated Hanford low-activity aqueous waste and glass precursor additives were heat treated at various temperatures up to 1000°C and characterized using X-ray diffraction, X-ray fluorescence spectroscopy, scanning electron microscopy, gas chromatography mass spectroscopy, and inductively coupled plasma – mass spectroscopy. It was found after heating the simulant mixture up to 700°C, the majority of the sodium nitrite present in the original waste reacted with the silica and perhaps boron species in the additives to form C– and N-containing gases, oxygen gas, and lower melting phases which contributed to the observed batch volume expansion beginning at these temperatures. Elemental analyses of minor components in the simulant mixture showed that I, Cl, and S exhibited significant losses during heating, presumably due to volatilization or entrainment. For the case of I, nearly 90% on an elemental basis was lost from the simulant mixture after processing to 1000°C, whereas Re exhibited less than 40% loss.
Processing of large-Scale radwaste-containing blocks using exothermic metallic mixtures
Authors: Ojovan M. I. , Petrov G. A. , Stefanovsky S. V. , Nikonov B. S.
Published: April 2011

The parameters of processing of large-scale (50-100 kg) radwaste-containing blocks were studied. Batches consisted of exothermic mixture, oxidizer, mineral additive, and radwaste surrogate. Such mixtures provide process temperature up to ˜2400 K. In the present work exothermic mixtures composed of Ca-Fe silicide, zirconium, silicon, and boron were intermixed with oxidizer (mixture of KMnO4 and V2O5), natural zircon concentrate as mineral additive, and cerium oxide (actinide surrogate). Tests were carried out in a bench-scale plant. Burning of the reaction mixture was remotely initiated from electric supply unit. Melt formation started in the nearbottom area sustained upwards frontally, The melt temperature ranged between 1600 and 1900 K. The melt formation rate during the tests ranged between 0.7 and 2.5 kg/s. Melt cooling after reaction completion was kept for about 15-18 hours. Volatile aerosols were essentially retained in the bulk. The initial volume of the mixture was reduced after reaction by factors of 2 to 3. The densities of the blocks were ranged between 3.2 and 3.9 kg/dm3.

Samples of the materials obtained were examined by optical microscopy, X-ray diffraction, and scanning electron microscopy. The product consisted of glass-like body with metallic spheres (˜0.5-1.5 mm) distributed within. The base was composed of glass (˜60-80 vol.% of total) and crystalline phases (mainly zirconia). Monoclinic baddeleyite was found to be the major crystalline phase. The minor phase was tetragonal zirconia. Cerium (actinide surrogate) was partitioned among the glass and zirconia. The Ce2O3 concentration in glass ranged between ˜0.4 wt.% and ˜0.8 wt.% whereas its content in zirconia was found to be lower (up to ˜0.3 wt %). The metallic constituent consisted of vanadium and Fe-V-Si alloy whose average composition wasFe7O2V19.3Mn1.1 Si9.4.
Nonisothermal kinetics of spinel crystallization in a HLW glass
Authors: Casler D. G. , Hrma P.
Published: April 2011

Nonisothermal kinetics of spinel crystallization in a high-level waste (HLW) glass was predicted using Mehl-Avrami-Johnson-Kolmogorov equation coefficients from isothermal data. The volume fraction of spinel was determined as a function of time, temperature, and cooling rate. The results were verified experimentally. Also predicted was the spatial distribution of spinel in a HLW glass canister. Finally, a parameter study was performed, and an empirical equation was proposed relating the final spinel volume fraction in glass to dimensionless numbers for cooling rate, phase equilibrium, and crystallization kinetics.
Partitioning of plutonium between glass, secondary phases, and contact refractories
Authors: Muller I. S. , Buechele A. C. , Perez-Cardenas F. , Gan H. , Pegg I. L.
Published: April 2011

A wide variety of plutonium-containing wastes have been generated from both defense and commercial nuclear programs. The Advanced Mixed Waste Treatment Project (AMWTP) involves the privatized treatment of 65,000 m3 of mixed TRU and low-level waste that is in storage at DOE's INEEL site. BNFL Inc. leads the commercial team including GTS Duratek, Inc., that has been selected to design, construct, and operate a facility to perform that service. Since vitrification is a potential treatment option, the current study was conducted to investigate issues surrounding the incorporation of the approximately 700 kg of plutonium that is present in the AMWTP waste. A series of glasses were prepared with plutonium (0.5 wt%), with cerium (as a potential surrogate), with both, and with neither. Scanning electron microscopy coupled with energy dispersive x-ray spectroscopy (SEM-EDS) and optical analyses confirmed that both elements were completely dissolved into the as-melted glasses. Heat treatments forced the formation of secondary phases, which were analyzed for Pu and Ce incorporation. Plutonium-containing glasses were spiked with sulfates and remelted to force the formation of a sulfate salt phase, which was analyzed to determine partitioning of Pu and Ce. Coupons of Monofrax K-3 glass contact refractory were exposed to melts of these glasses for six days at 1208°C using both standard and modified versions of ASTM procedure C621-84. Thickness loss, microstructural alteration, and the extent to which Pu and Ce were incorporated into the material were then determined by methods employing SEMEDS\ wavelength dispersive x-ray spectroscopy (WDS). Extreme heat treatments were found to lead to the formation of small amounts (< 0.1 vol%) of either (Pu, Ce)O2, or PuO2, depending upon whether Ce was present or absent, respectively. The Pu level in the sulfate phase was at least a factor often below that of the parent glass, and no tendency to concentrate Pu in the K-3 refractory above the level in the parent glass was found.
The Effects of Uranium on the Structure of Iron Phosphate Glasses
Authors: Badyal Yaspal , Karabulut Mevlut , Marasinghe Kanishka , Saboungi Marie-Louise , Haeffner Dean , Shastri Sarvjit , Day D. E. , Ray C. S.
Published: April 2011

Because of their high chemical durability and waste loading capacity, iron phosphate glasses are a natural candidate for a nuclear waste disposal medium. We have studied the effects of uranium on the structure of iron phosphate glasses with both neutron and high-energy x-raydiffraction. The results of neutron scattering, which is mostly sensitive to pair correlations involving light atoms i.e. O-O, Fe-O and P-O, indicate the main structural features of the base glass are largely unaffected by the addition of UO2. The nearest-neighborbour P-O, Fe-O and O- O peaks remain at the same position in real space and their intensities scale approximately with concentration. These findings are consistent with earlier results using Raman scattering and EXAFS on the Fe-K edge, where in both cases the spectra remain similar to the base glass. The results of high-energy x-ray scattering, which is sensitive to correlations involving the heavier atoms and thus complements the neutron measurements, are also consistent with the overall picture of uranium occupying interstitial sites in the relatively undisturbed base glass structure. Combining the neutron and x-ray data for a 10 mol% UO2 glass suggests the intriguing possibility of a U6+ uranyl ion configuration although further work is needed to establish the precise local structure and valence state of uranium in these glasses.
Fundamental and Technological Aspects of Actinide Oxide Pyrochlores: Relevance For Immobilization Matrices
Authors: Raison P. E. , Haire R. G. , Sato T. , Ogawa T.
Published: April 2011

Polycrystalline pyrochlore oxides consisting of selected f elements (lanthanides and actinides) and Zr and Hf have been prepared and characterized. Characterization to date has been primarily by X-ray diffraction, both at room and at elevated temperatures. Initial studies concentrated on selected lanthanides and the Np, Pu and Am analogs (reported here) but have been extended to the other actinide elements through Cf. Data from these studies have been used to establish a systematic correlation regarding the fundamental materials science of these particular pyrochlores and structurally related fluorite-type dioxides. In addition to pursuing their materials science, we have addressed some potential technological applications for these materials. Some of the latter concern: (1) immobilization matrices; (2) materials for transmutation concepts; and (3) special nuclear fuel forms that can minimize the generation of nuclear wastes. For f elements that display both a III and IV oxidation state in oxide matrices, the synthetic path required for producing the desired pyrochlore oxide is dictated by their pseudo-oxidation potential the stability of the compound towards oxygen uptake. For the f elements that display an oxidationreduction cycle for pyrochlore-dioxide solid solution, X-ray diffraction can be used to identify the composition in the oxidation-reduction cycle, the oxygen stoichiometry and/or the composition. This paper concentrates on the Np, Pu and Am systems, and addresses the above aspects, the role of the crystal matrix in controlling the ceramic products as well as discussingsome custom-tailored materials.
Chemistry of Rare Earth Oxalate Vitrification
Authors: Vienna J. D. , Peeler D. K. , Darab J. G. , Zamecnik J. R. , LI H. , Marra J. E.
Published: April 2011

Mixtures of rare earth and actinide oxalates will be vitrified into boro-aluminosilicate-based glasses for intermediate term stabilization according to current plans. The reaction chemistry involved with converting these oxalate feed stocks into glass products determines the potential for foaming, redox, and other melt and off gas related phenomena associated with this process. We've undertaken a detailed study of this conversion process using a variety of complementary techniques. A closed quartz crucible contained in a vertical furnace equipped with a quartz window and video camera was used to study volume expansion of the feed/melt during heating while monitoring the off-gas using a gas chromatograph-mass spectrometer. Simultaneous thermogravimetric and differential thermal analyses were conducted on small samples of feed and frit mixtures. Samples containing Ce were analyzed using established wet chemical techniques to determine Ce3+/Ce4+ ratio (redox) as a function of temperature. We evaluate the results and provide a description of the reaction chemistry of these oxalate feeds during vitrification.
NMR Characterization of Simulated Hanford Low-Activity Waste Glasses and its use in Understanding Waste form Chemical Durability
Authors: Darab J. G. , Linehan J. C. , McGrail B. P.
Published: April 2011

Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopy has been used to characterize the structural and chemical environments of B, Al, and Si in model Hanford low-activity waste glasses. The average 29Si NMR peak position was found to systematically change with changing glass composition and structure. From an understanding of the structural roles of Al and B obtained from MAS-NMR experiments, we first developed a model that reliably predicts the distribution of structural units and the average 29Si chemical shift value, δ, based purely on glass composition. A product consistency test (PCT) was used to determine the normalized elemental release (NL) from the prepared glasses. Comparison of the NMR and PCT data obtained from sodium boro-aluminosilicate glasses indicates that a rudimentary exponential relationship exists between the 29Si chemical shift value, and the boron NL value.
Effects of Halides and Sulphates in Synroc-C
Authors: Vance E. R. , Hartman J. S.
Published: April 2011

Halides can be retained in bellows hot-pressed Synroc-C. Loadings of 0.2 wt% of fluoride or 2 wt% of sulphate did not appear to exert deleterious effects on the aqueous durability of the Synroc, but 0.2 wt% of chloride or 0.5 wt% of fluoride increased the Cs leach rate by a factor of ˜10. Fluoride was present principally as CaF2 in both calcined and hot-pressed Synrocs containing (a) ˜2 wt% of F as the only waste or (b) ˜2 wt% of F plus 20 wt% of simulated PW-4b Purex reprocessing waste. The speciation of Cl- in Synroc is not clear, though there was some evidence of CsCl from nmr and a compound giving a similar X-ray pattern to that of TiCI3. Iodine can retained in the hot-pressed Synroc containing the full complement of fission products, probably as insoluble Ag, Cr or Pd group iodides.
α-Radiolysis and α-Radiation Damage Effects on uo2 Dissolution Under Spent Fuel Storage Conditions
Authors: Rondinella V. V. , matzke Hj. , Cobos J. , Wiss T.
Published: April 2011

α-decay will constitute almost entirely the radiation field in and around spent nuclear fuel after a few hundred years in a geological repository. Pellets of UO 2 containing ˜0.1 and ˜10 wt. % 238Pu were fabricated using a sol-gel method and characterized, comparing their properties to those of undoped UO2. The α-radiation fields of different types of commercial LWR spent fuel are of the same order of magnitude as the fuel with the lower Pu-concentration used in this work. The results of static batch leaching tests at room temperature in demineralized water under anoxic atmosphere showed that the amounts of U released during leaching were higher in the case of UO2 containing 238pu than for undoped UO2. Relatively large amounts of Pu were released after the longest leaching times. Lattice parameter measurements using XRD and hardness measurements by Vickers indentation showed a relatively rapid build-up of α-decay damage in the material stored at ambient temperature with the higher concentration of dopant, while for the material with ˜0.1 wt. % Pu no clear variations were detected during the same time intervals.
Modelling the Oxidative Dissolution of UO2
Authors: King Fraser , Kolar Miroslav , Shoesmith David W.
Published: April 2011

An electrochemically based model for predicting the effects of α-radiolysis, the precipitation of U(VI) corrosion products and redox processes with Fe and Fe(II) on the dissolution of UO2 is described. Various aspects of the model are presented, including: the underlying mechanism, the reaction-diffusion equations used to describe the mass transport and homogeneous reactions of the various species considered in the model, the geometrical grid used to simulate both experimental and used fuel/container geometries and the electrochemical boundary conditions used for the numerical solution of the reaction-diffusion equations. The results of preliminary simulations are also discussed.
Zirconia – A Ceramic for Excess weapons Plutonium Wastes
Authors: Gong W. L. , Lutze W. , Ewing R. C.
Published: April 2011

We synthesized a ceramic containing simulated excess weapons plutonium waste in solidsolution with zirconia (ZrO2)ss. ZrO2 has a large solubility for other metal oxides. More thantwenty binary systems AxOy- ZrO2 have been reported in the literature, including PuG2, rare earth elements, and oxides of metals contained in weapons plutonium wastes. We show that significant amounts of gadolinium (neutron absorber) and yttrium (stabilizer of the cubic modification) can be dissolved in ZrO2, together with plutonium (simulated by Th4+, Ce4+, or U4+) and impurities (e.g., Ca Mg, Fe, Si). Sol-gel and powder methods were applied to make homogeneous, single phase zirconia solid solutions. Pu waste impurities were completely dissolved in the solid solutions. In contrast to other phases, e.g., zirconolite and pyrochlore, yttrium stabilized cubic zirconia does not undergo amorphization upon irradiation.
A Consistance Model for Anion Exclusion and Surface Diffusion
Authors: Lehikoinent J. , Muurinen A. , Valkiainen M.
Published: April 2011

A decomposition of the diffusion flux equation for an electrostatically bound and mobile hydrated ion into two components is proposed. The first component includes the effects arising solely from the irregular pore shape and increase in solvent viscosity in the proximity of negatively charged pore walls. Apart from these effects, the second flux component includes an additional contribution from an increased (decreased) concentration for cations (anions) close to the pore walls. Defining the distribution coefficient, Kd, in a fashion that allows negative values for co-ions readily accounts for their exclusion without the need to introduce somewhat artificial quantities like the “effective co-ion porosity”. In this study, it is thus possible to retain the purely volumetric meaning of the porosity and to maintain consistency throughout the conceptualization for anions, cations and electrically neutral species. Furthermore, the decomposition of the flux equation provides support for surface diffusion, a subject of great controversy and lively debate in the literature. In this connection, the role of concentration to regulate the diffusive flux for ions in relation to neutral species is emphasized. Implications for the theoretical apparent and effective diffusivities in compacted montmorillonite clay are also discussed and a modified form of the macroscopic theory is proposed.
New Crystalline Silicotitanate (CST) Waste Forms: Hydrothermal Synthesis and Characterization of CS-SI-TI-O Phases
Authors: Nyman M. , Nenoff T. M. , Su Y. , Balmer M. L. , Navrotsky A. , Xu H.
Published: April 2011

The radioactivity of the Hanford site waste tanks is primarily from 137Cs and 90Sr, of which can both be selectively removed from solution using a crystalline silicotitanate (CST) ion exchanger. We are currently seeking waste forms alternative to borosilicate glass for Cs-CSTs. In order to obtain a fundamental basis for the development of an alternative waste form, we are investigating synthesis and characterization of CST component phases, namely Cs-Si-Ti-O phases. Two novel Cs-Ti-Si-O phases (one porous, one condensed) have been hydrothermally synthesized, characterized and evaluated as waste form candidates based on chemical and thermal stability, leachability, and ion exchange capabilities.
Deposition Behavior of Latex Particles In Filtration Process Through Glass Packed Column
Authors: Chinju H. , Nagasaki S. , Tanaka S. , Tanaka T. , Ogawa H.
Published: April 2011

The deposition behavior of colloids from a flowing suspension onto a solid collector surface infiltration process was observed by conducting column experiments with polystyrene latex particlesand glass beads packed columns. From the observed particle breakthrough curves, single collectorefficiencies were calculated. By comparing the single collector efficiency obtained in the columnexperiment with the existing approximate expression, the effects of flow velocity on the depositionbehavior was discussed.
Evaluation of thermally converted silicotitanate waste forms
Authors: SU Y. , Balmer M. L. , Wang L. , Bunker B. C. , Nyman M. , Nenoff T. , Navrotsky A.
Published: April 2011

Crystalline silicotitanate ion exchangers are highly selective for separating Cs from Narich waste streams. However, use of these ion exchangers for removal of Cs from radioactive tank waste will result in large volumes of secondary wastes. Thermal conversion of silicotitanates produces a durable waste form with reduced volumes up to 40%. Leach tests (MCC-l and PCT) have shown that Cs leach rates of IE-91 1-Na (heat treated at 900°C for an hour) are extremely low, ranging from 0.1 to lwt% Cs loss in Cs fraction release, or 10-1 to 10-8g/m2day in normalized Cs mass loss. These are several orders of magnitude lower than that of borosilicate glass. In order to understand the interplay between the structure and high Cs durability, X-ray diffraction, 133Cs NMR, and thermogravimetric analysis have been used to identify phase(s) responsible for trapping Cs in these silicotitanates. Results indicate that Cs is likely to be contained in a crystalline silicate phase.
Study of Natural Zircon-Xenotime Assemblages for Estimation of the Actinide Waste Forms Stability
Authors: Lapina M. I. , Yudintsev S. V.
Published: April 2011

In order to estimate compositions and structural stability of YPO4-ZrSiO4-USiO4- ThSiO4 solid solution, natural specimens of (U,Th)-enriched zircon-xenotime assemblages were studied. The samples were picked from granitic pegmatites of North Karelia of about 1,850 Ma age. TEM data show that amorphization of the zircon structure is not complete. Xenotime has typical crystal structure. ASEM and EPMA results indicate that zircon and xenotime are micro-heterogeneous in there compositions. Numerous micro-inclusions (<5 μm in diameter) of xenotime and uranium oxide were found within the zircon grains. Xenotime is free from inclusions. The results give rise to the conclusions that xenotime-zircon solid solution is restricted due to high difference in ionic radii (y3+ vs. Zr4+ and p5+ vs. Si4+). Data on high amounts of rare earth elements and phosphorus in natural zircons are result from REE-phosphate inclusions in analyzed zircons. Content of U in the zircon allowed the estimation of the solubility of coffinite end member to be no less than 5 mol.% "USiO4".
Vegetation-Derived Insights on the Mobilization and Potential:Transport of Radionuclides from the Nopal I Natural Analog Site, Mexico
Authors: Leslie Bret W. , Pickett David A. , Pearcy English C.
Published: April 2011

The Nopal I uranium (U) deposit, Peñia Blanca, Mexico is a source term and contaminant transport natural analog to the proposed high-level nuclear waste repository at Yucca Mountain, Nevada. In an attempt to characterize the mobilization and potential transport of adionuclides in the unsaturated zone at the Nopal I deposit, vegetation growing on ore piles was analyzed for238U,235U, and232Th decay-series isotopes. Specimens ofPhacelia robustagrowing on highgrade piles of U ore were collected and analyzed by alpha autoradiography, and by alpha and gamma spectrometry. Activities for U, thorium (Th), and radium (Ra) isotopes (Bq/kg dried plant) were 300, 1000, and 7000 for238U,230Th, and226Ra, respectively. The226Ra activities in these specimens are among the highest ever measured for plants; furthermore, the plant-to-soil226Ra concentration ratio is higher than expected. These results demonstrate the large mobility and bio-availability of Ra in the Nopal I environment, and support previous indications of recent loss of226Ra from the ore body. Comparison between the activities of238U and232Th decay-chain Th isotopes in the plants and in the ore substrate indicate that relative mobilization into pore solutions of228Th >230Th >232Th, in a ratio of about 50 – 25:4:1, respectively. The similarity of the plant's234U/238U activity ratio (˜1.2) to that of a caliche deposit that formed adjacent to the Nopal ore body around 54 ka suggests the234U/238U activity ratio of U released from the ore is approximately 1.2. The U and236Ra isotope activities of the plants and ore substrate, and solubility considerations, are used to assess a source term model of the potential Yucca Mountain repository. These results suggest the use of a natural analog source term model in performance assessments may be non-conservative.
Development of Integrated Mechanistically-Based Degradation-Mode Models for Performance Assessment of High-Level Waste Containers
Authors: Farmer J. C. , Mccright R. D. , Estill J. C. , Gordon S. R.
Published: April 2011

Alloy 22 [UNS N06022] is now being considered for construction of high level waste containers to be emplaced at Yucca Mountain and elsewhere. In essence, this alloy is 20.0–22.5% Cr, 12.5–14.5% Mo, 2.0–6.0% Fe, 2.5–3.5% W, with the balance being Ni. Other impurity elements include P, Si, S, Mn, Co and V. Cobalt may be present at a maximum concentration of 2.5%. Detailed mechanistic models have been developed to account for the corrosion of Alloy 22 surfaces in crevices that will inevitably form. Such occluded areas experience substantial decreases in pH, with corresponding elevations in chloride concentration. Experimental work has been undertaken to validate the crevice corrosion model, including parallel studies with 304 stainless steel.
The Stress Corrosion Cracking of Copper:Nuclear Waste Containers
Authors: King F. , Litke C. D. , Ikeda B. M.
Published: April 2011

The extent of stress corrosion cracking (SCC) of copper nuclear waste containers is being predicted on the basis of a “limited propagation” argument. In this argument, it is accepted that crack initiation may occur, but it is argued that the environmental conditions and material properties required for a through-wall crack to propagate will not be present.

In this paper, the effect of one environmental parameter, the supply of oxidant (Jox), on the crack growth rate is examined. Experiments have been conducted on two grades of Cu in NaNO2 environments using two loading techniques. The supply of oxidant has been varied either electrochemically in bulk solution using different applied current densities or by embedding the loaded test specimens in compacted buffer material containing O2 as the oxidant. Measured and theoretical crack growth rates as a function of Jox are compared with the predicted oxidant flux to the containers in a disposal vault and an estimate of the maximum crack depth on a container obtained.
Phase Stability and Mechanical Properties of C-22 Alloy Aged in the Temperature Range 590 to 760°C for 16,000 Hours
Authors: Summers Tammy S. Edgecumbe , Wall Mark A. , Kumar Mukul , Matthews Steven J. , Rebak Raú B.
Published: April 2011

The phase stability of C-22 alloy (UNS #N06022) was studied by aging samples at 593, 649, 704 and 760°C for 2000 h (2.7 mo) and 16,000 h (1.8 yr). The tensile properties and the Charpy impact toughness of these samples were measured in the mill annealed condition as well as after aging. The microstructures of samples aged 16,000 hours were examined using scanning and transmission electron microscopy (SEM and TEM). Preliminary TEM results suggest that μse forms at all temperatures investigated. Discrete carbide particles in addition to a film with very uniform thickness which appears to be μ phase formed on grain boundaries in the sample aged at 593°C. The ordered Ni2(Cr, Mo) phase was also seen in this sample. At the higher aging temperatures, mainly μ phase forms covering all the grain boundaries and also distributed throughout the bulk. Although strength increased somewhat with aging. the ductility decreased due to the formation of these grain boundary precipitates and brittle intermetallics.
The Role of H in the Growth Mechanism of PECVD a-Si:H
Authors: Sanden M.C.M. Van de , Kessels W.M.M. , Smets A.H.M. , Korevaar B.A. , Severens R.J. , Schram D.C.
Published: April 2011

This paper describes an extension of the silyl radical based kinetic growth model by atomic hydrogen induced surface hydrogen abstraction processes. It is shown that by including this direct abstraction process several problems of the SiH 3 based model are resolved. The defect density can be predicted with the proper temperature dependence and order of magnitude. The implications for high rate deposition of a-Si:H are discussed
High Quality a-Si:H Films Grown at High Deposition Rates
Authors: Lubianiker Yoram , Tan Yanyang , Cohen J. David , Ganguly Gautam
Published: April 2011

Intrinsic a-Si:H samples were grown with and without hydrogen (H2) dilution of silane at different growth rates. We find that the dilution leads to a considerable reduction in the defect density, in particular at high growth rates. The defect density is particularly low for samples grown using H2 dilution conditions at growth rates as high as 10 Å/sec. Using transient photocapacitance measurements we find evidence for a small concentration of microcrystallites embedded in the amorphous films. An increase in the microcrystalline fraction correlates with a decrease in the defect density.
Very Wide-Gap and Device-Quality a-Si:H from Highly H2 Diluted SiH4 Plasma Decomposed by High RF Power
Authors: Terada N. , Yata S. , Terakawa A. , Okamoto S. , Wakisaka K. , Kiyama S.
Published: April 2011

The H2 dilution technique at a high deposition rate (RD) was investigated by depositing hydrogenated amorphous silicon (a-Si:H) under a high if power density of 750 mW/cm2, which is 20 times as large as that of conventional conditions. It was found that the H2 dilution ratio γ ( = [H2 gas flow rate] / [SiH4 gas flow rate]) tendency of the film properties, such as the H content (CH), optical gap (Eopt), SiH2/SiH and photoconductivity (σph) of a-Si:H is different for the high rf power (750 mW/cm2) and the medium rf power (75 mW/cm2) conditions. Under medium rf power, the CH, Eopt and SiH2/SiH decrease as γ increases. Under the high if power, on the contrary, the CH and Eopt, monotonously increase while maintaining a low SiH2/SiH and a high σph of 10-6 S/cm as γ increases. These results suggest that increasing the rf power enhances the H incorporation reactions due to H2 dilution. It is thought that a high rf power causes the depletion of SiH4 and hence the extinction of H radicals, expressed by SiH4 + H* → SiH3* + H2, is suppressed. A high H radical density enhances the incorporation of H into a-Si:H, resulting in very wide-gap a-Si:H with a high CH, Consequently, very wide-gap a-Si:H with device-quality (Eopt of 1.82 eV with an (αhv)1/3 plot, corresponding to > 2.1 eV with Tauc's plot, and σph of 10-6 S/cm) can be obtained at a high RD of 12 Å/s without carbon alloying.
Electrochemical Tailoring and Optical Investigation of Advanced Refractive Index Profiles in Porous Silicon Layers
Authors: Zangooie S. , Jansson R. , Arwin H.
Published: April 2011

Porosity depth profiles with exponential or sinusoidal shape were fabricated electrochemically in crystalline silicon using time-variable current densities and studied employing variable angle spectroscopic ellipsometry. Since volume porosity in porous silicon depends on the current density, it was possible to electrochemically tailor porosity depth profiles, which in a first approximation resembled the time modulation of the applied current. Optical characterization of the samples were realized using multilayer optical models and the Bruggeman effective medium approximation allowing variations of the index of refraction according to the applied current density profiles. The analysis also revealed deviations from desired profiles in terms of in-depth inhomogeneities.
Theory of Hydrogen Interactions with Amorphous Silicon
Authors: Walle Chris G. Van De , Tuttle Blair
Published: April 2011

We present an overview of recent results for hydrogen interactions with amorphous silicon (a-Si), based on first- principles calculations. We review the current understanding regarding molecular hydrogen, and show that H2 molecules are far less inert than previously assumed. We then discuss results for motion of hydrogen through the material, as relating to diffusion and defect formation. We present a microscopic mechanism for hydrogen-hydrogen exchange, and examine the metastable ≠ SiH2 complex formed during the exchange process. We also discuss the enhanced stability of Si-D compared to Si-H bonds, which may provide a means of suppressing light-induced defect generation.
Intrinsic, n- and p-Doped a-Si:H Thin Films Grown by DC Magnetron Sputtering with Doped Targets
Authors: Johansson Å.A. , Jårrendahl K. , Birch J. , Hjörvarsson B. , Arwin H.
Published: April 2011

Intrinsic, n- and p-type a-Si:H films were deposited by dc magnetron sputtering and analyzed with several techniques. The films were synthesized in a reactive Ar-H2 atmosphere giving H contents in the range of 3-20 at %. The films were sputtered from pure silicon targets and doped silicon targets with 1 at % B or P. Doping by co-sputtering from composite Si/B4C targets was also explored. The doping concentrations were 3 × 1020 − 2 × 1021 cm-3 for the p-type films and 2.6-2.9 × 1019 cm-3 for the n-type films. The conductivity was in the range 10-2−10-4 Ω-1 cm-1 for p-doped films and 10-5 Ω-1 cm-1 for the best n-doped films. Band gap estimations were obtained from dielectric function data and showed an increase with hydrogen content. A comparison to device quality PECVD-samples was also made.
Photoinduced Expansion in Hydrogenated Amorphous Silicon
Authors: Nonomura S. , Gotoh T. , Nishio M. , Sakamoto T. , Kondo M , Matsuda A. , Nitta S.
Published: April 2011

The structural aspect of photodegradation effect in hydrogenated amorphous silicon has been investigated by the use of the simple and sensitive detection technique, the laser optical-lever bending method, for a small expansion or extraction in thin films. The volume change induced by the thermal expansion due to the photothermal effect and the residual expansion was observed in hydrogenated amorphous silicon prepared by PECVD. The latter residual expansion was persistent after light soaking and was recovered by thermal annealing at 200°C.The time dependence of the volume expansion with light soaking shows the same time dependence of photoinduced defect density. The photoinduced volume changes normalized by the initial volume are the order of 10-5~10-5, which values are two orders smaller than chalcogenide glasses such as a-As2S3. The normalized volume change of a-Si:H with the different sample preparation conditions of PECVD such as the hydrogen dilution ratio r (r = SiH4/H2) and substrate temperature is shown. Also it is demonstrated that the photoinduced expansion is observed in hydrogenated amorphous silicon prepared by photo CVD and hot-wire CVD methods. The spatial extent related to a photoinduced defect creation in a-Si:H is estimated.
Slow Degradation of Hydrogenated Amorphous Silicon Photoconductivity Under Pulsed Illumination
Authors: Heck Stephan , Branz Howard M.
Published: April 2011

We degraded hydrogenated amorphous silicon (a-Si:H) using red light pulses of 32 microseconds to 2 milliseconds. These metastable photoconductivity degradations were compared to degradation with continuous light of the same intensity and same exposure time. For a given integrated exposure time we observe higher degraded photoconductivities (by up to 40 %) as we shorten the pulses. For example, to obtain the same amount of degradation with 120 microsecond pulses as with continuous illumination, the integrated sample exposure time must be doubled. Experiments were conducted to exclude thermal effects. Our result cannot be explained with a simple recombination-driven degradation mechanism, because electron and hole populations rise and fall to steady-state values in only a few microseconds. We conclude that carrier recombination creates a more proximate precursor to metastable degradation of a-Si:H. When illumination begins, this precursor's density rises more slowly than the density of photocarriers and therefore degradation is delayed.
Light-Induced Increase in Two-Level Tunneling States in Hydrogenated Amorphous Silicon
Authors: Liu Xiao , Pohl R.O. , Crandall R.S.
Published: April 2011

We observe an increase of the low-temperature internal friction of hydrogenated amorphous silicon prepared by both hot-wire and plasma-enhanced chemical-vapor deposition after extended light-soaking at room temperature. This increase, and the associated change in sound velocity, can be explained by an increase of the density of two-level tunneling states, which serves as a measure of the lattice disorder. The amount of increase in internal friction is remarkably similar in both types of films although the amount and the microstructure of hydrogen are very different. Experiments conducted on a sample prepared by hot-wire chemical-vapor deposition show that this change anneals out gradually at room temperature in about 70 days. Possible relation of the light-induced changes in the low-temperature elastic properties to the Staebler-Wronski effect is discussed.
A Comparison of the Degradation and Annealing Kinetics in Amorphous Silicon and Amorphous Silicon-Germaniumsolar Cells
Authors: Carlson D. E. , Chen L. F. , Ganguly G. , Lin G. , Middya A. R. , Crandall R. S. , Reedy R.
Published: April 2011

The degradation and annealing kinetics of both a-Si:H and a-SiGe:H single-junction solar cells were investigated under varying conditions. In every case, the kinetics associated with degradation and annealing were slower for a-SiGe:H cells than for a-Si:H cells. Since deuterium diffusion studies indicate that the hydrogen in our a-SiGe:H films diffuses more slowly than that in the a-Si:H films, hydrogen motion may play a role in determining both the degradation and annealing kinetics of the devices.
A Molecular Dynamics Study of Band Tails in a-Si:H
Authors: Fedders Peter A. , Drabold D. A.
Published: April 2011

Band tail states are routinely invoked in models of a-Si:H including defect pool models and models of light induced defects. These models describe the band tail states as being localized on a single stretched bond. However to our knowledge, there is no theoretical or experimental work to justify these assumptions. In this work we use ab initio calculations to support earlier tight-binding calculations that show that the band tail states are very delocalized ---- involving large numbers of atoms as the energy is varied from miidgap into the tails. Our work also shows that the valence band tail states are statistically associated with short bonds (not long bonds) and the conduction-band states with long bonds. We have slightly modified a 512 atom model of a-Si due to Djordjevic et al. to produce a large, defect free model of a-Si:H with realistic band tails.
Atomic Force Microscopy Study of Initial Nucleation in the Deposition OF μc-Si:H
Authors: Brogueira P. , Chu V. , Conde J.P.
Published: April 2011

The initial stages of microcrystalline silicon growth of n+ doped films prepared by rf plasma enhanced chemical vapor deposition (PECVD) and of intrinsic films prepared by hot-wire chemical vapor deposition (HW-CVD) are studied using atomic force microscopy, Raman spectroscopy and parallel dark conductivity measurements. The effect of the use of a plasma hydrogen treatment, of chamber conditioning prior to this treatment, of the type of substrate (glass or c-Si) used and the effects of a seed layer on the film properties are discussed.
An Optical Gap Calibration Applied to the Case of Hydrogenated Amorphous Silicon
Authors: Sweenor D. E. , O'Leary S. K. , Foutz B. E.
Published: April 2011

There are many different empirical means whereby the optical gap of an amorphous semiconductor may be defined. We analyze some hydrogenated amorphous silicon data with respect to a number of these empirical measures for the optical gap. By plotting these various gap measures as a function of the breadth of the optical absorption tail, we provide a means of relating these disparate measures of the optical gap. The applicability of this calibration to another set of hydrogenated amorphous silicon data is investigated.
Amorphous Silicon Precipitates in (100) c-SI Films Grown by ECRCVD
Authors: Birkholz M. , Platen J. , Sieber I. , Bohne W. , Röhrich J. , Fuhs W.
Published: April 2011

Silicon films were grown on (100) n-Si with an electron-cyclotron resonance chemical vapor deposition (ECRCVD) system by decomposition of SiH4 at 325°C. Structure and composition of thin films were investigated by SEM, Raman spectroscopy, elastic recoil detection analysis (ERDA) and TEM. Excellent epitaxial growth was achieved for some hundred nm thickness. For more than 1 μm thick films, however, SEM revealed the occurrence of conical structures orientated upside-down with their basal plane in the film surface. Depth-profiling of the elemental composition of thin films by means of ERDA showed the hydrogen content CH to exhibit a pronounced increase with increasing film thickness. Raman spectroscopy evidenced the coexistence of c-Si and a-Si:H by the occurrence of two bands at 520 and 480 cm-1, the ratio of which was found to depend sensitively upon the position of the laser spot on the sample. All experimental results could be consistently explained by assuming the conical precipitates to consist of a-Si:H which was finally proven by coherent electron beam diffraction (CEBD).
Microcrystalline Silicon Tunnel Junctions for Amorphous Silicon-Based Multijunction Solar Cells
Authors: Ferlauto A. S. , Koh Joohyun , Rovira P. I. , Wronski C. R. , Collins R. W.
Published: April 2011

The formation of tunnel junctions for applications in amorphous silicon (a-Si:H) based multijunction n-i-p solar cells has been studied using real time optics. The junction structure investigated in detail here consists of a thin (~200 Å) layer of n-type microcrystalline silicon (μc-Si:H) on top of an equally thin layer of p-type μc-Si:H, the latter deposited on thick (~2000 Å) intrinsic a-Si:H. Such a structure has been optimized in an attempt to obtain single-phase μc-Si:H with a high crystallite packing density and large grain size for both layers of the tunnel junction. We have explored the conditions under which grain growth is continuous across the p/n junction and conditions under which renucleation of n-layer grains can be ensured at the junction. One important finding of this study is that the optimum conditions for single-phase, high-density μc-Si:H n-layers are different depending on whether the substrate is a μc-Si:H p-layer or is a H2-plasma treated or untreated a-Si:H i-layer. Thus, the top-most μc-Si:H layer of the tunnel junction must be optimized in the multijunction device configuration, rather than in single cell configurations on a-Si:H i-layers. Our observations are explained using an evolutionary phase diagram for a-Si:H and μc-Si:H film growth versus thickness and H2-dilution ratio, in which the boundary between the two phases is strongly substrate-dependent.
Role of Bandgap Grading for the Performance of Microcrystalline Silicon Germanium Solar Cells
Authors: Krause M. , Carius R. , Stiebig H. , Finger F. , Lundszien D. , Wagner H.
Published: April 2011

The optical absorption of microcrystalline silicon germanium alloys (μc-Si1-xGex:H) increases in the near infrared region with increasing germanium content. Therefore, these alloys are promising candidates for the application as intrinsic absorber material in thin film solar cells with tandem and triple structure or IR-detectors. The material properties for a germanium content (x) up to 0.6 and the performance of solar cells based on this material were investigated. Graded bandgap structures are used to optimize the cell performance. For cells with x > 0.3 a continuously graded bandgap in the rear 20 nm of the i-layer (at the i/n interface) results in an enhancement of the open circuit voltage by 40-80 mV compared to an abrupt bandgap discontinuity. When the design of the p/i interface region is changed in a similar way no effect on Voc is observed.
High Rate Deposition of Microcrystalline Silicon Using Resonance Plasma Source (HELIX) – Plasma Properties and Deposition Results
Authors: Grtiger H. , Terasa R. , Haiduk A. , Kottwitz A.
Published: April 2011

Microcrystalline silicon layers have been deposited by PECVD using a resonance plasma source (Helix) operating at frequencies of 46, 68, 113 and 163 MHz. The plasma discharges in hydrogen and various hydrogen/silane mixtures were investigated by optical emission spectroscopy (OES) and mass spectroscopy (MS). Growth rate, crystalline fraction and hydrogen content of the layers were studied for different gas compositions, excitation frequencies and plasma powers.

Plasma monitoring revealed preferably the formation of positive ions. The density of positive hydrogen ions increased steadily 15 times by raising the frequency from 46 MHz to 163 MHz, whereas the ion energy was reduced from 75 eV to 23 eV and the radiation from hydrogen decreased to 50 %. Growth rates up to 1.5 μm/h have been achieved for microcrystalline layers at 230 °C deposition temperature. The content of hydrogen was below 15 at.%. Raman spectroscopy measurements reveal that 20 to 70 % of the silicon was crystalline depending on the silane concentration.
Integrated Amorphous and Polycrystalline Silicon TFTs with a Single Silicon Layer
Authors: Pangal K. , Chen Y. , Sturm J.C. , Wagner S.
Published: April 2011

Selective exposure of an a-Si:H film to a room temperature hydrogen plasma using a patterned SiNx, capping layer and a subsequent anneal at 600°C, resulted in polycrystalline and amorphous silicon regions in a single silicon layer on the same glass substrate. Top-gate non-self-aligned TFTs were fabricated in both the amorphous and polycrystalline regions with all shared processing steps and no laser processing using a re-hydrogenation step. The TFTs had good characteristics, with field-effect mobilities upto 1.2 cm2/Vs and 15 cm2/Vs for the a-Si:H and the poly-Si TFTs, respectively, and ON/OFF ratios >105 in either case.
A New Method to Characterize TCO/P Contact Resistance in a-SI Solar Cells
Authors: Hegedus Steven S. , Gibson Michael , Ganguly Gautam , Arya Rajeewa
Published: April 2011

A method is presented to characterize the TCO/p contact and the TCO sheet resistance in a-Si based p-i-n superstrate devices. It requires having scribed TCO strips, which are electrically isolated before a-Si deposition, then fabricating rows of individual devices on each strip. Analysis of 4-terminal measurements in different V-sensing configurations yields the TCO/p contact and TCO sheet resistance in a straightforward manner. The method is applied to devices fabricated on 3 brands of commercial SnO2 substrates. The TCO/p contact resistance is found to be ~2 Ω-cm2 and the sheet resistance decreases by 2-4 Ω/sq after a-Si deposition on all 3 brands of SnO2 substrates.
Low Temperature Deposition of Amorphous Silicon Based Solar Cells
Authors: Koch Christian , Ito Manabu , Schubert Markus , Werner Jurgen H.
Published: April 2011

Our investigations study the influence of various deposition parameters on the structural and optoelectronic properties of a-Si:H deposited at temperatures of 100 °C and below. Despite a significant material quality deterioration at low substrate temperatures, we observe remarkable improvements of charge carrier transport properties due to an increased H2-dilution of the process gases. In case of doped layers, we restore the electrical conductivity of low temperature n-type a-Si:H to standard values, whereas the p-layer quality is still inferior. We incorporate our optimized low temperature layers into a variety of solar cell types, including single and tandem cell structures. In addition to our conventional pin-structures we also successfully develop nip-cells for growth on opaque polymer substrates. From pinpin tandem cells, we record initial efficiencies of 6.0 % at a deposition temperature of 100 °C and 3.8 % at 75 °C. Interestingly, our nipnip tandem structures attain similar values which offers the possibility for deposition on low-cost plastic substrates. The mechanical flexibility of such substrates offers a wide variety of novel applications.