bluuebunny/crossref_metadata_2025 huggingface dataset:
Finite Element Analysis of Nanoscale Thermal Measurements of Superlattices
Authors: Foley Jason R. , Thomas Avedisian C.
Published: April 2011

A finite element analysis applicable to two- and three-dimensional heat flow in samples of arbitrary geometry and composition is presented for use in a thermal wave experiment. The finite element formulation is summarized, including the use of symmetry to simplify the problem, and the governing differential equations for the heat transport are found to be in the form of the Helmholtz equation for the specific case of a modulated heat source. Simulated data for a Nb/Si superlattice is calculated using the finite element code and is shown to agree with predictions from an analytical model, validating the approach taken.
Application of a Focused Ion Beam System to Nanolithography
Authors: Langford Richard M. , O'Reilly Shamus , McEwen Iain J.
Published: April 2011

Nano-imprint lithography (NIL) and micro-contact printing (MCP) are currently receiving considerable attention as techniques that can be used for low cost nanolithography. Here the application of a focused ion beam (FIB) system for the analysis of the elastomer stamps and imprinted patterns which are used in these nanolithography techniques is discussed. It is shown that the ‘lift-out’ technique can be used to prepare cross-sections of both the elastomer poly(dimethylsiloxane) (PDMS) stamps and the imprinted poly(methylmethacrylate) (PMMA) patterns. In addition, the use of the FIB system to prepare masters such as gratings and structures with curved shapes that would be difficult to fabricate using conventional processing techniques is discussed.
Dendrimer Mediated ‘Bricks and Mortar’ Self-Assembly of Nanoparticles
Authors: Frankamp Benjamin L. , Boal Andrew K. , Rotello Vincent M.
Published: April 2011

Control of particle-particle spacing is a key determinant of optical, electronic, and magnetic properties of nanocomposite materials. We have used poly(amidoamine) (PAMAM) dendrimers to assemble carboxylic acid-functionalized mixed monolayer protected clusters (MMPCs) through acid/base chemistry between particle and polymer. IR spectroscopy and selective dendrimer staining, observed by Transmission Electron Microscopy (TEM), establish that the PAMAM dendrimers are the mortar in the assembly and act to space the MMPCs in the resulting aggregates. Small angle X-ray scattering (SAXS) was then used to establish average interparti cle distances; five generations of PAMAM dendrimer (0, 1, 2, 4, 6) were investigated and monotonic increase in interparticle spacing from 4.1 nm to 6.1 nm was observed.

Initial studies involving the application of this methodology to control the magnetic properties of 3-iron oxide nanoparticles have been completed. γ-Iron oxide nanoparticles (6.5 nm in diameter) have been assembled with PAMAM dendrimers generations 2.5, 4.5, and 6.5. The resulting aggregates were characterized with SAXS and magnetization obtained on a super conducting quantum interference devise (SQUID). An observed correlation between the blocking temperature (TB) and the average interparticle spacing suggests that our methodology could be used to tailor the magnetic profile of the nanoparticles.
MBE Growth and Optical Properties of ZnSeO
Authors: Nabetani Y. , Mukawa T. , Ito Y. , Kato T. , Matsumoto T.
Published: April 2011

ZnSeO alloy was successfully grown by molecular beam epitaxy on GaAs substrate using RF plasma. The crystal structure of epitaxial ZnSeO alloy was zincblende. No phase separation was observed by in-situ reflection high energy electron diffraction and X-ray diffraction measurements. O composition was estimated by lattice constant assuming Vegard's law. Photoluminescence intensity was larger than that of ZnSe. The peak energy shifted toward lower energies by increasing O composition. The band gap energy determined by photoluminescence excitation decreased with increasing O composition. The bowing parameter was obtained as high as 8.2eV. This large band gap bowing widens controllable energy range of II-VI semiconductor.
Screen Printable Doped Self-Aligned Metallization for Solar Cells
Authors: Addo Ernest A. , Shah Ismat , Opila Robert , Barnett Allen. M. , Allison Kevin , Sulima Oleg
Published: April 2011

Alloyed metal contacts to <100> and <111> p-type silicon (Si) were investigated using doped self-aligning metallization techniques [1]. The contact allows the formation of a pn-junction and provides metallization for photovoltaic applications via thick film technology [2]. A formulated screen-printable thick film was annealed above Ag/Si eutectic temperature of 830°C. The annealing process resulted in a junction depth of 0.3–1.2 μm and improved adhesion by the reduction of native oxide layer through use of a wetting agent. The technique inhibits shunts (high conductivity paths through the solar cell pn-junction caused by excessive metallization penetration). The technique also reduces the interfacial resistance due to dissimilar materials in contact – a parasitic resistance that also limit solar cell performance. The use of metallic Mg as a wetting agent additive in the thick film silver matrix was explored. Studies were carried out on screen printable thick films and the reducing effects of Mg were investigated using both air and reducing atmosphere (H2) annealing furnaces. We observed a correlation between increased alloying with reduced series resistance.
Characterization of Sputter Deposited PbTe on Si (111) for Optoelectronic Applications
Authors: Jdanov Alexey , Dashevsky Zinovi , Pelleg Joshua , Shneck Roni
Published: April 2011

IV-VI compound semiconductors are of interest due to their potential application as thermoelectric material, infrared detectors and semiconductor lasers. The use of PbTe based semiconductors is usually in the middle and far infrared region. Magnetron sputtering of PbTe thin films from a single target on Si (111) was performed under various conditions. Characterization of the films shows that PbTe films on Si (111) substrate are suitable for preparation of infrared (IR) detectors. By heat treatment novel IR detectors can be developed in the electron absorption region of 4 – 10 μm.
Far-infrared dielectric function and phonon modes of spontaneously ordered (AlxGa1-x)0.52In0.48P
Authors: Hofmann Tino , Gottschalch Volker , Schubert Mathias
Published: April 2011

We present a far-infrared spectroscopic ellipsometry study of the phonon properties of partially CuPtB -ordered (AlxGa1-x)0.52In0.48P with x =0, 0.32, 0.7, and 1.0 and degrees of ordering η = 0 … 0.63, determined from generalized ellipsometry measurement of the near-band-gap order birefringence. An anharmonic oscillator approach is used to model the ordinary and extraordinary dielectric functions and determine the infrared-active longitudinal and transversal phonon modes of the thin film samples. Besides the isotropic GaP-, InP- and AlP-like phonon modes, we observe alloy-induced modes with low polarity for η ∼ 0. The evolution of lattice like and alloy-induced modes with increasing η is reviewed for Ga0.52In0.48P. With increasing η all phonon modes split in E- and A1 -type vibrations with polarization perpendicular and parallel to the sublattice ordering direction, respectively, and the mode frequencies shift continuously. Our observations extend recently published phonon frequencies from experiment and theory for CuPt-ordered Ga0.52In0.48P, which are compared and discussed. The phonon modes of highly ordered (AlxGa1-x)0.52In0.48P, η ∼ 0.6, are compared to the modes observed in the highly disordered quaternary solid solution. We propose the far-infrared dielectric anisotropy as a sensitive measure for sublattice ordering in multinary III-V alloys.
Electrodeposition of CU2SE thin films by Electrochemical Atomic Layer Epitaxy (EC-ALE).
Authors: Vaidyanathan Raman , Mathe Mkhulu K. , Sprinkle Patrick , Cox Steve M , Happek Uwe , Stickney John L
Published: April 2011

Electrochemical atomic-layer epitaxy (EC-ALE) is an approach to electrodepositing thin-films of compound semiconductors. It takes advantage of underpotential deposition (UPD), deposition of a surface limited amount (a monolayer or less) of an element at a potential less negative than bulk deposition, to form a thin-film of a compound--one atomic layer at a time. Ideally, the 2-D growth mode should promote epitaxial deposition.

We report the formation of compound Cu2Se, at room temperature by electrochemical atomic layer epitaxy (EC-ALE). Cyclic voltammograms were used to determine the deposition potentials of each element. An automated deposition program was used to form 750 cycles of Cu2Se thin films. Electron probe microanalysis was done to determine the stoichiometry of the thin films. X-ray diffraction of the 200 cycle deposit indicated the presence of polycrystalline Cu2Se. The atomic ratio of Cu/Se in the thin films was found to be 2. Band gap of the thin films were determined by reflection absorption measurements. The band gap of the 200 cycle Cu2Se films was found to be 1.6 eV. X-ray diffraction of 350 and 750 cycle Cu2Se films, indicated the deposits consisted of Cu3Se2 and Cu2Se.
Native Point Defect Interactions in ZGP Crystals under Influence of e-Beam Irradiation.
Authors: Gribenyukov A.I. , Verozubova G.A. , Trofimov A. Yu. , Vere A.W. , Flynn C. J.
Published: April 2011

Optical absorption in the defect-related region of highly efficient non-linear ZnGeP2 crystals under e-beam irradiation and post-irradiation anneals has been investigated.

Partially irreversible changes of the absorption were found in the spectral range 0.9–2.5 μm (0.5–1.3 eV) after irradiation and subsequent low-temperature anneals. Data obtained do not support the vacancy model for ZnGeP2 absorption in the 0.5–1.3 eV range.

The least squares fit for the parameters of the theoretical dependence of optical absorption cross-section to the experimentally measured ZnGeP2 optical absorption coefficient spectra show that the defect-related absorption in 0.5–1.3 eV region is caused by deep donor levels with energy position E=Ev+(0.85–0.90) eV.

Significant changes in the energy spectrum of the dominant optically active centers have been observed under influence of e-beam irradiation and post-irradiation anneals.

Based on the optical absorption measurements obtained for as-grown, annealed and e-irradiated ZnGeP2 crystals, a model of point defect interactions has been proposed. This takes into account both the reversible interactions, such as the formation of donor-acceptor pairs, and the irreversible interactions of a quasi-chemical type.

The behavior of the energy spectrum of the optically active defects is discussed in terms of the modes of interaction between the initial point defects and those generated by irradiation. The analysis performed showed that the best agreement with experimental data is reached when it is assumed that optical defect-related absorption in the 0.5–1.3 eV range related mainly to the disordering defect in the cation sublattice of ZnGeP2, namely, to atoms of Ge substituting for Zn.

Defect concentration profiles created by e-irradiation in ZGP crystals of different thickness were calculated. The optimum conditions for providing a uniform defect distribution with depth in irradiated ZnGeP2 samples were determined.

The optimal e-beam irradiation fluences, giving maximum ZnGeP2 enlightenment, allowed us to reduce the defect-related absorption down to a value of 0.01 cm-1 at 2 μm.
Interdiffusion in Semiconductor Quantum Dot Structures
Authors: Lever P. , Fu L. , Jagadish C. , Gal M. , Tan H.H.
Published: April 2011

The potential profile of InGaAs quantum dots (QDs) was shown to be easily modified with annealing. We also demonstrate the use of spin-on-glass to create interdiffusion in QDs but the degree of interdiffusion was strongly dependent on the properties of the oxide. By using TiO2 significant suppression of thermal diffusion of the quantum dots could be achieved. On the other hand, very large additional blue shifts (in excess of 120 meV) could be obtained with both H and As implantation. The different nature of defects created by both ions and how they affect the interdiffusion of quantum dots were illustrated. In the dose and annealing temperature range studied, the degree of interdiffusion ultimately depends on the availability of free point defects or point defects liberated from clusters/extended defects to diffuse across the quantum dots.
Defects, Tunneling, and EPR Spectra of Single-Molecule Magnets
Authors: Park Kyungwha , Novotny M. A. , Dalal N. S. , Hill S. , Rikvold P. A. , Bhaduri S. , Christou G. , Hendrickson D. N.
Published: April 2011

We examine theoretically electron paramagnetic resonance (EPR) lineshapes as functions of resonance frequency, energy level, and temperature for single crystals of three different kinds of single-molecule nanomagnets (SMMs): Mn12 acetate, Fe8Br, and the S = 9/2 Mn4 compound. We use a density-matrix equation and consider distributions in the uniaxial (second-order) anisotropy parameter D and the g factor, caused by possible defects in the samples. Additionally, weak intermolecular exchange and electronic dipole interactions are included in a mean-field approximation. Our calculated linewidths are in good agreement with experiments. We find that the distribution in D is common to the three examined single-molecule magnets. This could provide a basis for a proposed tunneling mechanism due to lattice defects or imperfections. We also find that weak intermolecular exchange and dipolar interactions are mainly responsible for the temperature dependence of the lineshapes for all three SMMs, and that the intermolecular exchange interaction is more significant for Mn4 than for the other two SMMs. This finding is consistent with earlier experiments and suggests the role of spin-spin relaxation processes in the mechanism of magnetization tunneling.
Origin of the Magnetic Proximity Effect
Authors: Kiwi Miguel
Published: April 2011

The magnetic proximity effect (MPE) has attracted the attention of theorists and experimentalists for at least three decades. Lately, the relevance of the effect for the development of nanodevices has revived interest on the subject. Here we review how the field has evolved, centering our attention on metal-metal and metal-insulator systems. We describe, and critically compare, the different theoretical approaches that have been put forward, as well as their limitations. An evaluation of the relationship between existing theories and available experimental results is also attempted.
Double Quantum Wire Magnetic Response
Authors: Smirnov Anatoly Yu. , Mourokh Lev G. , Horing Norman J.M.
Published: April 2011

The induced magnetic moment of a biased semiconductor tunnel-coupled parallel double quantum wire system is examined here. The wires are in a series arrangement with tunnel coupling to each other and to leads. Their parallel lengths and associated continuous spectrum are taken in the direction perpendicular to the lead-to-lead current. The equations of motion for the double-wire electron Green's function are formulated and analyzed using the transfer-tunneling Hamiltonian formalism. We determine the average magnetic moment of the double-wire system induced by a magnetic field applied perpendicular to the plane of the structure and we show that there are crossovers between diamagnetic and paramagnetic behavior, depending on the bias voltage, equilibrium chemical potential of the leads and temperature.
‥Recent Progress in Ferroelectic-gate FETs
Authors: Ishiwara Hiroshi , Park Byung-Eun
Published: April 2011

Recent progress in the research of ferroelectric-gate FETs is reviewed mainly from a view-point of the data retention characteristics. First, importance of insulator-inserted gate structures such as an MFIS (M; metal, F; ferroelectric, I; insulator, S; semiconductor) or MFMIS structure is described and the necessary conditions for the insulating buffer layer and the ferroelectric film are discussed. Then, experimental results for the SiO2 and high-k dielectric buffer layers combined with such ferroelectric films as SBT (SrBi2Ta 2O9) and BLT ((Bi,La)4Ti3O12) are presented, in which particular attention is paid to the discussion on the induced charge matching between the buffer layer and the ferroelectric film. As an example, it is shown in a Pt/BLT/LaAlO3/Si diode that the high and low capacitance values written by positive and negative pulses can be retained at least for 3 days. Finally, recent experimental results on the data retention characteristics of the ferroelectric-gate FETs are presented.
Photo-Induced Insulator-Semiconductor Transition in 12CaO·7Al2O3 (C12A7)
Authors: Hayashi Katsuro , Matsuishi Satoru , Kamiya Toshio , Hirano Masahiro , Hosono Hideo
Published: April 2011

We report the first electronic conduction in main group metal oxides (MGOs) or light-metal oxides, which are represented by alkaline-earth oxides, alumina, and silica. They are believed to be never converted to an electronic conductor. One of the MGOs, 12CaO·7Al2O3 (C12A7), has optical transparency, but electrical insulation limits intrinsic form. It is characterized by sub-nanometer sized cages in the lattice framework. Hydride ion, H- was incorporated into the cage by a thermal treatment of C12A7 single crystals or ceramics in hydrogen atmosphere. The product, C12A7:H, was colorless, transparent and a good insulator having electronic conductivity less than 10-10 S·cm-1. We found that the C12A7:H exhibits a coloration of yellowish green corresponding to optical absorptions at 2.8 and 0.4 eV with simultaneous conversion into an electronic conductor with 0.3 S·cm-1 at 300 K upon a irradiation of ultraviolet light. The conductive state continued even after the irradiation was stopped. Inversion to the insulator occurred when heated more than ∼320°C accompanying with rapid decay of the optical absorptions. When temperature rose above 550°C, H2 gas was released from the sample and the photosensitivity was lost. We consider that high concentration of F+-like centers are created by photo-released electrons from the H- anions being captured by empty cages. Further, a migration of the electrons at the F+-like centers may be responsible for the conduction. The visible light absorption loss is estimated to be only 1% for a 200 nm thick conductive C12A7:H films. The present properties provides novel applications such as direct optical writing of conducting wires on insulating transparent media.
Ferroelectric Field Effect Device
Authors: Schrott A. G. , Misewich J. A. , Ramesh R. , Nagarajan V.
Published: April 2011

A ferroelectric field effect transistor with an oxide channel layer and a lead zirconate titanate gate oxide has been fabricated. The channel is a strontium ruthenate/titanate solid solution with n type semiconducting behavior, which has sufficient OFF-state free carrier concentration to provide proper balancing charge for ferroelectric stability. The dependence of channel resistance with gate voltage at room temperature yields a hysteresis curve with two state at zero volts with a ΔR/R of 75% and a coercive voltage of 3 volts. The device was subjected to more than 1010 cycles with no degradation and was also operated at 60° C with a only a slight reduction in the switching ratio.
Growth and Characteristics of Single Crystals of Lithium Niobate
Authors: Batra A. K. , Gebre T. , Stephens J. , Aggarwal M. D. , Lal R. B.
Published: April 2011

Lithium niobate single crystal is an excellent material for various optical applications such as frequency conversion, optical switches, optical modulators and others. An automatic diameter control Czochralski crystals growth system has been designed and fabricated. A brief description of the entire system along with software developed has been described. With optimized growth parameters, pure and Fe/Mn doped crystals have been successfully grown using this system. Preliminary characterizations of these crystals have also been presented.
Engineering ZnO/GaN Interfaces for Tunneling Ohmic Contacts to GaN
Authors: Kaminska E. , Piotrowska A. , Golaszewska K. , Kruszka R. , Kuchuk A. , Szade J. , Winiarski A. , Barcz A. , Jasinski J. , Liliental-Weber Z.
Published: April 2011

We have investigated two approaches for the fabrication of thin ZnO films: sputter deposition from the ZnO target and thermal oxidation of vacuum deposited Zn. The microstructure and electronic properties after consecutive steps of the formation of n-ZnO/p-GaN contacts have been studied using electron transmission microscopy and x-ray photoelectron spectrometry. We have achieved ohmic contacts by Zn oxidation and explain their ohmic behaviour in terms of a tunnel n+-ZnO - p-GaN junction.
Texture Evolution During Crystallization Of Thin Amorphous Films
Authors: Liu Q.K.K. , Schumacher G.
Published: April 2011

Stress and energy distributions for crystallization of a thin amorphous film are calculated by means of 3D finite element method. The changes in energy are caused by elastic strain induced by different thermal expansion of the film and the substrate and by different mass densities of the crystal and the surrounding amorphous matrix. The calculations were performed for cubic crystal structure and for disc shaped crystals. Three crystal orientations (001), (011) and (111) were considered. Based on strain energy considerations the (001) orientation of crystals with respect to the film plane is energetically more favorable than (011) and (111) orientations. Interfacial and surface energies are certain to play a part in these effects as well.
Mo-Si Interface Formation by Ion Beam Sputter Deposition
Authors: Köhler A. , Gerlach J.W. , Höche T. , Chassé T. , Neumann H. , Frank W. , Wagner G. , Rauschenbach B.
Published: April 2011

Molybdenum-silicon (Mo-Si) multilayers for EUV lithography were prepared by ion beam sputter deposition at room temperature. The multilayer structure was determined by X-ray-diffraction and transmission electron microscopy. Textured molybdenum layers with preferential (110) orientation normal to the surface are observed. At the interfaces of all Mo-Si and Si-Mo pairs additional intermixing resulting in molybdenum silicide layers were noticed. These layers have a thickness of about 0.7 and 1.5 nm each, respectively. Due to this intermixing, the nominal thickness of the Mo and Si layers is reduced. The optical index contrast at the interface is also expected to decrease. This is in accordance with the obtained EUV reflectivity results of 64–65% at 13.4 nm which remains below the theoretical value of 74%. The formation of the silicide interface layers is discussed and an optimized deposition process focused on narrowing these transition layers is suggested.
Improvement of surface roughness by ultra-thin film deposition with oxygen cluster ion beam assist deposition
Authors: Toyoda Noriaki , Yamada Isao
Published: April 2011

Ta2O5 films were deposited on a rough surface (average roughness 1.3nm, peak-to-valley 14nm) and surface roughness evolutions and improvements by O2 gas cluster ion beam (O2-GCIB) assisted deposition was studied. The average roughness dramatically decreased from 1.3nm to 0.5nm after deposition of Ta2O5 films 20nm in thickness with 7 keV of O2 cluster ion beams. As there was no etching or sputtering of Ta2O5 film by 7keV O2-GCIB irradiations, O2-GCIB assist deposition realized significant improvement of surface roughness by additional deposition of Ta2O5 film whose thickness was close to the peak-to-valley of original surface. It is expected that morphological evolution of the film by GCIB assist deposition becomes completely different from conventional ion assist deposition due to energetic cluster ion impacts.
Diamond Synthesis From Acetone Vapor And Impurity Control
Authors: Gyoda Kaoru , Tanaka Yuki , Takagi Yoshiki
Published: April 2011

We synthesized diamond via gaseous phase of vaporized acetone. Molecular acetone decomposes to two methyl radicals with thermal activation. We propose here a new method for diamond synthesis with these methyl radicals from molecular acetone. With this method, we successfully synthesized diamond particles on Si substrates with shorter experimental time and lower energy consumption than conventional methods. With liquid carbon sources, such as acetone, impurity elements will be easily substituted in synthesized diamond thin film, which has wide applications for the future electronic devices. Nickel, Boron and Phosphorus doping results will be presented.
Properties of Titanium Oxide Thin Film Prepared with E-beam Evaporation
Authors: Hou Ping , Sun Lianchao
Published: April 2011

The optical and mechanical properties of TiO2 film prepared by ion-assisted e-beam evaporation have been examined in this research. Spectroscopic ellipsometry analysis revealed an inhomogeneous behavior in both optical property and growth structure, vertically from substrate to the top surface of the film. This phenomenon was further confirmed with the electron microscopic analyses. The effects of deposition rate, chamber pressure, anode voltage and current on the stress of TiO2 films were also investigated and reported. Further study showed that a structural homogeneous film could be obtained through TiO2-SiO2 co-evaporation.
Study of Pore Architecture in Silicon Oxide Thin Films by Variable-energy Positron Annihilation Spectroscopy
Authors: Ito Kenji , Kobayashi Yoshinori , Yu Runsheng , Hirata Kouichi , Togashi Hisashi , Suzuki Ryoichi , Ohdaira Toshiyuki , Egami M. , Arao H. , Sakurai C. , Nakashima A. , Komatsu M.
Published: April 2011

Application of porous silicon oxide thin films to nanotechnology is under intensive investigation. Introducing a large amount of nano pores into a silicon oxide matrix is important to develop low-k dielectrics for future ultra-large-scale integrated circuits (ULSI). In this work, we applied variable-energy positron annihilation to the characterization of porous silicon oxide thin films fabricated on silicon wafers by sputtering and spincoating. It was found that the sputtered film has higher open pore connectivity than that of the spincoated low-k film.
Surface Electrical Measurements of Photo-catalysis on Rutile TiO2(11O)
Authors: Kaisei Kiyohiro , Ishida Kenji , Horiuchi Toshihisa , Matsushige Kazumi
Published: April 2011

We have studied the electronic properties of single crystal rutile TiO2(110) with surface electrical techniques; impedance spectroscopy and high sensitivity DC measurements. In the impedance spectroscopy we observed a spectrum involved in the dissociation of pure water on the TiO2 surface under UV irradiation. In the high sensitivity DC measurements we observed oscillations of the surface photo-current with three different long periodicities, which might have originated from the oxidation and reduction processes of water on TiO2 via photo-catalytic reactions induced by UV irradiation.
TEM Investigation of the Core/Cladding Interface of La2O3-Al2O3-SiO2 Glasses for High Power Fiber Lasers
Authors: Cheng Shang-Cong , Dejneka Matthew J.
Published: April 2011

For fabricating Yb3+ doped La2O3-Al2O3-SiO2 glasses for high-powered fiber lasers, it is critical to choose the right core glass composition to obtain a high numerical aperture and to avoid phase separation. TEM techniques were used to study the relationship between the core composition and phase separation. In the study, inter-diffusion between the core and cladding glasses was found. The inter-diffusion caused large density fluctuation in the region of the core/cladding interface in fibers containing relatively high concentrations of La2O3. The TEM results were used to optimize the chemical composition of the core glass for high-power fiber lasers.
Study of The Quasicrystal Formation in Zr46.8Ti8.2Cu7.5Ni10Be27.5 Bulk Metallic Glass
Authors: Soubeyroux J.L. , Pelletier J.M. , Van de Moortèle B. , Epicier T.
Published: April 2011

The formation of the primary crystals appearing during isothermal annealing of the Zr46.8Ti8.2Cu7.5Ni10Be27.5 (Vit4) bulk metallic glasses has been studied by in-situ neutron diffraction, DSC, high resolution TEM and electron diffraction. We have evidenced that the primary phase appearing on the isothermal plateau at 620 K is quasicrystalline and is beryllium free. Moreover, we have prepared an alloy with the composition determined by EDX (without beryllium), and studied the formation of quasicrystals in this alloy. A comparison is done between the quasicrystalline phases appearing during the annealing of Vit4 and the quasicrystals prepared without beryllium in an amorphous alloy prepared by melt spinning. In particular they have the same intensities and peak positions in the diffraction studies.
Characterization of the Structure of a Novel Refractory Alloy Glass – Ni60Nb37Sn3
Authors: Tokarz Michelle L. , Daniels Matt , Bilello John C. , Rek Zofia
Published: April 2011

Bulk forms of Refractory Alloy Glasses (RAGs) of the composition Ni60Nb37Sn3 have recently been synthesized as part of a larger project for potential use in structural applications. Modeling efforts of such metallic glasses have traditionally involved the use of hard sphere models without regard to potential electron density fluctuations of individual components. X-ray characterization of these materials (in both reflection and transmission modes) provided scattering data necessary for subsequent radial distribution analysis, which gives structural information such as nearest neighbor distances and packing characteristics. A model radial distribution function (RDF) was constructed based upon a hard sphere space filling assumption and compared to the RDFs obtained from experimental scattering data. The experimental RDFs showed no difference from the model RDF within the limit of experimental error, with regard to nearest neighbor distances and coordination numbers of the first two nearest neighbors.

Additionally, transmission mode scattering experiments from a white beam x-ray source (E = 2 to 40 keV) demonstrated a through thickness amorphous structure of 2mm thick samples. Converted line profiles from the two-dimensional diffraction patterns from this experiment also showed agreement with reflection mode experiments. Samples of Vitreloy-106 (Zr57Nb5Cu15.4Ni12.6Al10) of similar thicknesses were used as standards due to the well-known behavior of this particular class of metallic glasses.
Effect of Loading Rate on Failure in Bulk Metallic Glasses
Authors: Jiao T. , Fan C. , Kecskes L.J. , Hufnagel T.C. , Ramesh K.T.
Published: April 2011

We have investigated failure in bulk metallic glass-forming alloys under dynamic compression. We implemented a recovery technique for the compression Kolsky bar to obtain dynamically deformed, intact specimens at various stages of deformation; this allows us to characterize the development of failure. We have also used high-speed photography to examine the failure process during the recovery experiments. The experimental results indicate that the failure under dynamic loading is somewhat different from that under quasi-static loading. Specimens subjected to quasistatic deformation developed multiple shear bands and substantial plastic deformations, while specimens subjected to dynamic (—strain rate ∼103 s-1) compressive loading fail by fracture along one dominant shear band. The mechanisms of dynamic failure in bulk metallic glasses are discussed on the basis of these experimental results.
Obsevations Ofstress-Induced Structuraldisorder and Fictive Stress in Bulk Metallic Glasses
Authors: Chen Ho-Sou , Kato Hidemi , Inoue Akihisa
Published: April 2011

The transformation from Newtonian to non-Newtonian viscous flow at the glass transition of Pd- and Zr-based alloy glasses has been investigated in compressive tests under either a constant strain rate, or a constant load. The transition occurs at a critical stress being nearly independent of temperature. The mechanism of the transition thus has been attributed to the stress-induced structural relaxation. This paper describes the evidence of stress-induced disorder as indicated by the change in the viscosity with stress and the evolution of specific heat of the alloy glasses subjected to non-Newtonian steady-state viscous flow. Also presented in this paper is the in-situ observation of structural disorder, by direct measurements of the temperature change of sample, in particular the soften process during a constant load deformation. The heat of evolution is then calculated, and found to scale as the logarithm of the normalized viscosity during entire deformation. This result implies that the relationship between the structural disorder, as indicated by the enthalpy change and viscosity is the same in the transient state during deformation as well as in the steady-state flow process. This is conceptionally very important in that it enables us to introduce a fictive stress which indirectly represents the glass structure.
High Temperature Deformation in the Amorphous or Partially Crystallized Zr41.2Ti13.8Cu12.5Ni10Be22.5 Bulk Metallic Glass
Authors: Wang Q. , Blandin J.J. , Suery M. , Pelletier J.M.
Published: April 2011

The high temperature deformation of the Zr41.2Ti12.5Cu13.8Ni10Be22.5 bulk metallic glass (BMG) is studied in the supercooled liquid region. Both fully amorphous and partially crystallized states are investigated. In the studied experimental domain, the amorphous alloy exhibits a Newtonian behavior at high temperature and/or low strain rate whereas a transition to non-Newtonian behavior is observed when the temperature is decreased and/or the strain rate is increased. In the Newtonian domain, the dependency of the viscosity upon temperature can be described by an Arrhénius law. As far as the as-received alloy is maintained at high temperature for which phase separation and primary crystallisation is expected, the flow stress continuously increases, which is at least partly attributed to a change in the residual amorphous phase.
Yucca Mountain Project Status
Authors: Kiess Thomas E. , Hanauer Stephen H.
Published: April 2011

The Yucca Mountain site was designated in July 2002 as the United States' location for a geological repository for spent nuclear fuel and other high-level radioactive wastes. This site designation was a watershed event in the history of the project, enabling the U.S. Department of Energy to seek a license from the U.S. Nuclear Regulatory Commission to construct and operate a geologic repository. Summarized below are the history and technical basis for this site designation and some key anticipated future events. Many of the significant events to date have been framed by the Nuclear Waste Policy Act (and Amendments) and the requirements of the regulatory standard.
Modelling the Sorption of Actinides onto Cement: An Approach with the Surface Co-Precipitation Model
Authors: Sugiyama Daisuke , Fujita Tomonari
Published: April 2011

An approach, assuming that co-precipitation on the surface of the cement phase dominates ‘sorption’, is proposed and discussed to interpret the sorption behaviour of the cement-complex system. This model takes account of the mineralogical composition of Ordinary Portland Cement (OPC) and assumes that Calcium Silicate Hydrates (C-S-H) dominates the sorption (fixation) of actinide onto OPC and that a small amount of surface co-precipitation of actinides with calcium hydroxide in the structure of C-S-H is formed during the late period of the fixation process. Therefore, this will be referred to as the ‘Surface Co-Precipitation Model (SCPM)’. The modelling results show generally good agreement between the predicted and measured distribution ratios of plutonium, thorium and neptunium, of which the dominant species in high pH solution are neutral species, on OPC.
Phases Formation Rate at Synthesis of Actinide Waste Forms
Authors: Laverov N. P. , Yudintsev S. V. , Lapina M. I. , Stefanovsky S. V. , Chae S. C. , Ewing R. C.
Published: April 2011

Rate of zirconolite (CaZrTi2O7) and pyrochlores (Gd2Ti2O7, CaCeTi2O7, and Gd2Zr2O7) formation by cold pressing and sintering (CPS) was studied. Batches were prepared from CaCO3, ZrO2, TiO2, CeO2, and Gd2O3 milled to size of 20–30 micrometers. Powders were compacted and sintered in air (oxygen) at 800–1600 °C for 0.5–55 hours. Samples were examined with XRD, SEM, and TEM. Phase formation rate was the fastest in Gd-Ti-O and Ca-Ce-Ti-O systems and the reactions were the slowest in Gd-Zr-O system. In zirconolite-based system reaction rate had intermediate value. For three systems at 1400 C equilibrium was reached in 3–5 hours, while in Gd-Zr-O system required about ten time longer reaction time. High formation rate was evidently one of the reasons to select the pyrochlore (Ca,Gd,U,Pu)2(Hf,Ti)2 O7 as a host for Pu in the USA.

We have also studied an effect of titanium on pyrochlore formation in the Gd-Zr-Ti-O system. The batches with nominal composition Gd2Ti0.4Zr1.6O7, Gd2Ti0.2Zr1.8O7, and Gd2Ti0.1Zr1.9O7 were compacted and annealed at 1500–1600 °C for 3–98 hours. In most of them along with pyrochlore non-reacted zirconium and gadolinium oxides were found. Their content reduced with temperature and sintering duration increasing. Pyrochlore composition was varied in different parts of the samples. Variation range decreased with rise of temperature, runs duration, and titanium content in the precursors. Partial substitution of Ti for Zr increased rate of pyrochlore formation. However, even at the highest titanium content (0.4 formula units) and temperature of synthesis (1600 °C) too long sintering duration in tens of hours is required to form a single-phase pyrochlore matrix. This makes their production from oxide precursor via CPS route inefficient. Inductive melting in a cold crucible or self-sustaining high-temperature synthesis are more promising methods for fabrication of the zirconate pyrochlores.
Effect of gas generation in matrices containing Ra-226 sources
Authors: Ojovan M. I.
Published: April 2011

Spent sealed radiation sources containing Ra-226 are immobilised either by the embedding into metal matrix materials or by welding enclosure in metal hermetic capsules. Encapsulation of sources ought provide safe conditions for storage for a long time period. Gaseous products (helium and radon) are generated due to the decay of Ra-226. The gas generation of Ra-226 sources can cause their rupture and consequent leakages of radionuclides. The higher the radioactivity of sources the higher the intensity of gas generation. The pressure of accumulated gas depends on many other factors therefore necessary limitations ought be determined for the encapsulation procedures. The process of gas generation is described herein in order to reveal the conditions of safe immobilisation of radium sources. A simple model describing immobilisation of sources in a metal host is proposed. Both helium and radon generation is considered followed by accumulation of gases near sources. Partial pressures of helium and radon are found depending on time. The diffusion of helium in the metal and decay of Rn-222 are taken into account. It is shown that the pressures of both radon and helium increase until some maximum values, after that the pressures slowly fall down. The maximum radon pressure being not very high is achieved during 65 days after immobilisation whereas helium increases its pressure during an extended period of time lasting for tens of years (137 years in the glass matrix) reaching very high magnitudes for powerful sources (137.5 MPa for a 100 GBq source in a glass capsule). Safe conditions of immobilisation of radium sources can be determined on the base of obtained data.
Analytical Electron Microscopy Study of Electron Radiation Damage in Iron Phosphate Glass Waste Forms
Authors: Sun K. , Wang L. M. , Ewing R. C.
Published: April 2011

A series of iron phosphate glass waste forms with compositions of 45Fe2O3-55P2O5, 20Fe2O3-80P2O5, and 20Fe2O3-20Na2O-60P2O5, namely FeP-1, FeP-2 and FeP-3, was studied by analytical electron microscopy (AEM). Transmission electron microscopy (TEM) bright-field (BF) imaging showed that under the electron irradiation, phase segregation occurred in both the FeP-1 and FeP-2 samples at high electron doses (3.84×1026 e/m2). In contrast, bubbles formed in the FeP-3 sample, even at a relatively low dose (2.88×1025 e/m2), which may be attributed to the migration of Na under irradiation as in the case in sodium borosilicate glass. Series electron energy-loss spectroscopy (EELS) analysis showed that the glass materials experienced mass-loss and composition variation. No obvious Fe valence state changes under irradiation were observed within the irradiation period.
Understanding the Behavior and Stability of Some Uranium Mineral Colloids
Authors: Mertz Carol J. , Fortner Jeff A. , Tsai Yifen
Published: April 2011

Colloids are believed to be a dominant mode of transport for radionuclides in the unsaturated zone and thus the characterization of the attributes of the colloids is important for understanding and predicting subsurface transport. As uranium-based spent nuclear fuels will be prevalent at the high-level waste repository at Yucca Mountain, we have examined the colloidal properties of a mixture of two uranium minerals. This paper presents results of analyses on colloidal suspensions of meta-schoepite, (UO2)4)(OH)6·5H2O, and UO2+x, in 10 mM uranyl nitrate or J-13 groundwater (from Yucca Mountain, NV). A suite of techniques was used to characterize the colloids. The colloids detected by dynamic light scattering in the 10 mM uranyl nitrate solution exhibited a mean size of ∼200 nm and a colloid concentration on the order of 1012particles/L. Likewise, large 200 nm colloids of meta-schoepite and UO2+xwere dominant in the transmission electron microscopy analyses. The colloids detected in the J-13 groundwater exhibited a bimodal distribution; large globular 100–200 nm UO2+xcolloids and needle-like colloids of meta-schoepite were observed (with extremely fine microstructure exhibited for the meta-schoepite colloids). Electrophoretic mobility measurements of the meta-schoepite and UO2+xsuspension in 10 mM uranyl nitrate indicated that the colloids are not stable at pH values less than or equal to five. While at pH values ≥ 5.5, the colloids of meta-schoepite and UO2+xin 10 mM uranyl nitrate are stable, exhibiting zeta potentials of -30 to -60 mV. The results in this paper show that stable colloidal suspensions of uranium substrate colloids are possible under aqueous, oxidizing conditions.
As, Se, and Re Sorption by Mg-Al Layered Double Hydroxides
Authors: Bryan Charles R. , Wang Yifeng , Xu Huifang , Braterman Paul S. , Gao Huizhen
Published: April 2011

Layered double hydroxides (LDHs) have high anion exchange capacities and are easily synthesized in the laboratory, giving them considerable potential as adsorbents for anionic contaminants in the environment. In this study, sorption of arsenate, selenate, and perrhenate by uncalcined Mg-Al LDH (Mg:Al ratio 2.4:1) was evaluated. To investigate sorption mechanisms, the substrate was added to the solution both as a pre-wetted slurry (allowing 24 hours for rehydration), and as a dry powder. Sorption results varied for different anionic complexes. For selenate and perrhenate, an incubation time was required for the dry material to achieve the same Kd value as the pre-wetted material. For arsenate, however, both wetted and dry materials initially had high Kds, which decreased with time. These behaviors are attributed to the structural reconstruction of the calcined LDH material in solution. Se and Re are not highly sorbed by the phases present in the calcined material, but are incorporated as interlayer anions in the LDH structure upon hydration and reconstruction, while As may be sorbing onto the dehydration products, and being partially released during rehydration.
The Behavior or Light Water Reactor Fuel after the Cladding is Breached under Unsaturated Test Conditions
Authors: Cunnane J. C. , Fortner J. A. , Finch R. J.
Published: April 2011

Experiments were conducted to examine the corrosion behavior of fuel and cladding under test conditions selected to determine how fuel with breached cladding may behave under unsaturated repository conditions. The results discussed here were obtained from two test samples, each consisting of ∼3.5-in. segments of ATM103, a moderate-burnup (∼30 GWd/MtU) PWR fuel, exposed to humid air at 175°C. Visual examination of the samples after 540 days revealed that each had developed an axial crack that passed through a drilled hole in the cladding and ran the full length of the sample.

Destructive examination of the fuel and cladding showed that the cladding had experienced extensive fuel-side corrosion. Metallographic examination of the cladding shows the hydride distribution near the drilled hole, the fracture surfaces, and the fuel-side corrosion products. Electron microscopy (SEM, TEM) and electron diffraction analyses were used to characterize the corrosion products. The results indicate that the fuel-side corrosion of the cladding and the specific volume increase associated with the formation of the corrosion products caused the hoop stresses that resulted in the observed axial splitting. The implications of these results for the expected evolution of the spent fuel cladding after the cladding is initially breached in an unsaturated repository are discussed.
Observation of Studtite and Metastudtite on Spent Fuel
Authors: McNamara Bruce , Buck Edgar , Hanson Brady
Published: April 2011

We have characterized uranyl peroxide phases on commercial spent nuclear fuel (SNF) samples formed under immersion conditions. At short times, crystallites of metaschoepite were observed on the hydrated fuel particles. Over a two-year period, all evidences of metaschoepite disappeared and in many samples, the fuel particles appeared to be coated by a new alteration phase. Additionally, milligrams of corroded fuel aggregates were observed at the sample air-water interface in each sample. The corrosion phases on bulk fuel and on the suspended materials were examined by SEM, EDX, and XRD and were identified as studtite (UO4·4H20) and metastudtite (UO4·2H20), respectively. The reason for the partitioning of the two phases is unclear at this time. SEM micrographs of the bulk powders indicated extensive surface corrosion and fragmentation of particles.
Programmable Technologies for Micro- and Nano-Scale Pattern and Material Transfer and Possible Applications for Control of Self-Assembly
Authors: Nagel David J.
Published: April 2011

Programmable methods for transferring materials to surfaces in patterns can produce structures with micrometer and nanometer scale features. All such technologies involve combinations of information, materials and energy. The materials in additive technologies can originate in the vapor phase, as liquids or suspensions, or as solids. The energy can come from laser, electron or ion beams, or the pressures used in writing, dispensing, jetting or flow methods. Many of the programmable techniques do not require high temperatures, so they can be used to make fine-scale structures of organic and bio-materials, and even live biologicals. Quantitative comparisons of both additive and subtractive programmable methods show that only a few, based on electron or ion beams, or on proximal probes, are capable of making nanometer-scale structures. Assembly methods, notably self- and directed-assembly, should prove to be central to the realization of manufacturable nanotechnology. Programmable deposition technologies may be used to supply materials for, and otherwise control self-assembly processes. The four sets of technologies, namely masked lithography, direct-write techniques, self-assembly and directed-assembly, provide a versatile and powerful toolbox for making micro-and nano-meter scale devices and systems.