bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical Amplification In Nanocrystalline Silicon Superlattices
Authors: Ruan J. , Chen H. , Fauchet Philippe M.
Published: April 2011

Today, the overall performance of a multi-chip computing system is limited by the interconnection delay between chips. Conventional interconnects based on metal lines are expected to cause unmanageable problems with speed and power dissipation. Optical interconnects provide a solution to these problems. However, the low quantum efficiency associated with radiative recombination in silicon has so far prevented the demonstration of a practical laser. Recently stimulated emission has been demonstrated in Si nanocrystals prepared by ion-implantation. The reported material gain is high enough to realize a practical Si based laser. However, the optical filling factor in these samples was less than 10% due to the poor wave guiding nature of these structures.

In this work, we explore a possible way to achieve optical gain in nanocrystalline silicon superlattices. The samples are produced by growing alternating layers of amorphous silicon and SiO2 and then using a two step crystallization method to transform each amorphous silicon layer into a high density array of silicon nanocrystals having identical size. The waveguide structure is formed by sandwiching the superlattice between cladding layers. To measure optical gain we use the variable stripe length method where the amplified spontaneous emission emitted from the edge is measured as a function of the excitation length. Tuning from loss to gain was observed by just varying the pump power.
Graphyne nanotubes: New Families of Carbon Nanotubes
Authors: Coluci Vitor R. , Braga Scheila F. , Legoas Sergio B. , Galvão Douglas S. , Baughman Ray H.
Published: April 2011

Fundamentally new families of carbon single walled nanotubes are proposed. These nanotubes, called graphynes, result from the elongation of covalent interconnections of graphite-based nanotubes by the introduction of yne groups. Similarly to ordinary nanotubes, armchair, zig-zag, and chiral graphyne nanotubes are possible. We present here results for the electronic properties of graphyne based tubes obtained from tight-binding and ab initio density functional methods.
Evolution of Carbon Self-Assembly in Colloidal Phase Diagram
Authors: Bouda V.
Published: April 2011

The growth of the self-assembled structure of carbon colloidal particles has been studied [1]. The system of carbon particles was processed in electrical field in polymer melt with controlled ionic concentration. The interpretation of the complex evolution of the self-assembled structure of carbon particles was given in terms of phase transitions of colloidal systems of carbon particles.

Interactions between doublets of carbon black (CB) particles are interpreted in terms of DLVO approximation of interaction energy as multiples of average thermal fluctuation kT. Plots of the sum of energy of electrostatic repulsion and energy of van der Waals attraction versus separation between the doublets show the energy barriers to coagulation of high B and the energy wells with the secondary minima of depth W. The colloidal phase transitions appear at critical conjuncture of the concentration of ions in the medium and surface potential on the colloids. Six transition lines determine five phases of the assembly of carbon colloids in the proposed colloidal phase diagram: lateral vapor + axial vapor (vapor), lateral liquid + axial vapor (columnar liquid crystal), lateral liquid + axial liquid (smectic LC), lateral liquid + axial solid (nematic LC), and lateral solid + axial solid (solid).

The diagram provides a tool to control the evolution of carbon self-assembly. The eventual morphology depends on the route of the steps of the processing. During the time elapsed in the LC state, the structure can reorganize and the eventual coagulation produces various crystals. On the contrary, the route outside the LC state can produce glass.
Dramatic Effect of Temperature on Metal-oxide Nanostructures: Oxidation of Cu Films by In situ UHV-TEM
Authors: Zhou Guangwen , Yang Judith C.
Published: April 2011

We investigated the temperature effect on the Cu2O morphology by oxidizing Cu(100) thin films at the temperature ranging from 350°C to 1000°C. We demonstrated that dramatically different morphologies of oxide nanostructures can be achieved by modifying the oxidation temperature. Quasi-one-dimensional Cu2O structures with aspect ratios as large as 40:1 were formed at the oxidation temperature of 600°C. The in situ observation data on the elongation of Cu2O islands agree with the energetic calculations based on the balance between surface and interface energies and the elastic stress relaxation in the three dimensional islands.
Dilute Group III-AsN: Bonding of Nitrogen in GaInAsN and AlGaAsN on GaAs and Realization of Long Wavelength (2.3 μm) GaInAsN QWs on InP
Authors: Serries D. , Geppert T. , Köhler K. , Ganser P. , Wagner J.
Published: April 2011

Recent results on the local bonding of nitrogen in dilute GaInAsN and AlGaAsN on GaAs are reviewed, revealing that bonding of nitrogen in GaInAsN is controlled by an interplay between bond cohesive energy and reduction of local strain. Thus, III-N bonding in GaInAsN can be changed from Ga-N to In-N by post-growth thermal annealing. In AlGaAsN, in contrast, nitrogen bonds preferentially to Al, i.e. Al-N bonds are formed, due to the much larger cohesive energy of the Al-N bond. Further, results on indium-rich highly strained GaInAsN quantum wells on InP substrate are reported, showing room-temperature photoluminescence at wavelengths up to 2.3 μm. This result demonstrates the potential of high indium content dilute GaInAsN for InP-based long wavelength diode lasers.
Composition of β-FeSi2 Thin-Films Grown by a Pulsed Laser Deposition Method
Authors: Uekusa Shin-ichiro , Yamamoto Masahiro , Tsuchiya Keiichi , Miura Noboru
Published: April 2011

We prepared othorhombic β-FeSi2 thin-films on Si(111) substrates by a pulsed laser deposition method (PLD) and studied the relationship between bandgap energy and composition ratio by themal annealing of β-FeSi2 thin-films on Si substrate. When the substrate temperature was 500·°C, β-FeSi2 thin-films were grown on Si(111) substrates. The sample prepared on Si(111) substrate was a monocrystalline structure since only (220) or (202) signals of β-FeSi2 were observed. A large amount of defects existed in the thin film. These defects were reduced by using high temperature and long time annealing. Heat treatment of 20 hours or more time was effective for reducing diffect. The iron diffusion of β-FeSi2/Si interface was observed by RBS spectra. Composition ratio of the prepared β-FeSi2 was Fe:Si=30:70. As-deposition β-FeSi2 was Si-rich. The p-n diode characteristics of these heterostructure diodes were investigated by C-V measurements. β-FeSi2/Si interface prepared by 400· °C was an ideal one-side step junction. The surface roughness was decreased by annealing.
Characterization of Metallic and Metal Oxide Nanoparticles Produced by Electrothermal-Chemical Synthesis
Authors: Schroder Kurt A. , Wilson Dennis E. , Kim Kyoungjin , Elliott Henry E.
Published: April 2011

Electrothermal-Chemical Synthesis (ETCS) is a relatively new process for the synthesis of nanocrystalline powders. With ETCS nanoparticles are formed through the controlled chemical reaction of a metal plasma, produced by cathode erosion, with a gas. By controlling the quench rate of the plasma precisely, the average particle size can be varied from as small as 6–8 nm up to 100 nm and above. The process is quite versatile and can be used to produce a wide variety of nanopowders including metals, oxides, and nitrides. However, before the nanoparticles produced by this process (or any other process for that matter) can find wide scale commercial acceptance, their physical properties must be characterized. Among many characterization techniques, electron microscopy can provide structural, chemical, and morphological information on nanoparticles. In this paper, we will discuss the SEM and TEM characterization of metal and metal oxide nanoparticles powders produced by ETCS.
Yellow-green emission for ETS-LEDs and lasers based on a strained–InGaP quantum well heterostructure grown on a transparent, compositionally graded AlInGaP buffer
Authors: McGill Lisa , Wu Juwell , Fitzgerald Eugene
Published: April 2011

Epitaxial-transparent-substrate light emitting diodes with a primary emission peak at 590nm and a secondary peak at 560nm have been fabricated in the indium aluminum gallium phosphide (InAlGaP) system. The active layer consists of an undoped, compressively strained indium gallium phosphide (InGaP) quantum well on a transparent In0.22(Al0.2Ga0.8)0.78P/ ∇x[1nx(Al0.2Ga0.8)1-xP] /GaP virtual substrate. Theoretical modeling of this structure predicts an accessible wavelength range of approximately 540nm to 590nm (green to amber). Emission with a peak wavelength of 570nm has been observed via cathodoluminescence studies of undoped structures with a quantum well composition of In0.35Ga0.65P. Light emitting diodes have been fabricated utilizing simple top and bottom contacts. The highest LED power of 0.18μW per facet at 20mA was observed for a quantum well composition of In0.32Ga0.68P and a bulk threading dislocation density on the order of 7×106 cm-2. The spectrum of this device was composed of two peaks: a weak peak at the predicted 560nm wavelength and a stronger peak at 590nm. Based upon superspots present in electron diffraction from the quantum well region, we believe that the observed spectrum is the result of emission from ordered and disordered domains in the active region. The same device structure grown with a bulk threading dislocation density on the order of 5×107 cm-2 exhibited an identical spectral shape with a reduced power of 0.08μW per facet at 20mA. For a quantum well composition of In0.37Ga0.63P and an overall threading dislocation density on the order of 5×107 cm-2, a single peak wavelength of 588nm with a power of 0.06μW per facet at 20mA was observed.
Lanthanide Doped Cubic Boron Nitride
Authors: Vetter U. , Taniguchi T. , Wahl U. , Correia J. , Müller A. , Ronning C. , Hofsäss H. , Dietrich M. , Collaboration Isolde
Published: April 2011

First basic investigations on lanthanide implanted cubic boron nitride were performed with focus on the site of the implanted ions as well as their luminescence behaviour. In order to investigate the lattice sites of lanthanides in c-BN, we implanted single crystal c-BN with radioactive 139Ce ions using the on-line isotope separator ISOLDE at CERN. After the samples were annealed at 1193 K, direct lattice location studies were performed using the conversion electron emission channeling technique and showed a near-substitutional fraction of the implanted ions of at least 10 %. Additionally, poly crystalline c-BN samples were implanted with stable Eu and Tm ions in order to investigate the luminescence properties of triply ionised lanthanides in the c-BN host. It is found that the annealing of the implanted samples at high temperature and high pressure results in strong luminescence of the Eu3+ ions in the orange and moderate luminescence of the Tm3+ ions in the blue. In case of the Eu implanted samples, the luminescence is assigned to the 5D0-7FJ intra-4f electron transitions of Eu3+, in case of the Tm implanted samples, detectable transitions are 1D2-3H6, 1D2-3F4, 1G4-3H6. of Tm3+.
Growth and characterization of epitaxial films of ZnGeP2.
Authors: Verozubova G.A. , Gribenyukov A. I. , Ohmer M.C. , Fernelius N.C. , Goldstein J.T.
Published: April 2011

Thermodynamic analysis of the vapor phase over ZnGeP2 in Zn-Ge-P-Cl system has been carried out. The analysis showed that this system can be used for the vapor growth of ZnGeP2. Homoepitaxial layers of ZnGeP2 were grown in a closed system using chemical vapor transport. Electrical and photoluminescence properties of the layers were studied, and crystal lattice parameters were measured. Comparison of properties for bulk and vapor grown ZnGeP2 crystals were carried out. It was found that the vapor grown crystals have more perfect structure than the bulk ones, particularly, they have significantly lower vacancy concentration.
Domain Wall Magnetoresistance and Complex magnetic Response in Antiferromagnetically Coupled Fe/Cr Multilayers
Authors: Aliev F. G. , Villar R. , Schad R. , Martinez J. L.
Published: April 2011

For antiferromagnetically coupled Fe/Cr(100) multilayers the low field contribution to the resistivity, which is caused by the domain walls (DWs), is strongly enhanced at low temperatures. The low temperature resistivity increases approximately according to a power law with the exponent 0.7–1. This behaviour can be explained by the suppression of anti-localization effects by the nonuniform gauge fields caused by the domain walls. Analyses of complex low frequency magnetic susceptibility shows an enhancement of the magnetic losses at low magnetic fields, which may be related to the AC field induced DWs movement. At low temperatures (T<100K) DWs become pinned. For frequencies (102<f< 103) Hz at temperatures below 10K, this hysteretic low field peak in the magnetic losses transforms to a non-hysteretic dip for |H|< 20 Oe, indicating a possible qualitative change in the dynamics of the DWs. The frequency dependence of the dissipation at 2K, may be reasonably well fitted by the expression that describes the losses of a damped oscillator with a single relaxation time of about 10-4 sec.
Ab initio study of CPP transport in Fe/Cr/Fe trilayers: influence of transition metal impurities
Authors: Herper Heike C. , Entel Peter , Szunyogh Laszlo , Weinberger Peter
Published: April 2011

The transport properties of Fe(001)/Cr/Fe(001) trilayers are discussed with respect to the influence of transition metal impurities in form of layers. We are able to show that the periodicity of the giant magnetoresistance is directly influenced by the interlayer exchange coupling (IEC). Furthermore, it is observed that the behavior of the IEC strongly depends on whether an impurity overlayer of Mn or V is used. It turns out that the size of the GMR is only little effected by 3d-transition metal impurities, which is in agreement with the experimental findings. The electronic and magnetic properties of the trilayers have been investigated within the fully relativistic, spin-polarized SKKR method and the LDA. The transport properties of the Fe/Cr/Fe systems have been derived from the fully relativistic spin-polarized Kubo-Greenwood equation.
In-Plane Transport of Doped Manganite Trilayers
Authors: Alldredge Lisa Berndt , Suzuki Y.
Published: April 2011

We have synthesized and characterized doped manganite trilayers composed of La0.7Sr0.3MnO3 (LSMO) and La0.7Ca0.3MnO3 (LCMO). Because of the isostructural nature of the Sr and Ca doped manganites, trilayers exhibit good epitaxy as observed in X-ray diffraction. Magnetization measurements reveal magnetization values consistent with bulk values. In-plane transport of these trilayers reveals anisotropic magnetoresistance and high field negative magnetoresistance attributed to the suppression of spin fluctuations at high fields.
Electro-structural and Film Growth Properties of Room-temperature Deposited Indium-Tin-Oxide on Polymer Substrates
Authors: Park Sung Kyu , Han Jeong In , Moon Dae Gyu , Kim Won Keun , Kwak Min Gi
Published: April 2011

Electrical and structural properties of indium-tin-oxide (ITO) films on flexible polymer substrates were investigated. Room temperature sputtered ITO films on polymer substrates are initially amorphous but become partially crystalline as oxygen partial pressure decreases under 1.5%. The crystallinity shows more (400) and (222) preferentially oriented textures as decreasing of oxygen pressure resulting in lower resistivity. Moreover, an interesting growth property similar to 3D growth mechanism including larger grain size and columnar-like grain structure was also observed. It is considered that the columnar-like grain structure is probably attributed to the poor surface morphology of polymer substrates Based on the experiments, we obtained high performance ITO films on a polycarbonate substrate including 3.8 × 10 –4 Ω-cm in resistivity and transmittance above 80% in the visible ranges at 0.2% oxygen partial pressure.
Reactive Solid-Phase Epitaxy: ∼ A novel growth method for single-crystalline thin films of complex oxides with superlattice structure ∼
Authors: Ohta H. , Nomura K. , Hiramatsu H. , Suzuki T. , Ueda K. , Orita M. , Hirano M. , Ikuhara Y. , Hosono H.
Published: April 2011

We have developed a novel growth method for single-crystalline film of natural superlattice oxides and named the method “Reactive Solid-Phase Epitaxy (R-SPE).” Single-crystalline thin films of homologous series In-GaO3(ZnO)m (m=integer) are fabricated by the R-SPE method and its growth mechanism, especially a role of ZnO epitaxial layer, is clarified. High-temperature annealing of bi-layer films consisting of an amorphous InGaO3(ZnO)5 layer deposited at room temperature and an epitaxial ZnO layer on YSZ substrate allows for the growth of single-crystalline film with a controlled chemical composition. The ZnO layer plays an essential role in determining the crystallographic orientation, while the thickness ratio between the two layers controls the film composition.
The Interactions between SBT and Transition Metal Studied by DV-Xa Method
Authors: Liu Hanxing , Min Xinmin , Sun Xiaoqin , Xiao Jing , Ouyang Shixi
Published: April 2011

In the present study electronic structures of the interface between SBT and transition metal including Au, Ag, Cu, Ni, Pt were studied by employing DV-Xα method. The calculations were performed on cluster models of the interface which were SrTa8O36-M2 (M=Au, Ag, Cu, Ni, Pt, and Bi). The calculations results including Fermi level, bond order, atomic charge distribution shown that there existed significant hybridization between metal d states and the O2p–Ta3d states. The Fermi levels of the metals located in the gap of SBT and metal-induced states, and the position of the Fermi levels did not change greatly due to the different transition metals. The charge of Sr and the transition metal atom transferred to oxygen atom, so that the bonding between Sr (or metal atom) and O was almost ionic bonding. The interactions between transition metal and SBT mainly came from the Coulomb interaction.
A Comparison of Tunneling Through Thin Oxide Layers on Step-free and Normal Si Surfaces
Authors: Oliver Antonio C. , Blakely Jack M.
Published: April 2011

Surface and interface morphology may play an important role in the electrical performance of metal-oxide-semiconductor (MOS) devices with small characteristic dimensions. In previous work we showed how steps on the silicon surface influence the Si-SiO2 interface morphology and the outer oxide surface morphology following thermal oxidation [1]. The Si-SiO2 interface morphology is largely determined by the starting silicon substrate step distribution and atomic steps at the Si surface cause an inherent variation in oxide thickness after thermal oxidation. In the present study we report how roughness caused by increased interfacial step density may affect the electronic tunneling characteristics of an MOS device structure. To determine the extent to which the step morphology plays a role in the tunneling behavior of such devices, similar arrays of capacitors were fabricated on both Si surfaces with reduced step density and surfaces which had not undergone any special surface step removal treatment. The leakage currents due to tunneling for the two types of capacitors were measured and compared. Atomic steps cause an effective decrease in oxide thickness in those capacitors without reduced step density and this leads to increased leakage current.
A Structural Study of the Amorphous to Crystalline Transformation in In2O3 Thin Films
Authors: Yaglioglu Burag , Yeom Hyo-Young , Chason Eric , Paine David C.
Published: April 2011

We have investigated the structure of sputter deposited amorphous and crystalline indium oxide films by electron diffraction. Selected area diffraction patterns were recorded for both states from which radial density functions were derived. The comparison of the crystalline radial density function to the amorphous one shows that the first nearest neighbor distance corresponding to the In-O bond length is 2.2Å and is the same for both states. A model density function for crystalline structure was used to explore the difference in higher order peak positions. We report that the In-In separation in amorphous state has a single characteristic distance of 3.6Å compared to the crystalline state which has two non-equivalent In sites and consequently different separations as In(1)-In(2) at 3.4Å and In(2)-In(2) at 4.3Å.
Heteroepitaxial Growth of a Wide Gap P-type Oxysulfide, LaCuOS
Authors: Hiramatsu Hidenori , Ueda Kazushige , Ohta Hiromichi , Hirano Masahiro , Hosono Hideo
Published: April 2011

Epitaxial films of LaCuOS, a wide gap p-type semiconductor, were grown on yittria- stabi-lized-zirconia (YSZ) (001) or MgO (001) substrates by a reactive solid phase epitaxy (R-SPE) method. Crystal quality, electrical and optical properties on the epitaxial films on each substrate are examined in this paper. Achievement of the heteroepitaxial growth of LaCuOS on the MgO (001) substrate improves optical properties of LaCuOS such as spectral bandwidths and emission intensity, suggesting that the MgO (001) substrate is more preferable than the YSZ (001) for epitaxial growth substrate for LaCuOS.
Comparison of Growth Morphology in Ge (001) Homoepitaxy Using Pulsed Laser Deposition and MBE
Authors: Leonard John P. , Shin Byungha , McCamy James W. , Aziz Michael J.
Published: April 2011

Differences in the homoepitaxy of Ge(001) are explored using a dual MBE/PLD deposition system. With identical substrate preparation, temperature calibration, background pressure and analysis, the system provides a unique comparison of the processes arising only from kinetic differences in the flux and at the surface. All films show mounded growth. At substrate temperatures below 200°C, PLD films are smoother than MBE films, whereas they are similar at higher temperatures.
X-Ray Characterization of Nanostructured Semiconductor Short-Period Superlattices
Authors: Li Jianhua , Moss S. C. , Holy V. , Norman A.G. , Mascarenhas A. , Reno J.L.
Published: April 2011

Spontaneous lateral composition modulation during semiconductor thin film growth offers a particularly versatile and cost-effective approach to manufacture nanoscale devices. Recent experimental and theoretical studies have revealed that regular lateral composition modulation can be achieved via MBE growth of the so-called short-period superlattices and can be optimized via appropriate control of the global strain, substrate surface, and processing conditions. To characterize this phenomenon, we used synchrotron x-ray scattering to identify the interfacial morphology and laterally modulated composition profile of nearly strain-balanced InAs/AlAs short-period superlattices. Our results were compared with a theoretical model. It is shown that the lateral composition modulation is predominately caused by a vertically correlated morphlogical undulation of the superlattice layers.
Motions of Si(100) Steps at Low Temperature. A Study by Molecular Dynamics
Authors: Mazzone A.M.
Published: April 2011

An isothermal Molecular Dynamics simulation method has been applied to the study of motions of steps on Si(100). The steps have a monolayer height and their temperature is below the roughening transition (i.e. T≤ 1000K). The calculations predict spontaneous motions even in this low temperature range and describe the functional dependence of the displacements of step-edge atoms on the step geometry, size and temperature and on the nature of the interatomic forces. Owing to the use of the Fourier analysis, these results can be applied to structures of realistic size by a proper scaling of the wavevectors.
Time Evolution of the Microstructure of VO2(B) Films Deposited on Glass by MOCVD
Authors: Sahana M. B. , Subbanna G. N. , Shivashankar S. A.
Published: April 2011

Thin films of VO2(B), a metastable polymorph of vanadium dioxide, have been grown on glass by low-pressure metalorganic chemical vapor deposition (MOCVD). The films grown for 90 minutes have atypical microstructure, comprising micrometer-sized, island-like entities made up of numerous small, single-crystalline platelets (≅1 μm) emerging orthogonally from larger ones at the center. Microstructure evolution as a function of deposition time has been examined by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The metastable VO2(B) transforms to the stable rutile (R) phase at 550°C in inert ambient, which on cooling convert reversibly to M phase. Electron microscopy shows that annealing leads to the disintegration of the VO2(B) platelets into small crystallites of the rutile phase VO2(R), although the platelet morphology is retained. The magnitude of the jump in resistance at the semiconductor-to-metal, VO2(M)→VO2(R) phase transition depends on the arrangement of polycrystalline platelets in the films.
Characterization of Eu- and Y-polytantalate Films Deposited by RF Diode Sputtering
Authors: Vasilyev Vladimir S. , Drehman Alvin J. , Bouthillette Lionel O.
Published: April 2011

Using radio frequency (RF) diode sputtering of sintered stoichiometric (Eu,Y)2O3:7Ta2O5 ceramic targets, thin films (0.2–0.6 m) of Eu- and Y-polytantalates, and their solid solution EuxY1-xTa7O19 were deposited on fused silica, Si (100) and sapphire (001) substrates under various sputtering conditions. As-grown films were amorphous, and were crystallized by post-annealing in oxygen at 900 to 1000 °C. The influences of deposition gases (Ar and O2-partial pressure) substrate material, deposition time, substrate temperature, and post-annealing time and temperature on the structural, morphological and photo-luminescence (PL) properties of Eu 3+-ions in the films have been studied and compared to properties of sintered (Eu,Y)Ta7O19 ceramic and EuTa7O19 single crystals.
Crystal Phases of Glass-Forming Mixtures
Authors: Fernández Julián R. , Harrowell Peter
Published: April 2011

We compare the potential energy at zero temperature of a range of crystal structures for a glass-forming binary mixture of Lennard-Jones particles. The lowest energy ordered state consists of coexisting phases of a single component face centered cubic structure and an equimolar cesium chloride structure. An infinite number of layered crystal structures are identified with energies close to this groundstate. We demonstrate that the finite size increase of the energy of the coexisting crystal with incoherent interfaces is sufficient to destabilize this ordered phase in simulations of typical size. Specific local coordination structures are identified as of possible structural significance in the amorphous state. We observe rapid crystal growth in mixtures near the equimolar composition.
Nanocrystalline ZrN particles embedded in Zr-Fe-Cu-Al-Ni amorphous matrix.
Authors: Bab Marisa A. , Damonte Laura C. , Mendoza-Zélis Luis , Deledda Stefano , Eckert Jurgen
Published: April 2011

Melt-spun Zr64Al7Cu17Ni10Fe2 amorphous ribbons were milled under nitrogen atmosphere for different times. The resulting nitrided powders were studied by x-ray diffraction, Mössbauer spectroscopy and differential scanning calorimetry. The formation of nanosized crystalline particles, with cubic δ-ZrN structure, dispersed in the amorphous matrix was observed along with a change in the composition of the amorphous phase. Prolonged milling leads to the additional precipitation of late transition metals (Fe,Ni,Cu). The nitride particles affect the crystallization behavior and modify the thermal stability of the amorphous alloy.
Alloy Phase Formation in Isolated Nanometer-sized Particles in the Au-Sn and Sn-Bi Systems
Authors: Lee Jung-Goo , Mori Hirotaro , Yasuda Hidehiro
Published: April 2011

Alloy formation in nanometer-sized particles has been studied as a function of the particle size by in-situ transmission electron microscopy using particles in the Au-Sn and Sn-Bi systems. When the size of particles is larger than a critical value, essentially similar phase equilibrium was observed in nanometer-sized particles and bulk materials in the both systems. But a solid amorphous phase was formed over a compositional range near the eutectic composition when the size of particles was smaller than about 7 nm in diameter in the former system, whereas a fluid amorphous phase was formed when the size of particles is smaller than about 10 nm in diameter in the latter system. These amorphous phases directly changed into liquid phases upon heating and solidified into amorphous phases upon subsequent cooling. The formation of these thermodynamically stable amorphous phases can be attributed to the the suppression of the eutectic temperature (Teu) associated with size reduction being so large that Teu is below the glass transition temperature (Tg).
Electron Irradiation Effect on the Microstructure and Phase Stability of Fe-Based Fe71.0Zr9.0B20.0 Metallic Glass with a Wide Supercooled Liquid Region
Authors: Nagase T. , Umakoshi Y. , Sumida N.
Published: April 2011

The effect of electron irradiation on the microstructural change and phase stability of melt-spun Fe71.0Zr9.0B20.0 metallic glass having a wide supercooled liquid region of 71 K was examined. Crystallization from the amorphous phase was accelerated by electron irradiation, and this irradiation was effective in producing a nanocomposite microstructure.
Atomistic Theory of Bulk Metallic Glass Formation
Authors: Egami T.
Published: April 2011

Bulk metallic glass can be formed only when the critical cooling rate for glass formation is reduced to 100–2 K/sec. However, a cooling rate achievable with molecular dynamics simulation is higher by many orders of magnitude, so the gap has to be abridged by analytical theories. We propose a theory of bulk metallic glass formation based upon our early theories of glass formation composition. The critical concepts include the idea of local glass transition, distributed local glass transition temperatures and coincident local fluctuation for atomic transport. Strong repulsion between small atoms was recognized for the first time as the necessary condition for bulk glass formation.
Two-liquid Separation in Undercooled Co-Cu Alloys
Authors: Lu X. Y. , Kolbe M. , Herlach D. M.
Published: April 2011

Electromagnetic levitation and drop tube processing were used to investigate the solidification behaviour of Co50Cu50 alloys. Deep undercooling was achieved down to 241 K, the alloy melts separated into Co-rich L1-phase and Cu-richL2-phase before solidification. In drop tube processing, the Cu-rich phases disperse morehomogeneously than in EML. This is due to the reduced gravity and high cooling rate during free fall and solidification.
Glass-Forming Ability and Crystallization Behavior in High-Density Bulk Metallic Glasses
Authors: Kecskes Laszlo J. , Trevino Samuel F. , Woodman Robert H.
Published: April 2011

Alloys of composition (Hfx,Zr1-x)57Ti5Cu20Ni8Al10 and (Hfx,Zr1-x)52.5Ti5Cu17.9Ni14.6Al10, where x = 0, 0.2, 0.4. 0.6, 0.8, and 1.0, were evaluated for glass-forming ability, and the devitrification behavior on heating of the (Hfx,Zr1-x)52.5Ti5Cu17.9Ni14.6Al10 glasses was examined. Glass-forming ability was determined by suction casting 3-mm-diameter rods, followed by neutron diffraction examination. Results show that substitution of Hf for Zr in these alloys degrades glass-forming ability, and that this effect is more pronounced in compositions of the (Hfx,Zr1-x)57Ti5Cu20Ni8Al10 type. To examine devitrification behavior, thermal analysis was used to identify temperatures at which exothermic events occurred on heating in the (Hfx,Zr1-x)52.5Ti5Cu17.9Ni14.6Al10 series. The amorphous alloys were then subjected to annealing treatments corresponding to the exothermic events. Crystal structures formed during annealing were probedby neutron diffraction, and the size scale of features developed examined by small-angle neutron scattering. Results show that the initial decomposition produces a structure witha characteristic length scale, but it is not yet possible to comment on the mechanism.
Relaxation mapping analysis with an hyperbolic heating rate
Authors: Dias C. J.
Published: April 2011

Thermally stimulated discharge currents (TSDC) together with the Relaxation Map Analysis (RMA) method is a convenient method to study relaxation processes in complex materials. In the RMA technique, one performs repeated TSDC runs while selecting polarization states through the use of a fixed polarization time schedule and a sucessively higher polarization temperature. Each peak has an associated polarization strength and a pair of activation parameters which can be the Gibbs free activation energy and the activation entropy. In the present paper we propose a different approach to RMA. It consists of a global heating of the sample in an hyperbolic manner to determine the activation parameters as a function of temperature. This determination is done by first selecting a value for the activation entropy for each temperature and then calculating the implied activation enthalpy or Gibbs free energy.

We have reasoned that normal relaxation processes usually proceed with a null or close to zero activation entropy and that only relaxations associated to the glass transitionand/or cooperative phenomena involve large values of the activation entropy. In those cases and based on the curvature of the TSDC curve one can select a minimum activation entropy value which will fit the TSDC curve at a particular temperature. Using this data one determines the polarization strength associated to each pair of activation parameters. Results for the TSDC relaxation parameters spectra of a liquid crystal polymer will be presented together with comparisons between the thermal sampling method and the proposed method.
Thermodynamics of supercooled liquid silicon and its glass transition
Authors: Miranda Caetano R. , Antonelli Alex
Published: April 2011

The thermodynamic properties of various phases of silicon, namely, crystalline, amorphous, and liquid, have been studied using the Reversible Scaling method within Monte Carlo simulations. The recently proposed Environment Dependent Interatomic Potential was employed to model the atomic interactions. The calculated Gibbs free energy and entropy of both crystalline and liquid phases are in good agreement with available experimental data. The glass transition is continuous, taking place at 1150 K. We have also determined the Kauzmann temperature, TK = 955 K, the thermodynamic fragility, F3/4 = 0.64, which indicates a fragile thermodynamic character to l-Si, and the configurational entropy of the amorphous phase, Sconf ≈ 1.2 kB/atom.
The electronic and magnetic structure of Fe-based bulk amorphous metals: An ab-initio approach
Authors: Wang Yang , Widom Mike , Nicholson Don , Mihalkovic Marek , Naidu Siddartha
Published: April 2011

We applied the locally self-consistent multiple scattering (LSMS) method to the study Fe-based bulk amorphous metals. The LSMS method is an order-N approach to the electronic structure calculation for solid state materials based on density functional theory and local density approximation. Using LSMS method, we performed electronic structure calculations for the supercell samples generated by ab-initio molecular dynamics simulation. The equilibrium atomic volume and the bulk modulus are calculated based on the energy versus volume curve. The magnetic moment distribution in the samples is determined for both collinear and noncollinear cases. A comparison with the experimental results is also made.
Local Order in Amorphous Fe-alloys
Authors: Louca Despina , Ahn Kyungsoo , Ponnambalam V. , Poon S. J.
Published: April 2011

The pair density function analysis of neutron diffraction data of Fe-based metallic glasses of the Zr and Mo series shows how the local atomic structure changes by chemical substitution. The results provide evidence for short-range chemical reorganization accompanied by a volume contraction that could in turn be associated with stronger glass forming ability. While the existence of chemical short-range topological ordering is enhanced in both systems by alloying with a transition metal such as Mn, locally, the atomic structure changes in a way that corresponds to an increase in bonding interactions. The shortening of bonds is also related to volume contraction that can in turn be associated with a reduction of the ferromagnetic coupling of the Fe sublattice and to a lower Curie transition temperature.
Fracture Toughness of Amorphous Metals and Composites
Authors: Lewandowski J.J. , Thurston A.K. , Lowhaphandu P.
Published: April 2011

The effects of changes in notch radius on the toughness of two different Zr-based bulk metallic glasses have been determined. It is shown that increases in notch radius produce large increases to the toughness, accompanied by extensive shear banding and crack bifurcation. The fracture toughness of twenty (20) fatigue precracked specimens exhibiting planar crack growth were in the range 20.3 ± 6.7 MPa√m for the two Zr-based glasses. Increasing the notch radius to 110 μm produced notch toughness in the range 95.3 ± 8.3 MPa√m for nine (9) tests on Vitreloy I, well in excess of that typically observed in most structural materials. Toughness tests conducted on two fatigue precracked specimens of Vitreloy I at 77 K produced values for fracture toughness that were in the range 17.9 ± 2.7 MPa√m, similar to that obtained at 298 K. The fatigue precracked fracture toughness of metallic glass composites containing large crystalline regions of a body centered cubic Zr-Ti-Nb alloy were in the range 29–42 MPa√m, but the values were temperature dependent over the range 148 K to 500 K. Fracture surfaces were analyzed via Scanning Electron Microscopy (SEM).
Consequences of the Anticipated Long-Term Evolution of Spent Nuclear Fuel for the Assessment of the Release Rate of Radionuclides.
Authors: Poinssot Christophe , Lovera Patrick , Ferry Cécile , Gras Jean-Marie
Published: April 2011

The research conducted in the framework of the French research project on spent nuclear fuel (SNF) long - term evolution (PRECCI Project) has enlightened the potential significance of spent nuclear fuel intrinsic evolution in closed system for the assessment of radionuclide (RN) source term in long-term storage or geological disposal. Beyond others, alpha self-irradiation enhanced diffusion and evolution of the grain boundaries cohesion are two major processes which have to be accounted for in view of the RN source term models development. Accounting for these processes, operational models are developed, the aim of which is to quantitatively define the RN release rates from SNF in long-term storage or geological disposal. They distinguish basically an instantaneous contribution (IRF in geological disposal) and a time-dependent contribution (matrix oxidation or alteration). RN inventories associated to these two different processes have to be modeled since they are time-dependent due to the RN diffusion within the pellet. The present paper details the models that are developed in France in terms of assumptions, conservatism and robustness. It comes out from this work that for the instant release fraction, we have to consider a much higher instant release fraction than classically assumed (5–6% in geological disposal) in particular for geological disposal.
Effects of Volatilization on Groundwater Chemistry
Authors: Pulvirenti A. L. , Needham K. M. , Adel-Hadadi M. A. , Bishop E. J. , Barkatt A. , Marks C. R. , Gorman J. A.
Published: April 2011

Both concentrated and dilute simulated solutions of saturated J13 and unsaturated UZ pore water were concentrated through distillation of the solutions under atmospheric pressure. It was observed that condensed vapors from the pH of J13 waters steadily rose during the distillations to a value of 10, while the pH of UZ waters remained steady until 90% of the volume of the solution had been distilled, after which the pH of the condensed vapors dropped precipitously, often below 1. Residual solutions analyzed when most of the solution had been distilled away were also found to be extremely acidic. The temperature of these residual solutions was around 144°C due to their high solute content causing boiling point elevation. All experiments were performed with the condenser open to ambient air at atmospheric pressure. The pH drop during the distillation of UZ water is attributed largely to the presence of large amounts of magnesium. Specimens of Alloy 22 tested in the residual solutions of at their boiling temperature (around 144°C) showed significant rates of general corrosion over a broad range, often approaching 1 mm/year. Similarly high corrosion rates were also observed in tests on Alloy 22 specimens in condensates obtained during the late stages of the distillation. These tests were performed eitherin situat 75–80°C using a Soxhlet extractor, or in separate pressure vessels at temperatures between 90 and 130°C.
Cesium and Niobium transport through poorly cemented sandstone from Krasnoyarsk-26 (Russian Federation): From Batch to Transport experiments
Authors: Casas Ignasi , Giménez Javier , Merino Joan M. , Arasanz Emilio , de Pablo Joan , Torras Josep , Rovira Miquel
Published: April 2011

We have studied the sorption of Cs and Nb onto poorly cemented sandstone (drillcore from – 250 m depth) from the radioactive waste disposal site at Krasnoyarsk-26 (Russian Federation). Results were evaluated in terms of KD-values and Langmuir isotherms. These values have been used to model the transport of Cs and Nb through a sandstone column using the RETRASO code (REactive TRAnsport of Solutes), which is able to simulate simultaneously groundwater flow, heat transport and multicomponent reactive solute. Good agreement between experimental data and modeling has been achieved for both elements. Cs transport has been modeled using rapid sorption equilibrium, while for Nb a kinetically controlled transport has been considered. The results obtained in this work have been used to predict the 1D transport of Cs and Nb taking into account some parameters of the hydrogeology of Krasnoyarsk-26.
Sensitivity Studies of Waste Package Performance in High-Level Waste Management
Authors: Rivera-Feliciano N. , Bloomer T. , Ahn T. , Pensado O.
Published: April 2011

An important attribute of the high-level waste (HLW) disposal program in the United States (U.S.) is the estimated long life of waste packages (WPs). The outer container of the WP is proposed to be constructed of a nickel-based alloy, Alloy 22 (58Ni-21Cr-14Mo-3W-4Fe), which is highly corrosion resistant. Under nominal conditions of the proposed repository at Yucca Mountain (YM), Nevada, WPs are anticipated to have a long life because of their extremely slow uniform corrosion rate. The technical basis for this expected long life of geological scale is still subject of discussion. In this paper, the NRC Total-system Performance Assessment (TPA) Code is used to evaluate uncertainties associated with the WP performance within a 10,000-year period. Early WP failure may occur due to localized corrosion or high frequency of manufacturing defects. Localized corrosion may occur under mixed salt deposits on the WP surface at temperatures above 100°C. High frequencies of WP juvenile failure are considered to account for the limited industrial experience with Alloy 22 and uncertainty in the long-term performance of Alloy 22 as container material. The localized corrosion and the high frequency of manufacturing defects were simulated with the TPA code by lowering the critical relative humidity for the onset of aqueous corrosion, RHc, and increasing the fraction of initially defective WPs. By sampling RHc from various normal distributions, the mean annual total effective dose equivalent (TEDE) approached 1 mrem/year at 10,000 years. Assuming a fixed fraction of initially defective WPs (1 percent), the mean annual TEDE approached 0.07 mrem/year at 10,000 years.
Long-Term Extrapolation of Passive Behavior of Alloy 22
Authors: Pensado Osvaldo , Dunn Darrell S. , Cragnolino Gustavo A.
Published: April 2011

Common assumptions to extrapolate the lifetime of proposed high-level waste disposal containers made of Ni-Cr-Mo alloys, in the absence of environmental and electrochemical conditions leading to localized corrosion and stress corrosion cracking, are evaluated based on a mechanistic model for passive dissolution. The predominant charge conduction mechanism through the oxide film formed on Ni-Cr-Mo alloys is hypothesized to be interstitial transport of metal cations. Dissolution of the alloy and conduction of interstitial species through the film create vacancies in the alloy. The anodic current density under potentiostatic control decreases as a function of time, and potentiostatic decays in the current density are rationalized on the basis of vacancy accumulation at the metal-oxide interface. It is concluded that the dissolution process is regulated by vacancy-enhanced diffusion of the elements in the alloy. Long-term stoichiometric dissolution arises if the diffusion coefficients of the alloying elements are similar. No credible scenario is envisioned by which catastrophic failure may occur as a result of longterm passive dissolution.
Cold Crucible Vitrification of NPP Operational Waste
Authors: Lifanov Feodor A. , Ojovan Michael I. , Stefanovsky Sergey V. , Burcl Rudolf
Published: April 2011

Operational radioactive waste is generated during routine operation of nuclear power plants (NPP). This waste must be solidified in order to ensure safe conditions of storage and disposal. Vitrification of NPP operational waste is a relative new solidification option being developed for last years. The vitrification technology comprises a few stages, starting with evaporation of excess water from liquid radioactive waste, followed by batch preparation, glass melting, and ending with vitrified waste blocks and some relative small amounts of secondary waste. Application of induction high frequency cold crucible type melters facilitates the melting process and significantly reduces the generation of secondary waste. Two types of glasses were designed in order to vitrify operational waste depending on the reactor type at the NPP. For the NPP with RBMK-type reactors the glass 16.2Na2O 0.5K2O 15.5CaO 2.5 Al2O3 1.7Fe2O3 7.5B2O3 48.2SiO2 1.1 Na2SO4 1.2NaCl (5.7 others) was produced. For NPP with WWER reactors the glass 24.0Na2O 1.9K2O 6.2CaO 4.3Al2O3 1.8Fe2O3 9.0B2O3 46.8SiO2 0.8Na2SO4 0.9NaCl (4.3 others) was produced. The melting temperatures of both glass formulations were 1200–1250 C, specific power consumption was 5.2 ± 0.8 kW h/kg, 137Cs loss was within the range 3 - 4 %. The specific radioactivity of glass reached 7.0 MBq/kg. Glass blocks obtained were studied both in laboratory and field conditions. Long-term studies revealed that vitrified NPP operational waste has the minimal impact onto environment. Since the glass has excellent resistance to corrosion it gives the basic possibility of maximal simplification of engineered barrier systems in a disposal facility. The simplest disposal option for vitrified NPP waste is to locate the packages directly into earthen trenches provided the host rock has the necessary sorption and confinement properties.