bluuebunny/crossref_metadata_2025_split huggingface dataset:
Chemical-Mechanical Polishing and Rapid Thermal Annealing of SiC: Raman Spectroscopy and ESCA (XPS) Studies
Authors: Roughani Bahram , Ramabadran Uma , Phillips Diana , Mitchel W. C. , Neslen C. L. 
Published: April 2011

ABSTRACTThe effects of Chem-Mechanical Polishing (CMP) and Rapid Thermal Annealing (RTA) on n-type 4H:SiC samples doped with nitrogen were investigated using Raman scattering and X-ray Phtoelectron Spectroscopy (XPS a.k.a. ESCA) measurements. A comparison of the Raman spectra from Mechanically Polished (MP) SiC annealed at 600°C and 800°C displays a frequency shift in the coupled plasmon LO-phonon mode. Since the coupled mode frequency is a direct measure of the free carrier concentration, this observation may suggest the removal of polishing induced carrier traps with increasing annealing temperature. The CMP samples did not show this frequency shift, thereby indicating that such polishing traps were not created in that process. The Si-peak observed in the XPS spectra of the unannealed CMP sample indicates primarily a Si-C bonding, while that for the MP sample is more complex, indicating other bonds beside Si-C. Drastic changes in O, C, Si surface content were observed for annealing between 1000°C and 1100°C. The peaks in the XPS spectra associated with the chemical environment for C, O, and N are complex and may be explained as silicon oxycarbide type structures near the surface or possibly around the interface of the SiC substrate with a thin surface oxide layer.
An Analytical Approach for Computing the Energy Structure of InAs Quantum Dots
Authors: Stoleru Valeria G. , Pal Debdas , Towe Elias 
Published: April 2011

ABSTRACTThe experimental and theoretical results of the electronic structure of self-assembled InAs/GaAs quantum dots are presented. We have performed analytical calculations to obtain the spatial strain distribution in truncated pyramidally-shaped InAs quantum dots grown on (001) GaAs substrates by molecular beam epitaxy. The model reproduces our cross sectional high- resolution transmission electron microscopy observations very well. We further take into account the microscopic effects of the spatial strain distribution on carrier confinement potentials. The effects of piezoelectricity have been neglected in our calculations since they minimally affect the energy levels involved in optical transition. The dots considered here are assumed to be in the strong confinement regime; so Coulomb interaction is neglected. Our calculations of the peak luminescence energies are in good agreement with our experimental results and those of others. The calculated energy separation between the ground and the excited states for electrons and for heavy-holes is in very good agreement with the available photoluminescence, capacitance and far- infrared absorption data.
Potential industrial applications of Al-based quasicrystals: plasma sprayed vs. HVOF sprayed coatings
Authors: Lee S.M. , Fleury E. , Kim J.S. , Kim Y.C. , Kim D.H. , Kim W.T. , Ahn H.S. 
Published: April 2011

AbstractQuasicrystalline coatings with various aluminum based systems were deposited onto 304 stainless steel substrates using two thermal spraying techniques; plasma spraying and high velocity oxy-fuel spraying. The friction and wear performances of the coatings were evaluated using two different testing devices, varying testing conditions and counterpart materials. Values of the friction coefficient were found to be strongly dependent on the testing devices and the counterpart materials, with values ranging from 0.15 to 0.4. In testing conditions corresponding to high sliding velocity, this study showed that the contact problem was posed over a third body system due to the formation of an intermediate layer. Values of the coefficient of friction were found to be approximately the same for all coating layers regardless of the thermal spraying techniques used, however, larger differences were obtained in the wear performance. The tribological properties were also evaluated at high temperature. It is noted that quasicrystal coating layers exhibit better friction and wear performances at 450oC than at room temperature. In comparison to potential coating candidate such as Cr2O3 for piston rings in automotive engines, tribological property of quasicrystalline coating layers seems to be promising one, however, wear performance need to be improved.
Quasicrystalline Catalyst for Steam-Reforming of Methanol
Authors: Tsai A.P. , Yoshimura M. 
Published: April 2011

AbstractSteam reforming of methanol (CH3OH + H2O -> 3H2 + CO2) has been performed on a number of Al-Cu-based stable quasicrystals and related alloys. Alloys leached in NaOH aqueous solution exhibit the highest activity for steam-reforming of methanol. Among the alloys, AlCuFe quasicrystal, after leaching treatment, reveals excellent activity. The production rate of H2 reaches 235 l/kgmin at 573K for steam reforming of methanol. The activity is due to Cu nanoparticles at the surfaces of quasicrystalline grains which are generated by leaching treatment. The quasicrystals have two advantages: one is their brittle nature, which allows them to be crushed efficiently; the other is the involvement of Fe, which suppresses the sintering of Cu particles.
Transformation In Hydrogenated ZR-CU-NI-AL Quasicrystals
Authors: Zander D. , Köster U. , Khare V. 
Published: April 2011

AbstractThe high number of potential interstitial sites suitable for hydrogen and the favorable hydrogen-metal chemistry make quasicrystalline alloys candidates for hydrogen storage applications. Hydrogen charging was performed electrochemically in a 2:1 glycerin-phosphoric acid electrolyte. The microstructure of quasicrystals in Zr69.5Cu12Ni11Al7.5 was investigated by x-ray diffraction and TEM, the kinetics of transformations by DSC and TDA.Hydrogen was observed to exhibit a significant effect not only on the formation of Zr-based quasicrystalsfrom the glassy precursor material, but also on their stability. Ribbons with an icosahedral microstructure were found to absorb hydrogen up to H/M = 2.0. With increasing hydrogen content an expansion of the quasilattice constant was observed, followed by the formation of approximants and finally amorphization.Only partial desorption of hydrogen can occur prior to the decomposition of the quasicrystalline phase. Desorption at lower temperatures is hindered by the formation of a thin ZrO2 barrier. Pd plating, often used to overcome such a barrier, improves the absorption kinetics as well as the desorption behavior significantly. DSC of uncoated Zr-based quasicrystals reveals that at low hydrogen contents the decomposition ofthe icosahedral quasicrystals is shifted to lower temperatures; at high hydrogen contents the formation of ZrH2-x was revealed. In the Pd-coated ribbons fully desorption is observed even below 300°C, butonly after hydrogenation up to H/M = 0.5; higher hydrogen contents lead to irreversible changes.The micromechanism of transformations during hydrogenation as well as the decomposition during annealing of hydrogenated uncoated as well as Pd coated Zr-based quasicrystals are discussed in some detail.
Enhancement of the Icosahedral Quasicrystalline Phase Forming Ability with Be Addition to an Al62.5Cu25.5Fe12 Alloy
Authors: Lee S.M. , Kim B.H. , Kim W.T. , Kim D.H. 
Published: April 2011

AbstractThe quasicrystalline phase forming ability, solidification structure, thermal stability, and phase relationships of a series of conventionally-cast Al62-xCu25.5Fe12.5Bex (x=0,1,3,5,7) alloys were investigated. Single icosahedral quasicrystalline phase was successfully obtained by partial replacement of Al with Be (7 at.%) in an Al62Cu25.5Fe12.5 alloy during crucible cooling. The I-phase in an Al55Cu25.5Fe12.5Be7 alloy has the same face-centered icosahedral structure as that in an Al62Cu25.5Fe12.5 alloy, with less phason strain. Hardness of the I-phase increases with x, i.e. from Hv 710 for x=1 to Hv 809 for x=7.
Modulated Structures, Nanocrystals and Quasicrystals in Mg-Zn-Y Alloys
Authors: Subramaniam Anandh , Ranganathan S. 
Published: April 2011

AbstractThe formation of the icosahedral phase, BCC (Body centred cubic) phase and nanocrystals are seen in the as-cast alloy with nominal composition of Mg4Zn94Y2. FCC (face centred cubic) phase and modulated structures are formed in the alloys with higher Y content (10% and 25% Y respectively). These phases are analysed keeping in view their relation to the quasicrystals of the Mg-Zn-Y system.
Modelling Quasicrystal Plastic Deformation By Means of Constitutive Equations
Authors: Feuerbacher M. , Schall P. , Estrin Y. , Bréchet Y. , Urban K. 
Published: April 2011

AbstractThe interpretation of plastic deformation experiments on quasicrystals is a challenging task due to the occurrence of changes of the structure during deformation. In this paper, we present a quantitative model for quasicrystal plasticity on the basis of a constitutive-equations Ansatz, which takes these effects into account. A single-internal-variable model of the kind commonly used for describing crystal plasticity, is adapted for the description of the dislocation density evolution in a quasicrystal. In addition, we introduce a structural parameter that accounts for the evolution of order in the course of plastic deformation. The numerical solution of the resulting set of evolution equations yields the flow stress and the dislocation density as a function of strain, which can be directly compared to corresponding experimental curves obtained on icosahedral Al-Pd-Mn. An excellent agreement between experiment and the calculated curves obtained using our model is found.
The Thermal Stability of a Single-Grain Mg-Zn-Y Icosahedral Quasicrystal
Authors: Luo Z.P. , Tang Y.L. , Miller D.J. , Kramer M.J. , Fisher I.R. , Canfield P.C. 
Published: April 2011

AbstractThe stability of the Mg-Zn-Y icosahedral quasicrystal (IQC) has been studied by long-term annealing of a single grain IQC in quartz tubes. Decomposition of the IQC was observed after annealing at high temperatures (T≥773 K) sealed in Ar. During the decomposition process, the quasilattice parameteraRwas found to decrease, associated with a decrease in Mg content of the IQC phase as confirmed by quantitative x-ray energy dispersive spectroscopy analyses. In addition, a new cubic approximant has been found in the annealed samples. This cubic approximant has a face-centered cubic (fcc) structure with lattice parameter ofa= 1.276 nm, which is about (1/τ) times smaller than that of the fcc W'-(MgZnY) witha= 2.05 nm reported previously (where τ is the golden ratio).
The Model for Icosahedral Al-Pd-Mn phase based on theT*(2F)Canonical Tiling
Authors: Kasner Gerald , Paradopolos Zorka 
Published: April 2011

AbstractThe icosahedral canonical tiling of the three-dimensional space by six golden tetahedraT*(2F)[1] is decorated for physical applications by the Bergman polytopes [2]. The model can be also formulated as the “primitive) tilingTP[3] decorated by alternating Bergman symmetry axis of and icosahedron, there appear the plans on three mutual distances following the rule of a decorated Fibonacci sequence. All these three distances among the terraces (mutually scaled by a factor τ) have been recently observed by shenet al. [5]. In particular they have measured also the shortest distance of 2.52Å that breaks the Fibonnaci-sequence of terrace like surfaces measured previously by schaubet al. [6]. We predict the frequencies for the appearance of the terraces of different heights in the model under the condition that the model of Boudardet al. [7.8], we decorate the atomic positions by Al, Pd and Mn. We present images of the predictedpossibleterrace-like surfaces on three possible distances in the fully decorated model by the atomic species.
Internal Friction and Elastic Modulus Change Associated with Structural Relaxation and Crystallization in Amorphous Mg-Ni-Y Alloys.
Authors: Soifer Y. M. , Kobelev N. P. , Korin E. , Soifer L. , Mogilyanskii D. , Bettelheim A. 
Published: April 2011

AbstractMechanical spectroscopy technique has been applied to investigation of structural relaxation and crystallization of Mg-Ni-Y amorphous alloys of different composition. The composition of the alloys was varied due to change of yttrium concentration from 0 to 7 at.%. The internal friction (IF) and Young's modulus have been measured by a vibrating reed method at frequency of ∼200 Hz at heating and cooling runs in the temperature range from 300K to 650K. Crystallization kinetic studies of the alloys by DSC and XRD showed that the transition from amorphous to crystalline state in the Mg-Ni-Y alloys has gone through several stages with origination of intermediate phase. IF spectra and Young's modulus of the alloys have been analyzed and the effect of yttrium content on the stability of intermediate phase was investigated.
Structural and Phase Transformations During Crystallization of Pd43Ni10Cu27P20 Metallic Glass
Authors: Pekarskaya Evgenia , Schroers Jan , Johnson William L. 
Published: April 2011

AbstractCrystallization of the Pd43Ni10Cu27P20 amorphous alloy during isothermal annealing in the undercooled temperature region is studied by electron microscopy and diffrential scanning calorimetry (DSC). It is established that different crystallization processes take place above and below the nose temperature of the time-temperature-transformation (TTT) diagram. Detailed analysis of the microstructural evolution at the early stages of the crystallization is performed. In addition, the stable phases in the Pd-Ni-Cu-P system are identified.
Molecular Dynamics Simulations of Supercooled Liquid Metals and Glasses
Authors: Lee Hyon-Jee , Qi Yue , Strachan Alejandro , Cagin Tahir , Goddard William A. , Johnson William L. 
Published: April 2011

AbstractThe thermodynamic, transport and structural properties of a binary metallic glass former in solid, liquid, and glass phases were studied using molecular dynamics simulation. We used a model binary alloy system with a sufficient atomic size mismatch and observed a glass transition in a quenching process. The diffusivity and viscosity were calculated in the liquid state and the super-cooled liquid state. The smaller atom showed higher diffusivity and more configurational randomness compared to the larger atom. The viscosity increased abruptly around the glass transition temperature. The solvent/solute concentration effect on the glass transition was examined in terms of a packing fraction. We find that the glass forming ability increases with the packing fraction in the liquid state because the densely-packed material requires more time to rearrange and crystallize.
Crystallization of Bulk Zr48Nb8Cu14Ni12Be18Metallic Glass
Authors: Zhuang Y.X. , Gerward L. , Jiang J.Z. , Olsen J.S. , Zhang Y. , Wang W.H. 
Published: April 2011

AbstractThe crystallization of bulk Zr48Nb8Cu14Ni12Be18metallic glass has been investigated by differential scanning calorimetry (DSC) and X-ray powder diffraction. The activation energies of glass transition and crystallization for the glass obtained using Kissinger analysis from the shift of the peak temperature in the DSC curve are 470 and 235 kJ/mol, respectively. The effect of applied pressure on crystallization is studied by in situ high-pressure and high-temperature X-ray powder diffraction using synchrotron radiation. It is found that the crystallization temperature increases with pressure having a slope of 9.5 K/GPa in the range of 0-4.4 GPa.
Spatial Localization of the Nucleation Rate in Deeply Undercooled Liquids
Authors: Bossuyt S. , Johnson W. L. 
Published: April 2011

AbstractIn bulk metallic glass forming alloys cooled close to the critical cooling rate for glass formation, the nucleation density is observed to be spatially inhomogeneous; in an amorphous matrix there are spherical clusters with a high density of nanocrystals. This is attributed to the combined effect of recalescence due to the heat of crystallization and the fact that in deeply undercooled liquids the nucleation rate increases with increasing temperature.A linear stability analysis of the non-linear differential equations describing nucleation and growth reveals that in the early stages of crystallization, stability is determined by the temperature dependence of the nucleation rate. When the nucleation rate increases with increasing temperature, fluctuations are amplified, resulting in “hot spots” where both the nucleation rate and the growth rate are higher.
Changes in Crystal Structure and Microhardness During Thermal Treatment of a NiCoCrAlY Alloy.
Authors: Rehfeldt Thomas , Schumacher Gerhard , Klingelhoeffer Hellmuth 
Published: April 2011

ABSTRACTThe stability of microstructure and the microhardness of a NiCoCrAlY alloy was studied after thermal treatment at high temperatures and subsequent quenching into ice water. The alloy revealed mainly two ordered phases, a γ' phase with L12 crystal structure and a β phase with B2 structure. The γ' phase is shown to become unstable in the temperature range between 1073 K and 1373 K where it undergoes an order-disorder phase transformation. The low transformation temperature compared to pure Ni3Al is ascribed to deviations from the Ni3Al stoichiometry and to the additional alloying elements Co and Cr. The γ'-γ order-disorder transformation is shown to be attended by a decrease in microhardness of about 30 %.
Compression Behavior of L12 Modified Titanium Trialuminides Alloyed with Chromium and Iron
Authors: Takahashi Tohru , Hasegawa Tadashi 
Published: April 2011

ABSTRACTL12 modified titanium trialuminides have been prepared by replacing 9at.% of the aluminum in Al3Ti with chromium and/or iron. The materials were recrystallized into single phase polycrystals after isothermal forging resulting in an average grain diameter of about 40μm. Lattice parameter of the material containing 9 at.% chromium or 9 at.% iron, are 0.3959nm and 0.3939nm, respectively. The lattice parameters varied linearly with composition between these values for additions of both chromium and iron. Uniaxial compression tests were performed at temperatures ranging from 293K to 1300K. The yield strength is not sensitive to chemical composition within the range of compositions tested. Flow stress serrations of a few % were observed at temperatures around 600K, where intermittent drops in flow stress started immediately after yielding and continued to the end. These serrations were observed up to about 800K. At 900K and above the materials became fully deformable. Quasi steady state flow and strain softening were observed at 1200K and 1300K, respectively, due to dynamic recrystallization.
Dislocation Interaction with Point Defects in Transition-Metal Disilicides
Authors: Yoo Man H. 
Published: April 2011

ABSTRACTEnergetics of the formation of jog-pairs and kink-pairs on a straight dislocation are analyzed using anisotropic elasticity theory for the equivalent slip systems of seven transition-metal disilicides. While glide loops of the active slip systems are stable in all cases, having positive line tension, the interaction energies of two opposite segments in kink-pairs and jog-pairs are found to be very anisotropic with respect to dislocation orientation. The anisotropic interaction plays an important role in the glide resistance due to dislocation-point defect interactions. A dislocation model is proposed for the glide resistance on edge and near edge dislocations based on jog-pairs resulting from the contact interaction between dislocations and intrinsic point defects. The available data of yield strength anomaly and dislocation structures of disilicide crystals are discussed in view of the proposed model for jog-pair pinning and dynamic breakaway.
Mechanical Behavior of Molybdenum Disilicide-Based Alloys
Authors: Misra A. , Sharif A.A. , Petrovic J. J. , Mitchell T. E. 
Published: April 2011

ABSTRACTWe have investigated the mechanical behavior of the following single-phase polycrystalline alloys with the MoSi2 body-center tetragonal structure: MoSi2 alloyed with ∼2.5 at.% Re, MoSi2 alloyed with 2 at.% Al, MoSi2 alloyed with 1 at.% Nb, and MoSi2 alloyed with 1 at.% Re and 2 at.% Al. Several anomalies in the mechanical behavior of alloyed materials were observed. For example, (i) addition of only ∼2.5 at. % Re results in an order of magnitude increase in compressive strength at 1600 °C, (ii) additions of Nb and Al cause solution softening at near-ambient temperatures, and (iii) quaternary MoSi2-Re-Al alloys show strengthening at elevated temperatures and reduction in flow stress with enhanced plasticity at near-ambient temperatures in compression. The mechanisms of anomalous solution hardening and softening are discussed.
Multi-Modal “Order-Order” Kinetics in Ni3Al Studied by Monte Carlo Computer Simulation
Authors: Oramus P. , Kozubski R. , Pierron-Bohnes V. , Cadeville M.C. , Pfeiler W. 
Published: April 2011

ABSTRACT“Order-order” relaxations in γ-Ni3Al previously extensively studied by means of resistometry, are simulated within a model of vacancy mechanism of atomic migration in a superstructure implemented with Monte Carlo technique and the Glauber algorithm. The observed operation of two simultaneous relaxation processes showing different rates, as well as the theoretically predicted effect of vacancy ordering have been definitely reproduced and analysed in detail in terms of the dynamics of particular kinds of atomic jumps. The proposed model scenario for the creation and elimination of antisite atoms in the relaxing L12-type superstructure shows that the experimentally observed features of the “order-order” processes in Ni3Al follow from an interplay between two dominating and coupled modes of long- and short-range ordering: the creation/elimination of nn pairs of antisites (SRO) and the change of the “overall” number of antisites (LRO). High profile of the first process results in a high contribution of the fast component of LRO kinetics.
First Principles Calculation of Cooperative Atom Migration in L12 Ni3Al
Authors: Schweiger H. , Podloucky R. , Wolf W. , Püschl W. , Pfeiler W. 
Published: April 2011

ABSTRACTRecent Monte-Carlo simulations of order relaxations in L12-ordered Ni3Al reproduced the simultaneous action of two processes as experimentally observed by residual resistometry. It was shown that the fast process is related to the fast annihilation/creation of nearest neighbour antisite pairs. These findings are now strongly corroborated by a new supercell approach of ab initio quantum mechanical calculations describing the simultaneous displacement of Ni and Al atoms on their way to their respective antisite positions. Studies of single jumps suggest that such a cooperative migration of Ni and Al is necessary in order to prevent Al antisites from jumping back into their regular position. Relaxation of neighbouring atoms was taken into account. Thus, a minimum migration barrier of about 3 eV was derived which together with the calculated formation enthalpy of a Ni vacancy of 1.5 eV amounts to 4.5 eV, in remarkable agreement with the high activation enthalpy of 4.6 eV as observed experimentally.
Ni3Al Thin Foil by Cold Rolling
Authors: Hirano Toshiyuki , Demura Masahiko , Kishida Kyosuke , Suga Yozo 
Published: April 2011

ABSTRACTThin foils of stoichiometric Ni3Al below 100 μm in thickness were successfully fabricated by cold rolling of the sheets which were sectioned from directionally solidified ingots. Maximum rolling reduction in thickness amounted to 96%, irrespective of the initial orientation or the existence of columnar grains in the starting sheets. The as-rolled foils were characterized in terms of microstructures, textures and dislocation structures. The deformation microstructures were of a dual banded structure composed of two different {110} textures in the case of <001> rolling direction, while a rather homogeneous structure with a single {110} texture resulted in the case of <112> rolling direction. TEM observation revealed homogenous dislocation structures in either case without cell formation, accompanied by very fine grained-regions at higher reduction.