bluuebunny/crossref_metadata_2025 huggingface dataset:
Planar Synthesis of Anisotropic Nanoparticles
Authors: Khomutov Gennady B. , Gaynutdinov Radmir V. , Gubin Sergey P. , Obydenov Alexander Yu. , Soldatov Eugene S. , Tolstikhina Alla L. , Trifonov Artem S.
Published: April 2011

A novel method of two-dimensional (2-D) synthesis of anisotropic nanoparticles have been developed in which nanoparticle growth is an example of 2-D process where true 2-D diffusion of precursor molecules and active intermediates, metal atoms and its complexes, nucleus and growing nanoparticles, surfactants and additives occurs only in the plain of a monolayer at the gas/liquid interface. Nanoparticles were generated via ultraviolet decomposition of a volatile insoluble metal-organic precursor compound (iron pentacarbonyl) and by chemical reduction of palladium from Pd3(CH3COO)6 molecules in a mixed Langmuir monolayer with stearic acid, arachidic acid or octadecyl amine on the aqueous subphase surface. The properties of such surfactants to form Langmuir monolayer and to prevent aggregation of nanoparticles were here combined successfully. Atomic force microscopy, scanning tunneling microscopy and transmission electron microscopy techniques were used to study morphology of deposited nanoparticulate monolayers. The size and shape of nanoparticles were dependent substantially on the monolayer composition and state during the synthesis process. We demonstrate that planar synthesis in a monolayer at the gas/liquid interface allows to produce anisotropic extremely flat nanoparticles with very high surface to volume ratio and unique morphologies such as iron-containing magnetic nano-rings.
Multi-Layer Coating of Ultrathin Polymer Films on Nanoparticles of Alumina by a Plasma Treatment
Authors: Shi Donglu , Yu Zhou , Wang S. X. , Ooij Wim J. van , Wang L. M. , Zhao J. G.
Published: April 2011

Multi-layer ultrathin polymer films have been deposited on the surfaces of nanoparticles of alumina using a plasma polymerization treatment. The nanoparticles ranged from 10-150 nm in spherical shapes. High-resolution transmission electron microscopy (HRTEM) experiments showed that an extremely thin film of the pyrrole layer (10-20 Å) was uniformly deposited on the surfaces of the nanoparticles. In particular, the particles of all sizes (10-150 nm) exhibited equally uniform ultrathin films indicating well-dispersed nanoparticles in the fluidized bed during the plasma treatment. After single layer coating, hexamethyldisiloxane (HMDSO) was coated again as a second layer onto the surface of pyrrole. Subsequently, a third layer of pyrrole was coated on the top of HMDSO film completing the multi-layer coating process. Time-of-Flight Secondary ion mass spectroscopy (TOFSIMS) experiments confirmed the deposition of these multi-layer thin films on the nanoparticles. The deposition mechanisms and the effects of plasma treatment parameters are discussed.
High Spatial Resolution Assessment of the Structure, Composition, and Electronic Properties of Nanowire Arrays
Authors: Sander M.S. , Prieto A.L. , Lin Y.M. , Gronsky R. , Stacy A.M. , Sands T.D. , Dresselhaus M.S.
Published: April 2011

We have employed transmission electron microscopy (TEM) and analytical electron microscopy to perform preliminary assessment of the structure, composition and electronic properties of nanowire arrays at high spatial resolution. The two systems studied were bismuth and bismuth telluride nanowire arrays in alumina (wire diameters ~40nm), both of which are promising for thermoelectric applications. Imaging coupled with diffraction in the TEM was employed to determine the grain size in electrodeposited Bi2Te3 nanowires. In addition, a composition gradient was identified along the wires in a short region near the electrode by energy-dispersive x-ray spectroscopy. Electron energy loss spectroscopy combined with energy-filtered imaging in the TEM revealed the excitation energy and spatial variation of plasmons in bismuth nanowire arrays.
Generalized Ellipsometry Using a Rotating Sample
Authors: Xu Weiliang , Wood Lowell T. , Golding Terry D.
Published: April 2011

We propose a generalized ellipsometric technique using a rotating sample. The ellipsometer consists of a polarizer, a rotatable sample holder, an analyzer, and a detector. Fourier coefficients are measured and used to extract the system’s dielectric tensors and film thicknesses. The main advantage of the technique is that all parts of the ellipsometer are fixed except the sample, whose azimuth angle can be modulated. We show calculated responses to isotropic and anisotropic materials as well as superlattices. Potential applications for characterizations of anisotropic nanostructures are discussed.
Effective Medium Theory of DNA-linked Gold Nanoparticle Aggregates:Effect of Aggregate Shape
Authors: Lazarides Anne A. , Kelly K. Lance , Schatz George C.
Published: April 2011

We present a dynamical effective medium theory (EMT) of the dielectric properties of nanoparticle aggregates formed from DNA-linked gold nanoparticles. Experimental measurements show that such aggregateshave reduced UV extinction and plasmon bands that are considerably red-shifted and broadened relative to the plasmon absorption feature observed in spectra of dispersed colloid. The EMT, which can be used to reproduce the observed spectral changes, is tested by comparing aggregate spectra calculated using the EMT dielectric function with spectra from explicit coupled particle calculations, and good agreement is found. The EMT dielectric function is used as well in discrete dipole calculations to calculate extinction spectra for a variety of aggregate shapes not amenable to analytic solution, and the sensitivity of the spectra to aggregate shape is examined. We find that the spectra are only weakly sensitive to aggregate shape, and conclude that, when calculating extinction of the DNA-linked aggregates for comparison with experiment, spherical shapes can be assumed.
Tunable Porous Silicon Photonic Band Gap Structures
Authors: Lugo J. Eduardo , Lopez Herman A. , Chan Selena , Fauchet Philippe M.
Published: April 2011

The tuning of one-dimensional photonic band gap structures based on porous silicon will be presented. The photonic structures are prepared by applying a periodic pulse of current density to form alternating high and low porosity layers. The width and position of the photonic bandgap are determined by the dielectric function of each layer, which depends on porosity, and their thickness. In this work we show that by controlling the oxidation of the porous silicon structures, it is possible to tune the photonic bandgap towards shorter wavelengths. The formation of silicon dioxide during oxidation causes a reduction of the refractive index, which induces the blue shift. The photonic band gap is determined experimentally by taking the total reflection of the structures. In order to understand the tuning of the photonic band gap, we developed a geometrical model using the effective medium approximation to calculate the dielectric function of each of the oxidized porous silicon layers. The two key parameters are the porosity and the parameter β, defined as the ratio between the silicon dioxide thickness and the pore radius before oxidation. Choosing the parameter β, to fit the experimental photonic band gap of the oxidized structures, we extract the fraction of oxide that is present. For example, the measured 240 nm blue shift of a photonic bandgap that was centered at 1.7 microns corresponds to the transformation of 30% of the structure into silicon dioxide. A similar approach can be used for oxidized two-dimensional porous silicon photonic structures.
Inverse-Computation Design of a SiC Bulk Crystal Growth System
Authors: Kulik Alexey V. , Demina Svetlana E. , Kochuguev Sergey K. , Ofengeim Dmitry Kh. , Karpov Sergey Yu. , Vorob'ev Andrey N. , Bogdanov Maxim V. , Ramm Mark S. , Zhmakin Alexander I. , Alonso Anna A. , Gurevich Sergey G. , Makarov Yuri N.
Published: April 2011

Inverse modeling was applied to the optimization of a crucible design for SiC sublimation growth. We found a crucible shape providing the optimal temperature distribution in terms of the powder source stability during long-term operation and of the convex crystal shape. Considerable improvement of temperature uniformity throughout the powder charge was achieved. The results obtained show selective sensitivity of the thermal field inside the crucible to modification of the crucible design. The inverse problem approach is easy-to-adapt to various optimization criteria and seems to be especially effective in the case of multi-factor optimization.
Surface Morphology of 6H-SiC on various a-plane using Si2Cl6+C3H8+H2 by Chemical Vapor Deposition
Authors: Nishino Shigehiro , Masuda Yasuichi , Ohshima Satoru , Jacob Chacko
Published: April 2011

We have grown epitaxial layer introducing buffer layer using N2 doping on 6H-SiC (1120) and (1100) substrate. The improvement of morphology could be obtained for (1120) and (1100) epilayers. Morphologies of (1120) epilayers were independent on off-orientations, Morphologies of (1100) epilayers were very sensitive to the off-orientations. The quality of epilayer, and impurity incorporation for a-plane were very influenced by the surface treatment before CVD growth compared to (0001) epilayers.
Bonding, Defects, And Defect Dynamics In The Sic-SiO2 System
Authors: Pantelides S. T. , Buczko R. , Ventra M. Di , Wang S. , Kim S.-G. , PennycooK S. J. , Duscher G. , Feldman L. C. , Mcdonald K. , Chanana R. K. , Weller R. A. , Williams J. R. , Chung G. Y. , Tin C. C. , Isaacs-Smith T.
Published: April 2011

This paper presents a review of new results obtained by a combination of first-principles theory, Z-contrast imaging, and electron-energy-loss spectroscopy in the context of a broader experimental/theoretical program to understand and control the atomic-scale structure of SiCSiO2 interfaces. The ultimate purpose is to achieve low interface trap densities for device applications. Results are given for global bonding arrangements in comparison with those of the Si-SiO2 interface, the mechanism of the oxidation process, the nature of possible interface defects and their passivation by N and H, and the formation and dissolution of C clusters in SiO2 during oxidation and reoxidation.
Silicon Carbide bipolar power devices - potentials and limits
Authors: Singh Ranbir
Published: April 2011

Bipolar devices made with SiC offer 20–50X lower switching losses as compared to conventional semiconductors, and a comparable on-state voltage drop at sufficiently high current densities. To exploit the tremendous advantages offered by SiC for bipolar power devices, it is important to understand the relevant voltage/current range, fundamental limits and technological challenges in order to develop this technology commercially. The opportunity of operating a device at a high current density (>300 A/cm2) to increase total current with reasonable yield, the poor reliability of MOS at high temperatures, and the relatively low channel mobilities obtained in 4H-SiC MOSFETs may make certain bipolar devices more attractive even as low as 1700 V. The total power loss in various bipolar devices is analyzed and compared to fundamental operational limits in order to find the applicability of various devices to advanced applications.
Design and Process Issues for Silicon Carbide Power DiMOSFETS
Authors: Ryu Sei-Hyung , Agarwal Anant K. , Saks Nelson S. , Das Mrinal K. , Lipkin Lori A. , Singh Ranbir , Palmour John W.
Published: April 2011

This paper discusses the design and process issues of high voltage power DiMOSFETs (Double implanted MOSFETs) in 4H-silicon carbide (SiC). Since Critical Field (EC) in 4H-SiC is very high (10X higher than that of a Si), special care is needed to protect the gate oxide. 2D device simulation tool was used to determine the optimal JFET gap, which provides adequate gate oxide protection as well as a reasonable JFET resistance. The other issue in 4H-SiC DiMOSFETs is extremely low effective channel mobility (μeff) in the implanted p-well regions. NO anneal of the gate oxide and buried channel structure are used for increasing μeff. NO anneal, which was reported to be very effective in increasing the μeff of SiC MOSFETS in p-type epilayers, did not produce reasonable μeff of SiC MOSFETs in the implanted p-well. Buried channel (BC) structure with 2.7×1012 cm−2 charge in the channel showed high μeff utilizing bulk buried channel, but resulted in a normally-on device. However, it was shown that by controlling the charge in the BC layer, a normally off device with high μeff can be produced. A 3.3 mm × 3.3 mm DiMOSFET with BC structure showed a drain current of 10 A, which is the highest current reported in SiC power MOSFETs to date, at a forward drop of 4.4 V with a gate bias of only 2.5 V.
Comparison of Current – Voltage Characteristics of N and P Type 6H-SiC Schottky Diodes
Authors: Zhang Q. , Madangarli V. , Gao Y. , Sudarshan T. S.
Published: April 2011

Forward and reverse current – voltage (I–V) characteristics of N and P-type Schottky diodes on 6H-SiC are compared in a temperature range of room temperature to 550K. While the room temperature I–V characteristics of the N-type Schottky diode after turn-on is more or less linear up to ∼ 100 A/cm2, the I–V characteristics of the P-type Schottky diode shows a non-linear behavior even after turn-on, indicating a variation in the on-state resistance with increase in forward current. For the first time it is shown that at high current densities (> 210 A/cm2) the forward voltage drop across P type Schottky diodes is lower than that across N type Schottky diodes on 6H-SiC. High temperature measurements indicate that while the on-state resistance of N type Schottky diodes increases with increase in temperature, the on-state resistance of P type Schottky diodes decreases with increase in temperature until a certain temperature. While the N-type diodes seem to have soft breakdown characteristics, the P-type diodes exhibit more or less abrupt breakdown characteristics.
Calculation Of Positron Characteristics In Silicon Carbide
Authors: Barbiellini Bernardo , Kuriplach Jan , Anwand Wolfgang , Brauer Gerhard
Published: April 2011

Positron affinity calculations performed by a first-principles approach based on density functional theory reveal, contrary to many other semiconductors, that free positrons and positronium atoms can escape from SiC. It is found that the treatment of the electronpositron interaction plays a crucial role when calculating the annihilation characteristics. These characteristics originating from both valence and core electrons, combined with the corresponding measurements, yield a very useful tool for surface studies and point defect identification in the bulk. Calculations will be compared with available experimental data.
Effect of C/B Sequential Implantation on the B Acceptors in 4H-SiC
Authors: Nakano Yoshitaka , Kachi Tetsu , Tadano Hiroshi , Malhan Rajesh Kumar
Published: April 2011

We have systematically investigated the effect of C/B sequential implantation on the B-related acceptors and deep levels in 4H-SiC using thermal admittance spectroscopy. With increasing concentration of co-implanted C, the density of deep levels was found to start decreasing and to be completely suppressed for the C and B ratio of 1:1. Moreover, the density and ionization energy of B acceptors were seen to start increasing and lowering, respectively, with increasing C concentration. However, we found that excess C-content leads to the formation of a new complex defect. The C-V results also support the expected increase in the total hole concentration with increasing concentration of the co-implanted C-atoms, which is followed by a decrease in the concentration under C-rich condition. This is in reasonable agreements with the behavior of the B acceptors and deep defect levels. Therefore, the concentration of co-implanted C-atoms is considered to be very sensitive to the formation of the B acceptor levels.
Comparison of 3C-SiC Films Grown By CVD on Si(111) and Si(211) Substrates
Authors: Grayson B. A. , Wolan J. T. , Graves M. , Bledsoe M. , Kirchner K. , Saddow S. E.
Published: April 2011

In this study, the near-surface region of air-exposed thin 3C-SiC films grown on 50-mm (2-in.) diameter Si(111) and Si(211) substrates have been investigated. Carbonization to create the film in a radio-frequency (RF) induction-heated horizontal atmospheric-pressure chemicalvapor- deposition reactor utilized a propane-hydrogen mix (3% C3H8 in ultra-high purity hydrogen) with a hydrogen carrier. Elemental and chemical-state identification of the thin-films are presented. Several structure sensitive techniques including X-ray diffractaion (XRD) in ω-2θ as well as scanning electron microscopy (SEM) to examine crystal structure, surface morphology and film thickness are included.
Effects of Surface and Interlayer Processing Conditions on Selected Ohmic Contact Metallizations for p-type Silicon Carbide
Authors: Porter L. M. , Jang T. , Worren T. , Chang K. C. , Papanicolaou N. A. , Erickson J. W.
Published: April 2011

A comparative study of Pt and Pt/Si contacts to p-type 6H-SiC in terms of various processing conditions and interlayer specifications was performed. Deposition temperature, the thickness of the Si layer, and B-dopant incorporation in the Si were found to significantly affect the specific contact resistivity (SCR) values. In addition, pre-etching of the SiC surface in SF6 + Ar was found to consistently reduce the SCR's. The lowest average SCR values were 3 × 10−5 Ωcm2 for Pt/Si/SiC contacts deposited on pre-etched SiC surfaces (7.0 × 1018 cm−3 doping concentration) and annealed at 1100 °C for 5 min.

Aluminum-titanium contacts also showed dependence on the thicknesses of the Al and Ti layers and on the locations of the layers. Differences in both the SCRs and surface morphology are presented.
Decoherence Effects in Arrays of Coupled Quantum Dots
Authors: Cota E. , Rojas F. , Ulloa S. E.
Published: April 2011

We study the dynamics of the charge distribution (polarization) in an array of four quantum dots located at the corners of a square. The array has two extra electrons in a neutralizing background. We take into account electron-phonon interaction effects using a quantum Markovian master equation approach. Results show that dissipative effects are determinant in the dynamical response of a quantum-dot cell close to a nearby driver cell with prescribed polarization varying with time. The response is shown to depend strongly on temperature.
The Assembly of Nano-Materials Using Bio-Scaffolding
Authors: Yun C. Steven , Major Jody L. , Strouse Geoffrey F.
Published: April 2011

Bio-assembly strategies may allow formation of high-density nano-electronic architectures possessing unique electronic properties. These bio-nanomaterial hybrids should exhibit bio- compatibility and bio-viability. Therefore, contingencies of maintaining tertiary structure in the presence of nanometer-sized particles are a potential fatal flaw in bio-scaffolded assemblies. In this manuscript we investigate the assembly of bio-scaffolded nanomaterial constructs, specifically polypeptides, while probing the issue of bio-compatibility and viability by TEM imaging.
Atomistic Study of Strain Profiles in Semiconductor Quantum Dot Structures
Authors: Shintani K. , Sugii H. , Kikuchi Y. , Kobayashi M.
Published: April 2011

The minimum energy atomic configurations of stacked GaAs/InAs/GaAs quantum dot structures are calculated by using the conjugate gradient energy minimization with the Stillinger-Weber potentials. The islands are assumed to be either of pyramidal shape or of trapezoidal shape. The numerical results for the five-layer stacked structures show that the normal strains exhibit stepwise up-and-down profiles through the vertical centerlines of the islands and intermediate layers. Next, the molecular dynamics method with the Tersoff potential is applied to single Ge/Si and Si/Ge/Si structures with pyramidal islands in order to investigate the effect of temperature. It is found that there is a considerable difference between the normal strain in the direction perpendicular to the island base obtained by the conjugate gradient minimization with the Stillinger-Weber potential and that obtained by the molecular dynamics method at 800 K.
Irreversible Structural Transformation of five fold i-AlPdMn Quasicrystals after Ion Bombardment and Annealing
Authors: Schmithüsen F. , Cappello G. , Decossas S. , Torricelli G. , Lee T.-L. , Boissieu M. de , Calvayrac Y. , Lograsso T. , Comin F. , Chevrier J.
Published: April 2011

Five fold i-AlPdMn surface prepared under UHV by ion bombardment and annealing was so far considered to be bulk terminated. This result was substantially based on a quantitative LEED analyses [1]. Analysis of the specular rod in a X ray diffraction experiment at grazing incidence supported this result [2]. We present a new study of this surface by high resolution X ray diffraction at normal incidence. In this Bragg configuration the diffraction peak 18 – 29 for instance is at a photon energy of 2.873keV, the 72 – 116 reflection at 5.725keV. This results in an analyzed thickness of the sample surface of a few micrometers.

The surface was cleaned by ion bombardment. During annealing (T≅880K), we clearly observed the progressive disappearance of the initial Bragg peak characteristic of the as cast bulk sample. Conversely a new Bragg peak grows at an energy position shifted by 1eV compared to the position of the original Bragg peak. This is a clear signature for an irreversible structural transformation which takes place on at least the micron thickness. On the transformed surface, both, a LEED pattern and a RHEED pattern, characteristic for a five fold surface were easily obtained.

This high resolution experiment (the relative Bragg peak shift is 3ׁ10−4) was reproduced on samples from different initial compositions. This shows that five fold i-AlPdMn surface changes after preparation by ion bombardment and annealing at 900K on a micrometer thickness. This is not consistent with the conclusion that the surface is simply terminated by a cut of the original bulk. We conclude that a reorganization process of the quasicrystalline structure during annealing proceeds in the surface vicinity (probed depth is close to a few microns).
TDS Study of Potassium Adsorption on Al(111) and fivefold Al-Pd-Mn
Authors: Wehner Barbara I. , Anderegg James W. , Jenks Cynthia J. , Ross Amy R. , Lograsso Thomas A. , Diehl Renee D. , Thiel Patricia A.
Published: April 2011

We have used thermal desorption spectroscopy to carry out a comparative study of potassium adsorption on Al(111) and on the fivefold Al-Pd-Mn surface. Potassium adsorption on the quasicrystal was found to be different than on Al(111). The potassium monolayer desorbed from fivefold Al-Pd-Mn at lower temperatures than from Al(111). Potassium is known to form a dense monolayer on Al(111), with an ideal coverage of 0.33, but for the monolayer on fivefold Al Pd Mn we find that the saturation coverage is only one twelfth.
Surfactant-Mediated Growth of Epitaxial Films on Quasiperiodic Surfaces
Authors: Shimoda Masahiko , Guo Junqing , Sato Taku J. , Tsai An Pang
Published: April 2011

We have succeeded for the first time in inducing epitaxial growth of metal films on quasiperiodic surfaces. Depositions of Au onto the tenfold surface of a decagonal Al-Ni-Co quasicrystal were originally attempted to create a quasicrystal of a single element as an epitaxial film. Although the desired film was not obtained on the clean surface, a multiply twinned AuAl2 crystalline layer with a (110)-oriented surface was formed with the aid of an In pre-deposition layer. The presence of pre-deposited In also worked as a surfactant for the case of Au-deposition onto the fivefold surface of an icosahedral Al-Pd-Mn quasicrystal and promoted the formation of an ordered film of Au-Al alloy. X-ray photoelectron diffraction images from this ordered film show that the alloy has an icosahedral structure as a long-range order, suggesting the formation of a quasicrystalline film.
About the Metal-Insulator Transition in Quasicrystals
Authors: Delahaye J. , Berger C. , Grenet T. , Fourcaudot G.
Published: April 2011

Electronic properties (conductivity and density of states) of quasicrystals present strong similarities with disordered semiconductor based systems on both sides of the Mott-Anderson metal-insulator (MI) transition. We revisit the conductivity of the i-AlCuFe and i-AlPdMn phases, which has temperature and magnetic field dependence characteristic of the metallic side of the transition. The i-AlPdRe ribbon samples can be on either side of the transition depending on their conductivity value. In all these i-phases, the density of states at the Fermi level EF is low. Its energy dependence close to EF is similar to disordered systems close to the MI transition where it is ascribed to effects of interactions between electrons and disorder.
Beyond the Usefulness of Quasicrystals
Authors: Dubois J.M.
Published: April 2011

For more than a decade, quasicrystals were considered to exhibit only fundamental interest in so far that they have raised basic questions regarding the organization and stability of condensed matter. As well, they addressed the mechanisms of wave propagation in aperiodic solids. In more recent years, they turned to become useful materials in various mechanical devices, in solar absorbers or in heat insulation coatings. Nowadays, the science of quasicrystals, and especially the relevant high-dim crystallography techniques, have issued quite useful concepts and methods that prove useful in neighboring domains of condensed matter physics, of mechanics, of low temperature physics, etc. Hence, this article puts more emphasis on a few of the new subjects that quasicrystals science has helped us to study rather than on the few possible breakthroughs of quasicrystalline materials identified yet on the market.
Interralation Between Hume-Rothery Mechanism, Hybridization and Covalent Bonds in Aluminum- and Boron-based Icrosahedral Approximants and Quasicrystals
Authors: Kimura Kaoru , Kirihara Kazuhiro , Fujumori Masaaki , Nakayama Takahiro , Takata Masaki , Sakata Makoto
Published: April 2011

Metallic-covalent bonding conversion in icosahedral clusters of Al and B by small changes of the structure was demonstrated by molecular orbital calculations. According to the electron density distribution obtained using the maximum entropy method and the Rietveld method, the bonding conversion phenomenon occurs even in cluster solids such as some Al- and B-based icosahedral approximant phases (Al12Re, α-AlMnSi, α-rhombohedral boron). The multiple-shell atomic structure, the electrical and optical conductivity are compared for metal doped β-rhombohedral boron and AlLiCu or AlPdRe icosahedral quasicrystal. Photoemission and electron energy loss spectroscopy investigations for V doped β-rhombohedral have been discussed. Conclusions are the following two. It is difficult to distinguish a Hume-Rothery, i.e. structure-induced, pseudogap, and a covalent bonding, i.e. hybridization, pseudogap for materials with a highly symmetric Brillouin zone and a strong potential for valence electrons. Because hybridization is necessary not only for covalent bonds but also for metallic ones, it is better to use the covalent bonding pseudogap than the hybridization pseudogap.
Stable Icosahedral Quasicrystals in the Cd-Based Alloys
Authors: Guo J. Q. , Abe E. , Tsai A. P.
Published: April 2011

Stable icosahedral quasicrystals have been found in ternary Cd65Mg20RE15 (RE=Y, Gd, Tb, Dy, Ho, Er, Tm, Lu, Yb and Ca) and binary Cd85Ca15 and Cd84Yb16 alloys. These icosahedral quasicrystals have a primitive quasilattice. Their quasilattice parameters range from 0.5571 to 0.5731 nm. Formation mechanism of the quasicrystals is discussed.
Unraveling the Structure of Decagonal Approximants by “Brute Force” Deconvolution of the Experimental Autocorrelation Function
Authors: Estermann Michael A. , Lemster Katja , Steurer Walter
Published: April 2011

Methods for theab initiostructure analysis of periodic approximant phases from single- crystal X-ray diffraction data are presented. These methods are particularly suited to complex approximant structures with large unit cells and strong pseudosymmetry (where routine X-ray structure solution tools fail) and are based on the “brute-force” deconvolution of the experimentally measured autocorrelation function. This function is obtained directly by a simple Fourier transform of the measured X-ray diffraction intensities. Sub-optimal diffraction data from twinned, nanodomain and polycrystalline specimens can be processed despite the inevitable lack of information due to reflection overlap and limited resolution. The deconvolution process allows complex approximant structures to be unraveled without prior knowledge about the structure-building motifs. Examples are presented for the systems Al-Co-Ni and Al-Co-(Ta).
High Temperature Plastic Behaviour of Icosahedral AlCuFe Quasicrystals.
Authors: Fikar Jan , Bonneville Joël , Baluc Nadine , Guyot Pierre
Published: April 2011

Icosahedral AlCuFe poly-quasicrystalline specimens were deformed in constant strain rate compression tests at temperatures ranging between 300K - 1020K. Below nearly 0.7 Tm (Tm is the melting temperature) the specimens were brittle. Above the brittle-to-ductile transition temperature, after the elastic stage the stress-strain curves exhibit a marked yield-point followed by a stage of strain softening only. Transient creep tests were performed at different given stress/strain levels after interrupting the constant strain-rate deformation tests. After the transient tests, the flow strength of the specimens was investigated anew at constant strain rate. The results are interpreted in the framework of a dislocation model, where two effects opposing dislocation movement are considered: firstly, the usual elastic dislocation interaction, yielding a work-hardening contribution, and, secondly, a friction stress specific to the quasiperiodic lattice, leading to a softening effect.
Low Temperature Plastic Behaviour of Icosahedral AlCuFe Quasicrystals
Authors: Fikar Jan , Bonneville Joël , Rabier Jacques , Baluc Nadine , Proult Anne , Cordier Patrick , Stretton Iona
Published: April 2011

The mechanism by which dislocations move in the icosahedral quasicrystalline structure, i.e., glide or climb, is still an open question. In order to check whether pure dislocation glide occurs in this quasi-periodic structure, low temperature deformation tests have been performed under confining pressure conditions. These experimental techniques, which superimpose a shear stress on an isostatic component, enable the brittle-to-ductile transition temperature to be shifted to temperatures at which diffusion processes can be assumed to be negligible. Such techniques have been applied to deform plastically AlCuFe poly-quasicrystals at low and intermediate temperatures, using both gas and solid-confining media. Mechanical data as well as microstructural observations associated with this low temperature deformation range are reported. The first results provide new insights into the deformation mechanisms that control plasticity in the icosahedral quasicrystalline phase.
Estimation of Back Stress Produced by Dislocation Interaction in Icosahedral Al-Pd-Mn
Authors: Hirai Hisatoshi , Kitahara Akira , Yoshida Fuyuki , Nakashima Hideharu
Published: April 2011

We attempted to calculate the breakaway stress σb of dislocation from attractive junction made by reaction of dislocations. Assuming that the force f acting on the unit length of dislocation with the Burgers vector B under a shear stress τa is f τ∣b˝∣ where b˝ is the phonon component of B, and that the elastic energy per unit length of dislocation W is approximated by W = G(∣b˝∣2 + c2 ∣b˔∣2) where G is the shear modulus, b˔ the phason component of B and c2 a coefficient of about 3.1 × 10−3. Using the values G = 48.4 GPa at 1070 K, the Taylor factor M = 3 and the measured dislocation density of 1.8 × 1013 m−2, we calculated σb for 21 possible dislocation reactions. Picking up the most possible dislocation reactions, σb distributed between 50 and 80 MPa, and the average of them was 64 MPa. This result strongly suggested the possibility that the main part of the internal stress of the high-temperature deformation of icosahedral Al-Pd-Mn is explained by σb.
Influence of Chemical Disorder, Magnetic Field and Phason Flipd on the Localization of Electronic States in a Regular Icosahedral Quasicrystal
Authors: Vekilov Yu.Kh. , Isaev E.I. , Arslanov S.F.
Published: April 2011

The influence of substitutional chemical disorder, magenetic field and phasons, on the electronic spectrum and wave functions of icosahedral quasicrystals is investigated by means of tight-biding approximation and level statistic method. The results show that the localization of electronic states in an ideal quasictystal exists due to their coherent interfernce at the Fremi level which is caused by the symmetry and aperiodic long-range order.
Diffraction Study of The Transformation of an Al-Cu-Fe Atomized Powder Upon Annealing at 500°C
Authors: Weisbecker P. , Bonhomme G. , Cael A. , Zhang L. , Dubois J.M.
Published: April 2011

Using powder X-ray diffraction, we have studied the transformation of an atomized powder of nominal composition Al62Cu25.5Fe12.5 upon annealing at 500°C in air or in vacuum. The initial mixture of icosahedral and B2 cubic phases transforms within 2 hours into a nearly pure icosahedral compound. We observe however that a small amount of residual cubic phase is still present after annealings as long as 65 hours. Furthermore, the initial cubic phase splits in two components, a CsCl-type and a disordered bcc cubic phase. By comparison of the heat treatments in air and in vacuum, we point out that oxygen atoms diffuse into the icosahedral lattice.
Local Atomic Structures in Amorphous and Quasicrystalline Zr70Ni10Pt20and Zr80Pt20Alloys by the Anomalous X-ray Scattering Method
Authors: Matsubara Eiichiro , Nakamura Takahiro , Sakurai Masaki , Imafuku Muneyuki , Sato Shigeo , Saida Jyunji , Inoue Akihisa
Published: April 2011

Local atomic structures around Zr and Pt in a quasicrystalline Zr80Pt20alloy, and amorphous and quasicrystalline Zr70Ni10Pt20alloys have been determined by the anomalous x-ray scattering (AXS) method. A distinct prepeak observed in an intensity profile of the amorphous Zr70Ni10Pt20alloy indicates existence of strong chemical short-range order (CSRO) clusters in the amorphous phase. Total coordination numbers around Zr and Pt in a nearest neighbor region in both alloys have been evaluated. The values around Pt are almost equal to 12 in the amorphous and quasicrystalline states suggest formation of icosahedral clusters around Pt. Some of crystalline structures formed from the quasicrystalline phases by annealing consist of icosahedral clusters of Zr and Ni atoms, or polyhedral clusters of Zr and Pt atoms similar with icosahedral clusters. The present result appears to support that the phase transformation from the amorphous to the crystal through the quasicrystal is explained by the icosahedral CSRO clusters in the amorphous and quasicrystalline states.
Study of the Electrical Resistivity Change upon Phase Transformation in Zr-Based Metallic Glass
Authors: Komatsu Keiko , Tamura Ryuji , Takeuchi Shin , Shibuya Tadaharu
Published: April 2011

In order to gain insight into the influences of formation of an icosahedral phase in Zr-based metallic glasses on the physical properties, thermal and electrical properties of Zr70Ni10Pd20 metallic glass were investigated. From DSC analyses, activation energies for the phase transition from an amorphous to an icosahedral phase and from the icosahedral to a crystalline (Zr2Ni) phase were estimated to be 3.42 and 3.07 eV, respectively. The effect of the phase transformations on the electrical resistivity was successfully observed; the resistivity clearly exhibits an increase and a decrease corresponding to the transitions from the amorphous to the icosahedral phase and from the icosahedral to the crystalline Zr2Ni phase, respectively. It is concluded that the increase of the resistivity upon the precipitation of the icosahedral phase is due to an increase of the volume fraction of the icosahedral grains, which possess higher resistivity.
Structural Study of Fe-Based Glassy Alloys with a Large Supercooled Liquid Region
Authors: Imafuku M. , Sato S. , Nakamura T. , Koshiba H. , Matsubara E. , Inoue A.
Published: April 2011

The thermal stability and local atomic structures of glassy Fe70M10B20 (M = Hf, Zr, Nb, W and Cr) alloys were analyzed by DSC, ordinary X-ray diffraction and AXS measurements. The random network of the trigonal prism-like structure of (Fe,M)3B with edge-sharing, was identified in all the Fe70M10B20 (M = Hf, Zr, Nb, W and Cr) alloys in spite of the wide variety of thermal stability upon heating. Several unique primary precipitated crystalline phases, such as Fe23B6 type and Fe-M phases, were observed in the alloys exhibiting a high thermal stability. These crystallization reactions require relatively long range rearrangements of the constituents and hence the thermal stability of the glassy phase increases, leading to the appearance of a large supercooled liquid region upon heating. These phenomena may be originated from the difference in the chemical affinity and the atomic size mismatch between M and Fe or B.
Plastic Deformation of Bulk Amorphous Alloys
Authors: Xing L.Q. , Hufnagel T.C. , Ramesh K.T.
Published: April 2011

We have studied plastic deformation, including “serrated flow,” of bulk metallic glasses under quasi-static uniaxial compression. The deformation response is essentially elastic-perfectly plastic, but the “plastic” deformation actually consists of sections of elastic loading separated by abrupt load drops. The load drops are due to the formation of shear bands, which represent the primary mechanism of plastic deformation

In Zr-Ti-Cu-Ni-Al bulk metallic glasses, fracture occurs after about 1-2% plastic strain, but in Zr-Ta-Cu-Ni-Al metallic glass the plastic strain to failure can be as large as 6-7%. The difference appears to be due a strong tendency for the shear bands in this alloy to branch. The branching presumably reduces the stress concentration on the shear bands, retarding the onset of fracture. No evidence is seen for the formation of crystalline phases in this alloy.
Correlation Between Resistivity Characteristics and Electronic Structure Parameters of the Ni-Pd-P Amorphous Alloys
Authors: Babich Mykola , Nakonechna Olesya , Semen'ko Mykhailo , Zakharenko Mykola , Yeremenko Gennagy , Yavari Alain R.
Published: April 2011

The temperature dependencies of resistivity of the Ni-Pd-P-based amorphous alloys (P content was varied from 14 to 23 at. %) have been measured in a wide temperature range using the standard four-probe method. The temperature coefficients of resistivity (TCR) have been calculated. TCR value is decreased with P content increasing and becomes negative for Ni46Pd31P23 alloy. On the other hand, TCR value is increased with Pd content increasing while P content is not changed. Such behavior could be attributed to the electronic transfer effect from Ni to Pd and from P to Ni. These lead to the Fermi level shift to d-band edge and to the increasing influence of the Mott's localized state. The peculiarities caused by structural relaxation processes have been observed at the temperature range of 550 – 620 K. The relaxation process was shown to consist at least of two stages. The kinetics of the crystallization process has been analyzed using the Kissinger method.
Structure and Soft Magnetic Properties of Bulk Fe-Al-Ga-P-C-B-Si Glassy Alloys Prepared by Consolidating Glassy Powders
Authors: Inoue Akihisa , Yoshida Shoji , Mizushima Takao , Makino Akihiro
Published: April 2011

With the aim of developing a bulk glassy Fe-based alloy with good soft magnetic properties by the powder metallurgy technique, we have applied the pulse current sintering technique to a Fe70Al5Ga2P9.65C5.75B4.6Si3 glassy alloy powder with a large supercooled liquid region of 60K before crystallization. The existence of the supercooled liquid region was found to enable us to form a bulk glassy alloy with a very high relative density of 99%. The resulting bulk glassy alloy exhibits good soft magnetic properties, i.e., 1.17T for flux density at a field of 800A/m, 12.0A/m for coercive force and 8000 for maximum permeability which are much superior to those for the bulk amorphous Fe-Si-B alloy prepared by the same sintering method. The much better soft magnetic properties for the multicomponent Fe-based bulk alloy are attributed to the combination of the high relative density and the unique amorphous structure with the features of high packing density and long-range homogeneous atomic configurations. The first success of forming the bulk amorphous alloy with good soft magnetic properties by the powder metallurgy technique is expected to enable us to use as practical soft magnetic materials.
Exafs and Exelfs Study of the Structure of Pd-Ni-P Bulk Metallic Glasses
Authors: Alamgir Faisal M. , Jain Himanshu , Williams David B. , Hug Gilles , Schwarz Ricardo B. , Jin Ou
Published: April 2011

We have explored the short-range order around all three constituent atoms in (Pd-Ni)80P20 bulk metallic glasses (BMGs), a system that is a prototype for a whole class of BMG formers containing 80% transition metal and 20% metalloid. We have examined the changes in the nearest neighbor environments around the transition metals in (Pd-Ni)80P20 glasses using extended X-ray absorption fine structure (EXAFS) in comparison to their binary counterparts. We have done similar studies around the coordination of P using extended energy loss fine structure (EXELFS). The environment around the all the atoms in Pd60Ni20P20 and Pd30Ni50P20are very similar to those of the binary phosphides at the ends of the composition range. However, the (Pd-Ni)80P20 glasses are not simply solid solutions of the phases. The nearest neighbor distances of the metals are reduced in the ternary alloys with respect to those of the binary phosphides. The best glass former in this series, Pd40Ni40P20 is nearly isostructural to Pd30Ni50P20 but shows shorter distances at the 2nd and 3rd coordination shells, which we believe is due to more efficient packing in this glass. The metal environments in Pd40Ni40P20, on the other hand, are better described by a weighted average of those of Pd30Ni50P20 and Pd60Ni20P20.
Formation of Nanostructured Alloys by Liquid Phase Spinodal Decomposition
Authors: Guo W.H. , Kui H.W.
Published: April 2011

A molten Pd82Si18 alloy, free of potent heterophase impurities and enclosed in a fused silica tube of conical shape, was quenched in water. Four regions classified according to their microstructures can be distinguished. They are: region I, amorphous; region II, a mixture of an amorphous nanostructure and a nanocrystal; region III, a mixture of two kinds of nanocrystals; region IV, spinodal networks; and region V, conventional eutectic structure. The detailed microstructures in each region are discussed.
Processing and Hard Magnetic Properties of Nanocrystalline Sm(Co,Zr)7 Magnet Powders
Authors: Tang H. , Zhou J. , Liu Y. , Sellmyer D.J.
Published: April 2011

Isotropic nanocrystalline Sm(Co,Zr)7 permanent magnet powders of the TbCu7 type structure and with high coercivity and enhanced remanent magnetization has been synthesized by mechanically milling Sm12.5Co87.5-xZrx alloys (x = 0, 1, 2, 3) alloys and subsequently appropriate annealing. The mechanical-milling process of the alloys and the formation of nanostructured Sm(Co,Zr)7 magnet powders have been investigated with respect to hard magnetic properties. Hard magnetic properties are found to be strongly dependent upon the processing condition (like milling time, annealing temperature, etc.). Optimal coercivity Hci of value above 21 kOe has been obtained in Sm12.5Co85.5Zr2 magnet powders subjected to milling for 5 hr and annealing at 600°C for 20min. Optimal remanent magnetization Mr of 73.4 emu/g, remanence ratio Mr/Ms of 0.71, and maximum energy products (BH)max of over 13 MGOe have been realized in Sm12.5Co87.5-xZrx (x = 1, 2) magnet powders with grain size of 15-20 nm. The hard magnetic properties are ascribed to the nanosized Sm(Co,Zr)7 phase of the TbCu7 type structure with grain size of 10-20 nm. The enhancement of remanent magnetization may be contributed from the enhanced exchange-coupling interaction between nanosized grains.
Microstructures and Properties of Metal Reinforced Zr57Nb5Al10Cu15.4Ni12.6Bulk Metallic Glass Composites
Authors: Choi-Yim Haein , Conner Robert D. , Johnson William L.
Published: April 2011

To increase toughness of a metallic glass with the nominal composition Zr57Nb5Al10Cu15.4Ni12.6, it was used as the matrix in the composites reinforced with W fiber, W particles, and W/Re particles. Microstructures of the composites are studied by scanning electron microscopy and electron microprobe. Compression results are presented. Dynamic deformation behavior of the composites is studied in ballistic tests. The tests were performed firing composite rods into 6061 T651 aluminum target and 4130 steel targets. The composite rods failed by localized adiabatic shear banding and exhibited self-sharpening behavior.
Fracture and Fatigue Crack Growth of Bulk Metallic Glass Alloys and their Composites
Authors: Flores Katharine M. , Dauskardt Reinhold H.
Published: April 2011

Bulk metallic glasses exhibit extraordinary strength and elastic properties, but the lack of extensive plastic deformation and the catastrophic nature of failure limit the usefulness of the monolithic alloy. This work presents an overview of the results of fracture and fatigue crack growth experiments for a bulk metallic glass matrix composite with a Zr-Ti-Nb reinforcement phase precipitated in situ. The composite exhibits stable crack growth at stress intensities nearly double the fracture toughness of the monolithic glass. This is associated with the formation of a distributed damage zone at the crack tip. While the second phase had a marked effect on the fracture behavior, there was very little change to the fatigue crack growth results. Fractography suggests that the second phase particles are too small to cause crack bridging or significantly change the crack path.
Quality Assurance Evaluation of Thermal Barrier Coatings by Electrochemical Impedance Spectroscopy
Authors: Zhang Jianqi , Tamboli Danyash , Desai Vimal
Published: April 2011

The technique of electrochemical impedance spectroscopy (EIS) was used to examine the behavior of intact thermal barrier coatings (TBC) at ambient temperature. Cross-sectional morphological examination of TBC was conducted by scanning electron microscope (SEM). By correlating the SEM visual examination with EIS data, TBC was characterized non-destructively. A model of EIS alternative current (AC) equivalent circuit was proposed to establish the relationship between the EIS elemental parameters in the circuit and the microstructural characteristics of TBC. A linear relationship was found to exist between the electrical impedance of TBC topcoat and the thickness of the topcoat. The porosity of TBC top coat showed a linear relationship with the capacitance of ceramic TBC, and the pore shape in the TBC topcoat was represented by the value of the electrical impedance of the pore. The result in the study has demonstrated that EIS can be used as a non-destructive evaluation (NDE) technique for quality assurance of TBC.