bluuebunny/crossref_metadata_2025 huggingface dataset:
Physical Characterization of Residual Implant Damage in 4H-SiC Double Implanted Bipolar Technology
Authors: Wright N. G. , Johnson C. M. , O'Neill A. G. , Horsfall A. , Ortolland S. , Adachi K. , Phelps G. J. , Knights A. P. , Coleman P. G. , Burrows C. P.
Published: April 2011

The effects of post-implant anneal conditions on the level of residual damage resulting from nitrogen and boron implants after different anneal processes are investigated using the Positron Annihilation Spectroscopy (PAS) technique. It is shown that after implantation there is a substantial defect concentration significantly below the range of the implants. However such damage is almost completely recovered after anneal in contrast with the damage close to the implant range point. Such residual damage has a strong effect on the electrical characteristics of double implanted bipolar transistors - principally though reduction in carrier mobility and lifetime. It is shown that the precise implant and anneal conditions play a strong role in the level of such damage and the subsequent electrical performance of bipolar devices.
Observation of a Non-stoichiometric Layer at the Silicon Dioxide – Silicon Carbide Interface: Effect of Oxidation Temperature and Post-Oxidation Processing Conditions
Authors: Chang K. C. , Wahab Q. , Porter L. M.
Published: April 2011

Thermal oxides were grown on n-type 6H-SiC(0001) at 1100 °C for 2 hrs in a wet oxygen ambient after the substrates were cleaned using the complete RCA cleaning process. Metal-oxide-semiconductor (MOS) diodes were then fabricated and subsequently cleaned under different annealing conditions, including re-oxidation-, NO-, and forming gas (10% H2 + 90% N2)-annealing at 950 °C for one hour. Measurements of the interface state densities (Dit) at room temperature showed that post oxidation annealing (POA) reduced the Dit values to a varying degree depending on the specific annealing condition. Annealing in NO and forming gas resulted in the largest reduction in Dit values.

Oxides were grown at 950, 1100, and 1250 °C for varying amounts of time without receiving POA. A non-stoichiometric (SixC, x>1) transition layer adjacent to the SiO2/SiC interface has been observed by electron energy loss spectroscopy (EELS). The thickness of this layer was found to increase with oxidation temperature and was not observed at all for a thin oxide grown at 950 °C. The Dit values (close to the conduction band) for diodes with oxides grown at 950°C without POA were lower than the Dit values for the samples oxidized at 1100 °C with any of the POA treatments. While the thickness of the transition layer was found to be dependent on temperature, our results indicate that it is independent of oxide thickness. This transition layer may be associated with the high Dit values and low channel mobilities for SiC MOSFETs.
Structural Properties of 3C-SiC Layers Grown on Si Substrates by Electron Cyclotron Resonance CVD Technique
Authors: Giorgis F. , Chiodoni A. , Cicero G. , Ferrero S. , Mandracci P. , Pirri C. F. , Barucca G. , Calcagno L. , Foti G. , Musumeci P. , Reitano R.
Published: April 2011

In this work we mainly report on the analyses of polycrystalline silicon carbide films grown by Electron Cyclotron Resonance Chemical Vapor Deposition (ECR-CVD) on Si (100) and Si (111) substrates. Structural properties of the films have been analyzed by X-ray diffractometry, transmission electron microscopy and micro-Raman spectroscopy. Samples deposited with optimized deposition conditions, show a polycrystalline columnar structure with lateral crystal dimensions ranging from 300 up to 1400 Å and an orientation close to that of the Si substrates.
Ion Implantation for Silicon Carbide Electronic Devices
Authors: Capano Michael A.
Published: April 2011

As the demands on electronic systems intended for high temperature, high power or high frequency operation increase, silicon-based electronics are being pushed to its fundamental material limits. Consequently, continued improvement in system level performance for these applications requires new semiconductor materials. Silicon carbide (SiC) is a candidate material for the applications listed above, but considerable materials, processing, and device research are still needed before SiC devices are brought to market.

This paper concentrates on ion implantation processing of SiC, and attempts to illustrate specific problems associated with ion implantation. Data from a series of experiments relevant to SiC MOSFETs are presented for this purpose. Ion implants into SiC are discussed first to show how a strategy for improving the acceptor activation ratio from less than 1% to nearly 100% is developed. The annealing temperatures (>1600°C) needed to attain high activation ratios lead to severely roughened surfaces. Results from experiments designed to preserve high-quality surfaces have been encouraging from a purely materials perspective. However, these solutions do not translate into improved transport characteristics for SiC MOSFETs. An investigation of inversion layer mobility in 4H-SiC MOSFETs is presented to illustrate this point.
Structural Defects in Ion Implanted 4H-SiC Epilayers
Authors: Persson P. O. Å. , Skorupa W. , Panknin D. , Kuznetsov A. , Hallén A. , Hultman L.
Published: April 2011

Transmission electron microscopy (TEM) was used to investigate Al, Ar, C and Si ionimplanted 4H-SiC epilayers. After the implantation the samples were thermally annealed for 30 minutes at 1700°C. During the annealing process dislocation loops are formed and the generation of such dislocation loops upon annealing, is investigated with respect to dopant electrical activation, peak ion concentration and calculated interstitial/vacancy concentrations. It is concluded that the dislocation loops are generated as the result of a combination of residual damage and excess interstitials generated in a “plus one” (+1) process.
Optical Properties of Aluminium and Nitrogen in Compensated 4H-SiC Epitaxial Layers
Authors: Syväjärvi Mikael , Yakimova Rositza , Kakanakova-Georgieva Anelia , Henry Anne , Janzén Erik , Linnarsson Margareta K.
Published: April 2011

Compensating impurities affect the device performance but still the fundamental knowledge of this effect in SiC is poor. We demonstrate that the optical properties are affected by compensation effects between the nitrogen donor and aluminium acceptor in 4HSiC epilayers. As demonstrated using low-temperature photoluminescence measurements, the interaction occurs between the impurity bound excitons caused by changes in the aluminium concentration while the nitrogen concentration is constant. The variation is observed over two orders of magnitude in the aluminium concentration. The interplay is also observed in cathodoluminescence measurements between the band edge luminescence and nitrogen-aluminium donor-acceptor pair emission. These observations may be possible to utilize for a non-destructive characterization of compensated epilayers.
Engineering the Al-Ti/p-SiC Ohmic Contact for Improved Performance
Authors: Lin J. Y. , Mohney S. E. , Smalley M. , Crofton J. , Williams J. R. , Isaacs-Smith T.
Published: April 2011

The influence of composition on Al-Ti ohmic contacts to 4H p-SiC was studied. When NA was 7 x 1018 cm−3, contacts with 70 wt.% or more Al became ohmic when annealed at 1000°C for 2 min, whereas when there was 60 wt.% or less Al, the contacts did not become ohmic even when annealed under more severe conditions (longer times and/or higher temperatures). Spiking of the contact metallization always accompanied ohmic behavior and could be correlated with Al-Ti compositions that contain both an Al-rich liquid and solid TiAl3 at 1000°C prior to reaction with SiC or evaporative loss of Al. For the 70 wt.× contacts, a specific contact resistance of 1.5 × 10−4 Ω cm2 was measured along with spiking of the metallization into the SiC with a room-mean-square interfacial roughness of 150 Å and a maximum spiking depth of 1200 Å. Although still a concern, this spiking was less severe than observed for the 90 wt.× composition. A conductive CrB2 cap layer was next demonstrated to retard evaporation of Al during annealing of the Al-Ti contacts with 70 wt.× Al. The cap allowed use of thinner contact layers, reducing the depth of spiking and improving the surface morphology and edge definition of the ohmic contacts, with a one order of magnitude penalty in the specific contact resistance.
Reaction Kinetics of Thermally Stable Contact Metallization on 6H-SiC
Authors: Okojie Robert S. , Lukco Dorothy , Chen Yuan L. , Spry David , Salupo Carl
Published: April 2011

The growth kinetics of thermally stable Ti(100nm)/TaSi2 (200nm)/Pt (300nm) metallization on 6H-SiC was studied after heat treatment in air up to 700°C. Scanning electron microscopy (SEM) of the contact surface morphology reveals a two-dimensional network of features that is attributed to non-uniform oxide growth associated with the multigrain structure of the platinum overlayer. Auger electron spectroscopy (AES) and high-resolution transmission electron microscopy (HRTEM) identified three important reaction zones after initial 30-minute anneal at 600°C in nitrogen. One is the formation of a platinum silicide overlayer resulting from TaSi2 decomposition. The second is titanium silicide formation adjacent to the decomposed TaSi2. The third is pseudo-epitaxial Ti5Si3 at the SiC interface. Specific contact resistance values ranging from 10−4–10−6 Ω-cm2, remained stable after 200 hours at 600°C in air. Activation energies of 1.03eV for platinum silicide oxidation and 1.96eV for Ti5Si3 are obtained from Arrhenius plots.
Comparison Of F2 Plasma Chemistries For Deep Etching Of SiC
Authors: Lee K. P. , Leerungnawarat P. , Pearton S. J. , Ren F. , Chu S. N. G. , Zetterling C.-M.
Published: April 2011

A number of F2-based plasma chemistries (NF3, SF6, PF5 and BF3) were investigated for high rate etching of SiC. The most advantageous of these is SF6, based on the high rate (0.6 μm · min−1) it achieves and its relatively low cost compared to NF3. The changes in electrical properties of the near-surface region are relatively minor when the incident ion energy is kept below approximately 75 eV. At a process pressure of 5 mTorr, the SiC etch rate falls-off by ∼15 % in 30 μm diameter via holes compared to larger diameter holes (> 60 μm diameter) or open areas on the mask.
Quantum-Confined Electron Transitions in CdSe Nanocrystals
Authors: Ginger D. S. , Dhoot A. S. , Finlayson C. E. , Greenham N. C.
Published: April 2011

We present quasi-steady-state photoinduced absorption measurements on thin films of colloidal CdSe nanocrystals. We observe an intense, size-dependent absorption peaking in the mid-infrared when the samples are irradiated with visible light. The absorption is accompanied by a bleach in the visible near the position of the first excitonic absorption and persists with a lifetime of milliseconds at room temperature. We characterize the lifetime of the electron state and its cross-section. We attribute this feature to the 1S-1P transition of a delocalized electron. We report a bleaching of the first excitonic transition in electrically excited nanocrystal-based diode structures, and attribute it to charging of the 1S electron state.
Bioconjugates of Luminescent CdSe-ZnS Quantum Dots with Engineered Recombinant Proteins: Novel Self-Assembled Tools for Biosensing
Authors: Goldman Ellen R. , Mattoussi Hedi , Tran Phan T. , Anderson George P. , Mauro J. Matthew
Published: April 2011

Colloidal semiconductor quantum dots (QDs) are luminescent nanoparticles with size- dependent emission spectra spanning a wide range of wavelengths in the visible and near IR. This property, as well as their higher resistance to photo-degradation compared to organic dye labels, makes QDs potentially suitable for certain biomolecule tagging and multiplexing applications. We describe an electrostatic self-assembly approach for conjugating highly luminescent colloidal CdSe-ZnS core-shell QDs with engineered two-domain recombinant proteins to form conjugates for sensing and imaging applications. The design, preparation, and characterization of high quantum yield IgG antibody-binding protein G bioconjugates using luminescence, electrophoretic gel shift, and affinity assays is reported.
InGaAs Quantum Dots Embedded in p-n Junction on GaAs(311)B Substrate
Authors: Akahane Kouichi , Song Haizhi , Okada Yoshitaka , Kawabe Mitsuo
Published: April 2011

We have evaluated the photoluminescence (PL) properties of quantum dots (QDs) embedded in a p-n junction on GaAs(311)B and were compared with the identical structure grown on GaAs(001). The QDs on these two surfaces were fabricated with a similar size and density by optimizing the growth conditions. The PL emission from the QDs grown on GaAs(001) rapidly disappeared when the excitation power was decreased below 0.4W/cm2, while that from on GaAs(311)B was clearly observable even at a lower excitation power of 0.1W/cm2. From the measurements of PL intensity dependence on temperature and applied electric field, it was found that there are clear differences between the QDs grown on (001) and the QDs grown on (311)B in the recombination rates through non-radiative processes and/or the escape rates of carriers out of QDs to the GaAs host through a tunneling process.
Effects of Spacer Thickness on the Performance of InGaAs/GaAs Quantum Dot Lasers
Authors: Yeh Nien-Tze , Liu Wei-Shen , Chen Shu-Han , Chyi Jen-Inn
Published: April 2011

It is found that the performance of self-assembled In0.5Ga0.5As/GaAs multi-stack quantum dot lasers is sensitive to the GaAs spacer thickness between the dots. Reducing the spacer thickness from 30 nm to 10 nm leads to narrow photoluminescence linewidth, low threshold current, high characteristic temperature and high internal quantum efficiency. This behavior is attributed to inhomogeneous broadening caused by dot size fluctuation related to spacer thickness.
Growth, Structure, Optical and Ultrasonic Investigations of Icosahedral Zn-Mg-Y Quasicrystals
Authors: Sterzel R. , Bruls G. , Kounis A. , Miehe G. , Saitoh K. , Fuess H. , Karpus V. , Babonas G.-J. , Reza L. , Assmus W.
Published: April 2011

It is shown how the history of the growth of an icosahedral Zn-Mg-Y single grain can be determined by measuring the yttrium distribution. The growth mechanism and the stabilization of the icosahedral Zn-Mg-Y, RE (RE = rare earth: Ho, Er, Dy, Gd, Tb) quasicrystals are discussed with respect to structural investigations on related crystalline phases. We also show results of optical and ultrasonic investigations on icosahedral Zn-Mg-Y single crystals. They fit well to the discussed growth and stabilization mechanism.
The Influence of Growth Rate on Porosity in Al-Pd-Mn Icosahedral Quasicrystals.
Authors: Ross Amy R. , Fisher Ian R. , Canfield Paul C. , Lograsso Thomas A.
Published: April 2011

Growth experiments have been carried out to characterize the occurrence and development of porosity in Bridgman and flux grown Al-Pd-Mn icosahedral quasicrystals. The porosity level has been observed to fluctuate between values of 0.0 and 3.75 percent along the length of Bridgman single crystals implying that the development of porosity is affected by the local growth conditions. Experiments were conducted to evaluate the influence of the rate of solidification on the occurrence of porosity. Alloys were solidified with different growth rates, 1mm/hr and >10 mm/hr, using the Bridgman configuration and at different cooling rates, ranging from 0.29°C/hr to 10°C/hr, using the flux growth method. Porosity levels were analyzed via optical image analysis. These experiments indicate that porosity percentages are greatly influenced by cooling rates and crystal size.
Low-Energy Ion Scattering Measurements from an Al-Pd-Mn Quasicrystal
Authors: Bastasz R. , Jenks C. J. , Lograsso T. A. , Ross A. R. , Thiel P. A. , Whaley J. A.
Published: April 2011

Energy-angle distributions of low-energy inert-gas ions scattered from surfaces provide information about surface composition and structure. We have measured energy spectra of He+ scattered from an Al71Pd20Mn9 quasicrystal, which was oriented perpendicular to the 5-fold axis, along various azimuthal directions. Strong scattering signals are seen from Al and Pd, but only a weak Mn signal is observed. From measurements made of He+ at an oblique angle of incidence scattered in the forward direction, we observe a 72° periodicity in the azimuthal dependence of the scattering signal intensity from Al surface atoms. The effect arises from shadowing effects involving neighboring surface atoms and provides direct evidence that Al surface atoms exist in a local environment with 5-fold symmetry. In addition, measuring the variation of the signal intensity with incidence angle provides information about neighboring atom distances, which compare favorably with a model of the quasicrystal surface derived from the bulk structure.
Development of Al-Cu-Fe Quasicrystal-Poly(p-phenylene sulfide) Composites
Authors: Bloom Paul D. , Baikerikar K.G. , Anderegg James W. , Sheares Valerie V.
Published: April 2011

Quasicrystalline Al-Cu-Fe powders were used as a novel filler material in poly(p-phenylene sulfide) (PPS). These polymer/quasicrystal composites showed useful properties that may be beneficially exploited in applications such as dry bearings and composite gears. Al-Cu-Fe quasicrystalline filler significantly improved wear resistance to volume loss in polymer-based composites. In addition to improving the composite wear resistance, the Al-Cu-Fe filler showed low abrasion to the 52100 chrome steel counterface. Furthermore, mechanical testing results showed a two-fold increase in the storage modulus of the reinforced composites compared with the polymer samples. In addition, the Al-Cu-Fe filler was compared to its constituent metals, aluminum oxide, and silicon carbide in PPS. Chemical analysis of the wear interface by X-ray spectroscopy indicated the generation of a third body oxide layer during wear. The fabrication in addition to the thermal, mechanical, and wear properties of these unique materials is described.
Synthesis of Bulk Quasicrystals by Spark Plasma Sintering
Authors: Fleury E. , Lee J.H. , Kim S.H. , Song G.S. , Kim J.S. , Kim W.T. , Kim D.H.
Published: April 2011

Spark plasma sintering method was applied to Al-Cu-Fe and Al-Si-Cu-Fe gas-atomized powders to prepare almost pore-free cylindrical specimens with icosahedral and 1/1 cubic approximant phases, respectively. This investigation has revealed that a high density could be obtained despite the short period and low temperature imposed during spark plasma sintering. In comparison to hot press technique, these conditions are favorable since they limit the formation of secondary phases and avoid exaggerated grain growth. The Vickers microhardness and fracture toughness of these two alloy systems were found to be larger than those obtained from cast and hot pressed samples, which could be attributed to a strong bonding between powder particles and the small-grained microstructure of the bulk SPS quasicrystalline specimens.
On the Corrosion Mechanisms of Al-Cu-Fe Quasicrystals
Authors: Rüdiger A. , Köster U.
Published: April 2011

The mechanisms of dissolution and passivation of bulk polycrystalline icosahedral Al63Cu25Fe12 during electrolytic corrosion in 0.1 N NaOH and 1 N H2SO4 were studied in detail. In 1 N H2SO4 selective dissolution of Al and Fe occurs at the open circuit potential, which leads to a porous layer of recrystallized fcc Cu; after anodic polarization dissolution of the alloy is followed by redeposition of Cu and formation of Cu2O. In 0.1 N NaOH selective dissolution of Al was observed and a nanocrystalline layer consisting of Cu2O and FeOOH forms at the open circuit potential as well as during anodic polarization up to -150 mVSHE. At higher potentials Cu-ions dissolve into the electrolyte, which allows building up an Al-hydroxide layer. Presumably due to the loss of Al during polarization phase transforma-tions of the quasicrystals (e.g. continuous transformation or precipitation) were observed by transmission electron microscopy.

In order to clarify the influence of the quasicrystalline structure on the corrosion, open circuit potentials and current densities of the quasicrystalline and three ternary crystalline phases with different Al-content, but approximately constant ratio Cu:Fe of 2:1 were compared. Lower current densities, but a less noble open circuit potential for the quasicrystals than expected from the trend over the Al-content indicate a small influence of the quasicrystalline structure.
Phase Equilibria at Internal Interfaces of Icosahedral Al–Mn–Pd
Authors: Lück Reinhard , Gödecke Tilo , Beeli Conradin
Published: April 2011

Highly faceted microholes (voids) in icosahedral Al-Mn-Pd quasicrystals form during annealing at temperatures between 750 and 830°C via classical Ostwald ripening. The specimens were single-phase icosahedral with a composition of Al71Mn9Pd20. If such a specimen containing voids is cooled to room temperature with a constant cooling rate of typically 0.6 to 5 K/min, the facets of the voids are frequently found to be decorated by a different material. The decorations have typically a morphology similar to fractals. Specimens rapidly quenched after isothermal heat treatments around 830°C never showed similar decorations. By X-ray microanalysis in a scanning electron microscope it has been found that this decoration material is considerably poorer in Al-content that the icosahedral phase, namely: Al60Mn10Pd30. Accordingly, these decorations are precipitations at the surfaces of voids inside a single-phase icosahedral matrix. This implies that these decorations are formed at the void surfaces bya diffusion induced local equilibrium. The local equilibrium can be understood on the basis of the equilbrium results in a single icosahedral phase.
Solidification and Microstructure Control of Mg-rich Alloys in the Mg-Zn-Y Ternary Systemya
Authors: Park E.S. , Yi S. , Ok J.B. , Bae D.H. , Kim W.T. , Kim D.H.
Published: April 2011

Solidification microstructures of Mg rich Mg-Zn-Y alloys were studied by using optical microscopy, scanning electron microscopy and transmission electron microscopy. Pseudo eutectic reaction (Liquid→α-Mg + icosahedral phase) takes place during solidification. Alloys containing high Mg content solidifies by primary crystallization ofα-Mg followed by the eutectic reaction at interdendritic region. Mg68Zn28Y4 alloy solidifies by primary crystallization of I-phase followed by theeutectic reaction into a mixture of α-Mg and I-phase. Occasionally D-phase and Mg4Zn7 phases were observed to form with orientation relationships with previously formed I-phase and D-phase, respectively. Thestrength of the alloys increased with increasing the volume fraction of I-phase.
Approximants — the Key to the Structure of Quasicrystals
Authors: Steurer Walter
Published: April 2011

The relationship between approximant structures and quasicrystals is discussed on the example of quantitatively known decagonal quasicrystal and approximant structures in the thoroughly studied system Al-Co-Ni. The binary Al-Co as well as the ternary Al-Co-Ni approximants consist of the same basic pentagonal bipyramidal structure elements as the decagonal phase. The binary Al-Ni approximants, on the other hand, are vacancy ordered superstructures of the AlNi §-phase, which itself is related to the periodic average structure of decagonal Al-Co-Ni. Thus, the decagonal phase may be considered as a compromise between the competing driving forces to form pentagonal structural units on the one side and vacancy ordered superstructures of the AlNi §-phase on the other side. The broad stability range of decagonal Al-Co-Ni can be interpreted in terms of entropy gain by Co/Ni substitutional disorder and phasonic disorder of a part of the Al atoms.
Analysis of Gas Phase Clusters Made from Laser-Vaporized Icosahedral Al-Pd-Mn
Authors: Barrow J.A. , Rexer E.F. , Sordelet D.J. , Besser M.F. , Jenks C.J. , Riley S.J. , Thiel P.A.
Published: April 2011

Laser vaporization of an icosahedral Al-Pd-Mn sample with detection by time-of-flight mass spectrometry is used to probe metal clusters made from the alloy. After sample vaporization, clusters form by gas aggregation and may contain several to hundreds of atoms. Multi-photon ionization/fragmentation of these clusters yields mass spectra showing many cluster sizes with enhanced intensity. Clusters are identified at masses near those of pseudo- Mackay and Bergman clusters; however, these clusters do not appear special relative to neighboring clusters. Results of this study and its relationship to the proposed cluster structures in quasicrystalline materials are discussed.
High Pressure Studies of Quasicrystals
Authors: Sadoc Anne , Itié Jean-Paul , Polian Alain , Kelton Kenneth F.
Published: April 2011

High pressure X-ray diffraction studies of quasicrystals are presented and discussed for icosahedral Al-Cu-Ru and Ti-Zr-Ni alloys. The results in hydrogenated Ti-Zr-Ni show that Ti- Zr-Ni may be more compressible after hydrogenation.
Quasicrystalline Films and their Amorphous Precursors
Authors: Haberkern Roland , Barzola-Quiquia Jose , Madel Caroline , Häussler Peter
Published: April 2011

Binary and ternary aluminum based amorphous precursor films have been prepared by the sequential-flash evaporation technique in the case of Al-Mn, Al-Fe, Al-Cu-Fe and Al-Pd-Mn and by co-sputtering in the case of Al-Pd-Re. For well adjusted compositions and an appropriate heat treatment films can be transformed into polycrystalline single phased quasicrystals for all of the systems mentioned above. The transition can be monitored by in situ measurements of electronic transport properties. The structure factors as a function of composition and annealing state has been measured by electron diffraction. They show a unique behavior if the composition is chosen to that of the related quasicrystal. For high annealing states of the amorphous phase the structure factor becomes more intense at the peak positions of the following i-phase. This is interpreted as a unique radial near- and medium-range order of both the i-phase and its amorphous precursor. The final transformation into the quasicrystal is sharp for Al-Cu-Fe and is broad in the case of Al-Pd-Mn and Al-Pd-Re. The transport properties of the resulting single phased films are similar to that of the related bulk samples and allow systematic studies as a function of composition. Some of the i-Al-Pd-Re films are on the insulating side of the metal-insulator transition.
AC Conduction in Quasiperiodic Lattices
Authors: Wangs Chumin , Oviedo-Roa Raúl , Śnchez Vicenta , Pérez Luis A.
Published: April 2011

The electronic transport in Fibonacci lattices at zero temperature is studied by means of the Kubo-Greenwood formula within the tight-binding scheme, where a renormalization process capable to address the electrical conductivity in macroscopic quasiperiodic systems is used. The effects of the Fermi-energy location on the ac conductivity are analyzed in detail for a wide range of the system sizes. Special attention is paid to the transparent states, whose transmission coefficient is unity. The results show a rapid decay of their ac conductivity as the frequency increases in comparison with that of periodic systems, and the spectra scale with the inverse of the system size as occur in periodic ones, where analytical results are obtained. Furthermore, a new low-frequency minimum appears when the inhomogeneity of the Fibonacci lattice grows.
Crystallization Behavior of Zr65−×−yTixAlyCu22Ni13 Bulk Metallic Alloys by in-situ Neutron Diffraction
Authors: Soubeyroux Jean-Louis , Claret Nicolas
Published: April 2011

Several alloys of the Zr65×−yTixAlyCu22Ni13 system were found to be bulk metallic glasses. They have been investigated by DSC and in-situ neutron diffraction. DSC and neutron studies were performed at the same heating rates in order to correlate the evolution of the characteristic temperatures to the crystallization of phases. The first phase appearing at crystallization is dependant on the titanium content, being Zr2Ni for Ti ≈ 5% and a quasi- crystalline phase for Ti ≈ 2.5%. These phases are metastable and transform to Zr2Cu and other phases.
Structural and Electronic Properties of Zr-Ti-Cu-Ni-Be Alloys
Authors: Löffler Jörg F. , Tang X.-P. , Wu Yue , Johnsona William L.
Published: April 2011

We present crystallization studies on Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit1) and on other alloys, where the (Zr,Ti) and (Cu,Be) contents, along the line in composition space connecting Vit1 and Zr46.8Ti8.2Cu7.5Ni10Be27.5 (Vit4), were varied. Results from x-ray diffraction (XRD), small-angle neutron scattering (SANS) and differential scanning calorimetry (DSC) are combined to describe the crystallization behavior of these alloys at deep undercooling. SANS gives evidence for decomposition and the formation of nanometer sized crystals below a critical temperature Tc, which varies drastically as a function of composition. When Tc intersects with the glass transition temperature Tg, changes in the crystallization behavior are observed by DSC and XRD. At annealing temperatures near Tg, XRD resolves quasicrystalline phases for all alloy compositions from Vit1 to Vit4. From 9Be nuclear magnetic resonance (NMR) experiments performed on Vit1 upon annealing, we obtain information about the electronic structure and volume fraction of Be containing crystalline and quasicrystalline phases.
Local Atomic Structures and Plastic Deformation Modes in the Supercooled Liquid State of La55Al25Ni20
Authors: Ohkubo Tadakatsu , Hirotsu Yoshihiko , Inoue Akihisa
Published: April 2011

An in-situ electron diffraction structure analysis of La55Al25Ni20in the supercooled liquid temperature range (ΔTx) revealed a gradual structural change with a local phase separation due to strong atomic correlations especially of La-La and Al-Ni. On the basis of the structural data, structure models corresponding to the structures at temperatures between Tg and Tx were constructed using molecular dynamics (MD) simulation technique. These constructed structure models were then utilized for MD simulations to understand plastic deformation in the supercooled liquid state under a constant strain-rate. A larger elongation was obtained for the structure in the middle of the ΔTx range, but for the structure near Tx the elongation was relatively smaller which was supposed to be due to the local phase separation. These simulation results correspond well to the experimental elongation behaviors in the superplastic deformation that the elongation is enhanced in the middle of ΔTx but suppressed as the temperature is increased towards Tx.
Hydrogen-doped Bulk Metallic Glasses as High Damping Material
Authors: Yagi Teruaki , Oguro Rikiya , Tamura Ryuji , Takeuchi Shin
Published: April 2011

Bulk metallic glasses have extremely high strength and high ductility and are quite useful as the structural material. Many of bulk metallic glasses are based on Zr, Ti and Pd; these elements have a high affinity with hydrogen and hence the bulk metallic glasses can contain a large amount of hydrogen atoms. It is known that hydrogenized amorphous metals, as well as hydrogenized metallic crystals, exhibit Snoek-type relaxation, and hence metallic glasses containing a high density of hydrogen can have a high internal friction.

In the present experiments, internal friction measurements have been performed for bulk metallic glasses doped with a variety of hydrogen concentrations. It is shown that the peak value of the internal friction reaches the order of 10-2 in Zr-based bulk metallic glasses which have the fracture strength as high as 1.5 GPa. Thus, hydrogen-doped bulk metallic glasses can be used as the high-strength, high-damping material.
The Shear Modulus of Vit 1 in the Supercooled Liquid and Glassy State
Authors: Bains Alijeet S. , Gordon Craig A. , Granato Andrew V. , Lebedev Alexander B. , LaMadrid Marissa A. , Johnson William L.
Published: April 2011

Using a non-contact electromagnetic-acoustic transformation (EMAT) technique, we have measured the temperature dependence of the infinite frequency shear modulus of Vit 1 at constant heating rate in the glassy and supercooled liquid states. Values of the shear softening fragility parameter -dln(G/Gg/d(T/Tg) are compared with those obtainable from specific heat and viscosity measurements, using the interstitialcy theory of condensed matter states. There is overall agreement found between these independently measured values.
Synthesis and Viscoelasticity of Zr-based Bulk Glassy Alloy Containing ZrC Particles
Authors: Kato Hidemi , Hirano Tomoya , Inoue Akihisa , Kawamura Yoshihito , Chen H. S.
Published: April 2011

Cylindrical composite Zr55Al10Ni5Cu30 bulk glassy alloys containing up to 17.5 vol.% ZrC particles were prepared with using an in-situ reaction forming ZrC between graphite particles with diameters of ∼10 m and Zr metal in molten alloy during arc-melting followed by copper mold casting. Because the in-situ reaction suppresses an agglomeration and segregation among dispersoids in the glassy matrix, mechanical properties of the composite materials at room temperature, especially toughness and plastic elongation, were significantly improved from the Zr55Al10Ni5Cu30 single glassy phase. In order to demonstrate the influence of the dispersed ZrC particles on workability of the supercooled liquid of the alloy, viscous flow behaviors under constant strain-rates were investigated. Although the Newtonian viscosity becomes larger, the composite material has the same viscoelastic properties, i.e., “shear thinning”, and excellent workability as the single glassy phase in spite of the dispersed particles.
Intermetallic Phases in Ti-Ag-Zr-Ni Alloys
Authors: Nicula Radu C. , Jianu Adrian D. , Bunescu Mihaela C. , Ponkratz Ulrich E. , Stir Manuela E. , Schick Christoph
Published: April 2011

Titanium and zirconium based metallic glasses and crystalline intermetallic compounds with improved mechanical properties are presently a field of intense application-oriented research. The formation and stability of nanocrystalline phases in Ag-substituted Ti-Zr-Ni alloys was followed usingin-situhigh-temperature synchrotron radiation diffraction experiments. The substitution of Ti with Ag enhances the formation of amorphous or nanostructured phases during rapid solidification. High-resolution powder diffraction and electron microscopy investigations indicate that the as-quenched alloys are either amorphous or in a mixed nanostructured state. Upon heating, the alloy structure transforms to a fine mixture of icosahedral and crystalline phases depending on the alloy composition and thermal processing parameters. Differential scanning calorimetry studies were performed in order to identify the nature and sequence of the observed structural phase transitions. The experimental results are discussed with respect to the non- equilibrium synthesis and processing of bulk amorphous and bulk nanocrystalline materials in Ti/Zr-based alloys.
Nanometer scale intermixing mechanism in Ag-based nanocomposites
Authors: Jianu Adrian D. , Kuncser Victor E. , Nicula Radu C. , Palade Petre , Stir Manuela E. , Bunescu Mihaela C. , Burkel Eberhard
Published: April 2011

A new Ag-Sm-Fe nanocomposite material was obtained by high-energy ball-milling of elemental μ-Fe powder and melt-spun Ag-Sm flakes. Magnetic Fe nanocrystals were thus embedded in a paramagnetic Ag-based matrix. Synchrotron radiation powder diffraction and temperature dependent Mössbauer spectroscopy were used to study the nanometer scale Fe/Ag(Sm) intermixing mechanism. The evolution of the average grain size and microstrains in the Ag-based matrix vs. the milling time were obtained from the analysis of the X-ray diffraction linewidths. For reference Fe powder samples, a bimodal grain size distribution was evidenced by Mössbauer spectroscopy. The magnetic behaviour of the Fe nanocrystals was analysed with respect to the superparamagnetic relaxation effect and collective excitations. The nanometer scale intermixing of Fe nanocrystals in the Ag-based matrix is completed when the Fe and matrix grains have comparable dimensions.
Diffusion in Metallic Glasses and Supercooled Melts
Authors: Faupel F. , Rätzke K. , Ehmler H. , Klugkist P. , Zöllmer V. , Nagel C. , Rehmet A. , Heesemann A.
Published: April 2011

Diffusion in metallic glasses and in the supercooled liquid state is of considerable interest not only from the technological point of view but also in terms of fundamental science, particularly in connection with the glass transition. Within the framework of the mode coupling theory the glass transition is a kinetic phenomenon characterized by the arrest of viscous flow at a critical temperatureTcwell above the calorimetric glass transition temperatureTg. BelowTcthe theory predicts cooperative hopping processes. We present results from isotope effect measurements which indeed confirm the highly collective nature of diffusion in metallic glasses and suggest cooperative hopping processes to be closely related to the universal low-frequency excitations as observed in recent molecular dynamic simulations. In accord with the mode coupling scenario these cooperative hopping processes are also observed in the supercooled liquid state of the new bulk metallic glasses well aboveTg. The reported kink in the Arrhenius plot for diffusion of various elements is shown to be related to structural changes aboveTg, e.g., an increase in free volume as probed by positron annihilation, but not to a change in the diffusion mechanism. Measurements of the activation volume of diffusion indicate that, depending on the structure of the glass, cooperative hopping may take place without assistance of thermally generated defects or via delocalized thermal defects. Moreover, we provide evidence of the existence of an opposite Kirkendall effect in interdiffusion between certain amorphous alloys that combine slow diffusion via thermal defects and fast direct diffusion.
Glass-Forming Ability and Crystallization of High Purity Pd-Cu-Ni-P Alloy
Authors: Nishiyama Nobuyuki , Matsushita Mitsuhide , Inoue Akihisa
Published: April 2011

Glass-forming ability, thermal stability and nucleation behavior of a Pd40Cu30Ni10P20 alloy prepared using a high purity polycrystalline phosphorus are investigated. The critical cooling rate for glass formation for the high purity alloy is the same as that for the previous result, but the improvement of undercooling reaches about 80 K as compared with the fluxed ordinary alloy. In comparison with the non-fluxed alloy, the solidified structure of the present highly purified alloy is significantly different. The non-fluxed sample shows the characteristic “island-like” structure consisted of acicular fcc-Pd2Ni2P solid solution and Cu3Pd intermetallic compound. These acicular phases appear to be caused by the growth of quenched-in nuclei. In the isothermal experiment, nucleus density exhibits time dependence even at 683 K near the nose temperature. It is assumed that the crystallization behavior for the highly purified alloy is closer to homogeneous nucleation from quenched-in nuclei dominant behavior. In order to investigate the nucleation behavior, in-situ TEM observation was carried out. Spherical Pd15P2 particle with a diameter about 15 nm is observed, and this spherical region repeats generation and annihilation during isothermal annealing. The reason for the high glass-forming ability is discussed on the basis of the obtained results.
Crystallization in Zr-Cu-Ni-Al Metallic Glasses
Authors: Köster U. , Zander D. , Janlewing R.
Published: April 2011

Zr-Cu-Ni-Al exhibits one of the best glass forming alloys known. In a narrow concentration range, however, annealing leads to the formation of a metastable icosahedral phase. The aim of this paper is to investigate in detail the influence of alloying with Hf or Si, Sn, Y, Cr and Mo on the quasicrystal formation in Zr69.5Cu12Ni11Al7.5 glasses.

Whereas Y addition leads to loss in thermal stability and pronounced formation of quasicrystals, the addition of Si, Sn or Mo does not deteriorate thermal stability. Sn and Si were observed even to improve thermal stability significantly, but also to change phase selection during the early stages of crystallization. Si addition results in the formation of the tetragonal Zr2Ni phase and Mo as well as Cr to the formation of the fcc, so-called big-cube phase.

Nucleation rates for the quasicrystal formation were measured by means of crystallization statistics. By modeling the obtained nucleation rates in the framework of diffusion controlled classical nucleation the interface energies (between quasicrystals and amorphous matrix) as well as metastable melting temperatures for the quasicrystalline phase could be derived. In all cases nucleation could be described best with a homogeneous model with transient rates.

In order to understand the formation of the icosahedral structure also data on the influence of oxygen and hydrogen contamination as well as recent results of other authors on the influence of noble metals such as Ag, Pd, Au and Pt will be discussed in detail. There is some evidence that the quasicrystals are a hybrid of two structural elements, i.e. the tetragonal Zr2Ni (Al2Cu) and the tetragonal Zr2Cu (MoSi2) structure. It is further assumed that an icosahedral short-range order of the melt is stabilized by the addition of elements like Hf, probably Y or by contamination with very small amounts of oxygen, thus leading to a reduced interfacial energy between the quasicrystals and the amorphous matrix and therefore to a lower nucleation barrier.
Core Losses and Soft Magnetic Properties of Nanocrystalline Fe-Zr-Nb-B Alloys with Zero-Magnetostriction
Authors: Makino Akihiro , Inoue Akihisa , Masumoto Tuyoshi
Published: April 2011

The structure, the soft magnetic properties and the core losses for Fe-Zr-Nb-B(-Cu) nanocrystalline alloys were investigated. It has been already reported that the typical ternary nanocrystalline Fe90Zr7B3 and Fe84Nb7B9 alloys exhibit good soft magnetic properties and a small negative and a small positive magnetostriction (λs), respectively. The soft magnetic properties of the nanocrystalline Fe84–90(Zr, Nb)7B3–9 quaternary alloys with mixed compositions of Fe90Zr7B3 and Fe84Nb7B9 cannot be improved whereas very small magnetostrictions are obtained. The quaternary alloys with 7 at% (Zr + Nb) have structures of an amorphous with α-Fe phases on the free and roll-contacted surfaces at an as-quenched state. The crystalline surface layers deteriorate the soft magnetic properties at a crystallized state. The high saturation magnetic induction of 1.64 T, the high permeability of 60,000 at 1 kHz and the very low core loss of 0.9 W/kg at 1.4 T and 50 Hz are obtained for the Fe85.5(Zr1/3Nb2/3)6B8.5 alloy containing 6 at% of (Zr + Nb) content with nearly zero-λs produced by crystallizing the single amorphous phase without the crystalline surface layers. The thermal stability of the core loss of the quaternary alloy is significantly higher than that of the Fe78Si9B13 amorphous alloy. The crystalline surface layers of the Fe84–90(Zr, Nb)7B3–9 quaternary alloys disappear by 1 at% Cu addition, which results in significant improvement of the soft magnetic properties at a crystallized state.
Preparation and Characterization of the Soft Magnetic FeCo-based Amorphous Alloy with Enhanced Magnetic Properties and Thermal Stability
Authors: Hollmark Martin , Tkatch Victor , Khartsev Sergey , Grishin Alex
Published: April 2011

A glassy structure was formed in the Fe40Co40P14B6 alloy by melt-spinning technique. The as-quenched 2-8 mm wide and 15-30 [.proportional]m thick ribbons exhibit good soft magnetic properties: the saturation magnetization of 1.45 T, the coercive force of 4 A/m and maximum differential permeability at 60 Hz of about 90000. The FeCo-based glass crystallizes via eutectic reaction into a mixture of an austenite and a b.c. tetragonal Fe3P-like phase similar to that of the well-known Fe40Ni40P14B6metallic glass, but at temperatures about 60 K higher than the latter. The evaluation of the thermodynamic and kinetic parameters of crystallization process brought us to the conclusion that the improved thermal stability of the Fe40Co40P14B6 glass is caused by the enhanced interfacial nucleus-glass energy.
Internal Stresses in Bulk Metallic Glass Matrix Composites
Authors: Üstündag Ersan , Dragoi Danut , Clausen Bjorn , Brown Donald , Bourke Mark A. M. , Balch Dorian K. , Dunand David C.
Published: April 2011

Composites consisting of a bulk metallic glass (BMG) matrix and metallic fibers or particulates have been shown to exhibit superior mechanical properties as compared to monolithic BMGs. To understand the role of reinforcements in this improvement, it is necessary to investigate the state of internal stresses in these composites. These stresses arise from the thermal expansion mismatch between the reinforcement and the matrix, as well as the elastic and plastic incompatibilities between the two phases. Neutron diffraction and synchrotron X-ray diffraction were used to measure these mismatch-induced stresses in BMG-matrix composites with various reinforcements: continuous W fibers, W or Ta particles, and dendritic, in-situ formed precipitates. The results are compared to numerical and analytical predictions of internal stresses.
Failure of a Thermal Barrier System Due to a Cyclic Displacement Instability in the Thermally grown Oxide
Authors: Mumm Daniel R. , Evans Anthony G.
Published: April 2011

The mechanism controlling the cyclic failure of a commercial thermal barrier system has been investigated. The system comprises an electron-beam physical vapor deposited (EB-PVD) yttria-stabilized zirconia thermal barrier coating (TBC), deposited on a (Ni Pt) Al bond coating. The thermally grown oxide (TGO) layer that forms between the TBC and bond coat at high temperature is unstable with respect to out of plane displacement, provided initial perturbations are present. With cyclic thermal exposure, the TGO displaces into the bond coat at periodic interfacial sites. The out-of-plane displacements induce strains above the TGO, normal to the interface, that cause cracking. The cracks nucleate either within the TBC layer or at the TBC/TGO interface, and extend laterally until they coalesce with cracks from other sites and coating failure occurs by large scale buckling. The TGO displacements are accommodated by visco-plastic deformation of the underlying bond coat, and are driven by a lateral component of the growth strain in the TGO. The susceptibility of the TGO to out-of-plane displacement depends critically upon the initial morphology of the metal/oxide interface. The observed material responses are compared with predictions of a ‘ratcheting’ model.
Microstructural Characterization of Yttria-Doped Zirconia Coatings with Electron Microprobe Wavelength Dispersive Compositional Mapping
Authors: Marinenko Ryna B. , Verkouteren Jennifer R. , Bright David S.
Published: April 2011

The use of digital electron microprobe x-ray compositional mapping with wavelength dispersive spectrometers to understand the microstructure of yttria stabilized zirconia thermal barrier coatings is described. Data from quantification of element x-ray maps can be utilized to infer what phase or phases are present. Analysis of a plasma-sprayed coating prepared from a fused and crushed feedstock is compared to an annealed specimen of the same material.