bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of TMG Addition on the Epitaxial Growth of 3C-SiC on Si(100) and Si(111) Using Hexamethyldisilane
Authors: Lee Ki Hoon , Teker Kasif , Zhang Ming , Chung Juyong , Pirouz Pirouz
Published: April 2011

Epitaxial and crack-free 3C-SiC film was successfully grown on Si(100)/(111) by a one step process without separate nucleation or carbonization steps at a low temperature of 1200°C by MOVPE. The growth was achieved by using hexamethyldisilane (HMDS) with the addition of a small amount of trimethylgallium (TMG) (0.5 sccm) with dilute hydrogen (12% H2 + Ar) as a carrier gas. Without the addition of TMG during growth, epitaxial growth of SiC on Si was only possible at temperatures above 1300°C following a nucleation step at 1200∼1250°C. After growth, all the films were analyzed by using cross-sectional transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and Nomarski differential interference microscopy (NDIM). It was observed by XPS that the SiC film contained only a small amount of Ga, which means the Ga component of TMG does not much accumulate in SiC, in spite of a relatively high ratio of TMG/HMDS. To investigate the effect of TMG flow rate, various amounts of TMG (0.5 - 5sccm) were added during SiC deposition. Below 0.5 sccm of TMG flow rate, the SiC film showed epitaxial growth, observed by TEM and XRD. However, with an increasing flow rate of TMG above 1 sccm, the XRD peak of 3C-SiC becomes smaller and slightly shifted. At 5 sccm, the film was polycrystalline possibly implying that the SiC lattice is being strained gradually with TMG addition. It is also observed by AFM that, at the initial stages of growth, nucleation is much facilitated with the addition of TMG. The deposition rate with the addition of TMG at 1200°C is about 650 nm/hr, which is much faster than the rate without TMG addition (480 nm/hr). Thus, it is believed that TMG addition has a positive effect on the nucleation and HMDS decomposition during deposition. Also, with the addition of a small amount of Ar+10%HCl during growth, one-step deposition was possible helped by stress release due to void formation at the SiC/Si interface.
Ab Initio Study of SiC/Metal Polar Interfaces: Relation Between Interface Structure and Schottky-Barrier Height
Authors: Tanaka Shingo , Kohyama Masanori
Published: April 2011

Ab initio calculations of the polar interfaces between thin films of titanium and cubic silicon-carbide (SiC) have been performed by using the first-principles molecular dynamics method. Stable configurations, adhesive energies and Schottky-barrier height (SBH) for the Si-terminated and the C-terminated interfaces are obtained. The C-terminated interface has covalent C-Ti bonds, while the Si-terminated interface has shown metallic nature. Adhesive energy/SBH of the C-terminated interface is larger/smaller than that of the Si-terminated one, respectively. In order to examine a conventional SBH model, work functions of SiC slab with Si and with C surface and Ti slab have been calculated and SBHs have been estimated from the difference of work functions. In estimated SBHs between the interfaces, the relationship depend on the crystal orientation as (111) and (001).
Low Temperature (300°C) Formation of Thermodynamically Stable NiSi2 Contacts to SiC
Authors: Deeb C. , Kahn H. , Milhet X. , Zorman C. , Mehregany M. , Heuer A. H.
Published: April 2011

Thermodynamically stable, low specific contact resistance electrical contacts to SiC are essential to the application of such devices in high power or high temperature applications. However, thermal budgets impose processing constraints due to interfacial reactions and dopant profiles during device fabrication. Nickel disilicide (NiSi2) is useful for metallization of SiC, as stable contacts can be formed on SiC by reacting a fugitive layer of amorphous silicon (a-Si) with sputtered nickel at 300°C. NiSi2 forms directly upon reacting nickel with a-Si; this is not the case when nickel is reacted with crystalline silicon, where more nickel-rich silicides form first. Specific contact resistances as low as 5.6×10−5 Ωcm2 have been measured using NiSi2 circular TLM contacts so formed on APCVD 3C-SiC thin films deposited on a silicon wafer.
Structural, optical and chemical characterization of 300°C, MeV ion implanted and 1700°C annealed 6H-SiC
Authors: Nipoti R. , Carnera A. , Mattei G.
Published: April 2011

This work deals with the structural, optical and chemical characterization of ion implanted and annealed crystalline SiC. N+ or Al+ ions with various MeV energy and fluence values were implanted at 300°C in 6H-SiC wafers. These processes produced about 1 μm thick doping profiles characterized by a sequence of peaks and valleys across an average plateau value. The annealing process was done in a radio frequency furnace at 1700°C for 30 min in high purity Ar atmosphere. The hot implanted layers show the presence of a weak damage amount, whose profile was estimated by confocal micro-Raman measurements. The annealed samples, in spite of a good crystal recovery, showed the formation of secondary defects as inferred by the Rutherford Back Scattering-Channeling spectrometry. The relative magnitude and position of peaks and valleys in the doping profiles were preserved by the high temperature annealing but the annealed doping profile was displaced towards the surface with respect to the as-implanted profile as shown by the Secondary Ion Mass Spectrometry measurements. No material loss but material densification due to the reordering of the crystalline structure can explain the trend of the chemical profiles.
Interface Effects on the Raman Spectra of Si/3C-SiC Superlattices
Authors: Bezerra E. F. , Filho A. G. Souza , Filho J. Mendes , Lemos V. , Freire V. N. , Ikoma Y. , Watanabe F. , Motooka T.
Published: April 2011

Theoretical calculations are performed on the role of smooth interfaces in the Raman spectra of Si/3C-SiC superlattices. The dispersion relations were obtained using a linear chain model with the alloyed interface δ-(3C-SiC)0.5(Si)0.5 described in the virtual crystal approximation. A modified bond-polarizability model was used to calculate the Raman spectra. The main results are the enhancement of the Raman spectra and the appearance of new peaks in between those related to the Si quasi confined and the 3C-SiC-confined modes with increasing number δ?of interfacial monolayers (3C-SiC)0.5(Si)0.5. Some of the smooth interface related Raman peaks have intensity comparable with those of an abrupt Si/3C-SiC superlattice.
Heat and mass transfer modeling for a better knowledge of the large-area growth of homoepitaxial SiC by CVD
Authors: Pons Michel , Mezière Jerome , Dedulle Jean Marc , Kuan Stephane Wan Tang , Blanquet Elisabeth , Bernard Claude , Ferret Pierre , Cioccio Lea Di , Billon Thierry , Madar Roland
Published: April 2011

The growth of thick epitaxial 4H-SiC layers with low defect density is an essential step for the fabrication of SiC based devices. Cold- and hot wall reactors using silane and propane diluted in hydrogen were used in this study. The typical growth temperature range is 1700–1900 K and total pressure range 10–100 kPa. The resulting epilayers exhibit low background doping, low defect density and good thickness uniformity. The main problem is that it is difficult with this first generation of reactors to ensure a constant temperature over large wafer. A 3D simulation approach of heat and mass transfer was used with three objectives. The first one is to have a visualization of the flow, temperature and gaseous species fields in the standard reactor. The second one is to propose solutions for the optimal control of the temperature field and the subsequent uniformity of the epilayers over large dimensions. The third one is to improve the kinetic databases in this temperature range which has been very little investigated.
Investigation of Factors Limiting the High Temperature Stability of W/WC/TaC/SiC Ohmic Contacts to n-type 6H-SiC
Authors: Jang T. , Odekirk B. , Porter L. M.
Published: April 2011

The electrical properties and thermal stability of TaC ohmic contacts with W/WC overlayers were investigated on n-type 6H-SiC (0001) substrates. The specific contact resistance (SCR) was measured after annealing at 600 °C and 1000 °C for 50 ∼ 1000 h. The SCRs (3.0 ∼ 3.6 ×10−5 Ωcm2) of contacts annealed at 600 °C for 1000 h remained constant within the experimental error. Significant increases in the SCR were not observed until the samples were annealed at 1000 °C for several hundred hours.

No reaction of the film with the SiC substrate was observed after annealing at 600 °C for 1000 h; however, atomic-scale reactions appeared to be concentrated adjacent to grain boundaries in the reacted film. From Auger depth profiles, it was found that W and WC reacted to form W2C on TaC after annealing.

According to SIMS analysis, after annealing at 1000 °C for 600 h, small but measurable changes in the electrical characteristics were associated with O incorporation at the interface between TaC and SiC. Investigation of the W/WC/TaC/SiC interface by TEM indicated that a reaction between the metal layers had occurred, but there was no observed reaction with the SiC substrate. After annealing for 1000 h, substantial changes in the contacts were observed. The findings indicate that both oxidation and metallurgical reactions have important implications on the current operating limits for SiC high temperature devices.
Synthesis of Self-Assembled Metal-Oxide Nanostructures in a Diblock Copolymer Matrix and Integration onto Semiconductor Surfaces
Authors: Mulligan Robert F. , Iliadis Agis A. , Lee U. , Kofinas Peter
Published: April 2011

The synthesis of self-assembled ZnO nanostructures at room temperature using a microphase separated diblock copolymer as a template is reported. Poly(norbornene) / poly(norbornene-dicarboxylic acid) diblock copolymers were synthesized using Ring Opening Metathesis Polymerization (ROMP). The polymers were dissolved and the solutions were doped with ZnCl2. Films were formed from this solution, and subsequently reacted with NH4OH. This converted the ZnCl2 into ZnO contained within the microphase-separated nanodomains of the block copolymer. Fourier Transform Infrared Spectroscopy verified the association of the metal to the second block of the polymer, and X-ray Photoelectron Spectroscopy verified the conversion of the salt to ZnO nanoclusters. The development of such ZnO - block copolymer nanocomposites is targeting the functionalization of nanostructures into device technologies.
Determination of Spatially Hybridized Charge Distribution and its Effect on Electron Transport in the Al-Cu-Ru-Si 1/1-Approximant ---Theoretical Basis for the Hume-Rothery rule---
Authors: Mizutani U. , Takeuchi T. , Banno E. , Fourneé V. , Takata M. , Sato H.
Published: April 2011

The origin of the pseudogap across the Fermi level was investigated by analyzing the electronic structure calculated in the framework of the LMTO-ASA method for the RT-type Al-Mg-Zn and MI-type Al-Cu-Ru-Si 1/1-approximants. The pseudogap in the former is proved to originate from the interaction of electronic states with the Brillouin zone planes associated with reciprocal lattice vectors matching with the Fermi sphere in the extended zone scheme. In the latter, the Fermi surface-Brillouin zone interaction coupled with the hybridization effect between the Al-3p and transition metal d-states produces a deep pseudogap at the Fermi level. The real-space charge distribution for electrons at the Fermi level is calculated for the Al-Cu-Ru-Si 1/1-approximant. The charge distribution thus obtained could explain not only the possession of a large resistivity of this approximant but also evidenced that the icosahedral clusters play a key role in reducing the electronic energy in favor of quasicrystalline and approximant phases.
Electrical Resistivity of an Al-Re-Si Cubic Approximant Phase and Role of Local Environment in Electronic Transport
Authors: Tamura Ryuji , Asao Takayuki , Tamura Mutsuhiro , Takeuchi Shin
Published: April 2011

In order to gain an insight into the role of the local atomic environment in the electronic transport of the icosahedral quasicrystal, the electrical resistivity of α-AlReSi, which is the (1/1,1/1,1/1) approximant of the icosahedral phase, has been investigated. Very high resistivity and its pronounced negative temperature dependence have been observed, indicating that the electronic states of the 1/1 cubic approximant are quite similar to those of icosahedral phases. In order to further elucidate which structural entity is responsible for such anomalous transport, a comparison of the electrical resistivity between (1/1,1/1,1/1) and (1/0,1/0,1/0) approximants has been made. The typical transport behavior of icosahedral phases which is also seen in 1/1 and higher-order approximants was not observed in any of the studied 1/0 cubic approximants. The result can be regarded as an implication that the intercluster distance between the TM clusters plays a significant role in the confinement of electronic states.
Crystal Structure of New Approximant Phase in Al-Fe-V-Si System
Authors: Ezersky Vladimir I. , Rochman Avi D. , Talianker Michael M.
Published: April 2011

A model proposed for the atomic structure of the new hexagonal Al-Fe(V)-Si phase which was found in rapidly solidified Al-Fe(V)-Si alloys is discussed in terms of relation this phase to approximants. The structure of this new phase was described as hexagonal packing of double Mackay icosahedral units similar to those of which the cubic α-(AlFeSi) approximant phase is built up. On the basis of this model the experimental high resolution electron microscopic (HREM) images of the hexagonal phase were successfully interpreted. Like the á phase this structure is characterized by the local icosahedral ordering and can be regarded as an approximant.
Characterization of Ga-Mn Decagonal Quasicrystals in GaAs
Authors: Sun K. , Browning N. D.
Published: April 2011

Ga-Mn decagonal quasicrystals (DQC), as well as a Ga-Mn approximant and a normal crystal in GaAs are investigated by electron energy-loss spectroscopy (EELS) and energy dispersive X- ray spectroscopy (EDS) combined with Z-contrast imaging. Plasmon peak positions (Ep), full- width-half-maxima (FWHM) and Mn L3/L2ratios of these three phases are derived from their low-loss spectra and core-loss spectra respectively. Mn, Ga and As distributions in ion implanted GaAs layers are characterized by EDS at line-scan mode. These results show that the Ga-Mn DQC has higher Ep and FWHMs than those of its normal crystal counterpart, as well as all other reported QCs. The much larger L3/L2 intensity ratio of the Ga-Mn DQC over that of the Al-Mn icosahedral quasicrystals (IQC) may suggest Mn atoms in the Ga-Mn DQC have much larger local magnetic moments.
The Effect of Swift Heavy Ion Irradiation on Quasicrystals
Authors: Chatterjee Ratnamala , Kanjilal Aloke , Dunlop A.
Published: April 2011

The effect of pure electronic excitations (by ∼ 835 MeV Kr irradiation) on a stable icosahedral quasicrystal is compared with the effect of electronic excitations combined with the nuclear collisions (by 100 MeV Ni irradiation) occurring in a similar quasicrystal. The critical stopping power was kept at 1300 eV/Å for both the experiments. Under the pure electronic excitations, the R(φ)/R(0) [ratio of resistance at fluence φ and the resistance on the same piece with zero fluence) of quasicrystal goes through oscillatory changes, until at ∼ 1 × 1012 ions/cm2where this ratio drops considerably. The ex-situ XRD on the irradiated sample does show the evidence of degradation of the structure. In contrast, the Ni- irradiated sample shows an increase in resistivity after a critical fluence of 2.5 × 1013 ions/cm2, which remains constant for higher fluences. Corresponding defect annihilation effects are observed in the XRD of Ni- irradiated samples.
Dislocation Dynamics in Icosahedral Al-Pd-Mn Single Quasicrystals
Authors: Messerschmidt Ulrich , Bartsch Martin , Geyer Bert , Ledig Lars , Feuerbacher Michael , Wollgarten Markus , Urban Knut
Published: April 2011

The paper reviews results from in situ straining experiments on Al-Pd-Mn single quasicrystals in a high-voltage electron microscope. Slip planes were determined from the orientation and width of slip traces. Dislocations are generated by a specific cross slip mechanism. On some slip traces, dislocations move at two distinctly different velocities. A stress exponent was determined on a single dislocation by observing its displacement under decreasing load. The in situexperiments reveal the behaviour of individual dislocations in a temperature range where the deformation of bulk specimens is strongly affected by recovery.
First Results on the Electronic Distributions of Zn-Mg-Y Alloys as Investigated by Soft X-ray Spectroscopies
Authors: Esther Belin-Ferré , Vincent Fournée , Anne Sadoc , Marie Françoise Fontaine , Zoltan Dankhazi , Herbert Müller , Pang Tsai An
Published: April 2011

We report our first results on the electronic structure of icosahedral Zn55Mg35Y10as compared to hexagonal Zn65Mg28Y7and to the conventional Zn2Mg alloy. The occupied Mg 3s,d, and Mg 3p on the one hand and empty Mg p, Zn d-s and Y d-s states on the other hand have been investigated using soft X-ray emission and photoabsorption spectroscopy, respectively. The various sub-bands presented for Mg are adjusted in the binding energy scale whereas for the other elements they are plotted in the X-ray transition energy scale. We observe that the metallic character of the samples progressively decreases when going from the pure metals to the icosahedral compound.
Formation of Icosahedral Quasicrystals in Mg-Al-X (X=Transition Metal) Ternary Systems
Authors: Koshikawa Naokiyo , Yoda Shinichi , Edagawa Keiichi , Takeuchi Shin
Published: April 2011

Formation of icosahedral quasicrystalline phase (i-phase) was investigated for Mg-Al-X (X=Co, Ni, Cu, Rh, Ag, Pt and Au) ternary systems. A Mackay-icosahedron (MI) type i-phase was found to form in the Mg-Al-Rh system at the composition Mg8Al68Rh24. The i-phase has the F-type ordered structure and the quasilattice constant is 0.447nm. Differential thermal analysis (DTA) studies showed that the i-phase has a high stability. On the other hand, the formation of a Frank-Kasper (FK) type i-phase was found in the Mg-Al-Pt system at the composition Mg45Al42Pt13. The conditions necessary for the formation of the i-phases are discussed in terms of the atomic radius ratio and the electronegativity as well as the electron-per-atom ratio.
Short Range Chemical Ordering in Bulk Metallic Glasses
Authors: Sterne P. A. , Asoka-Kumar P. , Hartley J. H. , Howell R. H. , Nieh T.G. , Flores K. M. , Suh D. , Dauskardt R. H.
Published: April 2011

We provide direct experimental evidence for a non-random distribution of atomic constituents in Zr-based multi-component bulk metallic glasses using positron annihilation spectroscopy. The Ti content around the open-volume regions is significantly enhanced at the expense of Cu and Ni, indicating that Cu and Ni occupy most of the volume bounded by their neighboring atoms while Ti and Zr are less closely packed and more likely to be associated with open-volume regions. Temperature-dependent measurements indicate the presence of at least two different characteristic sizes for the open volume regions. Measurements on hydrogen- charged samples show that the larger open-volume regions can be filled by hydrogen up to a critical density. Beyond this critical density, local atomic-scale open-volume damage is created in the sample to accommodate additional hydrogen. The onset of this local damage in positron annihilation data coincides with the onset of volume expansion in X-ray diffraction data.
In-Situ Observations of Shear Band Development during Deformation of a Bulk Metallic Glass
Authors: El-Deiry Paul A. , Vinci Richard P. , Barbosa Nicholas , Hufnagel T. C.
Published: April 2011

We have studied the development of shear band structure in a Zr57Ti5Cu20Ni8Al10 bulk metallic glass during deformation. In order to investigate the relationship between shear band development and serrated flow, we performed uniaxial compression tests in an environmental scanning electron microscope (ESEM). During the deformation, load-time data and surface images were simultaneously recorded. In the stress-time data, stress drops or “serrated flow” appear to correlate to new shear band formation. The majority of the shear bands we observed were at an angle of 45° with respect to the compression axis. In addition, we measured shear offset as a function of position along the shear band. We observed two different offset behaviors: Consistent offset along the length of a shear band, and offset that is localized into part of the band. The localized offset behavior could be evidence for dislocation-like displacement increments, or could be the result of a transition in the failure mode.
On the High Temperature Creep and Relaxation Behaviour of Zr-based Bulk Metallic Glasses
Authors: Daniel B. S. S. , Heilmaier M. , Reger-Leonhard A. , Eckert J. , Schultz L.
Published: April 2011

Creep tests under constant load as well as constant true strain rate were carried out at near the glass transition temperatures (Tg) to study the time dependent flow behaviour of a Zr-based bulk metallic glass (BMG). The strain rate - stress relation over a wide strain rate-range (10-7 to 10-2 s-1) was established for different temperatures. The high temperature deformation behaviour is explained on the basis of stress induced creation of free volume versus diffusion controlled annihilation processes. It was found that the creep kinetics near Tg is controlled by the mobility of atoms with an activation energy value Q =410kJ/mol.
Effect of Preparation on Glass Formation and Magnetic Properties of Nd-Fe-Co-Al-B Alloys
Authors: Kumar G. , Eckert J. , Xing L.Q. , Güth A. , Roth S. , Löser W. , Ram S.
Published: April 2011

The effect of alternative preparation methods (copper mold casting, melt spinning, and mechanical attrition) on amorphization and properties of Nd57Fe20Co5Al10B8 and Nd40Fe40Co5Al8B7 alloys has been investigated. For all differently prepared samples an amorphous phase is formed upon solidification or solid sate reaction. However, the samples prepared by different processing routes exhibit different transformation behavior in thermal analysis. The cast Nd57Fe20Co5Al10B8 rod exhibits crystallization at 790 K followed by melting at 810 K. Neither appreciable endothermic reaction due to a glass transition nor a supercooled liquid region have been observed. Mechanically alloyed powders and ball-milled prealloys reveal two exothermic DSC peaks in the range of 650-850 K. The J-H hysteresis loops of samples synthesized by different routes show that the unique atomic order responsible for hard magnetic properties can only be accessed at moderate cooling rate of the melt as realized in copper mold casting. Rapidly quenched ribbons, mechanically alloyed powders and ball-milled ingots do not show hard magnetic properties at room temperature. These results indicate that amorphous samples with different local atomic order can be prepared by different processing routes.
Preparation and Mechanical Properties of Hafnium-based Bulk Metallic Glasses
Authors: Gu Xiaofeng , Xing Li-qian , Hufnagel T. C.
Published: April 2011

We have prepared bulk metallic glasses of composition (HfxZr1-x)52.5Cu17.9Ni14.6Al10Ti5 (with x=0-1) by an arc melting/suction casting method. The density of these alloys increases by nearly 67% with increasing Hf content, which is advantageous for their potential use as kinetic energy armor-piercing projectile materials. The glass transition temperature and the melting temperature increase linearly with increasing Hf content. The reduced glass transition temperature (Tg/Tm) decreases from 0.64 (x=0) to 0.62 (x=1), indicating reduced glass-forming ability for the Hf- based alloy. The fracture strength in uniaxial compression at quasi-static strain rates also increases with increasing Hf content, reaching ∼ 2.2 GPa for Hf52.5Cu17.9Ni14.6Al10Ti5.
Thermal Expansion of Glass-Forming Zr-based Alloys in the Melt, the Undercooled Liquid and the Different Solid States
Authors: Samwer K. , Damaschke B. , Krause M. , Ryder P.
Published: April 2011

The thermal expansion coefficients of glass-forming Zr-based alloys were measured in the melt, the undercooled liquid and the glassy/crystalline state. Due to the high reactivity of the liquid material the experiments were performed containerlessly in an electrostatic levitator. We used an optical method where the samples were imaged with a high-resolution CCD- camera and the volume of the samples was evaluated by digital image processing. The coefficients of thermal expansion in the liquid and in the solid state could be determined from the volume versus temperature curves. The results can be compared with measurements in the electromagnetic levitation facility TEMPUS performed under microgravity conditions in the mission MSL-1 and ground based DMA-measurements. The thermal expansion data can be interpreted in terms of the free volume model.
Glass and Quasicrystal Formation in a Zr-based Multicomponent Alloy
Authors: Kühn U. , Eckert J. , Mattern N. , Schultz L.
Published: April 2011

The phase formation of a Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5alloy has been investigated upon cooling from the melt at different quenching rates as well as upon annealing of as-cast specimens. The different samples are characterized by x-ray diffraction, transmission electron microscopy and differential scanning calorimetry. Rapid quenching using the melt spinning technique results in amorphization. Slower cooling as realized upon copper mold casting leads to an icosaheadral quasicrystalline phase coexisting with a small amount of amorphous phase. The primarily formed quasicrystals have a grain size of about 1 νm. Upon annealing, the amorphous phase formed upon melt spinning precipitates quasicrystals in the first step of a series of transformations to the crystalline equilibrium compounds. The quasicrystals formed by annealing do not exceed a size of 5 to 10 nm.
Self-diffusion in Zr55Al10Ni10Cu25 and Pd40Cu30Ni10P20 Bulk Metallic Glass
Authors: Nakajima H. , Kojima T. , Nonaka K. , Zhang T. , Inoue A. , Nishiyama N.
Published: April 2011

Self-diffusion coefficients of Ni in Zr55Al10Ni10Cu25and of Ni and Pd inPd40Cu30Ni10P20 bulk metallic glass below and above the glass transition temperature Tg have been measured with an ion-beam sputter-sectioning technique using the radioactive isotope 63Ni and 103Pd. The diffusion coefficients in the supercooled liquid region are much higher than those extrapolated from low temperature data in the amorphous region. The temperature dependence of the diffusion coefficients exhibits non-Arrhenius behaviour in the supercooled liquid phase. Such a deviation from the Arrhenius plots is interpreted by a cooperation of moving tracer atom with the surrounding atoms. The difference of the diffusion behaviour in between Zr-Al-Ni-Cu and Pd-Cu-Ni-P glasses is attributed to the different structural stability of both glasses.
Controlled Crystallization of Metallic Glasses Through Joule Heating
Authors: Knobel Marcelo , Piccin Rafael , da Silva Fábio C.S. , Filho Walter J. Botta , Yavari Alain Reza
Published: April 2011

This work reviews the basic aspects of an alternative thermal treatment of metallic glasses by Joule heating, which explores the heat produced by an electrical current flowing through samples. The advantages of this annealing method are discussed, and a brief description of available models in a variety of materials is given. The experimental procedures are discussed in detail, with emphasis in a new variant of Joule heating, known as “linear varying current Joule heating”. This method allows one to precisely monitor the structural transformations that take place during annealing, by interrupting the thermal treatment at any desired point, leading to samples with controlled micro and/or nanostructures. Some examples of Joule heating curves in novel bulk amorphous samples are shown, to illustrate the excellent perspectives of this annealing technique.
Effect of Boron on Crystallization Process, Forming Ability and Magnetic Properties of Amorphous Fe91−xZr5BxNb4 Alloy
Authors: Yao B. , Hu X. , Si L , Tan H , Li Y.
Published: April 2011

Effects of boron (B) on crystallization, glass forming ability (GFA) and magnetic properties of amorphous Fe91−xZr5BxNb4 (FZBN) were studied in the B content ranging from 0 to 36 at%. The amorphous FZBN crystallizes to μ-Fe in primary crystallization mode at B≤ 20 at% and to Fe-Zr-B-Nb cubic phase with lattice constant of 1.0704 nm and μFe in eutectic mode in the range from 22 at% to 30 at% B. The change of crystallization mode is accompanied by the sharp enhance of glass transition and crystallization temperatures. The GFA of FZBN alloys changes with B content and reaches the largest near 27.5 at % B. Curie temperature (Tc) and magnetization at room-temperature (MRT) and 78K (M78K) for the amorphous FZBN increase with increasing B content. However, the change laws of the Tc and the magnetizations are somewhat different in the composition ranges of 5 to 20 at% B and 22.5 at% to 30 at% B.
Domain Structure and Thermal Dependence of the Coercive Field in Nanocrystalline FeZrBCu
Authors: Hernando A. , Arcas J.
Published: April 2011

Nanocrystalline Fe85Zr7B6Cu2 and Fe87.2Zr7.4B4.3Cu1.1 (at. %) samples have been obtained by annealing Melt-spun ribbons for 1hr at various temperatures in the range 593K to 953K. Structural characterization by means of X-ray diffraction and thermomagnetic analyses reveal the manifestations of Fe nanocrystals embedded in an amorphous matrix. The coercive field has been measured by a Förster coercimeter at temperatures ranging between 50 K and 300 K, and the room temperature domain structure has been monitored by magneto-optical Kerr effect. Both, the as cast as well as the sample annealed above 813K display soft magnetic properties at room temperature, exhibiting a coercive field below 10 A/m, and wide regular domains. In contrast, the samples annealed at 750 K, corresponding to the beginning of the crystallization process, undergo a magnetic hardening, showing higher coercive fields (up to 150 A/m for Fe87.2Zr7.4B4.3Cu1.1) and a dull domain pattern. When these magnetically harder samples are cooled, their coercivity is reduced. Thus, the magnetic hardening is attributed to the exchange decoupling between crystallites, due to the proximity of the Curie temperature of the amorphous phase.
Transmission Electron Microscopy Investigations of an As-Processed Yttria-Stablilized zirconia on Platinum Aluminide Bond Coat
Authors: Yang Judith C. , Nuhfer Noel T.
Published: April 2011

We examined an as-processed yttria-stabilized zirconia (YSZ) on platinum aluminide bond coat (BC), produced by electron beam physical vapor deposition, with transmission electron microscopy, including energy dispersive X-ray spectroscopy and hollow-cone diffraction. Columnar α-Al2O3 grains (∼100nm) formed at the interface between the BC and YSZ. A thin intermix (∼50nm) region was observed between the α-Al2O3 and YSZ. Hollow cone diffraction showed that the α-Al2O3 grains and the small-grained (∼10nm) YSZ near the α-Al2O3 are randomly oriented, without preferential texturing. No evidence of spinel formation was noted.
Recent Advances in Development and Processing of Titanium Aluminide Alloys
Authors: Appel Fritz , Clemens Helmut , Oehring Michael
Published: April 2011

Intermetallic titanium aluminides are one of the few classes of emerging materials that have the potential to be used in demanding high-temperature structural applications whenever specific strength and stiffness are of major concern. However, in order to effectively replace the heavier nickel-base superalloys currently in use, titanium aluminides must combine a wide range of mechanical property capabilities. Advanced alloy designs are tailored for strength, toughness, creep resistance, and environmental stability. Some of these concerns are addressed in the present paper through specific comments on the physical metallurgy and technology of gamma TiAl-base alloys. Particular emphasis is placed on recent developments of TiAl alloys with enhanced high-temperature capability.
Refinement of Crystallographic Parameters in Refractory Metal Disilicides
Authors: Tanaka K. , Nawata K. , Inui H. , Yamaguchi M. , Koiwa M.
Published: April 2011

The crystallographic structures of seven refractory metal (Ti, V, Cr, Nb, Mo, Ta and W) disilicides with the C11b, C40 and C54 structures have been refined through analysis of single-crystal X-ray diffraction data. Crystallographic parameters refined are lattice constants, atomic parameters and the space group of the C40 disilicides. In most of previous studies, silicon atoms have been considered to locate at the ideal positions so that the refractory metal atoms are perfectly six-fold coordinated in RSi2 layers prevailing in all the three structures. The present analysis shows that the silicon atoms are displaced from the ideal positions. The magnitude of such displacement is found to be closely related to the interatomic distance in these pseudo-hexagonally arranged RSi2 layers. The space group of three of the four C40 disilicides, VSi2, CrSi2 and TaSi2, is determined to be P6422, which is of chirality with respect to that (P6222) assigned in the previous studies.
Effect of E21-Fe3AlC Precipitation on Mechanical Properties of γ -Austenite Stabilized by Addition of Mn and Co
Authors: Miura Seiji , Ishii Hiroaki , Mohri Tetsuo
Published: April 2011

Effects of Co addition on the phase relation and mechanical properties of an E21-Fe3AlC phase in the Fe-Mn-Al-C system are investigated. The relation between the γ-austenite and E21 phases is the same with Co-free alloys. Two kinds of precipitation processes of E21 phase from γ-austenite matrix are observed: a fine E21 phase precipitation and a cellular structure formation from grain boundaries. The cellular structure formation is explained in relation to the lattice mismatch evaluated by XRD.

The lattice mismatch is estimated to be ∼3%, rather higher than that of γ/γ′ in Ni-based superalloys, and the volume fraction of the E21-Fe3AlC phase precipitated in the γ-austenite phase is as large as 30%. The micro-vickers hardness increases with increasing Al or C, caused by the increase in the volume fraction of the E21-Fe3AlC phase.
Crystal Growth of RuAl-base Alloys
Authors: Rosset Sebastien , Cefalu E. , Varner Louis W. , Johnson David R.
Published: April 2011

Different techniques to produce single crystals of RuAl were investigated. Processing from the melt is problematic due to the rapid loss of Al. However, very large grained RuAl specimens can be produced from the solid-state by arc-zone melting. Directional solidification of eutectic alloys is less problematic due to the lower melting temperature. Alloys of RuAl-Mo were found to be eutectic, and the eutectic composition was determined to be RuAl-51 wt% Mo.
Effect of surface defects on the fatigue behavior of a cast TiAl alloy.
Authors: Nazmy M. , Staubli M. , Onofrio G. , Lupinc V.
Published: April 2011

The effect of surface defects on the performance of TiAl-base alloys is an issue of importance in contemplating their application into engine components. Due to the relatively low ductility and low impact resistance of gamma alloys the validation of models for estimating economic life and for safe-life approaches employed for components becomes of great importance. Surface defects can be attributed to various sources during the manufacturing or handling of the components. In fact, little is known about the detrimental effects of surface defects on gamma alloys. In the present study, the effect of artificially introduced surface defects, on the high cycle fatigue behavior of the Ti-47Al-2W-0.5Si, will be investigated and correlated with the crack growth behavior at 700°C. The results are reported in the form of the Kitagawa diagram in which the safe and unsafe zones for crack advance and fracture are defined.
Control of Lamellar Orientation in γ-TiAl Based PST Crystal by Using Seed Crystals
Authors: Yamamoto Y. , Morishima H. , Koike K. , Takeyama M. , Matsuo T.
Published: April 2011

Unidirectional solidification of Ti-48Al binary alloy using γ-TiAl single-phase seed crystals has been carried out by an optical floating zone method. The lamellar orientation of the grown PST crystal follows the orientation of the Ti-57Al seed crystal, while it fails to follow that in the case of the Ti-53Al seed. Microstructure analysis reveals that the seed crystal of Ti-57Al exhibits a flat liquid/solid interface in melting (γ→+L) even after making contact with 48Al to grow, whereas the seed of Ti-53Al shows a cellular interface due to the peritectic reaction in melting (γ→α+L). At the 57/48 interface, an abrupt change of Al concentration was detected from the seed to the grown crystal, indicating an occurrence of composition travel to skip the peritectic reaction, which is responsible for the control of lamellar orientation of the grown PST crystals. The same attempt has been made by using the 57Al single crystal seed with a different orientation, and the lamellar orientation of the grown PST crystal was confirmed to follow the orientation of the seed.
The Effect of Cu-Macroalloying Additions to Rapidly Solidified NiAl Intermetallic Compound
Authors: Colin J. , Campillo B. , Gonzalez C. , Alvarez-Fregoso O. , Juarez-Islas J. A.
Published: April 2011

The effects of two variables on the NiAl intermetallic compound were studied: 1) copper macroalloying additions and 2) rapid solidification processing. For that purpose, several NiCuAl alloys were vacuum induction melted and rapidly solidified by using a copper wheel, rotating at 15 m/s, under an argon atmosphere. Chemical analysis of as-rapidly solidified ribbons indicated, that four alloy compositions lie in the β-(Ni, Cu)Al field, one alloy composition lie in the boundary of the β-(Ni, Cu)Al/(Ni, Cu)2Al3 fields, one alloy composition lies in the boundary of the β-(Ni, Cu)Al/β-(Ni, Cu)Al + (Ni, Cu)3Al fields and two alloy compositions lie in the β-(Ni, Cu)Al + (Ni, Cu)3Al field. Transmission electron microscopic observations carried out in as-rapidly solidified ribbons, revealed the presence of at least three main structures: i) β-(Ni, Cu)Al, ii) β-(Ni, Cu)Al + martensite (Ni, Cu)Al and iii) (Ni, Cu)3Al + martensite (Ni, Cu)Al. Microhardness Vickers and tensile test data indicated that alloys with a β-(Ni, Cu)Al + martensite (Ni, Cu)Al microstructure have improved room temperature ductility, reaching values of elongation up to 3.28 %.
Microstructure and Property Change of Ti-49Al Induced by Hydrogen Charging
Authors: Abe E. , Gao K.W. , Nakamura M.
Published: April 2011

We have investigated an effect of hydrogen gas-charging on the microstructure and the mechanical property of a Ti-49at.%Al alloy. After hydrogen-charging performed under an atmospheric pressure of hydrogen gas at 1023K for 3 hours, the alloy with γ-single phase has become completely brittle, while this hydrogen-induced embrittlement is suppressed for that with (γ+α2) two-phase microstructure composed of lath-precipitates in the γ matrix. A significant microstructural change was found to occur for the two-phase alloy (approximately 340ppm hydrogen in the alloy); a thin amorphous layers with a few nm thickness appear at the preexisting γ/α2 interfaces in the lath-precipitates after hydrogen-charging. In-situ TEM observation confirmed that the amorphous region transforms to a nano-crystalline state after heating to 1000K at which the hydrogen could be removed (degassed), indicating that the amorphous phase is not a binary Ti-Al phase but a ternary Ti-Al-H one. This, in turn, suggests that the γ/α2 interface in the lath packets act as the most preferential sites for hydrogen storage. Therefore, the scavenging is expected to occur effectively for the microstructure composed of γ-α2 fine lamellae in which a large number of γ/α2 interfaces exist. It is worthwhile mentioning that the fine-scale of the lamellae makes it possible to have a large number of interfaces for a given volume of the α2 phase.
Mechanical Properties of γ-TiAl Based Alloys at Elevated Temperatures
Authors: Weller M. , Chatterjee A. , Haneczok G. , Appel F. , Clemens H.
Published: April 2011

Mechanical loss (internal friction) and creep experiments were carried out on specimens of a Ti-46.5at.%Al-4at.%(Cr,Nb,Ta,B) alloy with differently spaced fully lamellar microstructures. The creep tests were performed in a temperature range of 970 K to 1070 K at 175 MPa. For the mechanical loss measurements a low frequency subresonance torsion apparatus was applied, operating in the frequency range of 0.01 Hz to 10 Hz. The mechanical spectra show two phenomena: (i) A loss peak of Debye-type at 900 K (0.01 Hz) which is controlled by an activation enthalpy of 3.0 eV. The loss peak is related to thermally activated (reversible) motion of dislocation segments which are pinned at the lamellae interface and within gamma lamellae. (ii) A viscoelastic high temperature background above 1000 K with an activation enthalpy of 3.8 eV. This value agrees well with the activation enthalpy of 3.6 eV from creep experiments. Both high temperature background as well as creep are assigned to diffusion controlled climb of dislocations.
Phase Stability and High Temperature Tensile Properties of W doped gamma-TiAl
Authors: Hashimoto Keizo , Hirata Hirohiko , Mizuhara Youji
Published: April 2011

Tungsten (W) doped γ-TiAl is one of promising alloys among many other proposed TiAl base alloys, for the purpose of structural applications at elevated temperatures. Ingots of W doped γ-TiAl were produced by plasma arc melting, followed by homogenizing heat treatment and isothermal forging to control their microstructures. The phase stability of W doped γ-TiAl has been studied quantitatively, using the specimens quenched from 1273 K. Equilibrium compositions of consisting phases were analyzed by means of EDS analysis in a TEM. An isothermal cross section of the Ti-Al-W ternary phase diagram at 1273K has been proposed based on the experimental observations. Small amounts of W addition (< 1at%) to Ti-48at%Al cause a phase shift from α2+γ to α2+β+γ, which suggests that W is the strongest β stabilizer among transition metals, such as Cr and Mo. Mechanical property measurements of W doped γ-TiAl show that the high temperature tensile strength has been improved by the W addition. Relationships between the microstructures and the mechanical properties of W doped γ-TiAl have been discussed.
Lithium nanocluster formation in Li+ - ion implanted MgO
Authors: van Veen A. , van Huis M.A. , Fedorov A.V. , Schut H. , Falub C.V. , Eijt S.W.H. , Labohm F. , Kooi B.J. , Hosson J.Th.M. De
Published: April 2011

Photon absorption (PA), Positron Beam Analysis (PBA) and Neutron Depth Profiling (NDP) is applied to study the relation between photon absorption behavior and the precipitates formed by ion implantation and thermal annealing. Monocrystals of MgO(100) were implanted with 1.0×10166Li ions cm−2 at an energy of 30 keV. The samples were thermally annealed in air in steps up to 1200 K. After each step Doppler broadening Positron Beam Analysis (PBA) was applied to monitor the depth profile of the implantation defects. The evolution of the depth profile of lithium was followed with the aid of NDP. During the annealing there is hardly any change in the location of the lithium implantation peak at 150 nm (peak concentration 2 at. %). Only after annealing to 1200 K the majority of the lithium has left the crystal and optical absorption effects have disappeared. During annealing at 750 K an absorption band develops between 400 and 600 nm; at 950 K the maximum absorption is centered at 450 nm corresponding to Mie absorption and scattering by lithium nanoclusters. Positron beam analysis shows a considerable increase of annihilations with low momentum electrons in the implanted zone. A positron method for measuring electron momentum distributions (2D-ACAR) coupled to an intense positron beam gave evidence for the presence of semi-coherent metallic lithium inclusions.
Influence of carbon on erbium lattice location in Si:Er
Authors: Ren X. T. , Huang M. B.
Published: April 2011

The 1.5 µm luminescence from Si:Er is known to strongly depend on impurities (e.g. carbon) in silicon. In this work, we investigate the effect of carbon co-doping on the lattice location of Er atoms in Si by Rutherford backscattering(RBS)/channeling techniques. A float-zone (FZ) Si (100) wafer was first amorphized to a depth of ~ 0. 3 µm by Si ions implanted to a dose of ~ 1×1015cm2 at liquid nitrogen temperature. Carbon ions were then implanted into the amorphous silicon, which was recrystallized via solid phase epitaxial growth (SPEG) at 600°C following C implant. Finally Er ions were implanted into the C-doped and C-free Si crystals, with the substrate temperature at 25°C and 300°C respectively. The RBS/channeling results show that the Er redistribution and Si crystallinity are strongly affected by C co-doping. The incorporation of C into Si can significantly suppress Er surface segregation, and modify the lattice location of Er atoms in Si. We discuss these effects in terms of the formation of Er-C defect complexes.
tudies on Titanium Nitride Coatings - Effect of Ion Bombardment
Authors: Vargheese K. Deenamma , Rao G. Mohan
Published: April 2011

Ion bombardment during thin film growth is known to cause structural and morphological changes in the deposited films and thus affecting the film properties. These effects can be due to the variation in the bombarding ion flux or their energy. We have deposited titanium nitride films by two distinctly different methods, viz. Electron Cyclotron Resonance (ECR) plasma sputtering and bias assisted reactive magnetron sputtering. The former represents low energy (typically less than 30 eV) but high density plasma (1011cm−3), whereas, in the latter case the ion energy is controlled by varying the bias to the substrate (typically a few hundred volts) but the ion flux is low (109cm−3). The deposited titanium nitride films are characterized for their structure, grain size, surface roughness and electrical resistivity.
Investigation of Irradiation Damage in Silicon Dioxide Polymorphs using Cathodoluminescence Microanalysis.
Authors: Stevens-Kalceff Marion A.
Published: April 2011

Cathodoluminescence (CL) Microanalysis (spectroscopy and microscopy) provides unique high sensitivity, high spatial resolution information about the defect structure and distribution of defects in wide band gap materials and therefore is an ideal technique with which to investigate the microstructural processes induced by irradiation. CL microanalytical techniques allow the in situ monitoring and post irradiation assessment of electron irradiation induced damage. Changes in the defect structure and surface topography of electron irradiated silicon dioxide polymorphs and related silicates including pure crystal quartz, pure silica glasses, pure amorphous fused quartz and alkali-borosilicate glasses, have been investigated and compared using CL microanalysis and Scanning Probe Microscopy (SPM) techniques. CL and SPM evidence shows all specimens are sensitive to electron irradiation. CL evidence is consistent with the production and micro-segregation of irradiation induced defects. The observed damage is highly correlated with the electron irradiation induced changes in the surface topography of the investigated specimens.
Ion Implanted Er and Tb in SiO2 for Electroluminescence in MOS Diodes
Authors: Buchal Ch. , Coffa S. , Wang S. , Carius R.
Published: April 2011

Efficient infra-red and visible electroluminescence(EL) has been obtained from implanted rare earth ions in the SiO2 of a silicon-metal-oxide-semiconductor (MOS) diode structure at room temperature. The rare earth ions are excited by the direct impact of hot electrons tunneling through the oxide at electric fields larger than 6 MV/cm. The internal quantum efficiencies of Er and Tb implanted MOS diodes are estimated to be 10 % and 3 %, respectively. The hgh quantum efficiency is due to the high impact excitation cross-section of more than 10− 15cm2. These observations on MOS structures are an experimental proof for efficient light generation by hot electron impact.