bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical Studies of MOCVD InxGa1-xN Alloys
Authors: Little B.D. , Shan W. , Song J.J. , Feng Z.C. , Schurman M. , Stall R.A.
Published: April 2011

We present the results of optical studies of InxGa1-xN alloys (0<x<0.2) grown by metalorganic chemical vapor deposition on top of thick GaN epitaxial layers with sapphire as substrates. Photoluminescence (PL) and photoreflectance (PR) measurements were performed at various temperatures to determine the band gap and its variation as a function of temperature for samples with different indium concentrations. Carrier recombination dynamics in the alloy samples were studied using time-resolved luminescence spectroscopy. While the measured decay time for the alloy near-band-edge PL emissions was observed to be generally around a few hundred picoseconds at 10 K, it was found that the decay time decreased rapidly as the sample temperatures increased. This indicates a strong influence of temperature on the processes of trapping and recombination of excited carriers at impurities and defects in the InGaN alloys.
Strain Effects on Excitonic Transitions in GaN
Authors: Shan W. , Hauenstein R.J. , Fischer A.J. , Song J.J. , Perry W.G. , Bremser M.D. , Davis R.F. , Goldenberg B.
Published: April 2011

We present the results of experimental studies of the strain effects on the excitonic transitions in GaN epitaxial layers on sapphire and SiC substrates. Photoluminescence and reflectance spectroscopies were performed to measure the energy positions of exciton transitions and X-ray diffraction measurements were conducted to examine the lattice parameters of GaN epitaxial layers grown on different substrates. Residual strain induced by the mismatch of lattice constants and thermal-expansion between GaN epitaxial layers and substrates was found to have a strong influence in determining the energies of excitonic transitions. The overall effects of the strain generated in GaN is compressive for GaN grown on sapphire and tensile for GaN on SiC substrate. The uniaxial and hydrostatic deformation potentials of wurtzite GaN were derived from the experimental results. Our results yield the uniaxial deformation potentials b1≈−5.3 eV and b2≈2.7 eV, as well as the hydrostatic components a1≈−6.5 eV and a2≈−11.8 eV.
Investigation of Vacancies in GaN by Positron Annihilation
Authors: JØrgensen L. V. , Kruseman A. C. , Schut H. , Van Veen A. , Fanciulli M. , Moustakas T. D.
Published: April 2011

Positron beam analysis has been performed on autodoped n-type, semi-insulating and Mg doped p-type epitaxially grown layers of GaN on sapphire. Doppler Broadening measurements clearly indicate the presence of vacancies in the intrinsically autodoped n-type GaN by an increase in the annihilation Doppler lineshape S-parameter of 1.04 relative to the value for the high resistivity sample. This value is typical for vacancy-type defects in compound semiconductors such as GaAs. Results of experiments with higher sensitivity to core-electrons are also presented. These two detector coincidence measurements yield information on the chemical environment surrounding the vacancies. The results are consistent with the presence of Ga vacancies in the autodoped n-type sample.
The Structure and Texture of Y2O3:Tb Nanocrystals
Authors: Taylor J. , Libera M. , Goldburt E. , Bhargava R.
Published: April 2011

This paper presents the results of microstructural studies of terbium-doped yttria quantum nanocrystals by imaging and diffraction in a transmission electron microscope. The nanocrystals are typically found in clusters of varying size. Electron energy-loss spectroscopy confirms that the particles consist of yttrium and oxygen. Analysis of selected-area electron diffraction patterns shows that the nanocrystals largely have the cubic structure found in bulk yttria. These patterns furthermore suggest that within each cluster the nanocrystals align themselves with a preferred orientation relative to the incident electron beam as well as to each other.
Characterization of Light Emitting Porous Polycrystalline Silicon Films
Authors: Poon M. C. , Han P. G. , Sin J. K. O. , Wong H. , Ko P. K.
Published: April 2011

Polycrystalline silicon (poly-Si) thin films (∼700nm) were deposited by LPCVD, doped with 950°C phosphorous diffusion, and rendered porous by anodization and stain etching. From x-ray photoelectron spectroscopy, poly-Si films have atomic concentration of C(ls):0(ls):Si(2p) = 6%:15%:79%. However, porous poly-Si (PPS) films with weak photoluminescence (PL) have C:O:Si of 20%:38%:42%. For PPS films with strong PL, C:O:Si is 11%:38%:51%. From micro-Raman, scattered spectra for 632nm laser source has peak at 735nm and full wave half maximum (FWHM) of 76nm, and is similar to the PL spectra excited by 400nm uv laser source. High resolution transmission electron microscopy (TEM) study shows that PPS film is of complex structure and composes of numerous Si nano-crystals (1∼10nm) surrounded by amorphous materials.
Tem Study of Porous Silicon Fabricated from N- and P-Type Doped Polycrystalline Films
Authors: Haji L. , Le Thomas Y. , Chane Che Lai F. , Joubert P.
Published: April 2011

The formation of porous silicon (PS) from n/p, n+/p and p+/n structures carried from polycrystalline silicon films (poly-Si) deposited on single crystal silicon (c-Si) substrates was studied by cross-sectional transmission electron microscopy. Our results clearly show that the pore formation in such structures involve the extended defects of the poly-Si film. The role played by these defects depends on the doping type and level, and on whether the anodization is performed under illumination or not.
Optical Sensors based on Porous Silicon Multilayers: a Prototype
Authors: Theiβ W. , Arens-Fischer R. , Hilbrich S. , Scheyen D. , Berger M. G. , Krüger M. , Thönissen M.
Published: April 2011

Porous silicon multilayers are easily produced by a variation of the current density during the electrochemical etching process. The obtained porosity profile in depth is equivalent to a refractive index profile. Preparing appropriate multilayer systems one can design optical filters with sharp reflectivity peaks from the infrared through the whole visible spectral range (dielectric mirrors). Stability in time is achieved applying a suitable oxidation process. In many cases of optical process and material control the required spectral decomposition of the probing light can be done with sufficient accuracy by a set of porous silicon optical filters whose spectral characteristics can lbe adapted easily to the given problem. In this work we present the prototype of an optical sensor that is a low-cost alternative to ‘real’ (and expensive) spectrometers. Application examples in the infrared and visible are given to demonstrate the sensor performance;.
Fabrication of 10-nm Gratings Using STM Chemical Vapor Deposition
Authors: Archer A. , Hetrick J. M. , Nayfeh M.H
Published: April 2011

Using STM assisted chemical vapor deposition (STM-CVD), we have successfully fabricated what we believe are the finest gratings to date. Our best grating consists of half-micron long evenly spaced lines which have a width of 10 nm and a 10 nm interspacing. The gratings were fabricated on passivated p-type silicon <111> using trimethylaluminum (TMA) as a precursor gas. Although the composition of the nanostructures is not exactly known at this time, we believe that the lines are partially metallic. We made some systematic investigations to optimize the resolution, including studies of the effect of tunneling current, tip speed, tip-sample biasing voltage, and the effect of formation of very fine TMA whiskers on the tip.
Enhancement of the Spontaneous Emission Rates in all Porous Silicon Optical Microcavities
Authors: Pavesi L. , Cazzanelli M. , Bisi O.
Published: April 2011

The emission properties of a porous silicon layer placed in an optical microcavity is investigated by photoluminescence and time resolved photoluminescence measurements. The microcavity is formed by an all porous silicon Fabry-Perot filter made by two distributed Bragg reflectors separated by a λ or λ/2 porous silicon layer. Our main findings are that the spontaneous emission spectrum is drastically modified: the linewidth is narrowed, the time decay of the emission is shortened by a factor of about 2/3 at room temperature and the peak emission intensity is increased by a factor of more than 10. These facts are caused by the redistribution of the optical modes in the microcavity due to the presence of the optical resonator and to the variation of the dielectric environment.
Real-Time Monitoring of Hydrogen Elimination Processes in Pulsed-Gas PECVD Using in Situ Mass Spectroscopy
Authors: Srinivasan Easwar , Bordeaux Jeremy S. , Parsons Gregory N.
Published: April 2011

In situ mass spectroscopy is used to monitor and analyze the hydrogen elimination reaction products during cyclical exposure of thin films of amorphous silicon to a flux of atomic deuterium. Mass spectroscopy results that atomic deuterium etches deposited silicon forming SiD4 and abstracts hydrogen bonded to silicon in the film to form HD. The relative signal intensities show that abstraction is the primary hydrogen elimination mechanism. The energy of activation for the abstraction reaction is obtained from the mass spectroscopy signals through a first order kinetic analysis and is found to be approximately zero, indicating that abstraction is not thermally activated.
Role of SiH2 in 1H Nmr of μc-Si:H Deposited with Different Plasma Excitation Frequencies and Silane Concentrations
Authors: Hari P. , Taylor P. C. , Finger F.
Published: April 2011

Previous lH NMR and IR studies of six samples of μc-Si:H prepared under plasma excitation frequencies ranging from 13 MHz to 95 MHz and silane concentrations ranging from 3% to 8% revealed three important results: (1) for a fixed plasma excitation frequency (95 MHz) the hydrogen content increases with silane concentration; (2) for fixed silane concentration the hydrogen content is roughly constant over a wide range of plasma excitation frequencies; and (3) the 1H NMR free induction decay exhibits beat frequencies which correspond to the calculated frequencies due to SiH2 in microcrystalline silicon. In this study we investigate the role of SiH2 in the μc-Si:H structure using 1H NMR measurements. We studied two samples prepared at two different plasma excitation frequencies (13 MHz and 95 MHz). The lU NMR lineshapes of these samples were measured at 300 K and 77 K. The motionally narrowed component of the 1H NMR is not as rapid at room temperature as that observed previously, but the linewidth of this component increases significantly at 77 K. Because of this difference between our results and those reported previously, it is possible that H2 molecules are not responsible for the motional narrowing in our samples. We present plausible arguments for the hindered motion of SiH2 groups in our μc-Si:H samples.
Hall-Effect Studies on Microcrystalline Silicon with Different Structural Composition and Doping
Authors: Backhausen U. , Carius R. , Finger F. , Hapke P. , Zastrow U. , Wagner H.
Published: April 2011

Hall-effect experiments on -type microcrystalline silicon samples with a wide range of structural composition and doping have been performed. For highly doped samples the conductivity Σ and the mobility μ show a non-singly activated behaviour while the carrier density is almost temperature independent. The comparison of the carrier density with the phosphorous concentration in conjunction with the conductivity gives strong evidence that the Hall-effect data have to be corrected with the crystalline volume fraction Xc. Furthermore, the increase of the mobility with Xc, which is linked in our case to the grain size, can be explained when the length of the transport paths is taken into account. Our results will be discussed in the framework of different transport models. It is concluded that transport in μc-Si:H can not be explained in terms of thermionic emission over barriers with a well defined barrier height; instead a distribution of barrier heights have to be considered. A transport model is suggested where μc-Si:H is viewed as an interconnected network.
The “Micromorph” Cell: a New Way to High-Efficiency-Low-Temperature Crystalline Silicon Thin-Film Cell Manufacturing?
Authors: Keppner H. , Torres P. , Meier J. , Platz R. , Fischer D. , Kroll U. , Dubail S. , Anna Selvan J. A. , Pellaton Vaucher N. , Ziegler Y. , Tscharner R. , Hof Ch. , Beck N. , Goetz M. , Pernet P. , Goerlitzer M. , Wyrsch N. , Veuille J. , Cuperus J. , Shah A. , Pohl J.
Published: April 2011

In the past, microcrystalline silicon (μc-Si:H) has been successfully used as active semiconductor in entirely μc-Si:H p-i-n solar cells and a new type of tandem solar cell, called the “micromorph” cell, was introduced [1]. Micromorph cells consist of an amorphous silicon top cell and a microcrystalline bottom cell. In the paper a micromorph cell with a stable efficiency of 10.7 % (confirmed by ISE Freiburg) is reported.

Among sofar existing crystalline silicon-based solar cell manufacturing techniques, the application of microcrystalline silicon is a new promising way towards implementing thin-film silicon solar cells with a low temperature deposition. Microcrystalline silicon can, indeed, be deposited at temperatures as low as 220°C; hence, the way is here open to use cheap substrates as, e.g. plastic or glass. In the present paper, the development of single and tandem cells containing microcrystalline silicon is reviewed. As stated in previous publications, microcrystalline silicon technique has at present a severe drawback that has yet to be overcome: Its deposition rate for solar-grade material is about 2Å/s; in a more recent case 4.3 Å/s [2] could be obtained. In the present paper, using suitable mixtures of silane, hydrogen and argon, deposition rates of 9.4 Å/s are presented. Thereby the dominating plasma mechanism and the basic properties of resulting layers are described in detail. A first entirely microcrystalline cell deposited at 8.7 Å/s has an efficiency of 3.15%.
Crystallization and Evaluation of Non-Stoichiometric Silicon-Carbon Films for Hetero Thin-Film Transistors
Authors: Choi Kwangsoo , Matsumura Masakiyo
Published: April 2011

Experimental works have been reviewed on poly-Si/poly-SiCx hetero TFTs aiming at extremely low off-current even under intense light illumination conditions with reasonable field-effect mobility. The results indicated that the hetero TFT having the stacked poly-Si/poly-SiCx layers is promising as a switching device in matrices
Large Crystallite Polysilicon Deposited Using Pulsed-Gas PECVD at Temperatures Less Than 250°C
Authors: Srinivasan E. , Ellis S. J. , Nemanich R. J. , Parsons G. N.
Published: April 2011

A pulsed-gas intermittent deposition technique is used to deposit high crystallinity hydrogenated micro- or poly-crystalline silicon using silane and hydrogen. This method has been used to deposit crystallites that are comparable to those obtained using PECVD of fluorinated silanes. RHEED and TEM have been used to understand the nucleation process. The pulsed-gas method is promising for depositing polycrystalline silicon and subsequent use in thin film transistor applications.
Rapid Synthesis of Nanostructural Intermetallics and their Bulk Properties
Authors: Pickard S. M. , Ghosh A. K.
Published: April 2011

A rapid physical vapor deposition process (PVD) utilizing a high speed rotating substrate and small substrate-to-source spacing has been used to produce bulk sheet of Ti-Al alloys in the compositional range Ti-12% Al to Ti-75% Al1 at a rate of 1–3 μm/minute. Microstructural architectures produced by the method comprise of either fully homogenous phase mixtures of nano-grains, or nanolaminated material, depending on the substrate rotational rate, with lower rotational rate producing a layered microstructure. Defect populations within the as-deposited material are characterized by TEM and SEM, and hot pressing consolidation of the as-deposited material, which retains a grain size < 1000 nm, has been investigated. While indentation hardness of α2+γ(2 phase) alloys exceeded 7 GPa, brittle failure occurred in the elastic regime at nominally lower tensile stress than that for conventionally produced alloys containing Nb and Cr as solute elements. α2+γ alloys can exhibit tensile elongations of more than 100% at 850°C with retention of fine grain size. Elevated temperature failure occurs by the formation of voids in regions of compositional variability in the composite where single phase α2-Ti3Al structure was present.
Three Dimensional Simulation of the Microstructure Development in Ni-20%Fe Nanocrystalline Deposits
Authors: Li Hualong , Czerwinski F. , Szpunar J. A.
Published: April 2011

A Monte-Carlo computer model was applied to simulate a development of the three dimensional microstructure during electrodeposition of nanocrystalline alloys. The driving force for this process was the minimization of free energy of the system. For a particular deposit of Ni-20%Fe, the influence of the overpotential and current density on the grain size was tested. A strong decrease in grain size with increasing overpotential and current density obtained from the simulation is in qualitative agreement with the experimental data.
Magnetic Properties and Thermal Stability of Graphite Encapsulated Cobalt Nanocrystals
Authors: Host J. J. , Dravid V. P.
Published: April 2011

Small particles are of interest for magnetic study due to their range of magnetic properties with particle size and unusual magnetic properties. In this study we produced graphite encapsulated magnetic cobalt particles and measured their structural and magnetic properties both before and after annealing at 550° C for 6 hours. No large change in the structural properties (particle size, shape, and lattice parameter) was observed as a result of the annealing. There were, however, significant changes in the magnetic properties. The coercivity was found to decrease at all temperatures, especially at temperatures below 100° K. The possible reasons for this reduction in coercivity are examined, with the change in the shape of the particles appearing most likely.
Growth of BaTiO3 in Hydrothermally Derived (<100° C) BaTiO3/Polymer Composite Thin Films
Authors: Collins David E. , Slamovich Elliott B.
Published: April 2011

Composite BaTiO3/polymer films (<1μm thickness) were processed by the in-situ growth of BaTiO3 particles in a polymer matrix. A solution of a polybutadiene/polystyrene triblock copolymer and titanium diisoproxide bis(ethlyacetoacetate) dissolved in toluene was cast onto a Ag-coated substrate. Subsequent hydrothermal treatment of the films in 1.0 M Ba(OH)2 solutions at 80° C resulted in the nucleation and growth of BaTiO3 within the polymer matrix. The volume fraction/connectivity of BaTiO3 was controlled by varying the relative amounts of titanium precursor and polymer in solution. Growth of BaTiO3 within the polymer was examined by infrared spectroscopy and electron microscopy. The dielectric constant of the composite films increased with BaTiO3 content.
Sol-Gel Routes towards Magnetic Nanocomposites with Tailored Microwave Absorption
Authors: Colomban Ph. , Vendange V.
Published: April 2011

Ceramic microwave absorbents are of considerable interest for the structural applications with the tailored radar cross-section. The main parameters governing the electromagnetic cross-section are discussed in this paper with the specific interest of the dispersion of Co, Fe (and their alloys) particles in a dielectric aluminosilicate matrix (high Curie temperature, significant absorption in the 0.1–15GHz range) and the advantages of the sol-gel routes for the materials preparation. Three methods have been examined: (i) the first one is the mixing of the (sub)micronic metal powder within a liquid matrix precursor; (ii) the second is the mixing of the alkoxides with an aqueous solution of metal ions; and (iii) the third is the preparation of a porous host matrix impregnated by a concentrated solution of transition metal nitrates. Nanocomposites with diameter 20–200nm were achieved by heating at temperatures between 600 and 1100°C under a H2 atmosphere. The samples were characterized by SEM, TEM, magnetic hysteresis, Mössbauer and Raman spectroscopies and microwave absorption.
Mechanical Anisotropy in Sheets of γ-TiAl Alloys
Authors: Bartels A. , Clemens H. , Hartig C. , Mecking H.
Published: April 2011

At room temperature sheets of γ-TiAl exhibit a higher yield stress in the rolling direction than in the transverse direction. Around 700°C the opposite behavior is observed. The texture mainly consists of a modified cube component. The tetragonal c-axis (001) is aligned in the sheet plane transversely to the rolling direction. Taken into account this special texture and the single crystal yield surface of γ-TiAl we conclude that around 700°C the CRSS of super-dislocations is higher than the CRSS of ordinary dislocations. At RT the relation changes to the opposite.
Influence of Microstructure Refinement on Strength, Ductility and Toughness of TiAl Alloys
Authors: Morris M. A. , Leboeuf M.
Published: April 2011

Different Ti-Al alloys with grain sizes ranging from 7μm to 320 nm have been prepared by consolidation of atomised and mechanically milled powders of nominal composition Ti-48Al-2Mn-2Nb and containing carbon additions. The materials obtained consist of equiaxed γ grains containing a distribution of α2 phase mostly along grain or powder boundaries. The high flow strength and low fracture toughness obtained from these materials have been correlated to the fine scale of the microstructure and the transfer of slip across adjacent γ grains. The existence of stress concentrations at boundaries between γ grains leads to emission of curved dislocations and twins. The low mobility of emitted dislocations is responsible for the lack of tensile ductility in these alloys which fail by microcrack formation at grain boundaries.
Hall-Petch and Multiple Linear Regression Equations for the Prediction of Mechanical Properties in Gamma-Based Titanium Aluminides
Authors: Soboyejo W. O. , Soboyejo A. B. O. , Ni Y. , Mercer C.
Published: April 2011

In a recent paper, Mercer and Soboyejo [1] demonstrated the Hall-Petch dependence of basic room- and elevated-temperature (815°C) mechanical properties (0.2% offset strength), ultimate tensile strength, plastic elongation to failure and fracture toughness) on the average equiaxed/lamellar grain size. Simple Hall-Petch behavior was shown to occur in a wide range of extruded duplex α2-γ alloys (Ti-48A1, Ti-48Al-1.4Mn Ti-48Al-2Mn and Ti-48Al-1.5Cr). As in steels and other materials [2–5], simple Hall-Petch equations with were derived for the above properties [1]. However, the Hall-Petch equations did not include the effect of other variables that can affect to the basic mechanical properties of gamma alloys. Multiple linear regression equations for the prediction of the combined effects of several (alloying, microstructure and temperature) variables on basic mechanical properties temperature are presented in this paper.
Processing, Properties and Applications of Gamma Titanium Aluminide Sheet and Foil Materials
Authors: Clemens H. , Glatz W. , Eberhardt N. , Martinz H.-P. , Knabl W.
Published: April 2011

Intermetallic γ-TiAl based alloys (”γ-alloys”) have a great potential to become important materials for advanced applications in aerospace, automotive and related industries. Research and development on γ-alloys have progressed significantly within the last decade. This research has led to a better understanding of the fundamental correlations between alloy composition and microstructure, processing behaviour and mechanical properties. This paper describes the progress in sheet rolling of γ-TiAl based alloys on industrial scale. Employing an advanced hot-rolling process sheets with lengths >1000 mm have been rolled. Furthermore, first results of foil rolling are presented. The mechanical properties of γ-TiAl sheet material with regard to processing route, alloy composition and microstructure are summarized and discussed. Sheet forming by means of superplastic forming and conventional metal forming techniques has successfully been conducted. Different joining techniques have been studied for γ-alloys including solid-state diffusion bonding. The oxidation resistance of γ-alloys is higher than that of Ti-alloys, however, for long-term applications at temperatures >700°C the need for reliable oxidation protective coatings is anticipated. Recent results of cyclic oxidation tests on coated γ-TiAl sheet are presented. Finally, the results of a stability test conducted on a γ-TiAl panel at 750°C are summarized.
Low Cycle Fatigue of FeAl (42 at. % Al) at Room Temperature
Authors: Hanes D. B. , Gibala R.
Published: April 2011

The monotonie mechanical behavior in tension and compression of FeAl has been well documented. However, very little work has been done on the cyclic deformation behavior of this material. In this work, the behavior of FeAl (42 at. % Al) under low cycle fatigue was studied, including the effects of test environments and surface coatings. It was found that the fatigue life of this alloy is limited by environmental embrittlement. This embrittlement process can be equally well prevented by deformation in an oxygen environment or by coating the alloy with a protective film. The type of film applied appears to have little effect. Similar results were seen in monotonie testing.
Mechanical Properties of Binary Ni3Al Single Crystals
Authors: Bonneville J. , Martin J.-L. , Spätig P. , Viguier B. , Matterstock B.
Published: April 2011

The mechanical properties of binary Ni76.6Al23.4 single crystalline specimens have been studied in compression test over a wide range of temperature (293–1100K). The resolved proof stress (τ0.2%) and the corresponding work-hardening rate (θ0.2%) have been measured as a function of temperature. In addition, a technique of repeated stress relaxations has been used to investigate the related microscopic activation volume (V0.2%) and to characterize the variation in the density of mobile dislocations that occurs during such transient tests. It is observed that not only τ0.2% exhibits an anomalous behavior with temperature but also θ0.2%, while the stress dependence of V0.2% may be considered as normal, i.e. V0.2% is a monotonie decreasing function of stress.
Ductility Response of Ni3Al-Zr-B Base Alloys with Ternary Elements to Strain Rate and High Temperature
Authors: Wang Yinmin , Lin Dongliang , Lin T. L. , Zhang Yun
Published: April 2011

The compressive ductilities of Ni3Al-Zr-B base alloys with sole addition of magnesium (0.02∼0.06wt.%) and combined addition of magnesium(0.02wt.%) and silicon(0.54∼1.08wt.%) respectively responding to strain rate rising from 10-4sec-1 to 10-1 sec-1 have been studied in a high temperature range of 1073∼1273K. The results show that the compressive strains at rupture(CSR) of the alloys have been greatly improved by sole addition of magnesium and the alloys with combined addition of magnesium and silicon reveal even higher CSR values, furthermore, at temperatures of 1073K and 1273K, the strain rate dependence of CSR reveals to be anomalous, i.e, the CSR value increases as the strain rate rises, and then declines until it surpasses the peak value, which is corresponded to the strain rate of 10-3 sec-1 and 10-3 sec-1 respectively.

The beneficial effect of magnesium and silicon exists in their competence of reducing strain rate sensitivity exponent values. The mechanisms of the anomalous ductilizing behavior in the Ni3Al as affected by ternary elements are discussed.
Modeling of the Dislocation Dynamics in Ni3Al and the Flow Stress Anomaly
Authors: Devincre B. , Veyssiere P. , Kubin L. , Saada Lem G.
Published: April 2011

Ni3Al single crystals are known to exhibit a flow stress anomaly between 200 and 800K. The purpose of our work is to examine such an anomaly by means of a simulation of the dislocation dynamics at a mesoscopic scale. The simulation basic rules are: i) the dislocation glide in {111} octahedral planes, ii) the conditions at which screw lines are locked and unlocked by the formation of Kear-Wilsdorf locks, iii) the mobility of jogs in the {100} cube plane. Our results suggest that two different temperature regimes occur in the domain of the anomaly. At low temperatures, the plastic flow is governed by kink bow-out, itself a function of the kink length. At high temperatures, the plastic flow is governed by the unlocking of the weakest Kear-Wilsdorf locks in the microstructure. These outcomes of the simulation are discussed in relation with the existing theoretical models of the flow stress anomaly.
Fabrication of NiAl Intermetallic from Dense Elemental Powder Blends VIA Solid State Reactions
Authors: Farber L. , Gotman I. , Gutmanas E. Y.
Published: April 2011

Dense NiAl intermetallic was synthesized from very fine elemental powders via solid state reactions. Homogeneous blends of micron size Ni and Al powders were consolidated to full density and heat treated in a 425–800°C temperature range. During heat treatment, formation of various intermediate intermetallics phases: NiAl3, Ni2A13, Ni3Al and NiAl was observed. The sequence and kinetics of these phase formations at different temperatures were studied employing X-ray diffraction analysis (XRD). A model for a description of synthesis reaction kinetics in Ni-Al blends was developed. Based on the obtained results, the synthesis of NiAl was performed in two stages : reactions in 425–550°C range with consumption of Al, followed by a reaction at up to 800°C. It allowed uncontrolled SHS (self propagating high temperature synthesis, resulting in the occurrence of liquid phases and in formation of reaction products in a very fast /explosive manner) to be avoid. The synthesis temperatures are considerably lower than those used currently in processing of NiAl.
Anomalous Temperature Dependence of Yield Stress and Work Hardening Coefficient of B2-Stabilized NiTi Alloys
Authors: Hosoda Hideki , Mishima Yoshinao , Suzuki Tomoo
Published: April 2011

Yield stress and work hardening coefficient of B2-stabilized NiTi alloys are investigated using compression tests. Compositions of NiTi alloys are based on Ni-49moJ.%Ti, to which Cr, Co and Al are chosen as ternary elements which reduce martensitic transformation temperatures of the B2 phase. Mechanical tests are carried out in liquid nitrogen at 77K, air at room temperature (R.T.) and in an argon atmosphere between 473K and 873K. Only at 77K, some alloys show characteristic stress-strain curves which indicate stress induced martensitic transformation (SIMT), but the others do not. Work hardening coefficient is found to be between 2 and 1 lGPa in all the test temperature range. The values are extremely high compared with Young's modulus of B2 NiTi. Yield stress and work hardening coefficient increase with test temperature between R.T. and about 650K in most alloys. The anomalous temperature dependence of mechanical properties is not related to SIMT but to precipitation hardening and/or anomalous dislocation motion similar to B2-type CoTi. Solution hardening by adding ternary elements is evaluated to be small for Cr and Co additions, and large for Al addition, depending on difference in atomic size of the ternary element with respect to Ni or Ti.
Microstructure of Thermally Crosslinkable Poly(Ethylene Terephthalate) (Pet-co-Xta) Benzocyclobutene Functionalized Copolymers
Authors: Foran Brendan J. , Pingel Elizabeth , Spilman Gary E. , Markoski Larry J. , Jiang Tao , Martin David C.
Published: April 2011

The microstructure and thermal properties of copolymers of polyethylene terephthalate (PET) containing a crosslinkable terephthalic acid, 1,2-dihydrocyc Iobutabenzene 3,6 dicarboxylic acid (XTA) are reported. Wide angle x-ray scattering (WAXS) show that the addition of XTA does not alter the PET crystal structure in copolymers at low XTA contents. However, the degree of crystallinity drops for higher XTA levels. WAXS profiles show that PET-co-XTA 50% is amorphous, and that PEXTA homopolymer has a different crystal structure. Thermal data from DSC and TGA show that crosslinking of the benzocyclobutene groups (∼350°C) occurs at temperatures between melting (∼250°C) and degradation (∼400°C), making it possible to melt process the copolymers into fibers before the onset of crosslinking. Limiting oxygen index (LOI) measurements show that increased oxygen concentrations are required to sustain a stable flame in PET-co-XTA copolymers; whereas unmodified PET had an LOI value of -18%, the copolymers had LOI values near 32%. Further, while unmodified PET melts and drips as it burns, XTA copolymers formed a stable char that inhibiting flame propagation. An increased char was observed in optical micrographs for XTA containing polymers, and crystalline domains were observed near the burn surface in transmission electron micrographs.
A Problem of Maya Metallurgy in the Medium Grijalva, Central Guatemala, and the Yucatan Peninsula: Copper-Lead Alloy
Authors: Torres Luis M. , Franco Francisca V. , Navarrete Carlos C.
Published: April 2011

The study of 4 metallic artifacts from the Medium Grijalva, two bells, a bipointed awl, and a needle, and one bell from Huehuetenango, Guatemala, were studied using of elemental analysis by flame atomic absorption spectroscopy (FAAS), metallographic examination and typological classification. Several problems arose, including the need to have a clearer definition of bell types, better knowledge of the technology of production of needles and awls, and problems with the Cu-Pb alloys, especially if a how high a lead content forms a Cu-Pb alloy solution.

Early analyses made of bells and copper artifacts from the Maya area, Western Mexico and Southwest USA shows variable but important amounts of lead. The Cu-Pb phase diagram shows a monotectic reaction where the metals have poorcapability of alloying or mixing, forming a eutectic at very low lead concentrations. Few metalographic studies of these alloys are reported in papers about historic metals. However, some ternary Cu-Pb alloys are found in recent literature. Does the low arsenic content as in the analysis of the Cu-PbMayan bells influence the properties of the alloys? The importance of the distribution of types of bells made of Cu-Pb could clarify possible cultural connections and trade among the Maya area, western Mexico and southwest United States. The possible introduction of metallurgy from and trade with South America makes this search of even more importance.
ICP-MS Isotopic Signatures of Lead Ceramic Glazes, Rio Grande Valley, New Mexico, 1315–1700
Authors: Pingitore Nicholas E. , Hill David , Villalobos Joshua , Leach Jeff , Peterson John A.
Published: April 2011

ICP-MS isotopie analysis of lead ceramic glazes suggests at least two sources were exploited by Ancestral Pueblo potters to obtain the lead raw material, presumably galena (PbS). Five Rio Grande lead glazeware sherds from the Sandia area and two found at Socorro share a common isotopie fingerprint. The temper of one of the Socorro sherds confirms an origin in the Sandia area; petrography of the temper of the second sherd does not tie to any known Socorro source rock. Two other glazeware sherds from Socorro have a distinctly different lead isotopie signature. A fifth Socorro glaze may be a mixture of the Sandia and Socorro lead source materials. The differences in lead isotopie signature thus accord well with mineralogical differences in the ceramic pastes. Lead isotopie signatures generated by ICP-MS analysis are a powerful new tool for grouping glazeware sherds, classifying individual samples, defining lead sources, and delineating trade routes.
In Situ Conservation of the Roman Mosaics at the Villa of Boscéaz (Orbe, Switzerland): Diagnosis of Risks Involved
Authors: Flatt R.J. , Girardet F.J. , Weidmann D.C.
Published: April 2011

In order to maintain the archaeological integrity of the site of Boscéaz, one of the most remarkable north of the Alps because of the number and quality of its roman mosaics, it is intended to maintain these in situ and on their original support. This mode of conservation implies many risks, mainly linked to the climatic variations and to the presence of water and salts in the ground and the mosaics. The high degree of damage observed on these mosaics discovered more than 150 years ago illustrates this fact.

The aim of this study has been to establish a diagnosis of the risks linked to the uncovering and the in situ conservation of a mosaic recently discovered (currently covered with protective materials). This diagnosis has been established on the basis of the measurement of the physical and chemical properties of the mosaic and the ground.

It turned out that the rising of salts from the ground to the mosaic would be the major problem and that consequently a dessalaision will have to be carried out. Concerning the choice of the climatic conditions for conservation, the critical zone in which the salts present crystallize was determined. The thermal dilatation of the materials appeared relatively small, but might become damageable in the long term because of a possible hysteresis. The hydrie dilatation turned out to be negligible.
Polymers in Preservation and Restoration: Two Examples
Authors: Merrill E.W.
Published: April 2011

Two ‘families’ of polymers applicable to preservation and restoration are discussed: (i) polysiloxane networks generated by platinum catalyzed addition of vinyl silane precursors to hydrogenosilane precursors; and (ii) acrylic compositions generated by redox initiated polymerization of acrylic monomers in the presence of fine spherical acrylic polymer particles. In both cases the end result is achieved at temperatures between ambient and 60°C, and with negligible volumetric shrinkage.
Photoexcited Carrier Effect on Imaging of Organic Molecules On Si(100)2×l Surfaces
Authors: Maeda Y , Matsumoto T , Kawaj T.
Published: April 2011

Photocurrent images of zinc-phthalocyanine (ZnPc) and coronene molecules adsorbed on Si(100)2×1 surface have been obtained by scanning tunneling microscopy. The ZnPc molecules are observed as bright images indicating large local density of state (LDOS) at the bias voltage. However, coronene molecules show wide variety of contrast in their images. This difference is the result of variations of LDOS on molecules which are attribute to the difference of energy gap between highest occupied and lowest unoccupied molecular orbitals
Atomic Force Microscopy Study of Stabilized Quasi-Amorphous Carbon in the Range of Thickness from 5 nm To 300 μm
Authors: Pollak Fred H. , Wan J. Z. , Dorfman Benjamin F.
Published: April 2011

A new hard sp2-sp3 carbon stabilized by non-stoichiometric silica SiOx has been fabricated which combines a graphite-like layered structure with diamond-like mechanical properties, thermal stability and a very low density (≈ 1.6 g/cm3). This substance, which we refer to a quasi-amorphous material (QUASAM), has considerable potential applications as either thin films or bulk material since it can be grown as thick as 300 μm. Not only is the fine surface structure of crucial importance for thin films, but also the steadiness of a smooth growth front is a key factor for bulk material synthesis. Using atomic force microscopy (AFM) we have undertaken study of the surfaces/interfaces of such material over a wide range of thicknesses (5 nm < tf < 300 μm), including two free standing films.
Surface atomie resolution microscopy of sulfated-zirconia and Fe/Mn promoted sulfated zirconia
Authors: Benaissa M. , Santiesteban J. G. , Diaz G. , José-Yacamán M.
Published: April 2011

By using high-resolution transmission electron microscopy (HRTEM), it was possible to elucidate edge-on views of the surface atomic structure of sulfated zirconia and Fe/Mn promoted sulfated-zirconia catalysts. The results indicate that the presence of sulfate groups and Fe/Mn species not only stabilizes the tetragonal zirconia phase, but also induces the formation of well-faceted small zirconia crystallites. The shape of the sulfated zirconia crystallites exhibits two types of surfaces: long smooth and relatively few short rough planes, with the long smooth (110) plane being clearly predominant. HRTEM images of this plane revealed the presence of an adsorbed sulfate layer. On the other hand, the presence of Fe and Mn species were apparent on the (001) and (110) surfaces of zirconia particles.
The Importance of the Fringing Field Surrounding a Tem Foil to the Quantification of Phase Contrast at a P-N Junction
Authors: Dunin-Borkowski R. E. , Saxton W. O.
Published: April 2011

The space charge contribution to the electrostatic potential both inside and outside a dielectric slab containing a p-n junction is calculated using classical electrostatics, with particular reference to the use of phase contrast techniques in transmission electron microscopy for the characterisation of the carrier distributions present at such layers.
Lattice Disorder Effects on The Vacancy-Oxygen Centre in Ion-Irradiated Silicon
Authors: Keskitalo N. , Hallén A. , Lalita J. , Svensson B. G.
Published: April 2011

The Vacancy-Oxygen (VO) centre is one of the most prominent defects appearing in silicon after irradiation with energetic particles and gamma rays. It is formed when migrating vacancies are trapped by interstitial oxygen atoms. It gives rise to a deep level in the upper part of the silicon bandgap at 0.164 eV below the conduction band edge that can be conveniently studied by deep level transient spectroscopy (DLTS). It is furthermore a very important defect from a technological point of view since it normally controls the charge carrier lifetimes in silicon power devices at high injection levels.

Various doped n-type float zone Si samples have been irradiated with MeV ions at low doses. DLTS measurements of the charge carrier capture kinetics reveal lower capture rates if heavier ions, i.e. larger collision cascades, have been used to generate the defects. This effect can be interpreted as a local lowering of the Fermi level following lattice disorder in regions with a high elastic energy deposition. It will be shown that the VO is also very sensitive to lattice disorder in a similar way as previously reported for the divacancy centre.

In the DLTS spectra another defect, originating from an interstitial-carbon—substitutional-carbon pair (Cj-Cs), overlaps with the VO-peak. The Ci-Cs pair has been observed to posses bistable properties and in this paper we will further elucidate the various contributions from the two defects to the Ec-0.164 eV peak. In particular, the temperature dependence of the electron capture cross section of the Cj-Cs level will be discussed.
Fundamental Modeling of Transient Enhanced Diffusion through Extended Defect Evolution
Authors: Gencer A. H. , Chakravarthi S. , Clejan I. , Dunham S. T.
Published: April 2011

Prediction of transient enhanced diffusion (TED) requires modeling of extended defects of many types, such as {311} defects, dislocation loops, boron-interstitial clusters, arsenic precipitates, etc. These extended defects not only form individually, but they also interact with each other through changes in point defect and solute concentrations. We have developed a fundamental model which can account for the behavior of a broad range of extended defects, as well as their interactions with each other. We have successfully applied and parameterized our model to a range of systems and conditions, some of which are presented in this paper.
Influence of The Local Microstructure on The Macroscopic Properties of Si1–x–yGexCy, Alloys
Authors: Windl W. , Kress J. D. , Voter A. F. , Menéndez J. , Sankey O. F.
Published: April 2011

We performed a theoretical study of the microscopic arrangement of the C atoms in Si1–x–yGexCy, alloys using the Sankey local-orbital density-functional formalism. Our first-principles calculations show that in the dilute limit, the lattice constant in Si1–yCy, alloys decreases much more (by about 2/3) with concentration than predicted by Vegard's law. We show thatthis result is consistent with Raman and infrared experiments and with previous empiricalcalculations. As the C concentration increases, the interactions between the C atoms become important: Interstitial C becomes more abundant and increases the lattice constant, and the substitutional C atoms array under appropriate growth conditions. The effect of this ordering on the lattice constant is small, but it can be seen in the Raman spectra of layer-by-layer grown samples.
The Influence of Cavities and Point Defects on Cu Gettering and B'Diffusion in Si
Authors: Wong-Leung J. , Williams J. S. , Petravić M.
Published: April 2011

Cavities, formed in Si by hydrogen implantation and subsequent annealing, can provide ideal gettering sites for metal impurities. In this study, we have observed large differences in the accumulation of Cu at cavities depending on whether Cu was introduced into Si during cavity formation or into wafers with pre-formed cavities. The observed behaviour is consistent with a high flux of Si interstitials emitted during cavity formation which induce the dissolution of Cu3Si and the enhanced transport of Cu to cavities. In further studies, boron implantation was carried out into wafers containing pre-formed cavities and transient enhanced diffusion (TED) of boron was suppressed duringsubsequent annealing.
Atomistic Mechanisms of Dislocation Mobility in Silicon
Authors: Justo J. F. , Bulatov V. V. , Yip S.
Published: April 2011

We study the leading mechanisms of kink mobility of 30° and 90°-partial dislocations in the glide set {111} of silicon. The calculations are performed using a new empirical potential for Si, which has been shown to give an accurate description of core properties of partial dislocations [1], to study mechanisms of kink formation and migration. In the case of 30°-partial dislocation, two kinds of kinks are identified, left and right, which have different structures, formation energies, and mobilities.
Extracting the Grain Boundary Character/Free Energy Relationship from the Microstructure: Pure <100> and <111> Tilt Boundaries
Authors: Adams B. , Kinderlehrer D. , Ta'asan S. , Mullins W.W. , Costiner S.
Published: April 2011

Analysis is described to extract the excess free energy of grain boundaries from the mesotexture of well-equilibrated polycrystalline samples. The approach is based upon the force and torque balances at triple junctions described in the classical work of Herring [1]. The main advantage of the approach is that the free energy function is obtained over the full fundamental zone of grain boundary types. For the purposes of exposition, the method is described for specified two-parameter hypersurfaces in the fundamental zone for cubic polycrystals (the sets of pure <100> and <111> tilt boundaries) and the free energy is assumed to depend only upon the lattice misorientation. Results from testbed simulations are presented.