bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultra-Thin Carbon Coatings for Head-Disk Interface Tribology
Authors: Glosli J. N. , Belak J. , Philpott M. R.
Published: April 2011

Molecular dynamics computer simulations of the growth processes and microstructural properties of amorphous carbon (a:C) and amorphous hydrogenated carbon (a:CH) ultra-thin films have been performed. Films 1 to 10 nm thick were grown on a diamond (100) surface using Brenner’s[1–2] bond-order potential for hydrocarbons. The stoichiometry, radial distribution function, chemical bonding (amount of sp2 and sp3 hybridization) and residual stress are presented.
Wear and Adhesion of thin Carbon Coatings
Authors: Wang Hsin-Fu , Nelson John C. , Lin Chien-Li , Hoehn Joel W. , Gerberich William W.
Published: April 2011

The ability of a hard coating to resist delamination in wear applications is found to depend on the substrate crystal orientation. A variety of alloys with and without a carbon coating were tested by microscratching with a diamond stylus. The hardness of the carbon coating was characterized by nanoindentation techniques before the microscratch tests. As the diamond passed from one crystal to another differing amounts of damage were observed for each crystal orientation. The width of the delaminated region was found to be dependent on the extent of the slip line fields in the substrate generated by the scratching. To eliminate the possibility of differing adhesion on different crystal faces, a nickel aluminide intermetallic with the two crystals rotated about the same pole was tested. In the uncoated state when scratched across the grain boundary the slip line fields were extensive for one crystal but limited for the other crystal. Similar fixed-load scratches on the carbon coating produced delamination on the easy slip orientation but only a slight deformation of the coating on the other crystal. The observed failure appears to be due to the deformation of the substrate and not inferior adhesion of the coating to the crystal face. This finding should have important ramifications in the design and failure analysis of systems, such as magnetic recording media, medical implants, etc.
Thin Film Adhesion Characterization by Microwedge Scratching of Precracked Fine Lines
Authors: de Boer M. P. , Huang H. , Gerberich W. W.
Published: April 2011

A new procedure for obtaining accurate work of adhesion values for stiff films as thin as 0.5 µm or less is presented in this paper. The technique is based on fabrication of a two-dimensional test structure using a photolithographic alignment sequence. With precracks of variable length at the end of a fine line, a well known stress state is imposed before delamination. The test structure allows simple analytical equations to be applied to approximate the strain energy release rate, G. Finite element method is used to obtain more accurate values of G as well as of mode mixity. For short crack lengths, no buckling occurs, and the phase angle is near mode II. Kinking into the glass substrate is observed when the measured critical strain energy release rate, Gc, is greater than glass, but not when Gc is less than glass. When the precrack is lengthened, buckling may occur before delamination. In this case the phase angle has a large mode I component, and the crack remains in the interface even in the case of the strong interface.
Nonhydrostatic Stress Effects on Solid Phase Epitaxial Growth in Silicon
Authors: Carter William B. , Aziz Michael J.
Published: April 2011

The dependence of solid phase epitaxial growth in Si on uniaxial compression applied perpendicular to the amorphous-crystal interface is investigated. Long, thin pure Si bars of square cross section are ion-implanted to produce amorphous layers on the end faces. The bars are placed end-to-end and uniaxially loaded at temperature to partially regrow the amorphous layers. The resulting growth rates are measured ex situ by re-heating the samples on a hot stage and using time-resolved reflectivity to deduce interface depths. Preliminary results are that uniaxial compression is more effective than hydrostatic pressure for enhancing the growth rate, in qualitative but not quantitative agreement with previously made predictions.
Xps Study of Ni-Fe Manganite Thermistor Material
Authors: Braski D. N. , Osborne N. R. , Zurbuchen J. M.
Published: April 2011

The resistivity of the as-fabricated thermistor material, nickel-iron-manganite, changes during initial aging in the temperature range of 150-300ºC before becoming stable.X-ray photoelectron spectroscopy (XPS) was used to determine if any valency change or chemical shift of the cations or oxygen occurred during aging. The goal of the study was to identify any ionic changes that might affect thermistor stability. The only observed changes in 2p3/2 peaks due to aging were those related to Ni ions; the same peaks for Mn, Fe, and the 0-Is peak were unchanged. The changes in the Ni 2p3/2 peak may possibly be related to: (a) the migration of Ni2+ ions from octahedral to tetrahedral sites, (b) subtle changes in the energy states of Ni2+ which promoted a more stable ionic structure, or (c) the presence of Ni3+ ions, some of which revert to Ni2s+.
Compatibility of BaTiO3 and SrTiO3 with Shape Memory Alloy TiNi
Authors: Mercado Peter G. , Jardine A.Peter
Published: April 2011

Thin film layers of shape memory alloys and ferroelectric ceramics can produce a family of ‘smart’ heterostructures capable of performing both sensing and actuating functions. The compatibility of sol-gel processed BaTiO3 (BTO) and SrTiO3 (STO) thin films with shape memory effect TiNi substrates were investigated. Important issues in the synthesis of these active structures are the ability to generate the appropriate crystalline phases of each material while producing defect-free homogeneous high quality films.
Porous TiNi Synthesis from Elemental Powders
Authors: Hey Janet C. , Jardine A. Peter
Published: April 2011

The shape memory alloy TiNi is produced by either Vacuum Induction Melting (VIM) or by Vacuum Arc Remelting (VAR) of pure metal ingots for actuator applications. Powder metallurgy techniques provide an alternative fabrication route for their use in passive damping applications where porosity and deviations in chemical homogeneity can be tolerated. In this study TiNi compacts were cold pressed from the blended elemental powders and sintered in vacuum for varying times at temperatures from 800°C to 1000°C. Two heating rates were used, 5 K/min. and 10 K/min. A TiNi microstructure could be produced after annealing at 1000°C for 6 h., although some TiNi3 was still observed. Sintering above 900°C produced porous microstructures with green densities approaching 3.5 gm/cm3.
The Dynamics of SAW Streaming and its Application to Fluid Devices
Authors: Shiokawa Showko , Matsui Yoshikazu
Published: April 2011

Various liquid motions such as streaming and droplet formation on the surface of surface acoustic wave (SAW) propagating substrate are called SAW streaming. In this paper, the liquid dynamic motions excited by the SAW fabricated on 128° rot. Y-X LiNb03 at the frequency of 50 MHz with various pulse widths are demonstrated. The SAW streaming system are also tested for use as a micropump. Theoretically, SAW streaming force can be derived based on the acoustic streaming theory. The calculated SAW streaming force is found to be very large and strong enough to expel a droplet from a liquid surface.

Nonlinear behavior in a water drop vibration induced by the SAW streaming force is also experimentally investigated as a model of inverse bubble oscillation. We succeed in detecting the transformation of the water drop vibration with a single pulse frequency spectrum to 1/2 and its harmonics.
The Effects of Processing Variables on Reaction Synthesis of Fe-A1 Alloys
Authors: Joslin D. L. , Easton D. S. , Liu C. T. , David S. A.
Published: April 2011

The effects of alloy composition and reaction atmosphere on reaction synthesis of binary Fe-Al alloys were studied. Reactions were observed in an open (air) furnace, under static vacuum (in an evacuated quartz tube) and in a dynamic vacuum furnace. High-speed videotapes of reaction syntheses of compacts formed from 45-μm Fe and 10-μm Al powders reacted in air and under static vacuum revealed that an unusual “two-stage” reaction exists in this system under these conditions. The first stage of the two-stage reaction lasts several seconds and starts at around 650°C. The second stage begins at about 900°C, reaching temperatures between 1250 and 1350°C. The progress of the reaction to the second stage is sensitive to the alloy composition and reaction environment. The reaction behavior is explained in terms of thermodynamics and heat transfer, which control the delicate balance between heat accumulation and heat loss during reaction synthesis.
High Temperature Mechanical Behavior of Nb5Si3/Nb Laminates
Authors: Provancher William , Ghosh Amit K.
Published: April 2011

Two phase alloys based on the intermetallic system Nb5Si3/Nb system provide potentially superior combination of strength and toughness over their monolithic counterparts at high temperatures. To gain fundamental knowledge of the mechanical behavior of these materials, initial focus has been on the mechanical behavior of Nb5Si3/Nb laminates and composites at elevated temperatures. In this study, laminates have been fabricated at 32 at% Si with Nb, and a total laminate thickness of 2.1mm. Tests are being conducted on notched and unnotched tensile specimens. The unnotched specimens provide the overall strength and ductility of the laminate and a basis for comparison with notched specimens which provide information on crack propagation and ductile phase toughening effects. Results from samples with a variety of laminate spacings are being explored, and preliminary data will be presented.
Chemisorption of Moisture-Induced Hydrogen and the Embrittlement Effects in Transition Metal Aluminides
Authors: Wang Jian-Sheng
Published: April 2011

Based on simple thermodynamic models free energies of chemisorption of water vapor in transition metal aluminides and enthalpies and entropies of formation of solid solutions of hydrogen in the aluminides are derived. Susceptibilities of the aluminides to moisture-induced embrittlement are evaluated with these thermodynamic properties and compared with the susceptibilities to hydrogen-induced embrittlement.
Deformation of C15 Laves Phase Alloys
Authors: Chu F. , Pope D.P
Published: April 2011

Details of the structure and previous work on the deformation of C15 Laves phases are reviewed. The phase diagram of the Hf-V-Nb system, some metallurgical and physical properties, mechanical behavior, and the deformation mechanisms of HfV2+Nb (C15 HfV2+Nb and V-rich bcc solution) are presented based on our previous work. Theoretical approaches to understanding the results of these studies are discussed.
Dislocation Dynamics and Yield Stress Anomalies in Intermetallic Alloys
Authors: Caillard Daniel
Published: April 2011

Many intermetallic alloys with L12, L10and BCC related structures exhibit yield-stress anomalies, which have strong influence on their mechanical properties at intermediate temperatures. A short review is made of the anomalous glide systems in various intermetallics. They are shown to exhibit several common characteristics, which allow it to classify them into three groups with possibly similar dislocation mechanisms. In the second half, the well-known case of octahedral glide in Ni3Al is considered again, and a new model is proposed, based on the formation and the unlocking process of incomplete Kear-Wilsdorf locks by a double cross-slip mechanism. This model is supported by several microscopic and macroscopic observations including “in situ” straining experiments in a TEM. It accounts for the positive temperature dependence of the yield-stress with fairly good orders-of-magnitude estimates of the stress. It also explains satisfactorily the small stress-strain rate sensitivity and the transient effects.
Reaction Process of αəγ Massive Transformation in Ti-Rich TiAl Alloy
Authors: Kumagai T. , Abe E. , Takeyama M. , Nakamura M.
Published: April 2011

Reaction sequence of the massive transformation from the high-temperature α-Ti phase to the γ-TiAl phase (αm) in a Ti-48at.% Al alloy has been examined in terms of optical and transmission electron microscopes. Both transformed and untransformed regions were macroscopically observed in the sample quenched from the high-temperature α phase field, when the sample was held there for a extended period of time prior to quenching. The transformed region consists of randomly oriented fine α single phase grains, in which many thermal anti-phase domains (TAPDs), together with a number of stacking faults were observed. In contrast, the untransformed region comprises extremely fine lamellae of the α and α2-Ti3Al phases, and the α plates were found to run through the TAPDs caused by α ə α2 ordering. Subsequent aging at 1273K causes the microstructure change in the untransformed region from α2/γ lamellae to γ/γ lamellae spontaneously and expands the γm region. These observations suggest that the α ə γm transformation proceeds through formation of fine γ plates.
1200 K Compressive Properties of N-Containing Nial
Authors: Whittenberger J. Daniel , Noebe R. D. , Wheeler D. R.
Published: April 2011

As part of a series of experiments to understand the role of N on the strength of NiAl, a heat of NiAl was enriched with N by melting and atomization to powder in a nitrogen atmosphere. Following consolidation of the powder by hot extrusion, 1200 K compressive properties were measured in air. Within the range of strain rates examined, 10−3 to 10−9 s−1, the strength of the N-enriched NiAl was greater than that of a simple 15 µm grain size polycrystalline, binary NiAl alloy. For the most part the overall improvement in strength is ascribed to the fine grain size of the N-doped NiAl rather than the alloy chemistry; however, the alloy displayed a complex behavior exhibiting both weakening effects as well as strengthening ones.
Gamma Titanium Aluminide Alloys
Authors: Yamaguchi M. , Inui H. , Kishida K. , Matsumoro M. , Shirai Y.
Published: April 2011

Extensive progress and improvements have been made in the science and technology of gamma titanium aluminide alloys within the last decade. In particular, our understanding of their microstructural characteristics and property/microstructurc relationships has been substantially deepened. Based on these achievements, various engineering two-phase gamma alloys have been developed and their mechanical and chemical properties have been assessed. Aircraft and automotive industries arc pursuing their introduction for various structural components. At the same time, recent basic studies on the mechanical properties of two-phase gamma alloys, in particular with a controlled lamellar structure have provided a considerable amount of fundamental information on the deformation and fracture mechanisms of the two-phase gamma alloys. The results of such basic studies are incorporated in the recent alloy and microstructure design of two-phase gamma alloys. In this paper, such recent advances in the research and development of the two-phase gamma alloys and industrial involvement are summarized.
Strain Hardening by High Density SISFs During Low Cycle Fatigue in Ni3(Al,Zr) Single Crystal
Authors: Gu Yuefeng , Liu Yi , Guo Jianting , Lin Dongliang
Published: April 2011

Stress response and its correlation with dislocation substructures in Ni3(Al,Zr) single crystal fatigued at room temperature have been studied. Cyclic strain hardening was found to be asymmetric and increased with increasing applied cyclic strain. Transmission Electron Microscope (TEM) observation showed that there are a profusion superlattic intrinsic stacking faults (SISFs) in fatigued Ni3(Al,Zr) single crystal samples. The cyclic strain hardening and stress asymmetry are explained by the movement of the SISF.
In-SituHeating Observations on Superlattice Dislocation in A Ni3(Al, Zr) Single Crystal
Authors: Liu Yi , Gu Yuefeng , Lin Dongliang , Chen Shipu , Zhao Xiaoning , Hong Jianming
Published: April 2011

The TEM weak-beam technique has been used to investigate the behavior of dissociated superlattice dislocation in Ni3Al single crystal as a function of temperature. The observed dislocation with the Burgers vector of [110] partly dissociated on the (001) plane forming Kear-Wilsdorf (KW) lock. The dissociated pair did not indicate significant variation of separation in the temperature range from room temperature to 773K. but turned to form a jog at 773K. At 898K, which is near the peak temperature, the dissociated segment constricted completely. The experimental observations are discussed.
Formation and Growth of Antiphase Domains During Recrystallization of Cold-Rolled Cu3Au
Authors: Yang Rui , Cahn Robert W.
Published: April 2011

An experimental study by TEM was made of the morphology of the antiphase domains formed when heavily rolled Q13AU is annealed at a temperature slightly below the critical temperature for ordering, Tc. Domains are formed at the advancing grain boundary with extremely small size and grow as recrystallization proceeds. From an early stage, domain walls show a preference for (100) orientation. The key question is raised whether domain formation during recrystallization entails the presence of a disordered zone at a moving grain boundary near Tc, and the conclusion is that such a zone is probably present. A provisional theory is constructed for the genesis of domains during recrystallization, taking into account the dragging force which newly formed domains exert on a moving grain boundary thereby diminishing the effective driving force for grain boundary motion, and a critical domain size is estimated which should completely inhibit grain-boundary motion. The intriguing fact that no domains at all are formed during the recrystallization of strongly ordered intermetallics such as Ni3Al is briefly discussed and a reason is proposed.
Determination of Site-Occupancies in Aluminide Intermetallics by Alchemi
Authors: Anderson I. M. , Duncan A. J. , Bentley J.
Published: April 2011

The site-distributions of Fe in four B2-ordered NiAl-based alloys with Fe concentrations of 10%, 2%, and 0.5% have been determined by ALCHEMI (atom-location by channeling-enhanced microanalysis). Site-distributions have been extracted with standard errors between ∼1.5% (10% Fe concentration) and ∼6% (0.5% Fe concentration). The results show that Fe has no strong site-preference in NiAl and tends to reside on the site of the stoichiometrically deficient host element.

An improved ALCHEMI analysis procedure is outlined. The analysis explicidy addresses the phenomenon of ionization delocalization, which previously complicated the determination of site-distributions in aluminide intermetallics, leading to inaccurate and oftentimes nonphysical results. The improved ALCHEMI analysis also addresses the presence of anti-site defects. The data acquisition conditions have been optimized to minimize the sources of statistical and systematic error. This optimized procedure should be suitable for all analyses of B2-ordered alloys. Several analyses at different channeling orientations show that the extracted site-occupancies are robust as long as the data are acquired at orientations that are remote from any major pole of the crystal.
Normal and Anomalous Yield Stresses in a Tial Single Crystal and the Dominating Dislocation Mechanisms.
Authors: Bird N. , Taylor G. , Sun Y. Q.
Published: April 2011

Single crystal γ-TiAl with axial orientation [3 16 15] has been tested in compression between 4K and 1048K and the dislocation structures observed in TEM. The slip plane was found to be (111) over the entire temperature range tested. Three regimes exist in the variation of the yield stress with temperature, whereas the dislocation substructures are of two types, dominated by 30° 1/3[112] and 1/2 < 110] dislocations respectively. The anomalous yield stress is associated with 1/2 < 110] dislocations undergoing frequent cross-slip off the (111) plane.
Strain Rate Dependence of the Flow Stress and Work-Hardening of Single Crystals of NI3(Al,Hf)B
Authors: Ezz S. S. , Sun Y. Q. , Hirsch P. B.
Published: April 2011

The strain rate sensitivity ß of the flow stress τ is associated with workhardening and β=(δτ/δln ε) is proportional to the workhardening increment τh = τ - τy, where τy is the strain rate independent yield stress. The temperature dependence of β/τh reflects changes in the rate controlling mechanism. At intermediate and high temperatures, the hardening correlates with the density of [101] dislocations on (010). The nature of the local obstacles at room temperature is not established.
Dislocation Annihilation in L12 Alloys
Authors: Shi Xiaoli , Saada Georges , Lem Patrick VeyssiÈre
Published: April 2011

A transmission electron microscope (TEM) study of dislocation reactions that take place during the first few percents of permanent strain at room temperature is presented. The nature of the dipolar segments, a noticeable feature within the deformation microstructure, is elucidated. It is determined that antiphase boundary (APB) tube formation is unlikely to stem from the annihilation between a mobile superdislocation and an immobilized Kear-Wilsdorf (KW) configuration, at variance from what has been expected so far. A clear relationship between APB tubes and superlattice stacking fault (SSF) dipoles is pointed out.
Temperature Dependence of Dislocation Mobility in Ni3Al
Authors: Nadgorny Edward M. , Iunin Yu. L.
Published: April 2011

The stress and temperature dependence of the dislocation velocity v(σ, T) has been studied experimentally in Ni3AI single crystals. The mobility of individual dislocations was measured by the stress pulse-etching technique at temperatures between 77 and 873 K at the resolved shear stresses between 25 and 150 MPa over a range of dislocation velocities from 3×l0−7 to 3×10−5 m/s. Fresh dislocations were introduced and revealed by a selective etchant on {001} surfaces of electropolished specimens annealed at 1473 K for 120 hours in argon. The specimens were deformed in three point bending around <110> bending axis at a constant temperature. The dislocation arrays moved in two acting <110> {111} primary glide systems. The motion occurred over a range of applied stresses close to the macroscopical yield stresses y The dislocation velocity decreased as temperature increased, in perfect analogy to the experimental data on y obtained before 1 on the same Ni3Al.. No detectable violation of the Schmid law was observed. The stress dependence of the dislocation velocity was rather steep resulting in a relatively large activation area of A=100-200b2. Such an activation area is greater than that one could expect in a kink-mode crystal with predominant Peierls mechanism for dislocation motion, although it is considerably less than A measured in other materials controlled by local obstacles.
Microstructure/Property Studies Supporting Development of Low Cost Processes for TiAl Automotive Valves
Authors: Jones P. E. , Porter W. J. , Keller M. M. , Eylon D.
Published: April 2011

Lightweight gamma titanium aluminide automotive valves may significantly improve engine performance and fuel economy. However, low cost mass production processes must be developed to realize these gains. Permanent mold casting processes and low cost heat treatments are being developed to enable commercial introduction of TiAl automotive valves. Microstructural development, tensile, creep, and high cycle fatigue properties of permanent mold Ti-47Al-2Nb-1.75Cr (at%) castings are compared with investment castings of similar size. A recommendation is made to use as-HIP castings for components not designed to damage tolerant criteria. This work was performed as part of a vertically integrated team effort to transfer aerospace TiAl technology to the automotive market.
Growth and Facet Orientations in Single-Phase TiAl Single Crystals
Authors: Mahapatra R. , Lin H. , Pope D. P. , Chou Y. T.
Published: April 2011

Single crystals of single-phase TiAl alloy (Ti-56 at. % Al) have been grown at lOmm/h solidification rate in an ASGAL FZ-SS35W Optical Floating Zone Furnace. The orientations of the resulting single crystals have been determined using the Laue X-ray diffraction, and Electron Backscattering Pattern (EBSP) methods. A correlation between orientations of the crystal growth and grown-in facets has been established.
Pressureless Sintering of MoSi2 Materials
Authors: Scholl Roland , JÜngling Thomas , Kieback Bernd
Published: April 2011

Various powder mixtures were prepared by a modified mechanical alloying technique. Starting from elemental Mo-, Si- and C-powders the influence of milling conditions on phase formation during the milling process and the subsequent heat treatment was investigated. Phase formation during sintering and sintering kinetics of activated starting mixtures were studied by differential scanning calorimetry (DSC), thermal graphimetry (TG), X-ray diffraction (XRD) and dilatometry. The results show that phase formation during milling or sintering strongly depends on milling conditions. Optimized powder mixtures of single phase and reinforced molybdenum silicides show high densities up to 98,5 % TD by pressureless sintering in various atmospheres. Full density is possible by post-HIP because the samples show only closed porosity. The microstructure was studied in dependence of sintering parameters. The level of impurities, i.e. C, O2 was determined. Hardness, fracture toughness and bending strength were measured for single phase and particle reinforced materials.
Properties of SiC-SiC Composites Produced Using CVR Converted Graphite Cloth to SiC Cloth
Authors: Kowbel W. , Kyriacou C. , Gao F. , Bruce C. A. , Withers J. C.
Published: April 2011

Nicalon fiber is the primary reinforcement in SiC-SiC composites currently produced by a variety of techniques including CVI and polymer infiltration. Low strength retention at high temperatures of the Nicalon fibers limits the choice of manufacturing processes which can be employed to produce low cost SiC-SiC composites. MER has developed a new SiC reinforcement based upon a conversion of low cost carbon fabric to SiC via a Chemical Vapor Reaction (CVR) process. This new SiC filaments exhibit an excellent creep resistance at temperatures up to 1600°C. Several SiC-SiC composites were fabricated using graphite fabric converted to SiC fabric utilizing the CVR process combined with a slurry infiltration and CVI densification. A correlation between processing conditions, microstructure and properties of the SiC-SiC composites are discussed in detail.
Fiber Twist Test: A New Test Technique to Measure Composite Interface Properties
Authors: Chepolis William M. , Ertürk Turgay
Published: April 2011

This paper discusses the work accomplished, thus far, in the development of the Fiber Twist Test (FTT) as an alternative and improved technique for testing the interface properties of continuous fiber composite materials. The unique features of the fiber twist test include the absence of Poisson effects during testing. This eliminates the nonlinear variation, with changing embedded fiber lengths, of fracture modes and accompanying debond energy, and of friction along the interface. Since the fiber is not removed from its surrounding matrix during testing, the interface area remains constant. This facilitates a more accurate measurement of friction properties at the interface.
Rapid Processing of Carbon-Carbon Composites by Forced Flow-Thermal Gradient Chemical Vapor Infiltration (FCVI)
Authors: Vaidyaraman S. , Lackey W. J. , Agrawal P. K. , Freeman G. B. , Langman M. D.
Published: April 2011

Carbon fiber-carbon matrix composites were fabricated using the forced flow-thermal gradient chemical vapor infiltration (FCVI) process. Preforms were prepared by stacking 40 layers of plain weave carbon cloth in a graphite holder. The preforms were infiltrated using propylene, propane, and methane. The present work showed that the FCVI process is well suited for fabricating carboncarbon composites; without optimization of the process, we have achieved uniform and thorough densification. Composites with porosities as low as 7% were fabricated in 8–12 h. The highest deposition rate obtained in the present study was ∼3 μm/h which is more than an order of magnitude faster than the typical value of 0.1–0.25 μm/h for the isothermal process. It was also found that the use of propylene and propane as reagents resulted in faster infiltration compared to methane.
Fiber Coatings and the Fracture Behavior of a Continuous Fiber Ceramic Composite
Authors: Miller J. H. , Lowden R. A. , Liaw P. K.
Published: April 2011

Fiber coatings have been used to modify fiber-matrix interfacial forces, and thus control mechanical properties of continuous fiber ceramic composites. It has been shown that the properties and thickness of the interlayer influence composite properties such as matrix cracking and ultimate strength, toughness and interlaminar shear. The effects of fiber coating properties and thickness on fiber-reinforced SiC matrix composites fabricated employing CVI techniques have been examined. Correlations between interface condition, mechanical properties and failure mechanisms have been made.
Elevated Temperature Static Fatigue of a Nicalon Fiber-Reinforced SiC Composite
Authors: Lin Hua-Tay , Becher Paul F. , Tortorelli Peter F.
Published: April 2011

Static fatigue tests of a Nicalon fiber-reinforced SiC matrix composite were conducted in four-point bending over a temperature range of 425° to 1150°C in air at selected stress levels. The composite consisted of a Nicalon cloth with a 0.3 μm graphite interfacial coating and a Forced Chemical Vapor Infiltration (FCVI) SiC matrix composite; samples were tested with or without a final protective SiC seal coat. The results indicated that the fatigue life of the Nicalon-SiC composite decreased with an increase in either applied stresses or test temperatures. However, the composite exhibited a fatigue limit of ∼ 100 MPa at temperatures ≤ 950°C which decreased to ∼ 70 MPa at 1150°C. Both electron microscopy and thermogravimetric studies suggested that the lifetime of the composites was dictated by the oxidation of graphite interfacial layer at temperatures ≤700°C and by oxidation of graphite coating accompanied by formation of silicate interfacial layer via oxidation of the Nicalon fiber (and the SiC matrix) at temperatures ≥ 950°C. Use of a SiC seal coat effectively retarded the oxidation reactions and increased the lifetime by at least one order of magnitude at 425°C. On the other hand, the SiC seal coat made little (if any) difference in fatigue life at 950 °C.
Reaction Synthesis of MoSi2-Al2O3 Composite Using MoO3, Al and Si Powders
Authors: Deevi Seetharama C. , Deevi Sarojini
Published: April 2011

In-situ synthesis of a composite of MoSi2-Al2O3 was carried out by reacting a thermite mixture consisting of MoO3, Al, and Si powders. The reaction was found to be extremely fast and violent, and a diluent was required to moderate the reaction. Thermal behavior of the thermite mixture was studied using DTA at different heating rates, and DTA was interrupted at different temperatures to determine the reaction mechanism. X-ray characterization of the products obtained at different temperatures reveals that the mechanism consists of a reduction of MoO3 by Al to MoO2 followed by a simultaneous oxidation of Al to Al2O3 and synthesis reaction between reduced Mo and Si to form MoSi2. The rate determining step is found to be reduction of MoO2 by Al and oxidation of Al to Al2O3. The thermite reaction was moderated by adding Mo and Si to the mixture of MoO3, Al, and Si such that the ratio of MoSi2 to the thermite was in the range of 60:40 to 90:10.
Radiation Enhanced Diffusion in Mgo
Authors: Van Sambeek Andrew I. , Averback R.S. , Flynn C.P. , Yang M.H.
Published: April 2011

Measurements of marker spreading following 2.0 MeV Kr+irradiation at 25 to 1300°C have been performed on MgO samples containing O18, Ca and Zn buried tracer atoms. Ion beam mixing at room temperature on both sublattices was approximately 2.0 to 3.0 A5/eV. From 600 to 1000°C, the apparent activation enthalpy for diffusion on the anion sublattice (O18) was 0.35 eV and the diffusion coefficient was linear in the irradiation flux. From 1150 to 1300°C the measured activation enthalpy was 4.1 eV and the diffusion coefficient was proportional to the square root of flux. The measured activation enthalpy on the cation sublattice was roughly 0.30 eV up to 700°C for both Ca or Zn doped samples. Measurable extrinsic thermal diffusion from vacancies present for trivalent impurity charge compensation occurred above this temperature, complicating the analysis at higher temperatures. The observed kinetics in the lower temperature range are most likely controlled by interstitial loop formation. In the higher temperature range, vacancy traps with a binding energy of approximately 2 eV could account for the high activation enthalpy.
Electron Beam Irradiated Effects of SiO2 in Stem
Authors: Lee Tan-Chen , Silcox John
Published: April 2011

Electron beam irradiation effects in SiO2 have been studied by STEM (Scanning Transmission Electron Microscopy). Oxygen loss and the corresponding transformation from SiO2 to Si in SiO2 are confirmed and consistent with previous reports 1,2. A “flower-like” Si rich area, which might not be observed in STEM BF (Bright Field) or ADF (Annular Dark Field) images, was found in Si plasmon energy filtered images. Quantification of the observations and the formation mechanisms leading to the Si-rich area are discussed.
Application of Channeling Techniques and High Resolution Transmission Electron Microscopy to Ion-Beam Damaged Zircon
Authors: Bordes N. , Ewing R.C.
Published: April 2011

Zircon (ZrSiO4) samples were irradiated at 100K with 400 keV Ar+ and Xe+ ion beams to fluences ranging from 5x1013 to 5x1015 ions/cm2. Rutherford backscattering spectroscopy (RBS) experiments were completed to study the dechanneling of He ions in the irradiated zircons. Cross-sections of some irradiated samples were prepared, and the zircon microstructure was examined by highresolution transmission electron microscopy (HRTEM). At doses greater than 8x1014 ions/cm2 or 0.8 dpa (displacements per atom), RBS channeling experiments showed the presence of a disordered or amorphous layer. The electron microscopy confirmed the presence of an amorphous layer extending over a depth of 300 to 350 nm (Ar+ irradiation) and 200 nm (Xe+ irradiation) in agreement with the damage layer depth calculated by TRIM. At depths extending beyond the damage peak, TEM reveals an amorphous layer with “islands” of crystalline material of σ 30 nm in size. These experiments show that RBS and TEM are complementary techniques in investigating radiation effects in irradiations of bulk ceramic.
Room Temperature Damage, Annealing and Dislocation Growth in Silicon
Authors: Elliman R.G. , Mitchell I.V.
Published: April 2011

The concentration of residual defects produced by self ion implantation of silicon has been shown to be a sensitive function of implantation temperature at temperatures near room temperature. In this study samples were heated to temperatures of 20°C and 60°C and implanted with 540 keV Si ions to a fluence of 2x1015Si.cm-2 using a constant scanned ion flux of 0.2 μA.cm-2. The resultant primary defect concentrations, measured by Rutherford backscattering spectrometry and channelling (RBS-C), were 2.3±0.1x1022 cm-l and 1.8±0.2x1021 cm-3, respectively, i.e. a reduction by a factor of σ13 for a temperature increase of 40°C. Such differences were not evident in the concentration of secondary defects formed by annealing these samples at 900°C for 15 minutes: the defect concentrations were equal within the experimental uncertainties of the RBS-C and transmission electron microscopy (TEM) measurements. This result appears to lead to the surprising conclusion that the number of displaced atoms that survive high temperature annealing to form extended defects is largely independent of the dynamic annealing processes operating during implantation but depends instead on parameters which scale with the ion fluence.
Effect of Copper and Nickel on the Neutron Irradiation Damage in Iron Alloys
Authors: Hoelzer D.T. , Ebrahimi F.
Published: April 2011

Iron alloys containing copper and nickel were irradiated at 288°C to a fluence of 4.63 × 1019 neutrons/cm2. Neutron irradiation produced defects which were observable by TEM in all of the iron alloys studied. The TEM analysis of the defects showed them to be interstitial dislocation loops with a < 100 > and a/2 < 111 > Burgers vectors. The size, the number density, and the Burgers vector of dislocations were affected by the alloy composition. The addition ofcopper and nickel decreased the dislocation loop size and increased the fraction of a/2 < 111 > loops. No voids or vacancy loops were observed in the irradiated iron alloys. The results are discussed in terms of dislocation loop nucleation and growth.
Development of Neutral-Density Infrared Filters Using Metallic Thin Films
Authors: Zhang Z. M. , Dada R. U. , Hanssen L. M.
Published: April 2011

Broadband infrared filters with uniform spectral transmittance are used for spectrometer calibration and other applications. However, commercially available neutral-density filters with optical density (GD) greater than 2 exhibit significant variations in OD over the wavelength region from 2 μm to 25 μ m. In this work, we found a single-layer alloy film that, for appropriate thicknesses, yields a flat transmittance for OD near 3 and 4. The transmittance and reflectance of the filters were measured using a Fourier-transform infrared spectrometer. The optical constants of the alloy films were obtained from transmittance and reflectance measurements, which can be used for future design optimization.
Surface Reaction Mechanism During Deposition of a-Si: 11 by PECVD
Authors: Maeda Keiji , Kuroe Atsushi , Umezu Ikurou
Published: April 2011

Hydrogen contents were investigated in a-Si:H films prepared by PECVD under various conditions. The H atom contents were divided into SiH and SiH2 densities by spectral deconvolution of the IR absorption. Dehydrogenation of SiHo simultaneously occurs with Si-Si bonding in the surface reaction. The mechanisms of surface reaction were proposed based on an analysis of the variation in S1H2 density with the deposition condition. There are two kinds of processes, the fast process and the slow process. The fast process incorporates SiH2 into the network. The activation energy of the slow process is 0.3 eV, which is due to the surface migration of adsorbed radicals.
The Limitations of the Constant Photocurrent Method for Determining Subgap Absorption
Authors: Stradins P. , Fritzsche H. , Tran M.
Published: April 2011

The subgap absorption αcpm measured by the constant photocurrent method (CPM) was studied between 4.2K and 300K for different dopings and defect concentrations N in hydrogenated amorphous silicon (a-Si:H). We found that αcpm is reduced by up to a factor 100 between 50K and 200K due to infrared selfquenching which changes the charge state of the defects and enhances recombination. The effect is diminished at high N and large doping. The relative change Δα/α reflects ΔN/N both at very low and high temperatures. For undoped samples αcpm is about four times larger at 4.2K than at 300K. For doped samples the two values are essentially the same.
Structural Defects in thin Film Amorphous Silicon Films Deposited on Textured TCO Material
Authors: Knoesen D. , Schropp R. E. I. , Van Der Weg W. F.
Published: April 2011

A cross sectional TME study has been conducted into the structure and morphology of p- and i-type a-Si:H layers of device quality deposited on textured TCO on glass. The layer thickness over peaks is shown to be equivalent to that for flat regions, while defective regions are found in narrow valleys, initiating from the pit of the valleys. These regions may act as regions of excessive recombination and/or shunting regions, thus leading to a reduced Voc and fill factor in thin solar cells. A cosine relationship was found between the deposited thickness and the facet angles of the surface TCO crystals. It is concluded that for best performance of the deposited layer, the deposition has to be completely isotropie, and that the preferred surface morphology of textured TCO be sharp peaks with wider valleys.
The Effect of ZnO Sputtering Deposition Parameters on the Performances of Back Reflector Enhanced Amorphous Silicon Solar Cells
Authors: Terzini E. , Rubino A. , De Rosa R. , Addonizio M. L.
Published: April 2011

Low resistivity (5×10−4Ω cm) and high transmittance (T > 82 %) ZnO films have been obtained by RF magnetron sputtering deposition of ZnO/Al2O3 (2.5 % wt.) target. The doped oxide films have been investigated by optical and electrical measurements and by scanning electron microscopy. The optimized recipes have been applied to the realisation of amorphous p-i-n solar cells back reflector in the structure: TCO/pin/ZnO/Ag.

The application of ZnO/Ag leads to an increase of the Jsc current of about 13% but a poorer fill factor steadily affects the characteristics of the devices. We demonstrate that a thicker n+ layer of about three times the standard one is effective in driving the FF toward the normal values.

The cells' behaviour has been ascribed to a thinning of the n+ layer resulting from a sputtering action of energetic oxygen atoms during the ZnO deposition process. This effect has been evidenced by measuring the absorbance of SnO2/a-Si (20 nm) /ZnO in comparison with SnO2/a-Si (20 nm) structure.
A New Linear Array Thin Film Position Sensitive Detector (LTFPSD) for 3D Measurements
Authors: Fortunato E. , Soares F. , Lavareda G. , Martins R.
Published: April 2011

A Linear array Thin Film Position Sensitive Detector (LTFPSD) based on hydrogenated amorphous silicon (a-Si:H) is proposed for the first time, taking advantage of the optical properties presented by a-Si:H devices we have developed a LTFPSD with 128 integrated elements able to be used in 3D inspections/measurements. Each element consists on an one-dimensional TFPSD, based on a p.i.n diode produced in a conventional PECVD system, where the doped layers are coated with thin resistive layers to establish the required device equipotentials. By proper incorporation of the LTFPSD into an optical inspection camera it will be possible to acquire information about an object/surface, through the optical cross-section method1. The main advantages of this system, when compared with the conventional CCDs, are the low complexity of hardware and software used and that the information can be continuously processed (analogue detection).
Electron Spin Resonance Evidence for a Localized EP (E’δ-Like) Defect Structure
Authors: Conley John F , Lenahan PM
Published: April 2011

We report the first observation of a hydrogen complexed EP (E’δ) center. Comparing EP/H and E’γ/H electron spin resonance spectra, we provide the first strong evidence that EP (E’δ) centers do not have a delocalized structure.
Analysis of Hydrogen Passivation Mechanisms in Poly-Si TFT’s by Employing R.F. Plasma
Authors: Choi K. Y. , Kim Y. S. , Choi H.S. , Bae B.S. , Han M. K.
Published: April 2011

The dominant pathways for hydrogen diffusion in poly-Si TFT’s are identified by analyzing the hydrogenation effect on the various device geometries. It is observed that the gate poly-Si thickness and channel width didn’t affect the hydrogenation. As the channel length was decreased down to 3 µm, threshold voltage was reduced and field effect mobility was increased significantly with hydrogeantion time. In the thick gate oxide (2000 Å, 4000 Å) poly-Si TFT’s, the device characteristics have been improved rapidly with hydrogenation time. The tail state density of thin gate oxide TFT wasn’t change by hydrgenation while that of thick gate oxide TFT was significantly reduced. Our experimental results may support the model that hydrogen atoms diffuse into the bulk of the active channel layer through the gate oxide and passivate the grain boundary and intragranular defects limitedly by gate oxide area.
Hydrogenation of Multicrystalline Si-Materials for Solar Cells: Discrimination Between Effects in the Intra-Grain and Grain Boundary Regions
Authors: Poortmans J. , Rosmeulen M. , Kaniava A. , Vanhellemont J. , Elgamel H. , Nijs J.
Published: April 2011

In this paper we describe the results of a study on the hydrogenation treatment of multicrystalline substrates by an RF-plasma with emphasis on discriminating between effects on the intra-grain material and grain boundary regions. For this purpose small mesa-type diodes were processed. Two types of multicrystalline material are being compared in this study. The main difference between these materials is their oxygen and metallic impurity content. The effects of the hydrogenation treatment were studied by means of I-V and DLTS-measurements. Finally, we will present data on small multicrystalline solar cells and the effect of hydrogenation on the main parameters of this device to illustrate the correlation and the differences between the measurements on small-scale diodes and the effects on the macroscopic device which is the solar cell.
Characterization of Growth Defects in CdZnTe Single Crystals by Synchrotron White Beam X-ray Topography
Authors: Chung H. , Raghothamachar B. , Wu J. , Dudley M. , Larson D. J. , Gillies D. C.
Published: April 2011

Synchrotron white beam X-ray topography has been used to characterize structural defects in microgravity grown CdZnTe single crystals. Defects such as dislocations, slip bands, 180° rotation twins, precipitates and subgrain boundaries are observed but their density is much lower than those in crystals grown under normal gravity. The observed results also indicate that the defect structures of the as grown crystals are strongly influenced by cooling rates. X-ray transmission topographs recorded from regions grown at different cooling rates show that the dislocation density in rapidly cooled regions is higher than that in slowly cooled regions. The formation of dislocations is presumably attributed to the thermal stress caused by accelerated cooling rates, which is greater than the critical resolved shear stress. As the cooling rate is accelerated, the magnitude of thermal stress is increased and more dislocations are formed to relieve the accumulated lattice strain. In addition, if the cooling rates are increased further, the accentuated thermal stresses can give rise to more pronounced deformation processes, comprising the formation of dislocation slip bands, as confirmed by the extensive slip bands revealed by the X-ray reflection topographs.
Large Persistent Photochromic Effect Due to DX Centers in AlSb Doped with Selenium
Authors: Becla P. , Witt A. G. , Lagowski J. , Walukiewicz W.
Published: April 2011

A large photochromic effect has been observed in bulk AlSb crystals doped with Se. Illumination with the light of energy higher than 1 eV leads to an increase of the absorption coefficient in the spectral range 0.1 eV to 1.6 eV. The enhanced absorption is persistent at the temperatures below about 100 K. The effect is a manifestation of a DX-like bistability of Se donors. The illumination transfers the electrons from the DX center to a metastable hydrogenic level. The increased absorption with peaks around 0.2 eV and 0.5 eV is due to photoionization from the donor level to X1 and X3 minima of the conduction band