bluuebunny/crossref_metadata_2025 huggingface dataset:
Crystal Structure of Endohedral Metallofullerene La@C82
Authors: Suematsu H. , Murakami Y. , Kawata H. , Fujii Y. , Hamaya N. , Shimomura O. , Kikuchi K. , Achiba Y. , Ikemoto I.
Published: April 2011

Since the success of synthesis the molecular structure of the endohedral metallofullerene is a most interesting subject in relation to the electronic state of metal and carbon cage. Highquality crystals of endohedral metallofullerene La@C82 have been synthesized by the arc discharge and HPLC method. The x-ray powder diffraction and the precession photograph of a tiny single crystal have revealed the cubic structure with the space group I43d and the lattice constant ao = 25.72 ± 0.007 Å at room temperature. The structure is characterized by the molecular arrangement in which the molecules align in the [111] direction with the molecular axis orienting in the same [111] direction; this structure suggests the existence of a dipole interaction between the molecules. The metal position in the cage is also discussed on the basis of an ellipsoidal model of molecule.
Controlled Formation of Buried Layers of Carbon
Authors: Ila D. , Zimmerman R. L. , Jenkins G. M.
Published: April 2011

Partially cured and cured PF-resin samples were prepared at 150°C, 170°C and at 200°C in an inert environment and then bombarded by MeV ion beams using protons, alphas and nitrogen. Using low ion beam current density, 100 to 500 pA/mm2 for the nitrogen ions, 10 to 20 nA/mm2 for the alpha ions, and 50 to 500 nA/mm2 for protons, we have produced buried carbon layers without breakdown i.e., crack formation. The thicknesses of the carbon layers produced were of the order of a few tens of nanometers at a depth of a few nanometers to several micrometers depending on the energy, the type of bombarding ions and the curing temperature of the precursor. The electrical resistivity of these layers was measured in situ and was reduced from 109 Ω-cm to 10 Ω-cm. The lowest resistivity, 10 Ω-cm, was measured in the alpha bombarded, 150°C heat-treated resin. The carbonized volumes were analyzed by Raman microprobe spectroscopy which showed that the strongest graphitic (G-line) and distorted (D-line) Raman signals observed were from the nitrogen and alpha irradiated samples.
Photocarrier Generation in Fullerene Films
Authors: KÖnenkamp R. , Erxmeyer J. , Weidinger A.
Published: April 2011

We present results from a detailed study of photoconductivity in thin C60 films. It is shown that the generation of mobile charge carriers involves molecular electronic transitions coupled to molecular vibrations. The vibrational bands broaden with temperature and shift at the fcc-to-scphase transition. For excitations with energy below ∼1.8 eV photoconduction appears to involve a tail of defect states whose distribution is somewhat dependent on sample preparation.
Nucleation and Growth Processes During The Chemical Vapor Deposition of Diamond
Authors: Angus John C. , Evans Edward A.
Published: April 2011

The nucleation of independent diamond crystals can occur through graphitic, sp2, intermediates. Evidence from several sources indicates that diamond growth may take place through sp2 surface layers. Steady state analysis of simple reaction networks leads to expressions for the diamond growth rates, the concentration of sp2 impurities in the diamond, and for the nucleation rate of new crystals.
A Comparative Study of Nucleation Enhancement Techniques for Plasma Cvd of Diamond Thin Films
Authors: Meilunas R.J. , Tobin A.
Published: April 2011

Three methods recently proposed for enhancing the nucleation density of thin film diamond on non-diamond surfaces during microwave plasma assisted chemical vapor deposition are investigated. The results of a series of nucleation and growth studies utilizing a dc biasing technique, carbon cluster (C70) thin film overlayers, and thin film metal (Fe) overlayers for diamond nucleation enhancement are presented. The influence of the substrate and plasma processing parameters under which the above nucleation enhancement effects occur has been determined for the three respective techniques.
Preparation of High Density Graphite Materials from Coal Tar Pitch
Authors: Park Yang-Duk , Kim Chul-Woo , Seo Young-Dae
Published: April 2011

Preparation of high density graphite materials from coal tar pitch was investigated. The effect of β-resin content on the mechanical properties of graphite solid prepared from semi-coke, which was prepared by wet milling method, was examined. β-Resin content was effective for fabrication of green bodies without lamination and for improving the mechanical properties of graphite materials.
Parameter Optimization for the Arc-Adlc-Deposition Process
Authors: Knotek O. , Löffler F. , Barimani C. , Brand J.
Published: April 2011

One method used by the industry for the deposition of thin carbon films is the dc cathodic arcion plating process. The evaporation of a graphite cathode by a high energy arc beam results in the production of carbon atoms, ions and macroscopic graphitic fragments. All these particles influence the film growth, the structure and therefore the film properties. The incorporation of the macroscopic graphitic fragments can be avoided by the use of mechanical or magnetic filters in front of the cathode. These filters prevent the direct bombardment of the substrates with particles. This particle separation involves decreased substrate current densities and this means lower deposition temperature (< 200°C) and growth rates. With the addition of inert (argon) and reactive gases (nitrogen and methane) and the use of a bias voltage on the substrate economical deposition rates of 2-4 μm/h and good adhesion on various materials can be reached.

This paper describes the production of amorphous hydrogen and hydrogen-free carbon films by a modified cathodic arc evaporation process using a simple mechanical filter for particle reduction. It shows the influence of some process parameters on the optimization of the process stability and process efficiency. Some characteristic properties of these coatings are shown by an example.
Atomic Structure Modifications of Diamond-Like Nanocomposite Films: Observation by Raman Spectroscopy, FTIR and STM
Authors: Dorfman B. , Abraizov M. , Pollak Fred H. , Yan D. , Strongin M. , Yang X.-Q. , Rong Z.-Y.
Published: April 2011

Raman spectroscopy, FTIR and scanning tunneling microscopy (STM) were used to study diamond-like nanocomposite (DLN) and metal containing DLN (Me-DLN) films. The FTIR spectra showed no appreciable absorption from the C-H stretch vibration band in DLN when 1 kV rf bias voltage was applied. Thermal annealing (450°C for 2 hrs) of (a) DLN and Cr-DLN films caused no change in the Raman spectra while (b) for Pt-DLN films there was a blue shift of both the crystalline (G) and microcrystalline (D) graphite-like features, an increase in the ID/IG intensity ratio and a decrease in the linewidths. The changes observed were more pronounced in the film with the highest Pt concentration. The STM image of this Pt-DLN film revealed a structure of aromatic graphite rings.
Preparation and Characterization of Aerogel-Based Carbon Nanocomposites
Authors: Cao Wanqing , Song Xian Yun , Hunt Arlon J.
Published: April 2011

Aerogels are highly porous solids prepared by sol-gel processing and supercritical evacuation. Because of their high surface area, aerogels can be used as an effective catalyst for the thermal decomposition of many gaseous compounds. A variety of hydrocarbon gases have been chosen to deposit carbon in the aerogel matrix, with the deposition temperature varying from 500° to 850°C depending on the hydrocarbon used. The amount of carbon that can be deposited in the aerogel is surprisingly large, reaching up to 10 times the original weight after extensive deposition using acetylene. Overall, the aerogel composites prepared have a uniform microstructure with the average particle size in the nanometer range. In addition, we have observed some interesting graphitic structures including carbon nanotubes and rings of various shapes. Carbon deposited in the aerogel can reduce infrared transmission of the material as well as volume shrinkage at elevated temperatures, thereby improving its thermal performance.
Computer Simulation of Creation and Motion of Edge Dislocations in Face Centered Crystals
Authors: Tajima N. , Nozaki T. , Hirade T. , Kogure Y. , Doyama Masao
Published: April 2011

Complete and dissociated edge dislocations were created near the center of the surface (101) of aluminum small crystals whose surfaces are (111), (111), (101), (101). (121) and (121). Molecular dynamics with N-body embedded atom potentials were used. Higher stress is needed to create a complete edge dislocation than to create a dissociated dislocation.
Stress Evolution During the Formation and Transformation of Titanium Silicide
Authors: Svilan V. , Harper J. M. E. , Cahral C. , Clevengeri L. A.
Published: April 2011

The evolution of stress vs. temperature was measured during the formation of TiSi2 in the reaction of Ti with (100) Si and with polycrystalline Si, and during the phase transformation from C49 to C54 TiSi2. The formation of C49 TiSi2 causes an increase in compressive stress, followed by relaxation before the transformation to C54 TiSi2, which causes no significant stress change. C54 TiSi2 is shown to be elastic in the temperature range of 750–860 °C. This difference in the deformation mechanisms of C49 and C54 TiSi2 affects the morphological stability of TiSi2 in fine line structures.
Growth of Hydrogenated Amorphous Silicon (A-Si:H) on Patterned Substrates for Increased Mechanical Stability
Authors: Hong Wan-Shick , Delgado J. C. , Ruiz O. , Perez-Mendez V.
Published: April 2011

Residual stress in hydrogenated amorphous silicon (a-Si:H) film has been studied. Deposition on square island pattern reduced the stress when the lateral dimension of the islands became comparable to the film thickness. The overall stress was reduced by approximately 40% when the lateral dimension was decreased to 40 μm, but the adhesion was not improved much. However, substrates having a 2-dimensional array of inversed pyramids of 200 μm in lateral dimension produced overall stress 3∼4 times lower than that on the normal substrates. The inversed pyramid structure also had other advantages including minimized delamination and increased effective thickness. Computer simulation confirmed that the overall stress can be reduced by deposition on the pyramidal structure.
Dislocation Distributions in Cd1-xHgxTe/CdTe and Cd1-xHgxTe/Cd1-yZnyTe Grown by Liquid Phase Epitaxy
Authors: Watson C. C. R. , Durose K. , O’Keefe E. , Hudson J. M. , Tanner B. K.
Published: April 2011

Epilayers of LPE Cdo.24Hgo.76Te grown on (111)B CdTe and Cdi-xZnxTe substrates have been examined by defect etching and triple axis x-ray diffraction. Defect etching of bevelled layers has shown the threading dislocation density to fall with increasing distance from the heterointerface, for distances <6μm. In thicker regions however a constant ‘background’ dislocation density is observed. Background dislocation densities of ∼ 3 x 105cm-2 and 9 x 104cm-2 have been measured for layers grown on CdTe and Cdo.96Zn0.04Te respectively, this is compared with a substrate dislocation density of ∼ 3 x 104cm-2 measured in both types of substrates. The increase in the dislocation density within the epilayers compared with the corresponding substrate is discussed. An explanation is also given for the displacement of the peak dislocation density, from the interface to within the layer, observed in the Cd0.76Hg0.24Te / Cd0.96Zn0.04Te system.
X-Ray Measurements of Deformations in Films and Substrates in Heteroepitaxial SYSTEM GaAs/Ge
Authors: Burle N. , Pichaud B. , Thomas O.
Published: April 2011

X-Ray Transmission and Reflection Topography settings have been used to perform stress and strain measurements in the epitaxial system GaAs/Ge : radius of curvature of the whole sample is obtained from the Lang (transmission) set-up ; tetragonal distortion in the layer is deduced from the Berg-Barrett (reflection) setup. Comparison of these results and correlation with dislocations densities estimated from topographs enable the elastic state of the system to be known.
Mechanical Properties and Microstructure of AL(1wt%SI) And AL(1wt%SI, 0.5wt%CU) Thin Films. The Role of Diffusional Creep in the Tensile Stress Regime
Authors: Bader S. , Kalaugher E. M. , Arzt E.
Published: April 2011

The microstructure and mechanical properties of hot (h) and cold (c) sputtered Al-lwt%Si and Al-lwt%Si-0.5wt%Cu films were studied using transmission electron microscopy and wafer curvature stress measurements.

Stress/temperature curves of all films showed only slight differences in compression on healing once a stable grain size was established. However, on cooling several remarkable differences were observed. These observations cannot be explained by assuming dislocation glide/climb as the dominant relaxation mechanism. The results will be discussed in terms of grain boundary diffusional relaxation (Coble creep), which occurs in addition to dislocation glide.
Thin Film Fine Line Work of Adhesion by Microwedge Indentation
Authors: de Boer M. P. , Moody N. R. , Huang H , Gerberich W.W.
Published: April 2011

The mechanics for plane strain geometry are presented for thin film fine lines by considering the case of microwedge indentation of a fine line subject to no residual stress. The analysis is separated into three parts depending on the absence or presence of buckling. During indentation, unbuckled or double-buckled configurations may exist, while single-buckling may occur when the indenter tip is removed. A microwedge tip was employed in the indentation testing of unstressed rf-sputtered tungsten fine lines to demonstrate some of the analysis.
Scanning Scratch Tests for Evaluating the Adhesion of Thin Oxide Films on Stainless Steel
Authors: Haanappel V. A. C. , van Corbach H. D. , Fransen T. , Gellings P. J.
Published: April 2011

Amorphous alumina films were deposited by metal-organic chemical vapour deposition (MOCVD) on stainless steel, type AISI 304. The MOCVD experiments were performed in nitrogen at low pressure (0.17 kPa (1.25 torr)).

The effect of deposition temperature (200 − 380 °C), growth rate, film thickness, and post-deposition thermal treatment on the mechanical properties was studied. The experiments were performed with a scanning-scratch tester. The experiments are based on the estimation of the film adhesion to the substrate by determining a critical load, Lc: the load where the film starts to spall or to delaminate.

The best mechanical properties were obtained with unannealed samples. After thermal annealing the critical load decreases. Regarding the unannealed samples, the critical load increased with increasing film thickness. The deposition temperature and the growth rate had no effect on the critical load.
Manipulation of Stresses in Metallic Thin Films by Alloying
Authors: Nandedkar A. S.
Published: April 2011

Atomistic simulations were used to study the configurations of defects in copper aluminum alloy (2% copper, 98% aluminum). In the presence of free surface, the copper atoms migrated towards the surface. When the aluminum cell (about 2000 atoms) contained a dislocation, copper atoms segregated near the dislocation core on the compressional side. In presence of a grain boundary, copper atoms moved into the boundary plane. The segregation in these simulations resulted from reduction in localized strain near the structural defects.
Neural Network Processing of Optical Fiber Sensor Signals for Impact Location
Authors: Schindler Paul M. , Shaw John K. , May Russell G. , Claus Richard O.
Published: April 2011

A system to detect and locate impacts by foreign bodies on a surface was developed and tested. Fiber optic extrinsic Fabry-Perot interferometer (EFPI) strain sensors were attached to or embedded in the surface, so that stress waves emanating from an impact could be detected. By employing an artificial neural network to process the sensor outputs, the impact location could be inferred to centimeter range accuracy directly from the arrival time data. In particular, the network could be trained to determine impact location regardless of material anisotropy. Results demonstrate that a back-propagation network identifiesimpact location for an anisotropic graphite/bismaleimide plate with the same accuracy as that for an isotropic aluminum plate.
Thermo-Electric Properties of Thin-Film Refractory Metal Thermocouples
Authors: Schinazi Robert G. , Lu Guo-Quan , Spaepen Frans
Published: April 2011

Thin-film thermocouples are well suited for “smart” processes where the mass of traditional two-lead thermocouples inhibits their wide utilization. However, currentthin-film thermocouples lack the robustness desired for abrasive environments. To addressthis issue, thin-film refractory metal thermocouples consisting of combinations of molybdenum, tantalum, tungsten, vanadium, or nickel were produced by ion-beam sputtering on an alumina substrate. To speed up the process of sampling a large number of material combinations, the thermocouples were made small, about 200 nm thick, 2.5 mm wide, and 12 mm long. An apparatus was fabricated to measure the thermo-electric responses of these short-length thermocouples. Materials design and heat-transfer issues relating to the applicationsand testing of these thermocouples are discussed.
Ba/sr Doped Electrostrictive Compositions for Actuator Design
Authors: Zhuang Zhiqiang , Jan Lijun , Kang S.B. , Gallois B. , Yang Z. , Du H.
Published: April 2011

Solid solutions of lead zirconate titanate (PZT) and lead magnesium niobate (PMN) doped with barium/strontium were developed for use in miniature electrostrictive actuators. The microstructural features of the materials were determined by high-resolution transmission electron microscopy (HREM) and by x-ray diffractometry. The response and the strain hysteresis were measured by applying a sinusoidal or a rectangular voltage pulse. Pulses of variable amplitude and width were applied to the elements to study the switching behavior of 90° domain walls in the materials and to elucidate the initial zero-position “walk off” mechanism. The strain hysteresis or remanent strain of PZT decreased with increasing PMN concentration. A barium-doped PZT (Ba-PZT) composition containing 4 mole% of PMN exhibited ferroelectric relator behavior with a field-induced strain much larger than those of lead magnesium niobate (PMN) electrostrictive materials and a small remanent strain hysteresis.
Stresses Near the End of an Internal Electrode in Multilayer Electrostrictive Ceramic Actuators
Authors: Gong Xiao-Yan
Published: April 2011

Stresses near the end of an internal electrode in a multilayer electrostrictive ceramic actuator are studied in detail. A finite element program capable of overcoming two major difficulties is developed. The program solves both the mechanical and electrical coupling problem and the nonlinear electric field and electric displacement relationship for these materials. Results indicate that the stress difference between the coupled and the uncoupled cases can only be distinguished when a stress singularity is present. Tensile stresses are found both in front, and behind, the end of an internal electrode. The magnitude of the stresses is predetermined by the material constants.
In-Situ Observation of Microcracking in a Tial-Cr-V Alloy Under Compressive Stress
Authors: Pu Z. J. , Liu Y. Q. , Wu K. H.
Published: April 2011

The entire process of microcrack nucleation in a TiAl-Cr-V alloy with different microstructures is observed in situ by a scanning electronic microscope (SEM) during compressive deformation. The main results are summarized as follows: (1) In the fully lamellar microstructure, the microcracks nucleate mainly at the lamellar grain boundaries between soft-orientation and hard-orientation grains. (2) In the Widmanstatten structure, the microcracks distribute mainly at the intersection between two Widmanstatten plates. (3) In the duplex structure, the microcracks form at the lamella/ lamella and lamella/gamma boundaries. (4) In the nearly gamma structure, microcracks nucleate at gamma/gamma grain boundaries.
Tem and Computer Image Simulation Studies on the Dissociation of Superdislocations in Ll2 Al3Ti Alloy
Authors: Rong Yonghua , Wu Xiaohua , Chen Xiaofu , Chen Shipu , Hu Gengxiang
Published: April 2011

TEM investigation associated with computer image simulations were conducted to determine the dislocation core structure in the Ll2 Al67MnsTi25 alloy deformed at room temperature. It is identified that the a<110> superdislocations dissociated into pairs of a/3<112> superpartials with SISF between them. The theoretical and experimental micrographs are in good agreement under various diffraction conditions. The unusual temperature dependence of flow stress at low temperature can be related to the SISF-type dissociation.
Structure-Property Relationships and Deformation Mechanisms in an Orthorhombic Based Ti-25Al-17Nb Alloy
Authors: Majumdar B. S. , Boehlert C. , Rai A. K. , Miracle D. B.
Published: April 2011

The microstructure-property relations were evaluated for a nominally Ti-25Al-17Nb orthorhombic alloy, in terms of the deformation and failure mechanisms of the constituent ordered phases (orthorhombic, alpha-2, and B2). The mechanisms were characterized through observation of slip traces, crack initiation sites, and TEM. Properties of interest were the room temperature elongation, fatigue crack growth (FCG) resistance, and creep resistance.
Localized Oxidation Near Cracks and Lamellar Boundaries in a Gamma Titanium Aluminide Alloy
Authors: Kameda J. , Gold C. R. , Lee E. S. , Bloomer T. E. , Yamaguchi M.
Published: April 2011

Small punch (SP) tests on single grained titanium aluminide (Ti-48 at.%Al) specimens with 12° and 80° lamellar orientations with respect to the tensile stress axis were conducted at 1123 K in air. Brittle cracks readily extended through the thickness in the 80° lamellar structure. In a SP specimen with the 12° lamellar structure load-interrupted at the strain of 0.43%, surface cracks with the depth of 15–25 μm were formed along lamellar boundaries. Local oxidation behavior on partly sputtered surfaces in the load-interrupted 12° lamellar specimen was examined using scanning Auger microprobe (SAM). Oxygen enriched regions were observed near cracks and some lamellar layers. The mechanisms of high temperature oxygen-induced cracking are discussed in terms of the local oxidation near cracks and lamellar boundaries.
Microstructure and Mechanical Properties of The L12/L21 Two-Phase Alloys In The Quaternary Co-Al-Ni-Ti System
Authors: Matano Takeo , Kimura Yoshisato , Miura Seiji , Mishima Yoshinao
Published: April 2011

An attempt is made to develop a two-phase alloy consisting of the Ll2 and L21(Heusler) phases in the Co–Al–Ni–Ti quaternary sytem exhibiting a high elevated temperature strength as well as some room temperature ductility as a new class of heat resisting structural materials. The idea behind this approach is expectations for the L21 phase to provide high elevated temperature strength, whereas the Ll2 phase provides some room temperature ductility. Compositional optimization in the room temperature ductility of the Ll2 (Co,Ni)3(Al,Ti) is first carried out and then, based on the result, several L12/L21 two phase alloys are designed. It is found that a few to several per cent room temperature bend ductility is obtained in such two-phase alloys.
First-Principles Study of Intermetallic Phase Stability in the Ternary Ti-Al-Nb Alloy System
Authors: Asta Mark , Ormeci Alim , Wills John M. , Albers Robert C.
Published: April 2011

The stability of bcc-based phases in the Ti-Al-Nb alloy system has been studied from first-principles using a combination of ab-initio total energy and cluster variation method (CVM) calculations. Total energies have been computed for 18 binary and ternary bcc superstructures in order to determine low temperature ordering tendencies. From the results of these calculations a set of effective cluster interaction parameters have been derived. These interaction parameters are required input for CVM computations of alloy thermodynamic properties. The CVM has been used to study the effect of composition on finite-temperature ordering tendencies and site preferences for bcc-based phases. Strong ordering tendencies are observed for binary Nb-Al and Ti-Al bcc phases as well as for ternary alloys with compositions near Ti2AlNb. For selected superstructures we have also analyzed structural stabilities with respect to tetragonal distortions which transform the bcc into an fcc lattice. Instabilities with respect to such distortions are found to exist for binary but not ternary bcc compounds.
Site Occupation of Excess Al Atoms in Off-Stoichiometric γ-Tial: Measurement of Debye-Waller Factors
Authors: Swaminathan S. , Jones I. P. , Maher D. M. , Fraser H. L.
Published: April 2011

In recent years, the determination of x-ray structure factors and hence the electron charge density distribution in TiAl has received considerable attention. Experimental assessment of electron charge densities requires accurate values of the Debye-Waller (D-W) factors. This work attempts to determine the Debye-Waller factors from measurements of an Al rich off-stoichiometric single crystal of TiAl using the four circle x-ray diffraction method. The results suggest an ordered substitution of the excess Al atoms on the Ti-sublattice. This result is consistent with previous site occupancy studies.
Evolution of Dislocation Structures in Cyclically Deformed NiAl-Fe
Authors: Kallingal C. G. , Matsugi K. , Stoloff N.S. , Rajan K.
Published: April 2011

The microstructures of NiAl-Fe (Ni, 50.3at % Al, 0.28 at% Fe) polycrystals tested in HCF (high cycle fatigue) below and above the DBTT (ductile to brittle transition temperature) viz., 673 K, 823 K, 873 K and 928 K at different stress amplitudes were observed by transmission electron microscopy. The microstructure consisted of low energy dislocation networks and dislocation cells. The fundamental features of the dislocation structures are described. Misorientation angles between cells were measured from Kikuchi patterns obtained from cells through microdiffraction. The misorientations across the cell walls were found to increase with increase in the stress amplitude to which the material was subjected during cycling. The mechanisms for the formation and evolution of these structures are discussed on the basis of existing theoretical models. The implications of these substructures on the mechanical properties are also discussed.
Phase Decomposition and Deformation of L12-Ordered Co-Rich Co3Ti
Authors: Nemoto M. , Tian W. H. , Hayashi K.
Published: April 2011

The Co3Ti phase hardens appreciably by the fine precipitation of disordered fee Co-rich phase upon aging after quenching from solution annealing temperature. Transmission electron microscope(TEM) observations revealed that the precipitates are platelet in shape, lying nearly parallel to the {100} planes of the L12-ordered matrix, and perfectly coherent with the matrix lattice at the beginning of aging. The high temperature strength increases appreciably with the fine precipitation of disordered Co-rich phase over the whole temperature range investigated. TEM observations of the underaged and deformed alloys revealed that superdislocations are pinned by precipitates indicating an attractive interaction between dislocations and precipitates. At the overaged state, thin twins are introduced in the fee Co-rich precipitates during deformation.
Simulations of Creep in Ductile-Phase Toughened Nb5Si3/Nb In-Situ Composites
Authors: Henshall G. A. , Strum M. J. , Subramanian P. R. , Mendiratta M. G.
Published: April 2011

The primary and steady-state creep behavior of ductile-phase toughened Nb5Si3/Nb in-situ composites has been simulated using analytical and finite element (FE) continuum techniques. The microstructure of these composites is complex, consisting of large, elongated primary dendrites of the ductile (Nb) solid-solution phase in a eutectoid matrix with the silicide as the continuous phase. This microstructure has been idealized to facilitate the modeling; the effects of these idealizations on the predicted composite creep rates are discussed. Further, it has been assumed that the intrinsic creep behavior of each phase within the composite is the same as that of the corresponding bulk material. Thus, the experimentally measured creep properties of the bulk Nb5Si3 and (Nb) phases have been analyzed to provide the required material constants in the creep constitutive equation. Model predictions of the steady-state composite creep rate have been compared with the experimental results for a Nb-10 at.% Si alloy. While accurate at low stress, the models underpredict the composite creep rate at large stresses because the composite stress exponent is underpredicted. In the case of primary creep, the models somewhat over-predict the composite creep strain but are reasonably accurate given uncertainties in the primary creep data. Finally, FE predictions of the tensile stress distributions within the composites have been shown to be qualitatively consistent with the cracking observed experimentally during tertiary creep.
Microstructure and Growth Mechanism of Al2O3/Al Ceramic Matrix Composite Made by Directed Oxidation of Al Alloys
Authors: Qin C-D. , Lai M.O. , Cotterell B. , Shi S.
Published: April 2011

Since the Lanxide process was advanced for forming of Al2O3 ceramic composite by directed oxidation of Al alloys, much work has been done with various mechanisms being proposed. The mechanisms have claimed that only certain dopants are essential to the growth process. Nevertheless, no united consensus has yet been reached. In the present work, Al alloy containing 5% Mg was oxidised in air for 12 hours at 1150°C with or without surface dopants of MgO or Pd. The resultant composites showed very different microstructures. Without any surface doping, the alloy did not develop any portion of composite as the initial intimate oxide film stops further oxidation. This intimate oxide film can either be broken off by mechanical means or penetrated by reaction with surface dopants, so that the composite can grow and develop. The results show that the previously reported incubation time is not only related to reaction processes but also to the initial mechanical disturbances. Doping with Pd made the composite darker in colour as the grains of the alumina ceramic matrix and inclusions of Al metal are finer. This shows that Pd may make the top oxide layer less intimate, and more nucleation sites are therefore available for oxidation. A new model is presented for oxide sustained growth based on the existence of oxygen active top surface layer and the capillary flow of molten metal around ceramic phase.
Modeling the Flow of Molten Silicon in Porous Carbon Preforms and the Subsequent Formation of Silicon Carbide
Authors: Rajesh G. , Bhagat Ram B. , Nelson Emily
Published: April 2011

Ceramic matrix composites (CMCs) are being considered for a broad range of aerospace applications that include various structural components for the aircraft engine and the space shuttle main engine. Use of silicon-based CMCs which have high thermal conductivity, allows for improvements in fuel efficiency due to increased engine temperatures and pressures, which in turn generate more power and thrust. Furthermore, CMCs offer significant potential for raising the thrust-to-weight of gas turbine engines by tailoring directions of high specific reliability using design-based fiber architecture. One of the low-cost processing techniques for the silicon-based CMCs is the reactive melt infiltration [1] of silicon into the preform of carbon-coated silicon carbide fiber. However, fabrication of high performance SiC/SiC composites requires a deeper understanding of the infiltration kinetics such that fibers are protected from adverse reaction with the molten metal, that the preform is thoroughly infiltrated, and that there is no residual silicon left unreacted.
Hot-Isostatic Pressing of Silicon Carbide Based Multiphase Composed Materials
Authors: Jiang Dongliang , She Jihong
Published: April 2011

Silicon carbide (SiC) based ceramic composites with improved fracture toughness and increased flexure strength have been developed by incorporating some other non-oxide and oxide particles or some whiskers and fibres. Hot-Isostatic Pressing (HIP) has been identified as an important technology for strengthening carbide by surface modification. In this paper, Hot-pressed SiC-TiC with different densities and HIP-SiC/SiC(w) composites were post HIPped under a N2-pressure of 200MPa at 1850°C for lh. The results showed that the open pores were closed and physical and mechanical properties such as density, flexure strength and toughness were obviously improved. For the SiC-TiC composites, the final density can be reached above 98% theoretical density, flexure strength and fracture toughness were increased by 100% and 30-50%, respectively. For the SiC/5vol%-SiC(w) composites, the final flexure strength and fracture toughness were increased from 595 MPa and 6.7 MPa m1/2 to 920MPa and 8.5 MPa m1/2 separately. A possible reaction-HIP densification mechanism for SiC ceramics with open pores is proposed.
Forced Chemical Vapor Infiltration of Tubular Geometries: Modeling, Design, and Scale-Up
Authors: Stinton D.P. , Besmann T.M. , Matlin W.M. , Starr T.L. , Curtain W.A.
Published: April 2011

The development of thick-walled, tubular ceramic composites has involved investigations of different fiber architectures and fixturing to obtain optimal densification and mechanical properties. The current efforts entail modeling of the densification process in order to increase densification uniformity and decrease processing time. In addition, the process is being scaled to produce components with a 10 cm outer diameter.
Experimental Investigation of Interface Properties in SiC Fiberreinforced Reaction-Bonded Silicon Nitride Matrix Composites
Authors: Eldridge J.I. , Bhatt R.T.
Published: April 2011

Interfacial properties of 1-D SiC/RBSN composites were measured by the matrix crack spacing method and by the fiber push-out method, and the results were compared. The composites consisted of 8 to 33 vol% of aligned SCS-6 SiC fibers (142 μm diameter) in a relatively porous (20 to 40 vol%) Si3N4 matrix. The effects of fiber volume fraction and test temperature on the interfacial properties have been investigated. The advantages and limitations of both methods in evaluating the interface properties of fiber-reinforced ceramic matrix composites and the factors influencing the interfacial measurements are discussed.
Effect of Oxidation on the Mechanical Properties of Nextel™ 312/BN/Blackglas™ Composites
Authors: Vaidyanathan K. Ranji , Cannon W. Roger , Danforth Stephen C. , Tobin Albert G. , Holmes John W.
Published: April 2011

Preceramic polymers are attractive precursor materials for the production of low cost continuous fiber reinforced ceramic composites (CFCC) into near net shape components. Currently, CFCC components based upon a Blackglas™ matrix reinforced with Nextel™ 312 fibers with a BN rich surface layer are being investigated for gas turbine engine applications. The effects of oxidation on tensile and bend properties were investigated after exposure to flowing air at 600°C for 20–1000h. A significant reduction in the tensile properties accompanied by increases in fiber pull-out was observed after oxidation for 96 hours. After 500 hours oxidation, strength decreased by 50% relative to as-prepared composites. These results indicate that oxidation beyond 200 hours may be embrittling the composite.
Nickel-Alumina Composites: In Situ Synthesis by a Displacement Reaction, and Mechanical Properties
Authors: Jones Steven A. , Burlitch James M. , Üstündag Ersan , Yoo Jeannie , Zehnder Alan T.
Published: April 2011

Nickel-alumina composites have the potential to be high performance materials. Alumina, with its excellent oxidation resistance, combined with a ductile phase such as nickel may provide a tough material with a lower density and higher Young's modulus, overall, a higher specific modulus than typical Superalloys. Dense, interpenetrating Ni-Al2O3 composites were synthesized using a displacement reaction between NiO and aluminum. The resulting composites were characterized in terms of their mechanical properties such as hardness, flexure strength, fracture toughness and elastic constants. The synthesis, characterization, and mechanical properties, as well as the effect of the interpenetrating microstructure on the toughening mechanisms and other properties will be discussed.
Proton Irradiation Induced Grain Boundary Segregation in Austenitic Stainless Steels
Authors: Damcott D.L. , Was G.S. , Bruemmer S.M.
Published: April 2011

Radiation induced segregation (RIS) has been implicated as a mechanism for irradiationassisted stress corrosion cracking (IASCC) in reactor core components. Proton irradiation has been shown to be useful in creating grain boundary chemistries similar to those found in neutron and charged particle irradiated materials for accelerated testing of IASCC susceptibility. This work quantifies grain boundary RIS as a function of proton irradiation dose (0.1-3.0 dpa), temperature (200°−600°C), and alloy composition (20Cr-9Ni, 24Cr-19Ni, and xCr-24Ni, x=16, 20,24). Auger electron spectroscopy revealed Cr depletion and Ni enrichment under all irradiation conditions. As a function of dose, the degree of segregation increased rapidly to near saturation prior to 1 dpa, with a boundary composition of 12.1 at.% Cr and 36.0 at.% Ni at 1 dpa. Segregation peaked at approximately 500°C with 13.0 at.% Cr and 38.6 at.% Ni at the grain boundary at 0.5 dpa; very little segregation was observed at or below 300°C or at 600°C. The trends in segregation as a function of dose agreed well with the Perks' model predictions with the exception of the measurement at 600°C, which showed the sharp decrease in segregation predicted for a higher temperature (700°C-800°C). For alloys containing constant bulk Cr but varying Ni, the Perks' model agreed well with the observed segregation trend; however, for alloys containing constant bulk Ni and varying Cr, agreement was achieved only through the use of composition dependent diffusion parameters.
MÖssbauer Study of Zircaloy After Neutron Irradiation at Power Reactor Temperatures
Authors: Sawicki Jerzy A.
Published: April 2011

57Fe-Mössbauer spectroscopy was used to examine a specimen of Zircaloy-4 from a pressure tube extracted from a high-flux fuel channel of a CANDU* reactor. During 25 years of service the alloy was exposed to a fast-neutron fluence of 5.6×1021 n/cm2. The obtained spectra of unirradiated and neutron-irradiated specimens differ considerably. This is attributed to the radiation-induced dissolution and crystalline-to-amorphous transformation of Zr(Cr,Fe)2 precipitates, accompanied by reprecipitation and partial recrystallization of metastable Zr2Fe particles in the zirconium matrix. The correspondence between Mtssbauer and electron microscopy observations, as well as the similarity between neutron-irradiated and ion-implanted specimens, is discussed.
A Thermal Spike Model of the Amorphization of Insulators by High-Energy Heavy-Ion Irradiation
Authors: Szenes G.
Published: April 2011

A model based on the assumption of a Gaussian temperature distribution in the thermal spike well accounts for the basic features of latent track formation. In good agreement with the observations it predicts a logarithmic variation of the track size for 2.7≥Se/Set≥1 and a linear variation for Se/Set≥2.7, where Se is the electronic stopping power and Set is a threshold value. The model also provides a quantitative relation between Set and the thermal properties of the target. A weak temperature dependence of track formation is predicted in agreement with the experiments.
Amorphization of Graphite Under ion or Electron Irradiation
Authors: Abe Hiroaki , Naramoto Hiroshi , Kinoshita Chiken
Published: April 2011

A transmission electron microscope interfaced with ion accelerators (TEM-Accelerator Facility) in JAERI-Takasaki has been used to get insights into the irradiation-induced amorphization of highlyoriented pyrolytic graphite. Amorphization is induced through lattice disorder under irradiation with electrons or ions at room temperature. The displacement threshold energy is determined as 27 eV and 28 eV from the electron-energy dependence of the amorphization fluence at room temperature and 373 K, respectively. The amorphization fluences in dpa are almost identical for irradiation with various kinds of ions within the experimental error. It indicates the thermal spikes do not have an important role for the amorphization over the energy density between 10-4to 10-2eV/atom.
An Examination of the Factors Which Determine the Thermal Spike Lifetime of a Displacement Cascade
Authors: Tappin D.K. , Robertson I.M. , Kirk M.A.
Published: April 2011

The thermal spike phase of a displacement cascade is thought to be of fundamental importance to a number of issues in radiation effects. For example, the formation of stable vacancy clusters and the efficiency of ion mixing are thought to be directly controlled by the duration of the liquid-like state during the thermal spike. A number of factors have been proposed to be of significance to the lifetime of the thermal spike. For example, materials which have a high melting temperature, or which exhibit strong electron-phonon coupling, which provides an efficient method of energy transport to the surrounding material, are expected to have short lifetimes. We have examined these factors in detail and demonstrated that, in both elemental metals and alloys, electron-phonon coupling provides the most consistent explanation for variations in the thermal spike lifetime and, hence, the efficiency of ion mixing and the number of stable vacancy defect clusters formed.
Analytical Electron Microscopy of Precipitates in Ion-Implanted Mgal2O4 Spinel
Authors: Evans N. D. , Zinkle S. J. , Bentley J.
Published: April 2011

Analytical electron microscopy (AEM) has been used to investigate precipitates in MgAl2O4 spinel implantated with Al+, Mg+, or Fe2+ ions. Experiments combining diffraction, energy dispersive X-ray spectrometry (EDS), electron energy-loss spectrometry (EELS), and energy-filtered imaging were employed to identify and characterize precipitates observed in the implanted ion region. Diffraction studies suggested these are metallic aluminum colloids, although EELS and energy-filtered images revealed this to be so only for the Al+ and Mg+ implantations, but not for Fe2+ ion implantations. Multiple-least-squares (MLS) fitting of EELS plasmon spectra was employed to quantify the volume fraction of metallic aluminum in the implanted ion region. Energy-filtered plasmon images of the implanted ion region clearly show the colloid distribution in the Al+ and Mg+ implanted spinel. Energy-filtered images from the Fe2+ ion implanted spinel indicate that the features visible in diffraction contrast cannot be associated with either metallic aluminum or iron-rich precipitates.
Materials for Reverse Saturable Absorption Optical Limiters
Authors: Shirk James S. , Pong Richard G. S. , Flom Steven R. , Boyle Michael E. , Snow Arthur W.
Published: April 2011

Systematic studies of the nonlinear optical properties of metallo-organic materials have led to the development of promising new phthalocyanine materials for optical limiting. Several heavy metal substituted phthalocyanines exhibit a strong nonlinear absorption that is useful for optical limiters in the visible. In fast optical systems, other mechanisms, such as the thermal refraction, contribute to the limiting. The spectral window for limiting can by modified by altering the molecular structure.
Nonlinear Absorption in Organic Dyes
Authors: Welford K. R. , Swatton S. N. R. , Hughes S. , Till S. J. , Spruce G. , Hollins R. C. , Wherrett B. S.
Published: April 2011

The non-linear optical properties of chloro-aluminium phthalocyanine and 1,3,3,1′,3′,3′- hexamethylindotricarbocyanine have been studied as model systems using z-scan and time resolved experimental techniques. Excited state lifetimes and absorption cross sections have been measured as well as the spectral response of the induced absorption. Excited state lifetimes are critical in determining the performance of a reverse saturable absorber. Even though a first excited state absorption cross section may be large compared with the ground state absorption cross section the lifetimes and transition probabilities of higher lying states can lead to constraints upon how effective organic dyes are as optical limiting materials.

Both novel saturation of the induced absorption at high laser fluences and intermolecular quantum beats in 1,3,3, 1′,3′,3′-hexamethylindotricarbocyanine are demonstrated experimentally and accounted for theoretically. Quantum chemical calculations have been developed to propose a probable inter-molecular mechanism for the observed quantum beats.
Metal Organic Complexes for Optical Power Limiting
Authors: Lawson Chris M. , Zhai Tianyi , Gale David C. , Gray Gary M.
Published: April 2011

Nonlinear optical properties of transition metal-phosphine complexes have been measured at 532 nm by degenerate four-wave mixing. Large second-order molecular hyperpolarizabilities, γ, have been found for complexes containing two phosphine ligands. The measured γ values are closely related to the type and coordination geometry of the phosphine ligands, and the γ values vary as the fifth power of the number of substituents with π-electrons structure.
Thin Film Optical Switching Materials
Authors: Natale J. F. DE
Published: April 2011

The oxides of vanadium, VO2 and V2O3, are attractive materials for optical switching applications due to their large excursions in both electrical and IR optical properties. These materials can be prepared in thin film form by a number of deposition techniques, making them compatible with optical and electrical device applications. The performance of these films is a sensitive function of microstructure and processing conditions. The effects of these variables are discussed.
Is Thermalization Due to Electronic Self-Trapplng?
Authors: Kemp Mathieu
Published: April 2011

The interpretation that the red shift of the a-Si:H photoluminescence spectrum is due to carrier hopping is incompatible with Monte-Carlo simulations. We suggest that thermalization is instead the result of slow, gradual, electronic self-trapping. We show how this model reconciles the narrowness of the photoluminescence lifetime distribution with the observed red-shift of the energy spectrum.
Characterization of Localized Density of States in Intrinsic a-Si and poly-Si Films by Transient Voltage Spectroscopy
Authors: Kawachi G. , Ishii M. , Tanaka T. , Konishi N.
Published: April 2011

The localized density of states (LDOS) at interfaces between intrinsic silicon and silicon nitride (Si3N4 films are studied using transient voltage spectroscopy (TVS). In the TVS technique, the transient of the voltage across a MIS-diode after a trap filling voltage pulse is measured using a high-impedance voltage probe. This allows us to make a precise measurement of the LDOS at undoped Si/insulator interfaces. The LDOS in a-Si:H/Si3N4systems has a broad peak around the energy of 0.9 eV below the conduction-band edge. A modification of the LDOS at a-Si:H/Si3N4 interfaces by bias-annealing is clearly observed using this technique. The results are consistent with the defect pool model. The LDOS in laser annealed poly-Si/Si3N4 systems has a peak centered 0.6eV below the conduction-band edge, which seems to be the Si dangling bond states in the poly-Si films.