bluuebunny/crossref_metadata_2025_split huggingface dataset:
Hot Oxidation Resistance of an Intermetallic Nb-Ti-Al Alloy
Authors: Bannykh Oleg A. , Sheftel Elena N. , Usmanova Galina S. , Sharapov Anatoly A. 
Published: April 2011

AbstractStructure, phase composition and air oxidation behavior in the temperature range 800 to 1400°C of a Nb-Ti-Al-based intermetallic alloy with the chemical composition (wt %): Nb-47.0; Ti-23.9; Al-21.0, V-4.4, and Cr-4.1 have been studied. The alloy structure is two-phase -σ (Nb2Al type) and γ (TiAl type). Preliminary air oxidation at 1400°C decreases the oxidation rate at 1150°C by a factor 2 to 3. It is connected with the formation of a protective scale (rutile and corundum with chromium and vanadium additions), refining of the alloy structure, and the formation of an internally oxidized underscale.
Directionally Solidified Mullite Fibers
Authors: Sayir Ali , Farmer Serene C. 
Published: April 2011

ABSTRACTDirectionally solidified fibers with nominal mullite compositions of 3Al2O3·2SiO2 were grown by the laser heated float zone (LHFZ) method at NASA Lewis. High resolution digital images from an optical microscope evidence the formation of a liquid-liquid miscibility gap during crystal growth. Experimental evidence shows that the formation of mullite in aluminosilicate melts is in fact preceded by liquid immiscibility. The average fiber tensile strength is 1.15 GPa at room temperature. The mullite fibers retained 80 % of their room temperature tensile strength at 1450 °C. SEM analysis revealed that the fibers were strongly faceted and that the facets act as critical flaws. Examined in TEM, these mullite single crystals are free of dislocations, low angle boundaries and voids. Single crystal mullite showed a high degree of oxygen vacancy ordering. Regardless of the starting composition, the degree of order observed in polycrystalline fibers was lower than that observed in the mullite single crystals.
A Numerical Analysis of Crack Growth in Microcracking Ceramic and Ceramic Composites
Authors: Biner S. B. 
Published: April 2011

ABSTRACTA set of numerical analyses of crack growth was performed to elucidate the influence of microcracking on the fracture behavior of microcracking ceramic and ceramic composites. The random nucleation, orientation and size effects of discrete nucleating microcracks and resulting interactions are fully accounted for in a hybrid finite element model. The results obtained from the finite element analysis are compared with the continuum description of the microcracking. Although continuum description can provide a reasonable estimation of shielding, it fails to resolve the details of the micromechanism of toughening resulting from microcracking, since not every shielding event during the course of crack extension corresponds to an increase in the Rcurve. Moreover, as seen in the composite cases, the local events leading to toughening behavior may not be associated with the microcracking, even in the presence of large population of microcracks.
Stress Transfer from Axially Loaded Fiber to Matrix in a Microcomposite
Authors: Hsueh Chun-Hway 
Published: April 2011

ABSTRACTThe shear lag model has been used extensively to analyze the stress transfer in a singe fiberreinforced composite (i.e., a microcomposite). To achieve analytical solutions, various simplifications have been adopted in the stress analysis. Questions regarding the adequacy of those simplifications are discussed in the present study for the following two cases: bonded interfaces and frictional interfaces. Specifically, simplifications regarding (1) Poisson's effect, and (2) the radial dependences of axial stresses in the fiber and the matrix are addressed. For bonded interfaces, the former can be ignored, and the latter can generally be ignored. However, when the volume fraction of the fiber is high, the radial dependence of the axial stress in the fiber should be considered. For frictional interfaces, the latter can be ignored, but the former should be considered; however, it can be considered in an average sense to simplify the analysis. Comparisons among results obtained from analyses with various simplifications are made.
Single-Fiber Push-In vs. Single-Fiber Push-Out: A Comparison Between Two Test Methods to Determine the Interfacial Properties of Brittle Matrix Composites
Authors: Ferber M. K. , Lara-Curzio E. , Russ S. E. , Chawla K. K. 
Published: April 2011

ABSTRACTInterfacial properties of Nicalon™-reinforced brittle matrix composites were determined from single-fiber load-controlled push-in tests using a Mechanical Properties Microprobe (Nanoindenter) and single-fiber displacement-controlled push-out tests using the Interfacial Test System (ITS). A comparison between the results from these two tests is presented along with a discussion of data analysis techniques.
Microstructures of BN/SiC Coatings on Nicalon Fibers
Authors: Dickerson R.M. , Singh M. 
Published: April 2011

ABSTRACTThe microstructures of Nicalon silicon carbide (SiC) fibers and layered coatings of boron nitride (BN) followed by chemical vapor infiltrated silicon carbide (CVI-SiC) were characterized using optical and electron microscopy. Two different precursors and reactions were used to produce the BN layers while the deposition of CVI silicon carbide were nearly identical. Coated tows were examined in cross-section to characterize the chemistry and structures of the constituents and the interfaces. One BN precursor yielded three sublayers while the other gave a relatively homogeneous nanocrystalline layer.
Thermal Shock Behavior of Fiber-Reinforced Ceramic Composites
Authors: Singh Raj N. , Wang Hongyu 
Published: April 2011

ABSTRACTThe influence of fiber type and method of composite fabrication on the thermal shock behavior of 2-D fiber-reinforced ceramic composites is studied. Thermal shock tests are performed using a water quench technique, and thermal shock damage is characterized by both destructive and nondestructive techniques. It is shown that the composites possessed superior resistance to thermal shock damage than the monolithic ceramics. Catastrophic failure due to severe thermal stresses is prevented in composites and a significant portion of their original strength is retained at a quench temperature difference up to 1000°C. These results along with an analysis of the thermal shock damage mechanism based on the destructive and nondestructive tests is described.
High-Temperature Flow of SiC Continuous Fiber-Glass Ceramic Matrix Composites: The Effect of Interface/Interphase Ductility
Authors: Nair B.G. , Cooper R.F. , Almquist J.N. , Plesha M.E. 
Published: April 2011

ABSTRACTThe elevated temperature rheology of continuous SiC (Nicalon®) calcium aluminosilicate glass-ceramic matrix composites is evaluated in uniaxial compression creep experiments (-σ1 = 20-to-40 MPa; T = 1300–1320°C). The steady state strain rate is demonstrated.to be highly sensitive to the orientation of the reinforcement relative to the maximum compressional stress, with highest bulk specimen strain rates noted for conditions in vWhich the sliding between the fiber and the matrix is optimized as a kinetic flow response (i.e., a fiber orientation of approximately 40-50° from σ1). One further discovers that the temperature sensitivity (i.e., activation energy) of flow increases as the amount of interface flow/sliding increases. The experimental results suggest that the high-temperature, low-stress interface response in this composite system is related to the ductile flow of the “Planar” SiO2 reaction-layer interphase that exists (in addition to the well-recognized planar carbon interphase) in these materials. The results of these simple experiments are used to calibrate a microscale-to-macroscale rheologic model in which the fibermatrix interface is described by a viscous constitutive relationship.
Effects of Helium Pre-Implantation on the Microstructure and Mechanical Properties Of Irradiated 316 Stainless Steel
Authors: Toloczko Mychailo B. , Tedeski Gregg R. , Lucas Glenn E. , Odette G. Robert , Stoller Roger E. , Hamilton Margaret L. 
Published: April 2011

AbstractTransmission electron microscopy (TEM) specimens of a First Core heat of 316 stainless steel, in both the solution annealed and 20% cold worked condition, were irradiated to 46 dpa at 420°C, to 49 dpa at 520°C, and to 34 dpa at 600°C in FFTF/;MOTA. Prior to irradiation, about half of the specimens were pre-implanted with approximately 100 appm of helium, and of these, several of the solution annealed and pre-implanted specimens were aged at 800°C for 2h. Post-irradiation density measurements showed small differences in density between the unimplanted alloys and their helium implanted counterparts. Microstructural observations on specimens irradiated at 420°C and 520°C showed relatively minor differences in defect distributions between the unimplanted and the helium implanted materials; in all cases the defect sizes and number densities were consistent with data in the literature. Where possible, irradiation hardening of the alloys was experimentally evaluated by microhardness and shear punch; experimentally obtained values were compared to values calculated using a computer model based on barrier hardening and the microstructural data. All methods indicated relatively small effects of helium implantation, and both measured and calculated values were in agreement with the range of values reported in the literature.
Amorphization and the Effect of Implanted Ions in Sic
Authors: Snead L. L. , Zinkle S. J. 
Published: April 2011

AbstractThe effects of implanted ion chemistry and displacement damage on the amorphization threshold dose of SiC were studied using cross-section transmission electron microscopy. Room temperature as well as 200 and 400°C irradiations were carried out with 3.6 MeV Fe, 1.8 MeV Cl, 1 MeV He or 0.56 MeV Si ions. The room temperature amorphization threshold dose in irradiated regions well separated from the implanted ions was found to range from 0.3 to 0.5 dpa for the four different ion species. The threshold dose for amorphization in the He, Si and Fe ion-implanted regions was also σ0.3 to 0.5 dpa. On the other hand, the amorphization threshold in the Climplanted region was only about 0.1 dpa. The volume change associated with amorphization was σ17&. No evidence for amorphization was obtained in specimens irradiated at 200 or 400°C.
Ion Beam Modification of TI-Ba-Ca-Cu-O Type High Temperature Superconductors During Irradiation
Authors: Newcomer P.P. , Wang L. M. , Morosin B. 
Published: April 2011

AbstractMicrostructural modification of high temperature superconductor (HTS) single-crystal plates of T1-1212 and T1-2212 (numbers designate the Tl/Ba/Ca/Cu cation ratio) was studied during 1.5 MeV Kr+ and Xe+ ion irradiation with in-situ electron diffraction and after ion irradiation with high resolution TEM (HRTEM). Similar in-situ temperature dependence effects are seen for both phases. During irradiations from 22K to 673K, an amorphous halo develops after very low ion dose or fluence (l.7X1012 ions/cm2). During irradiation at 100K and 300K, complete amorphization is obtained, while at 22K and ≥533K, the halo fades slightly and a polycrystalline ring pattern develops, indicating ion irradiation induced crystallization occurred. After a low ion dose (8.5XlO12ions/cm2) at 100K and 300K, HRTEM reveals amorphous regions 5 -20 nm in size which are not columnar and do not all penetrate the entire sample thickness. At 22K and ≥533K, Moire fringes and misoriented crystallites of cascade size are observed. The 4 - 6nm crystallites are thallium-rich.
Production of Defects in Metals by Collision Cascades: Tem Experiments
Authors: Kirk M. A. 
Published: April 2011

AbstractI will review our experimental TEM data on the production of dislocation loops by low energy ion bombardment to low doses, as simulations of similar collision cascades produced by fast neutron irradiation, in various metals and alloys. The dependence of vacancy dislocation loop formation on recoil energy, sample temperature, and specific metal or alloy will be examined. Special emphasis will be placed on the effects of dilute alloy additions. A model for cascade melting will be employed to understand these effects, and will require an examination of the role of electron-phonon coupling in cascade cooling and recrystallization. The formation of interstitial dislocation loops as cascade defects, and the influence of the nearby surfaces in these experiments will be briefly discussed.
Cascade Damage in the Ordered Alloy Ni3AI
Authors: Bui Tho X. , Robertson I.M. , Kirk M. A. 
Published: April 2011

AbstractThe damage (disordered zones and vacancy loops) produced in the ordered alloy Ni3AI by irradiation with different ions (Ar+, Kr+, Xe+), ion energies (30 and 50 keV), and at 30 and 300 K has been investigated by using transmission electron microscopy. At room temperature and at constant ion energy, the average size of the disordered zones increases as the ion mass increases from Ar+ to Kr+and to Xe+ ions. This trend is opposite to that of the collisional volume which decreases with increasing ion mass and highlights the importance of the thermal spike in causing disorder. At room temperature and at constant ion mass, the zone size increases with increasing ion energy. At 30K, for the same ion mass and ion energy, the size of the disordered zones is smaller than at room temperature. At both irradiation temperatures, the dislocation loop yield increases with increasing ion energy and ion mass. The loop yield was lower for irradiations at 30K compared to at 300 K. Comparison of the dislocation loop yield and the size of the disordered zones indicates that the probability for forming a loop increases with the size of the disordered zone.
Excited State Absorption in Cubane-Like Transition Metal Clusters
Authors: Ji W. , Du H. J. , Shi S. 
Published: April 2011

AbstractWe report an investigation of excited-state absorption in cubane-like transition metal clusters. The fluence-dependent transmittances of the clusters have been measured with using 8-ns laser pulses. Time-resolved transmission measurements show that the triplet-triplet absorption occurs within a few nanoseconds. We have also developed a five-level (two singlet states, two triplet states, and one ionized state) model to simulate the excited-state absorption in these cluster compounds. The comparison between the model and the experimental measurements indicates that the population of the triplet states is created mainly by an ionization-recombination process and the triplet-triplet transitions are responsible for the observed nonlinear absorption.
Spectral Properties and Chi(3) of Metallo Meso-Tetra-Substituted Tetrabenzporphyrins
Authors: Nakashima Masato , Roach Joseph F. , Yeomans Walter G. , Alabran David M. 
Published: April 2011

AbstractWe have reported high Chi(3) values observed for metallo meso-tetra (substituted phenyl) tetrabenzporphyrins (metallo TPTBP). In order to find the relationship between the molecular structures and the Chi(3) values, we have attempted to find relationships between these values and the electronic spectral properties of the compounds. For a series of Zn TPTBPs, we found that the B and Q band peak wavelengths and the peak height ratio between these two bands are related as one would expect from such a series of porphyrins. The Chi(3) values appear to be directly related to the Q/B peak height ratio. For a series of metallo meso-tetra(p-methoxyphenyl) tetrabenzporphyrins, such a correlation of Chi(3) with the spectral properties of various metal species was not apparent. Strategies to increase the Chi(3) values of metallo TPTBP on the basis of these observations are discussed.
Optical Limiting in Protonic Doped Bis-Benzothiazole
Authors: Tang N. , Hellwarth R. W. , Partanen J. P. , Sun S. , Dalton L. R. 
Published: April 2011

AbstractA ∼13% increase in the absorption coefficient is seen by a weak probe beam to build-up exponentially in ∼50 ps and decay in ∼1.4 ns after a ∼30 ps intense pump pulse (∼220 mJ/cm2 ) at 532 nm goes through a bis-benzothiazole/H2SO4 solution. We propose a three-level model to explain the effect and obtain the basic parameters by fitting our data to the model.
Approaches for Optimizing and Tuning the Optical Limiting Response of Phthalocyanine Complexes
Authors: Perry J. W. , Mansour K. , Marder S. R. , Chen C.-T. , Miles P. , Kenney M. E. , Kwag G. 
Published: April 2011

AbstractRecent progress on the use of molecular engineering approaches for the enhancement and spectral tuning of the optical limiting responses of phthalocyanine complexes is presented. Examples are given involving the use of the heavy-atom approach to enhance the limiting response of naphthalocyanines and of donor substitution of naphthalocyanines to red shift the optical limiting response, as demonstrated for indium and tin octabutoxynaphthalocyanines. These approaches have led to new materials for optical limiting with potential for high performance. An experimental demonstration of the “bottleneck” limiter concept, in which the optical-limiting material is distributed nonhomogeneously to permit strong pumping of excited-state population throughout the interaction region, is presented and shows an order-of-magnitude increase in pulse suppression compared to a homogeneous distribution, without an increase in linear absorbance.
Nonlinear Optical Properties of Tetrabenzporphyrins
Authors: Wood Gary L. , Mott Andrew G. , Miller Mary J. , Nakashima Masato , Roach Joseph F. 
Published: April 2011

AbstractOptical limiting in a solution of tetrabenzporphyrin (TBP) dissolved in tetrahydrofuran (THF) was measured in an f/5 optical focusing arrangement using 532 nm wavelength laser light with a 9 ns pulse duration. The performance of this solution was compared to a variety of other materials. In order to better understand the nonlinear behavior of this material, apertured and unapertured Z-scans were performed. From these measurements it was determined that the excited-state cross section and its ratio to ground-state cross section were large. The nonlinear refraction of this material is made up of contributions from thermal density change and population redistribution. Limiter performance was modeled in an f/64 limiter. The limiter behavior of a slightly modified compound and an additional solvent were measured.
Thermally Activated Transmissive Optical Elements for Rejection of Laser Radiation
Authors: Marcy H. O. , Warren L. F. , Cunningham P. H. 
Published: April 2011

AbstractThe sacrificial thermal shutter/reflector concept is suitable for optical power limiting in optical systems containing an intermediate focus and uses a thermally activated chemical process to form a mirror that reflects a large fraction of the incident laser light. Under normal illumination, the thermal reflector component is a transmissive optical element at or near a focal plane of an optical system. A pulse from an incident laser beam heats a film of precursor chemicals on the optical element and thereby converts it to a localized reflective metallic mirror. For visible laser pulses longer than 0.1 ms, the thermal shutter provides optical limiting at energies below the ANSI Maximum Permissible Exposure (MPE) standard for laser eye protection. For shorter pulses, near-MPE limiting is achieved. See-through during exposure is also possible with these elements. Various thermal shutter chemistries and efforts to form optically clear thin precursor films are described. The laser testing results highlight the importance of the substrate and absorber in these devices.
U.V Induced Visible Band Excited State Absorption in Liquid Crystal Media.
Authors: McEwan Kenneth J. , Hollins Richard C. 
Published: April 2011

AbstractThe transient absorption, in the visible band, induced by a UV laser pulse at 337nm in three different liquid crystal materials is studied. Two decay channels are observed and assigned to the relaxation of the singlet and triplet excited state populations. The fluorescence is also measured under both UV and two-photon (590nm) excitation and the rate of decay is found to compare well with the rapidly relaxing component of the induced absorption. A strong temperature dependence on both the fluorescence and absorption transients is observed and this is discussed in the context of excimer formation.
Polymer Hosts for Non-Linear Absorbing Materials
Authors: McGeoch Stephen P , Christie Alan , Bahra Gurmit S , Welford Kevin R 
Published: April 2011

AbstractThere is a recognised requirement for solid-phase, non-linear absorbing devices. We present data for a range of non-linear absorbing dyes in polymer hosts. We identify polycarbonate as a useful host material. We recognise that progress will be required to increase the laser-induced damage threshold of the dyed polymers if practical devices are to be realised.
Novel Polycarbonate Diacetylene Materials for Optical Limiting
Authors: Sandman Daniel J. , Conroy Jeffrey L. , Piche Joseph 
Published: April 2011

AbstractIn the interest of finding useful materials systems for optical limiting, we have sought to combine the attractive third order nonlinear optical properties of polydiacetylenes (PDA) with the desirable ballistic and mechanical properties of polycarbonates in our study of two different systems. Composites of Lexan polycarbonate with the PDA from 1,6-di-N-carbazolyl-2,4-hexadiyne have attractive laser power transmission characteristics. Macromonomers derived from the chloroformates of bis -phenols A and F and diacetylene diols were synthesized, their spectral, molecular weight, and thermal properties were studied, and the crosslinking of the macromonomers to PDA structures was investigated.
Grain Formation in Polycrystalline Silicon Films Deposition on SiO2 at Very Low Temperatures
Authors: Wang Kun-Chih , Wang Ruo-Yu , Yew Tri-Rung , Loferski Joseph J. , Hwang Huey-Liang 
Published: April 2011

ABSTRACTThis paper describes the grain formation in very low temperature polycrystalline silicon (poly-Si) growth on SiO2. The silicon films were deposited by electron cyclotron resonance chemical vapor deposition with hydrogen dilution at 250°C and without any thermal annealing. The largest grain sizes observed in the poly-Si film is about 1 μm. The grains have a leaf-like shape as observed in plan-view transmission electron microscopy. The grain morphologies were determined by cross-sectional transmission electron microscopy and atomic force microscopy. Raman scattering spectrum was used to determine the crystalline fraction. X-ray diffraction patterns were used to study the film crystallinity. A simple model of grain formation is proposed.
Defects, Energies, Hydrogen, and Structural Properties of Molecular-Dynamics Modeled a-Si:H
Authors: Fedders P. A. 
Published: April 2011

ABSTRACTWe use ab initio pseudopotential local-density-approximation methods to create and study related supercells containing 62 Si atoms and from 5 to 7 H atoms. In particular, we obtain energies and structural properties of H in different charge states that are passivating dangling bonds, in bond centered positions, and in other interstitial sites. The most striking result found is the rather large (of order 1 eV) spread of energies for a given type of defect, depending on its surroundings. We also find that changes in single particle energies or energy eigenvalues from defect to defect are not particular close to changes in the total energy and that the distinction, between bond centered H and a dangling bond plus an Si-H bond, is not as clear cut as one might think. The calculations also provide insights into possible migration mechanisms and energies for H movement.