bluuebunny/crossref_metadata_2025_split huggingface dataset:
Comparative Studies of Optical Spectra in PbF2 at Different Excitations
Authors: Egorov V. K. , Klassen N.V. , Negrii V.D. , Prokopenko V.M. , Shmurak S.Z. , Sinitzin V.V. , Solov'ev A.V. 
Published: April 2011

ABSTRACTThe spectral characteristics for cubic and orthorhombic lead fluoride excited by light with energies of 5–2.5 eV, X-ray, and α-particles were studied. It is shown that the room temperature luminescence in orthorhombic lead fluoride is connected with resonance excitations of some luminescence centers. Possible models of the centers responsible for this phenomenon are proposed.
Suppression of BaF2 Slow Component of X-RAY Luminescence in Non-Stoichiometric Bao.9R0.1F2.1 Crystals (R=Rare Earth Element)
Authors: Sobolev B.P. , Krivandina E.A. , Derenzo S.E. , Moses W.W. , West A.C. 
Published: April 2011

ABSTRACTA series of BaF2 crystals doped with 10% mole fraction of each rare earth element has been grown, and the effect of these dopants on the slow luminescence (due to anionic self trapped excitons) and fast luminescence (due to core-valence transitions) has been studied. While significant suppression was observed for both components, the best dopants for suppressing the slow component of barium fluoride (up to 25-50%) while maintaining the luminosity of the fast component are La, Y, and Lu. The luminescence of Ba0.9Eu0.1F2.1 is almost entirely fast (>90%), but with low light output. For two rare earth dopants (La and Gd), the effect of slow and fast component suppression was studied as a function of concentration (Ba1-xRxF2+x with x≦0.5 for R=La and x≦0.3 for R=Gd). The suppression is non-linear with dopant concentration, with the relative degree of slow component suppression correlated with the melting point of these samples.
The Scintillation Mechanism in PbWO4 Crystals
Authors: Korzhik M.V. , Pavlenko V.B. , Katchanov V.A. , Peigneux Jean-Pierre , Poulet Maurice 
Published: April 2011

ABSTRACTA mechanism of scintillations based on sensitisation of radiating centres luminescence by Pb2+ ions is proposed for PbWO4 crystals.
Monte-Carlo Study of Energy Losses in Hot Stage of Electronic Excitation Relaxation in Scintillators
Authors: Glukhov Roman A. , Vasil'ev Andrey N. 
Published: April 2011

ABSTRACTThe results of computer simulation of the fast stages of energy relaxation in insulators after the VUV or XUV photon absorption are presented. The simulation involves two stages:the inelastic scattering of excitations with production of secondary excitations and thermalization through phonon emission. The main attention is focused on the spatial distribution functions of excitations, i.e. one-particle and two-particle distribution functions.The latter Function determines the energy transfer at final stages of energy relaxation and is important for different quenching processes and the acceleration of the luminescence decay. The Monte-Carlo simulation was carried out for BaF2 crystal for photon energies from 20 eV to 100 eV. The simulation shows that the two-particle distribution functions and thus the kinetics depend on the energy of excitation.
The Mechanism of Slow Component Suppression in Lanthanum Doped Barium Fluoride Crystal
Authors: Mu Gu , Lingyan Chen , Qing Li , Liming Wang , Kaihua Xiang 
Published: April 2011

ABSTRACTThe electronic structures of pure BaF2 crystal and lanthanum doped BaF2 crystal have been calculated in a self-consistent molecular-cluster model. The cluster is embedded in the crystal lattice and the entire system treatediteratively in the Hartree-Fock-Slater local-density theory. As lanthanum doped BaF2 is concerned, the obtained results revealed that the F1–i which is introduced by the lanthanum may contribute to the suppression ofthe slow component in the scintillation light of BaF2 crystal.
Low-Voltage Cathodoluminescent Properties of Europium-Activated Anion-Deficient Fluorites
Authors: Anderson Mark T. , Phillips Mark L. F. , Walko Robert J. 
Published: April 2011

ABSTRACTWe have examined the optical bandgap, particle size distribution, photoluminescence spectra, and cathodoluminescent response of fifteen phosphors that contain transition-metal and main-group sensitizers. We determined luminance versus applied voltage curves for electron energies from 5 to 1000 eV. Seven phosphors exhibit threshold voltages less than 110 V. Y1.96Eu0.04O3 exhibits a threshold voltage of 13 V and, at 300 V, displays a luminance of 25 fL and a luminous efficiency of 0.02 lm/W.
Particle Size Effects in YAG:CR Phosphors
Authors: Mckityrick J. , Hoghooghi B. , Dubbelday W. , Kavanagh K. , Kinsman K. , Shea L. , Sluzky E. 
Published: April 2011

ABSTRACTChromium doped yttrium aluminum garnet (Y3Al5O12:Cr or YAG:Cr) phosphors were synthesized by hydrolysis of aqueous nitrate solutions. The resulting as-synthesized powder was amorphous and crystallized into the YAG structure at ~1000°C. The light output as a function of annealing temperature was found to increase with increasing temperature. Particle size, adsorbed surface species or residual impurities, lattice stability and chromium site occupation were investigated as possible explanations. It was found that there was a small decrease in lattice parameter upon annealing, the particle size increased roughly five times, and residual or adsorbed surface species were present for the low temperature annealed powders. There was no evidence that kinetic barriers to diffusion of Cr ions into the luminescent sites occurred.
The Effect of Elevated Temperature Exposure of Composites on the Strength Distribution of the Reinforcing Fibers
Authors: Gambone M. L. , Wawner F. E. 
Published: April 2011

AbstractUnidirectionally-reinforced Timetal® 21S composite specimens were subjected to elevated temperature heat treatments. The SiC fibers were then chemically extracted from the matrix, and their tensile strengths were measured at room temperature. A Weibull statistical analysis of fiber strength distribution was performed to compare the Weibull parameters of fibers from the as-consolidated and heat-treated composites. Fractographic analysis of the tested fibers was used to identify the flaws which caused failure in each condition. Surface flaws were found to initiate low strength failures in all conditions, and the number of surface initiated failures increased with an increase in severity of heat-treatment. A relationship between the fiber/matrix chemical reaction and surface flaw development is demonstrated. A fracture mechanics analysis that explains the relationship between surface flaw size, fiber fracture toughness, and the measured tensile strengths is suggested.
Mechanical Behavior of Pressure-Cast Al2O3/NiAlFe Composites
Authors: Liu Cyrus , Jeng S. M. , Yang J.-M. , Slate E. , Cook A. 
Published: April 2011

AbstractThe microstructure and mechanical behavior of Al2O3 fiber-reinforced NiAlFe composites fabricated using the pressure casting technique were investigated. The mechanical response and fracture behavior of these unidirectional composites under tensile and three-point flexural loading were characterized. The results were compared to those measured for Al2O,/NiAl-based composites fabricated by powder metallurgy techniques. Preliminary results indicate that pressure casting appears to be a viable processing route for fabricating continuous fiber-reinforced NiAl-based composites.
Synthesis and Mechanical Properties of Nano-Scale NiAl and NiAl/Al2O3 Composites
Authors: Smith Timothy R. 
Published: April 2011

AbstractThis work reports on the synthesis, characterization and preliminary mechanical properties of nanocrystalline NiAl/Al2O3 composite materials. Nano-scale crystallites of NiAl were formed by high energy milling of pre-alloyed powder with and without Al2O3 under an Ar atmosphere. Consolidation of the nano-scale powders was done by sinter-forging at < 0.4 Tm in air. Characterization of the powders and consolidated microstructures and hardness data are reported.
Fatigue and Fracture of Nickel Aluminide Composites
Authors: Ramasundaram Padu , Bowman Randy , Soboyejo Wole 
Published: April 2011

AbstractThe results of preliminary investigations of the room-temperature fatigue and fracture mechanisms in model NiAl composites are presented. The composite systems studied include: NiAl reinforced with ductile second phase particles (Mo); ductile fibers (Mo); brittle second phase particles (zirconia partially stabilized with yttria) and brittle fibers+particles (Al2O3 + zirconia). Mechanisms of fatigue crack growth in heat treated specimens of the fiber-reinforced composites are also elucidated. The investigations indicate that both ductile and brittle reinforcements can enhance the toughness of NiAl significantly, and the ductile phase particulate reinforcement may even contribute to room-temperature ductility in the composite.
Atomistic Simulation of Dislocation Motion as Determined by Core Structure
Authors: Ternes Kevin , Farkas Diana , Xie Zhao-Yang 
Published: April 2011

AbstractTwo different interatomic potentials of the embedded atom type were used to study the relationships between dislocation core structure and mobility. Core structures were computed for a variety of dislocations inB2 NiAl. Several non-planar cores were studied as they reacted to applied stress and moved. The results show that in some cases, the dislocation core transforms to a planar structure before the dislocation glides, whereas in some other cases the core retains the non-planar structure at stresses sufficient to sustain glide. The effects of stoichiometry deviations on the core structure and motion were also studied.
Bio-Mimetic Composites
Authors: Burdon Jeremy , Szmania Jeffrey , Calvert Paul 
Published: April 2011

ABSTRACTThe properties of bone, as a polymer reinforced with nanometer-sized ribbon-shaped crystals of mineral, are compared with the properties of synthetic polymer composites. Bone does show some superiority to existing composites. The improvements can be attributed to the microstructure. Methods for reproducing this structure in a synthetic material are discussed.
Frequency-Dependent Second Harmonic Generation in Acentric Chromophoric Self-Assembled NLO Materials
Authors: Yitzchaik Shlomo , Lundquist Paul M. , Lin Weiping , Kanis David R. , Ratner Mark A. , Marks Tobin J. , Wong George K. 
Published: April 2011

ABSTRACTAn attractive and challenging approach to the construction of robust, thin film materials with large second-order optical nonlinearities is the covalent self-assembly of aligned arrays of high-β molecular chromophores into multilayer superlattices. In this paper, we describe the dispersion of second harmonic generation (SHG) in a self-assembled (SA) monolayer containing a stilbazolium chromophore. The frequency-dependent measurements were performed on 25 Å thick monolayers on glass using a tunable (0.4–2 μm) light source based on optical parametric amplification (OPA). The SHG spectrum contains a clear two-photon resonance at hω = 1.3eV. The maximum in the second-order susceptibility coincides with a low energy chromophore-centered charge-transfer excitation at 480 nm. The experimental SHG dispersion values compare favorably with theoretical results computed using a sum-over-states (SOS) formalism. However, the measured values exhibit a somewhat broader band response than the theoretical curve, and the origin of this behavior is discussed.
Optical Studies of Silicon Nanocrystals in Colloidal and Sol-Gel Matrices
Authors: Lee Howard W. H. , Davis Jeffrey E. , Olsen Margaret L. , Kauzlarich Susan M. , Bley Richard A. , Risbud Subhash H. , Duval David J. 
Published: April 2011

ABSTRACTWe studied the optical properties of silicon nanocrystals incorporated into colloidal and solgel matrices. The silicon nanocrystals are produced by ultrasonic dispersion of porous silicon layers. We report results on the dependence of the photoluminescence (PL) spectra with excitation intensity. The PL shows a blue peak (at ∼ 415-460 nm.) and a red peak (at ∼ 680 nm). This PL spectrum shows a remarkable dependence on the excitation intensity. As the intensity is increased, the blue peak grows at the expense of the red. A model is suggested for this behavior. We also report on the excitation intensity dependence and the emission wavelength dependence of the PL decay at low (1 kHz) and high (82 MHz) repetition rates of optical excitation. When low repetition rate excitation is used, the PL decay times are all exponential, short (ns), and appear to vary little with emission wavelength. This sharply contrasts with what is observed in porous silicon. With high repetition rate excitation, both red and blue peaks show long (100's ns) and short (ps-ns) lifetime components. We contrast the different optical properties of these silicon nanocrystals with that observed in porous silicon.
Relations Between Structure and Texture in Nano-Sized Aluminas
Authors: Coster Dominique J. , Levitz Pierre , Fripiat Jose J. 
Published: April 2011

ABSTRACTAlumina xerogels were prepared by a two step hydrolysis in non-aqueous media. This synthesis produced high surface area materials with controlled amount of pentacoordinated aluminum. The effect of heat treatment and of the preparation procedure on the physical characteristics of the powder was determined by nitrogen adsorption and X-Ray diffraction, whereas the aluminum coordination was studied by 27Al MAS NMR. The pore size distribution and the pentahedral aluminum content (up to 700°C) are closely related to the degree of condensation of the prehydrolysed oligomer species.
Synthesis and Characterization of Star Polypeptide Nonlinear Optical Materials
Authors: Cooper Thomas M. , Su Weijie , Tokarski Zbigniew , Adams W. Wade 
Published: April 2011

ABSTRACTTo develop novel polypeptide-based thin films, a series of star polypeptides modified with nonlinear optical chromophores has been synthesized. Using amino-substituted tetraphenyl porphyrin as an initiator, the N-carboxy anhydride of γ-benzyl L-glutamic acid was polymerized onto the porphyrin at a monomer to initiator ratio 20:1. The resulting four-branch star was modified with a selection of dyes. Dyes that modified both the N-terminus and benzyl side chain were used. We demonstrated feasibility of insertion of a metal ion into the polypeptide porphyrin core. The polypeptide series was characterized by UV/VIS, FTIR, and CD. The UV/VIS data suggested ease of modification of both the N-terminus and side chains. The FTLR and CD data show the resulting polypeptides were ∝-helical. The results demonstrate the feasibility of modifying the optical properties of a porphyrin by three approaches: insertion of metal, attachment of dye to N-terminus or modification of γ-benzyl L-glutamate side chain.
Molecular Sieve Based Chemical Sensors
Authors: Sottile Laura J. , Balkus Kenneth J. , Riley Scott J. , Gnade Bruce E. 
Published: April 2011

ABSTRACTBy virtue of their shape selectivity and stability, molecular sieves are ideal components for discriminating chemical sensors. In this paper we report the preparation of capacitance type sensors based on aluminum phosphate molecular sieves. Thin films of the molecular sieves AlPO4-5, A1PO 4-H3 and AlPO4-H1, which cover a range of pore dimensions, were deposited on titanium nitride coated silicon wafers by laser ablation. A subsequent hydrothermal treatment followed by a Pd/Au coating and the application of standard photoresist/masking techniques were used to generate the capacitors. The molecular sieves exhibit significant changes in capacitance upon exposure to target molecules, including CO2, CO, N2, H2O and toluene.
Self-Assembly of Oriented Metal Bisphosphonate Multilayers with Potential Nonlinear Optical Properties
Authors: Neff Grace Ann , Zeppenfeld Astrid C. , Klopfenstein Bethany , Page Catherine J. 
Published: April 2011

ABSTRACTWe are investigating a method for synthesizing oriented layered thin film structures that exhibit nonlinear optical properties. This method involves self-assembly of metal bisphosphonate multilayers on gold and silicon substrates. Multilayers are characterized via optical ellipsometry, diffuse reflectance and attenuated total reflectance FTIR, and grazing angle X-ray diffraction.To incorporate nonlinear optical activity into self-assembled films, we use a,w-bisphosphonates containing aromatic π systems sandwiched between donor and acceptor groups. In order to maximize the overall polarizability of the film, the polarizable bisphosphonate molecules must preferentially bind in only one orientation. To accomplish this, one of the terminal phosphonate groups of the molecule is protected (in ester form). After binding the free phosphonic acid end, the terminal ester is converted to a phosphonic acid group by hydrolysis with bromotrimethylsilane (BTMS). Layering is continued by binding a metal layer and repeating the cycle. Films prepared via this route do exhibit second harmonic generation, but apparently incomplete deprotection of each layer leads to increasing disorder with increasing number of layers.
Characterization of Silicon Nanoparticles Prepared from Porous Silicon
Authors: Bley Richard A. , Kauzlarich Susan M. , Lee Howard W. H. , Davis Jeffrey E. 
Published: April 2011

ABSTRACTNanometer sized silicon particles have been produced by ultrasonic dispersion of thin sections of porous silicon in organic solvents. High resolution TEM and FTIR have been used to establish the size range and surface structure/composition of these particles. The larger particles, which range in size from 20 to 50 nm, are made up of a conglomeration of smaller particles with a diameter of a few nanometers. The HRTEM shows an amorphous layer on the surface of many of the clusters. FTIR data suggest this amorphous layer is silicon-dioxide which may also have organic constituents.
A Low Temperature, Solution Phase Synthesis of III-V Semiconductor Nanocrystals
Authors: Kher Shreyas S. , Wells Richard L. 
Published: April 2011

ABSTRACTNanocrystalline materials have been intensely investigated in the recent past due to the novel properties associated with size-quantized particles. We have developed a new method for high yield, solution phase synthesis of nanocrystalline III-V semiconductors which eliminates the use of substituted or unsubstituted Group V hydrides and Group III alkyls. Our approach consists of in situ syntheses of (Na/K)3E (E = P, As, Sb) in aromatic solvents and subsequent reactions of these pnictides with Group III halide solutions in coordinating solvents. The nanocrystalline III-V semiconductors GaP, GaAs, GaSb, InP, InAs and InSb are readily prepared in a wide range of particle sizes (4–36 nm) and in high yields. The resultant Ill-V materials have been characterized by XRD, EDXA, TEM and elemental analyses.
Molecular Precursors to Functional Materials
Authors: Herron Norman , Thorn David L. , Harlow Richard L. 
Published: April 2011

ABSTRACTThe design of molecular precursors to materials for catalysis will be described. Aluminum fluoride is an important catalytic material for fluorocarbon transformations which are key to the production of CFC alternatives. Catalytic activity is closely related to the crystalline phase of the bulk AIF3 and we show how this phase chemistry can be precisely controlled using molecular precursors to produce pure known phases and also to extend to previously unknown phases. A similar approach to vanadyl phosphate oxidation catalysts will be described where cluster chemistry can be invoked to generate isolated fragments of a catalyst's structure and then used either to explore mechanism or be used as precursors to functional catalyst materials. The underlying themes of controlled molecular precursor synthesis, resultant processability and eventual easy conversion to useful materials are emphasized throughout.
Interfacial Structure of Electrically Conducting Adsorbed Multilayers
Authors: Kellogg G. J. , Mayes A.M. , Stockton W. B. , Ferreira M. , Rubner M.F. , Satija S.K. 
Published: April 2011

ABSTRACTThe multilayer organization of sulfonated polyaniline/polyallylamine (SPAN/PAH) films was investigated by neutron reflectivity. Films were prepared by sequential adsorption of polycations and polyanions from dilute solution. Scattering contrast was achieved by selective deuteration of blocks of bilayers at varying intervals along the films. The internal organization of the multilayer structure was found to decay significantly with increasing number of bilayer depositions. In addition, a high degree of roughness was determined at the free surfaces of the films. Together, these results suggest that both conformal and nonconformal roughness are present in the films, i.e., defects introduced during individual layer depositions are to some degree transferred to the surface of subsequently deposited layers.
Determination of the Bond Strength Between Nanosized Particles
Authors: Weber Alfred P. , Friedlander Sheldon K. 
Published: April 2011

ABSTRACTA method has been developed for determining the bond energies between nanosized particles from the kinetics of the rearrangement of aerosol agglomerates. The method of calculation is based on the change in Gibbs' free energy during restructuring. For Ag and Cu agglomerates, bond energies between the nanosized particles are of the order of magnitude calculated from bulk Hamaker constants.
Facet Formation of Lineshaped Silicon Mesas Grown with Micro Shadow Masks
Authors: Gossner H. , Fehlauer G. , Kiunke W. , Eisele I. , Stolz M. , Hintermaier M. , Knapek E. 
Published: April 2011

ABSTRACTAs reported previously, perfect facets can be achieved at the side walls of submicron silicon mesa structures grown by molecular beam epitaxy (MBE) with micro shadow masks [1]. An essentially self organizing, three-dimensional growth was observed. In this paper we present the results of the epitaxial growth on (001) substrates using long (≥ 1μm), lineshaped mask apertures, which put constraints on the formation of facets. At a growth temperature of 500°C {111} facet formation is observed for lineshaped mesas oriented along the <110> direction of the substrate. Side walls with a length of I μm are perfectly plane, while mesas with a length of 10 μm and more show rough sidewalls. This is explained by a limited silicon adatom diffusion on the facet. For higher flux rates the facet formation is suppressed. This can be understood in terms of a reduced adatom diffusion.A crossover from {111} to {113} facet formation is observed at growth temperatures above 500°C. A model for the temperature dependent formation of {111} and {113} facets is given.
Blue Light Emission from Silicon Ultrafine Particles
Authors: Nozaki Shinji , Sato S. , Ono H. , Morisaki H. 
Published: April 2011

ABSTRACTThe oxygen-containing silicon (Si) ultrafine particles have been deposited onto Si and SiO2 substrates by evaporation of Si powder in an oxygen-containing argon atmosphere. The asdeposited Si ultrafine particles exposed to the ultraviolet light emit blue light, which is strong enough to be seen with the naked eye. The blue light emission is associated with a broad photoluminescence (PL) peak at 2.7 eV, which is attributed to radiative recombination via a radiative recombination center. The proposed model with one radiative and two nonradiative recombination centers well explains the temperature-dependent PL peak intensity.