bluuebunny/crossref_metadata_2025 huggingface dataset:
Electromagnetic properties of photonic crystals with diamond structure containing defects
Authors: Kanehira Shingo , Kirihara Soshu , Miyamoto Yoshinari , Sakoda Kazuaki , Takeda Mitsuo Wada
Published: March 2008

Three-dimensional photonic crystals with a diamond structure, which are composed of the TiO2-based ceramic particles dispersed in an epoxy lattice, were fabricated by stereolithography. The diamond structure showed a photonic band gap in the 14.3–17.0 GHz range along the Γ-K 〈110〉 direction, which is close to the band calculation using the plain wave expansion method. Two types of lattice defects—air cavity and dielectric cavity—were introduced into the diamond structure by removing a unit cell of diamond structure or inserting a block of the lattice medium into the air cavity. The transmission of millimeter waves affected by multiple reflections at the defects was measured in the photonic band gap. Resonant frequencies in the defects were calculated and compared with the measurement results.
Effect of internal stress on elemental diffusion and crystallization of electroless Ni–Cu–P deposit on Al
Authors: Hsu Jen-Che , Lin Kwang-Lung
Published: March 2008

The type and magnitude of stress in electroless Ni–Cu–P deposits on Al were manipulated by controlling the concentration of saccharin in the plating solution. Tensile, zero, and compressive stress of the electroless Ni–Cu–P deposits was obtained with 0, 8, and 10 g/l saccharin for studying the effect of stress on the diffusion and crystallization behavior of the deposit. The effect of stress on the diffusion behavior of Cu, Ni, and Al elements during annealing was investigated. Interdiffusion between Al and Ni in an amorphous Ni–Cu–P/crystal Al diffusion couple is abated by the effects of amorphous structure, atomic affinity, and backstress. Therefore, the effect of stress on diffusion is manifested by Cu elemental diffusion. The tensile stress promotes the formation of Ni3P and the diffusion of Cu into the substrate.
Nanoindentation measurements on Cu–Sn and Ag–Sn intermetallics formed in Pb-free solder joints
Authors: Chromik R. R. , Vinci R. P. , Allen S. L. , Notis M. R.
Published: March 2008

Nanoindentation testing has been used to measure the hardness and elastic modulus of Ag3Sn, Cu6Sn5, and Cu3Sn intermetallics, as well as Sn–Ag–Cu solder and pure Sn and Cu. The intermetallics were fabricated by solid-state annealing of diffusion couples prepared from a substrate (Cu or Ag) and a solder material (Sn or Sn–Ag–Cu solder), providing geometries and length scales as close as possible to a real solder joint. Nanoindentation results for the intermetallics, representing penetration depths of 20–220 nm and loads from 0.7 to 9.5 mN, reveal elastic/plastic deformation without evidence of fracture. Measured hardness values of Cu6Sn5 (6.5 ± 0.3 GPa) and Cu3Sn (6.2 ± 0.4 GPa) indicate a potential for brittle behavior, while Ag3Sn (2.9 ± 0.2 GPa) appears much softer and ductile. Using a bulk Cu6Sn5 sample, Vickers hardness testing revealed an indentation size effect for this compound, with a hardness of 4.3 GPa measured at a load of 9.8 N. An energy balance model is used to explain the dependence of hardness with load or depth, where the observation of an increasing amount of fracture with applied load is identified as the primary mechanism. This result explains discrepancies between nanoindentation and Vickers results previously reported.
Mechanical and wear properties of Si3N4–W composites using tungsten boride powder
Authors: Hyuga Hideki , Jones Mark I. , Hirao Kiyoshi , Yamauchi Yukihiko
Published: March 2008

Silicon nitride–tungsten (Si3N4–W) composites were fabricated by reduction of tungsten boride under hot-press sintering in a nitrogen atmosphere. The fabricated composite consisted of mainly β–Si3N4 and W. The Si3N4 matrix grains were composed of an elongated and bimodal structure similar to conventional Si3N4. The mechanical properties of the composites in terms of fracture toughness and strength were almost the same as those of a monolithic Si3N4 produced under the same sintering conditions. The sliding wear properties of the composites were evaluated using a ball-on-disk machine under unlubricated sliding conditions against a commercial Si3N4 ceramic ball. The tungsten (W) content had a significant effect on the composite wear properties. In particular, for a composite disk with a W content of 8 vol% the specific wear rate of the opposing ball was decreased around ten times compared to the monolithic Si3N4. The composites had higher wear resistance compared with the conventional silicon nitride, which was attributed to the formation of debris consisting of W, Si, and O. The debris restricted the adhesion of the two surfaces.
Quantitative study of water transport during the hydrolysis of polymer coatings exposed to water vapor
Authors: Lee Sanboh , Nguyen Tinh , Byrd Eric , Martin Jon
Published: March 2008

Thermoset acrylic–melamine resins are widely used for automobile exterior coatings. These materials are formulated by reacting an acrylic polyol with an alkylated melamine. Because the reactions are reversible, acrylic–melamine coatings tend to hydrolyze during exposures in moist environments. During hydrolysis, water in the coating film is consumed. To keep the moisture content in the film in equilibrium, water must be transported from regions of high water concentration to regions of low water concentration. An approach based on Fourier transform infrared (FTIR) spectroscopy analysis of the coating degradation fitted to a transport model is presented to estimate the diffusion coefficients and velocities of water transport during the hydrolysis of an acrylic–melamine coating exposed to different relative humidities (RHs). Theoretical prediction agreed well with the experimental FTIR data of coating hydrolytic degradation. Generally, both the diffusion coefficient and velocity of water transport in the coating increased with increasing RH. Since water transport resulting from the hydrolysis reactions is a very slow and complex process, the approach presented here provides a viable means for obtaining valuable data for quantitative analyses of coating hydrolytic degradation at different RHs.
Deformation characteristics of tin-based solder joints
Authors: Antoniou Antonia , Bastawros Ashraf F.
Published: March 2008

A novel experimental configuration was devised to measure the evolution of the deformation field and the corresponding toughness in solder joints for microelectronic packaging. The utilized material system comprised a ductile layer of tin-based solder encapsulated within relatively hard copper shoulders. The experimental configuration provided pure shear state within the constrained solder layer. Different Pb/Sn compositions were tested with grain size approaching the film thickness. The in-plane strain distribution within the joint thickness was measured by a microscopic digital image correlation system. The toughness evolution within such highly gradient deformation field was monitored qualitatively through a two-dimensional surface scan with a nanoindentor. The measurements showed a highly inhomogeneous deformation field within the film with discreet shear bands of concentrated strain. The localized shear bands showed long-range correlations of the order 2–3 grain diameters. A size-dependent macroscopic response on the layer thickness was observed. However, the corresponding film thickness was approximately 100–1000 times larger than those predicted by nonlocal continuum theories and discreet dislocation.
Chemical solution deposition of biaxially oriented (Ba,Sr)TiO3 thin films on 〈100〉 Ni
Authors: Ong R. J. , Dawley J. T. , Clem P. G.
Published: March 2008

Biaxially oriented Ba1−xSrxTiO3 (BST) thin films were fabricated under a highly reducing atmosphere (oxygen partial pressure ∼10−18 atm) on base metal substrates (〈100〉 Ni) using a fluorinated chemical solution deposition method. The degree of film orientation was investigated with respect to film annealing temperature and composition (x = 0, 0.33, 0.5, 0.67, 1). The solution synthesis route included fluorinated solvents, donor-dopants, and chelating agents for control of orientation, defect chemistry, and morphology. Free-energy and phase diagrams guided solution development using halide addition to avoid intermediate BaCO3 formation and instead produce direct crystallization of BaTiO3-type materials from BaF2. The degree of (200) orientation of BST films deposited on 〈100〉 Ni substrates displayed a compositional dependence for 0 < x < 1, with a maximum orientation Lotgering factor approaching unity. Low-temperature (500 °C) crystallization of highly oriented films was demonstrated for solution-derived BST on Ni, which may have enabling materials integration implications for capacitors and other applications.
Investigation of mechanically activated field-activated pressure-assisted synthesis processing parameters for producing dense nanostructured FeAl
Authors: Paris S. , Valot Ch. , Gosmain L. , Gaffet E. , Bernard F. , Munir Z.
Published: March 2008

The parameters of the mechanically activated field-activated pressure-assisted synthesis (MAFAPAS) process, which were recently developed and patented for producing dense nanostructured materials, were studied in the case of the B2-FeAl intermetallic. Based on x-ray diffraction (XRD) experiments, residual stresses XRD analysis, relative density measurement, and secondary-electron microscopic observations, the optimal synthesis conditions (time, current intensity, and pressure) were studied. Fe + Al powders were comilled in a specially designed planetary mill to obtain a mixture of reactants at the nanoscale without the formation of any product. The milled mixtures were then subjected to a high density of alternating current (60 Hz ac, total current 1250 or 1500 A), a uniaxial pressure (70 or 106 MPa), and different times (from 2 to 5 min). This work confirms the reproducibility of the MAFAPAS process, showing the essential role of the mechanical activation step to produce a pure nanostructured material. In addition, the composition and the microstructure of MAFAPAS end-products depended on the processing parameters (time, current density, mechanical pressure). In particular, it was observed that the process of simultaneous synthesis and consolidation of the product introduced a high level of residual stresses.
Kinetic and characterization studies of the formation of barium monomolybdate in equimolar powder mixture of BaCO3and MoO3
Authors: Al-Hajji Latifa A. , Hasan Muhammad A. , Zaki Mohamed I.
Published: March 2008

The formation of barium monomolybdate (BaMoO4) in inequimolar powder mixtures of BaCO3and MoO3was examined under isothermal and nonisothermal conditions upon heating in air at 25–1200 °C, using thermogravimetry. Concurrence of the observed mass loss (due to the release of CO2) to the occurrence of the formation reaction was evident. Accordingly, the extent of reaction (x) was determined as a function of time (t) or temperature (T). Thex-tandx-Tdata thus obtained were processed using a well-established mathematical apparatus and methods to characterize the nature of the reaction rate-determining step and derive isothermal and nonisothermal kinetic parameters (rate constant, frequency factor, reaction order, and activation energy). Moreover, the reaction mixture quenched at various temperatures (450–575 °C) in the reaction course was analyzed by various spectroscopic (x-ray diffractometry, infrared spectroscopy, and laser Raman spectroscopy) and microscopic (scanning electron microscopy and x-ray energy dispersive spectroscopy) techniques for material characterization. The results obtained indicated that the reaction rate may be controlled by unidirectional diffusion of MoO3species through the product layer (BaMoO4), which was implied to form on the barium carbonate particles. The nonisothermally determined activation energy (156 kJ/mol) was found to be close to the isothermally determined one (164–166 kJ/mol)
Low-temperature route to nanoscale P3N5 hollow spheres
Authors: Gu Hongzhou , Gu Yunle , Li Zhefeng , Ying Yongcheng , Qian Yitai
Published: March 2008

Nanoscale hollow spheres of amorphous phosphorus nitride (P3N5) were synthesized by reacting PCl3 with NaN3 at 150–250 °C. Transmission electron microscope images show that the hollow spheres have a diameter of 150–350 nm, and the thickness of the shell is 20 nm. A very small amount of curly films were also found in the sample prepared at 150 °C. The infrared spectrum indicates a high degree of purity. X-ray photoelectron spectroscopy indicates the presence of P and N, with a molar ratio of 1:1.62 for P:N. Ultraviolet-visible absorption spectroscopy shows an absorption band at 265–315 nm. Under photoluminescent excitation at 230 nm, the P3N5 emits ultraviolet light at 305 nm. With a band gap of 4.28 eV, the products may be a wide gap semiconductor. A possible mechanism and the influence of temperature on the formation of the hollow spheres are also discussed.
Deposition and characterization of YBa2Cu3O7−δ/LaMnO3/MgO/TiN heterostructures on Cu metal substrates for development of coated conductors
Authors: Cantoni C. , Christen D. K. , Varela M. , Thompson J. R. , Pennycook S. J. , Specht E. D. , Goyal A.
Published: March 2008

In this paper a novel buffer layer architecture consisting of LaMnO3/MgO/TiN is proposed as a suitable structural and chemical template for the epitaxial growth of high-transition temperature (Tc) superconductors on Cu metal surfaces. Using techniques such as high-energy electron diffraction and scanning transmission electron microscopy, we present in situ and ex situ analyses of the buffer-layer and superconductor growth with focus on structural properties of the interfaces formed. While MgO is a good barrier to oxygen diffusion, we find that MgO alone is not a suitable buffer layer due to rapid Cu diffusion. Further, growth of MgO with a single epitaxy can be hindered by the presence of impurities such as S, which form strongly bonded superstructures on the metal surface. With the addition of a TiN layer as a barrier to Cu diffusion, oxide formation is suppressed, interfaces are clean, and a single cube-on-cube epitaxy is observed. While the Cu/TiN and TiN/MgO interfaces are rough, the MgO and LaMnO3 layers planarize the material, leading to growth of smooth YBa2Cu3O7−δ (YBCO). Residual strain in the YBCO film is 0.25% or less and does not lead to apparent cracking. The superconducting properties of the samples were investigated by electrical transport and magnetization measurements. For the first time, high critical current density (Jc) values are reported for YBCO films grown on (001) single-crystal and 100‹100›?textured Cu surfaces without intervening metal coatings. Jc on single crystal-like substrates is as high as 3.5 MA/cm2. Reduced Jc of approximately 1 MA/cm2 on rolled Cu tapes is limited by damage to the tape surface during the rolling process.
Phase transformation of Er3+-doped Al2O3 powders prepared by the sol-gel method
Authors: Wang X. J. , Lei M. K. , Yang T. , Yuan L. J.
Published: March 2008

Er3+-doped Al2O3 powders were prepared by the sol-gel method, using aluminum isopropoxide [Al(OC3H7)3]-derived Al2O3 sols. The phase content diagram for the Er-doped Al–O system with the doping concentration up to 20 mol% is presented for sintering temperature from 550 to 1250 °C. The different crystalline types of (Al,Er)2O3 phases—γ, θ, α—and two Er-Al-O phases—ErAlO3 and Al10Er6O24—were detected. The Er3+ doping in the Al2O3 phases has a significant effect on phase transformations for γ → θ and θ → α, indicating the enhancement for stability of the γ and θ phases. Compared with the γ–Al2O3 phase, the γ–(Al,Er)2O3 phase can be obtained at high temperatures up to about 1100 °C. The θ–(Al,Er)2O3 phase, together with the α–(Al,Er)2O3 phase, was still observed at 1200 °C under the high Er3+ doping concentration. The Er3+ doping suppressed the crystallization of the γ and θ phases. At a low Er3+ doping concentration from about 2 to 5 mol%, the ErAlO3 phase precipitated out around 1100 °C. The Al10Er6O24 phase was observed at all sintering temperatures; the precipitation was carried out to all Er3+ doping concentrations with increasing sintering temperatures up to about 1200 °C.
Structural, magnetic, and transport properties of pulsed-laser deposition La0.65Ca0.35MnO3 films grown under different substrate arrangements in the laser plume
Authors: Belevtsev B. I. , Naugle D. G. , Rathnayaka K. D. D. , Chukanova I. N. , Ross J. H. , Ishchuk V. M.
Published: March 2008

The structural, transport, and magnetic properties of La0.65Ca0.35MnO3 films grown on MgO substrates by pulsed-laser deposition under different substrate arrangements in the laser plume were studied. In addition to the standard substrate arrangement in the main stream of particles, substrate arrangements in the periphery area of the laser plume and in an area shielded from the plume were used. In the latter case, the deposition occurs from the flux that is reflected from a side screen. It was found that the substrate arrangement influences considerably the surface topography, structure, chemical composition, magnetic, transport, and magnetoresistance properties of the films. It was found that the substrate arrangements in the periphery area of the laser plume and in the shielded area of the plume eliminate deposition of large particulates. The latter arrangement is the most effective in elimination of particulate deposition. In this case, the film has the smoothest surface with root mean square roughness of about 2 nm. The data obtained revealed interesting correlations between the structural and transport properties as well. In particular, films deposited in the periphery and shielded areas of the laser plume have increased nonstoichiometry (La deficiency), higher resistivity, and broader magnetic transitions. The polycrystalline nature of the films studied is taken into account in the discussion of their transport properties.
Tensile strength of zinc oxide films measured by a microbridge method
Authors: Ong C. W. , Zong D. G. , Aravind M. , Choy C. L. , Lu D. R.
Published: March 2008

Double-layered ZnO/silicon nitride microbridges were fabricated for microbridge tests. In a test, a load was applied to the center of the microbridge specimen by using a microwedge tip, where the displacement was recorded as a function of load until the specimen broke. The silicon nitride layer in the structure served to enhance the robustness of the specimen. By fitting the data to a theory, the elastic modulus, residual stress, and tensile strength of the ZnO film were found to be 137 ± 18 GPa, −0.041 ± 0.02 GPa, and 0.412 ± 0.05 GPa, respectively. The analysis required the elastic modulus, internal stress, and tensile strength of the silicon nitride layer. They were measured separately by microbridge tests on single-layered silicon nitride microbridges. The measured tensile strength of the ZnO films represents the maximum tolerable tensile stress that the films can sustain when they are used as the functional component in devices.
Growth and properties of KBe2BO3F2 crystal
Authors: Wang Jiyang , Zhang Chenqian , Liu Yaogang , Zhang Jianxiu , Hu Xiaobo , Jiang Minhua , Chen Chuangtian , Wu Yicheng , Xu Zuyan
Published: March 2008

Single crystals of KBeBO3F (KBBF) up to 2 mm in thickness were grown by using a spontaneous nucleation method. The flux was composed of KF and B2O3, and the optimal ratio for growing KBBF single crystal thicker than 1 mm is suggested to be (1.0–1.5):(5.0):(0.7–1.2) (KBBF:KF:B2O3) in molar ratio. The ideal morphology of.KBBF is two sets of tabular facets and a set of rhombic facets. Surface morphology.was characterized both by optical microscopy and atomic force microscopy. Transmission spectra showed that high transmission was maintained up to 170 nm. Based on refractive index measurements, phase matching curves for types I and II were determined. Using a prism coupling method, the effective fourth harmonic generation for an Nd:YAB laser was realized in an 0.6-mm-thick KBBF sample. The output energy of the 266-nm beam was 0.282 mJ, and the conversion efficiency was 11.8%. A 183.6-nm laser over 1 mW was obtained from a Ti:sapphire second harmonic generation beam in an 0.8-mm KBBF sample. The conversion efficiency reached 2.3%. A 179.4-nm laser was also observed from a 358.7-nm laser by directly frequency doubling. This is the shortest wavelength obtained through nonlinear optical processes in a solid-state laser.
Thermal and structural properties of B2O3–H2O glasses
Authors: Brüning R. , Patterson S.
Published: March 2008

B2–2xO3–2xH2x glasses were prepared by quenching the melt contained in sealed tubes. The glass-forming range extends from x = 0 to 0.50 (equal to the stoichiometry of metaboric acid, HBO2). The glasses were characterized by differential scanning calorimetry and x-ray scattering. With increasing water content, the glass-transition temperature, Tg, decreases from 553 to 333 K. The specific heat of water-rich samples shows an unusual peak just above Tg. The origin of this peak, which is seen upon heating and cooling, has not been identified. Unlike the composition dependence of Tg, the x-ray structure factors depend for the most part linearly on the composition. In analogy with the crystalline layer compounds α-HBO2 and B(OH)3, the x-ray scattering data show evidence for layering in the medium-range order of water-rich glasses.
Enthalpies of formation of LaBO3perovskites (B = Al, Ga, Sc, and In)
Authors: Cheng Jihong , Navrotsky Alexandra
Published: March 2008

Enthalpies of formation from constituent oxides and elements at 298 K were determined by high-temperature oxide melt solution calorimetry for a group of technologically important perovskites LaBO3(B = La, Ga, Sc, and In). Enthalpies of formation from oxides of LaAlO3and LaGaO3are −69.61 ± 3.23 kJ/mol and −52.39 ± 1.99 kJ/mol, respectively. The data were consistent with literature values obtained using other methods. The enthalpies of formation of LaScO3and LaInO3from oxides were reported for the first time as −38.64 ± 2.30 kJ/mol and −23.99 ± 2.31 kJ/mol, respectively. As seen for other perovskites, as the tolerance factor deviates more from unity (in the order Al, Ga, Sc, In), the enthalpy of formation from oxides becomes less exothermic, indicating a less stable structure with respect to the constituent oxides.
Interface microstructures between Ni-P alloy plating and Sn–Ag–(Cu) lead-free solders
Authors: Hwang Chi-Won , Suganuma Katsuaki , Kiso Masayuki , Hashimoto Shigeo
Published: March 2008

The interface microstructures of Sn-Ag and Sn-Ag-Cu solders with Au/Ni-6P plating were studied primarily using transmission electron microscopy. During soldering at 230°C, Au dissolved into molten solder, and double reaction layers of Ni3Sn4/η–Ni3SnP formed between Sn-3.5Ag solder and Ni-6P layer. P content increases in the surface region of the Ni-6P layer due to the depletion of Ni diffused into molten solder, resulting in the formation of Ni3P+Ni layer. For Sn-3.5Ag-0.7Cu solder, an η-(Ni,Cu)3Sn2 single layer, containing Cu of about 50 at.%, formed as a reaction layer.
Electromigration failure in flip chip solder joints due to rapid dissolution of copper
Authors: Hu Y.C. , Lin Y.H. , Kao C.R. , Tu K.N.
Published: March 2008

An electromigration failure mechanism in flip chip solder joints is reported in this communication. The solder joints failed by a very rapid, asymmetrical, and localized dissolution of the Cu metallization on the cathode side. The average dissolution rate was about 1 μm/min. The dissolved Cu included not only the Cu under bump metallurgy but also the on-chip Cu conducting trace. From the location and geometry of the dissolved Cu, it can be concluded that current crowding plays a critical role in the rapid dissolution. The dissolved Cu atoms were driven to the anode side by electromigration, and a large amount of Cu6Sn5 was formed there.
Polymer-derived SiCN composites with magnetic properties
Authors: Saha Atanu , Shah Sandeep R. , Raj Rishi , Russek Stephen E.
Published: March 2008

Composites consisting of particles of α-iron dispersed in silicon carbonitride (SiCN) were fabricated by a polymer route. The composites had iron inclusions with the same magnetization as bulk iron, but they resisted oxidation up to 500°C and had a hardness of 5-7 GPa. The composites behaved as ferromagnets, albeit with a low susceptibility attributed to the pinning of the domains by imperfect interfaces and to the elastic resistance from the SiCN matrix. This low-cost, low-temperature processing method can be used to make different kinds of ceramic composites with multifunctional properties.
Growth and Ripening of (Au,Ni)Sn4 Phase in Pb-Free and Pb-Containing Solders on Ni/Au Metallization
Authors: Lee K.Y. , Li M. , Tu K.N.
Published: March 2008

The evolution of an interfacial microstructure between Cu/Ni/Au bond-pad metallization and Pb-free or Pb-containing solder bumps during solid-state aging was investigated. A distinctive difference between the Pb-containing and Pb-free solder bumps was observed. A continuous (Au,Ni)Sn4 intermetallic compound layer was readily detected in the Pb-containing solder bump on the bond-pad. No such layer exists in the Pb-free case; instead a large number of (Au,Ni)Sn4 particles scatter in the bulk of the solder. The different morphologies of the (Au,Ni)Sn4 have been explained on the basis of the contact angle in Young's equation and of the growth of a flat layer by ripening. A simple kinetic analysis of ripening between a set of monosize spheres and a flat surface is given to describe the layered growth having a time dependence around 0.5.
Synthesis and characterization of new inorganic polymeric composites based on kaolin or white clay and on ground-granulated blast furnace slag
Authors: Lecomte I. , Liégeois M. , Rulmont A. , Cloots R. , Maseri F.
Published: March 2008

Alkali activation of dehydroxylated kaolin or clay yielded high-strength polymeric materials, so-called geopolymers. They were synthesized by mixing the aluminosilicate with solutions of sodium metasilicate and KOH followed by adding 45 wt.% of ground-granulated blast furnace slag. The influence of the aluminosilicate source, its activation temperature, and the order of mixing raw materials were studied on the workability of the blending paste, the microstructure, and the Vickers hardness of the geopolymer samples. The polymeric material is completely amorphous according to x-ray diffraction. Solid-state27Al and29Si magic-angle-spinning nuclear magnetic resonance showed that the geopolymer consists of AlO4and SiO4tetrahedra linked together through a polymeric network constituted by branched entities SiQ4(4Al) and SiQ4(3Al), but also by less-polymerized silicates SiQ1and SiQ2. Scanning electron microscopy showed a homogeneous polymeric gel matrix containing unreacted slag (and quartz) grains; thermogravimetric analysis and differential scanning calorimetry exhibited a high content of water and an elevated melting point (1260°C). Vickers hardness values are in the range of 200 MPa.
Interface characteristics affecting electrical properties of Y-doped SiC
Authors: Siegelin F. , Kleebe H-J. , Sigl L.S.
Published: March 2008

Liquid-phase sintered SiC, doped with 3 vol% AlN, Al2OC, Y3Al5O12, revealed a variation in electrical resistivity of more than five orders of magnitude (<102-107Ω cm) upon slight variations in the sintering process. The materials were characterized using various transmission electron microscopy techniques such as high-resolution transmission electron microscopy (HRTEM), Fresnel fringe imaging, analytical electron microscopy, and electron holography. The main focus of this study was to verify whether there is a correlation between interface structure and electrical resistivity. Scanning electron microscopy (SEM) of polished and plasma-etched surfaces showed interface features similar to those observed in Si3N4ceramics containing amorphous grain-boundary films. Such films are expected to act as an insulating barrier for electric current. However, in contrast to the SEM results, HRTEM of SiC grain boundaries revealed no intergranular film in any of the SiC materials studied. Elemental analysis (i.e., energy dispersive x-ray and electron energy loss spectroscopy) of these “clean” SiC interfaces showed the segregation of secondary phase elements at grain boundaries. Electron holography and the Fresnel fringe technique were used to determine the change in the mean inner potential across SiC interfaces, which could be associated with the spatial charge distribution of a double Schottky barrier. The height of the potential barrier correlates with the electrical resistivity recorded via impedance spectroscopy.
Island growth of Y2BaCuO5 nanoparticles in (211∼1.5 nm/123∼10 nm)×N composite multilayer structures to enhance flux pinning of YBa2Cu3O7−δ films
Authors: Haugan T. , Barnes P.N. , Maartense I. , Cobb C.B. , Lee E.J. , Sumption M.
Published: March 2008

A controlled introduction of second-phase Y2BaCuO5 (211) nanoparticles into YBa2Cu3O7−δ (123) thin films was achieved for the first time for the purpose of increasing flux pinning. The island-growth mode of 211 on 123 was utilized to obtain an area particle density >1011 cm-2 of 211 thick-disk-shaped nanoparticles in individual layers. Composite layered structures of (211y nanoparticles/123z)×N were deposited by pulsed laser deposition on LaAlO3 substrates, with N bilayers = 24 to 100, y thickness = 1 to 2 nm, and z thickness = 6 to 15 nm (assuming continuous layer coverage). With 211 addition, the critical current densities at 77 K were higher at magnetic fields as low as 0.1 T and increased as much as approximately 300% at 1.5 T. The superconducting transition temperature was reduced by approximately 2 to 4 K for 211 volume fraction <20%. Reinitiation of 123 growth after every 211 layer resulted in a smooth and flat surface finish on the films and also greatly reduced surface particulate formation especially in thicker films (∼ μm).
Characterization of the microstructure and phase equilibria calculations for the powder metallurgy superalloy IN100
Authors: Wusatowska-Sarnek Agnieszka M. , Ghosh Gautam , Olson Gregory B. , Blackburn Martin J. , Aindow Mark
Published: March 2008

The microstructure of the Ni-based superalloy IN100 processed by a powder metallurgy route was evaluated to reveal the structures, volume fractions, distributions, and chemistries of the various phases present. These data were compared with those predicted by computational thermodynamics. It is shown that the microstructural parameters expected on the basis of global equilibrium conditions differ significantly from those measured experimentally. However, modification of these calculations by use of constrained and successive equilibria compensated for kinetic effects and led to accurate (or better) predictions of phase volume fractions and chemistries in this alloy. This demonstrated that such modified phase equilibria calculations could be powerful tools for modeling microstructures, even in complex multicomponent alloys processed under nonequilibrium conditions.
Analytic method for the minimum time for binder removal from three-dimensional porous green bodies
Authors: Lombardo Stephen J. , Feng Z.C.
Published: March 2008

An analytical expression was derived to predict the heating profile that minimizes the cycle time for the thermal removal of binder from porous green ceramic bodies. The analytical equation was based upon the solution to a three-dimensional convective transport equation that describes flow in porous media arising from the thermal decomposition of binder. The solution to the transport problem was then combined with an algorithm derived from variational calculus. The analytical expression described the time for binder removal in terms of the body dimensions, isotropic permeability, volume fraction of binder, and threshold pressure within the green body.
Thermal shock of porous silicon nitride with preferentially aligned grains
Authors: Vandeperre L.J. , Inagaki Y. , Clegg W.J.
Published: March 2008

We studied the thermal shock resistance of a silicon nitride containing elongated and preferentially aligned grains with different volume fractions of pores, ranging from 0 to 0.27. It was found that an increase in the volume fraction of pores decreased both the strength and the temperature through which the sample must be quenched to cause cracking. However, at intermediate values of the porosity (0.07), the temperature change required to cause cracking was much smaller than predicted. Observations of the resulting damage suggested that this had occurred because of the formation of cracks just underneath and parallel to the cooled surface of the sample that were able to change the direction of their growth. The extent of cracking was found to be only very weakly dependent on the volume fraction of pores, consistent with calculations of the variation of crack driving force within the sample.
Synthesis of CuO and Cu2O crystalline nanowires using Cu(OH)2 nanowire templates
Authors: Wang Wenzhong , Varghese Oomman K. , Ruan Chuanmin , Paulose Maggie , Grimes Craig A.
Published: March 2008

Crystalline CuO and Cu2O nanowires with an average diameter of about 10 nm and lengths of several tens of microns were successfully synthesized, depending on synthesis conditions, using precursor Cu(OH)2 nanowires as templates. The crystallinity, purity, morphology, and structural features of the as-prepared nanowires were characterized by powder x-ray diffraction, selected-area electron diffraction, and high-resolution transmission electron microscopy. The results showed that the precursor polycrystalline Cu(OH)2 nanowires served as both reactants for the growth of CuO and Cu2O nanowires, and as templates controlling the size and shape of the resulting nanowires.
Explosion Phase Formation of Nanocrystalline Boron Nitrides Upon Pulsed-Laser-Induced Liquid/Solid Interfacial Reaction
Authors: Wang J.B. , Zhong X.L. , Zhang C.Y. , Huang B.Q. , Yang G.W.
Published: March 2008

Boron nitride (BN) nanocrystals with explosion (E) phase were prepared by a novel laser-assisted materials fabrication, i.e., pulsed-laser-induced liquid (acetone)/solid (hexagonal boron nitride bulk) interfacial reaction at normal temperature and pressure. Typical diameters of these synthesized quasi-spherical BN nanocrystals were in the range of 30 to 80 nm. Transmission electron microscopy, x-ray diffraction, and Fourier transformed infrared spectroscopy were used to identify the morphologies and structures of the synthesized nanocrystals. Additionally, we proposed the formation mechanism of cubic-BN and E-BN nanocrystals upon pulsed-laser-induced liquid/solid interfacial reaction, in which both liquid and solid were simultaneously involved.
Selective Modification of the Tribological Properties of Aluminum Through Temperature and Dose Control in Oxygen Plasma Source Ion Implantation
Authors: Bolduc M. , Terreault B. , Reguer A. , Shaffer E. , St-Jacques R.G.
Published: March 2008

Improvements in the tribological properties of pure aluminum and “aeronautical” alloy AA7075-T651 were obtained by oxygen-ion implantation [(0.7 to 5) × 1017 O/cm2, 30 keV] using our pulsed electron cyclotron resonance plasma source. This oxygen plasma source ion implantation process produced oxide nanoprecipitates that enhanced the hardness up to three times in the surface layer and caused reductions in the scratch depths and the friction coefficients by similar factors. A spectrum of tribological properties was obtained depending on temperature and ion dose. Temperature measurement and control were obtained through an integrated thermocouple and by changing the duty-cycle of the microwave source. The oxygen content and the depth-resolved chemical composition were measured and optimized using x-ray photoelectron spectroscopy (XPS) combined with Ar-ion etching. The tribological properties were investigated by (i) depth-sensing nanoindentation for hardness and Young's modulus, (ii) scratching and scratch-depth measurement via atomic force microscopy (AFM), and (iii) friction force measurements using AFM. Low-temperature (≤160°C) implantations with optimal O-ion doses produced, in both pure and alloyed Al, an approximately 50-nm-thick, smooth, and extremely fine-grained metal–alumina nanocomposite. The resulting surface was hard and stiff but nonbrittle and displayed high scratch resistance and low friction. High-temperature (~430°C) implantation had different effects on pure Al and AA7075. On pure Al, it produced a very hard but brittle Al2O3 layer for which yield points (displacement excursions) were observed at critical load values in the nanoindentation force–displacement curves. On AA7075, XPS chemical profiling revealed an effect of extreme Mg surface segregation and complete Al surface depletion; MgO crystallites formed a rather rough but surprisingly thick layer (>100 nm). The resulting AA7075 surface showed a hardness increase that was substantial but slightly smaller than that obtained at low temperature.
Shape-Controllable Synthesis of Indium Oxide Structures: Nanopyramids and Nanorods
Authors: Zhang Ye , Jia Hongbo , Yu Dapeng , Luo Xuhui , Zhang Zhensheng , Chen Xihong , Lee Cheoljin
Published: March 2008

We describe a vapor-phase route to the controllable synthesis of indium oxide micro-and nanopyramids on the silicon wafer via selective epitaxial vapor-solid growth by a methane-assist thermal reduction method. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy revealed that the pyramids were cubic single crystals with a tetragonal symmetry. The size, morphology, and density of pyramids could easily be controlled by tuning reaction parameters. The method has good compatibility with other procedures involved in the microfabrication processes. Laterally grown indium oxide nanorods on the silicon wafer were also prepared via a vapor-liquid-solid mechanism. Those crystalline In2O3 nanorods were about 100 nm in diameter and 1 μm in length. The as-synthesized indium oxide nanopyramids and nanorods could offer novel opportunities for both fundamental research and technological applications.
Thermal Modification of Microstructures and Grain Boundaries in Silicon Carbide
Authors: Zhang Xiao Feng , Jonghe Lutgard C. De
Published: March 2008

Polycrystalline SiC samples hot pressed with aluminum, boron, and carbon sintering additions (ABC-SiC) were characterized using transmission electron microscopy. The study focused on the effects of high-temperature treatment on microstructure. Three temperatures, at which considerable microstructural changes took place, were found to be critical. At a threshold temperature of approximately 1000°C, 1-nm-wide, amorphous intergranular films started to crystallize. At approximately 1300°C, lattice diffusion in SiC grains resulted in nanoprecipits, which could diffuse into grain boundaries and significantly altered composition. Quantitative microanalysis revealed doubled Al content in intergranular films after annealing at 1300°C. Except for crystallization in intergranular films and nano-precipitation in matrix grains, microstructure remained stable until 1600°C, when microstructural changes with volatile features occurred. A brief holding at 1900°C brought marked changes in microstructure, including structural change in intergranular films, dissolved nanoprecipitates, unit cell dilation, and cracking. The results indicate that ABC-SiC is highly promising in structural applications at up to 1500°C.
Generalized shear-lag model for load transfer in SiC/Al metal-matrix composites
Authors: Ryu Ho J. , Cha Seung I. , Hong Soon H.
Published: March 2008

The load-transfer efficiency of reinforcement, in cylindrical forms in metal-matrix composite (MMC), was analyzed based on the shear-lag model. Both the geometric shape and alignment of reinforcement were considered. The stress transferred to a misaligned whisker was calculated from differential equations based on the force equilibrium in longitudinal and transverse directions. A new parameter, defined as effective aspect ratio, was used to indicate the load-transfer efficiency of misaligned reinforcement. The effective aspect ratio was formulated as a function of aspect ratio and misorientation angle of reinforcement in MMC. A probability density function of misorientation distribution was used to estimate the strengthening effect of all misaligned whiskers distributed in the matrix. Considering the contributions of both effective aspect ratio and misorientation distribution on load-transfer efficiency, a generalized shear-lag model was proposed to explain the mechanical anisotropy of discontinuous reinforced MMC.
Heteroepitaxial growth of tungsten oxide films on silicon(100) using a BaF2 buffer layer
Authors: Doucette L.D. , Santiago F. , Moran S.L. , Lad R.J.
Published: March 2008

Multidomained heteroepitaxial WO3 films were grown on Si(100) substrates using a (111)-oriented BaF2 buffer layer at the WO3–Si interface. The 30-nm-thick BaF2 layer, grown by very low rate molecular-beam epitaxy, consisted of four equivalent crystalline domains oriented about the BaF2[111] axis, which provided templates for heteroepitaxial WO3 film growth. The WO3 films were grown by electron cyclotron resonance oxygen plasma-assisted electron beam evaporation of a WO3 source, and the temperature range was varied between 25°C and 600°C. At an optimal deposition temperature of approximately 450°C, monoclinic-phase WO3 films were produced, which consisted of coexisting (002), (020), and (200) in-plane orientations with respect to the BaF2(111)/Si(100) substrate. During growth, an interfacial barium tungstate (BaWO4) reaction product formed at the WO3–BaF2 interface. The {112} planes of this BaWO4 layer also have a multidomained heteroepitaxial orientation with respect to the BaF2(111) buffer layer. Postdeposition annealing experiments in air for 24 h at 400°C indicated that the heteroepitaxial BaWO4 and WO3 layers remain stable. A thermodynamic argument is used to explain the BaWO4 interfacial reaction during initial growth stages, and kinetically limited diffusion processes through the BaWO4layer coupled with lattice matching across the WO3–BaWO4 interface are proposed to be responsible for the formation of stable WO3 films at later growth stages.
Grain-refinement mechanisms in titanium alloys
Authors: Bermingham M.J. , McDonald S.D. , Dargusch M.S. , StJohn D.H.
Published: March 2008

Despite the importance of the prior-β grain structure in determining the properties of titanium-based alloys, there are few published studies on methods of controlling the size of these grains in commercial alloys. The existing research raises questions about the relative importance of solute elements in grain-refining mechanisms, particularly the common alloying elements of aluminum and vanadium. The effect of these elements was investigated by producing a series of castings in a nonconsumable arc-melting furnace, and the results were interpreted with the aid of available phase-diagram information and solute-based models of grain refinement. A small reduction in grain size was obtained with increasing solute additions; however, this was not expected from the theoretical analysis. Possible reasons for this discrepancy are discussed.
Piezoelectric properties of lead-free (Na0.5Bi0.5)TiO3–(Na0.5K0.5)NbO3–BaTiO3 ceramics
Authors: Lee Seung-Ho , Yoon Chang-Bun , Lee Sung-Mi , Kim Hyoun-Ee , Lee Kyung-Woo
Published: March 2008

The microstructural evolution and piezoelectric properties of lead-free ceramics (0.98-x)(Na0.5Bi0.5)TiO3–x(Na0.5K0.5)NbO3–0.02BaTiO3 (0 ⩽ x ⩽ 0.98, abbreviated as (0.98-x)NBT–xNKN–0.02BT) were investigated. The effects of the amount of NKN on the crystal structure, microstructural evolution, and piezoelectric properties were examined. The 0.93NBT–0.05NKN–0.02BT ceramics having a lower NKN content gave good performances with piezoelectric properties of d33 = 140 pC/N and kp = 21%, because of the soft additive Nb5+ ions at the B sites. However, a paraelectric cubic phase was observed in the wide range of compositions between x = 0.1 and x = 0.9. At a higher NKN content of x > 0.9, a morphotropic phase boundary (MPB) between the tetragonal and orthorhombic phases was found in the 0.015NBT–0.965NKN–0.02BT ceramics, and the piezoelectric properties were enhanced (d33 = 135 pC/N, kp = 29%). The piezoelectric properties of this system were closely related to its crystal structure.
Photoluminescence and I–V characteristics of ZnS grown on silicon nanoporous pillar array
Authors: Xu Hai Jun , Jia He Shun , Yao Zhi Tao , Li Xin Jian
Published: March 2008

Silicon nanoporous pillar array (Si-NPA) is a silicon hierarchical structure with regularly patterned surface morphology. Through a heterogeneous reaction process, zinc sulfide nanocrystallites (nc-ZnS) were grown onto Si-NPA and a unique heterostructure of ZnS/Si-NPA was obtained. The formation of wurtzitenc-ZnS was proved by x-ray diffraction, and the average grain size was evaluated to be ∼18 nm. X-ray photoelectron spectroscopy disclosed that as-grownnc-ZnS was well separated from Si-NPA by a SiO2thin layer of ∼1.3 nm. The photoluminescence (PL) spectrum of ZnS/Si-NPA showed that in addition to the two red PL bands peaked at ∼648 and ∼705 nm observed in Si-NPA, three other PL bands peaked at ∼365, ∼418, and ∼472 nm were observed and attributed to the PL fromnc-ZnS. It was also demonstrated that as-prepared ZnS/Si-NPA heterostructure could exhibit good rectification characteristic featured by a high forward current density of ∼75 mA/cm2at 2 V and high reverse breakdown voltage of ∼10 V. Our results indicated that ZnS/Si-NPA might be a valuable heterostructure nanosystem to be further probed for achieving enhanced optical and electrical properties.
Combustion joining of refractory materials: Carbon–carbon composites
Authors: White Jeremiah D.E. , Simpson Allen H. , Shteinberg Alexander S. , Mukasyan Alexander S.
Published: March 2008

Refractory materials such as carbon possess properties that make joining them difficult. In this work, bonding of a carbon–carbon composite is achieved by employing self-sustained, oxygen-free, high-temperature combustion reactions. The effects of several parameters, such as the composition of the reaction media, and the values of the applied current and pressure, on the mechanical strength of the joint were investigated. It was found that the C–C composite possesses a high activity with the reactive media layer, the level of electrical current used to initiate the reaction and the applied pressure do not need to be excessive to obtain a strong joint. Some aspects of the joining mechanism are discussed in detail.
In situ synchrotron radiation monitoring of phase transitions during microwave heating of Al–Cu–Fe alloys
Authors: Vaucher Sébastien , Nicula Radu , Català-Civera José-Manuel , Schmitt Bernd , Patterson Bruce
Published: March 2008

The effect of rapid microwave heating has so far been evaluated mainly by comparing the state of materials before and after microwave exposure. Yet, further progress critically depends on the ability to follow the evolution of materials during ultrafast heating in real time. We describe the first in situ time-resolved monitoring of solid-state phase transitions during microwave heating of metallic powders using wide-angle synchrotron radiation diffraction. Single-phase Al–Cu–Fe quasicrystal powders were obtained by microwave heating of nanocrystalline alloy precursors at 650 °C in <20 s.