bluuebunny/crossref_metadata_2025 huggingface dataset:
Evolution of mechano-chemistry and microstructure of a calcium aluminate-polymer composite: Part I. Mixing time effects
Authors: Gülgün M.A. , Kriven W.M. , Tan L.S. , McHugh A.J.
Published: September 2007

Paste development and evolution of microstructure, microchemistry, and mechanical properties of macro-defect-free composites were investigated. Mixing torque plots from a Banbury mixer showed a “window of processibility” within which an optimum polymer-particle network structure formed. This processing window can be controlled by mixing rate and temperature. Network development during paste formation governed the flexural strength and microstructure of the cured material. Electron microscopy studies of the composites corresponding to various points along the mixing torque curve revealed a matrix formation-destruction process that was linked to the mixing activity and the mechano-chemistry of the system. Microchemical evolution in the composite was studied by EDS. These studies suggested that a combined convection and diffusion mechanism was responsible for the migration of Al3+ and Ca2+ ions into the different regions of the microstructure. Increasing concentrations of these elements in the polymer regions stiffened the matrix, leading to complete degradation on further mixing.
Effect of substrates on the crystallinity and morphology of sol-gel-derived epitaxial LiNbO3 films
Authors: Terabe K. , Iyi N. , Kitamura K. , Kimura S.
Published: September 2007

Epitaxial thin films of LiNbO3 were prepared by the sol-gel method on (0001)-sapphire, (0001)-LiTaO3 and (0001)-5% MgO-doped LiNbO3, substrates. The precursor films crystallized with the highly preferred orientation on all substrates. When sapphire substrates, which have large discrepancies in the lattice constant and thermal expansion percentage with the film were used, the resulting films showed a low crystallinity after heat treatment at 500 °C and grain growth at 650 °C. On the other hand, when using LiTaO3 and 5% MgO-doped LiNbO3 substrates, with smaller discrepancies, the formed films, after heat treatment at 500 °C, showed better crystallinity with the smooth surface.
Composites of polypyrrole and carbon black: Part III. Chemical synthesis and characterization
Authors: Wampler Wesley A. , Rajeshwar Krishnan , Pethe R.G. , Hyer R.C. , Sharma S.C.
Published: September 2007

A new class of molecular composites of carbon black and an electronically conducting polymer, namely polypyrrole, has been synthesized by chemically polymerizing pyrrole in an aqueous dispersion of carbon black. The carbon black content of these composites can be varied from ∼5% to ∼85% (by weight). The surface area and density of these composites were compared to corresponding mixtures of carbon black and polypyrrole. The influence of carbon black on the efficiency of polymerization of pyrrole is described. The effect of carbon black content on the electronic conductivity of the composite has been mapped, and compared with the corresponding behavior of a mixture of carbon black and polyvinylchloride. The influence of the parent black characteristics (porosity, void volume, surface area) on the electronic conductivity of the resultant composite has been probed by comparing the behavior of composites derived from six commercial and experimental blacks. The temperature dependence of the composites has been studied as a function of the carbon black content. Finally, the application of these new materials is an environmental remediation scenario is demonstrated for Cr(vi) as a model pollutant.
Annealing of tweed microstructure in high Tc superconductors in the presence of impurities
Authors: Parlinski K. , Watanabe Y. , Ohno K. , Kawazoe Y.
Published: September 2007

A two-dimensional model of oxygen-deficit layer of superconducting material YBa2Cu3O7 has been simulated by the molecular-dynamics technique in order to study the influence of the impurities in the site of copper on the low-temperature microstructure. The microstructure pattern arises as a result of quenching the system from a high-temperature tetragonal phase to the low-temperature orthorhombic one and subsequent annealing. The potential of the impurity is modified in such a way that it promotes occupation of opposite nearest-neighbor sites around impurity by an oxygen and vacancy simultaneously. The simulations of the annealing processes showed that the domain pattern becomes very tiny with increased concentration of randomly distributed impurities. Domains of larger sizes would appear if the impurities were able to diffuse to the domain walls. This is confirmed by annealing the sample containing linear chains of impurities. The tweed microstructure depends on the magnitude of the force constants of the elastic subsystem, and at too large coupling the randomly distributed impurities are not able to pin the stiff domain walls. The results resemble the electron-microscope photographs made for cobalt in YBa2(Cu1−xCox)3O7−δ.
Doping and crystallization of amorphous SiGe films with an excimer (KrF) laser
Authors: Krishnan S. , Chaudhry M.I. , Babu S.V.
Published: September 2007

Amorphous silicon germanium (a-SiGe) films, deposited on silicon substrates at room temperature in a molecular beam epitaxy system, were transformed into a single-crystal film and doped with phosphorus by exposure to KrF laser pulses. Electron channeling patterns showed that laser exposure resulted in crystallization of the undoped a-SiGe films. The SiGe films were doped by laser irradiation, using a phosphorus spin-on-dopant. The sheet resistance of the doped films decreased with increasing numbers of pulses, reaching a value of about ∼ 5 × 104 ohms/□ after 15 pulses. I-V data from mesa-type n-SiGe/p-Si diode devices were used to determine the effect of laser processing on the quality of the SiGe films.
On the microstructure and thermal stability of rapidly quenched Fe–B alloys in the intermediate composition range between the crystalline and amorphous states
Authors: Fernández van Raap M.B. , Sánchez F.H. , Zhang Y.D.
Published: September 2007

The structure and the thermal stability of the Fe0.89B0.11 rapidly quenched alloy have been investigated. Transmission Mössbauer measurements were carried out as a function of temperature in the range from 148 K to 513 K. Room temperature x-ray diffraction and transmission and conversion-electron Mössbauer experiments, as well as 4.2 K spin-echo nuclear magnetic resonance measurements, were also performed after some selected thermal treatments for one hour between 523 K and 1273 K. Based on these experiments it is suggested that the alloy is inhomogeneous at nanoscopic scale and consists of a fine dispersion of a defective boride phase with an o-Fe3B-like short-range order, embedded in an α-Fe matrix. This result gives support to the models which indicate phase separation in the amorphous phase with o-Fe3B short-range order prevailing in the hypereutectic iron concentration range. This phase was found to be less stable than the undefective one present in the less boron concentrated alloys. The transformation into the equilibrium phases, analyzed with an Arrhenius-type temperature dependence for the increase of the relative fraction of Fe2B, led to an activation energy Ea = 1.38 ± 0.68 eV/atom.
Microstructures of thermomechanically treated eutectoid Zn–Al alloy (II)
Authors: Zhu Yao Hua , Goodwin Frank E.
Published: September 2007

The evolution of microstructure in a cast, extruded, and aged eutectoid Zn-Al-Cu alloy is described based on x-ray diffraction an micrographic observations. Whereas the three phases α′, β′, and η′ were found in the as-cast state, the three phases α, T′, and η′E were observed in this alloy after extrusion at 250 °C. The Al-rich fcc α phase appeared as isolated particles with clear boundaries. The Zn-rich η′E phase decomposed upon aging into α, T′, and η phases. It was found possible to control the phase makeup of the alloy by controlling extrusion temperature. At extrusion temperatures greater than 268 °C, α, E, and η′E were detected, whereas temperatures below this, the phases α, T′, and η′E were found. After prolonged aging, the phase makeup of the alloy was found to be the same regardless of prior treatment routes. Early shrinkage of the extruded alloy occurred after aging at 91 and 150 °C and was related to precipitation of aluminum from the η′E phase during its decomposition to α, T′, and η products.
Solute-atom segregation is high-angle (002) twist boundaries in dilute Au–Pt alloys
Authors: Udler D. , Seidman D.N.
Published: September 2007

Solute-atom segregation is studied by Monte Carlo simulations for three high-angle symmetrical (002) twist boundaries in Au-1 at. % Pt and Pt-1 at. % Au alloys at T = 850 K. It complements our previous study, that focused mainly on low-angle boundaries in the same alloys. Solute enhancement occurs on the Pt-rich side of the phase diagram, while on the Au-rich side net depletion in solute is observed. Following the trend observed for low-angle boundaries, Au as a solute prefers the structural units of the perfect crystal type, while Pt as a solute is depleted at those sites. The solutc concentration at structural units depends on the planar fraction of those units in the boundary.
Growth by molecular beam epitaxy of (rare-earth group V element)/III-V semiconductor heterostructures
Authors: Guivarc'h A. , Le Corre A. , Auvray P. , Guenais B. , Caulet J. , Ballini Y. , Gúcrin R. , Députier S. , Le Clanche M.C. , Jézéquel G. , Lépine B. , Quémerais A. , Sébilleau D.
Published: September 2007

This paper deals with the growth by molecular beam epitaxy of semimetallic (rare-earth group V element) compounds on III-V semiconductors. Results are presented, first on the Er-Ga-As and Er-Ga-Sb ternary phase diagrams, second on the lattice-mismatched ErAs/GaAs (δa ≈ +1.6%), YbAs/GaAs (δa/a = +0.8%), and ErSb/GaSb (δa/a ≈ +0.2%) heterostructures, and third on the lattice-matched Sc0.3Er0.7As/GaAs and Sc0.2Yb0.8As/GaAs systems (δa/a < 0.05%). Finally the growth of YbSb2 on GaSb(001) is reported. The studies made in situ by reflection high-energy electron diffraction (RHEED) and x-ray photoelectron diffraction and ex situ by x-ray diffraction, transmission electron microscopy, He+ Rutherford backscattering, and photoelectron spectroscopy are presented. We discuss the atomic registry of the epitaxial layers with respect to the substrates, the appearance of a mosaic effect in lattice-mismatched structures, and the optical and electrical properties of the semimetallic films. The problems encountered for III-V overgrowth on these compounds (lack of wetting and symmetry-related defects) are commented on, and we underline the interest of compounds as YbSb2 which avoid the appearance of inversion defects in the GaSb overlayers.
Regeneration of PAN-based activated carbon fibers by thermal treatments in air and carbon dioxide
Authors: Ko Tse-Hao , Chiranairadul P.
Published: September 2007

PAN-based activated carbon fibers were saturated by dye adsorption and then were regenerated by thermal treatment in carbon dioxide and in air. The dye adsorption and the regeneration were carried out in several cycles. The changes in fiber physical properties and the capacity of dye adsorption will be discussed. Activated carbon fibers regenerated in air had greater dye adsorption and weaker mechanical properties than those regenerated in carbon dioxide. The preferred orientation changed slightly during air reactivation, but it decreased gradually after carbon dioxide regeneration. The regeneration processes led to a decrease in the weight and degradation of mechanical properties, but the processes increased the capacity of dye adsorption. After the second regeneration, the dye adsorption capacity of activated carbon fibers that were recycled by air regeneration was 15% higher than those that were recycled by carbon dioxide regeneration. But, after the third regeneration, the fibers recycled by air regeneration lost their mechanical properties. For carbon dioxide regeneration, fibers retained satisfactory mechanical properties even after the fourth regeneration. This study indicates that multiple effective applications can be accomplished with carbon dioxide treatment in place of air regeneration. The structural changes of activated carbon fiber during different regenerations are proposed.
The effect of hydrogen on the formation of carbon nanotubes and fullerenes
Authors: Wang X.K. , Lin X.W. , Mesleh M. , Jarrold M.F. , Dravid V.P. , Ketterson J.B. , Chang R.P.H.
Published: September 2007

A novel method to synthesize “clean” carbon nanotubes with relatively high yield in a hydrogen arc discharge has been developed. The quality and yield of the tubes depend sensitively on the gas pressure in the arc discharge. Sharp, open-ended nanotubes with clear lattice fringes at the edges and empty interiors have been observed. The existence of these frozen-open-ended tubes as part of nanotube-bundles provides evidence for an open-ended growth model for nanotubes. Using time of flight mass spectrometry, it was found that fullerenes, such as C60 and C70, are almost absent from the soot collected in the hydrogen arc discharge. The effect of hydrogen on the formation of fullerenes, both in the laboratory and in space, will be discussed.
Chemisorptive electron emission and atomic force microscopy as probes of plastic deformation during fracture at a metal/glass interface
Authors: Nakahara Sumio , Langford S.C. , Dickinson J.T.
Published: September 2007

We examine the use of chemisorptive emission (electron emission accompanying the adsorption of a reactive gas on a metal surface) and atomic force microscopy as measures of plastic deformation during fracture along a metallic Mg/glass interface. Localized ductile deformation in the metallic phase enhances the fracture energy, exposes metallic Mg to the reactive O2 atmosphere, and produces intense emissions. The number of electrons emitted following fracture in low-pressure oxygen atmospheres is strongly correlated with the total energy expended during failure (peel energy). The presence of localized ductile deformation is verified by atomic force microscopy (AFM): voids are observed on surfaces yielding significant cmissions and enhanced fracture energies. These voids are not observed on samples yielding the lowest peel energies and emission intensities, i.e., where the contribution of deformation to the peel energy is negligible. Quantitative use of roughness data derived from the AFM images is, however, problematic. The potential for chemisorptive electron emission as a probe of deformation along interfaces involving Mg, Ti, Zr, and Al is promising.
X-ray studies on the morphology of zwitterionic polymers
Authors: Montiel R. , Cardoso J. , Manero O.
Published: September 2007

Wide Angle X-ray Scattering (WAXS) and Small Angle X-ray Scattering (SAXS) were applied to elucidate the microstructure and morphology of two zwitterionic copolymers. The first system is a sulfobetaine derivative of poly(4-vinyl pyridine) with an ionic content of 10% (10% of the degree of quaternization). The copolymer is almost amorphous, with a small degree of crystallinity (10%). The second copolymer is a N-oxide derivative of the poly(N,N dimethyl aminoethyl methacrylate) highly crystalline (90%). The latter forms superstructures of globular shape with crystalline domains of coordinated polymer-salt systems. X-ray data are in accordance with those published by Castaño et al. [J. Mater. Res. 5, 654 (1990)]. These authors reported TEM micrographs of zwittcrionic systems, where clusters with grainy appearance were observed within the length of scale range found in this work. Models for the morphology of these copolymers are proposed for each use.
Interpretation of mercury porosimetry applied to aerogels
Authors: Pirard R. , Blacher S. , Brouers F. , Pirard J.P.
Published: September 2007

The observation of aerogels submitted to a pressure of mercury indicates that this porous material is compacted and not intruded by the mercury. Consequently, the classical Washburn equation cannot be applied. A relation is established between the pressure P of compaction and the size L of the largest pores. The size of pores is estimated by using the nitrogen adsorption-desorption isotherms analysis and SEM measurements. A relation is found in which P is proportional to L−4 The new relation is applied to mercury porosimetry. Finally, a mechanical model is proposed that reproduces successfully the behavior of aerogels under high pressure of mercury.
The crystal structure of the high-temperature polymorph of α–hexathienyl (α–6T/HT)
Authors: Siegrist T. , Fleming R.M. , Haddon R.C. , Laudise R.A. , Lovinger A.J. , Katz H.E. , Bridenbaugh P. , Davis D.D.
Published: September 2007

α-hexathienyl (α–6T) is a highly promising material for application in thin film transistor devices. Recently, record high mobilities, together with record high current on/off ratios, have been reported.1 Thus far, structural information on this exciting material is sketchy. The crystal structures of several such hexamers have been investigated, but only with powder samples, since the crystal growth has proven exceedingly difficult.2-5 Powder Rietveld refinements on these materials are severely hampered by the large number of overlapping reflections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of α–6T (α–6T/HT), as determined from a single-crystal structure analysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Extended Hückel theory (EHT) band structure calculations show that α–6T/HT is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at Γ. The conduction and valence bands both show a remarkable degree of dispersion along X and Y for a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic superconductors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as κ−(ET)2 Cu(NCS)2.
Preparation of a-axis YBa2Cu3Ox epitaxial films using direct current-95 MHz hybrid plasma sputtering
Authors: Ito Wataru
Published: September 2007

The dc-95 MHz hybrid plasma magnetron sputtering has been newly developed for obtaining a-axis oriented YBa2Cu3Ox (YBCO) films with an excellent crystallinity. The crystallinity was found to be the best among the films reported so far: the full width at half maximum value of 0.027°in the rocking curve measurement through the film (200) diffraction peak and Xmin of 2% estimated from the barium signal behind the surface peak in Rutherford backscattering (RBS) measurement using a 1 MeV He+ ion. The success in the excellent crystallinity was explained from the ion acceleration model at the ion sheath formed near the substrate surface considering the high ion density, which was revealed to be a characteristic of hybrid plasma. Almost perfect epitaxial growth was also confirmed by transmission electron microscopy. A characteristic grain boundary structure depending on the substrate was observed for the films on NdGaO3 and SrTiO3 substrates. Twist boundary is dominant for the film on NdGaO3, while symmetrical tilt boundary and basal-plane-faced tilt boundary exclusively exist for the film on SrTiO3. The microstructure of the film on SrTiO3 is very resistive against film relaxation. Strain relief was observed by RBS channeling spectra for the relatively high superconducting films. The results of Raman spectroscopy and RBS oxygen resonant measurements indicated that the oxygen content is not a critical parameter for determining the superconductivity of the a-axis oriented YBCO films, but oxygen ordering in the plane of the Cu-O chain and relief of the film strain are important for the improvement of Tc.
Origin of the Y2Ba1Cu1O5 free region in melt-textured Y–Ba–Cu–O oxides
Authors: Kim Chan-Joong , Lee Hee-Gyoun , Kim Ki-Baik , Hong Gye-Won
Published: September 2007

In order to understand the formation mechanism of the Y2BaCuO5 free region, microstructures concerning incongraent mehing and peritectic reaction were studied in melt-textured Y–Ba–Cu–O oxides. It is found that spherical pores form during incongruent melting of the YBa2Cu3O7−y phase into Y2BaCuO3 and a Ba–Cu–O liquid phase. As the melting goes on, liquid phase flows into the pores and then produces spherical liquid pockets containing a few Y2BaCuO5 particles. During slow cooling of the sample from the peritectic temperature to the temperature where YBa2Cu3O7−y phase is formed, the liquid pockets are converted into YBa2Cu3O7−y phase containing a few Y2BaCuO5 particles. Sometimes, remnant Ba–Cu–O liquid phase is present at the center part of the Y2BaCuO5 free regions due to the incomplete peritectic reaction. It is concluded that formation of spherical pores during incongruent melting of YBa2Cu3O7−y is responsible for the formation of the Y2BaCuO5 free regions.
Effects of processing parameters on the levitation force of melt-processed YBa2Cu3Ox
Authors: Varanasi C. , McGinn P.J. , Pavate V. , Kvam E.P.
Published: September 2007

Melt processing of YBa2Cu3Ox (Y-123) with a NdBa2Cu3Ox-123 single-crystal seed has been used to grow large single-grain Y-123 samples. Processing parameters have been varied to observe the resulting differences in the levitation force of the samples. Increased hold time above the peritectic temperature and increased undercooling temperature have been observed to be beneficial in enhancing the levitation force of the samples. The levitation force measurements have been correlated with magnetization measurements on the crushed domains.
Reaction-mediated texturing of barium ferrite magnetic thin films on ZnO underlayer
Authors: Li Jinshan , Sinclair Robert , Rosenblum Stephen S. , Hayashi Hidetaka
Published: September 2007

Using facing target sputtering, crystalline magnetoplumbite-type barium ferrite (BaFe12O19 or BaM) thin films have been prepared in situ at a substrate temperature of 640 °C without postdeposition annealing. Using our facing target sputtering system, BaM thin films grow randomly if they are directly deposited onto Si or thermally oxidized Si substrates. However, deposited onto a sputtered ZnO layer (∼230 Å) on Si substrates, BaM thin films show excellent c-axis out-of-plane texture with a 0.2°c-axis dispersion angle, as indicated by x-ray diffraction (XRD). Cross-section transmission electron microscopy (XTEM) reveals that the textured films epitaxially grow on a transition layer, which is formed between BaM and ZnO. No direct epitaxial relation between BaM and ZnO was observed. This transition layer is identified by TEM and XRD as ZnFe2O4, which, from a structure point of view, reduces the lattice mismatch between BaM and ZnO, and also enhances the c-axis out-of-plane epitaxial growth. ZnFe2O4 is a reaction product of BaM and ZnO, as indicated by both TEM and XRD. After ex situ annealing the film in air at 800 °C, the ZnFe2O4 layer becomes thicker at the expense of BaM and ZnO. The lattice parameters of both BaM and ZnO decreased as annealing time increased.
Gas transport model for chemical vapor infiltration
Authors: Starr Thomas L.
Published: September 2007

A node-bond percolation model is presented for the gas permeability and pore surface area of the coarse porosity in woven fiber structures during densification by chemical vapor infiltration (CVI). Model parameters include the number of nodes per unit volume and their spatial distribution, and the node and bond radii and their variability. These parameters relate directly to structural features of the weave. Some uncertainty exists in the proper partition of the porosity between “node” and “bond” and between intra-tow and inter-tow, although the total is constrained by the known fiber loading in the structure. Applied to cloth layup preforms the model gives good agreement with the limited number of available measurements.
Deposition and characterization of crystalline conductive RuO2 thin films
Authors: Jia Q.X. , Song S.G. , Foltyn S.R. , Wu X.D.
Published: September 2007

Highly conductive metal-oxide RuO2 thin films have been successfully grown on yttria-stabilized zirconia (YSZ) substrates by pulsed laser deposition. Epitaxial growth of RuO2 thin films on YSZ and the atomically sharp interface between the RuO2 and the YSZ substrate are clearly evident from cross-sectional transmission electron microscopy. A diagonal-type epitaxy of RuO2 on YSZ is confirmed from x-ray diffraction measurements. The crystalline RuO2 thin films, deposited at temperatures in the range of 500 °C to 700 °C, have a room-temperature resistivity of 35 ± 2 μω-cm, and the residual resistance ratio (R300 k/R4.2 k) is around 5 for the crystalline RuO2 thin films.
Conventional and microwave sintering of condensed silica fume
Authors: Majling Jan , Znasik Peter , Agrawal Dinesh , Cheng Jiping , Roy Rustum
Published: September 2007

Condensed silica fume, a by-product from the production of silicon alloys, was sintered by (i) conventional heating in a dilatometric furnace, both at constant heating rate and isothermal heating, and (ii) by the microwave heating. The dense products with relative density up to 95% of theoretical can be obtained only by short runs at high heating rates, preferentially accomplished by the microwave treatment. Prolonged heating leads to the devitrification of the original glassy phase to cristobalite, accompanied by an arrest of densification.
Parametric study of in situ growth of NdCeCuO thin films by laser ablation
Authors: Lin Wen-Tai , Chen Guo-Ju
Published: September 2007

The stability fields of the NdCeCuO (NCCO) films and the Ce0.5Nd0.5O1.75 (CNO) phase grown by laser ablation from the stoichiometric targets, Nd1.85Ce0.15CuO4−8, are demonstrated, respectively. The superconducting NCCO films were grown in a specific field below and close to the CuO/Cu2O transition line, i.e., at 700–725°C in 20–50 mTorr, without vacuum annealing. On postdeposition cooling in lower oxygen pressures (<10 mTorr), the superconductivity of the NCCO films was significantly improved without vacuum annealing. The CNO phase usually appeared in the NCCO films at temperatures above 680 °C, and the higher the temperature the greater the CNO phase due to copper loss. A surplus of copper in the NCCO targets effectively suppressed the formation of the CNO phase, and thereby improved the superconductivity of the NCCO films.
Interface between gold and superconducting YBa2Cu3O7−x
Authors: Chan Siu-Wai , Zhao Lie , Chen C. , Li Qi , Fenner D.B.
Published: September 2007

Gold (Au) and silver (Ag) are known to be important contact metals on YBa2Cu3O7−x (YBCO). Both metals have been used as additives in fabricating tapes of YBCO and Bi2Sr2CaCu2O8 (BSCCO) materials, and have favorable results in improving not only the flexibility but also the weighted critical currents of the resulting composites. Previous results on superconductor/normal metal/superconductor junctions made using YBCO/Au/YBCO and YBCO/Au/Nb demonstrated that a supercurrent can be induced in the normal metal layers through the proximity effect. Our transmission electron microscopy study of the Au/YBCO interfaces shows a well-bonded interface with no extraneous phases present. Lattice fringes of the (001) plane in YBCO terminated at the interface abruptly. This observation supports previous results of contact resistance of x-ray photoelectron spectroscopy (XPS). Both (001) integral steps and multiples of 1/3 (001) steps were observed at the Au/YBCO interface. When the top gold layer was absent locally, surface degradation was observed as the (001) lattice fringes stopped short from the surface by 10 nm. Our results support that Au is a desirable contact metal and a dependable surface passivation material for YBCO.
Preparation and chemical analysis of high purity iron-zinc alloys
Authors: Grant R.G. , Cook P.S. , Cook D.C.
Published: September 2007

We have produced and chemically analyzed a series of high purity iron-zinc alloys with iron concentrations in the range 4-27 wt. %. The technique involved slow diffusion of high purity zinc with small particle iron powder. We produced alloys within the four main iron-zinc phases, Zeta, Delta, Gamma-1, and Gamma to aid in the identification of the intermetallic phases formed in commercially produced galvanneal steel coatings. The diffusion technique produces iron-zinc alloys which are suitable as instrument calibration standards for galvanneal producers and users. A chemical titration technique which measures iron concentration to within 0.5 wt. % was also refined. In determining the bulk iron concentrations of the samples, we have compared the accuracy of the wet chemical technique with Induction Coupled Plasma spectroscopy. Homogeneity was measured using an electron microprobe and a scanning transmission electron microscope. The data show that samples are homogeneous to greater than 98% of the mean iron concentration.
Investigation of SiC–AlN: Part III. Static and dynamic fatigue
Authors: Huang Jow-Lay , Jih Jyh-Ming
Published: September 2007

SiC/AlN composites with controlled interfacial solid solution were employed in this present work to investigate the effects of interfacial chemical composition and AlN polytypes on the fatigue properties. The dynamic strength of the SiC/AlN composite was found to decrease initially as the stressing rate decreased. However, the strength increased with a decrease in stress rate at a low stress rate region of below 0.01 MPa/s. Crack arrest could have occurred before exhibiting spontaneous failure at an intermediate stress rate of 0.01 MPa/s. It was found that both the interfacial bonding strength and testing technique had essential effects on the behavior of slow crack growth.
Diamond-graphite hybrids and the nature of amorphous carbon and diamond-like carbon
Authors: Sen Rahul , Sumathy R. , Rao C.N.R.
Published: September 2007

The nature of amorphous carbon has been explored by molecular mechanics by examining the structures of species such as C84Hx and C150Hx, wherein the percentage of sp3 carbons is progressively increased in a graphitic network. The nature of diamond-like carbon has been similarly investigated by examining the structures of C84Hx and C102Hx where the percentage of sp2 carbons is varied in an sp3 network. The dependence of the average coordination number as well as the sp3/sp2 atom ratio on the atom fraction of hydrogen has been investigated in light of the random covalent network model.
Autostoichiometric vapor deposition: Part II. Experiment
Authors: Chour Kueir Weei , Xu Ren
Published: September 2007

An autostoichiometric vapor deposition method is implemented to achieve deposition of stoichiometric LiTaO3 using LiTa(OButn)6 precursor. Precisely stoichiometric LiTaO3 can be grown by this method. A simple low pressure reactor is used to facilitate the autostoichiometric vapor deposition through hydrolysis polycondensation of double alkoxides in the vapor phase. Typical deposition and annealing conditions are described. X-ray diffraction, SEM, and compositional analyses are performed on the films grown on fused silica, Pt, (100) sapphire, and LiNbO3(001). Rocking curve measurements indicated excellent epitaxial growth of LiTaO3 on (100) sapphire. Compositional analysis by measurement of lattice parameters confirmed that the present method can produce high quality stoichiometric LiTaO3. The nonstoichiometry factor for LiTa(OButn)6 is negligible. Mass spectrometric study of the precursor compound LiTa(OButn)6 suggests that the volatile species is Li2Ta2(OButn)12.
Preferential orientation and dielectric/pyroelectric properties of sol-gel-derived Pb(Mg, Zn)1/3Nb2/3O3 thin films
Authors: Cho Mun K. , Kim K.S. , Jang Hyun M.
Published: September 2007

Thin films of Pb(Mg,Zn)1/3Nb2/3O3 (PMZN) were fabricated by spin casting the partially hydrolyzed Pb-Mg-Zn-Nb-O complex alkoxide sols on (111) Pt-coated MgO(100) planes. Analysis of the isothermal transformation data indicated that the growth of perovskite grains from the pyrochlore matrix is a diffusion-controlled process with an initial fast nucleation. The effect of water concentration in the sol on the preferential orientation of perovskite grains in the corresponding thin film was examined. A strong preferential orientation of (100) perovskite was obtained in the PMZN thin film derived from the sol having a molar ratio of H2O to total metal alkoxides (Rw) of 2. A small-angle x-ray scattering (SAXS) experiment in the Porod region indicated that weakly branched precursor systems led to highly (100)-oriented perovskite grains after thin-film formation. The pyroelectric coefficient and the corresponding figure-of-merit were controlled by suitably varying the degree of preferential orientation during the fabrication of thin films.
The influence of an electric field on the mechanism of combustion synthesis of tungsten silicides
Authors: Gedevanishvili S. , Munir Z.A.
Published: September 2007

The synthesis of tungsten silicides by self-propagating combustion has been successfully accomplished under the influence of an electric field. Materials with starting composition ranging from 6 to 30 wt. % Si were investigated by the method of field-activated combustion synthesis (FACS). A threshold field value was required to initiate a self-sustaining wave; the threshold value depended on composition. It was shown that the level of the applied field can influence the mechanism of silicide formation. The silicide W5Si3 could be formed only at relatively high field values while WSi2 can be formed at any field. The effect of the field on the silicide formation is discussed in terms of its role in liquid phase formation.
Cu microcrystals in sol-gel derived glasses
Authors: Nogami Masayuki , Abe Yoshihiro , Nakamura Arao
Published: September 2007

Cu2+-containing SiO2 and Al2O5 · 9SiO2 glasses were prepared by the sol-gel method and heated under reducing conditions to precipitate small-sized Cu and Cu2O erystals. Cu2+ ions incorporated in SiO2 glass were reduced by heating in N2 to precipitate Cu2O and in H2 to precipitate Cu microcrystals with diameters of about 5 to 15 nm. Microcrystalline Cu-precipitated glass showed an optical absorption band at 560 nm and its third-order nonlinear susceptibility was 1.25 × 10−10 esu, which originated from the enhancement by the surface plasmon resonance of Cu particles. In contrast, Cu2+ ions incorporated in Al2O3 · 9SiO2 glass remained unchanged after heating in the reducing gas atmospheres.
Synthesis and characterization of polysilane precursors for silicon carbide fibers
Authors: Cranstone Wayne R.I. , Bushnell-Watson Suzannc M. , Sharp John H.
Published: September 2007

A series of polysilanes was prepared by the alkali metal dechlorination of chlorosilane monomers, in which the overall functionality, F, of the reaction was varied. Starting monomers of functionality f = 2.0 and 3.0 were reacted together in various proportions to achieve values of F of 2.2, 2.35, and 2.5. In addition to varying the functionality of the reaction, three different difunctional monomers, dimethyldichlorosilane (DMDCS), diphenyldichlorosilane (DPDCS), and methylphenyldichlorosilane (MPDCS), and two trifunctional monomers, phenyltrichlorosilane (PTCS) and ethyltrichlorosilane (ETCS), were used. The effect of these changes on the yields of the polysilanes was determined, and the products were investigated by the use of thermogravimetry (TG), gel permeation chromatography (GPC), and thermomechanical analysis (TMA). The ability to spin a polysilane fiber was also assessed.
Elastic interaction between a moving screw dislocation and a surface crack
Authors: Tsai Yu-Zen , Hu C.T. , Lee Sanboh
Published: September 2007

A moving screw dislocation near a surface crack was investigated using dislocation modeling. Motion directions parallel (x direction) and perpendicular (y direction) to the crack surface were considered. Due to the free surface, the net Burgers vector inside the crack is zero. After obtaining the dislocation distribution in the crack, we calculated the stress field in the medium. Relative to a static screw dislocation, the magnitude of σyz due to the moving screw dislocation decreases with increasing velocity Vx. Generally, the effect of dislocation shielding on fracture is reduced if the velocity Vx increases. The magnitude of the image force of the dislocation also decreases with increasing velocity Vx. The effect of velocity along the y direction on the stress intensity factor and image force has the opposite trend to that along the x direction. The present result can reduce to a moving dislocation near a semi-infinite crack and a static dislocation near a surface crack.
Nanophase BaFe12O19 synthesized from a nonaqueous microemulsion with Ba- and Fe-containing surfactants
Authors: Chhabra Vishal , Lal Manjari , Maitra A.N. , Ayyub Pushan
Published: September 2007

Ultrafine BaFe12O19 with uniform particle size was synthesized from an alcohol-in-oil (nonaqueous) microemulsion system where the metal ions were supplied by the surfactant (metal di-2-ethylhexylsulfosuccinate) molecules themselves. A monodisperse, fine-grained precipitate (Ba-Fe oxalate) was ensured by the steric barrier provided by the surfactant monolayer, while the nonaqueous environment promoted stoichiometric coprecipitation. Pure BaFe12O19 was obtained by calcining the oxalate precursor at or above 950 °C. Structural and magnetic properties of the resulting ultrafine magnetic material are reported.
Mechanisms of intrinsic stresses generation in sputtered amorphous Si: H films
Authors: Takahashi Hiroki , Nagata Hirotoshi , Kataoka Haruki , Takai Hiroshi
Published: September 2007

The relation between stresses of sputtered a-Si: H films and the film deposition conditions are investigated. The film stresses change from a large compressive stress of 1000 MPa to an almost stress-free one. They arise from distortions of the Si network via the following two mechanisms. The first results from the inclusion of the Ar-sputtering gas into the films, which provides volume expansion of the film network. The other is due to structural disorders, such as a deviation of the Si bond angle which is generated during the deposition processes. Moreover, it is found that Si–H terminations in the films contribute to reducing the film stresses because the Si–H termination breaks and relaxes the Si network. These effects can be realized as long as the Si–H terminations are homogeneously distributed in the films.
Hydrothermally grown BaZrO3 films on zirconium metal: Microstructure, x-ray photoelectron spectroscopy, and Auger electron spectroscopy depth profiling
Authors: Fuenzalida V.M. , Pilleux M.E.
Published: September 2007

BaZrO3 films were grown on zirconium metal substrates by immersing thin Zr foils in an alkaline solution under hydrothermal conditions. The films were produced at temperatures ranging from 200 °C to 270 °C in a 0.25 M barium hydroxide solution for 3 to 8 h. The resulting films did not have visible pores or defects, and displayed a grain structure which depended on the treatment conditions, especially temperature. X-ray photoelectron and Auger spectroscopies revealed that (a) after removing the surface layer, films were clean, not displaying OH groups nor carbon contamination; (b) the Ba concentration steadily decreased as the depth increased, and did not behave as in BaTiO3 or SrTiO3 films prepared under similar conditions; and (c) the BaZrO3/Zr interface was very broad. Grazing angle x-ray diffraction analysis showed BaZrO3 (cubic), ZrO2 (hexagonal), and Zr (hexagonal), suggesting a layered structure: BaZrO3/ZrO2/Zr. The relative dielectric constant was ≍70 and was independent of the frequency between 100 Hz and 1 MHz. The dielectric loss factor (tan δ) was between 0.01 and 0.02. Dielectric breakdown occurred between 25 and 40 MVm−1.
The effect of stoichiometry on the microstructure and properties of lead lanthanum titanate thin films
Authors: Khan Ashraf R. , Desu Seshu B.
Published: September 2007

Thin films of Lead Lanthanum Titanate (PLT) corresponding to 28 at. % of La were prepared by the metal-organic decomposition (MOD) process. The films were fabricated from two solutions of different composition. The composition of the first solution was determined, assuming that the incorporation of La3+ in the PbTiO3 structure gives rise to A-site or Pb vacancies, whereas for the composition of the other solution the creation of B-site or Ti vacancies was assumed. The effect of excess lead on the microstructure and the optical and electrical properties was studied for 0% to 20% excess PbO. The x-ray diffraction patterns of all films at room temperature indicated a cubic structure with a lattice constant of 3.92 Å. Optical and electrical measurements showed the films made assuming B-site vacancies had better properties. In general, excess PbO was found to improve the optical transmittance as well as the electrical properties of films. However, in films assuming the formation of B-site vacancies, PLT showed improved electrical properties only up to 5–10% excess PbO, while higher PbO additions had a deleterious effect. The films had a high resistivity, good relative permittivity, low loss, very low leakage current density, and high charge storage density. A type-B film with 10% excess Pb had a relative permittivity of 1340 at 100 kHz and a charge storage density of around 16.1 μC/cm2 at a field of 200 kV/cm at room temperature.
Synthesis and characterization of LiTaO3 thin films deposited on Si by the sol-gel method
Authors: Retuert P.J. , Kneuer P.G. , Wittke O. , Avila R.E. , Piderit G.J.
Published: September 2007

Polycrystalline LiTaO3 (LT) thin films have been prepared on (001)Si substrates by the sol-gel method. A Li-Ta double alkoxide prepared from lithium methoxide and tantalum ethoxide precursors was spin coated on Si and heated up to 950 °C. The dependence of the film quality upon the process variables, alkoxide concentration before hydrolysis, the water to double alkoxide ratio, and the final double alkoxide concentration, has been established. Preferential alignment of the (104) LT planes was observed parallel to the (100)Si surface. Most films present resistivities of the order of 30 kΩ · cm and breakdown field in excess of 200 kV/cm.
Discussion on microstructure of chemical-vapor-deposited TiN films based on the calculated gaseous concentration distribution in the reactor
Authors: Yoshikawa Noboru , Kikuchi Atsushi
Published: September 2007

TiN films were ehemical-vapor-deposited on the inner wall of a tubular reactor. Films deposited in the upstream region of the reactor consisted of small and sharp crystals with (111)-preferred orientation or random orientation. On the other hand, films deposited in the downstream region or at lower partial pressure of TiCl4 consisted of columnar crystals with (110)-preferred orientation, having polyhedral shapes on the surface. For the films deposited under different conditions at different axial positions, relationships were investigated among the temperature, the calculated concentrations on the substrate, and the degree of preferred orientation of the films. As a result, it was shown that formation of films with (110)-preferred orientation is related to the conditions of high temperature and low partial pressure of TiCl4. Films deposited at the higher gas flow rate had lower degrees of (110)-preferred orientation. Decrease in partial pressure of TiCl4 along the axial direction in the reactor was calculated to be smaller at higher gas flow rate, and provided suitable conditions for deposition of films having small and sharp crystals.
Nanoscale carbon blacks produced by CO2 laser pyrolysis
Authors: Bi Xiang-Xin , Jagtoyen M. , Endo M. , Das Chowdhury K. , Ochoa R. , Derbyshire F.J. , Dresselhaus M.S. , Eklund P.C.
Published: September 2007

CO2 laser pyrolysis has been used to synthesize carbon black (particle diameter ∼30 nm) via a catalytically driven pyrolysis of benzene vapor. The H : C ratio is found to be ∼1 : 10, which is unusually high for carbon blacks. Subsequent heat treatment of the “laser black” to temperatures up to ∼2800 °C produces well-graphitized faceted particles with central polygonal cavities. High resolution TEM lattice imaging, Raman scattering, and x-ray diffraction have been used to characterize the morphological structure of these carbon particles in their as-synthesized and heat-treated forms. Furthermore, KOH treatment at ∼800 °C has been employed to activate the as-synthesized particles, producing a tenfold increase in the surface area from 50 to 700 m2/g. Possible pore structures generated during this activation process have been identified by high resolution TEM imaging.
Transmission electron microscopy observation of the interfacial reaction between a metal-organic chemical vapor deposition BaTiO3 thin film and a (100) MgO substrate
Authors: Hwang Cheol Seong , Vaudin Mark D. , Stauf Gregory T.
Published: September 2007

Cross-sectional and plan-view transmission electron microscopy were used to characterize a BaTiO3 thin film deposited on a (100) MgO single-crystal substrate at 1000 °C. The major observations were as follows: interdiffusion between the film and substrate; a large number of antiphase boundaries in the BaTiO3; a two-phase microstructure in the film composed of perovskite BaTiO3 and a second nonperovskite phase, Ba2MgTi5O13 (2:1:5); and a well-defined orientation relationship between the 2 : 1 : 5 and BaTiO3 phases. We propose a mechanism for the formation of the 2 : 1 : 5 phase based on the similarities between the crystal structure of this phase and the structure of (210) antiphase boundaries in BaTiO3.
On the elastic moduli of nanocrystalline Fe, Cu, Ni, and Cu–Ni alloys prepared by mechanical milling/alloying
Authors: Shen T.D. , Koch C.C. , Tsui T.Y. , Pharr G.M.
Published: September 2007

Young's moduli of nanocrystalline Fe, Cu, Ni, and Cu-Ni alloys prepared by mechanical milling/alloying have been measured by the nanoindentation technique. The results indicate that Young's moduli of nanocrystalline Cu, Ni, and Cu–Ni alloys with a grain size ranging from 17 to 26 nm are similar to those of the corresponding polycrystals. The dependence of Young's modulus of nanocrystalline Fe on grain size corresponds well to a theoretical prediction, which suggests that the change in the Young and shear moduli of nanocrystalline materials, free of porosity, with a grain size larger than about 4 nm, should be very limited (<10%). It is likely that reported large decreases in the Young and shear moduli of nanocrystalline materials prepared by gas-condensation/vacuum consolidation result from a relatively large volume fraction of pores.
Influence of whisker volume fraction on the creep behavior of alumina composites reinforced with silicon carbide
Authors: Xia Kenong , Langdon Terence G.
Published: September 2007

Four-point bending creep tests were conducted in air on two alumina matrix composites reinforced with 18 and 30 vol% of silicon carbide whiskers, respectively. In both materials, the SiC whiskers tended to form agglomerates. In the temperature range from 1673 to 1823 K, the stress exponents, n, were ∼3.9 and ∼6.3 and the activation energies for creep, Q, were ∼690-740 and ∼930-1010 kJ mol−1 for the composites containing 18 and 30 vol % of SiC, respectively. It is shown that the higher value of n in the composite with 30 vol % of SiC whiskers may be lowered to ∼3 by incorporating a threshold stress. The creep strength of both composites was enhanced by comparison with a similar composite containing 9.3 vol % of SiC whiskers, but there was only a very minor improvement in creep strength when the volume fraction of whiskers was increased from 18 to 30 vol %.