bluuebunny/crossref_metadata_2025 huggingface dataset:
The solidification of large sections in ceramic injection molding: Part II. Modulated pressure molding
Authors: Zhang T. , Evans J.R.G.
Published: September 2007

Cylindrical moldings of 20 and 40 mm diameter were injection molded with the application of modulated hold pressure using a well-characterized alumina-polypropylene suspension. The effect of frequency on sprue solidification was explored. For the smaller moldings, very little extension to sprue solidification time was obtained with pressures up to 140 MPa, and this is attributed to the low reciprocating volume flow. For the larger moldings, pressures of 98 MPa were sufficient to produce moldings with neither voids nor cracks, and the sprue solidification time corresponded to the time needed for solidification of the molding. The use of higher pressures resulted in internal residual stresses which were qualitatively detected by the defomation on annealing of polished diametral sections.
On the first-matrix-cracking stress in unidirectional fiber-reinforced brittle materials
Authors: Yang X.F. , Knowles K.M.
Published: September 2007

A simple model which is able to account for the distribution of matrix crack spacings as a function of applied stress in unidirectional fiber-reinforced brittle materials has been used to generate stress-strain curves for such materials up to the maximum stress level at which transverse matrix cracking can occur when a tensile stress is applied parallel to the length of the fibers. The results give an insight into how to tackle the experimental problem of determining accurately the level of first-matrix-cracking stress in such materials.
Hardness measurements of Ar+-beam treated polyimide by depth-sensing ultra low load indentation
Authors: Lee E.H. , Lee Y. , Oliver W.C. , Mansur L.K.
Published: September 2007

Polyimide Kapton and spin-cast polyamic acid (PAA) on sapphire have been implanted with 1 MeV Ar ions to a dose of 4.7 × 1015 cm−2 at ambient temperature. The properties of both pristine and implanted surfaces were characterized by a depth-sensing low-load indentation technique. Experiments were carried out to investigate the effects of substrate, indentation rate, relaxation, and indentation technique. The results showed that (1) hardness was depth-dependent and decreased with increasing indentation depth, (2) measurements of the ion beam hardened surface with the untreated material as a substrate underestimated the hardness while measurements over the sapphire substrate overestimated it, (3) the effects of loading/unloading rates were apparent in the load displacement results, and (4) hardness values measured using the force modulation technique showed very little depth dependence. The hardness value at 100 nm depth is used for comparison purposes since the hardness value at this depth was almost independent of substrate, indentation rate, and indentation method. The hardness of Kapton, which was measured using the techniques described herein, was increased by over 30 times after Ar implantation, from 0.43 to 13 GPa at 100 nm indentation depth. A similar increase in hardness was also observed for polyamic acid. This result suggests that spin-cast PAA film may have potential technological applications for protective coatings where hardness and wear resistance are required.
Containerless solidification of YBa2Cu3O7-x in a drop tube
Authors: Todt V.R. , Schmitz G.J.
Published: September 2007

YBa2Cu3O7-x was molten and resolidified by a containerless process under free-fall conditions in a drop tube. The resulting samples revealed four different microstructures originating either from a “high-temperature melt” (Tm ≥ 1550 K) or from a “low-temperature melt” (1288 K ≤ Tm ≤ 1550 K). All structures exhibited very fine, homogeneously dispersed structural constituents. One of these microstructures is not compatible with actual equilibrium phase diagrams and implies the kinetic suppression of the nucleation of both the Y2BaCuO5- and the Y2O3-phase.
Bonding-property relationships in intermetallic alloys
Authors: Eberhart M.E. , Clougherty D.P. , MacLaren J.M.
Published: September 2007

A definition for structure of atomic scale systems is introduced which extends the typical crystallographic description to include elements of the total charge density. We argue that the mechanical properties of intermetallic alloys are related to this extended structure. These relationships have their origin in the nature of the charge redistribution accompanying strain. The direction of this charge redistribution is determined solely by the extended structure, while its magnitude can be correlated with a quantification of this extended structure. We demonstrate these facts by determining the extended structure and nature of the charge redistribution resulting from uniaxial strain for two alloys with the L10 structure: CuAu and TiAl. While these alloys share the same crystallographic structure, their extended structures are different, with CuAu possessing the same extended structure as the allotropic fcc metals while TiAl does not. These different extended structures give rise to different charge redistributions, which are argued to be related to the intrinsically ductile behavior of CuAu and the tendency for TiAl to fail transgranularly.
Embedded atom calculations of the equilibrium shapes of 5–60 atom palladium nanocrystals
Authors: Sachdev A. , Masel R.I.
Published: September 2007

The embedded atom method (EAM) has been used to compare the stability of a series of small palladium clusters with 5–60 atoms and a variety of shapes. It is found that the 13- and 55-atom icosahedra and cubo-octahedra are stable at 0 K. However, other sized icosahedra and cubo-octahedra are unstable at 0 K. Upon annealing, the icosahedra and cubo-octahedra reconstruct into nonpolyhedral structures which are highly strained. The strained structures are much more stable than the icosahedron or cubo-octahedron except when there are 13 or 55 atoms in the cluster. Further, there are many disordered shapes which are within 0.01 eV of the minimum energy structures at all cluster sizes including 13 and 55 atoms. We observe transitions between these low energy structures in Monte Carlo calculations. These results suggest that at equilibrium one should rarely observe polyhedral palladium particles. Instead, most of the particles should be disordered. Further, there should be a wide distribution of particle shapes in agreement with experiment.
Evidence of Co/SiO2 reaction during rapid thermal annealing
Authors: Ho Herbert L. , Nguyen Tue , Chang Julius C. , Machesney Brian , Geiss Peter
Published: September 2007

Cobalt (Co)/silicon dioxide (SiO2) reactions during rapid thermal annealing (RTA) in an N2 ambient have been investigated. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that islands consisting of Co and the cobalt oxide phase, Co3O4, form during high-temperature anneals (30 s−10 min at 800 °C). After a selective wet etch to remove the islands, groove-like features are produced in the SiO2 layer. The morphologies of the islands and grooves are strikingly similar, which strongly suggest that Co and the underlying SiO2 layer have reacted, most likely to form Co3O4. We propose that small concentrations of O2 are necessary to promote Co/SiO2 interactions.
Relationship between the structural organization and the physical properties of PECVD nitrogenated carbons
Authors: Ricci M. , Trinquecoste M. , Auguste F. , Canet R. , Delhaes P. , Guimon C. , Pfister-Guillouzo G. , Nysten B. , Issi J.P.
Published: September 2007

By a Plasma Enhanced Chemical Vapor Deposition process (PECVD), we are able to prepare nitrogenated amorphous carbon materials around room temperature from methane and nitrogen gas as precursors. We have also used chlorine gas as an additive to reduce the hydrogen content of our samples. Starting from the “as-deposited” materials, we have investigated their thermal stability by successive heat treatments up to 1400 °C. These compounds suffer a weight loss mostly due to the hydrogen departure. They become nonfusible and it turns out that nitrogen, chemically bound to sp2 hybridized carbons, induces some changes in the physical properties. In order to understand the relationship between the local structural organization and the physical characteristics, we have investigated different spectroscopic techniques such as Nuclear Magnetic Resonance, IR Absorption, and X-ray Photoelectron Spectroscopy. We have also investigated several transport properties: (i) The dc electrical conductivity shows a kind of metal/insulator transition around 700 °C. The temperature dependence for the conductive samples gives evidence for a pseudogap associated with the presence of localized states, (ii) The thermal conductivity exhibits, for the as-deposited compound, a very low value varying slowly with temperature; its magnitude as well as its temperature dependence, characteristic of noncrystalline materials, are modified by the annealing process. Finally, an electronic band model is proposed, explaining the structural evolution through a kind of Mott–Anderson pseudotransition.
Image analysis of TEM pictures of fluorine-intercalated graphite fibers
Authors: Oshida K. , Endo M. , Nakajima T. , di Vittorio S.L. , Dresselhaus M.S. , Dresselhaus G.
Published: September 2007

A digitization of the TEM pictures of fluorine-intercalated graphite fibers has been used to carry out quantitative measurements of the defect structure of this material. Emphasis is given to both the computer analysis technique and to the characterization of the defects. The amount of intercalation-induced disorder increases with increasing fluorine concentration. The fast Fourier transform of the digitized TEM image exhibits two diffuse spots, corresponding to the c-axis repeat distance of the intercalation compound. The length and width of the spots are a measure of the out-of-plane and in-plane disorder present in the fibers. From the fast Fourier transform, the distribution of interlayer repeat distances and the fraction of unintercalated graphite regions throughout the material is obtained. By selecting a small range of repeat distances and carrying out an inverse fast Fourier transform, the spatial distribution of material with a given repeat distance is determined. Regions with the same repeat distance are found to form islands. This particular feature of fluorine graphite intercalation compounds, as well as the nature of the microscopic defects and the staging behavior of fluorine-intercalated graphite fibers, are discussed in connection with the dual covalent and ionic nature of the carbon-fluorine bond in fluorine-intercalated graphite.
Atomistic considerations on the fracture toughness of brittle materials
Authors: Sakai M.
Published: September 2007

A simple atomistic approach to the mechanical strength and the fracture toughness of brittle materials is made by the use of a universal expression for binding potential energy versus atomic separation curves. The scaling factors for the atomic separation and for the energy amplitude successfully apply to describing the intrinsic fracture toughness K* in a scaled dimensionless form. It is demonstrated that the intrinsic fracture toughness combined with a stress shielding coefficient (SSC) yields the fracture toughness of real materials. Microfracture mechanisms for crack-tip stress-shielding processes, as well as the interrelationship between the stress intensity- and the potential energy-derived fracture toughness, are addressed.
X-ray diffraction study of the effect of boron on lattice properties of Ni76Al24
Authors: Rao Mohan P.V. , Satyanarayana Murthy K. , Suryanarayana S.V. , Nagender Naidu S.V.
Published: September 2007

A small addition of boron is suggested to increase the ductility of the polycrystalline Ni3Al when the Al content is less than 25 at.%. Both metallographic and x-ray investigation have shown the alloys of Ni3Al (24 at.% Al) containing 0.20, 0.42, 0.79, 0.98, and 1.22 at.% B to be of single phase and that of 1.76 at.% B to be of two phase. With the addition of boron, the lattice parameter of the Ni3Al phase is found to increase. Microhardness measurements indicate that initially the hardness decreases for the alloy of 0.20 at.% B, while for the rest of the single phase alloys the hardness is found to increase with further addition of boron. The addition of boron increases the deformation stacking fault probability value except for the alloy with 0.20 at.% B.
X-ray tensile testing of thin films
Authors: Noyan I.C. , Sheikh G.
Published: September 2007

The “x-ray tensile test” is the combination of the standard uniaxial tensile test with x-ray diffraction techniques. In this test, in addition to the mechanical stress-strain values usually obtained from a tensile test, one measures the x-ray strain and stress in the diffracting regions of the sample. In multilayer thin films or in multiphase materials, x-ray diffraction enables the determination of strains and stresses in the individual layers or phases. Correlation of the x-ray data with the mechanical stress-strain values may be used to analyze strain and load partitioning within the specimen. In this paper an extended theoretical analysis of this technique and its application to evaporated Cu films on Ni substrates is presented.
Diamond nucleation from the gas phase: A kinetic approach
Authors: Molinari E. , Polini R. , Sessa V. , Terranova M.L. , Tomellini M.
Published: September 2007

Particle size distributions derived from SEM micrographs of well-separated diamond particles, deposited on Si(100) substrates by the hot filament CVD method, have been analyzed in connection with previous measurements of particle growth rates. Rates of nucleation have been obtained from this analysis in an extended range of deposition conditions. A previously developed nucleation model has been utilized, and the kinetic parameters provide a framework for modeling the nucleation process from the gas phase.
Epitaxial growth versus nucleation in amorphous Si doped with Cu and Ag
Authors: Custer J.S. , Thompson Michael O. , Eaglesham D.J. , Jacobson D.C. , Poate J.M.
Published: September 2007

The competition between solid phase epitaxy and random nucleation in amorphous Si implanted with Cu and Ag has been studied. At low metal concentrations, solid phase epitaxy proceeds with slight deviations from the intrinsic rate, with the impurity segregated and evenly distributed in the amorphous layer. At an impurity concentration of 0.12 at.%, rapid nucleation occurs, transforming the remaining layer into polycrystalline Si. The nucleation rate is ≥108 the intrinsic homogeneous rate. The effects of the metals on epitaxy scale with the amount of metal–Si interaction. Nucleation appears to occur when the metal impurities exceed their absolute solubility limit and begin to phase separate.
Formation of buried Sb dopant profiles in silicon by pulsed laser epitaxy
Authors: Carolissen R.J. , Knoesen D.K. , Sinke W.C. , Pretorius R.
Published: September 2007

In this investigation buried Sb dopant profiles in single crystal silicon have been formed from evaporated layers using laser annealing. For irradiations carried out in air, severe oxidation of the surface layers inhibited epitaxy. Oxygen concentrations as high as 5 × 1017 atoms/cm2 (equivalent to about 105 nm SiO2) were measured. It was found that both the thin (less than 3 nm) Sb layer and the free volume in the a-Si, deposited by evaporation onto a cold substrate, need to be present for this degree of oxidation to take place. However, when silicon was evaporated onto a substrate heated to 350 °C, dense packing of the silicon atoms was obtained and even for irradiations in air good epitaxy (minimum yield of 7%) and no oxidation occurred. To form buried Sb profiles, laser energies only slightly higher than the threshold for epitaxy were used to prevent excessive spreading due to an increase in liquid state diffusion obtained at higher energies. Under these conditions the width of the buried Sb profile was typically about 120 nm, and up to 90% of the Sb atoms were found to occupy lattice sites.
A study of the local atomic structure in Hg0.80Cd0.20Te using x-ray diffuse scattering
Authors: Quintana J.P. , Cohen J.B.
Published: September 2007

The local atomic arrangements in a commercial n-type wafer of Hg0.8Cd0.8Te were investigated by measuring the diffuse x-ray scattering in two volumes in reciprocal space. A change in contrast between the two measurements was achieved by making one volume measurement at 12037 eV and a second volume measurement at 12270 eV, i.e., near the HgLIII edge. The difference between these two measurements yielded intensity only due to Hg–Hg, Hg–Te, and Hg–Cd pair interactions. In all three patterns, peak-like features were apparent at the forbidden Bragg peak positions on thermal diffuse scattering ridges that joined major Bragg reflections; these are primarily attributed to second order displacement effects on the mixed cation sublattice. The first two Warren–Cowley short-range order parameters were determined to be α½½0 = −0.050(26) and α110 = 0.118(35). Simulations of the structure revealed small ordered regions with a preference for 3:1 Hg–Cd near-neighbor configurations. The near-neighbor Hg–Te bonds contract from that calculated from the average crystal's lattice parameter, and this Hg–Te distance is less than the distance in HgTe.
Properties and crystallization of Li2O–CaO–Al2O3–SiO2–TiO2glasses
Authors: Wang Moo-Chin , Hon Min-Hsiung
Published: September 2007

The addition of CaO to Li2O–Al2O3–SiO2–TiO2(LAST), forming the Li2O–CaO–Al2O3–SiO2–TiO2(LCAST) system, is used in the preparation of low themal expansion coefficient glass-ceramics. By a progressive weight percent substitution of CaO for SiO2, at constant ratios of concentration of Li2O, Al2O3, and TiO2, a number of properties of these glasses have been studied. The results indicated that these thermal properties increased progressively with increasing CaO concentration. X-ray diffraction analysis was utilized to identify the crystalline phase in glass-ceramics of the Li2O–CaO–Al2O3–SiO2-TiO2system. Thed-spacings of the major crystallites were precisely measured and fitted with those of β-spodumene. The minor crystalline phase of titanite, CaO · TiO2· SiO2, was also present. The average thermal expansion coefficients from 25 to 700 °C were 3.50 × 10−6/°C, 3.81 × 10−6/°C, and 3.91 × 10−6/°C for samples A, B, and C, respectively.
Effect of fiber coating on the mechanical behavior of SiC fiber-reinforced titanium aluminide composites
Authors: Jeng S.M. , Yang J-M. , Graves J.A.
Published: September 2007

The effects of fiber surface coatings on the mechanical behavior and damage mechanisms of SCS-6 fiber-reinforced titanium aluminide matrix composites have been studied. Two different coating layers are used as model material: a brittle TiB2 and a ductile Ag/Ta duplex layer. The role of the coating layer on the interfacial reaction, interfacial properties, and mechanical behavior of the composites was characterized. Results indicate that both TiB2 and Ag/Ta are effective diffusion barriers in preventing fiber/matrix interfacial reactions during composite consolidation. However, the deformation mechanisms and crack propagation characteristics in these two coated composites are quite different. The criteria for selecting an improved interlayer to tailor a strong and tough fiber-reinforced titanium aluminide matrix composite are also discussed.
Resistive transition broadening in two-phase polycrystalline YBaCuO
Authors: Abele J.C. , Bristol R.L. , Nguyen T.C. , Ohmer M.W. , Wood L.S.
Published: September 2007

A model proposed by Tinkham1to explain the resistance versus temperature broadening found in highTcsuperconductors in applied magnetic fields is extended to “foot and knee”-structured data taken on polycrystalline YBa2Cu3O6+δ. The proposed extension involves a series combination of two types of superconductors. For this series combination to result, a critical ratio of the two types of superconductors must be met—a result common to both percolation and randomized cellular autonoma theory. This critical ratio is investigated via statistical computer models of a polycrystalline superconductor having two phases of crystallites—one with substantially lowerJcthan the other.
Effect of single crystal substrates on the growth and properties of superconducting Tl2Ba2CaCu2O8 films
Authors: Holstein W.L. , Parisi L.A. , Flippen R.B. , Swartzfager D.G.
Published: September 2007

Superconducting Tl2Ba2CaCu2O8 films were prepared by a two-step postdeposition anneal process under identical conditions on a number of single crystal substrates: (100) LaAlO3, (100) NdGaO3, (100) LaGaO3, (100) SrTiO3, (100) MgO, and (0001) Al2O3. The properties of the resulting Tl2Ba2CaCu2O8 films were strongly dependent on the substrate. Films on all substrates were predominantly c-axis oriented. Films prepared on LaAlO3, NdGaO3, and LaGaO3 also exhibited in-plane epitaxy, while films on MgO, Al2O3, and SrTiO3 were not aligned with the substrates. Some a-axis oriented grains of Tl2Ba2CaCu2O8 formed on NdGaO3 and both a-axis oriented grains and cracks were present on LaGaO3. Large amounts of secondary phases, as indicated by additional peaks in the x-ray diffraction scans, were observed for films on Al2O3 and SrTiO3 and smaller amounts on each of the other substrates. Depth profiling by sputtered neutral mass spectroscopy (SNMS) confirmed the presence of strong film-substrate reactions on Al2O3 and SrTiO3, and also indicated the presence of a reaction on NdGaO3. The Tc onset of the films as measured both inductively and by ac magnetic susceptibility varied depending on the substrate. The highest quality films were prepared on LaAlO3. NdGaO3 may also prove to be an acceptable substrate if the nucleation and growth of a-axis oriented Tl2Ba2CaCu2O8 grains can be eliminated. The remaining substrates appear to be less suitable due to severe film-substrate interactions, lack of epitaxial growth, or crystallographic phase changes.
(110)-oriented Bi–Sr–Ca–Cu–O superconducting thin films prepared by metalorganic chemical vapor deposition
Authors: Kubota Nobuhiko , Sugimoto Tsunemi , Shiohara Yuh , Tanaka Shoji
Published: September 2007

We have studied the superconducting Bi–Sr–Ca–Cu–O thin films prepared by metalorganic chemical vapor deposition (MOCVD). The thin films fabricated on an MgO(110) single crystal substrate show a preferential (110) orientation. The fabrication of (110)-oriented film was dependent on the chemical composition and the existence of a Sr–Ca–Cu–O buffer layer. The zero resistance temperature (Tc0) was obtained at 60 K for the 1 μm thick film and 23 K for the 300 nm thick film. The critical current density (Jc) of this film at 4.2 K was about 1 × 104A/cm2for the film of 1 μm thickness.
Transmission electron microscopy study of Co/Pd and Co/Au multilayers
Authors: Hakkens F. , De Veirman A. , Coene W. , den Broeder F.J.A.
Published: September 2007

The structure of Co/Pd and Co/Au (111) multilayers is studied using transmission electron microscopy and high resolution electron microscopy. We focused on microstructure, atomic stacking (especially at the interfaces), and coherency, as these are structural properties that have considerable magnetic effects. A columnar structure with a strong curvature of the multilayer influenced by substrate temperature during growth is observed. High resolution imaging shows numerous steps at the interfaces of the multilayer structure and the presence of misfit dislocations. In bright-field images, periodic contrast fringes are observed at these interfaces as the result of moiré interference. These moiré fringes are used to study the misfit relaxation at the interfaces, whereas electron diffraction gives the average relaxation over the whole layer. Both measurements determined that, for Co/Pd as well as Co/Au multilayers, 80–85% of the misfit is relaxed and 20–15% remains in the form of strain, independent of the Co layer thickness in the regime studied.
High-temperature oxidation of ion-plated TiN and TiAlN films
Authors: Ichimura Hiroshi , Kawana Atsuo
Published: September 2007

The high-temperature oxidation of TiN, Ti0.9Al0.1N, and Ti0.6Al0.4N films which were deposited onto stainless steel substrates using an arc ion-plating apparatus was studied at temperatures ranging from 923 to 1173 K for 0.6 to 60 ks in air. The oxidation rate obtained from mass gain as a function of time was found to fit well to a parabolic time dependence. From their temperature dependence, the apparent activation energies of oxidation were determined. With increasing Al contents, the oxidation rate decreased, and the activation energies of oxidation reaction increased. Formed oxide layers were analyzed by XRD, SEM, and EPMA. With increased Al content in TiAlN films, the rate-determining step changes from oxygen ion diffusion in formed rutile to oxygen or aluminum ion diffusion in the formed Al2O3 layer.
Electrical properties of silica-alumina ceramics in nitrogen atmosphere
Authors: Hirata Yoshihiro , Kawabata Mitsunori , Ishihara Yoshimi
Published: September 2007

The SiO2–Al2O3 powders with compositions of 46.5 to 76.6 wt. % Al2O3, synthesized by the hydrolysis of metal alkoxides, were sintered at 1700 °C for 3 h in a reduced pressure of 40 Pa. After sintering, the samples were annealed at 1300 °C for 10 h in air. The densities of sintered samples were higher than 97.0% T.D. The electrical conductivities of the SiO2–Al2O3 ceramics in N2 atmosphere (>99.99%) were measured in the temperature range of 500° to 800 °C by using ac bridge circuit at 120 Hz to 1 MHz and dc circuit at 1 V. The electrical conductivities at 800 °C were 3.4 × 10−7 to 4.3 × 10−6 S cm−1, and decreased with increasing Al2O3 content. The SiO2–Al2O3 ceramics were mixed conductors of oxygen ions and electrons in nitrogen atmosphere. The activation energies of ionic and electronic conductivities were 102 to 143 kJ/mole and 102 to 123 kJ/mole, respectively.
Swelling behavior of montmorillonite cation exchanged for ω-amino acids by ∊-caprolactam
Authors: Usuki Arimitsu , Kawasumi Masaya , Kojima Yoshitsugu , Okada Akane , Kurauchi Toshio , Kamigaito Osami
Published: September 2007

Natural Na-montmorillonite was cation exchanged for the ammonium cations of various ω-amino acids [H3N+(CH2)n−1 COOH, n = 2, 3, 4, 5, 6, 8, 11, 12, and 18]. X-ray diffraction (XRD) results suggested that the chain axes of ω-amino acids with a carbon number of eight or less were parallel to the silicate layers, and that the chain axes of those with a carbon number of 11 or more were slanted to the layers. The cation-exchanged montmorillonites form intercalated compounds with ∊-caprolactam at 25 °C. The montmorillonites intercalated with both ω-amino acid and ∊-caprolactam were studied by XRD measurement at room temperature and 100 °C. We propose a model where amino acid molecules were arranged perpendicular to silicate layers and ∊-caprolactam molecules filled the space between them. When the ∊-caprolactam was melted at 100 °C, the basal spacing for the montmorillonite increased, in which the carbon number exceeds 11. This phenomenon will be applicable to obtaining the nylon 6-clay hybrid, a molecular composite of nylon 6 and montmorillonite.
Plastic deformation in an Al–Cu–Fe icosahedral alloy
Authors: Shield J.E. , Kramer M.J. , McCallum R.W.
Published: September 2007

Al—Cu—Fe quasicrystalline alloys have been deformed by high-temperature creep between 680 and 740 °C. Deformations greater than 30% were achieved without cracking. Analysis of the data in the quasi-steady state regime reveals power law behavior with a stress exponent of 2.5. The activation energy for deformation was determined to be 640 ± 20 kJ/mole in the temperature region investigated. Transmission electron microscopy revealed lamellar defects which appear similar to twins.
Rapidly texturedBi2Sr2CaCu2O8
Authors: Arrasmith S.R. , Kramer M.J. , Merkle B.D. , Holesinger T.G. , McCallum R.W.
Published: September 2007

Amorphous Bi2Sr2CaCu2O8 (Bi-2212) was crystallized under uniaxial loads (up to 1500 N) at temperatures up to 890 °C to induce texture. Well-textured samples (19 mm in diameter and 0.15 mm thick) were obtained for samples heated to 890 °C and quenched. Heated (10 °C/min) and quenched samples at 550, 750, 850, and 870 °C demonstrate that the crystallization path is Bi2Sr2CuO6 (Bi-2201) (550 °C) to Bi-2212 (850 °C), with texturing occurring during grain growth (T > 850 °C). Comparison of samples crystallized under load and no load demonstrates that the texturing is a result of the plane stress biasing the normal anisotropic grain growth normal to the applied stress direction.
Microscopic phase stability of the dilute magnetic semiconductor Cd1−xFexSe
Authors: Klein E. , Homyonfer M. , Giriat W. , Tenne R.
Published: September 2007

The solubility of Fe in CdSe crystal has been studied in detail in the past. Using powder diffraction and atomic absorption spectroscopy, it was concluded that the upper limit for the solubility of iron in CdSe is 12–15%, and the actual Fe concentration in a solid solution agrees with the nominal Fe concentration (calculated from the concentration of the reactants). In the present study this question is re-examined using techniques that probe the average properties of the solid solution, such as x-ray diffraction, and techniques that probe the local structure and composition of the solid solution on the micron scale, such as energy and wavelength x-ray dispersive microprobe analysis. It is shown that the solubility of Fe in the CdSe matrix is appreciably smaller than the nominal value, and that microsegregation is obtained at a much lower Fe concentration than previously concluded. The immiscibility of Fe in this matrix is consistent with the large deviation of the lattice constants from Vegard's law. It is attributed to the large strain exerted on the lattice upon substitution of Cd by the much smaller Fe ion.
Isothermal capacitance transient spectroscopy study on trap levels in polycrystalline SnO2 ceramics
Authors: Kim Myung Chul , Song Kug Hyun , Park Soon Ja
Published: September 2007

The unusual sigmoidal behavior between electrical resistivity and inverse temperature in polycrystalline porous SnO2 was intensively investigated by detecting the localized traps formed at the interparticle through the measurement of Isothermal Capacitance Transient Spectroscopy. A trap of 1.0 eV was evaluated at a higher temperature range of 460–500 °C, and 0.6 eV trap at a lower temperature range 200–280 °C. It seems that the trap of 1.0 eV originates from O2− and 0.6 eV from O− defect center. Time constants which were determined at the peak position of ICTS spectra, S(t), were increased with the increase of temperature at the intermediate temperature range, indicative of the competition among various kinds of traps. The active competition among various traps is one of the causes that result in prominent gas sensitivity in polycrystalline porous SnO2.
High-temperature Hf-site-interchange chemistry in LiNbO3 and LiTaO3
Authors: Birnie Dunbar P. , Catchen Gary L.
Published: September 2007

We explain observed Hf-site interchange in LiNbO3 and LiTaO3 at high temperatures using solid-state defect chemistry reactions. The model takes into account fully the effects of nonstoichiometry on the ferroelectric-to-paraelectric phase transition. Specifically, we use this model to interpret the temperature dependence of the Hf-site interchange that we measured using perturbed-angular-correlation (PAC) spectroscopy. In this context, the site interchange is an equilibrium, thermodynamic process that involves the partitioning of Hf ions between Li and group-V (Nb and Ta) sites. The Hf ions replace group-V ions by pushing them from their normal sites to the Li sublattice. Based on the temperature dependence of the site occupancy, this reaction requires approximately 2.2 to 2.3 eV.
Solid solutions of AlN and SiC grown by plasma-assisted, gas-source molecular beam epitaxy
Authors: Kern R.S. , Rowland L.B. , Tanaka S. , Davis R.F.
Published: September 2007

Solid solutions of aluminum nitride (AlN) and silicon carbide (SiC), the only intermediate phases in their respective binary systems, have been grown at 1050 °C on α(6H)-SiC(0001) substrates cut 3–4° off-axis toward [11$\overline 1$0] using plasma-assisted, gas-source molecular beam epitaxy. A film having the approximate composition of (AlN)0.3(SiC)0.7, as determined by Auger spectrometry, was selected for additional study and is the focus of this note. High resolution transmission electron microscopy (HRTEM) revealed that the film was monocrystalline with the wurtzite (2H) crystal structure.
Energetics of the oxygen vacancy order-disorder transition in Ba2In2O5
Authors: Prasanna T.R.S. , Navrotsky Alexandra
Published: September 2007

The heat capacity and the enthalpy associated with the reported oxygen vacancy order-disorder transition in Ba2In2O5 were measured by high temperature step scanning calorimetry. The transition temperature is 1205 ± 2 K. The transition appears first order or nearly so. The enthalpy and entropy of transition are 1.3 kJ/mol and 1.1 J/mol K, respectively. The latter is only 4.8% of the configurational entropy, arising from mixing one vacancy and five oxygens per formula unit, 22.5 J/mol K. This suggests that the transition involves only a small fraction of the oxygen vacancies and implies extensive short-range order, SRO, in the high temperature phase.
A time-resolved diffraction study of the Ta–C solid combustion system
Authors: Larson E.M. , Wong Joe , Holt J.B. , Waide P.A. , Nutt G. , Rupp B. , Terminello L.J.
Published: September 2007

The formation of TaC and Ta2C by combustion synthesis from their elemental constituents has been studied by time-resolved x-ray diffraction (TRXRD) using synchrotron radiation. The reactions have been followed with a time resolution down to 50 ms. Since the adiabatic temperatures for both reactions are well below any liquidus temperature in the Ta—C phase diagram, no melting occurs and these combustion reactions occur purely in the solid state. The phase transformations associated with these reactions are followed by monitoring the disappearance of reactant and appearance of product powder diffraction peaks in real time as the reaction front propagates through the combusting specimen. In the synthesis of TaC, the results show the formation of the subcarbide (Ta2C phase as an intermediate. In the synthesis of Ta2C, the reaction proceeds directly to the product with no discernible intermediate Ta–C phase within a 50 ms time frame. The chemical dynamics associated with the combustion synthesis of TaC may be described by an initial phase transformation to hexagonal Ta2C arising from carbon diffusion into the Ta metal lattice. As more carbon is available this intermediate subcarbide phase, which has one-half of its octahedral interstices occupied by the carbon, further transforms to the cubic TaC final product, in which all octahedral sites are now occupied. The time-resolved data indicate that the rate of formation of Ta2C is a factor of two faster than that of TaC.
High-pressure phase transformation of BeGa2O4
Authors: Nakamura K. , Machida M. , Brito M.E. , Tabata H.
Published: September 2007

High-pressure phase transformation of beryllium gallium oxide (BeGa2O4) has been studied. Applying high pressure at elevated temperatures to the original hexagonal BeGa2O4 (β–Si3N4-type structure), a high-pressure modification with orthorhombic structure (olivine-type structure) was obtained, i.e., o-BeGa2O4. Lattice parameters of the new phase were determined to be a = 0.5698, b = 0.9759, and c = 0.4551 nm. The pressure and temperature ranges where the high-pressure phase was observed are 3.5 to 7.5 GPa and 800 to 1600 °C, respectively. A tentative pressure-temperature phase diagram for BeGa2O4 was proposed. Transformation is not straightforward; decomposition of the original phase into single oxides and their recombination to form o-BeGa2O4 are necessary. This process seems to apply in both ways, formation and decomposition of the high-pressure phase. The stability of the high-pressure phase is explained in terms of the total molar volume for the phase, the result of summing up molar volumes of constituent compounds. This is the first known report on transformation of β–Si3N4-type structure into a denser structure.
Speculation on the reconstruction of vicinal {100} faces of vapor-deposited diamond
Authors: Tamor M.A. , Everson M.P.
Published: September 2007

Recent morphological studies of vapor-deposited diamond indicate (1) that growth on the diamond {100} surface is mediated by steps and (2) that some defects cause rapid initiation of new layers, resulting in characteristic pyramids with edges parallel to surface 〈110〉 directions. If growth on the diamond {100} surface does indeed occur at steps, knowledge of their atomic structure is essential to an understanding of diamond growth. We reexamine results of a recent STM study of homoepitaxial diamond films and suggest that the surfaces on which growth occurs consist of regularly spaced double height steps separated by integral numbers of dimer rows running parallel to the step edges.
Dislocation emission criterion: Grain boundary effect
Authors: Shiue Sham-Tsong , Zhang Tong-Yi , Lee Sanboh
Published: September 2007

Based on the results of Shiue and Lee [J. Appl. Phys. 70, 2947 (1991)], the effect of plastic zone and grain boundary on the dislocation emission criterion was investigated. The emission criterion is based on the concept of spontaneous emission. The critical stress intensity factor for dislocation emission increases with the increasing size of dislocation-free zone and the number of piled-up dislocations in the plastic zone, but decreases with increasing grain size. The ductile versus brittle behavior of material was determined by the competition of critical stress intensity factors for dislocation emission and crack propagation. A material with larger grain size is easier to emit dislocation and allows more dislocations to be piled up, so that it behaves more ductile.
Characterization of carbon aerogels by transport measurements
Authors: Fung A.W.P. , Wang Z.H. , Lu K. , Dresselhaus M.S. , Pekala R.W.
Published: September 2007

Carbon aerogels are a special class of low-density microcellular foams. These materials are composed of interconnected carbon particles with diameters of approximately 10 nm. The temperature dependence of the dc electrical resistivity and magnetic susceptibility (χ) from 4 K to room temperature, magnetoresistance (MR) in a magnetic field up to 15 T, and Raman scattering were measured as a function of aerogel density. While Raman scattering measurements are not sensitive to variations in density, the χ data show that there are more free carriers in samples of higher density. Aerogel samples with different densities all show a negative temperature coefficient of resistivity and a positive MR. The less dense samples exhibit a stronger temperature dependence of resistivity and a stronger field dependence of the MR, indicating that with decreasing density and increasing porosity, charge carriers are more localized. Data analysis precludes variable-range hopping in favor of nearest-neighbor hopping and fluctuation-induced tunneling as the most likely conduction mechanisms for carbon aerogels.
Generation and annihilation of antiphase domain boundaries in GaAs on Si grown by molecular beam epitaxy
Authors: Georgakilas A. , Stoemenos J. , Tsagaraki K. , Komninou Ph. , Flevaris N. , Panayotatos P. , Christou A.
Published: September 2007

A comprehensive investigation of antiphase domain boundaries (APB's) in GaAs-on-Si is presented. A comprehensive experimental approach, based on complementary electron microscopy (TEM and SEM) and chemical etch techniques, is developed and used in the study of the structural evolution of APB's on vicinal (001)Si substrates. The question of whether a GaAs selective nucleation or APB annihilation accounts for the absence of APB's in thick GaAs/Si films, grown on substrates misoriented from (001) toward (110), is addressed. APB's are revealed by two different TEM techniques to exist in the first interfacial layers of GaAs/Si even in samples considered to be “APB free”. The APB annihilation mechanism is illustrated in GaAs films grown on substrates misoriented toward (100), either directly, by cross-sectional TEM observations, or indirectly, by combined chemical etch/SEM experiments. In addition, the structural characteristics of APB's and their interaction with other extended crystal defects are clarified by XTEM and TEM observations. Finally, the influence of APB's on GaAs/Si surface morphology and their electrical activity are shown explicitly for the first time.
Electronic structure of high pressure phase of AlN
Authors: Pandey Ravindra , Sutjianto Amin , Seel Max , Jaffe John E.
Published: September 2007

Results of ab initio Hartree–Fock calculations for the electronic structure of aluminum nitride in the (high-pressure) rocksalt phase are reported. In the rocksalt phase, the calculated lattice constant is 3.982 Å with the bulk modulus of 329 GPa. The band structure is predicted to be indirect at the X point with a gap of 8.9 eV. In this phase, the bonding is shown to be essentially ionic between Al and N. The direct gap shows a stronger linear dependence on pressure with a pressure derivative of 68 meV/GPa compared to that of the indirect gap with a pressure derivative of 31.7 meV/GPa.
Macro-effects of resputtering due to negative ion bombardment of growing thin films
Authors: Kester Daniel J. , Messier Russell
Published: September 2007

Bombardment of a growing thin film by negative ions can lead to changes in the film through the process of resputtering. Macro-effects of resputtering (effects on the film thickness) include a slowing of the film growth rate and, in some cases, a complete suppression of the film growth as well as an etching of the substrate materials. To study this result of resputtering, rf-diode sputtering was used to deposit BaTiO3 films under a variety of conditions, varying deposition time, rf-power level, substrate-to-target distance, total gas pressure, and argon, oxygen, and hydrogen partial pressures. The effect resputtering had on the thickness was seen to be a result of the competition between deposition and etching of the thin film material. The relative influence of the various sputtering parameters and the effect each of these has on the thickness distribution were examined. It was found that the greatest influence was system geometry, followed by rf-power level. Various methods of controlling resputtering are discussed.
27Al MAS-NMR spectra of alumina aerogels
Authors: Mizushima Yasuyuki , Hori Makoto , Sasaki Minoru
Published: September 2007

The 27Al MAS-NMR spectra of alumina aerogels were determined. The spectra showed different profiles according to the supercritical methods used during drying. The alumina aerogel supercritically dried in a CO2 extractor (80 °C, 15.7 MPa) displayed a pentahedral-coordinated aluminum structure. On the other hand, the alumina aerogel supercritically dried in an autoclave (270 °C, 26.5 MPa) revealed no pentahedral-coordinated aluminum structure. The absence of pentahedral-coordinated aluminum was one factor for the lower temperature of transformation from the γ to the θ phase.
Solidification of Bi2Sr2CaCu2Oy and Bi2Sr1.75Ca0.25CuOy
Authors: Holesinger T.G. , Miller D.J. , Viswanathan H.K. , Chumbley L.S.
Published: September 2007

The solidification processes for the compositions Bi2Sr2CaCu2Oy (2212) and Bi2Sr1.75Ca0.25CuOy (2201) were determined as a function of oxygen partial pressure. During solidification in argon, the superconducting phases were generally not observed to form for either composition. In both cases, the solidus is lowered to approximately 750 °C. Solidification of Bi2Sr1.75Ca0.25CuOy in Ar resulted in a divorced eutectic structure of Bi2Sr2−xCaxOy (22x) and Cu2O while solidification of Bi2Sr2CaCu2Oy in Ar resulted in a divorced eutectic structure of Bi2Sr3−xCaxOy (23x) and Cu2O. Solidification of Bi2Sr1.75Ca0.25CuOy in O2 resulted in large grains of 2201 interspersed with small regions containing the eutectic structure of 22x and CuO/Cu2O. Solidification of Bi2Sr2CaCu2Oy in partial pressures of 1%, 20%, and 100% oxygen resulted in multiphase samples consisting of 2212, 2201, some alkaline-earth cuprates, and both divorced eutectic structures found during solidification in Ar. For both compositions, these latter structures can be attributed to oxygen deficiencies present in the melt regardless of the overpressure of oxygen. These eutectic structures are unstable and convert into the superconducting phases during subsequent anneals in oxygen. The formation process of the 2212 phase during solidification from the melt was determined to proceed through an intermediate state involving the 2201 phase.
Phase evolution in silver-doped BiPbSrCaCuO(2223)/Ag superconducting composites
Authors: Guo Y.C. , Liu H.K. , Dou S.X.
Published: September 2007

Silver doping into (Bi, Pb)2Sr2Ca2Cu3O10 superconducting composite tapes was found to accelerate the formation process of high-Tc (2223) phase owing to lowering the partial melting point of the samples. The differential thermal analysis (DTA) results showed that the partial melting temperature of the sample was lowered by about 10 °C from 850 °C to 840 °C by silver doping. However, with sufficient sintering both the silver-doped and undoped samples can reach a very high level of high-Tc phase fraction, suggesting that the silver doping only speeds up the rate of high-Tc phase formation, but does not change the final phase assemblage of the materials. The reaction kinetics was analyzed by using the Avrami equation, and the results indicated that the conversion process of low-Tc (2212) phase to high-Tc (2223) phase was a diffusion-controlled, two-dimensional reaction. The correlation of the phase evolution with electrical property inside the superconducting tape during the process of heat treatment was also discussed.
Nucleation of CVD-TiN on tungsten
Authors: Glejb⊘l K. , Pryds N.H. , Thölén A.R.
Published: September 2007

Using Chemical Vapor Deposition (CVD), TiN was deposited on sharp tungsten needles. The reactant gases were TiCl4, N2, and H2. A Transmission Electron Microscopy (TEM) investigation revealed that the first nuclei of the CVD–TiN coating on tungsten did not consist of δ–TiN, but were a mixture of α–TiN and δ–TiN. These results were also verified with x-ray measurements. From these experimental results a possible mechanism for the initial growth of TiN on tungsten is suggested. It may be that the change in relative concentrations of the different titanium nitrides suggested as mechanism of the initial growth of CVD–TiN can be applied in general for all TiCl4/H2/N2/metal systems where the original substrate surface material partly or completely consists of a metal with catalytic properties.