bluuebunny/crossref_metadata_2025 huggingface dataset:
Cleavage fracture in an Al3Ti-based alloy having the Ll2 structure
Authors: George E. P. , Porter W. D. , Henson H. M. , Oliver W. C. , Oliver B. F.
Published: September 2007

Selected area electron channeling patterns were used to identify the cleavage planes in a polycrystalline Al–23Ti–6Fe–5V alloy, which is an Al3Ti-based alloy having the Ll2 structure. Alloys like this one are of scientific and technological interest because they fracture by brittle transgranular cleavage, despite the availability of more than five independent slip systems in the Ll2 structure and their relatively low hardness when compared to ductile Ll2 alloys like Ni3Al. Homogenized bars of the levitation zone melted and directionally solidified alloy were fractured at room temperature in three point bending. The fracture surfaces, which consisted almost entirely of brittle transgranular cleavage facets, were examined in a scanning electron microscope (SEM) equipped to take channeling patterns. An optical microscope with a short depth of focus was inserted in the SEM column just below the objective lens, and by focusing it on several points on the cleavage facets, it was possible to orient the facets normal to the optic axis of the SEM prior to taking the channeling patterns. Of the eight cleavage facets examined in this study, six were of the {110} type and two were of the {111} type. Although it cannot be said that these are the only two cleavage planes in this alloy, the availability of more than one plane with low cleavage strength contributes to the brittleness of this alloy. These results are examined in the light of theoretical treatments of cleavage fracture, and comparisons are made with earlier studies on other Ll2 materials.
Surface phenomena in liquid metal alloys of arsenic: Vapor pressure reduction and wetting to refractory metals
Authors: Bozack M. J. , Swanson L. W. , Bell A. E.
Published: September 2007

The influence of surface segregation on wetting of Pd2As and Pd50B25As25 to refractory metals has been studied by sessile drop and surface analytical techniques. The results indicate that the wetting behavior of Pd2As to W is strongly influenced by surface segregation of low-level bulk impurities in the alloy. At melting, the segregating impurities form an inert shell about the liquid alloy which inhibits reaction between alloy and substrate. A poorly-wetted droplet with a large contact angle results. Wetting is elicited by fracture of the high-surface tension shell during heat treatment. The efflux of pure alloy material from the interior of the droplet wets refractory metals with a contact angle of near-zero. The shell remnant floats atop the wetted alloy. Addition of B to the alloy to form Pd50B25As25 results in rapid attack of a Re support. Preliminary studies of the vapor pressure above the alloy and mass analysis of field evaporated ions show that Pd2As may be used as a liquid metal ion source of As. This is possible by means of successful reduction in As vapor pressure by over 10 orders of magnitude.
Nanoindentation of silver-relations between hardness and dislocation structure
Authors: Pharr G. M. , Oliver W. C.
Published: September 2007

The depth dependence of hardness in a well-annealed single crystal of silver has been characterized in nanoindentation experiments. The work is based on similar experiments performed by Chen and Hendrickson, but extends their results to indent depths on the nanometer scale. The hardness is generally found to increase with decreasing depth, with a rather sharp increase observed at depths of less than 50 nm. Using etch pitting to reveal the surface dislocation structure after indentation, the sharp rise in hardness is found to be associated with the disappearance of dislocation rosette patterns and any signs of near-surface dislocation activity, thereby suggesting that very small scale indentation plasticity may take place by nondislocation mechanisms. However, order of magnitude calculations show that possible alternatives, specifically, diffusional mechanisms, are too slow to make significant contributions. It is suggested that for very small indents, either the surface dislocation debris is quickly annealed out before it can be observed or indentation plasticity is accommodated entirely by subsurface dislocation activity.
Elastic Green's function for a composite solid with a planar crack in the interface
Authors: Tewary V. K. , Wagoner R. H. , Hirth J. P.
Published: September 2007

The elastic Green's functions for displacements and stresses have been calculated for a composite solid containing a planar crack in a planar interface using the Green's function derived in a previous paper for a line load parallel to the composite interface. The resulting functions can be used to calculate the stress or displacement at any point in the composite for a variety of elastic singularities. As specific applications, the Mode I stress intensity factor of an interfacial crack was calculated as were the Green's functions for the semi-infinite antiplane strain case. The Mode I case shows the near-crack tip oscillations reported by other authors while the Mode III case does not. The newly devised Green's functions are shown to reproduce the results of other authors in the isotropic limit.
Orientation of rf-sputter-deposited MoS2 films
Authors: Bertrand P. A.
Published: September 2007

A theory to predict the orientation of radio-frequency (rf) sputter-deposited MoS2 films with respect to the substrate surface is presented. Oxygen-containing species such as H2O and -OH are postulated to be active sites that force crystallites to form with their basal planes perpendicular to the surface. X-ray diffraction is used to determine the crystallite orientation in films deposited on smooth, rough, or oxidized Si surfaces, as well as on fused silica or gold surfaces. The results support the theory. To produce MoS2 films with their crystallite basal planes parallel to a substrate surface (which has been suggested to be the low friction orientation), the active sites must be removed from the substrate by, for instance, heating in vacuum.
The surface composition of silicon carbide powders and whiskers: An XPS study
Authors: Taylor T. N.
Published: September 2007

The surface composition and bonding of a wide variety of silicon carbide powders and whiskers have been characterized by x-ray photoelectron spectroscopy (XPS). Ultrafine SiC powders, grown by a radio frequency plasma process, have been shown to exhibit graphitic carbon and a thin suboxide coating. Whiskers of SiC, grown in a vapor-liquid-solid or proprietary commercial process, were generally covered by heavier oxides than the powders and to a variable degree showed silica-like bonding. Most of the materials were subject to sample charging. Procedures were developed to estimate these charging effects and interpret the complete catalog of XPS spectra from these materials with respect to Fermi-level assignments. Charge independent quantities, such as oxygen Auger parameter and O(1s)–Si(2p) peak position difference, were found to agree with accepted values in the literature while exhibiting trends consistent with suboxide and silica bonding assignments. The data give a broad basis for understanding the feedstock surface chemistry which is involved during fabrication of monolithic or composite silicon carbide materials.
Cesium-antimony and cesium-arsenic intercalated graphite
Authors: Essaddek Abderrahim , Assouik Jamal , Lagrange Philippe
Published: September 2007

Liquid binary alloys Cs–Sb and Cs–As are able to intercalate into graphite, provided that their compositions are neither too rich nor too poor in cesium. Several phases, whose stage varies between 1 and 4 in the case of antimony, and between 1 and 3 in the case of arsenic, have been observed. These new ternary compounds have been characterized by x-ray measurements.
The effect of epitaxy on the upper critical fields of evaporated niobium films
Authors: Oya Gin-ichiro , Komukai Tetsuro , Sawada Yasuji
Published: September 2007

Two upper critical fields normal (Bc2⊥) and parallel (Bc2∥) to the surfaces of both single-crystal and polycrystalline Nb films are studied at 4.2 K in relation to the purity and morphology of the films, in order to make properties of the single-crystal films clear. High-quality single-crystal Nb films (with transition temperature Tc of ∼9.3 K and normal resistivity ρn of ∼0.2 μΩ · cm) are found to have magnetic properties characterized by Bc2⊥ of 0.31 T as low as Bc2 (bulk upper critical field) of bulk Nb, and Bc2∥/Bc2⊥ of ∼1.85 as high as Bc3/Bc2 (where Bc3 is the surface nucleation critical field) of pure Nb. These properties depend strongly on high purity and surface smoothness of the films, which is naturally accompanied by superconductivity. Polycrystalline Nb films have higher Bc2⊥ and lower Bc2∥/Bc2⊥, which result from a decrease in purity of the films and a presence of normal-conducting niobium-suboxide layers at the surfaces of the films, and thereby a decrease of the surface effect in the films.
Lattice statics Green's function method for calculation of atomistic structure of grain boundary interfaces in solids: Part II. Anharmonic theory
Authors: Tewary V. K. , Fuller E. R.
Published: September 2007

The lattice statics Green's function method for calculation of the atomistic structure of grain boundary interfaces in solids as described in Part I is extended to include anharmonic effects. It is shown that the ‘anharmonic’ response of a solid to ‘anharmonic’ forces can be represented in terms of the ‘harmonic’ response of the solid to an effective anharmonic force. The Green's function method then requires solving a finite order nonlinear matrix equation, which is done by using standard numerical methods. For the purpose of illustration, the method is applied to calculate the atomistic structure of a ∑5 tilt boundary in fec copper.
Characterization of diamond thin films: Diamond phase identification, surface morphology, and defect structures
Authors: Williams B. E. , Glass J. T.
Published: September 2007

Thin carbon films grown from a low pressure methane-hydrogen gas mixture by microwave plasma enhanced CVD have been examined by Auger electron spectroscopy, secondary ion mass spectrometry, electron and x-ray diffraction, electron energy loss spectroscopy, and electron microscopy. They were determined to be similar to natural diamond in terms of composition, structure, and bonding. The surface morphology of the diamond films was a function of position on the sample surface and the methane concentration in the feedgas. Well-faceted diamond crystals were observed near the center of the sample whereas a less faceted, cauliflower texture was observed near the edge of the sample, presumably due to variations in temperature across the surface of the sample. Regarding methane concentration effects, threefold {111} faceted diamond crystals were predominant on a film grown at 0.3% CH4 in H2 while fourfold {100} facets were observed on films grown in 1.0% and 2.0% CH4 in H2. Transmission electron microscopy of the diamond films has shown that the majority of diamond crystals have a very high defect density comprised of {111} twins, {111} stacking faults, and dislocations. In addition, cross-sectional TEM has revealed a 50 Å epitaxial layer of β3–SiC at the diamond-silicon interface of a film grown with 0.3% CH4 in H2 while no such layer was observed on a diamond film grown in 2.0% CH4 in H2.
Characterization of diamond films by Raman spectroscopy
Authors: Knight Diane S. , White William B.
Published: September 2007

As the technology for diamond film preparation by plasma-assisted CVD and related procedures has advanced, Raman spectroscopy has emerged as one of the principal characterization tools for diamond materials. Cubic diamond has a single Raman-active first order phonon mode at the center of the Brillouin zone. The presence of sharp Raman lines allows cubic diamond to be recognized against a background of graphitic carbon and also to characterize the graphitic carbon. Small shifts in the band wavenumber have been related to the stress state of deposited films. The effect is most noticeable in diamond films deposited on hard substrates such as alumina or carbides. The Raman line width varies with mode of preparation of the diamond and has been related to degree of structural order. The Raman spectrum of hexagonal diamond (lonsdaleite) is distinct from that of the cubic diamond and allows it to be recognized.
Kinetics of silicon nitride crystallization in N+-implanted silicon
Authors: Kaushik V. S. , Datye A. K. , Kendall D. L. , Martinez-Tovar B. , Simons D. S. , Myers D. R.
Published: September 2007

Implantation of nitrogen at 150 KeV and a dose of 1 ⊠ 1018/cm2 into (110) silicon results in the formation of an amorphized layer at the mean ion range, and a deeper tail of nitrogen ions. Annealing studies show that the amorphized layer recrystallizes into a continuous polycrystalline Si3N4 layer after annealing for 1 h at 1200 °C. In contrast, the deeper nitrogen fraction forms discrete precipitates (located 1μm below the wafer surface) in less than 1 min at this temperature. The arcal density of these precipitates is 5 ⊠ 107/cm2 compared with a nuclei density of 1.6 ⊠ 105/cm2 in the amorphized layer at comparable annealing times. These data suggest that the nucleation step limits the recrystallization rate of amorphous silicon nitride to form continuous buried nitride layers. The nitrogen located within the damaged crystalline silicon lattice precipitates very rapidly, yielding semicoherent crystallites of β–Si3N4.
The effect of carbon morphology on the combustion synthesis of titanium carbide
Authors: Mullins Michael E. , Riley Erica
Published: September 2007

Titanium carbide may be readily produced in high purity via direct reaction between the solid phases of titanium and carbon in the process of combustion synthesis, also known as self-propagating high-temperature synthesis (SHS). The high temperatures generated by this exothermic reaction (<3000 °C) melt the titanium phase which subsequently flows to and reacts with the solid carbon. As a result, with lightly or uncompressed green bodies a product is obtained which retains most of the carbon precursor powder morphology down to the μm scale. The effect of three distinctly different carbon precursor powders on the final titanium carbide morphology has been examined. Subsequent effects of particle size and microstructure on the quality of the titanium carbide product are also noted.
Thermodynamic partial properties of Na2O in Nasicon solid solution, Na1+xZr2SixP3−xO12
Authors: Kale G. M. , Jacob K. T.
Published: September 2007

The thermodynamic activity of sodium oxide (Na2O) in the Nasicon solid solution series, Na1+xZr2SixO12, has been measured in the temperature range 700–1100 K using solid state galvanic cells: Pt|CO2 + O2|Na2CO3∥Na1+xZr2SixP3−xO12∥(Y2O3)ZrO2∥In + In2O3|Ta, Pt for 1 ≤ ⊠ ≤ 2.5, and Pt∥CO2 + O2∥Na2CO3∥β-alumina∥Na1+xZr2SixP3−xO12∥Ar + O2∥Pt for x = 0, 0.5, 2.5, and 3. The former cell, where the Nasicon solid solution is used as an electrolyte along with yttria-stabilized zirconia, is well suited for Nasicon compositions with high ionic conductivity. In the latter cell, β-alumina is used as an electrolyte and the Nasicon solid solution forms an electrode. The chemical potential of Na2O is found to increase monotonically with x at constant temperature. The partial entropy of Na2O decreases continuously with x. However, the partial enthalpy exhibits a maximum at x = 2. This suggests that the binding energy is minimum at the composition where ionic conductivity and cell volume have maximum values.
The structure of sodium trisilicate glass via molecular dynamics employing three-body potentials
Authors: Newell R. G. , Feuston B. P. , Garofalini S. H.
Published: September 2007

Molecular dynamics simulations (MD) employing multibody potentials were used to simulate sodium trisilicate glass (Na2O·3SiO2). A multibody term has been added to a pair potential in order to incorporate the bond directionality which is expected for the partially covalent silicate structure. The structure of the glass was analyzed and results were compared to those found using two-body potentials and molecular statics, as well as to experimental results found using x-ray diffraction, XPS, NMR, and EXAFS. Current results compared favorably to experiment and showed improvement over results obtained using two-body potentials. Nearest neighbor distances and coordination numbers agreed well with published data. Although two-body potentials normally show overcoordinated silicon (>4.3) and broad O–Si–O tetrahedral angle distributions, in this study all silicon exhibited tetrahedral coordination (4.0) and the O–Si–O bond angle distribution was markedly sharpened. The number of nonbridging oxygens was shown to be nearly equal to the number of sodium ions present, and a reasonable distribution of Qc species was found. The overall structure closely resembled the modified network structure of glass proposed experimentally, with silicon tetrahedra remaining intact and sodium ions breaking up the network through the creation of nonbridging oxygens.
Physicochemical characterization of sputtered iridium oxide
Authors: Aurian-Blajeni B. , Boucher M. M. , Kimball A. G. , Robblee L. S.
Published: September 2007

In the present work we characterize sputtered iridium oxide films (SIROF) by differential scanning calorimetry (DSC), x-ray, and impedance spectroscopies. We show that a crystallization transition occurs at ca, 230 °C, and suggest a bilayer model for the sputtered film. The transition results in a crystalline mixture of iridium metal and iridium oxide; this suggests a decomposition-crystallization process of the type 2Ir2O3 ⇉ Ir + 3IrO2. In the bilayer model proposed by us, the layer closer to the substrate would reflect the combined influence of the sputtering conditions and of the substrate, while the properties of the second layer depend on the sputtering conditions alone. The bilayer structure is supported by results obtained by impedance spectroscopy.
Microstructure of Nb3 (AlSiB) superconducting tapes with extremely high critical current densities
Authors: Clapp Mireille Treuil , Jian Zhang , Manzur Tariq
Published: September 2007

Alloys of Nb73Al12Si14.5B0.5 were rapidly solidified into amorphous ribbons using the melt spinning technique. These were isothermally annealed at temperatures ranging from 660 to 780 °C. The A15 phase began to crystallize at 700 °C and small amounts of second phases appeared at the higher temperatures. Crystallization was dependent on quenching rate as well as annealing conditions. Below 750 °C nucleation was nonuniform and was enhanced by surfaces and quenched-in nuclei. Above 750 °C nucleation became more uniform and completely crystalline ribbons with equiaxed grains ∼30 nm in diameter were obtained. These ultra fine grained ribbons had extremely high superconducting critical current densities of 8 × 1010 A/m2 and 5 × 1010 A/m2 at magnetic fields of 0.5 and 15 tesla, respectively, at 4.2 K.
Effect of geometry and interfacial resistance on current distribution and energy dissipation at metal/superconductor junctions
Authors: Liu Meilin , De Jonghe Lutgard C.
Published: September 2007

Potential and current distributions and local energy dissipation due to Joule heating in metal-superconductor junctions have been computed as a function of geometric parameters and interfacial resistance. The primary current distribution and power dissipation are highly nonuniform in the system. The secondary current distribution and power dissipation, however, become more uniform as the interfacial resistance increases. Analysis indicates that zero contact resistance is not a stable situation since the primary distribution leads to local current densities exceeding the critical current density of the superconducting phase near the corner of the junction. Local contact failure might then initiate. A finite contact resistance is necessary for a practical application, and the minimum value of the contact resistance can be estimated from the operating current density (javg) of the device and the critical current density (jcri) of the superconducting phase. To obtain an optimum value of the contact resistance, however, one further has to take into consideration the stability and reliability of the device performance, which is, in turn, directly related to the uniformity of the current distribution and power dissipation, to temperature fluctuation of the superconducting phases brought about by local power dissipation, and to the thermal management of the system. Furthermore, a nonuniform contact resistance layer of appropriate profile can redistribute the current more effectively and more uniformly and hence reduce the total power dissipation in the system for a given jmax/javg ratio obtained by a uniform resistance layer.
Rapidly solidified Al–Cr alloys: Crystalline and quasicrystalline phases
Authors: Swamy V. T. , Ranganathan S. , Chattopadhyay K.
Published: September 2007

Rapidly solidified Al–Cr alloys up to 20 at. % Cr were studied to delineate the extent of crystalline and quasicrystalline phase formation in these alloys in comparison with as-cast alloys by using transmission electron microscopy and x-ray diffraction technique. The icosahedral quasicrystals are observed from 7 to 15 at. % Cr alloys, while equilibrium η–Al11Cr2 phase is completely absent. Both rapid solidification and subsequent thermal decomposition studies indicate that the main competing phase is θ–Al2Cr up to 15 at. % Cr. Beyond this composition ∊–Al4Cr is the dominant phase together with a small amount of γ4–Al7Cr3. We have shown that the electron diffraction patterns of Al–Cr quasicrystals are often associated with a diffuse intensity distribution, indicative of short-range order. The change in quasilattice constant with composition suggests the existence of structural vacancies. Further, a sudden change from coarse to ultrafine quasicrystalline grain structure in Al-7 at. % Cr alloy points to a change in nucleation mechanism from heterogeneous to homogeneous mode during the rapid solidification.
An embedded atom analysis of Au and Pt substitutional atoms in Ni
Authors: Ruppa P. , Gilmore C. M.
Published: September 2007

The Embedded Atom Model (EAM) was utilized to analyze the energy and strain surrounding Au and Pt atoms substituted into a Ni crystal. Au substituted into Ni has a large positive size difference and a positive heat of mixing; Pt substituted into Ni has a large positive size difference and a negative heat of mixing. The EAM predicted highly anisotropic strain fields around the substitutional atom with large positive strains in the [110] direction and small or negative strains in the [100] and [111] directions. This is in contrast to the normal assumption that the strain around a substitution atom is spherically symmetric. The cohesive energy of the atoms around the substitutional atom was found to depend upon the crystallographic direction relative to the substitutional atom in addition to the radial distance from the substitutional atom. The EAM predicted an energy increase for Au substituted into Ni and an energy decrease for Pt substituted into Ni. The EAM results were compared with an elastic analysis.
Stoichiometry effects: on the deformation of binary TiAl alloys
Authors: Hall Ernest L. , Huang Shyh-Chin
Published: September 2007

The room temperature deformation behavior and microstructure of Ti48Al52 and Ti52Al48 alloys are compared. The material was fabricated by rapid solidification melt spinning, and examined in both as-cast and consolidated forms. The Ti52Al48 alloy exhibited enhanced strength and ductility in both forms in bend tests compared with the Ti48Al52 alloy. The microstructure of the Ti52Al48 alloy was two-phase γ–TiAl and α2–Ti3Al. The Ti48Al52 alloy was single-phase γ–TiAl and had a larger grain size than the previous alloy. The microstructure of the Ti52Al48 alloy after room temperature deformation consisted primarily of {111} mechanical twins and a/2〈110〉 perfect dislocations. The comparable Ti48Al52 alloy microstructure contained fewer twins, and many more a〈101〉 and a/2〈112〉 superdislocations were present in addition to a/2〈110〉 dislocations. The superdislocations had dissociated and formed sessile faulted dipoles. The possible reasons for the differences in microstructure and mechanical behavior between these two alloys are discussed.
Effect of surface roughening on liquid-solid interface velocity
Authors: Richards Peter M.
Published: September 2007

The difference in density between liquid and solid is shown to be altered at the interface of a rough surface. Since it has been argued previously by the author that density difference plays a major role in interface velocity, I conclude that the rough (100) surface of silicon can have both a larger slope and less asymmetry in the freezing-melting velocity versus temperature relation than does the smooth (111) surface. This may provide partial explanation for some recent experimental results and apparent controversy.
Diamond synthesis from vapor phase and its growth process
Authors: Setaka Nobuo
Published: September 2007

Diamond synthesis from the vapor phase has been studied using hot filament assisted CVD, microwave plasma assisted CVD in an open system, and chemical transport process in a closed system using a gas mixture of hydrocarbon diluted with hydrogen gas. The deposited materials were identified as a cubic diamond with x-ray diffraction and Raman scattering measurement. The deposition process of diamond was considered on the basis of these experimental results and a review of the relevant field. The results suggest that the diamond synthesis proceeded by the deposition process, with the etching process operating simultaneously, and that atomic hydrogen played a very important role as the etching agent for non-diamond carbon. Also, the deposition process would be considered as the method that utilizes the bonding energy difference on each crystal surface of the diamond. The complicated problems on diamond synthesis originate from the fact that a third allotropic form exists in carbon, that is, the carbyne group.
Parameters affecting elastic properties of silica aerogels
Authors: Woignier T. , Phalippou J. , Vacher R.
Published: September 2007

The Young's modulus of silica aerogels is measured, using the three point flexural technique. Various parameters have been investigated, such as the concentration of silicon compounds as well as the catalyting conditions used to develop the initial alcogel and the heat treatment performed to densify the material. The elastic behavior of aerogels depends on the conditions of gel preparation, and the Young's modulus is shown to obey the power dependence E ≃ ρ3.7. This scaling exponent is compared to the percolation model predictions. On the other hand, the elastic behavior of different sets of aerogels is related to the spatial arrangement of the particles pointed out by fractal features.
Thermomechanical detwinning of superconducting YBa2Cu3O7−x single crystals
Authors: Kaiser Debra L. , Gayle Frank W. , Roth Robert S. , Swartzendruber Lydon J.
Published: September 2007

A method for the complete removal of twins from single crystals of superconducting YBa2Cu3O7−x is described. The process depends on ferroelastic behavior found to exist in the phase, and should be generally applicable to the layered perovskite-type phases containing accommodation twins resulting from a tetragonal-to-orthorhombic transformation on cooling. The twin-free, superconducting single crystals will enable investigation of a–b anisotropy of properties as well as crystal structure determination without complication by the presence of microtwins.
Electron microscopy study of Pb-substitution in bulk Bi–Sr–Ca–Cu–O superconductors
Authors: Werder D. J. , Chen C. H. , Jin S. , Sherwood R. C.
Published: September 2007

We present the results of an electron microscopy study of Pb-substitution in bulk Bi–Sr–Ca–Cu–O superconductors. We observe grains with c-axis lattice parameters of 30.8 Å (2212 phase), 36.7 Å (2223 phase), and 43.2 Å (2234 phase) containing no intergrowth defects. The existence of two separate incommensurate superstructures, Q1 and Q2, which were previously observed in Pb-doped ceramics and single crystals, has been confirmed in this study. It is found that superlattice reflections from the Q2,-modulated structure in the 2223 phase are much weaker than those from the Q2-modulation, whereas the diffraction intensities of the Q1- and Q2-modulations in the 2212 phase are similar. We also show that the periodicity along the b-axis of the Q2-modulation increases with increasing Pb-substitution, contrary to the implications of a previously suggested structural model.
Effect of atmosphere and rare earth on liquidus relations in RE–Ba–Cu Oxides
Authors: Ullman J. E. , McCallum R. W. , Verhoeven J. D.
Published: September 2007

In the processing of the high temperature superconductors RE1Ba2Cu3O7−x a knowledge of the liquidus temperatures is required in order to avoid liquid formation during the initial reactions of the starting materials. We have investigated the invariant points on the liquidus surface of the RE–Ba–Cu–O systems for RE = Y, Er, Gd, and Nd in oxygen, air, and argon, While the temperatures of the low melting reactions are almost independent of the rare earth species, they are heavily dependent on oxygen partial pressure. In addition, the peritectic decomposition temperature of the REBa2Cu3O7 phase was found to be a function of rare earth with a significantly higher value for the Nd compound than for the other rare earths.
Structure of composition-modulated Cu/Ni thin films prepared by electrodeposition
Authors: Yahalom J. , Tessier D. F. , Timsit R. S. , Rosenfeld A. M. , Mitchell D. F. , Robinson P. T.
Published: September 2007

Copper/nickel multilayered thin-films prepared by electrodeposition have been examined in cross section by electron energy loss spectroscopy and high-resolution transmission electron microscopy. The results of the examinations provide the first direct experimental evidence of the large composition modulation across successive layers in the thin-film structure and the coherent nature of Cu/Ni interfaces.
The crystal structure of the 1212 nonsuperconductor phase (Pb0.71Cu0.29)Sr2(Y0.73Ca0.27)Cu2O7
Authors: Lee J. Y. , Swinnea J. S. , Steinfink H.
Published: September 2007

The reaction product from an oxide mixture of 1Pb:2Sr:0.75Y:25Ca:2Cu at 960 °C was nearly single phase. The crystal structure of a single crystal, selected from the reaction product, was determined from three-dimensional x-ray diffraction data. The stoichiometry was found to be Pb0.71Cu0.29Sr2(Y0.73Ca0.27)Cu2O7. The tetragonal lattice constants are a = 3.8207(4) Å, c = 11.826(1) Å, space group P4/mmm, Mr = 655.6, Z = 1, Dx, = 6.31 g cm−3, λ(MoKα) = 0.71069 Å. R = 0.0269 for 179 unique observed reflections. The crystal structure is isomorphous with the 1212 phases. The Pb ion at the origin has an occupancy of 0.705 (8) and a Cu ion is located at 0.221, 0, 0 with an occupancy of 0.295. The oxygen ion in the plane of the Pb ions is disordered over a fourfold site with an occupancy of one. An ordered arrangement of this oxygen ion results in a distorted environment for periodically repeated Pb ions. The Pb–O bond lengths to the two oxygen ions above and below the Pb plane are 1.968(14) Å. The Cu ion, displaced from the origin, can be considered as tetrahedrally coordinated. The four Cu–O bond distances in the CuO2 plane are 1.917(6) Å and the bond length to a fifth oxygen atom is 2.304(14) Å at the apex of the square pyramid. A valence sum calculation for Cu in square coplanar coordination yields a value of 2.01. A similar calculation for Cu in the tetrahedral site yields 1.5. Sr is in a monocapped square antiprism coordination polyhedron. Y and Ca atoms occupy statistically the same site and are in cubic coordination. The structure can be described as consisting of CuO2–(Y, Ca)–CuO2 layers with PbO6 octahedra flanking the layers and Sr in the oxygen interstice. The compound is metallic at room temperature and shows no superconducting transition to 20 K.
Low temperature synthesis of YBa2Cu3O7−x thin films
Authors: Muroi Michihito , Matsui Toshiyuki , Koinuma Yuji , Okamura Yuko , Tsuda Koichi , Nagano Megumi , Mukae Kazuo
Published: September 2007

Thin films of YBa2Cu3O7−x (YBCO) have been prepared on MgO substrates at 650 °C by rf-planar magnetron sputtering using a single oxide target. Depositions at two different substrate positions were examined: (A) above the inner boundary of the erosion area of a target (position A), (B) right above the center of a target (position B). The films deposited at position A suffered a serious deficiency in the contents of Ba and Cu, especially in the low gas pressure region due to the bombardment of the growing films by high-energy particles. Though we could obtain almost stoichiometric films by raising the gas pressure to 15 Pa to reduce the bombardment, high temperature annealing at 950 °C was required to improve their crystallinity and to achieve high critical temperatures (Tc). On the other hand, those deposited at position B were more excellent in crystallinity than those at position A and showed high Tc above 80 K without any annealing. Critical current density C/r) at 77 K was improved from about 103 A/cm2 to about 104 A/cm2 by annealing at low temperature of 500 °C. It is concluded that the deposition with reduced bombardment by high-energy particles enables the fabrication of high Tc YBCO films without a high temperature process.
Formation of coherent twins in YBa2Cu3O7–δ superconductors
Authors: Jou C. J. , Washburn J.
Published: September 2007

A nucleation-and-growth mechanism for the twin formation in YBa2Cu3O7–δ superconductors based on the oxygen uptake rate curve and published transmission electron microscopic observations is proposed together with an oxygen-depleted twin boundary model. The difficulty of reaching stoichiometric YBa2Cu3O7 is explained.
Thin-film reactions of Al with Co, Cr, Mo, Ta, Ti, and W
Authors: Colgan E. G. , Mayer J. W.
Published: September 2007

The thin-film interactions of Al with refractory metals (Co, Cr, Mo, Ta, Ti, and W) have been investigated. The composition and thickness of the reacted aluminide layers were determined by Rutherford backscattering and phase identification was made by x-ray diffraction. Scanning electron microscopy was used to examine the lateral uniformity. The initial aluminide phases to grow are the Al-rich phases: Co2Al9, Cr2Al13, MoAl12, TaAl3, TiAl3, and WAI12. These are the most Al-rich phases on the phase diagrams. The reaction temperatures varied between 350 and 525 °C.
Pulsed-laser induced transient phase transformations at the Si–H2O interface
Authors: Polman A. , Sinke W. C. , Uttormark M. J. , Thompson Michael O.
Published: September 2007

Phase transformations at the Si–H2O interface, induced by nanosecond pulsed laser irradiation, were studied in real time. Si samples were irradiated using a 4 ns pulse from a Q-switched frequency-doubled Nd:YAG laser while immersed in the transparent liquid. Using time-resolved conductivity and reflectivity techniques, in combination with modeling of optical parameters and heat flow, transient processes in the Si, the H2O, and at the interface have been unraveled. In the liquid, local rapid heating occurs as a result of heat flow across the interface, and formation of a low-density steam phase occurs on a nanosecond timescale. Expansion of this phase is followed by a collapse after 200 ns. These rapid phase transformations in the water initiate a shock wave with a pressure of 0.4± 0.3 kbar. Transient phase transformations and the heat flow into the water during the laser pulse influence the energy coupling into the sample, resulting in an effective laser pulse shortening. The pulse shortening and the additional heat flow into the water during solidification result in a 30% enhancement of the solidification velocity for 270 nm deep melts. Cross-section transmission electron microscopy data reveal that the Si surface is planar after irradiation and is inert to chemical reactions during irradiation. Recent experiments described in the literature concerning pulsed-laser induced synthesis at the solid-liquid interface are reviewed and discussed in the context of the fundamental phenomena presently observed.
Glow-discharge synthesis of silicon nitride precursor powders
Authors: Ho Pauline , Buss Richard J. , Loehman Ronald E.
Published: September 2007

A radio-frequency glow discharge is used for the synthesis of submicron, amorphous, silicon nitride precursor powders from silanc and ammonia. Powders are produced with a range of Si/N ratios, including stoichiometric, Si-rich, and N-rich, and contain substantial amounts of hydrogen. The powders appear to be similar to silicon diimide and are easily converted to oxide by water vapor. The powders lose weight and crystallize to a mixture of α and β–Si3N4 after prolonged heating at 1600 °C. Studies of spectrally and spatially resolved optical emission from the plasma are reported.
Photochemical vapor deposition of silicon oxynitride films by deuterium lamp
Authors: Watanabe Junji , Hanabusa Mitsugu
Published: September 2007

Silicon oxynitride films have been grown by a photochemical vapor deposition process utilizing VUV light of a deuterium lamp from a gas mixture of Si2H6, NH3, and NO2 at the substrate temperature of about 330 °C. The deposition rate of the film varied with NO2 flow rate and also with the excitation light spectrum which was varied by a low-pass filter of a synthetic or fused silica plate. The composition of the films was sensitive to the NO2 flow rate which was smaller than that of NH3 by a factor of 103.
The Gibbs free energy of formation of a glassy alloy
Authors: Clavaguera-Mora M. T. , Clavaguera N.
Published: September 2007

A simple expression for the Gibbs free energy of formation of a pure component or a eutectic alloy glass, relative to the stable crystalline phase (or phases) at the same temperature is deduced by use of thermodynamic arguments. The expression obtained is supposed to apply to both monocomponent and multicomponent liquid alloys that might become glasses from the supercooled liquid state, irrespective of the critical cooling rate needed to avoid crystallization.
Glass formation and glass structure of BiO1.5–CuO–Ca0.5Sr0.5O system
Authors: Zheng Haixing , Xu Ren , Mackenzie J. D.
Published: September 2007

The glass-forming region of the BiO1.5–CuO–Ca0.5Sr0.5O system has been determined by melting 5 g batches. Glass rods 4 mm in diameter and 75 mm long have been made. The glass transition temperatures (TG) and the onset crystallization temperatures (Tx) are around 390 °C and 440 °C, respectively. Densities of these glasses range from 5.5 to 7.0 g/cm3. The glass structure consists of the [BiO3] and [BiO6] units, and the conversion between these two polyhedra mainly depends on the Ca0.5Sr0.5O amount. The molecular volume of the glasses indicated that increasing Ca0.5Sr0.5O caused loose glass structures.
Application of nuclear reaction analysis to trace oxygen analysis in metal fluoride materials
Authors: Hu B-L. , Jones K. W. , Tebbe F. N. , Firment L. E. , Brixner L. H.
Published: September 2007

Trace oxygen contamination of metal fluoride glasses degrades their performance in applications such as optical fibers or x-ray phosphors. Traditional methods for oxygen determination based on reaction with carbon to form carbon monoxide are slow and insensitive. We have used nuclear reaction analysis, in particular the 18O(p, α) reaction, to determine oxygen content in a variety of metal fluorides. Careful choice of the energy of the incident proton eliminates interference from fluorine. Standardless quantification of our measurements yields good agreement with the known oxygen content of mixtures of oxides and fluorides and with several (non-fluoride) NBS standards over four orders of magnitude. Measurement of 30 ppm oxygen has been demonstrated.
Reactions at the interface between multi-component glasses and metallic lithium films
Authors: Zavadil K. R. , Armstrong N. R. , Peden C. H. F.
Published: September 2007

The reactions of vacuum deposited thin films of lithium with various complex glasses have been explored using x-ray photoelectron spectroscopy (XPS). In contrast to lithium reactions with simple glasses such as silica or boron oxides, the reactions are predominantly those of the network modifiers such as sodium, potassium, and magnesium. XPS and x-ray induced Auger lineshapes indicate the migration of the network modifier to the near surface region followed by its reduction. In the case of magnesium, there is evidence for stable alloy formation with unreacted lithium following these migration and reduction steps.
Transparent silica gel–PMMA composites
Authors: Pope E. J. A. , Asami M. , Mackenzie J. D.
Published: September 2007

Transparent silica gel–polymer composites have been prepared by the impregnation of porous gels with organic monomer and polymerization in situ. The relative amount of each phase was adjusted by varying the porosity of the silica gel prior to impregnation. These materials constitute a new class of transparent composites. Properties, such as density, refractive index, modulus of rupture, compressive strength, abrasion rate, and Vickers hardness, have been measured over the compositional range of 100% silica to 100% polymethyl methacrylate (PMMA).
Relaxation phenomena in evaporated amorphous silicon films
Authors: De Avillez R. R. , Clevenger L. A. , Thompson C. V.
Published: September 2007

Power compensated differential scanning calorimetry, transmission electron microscopy, and density measurements were used to characterize relaxation phenomena associated with structural transitions in electron beam evaporated amorphous silicon thin films. Two transitions were observed in the amorphous films upon heating: a broad exothermic process below 600 K and an endothermic step beginning at 600 K. The broad exothermic process is due to the relief of bond distortions. The endothermic step is related to homogenization and densification of the structure of the amorphous silicon.
Effect of heat treatment time and temperature on the properties of YBa2Cu3O7−x
Authors: Bloom Ira , Tani B. S. , Hash M. C. , Shi Donglu , Patel M. A. , Goretta K. C. , Chen Nan , Capone D. W.
Published: September 2007

Two YBa2Cu3O7−x powders were investigated: one was nearly stoichiometric, and the other, CuO-rich. After being sintered at 950 °C, samples from both powders were heat-treated in oxygen at 845, 860, and 910 °C for up to 10 days. The samples were characterized by differential thermal analysis, x-ray diffraction, optical and scanning electron microscopy, iodometric titrations, and measurement of their superconducting properties (electrical resistivity, critical temperature, and critical current density). The electrical properties of the nearly stoichiometric material improved with time at all temperatures. On the other hand, the electrical properties of the CuO-rich material deteriorated with temperature, possibly owing to problems associated with grain growth at 910 °C.
Reaction pathways in the formation of the Tl–Ca–Ba–Cu superconducting phases
Authors: Ruckenstein E. , Cheung C. T.
Published: September 2007

The 2223, 1223, 1324, and 1122 phases in the Tl–Ca–Ba–Cu system were prepared from sample mixtures having the same stoichiometry as the final phase. Powder x-ray diffraction analysis was used to follow the formation of these phases at 867 °C for up to 2 h. The formation of the 2223 phase is shown to occur via the sequential transformations 2021 → 2122 → 2223. For the 1–2–2–3 and 1–3–2–4 samples, the sequential transformations 1021 → 2021 → 2122 → 2223 occur before the final 1223 and 1324 phases are obtained. For the 1122 phase the formation is preceded by the 1021 → 2021 → 2122 transformations. Using scanning electron microscopy, a phase with a glassy appearance was detected in many of the reacted samples. This “glassy” phase appears to be a precursor in the formation of the Tl–Ca–Ba–Cu phases.
Kinetic paths of B2 and DO3 order parameters: Theory
Authors: Anthony L. , Fultz B.
Published: September 2007

It is shown that a binary alloy with an AB3 stoichiometry on a bcc lattice may develop various combinations of B2 and DO3 order along its kinetic path toward equilibrium. The temporal evolution of these two order parameters is analyzed with an activated-state rate theory. Individual vacancy jumps are treated in a master equation formalism that involves first-nearest-neighbor (1nn) and second-nearest-neighbor (2nn) interactions. In our formulation, a set of coupled differential equations is obtained describing the time-dependence of six order parameters. These equations were integrated numerically for a variety of interatomic interactions and initial conditions. It was found that the relative rates of B2 and DO3 ordering, and hence the path of the alloy through the space spanned by the B2 and DO3 order parameters, depend on the relative strengths of the interatomic interaction potentials and on the temperature. For very strong 2nn interactions, a transient B32 structure was observed to develop at early times, although this phase disappeared as equilibrium was approached.
Optical and magnetic properties of MnBi formed directly by ion beam mixing
Authors: Kido Yoshiaki , Tada Masa-aki
Published: September 2007

Multilayers of Mn/Bi were irrradiated with 400 keV Xe+ at temperatures from 80 to 550 K. Ferromagnetic MnBi layers oriented perpendicular to the substrate surface were formed directly at mixing temperatures from room temperature (RT) to 500 K. The measurements of Curie point and optical Faraday rotation revealed that 60–70 at. % of MnBi ion-mixed at RT is the metastable phase (QIITP: quenched high temperature phase). The fraction of QIITP is reduced with increase in mixing temperature. It was also found that ion beam mixing yields a pronounced protection effect against the deterioration of MnBi in a moist atmosphere. A detailed discussion is given on the kinetics of ion beam mixing and the growth mechanism of the metastable MnBi grains.
Thermally induced hillock formation in Al–Cu films
Authors: Chang C. Y. , Vook R. W.
Published: September 2007

Isothermal annealing studies of hillocks formed on Al–15 wt.% Cu films, vapor deposited at 25 °C on oxidized silicon wafers, were carried out in situ in a scanning electron microscope. The original hillocks formed as a result of substrate-induced thermal expansion strains which caused material to diffuse out of the film to form the hillocks when the films were heated to the isothermal annealing temperatures. During isothermal annealing the hillock density decreased and the average size of the hillocks increased. Measurements of these quantities as a function of time were made at a series of temperatures ranging from 200 to 300 °C. The activation energies for these two cases were found to be 0.29 and 0.28 eV, respectively. X-ray energy spectroscopy analysis of the films showed that the hillocks were richer in copper than the matrix. Transmission electron microscopy showed that the average hillock and grain sizes in the variously annealed films were linearly related and of the same order of magnitude. The results were also analyzed using Chakraverty's models for surface and interfacial diffusion. It was concluded that the evidence clearly shows that the observed processes could be well characterized by a typical Ostwald ripening model.