bluuebunny/crossref_metadata_2025 huggingface dataset:
Long-range chemical ordering in Al-Cu-Fe, Al-Cu-Mn, and Al-Cu-Cr quasicrystals
Authors: Ebalard S. , Spaepen F.
Published: September 2007

The dependence of the F-type order and the stability of Al–Cu–Fe quasicrystals on quenching rate and composition are reported. In the Al–Cu–Mn and Al–Cu–Cr systems, where only metastable quasicrystals can be formed, a disordered icosahedral phase is obtained by melt-spinning, and P ⇉ F-type ordering is observed after annealing. Simultaneous evolution of the diffuse scattering and the ordering is observed as well. The quasicrystalline phases, P-type icosahedral as well as decagonal, in the ternary Al–Cu–TM alloys are structurally similar to those in the binary Al–TM alloys. Dislocations in the icosahedral phase have been imaged both by their strain field contrast and in high resolution.
Si diffusion coating on steels by SiH4/H2 treatment for high temperature oxidation protection
Authors: Cabrera A. L. , Kirner J. F. , Pierantozzi R.
Published: September 2007

The reaction of SiH4/H2 mixtures with iron and steels was studied at a total pressure of 1 atm and temperatures above 500 °C. When the amount of water vapor in the gas mixture is carefully controlled, a metal silicide diffusion coating forms at low temperatures (below 900°C). Composition and structure of the Si diffusion coatings were determined with Auger depth profiling and x-ray diffraction. Kinetics of the surface reaction between SiH4, and the metal substrate as well as the behavior of these films in severe environments at high temperatures were studied by a microgravimetric technique. Characterization of these Si coatings on iron, low carbon steel (1010), 9% Cr/1% Mo steel (alloy A182F9), and stainless steels (310) and their applications to reduce oxidation, nitriding, or coking at high temperatures or corrosion in mineral acids are described.
Fracto-emission from deuterated titanium: Supporting evidence for a fracto-fusion mechanism
Authors: Dickinson J. T. , Jensen L. C. , Langford S. C. , Ryan R. R. , Garcia E.
Published: September 2007

Measurements of the emission of charged particles, photons, and radio frequency signals accompanying the deformation and fracture of polycrystalline Ti metal and deuterated Ti are described. Preliminary evidence for charge separation created by crack propagation is presented which supports a proposed fracture mechanism to explain neutron bursts observed during treatment of deuterated metals.
MoS2−xOx solid solutions in thin films produced by rf-sputter-deposition
Authors: Lince Jeffrey R.
Published: September 2007

The chemical composition and structure of MoS2 solid lubricant films are intimately related to their friction and wear characteristics. We have conducted an x-ray photoelectron spectroscopy (XPS) study of 1-μm thick radio frequency (rf)-sputter-deposited MoS2 films to determine the chemical state of the films, focusing on the role of oxygen impurities. Concentrations of chemisorbed and bulk species were determined from the Mo 3d, S 2p, and O 1s peak shapes and intensities after annealing the films to temperatures from 425 to 975 K. Films deposited on substrates that were at ∼345 K [ambient temperature (AT) films] and on substrates heated to ∼525 K [high temperature (HT) films] both had ∼10% oxygen within the bulk of the films. The relative areas and shapes of the XPS peaks for the HT films at all annealing temperatures were consistent with the formation of a MoS2−xOx solid solution, where O atoms were probably substituted into S sites in the 2H–MoS2 crystal lattice. In AT films, this phase composition was stable only for annealing temperatures ≥725 K, in agreement with previous studies of the changes in crystal structure of AT films with annealing. The results are discussed in terms of previous studies of the structure and composition of sputter-deposited MoS2 films.
Analysis of stability of a planar solid-liquid interface in a dilute binary alloy
Authors: Laxmanan V.
Published: September 2007

The question of stability of a planar solid-liquid interface in undercooled pure and alloy melts has been reconsidered without the restrictive assumption of no heat flow in the solid made in earlier works. The modified analysis indicates that provided the thermal gradient on the solid side of the interface, Gs, is positive, stability can be achieved in an undercooled alloy melt for growth rates R>Ra, whereas a recent analysis by Trivedi and Kurz, which assumes Gs = 0, suggests that stability is possible only if R>Ra + Rat. Here Ra is the familiar absolute stability limit of Mullins and Sekerka and Rat, is the absolute stability limit in an undercooled pure melt, as identified by Trivedi and Kurz. The absolute stability criterion for steady-state planar growth in an undercooled alloy melt is thus the same as derived earlier by Mullins and Sekerka for directional solidification. Relaxing the restrictive assumption of Gs = 0 also reveals that there is a regime of stability for low growth rates and low supercoolings. Stability is possible under these conditions if Gs>0, and the bath undercooling ΔTb < ΔTO + ΔTh/2, where ΔTO is the freezing range of the alloy and ΔTh is the hypercooling limit for the pure melt. For large supercoolings, Gs < 0, and the interface will be unstable with respect to large wavelength perturbations, even if R > Ra + Rat.
Electron microscopy of the Pb-Sr-Ca-Er-Cu-O superconductor
Authors: Ramesh R. , Wang E. , Greene L. H. , Hegde M. S. , Tarascon J-M. , Kim Y.
Published: September 2007

The structure and microstructure of a solid state processed Pb-Sr-Ca-Er-Cu-O superconductor have been investigated by transmission electron microscopy. In addition to the majority superconducting phase, at least two other impurity phases have been observed. The superconducting phase is a layered structure similar to the Bi2Sr2CaCu2Oy compound, with an extra Cu atom between the two PbO layers. Stacking defects inside the grain have been observed. A grain boundary amorphous phase has also been observed. The steps in the resistivity-temperature plot and the consequent absence of Tc,0 above 8 K are attributed to either the presence of the Pb2Sr2Cu2Oy unit cell at the grain boundary and/or a local enrichment of oxygen at the grain boundaries. Superlattice spots in the [100] zone axis diffraction patterns from regions enriched in Ca and Er with respect to the nominal composition are interpreted as due to ordering of Sr and Ca/Er in the Sr sites.
Thermodynamic parallels between solid-state amorphization and melting
Authors: Wolf D. , Okamoto P. R. , Yip S. , Lutsko J. F. , Kluge M.
Published: September 2007

A thermodynamics-based description, in the form of an extended phase diagram, of melting and solid-state amorphization is proposed which brings out the parallels between these two phenomena and suggests that their underlying causes are apparently the same. Through molecular dynamics simulations we demonstrate that every crystal, in principle, can undergo two different types of melting transitions with characteristic features that are also observed in radiation- and hydrogenation-induced amorphization experiments on ordered alloys. The first type, defined in terms of free energies, is shown to involve the heterogeneous nucleation of the liquid or amorphous phase at extended lattice defects (such as grain boundaries, free surfaces, voids, or dislocations) and subsequent thermally-activated propagation of solid-liquid/amorphous interfaces through the crystal. The second type, arising from a mechanical instability limit described by Born, is homogeneous and does not require thermally-activated atom mobility. It is suggested that the role of chemical and structural disordering, a prerequisite for irradiation- but not hydrogenation-induced solid-state amorphization, is merely to drive the crystal lattice to a critical combination of volume and temperature at which the amorphous phase can form either heterogeneously or homogeneously.
Grain growth in boron doped LPCVD polysilicon films
Authors: Sitaram A. R. , Murarka S. P. , Sheng T. T.
Published: September 2007

Dopant induced grain growth in LPCVD polysilicon films has been investigated using BBr3 as the diffusion source at 900 and 950 °C. TEM and sheet resistance measurements indicate rapid growth under such doping conditions. Results are compared with the grain growth observed during (a) PBr3 doping of similar films, and (b) during anneals in ambients containing 1% oxygen in nitrogen, similar to that used during BBr3 or PBr3 doping. The results clearly demonstrate the rapid grain growth induced by both types of dopants, although phosphorus is more effective in inducing this grain growth.
Crystallization of A15 Nb3Ge from the amorphous state
Authors: Wardak Marcin , Clapp Mireille Treuil
Published: September 2007

The critical current density Jc of superconductors is strongly dependent on microstructure. The microstructure and crystallization of amorphous Nb75Ge24.5B0.5 melt spun ribbons were studied as a function of 24-h anneals between 680 and 860 °C. Metastable A15 was formed with a composition close to that of the starting amorphous material. As the temperature increased from 680 to 720°C, crystallization remained nonuniform and the average grain size decreased from 130 nm to 60 nm. Between 740 and 800°C crystallization was uniform and the average grain size was constant at 30 nm. Above 820°C grain growth occurred. In order to have high Jc's a uniformly crystallized small-grained material is desirable.
Brittle fracture in disordered materials: A spring network model
Authors: Curtin W. A. , Scher H.
Published: September 2007

A model for investigating the influence of distributed disorder on the failure of brittle materials is introduced. The model assumes that microstructural features of a material can be represented by simple linear springs with a failure threshold, and that the entire material can be represented by a connected network of such springs. Distributed disorder is introduced by allowing spring-to-spring variations in spring characteristics such as the modulus and the failure strain. The conditions under which such a spring network model is valid for studying failure are discussed. The consequences of distributed residual stress disorder on macroscopic mechanical behavior are then studied using the network model, and a brittle to ductile-like transition in the stress-strain behavior is observed with increasing disorder. All the qualitative features of the network results can be described theoretically by a statistical analysis of this problem. Finally, notch tests are performed to evaluate the strength and toughness of the ductile-like materials as compared to the uniform (no disorder) material, and the ductile-like material is found to have (i) a larger work of fracture, (ii) comparable strength in the presence of processing flaws, and (iii) the possibility of larger toughness. Based on these results, the possibility of observing such ductile-like behavior in real composite materials is discussed.
Diffusion-accommodated sliding of irregularly shaped grain boundaries
Authors: Schneibel J. H. , Hazzledine P. M.
Published: September 2007

The diffusion-accommodated sliding of irregularly shaped grain boundaries in two-dimensional bicrystals is considered. The following assumptions are made: the grains adjoining the boundaries are rigid, the boundaries do not support any shear stresses, sliding displacements are infinitesimal, and sliding is accommodated only by grain boundary diffusion. The solution to this problem is illustrated for a bicrystal with a grain boundary consisting of three segments. The results of calculations involving up to 35 segments agree with Raj and Ashby's theory for the sliding of periodic boundaries. The influence of boundary conditions on the normal stress distributions along grain boundaries is examined. Zero-flux conditions at the intersection of a grain boundary with a free surface, which correspond to low surface diffusivities, can lead to high normal grain boundary stresses. The stress distributions and sliding rates of boundaries containing randomly spaced equisized bumps or equispaced bumps of random size are compared to the periodic case (i.e., equispaced equisized bumps). Substantial normal stresses can build up at such nonperiodic grain boundaries.
Evidence of ionic aggregates in some ampholytic polymers by transmission electron microscopy
Authors: Castaño V. M. , González A. E. , Cardoso J. , Manero O. , Monroy V. M.
Published: September 2007

Transmission electron microscopy observations were conducted on some zwitterionic polymers. Our observations were concentrated on the morphology of the solid state of poly (4-vinyl pyridine-sulfopropyl bctaines), with different degrees of quaternization. Among the morphological features, a microstructure arising from aggregated regions in the range of 50 to 200 Å is observed. Quite striking is the fact that this structure is absent in the micrographs for the base polymer, indicating that the aggregates are due to the interactions of the ion pairs of the polyzwitterions, as in the ionomer case. It is also found that the density of the aggregates increases when we increase the zwitterionic content of the copolymers.
Yttrium enrichment and improved magnetic properties in partially melted Y-Ba-Cu-O materials
Authors: Hojaji Hamid , Barkatt Aaron , Michael Karen A. , Hu Shouxiang , Thorpe Arthur N. , Ware Matthew F. , Talmy Inna G. , Haught Debbie A. , Alterescu Sidney
Published: September 2007

Samples of Y-Ba-Cu-O materials with the formulation Y:Ba:Cu = x:2:3 and values of x ranging between 1 and 3 were prepared by partial melting at a maximum temperature of 1045 °C. Measurements of magnetic susceptibility and maximum (low-field) as well as remanent magnetization show highest values for x = 2. XRD and SEM/EDX analyses show that the corresponding structure involves numerous small crystals of Y2BaCuO5 (211) embedded in highly ordered assemblages of continuous YBa2Cu3O7−y (123) layers. The presence of these impurity sites is correlated with flux pinning capacity. Other impurity phases include CuO, a minor phase which shows an increase in amount when x increases, BaCuO3, a possibly detrimental minor phase which vanishes when x is raised from 1 to 2, and zones of intermediate composition between the 211 grains and the 123 layers, which are formed from the residual liquid upon cooling. These intermediate regions, like the 211 grains themselves, become gradually more important when x is increased from 1 to 3.
Glass transformation phenomena in bulk and film amorphous selenium via DSC heating and cooling scans
Authors: Yannacopoulos S. , Kasap S.O.
Published: September 2007

Recently there has been an emphasis on the importance of using cooling scans in DSC experiments in studying the glass transformation kinetics of glasses. The physical interpretation of the apparent activation energy from DSC heating scans has been questioned as not being meaningful. The present paper reports glass transition temperature (Tg) measurements derived from Differential Scanning Calorimetry (DSC) experiments on bulk and film amorphous selenium samples subjected to heating, at constant rate r, and cooling, at constant rate q, scans. Film samples were prepared by thermal evaporation techniques in vacuum. It is shown that for both bulk and film forms of a–Se, within experimental errors, log(r/T2gm) vs 1/Tgm plot where Tgm is the peak glass transformation temperature, and log(r) vs 1/Tgh plot, where Tgh is the glass transition onset temperature from DSC heating scans, are parallel to the log(q) vs 1/Tgc plot where Tgc is the glass transition temperature from cooling scans. Within the Hutchinson and Kovacs formulation of the glass transformation phenomenon, the results imply that the structural contribution to the mean retardation time, τ, is negligible in comparison with the temperature dependent part. The mean structural relaxation time for both bulk and film forms was found to exhibit a typical Vogel-Tammann-Fulcher type of temperature dependence. Furthermore, the structural relaxation rate was observed to be inversely proportional to the viscosity, η(T), implying that the mean structural retardation time is proportional to the viscosity, τ ∼ η. The results also confirm that the earlier studies of glass transformation kinetics in a–Se utilizing only DSC heating scans remain meaningful.
Instrumentation of a conventional hardness tester for load-displacement measurement during indentation
Authors: Pharr G. M. , Cook R. F.
Published: September 2007

A conventional microhardness tester has been instrumented with a piezoelectric load cell and capacitance displacement gages to measure load and displacement during indentation. As in other recently-developed load and displacing sensing indentation instruments, the new device can be used to measure a variety of mechanical properties, but has the advantage of being relatively inexpensive to assemble since many of its components are standard equipment. Tests were performed on soda-lime glass and an aluminum alloy, demonstrating the diversity of material elastic-plastic responses under indentation, particularly in the unloading cycle. The data suggest that models of elastic unloading based on invariant indenter-surface contact area may not be general, and may lead to underestimates of hardness and modulus.
Growth twins in Bi2Ca1Sr2Cu2O8 superconductor single crystals
Authors: Han P. D. , Asthana A. , Xu Z. , Payne D. A.
Published: September 2007

Large-scale twin structures in single crystal Bi2Ca1Sr2Cu2O8 (2122) are reported for the first time. Symmetrical 90° (i.e., a-b) twins with a [110] type twin boundary were observed. A characteristic layer-growth morphology and jagged twin walls suggest that twin formation occurred layer by layer during crystal growth; i.e., the twins were growth twins. Hot-stage optical microscopy, x-ray diffraction, and electron microscopy results are discussed with reference to twin morphology.
Mechanical loss in a glass-epoxy composite
Authors: Weller Manfred , Ledbetter Hassel
Published: September 2007

Using a computer-controlled inverted torsion pendulum at frequencies near 1 Hz, we determined the mechanical losses in a uniaxially fiber-reinforced composite. The composite comprised glass fibers in an epoxy-resin matrix. We studied three fiber contents: 0,41, and 49 vol.%. Three mechanical-loss peaks appeared: above 300 K, near 200 K, and near 130 K. They correspond closely to α, β, and γ peaks found previously in many polymers. We failed to see a mechanical-loss peak for either the glass or the glass-resin interface. Between 300 and 4 K, the torsion modulus increased in the resin by a factor of 3.30 and in the 0.49 glass-epoxy by a factor of 2.37.
Investigation of the effects of boron on Ni3Al grain boundaries by atomistic simulations
Authors: Chen S.P. , Voter A.F. , Albers R.C. , Boring A.M. , Hay P.J.
Published: September 2007

A series of simulations has been performed on grain boundaries in Ni and Ni3Al with and without boron doping using embedded atom-style potentials. A new procedure of obtaining “reference” data for boron related properties from electronic band structure calculations has been employed. Good agreement with existing experimental structural and energetic determinations was obtained. Boron is found to segregate more strongly to grain boundaries than to free surfaces. Adding boron to grain boundaries in Ni and Ni3Al increases their cohesive strength and the work required to pull apart the boundary. This effect is much more dramatic for Ni-rich boundaries than for stoichiometric or Al-rich boundaries. In some Ni-rich cases, adding boron increases the cohesive strength of the boundary to such an extent that the boundaries become stronger than the bulk. Bulk Ni3Al samples that are Ni-rich produce Ni-rich grain boundaries. The best cohesive properties of Ni3Al grain boundaries are obtained when the boundary is Ni saturated and also with boron present. Boron and nickel are found to cosegregate to the grain boundaries.
Electronic, elastic, and fracture properties of trialuminide alloys: Al3Sc and Al3Ti
Authors: Fu C. L.
Published: September 2007

The electronic mechanism behind the brittle fracture of trialuminide alloys is investigated using the full-potential linearized augmented plane-wave (FLAPW) total-energy method within the local density functional approach. To this end, the bulk phase stability, the elastic constants, the anti-phase boundary (APB) energy, the superlattice intrinsic stacking fault (SISF) energy, and the cleavage energy on different crystallographic planes have been determined. A small energy difference (=0.10 eV/unit formula) is found between the DO22 and L12 structures of Al3Ti. In general, the trialuminide alloys have large elastic modulus, small Poisson's ratio, and small shear modulus to bulk modulus ratio. An extremely high APB energy (=670 mJ/m2) on the (111) plane is found for Al3Sc, indicating that the separation between ½(110) partials of a (110)(111) superdislocation is small. Since the total superdislocation has to be nucleated essentially at the same time, a high critical stress factor for dislocation emission at the crack tip is suggested. The high APB energy on the (111) plane is attributed to the directional bonding of Sc(d-electron)-Al(p-electron). The same type of directional bonds is also found for Al3Ti. In addition, moderately high values of SISF energy (=265 mJ/m2) on the (111) plane and APB energy (=450 mJ/m2) on the (100) plane are found for Al3Sc. The brittle fracture of trialuminide alloys is attributed to the higher stacking fault energies and a lower cleavage strength compared to those of a ductile alloy (e.g., Ni3Al). While the (110) surface has the highest surface energy, the cleavage strength (=19 GPa) of Al3Sc is found to be essentially independent of the crystallographic planes. The directional Sc—Al bond becomes even stronger on the (110) surface, which may explain the preferred (110) type cleavage observed by experiment.
The role of radiation in melt stability in zone-melt recrystallization of SOI
Authors: Knapp J.A. , Thompson L.R. , Collins G.J.
Published: September 2007

Under circumstances in Zone-Melt-Recrystallization (ZMR) of Si-on-Insulator (SOI) structures where radiative heat loss is significant, the ∼50% decrease in emissivity when Si melts destabilizes the Si molten zone. We have demonstrated this both experimentally using a slowly scanned e-beam line source and numerically with a finite-element computational simulation. The resulting instability narrows the process window and tightens requirements on beam control and background heating uniformity, both for e-beam ZMR systems and optically-coupled systems such as a graphite strip heater.
Analytical investigation of the simultaneous internal gettering of iron and nickel in silicon
Authors: Sinha P.K. , Glaunsinger W.S. , Deng Ray-Chern
Published: September 2007

The simultaneous gettering of iron and nickel in a float-zone silicon wafer in the (100) orientation and implanted with 2.5 ⊠ 1015 argon ions/cm2 at 280 keV has been investigated. Iron was deposited on one half of the back surface of the wafer and nickel was deposited on the other half. Chemical analyses by secondary ion mass spectroscopy, energy dispersive x-ray spectroscopy, and high-resolution imaging by high-resolution electron microscopy revealed the gettering rate of iron was orders of magnitude lower than that of nickel. The results also suggested that gettering is a metal-diffusion-controlled process. The simultaneous gettering of iron and nickel results in a complex distribution of the two metals in the silicon wafer.
Chemical corrosion of highly radioactive borosilicate nuclear waste glass under simulated repository conditions
Authors: Werme Las , Björner Inga K. , Bart Gerhard , Zwicky Hans U. , Grambow Bernd , Lutze Werner , Ewing Rodney C. , Magrabi Claude
Published: September 2007

This review summarizes the results of the joint Japanese (Central Research Institute of Electric Power Industry, CRIEPI, Tokyo), Swiss (National Cooperative for the Storage of Radioactive Waste, NAGRA, Baden), Swedish (Swedish Nuclear Fuel and Waste Management Company, SKB, Stockholm) international ‘JSS’ project on the determination of the chemical durability of the French nuclear waste borosilicate glass, which was completed in 1988. Radioactive and nonradioactive glass specimens were investigated. A data base was created with results from glass corrosion tests performed with different water compositions, pH values, temperatures, sample surface areas (S), solution volumes (V), and flow rates. Glass corrosion tests were performed with and without bentonite and/or steel corrosion products present. Variation of the glass composition was taken into account by including the borosilicate glass ‘MW’ in the investigations, formulated by British Nuclear Fuels, plc. An understanding was achieved of the glass corrosion process in general, and of the performance of the French glass under various potential disposal conditions in particular. A special effort was made to establish a corrosion data base, using highS/Vratios in the experiments in order to understand the glass durability in the long term.
Photoluminescence in chemically polymerized SiO2 and Al2O3–SiO2 systems
Authors: Yoldas Bulent E.
Published: September 2007

It is observed that in sol-gel derived oxide systems, chemical processes lead to the formation of photoluminescent centers analogous to those produced by neutron or γ-irradiation of oxide crystals, fused silica, and other glasses. The initial observations about the phenomenon and its proposed mechanism are reported here.
Lanthanum doping of YBa2Cu3Ox
Authors: Hong D. J. L. , Mehta A. , Smyth D. M. , Chang E. K. , Kirschner M. J.
Published: September 2007

A wide variety of cation substitutions have been tried in the 1-2-3 compound, YBa2Cu3Ox, in the hope that its superconducting properties can be improved. While significant improvement has not been achieved, the response of the material to such substitutions can still be helpful in understanding its defect chemistry. In this paper, we report on measurements of the equilibrium oxygen content, by thermogravimetry, and the electrical conductivity of YBa2−zLazCu3Ox, with z = 0, 0.1, and 0.2. The results are consistent with the view that the stoichiometric composition for this system, YBa2Cu3O6, is a highly acceptor-doped material.
Crystallization behavior of some melt spun Nd–Fe–B alloys
Authors: Clavaguera-Mora M. T. , Baró M. D. , Suriñach S. , Clavaguera N.
Published: September 2007

The kinetics of crystallization of four amorphous (or partially amorphous) melt spun Nd–Fe–B alloys induced by thermal treatment is studied by means of differential scanning calorimetry and scanning electron microscopy, In the range of temperatures explored experimentally, the crystallization process is thermally activated and generally proceeds in various stages. The Curie temperature and the crystallization behavior have been measured. The apparent activation energy of crystallization of most of the crystallization stages has been determined for each melt spun alloy. The explicit form of the kinetic equation that best describes the first stage of crystallization has been found. It follows in general the Johnson-Mehl-Avrami-Erofe'ev model, but clear deviations to that model occur for one alloy. Scanning electron microscopy demonstrates that preferentially hetereogeneous nucleation occurs at the ribbon surface which was in contact with the wheel. From crystallization kinetics results the lower part of the experimental time-temperature-transformation curves for all studied alloys are deduced and extrapolated to the high temperature limit of their range of validity, also deduced.
Thermal decomposition of Fe(III) nitrate and its aerosol
Authors: Gadalla Ahmed M. , Yu Hsuan-Fu
Published: September 2007

The thermal analyses of iron nitrate in stagnant and flowing air using different heating rates were conducted to establish the intermediate phases and reactions during its decomposition. In view of the thermal results, the degree of pyrolysis of the aerosol formed by atomizing ferric nitrate aqueous solution was studied. Hollow spherical particles were obtained and varied from noncrystalline to crystalline materials, depending on the pyrolysis temperature used. The decomposition efficiency of resulting particles increased with the pyrolysis temperature.
Grain growth in donor-doped SrTiO3
Authors: Peng C-J. , Chiang Y-M.
Published: September 2007

The Nb donor concentration and cation stoichiometry dependence of grain growth in SrTiO3 has been studied. The so-called donor anomaly, where grain growth is enhanced at low donor but suppressed at high donor concentrations, is observed only in nonstoichiometric compositions with an excess of B-site cations. Scanning transmission electron microscopy (STEM) observations indicate that this phenomenon is entirely a result of changes in the distribution of residual silicate phases. Exaggerated grain growth at low donor concentrations results from the presence of a continuously wetting grain boundary silicate, whereas inhibited growth at higher donor concentrations occurs in microstructures where the silicate phase is nonwetting. In stoichiometric compositions, however, grain growth rates are both slower and independent of donor concentration. In this composition regime grain growth appears to be limited by solid solution drag. The presence of residual silicates only in nonstoichiometric compositions implies a strong stoichiometry dependence of the silica solubility.
Raman microprobe study of nanophase TiO2 and oxidation-induced spectral changes
Authors: Parker J. C. , Siegel R. W.
Published: September 2007

A Raman microprobe study of as-compacted nanophase TiO2 was carried out to investigate the spatial inhomogeneity of its anatase and rutile phases. Also, changes in the observed Raman spectra (line shifts and broadening) were investigated as a function of annealing at temperatures up to 600°C in argon or air. Microscopic phase inhomogeneity is observed and Raman spectral changes are shown to result from inhomogeneous oxygen deficiency in the nanophase TiO2. The line positions corresponding to the Raman active Eg modes in both anatase and rutile are found to be sensitive to this oxygen deficiency and are potential quantitative indicators of such deviations from stoichiometry.
Effect of grain alignment and processing temperature on critical currents in YBa2Cu3O7-δ sintered compacts
Authors: Tkaczyk J. E. , Lay K. W.
Published: September 2007

Magnetically aligned YBa2Cu3O7-δ ceramics show resistivities approaching that of single crystals and improved transport critical currents in a magnetic field. Reduced microcracking and increased transport along the ab-plane are believed responsible for the improved performance over nonaligned ceramics. Aligned and nonaligned samples were prepared in parallel using a range of sintering and annealing temperatures. A rapid rise in density for samples sintered above 900°C in oxygen is accompanied by rapid grain growth, improved alignment, a drop in the room temperature resistivity, and an increase in the critical current. The presence of low melting point (i.e., Ba–Cu–O-rich) phases at grain boundaries is believed responsible for the rapid densification. However, the presence of this phase does not appear to be the most important factor limiting Jc. A high temperature oxygen anneal at 900°C improved performance as compared to anneals at 500°C, possibly due to the removal of carbon. For aligned samples sintered at 980°C, critical currents are over 200 A/cm2 at 77 K, ¼ tesla, and room temperature resistivities are below 350 μΩ-cm.
The decomposition of PdSi by adding a small amount of Ag
Authors: Tsumori K. , Akimune H.
Published: September 2007

The decomposition of the intermetallic compound PdSi by adding a small amount of Ag has been investigated by thermal analysis, metallographic measurement, and x-ray photoemission spectroscopy (XPS). Within the range of Ag concentration from 0 to 6 at.%, PdSi is decomposed in linear proportion to the concentration of Ag into Si and the ternary compound Pd2Si(Ag), which has the ratio of 16 PdSi molecules per single Ag atom. The PdSi is fully decomposed at Ag 10 at.%. In the ternary alloy included in the Pd–Si–Ag alloy involving Ag below 12 at. %, the atomic ratio of Si to metal atoms, which are Pd plus Ag, is nearly the same as the ratio in Pd-rich Pd2Si. Metallographic and XPS measurements indicate that the Ag atom included in the PdSi behaves as an electron donor with a large radius of the wave function; the Ag electron transfers to the anti-bonding state of PdSi and acts to decompose the compound. The Ag atoms seem to occupy the sites of Pd atoms in Pd2Si(Ag) which has been decomposed from PdSi.
Evidence against surface nucleation following pulsed melting of Si
Authors: Peercy P. S. , Tsao J. Y. , Thompson Michael O.
Published: September 2007

Time-dependent measurements of the melt and solidification behavior of amorphous Si, formed by ion implantation of In, have been obtained following irradiation with ∼3 ns ruby laser pulses. The recently observed buried In sheets formed under such conditions are shown to result from internal nucleation of melt, contrary to previous interpretations. Under no conditions was surface nucleation of either amorphous or crystalline Si observed from a surface melt. These results resolve previous inconsistencies in the understanding of phase formation during rapid melting and solidification of Si.
Low dislocation density GaAs grown by the vertical Bridgman technique
Authors: Kremer R. E. , Francomano D. , Beckhart G. H. , Burke K. M.
Published: September 2007

We have developed a new growth process for GaAs that combines advantages found in several methods currently in commercial use, while at the same time minimizing many of the problems inherent in these presently used processes. The new technique, a form of vertical Bridgman (VB) growth, is capable of producing either doped (semiconducting) or undoped semi-insulating GaAs with very low dislocation density.
Thermodynamic analysis of chemical compatibility of ceramic reinforcement materials with niobium aluminides
Authors: Misra Ajay K.
Published: September 2007

Niobium aluminide-based intermetallic matrix composites are currently being considered as potential high-temperature structural materials. One of the key factors in the selection of a reinforcement material is its chemical stability in the matrix. In this study, chemical interactions between two niobium aluminides, Nb3Al and Nb2Al, and several potential ceramic reinforcement materials, which include carbides, borides, nitrides, and oxides, were analyzed from thermodynamic considerations. Several thermodynamically stable reinforcement materials have been identified for these two matrices.
Infrared properties of the highTcsuperconductor Tl2Ba2CaCu2O8
Authors: Ye H. J. , McCall R. P. , Leng J. M. , Li C. G. , Zhang Yu-Ling , Cao Ning , Zheng Jia-Gi , Xie S. S.
Published: September 2007

We report the infrared reflection and absorption spectra of the highTcsuperconductor Tl2Ba2CaCu2O8in the temperature range from 10 K to 300 K. For temperatures above 100 K, the reflection spectra show only small changes with temperature. Just below 100 K, the reflectance suddenly increases for energies below 210 cm−1with very little change as the temperature is further decreased. Several peaks corresponding to normal modes of the material are observed in both reflection and absorption spectra.
Polytypoids in high Tc thallium based superconducting materials
Authors: Singh A. K. , Imam M. A. , Sadananda K. , Qadri S. B. , Skelton E. F. , Osofsky M. S. , Le Tourneau V. , Gubser D. U.
Published: September 2007

Several high Tc compounds containing Tl (thallium) were prepared starting from different initial compositions. Superconducting properties and the structure were determined for each sample. Electron diffraction and transmission electron microscopy showed the existence of polytypic high Tc compounds with the same a- and b-axes but different c-axis values. The c-axis appears to increase approximately in integral multiples of 0.15 nm with varying composition and is associated with the insertion of Cu–Ca or Cu–Tl layers in each unit cell. Several random stacking faults were also noted, which give rise to diffuse streaking in the electron diffraction pattern.
Microstructure and morphology of surface oxide films on Ti–6A1–4V
Authors: Ask M. , Rolander U. , Lausmaa J. , Kasemo B.
Published: September 2007

The microstructure of anodic oxide films grown on Ti–6A1–4V in H2SO4 was investigated by SEM, TEM, STEM, EDX, and AES, as a complement to a recent surface spectroscopic investigation of the same oxides by XPS, AES, and SIMS. The anodic oxide films are heterogeneous and the texture reflects the duplex microstructure (α and β phases) of the underlying metal. Porous oxide regions are observed with different appearances on α-phase and mixed-phase regions. The oxide films are essentially amorphous in the investigated thickness range 60–300 nm (in contrast to as-grown anodic films on pure Ti), but crystallize to the anatase structure upon annealing. Considerable lateral variation of the V content in the oxide is observed, reflecting the corresponding variation in the underlying metal. The results are compared with a previous, similar investigation of anodic oxides on pure Ti.
Formation process of CuCl2–NiCl2–graphite intercalation compounds
Authors: Inagaki M. , Ohira M.
Published: September 2007

The intercalation reaction of NiCl2 from the mixture of CuCl2 and NiCl2 has been studied by using natural graphite and a stage 1 graphite intercalation compound with CuCl2 at different temperatures from 400 °C to 550 °C for a wide range of residence times up to 8 days. In the mixture of CuCl2 and NiCl2, CuCl2 was found to be intercalated preferentially in the beginning of the reaction. At low reaction temperatures, NiCl2 was intercalated with the increase in heating time, but its content was saturated at a rather low value, NiCl2/C = 25 wt.% after 3 days at 450 °C, for example. At temperatures as high as 550 °C, however, the content of CuCl2, which had been intercalated in the beginning of the reaction, was found to decrease and NiCl2 started to be intercalated by prolonged heating, suggesting the exchange of the former with the latter. From the reactions of a stage 1 CuCl2–GIC with the mixture of CuCl2 and NiCl2, it was clearly revealed that the intercalated CuCl2 was replaced successively by NiCl2 in keeping the stage 1 structure. The saturated content of NiCl2 in respect to graphite depended strongly on the reaction temperature, attaining NiCl2/C = 150 wt.% at 550 °C. As seen by energy dispersive x-ray spectroscopy, NiCl2 intercalation proceeds from the periphery of the host particles. To form a uniform distribution of both intercalates, CuCl2 and NiCl2, it took a long time of residence, more than 3 days at 550 °C.
Self-assembly of octadecyltrichlorosilane monolayers on mica
Authors: Carson George A. , Granick Steve
Published: September 2007

A method is described to deposit a securely attached, self-assembled monolayer of octadecyltrichlorosilane (OTS) on the surface of freshly cleaved muscovite mica. Comparison of the infrared methylene spectra with those of closely packed Langmuir-Blodgett films implies that the surface coverage of the OTS films was a fraction 0.8–0.9 that of films formed by Langmuir-Blodgett (LB) methods. However, LB monolayers are less securely attached to the substrate. The contact angle of water on these self-assembled monolayers remained over 100° for over 24 h and it suffered no noticeable degradation after prolonged reflux in cyclohexane. The method to form an OTS monolayer on mica involves three steps; first, ion exchange of the native K+ ions of cleaved mica for H+ ions; second, control of the quantity of resulting water on the mica surface; third, adsorption and surface polymerization of octadecyltrichlorosilane (OTS) by self-assembly from dilute cyclohexane solution.
Nanocrystalline titanium-magnesium alloys through mechanical alloying
Authors: Suryanarayana C. , Froes F. H.
Published: September 2007

The solid solubility of magnesium in titanium under equilibrium conditions is reported to be extremely small. Mechanical alloying of a mixture of titanium and magnesium powders resulted in the formation of nanocrystalline (10–15 nm in size) grains of Ti–Mg solid solution. This solid solution has a metastable fcc structure with a = 0.426 nm and contains about 3 wt.% (6 at.%) magnesium in it. It is suggested that the fcc structure has formed as a result of the heavy mechanical deformation of the hep structure introduced during milling. High temperature annealing of the metastable solid solution led to its decomposition forming the equilibrium phases, viz., elemental titanium and magnesium.
Neutron Rietveld refinement of the structure of the ternary silicide Ti4Ni4Si7
Authors: Horache E. , Feist T. P. , Stuart J. A. , Fischer J. E.
Published: September 2007

The structure of Ti4Ni4Si7 is studied using powder neutron diffraction and Rietveld analysis. Prccise determination of the crystalline parameters, atomic positions, and temperature factors is achieved. A criterion previously elaborated for the phase TiFeSi2 is used to determine the coordination number. From the arrangement of the atoms in the unit cell and their interbond distances, we find that Si atoms are nearest neighbors to Ti and Ni. An extension of the same criterion shows that Ti–Ni and Ni–Si hybridization is likely to be strong while Ti–Ni hybridization is weak. A comparison with mono- and disilicides is made.
Variation of dislocation morphology with strain in GexSi1−x epilayers on (100)Si
Authors: Kvam E. P. , Maher D. M. , Humphreys C. J.
Published: September 2007

A change in microstructure, including dislocation Burgers vector, length, and behavior, has been observed to occur when the epilayer mismatch is varied in GexSi1−x layers grown on (100) Si. At low mismatches (<1.5%), there is an orthogonal array of very long 60° misfit dislocations. At higher mismatches (>2.3%) there is an orthogonal array of short edge dislocations. At intermediate mismatches (1.5 to 2.3%) there is a mixture of 60° and edge dislocations. The nature of the microstructure has a pronounced effect on the density of threading dislocations in the epilayer, which increase by a factor of ∼60× through a relatively small range of mismatch (1.7 to 2.1%, corresponding to x ranging from 0.4 to 0.5). These morphologies are discussed in the light of recent work on the sources of misfit dislocations. While mechanisms for the introduction and propagation of dislocations at low mismatch have recently been observed and explained, the high misfit case is clearly very different; i.e., surface nucleation seems to be likely in the latter case as opposed to operation of an internal source in the former. A mechanism for edge dislocation formation is proposed.
Nucleation in the multicrystalline Polix silicon ingot
Authors: Andonov P. , Dervin P. , Lay P.
Published: September 2007

An analysis of nucleation from solid nuclei in Polix photovoltaic polycrystalline silicon is reported. The bulk texture is studied by neutron diffraction using a transmission method. The crystallographic orientation is determined for three adjacent samples along the growth axis in the lower central part of the ingot. The size and orientation of all crystallites are obtained in the domains showing different morphologies: a nucleation zone with equiaxed crystallites, an upper domain with columnar structure, and an intermediate part where both types of crystallization are present. The crystallites are counted in terms of their volume in the three parts. The mean grain size increases in the columnar domain, but a pronounced elimination of particular crystalline orientations is not observed as the morphology is changed.
Compatibility of Fe–40Al with various fibers
Authors: Draper S. L. , Gaydosh D. J. , Nathal M. V. , Misra A. K.
Published: September 2007

Chemical reaction can occur at the fiber/matrix interface of intermetallic matrix composites, leading to a degradation of mechanical properties. Fe–40Al matrix composites were fabricated using SiC, B, W, Mo-base, and Al2O3 fibers. Composite samples were heat treated up to 1500 K to study the reaction kinetics, and reaction rates were determined from reaction zone thickness measurements. The Al2O3 and W fibers were found to be compatible with the Fe–40Al matrix, while the Mo-based fibers reacted moderately and the B and SiC fibers reacted severely. Experimental results are compared to theoretical thermodynamic predictions.
High-resolution electron microscopy of metal/metal and metal/metal-oxide interfaces in the Ag/Ni and Au/Ni systems
Authors: Gao Y. , Merkle Karl L.
Published: September 2007

The atomic structures of heterophase interfaces with large misfits (>14% in Ag/Ni and Au/Ni) and with small misfits (∼2% in Ag/NiO and Au/NiO) have been studied by high-resolution electron microscopy (HREM). It is found that all interfaces are strongly faceted on (111) planes. This indicates that (111) interfaces have the lowest interfacial energy in both metal/metal and metal/metal-oxide systems. For the metal interfaces, this also agrees with determinations of interfacial energies by lattice statics calculations. The large misfit of Ag/Ni and Au/Ni interfaces is accommodated by misfit dislocations. Observations of misfit localization by HREM are in good agreement with images derived from computer simulation, based on relaxed structures, obtained in embedded atom calculations. All misfit dislocations at the Ag/Ni and Au/Ni interfaces lie exactly in the plane of the interfaces, while the dislocations at Ag/NiO and Au/NiO interfaces reside at a stand-off distance, 3 to 4 (111)Ag or (111)Au interplanar spacings from the interfaces.
Acoustic emission study of microcracking in 123-type ceramic superconductors
Authors: Richardson T. J. , De Jonghe L. C.
Published: September 2007

Acoustic emission from sintered ceramic YBa2Cu3O7−x (YBCO) superconductor pellets provides a direct measure of microcracking behavior during processing. By detection and statistical analysis of acoustic events, the effects of cooling rates, processing atmosphere, average grain size, additives, and grain alignment on microcracking in YBCO have been studied. The onset temperature and duration of acoustic emission during cooling correlate well with the oxygen partial pressure in the furnace. Rapid changes in oxygen partial pressure at constant temperature produce acoustic emission that is characteristic of microcracking. A reported critical grain size for microcracking in sintered polycrystalline YBCO of about 1 μm has been confirmed. Two superconducting compounds, YSrBaCu3O7−x and LaBaCaCu3O7−x with the 123 structure but with smaller crystallographic anisotropy were also examined. Recommendations are made for minimizing microcracking during processing of superconducting ceramics.
Deuterium clusters in a strained palladium lattice
Authors: Rosato Vittorio , Cleri Fabrizio
Published: September 2007

We report a first observation, of the behavior of clusters of deuterium atoms confined in a lattice vacancy in a Pd crystal, either at equilibrium or under the influence of externally applied lattice strains. Molecular dynamics simulations have been performed on both Pd perfect crystals and Pd crystals with a single vacancy, showing the energetic behavior and the dynamical values of the interatomic distance among deuterium atoms belonging to clusters of different sizes confined within the vacancy. At low temperatures, the smallest interatomic D-D distances are of the order of the D2 molecular bond length and are obtained during the transitory regimes induced by hydrostatic strains applied to the lattice.