bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of tensile stress on the annealed structure of a metallic glass
Authors: Vianco P. T. , Li J. C. M.
Published: September 2007

The low-temperature (120°–245°C) structural relaxation of Metglas¯ 2826B (Ni49Fe29P14B6Si2) amorphous alloy was investigated for samples subjected to a tensile stress in the range of 20–400 MPa during annealing. The stress-annealed samples demonstrated a much smaller increase of microhardness than was observed in similarly annealed ribbons without a stress. Further heat treatment of the stress-annealed specimens, this time without the stress, was capable of increasing the microhardnesses of only some ribbons to values equal to those of samples similarly heat treated initially without a stress. An additional exothermic peak in the differential scanning calorimetry (DSC) thermograms of the stress-annealed specimens indicated the presence of a more disordered structure at room temperature, which was found to correlate with the lower microhardness values. Otherwise, those artifacts of the DSC thermograms that were characteristic of samples annealed without a stress were still present in the stress-annealed ribbons. No effect on the crystallization temperature was noted but the glass transition temperature was increased in the stress-annealed case with respect to values attained when the stress was absent during heat treatment. A reduction in the degree of embrittlement of those samples annealed with a tensile stress was a further indication of more disorder in the stress-annealed ribbons.
Densification process in undoped zinc oxide
Authors: Kobayashi K. , Dordor P. , Bonnet J. P. , Salmon R. , Hagenmuller P.
Published: September 2007

An original preparation of ZnO is described where the powder is obtained by precipitaton from zinc nitrate in aqueous solution followed by a calcination. Scanning electron microscopy reveals a close relation between the initial density and the morphology of the powder, the latter being also strongly dependent on the calcining conditions. The investigation of the densification process shows the extreme sensitivity of the final density to sintering conditions. A physical model based on the competitive influences of shrinkage and evaporation successfully describes the observed phenomena.
Liquid-phase sintering of barium titanate with lithium fluoride
Authors: Potin A. , Ravez J. , Bonnet J. P.
Published: September 2007

Lithium fluoride added to BaTiO3 as a sintering additive can be worthwhile for realizing cheap multilayer ceramic capacitors. Such an addition leads to a decrease both in sintering and Curie temperatures from 1400°–930°C and from 120°–10°C, respectively. A transmission electron microscopy (TEM) study of thin samples quenched from intermediary stages has shown the microstructure of the sintered material to be complex. A liquid-phase sintering, mechanism occurring at temperatures slightly lower than 730°C has been proposed. Volatility accounts for partial or complete disappearance of the liquid during heat treatments.
Crystallization of ThSiO4 from structurally and/or compositionally diphasic gels
Authors: Vilmin Gabriel , Komarneni Sridhar , Roy Rustum
Published: September 2007

The use of structurally and/or compositionally diphasic gels has been shown earlier to have profound effects in lowering the crystallization temperature and enhancing densification in several systems. In this study of solid-state epitaxy the ThSiO4 composition has been investigated. The use of compositional diphasicity allows one to lower the huttonite (β-ThSiO4) crystallization temperature by as much as 200°C. Using crystalline thorite (α-ThSiO4) as second phase nuclei partially stabilizes this phase (hypothetically metastable), while the introduction of huttonite nuclei completely inhibits the formation of the α form, which appears in detectable quantities in the unseeded precursors. This confirms the structural (epitaxial) control of the solid-state reaction. The use of structural or compositional nanoheterogeneity clearly helps to lower the crystallization temperature and to stabilize a particular polymorph in ccramics processed by the sol-gel route, and this principle can certainly be utilized more generally.
Aperiodic stepwise growth model for the velocity and orientation dependence of solute trapping
Authors: Goldman L. M. , Aziz M. J.
Published: September 2007

An atomistic model for the dependence on interface orientation and velocity v of the solute partition coefficient k during rapid solidification is developed in detail. Starting with a simple stepwise growth model, the simple continuous growth model result is obtained for k(v) when the growth steps are assumed to pass at random intervals rather than periodically. The model is applied to rapid solidification of silicon. Crystal growth at all orientations is assumed to occur by the rapid lateral passage of (111) steps at speeds determined by the interface velocity and orientation. Solute escape is parametrized by a diffusion coefficient at the edge of the moving step and a diffusion coefficient at the terrace, far from the step edge. The model results in an excellent fit to data for the velocity and orientation dependence of k of Bi in Si.
Reply to comments of F. F. Morehead
Authors: Fair Richard B.
Published: September 2007

The expression used for electric field in the article being discussed is not appropriate for the experimental conditions, but there is a built-in electric field present that affects Sb diffusion. Thus the calculated values of D=i are reasonably within the experimental error of the data on which the calculations are based. Our conclusions that V− vacancies do not play an important role in Sb diffusion and that Sb–B pairs are responsible for retarded Sb diffusion in high-concentration B-doped Si are supported.
Contrast enhancements due to superconductivity in scanning electron microscopy from Y1−x BaCu2+x Oy high Tc superconductors
Authors: Millman W. S. , Ulla M. , Han Zhilan , Maclaurin M. , Tonner B. P.
Published: September 2007

Microscopy and x-ray emission analysis have been performed on Y–Ba–Cu–O superconductors as a function of temperature using a scanning electron microscope equipped with a low-temperature stage. The samples exhibit a superconducting phase transition above liquid nitrogen temperature as determined by resistivity measurements and a demonstrated Meissner effect. Strong contrast changes in the micrographs due to the superconducting phase transition are found, which have been used to identify normal and superconducting regions of the sample. These samples are found to consist of particles with superconducting shells surrounding a nonsuperconducting core.
An electron microscopic study of a rapidly solidified Al-5 wt. % Co alloy
Authors: Van Tendeloo G. , Menon Jyothi , Suryanarayana C.
Published: September 2007

An Al-5 wt. % Co alloy has been rapidly solidified from the liquid state, resulting in the formation of a slightly supersaturated aluminum solid solution and the monoclinic Al9Co2phase. High-resolution electron microscopy and electron diffraction have been extensively employed to characterize the as-solidified as well as the annealed foils. The high-resolution micrographs of the as-solidified solid solution showed the presence of clustering andD1a- type ordering on an extremely small microscopic scale. Annealing at temperatures above 623 K resulted in the precipitation of Al9Co2in the form of platelets arranged in a Widmanstätten pattern. Orientation relationships between the Al9Co2platelets and the matrix have been established. Guinier-Preston zonelike platelets on close-packed {111} planes, however, have also been observed upon electron irradiation in the annealed alloys, presumably due to the enhanced diffusivity of cobalt atoms.
Investigation of the dielectric and thermal properties of sintered Li1−x Ta1−3x Ti4xO3 solid solution ceramics
Authors: Deb K. K.
Published: September 2007

A series of twelve Li1−x Ta1−3x Ti4xO3 solid solution ceramics was sintered following the low-temperature metal alkoxide routes. Measurements of dielectric constants, dielectric losses, and heat capacities as functions of temperature and frequency are presented. The Curie temperature (Tc) in comparison to LiTaO3 was reduced from 620°C to about 358°C in the Li0.91 Ta0.73 Ti0.36O3 ceramics. The heat capacity data showed no dependence on the bulk density or grain size. Also the mechanical properties of the undoped crystals appear to stay undisturbed. On the other hand, an apparently abnormal dependence of the Curie temperature on density was found. Heat capacity and dielectric loss decreased while the dielectric constant increased with the addition of Ti+4 doping in the LiTaO3 structure. Overall, a good detector performance might indeed be obtained from these ceramic materials if they can be poled reproducibly.
Effect of halide ions on Li+ ionic conductivity in oxyhalo-borate glasses
Authors: Wang Yuhu , Osaka Akiyoshi , Miura Yoshinari , Takahashi Katsuaki
Published: September 2007

The electrical conductivity has been measured for the glasses in the system B2O3–Li2O–Li2X2(X = F, Cl, Br), where Li2O is replaced by Li2X2 with Li content kept constant. The conductivity decreases for F glasses and increases for Cl and Br glasses with increasing halide ion content, which is caused by the increase in the activation energy for F glasses and the decrease for Cl and Br glasses. The analysis of the activation energy in terms of the Anderson-Stuart model indicates that, with the increase in LiX, Δ Eb, (the electrostatic energy) increases for F glass, whereas ΔEs (the strain energy) and ΔEb decrease for Cl or Br glasses. The effects of F− and other halide ions are attributed to a stronger interaction between Li and F, and to expanded and weakened glass network due to accommodation of Br− and Cl− ions.
Theory of chemically induced kink formation on cracks in silica. I.3-D crack Green's functions
Authors: Thomson Robb , Tewary V. K. , Masuda-Jindo K.
Published: September 2007

This article is the first of a pair on a theory of chemically assisted fracture. It includes the Green's function analysis for a three-dimensional crack with a kink on it. Equations are developed for the activation energy for the motion and nucleation of such kinks using information to be found in the second article regarding the force laws appropriate for water attack of silica. The most general conclusion is that lattice trapping barriers to crack motion (including chemical effects) are associated with a narrow core region of the crack, which is in turn connected to the nature of the interatomic force laws of the material (including the modifications of these force laws induced by chemical reactions). Further, it is found that the force law must have a severely “snapping” characteristic in order to assure a narrow core, a feature not to be expected except under certain types of external chemical attack of the crack. Additional results are that the energy to nucleate a kink pair in silica under water attack is in the neighborhood of 2 eV and that the motion energy is of order 0.1 eV. Motion energies are expected to be considerably smaller than formation energies in general.
Defect model of melt-grown GaAs
Authors: Morrow Richard A.
Published: September 2007

A phenomenological model is developed to treat defect properties and defect chemistry in melt-grown GaAs. Defects are characterized by their charge and their stoichiometric signature (local deviation from stoichiometry); no specific atomic structure is assumed for them. Good fits to existing data are obtained for the room-temperature concentrations of the midgap donor (EL2), an unknown double acceptor, electrons, and holes as functions of melt composition. In obtaining these fits the role and importance of the (unknown) GaAs solidus is emphasized and it is demonstrated that many popular models of EL2 are consistent with the data analyzed. The model is extended to account for the observed decrease of EL2 concentration with increasing boron or silicon doping concentration, and here again attention is clearly focused on certain unknown information concerning the crystal formation process. Fundamental parameters in the model whose values are estimated through fits to the data include (1) the equilibrium constant in the defect reaction between EL2 and the double acceptor, (2) the equilibrium constant in the defect reaction between EL2, BAs and BGa, (3) the fractions of boron and silicon taken up on Ga sites during the crystal formation process, and (4) the proportionality factor between the melt composition and the crystal composition.
Thin-film synthesis of the high-Tc oxide superconductor YBa2Cu3O7 by electron-beam codeposition
Authors: Naito M. , Hammond R. H. , Oh B. , Hahn M. R. , Hsu J. W. P. , Rosenthal P. , Marshall A. F. , Beasley M. R. , Geballe T. H. , Kapitulnik A.
Published: September 2007

The successful synthesis of high-Tc YBa2Cu3O7 films by means of electron-beam codeposition are reported. Several important growth parameters have been surveyed in a preliminary way. The substrates investigated include Al2O3, ZrO2, MgO, and SrTiO3, The films were characterized by resistivity measurements, x-ray diffraction, microprobe, and Rutherford backscattering analysis. Some TEM and critical current density studies were also carried out. The best results to date have been obtained on SrTiO3 substrates with which polycrystalline epitaxial growth has been achieved. Resistive superconducting transitions with zero resistance at 89.5 K and a 2 K width have been observed in these films.
The role of boundaries in the superconducting properties of sintered YBa2Cu3O7−x
Authors: Camps R. A. , Evetts J. E. , Glowacki B. A. , Newcomb S. B. , Stobbs W. M.
Published: September 2007

Since the discovery of high critical temperature superconducting oxides, an important factor potentially limiting their technological application has been the typically low value of the critical current density that they can carry. Here are presented the results of a transmission electron microscopy study of sintered YBa2Cu3O7−x aimed primarily at the characterization of those elements of the microstructure responsible for the poor current-carrying capacity in the superconducting state. The observations are further discussed in relation both to the flux pinning characteristics required and to the prospect for controlling the microstructure in order to optimize the properties of this material for technological development.
Phase relations in pseudobinary Ba2M Cu3O6.5 + x (M = Y,Gd)
Authors: Hauck J. , Bickmann K. , Zucht F.
Published: September 2007

Studies show that Ba2M Cu3O6.5 + x can be treated like a pseudobinary oxide MeOx with metal framework Me = Ba2YCu3O6.5. An approximate phase diagram was derived by thermogravimetry, differential thermal analysis, strain gauge measurements, and annealing at varied oxygen partial pressure.
Interaction of a 10 eV silicon beam with the Si(111) surface: A molecular dynamics study
Authors: Dodson Brian W. , Taylor Paul A.
Published: September 2007

The interaction of a low-energy silicon beam with a silicon substrate has been simulated. The combined effects of vibrational lattice excitation and of covalent binding have been included for the first time by using a molecular dynamics technique and an empirical potential that accurately describes the covalent Si–Si interactions. A 10 eV silicon beam was directed normal to a silicon (111) substrate. Sticking ratio, penetration depth, substrate structure, and vibrational excitation of the substrate are quantitatively determined. The special features of such low-energy beam deposition relative to thermal deposition processes are discussed.
Crystallinity of rf-sputtered MoS2 films
Authors: Lince Jeffrey R. , Fleischauer Paul D.
Published: September 2007

The crystallinity and morphology of thin, radio-frequency (rf) -sputtered MoS2 films deposited on 440C stainless steel substrates at both ambient (∼70°C) and high temperatures (245°C) were studied by scanning electron microscopy (SEM) and by x-ray diffraction (Read thin-film photography and 0−20 scans). Under SEM the films exhibited a “ridgelike” (or platelike) formation region for thicknesses between 0.18 and 1.0 μm MoS2. X-ray diffraction was shown to give more detailed and accurate information than electron defraction, previously used for elucidating the structure of sputtered lubricant films. Read thin-film x-ray diffraction photographs revealed patterns consistent with the presence of polycrystalline films and strong orientation of the MoS2 crystallites. Correlation of those patterns with 0−20 scans of the films indicated that the basal planes of the MoS2 crystallites [i.e., the (001) planes] were perpendicular to the substrate surface plane, and that various edge planes [i.e., the (h k 0) planes] in the individual crystallites were parallel to the surface plane, in agreement with previous observations of thinner films. Sliding wear caused the crystallites to orient with their basal planes parallel to the surface plane. The crystallite lattices in all films in this study were shown to exhibit compressive stress (∼ 3%–5% with respect to natural molybdenite) in the direction perpendicular to the (h k 0) planes, and the worn films were expanded (i.e., exhibited tensile stress) perpendicular to the (001) plane. In addition, the shapes of the x-ray diffraction peaks were strongly influenced by the presence of oxygen impurities and/or sulfur vacancies in the MoS2 lattice, indicating that x-ray diffraction may provide a simple quality-control test for the production of a film with optimum lubricating properties.
Graphite-alkali metal-heavy metal ternary compounds: Synthesis, structure, and superconductivity
Authors: Lagrange Philippe
Published: September 2007

The main results regarding the synthesis of the graphite intercalation compounds containing a metallic alloy (constituted by an alkali metal and a heavy metal: mercury, thallium, or bismuth) are reported. The synthesis conditions are described: reaction temperatures, compositions of the reactive alloys, and reaction mechanisms. The differences of behavior of the various alloys with respect to the graphite are explained. The structure of the ternary phases is described, and their superconducting properties are reported.
Kinetics of surface area changes in glasslike carbon: reanalysis of the data
Authors: Lachter Jalil , Bragg Robert H.
Published: September 2007

Previous small-angle scattering measurements of the kinetics of changes of the surface area of pores in an annealed glasslike carbon have been reanalyzed in terms of the Lifschitz, Slyozov, and Wagner coarsening theory in which the surface area is used to obtain an average spherical pore size. When the data are corrected for the transient heat-up time to reach the annealing temperature, the reanalysis leads to an activation energy of 75 ± 7.2. kcal/mol, in agreement with recent results (75 ± 3.6 kcal/mol) obtained using direct measurements of changes in pore size as determined by the pore radius of gyration. The best value for the activation energy for vacancy migration in carbon is therefore estimated at 75 ± 3.2 kcal/mol.
Structures and physical properties of carbon fibers from coal tar mesophase pitch
Authors: Hamada T. , Nishida T. , Sajiki Y. , Matsumoto M. , Endo M.
Published: September 2007

Carbon fibers having various types of structures were prepared by spinning coal tar mesophase pitch, followed by thermosetting and heat treatment at high temperature. Two kinds of spinning—spinning with stirring the pitch above a capillary and without stirring—have been tried to form pitch fibers from coal tar mesophase pitch. Carbon fibers obtained from mesophase pitch and spun without stirring have a radial transverse structure where the graphite layers are arranged radially in the transverse cross section of the fibers. Carbon fibers made with a stirring system can have random, onion, and a novel “quasionion structure” by changing the spinning conditions. Carbon fibers spun with stirring are less graphitizable than those spun without stirring. No separation of the ten diffraction bands into 100 and 101 peaks and no appearance of a 112 peak were observed by x-ray diffraction when the fibers were heat treated at 2700°C, whereas carbon fibers spun without stirring show clear evidence of graphitization by heat treatment at 2700°C. Transverse magnetoresistance effects at 77 K, (Δρ/ρ)t have been measured to characterize the structure of the carbon fibers. The carbon fibers spun with stirring and heat treated at 2500°C generally exhibit a negative transverse magnetoresistance effect, whereas the carbon fibers spun without stirring exhibit a positive magnetoresistance. Good correlations are found among d002, Lc (002), transverse magnetoresistance, and resistivity at room temperature of carbon fibers spun under various conditions and heat treated at 2500°C. The tensile strengths (TS) of carbon fibers that are less graphitized are higher than those of carbon fibers with a higher degree of graphitization if tensile moduli (TM) are almost constant.
Optical transmission of graphite and potassium graphite intercalation compounds
Authors: Zhang J. M. , Eklund P. C.
Published: September 2007

The results of an optical transmission study through ∼ 1000 Å thick flakes of graphite and the potassium graphite intercalation compounds KC8 and KC24 in the photon energy range of 1–5.5 eV are reported. Structure in the transmission spectra associated with the Drude minimum, the threshold for inter-π-band and the graphitic M-point absorption are observed. The transmission spectra are found to be consistent with the dielectric function ∊(ω) obtained from previous reflectance studies. The influence of the electronic absorption in the quasi-two-dimensional carbon π bands on the general form of the transmission spectra of graphite intercalation compounds is also shown.
Fluoropolymer barriers to stress corrosion in optical fibers
Authors: Klinger Liliana , Griffith James R.
Published: September 2007

Some recent studies on fluoropolymers that exhibit substantially increased barrier properties against the migration of moisture when compared to conventional fiber coating materials, and as a result may offer unique advantages for use in optical fiber applications, are reported. Moisture vapor permeability studies ofcis-transfluoropolyol polyacrylate indicate that it can offer up to an order of magnitude reduction in H2O permeability relative to commercially available UV curable acrylate coatings. Experimentally determined permeability coefficients were of the order of 10−12g cm−2s−1cm/mm Hg and diffusion coefficients were of the order of 10−10cm2s−1. In addition to improved barrier properties, incorporation of fluorine into the polymer structure lowers the index of refraction of the polymer enabling a material to be tailored for use as a cladding in conjunction with silica glass. The index of refraction ofcis-transfluoropolyol polyacrylate was determined, using index matching oils, to be 1.437 (lower than the silica core) permitting its use as a cladding. Other material parameters relevant to fiber coatings have also been measured. The elastic modulusEof the material was determined to be 1010dyn/cm2with a slow drop to 108dyn/cm2at the glass transition temperature ∼45°C. In addition, an improved formulation ofcis-transfluoropolyol polyacrylate is presented that allowed in-line coating of the optical fiber in an oxygen environment.
In situ capacitance studies of thin polymer films during compressed fluid extraction
Authors: Ziger David H.
Published: September 2007

Supercritical and compressed liquid CO2 extraction of thin solid organic films was studied using in situ capacitance monitoring. Parallel plate and fringe field capacitors were fabricated using organic films as principal dielectric media. Supercritical and compressed liquid CO2 extraction of these films at 100 bar and 30–45°C caused compositional and physical changes in the host polymer matrix that were correlated to capacitance history. As a result of these studies, film damage during extraction was attributed to explosive decompression of the CO2 solvent. In addition, a mechanism for supercritical fluid extraction of nonvolatile solutes from thin films, which is consistent with these capacitance measurements, is discussed.
Precipitation phenomena in high-dose iron-implanted silica and annealing behavior
Authors: Perez A. , Treilleux M. , Capra T. , Griscom D. L.
Published: September 2007

The effects of high-dose iron implantation into high-purity fused silica have been investigated by conversion-electron Mössbauer spectroscopy, transmission electron microscopy, Rutherford backscattering, and optical spectroscopy. In addition to isolated Fe2+ ions, samples subjected to doses of 4⊠1016 and 6 ⊠ 1016 ions cm−2 were found to contain homogeneously dispersed, equidimensional, crystalline particles ⋍2 nm, similar to Fe3O4. Precipitated spherical particles of metallic α-Fe⋍4 nm were observed in samples receiving a dose of ⋍ 1017 ions cm−2; as the dose was raised to 2.5 ⊠ 1017 ions cm−2 the mean size of these particles reached ⋍ 30 nm. Annealing in air to 800°C resulted in the growth of acicular grains of α-Fe2O3 ⋍ 20–300 nm. The optical spectra of the implanted layers are compared with the predictions of small particle theory.
Crystal structure modification of the rhombohedral Mu phase in a C-276 alloy
Authors: Ayer Raghavan , Scanlon J. C. , Watkins M. , Vaughn G. A. , Steeds J. W.
Published: September 2007

A new hexagonal crystal structure modification of the rhombohedral Mu phase with A7B6 stoichiometry was observed in a cast alloy C-276. The hexagonal structure was determined to share the same lattice parameter along the a axis, while the dimension along the c axis was such that chex = 2/3crhomb. The space group of the hexagonal phase is suggested to be P 63/mmc and forms by a four-layer ABAB repeat, where A and B represent Zr4Al3 and MgCu2 structures, respectively. Details of the diffraction analysis are presented in this article.
Molybdenum effect on Fe–Cr–Ni-alloy elastic constants
Authors: Ledbetter H. M. , Kim S. A.
Published: September 2007

This study involved the ultrasonic measurement of the polycrystalline elastic constants of six face-centered-cubic Fe–Cr–Ni alloys, nominally Fe–19Cr–12Ni (at. %). In these alloys, Mo content ranged up to 2.4 at. %. Molybdenum lowers the Young and shear moduli, and it raises the Poisson ratio. Against expectation (because it increases volume), Mo raises the bulk modulus. Qualitatively, the results show that Ni raises the bulk modulus and Poisson ratio; but Ni lowers the Young and shear moduli. (Nickel decreases the alloy's atomic volume.) The discussion includes existing models based on 3d-electron theory.
The structure of thin films sputter deposited from a Ba2 Si2TiO8 ceramic target
Authors: Yamauchi H. , White R. J. , Ayukawa M. , Murray T. C. , Robinson J. W.
Published: September 2007

Thin films were sputter deposited from a Fresnoite (Ba2Si2TiO8) ceramic target at substrate temperatures lower than 175°C. The as-deposited thin films were near amorphous with a void network morphology. In spite of the fact that the film compositions were shifted from stoichiometry, x-ray diffraction studies showed that the films crystallized to form randomly oriented Fresnoite grains. The crystallization kinetics were quite sluggish and the resultant activation energy for the crystallization process was 370 ± 30 kJ/mol. Even after annealing for 10 h at 750°C an appreciable amount of amorphous material remained in the thin films. The short-range order in this amorphous material was changed from that of the as-deposited thin films. The overall devitrification kinetics of amorphous Fresnoite thin films at a fixed temperature were represented theoretically by an equation of Tool's type.
Dielectric, piezoelectric, and pyroelectric studies of LiTaO3-derived ceramics sintered at 900°C following the addition of (LiF + MgF2)
Authors: Ye Zuo-Guang , Von Der Mühll R. , Ravez J. , Hagenmuller P.
Published: September 2007

High-densified LiTaO3-type ceramics have been obtained by sintering at 900°C after the addition of small amounts of a LiF + MgF2 mixture. Partial hydrolysis of MgF2 and volatilization of either LiF, LiOH, or Li2O due to hydrolysis and partial substitution of F-ions by (OH−) occur during heat treatment. The ferroelectric Curie temperature Tc increases with the amount of additive. The ceramics have been polarized by heating to Tc and cooling down under a dc field. The piezoelectric and pyroelectric coefficients of such ceramics are approximately 30% and 20% of the corresponding values for a LiTaO3 single crystal.
Oxygen diffusion in La2−x Srx CuO4−y
Authors: Routbort J. L. , Rothman S. J. , Flandermeyer B. K. , Nowicki L. J. , Baker J. E.
Published: September 2007

Diffusion of 18O in the superconductor La2−x Srx CuO4−y has been measured as a function of x (0 to 0.20) between 300 and 500°C at an oxygen partial pressure of ≍ 1 atm. Concentration profiles were obtained using a secondary ion mass spectrometer. The diffusion coefficient decreases with increasing Sr additions from 0.1 to 0.2, primarily because of an increase in activation energy. This result, which is contradictory to the expectation that the diffusion coefficient should increase with increasing vacancy concentration caused by the added Sr, can be explained if oxygen vacancies are bound to Sr clusters. Measurements on samples with x = 0 or 0.05 were unsuccessful, probably because of porosity.
Anisotropic shrinkage of cordierite-type glass powder cylindrical compacts
Authors: Exner Hans Eckart , Giess Edward A.
Published: September 2007

The pronounced anisotropy of shrinkage in the axial and the radial direction, observed in earlier studies on the sintering behavior of cordierite-type glass powder, is investigated in detail. It is shown that a planar orientation of the pore/solid interface exists that gives rise to an oriented shrinkage in the radial direction. The results show that the axial-to-diametral shrinkage ratio increases from a value of 0.3 to approximately 0.7 with increasing density, and that particle size and particle shape of the glass powders have little influence on shrinkage anisotropy.
An investigation of hardness and adhesion of sputter-deposited aluminum on silicon by utilizing a continuous indentation test
Authors: Stone D. , LaFontaine W. R. , Alexopoulos P. , Wu T. -W. , Li Che-Yu
Published: September 2007

The hardness of aluminum films on silicon are measured as functions of depth of the indenter. The films have thicknesses of 0.25,0.5, and 1.0μm. The adhesion between one film and the substrate has been reduced through the prior deposition of a 10 nm layer of carbon. In each case the hardness is found to increase as the indenter approaches the film-substrate interface, but the rate of increase is greater for a film with good adhesion than for one with poor adhesion. It is suggested that this increase results from the constraint on deformation of the film by the substrate. A physical model is proposed whereby the yield stress of the film, σo, and an average effective shear strength τ of the indenter-film and film-substrate interfaces, may be determined from the data.
Interface modifications for improving the adhesion of a-C:H films to metals
Authors: Grill A. , Meyerson B. , Patel V.
Published: September 2007

The adhesion of diamondlike hard carbon films to silicide forming metals was improved by using an interfacial silicon film several atomic layers thick. The use of thicker (> 10nm) silicon layers results in a decrease in the adhesion, probably due to a degradation of the structural integrity by excessive silicide formation.
Growth and characterization of Al–Cu–Li quasicrystals
Authors: Parsey J. M. , Chen H. S. , Kortan A. R. , Thiel F. A. , Miller A. E. , Farrow R. C.
Published: September 2007

The alloy system Al–Li–Cu was investigated extensively over the composition range 5.8–8 Al–Cu–3Li to develop a detailed understanding of the formation and properties of the icosahedral phase, known as T2. Material from the various charges was analyzed by optical and electron microscopy, energy dispersive x-ray analysis, differential thermal analysis, and differential scanning calorimetry. The role of the melt composition and the solidification velocity were found to be crucial in determining the micro- and macrostructure and the existence of the icosahedral phase. A pseudobinary phase diagram for the region around Al6CuLi3, is presented based on these analyses. Based on this phase diagram the largest single icosahedral crystals of Al5.1CuLi3 yet reported, with diameters greater than 1 cm, were produced by Bridgman methods.
Co and Fe substitution in YBa2Cu3O7−δ
Authors: Tao Y. K. , Swinnea J. S. , Manthiram A. , Kim J. S. , Goodenough J. B. , Steinfink H.
Published: September 2007

The limiting substitution of Co or Fe for Cu in the orthorhombic superconducting phase YBa2Cu3O7−δ yields the tetragonal, semiconducting compositions YBa2Cu2CoO7.25 and YBa2Cu2.5 Fe0.5O7.19 after annealing at 400°C and slow cooling in O2. Single-crystal x-ray structural analyses, conductivity and magnetic-susceptibility measurements, and thermogravimetric analysis (TGA) data provide evidence for Co or Fe substitution at the Cu (1) position and intercalation of peroxide ions (O2)2− The two compositions decompose for disintercalation of oxygen beyond O6.7 for Co and O6.4 for Fe when heated in N2. The Cu(2) atoms of tetragonal YBa2Cu4O6 appear to be antiferromagnetic below a Tn ∼ 240 K, which is similar to comparable Cu–O planar nets in stoichiometric La2CuO4.
An examination of organometallic thermal stability and its relevance to low-temperature MOCVD growth of HgCdTe
Authors: Hoke W. E. , Lemonias P. J. , Korenstein R.
Published: September 2007

A well-established stability model for hydrocarbon molecules is reviewed and then applied to organometallic compounds used in the epitaxial growth of HgCdTe films. For hydrocarbon molecules, the strength of carbon-hydrogen bonds is modified by neighboring organic groups. The mechanism for this effect is delocalization of the free radical electronic charge by the neighboring groups. The delocalization effect is present in organometallic compounds and is illustrated for tellurium, mercury, and cadmium compounds. An important application of the delocalization effect is the development of a low-temperature approach for pyrolytic metalorganic growth of HgCdTe.
Physical properties of spinel iron oxide thin films
Authors: Ortiz C. , Lim G. , Chen M. M. , Castillo G.
Published: September 2007

This paper describes the complexity of the spinel iron oxides in thin-film configuration. First, the experimental deposition conditions are determined for the parameters of substrate temperature and oxygen flow such that only the “Fe3O4” phase is formed. Then a study is made of how the structural (grain size, lattice parameter, texture), magnetic (M), and optical (visible and infrared transmission) properties of the films depend on the deposition and postdeposition (air annealing) conditions. The experimental deposition region is defined where the films have the most similar physical properties to bulk Fe3O4 and subsequently, after annealing, to bulk gamma Fe2O3. Finally, a discussion is presented of a model that accounts for the anomalous, low values of the magnetic moment for the samples deposited at room temperature. The model proposes an overpopulation of the iron tetrahedral A sites.
Metastable solid solution extension of mullite by rapid solidification
Authors: Pollinger John P. , Messing Gary L.
Published: September 2007

The melt self-quenching technique has been used to examine the metastable solid solution extension of mullite formed from rapidly solidified Al2O3.SiO2 melts. Increasing melt cooling rates were seen to increase the Al2O3 content of mullite, decrease the amount of mullite precipitating, and decrease the melt compositional range over which mullite forms. The maximum mullite Al2O3 content achieved was 77.3 mol % for cooling rates between 103 and 105 K/s. The highest Al2O3 content mullite also exhibited very similar ao and bo lattice parameters indicating a structure close to tetragonal symmetry (equilibrium mullite is orthorhombic).
Coble creep in a powder-metallurgical nickel aluminide of composition Ni–22.8Al–0.6Hf–0.1B (at. %)
Authors: Schneibel J. H. , Porter W. D.
Published: September 2007

Coble creep, which is controlled by mass transport along grain boundaries, has been identified in a powder-metallurgically prepared nickel aluminide with the nominal composition Ni–22.8Al–0.6Hf–0.1B (at. %). Diffusional creep rates∊as a function of temperatureT, stressσ, and grain sizeLare well described by∊ = 33δbDbΩσ(kTL3), whereδbDb= 3 × 10−6m3s−1× exp [– (313 kJ/mol)/(RT) (δbis the diffusional grain boundary width,Dbis the grain boundary diffusivity,Ris the gas constant, andTis the absolute temperature). The activation energy of 313 kJ/mol is unusually high as compared to that for volume diffusion.
Scanning Auger microscopy analysis of 90 K Y–Ba–Cu–O superconductors
Authors: Cota L. , de la Garza L. Morales , Hirata G. , Martínez L. , Orozco E. , Carrillo E. , Mendoza A. , Albarrán J. L. , Fuentes-Maya J. , Boldú J. L. , Pérez-Ramírez J. G. , Pérez R. , Reyes Gasga J. , Avalos M. , José-Yacamán M.
Published: September 2007

The oxide superconductor Y–Ba–Cu–O is studied using Auger scanning microscopy. The chemical depth profiles of the samples were obtained. It is concluded that two phases are present in the sample, one corresponding to the standard composition and another that is Ba enriched. The first shows a platelet shape and the second a granular appearence that covers the surface of the sample.
Kinetics of the solid-state reaction for the formation of amorphous ZrCo studied by electrical conductance measurements
Authors: Schröder H. , Samwer K.
Published: September 2007

Thin-film reactions of Co with Zr have been studied in the temperature range between 473 and 523 K by electrical conductance measurements and cross-sectional transmission electron microscopy (CS-TEM). The reduction of the electrical conductance during the solid state reaction is explained by formation and growth of an amorphous phase at every Zr/Co interface. For long reaction times the growth of the layer thickness follows a shifted $\sqrt t$ law. For short reaction times the measurements show a linear time law, which is expected for an interface limited reaction.
Oxidation of Pb–2.9 at. % Sn alloys
Authors: Konetzki R. A. , Chang Y. A.
Published: September 2007

The oxidation behavior of a Pb-2.9 at.% Sn alloy was studied using Auger electron spectroscopy (AES) combined with sputter depth profiling. Samples were oxidized in air at temperatures less than 90 °C and at low oxygen pressures at room temperature. The oxide formed on the alloy after oxidation in air has an “oxide inversion layer,” where the amount of preferentially oxidized tin increases with depth into the sample, reaches a maximum, and then decreases to its bulk value. Internal oxidation of tin was also observed in this alloy, probably occurring along grain boundaries. Low pressure oxidation studies indicate that grain boundaries play an important role in the initial development of the oxide.
Effects of crystal bonding on brittle fracture
Authors: White G. S. , Freiman S. W. , Fuller E. R. , Baker T. L.
Published: September 2007

A series of unistructural materials was used to investigate dependence of fracture energy upon crystal bond ionicity. Fracture energy values obtained from indentation measurements were found to agree with those obtained from a simplistic sine function force law calculation. In addition, an indentation procedure to determine whether environments enhance fracture is discussed.
Time-resolved optical studies of oxide-encapsulated silicon during pulsed laser melting
Authors: Jellison G. E. , Lowndes D. H. , Sharp J. W.
Published: September 2007

Nanosecond time-resolved reflectivity and ellipsometry experiments have been performed on (100) Si wafers encapsulated by 5.5–76.2 nm thick thermal oxides, using pulsed KrF (248 nm) laser energy densities sufficient to melt the Si beneath the oxide. Post-irradiation nulling ellipsometry, optical microphotography, and surface profiling measurements were carried out. It was found that the threshold energy density required to melt the Si varies with oxide thickness; this is explained primarily by the reflective properties of the oxide overlayer. The time-resolved reflectivity and ellipsometry measurements show that rippling of the SiO2 layer occurs on the 20–40 ns timescale and results in a decrease in specular reflectivity of the rippled silicon surface beneath. Optical model calculations suggest that pulsed laser annealing through a thick oxide layer results in a damaged near-surface silicon layer (∼ 30 nm thick); this layer contains defects that are probably responsible for the degraded performance of devices.
A discussion of the chemical mixing process forin situpreparation of silicon carbide whiskers in silicon nitride powder
Authors: Yamada Shigehiko , Kimura Shiushichi , Eiichi Yasuda , Tanabe Yasuhiro , Asami Yuuichi
Published: September 2007

To produce a homogeneous composite of silicon carbide whiskers in a silicon nitride matrix, the whiskers were formedin situ. The resulting bulk density was higher than that obtained by a physical mixing of the components using a dispersion procedure. Bending strength at room temperature of thein situformed composite was about the same as that for the physically mixed composite when the small amount of carbon remaining in thein situformed composite was removed.
Hot forging characteristics of transformation-toughened Al2O3/ZrO2 composites
Authors: Kellett B. J. , Lange F. F.
Published: September 2007

Fine grain, transformation-toughened Al2O3/ZrO2 (tetragonal, 5–30 vol. %) composites can exhibit both large deformation strains (80%) and strain rates (> 0.001 s−1) without tearing, cavitating, or cracking during hot forging at 1500 °C. Deformability increases with ZrO2 content. The Al2O3 grains grow, develop a crystallographic orientation, and form column structures with increasing deformation. The deformation behavior of the composite series primarily depends on the content of the ZrO2 phase. In this regard, two deformation regimes were observed that appear to be separated by the volume fraction where the ZrO2 phase might form a connective network.
In situ phase equilibria determination of a manganese ferrite by electrical means
Authors: Yoo Han-Ill , Tuller Harry L.
Published: September 2007

Electrical conductivity and thermoelectric power have been measured for a series of MnZn ferrites as functions of the oxygen partial pressure under high-temperature equilibrium conditions. The isothermal variation of both properties was successfully correlated to the onset of phase transitions at characteristic Po2's. The ferrite 0.482MnO-0.518Fe2O3 was examined in some detail to locate the stability fields of the metallic alloy of iron and manganese, manganowustite, the spinel ferrite, and the hematitelike phase, and to extract the appropriate free-energy data. The results confirmed by x-ray diffraction are in satisfactory agreement with literature data.
Thermodynamics of Te80Ge20 − x Pbx glass-forming alloys
Authors: Battezzati L. , Greer A. L.
Published: September 2007

The specific heat of liquid and solid phases and the heats of crystallization and fusion have been measured by differential scanning calorimetry (DSC) for a series of Te80Ge20 − x Pbx alloys (0≤x≤20). The enthalpy, entropy, and free energy of the undercooled liquid are quantitatively assessed with reference to the crystal phases. The available formulas for computing the free energy of the liquid are compared, and their relative merits are discussed. The glass transition temperature is shown to depend strongly on the ratio of the average excess specific heat of the liquid to the entropy of fusion. An anomaly in the liquid specific heat, which is particularly important for Te80Ge20 and Te80Ge15Pb5, leads to very good glass forming ability for these alloys; this is demonstrated by preparing amorphous samples by means of fluxing.
Hardness of polymeric glasses in the TG region: a-Se
Authors: Kasap S. O. , Winnicka M. , Yannacopoulos S.
Published: September 2007

The Vickers hardness number Hv of a typical glassy inorganic polymer, a-Se, is studied as a function of temperature with the heating rate varied as a parameter from 0.032 to 3 °C/min, over two decades. It is shown that Hv(T), as a function of temperature, goes through a sharp drop in the glass transformation region following the similar drop for the shear modulus G(T) reported previously. By defining an empirical glass transition temperature TG at the inflection point of Hv vs T behavior, the heating rate dependence of TG is examined and interpreted via the kinetic structural relaxation model of glass transformation. It is shown that over the temperature range 36–50 °C the rate of structural relaxation processes controlling the mechanical properties obeys an Arrhenius type of temperature dependence with an activation energy ∼2.75 cV/atom. Furthermore, over the temperature range accessed, the structural relaxation rate seems to follow the viscosity-temperature behavior.
Chaotic dynamics in an atomistic model of environmentally assisted fracture
Authors: Markworth Alan J. , Kevin McCoy J. , Rollins Roger W.
Published: September 2007

A simple atomistic model of a crack tip is used to demonstrate the existence of chaotic motion of crack-tip atoms. The model, which has been developed in detail elsewhere in the literature, consists of a linear chain of four atoms. Nearest neighbors interact via Morse-function potentials, with environment-induced lattice decohesion simulated by reducing the strength of the inner bond. Dynamic calculations are carried out by allowing the two inner atoms to move freely, starting from rest in a given initial configuration, with the two end atoms being held rigidly in place, Under certain conditions, associated with large departures from minimum potential energy, the motion of the inner atoms is shown to be chaotic in a manner that is consistent with the Kosloff-Rice description of chaotic dynamics in a classical Hamiltonian system. Possible implications of the results relative to the fracture of actual materials are discussed.
Use of metastable equilibria for determination of Gibbs energy of solids
Authors: Jacob K. T. , Srikanth S.
Published: September 2007

Attempts are made to measure activities of both components of a binary alloy (A–B) at 650 K using a solid-state galvanic cell incorporating a new composite solid electrolyte. Since the ionic conductivity of the composite solid electrolyte is three orders of magnitude higher than that of pure CaF2, the cell can be operated at lower temperatures. The alloy phase is equilibrated in separate experiments with flourides of each component and fluorine potential is measured. The mixture of the alloy (A–B) and the fluoride of the more reactive component (BF2) is stable, while (A–B) + AF2 mixture is metastable, Factors governing the possible use of metastable equilibria have been elucidated in this study. In the Co–Ni system, where the difference in Gibbs energies of formation of the fluorides is 21.4 kJ/mol, emf of the cell with metastable phases at the electrode is constant for periods ranging from 90 to 160 ks depending on alloy composition. Subsequently, the emf decreases because of the onset of the displacement reaction. In the Ni–Mn system, measurement of the activity of Ni using metastable equilibria is not fully successful at 650 K because of the large driving force for the displacement reaction (208.8 kJ/mol). Critical factors in the application of metastable equilibria are the driving force for displacement reaction and diffusion coefficients in both the alloy and fluoride solid solution.