bluuebunny/crossref_metadata_2025 huggingface dataset:
Analytic embedded atom method model for bcc metals
Authors: Johnson R. A. , Oh D. J.
Published: September 2007

The requirements for fitting bcc metals within the EAM format are discussed and, for comparative purposes, the EAM format is cast in a normalized form. A general embedding function is defined and an analytic first- and second-neighbor model is presented. The parameters in the model are determined from the cohesive energy, the equilibrium lattice constant, the three elastic constants, and the unrelaxed vacancy formation energy. Increasing the elastic constants, increasing the elastic anisotropy ratio, and decreasing the unrelaxed vacancy formation energy favor stability of a close-packed lattice over bcc. A stable bcc lattice relative to close packing is found for nine bcc metals, but this scheme cannot generate a model for Cr because the elastic constants of Cr require a negative curvature of the embedding function.
Stability of As and B doped Si with respect to overlaying CoSi2 and TiSi2 thin films
Authors: Maex K. , Ghosh G. , Delaey L. , Probst V. , Lippens P. , den hove L. Van , De Keersmaecker R. F.
Published: September 2007

The thermodynamic equilibrium of structures consisting of a thin film silicide (TiSi2 or CoSi2) on doped Si (with As or B) is investigated. Isothermal sections of the ternary phase diagrams for Ti–Si–B, Co–Si–B, Ti–Si–As, and Co–Si–As have been evaluated, indicating the stability of high B concentrations in Si underneath a CoSi2 layer, the instability of high As concentrations in Si underneath a CoSi2 layer, and of B and As concentrations underneath a TiSi2 layer. The obtained thermodynamic predictions agree very well with experimental results (i) on the redistribution of dopants during silicide formation, (ii) on the diffusion of dopants from an ion implanted silicide, and (iii) on the stability of highly doped regions underneath the silicide, both for the case of TiSi2 and CoSi2. It is shown that even though the inaccuracy of reported thermodynamic data is substantial, thermodynamic calculations provide a useful guidance and are consistent with the experimental results.
Formation of a ternary silicide for Ni/Ti/Si (100) and Ni/TiSi2 structures
Authors: Setton M. , der Spiegel J. Van , Rothman B.
Published: September 2007

Phase formation was studied for Ni/Ti/Si and Ni/TiSi2 structures processed by vacuum RTP. Intermetallic compounds Ni3Ti and Ti2Ni form sequentially above 425 °C for metal bilayers Ni/Ti on Si, as Ni diffuses into Ti. When the temperature reaches 550 °C, Si becomes mobile and diffuses into the Ni–Ti compound, resulting in the growth of a ternary phase Ti4Ni4Si7, (V phase). If Ni is in excess with respect to this ternary silicide, a separate layer of Ni silicide grows between the substrate and the V phase, due to the fact that Ni is the main diffusing species. For the case of an excess Ti, the Si atoms are the most mobile species during Ti silicidation. Below 700 °C, TiSi2 grows with a C 49 structure whereas a mixture of TiSi2 C 54 and V phase forms at high temperature, without phase separation in distinct layers. Ni is also a fast diffuser in TiSi2. The activation energy for the diffusion along the grain boundaries of the Ti silicide is about 1.25 ± 0.2 eV. For these Ni/TiSi2 samples too, the same V phase starts to grow at the metal/silicide interface.
A study of 2 MeV oxygen implantation to form deeply buried SiO2 layers
Authors: Grob J. J. , Grob A. , Thevenin P. , Siffert P. , d'Anterroches C. , Golanski A.
Published: September 2007

Oxygen ions were implanted into (100) oriented single crystal Si at energies in the range of 0.6 to 2 MeV at normal and oblique (60°) incidences. Oxygen concentration profiles were measured using the 16O(d, α)14N nuclear reaction for 900 keV deuterons. The experimentally measured oxygen distributions were subsequently fitted to the theoretical profiles calculated assuming the Pearson VI distribution. The distribution moments (Rp, ΔRp, ΔR⊥ skewness, and kurtosis) were deduced as the best fit parameters and compared to the computer simulation results (TRIM 87 and PRAL). Whatever the calculation method, the measured Rp and ΔRp values are close to those predicted by the theory. Deeply buried SiO2 layers were formed using a single step implantation and annealing process. A dose of 1.8 × 1018/cm2 of 2 MeV O+ was implanted into the Si substrate maintained at a temperature of 550 °C. The implanted samples were characterized using the Rutherford backscattering (RBS)/channeling technique and cross-sectional transmission electron microscopy (XTEM). The implanted samples were subsequently annealed at 1350 °C for 4 h in an Ar ambient. The annealing process results in creating a continuous SiO2 layer, 0.4 μm thick below a 1.6 μm thick top single crystal silicon overlayer. The buried SiO2 layer contains the well-known faceted Si inclusions. The density of dislocations within the top Si layer remains lower than the XTEM detection limit of 107/cm2. Between the Si overlayer and the buried SiO2 a layer of faceted longitudinal SiO2 precipitates is present. A localized dislocation network links the precipitates to the buried SiO2 layer.
Properties of carbon films deposited by pulsed laser vaporization from pyrolytic graphite
Authors: Malshe A. P. , Chaudhari S. M. , Kanetkar S. M. , Ogale S. B. , Rajarshi S. V. , Kshirsagar S. T.
Published: September 2007

Amorphous carbon films have been deposited on silicon 〈111〉 and quartz substrates by pulsed ruby laser vaporization from pyrolytic graphite. Depositions have been carried out at different substrate temperatures, and the properties of the deposited carbon films have been studied using IR and UV–VIS transmission, ellipsometry, and laser-Raman spectroscopies. Chemical and electrical resistivity measurements have also been performed. It is shown that the film properties depend critically on the substrate temperature and that at the substrate temperature of 50 °C films with substantial proportion of sp3 hybridized orbitals are obtained.
Raman spectroscopy of nanophase TiO2
Authors: Melendres C. A. , Narayanasamy A. , Maroni V. A. , Siegel R. W.
Published: September 2007

Raman spectra are reported for consolidated nanophase TiO2 particles in their as-compacted state and after annealing at a variety of temperatures up to 1273 K. The Raman-active bands normally observed for the rutile form of TiO2 were present in as-compacted samples having average grain sizes in the range from about 10 to 100 nm. However, significant broadening of these bands was found, which was uncorrelated with initial grain size, but not necessarily with other synthesis-related factors. This broadening decreased upon isochronal annealing at elevated temperatures in air. Based upon these observations, it is concluded that nanophase TiO2 in the as-consolidated state contains significant defect concentrations within the rutile grains and that these intragrain defects and the grain-boundary regions as well have local atomic structures with the rutile symmetry, albeit with some short-range displacements. Some sporadic sample regions containing small amounts (<5%) of the anatase form of TiO2 were also found; these traces of anatase transformed to rutile upon annealing in air at temperatures above 883 K.
Photon emission as a probe of chaotic processes accompanying fracture
Authors: Langford S. C. , Zhenyi Ma , Dickinson J. T.
Published: September 2007

Photon emission accompanying the fracture of an epoxy and single crystal MgO is examined for evidence of deterministic chaos by means of the autocorrelation function, the Fourier transform, the correlation integral of Grassberger and Procaccia, and the fractal box dimension. A positive Lyapunov exponent is also obtained from the epoxy phE data. Each of these measures is consistent with a significant degree of deterministic chaos associated with attractors of relatively low dimension. A typical epoxy fracture surface was analyzed for fractal character by means of the slit island technique, yielding a fractal dimension of 1.32 ± 0.03. The fractal dimensions of the fracture surface and the photon emission data (box dimension) of the epoxy are in good agreement. These observations suggest that fluctuations in photon emission intensity during fracture reflect the production of fractal surface features as they are being produced and thus provide important information on the process of dynamic crack growth.
Synthesis of B2YCu4O8 by anionic oxidation-reduction
Authors: Kourtakis K. , Robbins M. , Gallagher P. K. , Tiefel T.
Published: September 2007

The superconducting phase Ba2YCu4O8 has been synthesized (as a bulk powder) in flowing oxygen. This was achieved by using the anionic oxidation/reduction method (or the SCD method), in which an aqueous solution containing oxidizing (NO−3 salts) and reducing components (e.g., acetate salts) is spray dried into an intimate mixture. At 300 °C, an internal exothermic oxidation/reduction reaction completely converts the spray-dried powder into the corresponding mixed oxides in one reaction step. Because the resultant mixed oxide powder is homogeneous on a fine scale, the final oxide phase can subsequently be produced at lower temperatures and shorter reaction times. Using this method, ∼93–94% phase pure Ba2YCu4O8 (as determined by x-ray diffraction) has been synthesized in flowing oxygen. Pellets of Ba2YCu4O4 fabricated from the mixed oxides were prepared and partially sintered by firing at 800 °C in flowing oxygen for 50 h. Resistivity and magnetic (AC) susceptibility measurements show onset Tc's of 82–84 and 72–73 K, respectively.
Stability of CsCl-type intermetallic compounds under ball milling
Authors: Hellstern E. , Fecht H. J. , Fu Z. , Johnson W. L.
Published: September 2007

We investigated the stability of the CsCl-type compounds AlRu, SiRu, NiTi, and CuEr under heavy deformation by high-energy ball milling. As a result, nanocrystalline intermetallic compounds have been synthesized with a microstructure showing a mixture of highly strained crystallites (5–12 nm) with random orientation. Thermal analysis of the samples indicates stored deformation energies of 5–10 kJ/mole stored by incorporation of lattice defects, chemical disorder and grain boundaries, and an increase of the heat capacity of up to 13%. Chemical disordering of the crystal is monitored by means of the long-range order parameter. The observed decrease in chemical order can be explained by the high grain boundary density of nanocrystalline metals.
Solid-state amorphization reaction by temperature-dependent ion mixing
Authors: Ma E. , Nieh C. W. , Nicdlet M-A. , Johnson W. L.
Published: September 2007

A layer of amorphous Mo–Al alloy is formed by ion-assisted solid-state interdiffusion reaction of a Mo/Al bilayer. Xe ion irradiation at 200 °C enhances the long-range diffusion of the dominant moving species while the formation of equilibrium compounds remains inhibited. The idea of using the temperature-dependent ion mixing at intermediate temperatures to promote a solid-state amorphization reaction opens the possibility of growing an amorphous phase in a system where both compound formation and interdiffusion are difficult
Rapidly quenched Bi-containing high Tc superconducting oxide compositions
Authors: Nassau K. , Miller A. E. , Gyorgy E. M. , Siegrist T.
Published: September 2007

Twin roller quenching from the melt at cooling rates of ∼107 K s−1 was used on several Bi-containing high Tc compositions. Pure glass was obtained in most compositions and the route of crystallization was studied by differential thermal and thermogravimetric analysis, x-ray diffraction, and magnetic measurements. Heating of the glasses produced poorly crystallized short-range-order-only as determined by x-ray diffraction, despite passing through several exotherms, some of which were very strong. This anomalous behavior deserves further investigation. On heating to higher temperatures the expected double-copper-oxide-layer high Tc phase formed sluggishly via intermediate CuBi2O4 and/or single-copper-oxide-layer Bi2(Sr, Cu)2CuO4 phases. Considerable oxygen absorption of about 0.5 O per Cu occurs during these steps, peaking at about 680 °C; some of this oxygen is lost again by the time the high Tc phase forms just below the melting point.
Boriding of Fe and Fe–C, Fe–Cr, and Fe–Ni alloys; Boride-layer growth kinetics
Authors: Brakman C. M. , Gommers A. W. J. , Mittemeijer E. J.
Published: September 2007

Specimens of pure Fe and of Fe-0.8 mass % C, Fe-0.5 mass % Cr, Fe-4.0 mass % Cr, Fe-4.0 mass% Ni, and Fe-10.0 mass% Ni alloys were borided in boriding powder. A boron-compound layer developed consisting of a surface-adjacent “FeB” sublayer on top of an “Fe2B” sublayer. Layer-growth kinetics were analyzed by measuring the extent of penetration of the “FeB” and “Fe2B” sublayers as a function of boriding time and temperature in the range 1025–1275 K. Layer growth is dominated by B diffusion through “FeB/Fe2B”. This diffusion process is of strongly anisotropic nature. Consequently, ragged interfaces occur between the substrate and the boride layers. The depths of the tips of the most deeply penetrated “FeB” and “Fe2B” needles have been taken as measures for diffusion in the easy [001] diffusion directions. Assuming unidirectional B diffusion and parabolic growth, a simple model of layer growth has been given. It accounts for the specific volume difference between “FeB” and “Fe2B”. In contrast with earlier work, the model provides values for the kinetic parameters for growth of each of the phases in the boron-compound layer.
Effect of simultaneous B+ and N+2 implantation on microhardness, fatigue life, and microstructure in Fe–13Cr–15Ni base alloys
Authors: Lee E. H. , Mansur L. K.
Published: September 2007

Microhardness and cantilever beam fatigue measurements were conducted on Fe–13Cr–15Ni base austenitic alloys that were implanted with boron and nitrogen ions either singly or simultaneously. The microstructure of the modified surface layer and dislocation slip modes after fatigue tests were investigated by optical and transmission electron microscopy. Both hardness and fatigue life were improved by ion implantation, but the greatest improvement was achieved when boron and nitrogen were implanted simultaneously. The degree of fatigue life improvement also varied with minor changes in the base alloying compositions: nitrogen was detrimental or ineffective in the presence of titanium, and boron was much more effective in the presence of molybdenum. Comparison of slip band morphology between the compression and tension cycles indicated that implantation improved the reversibility of surface slip and delayed crack initiation.
Metastable phase formation by ion mixing of Nb–Al multilayers
Authors: Pampus K. , Dyrbye K. , Torp B. , Bormann R.
Published: September 2007

The structure of Nb–Al thin films after ion mixing was studied for compositions from 20 to 85 at. % Al as a function of temperature in the range between 40 and 620 K. The phase formation was determined by transmission electron microscopy. At lower temperatures, only supersaturated bcc-solid solution, NbAl, and amorphous phase were found throughout the studied composition range. Besides these phases irradiation at temperatures above 470 K causes the formation of a metastable crystalline compound at an overall composition close to Nb25Al75, and for T = 623 K the equilibrium compound NbAl3 is formed. The other intermetallic phases Nb2Al and Nb3Al have not been observed at any irradiation temperature. Calculations of the Gibbs free energies of the various phases are presented, and the reliability of extrapolations to regions of metastability with respect to temperature and composition is commented on. The phase formation during heavy-ion irradiation is discussed in the context of the calculated free energies and kinetic constraints. For temperatures above 300 K, the attainment of a metastable phase equilibrium between the bcc solid solution and the amorphous phase is proposed due to the influence of radiation enhanced diffusion.
e-beam melted icosahedral Al–Mn surface layers: Microstructures and phason defects
Authors: Follstaedt D. M. , Knapp J. A.
Published: September 2007

The microstructure of icosahedral Al(Mn) in melt-quenched surface alloys has been characterized with TEM. Single-phase alloys with 20 at. % Mn show angular misorientations (up to ∼7°) across internal boundaries radiating from the nucleation centers of grains. Lowering the Mn content results in fcc Al at grain boundaries and between branches of the icosahedral grains; the presence of Al-rich liquid around growing grains apparently eliminates the misorientations seen with 20 at. % Mn. The grains have a dense distribution of dislocation-like defects, but have sufficient orientational order to exhibit well-defined diffraction contours. Weak reflections in diffraction patterns from small areas (0.7 μm) of grains exhibit deviations from exact icosahedral symmetry positions which were analyzed to evaluate a phason strain tensor and characterize misordering defects in the quasicrystalline structure. A negative matrix element (m ≍–0.05) for a tensor axis along a twofold icosahedral axis accounts for most of the distortions in these patterns and those from other melt-quenched icosahedral Al(Mn) and Al(Mn, Si) alloys; this dominant element lowers the symmetry to D2. This axis is nearly perpendicular to the growth direction, which suggests that the misordering is due to rapid growth during the quench; however, the D2 distortion may also result from an elastic instability predicted for icosahedral phases. Distortions in Al(Cr, Ru) can be fit with a similar tensor but with m>0. Thus the phason character identified here may apply to other aluminum-transition metal icosahedral phases as well.
Structurally-induced elastic anomalies in a superlattice of (001) twist grain boundaries
Authors: Wolf D. , Lutsko J. F.
Published: September 2007

It is suggested that the “supermodulus effect” observed for composition-modulated strained-layer superlattices may arise from the presence of the structurally disordered solid interfaces and not necessarily from electronic-structure effects. The latter are excluded by investigating the elastic properties of a so-called grain-boundary superlattice in which chemically identical materials are joined to form a three-dimensional superlattice. Both an embedded-atom-method and Lennard-Jones potential are employed in our zero-temperature atomistic calculations of the elastic constants and moduli of such a superlattice. They yield qualitatively similar results which, for large modulation wavelengths, can be represented by a mean-field model in which the interfacial regions are characterized by a set of effective elastic constants which are different from those of the bulk regions. The appearance of a maximum in the biaxial modulus and a minimum in the shear modulus is shown to arise from the interaction between interfaces. It is also shown that such extreme anomalies appear only in the moduli but not in the elastic constants of the grain-boundary superlattice.
Solid-state amorphization, interdiffusion, and ion-beam mixing in Au/Zr and Ni/Zr
Authors: Ding Fu-Rong , Okamoto P. R. , Rehn L. E.
Published: September 2007

Inert-gas markers, Rutherford backscattering, and x-ray diffraction were used to investigate solid-state interdiffusion in Ni/Zr and Au/Zr bilayer films as a function of temperature; microstructural studies during annealing were performed in situ, in a high-voltage electron microscope. Au, in contrast to Ni, is not an anomalously fast diffuser in crystalline Zr. Nevertheless, an amorphous product phase was found in both alloy systems for reaction temperatures  550 K; heterogeneous nucleation of the amorphous phase was observed in Au/Zr. The interdiffusion data reveal two distinct Arrhenius regimes, 330–∼470 K and ∼480–550 K, with quite different apparent activation enthalpies. These thermal interdiffusion results are compared with temperature dependent studies of ion-beam mixing in similar bilayer specimens. This comparison indicates that the enhanced efficiencies observed for ion-beam mixing above ∼480 K result from the as-prepared metastable microstructurc, and are not due to radiation-enhanced diffusion.
The influence of substrate bias on the morphology and charge capacity of rf-sputtered iridium oxide films
Authors: Klein J. D. , Clauson S. L. , Cogan S. F.
Published: September 2007

Iridium oxide films deposited on Ti-alloy stimulation electrode wires by rf sputtering exhibit markedly different surface morphologies and redox capacities in response to variations in applied substrate bias potential. Films deposited with a −20 volt bias were relatively smooth and featureless whereas those sputtered with a +20 volt bias were comprised of closely packed 1 micron long platelets. Intermediate substrate biases revealed a gradual progression from the smooth surface to one sparsely populated with particles to a morphology comprised of tightly packed platelets. The electrochemical properties of the films are strongly dependent on the substrate bias employed during deposition. As the DC bias was increased from −20 volts to +20 volts the anodic and cathodic charge capacities determined by cyclic voltammetry decreased linearly from 36 to 12 mC/cm2.
Electron diffraction and microscopy study of oxygen ordering in YBa2Cu3O7-δ
Authors: Hou C. J. , Manthiram A. , Rabenberg L. , Goodenough J. B.
Published: September 2007

Electron-diffraction techniques have been used to study the ordering of oxygen atoms within the Cu(1)O1-δ plane of YBa2Cu3O7-δ. The results show that ordering along a single axis and ordering along two orthogonal axes both occur and alternate systematically across the composition range 0.06 < δ < 0.75. Where the ordering is restricted to a single axis, it is found to be the crystallographic a axis. Various models of oxygen ordering are proposed to explain the observed diffraction phenomena. In addition to the ordering of oxygen atoms that give the diffuse diffraction spots at (h∗/2,0,0) and (0, k∗/2,0), a second type of ordering reflection that occurs at (h∗/3,0,0) and (0,k∗/3,0) was also observed. These sharper and more strongly streaked diffraction maxima are explained in terms of a defect in the cation sublattice.
Oxygen diffusion in Y1Ba2Cu3O7-δ grains: An experimental study of ozone and oxygen annealing processes
Authors: Song Yi , Chen Xiao-Dong , Gaines James R. , Gilje John W.
Published: September 2007

By monitoring the electrical resistivity of single phase polycrystalline Y1Ba2Cu3O7-δ samples while changing their oxygen content in both ozone and ordinary oxygen environments, we were able to investigate the correlation between their average oxygen content and the diffusion time for oxygen inside the grains. We model the resistivity time dependence as a two-step process and find that this explains our experimental results satisfactorily. From this model we are able to estimate a value of the oxygen diffusion coefficient from our data that agrees well with other measurements. We also conclude that while ozone and ordinary oxygen may have different effects in oxidizing the surface of grains, they show no observable differences in the oxygen diffusion process in the bulk Y1Ba2Cu3O7-δ material.
Determination of the angles between the quasicrystal zone axes of the decagonal phase in Al–Mn alloys
Authors: Mayer J. , Csanady A. , Urban K.
Published: September 2007

The angles between the zone axes of decagonal quasicrystals in two different Al–Mn alloys have been studied by means of precision tilt experiments in a transmission electron microscope. The results confirm that a tenfold symmetry axis is present in the reciprocal lattice. The angles between the principal axes in the decagonal phase do not coincide with those between the corresponding axes in the icosahedral phase. The presence of this distortion is independent of quenching rate and size of the decagonal quasicrystals.
Tensile fracture and fractographic analysis of 1045 spheroidized steel under hydrostatic pressure
Authors: Kao A. S. , Kuhn H. A. , Richmond O. , Spitzig W. A.
Published: September 2007

Void formation in tensile test under hydrostatic pressure is characterized through quantitative metallography, and the fracture mechanism under pressure is analyzed by fractography. Transition of the fracture surface from the cup-and-cone under atmospheric pressure to a slant structure under high pressure is explained on the basis of the void development leading to fracture and the concomitant change in fracture mechanism. The concept of “shear blocks” is introduced to illustrate the features observed on the fracture surface of specimens tested under high pressure. It is postulated that shear blocks evolve to connect the central crack regions with the shear crack initiated on neck surface due to the severe necking deformation under applied pressure.
Monte-Carlo simulations of spinodal ordering and decomposition in compositionally modulated alloys
Authors: Atzmon Michael
Published: September 2007

The early-stage kinetics of interdiffusion in compositionally modulated films have been studied by Monte-Carlo simulations on an Ising lattice in two and three dimensions, using nearest-neighbor interactions. For a negative heat of mixing and below the order-disorder transition temperature, if a short-wavelength modulation is along a direction that is not consistent with long-range order, then its temporal evolution does not follow the Cahn-Hilliard-Cook theory. The modulation amplitude decreases as a function of time, rather than increases; i.e., no one-dimensional spinodal ordering is observed. This disagreement with the theory implies that ordering in three dimensions cannot be described by a one-dimensional theory.
Characterization of ceramic/TA6V titanium alloy brazed joints
Authors: Peytour C. , Barbier F. , Revcolevschi A.
Published: September 2007

Al2O3and Si3N4ceramics were joined to titanium alloy (TA6V) and niobium by active brazing. Chemical reactions between the Cu-40Ag-5Ti braze alloy and these materials were examined, and interfacial reaction products were characterized by scanning electron microscopy and electron probe microanalysis. Results indicate the importance of studying the microstructural features of the joints in order to understand better their effect on the mechanical properties of the interface.
Laser synthesis and crystallographic characterization of ultrafine SiC powders
Authors: Fantoni R. , Borsella E. , Piccirillo S. , Ceccato R. , Enzo S.
Published: September 2007

High purity, ultrafine SiC powders have been produced from gas phase reactants (SiH4, C2H2) in a CO2 laser induced process. The flow reactor designed to operate with a medium power (10–50 W) continuous wave CO2 laser source is described. The mechanism of gas phase reactions involved has been investigated by means of on-line optical diagnostics. Powders produced have been characterized by means of conventional chemical and spectroscopic methods. The x-ray results point out a growth mechanism by coalescence; i.e., whole islands move in the flame to take part in binary collisions, analogously to that observed for particles produced by inert gas evaporation.
The mechanism of hydrolytic polymerization of zirconyl solutions
Authors: Clearfield Abraham
Published: September 2007

Solutions of zirconyl salts can be treated in a variety of ways to precipitate hydrous amorphous ZrO2. The phases formed upon heating these precipitates are described and rationalized based upon a theory of hydrolytic polymerization which had been proposed at an earlier stage.
Structural and optical transformations by laser irradiation of InSb-based thin films
Authors: Solis J. , Rubin K. A. , Ortiz C.
Published: September 2007

The effect of 647 nm Kr-ion laser irradiation on small, 1 μm diameter regions of (In0.43Sb0.57)0.87Ge0.13 and In0.43Sb0.57 amorphous as-deposited thin alloy films was determined. Laser irradiation times of 50 ns-20 μs produced three distinct reflectivity states which differed from the original as-deposited values for both alloys. Both alloys exhibited an amorphous state of increased reflectivity after short pulse (∼100 ns to ∼500 ns) laser irradiation which has not been previously observed. Slightly longer pulses caused surface corrugations to form in these high reflectivity regions. The formation of corrugations was consistent with stress relaxation. Further laser annealing caused crystallization and decreased reflectivity. The addition of Ge increased the minimum time to crystallize by one order of magnitude, inhibited the precipitation of pure Sb crystals, and caused crystallites to nucleate at the spot edge rather than from the center of the spot. Optical, microstructure, and electrical changes were correlated. SEM and TEM showed that regions of similar reflectivity could result from significantly different microstructures.
Refractive index gradient in the surface of a zirconium fluoride glass by exchange with chlorine
Authors: Ko Sen-Hou , Doremus Robert H. , Guo X. S. , Landford William
Published: September 2007

The refractive index of the surface of a zirconium fluoride glass was increased by exchanging fluorine in the glass with gaseous chlorine. The chlorine profile in the glass surface, as measured by Rutherford backscattering, was that expected for diffusion. The refractive index change was measured with ellipsometry, and compared to models of the concentration profile of chlorine.
Fracture mechanisms of quasi-brittle materials based on acoustic emission
Authors: Maji A. K. , Ouyang C. , Shah S. P.
Published: September 2007

Recently acoustic emission (AE) techniques have been used to study crack propagation in materials. The application of these techniques to heterogeneous, quasi-brittle materials such as concrete requires a better understanding of how the signal generated from a microfracture is transformed due to wave propagation and due to the transducer response. In this study, piezoelectric transducers were calibrated using displacement transducers. The validity of an elastodynamic Green's function approach was examined for cement-based materials. The acoustic emission source was characterized using moment tensor analysis. Acoustic emission measurements were analyzed for center-cracked-plate specimens of mortar and concrete. It was observed that, as expected, the dominant mode of cracking was mode I (tensile). However, mode II (shear) and mixed mode cracks also occurred, perhaps due to grain boundary sliding and interface debonding. Microfractures appear to localize prior to critical crack propagation. Mode I cracks generally required more energy release than mode II and a smaller inclusion provided a stronger interface bond than the larger ones.
Thin films of BiSrCaCu oxide prepared by laser evaporation
Authors: Perrière J. , Hauchecorne G. , Kerhervé F. , Rochet F. , Defourneau R. M. , Simon C. , Rosenman I. , Enard J. P. , Laurent A. , Fogarassy E. , Fuchs C.
Published: September 2007

We have prepared BiSrCaCuO superconducting thin films by the pulsed laser evaporation method, using a YAG or an ArF excimer laser. The as-deposited films, which were enriched in Cu with respect to the bulk target composition Bi2Sr2Ca1Cu2O8 (i.e., the “2212” phase), were converted into the superconducting phase by a subsequent high temperature annealing in oxygen. Resistivity measurements showed superconducting transitions near 90 K, and the highest zero resistivity temperature was 83 K. The influence of the deposition parameters (photon wavelength) and annealing conditions (temperature, duration, and atmosphere) have been studied. The composition and in-depth distribution of the various elements were determined by Rutherford backscattering spectrometry (RBS), and the surface morphology was studied by scanning electron microscopy (SEM).
Surface hardening of SUS304 by irradiation with a KrF excimer laser in SiH4 gas ambient
Authors: Sugioka Koji , Tashiro Hideo , Toyoda Koichi , Tamura Eiichi , Nagasaka Keigo
Published: September 2007

Surface hardening of SUS304 resulting from the process of doping and deposition of Si by irradiation of a KrF excimer laser beam in a SiH4 gas ambient is investigated, and variations of the surface hardness are examined for different numbers of laser pulses and the laser fluences. The hardening is due to Si incorporation in high concentration. The continuous distribution of Si atoms across the surface layer suggests that a very high adhesion strength of the deposited Si films can be formed. The specific process for surface modification is referred to as laser implant-deposition (LID).
Reaction-assisted shock consolidation of RSR Ti–Al alloys
Authors: Yu L. H. , Meyers M. A. , Thadhani N. N.
Published: September 2007

A new method for the shock consolidation of hard metallic powders has been successfully tested. This method extends the process developed by Sawaoka and Akashi for the processing of ceramics (U.S. Patent 4,655,830) to metallic powders. Shock-activated reactions between elemental mixtures of niobium and aluminum powders were used to chemically induce bonding between difficult-to-consolidate intermetallic TiAl compound powder particles. The highly exothermic reactions activated by the passage of shock waves form an intermetallic binder phase which assists in the consolidation of the very hard TiAl alloy powders. Shock impact experiments were carried out utilizing a twelve-capsule shock recovery system in which a plane wave generating lens is used for accelerating a flyer plate to velocities of 1.7 and 2.3 km/s. With these impact velocities, sufficient shock pressures are generated in the powders, contained in capsules, to result in shock-induced reactions between the elemental powders of the mix. Fully dense compacts were successfully recovered and were subsequently characterized by optical, transmission, and scanning electron microscopy, x-ray diffraction, and microhardness testing. Transmission electron microscopy revealed both microcrystalline and amorphous regions in the reaction zone. In one instance, the amorphous material crystallized under the heating effect of the electron beam. Shock induced reaction between elemental powders and with the TiAl powders, producing ternary compounds, was also observed.
Melting and possible amorphization of Sn and Pb in Ge/Sn and Ge/Pb mechanically milled powders
Authors: Jang J. S. C. , Koch C. C.
Published: September 2007

Mixtures of Ge–Sn and Ge–Pb powders were ball-milled to form a fine dispersion. After 32 h of milling the average diameter of the hard Ge particles embedded in the Sn (or Pb) matrix was about 10 nm. As the Ge concentration was increased in each system, the melting point,TM, and the enthalpy of fusion, ΔHM, of Sn (or Pb) decreased. Only small changes in ΔTMand ΔHMwere observed after heating cycles in the DSC to above the melting point. The melting endotherm measured by DSC disappeared for Ge-rich compositions (88 and 95 vol.% Ge for Ge–Sn; 93.5 vol.% Ge for Ge–Pb). It is suggested that atomic disorder/melting is nucleated at the Ge/Sn (or Ge/Pb) interfaces and the melting point and enthalpy of fusion decrease as the interfacial area increases. When the Ge volume reaches a value where essentially all the Sn (or Pb) atoms are adjacent to the Ge particle surfaces, the Sn is in a disordered–perhaps amorphous–state such that no melting transition is observed.
Low temperature epitaxial NiSi2 formation on Si(111) by diffusing Ni through amorphous Ni–Zr
Authors: de Reus R. , Tissink H. C. , Saris F. W.
Published: September 2007

Although amorphous alloys are known to be good diffusion barriers, amorphous nickel-zirconium is shown to react with Si at relatively low temperatures. Diffusion of Ni at 350°C through an amorphous Ni–Zr buffer layer leads to the formation of epitaxial NiSi2 on single crystal silicon substrates. Interplay of mobility and thermodynamics is applicable for epitaxial silicide nucleation and growth. Also, a one-step annealing process in oxygen ambient leads to bilayer formation of NiSi2/ZrO2 structures on silicon substrates.
Composition and structure of native oxide on silicon by high resolution analytical electron microscopy
Authors: Kim M. J. , Carpenter R. W.
Published: September 2007

Compositional analysis of thin nanoscale native oxide films formed on {001} silicon wafer surfaces at room temperature was done with an electron energy loss spectrometer coupled to an analytical electron microscope having a field emission source, with better than 4 nm spatial resolution. The electron energy loss spectra show a shift in the threshold onset energy of the Si–L edge of the native oxide from ∼99 eV loss corresponding to pure elemental silicon to ∼105 eV loss, and elemental analysis using the ionization regions of the core loss edges showed the composition to be SiO, within a few percent. Microdiffraction and high resolution electron microscopy (HREM) results showed that the native oxide was completely amorphous, and did not contain detectable nanocrystals. The native oxide can be removed from the Si surface by heating in UHV for a short time at 1000 °C. However, this procedure resulted in the formation of small amounts of a crystalline phase on the Si wafer surface, which was shown to be β-SiC by the same methods.
Structural characterization of damage in Si(100) produced by MeV Si+ion implantation and annealing
Authors: El-Ghor M. K. , Holland O. W. , White C. W. , Pennycook S. J.
Published: September 2007

Buried amorphous layers were produced by implantation of MeV Si+ions in silicon single crystal at room temperature and liquid nitrogen temperature. The damage is characterized structurally both in the as-implanted condition and after post-implantation furnace annealing. Growth of the amorphous layer during room temperature implantation is found to occur by a layer-by-layer mechanism with relatively sharp interfacial transition regions. A wide region ahead of the buried amorphous region extending to the surface is observed to be free of any extended defects. Recrystallization of the damaged region during thermal annealing occurs by solid-phase epitaxial growth at both interfaces. A lower growth velocity is found at the upper interface, which is attributed to a higher hairpin dislocation density grown-in at this interface. Results of irradiation at liquid nitrogen temperature, on the other hand, show that nucleation and growth of the amorphous damage occurs over a wide region and is not confined to the interfacial region. This results in a very diffuse upper interface composed of a mixture of amorphous and crystalline phases. Substantial reordering is observed in this mixed-phase region after 400°C annealing, even though this temperature is too low for normal interfacial solid-phase epitaxial growth. Cross-sectional transmission electron microscopy, as well as Rutherford backscattering spectroscopy, were used in this study.
Hydrogen-alkali-metal-graphite ternary intercalation compounds
Authors: Enoki Toshiaki , Miyajima Seiichi , Sano Mizuka , Inokuchi Hiroo
Published: September 2007

Alkali-metal-graphite intercalation compounds (alkali-metal-GIC's) absorb hydrogen in two ways: physisorption and chemisorption. Hydrogen uptake through the physisorption process occurs at low temperatures below about 200 K in higher stage alkali-metal-GIC's, where hydrogen molecules are stabilized to form a two-dimensional condensed phase in the galleries of the graphite sheets. The concentration of absorbed hydrogen molecules is saturated at a rate of H2/alkali metal atom ∼2. The hydrogen physisorption shows a strong isotope effect and a swelling effect on c-axis lattice expansion. In the case of hydrogen uptake through the chemisorption process, dissociated hydrogen species are stabilized in the intercalate spaces. The activity of the chemisorption increases in the order Cs < Rb < K. The introduction of hydrogen generates a charge transfer from the host alkali metal GIC's to the hydrogen since hydrogen has strong electron affinity. The hydrogenated potassium-GIC's have intercalates consisting of K+-H−-K+ triple atomic layer sandwiches which are inserted between metallic graphite sheets. The inserted two-dimensional hydrogen layer is suggested to consist of H ions with a weakly metallic nature. The superconductivity of the hydrogenated potassium-GIC is also discussed in terms of the change in the electronic and lattice dynamical properties by hydrogen uptake. The hydrogen-absorption in alkali-metal-GIC's is an interesting phenomenon in comparison with that in transition metal hydrides from the point of hydrogen storage. The hydrogen-alkali-metal-ternary GIC's obtained from hydrogen absorption have novel electronic properties and lattice structures which provide attractive problems for GIC research. The studies of hydrogen-alkali-metal ternary GIC's are reviewed in this article.
Strain-free, ultra-high purity ZnSe layers grown by molecular beam epitaxy
Authors: Park R. M. , Rouleau C. M. , Troffer M. B. , Koyama T. , Yodo T.
Published: September 2007

ZnSe layers have been grown by molecular beam epitaxy on high-purity, high-quality ZnSe wafers [(100) oriented] cut from ingots grown by the iodine vapor transport method. Photoluminescence (PL) analysis indicates the homoepitaxial ZnSe material to be of ultra-high purity as well as being strain-free relative to ZnSe/(100) GaAs layers which exhibit in-plane biaxial tension. The 10 K PL spectra recorded from homoepitaxial layers exhibit unsplit free- and donor-bound exciton transitions of comparable intensity together with a strong peak at 2.7768 eV believed to be the so-called Iv transition in relaxed ZnSc. The ultra-high purity nature of the homoepitaxial layers is attributed to the high purity of the substrate material in addition to the use of high purity Zn and Se source materials in this work.
Solid state amorphization reactions in deformed Ni-Zr multilayered composites
Authors: Wong G. C. , Johnson W. L. , Cotts E. J.
Published: September 2007

The mechanisms of metallic glass formation and competing crystallization processes in mechanically-deformed Ni-Zr multilayered composites have been investigated by means of differential scanning calorimetry and x-ray diffraction. Our investigation of the heat of formation of amorphous NixZr1−x alloys shows a large negative heat of mixing (on the order of 30 kJ/mole) for compositions near Zr55Ni45 with a compositional dependence qualitatively similar to that predicted by mean field theory. We find that the products of solid state reactions in composites of Ni and Zr can be better understood in terms of the equilibrium phase diagram and the thermal stability of liquid quenched metallic glasses. We have determined the composition of the growing amorphous phase at the Zr interface in these Ni-Zr diffusion couples to be 55 ± 4% Zr. We investigated the kinetics of solid state reactions competing with the solid state amorphization reaction and found the value of the activation energy of the initial crystallization and growth of the growing amorphous phase to be 2.0 ± 0.1 eV, establishing an upper limit on the thermal stability of the growing amorphous phase.
Amorphization and disordering of the Ni3Al ordered intermetallic by mechanical milling
Authors: Jang J. S. C. , Koch C. C.
Published: September 2007

The ordered fcc intermetallic compound Ni3Al was mechanically milled in a high energy ball mill. The severe plastic deformation produced by milling induced transformations with increasing milling time as follows: ordered fcc → 2; disordered fcc → 2; nanocrystalline fcc + amorphous. The milling time for complete disordering occurred at 5 h for stoichiometric Ni3Al milled at ambient temperature compared to 50 h for the first observation of an amorphous structure. The structural and microstructural evolution with milling time was followed by x-ray diffraction, TEM, hardness, and calorimetry. The major defect believed responsible for inducing the crystalline-to-amorphous transformation is the fine grain boundary structure with nanometer (∼2 nm diameter) dimensions. The calculated interfacial free energy of the grain boundaries is consistent with the estimated free energy difference between the fcc and amorphous phases in Ni3Al.
The molecular wedge in a brittle crack: A simulation of mica/water
Authors: Thomson Robb
Published: September 2007

This paper presents an atomic calculation of the wedging effect which occurs in a brittle crack when molecules of a chemisorbing species of molecules of sufficient size enter the crack mouth. A surface tension develops at the tip of the wedge caused by the difference between the covered and vacuum surface energies. This force draws the chemisorbing molecules toward the crack tip and distorts the crack faces, causing, in turn, a compensating elastic force on the molecules which tends to eject the molecules. We calculate the equilibrium penetration of the wedging molecules and the configuration of the crack and wedge by an atomistic calculation. We simulate mica/water chemistry by means of a simplification of the mica lattice and calculate interactions between the water and mica on the basis of Born–Mayer. Water is found to form a wedge tongue of two or three molecular thicknesses and a length of about 20 molecular distances, which penetrates into the crack tip cohesive zone. When strong wedging action occurs at a crack tip, crack advance near threshold loadings will be limited by molecular diffusion through the wedge tongue.
Structures and electric properties of pitch-based carbon fibers heat-treated at various temperatures
Authors: Hamada T. , Furuyama M. , Sajiki Y. , Tomioka T. , Endo M.
Published: September 2007

Four kinds of pitch precursor fibers possessing different mean sizes of mesophase domains in their transverse sections, but having the same chemical constituents, were prepared by controlling only the spinning conditions. The fibers were thermoset and heat-treated at various temperatures from 600°C to 3300 °C. Lc (002) or d002 vs THT curves showed that pitch precursor fibers with finer transverse microstructures have a lower degree of graphitizability. Further, these four kinds of fibers exhibited systematically different d002 vs Lc (002) curves. The resistivity at room temperature is dependent on d002, being independent of the mean size of the mesophase domains in the transverse section of the pitch precursor fiber. The transverse magnetoresistance at 77 K and 10 kG, (Δρ/ρ)t, also showed a universal dependence on d002 if (Δρ/ρ)t was positive. However, the (Δρ/ρ)t vs d002 curves seemed to be slightly split in the negative region of (Δρ/ρ)t for each of the four kinds of fibers.
Quantitative investigation of titanium/amorphous-silicon multilayer thin film reactions
Authors: De Avillez R. R. , Clevenger L. A. , Thompson C. V. , Tu K. N.
Published: September 2007

Growth of amorphous-titanium-silicidc and crystalline C49 TiSi2 in titanium/amorphous-silicon multilayer films was investigated using a combination of differential scanning calorimetry (DSC), thin film x-ray diffraction, Auger depth profiling, and cross-sectional transmission electron microscopy. The multilayer films had an atomic concentration ratio of 1Ti to 2Si and a modulation period of 30 nm. In the as-deposited condition, a thin amorphous-titanium-silicide layer was found to exist between the titanium and silicon layers. Heating the multilayer film from room temperature to 700 K caused the release of an exothermic heat over a broad temperature range and an endothermic heat over a narrow range. The exothermic hump was attributed to thickening of the amorphous-titanium silicide layer, and the endothermic step was attributed to the homogenization and/or densification of the amorphous-silicon and amorphous-titanium-silicide layers. An interpretation of previously reported data for growth of amorphous-titanium-silicide indicates an activation energy of 1.0 ± 0.1 eV and a pre-exponential coefficient of 1.9 × 10−7 cm2/s. Annealing at high temperatures caused formation of C49 TiSi2 at the amorphous-titanium-silicide/amorphous-silicon interfaces with an activation energy of 3.1 ± 0.1 eV. This activation energy was attributed to both the nucleation and the early stages of growth of C49 TiSi2. The heat of formation of C49 TiSi2 from a reaction of amorphous-titanium-silicide and crystalline titanium was found to be –25.8 ± 8.8 kJ/mol and the heat of formation of amorphous-titanium-silicide was estimated to be –130.6 kJ/mol.