Showing papers in "Journal of Materials Research in 1987"

Application of the embedded atom method to Ni 3 Al

Abstract: The embedded atom method [M. S. Daw and M. I. Baskes, Phys. Rev. B 29, 6443 (1984) used to calculate phase stability, lattice vibrational frequencies, point defect properties, antiphase boundary energies, and surface energies and relaxations for Ni3Al. The empirical embedding functions and core-core repulsions used by this method are obtained. The equilibrium phases for the Ni-rich half of the composition range of Ni–Al are determined for 1000 K and compared with experiment. The elastic constants and vibrational modes of Ni3Al are calculated and the elastic constants are compared with experiment. The formation energy, formation volume, and migration energies of vacancies are computed, and it is found that the formation energy of vacancies on the Ni sublattice is less than that on the Al sublattice. The (100) antiphase boundary is shown to be significantly lower in energy than the (111) antiphase boundary. The surface energies and atomic relaxations of the low index faces are computed, and it is shown that for the (100) and (110) faces that the preferred surface geometry corresponds to the bulk lattice with the mixed composition plane exposed.

Thin-film synthesis of the high-Tc oxide superconductor YBa2Cu3O7 by electron-beam codeposition

Abstract: 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.

Ni, Pd, and Pt on GaAs: A comparative study of interfacial structures, compositions, and reacted film morphologies

Abstract: The reactions between (100) GaAs and the near-noble metals Ni, Pd, and Pt have been investigated by application of high-resolution transmission electron microscopy (TEM), energy-dispersive analysis of x-rays in the scanning TEM and Rutherford backscattering spectrometry. Emphasis is placed on the evolution of the phase distributions, film compositions, and interface morphologies during annealing at temperatures up to 480°C. The first phase in the Ni/GaAs reaction is shown to have the nominal composition Ni3GaAs. Ternary phases of the type PdxGaAs are also found to be the dominant products of the Pd/GaAs reaction. Conversely, only binary phases result from the Pt/GaAs reaction. These observations are used to construct isothermal sections of the M-Ga-As thin-film phase diagrams. The behavior of a thin (1–2 nm) native oxide-hydrocarbon layer during the Ni/GaAs, Pd/GaAs, and Pt/GaAs reactions is also investigated. Only the Ni/GaAs reaction is noticeably impeded in some regions by this intervening layer. In contrast, the Pd/GaAs and Pt/GaAs reactions tend to mechanically disperse the native oxide layers.

Grazing incidence synchrotron x-ray diffraction method for analyzing thin films

Abstract: A method using synchrotron radiation parallel beam x-ray optics with a small incidence angle α on the specimen and 2Θ-detector scanning is described for depth profiling analysis of thin films. The instrumentation is the same as used for Θ:2Θ synchrotron parallel beam powder diffractometry, except that the specimen is uncoupled from the detector. There is no profile distortion. Below the critical angle for total reflection αc, the top tens of Angstroms are sampled. Depth profiling is controlled to a few Angstroms using a small α and 0.005° steps. The penetration depth increases to several hundred Angstroms as α approaches αc. Above αc there is a rapid increase in penetration depth to a thousand Angstroms or more and the profiling cannot be sensitively controlled. At grazing incidence the peaks are shifted several tenths of a degree by the x-ray refraction and an experimental procedure for calculating the shifts is described. The method is illustrated with an analysis of iron oxide films.

Spark erosion: A method for producing rapidly quenched fine powders

Abstract: Spark erosion is a method for producing fine powders of metals, alloys, semiconductors, and compounds. The technique involves maintaining repetitive spark discharges among chunks of material immersed in a dielectric liquid. As a result of the spark discharge there is highly localized melting or vaporization of the material. The powders are produced by the freezing of the molten droplets or the condensation and freezing of the vapor in the dielectric liquid. Since the powders are quenched in situ, they may be extremely rapidly cooled. Particles can be produced in sizes ranging from 5 nm to 75 μm. The average powder size and production rate depend on the power parameters, material used, and the dielectric liquid.

Crack resistance by interfacial bridging: Its role in determining strength characteristics

Abstract: An indentation-strength formulation is presented for nontransforming ceramic materials that show an increasing toughness with crack length (T curve, or R curve) due to the restraining action of interfacial bridges behind the crack tip. By assuming a stress-separation function for the bridges a microstructure-associated stress intensity factor is determined for the penny-like indentation cracks. This stress intensity factor opposes that associated with the applied loading, thereby contributing to an apparent toughening of the material, i.e., the measured toughness in excess of that associated with the intrinsic cohesion of the grain boundaries (intergranular fracture). The incorporation of this additional factor into conventional indentation fracture mechanics allows the strengths of specimens with Vickers flaws to be calculated as a function of indentation load. The resulting formulation is used to analyze earlier indentation-strength data on a range of alumina, glass-ceramic, and barium titanate materials. Numerical deconvolution of these data determines the appropriate T curves. A feature of the analysis is that materials with pronounced T curves have the qualities of flaw tolerance and enhanced crack stability. It is suggested that the indentation-strength methodology, in combination with the bridging model, can be a powerful tool for the development and characterization of structural ceramics, particularly with regard to grain boundary structure.

Constrained network model for predicting densification behavior of composite powders

Abstract: Inert particles that do not contribute to the densification of a composite powder compact are visualized as located on network sites; the network is defined by the distribution of the particles in the powder matrix. Because the distances between neighboring network sites are not identical, the strain produced by the sintering powder between all inert particle pairs cannot be the same as that for the powder compact without the inert particles. The constrained network model is based on the hypothesis that the densification of the composite will be constrained by the network and will mimic that of the network. The shrinkage of the network, and thus the densification of the composite, is estimated with a periodic network. A distance between the minimum and maximum site pairs within the unit cell defines the distance between site pairs in the random network where the powder between the particles densifies in the same manner as that for the powder without the inert particle. When the particles form a continuous touching network, composite shrinkage and densification is nil. The chosen lattice must also conform to this condition. A simple relation was developed relating the densification behavior of the composite to that of the matrix without the inert particles and the parameter associated with the chosen lattice. By choosing the lattice formed by the tetrakaidecahedron unit cell (volume fraction of particles for a touching network = 0.277), remarkable agreement was achieved for the experimental data concerning the densification behavior of the ZnO/SiC composite system reported by De Jonghe et al. [L. C. De Jonghe, M. N. Rahaman, and C. H. Hsueh, Acta Metall. 34, 1467 (1986)]. The universal nature of this lattice for other composites is discussed with respect to site percolation theory. The application of this concept to powder compacts containing either whiskers or agglomerates is briefly discussed.

Crystallinity of rf-sputtered MoS2 films

Abstract: 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.

The diffusion of hydrogen in silicon and mechanisms for “unintentional” hydrogenation during ion beam processing

Abstract: Experiments are described in which hydrogen is injected into silicon by various techniques and detected by the neutralization of boron acceptor sites. Wet chemical etching is shown to inject protons several microns in a few seconds; this experiment is used to set a lower limit on the diffusivity of hydrogen of ⋍2⊠10−11 cm2/s at 300 K, a number in reasonable agreement with prior estimates deduced by Van Wieririgen and Warmholtz from high-temperature permeation measurements. A number of experiments are reported to elucidate the mechanism for “unintentional” hydrogenation occurring during argon ion bombardment. The data suggest that this effect is caused by bombardment-induced injection of hydrogen from surface H2O/hydrocarbon contaminants.

Recrystallization of ion-implanted α-SiC

Abstract: The annealing behavior of ion-implanted α-SiC single crystal was determined for samples implanted with 62 keV 14N to doses of 55X1014/cm2 and 80X1016/cm2 and with 260 keV 52Cr to doses of 15X1014/cm2 and 10X1016/cm2 The high-dose samples formed amorphous surface layers to depths of 017 μm (N) and 028 μm (Cr), while for the low doses only highly damaged but not randomized regions were formed The samples were isochronically annealed up to 1600°C, holding each temperature for 10 min The remaining damage was analyzed by Rutherford backscattering of 2 MeV He+, Raman scattering, and electron channeling About 15% of the width of the amorphous layers regrew cpitaxially from the underlying undamaged material up to 1500°C, above which the damage annealed rapidly in a narrow temperature interval The damage in the crystalline samples annealed linearly with temperature and was unmeasurable above 1000°C

Crystal chemical incorporation of high level waste species in aluminotitanate-based ceramics: Valence, location, radiation damage, and hydrothermal durability

Abstract: A compilation is made of the crystallographic mechanisms whereby (simulated) radwaste species are incorporated in the “synroc” phases zirconolite, perovskite, hibonite, and “hollandite.” From these data and consideration of the crystal chemical criteria of valence and effective ionic radii, the most probable host phases for transuranic isotopes are identified. The distinction is drawn between incorporation of radwaste species as dilute homogeneous, continuous solid solutions and as heterogeneous, noncontinuous solid solutions. It is shown that compositional variations at the nanometer level are frequently accompanied by the generation of new interstices within extended defects, which are suitable for the location of radwaste. Nuclides that are unstable towards beta and gamma decay lead to transmutation-induced changes in stoichiometry. Crystallochemical mechanisms that minimize structural disruption during the formation of radiogenic species are discussed. Thermally promoted recovery of metamict phases, which were rendered aperiodic by direct atomic displacements of alpha-recoil nuclei, are examined in terms of recrystallization via the hierarchial arrangement of space groups. An evaluation is made of the hydrothermal durability of synroc phases under conditions likely to be experienced by the wasteform should the repository be breached by groundwaters shortly after disposal.

Properties and microstructure of thin LiNbO3 films prepared by a sol-gel process

Abstract: Thin LiNbO3 films were prepared from polymerized sol-gel precursor solutions having various concentrations and water:alkoxide ratios in an effort to investigate the effects of these and other processing variables on the resultant film properties and microstructure. Films deposted on silicon substrates were mostly amorphous when pyrolyzed at 435°C for 30 min. Randomly oriented polycrystalline films having distinctive microstnietures were produced using longer heating times or higher temperatures. All of the films exhibited low refractive indices due to porosity, which was attributed to the low level of hydrolysis water required to produce stable polymeric precursor solutions. When single-crystal LiNbO4 was used as the substrate, epitaxial growth of the film resulted. This ideal case establishes the feasibility of producing epitaxial films via sol-gel processing. All films were characterized by transmission electron microscopy (TEM) and thin-film x-ray diffraction patterns.

Effect of boron on the mechanism of strain transfer across grain boundaries in Ni 3 Al

Abstract: The effect of boron on the mechanism of strain transfer across grain boundaries in Ni/sub 3/Al has been investigated by dynamic recording of events occurring during in-situ straining in the transmission electron microscope. Boundaries in both doped and undoped material can act as effective barriers to dislocation motion, large numbers of dislocations being incorporated into the boundary without any plastic strain occurring in the adjacent grain. In the undoped material, the grain-boundary strain is relieved by the sudden failure of the grain boundary. In the doped material the strain is relieved by the sudden generation and emission of large numbers of dislocations from the grain boundary. This effect may be understood by boron either increasing the grain-boundary cohesion or reducing the stress required to operate grain-boundary dislocation sources, rather than easing the passage of slip dislocations through the grain boundary.

Elastic constants and Debye temperature of polycrystalline Y1Ba2Cu3O7−x

Abstract: Using ultrasonic methods, the quasi-isotropic elastic stiffnesses of void-containing Y1Ba2Cu3O7−x were determined By a composite-material model, these were corrected to the void-free state. From these, the Debye characteristic temperature was calculated. All the elastic stiffnesses fall well below those of polycrystalline BaTiO3, an approximate crystalstructural building block of Y1Ba2Cu3O7−x. The low apparent stiffness may result from oxygen vacancies, which soften interionic forces. Also, it may result from microcracks, which reduce elastic stiffness without lowering mass density.

Structures and physical properties of carbon fibers from coal tar mesophase pitch

Abstract: 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.

Solid-phase epitaxy of ion-implanted LiNbO 3 for optical waveguide fabrication

Abstract: A new technique for successfully fabricating high-quality optical waveguides in LiNbO/sub 3/ is reported. A high concentration of Ti is implanted with the substrate at liquid nitrogen temperature and an amorphous, Ti-rich, nonequilibrium phase is produced in the implanted, near-surface region. Subsequent thermal annealing in water-saturated oxygen atmosphere at up to 1000 /sup 0/C initiates solid-phase epitaxial regrowth onto the crystalline substrate. A high-quality single crystalline layer results that is rich in Ti and has excellent waveguiding properties.

Precipitation phenomena in high-dose iron-implanted silica and annealing behavior

Abstract: The effects of high-dose iron implantation into high-purity fused silica have been investigated by conversion-electron Mossbauer 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.

Mechanical properties of GaAs crystals

Abstract: Mechanical properties of GaAs crystals grown by the liquid encapsulated Czochralski technique and the boat technique are investigated by means of compression tests. Stressstrain characteristics of a GaAs crystal in the temperature range 400°–500°C are very similar to those of a Si crystal in the temperature range 800°–900°C. This seems to reflect the fact that the dislocation mobility in a GaAs crystal in the former temperature range is comparable to that in a Si crystal in the latter temperature range. Dislocations in GaAs crystals are found to be easily immobilized at an intermediate temperature due to gettering of impurities and/or impurity-point defect complexes. In comparison to a Si crystal, the surface of a GaAs crystal seems to involve irregularities that act easily as effective generation centers for dislocations. Thus the magnitude of the yield stress of an aged GaAs crystal is controlled by the surface condition and is not influenced by the density of dislocations involved in the crystal. The socalled steady state of deformation is realized in a GaAs crystal in the deformation stage after the lower yield point as in Si and Ge crystals. Dislocation distributions in a deformed GaAs crystal observed by transmission electron microscopy is very similar to those in deformed Si and Ge crystals.

Microstructural evolution in sintering of ALOOH gels

Abstract: Materials derived by precipitation or polymerization chemistry (e.g., “sol-gel” methods) are usually obtained in noncrystalline or otherwise metastable phases, and transformation to more thermodynamically stable phases generally occurs by a nucleation and growth process. In a fully reconstructive transformation, such as occurs in the alumina system, the activation energy for nucleation may be higher than that for simple short-range diffusion. Hence nucleation frequency can be a controlling factor in the development of microstructurc. The efficacy of seeding as a method of microstructural and phase control in solution-derived or so-called sol-gel materials has been clearly demonstrated for the alumina system. The epitaxial nature of this phenomenon is explored, using the polarizing microscope to follow the crystallographic orientation of the transformed material as the transformation proceeds, showing that this is epitaxial in nature, and that the nucleation frequency in unseeded material is relatively low (∼ 1010 cm−3). The microscope was then used to demonstrate the effect on nucleation frequency of seeding with materials selected to be isostructural, isotypic, and having little or no similarity to the corundum structure. Using these and other methods, the seeding phenomenon in alumina gels is shown to result from epitaxial growth of the stable corundum phase on isostructural or isotypic nuclei in the solid state. This approach is applied to formulate hypotheses for the mechanisms by which some of the previously reported effects of seeding, e.g., enhanced densification and microstructural refinement, can be understood and to formulate a set of generalizations for its potential application to other systems.

Metal site disorder in zinc tin phosphide

Abstract: The optoelectronic properties of the II-IV-V 2 semiconductor ZnSnP2 are studied as a function of the cooling rate of the crystal growth melt. The structure of the material, as studied by x-ray diffraction, is seen to change from chalcopyrite to sphalerite as the cooling rate is increased. Photoelectrochemical measurements show that the bandgap of the material decreases from 1.64 eV for the chalcopyrite to 1.25 eV as the structure approaches sphalerite. The 119Sn Mossbauer spectroscopy shows both an isomer shift and a broadening of the 119Sn resonance as a result of new tin environments produced by disordering of zinc and tin sites at the faster cooling rates. The 31P solid-state nuclear magnetic resonance spectroscopy clearly shows new resonances associated with the additional phosphorus environments produced by metal site disordering. A model based on zinc and tin site exchange with the introduction of compensating donor and acceptor states is proposed and discussed.

Aperiodic stepwise growth model for the velocity and orientation dependence of solute trapping

Abstract: 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.

The use of thermodynamic models in the prediction of the glass-forming range of binary alloys

Abstract: Analytical expressions describing the various thermodynamic phases in binary systems can be derived by the CALPHAD approach, whereby the parameters in these expressions are obtained from fits to measured phase equilibrium data. Extrapolations of these models can be used to predict metastable equilibria. Different thermodynamic models proposed for the Ni--Ti and the Cu--Ti systems are used to calculate the T/sub 0/ curves that are relevant to the prediction of the glass-forming range in rapidly cooled molten alloys. Although all the models give acceptable fits for the equilibrium range for which they were derived, significant differences are found for the metastable regime. The problems that need to be addressed in order to improve the extrapolations into the metastable regime are discussed. Simple thermodynamic considerations predict that the glass-forming range for rapidly quenched alloys exceeds that for amorphous alloys synthesized by isothermal solid-state interdiffusion reactions.

Oxygen content, microstructure, and superconductivity of YBa2Cu3O7−x

Abstract: The influence of preparation conditions and microstructure on the superconductive properties of single-phase poly-crystalline YBa2Cu3O7−x was investigated by electron probe microanalysis, transmission electron microscopy (TEM), and x-ray powder diffraction as a function of temperature in various ambients supplemented by resistivity and susceptibility measurements. Leaching of Ba was observed when samples were brought in contact with water. The TEM imaging revealed that individual grains have an extremely defect-rich outer shell and an inner core with a domain structure with a and b axes interchanged. The transition temperature Tc was found to decrease with increasing quench temperature in the range 400–900°C. The Tc was observed to be linearly proportional to the difference in the orthorhombic cell parameters (b-a). Further implications are discussed.

The crystal structure of YBa 2 Cu 3 O 6 , the x = 1 phase of the superconductor YBa 2 Cu 3 O 7− x

Abstract: Black crystals, displaying a micaccous habit, result from heating the 90 K superconducting phase YBa2Cu3O7−x above 1050°C. The crystals are tetragonal, P4/mmm, a = 3.863 (2) A, c = 11.830 (4) A, Z = 1, ρcal = 6.115 g/cm3. The 184 independent single-crystal reflections were used to refine the structure by least squares to R = 0.0374, wR = 0.0347. The structure contains Cu+ in twofold coordination and corrugated layers of Cu2+–O. The Ba2+ is in eightfold coordination between the Cu+ and Cu2+ layers and Y is in eightfold coordination between the Cu2+ layers.

Solidification of highly undercooled liquid silicon produced by pulsed laser melting of ion-implanted amorphous silicon: Time-resolved and microstructural studies

Abstract: Nanosecond resolution time-resolved visible (632.8 nm) and infrared (1152 nm) reflectivity measurements, together with structural and Z-contrast transmission electron microscope (TEM) imaging, have been used to study pulsed laser melting and subsequent solidification of thick (190–410 nm) amorphous (a) Si layers produced by ion implantation. Melting was initiated using a KrF (248 nm) excimer laser of relatively long [45 ns full width half maximum (FWHM)] pulse duration; the microstructural and time-resolved measurements cover the entire energy density (E 1) range from the onset of melting (at ∼ 0.12J/cm2) up to the onset of epitaxial regrowth (at ∼ 1.1 J/cm2). At low E 1 the infrared reflectivity measurements were used to determine the time of formation, the velocity, and the final depth of “explosively” propagating buried liquid layers in 410 nm thick a-Si specimens that had been uniformly implanted with Si, Ge, or Cu over their upper ∼ 300 nm. Measured velocities lie in the 8–14 m/s range, with generally higher velocities obtained for the Ge- and Cu-implanted “a-Si alloys.” The velocity measurements result in an upper limit of 17 (± 3) K on the undercooling versus velocity relationship for an undercooled solidfying liquid-crystalline Si interface. The Z-contrast scanning TEM measurements of the final buried layer depth were in excellent agreement with the optical measurements. The TEM study also shows that the “fine-grained polycrystalline Si” region produced by explosive crystallization of a-Si actually contains large numbers of disk-shaped Si flakes that can be seen only in plan view. These Si flakes have highly amorphous centers and laterally increasing crystallinity; they apparently grow primarily in the lateral direction. Flakes having this structure were found both at the surface, at low laser E 1, and also deep beneath the surface, throughout the “fine-grained poly-Si” region formed by explosive crystallization, at higher E 1. Our conclusion that this region is partially amorphous (the centers of flakes) differs from earlier results. The combined structural and optical measurements suggest that Si flakes nucleate at the undercooled liquid-amorphous interface and are the crystallization events that initiate explosive crystallization. Time-resolved reflectivity measurements reveal that the surface melt duration of the 410 nm thick a-Si specimens increases rapidly for 0.3E 1 <0.6 J/cm2, but then remains nearly constant for E 1 up to ∼ 1.0 J/cm2. For 0.3 < E 1 < 0.6 J/cm2 the reflectivity exhibits a slowly decaying behavior as the near-surface pool of liquid Si fills up with growing large grains of Si. For higher E 1, a flat-topped reflectivity signal is obtained and the microstructural and optical studies together show that the principal process occurring is increasingly deep melting followed by more uniform regrowth of large grains back to the surface. However, cross-section TEM shows that a thin layer of fine-grained poly-Si still is formed deep beneath the surface for E 1<0.9 J/cm2, implying that explosive crystallization occurs (probably early in the laser pulse) even at these high E 1 values. The onset of epitaxial regrowth at E 1 = 1.1 J/cm2 is marked by a slight decrease in surface melt duration.

An investigation of the Ni 5 Al 3 phase

Abstract: The crystallography of the phases obtained by heat treating a Ni-34.7 at. % Al alloy with 0.9 at. % Hf and 0.04 at. % B was studied in detail using x-ray diffraction techniques. Particular emphasis was placed on characterizing the Ni5Al3 phase, which is obtained by quenching the B2 NiAl phase and aging the resultant martensite. The structure of the Ni5Al3 phase is confirmed to be orthorhombic with space group D192h (Cmmm) and lattice parameters of a0 = 0.7475 nm, b0 = 0.6727 nm, and c0 = 0.3732 nm. The transformation from B2 NiAl to L10 martensite is shown to involve a Bain distortion of the c/a ratio from 0.707 to 0.853, and the further ordering transformation to Ni5Al3 involves a further increase of this c/a ratio to 0.900.

Low-voltage field emission from tungsten fiber arrays in a stabilized zirconia matrix

Abstract: Field emitter array cathodes were fabricated from unidirectionally solidified composites of tungsten fibers in an insulating yttria-stabilized-zirconia (YSZ) matrix. A close-spaced molybdenum gate film (extractor) was formed utilizing e-beam evaporation of alumina as an insulator, which was overlayed by the molybdenum extractor. The high resistivity of the composite matrix coupled with the alumina insulator resulted in low leakage current and permitted dc operation of the device. Emission testing demonstrated current densities of 1--5 A/cm/sup 2/ with leakage in the ..mu..A range for applied potentials of 125--200 V. Variation of emitter tip geometries from hemispheres to right circular cylinders to pointed cones produced increases in emission consistent with reduced tip radii.

X-ray photoemission spectroscopy of the 90 K superconductor Ba2YCu3O7−δ

Abstract: The x-ray photoemission spectroscopie (XPS) data from different pelletized samples of the 90 K superconductor Ba2YCu3O7−δ (where δ∼0.2) have been obtained. The valence band spectrum (recorded at 300 and 170 K), which is composed of contributions from both the Cu 3d and O 2p levels, is compared with the full potential linearized augmented plane wave (FLAPW) calculated electronic density-of-states (DOS) reported by Massidda et al. and Mattheiss and Hamann. The experimental data indicate a relatively low DOS at the Fermi level. Detailed measurements of the core level Cu 2p, O 1s, Ba 3d, 4d, and Y 3d spectra of the superconducting and related standard materials, are presented. Data for the superconducting material were recorded in the freshly prepared form as well as after scraping in situ. The Cu 2p core level, satellite, and Auger spectra for the various samples were carefully examined in order to assess the possibility of the presence of Cu3+ ions in Ba2YCu3O7-δ. It is observed that surface reaction in air to form carbonates and hydroxides occurs readily in the superconducting material.

Structural ceramics based on Si 3 N 4 –ZrO 2 (+ Y 2 O 3 ) compositions

Abstract: An investigation was made of the effect of Y2O3 in solid solution in ZrO2 on the phases that are formed when ZrO2 is reacted with either N2 or Si3N4. The results suggest that the Zr–oxynitride phase can be predcluded as a reaction product when the Y2O3 content of the ZrO2 is ≥4 mol %. Dense Si3N4/ZrO2(+ Y2O3) composites were fabricated, which did not degrade during oxidation at temperatures %le; 1000°C. Severe degradation was observed for composites containing the Zr-oxynitride phase. The fracture toughness of the Si3N4/ZrO2(+ Y2O3) composites increased with ZrO2 content.