bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanisms for nonthermal effects on ionic mobility during microwave processing of crystalline solids
Authors: Booske John H. , Cooper Reid F. , Dobson Ian
Published: September 2007

Models for nonthermal effects on ionic motion during microwave heating of crystalline solids are considered to explain the anomolous reductions of activation energy for diffusion and the overall faster kinetics noted in microwave sintering experiments and other microwave processing studies. We propose that radiation energy couples into low (microwave) frequency elastic lattice oscillations, generating a nonthermal phonon distribution that enhances ion mobility and thus diffusion rates. Viewed in this manner, it is argued that the effect of the microwaves would not be to reduce the activation energy, but rather to make the use of a Boltzmann thermal model inappropriate for the inference of activation energy from sintering-rate or tracer-diffusion data. A highly simplified linear oscillator lattice model is used to qualitatively explore coupling from microwave photons to lattice oscillations. The linear mechanism possibilities include resonant coupling to weak-bond surface and point defect modes, and nonresonant coupling to zero-frequency displacement modes. Nonlinear mechanisms such as inverse Brillouin scattering are suggested for resonant coupling of electromagnetic and elastic traveling waves in crystalline solids. The models suggest that nonthermal effects should be more pronounced in polycrystalline (rather than single crystal) forms, and at elevated bulk temperatures.
Modified resin–intermediate processing of perovskite powders: Part I. Optimization of polymeric precursors
Authors: Tai Lone-Wen , Lessing Paul A.
Published: September 2007

The formation of a polyester between citric acid (CA) and ethylene glycol (EG) was found to be a decisive factor for the foaming of resin intermediates in a Pechini-type powder process. This process was modified by changing the organic mass ratio of CA/EG which results in ceramic powders with different morphologies. The most porous resin intermediate (with or without chelated cations) was prepared using a polymeric gel made of equimolar citric acid and ethylene glycol. It was also found that a premixing of organic components, prior to adding constituent nitrate solutions, makes the whole process more controllable.
Insulator-metal transitions at microindentations
Authors: Gilman John J.
Published: September 2007

For all tetrahedrally bonded semiconductors (five group IV plus nine III-V compounds and nine II-VI compounds), it is shown that the critical pressure needed to transform the semiconductor into the metallic state correlates with the microindentation hardness number. The same is done for five alkaline earth oxides. The critical transition pressures have been estimated from Herzfeld's theory—that is, from the compression at which the dielectric constant diverges to infinity. Experimental transition pressures also correlate with hardness numbers, and they correlate with the activation energies for dislocation motion. Since these transitions are electronic they can be influenced by photons, doping (donors enhance while acceptors inhibit them), currents, surface states, etc. Microindentation also provides a simple experimental tool for observing pressure and/or shear induced transformations.
YBa2Cu3O6+δ microcrystals by extraction of a nonstoichiometric melt
Authors: Pellerin N. , Gervais M. , Odier P.
Published: September 2007

YBa2Cu3O6+δ microcrystals are synthesized by extraction of a nonstoichiometric melt similar to that employed for crystal growth of this compound. This method uses, first, initial compositions corresponding to the field YBa2Cu3O6+δ plus liquid, and second, the interaction properties of the flux with an appropriate substrate, i.e., Y2O3. Using other substrates like alumina produces 123 microcrystals, but embedded in the residual flux; in contrast yttrium oxide reacts with this one, and it is essential for its extraction. Choosing proper conditions makes it possible to obtain relatively pure YBa2Cu3O6+δ samples made of regular pellets (size: 10 to 20 μm thickness: 3 to 6 μm). The process is discussed using the phase diagram and the concept of dissolution-reprecipitation often involved in liquid phase sintering.
Single crystal electron paramagnetic resonance study of Y2BaCuO5, a common impurity in the high temperature superconductor, YBa2Cu3O7
Authors: Barham R.J. , Doetschman D.C.
Published: September 2007

Electron paramagnetic resonance (EPR) studies of pure Y2BaCuO5 in powder and single crystal forms and of YBa2Cu3O7−δ in powder and single crystal forms provide further evidence that it is Y2BaCuO5 that is the common green impurity found in many preparations of YBa2Cu3O7−δ as a powder or in pellet forms. Y2BaCuO5 tends to be excluded in the growth of YBa2Cu3O7−δ single crystals. A method is presented for the growth of Y2BaCuO5 crystals from a flux. An apparent discrepancy between the observed single crystal EPR anisotropy and the reported crystal structure is resolved in three independent ways from the Y2BaCuO5 Powder and single crystal EPR data. These results show that the EPR spectrum is a superposition of the spectra of the two differently oriented Cu sites in the unit cell and is not a spectral average of them. The temperature independence of the EPR spectrum between 150 K and 300 K is also consistent with there being no temperature dependent exchange averaging of the EPR spectra of the two sites in this range. The orientations of the Cu crystal field axes, as indicated by the g axes, are in agreement with the crystal structure. Crystal field splittings of the Cu d-orbitals are estimated from the measured g values and indicate an appreciable covalency in the Cu–O bonds. The linewidth and its anisotropy indicate a minor degree of exchange narrowing.
An investigation of the creep processes in tin and aluminum using a depth-sensing indentation technique
Authors: Raman V. , Berriche R.
Published: September 2007

Constant load creep experiments were conducted using a depth-sensing indentation instrument with indentation depths in the submicron range. Experiments were conducted on polycrystalline Sn and sputtered Al films on Si substrates. The results show that the plastic depth versus time curves and the strain rate versus stress plots from these experiments are analogous to those obtained from conventional creep experiments using bulk specimens. The value of the stress exponent for Sn is close to the reported values from uniaxial creep tests. Tests on Al films showed that the stress exponent is dependent on the indentation depth and is governed by the proximity to the film/substrate interface. Load change experiments were also performed and the data from these tests were analyzed. It is concluded that indentation creep experiments may be useful in elucidating the deformation properties of materials and in identifying deformation mechanisms.
Zirconia and organotitanate film formation on graphite fiber reinforcement for metal matrix composites
Authors: Subramanian R.V. , Nyberg Eric A.
Published: September 2007

The formation and characterization of zirconia coatings on graphite/carbon filaments were investigated. The objective was to eliminate or minimize degradative chemical reactions and improve bonding at the metal/carbon-fiber interface when the coated fiber is used as reinforcement in metal matrix composites. Thin, homogeneous films of zirconium oxide, ZrO2, less than 1μm thick, were formed on carbon monofilaments (35 μm diameter) from a zirconium oxychloride solution in water (less than 1.0 wt. % ZrO2) by dip coating and heating. Chemical changes during thermal decomposition and polycondensation were examined by FTIR spectroscopy. Through dynamic x-ray diffraction tests, the zirconia coating was found to transform first to a metastable tetragonal phase on heating to 330 °C, and then upon cooling, to a stable monoclinic structure. Organotitanate coatings were formed by electrodeposition of the ionizable organometallic complex, titanium di(dioctylpyrophosphate) oxyacetate (TDPA). Single fiber tests revealed a slight reduction in the strength of fibers with thicker coatings, probably due to crack initiation by brittle fracture of the ZrO2 coating. Thin coatings applied from 0.25% ZrOCl2 did not cause such strength reduction. Effective bonding of ZrO2 coatings to the filaments was revealed in single filament composite tests which showed the interfacial shear strength (IFSS) of the coated filaments in an epoxy matrix to be higher than that of the uncoated filaments. The IFSS of monofilaments electrocoated by the titanate was also higher. Examination of the fracture surfaces showed fiber pull-out associated with poor bonding in the case of specimens prepared from uncoated monofilaments. The coated monofilaments showed no fiber pull-out, suggesting that maximum fiber strength was achieved and transferred through the interface to the matrix. Finally, the chemical and thermal stability of the interfacial region of coated and uncoated graphite rods embedded in an aluminum matrix were evaluated. The uncoated rod showed little or no interfacial bonding to the metal matrix, suggesting poor wetting, while the dip-coated and electrocoated rods showed good wetting and compatibility with the metal matrix.
Synthesis of yttrium aluminum garnet by reverse strike precipitation
Authors: Apte Prasad , Burke Harry , Pickup Helen
Published: September 2007

Yttrium aluminum garnet (YAG) powders were synthesized by normal and reverse strike precipitation from a mixed solution containing aluminum and yttrium nitrates. The precipitates were characterized by FTIR, DTA, and TGA techniques. The amorphous precipitates were crystallized under various atmospheres (air, argon, hydrogen) at different temperatures and the product analyzed by x-ray diffraction. Only reverse strike precipitation followed by crystallization in hydrogen consistently yielded phase pure YAG.
Deposition of nitrides and oxides of aluminum and titanium by pulse laser irradiation
Authors: Nishitani S.R. , Yoshimura S. , Kawata H. , Yamaguchi M.
Published: September 2007

Deposits of nitrides and oxides of Al and Ti have been produced by laser irradiation of Al and Ti targets in air, N2, and NH3 + N2 gases. Microstructure and constituent phases in these deposits have been examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray diffractometry (XRD). The distribution of metalloid elements has been investigated by Rutherford backscattering spectrometry (RBS). On the basis of the results of these examinations, the nitride and oxide deposits have been shown to be formed by reactions between ambient gas and metal-melt or metal-vapor which take place during pulse laser irradiation.
Copper vanadates as candidate materials for phase change optical memory
Authors: Birnie D.P. , Weinberg J.D. , Swanson D.G.
Published: September 2007

Several copper vanadium oxide melts were tested for possible application as the active medium in phase-change optical data storage devices. These materials were melted in the bulk and then quenched. Their phase development was characterized to help determine their applicability to optical data storage. It was found that they satisfy many of the criteria necessary for successful phase-change data storage; further studies of their behavior in thin film geometry would be warranted.
Thermodynamic analysis for the chemical vapor deposition of composite coatings from the Al–B–Ti–N–H–Cl–Ar system
Authors: Moss T.S. , Hanigofsky John A. , Lackey W.J.
Published: September 2007

Thermodynamic calculations were performed for chemical vapor deposition in the Al–B–Ti–N–H–Cl–Ar system in order to determine the feasibility of multiphase deposition. Reagent species used were BCl3, AlCl3, TiCl4, NH3, H2, and Ar; B2 was substituted for BCl3 to determine changes in deposition efficiency. Temperature and input molar concentrations were varied over a range of values to establish relationships among solid deposits. Through deposition diagrams, molar efficiency plots, and partial pressure graphs, several two and three phase regions were found to exist. The calculations indicate that the following dispersed phase composites could be prepared: A1N + BN + TiN, BN + TiN, BN + TiB2, BN + TiB2 + TiN, and TiB2 + TiN.
Powder characteristics and sintering behavior of Ag-doped YBa2Cu3O7−x produced by aerosol decomposition
Authors: Ward Timothy L. , Kodas Toivo T. , Carim Altaf H. , Kroeger Donald M. , Hsu Huey
Published: September 2007

YBa2Cu3O7−x (1-2-3) powders and 1-2-3 powders doped with 14 wt. % Ag (AgYBa2Cu3O7−x) were produced using aerosol decomposition of nitrate solutions. Powder produced at T > 900 °C consisted of submicron particles and had Tc ≍ 92 K in magnetic susceptibility measurements. As-produced Ag-doped powder was a composite of nearly phase-pure 1-2-3 and crystalline Ag (by x-ray diffraction) for reactor temperatures between 900 °C and 950 °C, whereas powder produced at T≥ 970 °C contained significant amounts of Y2BaCuO5 which were not found in 1-2-3 synthesis in the absence of Ag. This implied that the melting of Ag (∼960 °C) or the Ag-O eutectic (∼940 °C) promoted decomposition of 1-2-3 during powder synthesis. Dilatometry showed that 1-2-3 and Ag/1-2-3 powders densified rapidly between 800 °C and 875 °C, achieving nearly 90% of theoretical density after heating to 875 °C at 5 °C/min in air. Pellets of the Ag-doped powder were also sintered for 2-60 h at 895 °C in air. Scanning electron and optical microscopy revealed that Ag grains remained fine and uniformly distributed, varying in size from ∼1 μm after 2 h to 3–7 μm after 60 h, while 1-2-3 grains became plate-shaped with thicknesses of 1–5 μm and lengths of 10–30 μm after 60 h. Thus, the use of aerosol Ag /1-2-3 powders allows the use of lower processing temperatures and shorter times to produce dense ceramics with smaller Ag and 1-2-3 grain sizes than can be obtained using solid-state reaction routes.
The crystallography of cleavage fracture in Al3Sc
Authors: Schneibel J.H. , Hazzledine P.M.
Published: September 2007

The intermetallic compound Al3Sc is a trialuminide with the L12 structure, which deforms easily in compression at room temperature, with yield stresses around 100 MPa. As shown by single crystal compression experiments, slip occurs on {111} planes. In tension Al3Sc fractures transgranularly in a brittle manner. The predominant cleavage plane is {011}. In a scanning electron microscope (SEM) numerous cleavage steps, which are aligned in three major crystallographic directions, are observed. The fracture surfaces almost always show microscopic waviness along the traces of intersecting slip planes. Regions that are flat within the experimental resolution of the SEM are only occasionally observed. Some of the cleavage steps consist of {111} or {001} planes, but others are not distinctly crystallographic. Plastic deformation involving dislocation motion or twinning may have occurred at these steps. Reactions among different types of steps are also observed. One type of cleavage pattern found is strikingly similar to the typical appearance of fracture surfaces of fcc or fcc related materials after stress corrosion cracking. However, this particular pattern is only rarely observed in Al3Sc and can therefore not be taken as evidence for stress corrosion cracking in this material. Our observations indicate that an interpretation of cleavage fracture in Al3Sc in terms of surface energies alone is unlikely to be successful. Similarly, any criterion that categorizes its fracture behavior into either fully brittle or fully ductile is faced with difficulties. A full understanding of the fracture morphology of Al3Sc will therefore require detailed atomistic simulations.
Morphological and structural evolutions of nonequilibrium titanium-nitride alloy powders produced by reactive ball milling
Authors: Sherif El-Eskandarany M. , Sumiyama K. , Aoki K. , Suzuki K.
Published: September 2007

Nonequilibrium titanium-nitride alloy powders have been fabricated by a high energetic ball mill under nitrogen gas flow at room temperature and characterized by means of x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and differential scanning calorimetry. Initial hcp titanium is completely transformed to nonequilibrium-fcc Ti–N after 720 ks of the milling time. The fcc Ti–N phase is stable at relatively low temperature and transforms at 855 K to Ti2N and δ phases. At the final stage of milling, the particle- and grain-sizes of alloy powders are 1 mm and 5 nm, respectively, and the lattice parameter is 0.419 nm.
Growth, structure, and microhardness of epitaxial TiN/NbN superlattices
Authors: Shinn M. , Hultman L. , Barnett S.A.
Published: September 2007

Epitaxial TiN/NbN superlattices with wavelengths/ranging from 1.6 to 450 nm have been grown by reactive magnetron sputtering on MgO(100). Cross-sectional transmission electron microscopy (XTEM) studies showed well-defined superlattice layers. Voided low-angle boundaries, aligned perpendicular to the film plane, were also present. High-resolution images showed misfit dislocations for Λ = 9.4 nm, but not Λ = 4.6 nm. Up to ninth-order superlattice reflections were observed in diffraction, indicating that the interfaces were relatively sharp. Analysis of the first-order x-ray superlattice reflection intensities indicated that the composition modulation amplitude increased and the coherency strains decreased for Λ increased from 2 to 10 nm. Vickers microhardness H was found to increase rapidly with increasing Λ, from 1700 kg/mm2 for a TiN–NbN alloy (i.e., Λ = 0) to a maximum of 4900 kg/mm2 at Λ = 4.6 nm. H decreased gradually for further increases in Λ above 4.6 nm, to H = 2500 kg/mm2 at Λ = 450 nm. The hardness results are compared with theories for strengthening of multilayers.
On the chemical diffusion in layered thin films containing amorphous Co–Zr, Ni–Zr, and Fe–Zr
Authors: Karpe N. , B⊘ttiger J. , Greer A.L. , Janting J. , Kyllesbech Larsen K.
Published: September 2007

The chemical diffusion in thin trilayer films of TM–TM100−xZrx–TM with an amorphous middle layer where TM = Co, Ni, or Fe and in amorphous Fe–Zr and Ni–Zr films with composition gradients has been investigated using Rutherford backscattering spectrometry. The growth of the amorphous layer in the trilayers, due to in-diffusion of cobalt and nickel, is initially found to be proportional to the square root of the time, t1/2, and subsequently found to level off before the compositions corresponding to metastable equilibria are reached. Irradiation, with 500 keV Xe+ ions, is found to promote the in-diffusion. This behavior is discussed in terms of structural relaxation effects and their influence on the metastable equilibrium. In amorphous Fe–Zr the chemical diffusivity is observed to be very sluggish. Contrary to the behavior in Co–Zr and Ni–Zr trilayers, the direction of the iron diffusion in Fe–Zr trilayers suggests a broad positive peak in the Gibbs free energy at a composition around 50 at. % Zr. It is argued that the sluggish chemical diffusivity of iron is directly related to the unusual composition-dependence of the Gibbs free energy for amorphous Fe–Zr.
Kinetics of short- and long-range B2 ordering in ternary alloys
Authors: Fultz B.
Published: September 2007

The kinetics of disorder → B2 order transformations in ternary alloys were studied in the pair approximation. The predictions of two sets of rate equations were compared, one based on an atom-atom interchange mechanism and the other based on an atom-vacancy interchange mechanism. Examples are presented to show how the evolution of order is affected by differences in interatomic potentials, differences in activation barrier heights, and effects of vacancy ordering. A wide variety of states of order are possible during the disorder → order transformation in ternary alloys, and qualitative features of the sequences of these states are discussed. The sequences of states, or “kinetic paths”, are much less intuitive than for binary alloys.
On controlled solidification studies of some TiO2 binary alloys
Authors: Yen C.T. , Nason D.O. , Tiller W.A.
Published: September 2007

TiO2 single crystal fibers in the 1 mm diameter range were pulled from different alloy melts using the laser heated pedestal melting technique. The alloying elements studied were CaO, MnO, MgO, SiO2, FeO, and Al2O3. Phase diagram solute partition coefficient, k0, maximum solid solubility limit, CS(max), eutectic concentration, CE, and eutectic temperature, TE, were determined for each of these alloys. Solute redistribution effects in the solid, controlled precipitation in the solid, smooth solid-liquid interfaces in the presence of high melt concentrations and substantial crystal broadening by fluid migration up the solid from the melt all indicated the existence of a very strong thermodynamic field and a large solid diffusion coefficient operating in the solid behind the solid/liquid interface.
Sol-gel processing of PbTiO3 and Pb(Zr0.52Ti0.48)O3 fibers
Authors: Selvaraj Ulagaraj , Prasadarao A.V. , Komarneni Sridhar , Brooks Keith , Kurtz Stewart
Published: September 2007

Homogeneous and stoichiometric PbTiO3 and PbZr0.52Ti0.48O3 gel fibers were drawn from viscous solutions prepared by sol-gel processing of alkoxide precursors. The fibrous gels on heating at 450 and 600 °C, respectively, formed the well-crystallized phases of PbTiO3 and PbZr0.52Ti0.48O3. Fibers heat treated at 700 °C are a few centimeters long and 50 to 150 μm in diameter. PbZr0.52Ti0.48O3 fibers subjected to heat treatment at 700 °C for 1 h consisted of fully dense submicron grains (0.15 to 0.2 μm). However, PbTiO3 fibers under analogous conditions showed a few micropores with grains of ∼0.1 μm. Dielectric constants of these PbTiO3 and PbZr0.52Ti0.48O3 fibers were approximately 300 and 800, respectively.
Production of amorphous metallic surfaces by means of a pulsed glow discharge electron beam
Authors: Mingolo N. , Rocca J.J.
Published: September 2007

A pulsed glow discharge electron beam has been used for the production of metallic amorphous surfaces in MgZn alloys. Electron beam pulses of 20 μs pulse width produced by a 40 A, 22.5 kV glow discharge were found to provide sufficient energy for melting the metallic surfaces; that due to the rapid cooling to the substrate yielded amorphous phases. The system allows control of the energy density, penetration, and pulse width of the heating pulse.
An Al13Co4 phase in the Al–Cu–Co alloy system
Authors: Grushko B. , Freiburg Ch.
Published: September 2007

A part of the Al–Cu–Co phase diagram adjacent to the Al13Co4 composition was studied at 800 °C by scanning electron microscopy and x-ray diffractometry. The homogeneity region of the Al13Co4 phase was found to extend up to 6 at. % Cu. At 800 °C this phase is in equilibrium with Al(Co, Cu), Al5Co2, Al9Co2, the decagonal phase, and a liquid phase. The existence of the Y-phase and Al3Co was not confirmed at this temperature.
A study of stress-driven diffusive growth of voids in encapsulated interconnect lines
Authors: Sauter Anne I. , Nix W.D.
Published: September 2007

Stress-driven diffusive growth of voids in encapsulated interconnect lines is studied. By calculating the rate of growth of a single void in a passivated line subjected to an initial hydrostatic tension stress and by assuming that failure occurs when the void reaches a critical size, a model for failure of encapsulated interconnect lines by stress voiding can be developed. The model for the prediction of void growth and failure is based on two limiting kinds of void growth. In one limit, which applies at short times, radial displacements occur by diffusional flow processes around the growing void and relax the local hydrostatic tension stress. In the long time limit, vacancies flow to the void from distant parts of the line by diffusion along grain boundaries, thereby relaxing the stress in a growing section of the line. A model based on a combination of these behaviors leads to a failure law for aluminum lines of the form tfσ2/d = 1019.2 exp(Q/RT) where tf is the failure time in seconds, σ is the initial hydrostatic tension stress in the line in Pa, d is the grain size in meters, and the activation energy, Q = 80.9 kJ/mol, is close to that for grain boundary diffusion in aluminum. The model predictions appear to be in good agreement with the few experiments on stress voiding that have been conducted.
On-line gas-phase optical diagnostics in plasma CVD deposition of carbon films
Authors: Fantoni R. , Giorgi M. , Moliterni A.G.G. , Berden W.C.M. , Lazic V. , Martini O. , Polla Mattiot F.
Published: September 2007

Space resolved, on-line spontaneous and stimulated emission, and CARS diagnostics have been employed on CH4/H2 mixtures excited by rf-discharge in order to investigate the chemical processes and the gas phase kinetics in the plasma. CH4 and H2 concentration and temperature have been monitored during the process as a function of main reaction parameters (rf-power, total pressure, CH4/H2 ratio). Formation of CH,H2, and H in excited states has been observed. On the basis of present spectroscopic data, a model for the gas-phase reactions accompanying the carbon film deposition is proposed.
Surface migration and volume diffusion in the AgGaSe2−Ag2Se system
Authors: Kim N-H. , Feigelson R.S. , Route R.K.
Published: September 2007

AgGaSe2 crystals are useful for nonlinear optical applications after a post-growth heat-treatment in the presence of Ag2Se to eliminate an unavoidable precipitate phase that degrades their optical properties. To understand better the heat-treatment procedure, surface migration and volume diffusion were investigated in the Ag2Se–AgGaSe2 system using reactive diffusion couples which were analyzed by x-ray diffraction, optical microscopy, and electron probe microanalysis. Surface diffusivities of all mobile species were found to be much larger than volume diffusivities. Specific values determined were DsAg = 5.43 × 10−4 exp(-0.46 eV/kT) and DvAg = 2.40 × 10−7 exp(-0.84 eV/kT), where DsAg and DvAg are effective surface diffusivity and effective volume diffusivity of Ag, respectively. The corresponding diffusivities for Ga and Se were found to be almost the same, indicating that Ga and Se move together with Ag to maintain binary (Ag2Se and Ga2Se3) stoichiometry and electroneutrality. These findings are consistent with the pattern of annihilation of the second phase precipitates in the AgGaSe2 matrix during heat treatment.
Optical and physical properties of sputtered Si:Al:O:N films
Authors: Knoll R.W. , Henager C.H.
Published: September 2007

Mechanical and optical properties and structural characteristics are described for Si:N films with Al and O additions (SixAl1−xOyN1−y) deposited by reactive RF diode sputtering on Si and SiO2 substrates. The thermal and intrinsic stress components, elastic stiffness, coefficient of thermal expansion (CTE), and refractive index were measured for films ranging in thickness from ∼2 μm to 50 μm. Some structural and microstructural data were obtained using x-ray diffraction, optical and scanning-electron microscopy, and surface profilometry. Alloying Si:N with Al to form Si:Al:N greatly reduced the compressive intrinsic and total stress found in pure Si:N films on Si. Addition of O to the Si:Al:N moderately increased the intrinsic stress, decreased the elastic stiffness, and produced a smoother, more glassy (amorphous) film.
Synthesis and characterization of ceramic (Sr1−xCax)2 CuO3
Authors: Xu Ming , Voiles E.T. , Chumbley L.S. , Goldman A.I. , Finnemore D.K.
Published: September 2007

Ceramic (Sr1−xCax)2CuO3 samples have been prepared with x = 0.00, 0.25, 0.50, 0.55, 0.75, and 1.00 in order to study the change in lattice constants with the ratio of Sr/Ca. This phase is frequently an impurity phase in the fabrication of Bi–Sr–Ca–Cu–O superconductors, and it is important to be able to connect the lattice constants with stoichiometry. The behavior of the lattice constants closely follows Vegard's law.
On diamond-hexagonal germanium
Authors: Xiao S-Q. , Pirouz P.
Published: September 2007

Diamond-hexagonal (dh) Ge has been produced by Vickers micro-indentation of a Ge single crystal and investigated by high resolution transmission electron microscopy (HREM). The results are consistent with the mechanisms that have been proposed earlier for the generation of dh Si. The hexagonal phase of these elemental semiconductors may be produced by two related mechanisms: (a) secondary twinning or (b) the intersection of two twins. The hexagonal germanium produced by both of these mechanisms has been observed and is presented in the present paper.
Investigation of growth rates and morphology for diamond growth by chemical vapor deposition
Authors: Everson M.P. , Tamor M.A.
Published: September 2007

We describe two complementary studies of diamond growth by chemical vapor deposition. In the first, the early stages of growth of randomly distributed nuclei on silicon are studied by scanning tunneling microscopy. For growth times from 1 to 30 min nearly all crystallites are three dimensional, and increase in volume as t1.5. Although this result could be interpreted in terms of diffusion limited growth, the conditions for diamond CVD are more consistent with rate limited growth where the crystals are expected to gain volume as t3. This anomaly can be explained in terms of a two-species growth mechanism in which the rate constant for carbon addition is proportional to the diffusion limited flux of atomic hydrogen. Other mechanisms giving rise to the observed t1.5 dependence are also considered. The second study uses both scanning electron and tunneling microscopies to examine the morphology of a boron-doped film homoepitaxial to the {100} surface of natural type 2a diamond. In regions distant from gross defects, this film is very smooth. However, gross defects appear to initiate growth of new epitaxial layers at a rate much higher than in defect-free regions. This observation suggests that diamond growth is promoted by “enabling defects” and that without such defects nucleation of new layers is a slow process and permits layer-by-layer growth at a much lower rate.
The sintering behavior of ultrafine alumina particles
Authors: Bonevich John E. , Marks Laurence D.
Published: September 2007

Ultrafine particles (UFPs) of aluminum oxide were prepared by an arc discharge, sintered in a custom-built ultrahigh vacuum (UHV) furnace system, and characterized in a high resolution electron microscope (HREM) operating under UHV conditions. The UFPs produced range in size from 20 to 50 nm and have highly faceted surfaces. The atomic structure of the UFPs corresponds to the cubic (γ) and orthorhombic (δ) variants of the spinel structure. Sintering in these UFPs demonstrates three major issues. Surface faceting plays a major role in determining the final sintering geometry with sintering occurring predominantly on the close-packed {111} facets. Surface diffusion is the predominant mechanism for sintering, as evidenced by the fact that many sintered particles have their initial adhesion structure ‘locked-in’ during sintering with no reorientation occurring. Furthermore, the necks formed during sintering have well-defined, atomically sharp contact angles which suggests that the neck growth process is controlled by the faceted structures and may be modeled by a mechanism similar to crystal growth due to ledges, grain boundaries, and twins. The driving force for sintering can be considered as a chemical potential difference between facet surfaces and the neck region.
The purification of elemental sulfur
Authors: Susman Sherman , Clark Rowland S. , Volin Kenneth J.
Published: September 2007

Sulfur has been purified by a dynamic pyrolysis procedure that takes into account the unique viscosity-temperature relationship of the liquid. An apparatus is described that incorporates gas-bubbling and rocking to overcome diffusion-limited kinetics. For the first time, a mechanism is proposed for the pyrolytic purification of sulfur based upon Carsul formation and the H2S ↔ H2Sx equilibrium. Infrared methods are delineated for qualitative analysis and quantitative estimates of anion contaminants, including the omnipresent hydrocarbon impurities. Our purified sulfur has a total carbon, oxygen, and hydrogen impurity level of <10 μg/g.
Interfacial structure and chemistry in a ceramic/polymer composite material
Authors: Popoola Oludele O. , Kriven Waltraud M.
Published: September 2007

The microstructure and microchemistry of ceramic/polymer interfaces in a calcium aluminate/polyvinyl alcohol composite material have been studied using transmission electron microscopy (TEM), high resolution electron microscopy (HREM), x-ray photoelectron spectroscopy (XPS), and energy dispersive spectroscopy (EDS). Interfaces consisted of an amorphous interphase layer, inside of which were dispersed metastably retained CaAl2O5 · 8H2O crystallites. The amorphous phase was a mixture of the hydration products of calcium aluminate and aluminum–crosslinked, polyvinyl alcohol. The crystalline hydration product CaAl2O5 · 8H2O was metastably retained due to polymer poisoning of nucleation sites and significant reduction of conversion kinetics.
Analysis of the accuracy of the bulge test in determining the mechanical properties of thin films
Authors: Small Martha K. , Nix W.D.
Published: September 2007

Since its first application to thin films in the 1950's the bulge test has become a standard technique for measuring thin film mechanical properties. While the apparatus required for the test is simple, interpretation of the data is not. Failure to recognize this fact has led to inconsistencies in the reported values of properties obtained using the bulge test. For this reason we have used the finite element method to model the deformation behavior of a thin film in a bulge test for a variety of initial conditions and material properties. In this paper we will review several of the existing models for describing the deformation behavior of a circular thin film in a bulge test, and then analyze these models in light of the finite element results. The product of this work is a set of equations and procedures for analyzing bulge test data that will improve the accuracy and reliability of this technique.
Effects of homoepitaxial surfaces and interface compounds on the in-plane epitaxy of YBCO films on yttria-stabilized zirconia
Authors: Fork D.K. , Garrison S.M. , Hawley Marilyn , Geballe T.H.
Published: September 2007

Control of the in-plane epitaxial alignment of c-axis YBa2Cu3O7−δ (YBCO) films on yttria-stabilized zirconia (YSZ) substrates is necessary for achieving optimal transport properties. We have used pulsed laser deposition to grow homoepitaxial YSZ and heteroepitaxial CeO2 on YSZ single crystal substrates. This procedure dramatically improves the epitaxy of YBCO and reduces the number of low and high angle grain boundaries. We have also studied the effects of preparing the YSZ growth surface with approximately monolayer amounts of CuO, Y2O3, BaO, and BaZrO3 to determine the effects these compositional variations have on the subsequent YBCO epitaxy. CuO, Y2O3, and BaZrO3 induce an in-plane crystallography of YBCO distinct from that initiated with BaO. Both homoepitaxy and monolayer depositions may be carried out in situ and are simple and effective for controlling the epitaxy and electrical properties of YBCO on YSZ. The effects of substrate temperature, oxygen pressure, and yttria content have also been studied.
Chemical vapor deposition of aluminum nitride thin films
Authors: Gordon Roy G. , Riaz Umar , Hoffman David M.
Published: September 2007

The atmospheric pressure chemical vapor deposition of aluminum nitride coatings from hexakis(dimethylamido)dialuminum, Al2(N(CH3)2)6, and ammonia precursors is reported. The films were characterized by ellipsometry, transmission electron microscopy, x-ray photoelectron spectroscopy, Rutherford backscattering, and forward recoil spectrometry. The films were deposited at 100–500 °C with growth rates up to 1500 Å/min. The films showed good adhesion to silicon, glass, and quartz substrates and were chemically inert. Rutherford backscattering analysis revealed that the N/Al ratio was 1.15 ± 0.05 for films deposited at 100–200 °C and 1.05 ± 0.05 for those deposited at 300–500 °C. Films deposited at 100–200 °C had refractive indexes in the range 1.65–1.80 whereas indexes for films deposited at 300–400 °C were 1.86–2.04. The films were transparent in the visible region. The optical bandgap varied from 5.0 eV for films deposited at 100 °C to 5.77 eV for those deposited at 500 °C. Films deposited at 100–200 °C were amorphous whereas those deposited at 300–500 °C were polycrystalline.
Determination of grain boundary geometry using TEM
Authors: Jang H. , Farkas D. , De Hosson J.T.M.
Published: September 2007

An experimental method to obtain the grain boundary geometry using the transmission electron microscope is presented. The method allows Σ determination including grain boundary plane orientation. In order to determine the specialness of the grain boundary, three different criteria for maximum allowable deviations from exact CSL misorientations were examined. We tested these three criteria from a statistical distribution of grain boundary types in terms of Σ. We compared grain boundary distributions from other studies in Ni3Al and found discrepancies among them. It seems that the discrepancy came from the different criteria for special boundaries in Σ determination and different experimental procedures they used. The statistical distribution of grain boundary plane orientations showed that low Σ boundaries (Σ < 11) were oriented to the plane of high density of coincident sites.
Observation and characterization of a twinned monoclinic phase as a product of the solid state decomposition of Nd2Fe14B
Authors: Rabenberg L. , Fidler Josef , Bernardi Johannes
Published: September 2007

A microtwinned monoclinic phase is observed to occur within the grains of the hard magnetic phase in Nd2Fe14B-based permanent magnet alloys. This phase seems to be the product of a displacive solid state phase transformation of tetragonal Nd2Fe14B, distinguished from Nd2Fe14B by the presence of an ordered substitution into alternating unit cells parallel to the crystallographic c-axis and by a small shear of the type 〈100〉 {011}. The four different orientational variants of this monoclinic phase with respect to the parent tetragonal phase are arranged into two distinct colonies of twin-related plates. The observation that Nd2Fe14B can undergo a displacive solid state transformation has broad implications for the entire field of Nd2Fe14B permanent magnets.
The influence of an oxidation inhibitor on the elevated temperature fracture resistance of carbon/carbon composites
Authors: Lucchesi Arthur J. , Hay Jack C. , White Kenneth W.
Published: September 2007

This fracture study evaluates the role of a fiber/matrix interfacial glass on the toughening of two different carbon/carbon (C/C) composites. Both composites incorporate a two-dimensional layup of 8-harness satin weave continuous fiber fabric, but differ in several aspects, the most significant of which is the presence of an oxidation inhibitor in one of these. The fracture behavior of both materials was determined in three-point flexure at 20 through 1650 °C. Microstructural studies indicate that the nonhomogeneous distribution of the oxidation inhibitor within the fiber bundles controls the fracture behavior. Electron microprobe results indicate a high concentration of the glass oxidation inhibitor associated with the inter-bundle matrix, while the intra-bundle matrix is composed primarily of carbon. Accordingly, debonding along the inter-bundle interfaces characterizes the oxidation inhibited composite, whereas the nonoxidation inhibited samples debond by individual fibers. Both materials exhibit strongly rising R-curves throughout the test temperature range. At the higher test temperatures the oxidation inhibited C/C shows the greatest cumulative toughening component, although at a lower value of the fracture toughness. This is consistent with the observed increase in the percentage of fibers that experience individual pullout at the higher temperatures.
Determination of particle size distribution in an Fe2O3-based catalyst using magnetometry and x-ray diffraction
Authors: Ibrahim Manjula M. , Zhao Jianmin , Seehra Mohindar S.
Published: September 2007

In this paper, the techniques of SQUID magnetometry and line broadening in x-ray diffraction are employed for determining an important parameter for catalysts, viz. the particle size distribution. Magnetization versus temperature (5 K–400 K) and magnetization versus field (up to 55 kOe) data are reported for an α–Fe2O3 based catalyst. After determining the region of superparamagnetism, the distribution function f(r) is determined assuming a log normal distribution and Langevin paramagnetism of superparamagnetic particles. The distribution is found to be fairly symmetric with center near 65 Å and range of 35 to 115 Å. From line-broadening of Bragg peaks in x-ray diffraction, particle radii varying between 75 Å and 110 Å are obtained. These results are compared with the reported Mössbauer measurements of Huffman et al. on the same sample.
Formation of mixed oxide powders in flames: Part I. TiO2−SiO2
Authors: Hung Cheng-Hung , Katz Joseph L.
Published: September 2007

Mixed oxide powders, e.g., Al2O3−TiO2, SiO2−GeO2, and TiO2−SiO2, are used in industry to produce ceramics, optical fibers, catalysts, and paint opacifiers. The properties of these products depend upon the morphology of the powders. Ceramics and optical fibers are produced using either a uniform mixture of multicomponent particles or a uniform solution. The desired morphology for catalysts is a high surface area and many active sites. TiO2 coated with a layer of SiO2 is the desired structure for use as a paint opacifier. In this paper, TiO2−SiO2 mixed oxide powders were synthesized using a counterflow diffusion flame burner. TiCl4 and SiCl4 were used as source materials for the formation of oxide particles in hydrogen-oxygen flames. In situ particle sizes were determined using dynamic light scattering. A thermophoretic sampling method also was used to collect particles directly onto carbon coated grids, and their size, morphology, and crystalline form examined using a transmission electron microscope. A photomultiplier at 90° to the argon ion laser beam was used to measure the light-scattering intensity. The effect of temperature and of Si to Ti concentration ratio on particle morphology was investigated. Strong temperature dependence was observed. At high temperatures, TiO2 particles were covered with discrete SiO2 particles. At low temperatures, the structure changes to TiO2 particles encapsulated by SiO2. TEM diffraction pattern measurements showed that the TiO2 is rutile and the SiO2 is amorphous silica. At high Si to Ti ratios, SiO2-encapsulated TiO2 particles form. At low Si to Ti ratios, one obtains TiO2 particles covered with discrete SiO2 particles.
Formation of mixed oxide powders in flames: Part II. SiO2−GeO2 and Al2O3−TiO2
Authors: Hung Cheng-Hung , Miquel Philippe F. , Katz Joseph L.
Published: September 2007

SiO2−GeO2 and Al2O3−TiO2 mixed oxide powders were synthesized using a counterflow diffusion flame burner. SiCl4, GeCl4, Al(CH3)3, and TiCl4 were used as source materials for the formation of oxide particles in hydrogen-oxygen flames. In situ particle sizes were determined using dynamic light-scattering. Powders were collected using two different methods, a thermophoretic method (particles are collected onto carbon coated TEM grids) and an electrophoretic method (particles are collected onto stainless steel strips). Their size, morphology, and crystalline form were examined using a transmission electron microscope and an x-ray diffractometer. A photomultiplier at 90° to the argon ion laser beam was used to measure the light-scattering intensity. The formation of the mixed oxides was investigated using Si to Ge and Al to Ti ratios of 3:5 and 1:1, respectively. Heterogeneous nucleation of the SiO2 on the surface of the GeO2 was observed. In Al2O3−TiO2 mixtures, both oxide particles form at the same temperature. X-ray diffraction analysis of particles sampled at temperatures higher than 1553 K showed the presence of rutile, γ–Al2O3, and aluminum titanate. Although the particle formation process for SiO2−GeO2 is very different from that for Al2O3−TiO2, both mixed oxides result in very uniform mixtures.
Growth of noncrystallographic dendrites
Authors: Fabietti L.M. , Sekhar J.A.
Published: September 2007

First time experimental evidence is presented for the steady state growth of dendrites away from their normal crystallographic growth directions. The evidence is shown for transparent dendrites in the succinonitrile system, which are induced to grow along wetting surfaces. A method for the quantitative characterization of the tips of such dendrites is described and the differences in the secondary arm formation between off-axis and regular dendrites are recorded. The scientific and technological implications of this finding are examined.
Grain size effects in nanocrystalline materials
Authors: Suryanarayana C. , Mukhopadhyay D. , Patankar S.N. , Froes F.H.
Published: September 2007

Nanocrystalline materials have a grain size of only a few nanometers and are expected to possess very high hardness and strength values. Even though the hardness/strength is expected to increase with a decrease in grain size, recent observations have indicated that the hardness increases in some cases and decreases in other cases. A careful analysis of the available results on the basis of existing models suggests that there is a critical grain size below which the triple junction volume fraction increases considerably over the grain boundary volume fraction and this is suggested to be responsible for the observed softening at small grain sizes.
AFM observations of the surface morphology of metallic glasses Fe78B13Si9 in the early stage of crystallization
Authors: Watanabe Yoshihisa , Nakamura Yoshikazu
Published: September 2007

The change in the surface morphology of metallic glasses Fe78B13Si9 during the early stage of the crystallization process has been studied mainly by atomic force microscopy (AFM). Specimens of 50-μm thickness have been heated up to 323 K, 373 K, 423 K, and 523 K with a heating rate of 30 K/min in vacuum, and then the surface morphology of each specimen has been observed by AFM in air. In the surface image of the specimen heated to 323 K, many holes are observed and inside the holes single or plural protrusions can be observed. Clusters composed of aggregated protrusions are also found like islands in the amorphous sea. The specimen heated to 323 K has also been observed by a high-resolution transmission electron microscope and crystalline structures among the amorphous matrix have been detected. From the selected area diffraction study, the crystalline structure is found to be α–Fe crystallites. In contrast, in the AFM image of the surface of the specimen heated to 423 K, no holes are seen and many protrusions are found to extend above the surface and form several parallel lines. The spatial density of protrusions above the surface becomes much higher in the specimen heated to 523 K. We propose that this change in the surface morphology during heat treatments indicates the process of nucleation and growth of α–Fe crystallites in the surface of metallic glasses. It is also found that the stage of many protrusions extending above the surface corresponds with the beginning of exoelectron emission from the surface. This result suggests that exoelectron emission and surface crystallization connect with each other.