bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear silicon oxide growth patterns in a gold-silicon system
Authors: Leclerc Gervais , Paquin Louis , Baratay Fabrice
Published: September 2007

Oxide films are grown on a silicon wafer at low temperatures through the catalytic action of a thin gold film. Our results indicate that the oxide film thickness and morphology vary with the initial gold film thickness but do not significantly depend on the temperature. A Fourier analysis of the film structure suggests that the growth mechanism may include a spinodal decomposition where a binary alloy undergoes a phase separation. It is argued that gold silicide is the most likely candidate for spinodal decomposition.
A study of the effect of composition on the microstructural evolution of a–SixC1−x: H PECVD films: IR absorption and XPS characterizations
Authors: Gat E. , El Khakani M.A. , Chaker M. , Jean A. , Boily S. , Pépin H. , Kieffer J.C. , Durand J. , Cros B. , Rousseaux F. , Gujrathi S.
Published: September 2007

Amorphous silicon carbide films (a–SixC1−x :H) deposited by the argon- or helium-diluted PECVD technique were studied as a function of their composition. Microstructural investigations were mainly achieved by means of FTIR and XPS techniques. Nuclear techniques were used to obtain precise information on the film hydrogen content. The Si–H IR-absorption band was deconvoluted in different monohydride and dihydride silicon environments. The existence of SiH2 bonds in the Si-rich composition was evidenced. From the analysis of the C–H and Si–H absorption bands it is shown that hydrogen atoms are preferentially bonded to carbon atoms. The deconvolution of the Si2p core level peak suggests that above a composition of x ∊ 0.5, the noncarburized (Si, Si, H) local environment contribution increases to the detriment of the hydrocarburized (Si, C, H) environments. From the evolution of the C1s peak, it can be deduced that there is a change in the carbon atom bonding states when the film composition is varied. These results are correlated and discussed in terms of the local bonding environments and their evolution with film composition.
Structural properties of epitaxial TiO2 films grown on sapphire (11$\overline 1$0) by MOCVD
Authors: Chang H.L.M. , You H. , Gao Y. , Guo J. , Foster C.M. , Chiarello R.P. , Zhang T.J. , Lam D.J.
Published: September 2007

Titanium dioxide thin films were grown on sapphire (11$\overline 1$0) substrates in a low-pressure metal-organic chemical vapor deposition system at temperatures ranging from 400 to 800 °C. Raman scattering, x-ray diffraction, transmission electron microscopy, and high resolution electron microscopy techniques were employed to characterize the structural properties of the deposited films. The resultant phases and structures of the deposited films depended on both the growth temperature and the substrate surface properties (surface imperfections, steps, etc.). At the growth temperature of 800 °C, single-crystal rutile films were obtained reproducibly with two possible epitaxial relationships. At lower temperatures (400 to 775 °C), the deposited films can be epitaxial or polycrystalline with highly oriented grains. The similarity between the atomic arrangements of the substrate and the film is discussed in detail to explain the observed epitaxial relationships and abruptness of the interfaces.
Preparation and characterization of NH4Bi3F10
Authors: Niznansky D. , Rehspringer J.L.
Published: September 2007

A new NH4Bi3F10 compound was prepared, which results from a reaction between Bi2O3 and NH4F in an argon stationary atmosphere. It has a cubic fluorite-related structure isotypic with KBi3F10 with space group Fm3m (a = 11.974 Å). Further heating leads to decomposition to BiF3. IR spectra at different temperatures show the disappearance of NH4 group characteristic vibrations. Indexed x-ray diffraction pattern and IR spectra at different decomposition temperatures are given.
Pulsed excimer laser deposition and characterization of ferroelectric Pb(Zr0.52Ti0.48)O3 thin films
Authors: Roy D. , Krupanidhi S.B.
Published: September 2007

Lead zirconate titanate (PZT) thin films were prepared by excimer laser ablation on platinum coated silicon substrates. The composition of the films showed dependence on the fluence at low energy densities (<2 J/cm2), and less dependence on the ablation fluence was observed beyond a fluence of 2 J/cm2. A correlation among the fluence, ablation pressure, and substrate temperature has been established. Crystalline perovskite PZT films showed a dielectric constant of 800–1000, a remnant polarization of 32 μC/cm2, and a coercive field of 130 kV/cm. Films showed fatigue behavior that may be used in a device, and a close comparison of fatigue behavior between the films deposited at different energy densities indicated a better fatigue behavior for a fluence of 4 J/cm2.
A small-angle x-ray scattering study of microstructure evolution during sintering of sol-gel-derived porous nanophase titania
Authors: Hackley Vincent A. , Anderson Marc A. , Spooner Steve
Published: September 2007

The evolution of microstructure as a function of firing temperature in sol-gel derived porous titania xerogels was investigated by small-angle x-ray scattering (SAXS). SAXS curves for xerogels fired below 550 °C exhibit a well-defined structure peak. This peak indicates the presence of a high degree of order in the electron density correlations associated with the interparticle structure factor. Results from electron microscopy and SAXS give a primary particle mean diameter of 50 Å, while scaling analysis of the scattered intensity at large momentum transfer values yields the Porod exponent 4, indicating a sharp transition zone between the solid and void phases. The pore volume fraction of the unfired xerogel is consistent with random close-packing of spheres. The internal surface area decreases almost linearly with increasing firing temperature. Rapid grain growth and pore coarsening begin near 90% of theoretical density, and lead to a breakdown in pore interconnectedness and the development of isolated pores. Observed enhanced sintering properties may be attributed primarily to the large surface-to-mass ratio of the sol-gel particles. The SAXS curves were adequately fit using a bicontinuous phase model developed for late-stage spinodal decomposition structures. Alternatively, the microstructure can be described by a hierarchical close-packing of spheres model. SAXS results are compared with data from gas adsorption and electron microscopy.
Morphological studies of nanoclusters on grid-supported polymer thin films
Authors: Spontak Richard J. , Burns Janet L. , Echer Charles J.
Published: September 2007

Modification of substrates by controlled deposition of nanometer-size particulates (nanoclusters) is an efficient means of fabricating materials designed for applications in which specific surface interactions play a vital role (e.g., molecular catalysis and microelectronics). We have found that highly dispersed nanoclusters form on thin films of poly(siloxaneimide) (PSI) copolymers supported on copper transmission electron microscopy (TEM) grids when subjected to long anneals at elevated temperatures. In this note, we report on the composition and source of these anomalous nanoclusters, as determined by a variety of electron microscopical techniques. Spectra obtained with parallel electron energy-loss spectroscopy (PEELS) indicate that these particulates, which typically measure 4–18 nm in diameter, are composed of copper with a mean valence of +1. Electron microdiffraction patterns reveal that the nanoclusters are polycrystalline, possessing lattice spacings similar to those of Cu2O. Mechanistic routes of formation are suggested based on experimental design, and factors influencing formation are also described.
Subcritical crack growth along epoxy/glass interfaces
Authors: Conley K.M. , Ritter J.E. , Lardner T.J.
Published: September 2007

Subcritical crack growth behavior along polymer/glass interfaces was measured for various epoxy adhesives at different relative humidities. A four-point flexure apparatus coupled with an inverted microscope allowed for observation in situ of the subcritical crack growth at the polymer/glass interface. The specimens consisted of soda-lime glass plates bonded together with epoxy acrylate, epoxy (Devcon), or epoxy (Shell) adhesives. Above a threshold strain energy release rate, the subcritical crack velocity was dependent on the strain energy release rate via a power law relationship where the exponent was independent of the adhesive tested and the test humidity (n = 3). However, the multiplicative constant A in the power law relation varied by over three orders of magnitude between the various adhesives with epoxy (Shell) having the smallest value and the epoxy (Devcon) the greatest value; in addition, A was very sensitive to humidity, decreasing by over two orders of magnitude from 80% to 15% relative humidity. At high strain energy release rates, the subcritical crack velocity reached a plateau at approximately 10−6 m/s. The use of this subcritical crack velocity data in predicting thin film delamination is discussed.
Fabrication of Ni–Al thin films by the pulsed laser deposition technique
Authors: Singh R.K. , Bhattacharya Deepika , Sharan S. , Tiwari P. , Narayan J.
Published: September 2007

We have fabricated Ni3Al and NiAl thin films on different substrates by the pulsed laser deposition (PLD) technique. A high energy nanosecond laser beam was directed onto Ni–Al (NiAl, Ni3Al) targets, and the evaporated material was deposited onto substrates placed parallel to the target. The substrate temperature was varied between 300 and 400 °C, and the substrate-target distance was maintained at approximately 5 cm. The films were analyzed using scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and Rutherford backscattering spectrometry. At energy densities slightly above the evaporation threshold, a slight enrichment of Al was observed, while at higher energy densities the film stoichiometry was close (<5%) to the target composition. Barring a few particles, the surface of the films exhibited a smooth morphology. X-ray and TEM results corroborated the formation of Ni3Al and NiAl films from similar target compositions. These films were characterized by small randomly oriented grains with grain size varying between 200 and 400 Å.
Structural evolution of the amorphous solids produced by heating crystalline MgHPO4 · 3H2O
Authors: Sales B.C. , Chakoumakos B.C. , Boatner L.A. , Ramey J.O.
Published: September 2007

The precise structural characteristics of the amorphous solids that are generated during the dehydration of crystalline MgHPO4 · 3H2O (newberyite) are reported. The results show that a dramatic change in the distribution of phosphate anions (i.e., chains of corner-linked PO4 tetrahedra) occurs during the process of dehydration and that anions up to 13 PO4 tetrahedra in length are formed. The distribution of phosphate anions formed at intermediate stages of the dehydration process was found to be in quantitative agreement with the theory of Parks and Van Wazer.
Comparison of Ba2YCu3O7−δ thin films grown on various perovskite substrates by coevaporation
Authors: Phillips Julia M. , Siegal M.P. , van Dover R.B. , Tiefel T.H. , Marshall J.H. , Brandle C.D. , Berkstresser G. , Strauss A.J. , Fahey R.E. , Sengupta S. , Cassanho A. , Jenssen H.P.
Published: September 2007

We have compared the quality of Ba2YCu3O7−δ (BYCO) films grown by the BaF2 process on (100) substrates of the perovskites LaAlO3, LaGaO3, NdAlO3, NdGaO3, LiBaF3, and SrTiO3. The films were grown by coevaporation of Y, Cu, and BaF2 followed by a two-stage anneal. The high temperature stage of the anneal, the part of the process during which the BYCO crystal structure and morphology develop, has been varied. LaAlO3 and SrTiO3 support much better films, both electrically and structurally, than the other substrates. Of the oxides, NdGaO3 supports the worst films, while films on LiBaF3 are nonconducting. These results emphasize the overriding importance of chemical compatibility in determining the suitability of a potential substrate material. Unless this criterion is satisfied, the issue of lattice matching is unimportant.
Microstructures of Bi-based oxide superconductors doped with Sb
Authors: Masuda Yoshio , Ogawa Rikuro , Kawate Yoshio , Hara Naohiro , Tateishi Tsuyoshi
Published: September 2007

Using Bi-based oxide superconductors of the Bi0.96Pb0.24-xSbxSr1.0Ca 1.1 Cu1.6Oy composition which were prepared by the sol-gel method, production of the high-Tc phase and also microstructures of the high-Tc and the low-Tc phases were investigated using the XRD method and the TEM technique. Two superconducting phases showed good linearities of shrinkages of the a, b, and c lattice parameters as the Sb content increased, and so it has been found that Sb can be included in superconducting crystal structures. However, all shrinkages were below 1%. The activation energy of Sb diffusion was estimated from time dependence of changes of the lattice parameters using the Arrhenius plots. Its energy was from 510 to 593 Kj/mol.
On the solidification of Al62Cu20Co15Si3 and Al61Cu19.5Co14.5Si5 alloys
Authors: Grushko B. , Wittmann R. , Urban K.
Published: September 2007

The solidification behavior of Al62Cu20Co15Si3 and Al61Cu19.5Co14.5Si5 alloys was studied by means of optical metallography, scanning and transmission electron microscopy, energy-dispersive x-ray analysis, powder x-ray diffraction, and differential thermal analysis. Slowly as well as rapidly cooled ingots of both alloys contained a decagonal quasicrystalline phase as the dominant phase with, additionally, several minor crystalline phases. The structure of the rapidly solidified Si-containing alloys was similar to that of the ternary Al65Cu20Co15 alloy. In the slowly solidified alloys the substitution of 3 at. % Al by Si did not change the basic phase constitution. Si was only partially incorporated in the decagonal phase and a significant quantity of Si was found in elemental form. The increase of Si concentration to 5 at. % resulted in the appearance of new minor phases.
Combustion synthesis of cubic Al3Ti alloys
Authors: Wright R.N. , Rabin B.H. , McFerran W.H.
Published: September 2007

Combustion synthesis of elemental powder compacts has been used to produce alloys with compositions corresponding to Al3Ti, Al73Ti24Cr3, and Al67Ti25Cr8. The binary composition exhibits a strong exothermic reaction at ∼665 °C, resulting in formation of single phase Al3Ti with the tetragonal DO22 structure. The heat of formation of the compound at 298 K, δHf(298), was estimated from peak reaction temperature measurements to be −35.0 kJ mol−1. Additions of Cr lowered the reaction initiation temperature and resulted in the formation of intermediate products attributed to Al–Cr reactions. The DO22 structure remained the primary reaction product in Cr-containing alloys. The Cr particles did not completely dissolve during initial reaction; an additional heat treatment at 1200 °C or above was required to homogenize the material, resulting in partial or complete transformation of the DO22 structure to the cubic L12 structure, depending upon Cr content. Homogenization at 1250 °C for 4 h produced material consisting approximately of a 50-50% mixture of the DO22 and L12 structures for Al73Ti24Cr3, and 100% L12 for Al67Ti25Cr8. In all cases, reaction at ambient pressure was accompanied by swelling, resulting in final densities less than 60% of theoretical. Application of external pressure, either during or subsequent to combustion synthesis, produced near theoretical density materials exhibiting a homogeneous and equiaxed microstructure.
A study of pest oxidation in polycrystalline MoSi2
Authors: McKamey C.G. , Tortorelli P.F. , DeVan J.H. , Carmichael C.A.
Published: September 2007

MoSi2is a promising high-temperature material with low density (6.3 g/cm3), high melting point (2020 °C), and good oxidation resistance at temperatures to about 1900 °C. However, in the intermediate temperature range between 400 and 600 °C, it is susceptible to a “pest” reaction which causes catastrophic disintegration by a combination of oxidation and fracture. In this study, we have used polycrystalline MoSi2, produced by arc-casting of the pure elements and by cold and hot pressing of alloy powders, to characterize the pest reaction and to determine the roles of composition, grain or phase boundaries, and physical defects on the oxidation and fracture of specimens exposed to air at 500 °C. It was found that pest disintegration occurs through transport of oxygen into the interior of the specimen along pre-existing cracks and/or pores, where it reacts to form MoO3and SiO2. The internal stress produced during the formation of MoO3results in disintegration to powder. Near the stoichiometric ratio, the susceptibility to pest disintegration increases with increasing molybdenum content and with decreasing density. Silicon-rich alloys were able to form protective SiO2and showed no indication of disintegration, even at densities as low as 60%.
On glass formation in rapidly solidified aluminum-based alloys
Authors: Li Q. , Johnson E. , Johansen A. , Sarholt-Kristensen L.
Published: September 2007

Thermodynamic properties of the melts of several Al–Y and Al–Fe–Y alloys are studied by means of calibrated differential thermal analysis. The results can be used to optimize process parameters of rapid solidification which are important for glass formation in the Al-based alloys. Close examinations of the melt-spun alloys show that the process parameters, particularly the temperature of the melts, will influence not only the amorphicity and the chemical short-range order but also the crystallization process of the glasses. A key point of glass formation in the Al-based alloys is found to be related to the content in the melts of a certain amount of the intermetallic compounds which are gradually dissolved in the premelted α Al matrix.
Synthesis of diamond films on Hastelloy
Authors: Godbole V.P. , Narayan J.
Published: September 2007

We have developed a two-step hot filament chemical vapor deposition method to form polycrystalline films of diamond on Hastelloy substrates. The first step at a lower temperature results in the deposition of a composite layer of carbon, diamond-like carbon, and diamond, which provide nucleation sites for diamond growth in the second step at a higher temperature. To obtain a cleaner amorphous carbon-free diamond film, we introduced an intermediate hydrogen etching step. Using this procedure, we have obtained high quality polycrystalline diamond film on Hastelloy substrates, as characterized by scanning electron microscopy and Raman measurements.
Mechanisms of orientation and graphitization of hard-carbon matrices in carbon/carbon composites
Authors: Rellick Gerald S. , Chang Dick J. , Zaldivar Rafael J.
Published: September 2007

It now appears generally accepted that the process of stress-induced orientation and graphitization of a thermoset-resin-derived matrix in a carbon-fiber/carbon-matrix (C/C) composite is principally a result of molecular orientation induced during the pyrolysis process as a consequence of the restraint of pyrolysis shrinkage at the fiber/matrix interface by attractive or frictional forces between fiber and matrix. We hypothesize that the critical factor for the formation of lamellar graphite (by subsequent high-temperature heat treatment), instead of fibrillar or isotropic glassy carbon, is a state of multiaxial deformation in the pyrolysis step. Finite-element stress analyses of the relative stresses in the regions of interfilament matrix, as the matrix pyrolyzes from polymer to carbon, reveal patterns of biaxial and triaxial stress consistent with experimental observations of lamellar graphite formation in the matrix by the techniques of optical microscopy, scanning electron microscopy, and transmission electron microscopy. The implications of localized matrix orientation and graphitization for C/C composite properties are discussed in terms of a “duplex” system of composite reinforcement. An example is presented showing crack deflection and blunting at the matrix/matrix-sheath interface produced as a result of such orientation and graphitization.
Epitaxial growth of Si1−xGex on Si(100)2 × 1: A molecular-dynamics study
Authors: Ethier Stéphane , Lewis Laurent J.
Published: September 2007

We use molecular-dynamics simulations to study the growth of pure Si, Si0.5Ge0.5, and pure Ge on the 2 × 1 reconstructed surface of Si(100) in a way appropriate to the fabrication of thin films by the method of molecular-beam epitaxy (MBE), namely sequential deposition of energetic atoms. The atoms interact with one another via effective potentials of the Stillinger–Weber form, with parameters adjusted such as to describe all possible types of triplet interactions. Motivated by numerous experimental studies of MBE-grown films, we investigate in particular the structure of the deposits as a function of substrate temperature. We find in all three cases that at low substrate temperatures, poorly ordered structures form, while at high substrate temperatures, epitaxial growth takes place. The presence of Ge limits the number of crystalline overlayers that form, even though it appears to favor a more-ordered structure in the initial stages of growth. For pure Ge epitaxy, in particular, only the first three layers are crystalline, after which growth appears to proceed by the formation of islands, reminiscent of the Stranski–Krastanow growth scheme, and in qualitative agreement with recent experimental and theoretical work. In all samples, annealing improves the quality of the films—at least when grown at sufficiently high substrate temperatures. The interdiffusion of the species at the substrate-deposit interface is also examined.
Epitaxial growth of Cu2O films on MgO by sputtering
Authors: Miller Dean J. , Hettinger Jeffrey D. , Chiarello Ronald P. , Kim Hyung K.
Published: September 2007

The epitaxial growth of Cu2O films is of significant interest for the unique potential they offer in the development of multilayer devices and superlattices. While fundamental studies may be carried out on epitaxial films prepared by any technique, the growth of artificially layered superlattices requires that films grow epitaxially during deposition. The present study examined the growth of Cu2O on MgO substrates directly during deposition by sputtering. Although epitaxial thin films of Cu2O could be produced, a mosaic structure was observed. The structure of the film may be related to the growth mechanism in which islands coalesce to form a continuous film.
Evolution of hardness and transmittance in irradiated LiF single crystals at elevated temperatures
Authors: Lin Hung-Ye , Tsai Yu-Zen , Lee Sanboh
Published: September 2007

The evolution of hardness and transmittance of irradiated LiF single crystals at elevated temperatures has been investigated. The Vickers hardness is proportional to φ1/3 where φ is the dosage of γ-rays. The thermal annealing of hardness which is analyzed according to a first-order kinetic process has one stage for φ = 100 KGy and two stages for φ ≥ 250 KGy. For a given dosage, the activation energy of stage I is larger than that of stage II. The transmittance of irradiated LiF crystals is attributed to F, M, R1, and R2 centers. The transmittance decreases with increasing time at the brief durations, then oscillates, and finally increases monotonically. The region in which the transmittance increases monotonically with time is simulated by a second-order kinetic process of the color center. The activation energy of optical absorption was determined. Comparing both activation energies of hardness and optical absorption, we find that the mechanism of the kinetic process for hardness differs from that for optical absorption.
Sol-gel synthesis of Ln2(Ln = La, Nd)Ti2O7
Authors: Prasadarao A.V. , Selvaraj Ulagaraj , Komarneni Sridhar , Bhalla Amar S.
Published: September 2007

Lanthanum and neodymium titanates were prepared by a sol-gel route. Synthesis of La2Ti2O7from pure alkoxide precursors yielded an intermediate perovskite type phase, La(1–x)TiO3, which partially transformed to La2Ti2O7on heating to 1500 °C. Substitution of the lanthanum acetylacetonate for alkoxide precursor yielded La2Ti2O7without any intermediate phase at a very low temperature of 700 °C. Sintering of the La2Ti2O7gel powder yielded a highly dense ceramic with ∼97% theoretical density. Similar sintering treatment resulted in ∼92% theoretical density for Nd2Ti2O7.
Elastic constants of polycrystalline Y1Ba2Cu3Ox
Authors: Ledbetter Hassel
Published: September 2007

We measured the elastic constants of polycrystalline Y1Ba2Cu3Ox with eleven oxygen contents varying from 6.2 to 6.9. Reported properties include sound velocities; bulk, shear, Young's moduli; Poisson ratio; Debye temperature. After correction to the void-free state, the measurements fail to show a systematic dependence on oxygen content. Focusing on the bulk modulus, all measurements fall below an ionic-crystal model prediction, by 5–50%. We attribute this macroscopic softness to either microcracks or weak grain-boundary mechanical linkages. For the case x = 6.9, we suggest a set of intrinsic elastic constants.
Thermochemistry of the Y2O3–BaO–Cu–O system
Authors: Zhou Zhigang , Navrotsky Alexandra
Published: September 2007

High temperature reaction calorimetry using molten lead borate as a solvent is used to study the thermochemistry of phases in the Y2O3–BaO–Cu–O system. Enthalpies of formation of YBa2Cu3Ox (6 < x < 7), YBa2Cu4O8, Y2BaCuO5, Y2Cu2O5, and BaCuO2.01 phases have been determined. The thermodynamic relationships among these phases at room temperature have been assessed. The results show that the 123 phase is not thermodynamically stable with respect to assemblages containing combinations of Y2BaCuO5, Y2Cu2O5, BaCuO2, Y2O3, and CuO. The fully oxidized 123 phase is not stable with respect to the 124 and the above assemblages. Reactions at room temperature are shown as follows: YBa2Cu3O7 (s) + CuO (s) → YBa2Cu4O8 (s); δH = −31.9 ± 22.3 kJ mol−1 and YBa2Cu3O6.76 (s) → ½YBa2Cu4O8 (s) + ¼Y2BaCuO5 (s) + ¾BaCuO2.01 (s); δH = −78.9 ± 21.0 kJ mol−1.
Interfacial reactions between Y1Ba2Cu3O7−x and Ag–Pd alloys during high temperature heat treatments
Authors: Allem R. , Lavallée F. , L'Espérance G. , Schulz R. , Trudeau M.L.
Published: September 2007

The interfacial reactions between the Y1Ba2Cu3O7−x (123) compound and a Ag70Pd30 alloy have been studied for several heat treatments below and above the melting point of the superconductor. Scanning and transmission electron microscopy, energy dispersive x-ray analysis, Auger spectroscopy, and x-ray diffraction have been used to characterize the reaction products. For the heat treatments below the melting point of the superconductor (950 °C), the Ba and Cu of the 123 compound migrate to the interface and react with the Pd to form a body-centered cubic phase Ba(Pd, Cu)O2. This structure is formed by substitution of Cu by Pd on the Cu+2 sites of the BaCuO2 structure (Im3m space group). CuO is also found in some areas as a product of the reaction. For the thermal treatments above the melting point of the superconductor, the reaction products are the same but, in this case, a large number of Y2BaCuO5 precipitates are found in the bulk of the superconductor near the interface. Pure Ag particles have also been observed in the 123 compound at distances relatively far from the interface.
Microstructure of melt-processed Bi2Sr2CaCu2Oy and reaction mechanisms during post heat treatment
Authors: Heeb B. , Oesch S. , Bohac P. , Gauckler L.J.
Published: September 2007

Phase compositions and microstructures of melt processed 2212 were studied. 2212 starting powder was cooled from temperatures between 910 °C and 1100 °C in air at rates ranging from 350 K/min to 0.083 K/min. The solidification sequence was established for all cooling rates. Under all conditions the Bi-free (Sr, Ca)CuO2 (01x1) is the primary phase. The one-layer solid solution 11905 nucleates on this phase. The residual liquid solidifies to a glassy state, decomposes into the eutectic of Cu2O and Bi2Sr2.1Ca0.9Ox, or reacts with the primary phase and the 11905 forming 2212 at high, intermediate, or low cooling rates, respectively. Post solidification heat treatment at 850 °C in air leads to partial remelting. The Cu-rich liquid reacts with 11905 and 01x1 forming 2212. Subsequent solid/solid reactions lead to a high volume fraction of 2212 with almost ideal 2 : 2 : 1 : 2 stoichiometry.
Characteristics of nanophase TiAl produced by inert gas condensation
Authors: Chang H. , Altstetter C.J. , Averback R.S.
Published: September 2007

Nanophase TiAl, with grain sizes in the range of 10–20 nm, was synthesized by magnetron sputtering in an inert gas atmosphere and consolidated, in situ, under vacuum. The properties of the powders and sintered compacts were studied by transmission electron microscopy, scanning electron microscopy, calorimetry, Rutherford backscattering, and x-ray diffraction. Samples compacted at 1.0 GPa at room temperature had a large fraction of amorphous phase, while samples compacted at the same pressure and 250 °C were predominantly the equilibrium γ phase. An enthalpy change of 22 kJ/g-atom was measured during a DSC scan over the temperature range 125–450 °C, which is approximately the range over which crystallization occurs. Nearly full density could be achieved by sintering at 450 °C without significant, concomitant grain growth. The Vickers microhardness of these samples at room temperature and at −30 °C revealed an inverse Hall–Petch relationship at small grain sizes, 10–30 nm, and the usual Hall–Petch behavior at larger grain sizes. A small component of indentation creep was also observed. The maximum hardness is 4 times larger than that of a cast TiAl specimen of the same composition. The Vickers hardness was also observed to decrease rapidly with temperature above 200 °C.
Processing of diamond/alumina composites for low wear applications
Authors: Chu May-Ying , Bhushan Bharat , De Jonghe Lutgard C.
Published: September 2007

A simple hot-pressing procedure for fabricating composites of diamond particulates in an alumina matrix at moderate applied pressures is described. Dense composites with up to 33 vol. % diamond particles are made by pressure-sintering at applied stress of 35 MPa in vacuum atmosphere. Preliminary wear tests of these composites on magnetic thin-film rigid disks show a low friction comparable to that of single crystalline diamond. Diamond/alumina composites can be an economical alternative to diamond or diamond coated materials for abrasion resistant applications.
Prediction of chemical vapor deposition rates on monofilaments and its implications for fiber properties
Authors: Gökoĝlu S.A. , Kuczmarski M. , Veitch L.C.
Published: September 2007

Deposition rates are predicted in a cylindrical upflow reactor designed for chemical vapor deposition (CVD) on monofilaments. Deposition of silicon from silane in a hydrogen carrier gas is chosen as a relevant example. The effects of gas and surface chemistry are studied in a two-dimensional axisymmetric flow field for this chemically well-studied system. Model predictions are compared to experimental CVD rate measurements. The differences in some physical and chemical phenomena between such small diameter (∼150 μm) fiber substrates and other typical CVD substrates are highlighted. The influence of the Soret mass transport mechanism is determined to be extraordinarily significant. The difficulties associated with the accurate measurement and control of the fiber temperature are discussed. Model prediction sensitivities are investigated with respect to fiber temperatures, fiber radii, Soret transport, and chemical kinetic parameters. The implications of the predicted instantaneous rates are discussed relative to the desired fiber properties for both batch and continuous processes.
Suppression of dislocation accumulation in GaAs film on Si substrate by combination of impurity doping and selective area growth
Authors: Ohashi Tetsuya
Published: September 2007

Dislocation accumulation in gallium-arsenide film, which is deposited on silicon substrate and cooled down from the deposition to room temperature, is examined by crystal plasticity simulation. A new approach to suppression of dislocation accumulation is proposed such that selective growth of the film and partial doping of impurities into it are combined. The results show the possibility to localize the plastic deformation near the film/substrate interface and to keep the surface region of the film almost dislocation-free. Geometry of the selectively grown film and a strategy for partial doping of impurities, which suppresses the dislocation accumulation in the surface region of the film, are considered.
Composition/structure/property relations of multi-ion-beam reactive sputtered lead lanthanum titanate thin films: Part I. Composition and structure analysis
Authors: Fox G.R. , Krupanidhi S.B. , More K.L. , Allard L.F.
Published: September 2007

Material properties are greatly dependent upon the structure of the material. This paper, the first of three parts, discusses how composition influences the crystallographic structure and microstructure of lead lanthanum titanate (PLT) thin films grown by the multi-ion-beam reactive sputtering (MIBERS) technique. A transmission electron microscopy (TEM) study detailing the relationship between crystallographic texturing and microstructure development will be presented in a second paper. The dependence of the ferroelectric properties on observed crystallographic structure and microstructure is presented in the third paper of this series. As-deposited PLT microstructures coincide with the structure zone model (SZM) which has been developed to describe the microstructure of thin films deposited by physical vapor deposition. The as-deposited PLT structures are altered during post-deposition annealing as a result of crystallization and PbO evaporation. Amorphous films with more than 10 mole % excess PbO become polycrystalline with porous microstructures after annealing. When there is less PbO in the as-deposited film, 〈100〉 texture and dense structures are observed. Porosity results from PbO evaporation, and 〈100〉 texture is inhibited by excess PbO.
Investigations of the reduction behavior of iron-impregnated alumina gels (Fe/AlOOH) and the formation of Fe0–Al2O3 metal-ceramic composites
Authors: Verelst M. , Kannan K.R. , Subbanna G.N. , Rao C.N.R. , Laurent Ch. , Rousset A.
Published: September 2007

Fe/AlOOH gels calcined and reduced at different temperatures have been investigated by a combined use of Mössbauer spectroscopy, x-ray diffraction, and electron microscopy in order to obtain information on the nature of the iron species formed as well as the various reduction processes. Calcination at or below 1070 K mainly gives reducible Fe3+ while calcination at higher temperatures gives substitutional Fe3+ in the form of Al2−xFexO3. The Fe3+ species in the calcined samples are, by and large, present in the form of small superparamagnetic particles. Crystallization of Al2O3 from the gels is catalyzed by Fe2O3 as well as FeAl2O4. Fe (20 wt. %)/AlOOH gels calcined at or below 870 K give FeAl2O4 when reduced in hydrogen at 1070 K or lower and a ferromagnetic Fe0–Al2O3 composite (with the metallic Fe particles >100 Å) when reduced at 1270 K. Samples calcined at 1220 K or higher give the Fe0–Al2O3 composite when reduced in the 870–1270 K range, but a substantial proportion of Fe3+ remains unreduced in the form of Al2−xFexO3, showing thereby the extraordinary stability of substitutional Fe3+ to reduction even at high temperatures. Besides the ferromagnetic Fe0–Al2O3 composite, high-temperature reduction of Al2−xFexO3 yields a small proportion of superparamagnetic Fe0–Al2O3 wherein small metallic particles (<100 Å) are embedded in the ceramic matrix. In order to preferentially obtain the Fe0–Al2O3 composite on reduction, Fe/AIOOH gels should be calcined at low temperatures (∼1100 K); high-temperature calcination results in Al2−xFexO3. Several modes of formation of FeAl2O4 are found possible during reduction of the gels, but a novel one is that involving the reaction, 2Fe3+ + Fe0 → 3Fe2+.
The amorphization of complex silicates by ion-beam irradiation
Authors: Eby Ray K. , Ewing Rodney C. , Birtcher Robert C.
Published: September 2007

Twenty-five silicates were irradiated at ambient temperature conditions with 1.5 MeV Kr+. Critical doses of amorphization were monitored in situ with transmission electron microscopy. The doses required for amorphization are compared with the structures, bond-types, compositions, and physical properties of the silicates using simple correlation methods and more complex multivariate statistical analysis. These analyses were made in order to determine which properties most affect the critical amorphization dose. Simple two-variable correlations indicate that melting point, efficiency of atomic packing, the dimensionality of SiO4 polymerization (DOSP), and bond ionicity have a relationship with critical amorphization dose. However, these relationships are evident only in selected portions of the data set; that is, for silicate phases with a common structure type. A clearer relationship between the silicate properties and critical amorphization dose was determined for the entire data set with multiple linear regression. Several regression models are proposed which describe the variation in amorphization dose. All regression models contain the following properties: (i) melting point; (ii) a structural variable (DOSP, elastic modulus, and/or atomic packing); and (iii) the proportion of Si–O bonding (instead of bond ionicity). The regression models are equivalent, because they represent combinations of similar properties. Notably, density and atomic mass are not controlling properties for the critical amorphization dose. Melting and amorphization by ion irradiation are apparently related processes. Neither melting point nor critical amorphization dose can be predicted by considering only the structure, composition, or bonding of a particular phase. The Si–O bond is the most covalent bond in silicates, and is the “weak link” in the structure with respect to amorphization. Thus, DOSP is also an important property, as the topology of these “weak links” influences a structure's ability to accumulate amorphous regions. The efficiency of atomic packing is related to the process of defect self-recombination during amorphization. The bulk modulus and shear modulus are important variables within the regression models because of their direct relationship to atomic packing.
Crystallization kinetics of binary borosilicate glass composite
Authors: Jean Jau-Ho , Gupta Tapan K.
Published: September 2007

Kinetics of cristobalite precipitation in a binary glass composite, containing a low-softening borosilicate (BSG) and a high-softening high silica (HSG) glass, have been investigated. XRD results show that the pure glasses do not crystallize under the sintering conditions used, but when mixed in appropriate proportions the cristobalite gradually precipitates out of the initial amorphous binary glass mixture as the sintering continues at temperatures ranging from 800 to 1000 °C. Average linear thermal expansion coefficient (TCE) results show that the TCE increases significantly with increasing precipitation of cristobalite as a function of sintering time. Comparing the experimental TCE results with those theoretically calculated, it is concluded that the precipitation originates most likely in the HSG rather than in the BSG. The precipitation kinetics follow the Avrami equation, and the results show an apparent activation energy of 82 kJ/mol which is close to those for the diffusion of alkali ions in silicate glasses, suggesting mass-transport controlled kinetics. The values of the Avrami exponent are 1.7–1.8, which could be interpreted as a 3-dimension diffusional growth at zero nucleation rate. The linear growth rates of cristobalite, calculated from the precipitation curve, are in the range of 4–8 × 10−5 μm/min, and show slight temperature dependence from 800 to 1000 °C. The linear growth rates of cristobalite are also calculated theoretically using the equation derived by Turnbull et al.,2 and the data are 0–3 orders of magnitude smaller than those observed experimentally. This disparity is attributed to the catalytic effect of the OH and O in air and in the glass network, as well as the diffusion of alkali ions from BSG to HSG.
Synthesis of YBa2Cu3O7−δ with clean grain boundaries by a modified aerosol decomposition process
Authors: Wang Yuan-Liang , Tan Z.Q. , Zhu Yimei , Moodenbaugh A.R. , Suenaga M.
Published: September 2007

A modified aerosol decomposition method was developed for producing sintered YBa2Cu3O7−δ (1-2-3) pellets having sharp superconducting transition Tc's at ∼90 K, fine grain sizes, and clean grain boundaries. This method used “precursor” powders, which were produced with an aerosol flow reactor at 550 °C, instead of in situ produced 1-2-3 powders commonly processed at 880–1000 °C. As determined by x-ray absorption and powder diffraction studies, the 1-2-3 powders produced by the modified route contain less cation disorders than the in situ produced powders. The disorder is speculated to be the cause of Tc-suppression and broadening. It cannot be removed unless high sintering temperatures were used (>950 °C), which resulted in large grains and impurities and cracks at grain boundaries.
TEM study of the early stages of the precipitation process in strip-cast Al3003 alloys
Authors: Cheng Y.F. , Hansen V. , Gj⊘nnes J. , Reine Wallenberg L.
Published: September 2007

The precipitation behavior, especially the early nucleation stages, of the industrial strip-cast Al3003 alloys was investigated by using transmission electron microscopy (TEM). An icosahedral quasicrystalline phase was found as secondary particles in these strip-cast alloys after heat treatment for a few seconds. Three different nucleation paths are proposed based on the TEM observations. They have the same origin, viz. (Mn, Fe)-containing Mackay icosahedra, and are governed by the composition of alloys, especially the Mn and Si content.
The study of pulsed laser deposited films from a pressed, sintered, W–C mixture at two different fluences
Authors: Ghaisas Smita , Vispute R.D. , Ogale S.B. , Choudhari S.M. , Kanetkar S.M. , Kulkarni S.K. , Mahamuni S. , Badrinarayan S. , Ghaisas S.V.
Published: September 2007

The reactive aspect of pulsed laser induced vaporization has been explored via synthesis of tungsten carbide films from an unreacted sintered mixture of the W–C system. Using low-angle x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS), it is shown that stoichiometry and the extent of chemical compound formation in the deposited films depend upon the laser fluence. Two cases with energy density 20 J/cm2 and 40 J/cm2 are discussed and compared. An attempt has been made to illustrate the mechanism behind the reactive aspect of the deposition.
Deposition of diamond onto aluminum by electron cyclotron resonance microwave plasma-assisted CVD
Authors: Eddy C.R. , Youchison D.L. , Sartwell B.D. , Grabowski K.S.
Published: September 2007

Diamond crystallites and thin films have been deposited onto polycrystalline aluminum substrates utilizing an electron cyclotron resonance microwave plasma-assisted chemical vapor deposition (ECR-PACVD) method. For all depositions, the substrates were biased to +40 V dc with respect to ground and their temperature was maintained at 500 °C. Similar deposits were obtained from two different feedgas systems at a total pressure of 1.33 Pa (10 mTorr). The first system consisted of a carbon monoxide (CO) and hydrogen (H2) mixture (CO:H2 = 20:80), and the second was a methane (CH4), oxygen (O2), and hydrogen (H2) mixture (CH4:O2:H2 = 21:10:69). The deposits were subsequently characterized by scanning electron microscopy, micro-Raman spectroscopy, and x-ray diffraction. The results of these analyses indicate that polycrystalline diamond was deposited onto aluminum substrates, as both individual crystallites and continuous films.
Defect density measurements in shocked single crystal ammonium perchlorate by x-ray photoelectron spectroscopy
Authors: Beard B.C. , Sandusky H.W. , Glancy B.C. , Elban W.L.
Published: September 2007

The linewidths of x-ray photoelectron spectra have been correlated with dislocation densities in a shock-damaged crystal of ammonium perchlorate (AP). A centimeter-size AP crystal was loaded at several sites with a diamond pyramid (Vickers) indenter, creating localized strain centers. The crystal was nonuniformly damaged by a rapidly decaying shock (peak pressure of 24.4 kbar at the entry surface), recovered intact, and cleaved through the indentations. The cleaved planes permitted interior analysis of the crystal by x-ray photoelectron spectroscopy (XPS) over a pattern of 1 mm by 1 mm areas. The linewidth of the Cl(2p3/2) spectra ranged from 1.70 eV for the region of greatest visible damage to 1.22 eV for the region of no visible damage, the same linewidth as that obtained for unshocked AP (control). The observed damage was compared to photographs in the literature of gamma-ray irradiated AP crystals, for which dislocation densities were reported. This provided an approximate correlation of dislocation density versus XPS linewidth. The correlation was refined by chemically etching and determining densities on another cleaved plane in the recovered crystal. By this technique, a ∼100X increase in dislocation density was determined for the region of greatest shock damage relative to an unshocked crystal. The strain fields associated with the impressions were found to enhance perchlorate decomposition when driven by shock. Distortion of the molecular lattice in the vicinity of a dislocation is the physical mechanism responsible for the broadening of the photoelectron lines. Ab initio calculations of the Cl(2p) energy level in the perchlorate anion predicted variations of 0.1 to 0.46 eV. Variations of this magnitude are sufficient to produce the observed linewidth broadening.
In situobservation of subcritical crystalline particles and their behavior on growth of KDP crystals from aqueous solution by light-scattering technique
Authors: Lian Li , Taijing Lu , Sakai Kazufumi , Ogawa Tomoya
Published: September 2007

By the 90° laser light-scattering technique, it was possible to visualize ultrafine crystalline particles and to investigate their behavior in relation to the growth of KDP crystals in an aqueous solution. It was found that the scattered microparticles move randomly and exist inhomogeneously in the solution. The density of microparticles increases with increasing supersaturation. Whereas in the bulk solution, the movements of the particles are slow, as soon as they approach the crystal, they are rapidly sucked into the surface to within a certain distance, corresponding to the thickness of the diffusion boundary layer. On the growing surface, they move or shake themselves for a while, and eventually become invisible. The crystal growth rate increases proportionally with increasing the density of the microparticles and the supersaturation. The results suggest that they are subcritical crystalline particles which are formed in the KDP solution, and contribute to the crystallization of KDP from the aqueous solution.