bluuebunny/crossref_metadata_2025 huggingface dataset:
Extension of Gladman's model for abnormal grain growth
Authors: Wörner C.H. , Romero S. , Hazzledine P.M.
Published: September 2007

We present a first principles model for abnormal grain growth in either two or three dimensions that has the virtue of simplicity on the one hand and of close agreement with recent computer simulations on the other. The model is based on Gladman's picture of one grain growing in a matrix of similar grains, retaining its shape as it grows. The grain must have a critical size advantage over its neighbors before the process is spontaneous. Once growth is under way the kinetics are identical to those in Hillert's classical model of grain growth. In the competition between abnormal growth of one grain and the normal growth in the matrix, the important parameters are the ratios γa/γm and μa/μm, where γ is the energy and μ is the mobility of a grain boundary and the subscripts a and m refer to abnormal and matrix, respectively. Values of these three factors: size advantage, energy advantage, and mobility advantage define the conditions under which abnormal grain growth may occur. The results agree closely with recent computer simulations and therefore permit interpolations to be made between computer answers. A hypothesis on the role of second phase particles as a trigger for abnormal grain growth is proposed.
A TEM study of microstructures of YBa2Cu3O7−x thin films deposited on LaAlO3 by laser ablation
Authors: Basu S.N. , Carim A.H. , Mitchell T.E.
Published: September 2007

The microstructures of YBa2Cu3O7−x thin films deposited by laser ablation on single crystal (001) LaAlO3 substrates have been investigated. The orientation of the YBa2Cu3O7−x layer next to the interface is found to be completely c-perpendicular, with a high degree of epitaxy between the film and the substrate. Misfit dislocations, with a periodic spacing of around 13 nm, are present at the interface. Two distinct interfacial structures are seen in these films. At a film thickness of around 400 nm, nucleation of c-parallel grains occurs, leading to a switchover from a c, and, and-perpendicular to a c-parallel microstructure. Amorphous particulates, ejected from the target during processing, lead to the formation of misoriented grains, giving rise to high-angle grain boundaries in the film.
Elstic energy changes accompanying gamma-prime rafting in nickel-base superalloys
Authors: Chang Julius C. , Allen Samuel M.
Published: September 2007

Eshelby's equivalent inclusion method is applied to the case of a single, inhomogeneous, ellipsoidal precipitate in an infinite matrix to study the morphological changes of the gamma-prime precipitates in nickel-base superalloys due to the influence of lattice constant misfit, elastic inhomogeneity and anisotropy, applied stress, and interfacial energy. The energy-minimizing inclusion shapes depend very sensitively on the degree of elastic inhomogeneity, on the sense and magnitude of the applied stress, and on the sense of the lattice constant misfit. The interfacial energy contribution can dominate that of elastic strain energy for small precipitate sizes, elastically compliant systems, nearly homogeneous alloys, and/or nearly isotropic materials. Calculations are carried out for two well-characterized nickel-base alloys: a Ni–13.5Al alloy (positive misfit, elastically hard inclusions) studied by Miyazaki et al. and CMSX-3 (negative misfit, elastically soft inclusions) studied by Pollock. The Eshelby energy calculations correctly predict the precipitate morphologies observed by Miyazaki et al. and by Pollock.
Interdiffusion reactions in Ni/Ta multilayers studied by x-ray diffraction
Authors: Hollanders Mark A. , Duterloo Caroline G. , Thijsse Barend J. , Mittemeijer Eric J.
Published: September 2007

Diffusion-induced phase transformations were studied in Ni/β-Ta multilayers between 523 K and 823 K, primarily using x-ray diffraction. The multilayers had a modulation length, A, of 20.3 nm and a composition of Ni48Ta52. They were polycrystalline without coherency between the Ni and Ta sublayers. Upon annealing at relatively low temperatures (up to 723 K) Ta dissolved in crystalline Ni, concurrently with the formation of an amorphous phase. The interdiffusion reactions did not take place only at the Ni/Ta interfaces, but also along the grain boundaries in the sublayers. The chemical diffusion coefficient in the amorphous phase was determined at 673 K, using a previously developed method. The results were compared with experiments on Ni/Ti multilayers, which show similar reactions. At 723 K and higher temperatures the fcc (Ni, Ta) solid solution transformed into the stable Ni3Ta compound.
Cobalt disilicide formed by rapid thermal annealing and throughmetal arsenic implantation
Authors: Burte Edmund P. , Ye Min
Published: September 2007

Cobalt disilicide CoSi2 of a specific resistivity of 23 μω was formed by the solid phase reaction of cobalt and silicon in the phase sequence of Co2Si, CoSi, and CoSi2 by use of rapid thermal annealing. The through-metal arsenic implantation caused the mixing of cobalt with the silicon substrate and the formation of cobalt silicides. A significant lateral growth of cobalt silicides was observed in samples subjected to one-step rapid thermal annealing process at 900 °C without through-metal ion implantation. Ion beam mixing reduced this lateral silicide growth efficiently, but resulted in a higher density of cobalt atoms remaining in the silicon oxide film than after rapid thermal annealing, as revealed by vapor phase decomposition atomic absorption spectroscopy.
Improvement ofa–Si1−xCx: H/a–Si: Hp/iinterface by hydrogen–plasma flushing studied by photoluminescence
Authors: Han Daxing , Pankove J.I. , Tsuo Y.S. , Qiu C.H. , Xu Y.
Published: September 2007

A hydrogen–plasma reactive flush of the glow discharge reactor after boron–dopeda–Si1−xCx: H film deposition has been used to reduce boron contamination of subsequently deposited intrinsica–Si:H. Photoluminescence studies ofp/istructures ina–Si: H show that the hydrogen–plasma process increases both the luminescence efficiency and the activation energy for the competing nonradiative recombination. The process also shifts the emission peak to higher energies by 25 meV.
An experimental technique for the evaluation of properties of ice
Authors: Chu M.L. , Scavuzzo R.J. , Srivatsan T.S.
Published: September 2007

The problem of ice accretion and accumulation on critical structural components of fixed wing and rotary wing aircraft structures has in recent years engendered much interest. However, the mechanical properties of the accreted impact ices are at present not adequately known and, therefore, analytical modeling of this particular material is not possible. This paper proposes a technique for experimentally determining both the modulus of elasticity and tensile strength of ice. The feasibility and accuracy of the test technique were verified by determining the properties of laboratory grown ice. Results reveal a slight degradation of ice tensile strength with an increase in test temperature. The degradation in ice tensile strength is rationalized on the basis of competing mechanistic effects involving an incipient melting of the ice at the higher test temperature and a concomitant intrinsic variation in microstructural features due to variations in freezing rate. Application of the test technique to impact ices will soon be conducted inside an icing wing tunnel.
Transmission electron microscopy of liquid phase densified SiC
Authors: Carpenter R.W. , Braue W. , Cutler Raymond A.
Published: September 2007

Transmission electron microscopy was used to characterize microstructures of SiC densified using a transient liquid phase (resulting from the reaction of Al2O3 with Al4C3) by hot pressing at 1875 °C for 10 min in N2. High resolution electron microscopy showed that the SiC grain boundaries were free of glassy phases, suggesting that all liquid phases crystallized upon cooling. Phases that might be expected due to reactive sintering (i.e., AlN, Al2OC, Al2O3, Al4O4C, Al3O3N, or solid solutions of SiC, AlN, and Al2OC) were not observed. However, significant Al, Si, O, and C concentrations were found at all triple junctions of these rapidly densified ceramics.
Nucleation and growth of Al2O3/metal composites by oxidation of aluminum alloys
Authors: Salas O. , Ni H. , Jayaram V. , Vlach K.C. , Levi C.G. , Mehrabian R.
Published: September 2007

The nucleation and growth mechanisms during high temperature oxidation of liquid Al−3% Mg and Al−3% Mg−3% Si alloys were studied with the aim of enhancing our understanding of a new composite fabrication process. The typical oxidation sequence consists of an initial event of rapid but brief oxidation, followed by an incubation period of limited oxide growth after which bulk Al2O3/Al composite forms. A duplex oxide layer, MgO (upper) and MgAl2O4(lower), forms on the alloy surface during initial oxidation and incubation. The spinel layer remains next to the liquid alloy during bulk oxide growth and is the eventual repository for most of the magnesium in the original alloy. Metal microchannels developed during incubation continuously supply alloy through the composite to the reaction interface. During the growth process, a layered structure exists at the upper extremity of the composite, consisting of MgO at the top surface, MgAl2O4(probably discontinuous), Al alloy, and finally the bulk Al2O3composite containing microchannels of the alloy. The bulk oxide growth mechanism appears to involve continuous formation and dissolution of the Mg-rich oxides at the surface, diffusion of oxygen through the underlying liquid metal, and epitaxial growth of Al2O3on the existing composite body. The roles of Mg and Si in the composite growth process are discussed.
A thermogravimetric study of the oxidative growth of Al2O3/Al alloy composites
Authors: Vlach K.C. , Salas O. , Ni H. , Jayaram V. , Levi C.G. , Mehrabian R.
Published: September 2007

The oxidation of liquid Al–Mg–Si alloys at 900–1400 °C was studied by thermogravimetric analysis (TGA). The development of a semi-protective surface layer of MgO/MgAl2O4 allows the continuous formation of an Al2O3-matrix composite containing an interpenetrating network of metal microchannels at 1000–1350 °C. An initial incubation period precedes bulk oxidation, wherein Al2O3 grows from a near-surface alloy layer by reaction of oxygen supplied by the dissolution of the surface oxides and Al supplied from a bulk alloy reservoir through the microchannel network. The typical oxidation rate during bulk growth displays an initial acceleration followed by a parabolic deceleration in a regime apparently limited by Al transport to the near-surface layer. Both regimes may be influenced by the Si content in this layer, which rises due to preferential Al and Mg oxidation. The growth rates increase with temperature to a maximum at ∼1300 °C, with a nominal activation energy of 270 kJ/mole for an Al−2.85 wt. % Mg−5.4 wt. % Si alloy in O2 at furnace temperatures of 1000–1300 °C. An oscillatory rate regime observed at 1000–1075 °C resulted in a banded structure of varying Al2O3-to-metal volume fraction.
Epitaxy of barium titanate thin films grown on MgO by pulsed-laser ablation
Authors: Norton M. Grant , Scarfone Christopher , Li Jian , Carter C. Barry , Mayer James W.
Published: September 2007

Thin films of barium titanate (BaTiO3) have been deposited by pulsed-laser ablation onto (001)-oriented MgO substrates. The films were epitactic with the c-axis perpendicular to the film-substrate interface, as evidenced by both transmission electron microscopy (TEM) and ion-channeling techniques. The elastic resonance of 3.045 MeV α-particles, generating the 16O(α, α)16O reaction was used to determine the oxygen stoichiometry of the film and the minimum yield based on the oxygen peaks, thereby enabling conclusions to be drawn about the crystalline perfection of the oxygen sublattice.
Grain growth kinetics and microstructure in the high Tc YBa2Cu3O7−δ superconductor
Authors: Shin M.W. , Hare T.M. , Kingon A.I. , Koch C.C.
Published: September 2007

The grain growth and microstructure development of YBa2Cu3O7−δ have been investigated utilizing two different starting particle size distributions (normal and bimodal). The grain growth exponent, n, was found to be about 0.21 for both normal and bimodal samples. An activation energy of 125 kJ/mole was calculated. The low value of n might be attributed to the high anisotropy of grain boundary energy in this system. Samples made from the bimodal powder were found to accelerate grain growth without introducing abnormal grain growth. Although most of the samples attained fractional densities greater than 0.95, the presence of various amounts of porosity (particularly in the case of the bimodal starting powder) did not affect the growth kinetics. The measured aspect ratio of grains did not significantly change during growth. A significant difference in aspect ratio was measured between samples made from the two different starting powders. Critical currents ranged from 10 to 120 A/cm2, but no concrete relationship with grain size was established. This implies that the grains produced by this experiment were in the size range where other factors, presumably microcracking, severely limited the current carrying capacity by the weak link effect.
Spatial variations in a.c. susceptibility and microstructure for the YBa2Cu3O7−x superconductor and their correlation with room-temperature ultrasonic measurements
Authors: Roth Don J. , DeGuire Mark R. , Dolhert Leonard E. , Hepp Aloysius F.
Published: September 2007

The purpose of this study was to (1) examine the spatial (within-sample) uniformity of superconducting behavior and microstructure in YBa2Cu3O7−x specimens over the pore fraction range 0.10–0.25 and (2) determine the viability of using a room-temperature, nondestructive characterization method (ultrasonic velocity imaging) to predict spatial variability. Spatial variations in a.c. susceptibility were observed for specimens containing 0.10 pore fraction. An ultrasonic velocity image constructed from measurements at 1 mm increments across one such specimen revealed microstructural variation between edge and center locations that correlated with variations in a.c. shielding and loss behavior. Optical quantitative image analysis on sample cross sections revealed pore fraction to be the varying microstructural feature.
Characterization of YBa2Cu4O8 high temperature electrical properties and thermodynamic stability
Authors: Hong B-S. , Mason T.O.
Published: September 2007

Via in situ electrical property measurements (conductivity, Seebeck coefficient) over the temperature range 500–800 °C and oxygen partial pressure range 10−4-1 atm, the equilibrium transport properties and stability range of YBa2Cu4O8 were determined. YBa2Cu4O8 behaves like the intrinsically mixed-valent compound, magnetite (Fe3O4), with small variations in electrical properties with changes in oxygen partial pressure. The decomposition boundary to YBa2Cu3O6+y (or YBa2Cu3.5O7.5±z) and CuO occurs at log(po2, atm) = −1.24 × 104/T(K) + 11.01(773 ⋚ T(K) ⋚ 1073).
A directionally-disordered precursor to amorphization in electron irradiated Cu4Ti3
Authors: Luzzi D.E.
Published: September 2007

Evidence of directional structural disordering prior to complete amorphization of Cu4Ti3 during 2 MeV electron irradiation is presented. It is shown that the observed disordering is intrinsic to the irradiated material and is incommensurate with the structure of the crystalline compound. The results point to the presence of significant anisotropy in the response of the material to irradiation. A model based on local rotation of clusters of atoms within a partially chemically-disordered matrix is developed. These results are discussed in relation to recent proposed mechanisms of amorphization based on volume expansion. The questions raised by this work indicate a need for detailed studies of the atomic level structural evolution of a material during solid state amorphization.
Synthesis of SiC/TaC ceramics from tantalum alkoxide modified polycarbosilane
Authors: Thorne Kevin , Liimatta Eric , Mackenzie John D.
Published: September 2007

The reaction between tantalum ethoxide and an inorganic, silicon-carbon based polymer known as polycarbosilane resulted in a modified polymer that could be thermally converted into a binary ceramic of SiC and TaC. In this report, the initial reaction of the precursors and the high temperature transformations that resulted in the mixed ceramic carbide are discussed. The synthesis of this modified polymer was characterized using 29Si, 13C NMR, and infrared spectroscopy. The reaction involved cross-linking of polycarbosilane through bridging carbon bonds and the formation of Si–OCH2CH3 ligands. According to these data and to the low-angle x-ray diffraction data, the structure of the reaction product can be described as a network of modified polycarbosilane with intimately dispersed tantalum oxide particles. The structural transformations that occurred during inert atmosphere pyrolysis of the polymer product were determined using 29Si, 13C MAS NMR, infrared and x-ray diffraction spectroscopy. Inert atmosphere pyrolysis at temperatures below 500 °C involved continued cross-linking of polycarbosilane through the endothermic formation of bridging carbon bonds. During pyrolysis at 500 °C, an exothermic reaction between the modified polycarbosilane and the intimately dispersed tantalum oxide particles was observed. This reaction involved the formation of an inorganic, amorphous oxycarbide phase that can be described as a continuous network of C–Si–O and C–Ta–O bonds. At pyrolysis temperatures exceeding 1000 °C, carbothermal reduction of the oxide constituents initiated. Further pyrolysis at temperatures exceeding 1200 °C resulted in the crystallization of zinc-blend β–SiC and NaCl structured TaC.
Magnetic microstructure of nanostructured Fe, studied by small angle neutron scattering
Authors: Wagner W. , Wiedenmann A. , Petry W. , Geibel A. , Gleiter H.
Published: September 2007

Small angle neutron scattering (SANS) was applied to achieve insight into the magnetic correlations in nanostructured Fe. The results confirm the expected microstructure involving ferromagnetic grains and a nonmagnetic or weakly magnetic interface region, the interfaces occupying about half the specimen volume. The SANS measurements further reveal that in nanostructured Fe the magnetic correlations are not confined to single grains, but are extended across the interfaces and result in the alignment of the magnetization over several hundreds of grains. An external field of 1.5 kOe is not sufficient for complete magnetic alignment of the entire specimen. However, the long-range magnetic correlations are considerably disturbed by this field. Reducing the external magnetic field to zero causes the magnetic correlations to resume microstructural characteristics similar to what they had in the original state.
Selective epitaxial growth of silicon
Authors: Osenbach J.W. , Schimmel D.G. , Feygenson A. , Bastek J.J. , Tsai J.C.C. , Praefcke H.C. , Bonato E.W.
Published: September 2007

We have studied selective epitaxial growth of silicon in the SiCl2H2/HCl/H2system. Our results on growth rate and selectivity are reported here. We found that the growth rate and selectivity are strong functions of the partial pressures of HCl, SiCl2H2, H2, exposed Si area, and wafer position. We propose that growth and selectivity are controlled by surface reactions between SiCl2H2and HCl and the SiO2and Si surfaces. Higher HCl flow rates decrease the adsorption coefficients and/or incorporation rates of the reactive SiCl2H2(g)species on both SiO2and Si. When either is zero for SiO2and both are greater than zero on Si, selective depositions occur. Finally, we present data on the effect various SiO2surface treatments have on the density and distribution of Si nuclei on this surface.
Thermochemistry in the system Cu–In–S at 298 K
Authors: Migge H.
Published: September 2007

A thermochemical analysis is performed in the system Cu–In–S at 298 K. Free energies of the compounds In6S7, In2.8S4, CuInS2, CuIn5S8, and of the recently discovered CuIn2 have been estimated, the numerical values (kJ/mol) of which are −1043 ± 21, −556 ± 8.8, −315 ± 54, −1238 ± 113, and −51 ± 26. A consistent set of data is used for the calculation of the Gibbs triangle as well as of the predominance area diagram. The results are in nearly complete agreement with the measurements published recently, in particular with those using the nuclear method of perturbed angular correlations (PAC). The compound CuInS2, one of the possible base materials for thin film solar cells, is shown to equilibrate with nearly all of the compounds of the system. A nonvariant four-phase equilibrium CuIn2−InS–In–CuInS2 is found at about 298 K. It is noted where more precise data are needed.
Acoustic wave stimulation of low temperature ceramic reactions: The system Al2O3−P2O5−H2O
Authors: Roy Rustum , Agrawal Dinesh K. , Srikanth V.
Published: September 2007

Acoustic energy has been primarily used in materials research to disaggregate solids and/or disperse them in fluids. It has also been used in a few studies to influence or cause new reactions in organic and organometallic systems. However, the potential of acoustic energy in the area of inorganic reactions and materials science has been virtually unexploited. We report herein new effects of acoustic waves in catalyzing reactions in the systems Al2O3 + H2O and Al2O3 + P2O5 + H2O. In this study the effect of different alumina precursors and their particle size on their reactivity with water and H3PO4 was investigated when exposed to 20 kHz waves.
Microstructure of entrained air voids in concrete, Part II
Authors: Rashed Ahmed I. , Williamson Robert Brady
Published: September 2007

The microstructure of air voids in both air-entrained and non air-entrained paste, mortar, and concrete containing silica fume has been studied at different ages (5 min to 60 days) in order to understand how air-entrained voids form in portland cement–silica fume systems. Scanning electron micrographs of air voids are presented for many different ages. The solidification process of portland cement–paste and mortar was frozen at different ages using both a low-temperature scanning electron microscope and freeze drying. During the course of these experiments, it was discovered that air voids can be considered “windows” in the microstructure in which one can observe early hydration features that give insight into the nature of portland-cement systems. A well-developed sheaf of wheat morphology has been observed in concretes incorporating silica fume, and a new interpretation of the microstructure of silica fume concrete is presented.
Electron channeling patterns of a-axis and c-axis YBa2Cu3O7−δ thin films
Authors: Young K.H. , Sun J.Z. , Kapolnek D. , Tarsa E.J. , Inam A. , Chai B.H.T.
Published: September 2007

We have studied the electron channeling patterns (ECPs) of both pure a-axis and pure c-axis epitaxial YBa2Cu3O7−δ thin films on various substrates. ECPs from three representative a-axis films [on PrBa2Cu3O7−δ buffered LaAlO3 (001), MgO (001), and SrLaAlO4 (001)] and three c-axis films [on LaAlO3 (001), NdGaO3 (001), and SrTiO3 (001)] are presented. The differences between both sets of ECPs are discussed and recorded as references for future identifications.
Grain boundary in textured YBa2Cu3O7−δ superconductor
Authors: Zhu Yimei , Zhang H. , Wang H. , Suenaga M.
Published: September 2007

The misorientations of over 200 pairs of adjacent grains separated by grain boundaries in textured YBa2Cu3O7−δ were measured using a transmission electron microscopy technique. The results indicate that there exist discrete preferred rotation angles and rotation axes. The existence of low-energy boundaries is inferred. The results are analyzed based on the Constrained Coincidence Site Lattice (CCSL) and O2-lattice theories and imply the applicability of such theories for the case of large-angle grain boundaries in a complex crystal structure such as YBa2Cu3O7−δ. The results of analysis also show that some boundaries are likely to be reduced in oxygen near the boundary to satisfy the constraint of the coincidence site lattice.
Green's function for generalized Hilbert problem for cracks and free surfaces in composite materials
Authors: Tewary V.K.
Published: September 2007

Green's function for a generalized vector Hilbert problem is calculated which can be used to solve the Hilbert problem with any integrable inhomogeneity. The Green's function is obtained by using a complex transform defined by eigenfunctions of the homogeneous Hilbert problem. This method should be particularly convenient for the stress analysis of anisotropic composite materials containing cracks and free surfaces. The method is illustrated by applying it to calculate the stress field in an anisotropic bimaterial composite containing an interfacial crack. It is found that, in agreement with the earlier published work on isotropic composites, the stress field is oscillatory but, except very near the crack tip, the oscillations are negligible. Numerical results are presented for a stress field in a uniformly loaded (shear and compressive) Cu/Ni layered composite containing an interfacial crack as well as cracked homogeneous copper.
Germanium supersaturation during the crystallization of amorphous Te–Ge–Sn thin films
Authors: Libera M. , Chen M. , Rubin K.
Published: September 2007

The structure and phase relations of Te–Ge–Sn thin films are examined with application to erasable optical storage media. Free energy data from the literature predict that the region of the Te–Ge–Sn phase diagram between Ge, Sn, and the TeGe–TeSn pseudobinary consists of one two-phase field [α–Ge and Te50 (GexSn1−x)50] and one three-phase field (α–Ge, β–Sn, and TeSn). Electron diffraction from five different Te–Ge–Sn films annealed at 623 K experimentally confirms this prediction. One composition from the two-phase field is deposited as a tri-layer film with the structure 150 nm SiO2/75 nm Te36.3Ge47.4Sn16.3/150 nm SiO2 on a grooved disk substrate, and the microstructure resulting from low-power (12 mW) CW and higher-power (∽50 mW) pulsed laser exposure is studied by transmission electron microscopy and electron diffraction. Of particular significance is that laser-induced crystallization produces a single-phase structure consisting of the Te–Ge–Sn compound phase which is supersaturated with respect to the excess Ge. This supersaturation leads to a disordering of the equilibrium NaCl-type structure of this phase. Crystallization of a micron-sized amorphous spot on a ∼200 ns time scale occurs by a diffusionless process. The fast erase times required by a phase-change optical recording application can thus be achieved in off-stoichiometric compound compositions by way of a nonequilibrium crystallization process.
Preparation and structural characterization of sputtered CoO, NiO, and Ni0.5Co0.5O thin epitaxial films
Authors: Carey M.J. , Spada F.E. , Berkowitz A.E. , Cao W. , Thomas G.
Published: September 2007

Single phase CoO, NiO, and Ni0.5Co0.5O epitaxial films have been prepared by reactive sputtering onto 〈0001〉 α−Al2O3 substrates maintained at 373 K. Epitaxy was confirmed by x-ray diffraction (XRD) and high resolution electron microscopy (HREM) techniques. XRD experiments indicate that these monoxide films are cubic and contain rotation twins with the twin axis parallel to 〈111〉. Lattice parameters for the CoO and NiO films are 0.4254 ± 0.0001 nm and 0.4173 ± 0.0006 nm, respectively, and agree with published values for the corresponding bulk oxides. The lattice parameter 0.4220 ± 0.0001 nm for the Ni0.5Co0.5O film lies between those of CoO and NiO and suggests that the mixed oxide film is compositionally homogeneous. Cross-sectional HREM images of the Ni0.5Co0.5O specimen show Σ3(12) twin boundaries perpendicular to the oxide-substrate interface. The twin regions are approximately 30 nm in size and are uniformly distributed throughout the film. The epitaxial orientation of the monoxide films with respect to the substrate can be summarized by the relationships [111] monoxide // [0001] α−Al2O3, [10] monoxide // [100] α−Al2O3, and [11] monoxide // [110] α−Al2O3.
In situ growth rate measurement and nucleation enhancement for microwave plasma CVD of diamond
Authors: Stoner B.R. , Williams B.E. , Wolter S.D. , Nishimura K. , Glass J.T.
Published: September 2007

Laser reflection interferometry (LRI) has been shown to be a useful in situ technique for measuring growth rate of diamond during microwave plasma chemical vapor deposition (MPCVD). Current alternatives to LRI usually involve ex situ analysis such as cross-sectional SEM or profilometry. The ability to measure the growth rate in ‘real-time’ has allowed the variation of processing parameters during a single deposition and thus the extraction of much more information in a fraction of the time. In situ monitoring of growth processes also makes it possible to perform closed loop process control with better reproducibility and quality control. Unfortunately, LRI requires a relatively smooth surface to avoid surface scattering and the commensurate drop in reflected intensity. This problem was remedied by greatly enhancing the diamond particle nucleation via the deposition of an intermediate carbon layer using substrate biasing. When an unscratched silicon wafer is pretreated by biasing negatively relative to ground while in a methane-hydrogen plasma, nucleation densities much higher than those achieved on scratched silicon wafers are obtained. The enhanced nucleation allows a complete film composed of small grains to form in a relatively short time, resulting in a much smoother surface than is obtained from a film grown at lower nucleation densities.
Chemical vapor deposition of copper via disproportionation of hexafluoroacetylacetonato(1,5 -cyclooctadiene)copper(I), (hfac)Cu(1,5-COD)
Authors: Jain A. , Chi K.M. , Hampden-Smith M.J. , Kodas T.T. , Farr J.D. , Paffett M.F.
Published: September 2007

Hot- and cold-wall chemical vapor deposition (CVD) using the volatile copper(I) compound (hfac)Cu(1,5-COD), where hfac = 1,1,1,5,5,5,-hexafluoroacetylacetonate and 1,5-COD = 1,5-cyclooctadiene, as a precursor was carried out in hot-wall and warm-wall, lamp-heated reactors using SiO2 substrates that had been patterned with Pt or W, over a temperature range 120 °C-250 °C. Deposition was observed onto Pt, W, and SiO2 over this temperature range at rates of up to 3750 Å/min to give copper films that contained no detectable impurities by Auger electron spectroscopy and gave resistivities of 1.9-5.7 μ ohm cm. The volatile by-products formed during deposition were 1,5-COD and Cu(hfac)2 and a mass balance was consistent with the quantitative disproportionation reaction: 2(hfac)Cu(1,5-COD) → Cu + Cu(hfac)2 + 2(1,5-COD). The measured activation energy for this CVD reaction was 26(2) kcal/mol. The absence of selectivity for metal surfaces in the presence of SiO2 is in contrast to CVD results for the related compounds (β-diketonate)Cu(PMe3) where β-diketonate = hfac, 1,1,1-trifluoroacetylacetonate (tfac), and acetylacetonate (acac).
Observations on the role of Mg and Si in the directed oxidation of Al–Mg–Si alloys
Authors: Nagelberg Alan S.
Published: September 2007

The initiation and growth of α–Al2O3/metal composites by the directed oxidation of molten Al–Mg, Al–Si, and Al–Mg–Si alloys at 1373 and 1523 K were investigated. Spontaneous initiation and growth occurred only on the ternary Al–Mg–Si alloy. Growth also occurred on the Al-Mg binary alloy, but in this case initiation required the mechanical disruption of the protective oxide formed during initial heating to the growth temperature. In addition, mechanical disruption of the protective oxide scale on the Al–Mg–Si alloy enhanced growth initiation. Growth on the Al–Si alloy could not be induced under any conditions. From these observations it is concluded that, in the directed oxidation of Al–Mg–Si alloys, Mg is essential to the accelerated oxidation reaction, while Si appears to play a role in promoting the breakdown of the protective oxide layers. The most uniform initiation and growth results were obtained by providing a thin layer of SiO2 particles at the initial growth surface of the alloy.
Preparation of Bi based high-Tc superconductors containing Pb and Sb by the sol-gel method
Authors: Masuda Y. , Ogawa R. , Kawate Y. , Tateishi T. , Hara N.
Published: September 2007

Preparation of Bi based high-Tc superconductors having Pb and Sb elements has been attempted using the sol-gel method. Production of the high-Tc phase was promoted by doping a little Sb (0.05 mol) together with Pb. The nominal composition of the system is Bi0.96Pb0.19Sb0.05Sr1.0Ca1.1Cu1.6Oy. It has been found that the reaction scheme of Sb doped samples is different from that of Sb nondoped samples.
Dislocations in plastically deformed L12 compounds based on Al3Ti
Authors: Morris D.G.
Published: September 2007

The fine structure of dislocations in lightly deformed samples of several cubic ordered alloys with composition based on Al3Ti has been examined by weak beam electron microscopy. For all the materials examined the dislocations tend to dissociate into two 1/2(110) partials separated by APB. Dislocation dissociation is not complete at very small strains and the strain required to dissociate, as well as the dissociation distance, varies from one alloy to another. Improvements in ductility achieved by alloying are directly related to the ease and extent of this dissociation.
Initial stage Fe-clustering in the Au–Fe spin-glass system
Authors: Chou Chen-Chia , Wayman C.M.
Published: September 2007

Aging experiments were conducted to study initial state Fe-clustering in Au–Fe alloys with Fe content from 10.7 to 33%. Information derived from experiments using transmission electron microscopy, electron diffraction, and high resolution electron microscopy suggests the coexistence of short-range-order and Fe-clusters. At early stages of aging, lobe-like and/or rod-shaped strain contrast images, identified as clusters, were revealed after specimens were further cleaned by an ion-miller. The (11/20) special point diffuse reflections were prominent in the as-quenched condition and/or early stage aging, and the intensities decrease gradually as aging proceeds. After a certain period, (11/20) diffuse reflections disappear but strain contrast images still remain similar. This suggests that the strain contrast images are not related to the (11/20) diffuse reflections. The cluster characteristics of Au–Fe alloys are differentiated from those of Al–Cu and Cu–Be based upon theoretical calculations.
Anisotropic thermal conductivity of rare earth–transition metal thin films
Authors: Shaw-Klein L.J. , Hatwar T.K. , Burns S.J. , Jacobs S.D. , Lambropoulos J.C.
Published: September 2007

Thermal conductivity measurements were performed on several amorphous rare earth transition metal thin films of varying microstructure. The thermal conductivity perpendicular to the plane of the film, measured by the thermal comparator method, was compared with the thermal conductivity value measured parallel to the plane of the film. The latter value was obtained by converting electrical conductivity values to thermal conductivity via the Wiedemann–Franz relationship. As expected, the columnar microstructure induced during the sputter deposition of the thin films causes an anisotropy in the thermal conductivity values, with the in-plane values consistently lower than the out-of-plane values. The effect is most pronounced for the more columnar films deposited at higher pressure, for which the in-plane thermal conductivity, 0.3 W/mK, is an order of magnitude lower than the out-of-plane thermal conductivity, 4.3 W/mK. The thermal conductivity out of the plane of the film decreased with increasing deposition pressure, due to the decreasing film density.
Imaging of stacking faults in highly oriented pyrolytic graphite using scanning tunneling microscopy
Authors: Snyder S.R. , Foecke T. , White H.S. , Gerberich W.W.
Published: September 2007

Scanning tunneling microscopy images of the (0001) plane of highly oriented pyrolytic graphite show defect regions consisting of an extensive network of partial dislocations that form extended and contracted nodes. The partial dislocations in hexagonal graphite enclose triangular regions (∼1000 nm on a side) of faulted material comprised of rhombohedral graphite. Electronic and elastic interactions of the tip with the HOPG surface are proposed to explain the observed image contrast between hexagonal and rhombohedral graphite.
Temperature dependence of epitaxial growth of Al on Si(111) by chemical vapor deposition
Authors: Nishikawa Satoshi , Tani Kouichi , Yamaji Tetsuo
Published: September 2007

Chemical vapor deposition of aluminum films using tri-isobutyl aluminum on Si(111) wafers has been studied from the viewpoint of structural and electrical properties of Al films as a function of substrate temperature(Ts).The epitaxial relation of Al on Si is found to be very sensitive toTs, thus changing from Al(100)))Si(111) with Al[1$\overline 1$0]))Si[11$\overline 1$] to Al(111)))Si(111) with Al[1$\overline 1$0]))Si[1$\overline 1$0] around 410 °C in the course of increasingTs. The epitaxial relation is mainly determined at the initial stage of the deposition, but in some cases the relation changes with increasing film thickness. Above 420 °C, single-crystalline Al(111) is grown on Si(111), which has resistivity as low as the bulk value, high reflectivity, and a very flat surface.
Diamond growth with locally supplied methane and acetylene
Authors: Yarbrough W.A. , Tankala K. , DebRoy T.
Published: September 2007

Experiments have been conducted to examine the relative importance of different gas feed geometries and molecular species in the hot filament assisted vapor phase deposition of diamond. Remote and local (relative to the substrate) gas feeds of methane and acetylene were tested and it was found that although dramatic effects on uniformity and rate of deposition can be observed using a gas feed of methane local to the substrate surface, little or no variation in growth rate or deposition uniformity was produced by feeding acetylene locally. The growth rate observed on a defined area of the substrate using a local gas feed of methane was 0.50 mgms/h versus 0.17 mgms/h for a local feed of acetylene. These results, as well as the patterns of deposition observed, suggest that the major species contributing to growth in the hot filament assisted CVD of diamond is the methyl radical. This species is readily formed in the gas phase from methane and atomic hydrogen by hydrogen abstraction. It is, however, further suggested that numerous different hydrocarbon species may be of importance, with the nature of the species contributing most to the growth dependent on the method used.
Morphological instabilities in the low pressure synthesis of diamond
Authors: Ravi K.V.
Published: September 2007

Morphological instabilities attending the high growth rate of diamond films are examined. Pertinent literature on morphological instabilities and microstructure evolution in vapor deposited films is reviewed and theoretical treatments related to the case of diamond growth are discussed. Diamond films of various thicknesses have been synthesized utilizing the combustion flame synthesis technique involving diamond growth rates of ∼1 μm/min. Films of thicknesses under 20 μm are found to be dense and the surface smoothness of such films is governed by facets on the individual crystallites that make up the film. Increasing film thicknesses, at high growth rates, results in extremely rough surfaces, the trapping of voids and discontinuities, and the incorporation of non-diamond phases in the growing film. These characteristics are typical of morphological instabilities when surface diffusion and re-evaporation processes are absent and instability is promoted by the high rate arrival of the appropriate species from the flame ambient to the surface. Factors contributing to morphological instabilities include competitive shadowing and nutrient starvation and growth anisotropy of the different crystallographic faces on individual diamond crystals. It is shown that surface temperature and the presence of oxidizing species in the flame ambient contribute to anisotropic growth of diamond crystals and hence to morphological instabilities in diamond films. An approach to avoiding these instabilities is briefly discussed.
Spatially and spectrally resolved cathodoluminescence of hot-filament chemical-vapor-deposited diamond particles
Authors: Robins Lawrence H. , Farabaugh Edward N. , Feldman Albert
Published: September 2007

Spectrally resolved cathodoluminescence (CL) images and spatially resolved CL spectra were obtained from two specimens grown by hot-filament chemical vapor deposition. Each specimen consisted of a large number of unconnected diamond particles with cubo-octahedral and pseudo-fivefold twinned growth habits. The growth temperature was nominally 600 °C for one specimen and 750 °C for the other. In the 1.5–3.5 eV range, the spectra are composed of four defect and impurity related bands: there are three bands with zero-phonon lines at 1.68 eV, 2.156 eV, and 2.325 eV, and one broad band centered at 2.85 eV. A weak peak at 5.27 eV, due to exciton recombination, was also observed. Spectrally resolved images of the two most intense CL bands, at 2.156 eV and 2.85 eV, were obtained for several particles. In the low-temperature specimen, bright regions in images of the 2.156 eV band are correlated with {111} facets; bright regions in images of the 2.85 eV band are correlated in some cases with the central regions of {100} facets and in other cases with {111} facets. In the intermediate-temperature specimen, bright regions in the images of both bands are correlated with {100} facets. A model of competing recombination at different types of CL centers and nonradiative centers is proposed to facilitate the interpretation of the experimental results. For the low-temperature specimen, the model suggests that the 2.156 eV CL centers are located primarily in {111} growth sectors and the 2.85 eV CL centers are distributed relatively uniformly; images of the two dominant CL bands are predicted to have a complementary relationship in particles where there are few competing nonradiative centers. For the intermediate-temperature specimens, the model suggests that nonradiative recombination is dominant, and that the CL image contrast arises primarily from a nonuniform distribution of nonradiative centers.
Characterization of chemical bonding and physical characteristics of diamond-like amorphous carbon and diamond films
Authors: Bhushan Bharat , Kellock Andrew J. , Cho Nam-Hee , Ager Joel W.
Published: September 2007

Diamond-like (amorphous) carbon (DLC) films were prepared by dc magnetron sputtering and plasma enhanced chemical vapor deposition (PECVD) and diamond films were prepared by microwave plasma enhanced chemical vapor deposition (MPECVD). For the first time, chemical and mechanical characterization of the films from each category are carried out systematically and a comparison of the chemical and physical properties is provided. We find that DLC coatings produced by PECVD are superior in microhardness and modulus of elasticity to those produced by sputtering. PECVD films contain a larger fraction of sp3-bonding than the sputtered hydrogenated carbon films. Chemical and physical properties of the diamond films appear to be close to those of bulk diamond.
Photoluminescence and optical absorption in CaS : Eu2+ : Sm3+ thin films
Authors: Zhang J-G. , Eklund P.C. , Hua Z.L. , Salamanca-Riba L.G. , Wuttig M. , Soltani P.K. , Storti G.M.
Published: September 2007

The optical absorption and photoluminescence of Eu2+ : Sm3+-doped CaS films have been investigated in this paper. The energy dependence of the refractive index and absorption coefficient of the film were obtained by analyzing transmission and reflection spectra of the thin film. The refractive index at low energies is very close to the bulk value for CaS (n = 2.03) and decreases with increasing energy in the range of 1.2 to 2.5 eV. The energy gap of CaS: Eu2+ : Sm3+ thin films is found to be a direct gap with value Eg = 4.48 eV. Photoluminescence studies on annealed CaS: Eu2+ : Sm3+ thin films showed a broad band at ∼1.92 eV identified with emission from Eu2+ ions, and a set of sharp lines corresponding to emission from Sm3+ ions. Most of the strong features presented in the room temperature spectra are found in good agreement with previous bulk studies of CaS: Eu2+ and CaS: Sm3+. The abnormal increase of the Sm3+ photoluminescence with increasing temperature is explained by the phonon-assisted energy transfer from Eu2+ ions to Sm3+ ions.
Low temperature synthesis of ultrafine strontium titanate (SrTiO3) powders
Authors: Potdar H.S. , Deshpande S.B. , Godbole P.D. , Gunjikar V.G. , Date S.K.
Published: September 2007

Ultrafine (≍0.4 μm) strontium titanate powder was synthesized using strontium nitrate and potassium titanyl oxalate as precursors. An aqueous (0.1 M) potassium titanyl oxalate solution was added dropwise to a (0.1 M) strontium nitrate solution with continuous stirring at room temperature to precipitate strontium titanyl oxalate (STO). The precipitate, after careful washing with distilled water several times, was air-dried and calcined at 550 °C/6 h to obtain homogeneous, spherical, stoichiometric powder of SrTiO3 ≍0.4 μm in size. The calcined powder was characterized using different techniques such as XRD, DTA/TG/DTG, SEM, and IR.
Modified resin–intermediate processing of perovskite powders: Part II. Processing for fine, nonagglomerated Sr-doped lanthanum chromite powders
Authors: Tai Lone-Wen , Lessing Paul A.
Published: September 2007

Nonagglomerated ceramic precursor for Sr-doped lanthanum chromite powder was prepared by a modified resin–intermediate process using equimolar citric acid, ethylene glycol, and metallic nitrates. Pure and well-crystallized submicron powders with a narrow particle-size distribution were calcined from an optimally pulverized resin at a temperature of 750 °C for 2 h in flowing air. The material cost for organics used in the investigated process was considerably lower than the original Pechini process.
Radiation degradation of poly(butene-1 sulfone) and poly(1,4-phenylene ether sulfone) in the soft x-ray region
Authors: Yates Brian W. , Shinozaki Douglas M.
Published: September 2007

The mass spectral fragment ion yields and total electron yields have been measured for the polymers poly(butene-1 sulfone) [PBS] and poly(1,4-phenylene ether sulfone) [PPES] around the sulfur 2p (LII) edge, using synchrotron radiation from the Canadian Synchrotron Radiation Facility [CSRF]. Ionization of the sulfur 2p orbitals of PBS produced a sharp decrease in the neutral yields of SO2 and 1-butene. Decreases in the SO2 and 1-butene neutral yield shapes “mirrored” the structure present in the total electron yield. Ionization of the sulfur 2p orbital does not appear to produce any enhanced cleavage of the C–S bonds in the polymer backbone. A G(olefin)/G(SO2) ratio of 0.96 ± 0.15 was determined at 21 °C from mass spectrometry, consistent with a depropagation reaction mechanism. PPES showed a much different behavior than PBS about the sulfur 2p edge, with no detectable yield of SO2.
Absorption coefficient of β–SiC grown by chemical vapor deposition
Authors: Solangi Attaullah , Chaudhry M.I.
Published: September 2007

The absorption coefficient of β–SiC is measured by the transmission technique. β–SiC films are grown on Si substrates by chemical vapor deposition using a buffer layer between β–SiC and the substrate. The absorption coefficient value ranges from 10 to 5.5 × 104 cm−1 in the energy range between 2.42 and 4.4 eV. The bandgap of β–SiC films investigated in this study ranges between 2.13 and 2.32 eV.