bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling of diamond growth from a microwave plasma: C2H as growth species
Authors: Rau Hans , Picht Friederike
Published: September 2007

Diamond growth experiments were performed in a microwave plasma ball reactor on silicon wafers or on a molybdenum sheet provided with cones (stamped into the sheet with a punch). All substrates had been treated by scratching with diamond powder in advance. The gas mixture used was CH4/H2, sometimes with the addition of CO. Substrate temperatures ranged from 953 to 1428 K, pressures from 100 to 400 mbar, and microwave powers from 250 to 700 W. A strong preference of diamond growth was observed on the cones in the molybdenum substrates. This is interpreted as being caused by gas transport hindrance. The resulting deposition coefficient of the “active” species is about 0.1 under all conditions investigated. The deposition experiments on silicon substrates are numerically modeled in two steps. In the first step, temperature fields and electron density and energy distributions in pure hydrogen are calculated following the method described previously. The output of this first simulation step is taken as input data for the second step. The condition is applied that chemical reaction rates due to thermal or electronic activation and diffusional flows compensate each other at every point of the reactor. In this way stationary concentrations of the 13 species in 29 elementary reactions are computed and, from these, the expected deposition profile of diamond on the silicon substrate, assuming one of the carbon-containing species to be the “active” one. When the experimental deposition profiles are compared with the calculated ones, C2H as the “active” species gives the best match to all the experimental results. CH3 and C2H2 (and perhaps others) might contribute to the diamond growth to a limited extent only.
Reaction study of cobalt and silicon nitride
Authors: Nguyen Tue , Ho Herbert L. , Kotecki David E. , Nguyen Tai D.
Published: September 2007

The interaction of cobalt (Co) and low-pressure chemical-vapor-deposited silicon nitride (LPCVD Si3N4) during anneals from 200 °C−1000 °C in vacuum, Ar, and Ar–H2 ambient (95% Ar and 5% H2) has been studied. After the anneals, reduction of Si3N4 by Co to form cobalt silicide and cobalt nitride phases has been observed. Reduction of Si3N4 initially occurs at 600 °C; however, gross physical damage occurs at temperatures of ∼900 °C in Ar. The addition of hydrogen to the ambient enhances the onset of physical damage to the nitride film by as much as 200 °C. Mechanisms governing the Co/Si3N4 reaction have been proposed.
Chemical reaction strengthening of Al/TiC metal matrix composites by isothermal heat treatment at 913 K
Authors: Mitra R. , Fine M.E. , Weertman J.R.
Published: September 2007

The effect of solid state heat treatment at 913 K on extruded XD Al/TiC metal matrix composite with 0.7 and 4.0 μm particle sizes has been investigated. The interfaces between Al and TiC after extrusion were atomically abrupt, as observed by HRTEM. On holding at 913 K, the composite with submicron particle size showed substantial changes in the phases present due to reaction between Al and TiC at 913 K. The stable reaction products are Al3Ti and Al4C3. A substantial increase in Young's modulus occurs. The room and elevated temperature strength and hardness of the composite with submicron particles also increase significantly with time of heat treatment, but at the expense of ductility. The effect of heat treatment over the time range investigated is limited to the interfaces for the 4.0 μm TiC particle size composite due to longer diffusion paths.
Interfaces in as-extrudedXDAl/TiC and Al/TiB2metal matrix composites
Authors: Mitra R. , Chiou W.A. , Fine M.E. , Weertman J.R.
Published: September 2007

A detailed study was conducted of the microstructure and particle-matrix interfaces in Al/TiCpmetal matrix composites prepared by theXDprocess and subsequent extrusion. A study of the morphology of the TiC particles showed that the surfaces are low index (111) and (200) planes, the former being more common. Direct contact on an atomic scale is established between Al and TiC, allowing chemical bonds to form. Young's modulus is in the range expected for a composite of Al and TiC with good interfacial bonding and load transfer to the particles. No third element has been detected at the interfaces, showing that they are clean. Both incoherent and semicoherent interfaces are seen. The interface character depends on the size of the particles and their orientation with respect to the neighboring Al grains. “Special” interfaces with evidence for nearly periodic dislocations were observed in bothXDAl/TiC and Al/TiB2composites, indicating the general tendency ofin situcomposites to lower their interfacial energy by forming such boundaries.
Epitaxical nucleation of polycrystalline silicon carbide during chemical vapor deposition
Authors: Sheldon Brian W. , Besmann Theodore M. , More Karren L. , Moss Thomas S.
Published: September 2007

Polycrystalline silicon carbide was deposited from methyltrichlorosilane in cold-walled and hot-walled reactors, on (100) SiC surface layers that were formed on (100) Si wafers. The initial stages of the process were studied by electron microscopy after relatively short deposition times. Submicron surface features nucleated with a specific crystallographic orientation with respect to the substrate, where {111} planes in the β–SiC substrate coincided with {0001} planes in the α–SiC features. These α–SiC features occurred only at twins on {111} planes of the β–SiC substrate. This demonstrates that nucleation under these conditions is controlled by defects in the substrate. Surface contamination and the reactor configuration also had substantial effects on nucleation.
An electron microscopy study of the precipitation of rhenium in the B2 nickel aluminide
Authors: Van Aken D.C. , Mason D.P. , Malhotra S.G. , Webber J.G.
Published: September 2007

Precipitation of rhenium in β–NiAl was studied by analytical and high resolution electron microscopy. Extended solid solutions were created by solidification processing, and the precipitation and hardening behavior were studied. Evidence suggests that rhenium is initially precipitated as disks parallel to {100} of β-NiAl. Subsequent growth produces either a rod morphology or a pair of twin-related rods that form the shape of a butterfly. The twin plane of the butterfly is {1011} and this plane is nearly parallel to {110} of β–NiAl. A twinning transformation given by K1 = {1011}, K2 = {1013}, η1 = 〈1012 〉, and η2 = 〈3032〉 was determined for the butterfly-shaped particles. All of the precipitates exhibited an orientation relationship consisting of parallel close-packed planes and directions, i.e., (101//(0001) and [111]//[1210]. Performing the twinning transformation on a rhenium precipitate produces a variant rather than a new orientation relationship. After elevated temperature aging, a rod morphology was observed with the rod axes aligned parallel with either 〈121〉 or 〈131〉 of the β–NiAl matrix. A total of 24 different variants are possible based upon the observed orientation relationship and the two observed growth directions.
First-principles study of phase stability of Ti–Al intermetallic compounds
Authors: Asta Mark , de Fontaine Didier , van Schilfgaarde Mark
Published: September 2007

Thermodynamic and structural properties of fcc- and hcp-based Ti–Al alloys are calculated from first-principles and are used to perform an ab initio study of phase stability for the intermetallic compounds in this system. The full potential linear muffin tin orbital method is used to determine heats of formation and other zero-temperature properties of 9 fcc- and 7 hcp-based intermetallic compounds, as well as of elemental fcc and hcp Ti and Al. From the results of these calculations, sets of effective cluster interactions are derived and are used in a cluster variation method calculation of the solid-state portion of the composition-temperature phase diagram for fcc- and hcp-based alloy phases. The results of our calculations are compared with those of experimental studies of stable and metastable phases in the Ti–Al system.
The influence of microstructure on the electrochromic properties of LixWO3 films: Part II. Limiting mechanisms in coloring and bleaching processes
Authors: Zhang Ji-Guang , Benson David K. , Tracy Edwin C. , Deb Satyen K.
Published: September 2007

Various factors affecting the coloring and bleaching processes of LixWO3 films have been studied. The rate of the coloring process is limited by the decreasing electromotive force in the LixWO3 film and by the components of the series circuit resistance, including the electrolyte resistance and the diffusion impedance within the film. The bleaching process in a thick film is limited by either the space charge or by the diffusion impedance, depending on the experimental conditions. A more complete and quantitative model of the coloring/bleaching process has been developed and shows good agreement with experimental results. Our analysis also indicates that the lithium concentration value of x near the LixWO3/electrolyte interface can greatly exceed its reversible limit during the coloring process, even though the average x value within the film remains much lower than the reversible limit. This phenomenon may introduce some irreversible structural changes in the film, which in turn may constitute one of the film's degradation mechanisms.
The influence of water of hydrolysis on microstructural development in sol-gel derived LiNbO3 thin films
Authors: Joshi Vikram , Mecartney Martha L.
Published: September 2007

The effect of water of hydrolysis on nucleation, crystallization, and microstructural development of sol-gel derived single phase LiNbO3 thin films has been studied using transmission electron microscopy (TEM), atomic force microscopy (AFM), x-ray diffraction (XRD), and differential scanning calorimetry (DSC). A precursor solution of double ethoxides of lithium and niobium in ethanol was used for the preparation of sol. DSC results indicated that adding water to the solution for hydrolysis of the double ethoxides lowered the crystallization temperature from 500 °C (no water) to 390 °C (2 moles water per mole ethoxide). The amount of water had no effect on the short-range order in amorphous LiNbO3 gels but rendered significant microstructural variations for the crystallized films. AFM studies indicated that surface roughness of dip-coated films increased with increasing water of hydrolysis. Films on glass, heat-treated for 1 h at 400 °C, were polycrystalline and randomly oriented. Those made with a low water-to-ethoxide ratio had smaller grains and smaller pores than films prepared from sols with higher water-to-ethoxide ratios. Annealing films with a low water concentration for longer times or at higher temperatures resulted in grain growth. Higher temperatures (600 °C) resulted in grain faceting along close-packed planes. Films deposited on c-cut sapphire made with a 1:1 ethoxide-to-water ratio and heat-treated at 400 °C were epitactic with the c-axis perpendicular to the film-substrate interface. Films with higher concentrations of water of hydrolysis on sapphire had a preferred orientation but were polycrystalline. It is postulated that a high amount of water increases the concentration of amorphous LiNbO3 building blocks in the sol through hydrolysis, which subsequently promotes crystallization during heat treatment.
Study of the anatase-rutile transformation in TiO2 powders obtained by laser-induced synthesis
Authors: Depero Laura E. , Bonzi Paolo , Zocchi Marcello , Casale Cristina , De Michele Gennaro
Published: September 2007

Powder samples of pure anatase were produced using laser-induced pyrolysis of titanium alkoxides, and the catalysts were prepared using conventional wet impregnation methods. The diffraction patterns were interpreted in microstructural terms by Fourier analysis of their peak profiles. The transition temperature for the anatase-rutile transition in these catalysts was found between 500° and 550 °C. For the reflections of the anatase phase, a decrease of their Bragg (2θ) positions was observed up to 550 °C when the presence of the rutile phase becomes important. The response of the anatase structure to the thermal treatment is anisotropic with the c-axis showing the highest sensitivity to the observed expansion of the lattice. The rutile Bragg reflections are sharper than those of the anatase phase. The corresponding microstructural parameters indicate that, in all cases, the transformation is accompanied by an increase of the crystallites and/or of the lattice perfection. The evolution of these parameters is influenced by the presence of vanadium. The V-treated surface layer must be particularly distorted and apparently act as a restraint to perfecting by thermal treatments. Only the transition to rutile is capable of overcoming that restraint by allowing crystallite growth at the expense of the smaller and distorted anatase crystallites.
Interaction between SiC and Ti powder
Authors: Gotman I. , Gutmanas E.Y. , Mogilevsky P.
Published: September 2007

The interaction between SiC and Ti powder at 1073–1523 K was investigated employing a combination of x-ray diffraction, scanning electron microscopy with EDS, Auger spectroscopy, and transmission electron microscopy. As a result of the interaction, a triple-layer reaction zone was formed. The most important part of the reaction zone was a mixed TiC–Ti5Si3(C) layer. Thin TiC sublayers were formed on both the inner and the outer sides of the mixed reaction layer. The reaction zone was found to grow by a parabolic law with the kinetic constant,k= 1.3 × 10−3exp (-21800/t) cm2/s. The growth process of the SiC/Ti reaction zone was assumed to be controlled by diffusion of all three components of the system: Ti, Si, and C. Thin reaction layers (<5 μm) obtained after short exposures at relatively low temperatures formed coatings on the SiC surface; thicker reaction layers spalled off the ceramic surface. Experiments with the samples partially immersed into the metal powder showed that interaction between SiC and Ti was very sensitive to the environment.
Grain size of MgO and polymorphic phases of ZrO2 in zirconia-toughened MgO
Authors: Ikuma Yasuro , Sugiyama Toshio , Okano Junko
Published: September 2007

Zirconia-toughened MgO was manufactured and examined by scanning and transmission electron microscopes. It was found that ZrO2 particles that are present on the MgO grain boundary limit the grain growth of MgO. The cooling rate has an effect on the ZrO2 phase in zirconia-toughened MgO fired in a cubic ZrO2–MgO field, but it does not have an effect on the ZrO2 phase in specimens fired in a tetragonal ZrO2–MgO field. Tetragonal ZrO2 was retained at room temperature in zirconia-toughened MgO.
Characterization of epitaxial SrTiO3/YBa2Cu3O7 layers deposited on (001) MgO by laser ablation
Authors: Scardi P. , Lutterotti L. , Correra L. , Nicoletti S.
Published: September 2007

We discuss the results obtained for SrTiO3/YBa2Cu3O7 layers deposited on (001) MgO substrates by UV pulsed laser ablation. Different samples were prepared to study both the growth of a thin (55 nm) layer of SrTiO3 on MgO and the successive epitaxy of a 220 nm YBa2Cu3O7 (YBCO) film on the SrTiO3 layer. An x-ray diffraction (XRD) texture analysis is reported for the bilayers together with resistivity versus temperature and critical current density (Jc) measurements of the superconducting films. The results show that YBCO grains grow with c-axis normal to the surface; the main in-plane orientations are [100] MgO // [100] SrTiO3 // [100] YBCO ([010] YBCO). The XRD line broadening analysis suggests that YBCO columnar grains grow along the whole thickness of the film, also evidencing dislocations and/or faulting separated by a mean distance of 80 nm. The values obtained for the critical current of the superconductor demonstrate the effectiveness of the SrTiO3 intermediate layer in improving the structural quality of the YBCO film.
The composition and structure of SIPOS: A high spatial resolution electron microscopy study
Authors: Catalano M. , Kim M.J. , Carpenter R.W. , Chowdhury Das K. , Wong Joe
Published: September 2007

The nanostructure and chemical distribution in semi-insulating polycrystalline oxygen-doped silicon (SIPOS) deposited on (001) Si and its isothermal transformation behavior at 900 °C were investigated by high resolution electron microscopy (HREM) and electron energy loss nanospectroscopy (EELS). The structure of the as-deposited film, which contained 15 at. % oxygen, was amorphous. No evidence for nanocrystalline second phases was found. It was similar in appearance to amorphous silicon. After annealing for 30 min at 900 °C in an inert environment (N2), a dispersion of small nanocrystals, identified as silicon by imaging, diffraction and EELS, formed in the amorphous SIPOS matrix, with a thin precipitate free zone (PFZ) adjacent to the Si substrate. The SIPOS matrix oxygen concentration increased to 36 at. % and the matrix remained amorphous after annealing. No other phases were observed in annealed specimens. Changes in Si–L near edge fine structure and low loss peaks in EELS spectra from SIPOS with increasing oxygen concentration indicated that it is a solid solution supersaturated with silicon. Microstructures indicated that the Si nanocrystals formed during a homogeneous precipitation reaction.
Characterization of the mechanical properties of a–SiC: H films
Authors: Loboda M.J. , Ferber M.K.
Published: September 2007

Amorphous hydrogenated silicon carbide (a–SiC: H) thin films (t < 1 μm) were grown from two different precursor gases, a methane/silane mixture and silacyclobutane (SiC3H8). Plasma enhanced chemical vapor deposition was used to deposit a–SiC: H thin films on silicon substrates at temperatures of 175 °C and 600 °C. These a–SiC: H films were characterized using the mechanical properties microprobe (nanoindenter) and by scratch testing. Data and mechanical properties information collected from these measurements have been correlated with film process conditions and materials characteristics. A simplified approach was used to calculate the average nanoindentation hardness from shallow indentations. Using this technique, results for a silicon wafer are in good agreement with that previously reported. Analysis of the substrate influence on the thin film nanoindentation data implies that the measured hardness is relatively unaffected by the substrate, while the measured elastic properties are somewhat influenced by the substrate. The a–SiC: H film hardness is shown to depend on the precursor gas and molecular bonding, while the elastic properties vary with precursor gas, composition, and density, as influenced by the plasma source deposition power. The MPM data and scratch test data show similar correlations to plasma source power, film structure, and film composition.
Recombination on fractal networks: Photon and electron emission following fracture of materials
Authors: Dickinson J.T. , Langford S.C. , Jensen L.C.
Published: September 2007

We report measurements and analysis of fracture-induced photon and electron emissions from several polymeric and inorganic systems on time scales of 10−2 to 103 s following fracture. The dominant mechanism for postfracture emission involves the recombination of mobile free carriers (usually electrons) with immobile recombination centers. The emission decays were modeled as (pseudo)unimolecular and bimolecular recombination on fractal lattices as described by Zumofen, Blumen, and Klafter.1 Although the decay kinetics shows a great deal of variability from material to material, this random walk description of the recombination process provides an excellent description of the emissions over long time scales. This analysis shows a strong correlation between the local structure at the fracture surface and the resulting decays.
Characterization of pulsed laser deposited MoS2 by transmission electron microscopy
Authors: Walek S.D. , Donley M.S. , Zabinski J.S. , Dyhouse V.J.
Published: September 2007

Molybdenum disulfide is a technologically important solid phase lubricant for vacuum and aerospace applications. Pulsed laser deposition of MoS2 is a novel method for producing fully dense, stoichiometric thin films and is a promising technique for controlling the crystallographic orientation of the films. Transmission electron microscopy (TEM) of self-supporting thin films and cross-sectional TEM samples was used to study the crystallography and microstructure of pulsed laser deposited films of MoS2. Films deposited at room temperature were found to be amorphous. Films deposited at 300 °C were nanocrystalline and had the basal planes oriented predominately parallel to the substrate within the first 12–15 nm of the substrate with an abrupt upturn into a perpendicular (edge) orientation farther from the substrate. Spherically shaped particles incorporated in the films from the PLD process were found to be single crystalline, randomly oriented, and less than about 0.1 μm in diameter. A few of these particles, observed in cross section, had flattened bottoms, indicating that they were molten when they arrived at the surface of the growing film. Analytical electron microscopy (AEM) was used to study the chemistry of the films. The x-ray microanalysis results showed that the films have the stoichiometry of cleaved single crystal MoS2 standards.
Thermodynamic stability field of the 123 and 124 phases in the Y2O3–BaO–Cu–O system
Authors: Zhou Zhigang , Navrotsky Alexandra
Published: September 2007

The partial molar entropy of solution of oxygen in YBa2Cu3Ox, δsO, has been computed using equilibrium data and the recently measured partial molar enthalpy of oxygen. While δhO is independent of oxygen content (6 < x < 7), δsO depends on x. The po2, T location of phase equilibrium between the 123 and 124 superconducting phases is calculated. The standard free energies of interactions of the superconducting phases with carbon dioxide (CO2) and water vapor (H2O) have been assessed. The free energies of carbonation of the fully oxidized 123 and 124 phases are −323 ± 22 kJ mol−1 and −291 ± 32 kJ mol−1, respectively. The free energies of hydration of the 123 and 124 are −397 ± 22 kJ mol−1 and −469 ± 32 kJ mol−1, respectively. These large exothermic values show that degradation of these superconductors at ambient conditions is thermodynamically favorable.
Effect of pulsed excimer laser irradiation on the magnetic anisotropy of iron-base and nickel-iron-base metallic glasses
Authors: Sorescu M. , Knobbe E.T.
Published: September 2007

The present work has examined the effect of pulsed excimer laser irradiation (λ = 308 nm, τ = 26 ns, E = 100 mJ/pulse, N = 1, 5, 10 laser pulses/spot, repetition rate 1 Hz) on the magnetic anisotropy of Fe81B13.5Si3.5C2, Fe78B13Si9, Fe40Ni38Mo4B18, and Fe40Ni40P14B6 metallic glasses. The transmission Mössbauer spectra yielded the intensity ratio R21 of the second to the first line as a function of the number of applied laser pulses. On these grounds, it was found that in the higher magnetostriction samples, controlled changes in the magnetic anisotropy could be induced without onset of bulk crystallization; in the lower magnetostriction samples, a random distribution of the magnetic moment directions was obtained. The conversion electron Mössbauer spectra were characteristic of the amorphous state but showed the same texture as the nonirradiated samples. SEM examinations of the laser-treated surfaces revealed the presence of molten zones subsequently solidified. It is suggested that the correspondingly induced stresses may account for the observed changes in the bulk magnetization orientation, and the model of closure domain structure is supported.
Investigation of the thermal anisotropy of unidirectional carbon fiber reinforced composite plates using optically generated thermal waves and a noncontact optical detection technique
Authors: Lauriks W. , Desmet C. , Glorieux C. , Thoen J.
Published: September 2007

Optically generated thermal waves have been used to measure the thermal diffusivity of a unidirectional carbon fiber reinforced composite plate (CFRC) both parallel and perpendicular to the fiber direction. The optically generated thermal waves have been used in combination with a noncontact optical detection technique. The diffusivity perpendicular to the fiber direction can also be determined by attaching a pyroelectric detector to the back of the sample. The value obtained this way agrees well with the results from the optical detection technique. An anisotropy factor of about 18 has been measured for a unidirectional CFRC, which agrees well with literature values obtained with completely different techniques.
Powder x-ray diffraction study of a cryptomelane-type manganic acid and its alkali cation exchanged forms
Authors: Tsuji Masamichi , Komarneni Sridhar
Published: September 2007

Cryptomelane-type manganic acid (CMA), H2Mn8O16 · 2.4H2O, with a theoretical cation exchange capacity of 2.70 meq/g, was prepared and its cation exchange properties were studied as a function of XM which is defined by the ratio of uptake in meq/g to the theoretical capacity. Plots of the corrected selectivity coefficients log KHM vs the fractional exchange M for alkali metal ions on a 2 × 2 type tunnel-structured manganic acid showed a gradual decrease in small XMM regions and a steep decrease in large XMM) were 0.92 for Li+, 0.74 for Na+ and K+, 0.67 for Rb+, and 0.44 for Cs+. These maximums have been attributed to steric limitation as well as a limit on the expansion of the crystal lattice. Some x-ray diffraction (XRD) data of alkali cation exchanged forms were in good agreement with the XRD data of synthetic alkali CMA phases as given in the cards by the Joint Committee on Powder Diffraction Standards (JCPDS). The lattice parameter a0 has been found to depend on the exchanged amounts and the nature of the cations involved, while the c0 value remained almost the same irrespective of the amount of exchange or the nature of the cations.
Formation of face-centered cubic titanium on a Ni single crystal and in Ni/Ti multilayers
Authors: Jankowski Alan F. , Wall Mark A.
Published: September 2007

The artificial layering of metals can change both physical and structural characteristics from the bulk. The stabilization of polymorphic metallic phases can occur on a dimensional scale that ranges from single overgrowth layers to repetitive layering at the nanoscale. The sputter deposition of crystalline titanium on nickel, as both a single layer and in multilayer form, has produced a face-centered cubic phase of titanium. The atomic structure of the face-centered cubic titanium phase is examined using high resolution electron microscopy in combination with electron and x-ray diffraction.
Point-defects in magnesium sulfide
Authors: Puntambekar Upendra , Veliah Sunder , Pandey Ravindra
Published: September 2007

The results of a study of point defects in MgS are presented. First we obtain empirical interionic potentials in the framework of a shell model and then calculate defect energies using the HADES and ICECAP simulation procedures. The calculated Schottky formation energy is 10.9 eV in comparison to the cation and anion Frenkel formation energies of 11.9 and 25.1 eV, respectively. The migration energy by the vacancy mechanism of the Mg2+ and S2− ions is predicted to be 2.5 and 3.4 eV, respectively. One-electron ICECAP calculations yield the optical absorption energy of 3.1 eV for the F+ center in MgS.
High temperature oxidation of ion-plated CrN films
Authors: Ichimura Hiroshi , Kawana Atsuo
Published: September 2007

The high-temperature oxidation of CrN films which were deposited onto stainless steel substrates using an arc ion plating apparatus was studied at temperatures ranging from 1023 to 1173 K for 0.6 to 480 ks in air. The oxidation rate obtained from mass gain as a function of time was found to fit well to a parabolic time dependence. Formed oxide layers were analyzed by XRD, SEM, and SAM. An activation energy of the oxidation of CrN was slightly lower than that of the self-diffusion coefficient of Cr ion in Cr2O3. It is concluded that the oxidation of CrN is controlled by the outward diffusion of Cr ions through the Cr2O3 layer formed on each CrN grain.
Effect of fiber coating on creep behavior of SiC fiber-reinforced titanium aluminide matrix composites
Authors: Chiu Hsing-Pang , Yang J-M. , Graves J.A.
Published: September 2007

The effect of fiber coating on the creep behavior and damage mechanisms of unnotched SCS-6 fiber-reinforced Ti3Al matrix composites under longitudinal and transverse loading was investigated at 700 °C. Stresses ranging from 700 to 900 MPa and 200 to 400 MPa were used for longitudinal and transverse loading, respectively. An Ag/Ta duplex layer was coated onto the SCS-6 fiber prior to consolidation via physical vapor deposition. The microstructure of the crept composites was examined to determine the creep deformation mechanisms. The creep cracking behavior of the notched composites was also studied at initial stress intensity factors, Ki, ranging from 15 to 20 MPa-m1/2. Microstructural observation revealed that multiple fiber fracture (at low to medium stress levels), microcracking along the reaction zone/matrix interface (at medium stress levels), and matrix cracking extending from the broken fiber ends (at high stress levels) were the major damage mechanisms during quasi-steady state creep under longitudinal loading. The results show that the Ag/Ta duplex coating significantly improved the creep resistance and flexural strength of the composite under transverse loading. The Ag/Ta duplex coating was also shown to significantly prolong the creep rupture life of SiC fiber-reinforced Ti3Al composites.
Anisotropies and mechanical properties of hot-pressed Al2O3 and SiC-whisker/Al2O3 composites
Authors: Akatsu Takashi , Tanabe Yasuhiro , Yasuda Eiichi
Published: September 2007

The anisotropies and mechanical properties of hot-pressed Al2O3 and SiC-whisker/Al2O3 composites were investigated. The c-axis of Al2O3 crystals in monolithic Al2O3 was oriented in the direction of hot-pressing. The same tendency was observed in Al2O3 matrices of composites, but orientation of Al2O3 crystals was smaller than that of monolithic Al2O3. SiC-whiskers obstructed plastic deformation and mass transfer of Al2O3 matrices. The twist mode crack deflection was remarkable on fractured surfaces of polycrystalline Al2O3 in which the main crack propagated perpendicular to hot-pressing direction. This was caused by the orientation of Al2O3 crystals. This anisotropy resulted in a small difference in fracture toughness; however, other mechanicl properties didn't change. The fracture mode of SiC-whisker/Al2O3 composites was obviously changed, depending on the direction of crack extension as compared with the changes observed in monolithic Al2O3. However, there was no difference in mechanical properties among the composites. Macroscopic fracture mode seemed to have less effect than microscopic fracture mechanism, such as whisker bridging and pull-out.
Diffusion barrier of sol-gel derived silica for sprayed tin oxide film on soda-lime glass
Authors: Davies B.M. , Pannell K.H. , Albright S.P.
Published: September 2007

Diffusion barriers of silica were deposited on soda-lime glass using a sol-gel technique. The migration of sodium from the glass through these layers and into a spray-pyrolyzed layer of fluorine-doped tin oxide was studied by SIMS depth profiling. The silica layer reduced sodium in the tin oxide overlayer by two orders of magnitude.
Characterization of pulsed laser deposited PbO/MoS2 by transmission electron microscopy
Authors: Walck S.D. , Donley M.S. , Zabinski J.S. , Dyhouse V.J.
Published: September 2007

Films of PbO/MoS2, grown by pulsed laser deposition, exhibit a significant improvement in tribological performance compared to MoS2 films grown by the same process. The microstructure and crystallography of PbO/MoS2 composite films were investigated using transmission electron microscopy (TEM) to identify the features responsible for this tribological improvement. Self-supporting samples were prepared from pulsed laser deposited, PbO/MoS2 thin films grown on single crystal sodium chloride substrates. Films deposited at room temperature exhibited a two-phase microstructure with one of the phases being amorphous. X-ray microanalysis results showed that the crystalline phase had significantly higher concentration ratios of Mo/Pb, Mo/S, and Pb/S than did the amorphous phase. Films grown at 300 °C were polycrystalline, with a grain size of about 20 nm, and had a NaCl type structure which was isomorphous to PbS. The grains had rectangular shape, and exhibited preferred orientation with the sodium chloride substrate. The concentration of S for these films was approximately 80% of the S concentration for films grown at room temperature. Both the high temperature and room temperature films had S concentrations which were higher than expected from the MoS2 in the target; this was attributed to gettering of the S in the vacuum chamber by Pb. The electron diffraction results, together with previously published results, suggest that the crystal structure of the phases in these films is not responsible for the improvement in tribological properties. However, the microstructural components formed during film growth do determine the wear-induced chemical reaction pathways.
Sintering behavior and dielectric properties of pyrochlore Pb2FeWO6.5
Authors: Lu Chung-Hsin , Mizutani Nobuyasu
Published: September 2007

Pyrochlore-type Pb2FeWO6.5 is difficult to be sintered without applied pressure at temperatures lower than its decomposition temperature. Through hot pressing or hot isostatic pressing processes, densification of specimens is greatly enhanced; moreover, grain growth during sintering is effectively suppressed. Densified Pb2FeWO6.5 exhibits paraelectric characteristic from −135°to −2 °C with low dielectric constants and low dissipation factors. The dielectric constants show rather weak dependence of temperature and frequency.
Influence of oxygen partial pressure on the kinetics of YBa2Cu3O7−x formation
Authors: Milonopoulou V. , Forster K.M. , Formica J.P. , Kulik J. , Richardson J.T. , Luss D.
Published: September 2007

The YBa2Cu3O7−x formation kinetics from a spray-roasted precursor powder containing Y2O3, BaCO3, and CuO was followed via in situ, time-resolved x-ray diffraction as a function of gas atmosphere and temperature. In inert atmospheres, BaCO3 and CuO form BaCu2O2 which subsequently reacts with Y2O3 to form YBa2Cu3O6. However, YBa2Cu3O6 decomposes at temperatures exceeding 725 °C with Y2BaCuO5 being one of the decomposition products. In oxidizing atmospheres, YBa2Cu3O7−x formation involves the BaCuO2. At high temperatures (800–840 °C), oxygen increases the yield of YBa2Cu3O6. A nuclei growth model assuming two-dimensional, diffusion-controlled growth with second-order nucleation rate fits the experimental data.
Mechanical strength and microstructure of oxygen ion-implanted Al films
Authors: Bader S. , Flinn P.A. , Arzt E. , Nix W.D.
Published: September 2007

The influence of finely dispersed, stable particles on the mechanical strength and microstructure of Al films on Si substrates has been studied. Aluminum oxide particles were produced in Al films by oxygen ion implantation, and the grain size was increased by a laser reflow treatment. Transmission electron microscopy (TEM) was employed to observe the oxide particles and the grain structure in the films after subsequent annealing, and the wafer curvature technique was used to study the deformation properties of the films as a function of temperature. Significant particle strengthening was obtained in the coarse-grained films in tension as well as in compression. In the as-deposited and ion-implanted films a very fine grain size of only 0.35 μm is stabilized after annealing which causes considerable softening of the film in compression at higher temperature because of the enhancement of grain boundary and volume diffusion controlled relaxation mechanisms. However, in tension at low temperature these films show high stresses comparable to those of the laser reflowed and ion-implanted films. The results are discussed in the light of TEM observations.
Amorphization phenomenon in Ni/amorphous Si multilayers
Authors: Wang W.H. , Wang W.K.
Published: September 2007

Interfacial reactions of Ni/amorphous Si(a-Si) multilayers are studied by means of transmission electron microscopy (TEM) and cross-sectional transmission electron microscopy (XTEM). Transformation from a crystalline to an amorphous structure has been observed in as-deposited Ni/a-Si multilayers with small modulation periods. This phenomenon is suggested to be due to interdiffusion-induced solid state amorphization which is facilitated by the high density of interface in the shorter modulation period multilayers. A thermodynamic and kinetic explanation is given for this phenomenon.
Epitaxial growth of sol-gel PLZT thin films
Authors: Yoon Dae Sung , Kim Chang Jung , Lee Joon Sung , Lee Won Jong , No Kwangsoo
Published: September 2007

Epitaxial lead lanthanum zirconate titanate [PLZT(9/50/50)] thin films were fabricated on various single crystal substrates using the spin coating of metallo-organic solutions. The films were heat-treated at 700 °C for 1 h using the direct insertion method. The films were epitaxially grown with (100), (100), and (110) being parallel to the SrTiO3(100), the MgO(100), and the sapphire (0112) substrates, respectively. The epitaxy of the films was investigated using x-ray diffraction, pole figures, rocking curves, and scanning electron microscopy.
Preparation, characterization, and sinterability of well-defined silica/yttria powders
Authors: Giesche Herbert , Matijević Egon
Published: September 2007

Dispersions of uniform submicron spherical particles consisting of silica cores and yttria coatings, or vice versa, were prepared by a precipitation technique. The overall size of the particles and the thickness of the shells could be varied over a wide range. Such powders were used to form green bodies by sedimentation, centrifugation, or pressure filtration, and the density and the pore size distribution of the resulting solids were evaluated. The green bodies were sintered and the changes in density, phases, and microstructure were followed with temperature. In general, the coated powders exhibited enhanced densification. On processing composite solids at temperatures <1000 °C, the formation of Y2Si2O7 took place, which caused a pronounced shrinkage of the samples. Powders of coated particles having the same silica/yttria ratios sintered at lower temperatures when the shell was composed of silica rather than of yttria. When either silica or yttria were in molar excess in the coated particles, the sintered products had a mixed composition of Y2Si2O7 and the component in excess. By terminating the sintering process before the grain growth started, the solids displayed a well-defined microstructure with a uniform distribution of areas of one phase in the matrix of the matter in excess. This property was mainly due to the uniformity of initial powders in terms of the particle size and the coating.
Oxygen availability in mixed cerium/praseodymium oxides and the effect of noble metals
Authors: Logan A.D. , Shelef M.
Published: September 2007

Oxyreduction studies of mixed Ce/Pr oxides have been carried out. Temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), and temperature-programmed oxidation (TPO) were used to study the uptake and release of the oxygen. Large amounts of oxygen, exceeding those in ceria, are accessible in the mixed metal oxides at moderate temperatures. The addition of small amounts of noble metals to the mixed oxides shifts the accessibility of the “stored” oxygen to still lower temperatures with the effect of Pd being more pronounced than that of Pt. In a sample containing 45 mol % ceria and 55 mol % praseodymia, a small addition of Pd (0.24 mol %) was found to lower the reduction temperature by more than 100 °C. The addition of Pt had a lesser effect. Similarly, in pure praseodymia (Pr6O11) Pd influences the reduction much more strongly than Pt. In the mixed samples, whether doped with a noble metal or not, the whole oxyreduction effect can be accounted for by the change in oxidation state of the praseodymium ions solely. This notwithstanding, the reduction of the mixed oxides, without noble metals or doped by Pt, is more facile than that of praseodymia. Only the incorporation of Pd makes the reduction of praseodymia proceed at a temperature below that registered for a mixed ceriapraseodymia sample.
CdTe microcrystallites doped in melt-quenched borosilicate glass
Authors: Fu Jie , Osaka Akiyoshi , Nanba Tokuro , Miura Yoshinari
Published: September 2007

CdTe microcrystallites were precipitated in 57.8CdO · 19.7SiO2 · 19.0B2O3 · 3.5Al2O3 glasses doped with 5.0 mol % CdTe. The microcrystallite size increased linearly with t1/3 (t: heat-treatment time), indicating diffusion-controlled growth mechanism, where the estimated activation energy for diffusion was 542 kJ/mol. The linear increase in the band gap energy with l/r2 (r: the crystallite radius) suggested the quantum size effect of the CdTe microcrystallites.
Interfacial stability and mechanical properties of Al2O3 fiber reinforced Ti matrix composites
Authors: Wang Hsin-Fu , Nelson John C. , Lin Chien-Li , Gerberich William W.
Published: September 2007

The mechanical properties of the interfaces in an Al2O3 fiber reinforced β-21S Ti alloy have been evaluated by using fiber pushout tests. The Al2O3 fibers were coated with a refractory metal and Y2O3 which served as a diffusion barrier during the HIPing used to produce the metal matrix composites. By doing fiber pushout tests, the interfacial fracture was found to occur at the interface between the refractory metal and the Y2O3. The interfacial shear strength and interfacial frictional stress were measured to be 323 and 312 ± 2 MPa, respectively. The interfacial frictional stress, which is due to asperity interlocking during the fiber sliding, was correlated to the surface roughness of the coated Al2O3 fiber obtained with the aid of an atomic force microscope. The measured surface roughness of 18.8 ± 2.2 nm was related to the frictional stress through Hutchinson's model.9 The frictional coefficient between the Al2O3 fiber and the Ti matrix is calculated to be 0.32 ± 0.02.
Epitaxial growth of Co/Cu[100] and [111] superlattices via rf sputtering on sapphire (1120)-substrates
Authors: Morawe Ch. , Abromeit A. , Metoki N. , Sonntag P. , Zabel H.
Published: September 2007

Co/Cu superlattices with total thicknesses ranging from 10 nm to 60 nm and with periodicities of 1.6–8.5 nm were sputtered on single-crystalline sapphire (1120)-substrates. Sputtering with low rates at room temperature yields samples of high epitaxial and crystalline quality. By careful choice of the sputtering parameters, either the fee [100] or the fcc [111] orientation can be selected as growth direction on one and the same substrate orientation. The preference for a particular film orientation appears to be kinetically driven. In all cases, the average lattice spacings d and the appearance of satellite reflections in x-ray Bragg-scans point to coherent growth up to thicknesses of 30 nm. X-ray small angle reflectivity measurements reveal clear oscillations and satellites indicative for smooth interfaces. Scanning electron microscope (SEM) and transmission electron microscope (TEM) observations supplement the characterization of the films.
Growth defects in CuInSe2 thin films
Authors: Wada Takahiro , Negami Takayuki , Nishitani Mikihiko
Published: September 2007

CuInSe2 thin film solar cells with an efficiency of about 10% were studied with a cross-sectional high resolution transmission electron microscope (HRTEM). The growth defects such as twins, stacking faults, and intergrowth phase in the CuInSe2 thin films were studied in detail. Polycrystalline CuInSe2 films were deposited on a Mo-coated glass substrate by using the three source evaporation system. The CuInSe2 film contains fivefold multiply twinned crystallites as well as a high density of twins in the {112} plane. The CuInSe2 film also contains intergrowth phase with a long range of periodicities of 10 Å parallel to the [112] direction of the chalcopyrite structure. The intergrowth phase composition is similar to the chalcopyrite phase. The structural model of the intergrowth phase is proposed on the basis of the high resolution electron micrograph.
Densification kinetics and modeling of glass-filled alumina composite
Authors: Jean Jau-Ho , Gupta Tapan K.
Published: September 2007

Kinetics and mechanism of sintering in a model borosilicate glass (BSG) + alumina composite system have been investigated isothermally at 600 to 1000 °C. A faster and greater densification is observed with higher sintering temperature, greater BSG content, and larger alumina particle size. Using measurable densification kinetics, it is found that the activation energy of densification (Q) increases continuously with increasing BSG content, from predominant <110 kJ/mol for BSG < 40 vol.% to 245 kJ/mol for BSG ≥ 90 vol.%. The rate-limiting step during densification is identified to be diffusion of alkali ions in BSG when BSG < 40 vol.% (Q = 110–170 kJ/mol), diffusion of both alkali and aluminum ions when BSG = 40–60 vol.% (Q = 110–170 kJ/mol), diffusion of aluminum ion in BSG when BSG = 60–80 vol.% (Q = 170 kJ/mol), and viscous flow of BSG when BSG ≥ 90 vol.% (Q = 245 kJ/mol). These observations are attributed to a chemical reaction taking place at the interface of alumina/BSG, resulting in a reaction layer adjacent to alumina. Since the composition of the reaction layer is known to be rich in aluminum and alkali ions and poor in silicon, the alkali ions content in BSG is continuously decreased during sintering. Accordingly, when the BSG content in the mixture is low, the resultant loss of alkali ions from BSG causes a rise in viscosity of BSG, thus slowing down the densification kinetics and changing the densification mechanism. As the amount of BSG is increased to ≥90 vol.% at the expense of alumina, the reaction as described above becomes less dominant and the sintering proceeds by viscous flow of glass.
Low temperature stabilization of zirconia by Mn through Co-precipitated hydroxide gel route
Authors: Keshavaraja A. , Ramaswamy A. V.
Published: September 2007

Stabilization of zirconia into cubic phase is achieved by partly substituting Zr4+ with Mn4+ ions (5–30 mole %) via hydroxide gel formation and subsequent calcination at 773 K and is supported by XRD and IR data. A linear correlation between the lattice parameter and the Mn content confirms the incorporation of Mn into ZrO2. The XPS and TPR results provide some evidence for the presence of Mn4+ ions in these samples which have a surface area of about 100 m2 g-1 and are stable in the cubic phase up to 973 K. On reduction above 973 K, the cubic phase is stabilized probably by Mn2+ ions.
Epitaxial LaNiO3 thin films: A normal metal barrier for SNS junction
Authors: Hegde M. S. , Satyalakshmi K. M. , Mallya R. M. , Rajeswari M. , Zhang H.
Published: September 2007

Epitaxial LaNiO3 thin films have been grown on SrTiO3 and several other substrates by pulsed laser deposition. The films are observed to be metallic down to 15 K, and the temperature dependence of resistivity is similar to that of bulk LaNiO3. Epitaxial, c-axis oriented YBa2Cu3O7-x films with good superconducting properties have been grown on the LaNiO3 (100) films. I-V characteristics of the YBa2Cu3O7-x-LaNiO3 junction are linear, indicating ohmic contact between them.