bluuebunny/crossref_metadata_2025 huggingface dataset:
Textures of laser ablated superconducting thin films of YBa2Cu3O7−δ as a function of deposition temperature
Authors: Heidelbach F. , Wenk H-R. , Muenchausen R.E. , Foltyn S. , Nogar N. , Rollett A.D.
Published: September 2007

The preferred orientation of a series of laser deposited superconducting thin films of YBa2Cu3O7−δ on LaAlO3 substrate has been examined. X-ray measurements (pole figures, χ-scans, ω-scans, rocking curves) reveal an increasingly strong preferred orientation of the polycrystalline material with c-axes perpendicular to the substrate surface as deposition temperature increases. At low temperatures c-axes are predominantly parallel to the substrate surface. Characteristic parameters of the texture types were derived from those measurements. With higher temperatures twinning on (110) was observed. The different texture types are interpreted in terms of a layered film structure.
Microstructure and properties of Cu-rich 123. Part I: Copper at the grain boundaries
Authors: Zhang J.P. , Li D.J. , Marks L.D. , Lin C.H. , Eades J.A. , Sodonis A. , Wolbach W. , Chabala J.M. , Levi-Setti R.
Published: September 2007

A range of copper-rich bulk YBa2Cu3+xO7+y superconductors have been prepared by mixing excess copper oxide in the initial material and characterized for their magnetic properties and microstructure. The microstructure of the materials exhibits a high density of planar defects at the grain boundaries and a grain boundary amorphous phase. There is a small increase in the magnetic Jc at 4.5 K but a decrease compared to a conventional material at 77 K, and there is no correlation of the magnetic Jc with the twin boundary density. The change in Jc can be understood if the grain boundary pinning is strong at 4.5 K but weak at 77 K. Data obtained using a very wide range of different microstructure characterization techniques clearly indicate the dangers of relying on only one technique to obtain a full picture of the material.
On the generality of the relationship among contact stiffness, contact area, and elastic modulus during indentation
Authors: Pharr G.M. , Oliver W.C. , Brotzen F.R.
Published: September 2007

Results of Sneddon's analysis for the elastic contact between a rigid, axisymmetric punch and an elastic half space are used to show that a simple relationship exists among the contact stiffness, the contact area, and the elastic modulus that is not dependent on the geometry of the punch. The generality of the relationship has important implications for the measurement of mechanical properties using load and depth sensing indentation techniques and in the measurement of small contact areas such as those encountered in atomic force microscopy.
A study of the mechanics of microindentation using finite elements
Authors: Laursen T.A. , Simo J.C.
Published: September 2007

In this paper the finite element method is used to explore the mechanics of the microindentation process. In the simulations discussed, aluminum and silicon are investigated both in their bulk forms and in thin film-substrate combinations. Among the quantities readily computed using this approach and given in this paper are hardness (computed using actual contact area), contact stiffness, effective composite modulus, and surface profile under load. Importantly, this investigation builds on previous work by providing a more critical examination of the amount of pileup (or sink-in) around the indenter in the fully loaded configuration, as well as the variation of the actual contact area during indenter withdrawal. A key conclusion of this study is that finite element simulations do not support the widely used assumption of constancy of area during unloading (for either bulk materials or thin film systems). Furthermore, the amount of pileup or sink-in can be appreciable. The implication of these findings is that for many situations the commonly used straight-line extrapolation of a plastic depth may render an estimate for the contact area that is quite distinct from the actual area. This assertion is demonstrated herein through comparison of hardnesses calculated using actual contact area with values calculated using the straight-line extrapolation of plastic depth.
The application of the analytic embedded atom method to bcc metals and alloys
Authors: Guellil A.M. , Adams J.B.
Published: September 2007

Johnson and Oh have recently developed Embedded Atom Method potentials for bcc metals (Na, Li, K, V, Nb, Ta, Mo, W, Fe). The predictive power of these potentials was first tested by calculating vacancy formation and migration energies. Due to the results of these calculations, some of the functions were slightly modified to improve their fit to vacancy properties. The modified potentials were then used to calculate phonon dispersion curves, surface relaxations, surface energies, and thermal expansion. In addition, Johnson's alloy model, which works well for fcc metals, was applied to the bcc metals to predict dilute heats of solution.
Electronic structure and total energy of diamond/BeO interfaces
Authors: Lambrecht Walter R.L. , Segall Benjamin
Published: September 2007

Electronic structure calculations are used to study the bonding at diamond/BeO interfaces. The {110} interface between zinc blende BeO and diamond is used as a representative model for general reconstructed interfaces characterized by an equal amount of Be–C and O–C bonds. The interface energy is calculated to be 2 J/m2 and combined with the estimated free surface energies to obtain an estimate of the adhesion energy. It is found to be close to the adhesion of BeO to itself, but somewhat lower than that of diamond to itself. The effects of the 7% lattice mismatch on the total energy and the band structure for a biaxially strained pseudomorphic diamond film are investigated. The effect of misfit dislocations, expected to occur for thicker films, on the adhesion energy is estimated to be lower than 10%. The bulk properties, such as equilibrium lattice constant, bulk modulus, cohesive energy, and band gap of BeO are shown to be in good agreement with experimental values and previous calculations. The valence-band offset is calculated to be 3.9 eV and found to take up most of the large band gap discontinuity. The nature of the bonding is discussed in terms of the local densities of states near the interface. The interface localized features are identified in terms of Be–C and O–C bonding and antibonding states.
Luminescence of Eu3+ ions during thermal densification of SiO2 gel
Authors: Campostrini R. , Carturan G. , Ferrari M. , Montagna M. , Pilla O.
Published: September 2007

This paper compares the luminescence spectra of Eu3+ in sol-gel derived silica samples heated at different temperatures, following the densification process from wet gel to compact silica glass. Lifetimes, linewidths, and Stark splittings of the transitions were used to study the structural evolution of the gel network.
The effect of crack growth stability induced by residual compressive stresses on strength variability
Authors: Tandon Rajan , Green David J.
Published: September 2007

Rising T- (or R-) curve behavior is increasingly being used in order to improve the mechanical reliability of ceramic materials. In this study, the possibility of inducing such behavior using residual compressive stresses is analyzed. The T-curves obtained for certain residual stress profiles induce crack stability when the stress minima (compressive stress maxima) lie away from the surface of the sample. The consequences of this stabilization on the strength characteristics are a significant reduction in the strength variability and strength insensitivity to the initial flaw size. In addition to these desirable features, considerable strengthening is also obtained. Hence, suitably engineered compressive stress profiles are shown to be a novel and alternative means of enhancing mechanical reliability.
Preparation of Pb(Zr1−xTix)O3 powders from complex alkoxide and their lower-temperature crystallization
Authors: Fukui Toshimi , Sakurai Chihiro , Okuyama Masahiko
Published: September 2007

Pb(ZrxTix)O3 (PZT) powders of 0.2–0.4 μm particle sizes could be obtained from the complex alkoxides synthesized from lead acetate, Zr(OBun)4, and Ti(OEt)4. The powders crystallized to well-crystalline perovskite phases at a temperature as low as 250 °C, independent of the compositional ratio of the B site (Zr/Ti). For the preparation of powders, the complexing of alkoxides, addition of the second solvent such as acetone, acetonitrile, or methyl acetate, and base-catalyzed hydrolysis with ammonia water were essential.
Excitation frequency effects on rf magnetron sputtering of YBa2Cu3O7−δ thin films
Authors: Homma N. , Takahashi H. , Okayama S. , Morishita T. , Tanaka S.
Published: September 2007

Effects of an excitation frequency on on-axis rf magnetron sputtering have been studied for fabrication of YBa2Cu3O7−δ thin films on MgO (100). The excitation frequencies examined are 13.56, 40.68, 67.80, and 94.92 MHz. Sputtering was performed in a pure argon gas in order to distinguish excitation frequency effects of oxygen emitted from a target from those of oxygen in a mixture atmosphere. Excitation frequencies higher than 40.68 MHz result in both a low self-bias voltage and a narrow distribution of the energy of sputtering ions, which reduce resputtering of a film surface caused by negatively charged oxygen from the target. Thin films of a-axis oriented YBa2Cu3O7−δ deposited at 94.92 MHz have shown better morphology and crystallinity than those at lower excitation frequencies under the same conditions. Furthermore, a-axis oriented superconductive YBa2Cu3O7−δ films have successfully sputtered in 200 mTorr atmosphere of argon and oxygen on SrTiO3 (100) by using an excitation frequency of 94.92 MHz, showing a full width at half maximum of 0.1° in the rocking curve through the (100) peak and a very smooth surface. The zero resistance temperature is, at most, 33 K.
Preparation of YBa2Cu3O7−x superconducting films through the sol-gel method using metal alkoxides as starting materials
Authors: Masuda Yoshio , Ogawa Rikuro , Kawate Yoshio , Matsubara Kazuo , Tateishi Tuyoshi , Sakka Sumio
Published: September 2007

YBa2Cu3O7−x superconducting films have been prepared on a YSZ ceramic substrate through the sol-gel method using metal alkoxides of Y, Ba, and Cu. A homogeneous solution containing the three metal alkoxides was prepared by dissolving the copper alkoxide, which is otherwise insoluble, in diaminoalcohol. The gel film, formed by dipping the substrate into the sol of 80 °C and withdrawing it up, was converted to a ceramic film consisting of YBa2Cu3O7−x phase by sintering at 850 °C for 30 min. The resulting film of about 10 μm in thickness showed zero resistivity (Tc end) at 54 K. Cold isostatic pressing of the film before sintering remarkably improved the Tc end to 85 K.
A study of Al2Ti precipitation in a Cu-modified Al3Ti alloy
Authors: Potez L. , Loiseau A. , Naka S. , Lapasset G.
Published: September 2007

An equilibrium precipitation of Al2Ti is shown to occur within a Cu-modified Al2Ti alloy having the L12 structure. This precipitation is analyzed by conventional and high resolution electron microscopy and some insights are given concerning the mechanisms of nucleation and growth of Al2Ti within the L12 matrix. In the meantime, Vickers microhardness tests have been performed as a first approach to the mechanical behavior of this alloy and the results are compared to measurements obtained in an actual single-phased L12 compound. The Al2Ti precipitation seems to have an important hardening effect.
Stability of tight-packed metals with the embedded-atom method
Authors: Johnson R.A.
Published: September 2007

Relationships between embedded-atom method parameters and the energies of fcc-hcp stability and intrinsic and extrinsic fcc stacking-faults were studied for Cu, Ag, Au, Ni, Pd, and Pt. It was found that the relative magnitudes of these energies for different metals are determined primarily by the physical input data and are almost independent of the cutoff distance and the functions used in the model. These energies increase with increasing vacancy formation energy, decrease with increasing atomic volume and shear modulus, and are almost independent of variations in the cohesive energy and the bulk modulus. However, the shape of the energy versus cutoff distance curve is almost the same for all six metals and is determined primarily by the cutoff distance and the functions used in the model. The shape for a given model is almost independent of the physical input parameters used for fitting to specific metals, can yield either positive or negative values (determined primarily by the cutoff distance), and is similar for all three energies.
Processing of TiAl–Ti2AlN composites and their compressive properties
Authors: Mabuchi H. , Tsuda H. , Nakayama Y. , Sukedai E.
Published: September 2007

Using elemental powders, combustion reaction was carried out to form intermetallic-ceramic composites in the Ti–Al–N system. Ti and Al powders reacted exothermically in gaseous nitrogen and formed a mixture product which had a fine distribution of the Ti2AlN particles in the matrix TiAl with a small amount of Ti3Al. Subsequently, these reacted products were arc-melted to obtain fully dense button ingots. The resulting composites had about 30 vol. % Ti2AlN, and the Ti2AlN particles were ellipsoidal or columnar in shape with sizes of 2–10 μm and appeared to be homogeneously distributed and well bonded to the matrix TiAl. It was found that such composite materials have a high strength at both room and elevated temperatures and some intrinsic compressive ductility at room temperature. Therefore, the processing technique in the present investigation is of interest as a new combustion reaction process to make intermetallic-based composite materials.
Preparation and characterization of uniform submicrometer metal niobate particles: Part II. Magnesium niobate and potassium niobate
Authors: Kim Myung Jin , Matijević Egon
Published: September 2007

Amorphous spherical particles of magnesium and potassium niobates were produced by homogeneous precipitation in solutions of magnesium and potassium salts, respectively, in the presence of polymeric niobium oxo-hydroxide ions. At pH 9 solids of definitive compositions were achieved and converted to crystalline powders of MgNb2O6 and KNbO3 on calcination. Chemical mechanisms for the formation and transformation of such colloidal particles are offered.
Metal-insulator transition in highly disordered carbon fibers
Authors: Kuriyama K. , Dresselhaus M.S.
Published: September 2007

The electronic transition from localized to delocalized states of carriers in a disordered carbon material is investigated by photoconductivity measurements. Phenol-derived activated carbon fibers, where the carriers are strongly localized due to disorder, are heat treated in the range 300–2500 °C to give rise to the insulator-metal transition. Dark conductivity, Raman spectra, and x-ray diffraction patterns are also measured to characterize their structural changes. As a result, the transition temperature was determined to be rather low, around 1000 °C, considering the rapid decrease in the photoconductivity above this temperature. This decrease was ascribed to a fast recombination between the photoexcited carriers and the delocalized carriers generated by heat treatment.
Electrical resistance of metallic contacts on silicon and germanium during indentation
Authors: Pharr G.M. , Oliver W.C. , Cook R.F. , Kirchner P.D. , Kroll M.C. , Dinger T.R. , Clarke D.R.
Published: September 2007

The effects of indentation on the electrical resistance of rectifying gold-chromium contacts on silicon and germanium have been studied using nanoindentation techniques. The DC resistance of circuits consisting of positively and negatively biased contacts with silicon and germanium in the intervening gap was measured while indenting either directly in the gap or on the contacts. Previous experiments showed that a large decrease in resistance occurs when an indentation bridges a gap, which was used to support the notion that a transformation from the semiconducting to the metallic state occurs beneath the indenter. The experimental results reported here, however, show that a large portion of the resistance drop is due to decreases in the resistance of the metal-to-semiconductor interface rather than the bulk semiconductor. Experimental evidence supporting this is presented, and a simple explanation for the physical processes involved is developed which still relies on the concept of an indentation-induced, semiconducting-to-metallic phase transformation.
Role of interfacial thermal resistance and laser energy density during laser processing of copper-sapphire couples
Authors: Godbole M.J. , Pedraza A.J. , Lowndes D.H. , Thompson J.R.
Published: September 2007

The effect of the interfacial thermal resistance and the laser energy density on film morphology and the extent of evaporation was studied in the excimer laser processing of copper-sapphire couples. Copper films of 80 nm thickness were sputter-deposited on sapphire substrates and laser-irradiated with energy densities in the range of 0.2 to 3.5 J/cm2. The changes in film morphology and thickness as a function of laser energy density were analyzed by energy dispersive x-ray spectroscopy. Four regimes can be established as a function of the laser energy density. First, for low energy densities up to a critical value, the film is partially removed by thermal stresses. Second, as the energy density is increased above that critical value, larger portions of the film remain attached to the substrate. In this regime adhesion enhancement takes place. Third, a further increase in the energy density results in film evaporation. Finally, the decrease in the specific mass removal rate of copper is related to the formation of a laser generated plasma that shields the sample from the incoming radiation. In this last regime, an intermediate compound may form at the substrate surface. The data were correlated with results from a computer model of the heat flow during laser processing of metal-ceramic couples.
Photoelastic and finite element analysis of different size spheres in contact
Authors: Shih C.W. , Schlein W.S. , Li J.C.M.
Published: September 2007

The photoelastic stress-freezing technique is applied to observe the stress distribution inside two spheres of different sizes compressed together elastically. After the stress is frozen in, thin slices of the material containing the symmetry axis are prepared for observation through a polariscope. The stress distribution is compared with both the finite element numerical analysis and the Hertz analytical theory which is limited to small deformations. Among the three, the agreement between the experimental results and the finite element analysis is the best. The deviation from the Hertz theory is less in the larger sphere contacting a smaller one than in the smaller sphere contacting a larger one.
Lattice statics of interfaces and interfacial cracks in bimaterial solids
Authors: Tewary V.K. , Thomson Robb
Published: September 2007

A method for calculating lattice statics Green's function is described for a bimaterial lattice or a bicrystal containing a plane interface. The method involves creation of two half space lattices containing free surfaces and then joining them to form a bicrystal. The two half space lattices may have different structures as in a two-phase bicrystal or may be of the same type but joined at different orientations to form a grain boundary interface. The method is quite general but, in this paper, has been applied only to a simple model bicrystal formed by two simple cubic lattices with nearest neighbor interactions. The bimaterial Green's function is modified to account for an interfacial crack that is used to calculate the displacement field due to an applied external force. It is found that the displacement field, as calculated by using the lattice theory, does not have the unphysical oscillations predicted by the continuum theory.
Equilibrium shapes of semicoherent inclusions
Authors: Rottman Craig
Published: September 2007

The equilibrium shapes of stress free inclusions with a small mismatch and possibly a small rotation from the matrix phase are studied. The model used includes both bond-breaking and elastic contributions to the interface energy. The unrotated shape contains only facets. Rotated inclusions sometimes contain smoothly curved parts as well. Interface phase diagrams are used to characterize the stable phase behavior of flat interfaces of fixed orientation.
Interdiffusion between the L12 trialuminides Al66Ti25Mn9 and Al67Ti25Cr8
Authors: Kumar K.S. , Whittenberger J.D.
Published: September 2007

Interdiffusion coefficients (D) were determined in the temperature range 1373 K–1073 K (0.85 Tm−0.65 Tm) from concentration-distance profiles that were generated from Al66Ti25Mn9/Al67Ti25Cr8 diffusion couples. As both of these materials are completely soluble in one another, the couples were treated as pseudobinaries (particularly because the Ti and Al levels were similar), and the Matano approach was used to calculate diffusion coefficients. The variation in D with composition was determined and a minimum was noted at intermediate temperatures and a relative Mn concentration of 0.5. A maximum in activation energy for interdiffusion was also noted at this concentration where Q ≍ 350 kJ/mol.
The 45° grain boundaries in YBa2Cu3O7−δ
Authors: McKernan Stuart , Norton M. Grant , Carter C. Barry
Published: September 2007

High-angle grain boundaries in YBa2Cu3O7−δthin films are of technological interest because of the weak coupling observed between the grains; however, not all high-angle grain boundaries show this weak-link behavior. The microstructure of both these boundaries is not understood, nor is the reason for the differing electrical transport properties. High-angle grain boundaries in YBa2Cu3O7−δthin films on MgO, where the angular misorientation between the grains is ∼45°, have been examined using high-resolution electron microscopy. The results show that the boundary structure can appear quite different even when the angular misorientation between the two grains is the same. The stability of the grain boundaries under the electron irradiation in the electron microscope was found to be a function of the accelerating voltage—400 kV leads to rapid disordering of the boundary region.
Ultrahigh vacuum STM studies of the Bi–O surface of Bi2212
Authors: Ikeda Kazuto , Takamuku Kenshi , Yamaguchi Koji , Itti Rittaporn , Koshizuka Naoki
Published: September 2007

Observations of the Bi–O surface of superconductive Bi2212 single crystals were carried out using an ultrahigh-vacuum scanning tunneling microscope (UHV-STM). In the atomic resolution images, surface corrugations, which correspond to the superstructure of the Bi–O surface in addition to Bi atom deficiencies, were observed. There were hollow lines along the ridges of the corrugations, which may be due to missing atom rows.
Shock-induced reaction synthesis (SRS) of nickel aluminides
Authors: Thadhani N.N. , Work S. , Graham R.A. , Hammetter W.F.
Published: September 2007

Shock-induced chemical reactions between nickel and aluminum powders (mixed in Ni3Al stoichiometry) are used for the synthesis of nickel aluminides. It is shown that the extent of shock-induced chemical reactions and the nature of the shock-synthesized products are influenced by the morphology of the starting powders. Irregular (flaky type) and fine morphologies of the powders undergo complete reactions in contrast to partial reactions occurring in coarse and uniform morphology powders under identical shock loading conditions. Furthermore, irregular morphology powders result in the formation of the equiatomic (B2phase) NiAl compound while the Ni3Al (L12phase) compound is the reaction product with coarse and regular morphology powders. Shock-induced reaction synthesis can be characterized as a bulk reaction process involving an intense “mechanochemical” mechanism. It is a process in which shock compression induces fluid-like plastic flow and mixing, and enhances the reactivity due to the introduction of defects and cleansing of particle surfaces, which strongly influence the synthesis process.
The effect of Cu on morphological instabilities in thin Al/Pt films
Authors: Colgan E.G. , Blanpain B.
Published: September 2007

Al films deposited on Pt layers developed voids after annealing at 250 °C. The amount of Al in the area surrounding the voids increased relative to the as-deposited film. The addition of 1 or 4% Cu to the Al suppressed the void formation and lateral Al migration. The void formation is related to compound formation with the Pt. The addition of Cu to the Al did not modify the Pt2Al3 formation.
Nucleation, growth, and structure of Al–Mn quasicrystalline thin films prepared by high-temperature vapor deposition
Authors: Barna P.B. , Csanády A. , Timmer U. , Urban K.
Published: September 2007

The nucleation and growth of quasicrystalline thin films during sequential vapor deposition of aluminum and manganese on various substrates have been studied at temperatures between 530 and 650 K. The films were analyzed by transmission electron microscopy, electron diffraction, energy dispersive x-ray analysis, replica techniques, and Auger depth profiling. The quasicrystalline phase is identified as icosahedral. It nucleates on the surfaces of the Al films. There is no indication of substantial bulk Mn diffusion. The growth process is governed by diffusion of Al to the quasicrystal surface where it reacts with the incident Mn.
Selective growth of polycrystalline diamond thin films on a variety of substrates using selective damaging by ultrasonic agitation
Authors: Ramesham R. , Roppel T.
Published: September 2007

Polycrystalline diamond thin films have been selectively grown on silicon, silicon dioxide, silicon nitride, tantalum, molybdenum, alumina, and sapphire substrates using selective damaging by ultrasonic agitation. Processes were developed to selectively damage the substrates by ultrasonic agitation in methanol containing diamond particles of typical size 90 μm. Optical and scanning electron microscopy is used to study selectivity and morphology of as-grown diamond thin films.
Structural and textural changes from polyimide Kapton to graphite: Part I. Optical microscopy and transmission electron microscopy
Authors: Bourgerette C. , Oberlin A. , Inagaki M.
Published: September 2007

Kapton films, 25 μm thick, were carbonized and then graphitized at various temperatures up to 3000 °C. They were studied by optical microscopy and transmission electron microscopy and were prepared by thin-sectioning for transmission electron microscopy. The residual embedded blocks were used as polished sections for optical microscopy. Kapton thin film heat-treated at 650 °C showed thin areas of preferred orientation at its two opposite faces. This orientation extends all over the film thickness between 800 °C and 1000 °C. It corresponds to a statistical orientation of small aromatic layer stacks, parallel to the film plane. From 1000 °C up to 2475 °C flattened pores develop everywhere. At 2475 °C, they suddenly collapse and partial graphitization occurs. The Kapton film is completely graphitized between 2820 °C and 3000 °C. The mechanism of graphitization is similar to that of anthracites and also to that of nongraphitizing carbons heat-treated under pressure.
MBE growth of compound semiconductors: Part II. Applications of the stochastic model
Authors: Venkatasubramanian R.
Published: September 2007

In this part of the work (Part II), two typical applications of the stochastic model to the MBE growth kinetic studies are presented. The applications are the MBE growth kinetics of a hypothetical compound semiconductor, ab, and diamond cubic alloy, ax. In this study, the effect of the surface diffusion process on the MBE growth kinetics is analyzed. In the case of the compound, ab, the results of the present stochastic model are compared with that of a Monte Carlo simulation study in the temperature range of 600–850 K. The results of the two studies agree qualitatively. Higher substrate temperatures result in higher growth rate and growth front smoothness due to higher surface diffusion. Beyond 800 K, the growth rate and the growth front smoothness become independent of temperature because of the saturation of the interlayer diffusion process. In the case of the alloy studies, the kinetics of a hypothetical diamond cubic alloy in which the thermodynamics favors phase separation, is studied in the temperature range of 573–898 K. Below 648 K, due to negligible surface diffusion, there is no clustering of the alloy, but the surface roughness is very large. In the intermediate temperature range of 573–798 K, with increasing temperature, the surface diffusion increases, resulting in more clustering and less surface roughness. Above 798 K, due to very high surface diffusion, complete phase separation of the alloy and a smooth surface result.
Solid state interfacial reactions of Ti3Al with Si3N4 and SiC
Authors: Chou T.C. , Joshi A.
Published: September 2007

Solid state interfacial reactions of Ti3Al with Si3N4 and SiC have been studied via both bulk and thin film diffusion couples at temperatures of 1000 and 1200 °C. The nature of reactions of Ti3Al with Si3N4 and SiC was found to be similar. Only limited reactions were detected in samples reacted at 1000 °C. In the Ti3Al/Si3N4, layered reaction products consisting of mainly titanium silicide(s), titanium-silicon-aluminide, and titanium-silicon-nitride were formed; in the Ti3Al/SiC, the reaction product was primarily titanium-silicon-carbide. In both cases, silicon was enriched near the surface region, and aluminum was depleted from the reacted region. Reactions at 1200 °C resulted in a drastic change of the Si distribution profiles; the enrichment of Si in near surface regions was no longer observed, and the depletion of Al became more extensive. Titanium nitride and titanium-silicon-carbide were the major reaction products in the Ti3Al/Si3N4 and Ti3Al/SiC reactions, respectively. Mechanisms of driving the variation of Si, N, and C diffusion behavior (as a function of temperature) and the depletion of Al from the diffusion zone are suggested. It is proposed that reactions of Ti3Al with Si3N4 and SiC lead to in situ formation of a diffusion barrier, which limits the diffusion kinetics and further reaction. The thermodynamic driving force for the Ti3Al/Si3N4 reactions is discussed on the basis of Gibbs free energy.
X-ray photoelectron spectroscopy of nickel manganese oxide thermistors
Authors: Hashemi T. , Brinkman A.W.
Published: September 2007

The major problem in the NiMn2O4 system has been the determination of the valencies and the cation distribution between the two sublattices of the inverse spinel structure, in which both manganese and nickel can adopt more than one valence state. X-ray photoelectron spectroscopy (XPS) was used to elucidate the valence distribution of the manganese and nickel ions. The results showed that identification of the three Mn and two Ni species in the nickel manganite phase is possible, enabling the validity of crystal structure configurations, proposed in the literature, to be assessed. A small amount of copper, added as CuO to the system, was found to be present in both monovalent and divalent states, the Cu+ ions inducing the creation of the large amount of tetravalent Mn observed. This led to a larger number of sites for electron hopping, the mechanism of conduction, and consequently the electrical conductivity of the Cu doped material is much higher than the undoped NiMn2O4.
On the atomic interdiffusion in Co/Pt superlattices
Authors: Yan X. , Egami T. , Marinero E.E. , Farrow R.F.C. , Lee C.H.
Published: September 2007

We have performed x-ray diffraction experiments on MBE grown 〈111〉 and 〈001〉 oriented Co (3 Å)/Pt (18 Å) superlattices with the scattering vector Q covering a plane that includes a major component perpendicular and a minor component parallel to the plane of the sample. The superlattice diffraction peaks were found to have a large width parallel to the plane of the film, indicating that there is strong in-plane disorder. These broad peaks were integrated to evaluate more accurately the amount of interdiffusion at the interface, which was found to be similar in both films, contrary to what the conventional x-ray diffraction experiment suggests.
Metastable copper-chromium alloy films
Authors: Payne A.P. , Clemens B.M.
Published: September 2007

Due to the large positive heat of mixing associated with the Cu–Cr binary system, solid solutions exist only as nonequilibrium states. In this study, a series of metastable Cu–Cr alloys ranging in composition from 14.1 to 75.4% copper was fabricated by sputter deposition. Symmetric, asymmetric, and grazing incidence x-ray diffraction geometries were used to trace the phase transition from bcc to fcc crystal structures with increasing Cu fraction. It is shown that the transition takes place not by a two-phase region suggested by equilibrium thermodynamics, but rather through gradual disordering of the bcc lattice as copper atoms are substitutionally accommodated. At a critical saturation near 71% Cu, the bcc structure becomes unstable relative to the fcc and a phase transition occurs. The free energies of the kinetically constrained Cu–Cr system are modeled and the results are found to agree well with observed behavior.
The formation of intermetallics in Cu/In thin films
Authors: Roy Rita , Sen S.K. , Sen Suchitra
Published: September 2007

The kinetics of the formation of intermetallics in the Cu–In bimetallic thin film couple have been studied from room temperature to 432 K by measuring the evolution of composite and contact electrical resistance with time and temperature. The resistivity measurements have been supplemented by x-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Copper reacts with indium even at room temperature to form CuIn intermetallic and assuming a model of defect assisted diffusion into the grains, the activation energy averaged over five different samples is found to be 0.40 eV. The grain boundary diffusion is found to occur with an average activation energy of 0.55 eV. XRD confirms the growth of CuIn intermetallic and on annealing at higher temperature, for copper-rich films copper further reacts with CuIn to form Cu9In4. Further evidences of solid state reactions and grain boundary diffusion through Cu grain boundaries have been obtained from SEM study. TEM indicates the growth of the grain size on annealing and confirms the presence of the CuIn phase.
A microstructural study of mechanical alloying of Fe and Sn powders
Authors: Le Caër Gérard , Matteazzi Paolo , Fultz Brent
Published: September 2007

Elemental Fe and Sn powders in the ratio of 1:2 were ball-milled for various times at room temperature, and phase transformations in the powders were studied by 57Fe and 119Sn Mössbauer spectrometries, transmission electron microscopy, and x-ray diffractometry. Although Fe-Sn alloys are not obvious candidates for solid-state amorphization reactions, an amorphous phase formed after only a few hours of ball milling. Nanocrystalline intermetallic FeSn2 particles nucleated and grew within the amorphous phase, and FeSn2 became the major phase after 10–20 h of ball milling. These small particles were superparamagnetic, and were above the blocking temperature at room temperature.
Exoelectron emission from unexcited metallic glasses Fe78B13Si9 during heat treatment
Authors: Watanabe Yoshihisa , Kubozoe Tadayoshi , Nakamura Yoshikazu
Published: September 2007

Exoelectron emission from the surface of unexcited metallic glasses Fe78B13Si9 during heat treatment has been studied under ultra high vacuum condition. In the first heating cycle, exoelectrons are emitted from the as-cast ribbon in the temperature range from approximately 423 K to 773 K (150 °C to 500 °C), although the surface of the specimen is not excited by ionizing radiation, chemical processes, or mechanical treatments prior to measurements. In the second and subsequent heating cycles, however, there is no anomalous emission observed in the same temperature range. In order to elucidate the mechanism of emission, the surface of the specimen is observed by the atomic force microscope (AFM) before and after measurements. In the AFM image, many crystallites in the amorphous matrix can be found in the surface of the heated specimen. These experimental results show that exoelectrons are emitted in the same temperature range as the early stages of crystallization on the surface of metallic glasses. We hypothesize that the two effects are correlated.
Efficiency of methane and acetylene in forming diamond by microwave plasma assisted chemical vapor deposition
Authors: Johnson Curtis E. , Weimer Wayne A. , Cerio Frank M.
Published: September 2007

Diamond films were grown by microwave plasma assisted chemical vapor deposition using mixtures of 1% 13CH4 and 0.5% 12C2H2 in H2, and stable gaseous products were analyzed by mass spectrometry. The major gaseous products are methane and acetylene, and scrambling of the 13C label can be controlled at relatively high gas flow rates. At the highest flow rate studied a diamond film was grown with 77% 13C incorporation, while the methane in the reactor exhaust gas at this flow rate contained 83% 13C. By comparing gaseous 13C compositions with that of the films, the efficiency of diamond growth from methane (possibly via the methyl radical) is estimated to be about ten times higher than that for acetylene.
Fracture strength measurement of filament assisted CVD polycrystalline diamond films
Authors: Cardinale G.F. , Robinson C.J.
Published: September 2007

The fracture strength of polycrystalline diamond films deposited by filament assisted chemical vapor deposition in the thickness range of 3.5 to 160 μm is investigated. Using a burst pressure technique, the fracture strengths of circular diamond film specimens are calculated. An average fracture strength of 730 MPa for nine samples was computed. This value is in good agreement with published strengths of microwave plasma deposited diamond films, comparable to other high strength materials, and within an order of magnitude of the fracture strength of bulk natural diamond. The average fracture strength of the fine-grained substrate interface appears consistently higher than that of the coarse-grained diamond growth surface.
CVD Si3N4 on single crystal SiC: Part II. High resolution electron microscopy and atomic models of the interface
Authors: Unal O. , Mitchell T.E.
Published: September 2007

Interface studies of CVD Si3N4 grown on (11$\overline 1$) SiC single crystal substrates have been made by transmission electron microscopy (TEM). It is found that there are two orientation relationships both of which involve the same (10$\overline 1$0)Si3N4//(11$\overline 1$)SiC planar relationship. However, the orientation relationships are not perfect and rotations of 2–6° are commonly seen between both directions and planes involved. High resolution electron microscopy (HREM) of the interfaces shows that the SiC and Si3N4 are continuous up to the interface and that no intermediate phases are formed. However, due to the small rotations, the HREM images are difficult to interpret directly in terms of atomic positions. Nevertheless, possible atomic models of the interface are proposed based upon the experimental findings. These models exploit the similarities between the [SiN4] and [SiC4] tetrahedra in Si3N4 and SiC, respectively. The observed orientation relationships appear to be due to matching of tetrahedra across the interface along with adequate lattice matches.
Substrate and temperature effects in lead zirconate titanate films produced by facing targets sputtering
Authors: Roy R.A. , Etzold K.F.
Published: September 2007

The growth of ferroelectric lead zirconate titanate (PZT) films by rf-sputtering using a facing targets geometry is described. This study focuses on the influence of the substrate on PZT thin film composition, structure, and electrical properties. The deposition temperatures ranged from room temperature to 700 °C and the process gas was a mixture of argon and oxygen. Effects of deposition conditions and post-deposition annealing on film composition, microstructure, and properties were evaluated using Rutherford backscattering spectroscopy (RBS), x-ray diffraction, electron microscopy, and measurements of the permittivity and polarization. The microstructure, composition, and permittivity of the films were found to be strongly dependent on the substrate temperature and on the preparation history of the films.
Diffusion bonding of nickel and zirconia: Mechanical properties and interfacial microstructures
Authors: Qin C-D. , Derby B.
Published: September 2007

Diffusion bonds of Ni/ZrO2 and Ni/NiO/ZrO2 fabricated in vacuum have been investigated using flexural 4-point bending tests and optical and electron microscopy. It is found that subsequent annealing in air after bonding improves bond strength, and annealing in vacuum reduces strength. This is attributed to the formation of a thin oxide layer during annealing in air which enhances adhesion to the ceramic, whereas annealing in vacuum creates debonding voids at the specimen edges. The transformation of NiO in vacuum to Ni explains why the strength of bonds using preoxidized Ni foil does not show any increase, as it is essentially still the diffusion bonding of Ni to ZrO2 in the configuration of Ni/NiO/Ni/ZrO2. The presence of extensive void necklaces on grain boundaries in the metal where they intersect the bonding interface shows the importance of the metal grain boundaries acting as vacancy sinks during diffusion bonding.
Dynamic consolidation of superhard materials
Authors: Yang Wenbo , Bond G.M. , Tan Hua , Ahrens Thomas J. , Liu G.
Published: September 2007

Shock consolidation experiments were conducted via flyer impact on synthetic diamond (6–12 μm) and cubic boron nitride (c-BN) (4–8 μm) admixed with SiC whisker (SCW), Si3N4 whisker (SNW), SiC powder, and Si powder contained in stainless steel capsules under the shock pressure range of 10–30 GPa. Scanning electron microscopy and transmission electron microscopy imaging of the samples revealed no plastic deformation or melting of diamond and virtually no deformation of c-BN, whereas the SCW and SNW were extensively melted and recrystallized into bundle-shaped crystallites. In contrast, SiC powder mixed with diamond was also melted but demonstrated equant grain growth. A new method to calculate the shock temperature and melt fraction is formulated on the basis of Milewski's sphere-rod packing data. The new method assigns excess bulk volume to the zone around whiskers and yields a better description of the energy deposition mechanism of the consolidation of powder-whisker systems. Some of the experiments employed Sawaoka's post-shock annealing technique, in which the sample is sandwiched between two layers of a mixture of titanium powder plus carbon. Very well consolidated samples were obtained with post-shock heating under shock pressures of only about 11 GPa. Micro-Vickers hardness values up to 27 GPa were obtained for c-BN plus SCW at a low impact velocity of 1.45 km/s with post-shock heating. This hardness is similar to that obtained at a higher impact velocity of 1.95 km/s without post-shock heating. To understand the post-shock heating process, one-dimensional time dependent temperature profile calculations were conducted for the sample and Ti + C layers. Post-shock heating appears to be very important in the consolidation of powder and whisker admixture. The calculated optimum Ti + C thickness is about 0.8–1.7 mm at a porosity of 40% for a typical sample thickness of 2 mm. The heating and cooling time is a few milliseconds. Good compacts with micro-Vickers hardness values up to 28 GPa were also obtained upon shock consolidation of diamond plus Si admixtures.
Highly stable fluoride glass
Authors: Elyamani Abdessamad , Snitzer Elias , Pafchek Robert , Sigel George H.
Published: September 2007

A new family of modified ZBLAN glass compositions has been synthesized which provides better stability against crystallization. A preliminary study of the Hruby factor HR shows higher values as compared to a ZBLAN glass prepared under the same conditions, utilizing similar high purity starting materials. Large rods of clear glass with a 1.5 cm diameter and up to 17 cm in length were easily obtained. Results will be presented on the physical properties and crystallization behavior of these high stability glasses.
Repulsive interaction between coplanar cracks in the double–cantilever geometry
Authors: Wan Kai-tak , Lawn Brian R. , Horn Roger G.
Published: September 2007

Experiments on thin mica sheets are used to demonstrate that coplanar cracks in double-cantilever beam specimens do not universally attract each other, as conventionally portrayed, but, at long range, actually repel. An elasticity analysis explains the repulsion in terms of a compression zone, ≍0.35 times the beam half-thickness ahead of the crack tip, generated by bending moments from the cantilever arms on the remaining specimen section.
Effect of Al2O3 additives on the formation of superconducting whiskers (2212 phase) in melt-quenched BiSrCaCu2Ox
Authors: Abe Yoshihiro , Hirata Kazumi , Hosono Hideo , Kubo Yukio
Published: September 2007

Whiskers of 2212 phase Bi2Sr2Ca1Cu2Ox were prepared by heating in air the powder-compacted pellets of melt-quenched glassy products in BiSrCaCu2Ox containing a small amount of Al2O3. The dissolved and/or physically mixed Al2O3 in the powder-compacted specimen is found to enhance the whisker formation remarkably.
Effect of the direction of field annealing on the stress + field induced magnetic anisotropy in Co–Fe–Ni amorphous alloys
Authors: González J. , Blanco J.M.
Published: September 2007

Compositional dependence of the stress + longitudinal field induced magnetic anisotropy (SLFA) in (Co1−xFex)75Si15B10 (0 ⋚ x ⋚ 1), (Co1−xNix)75Si15B10 (x = 0.22) and [Co1−x(Fe0.5Ni0.5)x]75Si15B10 (x = 0.20, 0.50, and 0.80) amorphous alloy ribbons has been studied. The magnetic field was applied parallel to the ribbon axis. Two mechanisms have been invoked in order to explain the microscopic origin of this anisotropy: the directional ordering of atomic pairs, and the tetrahedral holes like 3Co–1Fe and 1Co–3Fe. The last contribution is found to be similar both in its direction and its value with respect to that obtained for stress + transverse field induced anisotropy (STFA) in the same compositions containing Fe, while in the Co–Ni samples only the first contribution is responsible for the SLFA, depending on its direction for the field annealing.
Elastic moduli of MoSi2-based materials
Authors: Srinivasan S.R. , Schwarz R.B.
Published: September 2007

We prepared MoSi2, two-phase MoSi2/Mo5Si3, and (Mo, W)Si2 solid-solution alloy powders by mechanically alloying mixtures of elemental molybdenum, silicon, and tungsten. These powders were consolidated by hot-pressing them at 1500 °C in graphite dies. We measured the elastic moduli of these alloys by a recently developed technique for non-contact ultrasonic spectroscopy. Second-phase Mo5Si3 additions to MoSi2 result in decreased values for G and E, whereas alloying MoSi2 with WSi2 results in increased values for G and E. An analysis of these Young's moduli and of data from the literature for various intermetallic alloys suggests that for any given alloy system, the Young's moduli for its various intermetallics as a function of density fall on a straight line. Further, the lines for the different alloy systems are approximately parallel.