bluuebunny/crossref_metadata_2025 huggingface dataset:
TEM lattice imaging of the nanostructure of early-growth sputter-deposited MoS2solid lubricant films
Authors: Hilton M. R. , Fleischauer P. D.
Published: September 2007

Transmission electron microscopy (TEM) was used to investigate the nanostructure of sputter-deposited MoS2films; these films represent the early stages of growth under deposition conditions that produce a zone 2 columnar morphology. Analysis reveals that the early-growth film morphology consists of anisotropic (plate-like) islands in which the (001) basal planes are generally perpendicular to the substrate (forming “edge islands”) or parallel to the substrate (forming “basal islands”). Within the context of an active-sites nucleation model, localized regions of the substrate surface can lack the active sites needed to induce edge orientation. The edge islands evolve into the zone 2 morphology, shadowing and inhibiting the continued growth of the basal islands. Basal plane (002) lattice image curvature and kinking were observed in the edge islands. Dark-field analysis showed intensity variations within the edge islands. The edge island plates appear to be imperfect crystals of aligned basal planes. These crystals can bend, kink, or twist, apparently because of variations in local growth conditions. The ability of crystal growth to deviate has implications for the morphological evolution of thicker films. The ability of the MoS2crystal lattice to bend supports the idea that localized deviation can occur during sliding wear.
Anomalous solid state reaction between SiC and Pt
Authors: Chou T.C.
Published: September 2007

Periodic structures are generated by solid state reaction between platinum (Pt) and silicon carbide (SiC). At temperatures above 900°C, periodic structures consisting of alternating layers of platinum silicides and carbon are produced in the diffusion zone. The composition profile across the diffusion zone and the chemistry of the periodic structures are investigated by scanning electron microscopy (SEM), scanning Auger microscopy (SAM), x-ray diffraction (XRD), and laser Raman microprobe. The formation of the platinum silicides causes an interfacial melting between Pt and SiC. X-ray diffraction indicates that Pt3Si is formed at 900°C, while Pt2Si is formed at 1000 °C. Laser Raman spectroscopy indicates that carbon is in either an amorphous state or a highly ordered graphitic state, depending upon its location from the reaction interface. The mechanism of formation of the periodic structure is discussed in terms of the solubility of carbon in platinum silicide during the solidification process.
Sintering characteristics of nanocrystalline TiO2
Authors: Hahn H. , Logas J. , Averback R. S.
Published: September 2007

The microstructural development of compacted nanocrystalline TiO2 powder was studied as a function of sintering temperature up to 1000°C. Grain growth was monitored using x-ray diffraction and scanning electron microscopy. The specific surface area and total porosity were determined quantitatively using the nitrogen adsorption BET. The density was measured by gravimetry using Archimedes principle. The green body density of compacted n-TiO2 with average grain size of 14 nm is as high as 75% of theoretical bulk density. Low temperature surface diffusion leads to the disappearance of small pores, while noticeable densification commences at 600°C and reaches near theoretical values at 900°C. Grain growth also begins at 600°C, accelerating rapidly by 1000°C. Hot isostatic pressing is observed to enhance densification while suppressing grain growth.
The role of materials science after the Mexico City earthquakes in 1985
Authors: Castaño V. , Martinez L.
Published: September 2007

We review our field investigations of construction materials which were initiated after the 1985 Mexico City earthquakes. We report observations on reinforcing steel samples collected in the ruins of collapsed buildings and describe the experiences in the production and testing of HSLA steel reinforcing bars with mechanical and metallurgical properties suitable for earthquake resistant construction. We review some aspects of the cement and concrete industries of Mexico before 1985 and present a description of the properties of polymer modified cements considering the potential not only for construction but for many other applications.
Superconducting oxide YBa2Cu3O7−x prepared by the metal alkoxide method
Authors: Katayama Shingo , Sekine Masahiro
Published: September 2007

Preparation of the superconducting oxide, YBa2Cu3O7−x, was carried out according to the metal alkoxide method. To obtain a homogeneous solution of yttrium, barium, and copper alkoxide in an organic solvent, alkoxides of copper and yttrium were modified by the O-R interchange reaction with 2-dimethylaminoethanol and the structures of their modified alkoxides were studied by 1H-NMR, FT-IR, and EXAFS. The YBa2Cu3O7−x precursor, prepared by hydrolysis of the present solution of alkoxides, was calcined at 850°C for 3 h to give YBa2Cu3O7−x powder as a single phase. This powder was sintered between 900°C and 950°C and characterized for bulk density, microstructure, Tc, and Jc.
Defect structure of laser deposited Y-Ba-Cu-O thin films on single crystal MgO substrate
Authors: Ramesh R. , Hwang D. M. , Barner J. B. , Nazar L. , Ravi T. S. , Inam A. , Dutta B. , Wu X. D. , Venkatesan T.
Published: September 2007

Structural defects in thin films of nominal composition YBa2Cu3O7 (123) grown on single crystal MgO have been characterized. The main types of stacking defects correspond to the cationic stoichiometries of “248”, “247”, and “224”. Several types of edge dislocations have been observed. Due to the frequent changes in the stacking sequence, antiphase boundaries are created to accommodate the misfit across regions in which the repeat sequence is not identical. The films exhibit a mosaic microstructure due to the formation of grain boundaries in the a-b plane.
X-ray and electron diffraction study of single crystal Bi2Sr2CaCu2Ox
Authors: Kan X. B. , Kulik J. , Chow P. C. , Moss S. C. , Yan Y. F. , Wang J. H. , Zhao Z. X.
Published: September 2007

Using both x-ray and electron diffraction, we have studied the incommensurate modulation structure in single crystal Bi2Sr2CaCu2Ox. Electron diffraction clearly indicates a small a∗-axis component of the modulation wave vector k, along with the more familiar b∗-axis component (0.004a∗ + 0.212b∗). This tilt of the k-vector breaks the mirror symmetry perpendicular to a- and b-axes as well as the twofold symmetry about a- and b-axes. The modulated structure thereby has only a mirror plane normal to the c-axis. Intense x-ray streaking along the c-axis at several satcllite reflections was considerably more pronounced than at the fundamental reflections, indicating a stacking disorder along the c-axis predominantly in the modulated structure.
Characterization of (Bi, Pb)-Sr-Ca-Cu-O system by analytical electron microscopy
Authors: Inoue Yasuhide , Hasegawa Masahiro , Shichi Yushi , Munakata Fumio , Yamanaka Mitsugu , Hiyama Susumu , Nittono Osamu
Published: September 2007

Chemical compositions of superconductors of Bi1−xPbxSrCaCu1.8Oy (x = 0.1, 0.3, and 0.5) were investigated by analytical electron microscopy (AEM) supplemented by electron microprobe analysis. Samples were prepared by a solid state reaction method under oxygen partial pressures of 0.20 and 0.077 atm. The high-Tc phase appeared only in the samples of Bi1−xPbxSrCaCu1.8Oy (x = 0.1 and 0.3) prepared under an oxygen partial pressure of 0.077 atm. In the samples containing the high-Tc phase, stacking structures of 2, 3, and 4 perovskite layers were observed by transmission electron microscopy (TEM). From AEM analysis, it was shown that in order to make the high-Tc phase, the substitution ratio of Pb for Bi was about 0.2.
Temperature dependence of the amorphization of NiTi irradiated with Ni ions
Authors: Maziasz P.J. , Pedraza D.F. , Simmons J.P. , Packan N.H.
Published: September 2007

NiTi was irradiated with Ni ions at various temperatures in order to study the temperature dependence of the irradiation-induced crystalline-to-amorphous transition. The irradiations were conducted above theAftemperature, and thus the specimens contained only the ordered B2 (CsC1) phase. The irradiations to similar doses at 150, 200, and 250°C showed that the amorphization kinetics slow down appreciably as the temperature is increased in this range. No amorphization was detected at irradiation temperatures of 350°C or higher, even after doses of 4 dpa. The small volume fraction of amorphous material observed after irradiation to 0.67 dpa at 250°C indicates that the cutoff temperature for amorphization is in the vicinity of this temperature. The amorphous regions of partly amorphous samples are distributed in a nonuniform manner and exhibit a morphology similar to the martensitic microstructure that existed in the specimens before heating to the irradiation temperature. Large amorphous regions in these samples exhibit some very fine crystalline debris which tends to disappear with increasing irradiation dose. Post-irradiation annealing experiments indicated that no thermally activated crystallization occurred during irradiation at temperatures up to 250°C.
The anisotropic powder metallurgy of n-type Bi2Te2.85Se0.15 thermoelectric material
Authors: Wada Hideo , Sato Tetuji , Takahashi Kazuaki , Nakastukasa Norio
Published: September 2007

The anisotropic powder metallurgy of n-type Bi2Te2.85Se0.15 doped with bromines was studied by both x-ray diffraction and thermoelectric measurements. The statistical orientation of platelet-like grains in the samples was characterized using the orientation factor estimated by the Lotgering formula from (0,0,1) x-ray diffraction intensities. It is demonstrated that the orientation factor which is strongly influenced by the hot-pressing conditions and the particle size of the starting material has a strong effect on the thermoelectric properties of the samples in this system. This implies that the powder metallurgy has the additional freedom of controlling the thermoelectric properties in addition to the doping level in the grains.
A relation between the surface energy and the Debye temperature for cubic solids
Authors: Tewary V.K. , Fuller E.R.
Published: September 2007

It is shown that a phenomenological relation exists between the Debye temperature θ (in degree Kelvin) and the surface energy Γ (in ergs/cm2) of cubic solids: , where M is the atomic weight. This relation is derived theoretically in the Debye isotropic approximation by assuming that the interatomic potential is central. No restrictions are imposed on the range of the potential. The relation is obeyed very well by the observed values of θ and Γ in the case of many solids.
Nondestructive investigation of damage in composites using x-ray tomographic microscopy (XTM)
Authors: Kinney J.H. , Stock S.R. , Nichols M.C. , Bonse U. , Breunig T.M. , Saroyan R.A. , Nusshardt R. , Johnson Q.C. , Busch F. , Antolovich S.D.
Published: September 2007

X-ray tomographic microscopy (XTM), utilizing intense, highly collimated synchrotron radiation, has been used to nondestructively image materials structures in three dimensions. The spatial resolution of the technique approaches that of conventional optical microscopy, but XTM does not require polished surfaces or serial sections. We present the results of an XTM investigation of a composite material composed of silicon-carbide fibers in an aluminum matrix. The results reveal the aluminum/silicon-carbide interfaces and show microcracks running along many of the interfaces as well as in the matrix. Excellent contrast is observed between the silicon-carbide sheath of the fiber surrounding the graphite core and the coating at the fiber-matrix interface. The sensitivity to small changes in the linear absorption coefficient allows nondestructive imaging of variations in chemistry between graphite and silicon carbide and between silicon carbide and aluminum. The experimentally determined values of the absorption coefficients of these phases are identical to values published in the literature. For the first time, XTM will allow observation of damage accumulation and crack growth during deformation testing. The availability of such data will greatly improve our understanding of failure in advanced materials.
Thermal shock-induced fracture of ion-implanted LiF crystals
Authors: Gurarie V. N. , Williams J. S.
Published: September 2007

Monocrystals of LiF, ion implanted with Ar+, were exposed to thermal shock in a plasma of different intensities. Ion implantation substantially alters the fracture pattern and characteristics of the material, particularly in reducing the thermal shock resistance parameter, S′, and in increasing the damage resistance parameter, S″. The former parameter indicates that ion implantation allows fracture to be initiated at lower thermal shock temperature differences and the latter parameter is associated with higher crack densities and lower crack penetration depths. The increase in the parameter S″ indicates that ion implantation can result in a higher mechanical stability and greater durability of the crystals damaged by thermal shock. Surface melting at very high heat fluxes eliminates any effect of ion implantation.
Preferred orientation of pitch precursor fibers
Authors: Hamada T. , Furuyama M. , Sajiki Y. , Tomioka T. , Endo M.
Published: September 2007

Spinning condition dependences of the microstructures of a pitch precursor fiber which is a raw material for carbon fiber preparation were studied in detail. The degree of preferred orientation of the molecules in a pitch precursor fiber along the fiber axis was higher when spinning was done at a lower mesophase pitch viscosity. An intermediate diameter gave the highest degree of preferred orientation. The degree of preferred orientation of the molecules in an extruded pitch rod was especially low. The crystallite size Lc(002) of the pitch precursor fiber also was varied by controlling the spinning conditions, and exhibited a good correlation with the degree of preferred orientation. Mesophase pitch at a higher temperature possessed a smaller Lc(002) and a larger d002. The small Lc(002) observed at high temperature was maintained by a rapid cooling to room temperature, but the large d002 relaxed to a small value at room temperature, even with rapid cooling. A qualitative model for the formation of a preferred orientation through spinning is proposed.
Surface, interface, and thin-film magnetism
Authors: Falicov L. M. , Pierce Daniel T. , Bader S. D. , Gronsky R. , Hathaway Kristl B. , Hopster Herbert J. , Lambeth David N. , Parkin S. S. P. , Prinz Gary , Salamon Myron , Schuller Ivan K. , Victora R. H.
Published: September 2007

A comprehensive review and state of the art in the field of surface, interface, and thin-film magnetism is presented. New growth techniques which produce atomically engineered novel materials, special characterization techniques to measure magnetic properties of low-dimensional systems, and computational advances which allow large complex calculations have together stimulated the current activity in this field and opened new opportunities for research. The current status and issues in the area of material growth techniques and physical properties, characterization methods, and theoretical methods and ideas are reviewed. A fundamental understanding of surface, interface, and thin-film magnetism is of importance to many applications in magnetics technology, which is also surveyed. Questions of fundamental and technological interest that offer opportunities for exciting future research are identified.
Preparation of YBa2Cu3O6+x by a formic acid method
Authors: Otamiri J. C. , Andersson A.
Published: September 2007

Synthesis of YBa2Cu3O6+x from formic acid solutions of stoichiometric amounts of the initial materials has been investigated in the temperature range 750–950°C. The reaction is strongly influenced by the concentration of formic acid. Dilute solutions favor synthesis of purer samples under moderate conditions of temperature and sintering time. At lower temperature the same effect can be achieved if sintering time is longer, while at higher temperature, multiple grinding and heating at very short intervals are necessary to avoid formation of Y2BaCuO5. The observed effect of HCOOH concentration is explained by the different mechanisms involved in the decomposition of the formates obtained.
Microstructural development in the near-surface region during thermal annealing of Al2O3 implanted with cationic impurities
Authors: Farlow G. C. , Sklad P. S. , White C. W. , McHargue C. J.
Published: September 2007

Single-crystal Al2O3 was implanted with cationic impurities in the dose range 1–4 ⊠ 1016/cm2 and subsequently annealed in either an oxidizing or reducing environment. Following annealing at 1200°C or higher, crystalline precipitates or solid solutions are observed, which are consistent with what is expected from the equilibrium phase diagram.
Epitaxial growth of β–SiC on Si by low-temperature chemical vapor deposition
Authors: Chaudhry M. Iqbal , Wright Robert L.
Published: September 2007

In this paper we report the growth of β–SiC films on Si(100) substrates by low-temperature chemical vapor deposition. Single crystals of β–SiC are grown at temperatures as low as 1150 °C. Low-temperature growth β–SiC is achieved using a SiH4–C3H8–H2–Ar gas system. The growth rate of films grown at 1150 °C is 1 μm/h. Transmission electron microscopy and x-ray diffraction results indicate that the β-SiC films grown at and above 1150 °C are single crystals. Films grown at temperatures lower than 1150 °C are polycrystalline.
Surface resistance measurement of superconducting YBa2Cu3O7 in a magnetic field
Authors: Chin C. C. , Rainville P. J. , Drehman A. J. , Derov J. S. , Steinbeck J. , Dresselhaus G. , Dresselhaus M. S.
Published: September 2007

We report on the magnetic-field dependent surface resistance of polycrystalline YBa2Cu3O7 (Tc ≃ 92 K), measured using a brass cylindrical cavity resonator, operating at 16.5 GHz in the TE011 mode. A de magnetic field Happ is applied parallel to the superconducting sample surface, and the temperature dependence of the surface resistance is measured for four different values of Happ (0 T, 0.22 T, 1 T, 5 T). An effective medium theory and the two-fluid model are used to fit the surface resistance versus temperature measurements both in zero field and for various applied fields. These results are applied to characterize the microwave properties of a polycrystalline ceramic superconductor.
Characterization of spray-pyrolized superconducting YBaCuO thin films on single-crystal MgO by transmission electron microscopy
Authors: Golden S. J. , Isotalo H. , Lanham M. , Mayer J. , Lange F. F. , Rüble M.
Published: September 2007

Superconducting YBaCuO thin films have been fabricated on single-crystal MgO by the spray-pyrolysis of nitrate precursors. The effects on the superconductive behavior of processing parameters such as time and temperature of heat treatment and film thickness were investigated. The superconductive behavior was found to be strongly dependent on film thickness. Films of thickness 1 μm were found to have a Tc of 67 K while thinner films showed appreciably degraded properties. Transmission electron microscopy studies have shown that the heat treatments necessary for the formation of the superconductive phase (for example, 950 °C for 30 min) also cause a substantial degree of film-substrate interdiffusion. Diffusion distances for Cu in the MgO substrate and Mg in the film were found to be sufficient to explain the degradation of the superconductive behavior in films of thickness 0.5 μm and 0.2 μm. From the concentration profiles obtained by EDS analysis diffusion coefficients at 950 °C for Mg into the YBaCuO thin film and for Cu into the MgO substrate were evaluated as 3 × 10−19 m2/s and 1 × 10−17 m2/s, respectively.
Brittle cleavage of L12 trialuminides
Authors: George E. P. , Horton J. A. , Porter W. D. , Schneibel J. H.
Published: September 2007

Three trialuminide alloys, binary Al–25Sc, ternary Al–25Zr–6Fe, and quaternary Al–23Ti–6Fe–5V, all having the cubic L12 structure, were investigated. All three alloys fracture in a brittle manner (fracture toughness, 2–3 MPa m½), predominantly by transgranular cleavage. Of nineteen cleavage facets examined in binary Al3Sc, seventeen were of the {110} type and only two were of the {100} type, consistent with our earlier work which showed that the cleavage plane occurring most frequently in quaternary Al–23Ti–6Fe–5V is also {110}. The room-temperature hardnesses and yield strengths (100–200 DPH and 100–270 MPa, respectively) of all three alloys are quite low (comparable to ductile L12 alloys like Ni3Al), indicating that there is significant dislocation activity in these materials. Consistent with this, transmission electron microscopy identified several APB-coupled dislocations with b - a/2〈110〉 gliding on the {111} planes in both binary Al–25Sc and quaternary Al–23Ti–6Fe–5V. The separations between the superpartials in Al–25Sc and Al–23Ti–6Fe–5V were measured to be 3.7 and 4 nm, respectively, giving APB energies of 313 and 274 mJ/m2, respectively. Auger analyses failed to detect any impurities on the cleavage facets themselves, or on second phase particles (or other potential cleavage crack nucleation sites). It is therefore concluded that brittle fracture in these alloys is not impurity-induced. Based on all the results obtained to date we conclude that the unusual brittleness of L12 trialuminides is related to their intrinsically low cleavage strength. Possible reasons for their low cleavage strength are discussed.
The chemical compatibility and tensile behavior of an Ni3Al-based composite
Authors: Fuchs G. E.
Published: September 2007

The chemical compatibility and tensile properties of a powder processed intermetallic matrix composite (IMC) were investigated in the temperature range 298–1373 K. The matrix alloy selected for this study was the ORNL developed advanced nickel-aluminide IC-221 (Ni-16Al-8Cr-1Zr-0.05B at. %). The composite contained 25 vol. % TiC particulate reinforcement. TiC/IC-221 compatibility samples were heat treated at 1373 K for up to 1000 h. A layer enriched in Zr and Ti formed at the TiC/IC-221 interface after heat treatment. In addition, Ti was observed to diffuse into the matrix. The composite exhibited higher yield strength and lower ductility than a similarly processed matrix alloy at all test temperatures. Heat treatment of the composite improved the tensile properties due to particulate/matrix interaction, resulting in improved load transfer. SEM fractography revealed that tensile failure occurred at the matrix/prticulate interface. Remnants of the matrix were observed on TiC particles on the fracture surface, suggesting good matrix/particulate bonding. The mechanical properties of the composite were very competitive with Ni-base superalloys.
Correlation between structure, energy, and ideal cleavage fracture for symmetrical grain boundaries in fcc metals
Authors: Wolf D.
Published: September 2007

The misorientation phase space for symmetrical grain boundaries is explored by means of atomistic computer simulations, and the relationship between the tilt and twist boundaries in this three-parameter phase space is clucidated. The so-called random-boundary model (in which the interactions of atoms across the interface are assumed to be entirely random) is further developed to include relaxation of the interplanar spacings away from the grain boundary. This model is shown to include fully relaxed free surfaces naturally, thus permitting a direct comparison of the physical properties of grain boundaries and free surfaces, and hence the determination of ideal cleavage-fracture energies of grain boundaries. An extensive comparison with computer-simulation results for symmetrical tilt and twist boundaries shows that the random-boundary model also provides a good description of the overall structure-energy correlation for both low- and high-angle tilt and twist boundaries. Finally, the role of the interplanar spacing parallel to the grain boundary in both the grain-boundary and cleavage-fracture energies is elucidated.
Model for melting enthalpy of Sn in Ge–Sn composites
Authors: Turnbull D. , Jang J. S. C. , Koch C. C.
Published: September 2007

The finding of Jang and Koch that the melting enthalpy/mass, ΔHm, of a Sn matrix containing a fine dispersion of Ge particles sharply decreases with increasing Ge volume fraction, νGe, >0.5 and vanishes at νGe = ν°Ge ≍ 0.81, is accounted for by supposing that the Sn is distributed between an interfacial and bulk state. The interfacial statc is one in which the Sn is assumed to be in a disordered, possibly amorphous, structure coating the Ge particles uniformly to a constant thickness, δ. The remaining “bulk” Sn is assumed to exhibit the normal enthalpy of fusion, ΔH°m. The model accounts for the dependence of ΔHm on νGe within the experimental uncertainty. With the average width of Ge particles −10 nm, δ is estimated to be −0.23 nm; i.e., of the order of the thickness of one Sn monolayer.
Effects of lithium oxide on the electrical properties of CuO at low temperatures
Authors: Lanza F. , Feduzi R. , Fuger J.
Published: September 2007

We present an investigation of the influence of four dopant lithium concentrations on the electrical properties of CuO. X-ray measurements have revealed a single phase formed up to 4.2 at. % of Li, and a second phase formed, Li2CuO2, in the case of 10.5 at. % of Li concentration. The log(ρ/T) vs 1/Tdata are better represented by two straight lines than by one and show an initial strong decrease of the conduction activation energies for small contents of Li (<1.7 at. %), which becomes weak for larger contents. The change in slope observed in the resistivity measurements as a function of temperature has also been investigated by calorimetric measurements, revealing a specific heat anomaly which can be associated with the antiferromagnetic order transition.
A comparison of the modulated microwave absorption spectra of ceramic and powdered YBa2Cu3O7-δ samples
Authors: Rubins R. S. , Hutton Stuart L. , Drumheller John E. , Jeong D. Y. , Black T. D.
Published: September 2007

Flux trapping in the 9.3 GHz modulated microwave absorption spectra observed near 4 K from ceramic and powdered ceramic specimens of two separately prepared YBa2Cu3O7-δ samples has been used to separate the intergranular and intragranular contributions to the spectra. In the denser, glassy sample, a broad absorption with a peak near 400 Oe for forward sweeps was observed with appreciable intensity after the maximum flux was trapped. This spectrum is attributed to intergranular junctions, since its relative intensity was reduced on powdering and suspending in wax. In the less dense, more uniform sample, the latter spectrum was appreciably weaker in both ceramic and powder. Both types of junction appear to contribute to the narrow low-field absorption which was observed after zero field cooling in all the samples.
Effect of microcracking on the measured moduli of bulk YBa2Cu3Ox
Authors: Holcomb D. J. , Mayo M. J.
Published: September 2007

Elastic moduli of both as-hot-pressed and oxygenated YBa2Cu3Ox were measured, under isostatic pressures up to 1 GPa, using a combination of strain gage and ultrasonic techniques. Pressurization of the as-hot-pressed samples caused no significant change in moduli; however, pressurization of the oxygenated sample produced a 5-fold increase in bulk modulus. The results show that the significant decreases in moduli between the as-hot-pressed and the oxygenated state were due to extensive microcracking, of an extremely fine nature, that occurred during the oxygenating sequence.
Critical current density of (Bi, Pb)–Sr–Ca–Cu–O filament prepared by suspension spinning method
Authors: Goto Tomoko , Maruyama Tamotsu
Published: September 2007

A high Tc superconducting (Bi, Pb)–Sr–Ca–Cu–O filament was prepared using a combined technique of suspension spinning and densification of the pyrolyzed filaments by pressing and sintering. The effects of the Jc of the filament obtained on the spinning and densifying conditions were examined. The Jc was dependent on the composition of the starting powder and the kind of suspension medium. The Jc was strongly affected by the pyrolyzed condition and densification time. The highest Jc value at 77 K, 0 T of 1900 A/cm2 was attained for the filament sintered under the optimum sintering condition. The grains in the filament were oriented with the c axis perpendicular to the filament surface, and the coupling of the grains in the perpendicular to the filament surface was weaker than that for the parallel direction in the filament surface.
Formation route of higher member phases from lower ones in the Tl–Ca–Ba–Cu–O system
Authors: Cheung C. T. , Ruckenstein E.
Published: September 2007

An experiment was designed to follow the formation of higher member phases from lower member phases in the Tl2Can−1Ba2CunO2n+4 series. Coarse particles of the lower member phase were directly interfaced with various Ca–Cu–O sources and the pressed pellets were heat treated in air in a gold pocket at 850 °C for a period of 2–4 h. The reacted samples were polished and examined by scanning electron microscopy with energy dispersive x-ray spectrometry analysis (EDS). For the 2021 based reaction systems, the results suggest that the 2122 phase can be formed via reactions which involve the decomposition of the 2021 phase, the formation of a complex liquid phase, and finally the reaction between this liquid phase and a Ba/Ca compound. The formation of the 2223 phase from the 2122 phase is likely to follow a similar reaction route. The involvement of a liquid phase in the formation of the higher member phases may lead to a glassy phase, and hence to a barrier to the achievement of a high critical current density.
Kinetic mechanisms for mullite formation from sol-gel precursors
Authors: Li Dong X. , Thomson William J.
Published: September 2007

The reaction kinetics for the formation of mullite (3Al2O3 · 2SiO2) from sol-gel derived precursors were studied using dynamic x-ray diffraction (DXRD) and differential thermal analysis (DTA). The reaction kinetics of diphasic and single phase gels are compared and different reaction mechanisms are found for each gel. Mullite formation in the diphasic gel exhibits an Avrami type, diffusion-controlled growth mechanism with initial mullite formation temperatures of about 1250 °C and an activation energy on the order 103 kJ/mole. On the other hand, mullite formation from the single phase gel is a nucleation-controlled process with an initial formation temperature of 940 °C and a much lower activation energy of about 300 kJ/mole.
Solid state reactions between Ni3Al and SiC
Authors: Chou T. C. , Nieh T. G.
Published: September 2007

Solid state reactions between SiC and Ni3Al were studied at 1000°C for different times. Multi-reaction-layers were generated in the interdiffusion zone. Cross-sectional views of the reaction zones show the presence of three distinguishable layers. The Ni3Al terminal component is followed by NiAl, Ni5.4Al1Si2, Ni(5.4−x)Al1Si2+ C layers, and the SiC terminal component. The Ni5.4Al1Si2layer shows carbon precipitation free, while modulated carbon bands were formed in the Ni(5.4−x)Al1Si2+ C layer. The NiAl layer shows dramatic contrast difference with respect to the Ni3Al and Ni5.4Al1Si2layers, and is bounded by the Ni3Al/NiAl and Ni5.4Al1Si2/NiAl phase boundaries. The kinetics of the NiAl formation is limited by diffusion, and the growth rate constant is measured to be 2 ⊠ 10−10cm2/s. The thickness of the reaction zone on the SiC side is always thinner than that on the Ni3Al side and no parabolic growth rate is obeyed, suggesting that the decomposition of the SiC may be a rate limiting step for the SiC/Ni3Al reactions. The carbon precipitates were found to exist in either a disordered or partially ordered (graphitic) state, depending upon their locations from the SiC interface. The formation of NiAl phase is discussed based on an Al-rejection model, as a result of a prior formation of Ni–Al–Si ternary phase. A thermodynamic driving force for the SiC/Ni3Al reactions is suggested.
Tungsten wire for incandescent lamps
Authors: Walter John L. , Briant Clyde L.
Published: September 2007

Tungsten wire for incandescent lamp filaments must operate at high temperatures and for long times. To meet these requirements, the grain morphology of the wire must be controlled to reduce the propensity for grain boundary sliding. The morphology is a function of the distribution of very small pockets of potassium in the wire and the mechanical processing from ingot to wire. The behavior of the filament is directly related to the grain morphology. This paper describes the mechanism by which the potassium is incorporated into and distributed in the ingot. The elongation and spheroidization of the bubbles during hot rolling and swaging are also examined and related to the grain morphology of wire. Some indications of the relationship between grain morphology and filament behavior are also given.
Microstructural interaction of Y2Ba4Cu8O16 stacking faults within YBa2Cu3O7−x
Authors: Marshall A. F. , Char K. , Barton R. W. , Kapitulnik A. , Laderman S. S.
Published: September 2007

A transmission electron microscopy study of a post-annealed YBa2Cu3O7−x thin film shows that extra Cu–O planes within the structure can aggregate as stacking faults to form a defect microstructure rather than forming the well-ordered Y2Ba4Cu8O16 phase. Interaction of the stacking faults with the surrounding matrix results in strain effects and microstructural variations which may hinder ordering as well as influencing superconducting properties if occurring in higher concentration. When viewed normal to the plane of the film, the boundaries of the stacking faults can be imaged as dislocation-like defects, indicating the size and shape of the stacking faults and their relationship to other defects such as twins and second phase precipitates.
Methyl versus acetylene as diamond growth species
Authors: Harris Stephen J. , Martin L. Robbin
Published: September 2007

We have modeled plasma-assisted diamond growth on substrates placed in a high velocity 1-dimensional flow. The gas consisted of methane or acetylene injected into a flow of partially dissociated hydrogen gas at 800 °C. Diamond is formed only near the injector. More diamond is formed when methane is the additive, and Raman spectra show that the quality of the diamond films is also higher when methane is the additive. The model, which includes detailed chemistry, convection, concentration diffusion, and thermal diffusion, shows that with this experimental arrangement only methane and methyl radicals are present in significant quantities when methane is added, while only acetylene is present when acetylene is added. We conclude that (1) Diamond films can be grown directly from methyl radicals (or, possibly, from methane) and from acetylene. This suggests that a variety of hydrocarbons could act as growth species. (2) An environment containing methane and methyl is much more effective for growing diamond films than one containing acetylene. (3) The quality of the diamond film depends on the identity of the growth species, with acetylene producing lower quality films than methyl (or methane). (4) The fall-off in diamond formation with distance from the injector is due to destruction of species crucial to diamond growth on the silicon substrates.
Thermogravimetric analysis of the oxidation of CVD diamond films
Authors: Johnson Curtis E. , Hasting Michael A.S. , Weimer Wayne A.
Published: September 2007

Diamond films grown by microwave plasma assisted chemical vapor deposition (CVD) were studied by thermogravimetric analysis under an air atmosphere. Oxidation rates were measured between 600 and 750 °C to determine an activation energy of 213 kJ/mol which is similar to that reported for natural diamond. The rate of oxidation increases with increasing surface area and decreases with increasing humidity. The oxidation proceeds by etching pits into the film, creating a highly porous structure. Graphitization was not detected in partially oxidized samples by Raman or Auger electron spectroscopy. A film that was heated to 1170 °C under nitrogen remained IR transmissive.
Dislocations, twins, and grain boundaries in CVD diamond thin films: Atomic structure and properties
Authors: Narayan J.
Published: September 2007

We have used transmission electron microscopy techniques to study the nature of dislocations, stacking faults, twins, and grain boundaries in CVD (chemical-vapor-deposition) diamond thin films. Perfect a/2〈110〉 and partial a/6〈112〉 and a/3〈111〉 type dislocations are observed; the partial dislocations are also associated with twins and stacking faults. The most common defect in diamond thin films, particularly in 〈110〉 textured films, is Σ = 3 grain boundary or the primary twin. These twins in 〈110〉 textured films can lead to formation of fivefold microcrystallites. We have also investigated the splitting of Σ = 9 grain boundary (second order twin) into two Σ = 3 boundaries or primary twins via reaction Σ9 = 2Σ3. A rapid thermal annealing treatment has been shown to result in annealing of Σ = 3 boundaries and produce “defect-free” regions in thin films. A mechanism of annealing (removal) of Σ = 3 boundaries is discussed. Atomic structure and energetics of dislocations, twins, and grain boundaries are calculated using Tersoff potentials. The calculated atomic structure for Σ = 3 boundary is compared with high-resolution TEM images and a good agreement is obtained. These boundaries consist of periodic units of 5–7 rings which are similar to the core structure of 90° a/2〈110〉{001} dislocations. The energy of the 5–7 rings in the grain boundaries is considerably lower, due to overlapping and strain cancellation effects, than that associated with single dislocations. The 5–7 ring energy and consequently the boundary energy increases as the overlapping effects decrease. An interesting analogy between the diamond and silicon results is drawn.
Photoluminescence studies of polycrystalline diamond films
Authors: Freitas J. A. , Butler J. E. , Strom U.
Published: September 2007

The photoluminescence spectra of polycrystalline diamond films prepared by filament assisted chemical vapor deposition are dominated by a defect band with a strong zero phonon line near 1.68 eV and weak phonon replicas at lower energies. The 1.68 eV line is blucshifted from the 1.675 eV zero phonon line associated with the neutral vacancy in bulk diamond. The line shape and position of the 1.68 eV line are shown to depend on substrate material (Si, Mo, Ni). The 1.68 eV emission for Ni and Mo substrates is interpreted in terms of the stress shifted and broadened neutral vacancy emission. The broader 1.68 eV line observed for Si substrates may indicate the additional effects of Si absorption by the diamond films. Films prepared by an oxygen-acetylene flame technique exhibit two additional luminescence bands with zero phonon lines at 1.95 and 2.16 eV. These lines have been tentatively assigned to nitrogen-vacancy complexes. The temperature dependence (6 K–300 K) of the luminescence of a free-standing diamond film, which had been deposited on a molybdenum substrate, is comparable to similar observations reported for bulk diamond. We have also observed a strong dependence of the PL spectra radially across a given combustion film and associated this with details of the flame chemistry.
A spectroscopic study of optical centers in diamond grown by microwave-assisted chemical vapor deposition
Authors: Collins A. T. , Kamo M. , Sato Y.
Published: September 2007

Absorption and cathodoluminescence spectra have been recorded for single crystals of diamond and polycrystalline films of diamond, grown by microwave-assisted chemical vapor deposition (CVD) using methane and hydrogen. The investigation has been carried out to see to what extent the properties of CVD diamond are similar to those of conventional diamond, and to what extent they are unique. Studies have been made of the as-grown material, which has not been intentionally doped, and also samples that have been subjected to radiation damage and thermal annealing. The single crystals grown using methane concentrations of 0.5 to 1.0% exhibit bright blue “band A” emission and also intense edge emission, similar to the cathodoluminescence spectra of some natural type IIa diamonds. This implies that the crystals are relatively free from structural and chemical defects, a conclusion which is reinforced by the absence of any zero-phonon lines in the absorption spectra of crystals which have been subjected to radiation damage and annealing at 800 °C. Before radiation damage the spectrum does, however, reveal an absorption which increases progressively to higher energies, and which may be associated with sp2-bonded carbon. The Cathodoluminescence spectra after radiation damage indicate that the crystals contain some isolated nitrogen, and the detection of H3 luminescence, following thermal annealing at 800 °C, demonstrates for the first time that these samples contain small concentrations of nitrogen pairs. All of the polycrystalline films, grown using methane concentrations between 0.3 and 1.5%, have an absorption which increases progressively to higher energies, and which again is attributed to sp2-bonded carbon. This absorption is stronger in the films grown using higher methane concentrations. Films grown at a methane concentration of 0.3% also exhibit bright blue cathodoluminescence, although the edge emission is undetectably weak. The use of higher methane concentrations produces films with evidence in the cathodoluminescence spectra of nitrogen + vacancy and nitrogen + interstitial complexes, as well as optical centers unique to CVD diamond. One particular defect produces an emission and absorption line at 1.681 eV. By implanting conventional diamonds with 29Si ions it has been confirmed that this center involves silicon, and it has been shown that the 1.681 eV luminescence is relatively more intense in implanted diamonds which have a high concentration of isolated nitrogen.
Optical and tribological properties of heat-treated diamond-like carbon
Authors: Grill A. , Patel V. , Meyerson B. S.
Published: September 2007

Diamond-like carbon, or DLC, films, prepared by the rf plasma decomposition of acetylene, have been deposited at substrate temperatures of 100 to 250 °C, with the substrate at a negative bias of 80 V dc. The DLC films have been annealed in vaccum at temperatures up to 600 °C for 3–4 h. The optical properties of the as-deposited and annealed films have been characterized by ellipsometry and Fourier transform infrared spectroscopy. The wear resistance of the thin DLC films and their friction coefficients have been characterized by a specially designed tribotester. The stresses in the films have also been determined. No significant differences were found between the index of refraction and IR absorption spectra of the as-deposited films, or films annealed at up to 390 °C. The DLC films begin losing hydrogen after annealing above 390 °C and only sp2 bond carbon is observed after annealing at 590 °C. The wear behavior of the as-deposited films was identical for all deposition temperatures. DLC films deposited at 250 °C were more stable and could withstand higher annealing temperature than films deposited at lower temperatures, and remained wear resistant after anneling at 390 °C.
Ultralow-load indentation hardness and modulus of diamond films deposited by hot-filament-assisted CVD
Authors: Beetz C. P. , Cooper C. V. , Perry T. A.
Published: September 2007

Diamond films, ranging in thickness to approximately 35 μm, were grown on Si(100) substrates using hot-filament-assisted CVD. Two different CH4:H2 ratios were employed during deposition, and the effects on the film morphology and ultralow-load indentation hardness and modulus were investigated. Films deposited from a single, linear filament exhibited a nonuniform deposition thickness profile that can be described by a simple exponential function. Films deposited at lower methane concentrations, 0.11% CH4 in H2, had larger crystallite sizes of ∼5–8 μm, an average hardness of 31 GPa, and an average modulus of 541 GPa. A higher CH4 concentration of 0.99% in H2 resulted in the formation of finer crystallites of approximately 0.5 μm, an average hardness of 65 GPa, and an average modulus of 875 GPa. While these values lie on the low end or outside of the range reported for single crystal diamond, this study has demonstrated that CVD diamond films can be synthesized with ultrahigh or near ultrahigh hardness.
Wear property and structure of nitrogen implanted glassy carbon
Authors: Iwaki M. , Takahashi K. , Sekiguchi A.
Published: September 2007

A study has been made of the correlation between wear resistance and crystalline structure of ion implanted glassy carbon. Nitrogen ions were implanted in glassy carbon with fluences ranging from 5 × 1014 to 5 × 1016 ions/cm2 at an energy of 150 keV. The target temperature during ion implantation was maintained constantly at −70, 25, and 200 °C. Wear tests were carried out with the system of glassy carbon and polishing silk disk on which water including diamond slurry was poured, using a conventional polishing machine. The surface layer structure was investigated by means of laser Raman spectroscopy. Nitrogen implantation causes the wear resistance to improve, and the reduction of wear rate is dependent on the ion fluence and the target temperature during ion implantation. As the fluence increases and the target temperature is lower than room temperature, the wear rate decreases drastically. Raman spectra show that the structure of ion implanted layers becomes what is called amorphous-like as the fluence increases and the target temperature is lowered. In conclusion, the wear resistance of glassy carbon is improved owing to the change in structure followed by nitrogen implantation at a relatively high fluence and low target temperature.
Fabrication of textured YBa2Cu3Ox superconductor using directional growth
Authors: No Kwangsoo , Chung Dae-Shik , Kim Jae-Myung
Published: September 2007

Textured bulk YBa2Cu3Ox superconductor samples were fabricated using directional growth of superconductor grains reacted from Y2BaCuO5, BaCuO2, and CuO powder mixtures. The samples consisted of several mm long grains aligned parallel to the growth direction. The microstructure observation and x-ray diffraction analysis showed that the grains have a preferred orientation to improve critical current density.
Thermal behavior of radiation damage cascades via the binary collision approximation: Comparison with molecular dynamics results
Authors: Caro M. , Ardelea A. , Caro A.
Published: September 2007

Based on the profile of the energy deposition obtained using the binary collision model, we follow the diffusion of energy by solving a simplified version of the heat equation. An estimation of the molten zone compares very well with the molecular dynamics prediction for the same event. We discuss the reasons for this agreement and the relevance of such simplified procedure in terms of present-day computer limitations to simulate high energy cascades using molecular dynamics.
Preparation of high Tc Tl–Ba–Ca–Cu oxide superconducting filament by suspension spinning method
Authors: Goto Tomoko
Published: September 2007

High Tc Tl–Ba–Ca–Cu–O (A) and Tl–Ba–Sr–Ca–Cu–O (B) superconducting filaments were successfully prepared using a technique in which suspension spun and pyrolyzed filaments were densified by pressing and sintering. A zero resistance state was achieved at 112 K and 106 K, and the maximum Jc at 77 K, 0 T was 1200 A/cm2 and 2300 A/cm2 for Tl–Ba–Ca–Cu–O (A) and Tl–Ba–Sr–Ca–Cu–O (B) filaments, respectively. Although the starting powder had a high Tc phase of Tl2Ba2Ca2Cu3Ox, the structure of the filament with high Jc was a high Tc phase of Tl1Ba2Ca2Cu3Ox, due to the volatility of Tl oxide from the large surface area of the filament.