bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling of crack tip dislocation emission in B2 intermetallic alloys
Authors: Bartholomeusz Michael F. , Wert John A.
Published: September 2007

A model has been previously proposed to describe the energy associated with emission of dissociated superlattice dislocations from crack tips in ordered intermetallic alloys. In the present paper, the model is applied to several B2 intermetallic alloys. The results of the analysis reveal a correlation between the range of slip system orientations for which emission of a dislocation from a crack tip is energetically favorable and the macroscopic fracture mode of the alloy. Additionally, the effects of changing the active slip system, increasing the thermal energy available for thermally activated dislocation emission, and changing the {111} APB energy on the fracture mode of NiAl and FeAl are discussed.
Rate limitation in low pressure diamond growth
Authors: Rau Hans , Picht Friederike
Published: September 2007

We performed diamond deposition experiments from a gas phase containing H2, CH4, and sometimes CO, using a microwave plasma ball reactor operating at 400 mbar pressure. The molybdenum substrates were stamped with a suitable tool to form a number of flattened cones on its surface. A strong preference for crystal growth on top of the cones was observed. Numerical calculations were used to solve the underlying thermal conduction and diffusion problems. At the substrate, the flow of the active species entering by diffusion from the bulk of the gas phase was balanced by those leaving the system due to incorporation in the crystals. Comparison with the experiments showed that at least 10% of the active species striking the surface are incorporated. Thus, the limitation of diamond growth in our investigation lies in gas phase transport and not in incorporation difficulties at the growing surface.
Synthesis of high phase pure cuprate superconductors via xerogel precursors
Authors: Macho J. , Schaeffer R.W. , Myer G.H. , Salomon R.E. , Crow J.E.
Published: September 2007

Phase pure YBa2Cu3O7 high transition temperature superconductor powders were synthesized by a new precursor route. This new synthesis procedure is easy, leads to an atomic mixture of the relevant constituents, and is readily adaptable to many other systems or to the doping of existing systems. A gel of yttrium, barium, and copper salts was prepared using organic gelling agents. The gel was then dried and ground, leaving the xerogel precursor. These xerogels are perfectly stable even under high humidity atmospheric conditions and show an amorphous structure when characterized by x-ray diffraction, which is consistent with the existence of an atomic mixture of the three relevant metal salts. The organic gelling agent was selected after optimization studies focused on such issues as ash residue after calcination, solubility, and chemical interference with the metal salts. From a great variety of these gelling agents (agar, xanthan gum, gelatin, alginates, and others), gelatin was chosen as the ideal gelling agent for this specific application. The final product characterization shows very unexpected results. The crystalline phase purity of the YBa2Cu3O7 superconducting phase was about 99%, with no traces of barium carbonate, as expected from pseudo-organic precursors (xerogels). The product shows superconducting properties even before oxygen anneal (Tc ≍ 70 K), and after oxygen annealing of the powders the transition temperature was found to be 91 K, showing a linear behavior of the resistivity versus temperature before the drop at Tc, extrapolating to zero at 0 K. The particle size of these powders is smaller than 1 μm, as shown by SEM.
Mechanical and microstructural properties of nitrogen–high pressure melted Fe–Cr–Ni alloys
Authors: Rawers J. , Asai G. , Doan R. , Dunning J.
Published: September 2007

A series of iron-chromium-nickel alloys was melted under a nitrogen atmosphere at several different pressures. Nitrogen–high pressure melting (N–HPM) was conducted under pressures ranging from 0.1 to 200 MPa. The total nitrogen concentrations achieved in these alloys were proportional to the square root of the nitrogen pressure used during melting. Nitrogen took the form of soluble interstitial nitrogen and metal nitride precipitates, FexN and CrN. Tensile properties of N–HPM alloys were directly proportional to the nitrogen concentration in the alloy.
Microscratch analysis of the work of adhesion for Pt thin films on NiO
Authors: Venkataraman S. , Kohlstedt D.L. , Gerberich W.W.
Published: September 2007

The adhesion of as-sputtered Pt thin films to NiO single crystals has been characterized by a continuous microscratch technique. In these experiments, a conical indenter was driven into a 1.2 μm thick Pt film at a rate of 15 nm/s, and across the sample surface at a rate of 0.5 μm/s, until a load drop was observed indicating that the film had delaminated. Using the width of the scratch track at the point at which the film delaminated from the substrate, the critical load required for delamination, and the area of the delaminated region, a model has been developed to determine the work of adhesion of the Pt/NiO system. This model uses an elastic contact mechanics approach to relate the stresses acting in a scratch experiment to the strain energy released during film delamination. Using this model, the work of adhesion and hence the interfacial fracture toughness have been determined to be 0.023–0.06 J/m2 and 0.07–0.11 MPa$\sqrt m$, respectively. These values are in reasonable agreement with those determined by other methods for metal-ceramic systems.
Structural and textural changes from polyimide Kapton to graphite: Part II. Magnetoresistance and x-ray diffraction
Authors: Inagaki Michio , Hishiyama Yoshihiro
Published: September 2007

Structural and textural changes in carbon films prepared from the polyimide Kapton with heat-treatment temperatures up to 3000 °C were followed by measuring the magnetoresistance at 77 K, the ratio of electrical resistivity at room temperature to that at 77 K, and x-ray powder patterns. The maximum magnetoresistance (Δρ/ρ)max as a measure of graphitization changed its sign from negative to positive around 2500 °C and then increased its value abruptly. The resistivity ratio ρR.T./ρ77 K increased above 2500 °C treatment. The values of (Δρ/ρ)max and ρR.T./ρ77 K after 3000 °C treatment reached 275% and 0.93, respectively. The average interlayer spacing d002 after the 3000 °C treatment was 0.3354 nm, the same as natural graphite. The changes in structure and texture in the film were consistent with the model proposed in the previous paper from observation by transmission electron microscopy.
Optical properties of polycrystalline CdSe films
Authors: Gupta Tapan K. , Doh Jim
Published: September 2007

The optical properties of polycrystalline CdSe films prepared using electroless deposition techniques are discussed. Refractive index measurements are reported for the wavelength region 400–2000 nm. Extinction and absorption coefficient measurements are reported for the strong absorption regime (450–750 nm). UV reflectance measurements are also discussed.
Computer modeling of thermal shock-induced crack growth in brittle materials
Authors: Shimamura S. , Sotoike Y.
Published: September 2007

Crack morphology in brittle materials exposed to thermal shock is studied by means of Monte Carlo simulations based on simple three-dimensional lattice systems. Thermal shock-induced crack growth is modeled as a series of processes of storage, release, and transfer of strain energies in the system of grains. Special attention is devoted to the crack pattern on the surface exposed to thermal shock and the crack penetration in the direction perpendicular to the surface. The dependences of the crack pattern and the crack penetration on the grain boundary strength and the temperature difference of thermal shock are explored through the simulations. The results of the simulations are qualitatively consistent with crack features in available experimental reports.
Scanning tunneling microscope observations of polymer fracture surfaces
Authors: Kulawansa D.M. , Langford S.C. , Dickinson J.T.
Published: September 2007

Scanning tunneling microscope observations of gold-coated polymer fracture surfaces are reported. We compare nm-scale surface features of poly(methyl methacrylate) (PMMA) fractured under three different loading conditions: in tension at room temperature, in tension at liquid nitrogen temperature, and in the double torsion geometry at room temperature (slow crack growth). Fracture surfaces of polystyrene and polycarbonate loaded in tension at room temperature are also described. Each of these surfaces shows distinctive nm-scale features which we interpret in terms of the interaction between craze growth (fibril formation) and crack growth along the craze boundary. The resolution of these images is sufficient to greatly complement other fractographic studies.
Sol-gel processing of high-Tc superconductors in the Bi–(Pb)–Sr–Ca–Cu–O system
Authors: Ma Kunsong , Pierre Alain C.
Published: September 2007

Superconducting materials in the Bi–(Pb)–Sr–Ca–Cu–O system have been synthesized by sol-gel processing from nitrates by complexation with citric acid. The gelation process and the microstructures of the gels obtained have been studied through titration and SEM observation. Their phase transformations and microstructure development, during heat treatment, have also been investigated. The critical temperature Tc of superconductors by this sol-gel processing can reach above 100 K.
Radiation damage evolution and its relation with dopant distribution during self-annealing implantation of As in silicon
Authors: Lulli G. , Merli P.G. , Migliori A. , Brusatin G. , Drigo A.V. , Gerardi C. , Guerrieri A.
Published: September 2007

The evolution of radiation damage and of dopant profiles in Si samples subjected to self-annealing implantation with 160 keV As+ ions, under various transient heating conditions, depending on the beam current density, has been investigated. For temperatures in excess of 880 °C, the formation of voids is evidenced by Transmission Electron Microscopy observations. They are located in a layer extending from the surface over a depth of about 0.8 of the ion projected range, Rp (≃110 nm), while extended interstitial-type defects are observed in the region below. With increasing temperature (due to increasing beam power density and/or irradiation time), voids tend to anneal in the near surface region, while they survive and grow in a region about 40 nm thick, centered at a depth of about 50 nm, where an anomalous peak in the dopant profile is developed. The annealing of the extended interstitial-type defects at depths ≥Rp, which becomes appreciable for T ≥ 1050 °C, is coupled to a large enhancement of dopant diffusivity in the same region. It is argued that the local vacancy supersaturation, which leads to void formation, and the corresponding interstitial excess in the deeper region, which leads to extended defect formation, are a natural consequence of the collision kinetics of the displacement process, as suggested by Monte Carlo simulations reported in the literature. The evolution of damage and the anomalies in the redistribution of dopant, observed by increasing the temperature, are described as the result of the increase in the population of mobile point defects and of their influence on the mechanism of As diffusion in the different regions of the implanted layer.
Relationship between characteristics of modulation peaks and modulation wavelength, average composition, and interdiffusion tendency of multilayers
Authors: Bai H.Y. , Wang W.H. , Zhang Y. , Chen H. , Wang W.K.
Published: September 2007

It was found that the modulation wavelength and average composition of multilayers are related to the position, intensity, and diffraction order of x-ray modulation peaks for multilayers. The relation between the characteristics of the modulation peaks in different as-deposited multilayers and their interdiffusion capability is also discussed.
The properties of W–C:H films deposited by reactive rf sputtering
Authors: Wang M. , Schmidt K. , Reichelt K. , Jiang X. , Hübsch H. , Dimigen H.
Published: September 2007

Tungsten-containing amorphous hydrogenated carbon (W–C:H) films were prepared on silicon substrates by reactive rf sputtering (13.56 MHz). Elastic recoil detection (ERD) and Rutherford backscattering (RBS) of MeV He+ ions have been performed to determine the hydrogen concentration and mass density of the films, respectively. The mechanical properties, i.e., the microhardness, Young's modulus, and the adhesion on substrates, have been studied by depth-sensing indentation equipment (nanoindenter) and a scratch tester with an acoustic emission (AE) detector, respectively. The electric conductivity of the films was also measured. The results show that these properties depend mainly on the tungsten concentration. X-ray diffraction suggests that the W–C:H films consist of an a–C: H polymeric matrix with WC1−x(β) particles embedded. With increasing tungsten concentration the films change from polymeric a–C:H dominant to crystalline WC1−x(β) dominant W–C:H, resulting in different film properties.
Milling and mechanical alloying of inorganic nonmetallics
Authors: Kosmac T. , Courtney T.H.
Published: September 2007

The versatility of mechanochemical processing was investigated in a number of nonmetallic inorganic systems. It is shown that high energy grinding can be used to produce amorphous carbon from synthetic graphite and some forms of natural graphite. Elemental sulfur can likewise be amorphized by prolonged high energy grinding. Phase transformations of αFe2O3(hematite) and mechanochemical reactions of this phase with ZnO and NiO are strongly influenced by the presence of iron resulting from wear of the grinding media. Thus spinel type ferrites were obtained by grinding of such mixtures for short times (1–3 h in a Spex mill); however, longer grinding times resulted in the formation of FeO or (Fe,Zn)O (when grinding ZnO) or FeO or (Fe, Ni)O (when grinding NiO), presumably as a result of the reaction of mill wear debris with the mill charge. The suspected (Fe, Zn)O phase is most likely a nonequilibrium solid solution. Negligible were accompanied the mechanochemical synthesis of NiS and ZnS from elemental powders. These sulfides were formed for short milling times. In contrast, sulfides of tungsten were not formed even when rather long milling times were employed. The survey of mechanochemical reactions presented here further reinforces the concept that this low temperature synthesis method is a robust process route for production of a wide range of materials.
Reactive atmosphere synthesis of sol-gel heavy metal fluoride glasses
Authors: Mailhot A.M. , Elyamani A. , Riman R.E.
Published: September 2007

A glass based on ZrF4 was synthesized by a combination of sol-gel and reactive atmosphere processing. Infrared spectroscopy and combustion analysis show that the fluorination process reduces the concentration of organic and hydroxide impurities. Chemical changes have occurred during fluorination; this is indicated by different x-ray diffraction traces and crystallization characteristics for the fluorinated sample compared to the oxide gel product. Chemical analysis from x-ray fluorescence indicates that the fluorinated gel is chemically similar to a melted glass of the same composition. Differential scanning calorimetry gives a glass transition temperature of 290 °C and a crystallization temperature of 390 °C. These values are consistent with those expected for a fluoride glass prepared by conventional methods.
Crystallization of Bi–Sr–Ca–Cu–O glasses in oxygen
Authors: Holesinger T.G. , Miller D.J. , Chumbley L.S.
Published: September 2007

A detailed study of the crystallization process for compositions near Bi2Sr2Ca1Cu2Oy was undertaken using differential thermal analysis (DTA), transmission and scanning electron microscopy (TEM and SEM), and x-ray diffraction (XRD). Glasses prepared by a splat-quench technique were free of secondary phases in most cases. A two-step crystallization process in oxygen was observed in which partial crystallization of the glass occurs initially with the nucleation of “2201” and Cu2O, and is completed with the formation of SrO, CaO, and Bi2Sr3−xCaxOy. No specific thermal event could be associated with the formation of the “2212” phase. Rather, formation occurs via conversion of 2201 into 2212. This was a kinetically limited process at temperatures below 800 °C as other phases were found to evolve in addition to the 2212 phase during extended anneals. In contrast, a nearly full conversion to the 2212 phase occurred after only 1 min of annealing at 800 °C and above. However, changes in resistivity data, secondary phases, and the measured 2212 composition upon extended anneals at 865 °C showed that considerably longer heat treatments were necessary for the sample to reach its equilibrium state.
Analysis of extended dislocation faults
Authors: Kumar Arun M. , Hirth John P.
Published: September 2007

The relative stability of standard extended dislocation dipoles and some new stable defects with lozenge-shaped cross sections have been numerically estimated. An earlier study of these defects in an isotropic fee structure has been extended to the anisotropic case to assess the effect of anisotropy on the calculations. The study is further extended to the case of the L12 crystal structure of the ordered alloy Ni3Al, where the Burgers vectors are large. Results indicate that the introduction of anisotropy has a small effect in determining the relative stability of extended dislocation faults. The results also show that the large values of the Burgers vectors stabilize the arrays in Ni3Al and that the most stable defect favored is the screw lozenge array LD.
A review of the superconducting and normal state properties of Y1−xPrxBa2Cu3O7
Authors: Radousky H.B.
Published: September 2007

The properties of the Y1−xPrxBa2Cu3O7(YPrBCO) system are reviewed. These include superconducting, normal state, structural, chemical, optical, magnetic, and thermal properties. The destruction of superconductivity with Pr doping is discussed in view of possible models such as hole filling, localization, magnetic pair-breaking, and the role of hybridization. Applications to electronic devices using YBCO/PrBCO/YBCO multilayers are also reviewed.
A new class of composite materials: Matrix auto-reinforced organic material–MARIOM
Authors: Wyler A. , Markus R.T. , Wagner H.J. , de Castro B.
Published: September 2007

A sort of composite material can be produced from a single kind of natural organic fiber. This has been observed with bovine leather. During the formation process in a die particulated leather fibers were subjected for a period of about 6 min to a pressure of over 100 bar at a processing temperature of between 60 and 200 °C. In this way a portion of these collagenous fibers was plastically deformed and converted into continuous matrix material in which unconverted fibers act as matrix reinforcement. Round, collagenous fibers assumed an angular cross section and became the building blocks of the continuous matrix. This is clearly visible on pictures made with a scanning electron microscope. At a processing temperature of 70 °C and a pressure of 650 bar, applied for 6 min, the maximum compression strength at room temperature is found to be 185 MPa. At these processing parameters Young's modulus is about 2.4 GPa and the bulk density is 1350 kg/m3. Other natural fibers, to be used as organic raw material for the production of composites, are currently under investigation.
A critical analysis of existing models for plastic flow in Ni3Al: Comparisons with transient deformation experiments
Authors: Hemker K.J. , Mills M.J. , Nix W.D.
Published: September 2007

A review of the intermediate temperature creep properties of Ni3Al indicates that octahedral glide, the mechanism associated with the anomalous temperature dependence of yielding in this alloy, is exhausted during primary creep. Comparisons of this primary creep transient with the various models proposed to explain the anomalous yielding behavior suggest that additional transient experiments are needed to fully characterize octahedral glide in this alloy. Cottrell–Stokes type temperature change experiments, stress relaxation experiments, and deformation exhaustion/temperature drop tests have been conducted in an attempt to better characterize octahedral glide in Ni3Al. The results of these transient experiments indicate that octahedral glide is only partially a thermally reversible process and that the formation of dislocation substructure, KW locks, plays an important role in determining the flow strength of this alloy. These experiments also suggest that flow in Ni3Al should not be viewed as a viscous drag process, but can best be described as a “pure-metal-like” process involving the rapid motion of a small number of highly mobile dislocations. The stochastic nature of dislocation motion and the importance of substructure formation are emphasized and a description of octahedral glide that is consistent with the transient deformation experiments is proposed.
Plasma production of metallic nanoparticles
Authors: Chou ChinHao , Phillips Jonathan
Published: September 2007

Metallic iron and iron oxide particles were produced by injecting ferrocene into the afterglow region of a low pressure, low power, plasma generated using a microwave power source. This was done as part of an effort to explore the feasibility of using flow type microwave plasmas for the production of metal nanoparticles. It was found that two parameters had the largest impact on the particles: injection point and plasma composition. Analysis done using Mössbauer effect spectroscopy, transmission electron microscopy, and x-ray diffraction indicated that low yields of small particles (ca. 10 nm) resulted from injection into the afterglow region. Much higher yields of large particles (ca. 50 nm) formed if the ferrocene was injected through the coupler. In hydrogen plasmas the particles that were produced were metallic iron, whereas in oxygen and argon plasmas the particles were iron oxide. In all cases significant amounts of graphitic carbon formed around the metal particles.
Electron mobility in n-type Hg1−xCdxTe and Hg1−xZnx Te alloys
Authors: Patterson J.D. , Gobba Wafaa A. , Lehoczky S.L.
Published: September 2007

We have calculated the mobility of electrons in n-type Mercury Cadmium Telluride (MCT) and compared it to a calculation of the mobility of electrons in n-type Mercury Zinc Telluride (MZT) with nearly the same energy gap and with the same number of donors and acceptors. We also compared the results of the MZT calculation with experiment. We found for equivalent energy gaps that the mobilities in the two compounds (MCT, MZT) were nearly the same. The calculations for both MCT and MZT were based on the best set of material parameters that we could compile from the literature. Using these parameters, the comparison with experiment for MZT yielded good results. Since MZT is harder and structurally more stable with respect to Hg retention than MCT, the possibility of equivalent mobility for MCT and MZT is significant. This calculation is one of the first extensive calculations of the mobility of MZT, and we compared it with another which appeared to be less extensive. Our calculation involves scattering of the electrons by longitudinal optic phonons, acoustic phonons, ionized impurities, holes, and compositional disorder. Since not all of these interactions can be approximated by elastic scattering, the corresponding Boltzmann equation was solved by a variational principle. We also discuss directions for future work.
Continuous precipitation of monodispersed yttrium basic carbonate powders: Part III. Kinetics
Authors: Her Y-S. , Matijević E. , Wilcox W.R.
Published: September 2007

The reaction kinetics involved in the continuous precipitation of monodispersed spherical yttrium basic carbonate particles was studied. The amount of powder produced was a linear function of H2CO3 generated by the decomposition of urea in mildly acidic solutions. A model equation, containing an empirical constant F, was derived to describe the conversion yield as a function of reactant concentrations, reaction time, and temperature. The calculated and experimental data agree well for both the batch and continuous processes.
Thermodynamics of (Cr, Mo, Nb, Ta, V, or W)–Si–Cu ternary systems
Authors: Reid J.S. , Kolawa E. , Nicolet M-A.
Published: September 2007

The room-temperature and 700 °C tie lines of the early transition metal–Si–Cu ternary systems have been calculated for the metals Cr, Mo, Nb, Ta, V, and W. The tie lines are determined by considering only binary and elemental phases in observance of strict stoichiometry and zero solid solubility. We find that copper is stable with the majority of transition metal silicides under consideration at 700 °C. The result is relevant to the design of stable copper metallizations for silicon.
Indium tin oxide thin films prepared by thermal decomposition of ethylene glycol solution
Authors: Yamamoto Osamu , Sasamoto Tadashi , Inagaki Michio
Published: September 2007

Indium tin oxide (ITO) films were prepared on a glass substrate by thermal decomposition of ethylene glycol (EG) solutions which were prepared by dissolving indium and tin chlorides into ethylene glycol and then heating the solution at 200 °C. Homogeneous ITO film with a thickness of 1 μm without any cracks was obtained from the EG solution with a viscosity of about 50 cSt by spin coating, with the rotation of the substrate at 2000 rpm. The crystallization of the film thus obtained occurred above 400 °C. A well-crystallized ITO film was obtained by heat-treating at 600 °C. The resistivity of the ITO film obtained on a glass substrate showed a minimum value of 1.1 × 10−4 ohm cm at about 7 at.% Sn.
Thermal conductivity of high temperature superconductor substrate materials: Lanthanum aluminate and neodymium aluminate
Authors: Morelli Donald T.
Published: September 2007

The thermal conductivities of LaAlO3 and NdAlO3 have been measured for the first time between 10 and 300 K. These substrates can be classified as intermediate thermal conductors. The measured room temperature values of 9 W m−1 K−1 for LaAlO3 and 7.5 W m−1 K−1 for NdAlO3 are close to those predicted from the crystal structure of these materials, despite the fact that the crystals are twinned.
Preferred orientation of high performance carbon fibers
Authors: Hamada T. , Furuyama M. , Tomioka T. , Endo M.
Published: September 2007

The preferred orientation of polyacrylnitrile (PAN)-based carbon fibers, mesophase pitch-derived carbon fibers, and pitch precursor fibers was studied by using x-ray diffraction technique. The half width at half maximum (HWHM) intensity of the φ scan x-ray diffraction profiles of these fibers was a minimum at around 2θ = 26°. The result implies that a crystallite with a larger coherence length of crystallite size Lc(002) is better aligned along the fiber axis than that with a smaller Lc(002) in these fibers. Further, θ-2θ scan profiles depending on φ showed that a better aligned crystallite possesses a larger Lc(002) than a misaligned one. Lc(002) of a significantly misoriented crystallite remained constant at about 2 nm even after heat-treatment, though Lc(002) of a well-aligned crystallite was easily changed by heat-treatment for both PAN and pitch-based fibers. The pitch precursor fiber exhibited a clear peak at about 2θ = 7° in the θ-2θ profile and unusual ° scan profiles for 2θ around 7°, which were explained by assuming columnar structures formed by molecule stacking along the c-axis with periodic arrangements of the columns perpendicular to the c-axis. The periodic column stacking structure observed in the pitch precursor fiber was also detected in pitch-based carbon fibers heat-treated at lower temperatures.
Plasma activated sintering of additive-free AlN powders to near-theoretical density in 5 minutes
Authors: Groza Joanna R. , Risbud Subhash H. , Yamazaki Kazuo
Published: September 2007

AlN powders (particle size = 0.44 ± 0.08 μm) containing no deliberate sintering additives were consolidated to near theoretical density in 5 min at 2003 K (1730 °C) using a Plasma Activated Sintering (PAS) process. PAS is a novel consolidation method that combines a very short time at high temperature with pressure application in a plasma environment. The in situ cleaning ability of powder particle during plasma activated densification leads to enhanced particle sinterability. The densities of undoped AlN specimens that were PAS consolidated at 2003 K for 5 min under 50 MPa pressure ranged from 97.5 to 99.3% of theoretical. The initial submicron particle size of AlN powders was retained in the final microstructure that consisted of polycrystalline grains with an average size of ≍0.77 ± 0.1 μm.
Superconducting YBa2Cu3Ox films prepared by rf plasma flash evaporation
Authors: Yuhya S. , Kikuchi K. , Shiohara Y. , Terashima K. , Yoshida T.
Published: September 2007

Superconducting YBa2Cu3Ox films were prepared by rf plasma flash evaporation on MgO(100) single crystals. 4 MHz rf Ar–O2 plasma was used and Y–Ba–Cu–O (YBCO) powder as raw material was fed into the plasma. The growth rate of 730 nm/min was achieved. Texture of the grown films varied from a-axis orientation to c-axis with increasing growth temperature and decreasing growth rate. The films without heat treatment exhibited zero resistance temperature (Tc0) more than 77 K. The deposited films using the stoichiometric powder were slightly poor in Y. The sample using Y-rich powder revealed nearly stoichiometric composition and exhibited a Tc0 of 90.6 K and a critical current density (Jc) of 6.8 × 105 A/cm2 at 77 K in zero applied field.
Epitaxial growth of YbBa2Cu3O7−δ films on (100)-oriented MgO and SrTiO3 substrates by oxidation of a liquid alloy precursor
Authors: Merchant N. , Luo J.S. , Maroni V.A. , Gruen D.M. , Tani B.S. , Sinha S. , Sandhage K.H. , Craven C.A.
Published: September 2007

Textured superconducting films of YbBa2Cu3O7−δ were grown on single crystals of MgO (100) and SrTiO3 (100) by oxidation of a liquid alloy precursor. The substrates were coated by dipping them in molten YbBa2Cu3 (m.p. ~870 °C). After removal from the melt, the liquid layers on the substrates were oxidized in pure oxygen to form the tetragonal oxide phase, i.e., YbBa2Cu3O7−δ, then annealed at 500 °C to obtain the superconducting orthorhombic phase of the same compound. The microstructure of the films obtained in this way was found to be related to the nature of the substrate as well as to processing variables that included oxidation temperature and oxidation time. Films grown on MgO (100) showed c-axis texture as well as a random growth structure. Films prepared on SrTiO3 (100) showed either a c-axis texture or a mixture of c-axis and a-axis texture. The superconducting properties of the as-prepared films and the effects of key process parameters on film quality and microstructure are presented and discussed.
Mechanical properties and microstructures of metal/ceramic microlaminates: Part I. Nb/MoSi2 systems
Authors: Chou T.C. , Nieh T.G. , Tsui T.Y. , Pharr G.M. , Oliver W.C.
Published: September 2007

Artificial multilayers, or microlaminates, composed of alternating layers of Nb and MoSi2 of equal thickness were synthesized by d.c., magnetron sputtering. Four different modulation wavelengths, λ, were studied: 7, 11, 20, and 100 nm. The compositions, periodicities, and microstructures of the microlaminates were characterized by Auger electron spectroscopy and transmission electron microscopy. Structural characterization revealed that the as-deposited Nb layers are polycrystalline, while the MoSi2 layers are amorphous. The hardnesses and elastic moduli of the films were measured using nanoindentation techniques. Neither a supermodulus nor a superhardness effect could be identified in the range of wavelengths investigated; for each of the microlaminates, both the hardness and modulus were found to fall between the bounds set by the properties of the monolithic Nb and MoSi2 films. Nevertheless, a modest but a measurable increase in both hardness and modulus with decreasing wavelength was observed, thus indicating that behavior cannot be entirely described by a simple rule-of-mixtures. The hardness was found to vary linearly with Δ−1/2 in a manner similar to the Hall–Petch relationship. Annealing the microlaminates at 800 °C for 90 min produces significant increases in hardness and modulus due to chemical interaction of the layers.
Mechanical properties and microstructures of metal/ceramic microlaminates: Part II. A Mo/Al2O3 system
Authors: Chou T.C. , Nieh T.G. , McAdams S.D. , Pharr G.M. , Oliver W.C.
Published: September 2007

Synthetic multilayers consisting of periodic layers of the refractory metal Mo and the oxide ceramic Al2O3 have been produced by alternating d.c. and r.f. reactive sputter deposition. Microlaminates with four different modulation wavelength—5, 20, 30, and 100 nm—were investigated in this study. The compositions, periodicities, and microstructures of the microlaminates were characterized by Auger electron spectroscopy, low-angle x-ray diffraction, and transmission electron microscopy, including high resolution lattice imaging and microdiffraction. Transmission electron microscopy from the microlaminates indicated that the as-deposited Mo layers are polycrystalline, while the as-deposited Al2O3 layers are primarily amorphous. The Mo and Al2O3 layers are thermally compatible at 800 °C for 6 h, showing no evidence of atomic interdiffusion between the layers. The mechanical properties of the microlaminates, as well as those of monolithic films of Mo and Al2O3 (i.e., the baseline materials), were investigated using nanoindentation methods. A higher than expected modulus and hardness were observed for the microlaminate with the longest wavelength (100 nm); otherwise the mechanical properties are explainable by a rule-of-mixtures. The enhanced mechanical properties of the 100 nm microlaminate may be attributed to crystallization of the amorphous Al2O3 layers and the evolution of a structural texture within this phase.
Natural-abundance 13C dynamic nuclear polarization experiments on chemical vapor deposited diamond film
Authors: Lock Herman , Maciel Gary E. , Johnson Curtis E.
Published: September 2007

13C cross-polarization (CP) and direct-polarization (DP) spectra of an 83 mg sample of a chemical vapor deposited (CVD) diamond film (combined from 12 separate depositions) have been obtained via dynamic nuclear polarization (DNP) combined with magic-angle spinning (MAS). With DNP, the presence of unpaired electron spins in the sample, measured to be 2 × 1018 spins/g, provides a way to enhance the 13C or the “residual” 1H signal by irradiating the sample with microwaves at or near the electron spin resonance (ESR) Larmor frequency; the interactions between the unpaired electrons and protons or 13C spins lead to a transfer of polarization from the electron spin system to the 1H and/or 13C spin systems. No signal for sp2 hybridized carbons could be observed. The DNP-CP-MAS spectrum, obtained in an experiment in which the DNP-enhanced proton polarization is in turn transferred via CP to the 13C spin system, differs significantly from the DNP-DP-MAS spectrum, in which the 13C spins are directly enhanced.
Effective dispersants for concentrated, nonaqueous suspensions
Authors: Sushumna I. , Gupta R.K. , Ruckenstein E.
Published: September 2007

With the aim of identifying effective dispersants that would yield stable, high solids loading (≥60 vol.%) suspensions of oxides, carbides, or nitrides in nonaqueous carriers such as paraffinic oils, a number of dispersants were evaluated, using in most cases A16SG grade alumina from Alcoa as the filler. Among those evaluated were some common dispersants, such as menhaden fish oil and oleic acid, and commercial dispersants not commonly used in ceramic processing, such as polymeric fatty esters and petroleum sulfonates. More importantly, a few dispersants were synthesized and evaluated. The latter dispersants contained straight or cyclic (benzenic) side chains located far from the head group on 18 carbon main-chain fatty acid molecules. Among these, the dispersants with a 5–10 carbon side chain or with a benzenic side chain yielded very fluid suspensions (≥60 vol.%) compared to those with long polymeric or oligomeric side chains, or with no side chains, or the commercial dispersants; in some cases, for the same solid loading, the suspension viscosities were an order of magnitude lower with the synthesized side chain dispersants. These results indicate that molecules with an optimum side chain length located sufficiently far from the head group and an optimum backbone (main chain) constitute the most effective dispersants for concentrated suspensions. By combining the advantages provided by wider particle size distributions and by these effective dispersants, suspensions highly concentrated (up to 74 vol.%), and yet processable and “flowing” paste-like have been prepared.
A study of temperature and pressure induced structural and electronic changes in SbCl5 intercalated graphite: Part I. Structural aspects
Authors: Lelaurain M. , Marêché J.F. , McRae E. , Andersson O.E. , Sundqvist B.
Published: September 2007

We have studied the effects of temperature (10 ≤ T ≤ 295 K) and pressure (0 ≤ p ≤ 0.8 GPa) on the state of intercalate layer crystallization in SbCl5 graphite intercalation compounds of stages 2, 4, and 8. At room temperature (RT), the intercalate layer may in some second stage compounds be fully crystallized and lowering the temperature creates no further modifications. In all other cases, i.e., those in which the intercalate layer has only partial crystallization at RT, lowering T leads to the formation of new in-plane unit cells, the final state depending on the kinetics. Applying pressure to above 0.3–0.5 GPa results in crystallization in all cases, different from that induced simply by lowering of the temperature. We discuss the unit cells observed and the relationships they bear to each other in the light of other works on similar compounds.
A study of temperature and pressure induced structural and electronic changes in SbCl5 intercalated graphite: Part II. Experimental data for c-axis resistivity
Authors: Andersson O.E. , Sundqvist B. , McRae E. , Marêché J.F. , Lelaurain M.
Published: September 2007

We present experimental data for the c-axis resistivity ρc of stage s = 2, 3, 4, 5, and 8, SbCl5 intercalated graphite, over the temperature range 40 to 300 K and the pressure range 0 to 0.8 GPa (0–8 kbar). For most specimens studied, resistance anomalies are observed below 230 K at atmospheric pressure and at pressures up to 0.5 GPa at 293 K. These anomalies are explained in terms of structural changes from a disordered or partly crystallized in-plane intercalate structure at atmospheric pressure and T > 230 K to an almost completely crystallized structure below this temperature, or at elevated pressures, as discussed in a companion paper. In the crystallized phases ρc is approximately linear in T except for stage 8, for which a nonlinear behavior, with a negative temperature coefficient of resistivity below 200 K, is observed. The results are compared with previously available literature data, and the p-T phase diagram is briefly discussed.
Fracture behavior of short-fiber reinforced materials
Authors: Murat Michael , Anholt Micha , Wagner H. Daniel
Published: September 2007

A discrete model of springs with bond-bending forces is proposed to simulate the fracture process in a composite of short stiff fibers in a softer matrix. Both components are assumed to be linear elastic up to failure. We find that the critical fiber length of a single fiber composite increases roughly linearly with the ratio of the fiber elastic modulus to matrix modulus. The finite size of the lattice in the direction perpendicular to the fiber orientation considerably alters the behavior of the critical length for large values of the modulus ratio. The simulations of the fracture process reveal different fracture behavior as a function of the fiber content and length. We calculate the Young's modulus, fracture stress, and the strain at maximum stress as a function of the fiber volume fraction and aspect ratio. The results are compared with the predictions of other theoretical studies and experiments.
Effect of crystalline and amorphous phases on the transfer of polytetrafluoroethylene (PTFE) onto metallic substrates
Authors: Yang E-L.
Published: September 2007

The transfer of polytetrafluoroethylene (PTFE) onto stainless steel (AISI 316) and silicon wafers in a static contact was characterized at temperatures ranging from 21 °C to 300 °C. The amount of transferred PTFE on AISI 316 substrate was found to be strongly time- and temperature-dependent, and reached a steady state after a certain period of contact time. On testing the effect of temperature on this steady transfer state, an almost temperature independent amount of PTFE was observed to be transferred onto AISI 316 at temperatures between 130 and 180 °C. This steady state started at the glass transition at about 127 °C. This was significantly consistent with an almost constant amorphous-to-crystalline phase ratio at about the same temperature range as determined using infrared spectrophotometry (IR). In general, the amount of the transferred PTFE on AISI 316 depended on the relative amount of amorphous phase in the PTFE contact area. Thus, the amorphous phase in semicrystalline PTFE evidently plays an active role in the transfer process.
Solid krypton in MgO
Authors: Norton M. Grant , Carter C. Barry , Fleischer Elizabeth L. , Mayer James W.
Published: September 2007

Recent work by the authors has been extended to demonstrate the formation of solid krypton in single-crystal magnesium oxide. The solid inclusions, which were formed by ion implantation at room temperature, have been identified by electron diffraction. The formation of solid noble gas inclusions at room temperature indicates that they were under a high pressure. This pressure was determined, based on the measured lattice parameter, to be 1.7 GPa.
Hydrostatic and triaxial compression experiments on unpoled PZT 95/5–2Nb ceramic: The effects of shear stress on theFR1→AOpolymorphic phase transformation
Authors: Zeuch David H. , Montgomery Stephen T. , Keck Jeffrey D.
Published: September 2007

We conducted a series of hydrostatic and constant shear stress experiments at room temperature on three different sintering runs of unpoled, niobium-doped lead-zirconate-titanate ceramic (PZT 95/5–2Nb) in order to quantify the influence of shear stress on the displacive (possibly martensitic), first-order, rhombohedral → orthorhombic phase transformation. Inter- and intra-batch variations were detected, but some generalizations can be made. In hydrostatic compression at room temperature, the transformation began at approximately 260 MPa, and was usually incompletely reversed upon return to ambient conditions. Strains associated with the transformation were isotropic, both on the first and subsequent hydrostatic cycles. Results for the constant shear stress tests were very different. First, the confining pressure and mean stress at which the transition begins decreased systematically with increasing shear stress. Second, we observed that the rate of transformationdecreasedwith increasing shear stress and the associated elastic shear strain. This result contrasts with the typical observation that shear stresses increase reaction and transformation kinetics. Third, strain was not isotropic during the transformation: axial strains were greater and lateral strains smaller than for the hydrostatic case, though volumetric strain behavior was comparable for the two types of tests. However, this effect does not appear to be an example of transformational plasticity: noadditionalunexpected strains accumulated during subsequent cycles through the transition under deviatoric loading. If subsequenthydrostaticcycles were performed on samples previously subjected to shear stress, strain anisotropy was again observed, indicating that the earlier superimposed shear stress produced a permanent mechanical anisotropy in the material. The mechanical anisotropy probably resulted from a crystallographic preferred orientation that developed during the transformation under shear stress. Finally, in a few experiments on specimens from one particular sintering run, volume strain was often completely recovered and sporadic evidence for a shape memory effect was observed.
The yield stress of polycrystalline thin films
Authors: Thompson C.V.
Published: September 2007

In recent experiments it has been shown that the yield stress of polycrystalline thin films depends separately on the film thickness and the grain size. It was also shown that the grain size dependence varies as the reciprocal of the grain size. In this paper an analysis is presented which leads to these results and provides a more detailed understanding of the origins of the observed behavior.
Processing effects on high Tc properties of YBa2Cu3O7−x films from carboxylate solution precursors
Authors: Chu Peir-Yung , Campion Isabelle , Buchanan Relva C.
Published: September 2007

Heat-treatment temperature/time and ambient effects on the properties of YBa2Cu3O7−x superconducting films derived from carboxylate precursors were investigated. The thin films were processed under O2, N2, or N2/O2 mixed atmosphere. The use of N2 or N2-rich atmosphere facilitated the 123 phase formation, grain growth, and orientation due to liquid phase assist sintering. Thin films with Tc(zero) = 87.8 and Jc > 105 A/cm2 at 77 K were prepared under such atmosphere at T ≤ 920 °C. Higher temperatures were required for thin films processed under O2 atmosphere and inferior transport properties were obtained. Processing above the melting temperature resulted in highly oriented films with dense microstructures and large grains, but significant post annealing was needed to obtain good film properties.
Transmission electron microscope observations of rectangular dislocation networks in an Al70Co15Ni15 decagonal quasicrystal
Authors: Yan Yanfa , Wang Renhui
Published: September 2007

The electron diffraction contrast of two types of rectangular dislocation networks in an Al70Co15Ni15 decagonal quasicrystal has been analyzed. One type of dislocation network consists of two dislocation sets whose Burgers vectors are parallel to the tenfold axis A10 and a twofold axis A2D. The other type of dislocation network consists of two dislocation sets whose Burgers vectors are parallel to the A10 and the other twofold axis of A2P. The characteristics of the diffraction contrast of the dislocation networks in the Al–Co–Ni decagonal phase are similar to those in conventional crystals.
Hydride precipitation in vapor deposited Ti thin films
Authors: Peddada S.R. , Robertson I.M. , Birnbaum H.K.
Published: September 2007

Titanium hydrides having two different crystal structures were observed in α–Ti thin films grown epitaxially on sapphire substrates by e-beam physical vapor deposition. One of the hydrides (γ-hydride) had a face-centered tetragonal structure (c/a > 1) with an ordered arrangement of hydrogen atoms. The second hydride formed was the fcc δ-hydride. The γ-hydride grew as platelets in the α–Ti lattice with {10$\overline 1$0}Ti habit planes, whereas the γ-hydrides formed directly on the sapphire substrate parallel to the (0001)Ti. These hydrides are one of the principal causes of film decohesion.
Syntheses and some physical properties of solid solutions Sr2Cu1−xLixMO6 with perovskite type of structure
Authors: Jansen M. , Kucheiko S.
Published: September 2007

A number of the samples with general formula Sr2Cu1−xLixMO6−δ, where M = W, x ≤ 0.5 and M = Mo, x ≤ 1.0 was synthesized by means of solid state reactions at 800–950 °C in flowing oxygen atmosphere. The powder x-ray diffraction data show that most of the samples have a cubic structure. The results of magnetic measurements indicate a near Curie behavior for Sr2Cu0.5Li0.5WO6-δ. The effective number of Bohr magnetrons per copper atom for tungsten-containing samples decreases from 1.62 (x = 0) to 1.39 (x = 0.5) with increasing lithium content. The samples show electrical insulator behavior.