bluuebunny/crossref_metadata_2025 huggingface dataset:
The pyrolysis process of a polytitanocarbosilane into SiC/TiC ceramics: An XPS study
Authors: Sorarù G. D. , Glisenti A. , Granozzi G. , Babonneau F. , Mackenzie J. D.
Published: September 2007

The pyrolysis process of polytitanocarbosilane, precursor for SiC/TiC ceramics, has been followed step by step using x-ray photoclectron spectroscopy. This study shows that Ti–O bonds, present in the precursor, are stable up to 700 °C. Above this temperature, Ti–C bonds, precursor for TiC network, start to form. From a detailed analysis of Ti(2p) and Si(2p) peaks, the formation of intermediate species such as SiCxOy and TiCxOy has been detected.
Ion beam mixing of La(OH)3/Cu bilayers
Authors: Mathevet J. P. , Traverse A. , Chaumont J. , Gasgnier M. , Megtert S.
Published: September 2007

The evolution of atomic composition, atomic depth distribution, and structural states of La(OH)3/Cu bilayers prepared by electron gun evaporation and submitted to ion beam irradiation is described. Chemical reactivity of La with O and H is evidenced in the initially deposited La/Cu bilayer, despite the fact that the pure La layer is coated with a thick Cu layer. Ion beam mixing with energetic Au ions, at 300 K and 700 K, results in breaking down the La–O–H bonds, while Cu atoms are knocked into the layer. There is a depth redistribution of the different atomic species, with formation of nonidentified phases.
Amorphous lead iron tungstate prepared by twin-roller quenching
Authors: Kim N. K. , Payne D. A.
Published: September 2007

Amorphous lead iron tungstate was prepared from the melt by twin-roller quenching. The rapidly solidified material was characterized in terms of density, x-ray diffraction, differential thermal analysis, and electrical properties. Results are given for the amorphous-crystalline transformation.
A dislocation based analysis of continuum mechanical and microscopic local stresses during cyclic deformation of copper
Authors: Jain Mukesh
Published: September 2007

A relationship between continuum mechanical internal stress variables, kinetic back stress, isotropic drag stress, and microscopic local stresses in the dislocation cell interior and cell walls, is developed based upon Mughrabi's composite model of deformation of heterogeneous microstructure during cyclic deformation in cell forming metals. The experimental data on the evolution of kinematic back stress and isotropic drag stress during cyclic deformation of Cu along with TEM measurements of cell diameter and cell width are utilized to determine the evolution of mobile and immobile dislocation densities in the cell interior and cell walls, respectively, as a function of the number of cycles. The range of values obtained is in agreement with the available experimental data.
Silicide precipitation and silicon crystallization in nickel implanted amorphous silicon thin films
Authors: Cammarata R. C. , Thompson C. V. , Hayzelden C. , Tu K. N.
Published: September 2007

The nucleation and growth kinetics of NiSi2 precipitation in amorphous silicon thin films ion implanted with nickel was investigated using scanning transmission electron microscopy. It was found that the nucleation rate could be approximately described by a delta function at time t = 0 when the films were annealed between 325 and 400 °C. The growth kinetics of the precipitates at these temperatures were described by r ∝ tn, where r was the average radius and n was about 1/3. This behavior is consistent with models for growth of three-dimensional particles in a two-dimensional diffusion field. It was also found that the implanted amorphous films displayed an enhanced rate of single crystal silicon formation, apparently catalyzed by migrating silicide precipitates.
The mechanism of synthesis of titanium nitride by self-sustaining reactions
Authors: Deevi S. , Munir Z. A.
Published: September 2007

The mechanism of formation of titanium nitride by the self-propagating high-temperature synthesis was investigated. Samples were quenched in liquid argon at different stages of the reaction and analyzed by x-ray diffraction and electron microprobe methods. A mechanism is proposed in which the propagation of the reaction wave results in the formation of a thin surface layer of TiN and an unsaturated solid solution. The layer then dissolves to form a solid solution of nitrogen in titanium and then re-forms with continued reaction. On the basis of phase equilibria and diffusion information, an analysis is made in which the feasibility of the proposal mechanism is demonstrated.
Simulation of porosity reduction in random structures
Authors: Visscher P. B. , Cates Joseph E.
Published: September 2007

We describe an algorithm for computing the motion of a solid-liquid interface in 2D, which is applicable to geological pressure solution or to pressure sintering. The backward motion (toward the solid) of the interface is due to dissolution of the solid, and the forward motion (away from the solid) is due to the inverse process of reprecipitation. The interface velocity is assumed proportional to the difference between the solubility of the solid and the concentration of the solution. The former is dependent upon stress (the phenomenon of “pressure solution”), so our algorithm must also keep track of the stress. We use a Lagrangian grid, with constant-stress periodic boundary conditions. The method has been applied to porosity reduction in sandstone. It is applicable to other interface-following problems, such as freezing, if the motion is slow enough that heat transport is not rate-limiting.
Adhesion and short-range forces between surfaces. Part I: New apparatus for surface force measurements
Authors: Israelachvili Jacob N. , McGuiggan Patricia M.
Published: September 2007

A new miniature Surface Forces Apparatus (SFA Mark III) is described for measuring the forces between surfaces in vapors and liquids. The apparatus employs similar techniques to those used in current SFAs, but it is easier to operate and is generally more user-friendly. Four stages of increasingly sensitive distance controls replace the three control stages of previous apparatuses. The first three stages allow for rapid manual control of surface separation to within 10 Å, while the fourth piezo-control stage has a sensitivity of better than 1 Å. All four distance controls have been specially designed to produce perfectly linear displacements of the surfaces. In addition, the SFA Mk III is more robust, less susceptible to thermal drifts, easier to clean, and requires smaller quantities of liquid than conventional SFAs. The high performance of this new instrument is illustrated in the succeeding paper (Part II), which describes the subtle effects of surface lattice mismatch on the oscillatory forces in water in the distance regime from 0 to 10 Å.
On the deposition mechanism of Al2O3 in the CVI process for forming ceramic composites
Authors: Tai Nyan-Hwa , Chou Tsu-Wei
Published: September 2007

Chemical vapor infiltration (CVI) is a relatively new fabrication method for forming ceramic composites. The vapor deposition mechanism is determined by the reaction pressure and temperature, as well as the concentration and composition of the reactants. This commentary discusses two mechanisms for the deposition of alumina from AlCl3–H2–CO2 gas mixtures, and it explains that the reaction of the gas reactants on the substrate surface is a highly feasible deposition mechanism of the CVI process for forming alumina matrix composites. Comments are also made regarding a recent publication of Middleman in the Journal of Materials Research.
Hypothetical superhard carbon metal
Authors: Tamor M. A. , Hass K. C.
Published: September 2007

We describe a hypothetical new phase of carbon. The proposed structure is a rigid three-dimensional network with sp2 (threefold) coordination only. Tight-binding calculations indicate that the proposed material is a metastable metal significantly harder than diamond. Although it is unclear at present whether such a material can be produced in significant quantity, we speculate that it (and related structures) plays a role in the chemical vapor deposition of diamond films.
The effect of oxygen in diamond deposition by microwave plasma enhanced chemical vapor deposition
Authors: Liou Y. , Inspektor A. , Weimer R. , Knight D. , Messier R.
Published: September 2007

High quality diamond thin films were deposited on different substrates at temperatures from 300 to 1000 °C by the microwave plasma enhanced chemical vapor deposition (MPCVD) system. The quality of deposited diamond films was improved by adding oxygen in the gas mixtures. Different ratios of methane to oxygen concentration in hydrogen at different temperatures have been studied. At high temperatures (800–1000 °C), the addition of oxygen will not only enhance the growth rate of deposited films but also extend the region of diamond formation. At low temperatures (<500 °C), the oxygen plays an important role in diamond film growth by preferentially etching the non-diamond carbon. Without the addition of oxygen, the films deposited at high temperatures (>900 °C) were either graphitic or diamond containing a large amount of graphitic or amorphous carbon and at low temperatures (<500 °C) were white, soot-like coatings which were easily scraped off. The quality of the deposited films was characterized by Raman spectroscopy and scanning electron microscopy.
High growth rate diamond synthesis in a large area atmospheric pressure inductively coupled plasma
Authors: Cappelli M. A. , Owano T. G. , Kruger C. H.
Published: September 2007

A study of diamond synthesis in an atmospheric pressure inductively coupled argon-hydrogen-methane plasma is presented. The plasma generated has an active area of 20 cm2 and a free stream temperature of approximately 5000 K. Deposition experiments lasting 1 h in duration have been performed in both stagnation flow and flat plate parallel flow geometries. The diamond film deposited in both configurations are nonuniform yet fairly reproducible. The variation in the growth rates at various regions of the substrate is attributed to the variation in the surface atomic hydrogen flux. Growth rates are as high as 50 μm/h, in regions of the substrate where the atomic hydrogen flux is expected to be large. Little or no growth is observed in regions where the atomic hydrogen is expected to recombine within the thermal boundary layer before arriving at the surface. Individual particles are analyzed by micro-Raman spectroscopy. Large (50 μm) size well-faceted particles show little evidence of non-diamond carbon content and are found to be under a state of compression, displaying shifts in the principal phonon mode as great as 3 cm−1.
The nucleation and morphology of diamond crystals and films synthesized by the combustion flame technique
Authors: Ravi K. V. , Koch C. A. , Hu H. S. , Joshi A.
Published: September 2007

The nucleation and morphology of diamond crystals and films synthesized by the use of a combustion flame have been investigated. By operating an oxy-acetylene torch in a fuel-rich mode, diamond crystals and films have been deposited on mechanically abraded molybdenum, onin situcreated molybdenum carbide, and on thin diamond-like carbon (DLC) layers synthesized on molybdenum. Scanning electron microscopy, Auger and Raman spectroscopy have been used to characterize the films and crystals. Diamond is found to be uniformly deposited in the region of the substrate that intersects the inner, acetylene-rich region of the flame. The nucleation density, the growth rate, and the morphology of the diamond crystals and films are found to be strongly influenced by the surface condition of the substrate. On mechanically abraded molybdenum, abraded with 600 mesh silicon carbide, and on molybdenum carbide, well-formed cubo-octahedrons of diamond, up to 45 μm in diameter, are formed for deposition times of 90 min. Film formation is seldom observed under these conditions. To enhance nucleation, thin layers of DLC were formed on molybdenum substrates by reducing the O2/C2H2ratio in the gas mix to ∼ 0.75 for short periods of time under 30 s. This was followed by increasing the O2/C2H2ratio to conditions that produce diamond (an O2/C2H2ratio of ∼ 0.9). Under these conditions the nucleation density of diamond was increased by an order of magnitude and the growth rates by about 60%, as compared to diamond deposited on abraded molybdenum and molybdenum carbide. In addition, the morphology of the diamond crystals and films was substantially affected with indications of dendritic growth. The DLC layer is effective in promoting diamond nucleation due to the high surface defect density and the high hydrogen concentration of these films. The combination of surface defects, in the form of dangling bonds, and the evolution of hydrogen from the DLC layer during the diamond deposition process, which is characterized by higher temperatures, result in a high concentration of active surface sites for diamond nucleation. The nucleation density, the distribution on the substrate, and the morphology of diamond crystals and films are not driven by the transport of reactive specie in the flame to the substrate, but rather by nucleation processes, temperature distribution across the surface, and attendant surface phenomena.
Mechanism of diamond film growth by hot-filament CVD: Carbon-13 studies
Authors: Chu C. Judith , D'Evelyn Mark P. , Hauge Robert H. , Margrave John L.
Published: September 2007

Mixed carbon-12/carbon-13 diamond films were synthesized by hot-filament chemical vapor deposition, using mixtures of 13CH4 and 12CH4 or 12C2H2 in H2. The first-order Raman peak at 1332 cm−1 for 12C-diamond was found to shift by 50 cm−1 to 1282 cm−1 for pure 13C-diamond. For mixed-isotope films, the Raman peak frequency shifts linearly between these values as a function of the 13C mole fraction. The mechanism of diamond film growth by hot-filament CVD has been investigated by growth from mixtures of 13CH4 and 12C2H2, using the shifted Raman frequency to determine the relative incorporation rates of 13C and 12C into the film. The 13C mole fraction in the film agrees closely with the 13C mole fraction inferred for the methyl radical but differs substantially from that of acetylene, indicating that the methyl radical is the dominant growth precursor under the conditions studied.
Line shape analysis of the Raman spectrum of diamond films grown by hot-filament and microwave-plasma chemical vapor deposition
Authors: Robins Lawrence H. , Farabaugh Edward N. , Feldman Albert
Published: September 2007

Raman spectra were taken of 48 polycrystalline diamond films grown by hot-filament and microwave-plasma chemical vapor deposition (CVD), and one gem-quality diamond, and characterized by fitting the spectra to a model line shape function. The line shape function contains three components: a narrow symmetric line at ∼1333 cm−1, the Raman line of diamond; a band centered at ∼1525 cm−1 ascribed to sp2-bonded carbon; and a broad background due to photoluminescence (PL). Four spectral parameters were observed to change from one specimen to another in an interrelated manner: (1) the linewidth of the diamond Raman line, which varies from ∼3 to ∼25 cm−1; (2) the intensity in the tails of the diamond Raman line, several halfwidths away from the peak; (3) the peak intensity ratio of the sp2-bonded carbon band to the diamond Raman line, which varies from ∼0 to ∼1; and (4) the intensity ratio of the PL background to the diamond Raman peak, which varies from ∼0.03 to ∼9. We suggest that the interrelated changes in the Raman spectra are due to changes in the abundance of sp2-bonded defect structures within the diamond crystallites. Polynuclear aromatic clusters are a possible model for these defect structures. Differences in film morphology, which were observed by SEM imaging, appear to be correlated with the changes in the Raman spectra. The peak position of the diamond Raman line is shifted to higher wave number in the CVD-grown films than in the gem, by as much as 3 cm−1, but this shift is not correlated with the other changes in the Raman spectra. As a function of deposition conditions, the defect-related features of the Raman spectra generally increase with increasing methane concentration or substrate temperature, and decrease with increasing oxygen concentration. A cyclic dependence of the defect-related features on deposition time is observed for one set of films grown by hot-filament CVD.
Low friction coatings of diamond-like carbon with silicon prepared by plasma-assisted chemical vapor deposition
Authors: Oguri K. , Arai T.
Published: September 2007

Amorphous carbon-silicon (a–C–Si) coatings with smooth surface and adhesion to steel substrate were deposited at 550°C by a dc glow discharge method from reactant gases of CH4, SiCl4, H2, and Ar at a deposition rate of 1–3 μm h−1. Diamond-like carbon was detected by laser Raman spectroscopy in the coatings with high hardness of Hv 2000 and more than 70 at. % carbon. Ball-on-disk type and Ohgoshi-type apparatuses were used to measure the tribological properties. Ball-on-disk tests revealed that the a–C–Si coatings showed a friction coefficient as low as 0.04 against steel with no lubricant in an ambient atmosphere of 70% relative humidity, which was one-third that of an i–C coating. The wear rate of the steel ball against the coated disk was an order and three orders of magnitude smaller than that against i–C coated and uncoated disks, respectively. Both types of tests showed that the wear rate of the a–C–Si coating itself was also very small.
The effect of environment on the tribological properties of polycrystalline diamond films
Authors: Gardos M. N. , Soriano B. L.
Published: September 2007

Essentially pure, ∼1.5 μm thick, polycrystalline diamond films DC–PACVD deposited on polycrystalline α-SiC flats were friction and wear tested against similarly coated α-SiC pins. The oscillatory sliding tests were performed with a Knudsen cell-type, wide temperature range tribometer built into a scanning electron microscope. Experiments were completed in 1.33 × 10−3 Pa (1 × 10−5 Torr) and 13.3 Pa (1 × 10−1 Torr) Pair, at test flat temperatures cycled to 850°C and back to room temperature. The results are compared with similar tests previously completed with diamond versus bare α–SiC and diamond versus Si(100) sliding combinations. In the absence of in situ surface analytical capability in the SEM tribometer, the findings are interpreted based on relevant information collected from the literature. The data indicate that the friction of the respective, tangentially sheared interfaces depends on the generation and annihilation of dangling bonds. Desorption of hydrogen during heating and sliding under the electron beam, in vacuum, create unoccupied orbitals on the rubbing surfaces. These dangling bonds spin-pair with others donated from the counterface, leading to high friction. Resorption of adsorbates such as hydrogen (e.g., by cooling the tribosystem) lowers the interfacial adhesion and friction. At high temperatures, in vacuum, the interaction energy may also be reduced by surface reconstruction and graphitization. At high temperatures, in Pair, the coefficient of friction of diamond versus itself is substantially lower than that in vacuum. This reduction is most probably caused by the generation of oxidation products combined with the temperature-shear-oxygen-induced phase transformation of diamond to graphite. The wide temperature wear rates of pure, polycrystalline diamond, characteristic of our test procedure, varied from ∼4 × 1016 m3/N · m in 1.33 × 10−3 Pa vacuum to ∼1 × 10−15 m3/N · m in 13.3 Pair.
Finite temperature structure and thermodynamics of the Au Σ5 (001) twist boundary
Authors: Najafabadi R. , Srolovitz D. J. , LeSar R.
Published: September 2007

The structure and thermodynamic properties of a Σ5 (001) twist boundary in gold are studied as a function of temperature. This study was performed within the framework of the Local Harmonic (LH) model and employed an Embedded Atom Method (EAM) potential for gold. We find that for the Σ5 (001) twist boundary in gold, a distorted CSL structure is stable at low temperatures, but undergoes a phase transformation to a DSC related structure near room temperature. This transformation is shown to be first order. The temperature dependences of the excess grain boundary free energy, enthalpy, entropy, specific heat, and excess volume are calculated. Discontinuities are observed in the slope of the grain boundary excess free energy (versus temperature), in the value of the grain boundary excess specific heat and excess volume. The stable high temperature grain boundary structure has a smaller excess volume than does the lower temperature structure, and both structures have a coefficient of thermal expansion which is in excess of that for the perfect crystal.
Hydrothermal crystallization kinetics ofm-ZrO2andt-ZrO2
Authors: Denkewicz Raymond P. , TenHuisen Kevor S. , Adair James H.
Published: September 2007

The isothermal nucleation and crystallization kinetics of hydrothermally prepared monoclinic and tetragonal ZrO2have been determined at various pH conditions. It is shown that monoclinic ZrO2precipitates at low pH whereas at high pH tetragonal ZrO2crystallizes from an amorphous zirconium (hydrous) oxide, Zr(OH)xOy, precursor. At intermediate pH conditions mixtures of the polymorphs are formed suggestive of kinetically competing particle formation mechanisms. The data are explained by the proposed existence of three controlling regimes for the formation of crystalline ZrO2: dissolution/precipitation at low pH, a solubility controlled regime at intermediate pH values, and a gel structure controlled regime at high pH. Apparent activation energies for the nucleation and crystallization of monoclinic and tetragonal ZrO2formed under hydrothermal conditions are presented.
Ion beam etching of polytetrafluoroethylene
Authors: Torrisi L. , Foti G.
Published: September 2007

Cross-links among long chains have been observed in ion bombarded hydrocarbon polymers like polystyrene or polyethylene. Irradiation of fluoropolymers, instead, produces a strong sample erosion with emission of fragments produced along the ion track. Polytetrafluoroethylene foils of thickness ranging from 50 μm up to 2 mm were exposed to MeV helium and proton beams. The ion erosion rate was investigated by changing the target temperature and observing surface topography modifications, using the scanning electron microscopy technique. Etching was measured as removed thickness per irradiation time in the range values of 102−104 μm/h, corresponding to about 104−106 CF2 emitted group/ion. Target temperature (200–400 K), ion mass, and ion energy are the key parameters to follow the evolution of ion erosion of polytetrafluoroethylene.
The control of gas phase kinetics to maximize densification during chemical vapor infiltration
Authors: Sheldon Brian W.
Published: September 2007

A serious problem during the fabrication of composite materials by isothermal chemical vapor infiltration is that the matrix forms more rapidly at the external edges of the body and traps a large amount of porosity inside. In theory, this problem can be eliminated by controlling the gas-phase kinetics to obtain densification which is more rapid in the center of a preform than at its outer surfaces. An analysis of a first-order gas-phase reaction followed by a first-order deposition reaction indicates that improved infiltration is possible under a relatively narrow range of conditions.
Preparation of YBa2Cu3O7−x using barium hydroxide flux
Authors: Coppa N. , Kebede A. , Schwegler J. W. , Perez I. , Salomon R. E. , Myer G. H. , Crow J. E.
Published: September 2007

High quality bulk YBa2Cu3O7−x was synthesized by fusing stoichiometric amounts of yttrium and copper nitrates and barium hydroxide in air, using an ordinary Bunsen burner. The starting materials go through a short-lived liquid phase yielding a solid black product which was subsequently heat treated (900 °C, 18–24 h in air, followed by 500 °C, 5 h in O2). These materials were greater than 99% phase pure with CuO as the only other phase and they exhibited a transition temperature of 92 K, a 15.5% perfect diamagnet response (field cooled). This synthesis represents an improvement over the much more labor and time intensive conventional methods in that it allows high quality materials of various compositions to be prepared quickly.
Atomic structure of amorphous Al90FexCe10−x
Authors: Hsieh H. Y. , Toby B. H. , Egami T. , He Y. , Poon S. J. , Shiflet G. J.
Published: September 2007

The atomic structure of liquid-quenched amorphous Al90FexCe10−x(x= 5,7) was studied by pulsed neutron and x-ray scattering. The atomic pair-density function determined by pulsed neutron diffraction indicates that a significant portion of Al–Fe distances is anomalously short, while some part of the Al–Al distances is anomalously long. Both neutron and x-ray scattering showed the presence of a prepeak in the structure factor. These results suggest that a strong interaction between A1 and Fe modifies the structure of this glass, leading to chemical and topological short-range ordering.
1300 K compressive properties of a reaction milled NiAl–AlN composite
Authors: Daniel Whittenberger J. , Arzt Eduard , Luton Michael J.
Published: September 2007

Cryomilling (high intensity mechanical ball milling in a liquid nitrogen bath) of the B2 crystal structure nickel aluminide leads to a NiAl composite containing about 10 vol.% of AlN particles. This is the result of a reaction milling process, where nitrogen incorporated into the matrix during cryomilling reacts with Al during subsequent thermomechanical processing to form the composite. Compressive testing at 1300 K of such materials densified by 1505 K extrusion or isostatic pressing at 1323 K or 1623 K indicated that strength at relatively fast strain rates (>10−7 s−1) is slightly dependent on the method of consolidation. At slower rates, however, no clear dependency on densification technique appears to exist, and four different consolidation methods possessed similar creep strengths. In all cases deformation at 1300 K occurred by two distinct mechanisms: at high strain rates the stress exponent is greater than 11 while at slower rates (<10−7 s−1) a much lower stress exponent (∼6) was found. Comparison of density compensated creep strengths reveals that the properties of NiAl–AlN are similar to those of the single crystal Ni-base superalloy NASAIR 100.
The structure and fracture mode of rapidly solidified Pt3Ga
Authors: Briant C. L. , Taub A. I. , Hall E. L.
Published: September 2007

This paper reports a study of the structure and fracture mode of Pt3Ga. The results show that the structure of this compound is a tetragonal distortion of the L12 structure with lattice parameters of a = 5.47 Å and c = 7.89 Å. If the gallium concentration decreases from 22.8 at.% to 21.5 at.%, a disordered face-centered cubic phase also forms. This phase is Pt-rich and appears to form along the grain boundaries of the Pt3Ga phase. The fcc phase can also contain coherent Pt3Ga precipitates. Single phase Pt3Ga fails in a very brittle manner, and the fracture mode is intergranular. Boron additions have no effect on the fracture mode, since boron does not segregate to the grain boundaries. The fracture of an alloy containing a small amount of the disordered fcc phase is also brittle, but the fracture mode is predominantly transgranular.
Electrical resistivity and hydrogen-physisorption behavior of potassium-graphite intercalation compounds in the course of reactions with ammonia, water, and oxygen
Authors: Akuzawa N. , Amari Y. , Nakajima T. , Takahashi Y.
Published: September 2007

The electrical resistivity of potassium-graphite intercalation compounds (K–GICs) was measured in the course of reactions with ammonia, oxygen, water, etc, The hydrogen absorption behavior at 77 K was also investigated on K–GICs before and after the reactions. The electrical resistivity of KC8 increased by reactions with ammonia, furan, and water vapor, whereas almost no change was observed in the case of the reaction with oxygen. Molecules of ammonia, furan, and water are considered to penetrate into the KC8 interlayers, while oxygen draws potassium from interlayer spaces toward the surface with resultant potassium-diluted mixed stage compounds. The hydrogen absorption isotherms of K(NH3)xC31 (0 ≤ x ≤ 2.65) at 77 K showed that the saturated amount of absorbed hydrogen, (H2/K)sat, decreased linearly with increasing ammonia content, x, When x went up to 2, (H2/K)sat became zero. Similar behavior in the degradation of the hydrogen absorption capacity of K(H2O)xC25 (0 ≤ x ≤ 1.3) was observed. Contrary to such behavior, partially oxidized KC24 could not absorb hydrogen gas. These facts are also explained by taking into account the fact that ammonia and water molecules penetrate K–GICs, while oxygen draws potassium atoms toward the surface, as predicted from the electrical resistivity measurements. Successive oxidation and heat-treatment processes made KC8 more able to absorb hydrogen, while similar processes of ammoniation and hydration followed by heat-treatment did not.
The structure of carbon in chemically vapor deposited SiC monofilaments
Authors: Ning X. J. , Pirouz P. , Lagerlof K. P. D. , DiCarlo J.
Published: September 2007

The microstructures of carbon-rich regions in chemically vapor deposited SCS-6 SiC fibers have been studied by electron diffraction and high resolution transmission electron microscopy. The microstructures of the carbon in the substrate filament and the outer coating layers of the fiber are consistent with different distributions of Oberlin's model of Basic Structural Units (BSUs)1 while that of the inner substrate coating is consistent with turbostratic carbon (TC).
Physical properties of amorphous silicon-carbon alloys produced by different techniques
Authors: Carbone A. , Demichelis F. , Kaniadakis G. , Della Mea G. , Freire F. , Rava P.
Published: September 2007

Results of a study of compositional, optical, electrical, and structural properties of hydrogen amorphous silicon carbide (a-SiC:H) prepared, respectively, by glow-discharge (GD) and reactive sputtering (SP) techniques at power densities varying between 1.25 · 10−2 and 1.25 · 10−1 W · cm−2 for GD samples are presented. Measurements are reported on the composition, optical and IR spectroscopy, and on the temperature dependence of electrical conductivity. All experimental observations suggest that the power density only slightly affects the physical properties of GD silicon-rich samples, whereas those of the carbon-rich SP samples depend more strongly on this deposition parameter. Finally, it is shown that the GD technique can provide films with better characteristics, whereas samples of similar composition prepared by sputtering have higher compositional disorder and are more inhomogeneous.
Controlled crystallization of vaterite from viscous solutions of organic colloids
Authors: Pach Ladislav , Hrabe Zdenek , Komarneni Sridhar , Roy Rustum
Published: September 2007

Precipitation of calcium carbonate from hydroxyethyl cellulose (HEC) containing aqueous solutions of CaCl2 and Na2CO3 resulted in an uncommon polymorph, vaterite. In contrast to precipitations without HEC, crystallization in the presence of HEC led to a shell-shaped body consisting of organized vaterite platelets. The artificial shell is a composite of stacks of vaterite and about 2% HEC. In the experimental arrangement used, HEC controlled both nucleation and crystal growth of vaterite. Concentration of HEC also affected the platelet's thickness which in turn influenced the shell's morphology as well. These results demonstrate the importance of organic-inorganic interface in controlling crystallization.
Refractory semiconductor of boron phosphide
Authors: Kumashiro Y.
Published: September 2007

The single crystal growth of boron phosphide (BP) by employing the high pressure flux method and chemical vapor deposition (CVD) process is described together with characterization of the prepared BP and its electrical, thermal, semiconducting, and electrochemical properties. BP single crystals prepared by the high pressure flux method contain copper used as the flux, but they are promising for photocathode materials. BP single crystalline wafers prepared by the CVD process using Si wafer substrate contained autodoped silicon with the concentration of 1018−1020 atoms·cm−3, depending on the growth temperature and the substrate plane. The Si atoms which act as acceptors are incorporated at phosphorus sites in BP. The lattice constants determined by the Bond method explain the conduction type of BP. Some electronic transport properties such as donor and acceptor levels and lattice scattering process before and after thermal neutron experiments are clarified. The thermal conduction is limited by three-phonon processes. The formation of defects by thermal neutron irradiation and that of structural disorder by ion-irradiation are mentioned. Schottky diodes consisting of n–BP and Sb or n–BP and Au, which are denoted as n–BP–Sb and –Au, respectively, show excellent characteristics, and their barrier heights are independent of metals and two-thirds of energy bandgap, expected from the surface-state model. Finally, recent results on thermoelectric properties of sintered specimens are mentioned.
Formation threshold and structural evolution of molybdenum nanocrystals with sputtering pressure
Authors: Chow G. M. , Chien C. L. , Edelstein A. S.
Published: September 2007

The evolution of the size and shape of molybdenum nanocrystals fabricated by sputtering in a thermal gradient has been studied as a function of the argon gas pressure, p. For 4 < p < 100 mTorr, continuous Mo films are deposited. At p = 150 mTorr, isolated and well-faceted Mo nanocrystals of two sizes (20 and 5 nm average size) are formed. For 200 ≤ p ≤ 400 mTorr, the particle size decreases with increasing pressure and is about 7 nm at 400 mTorr. On increasing p further, larger particles start to form and at p = 700 mTorr, particle agglomerates are observed. Possible mechanisms leading to these results are suggested.
Electrical conduction in superconductive Nd2−xCexCuO4−y at high temperatures
Authors: Munakata Fumio , Kawano Takashi , Nozaki Ayumi , Yamauchi H.
Published: September 2007

The temperature dependence of electrical resistivity of Nd2−xCexCuO4−y (x = 0.0, 0.1, 0.15, 0.18) was measured at oxygen partial pressures of 2.08 × 10−1 1.8 × 10−2, 8.1 × 10−4, and 3.3 × 10−6 atm. The oxygen-partial-pressure dependence of resistivity indicated that the charge carriers in Nd2−xCexCuO4−y were electrons. According to the resistivity data of these compounds at temperatures above 770 K, Ce doping affected the relation between the oxygen deficiency and oxygen partial pressure in Nd2CuO4−y: the doping of Ce worked to hinder the formation of oxygen vacancies in the lattice. Moreover, the carrier density after Ce doping was found to be much less than the value anticipated from the amount of the dopant. This suggested that not all the doped Ce ions worked as donors. That is, the relative amount of Ce3+ ions compared to that of Ce4+ ions increased as the total amount of the doped Ce ions increased.
The ternary system: Aluminum–iron–praseodymium
Authors: Klesnar H. , Rogl P.
Published: September 2007

Phase equilibria in the ternary system Pr–Fe–Al have been established in an isothermal section at 800 °C from room temperature x-ray powder diffraction analysis of about 50 alloys, which were melted, annealed at 800 °C, and quenched. Phase equilibria are characterized by the formation of rather extended homogeneous regions, i.e., by a random substitution of Fe/Al in Pr(Al1−xFex)2, 0 ≤ x ≤ 0.15, in Pr2(Fe1−xAlx)17, 0 ≤ x ≤ 0.65, as well as by the formation of at least four ternary compounds. Whereas the existence of PrFe4Al8 with the CeMn4Al8-type structure has been confirmed, there were no indications for a compound “PrFe6Al6” earlier claimed to crystallize with the ThMn12-type structure. Pr6(Fe1−xAlx)14, 0.16 ≤ x ≤ 0.36 with a homogeneous region parallel to the Fe–Al binary, was found to be isotypic with the La6Co11Ga3-type of structure. Pr-rich alloys are liquid at 800 °C, and all the alloys Pr2(Fe1−xAlx)17 with aluminum concentrations less than 5 at.% Al (x ∼ 0.07) enter a two-phase equilibrium with the Pr-rich liquid. At temperatures below 800 °C, alloys with compositions close to 30 at.% Pr and 5 at.% Al show a further ternary phase on solidification, whose crystal structure is related to the La6Co11Ga3-type. PrFe2Al8 is a new representative of the CeFe2Al8-type structure. The crystal structure of the ternary compound richest in Al, PrFe2Al10, has not been solved yet.
Structural analysis of lanthanum gallate
Authors: Kobayashi J. , Tazoh Y. , Sasaura M. , Miyazawa S.
Published: September 2007

High temperature x-ray diffractometry (HT-XRD) and differential scanning calorimetry (DSC) measurements confirm that two kinds of structural phase transitions exist in LaGaO3. One is the first order transition from the orthorhombic to the rhombohedral structure at ∼150 °C. The other is the second order transition from the rhombohedral to the monoclinic structure from ∼750 °C to ∼1000 °C.
Atomic and molecular emission following fracture of alkali halides: A dislocation driven process
Authors: Dickinson J.T. , Jensen L.C. , Langford S.C. , Hirth J.P.
Published: September 2007

During and following fracture of a number of materials, the emission of photons, electrons, ± ions, and neutral species are observed; these emissions are collectively known as fracto-emission. In this work, we present measurements of the neutral particle emission following fracture of two single crystal fcc alkali halides: NaCl and LiF. We observe no measurable emission attributable to release during the fracture event itself. However, after relatively long time intervals of ∼0.5–250 ms, we observe rapid bursts of alkali atoms, as well as molecular species which include NaCl and (LiF)n where n = 1,2,3. Bursts of alkali containing species also occur during loading prior to fracture and for unloaded specimens during heat treatment. We argue that these bursts are due to energetic emergence (“popout”) of dislocations at free surfaces.
Raman microprobe studies of laser induced damage in dielectric films
Authors: White P.L. , Exarhos G.J. , Bowden M. , Dixon N.M. , Gardiner D.J.
Published: September 2007

Raman microprobe studies of pulsed laser damaged TiO2 films deposited using three different methods are reported. Phase transformation and redeposition of coating materials were observed in selected regions of amorphous films deposited by ion beam sputtering and electron beam evaporation. Preferential removal of a specific phase or transformation to a second phase were observed in reactively sputtered films. Some damage sites exhibited regions of stress heterogeneity which can be explained in terms of the return electron stream model of plasma/target interaction and rapid quenching.
Ion implantation, diffusion, and solubility of Nd and Er in LiNbO3
Authors: Buchal Ch. , Mohr S.
Published: September 2007

We have implanted Nd and Er ions into x- and z-cut LiNbO3 single crystals. Rutherford backscattering spectrometry and channeling shows the recrystallization of the host during annealing and the rare earth diffusion. Nd and Er have different solubilities and different diffusion constants in LiNbO3. The solubility is strongly temperature dependent. The diffusion is substitutional, fastest parallel to c-axis of the LiNbO3 crystal and characterized by an activation energy of approximately 3.6 eV.
High resolution electron microscopy study of the cationic disorder in Al2TiO5
Authors: Epicier T. , Thomas G. , Wohlfromm H. , Moya J.S.
Published: September 2007

As part of a research program devoted to the microstructural characterization of Al2TiO5-based compounds, high resolution electron microscopy (HREM) has been undertaken in order to study the crystallographic arrangement, especially ordering possibilities, of Al and Ti cations in the metallic sublattice of aluminum titanate. It is seen that adequate experimental conditions, mainly defocus setting, for a resolution of at least 2.5 Å point-to-point, enable the disordered model to be directly and unambiguously proved on 100-oriented micrographs.
Alkali-resistant oxynitride glasses
Authors: Holmes T.M. , Leatherman G.L. , El-Korchi T.
Published: September 2007

Oxynitride glasses in the Si–Zr–Na–Li–K–B–O–N system have been produced via incorporation of Si3N4into the glass structure. This system is the oxynitride analogue of commercially available alkali resistant (AR) glasses used for concrete reinforcement. Glasses with nitrogen contents up to approximately 4 at.% have been obtained. Hardness, fracture toughness, and chemical durability were found to increase with increasing nitrogen content. Fibers were drawn from the glasses containing approximately 4 at.% nitrogen and used to produce reinforced cement composites. The microstructure of the fiber-matrix interface was examined in these samples after aging and compared to that of oxide AR glass fiber-matrix interface. Measurement of the wetting behavior of aqueous solution as a function of the nitrogen content of the glass suggests that this difference in microstructure is the result of changes in physiochemical properties of the glass surface due to the incorporation of nitrogen into the glass structure.
Thermal behavior of optical fibers during the cooling stage of the drawing process
Authors: Papamichael Haris , Miaoulis Ioannis N.
Published: September 2007

A general method that predicts the thermal behavior of optical fibers during the cooling stage of the drawing process was developed. The method can be used for thin diameter D < 200 μm, medium (200 μm < D < 500 μm), and thick (0.5 mm < D < 2 mm) single as well as core-clad fibers. A two-dimensional analysis implementing a finite difference method combined with the Kármán–Pohlhausen technique was performed to obtain the temperature profiles in thick fibers. This method accounted for axial and radial heat conduction, and can also be applied to thin and medium fibers. The case of the core-clad fibers was investigated to obtain the temperature profiles both in the radial and the axial directions. All results are presented in graphical form and can be used for optimization of the drawing process.
Fractal character of crack propagation in epoxy and epoxy composites as revealed by photon emission during fracture
Authors: Zhenyi Ma , Langford S.C. , Dickinson J.T. , Engelhard M.H. , Baer D.R.
Published: September 2007

We examine the photon emission accompanying rapid crack growth in an unfilled epoxy resin and in the same resin filled with alumina particles. The alumina particles substantially increase the toughness of the material and increase the photon emission intensities at least tenfold. We attribute the increased photon emission in the filled material to high densities of broken bonds near the alumina particles. The photon emission signals from both filled and unfilled materials show nonintegral (fractal) dimensions which are insensitive to the presence of the particles at the level of precision employed. Fractal dimension measurements of the fracture surfaces are likewise relatively insensitive to the presence of the filler, despite marked variations in apparent surface roughness. The photon emission signals were examined for the presence of chaos. Computations of the correlation exponent of Grassberger and Procaccia indicate that the photon emission fluctuations are not noise-like in character, and suggest deterministic chaos. Lyapunov exponent estimates on photon emission signals confirm the presence of chaotic processes. X-ray photoelectron spectroscopy and electron microscopy of the fracture surface indicate very little interfacial failure; i.e., fracture proceeds predominantly through the epoxy matrix in both filled and unfilled materials. Consequently, the character of the polymer matrix dominates the fracture process and therefore determines the fractal nature of the surface and the chaotic nature of the photon emission intensities in each material.
Digital simulation of the aggregate–cement paste interfacial zone in concrete
Authors: Garboczi Edward J. , Bentz Dale P.
Published: September 2007

Researchers using backscattered scanning electron microscopy, along with quantitative image analysis techniques, have clearly demonstrated the existence of a highly porous interfacial region between aggregate particles and the cement paste matrix in ordinary Portland cement concrete. This paper presents the results of a digital-image-based simulation model of this interfacial zone. A dissolution-diffusion-reaction-like cycle of hydrating cement particles is directly simulated using cement particles packed around a simple nonreactive aggregate particle. The model is two-dimensional, as we are comparing to experimental results obtained on two-dimensional images of polished sections. The qualitative features seen experimentally, such as large amounts of porosity and calcium hydroxide in the interfacial zone, are accurately reproduced. A new mechanism, one-sided growth, is proposed, along with the more usual particle-packing ideas, as an explanation of the origin of the characteristic features of the interfacial zone.
A probabilistic approach to bone fracture analysis
Authors: Sadananda Ramana
Published: September 2007

Bones are biological structural materials made of dynamically adaptable tissues. They can be considered as complex natural composite materials with load bearing constituents such as osteons and interstitial lamellae cemented with weak bonding materials. In addition, they contain Haversian and Volkmann canals that are functionally needed but are structurally weak. Because of large variation in microstructure, the strength of a bone varies from bone to bone and animal to animal. In this study the applicability of Weibull statistics to fracture strength of bones has been evaluated. The statistics is based on the weakest link theory and has been used successfully for probabilistic design of critical engineering structural components. The analysis shows that the statistics is valid when applied to each type of bone and it differentiates data from different types of bones. The analysis provides an insight in terms of how nature designs its load bearing structures by the process of natural selection.
Studies of weight changes and oxygen diffusion in oxide superconductors
Authors: Lau S-F. , Rosenthal A.B. , Pyrros N.P. , Graham J.A. , Gheng H.N.
Published: September 2007

In sintering and annealing of ceramic superconductors, diffusion of oxygen and sample weight changes are two important parameters to understand. In this work, we have undertaken to make weight loss measurements at different temperatures. At 400–600 °C, the weight loss data can be simply interpreted via one diffusion process. The Arrhenius activation energy is found to be ca. 23 kcal/mole. At 500 °C, the isotropic diffusion constant was found to be 9.5 × 10−12cm2/s. Above 600 °C, the weight loss curves become complex and indicate the occurrence of two diffusion processes. The oxygen loss and the oxygen uptake data are interpreted via our present knowledge of this system.
Microstructural characterization of YBaCuO thin films deposited by rf magnetron sputtering as a function of annealing conditions
Authors: Ece M. , Vook R.W. , Allen John P.
Published: September 2007

Thin films of Y1Ba2Cu3O7−xhave been prepared on MgO, SrTiO3/Al2O3, and Al2O3substrates by rf magnetron sputtering. A buffer layer of SrTiO3was deposited on Al2O3by flash evaporation. The as-deposited films on MgO and SrTiO