bluuebunny/crossref_metadata_2025 huggingface dataset:
Texture development due to preferential grain growth of Ho–Ba–Cu–O in 1.6-T magnetic field
Authors: Holloway A. , McCallum R.W. , Arrasmith S.R.
Published: September 2007

It has been experimentally observed that the application of even a relatively weak magnetic field of 1.6 T during sintering of HoBa2Cu3O7-δ (hereafter HoBCO) results in a significant degree of grain alignment. The orientation of grains is found to be controlled by the direction and magnitude of a magnetic field. The degree of alignment was monitored by x-ray diffraction measurements on the flat surface of the samples and by metallography. It has been observed that the degree of alignment grows as the magnitude of the field increases between 0 and 1.6 T for a fixed temperature and processing time. The degree of alignment also increases when the processing temperature changes from 930 °C to 965 °C for a fixed field and time. It has also been observed that for both a fixed field and processing temperature, the alignment grows when the processing time increases between 16 and 72 h. Metallography measurements on the flat and cross-sectional parts of the samples showed that the texture propagates into the bulk of the samples. In the presence of a sufficient amount of the liquid phase, the enhancement of the grain growth in the direction favorable to the magnetic field produces rather large single crystals (0.3 to 0.5 mm linear size) within the sample.
Creep of a niobium beryllide, Nb2Be17
Authors: Nieh T.G. , Wadsworth J. , Chou T.C. , Owen D. , Chokshi A.H.
Published: September 2007

A niobium beryllide, Nb2Be17, has been prepared by powder-metallurgy techniques and the mechanical properties characterized both at room and elevated temperatures. Microhardness and fracture toughness were measured at room temperature. Hardness and hot-hardness test results indicated that, although the material was brittle at low temperatures, it became plastic at elevated temperatures (>1000 °C). Creep properties of Nb2Be17 were studied at temperatures from 1250 to 1350 °C and applied stresses from 10 to 90 MPa. The stress exponent, determined from stress-change tests, was about 3, and the activation energy, determined from temperature-change tests, was about 575 kJ/mol. The creep of Nb2Be17 at high temperature is apparently controlled by dislocation glide; this proposal was supported by transient creep experiments. Comparisons have been made between the creep properties of Nb2Be17 and other intermetallics.
Magnetic domains along shear bands in amorphous metals observed by spin-polarized scanning electron microscopy
Authors: Tseng W.J. , Koike K. , Li J.C.M.
Published: September 2007

A novel spin-polarized scanning electron microscope with capabilities of observing microscopic domain structures and determining in-plane magnetization directions has been used to investigate the stress-induced magnetic domains in the Fe78B13Si9 (Allied 2605-S2) metallic glass. The magnetic structure in the vicinity of shear bands that are produced near a Mode III crack has been examined. On the tensile side of bending, arrays of individual, discontinuous magnetic “islands” of similar shape and size (about 2–10 μm in length, 3 μm in width, and spaced about 2–4 μm apart) are uniformly distributed on one side of the shear bands. Their easy axis is about parallel to the shear bands. On the other hand, well-defined elliptical domains are found between shear bands on the compression side of bending. Their easy axes are approximately perpendicular to the shear bands. These results suggest the existence of isolated defects of similar stress fields located along the shear bands. The possibility that these defects are dislocations will be investigated next.
Photoconductivity of carbon aerogels
Authors: Hosoya M. , Reynolds G. , Dresselhaus M.S. , Pekala R.W.
Published: September 2007

Photoconductivity was measured on a series of carbon aerogels to investigate their electronic properties. Carbon aerogels are a special class of low-density microcellular foams, consisting of interconnected carbon particles (∼120 Å diameter) and narrow graphitic ribbons (∼25 Å width) intertwined within each particle. Both the dark- and photoconductivities show drastic changes in the temperature range 5–300 K, which are similar to those in a-Si and chalcogenide photoconductors. At high temperatures, the photoconductivity is dominated by the carrier recombination within each particle. The photoconductivity at low temperatures is dominated by the same carrier transport mechanism as that for the dark conductivity, which is based on hopping and tunneling transport. The activation energy values for transport and recombination identify the electronic structure of the particles among samples of different bulk density. The long decay time of the photoconductivity suggests a relaxation mechanism associated with the dangling bonds.
Observation, analysis, and simulation of the hysteresis of silicon using ultra-micro-indentation with spherical indenters
Authors: Weppelmann E.R. , Field J.S. , Swain M.V.
Published: September 2007

The recently reported hysteretic behavior of silicon under indentation (Clarke et al.1 and Pharret al.2-5) is investigated using an ultra-micro-indentation system with an 8.5 μm spherical-tipped indenter. The onset of “plastic” behavior during loading and hysteresis during unloading was readily observed at loads in excess of 70 mN. Cracking about the residual impression was observed only at loads of 350 mN and higher. An analysis of the data is presented that estimates the following: (1) the initial onset of deformation occurs at a mean pressure of 11.8 ± 0.6 GPa, (2) the mean pressure at higher loads is 11.3 ± 1.3 GPa, and (3) the hysteretic transition on unloading occurs at mean pressures between 7.5 and 9.1 GPa. These values are in good agreement with the accepted literature values for the known silicon transformation pressures. A simulation of the force-displacement data based on the analysis and model is presented and is found to fit the observations very well.
Extrinsic contributions to the grain size dependence of relaxor ferroelectric Pb(Mg1/3Nb2/3)O3: PbTiO3 ceramics
Authors: Randall C.A. , Hilton A.D. , Barber D.J. , Shrout T.R.
Published: September 2007

This paper addresses the observed grain size with dependence of the dielectric behavior for Pb(Mg1/3Nb2/3)O3: PbTiO3 ceramics grain sizes ≥ 1.0 μm. A combined transmission electron microscopy (TEM) analysis and dielectric characterization are modeled with a modified brick wall approach. From this model, it is possible to extrapolate information such as single crystal values of dielectric maximum, Kmax, the diffuseness coefficient, δ, and the average intergranular thickness for relaxor ceramics. The calculated intergranular thickness agrees well with TEM observations, ≍2.0 nm. This semi-empirical method may be potentially useful in development work of relaxor ceramics to predict the optimized dielectric properties obtainable within microstructural restrictions.
On the Maxwell–Garnett model of chiral composites
Authors: Lakhtakia Akhlesh , Varadan Vijay K. , Varadan Vasundara V.
Published: September 2007

The Maxwell–Garnett model for isotropic chiral spherical inclusions in free space has been briefly reviewed, and pertinent results for the effective intrinsic and extrinsic properties of the composite medium, along with useful Taylor expansions, have been obtained in the Drude–Born–Fedorov representation. It has been shown that this model does not yield the chirality parameter of the composite independently of the permeability and the permittivity, and treats the permeability and the permittivity as duals of each other. Finally, even if the inclusions are nonmagnetic, the composite medium may not be necessarily so. It is anticipated that the formulae derived here will not only assist in the formulation of more rigorous multiple scattering theories, but will also aid designers of chiral composites.
Ionic conductivity and thermal analysis of a high-temperature polymer
Authors: Pak Y.S. , Xu G.
Published: September 2007

A high-temperature polymer electrolyte based on lithium-doped pyromellitic dianhydride + Oxybis(aniline)/2,5-diaminobenzenesulfonic acid (PMDA-ODA/DABSA) copolymer has been synthesized and characterized by ac impedance analysis and differential scanning calorimetry. Its dc conductivity was found to reach 10−8 S/cm near 60 °C at ambient relative humidity and was confirmed to be of ionic nature. Besides the increase of the conductivity achieved by moisture uptake, the doping of lithium was found to yield enhancement of the ionic transport in the absence of water molecules as well.
Microstructural effects on surface mechanical properties of ion-implanted polymers
Authors: Rao G.R. , Wang Z.L. , Lee E.H.
Published: September 2007

Tefzel, a copolymer of tetrafluoroethylene and ethylene, was implanted simultaneously with 400 keV boron, 700 keV nitrogen, and 600 keV carbon to a dose of 3 × 1015 ions/cm2 for each ion. The implanted layer was examined using transmission electron microscope and compared with the pristine Tefzel for microstructural changes. The microhardness of the implanted and pristine Tefzel was determined using a nanoindentation technique. TEM bright-field images of the implanted layer show a patch-type contrast with distinguishable bright and dark regions. Electron energy loss spectroscopy (EELS) was used to show that the bright regions had a higher carbon concentration, as compared with the dark regions. The carbon-rich regions had an average size of approximately 40 nm. The pristine material showed a fairly featureless contrast with occasional local patchy regions. These were determined to be due to local thickness variations. The triple implantation improved the hardness of pristine Tefzel by over 66 times. The structure of the carbon-rich regions appears to be clusters of sp2 bonded C atoms with sp3 sites present and hydrogen preferentially bonded in the sp3 C configuration. It was speculated that the carbon-rich regions could be harder than the surrounding regions, but this could not be resolved due to the small size of the regions.
Wagner oxidation in the nonisothermal regime
Authors: Nánai László , Jelski Daniel A. , Hevesi Imre , George Thomas F.
Published: September 2007

A computational model for laser-induced Wagner oxidation processes on a vanadium metal surface is described and compared with the experiment. It is found that for thin oxide layers, the oxide is nearly transparent and the metal temperature rapidly increases. For thicker layers, the oxide is nearly opaque and the metal temperature slowly decreases. The Wagner rate law is found to hold for thicker oxide layers, but not at the beginning of the process.
Synthesis of microcrystalline BaZrO3 via molecular precursor route
Authors: Potdar H.S. , Deshpande S.B. , Godbole P.D. , Date S.K.
Published: September 2007

A molecular precursor barium zirconyl oxalate (BZO) was synthesized by exchange reaction between freshly generated water soluble sodium zirconyl oxalate and Ba-nitrate solutions at room temperature. The controlled pyrolysis of BZO in air at 800 °C/6 h resulted in the formation of single-phase cubic microcrystalline barium zirconate powder. These powders exhibit variable-shaped agglomerates with an average size ≍0.2 μm having a surface area ≍2.63 m2/gm.
Narrowing sputtered nanoparticle size distributions
Authors: Kaatz F.H. , Chow G.M. , Edelstein A.S.
Published: September 2007

By adjusting the sputtering rate and gas pressure, it is possible to form nanoparticles of different sizes, phases, and materials. We have investigated the spatial distribution of sputtered particle formation using a vertical, linear arrangement of substrates. Collecting the particles soon after they are formed, before they have time to grow and agglomerate, allows one to obtain a narrow size distribution. In the case of molybdenum, a narrow distribution of cubic particles is formed at relatively large distances (8 cm) from the source. These cubic particles collide and self-assemble in the vapor into arrays of larger cubic particles. The particle size histograms are fitted to lognormal distribution functions. How supersaturation occurs is discussed qualitatively as a function of the distance from the substrate, sputtering rate, and the mean free path in the vapor. This method of nanocrystalline particle formation has potential use in magnetic and opto-electronic (quantum dot) applications, where a narrow size distribution is required.
Nickel-alloyed tin-lead eutectic solder for surface mount technology
Authors: Kang Sung K. , Ference Thomas G.
Published: September 2007

A new ternary solder alloy of tin-lead-nickel was developed for solder joints. It has an optimum composition range of (60–65)% Sn, (35–40)% Pb, (0.5–1.0)% Ni by weight. The alloy exhibits a higher strength and longer fatigue life than the pure 63% Sn−37% Pb eutectic. Melting point, wettability, and fatigue life were the key properties used to determine this composition. In particular, the melting point of the alloyed eutectic solder did not change for rapidly solidified samples when up to 2 wt. % Ni was added. Instead of modifying the solution melting temperature, the majority of the Ni precipitated out as a fine dispersion of Ni3Sn4. These Ni3Sn4 particles help to strengthen the solder matrix. Reflow experiments on rapidly solidified material showed minimal coarsening and agglomeration of the Ni3Sn4 particles for up to 5 min of reflow cycles. The wettability of the solder, however, decreased significantly for contents of Ni above 1.5%. Low cycle fatigue tests show that for a solder with 0.5% Ni, the fatigue life was about twice as long as that of standard eutectic solder. Suggested processing for the alloy is rapid solidification to form the alloy powder which is incorporated into a reflowable paste.
Chemical vapor infiltration of SiC with microwave heating
Authors: Morell José I. , Economou Demetre J. , Amundson Neal R.
Published: September 2007

A mathematical model was developed to elucidate the interaction between transport/reaction processes and the evolution of porosity in chemical vapor infiltration with microwave heating (MCVI). The analysis included a set of partial differential equations describing the spatiotemporal variation of gaseous species concentration, composite temperature, porosity, and stress. Maxwell's equations were used to determine the distribution of power dissipated inside the composite. The deposition of silicon carbide was selected as a model chemical system to explore the general features of MCVI. MCVI can provide a favorable temperature distribution in the composite yielding an inside-out deposition pattern, thereby preventing entrapment of accessible porosity. For this temperature profile, tensile stresses develop at the outer regions and compressive stresses are found in the composite core. For a given system there exists a minimum value of the coefficient for heat transfer from the composite surface, h, below which accessible porosity is trapped within the composite. Similarly, there exists a maximum value of the incident microwave energy flux, I0, above which accessible porosity is trapped within the composite. I0 and h can be optimized for a given preform to achieve complete densification with minimum processing time. Using the technique of pulsed-power, the processing time can be reduced even further without compromising density uniformity. Power dissipation profiles in the composite depend strongly on preform thickness, microwave frequency, and relative loss factor.
Precipitation of Mo5Si3 in MoSi2
Authors: Maloy S.A. , Xiao S-Q. , Heuer A.H. , Garrett J.
Published: September 2007

Mo5Si3 precipitates in MoSi2 single crystals have been studied. The orientation relationship between the long rectangular lath-like precipitates and the matrix is [110] MoSi2[110] Mo5Si3 and (002) MoSi2(2$\overline 1$0)Mo5Si3. Two sets of misfit dislocations extend along the long dimensions of the precipitates.
Wear resistance of arc ion-plated chromium nitride coatings
Authors: Chiba Yuji , Omura Toshio , Ichimura Hiroshi
Published: September 2007

Wear resistance of arc ion-plated chromium nitride films has been studied. It has been found that texture and phases composing the films depend much on bias voltage and nitrogen gas pressure at the deposition. A phase diagram was constructed as a function of these two parameters, which indicated that three categories exist: CrN single, CrN and Cr2N dual, and CrN and Cr dual phased regions, respectively. Results of Falex No. 2 test showed that the wear resistance of CrN single phased films is superior to others, especially when (220) preferred orientation is developed. Since hardness and surface morphology do not differ much between these films, a high toughness of CrN single phased film is considered to make a difference by suppressing abrasion wear.
Inhomogeneous distribution of Cr impurities in α–Al2O3 during refractory aging
Authors: Galoisy Laurence , Calas Georges
Published: September 2007

Aged alumina refractories have been investigated by EPR (Electron Paramagnetic Resonance) and EPMA (Electron Probe Micro-Analysis). EPMA shows that Cr is homogeneously distributed inside the corundum grains within spatial resolution (1 μm). Cr3+ concentration at the atomic scale is obtained by EPR by the magnitude of dipolar broadening due to Cr-Cr interactions. This local concentration appears to be systematically higher (up to a factor of 10) than the average chromium content of the corundum crystals determined by EPMA. Cr3+ ions may be preferentially concentrated at the vicinity of extended lattice defects which help in accommodating the lattice strain due to the size difference between Al3+ and Cr3+ cations. A random location of these defects explains the apparent homogeneous chromium distribution found by EPMA in these aged refractories.
Mechanical properties of nylon 6-clay hybrid
Authors: Kojima Yoshitsugu , Usuki Arimitsu , Kawasumi Masaya , Okada Akane , Fukushima Yoshiaki , Kurauchi Toshio , Kamigaito Osami
Published: September 2007

Various nylon 6-clay hybrids, such as molecular composites of nylon 6 and silicate layers of montmorillonite and saponite, NCH's and NCHP's, respectively, have been synthesized. To estimate the mechanical properties of these hybrids, tensile, flexural, impact, and heat distortion tests were carried out. NCH was found superior in strength and modulus and comparable in impact strength to nylon 6. The heat distortion temperature (HDT) of NCH (montmorillonite: 4.7 wt. %) was 152 °C, which was 87 °C higher than that of nylon 6. In NCHP, saponite had a smaller effect on the increase of these mechanical properties. The modulus and HDT of NCH and NCHP increased with an increase in the amount of clay minerals. It was found that these properties were well described by the contribution of the constrained region calculated from the storage and loss modulus at the glass transition temperature. According to the mixing law on elastic modulus, the following expression was obtained between the modulus E at 120 °C and the fraction of the constrained region C, En = Ecn = C, where the values of n and Ec (modulus of the constrained region) were 0.685 and 1.02 GPa, respectively.
Reply to Comment of V. A. Shneidman
Authors: Shi Frank G. , Seinfeld John H.
Published: September 2007

We show that the assertions of Shneidman made in the previous Comment are incorrect.
NMR study of Ni3Al with B and C additions
Authors: Shinohara T. , Takasugi T.
Published: September 2007

NMR study has been carried out on Ni3Al with B and C additions. The 27Al-, 11B-, and 13C-spectra have been measured as a function of temperature for determining their Knight shifts. The Curie temperature and Knight shift at low temperature in Ni3Al with B and C additions decrease in the order of atomic number of additions, but the Knight shifts around room temperature are nearly the same as that of pure Ni3Al. It is suggested that the addition of B and C affects the electronic structure of Ni, and that B atoms and possibly C atoms occupy the octahedrally symmetric sites neighboring all the Ni atoms.
The mechanism of mechanical alloying of MoSi2
Authors: Patankar S.N. , Xiao S-Q. , Lewandowski J.J. , Heuer A.H.
Published: September 2007

An incubation time exists for the formation of MoSi2 by mechanical alloying (MA). For the particular milling conditions employed, the bulk of MoSi2 formation occurs between 3 h and 12 min and 3 h and 13 min. This abrupt formation of MoSi2 during MA suggests that the reaction occurs by a form of self-propagating high-temperature synthesis (SHS), when a critical amount of stored energy (in the form of interfacial energy and cold work) is introduced by MA, and increases the adiabatic reaction temperature sufficiently to make the reaction self-sustaining.
Tungsten nitride thin films prepared by MOCVD
Authors: Chiu Hsin-Tien , Chuang Shiow-Huey
Published: September 2007

Polycrystalline tungsten nitride thin films were grown by low pressure metallo-organic chemical vapor deposition (MOCVD) using (tBuN)2W(NHtBu)2 as the single-source precursor. Deposition of uniform thin films on glass and silicon substrates was carried out at temperatures 723–923 K in a cold-wall reactor, while the precursor was vaporized at 333–363 K. The growth rates were 2–10 nm/min depending on the condition employed. Bulk elemental composition of the thin films, studied by wavelength dispersive spectroscopy (WDS), is best described as WNx (x = 0.7–1.8). The N/W ratio decreased with increasing temperature of deposition. X-ray diffraction (XRD) studies showed that the films have cubic structures with the lattice parameter a = 0.414–0.418 nm. The lattice parameter decreased with decreasing N/W ratio. Stoichiometric WN thin films showed an average lattice parameter a of 0.4154 nm. X-ray photoelectron spectroscopy (XPS) showed that binding energies of the W4f7/2, W4f5/2, and N1s electrons were 33.0, 35.0, and 397.3 eV, respectively. Elemental distribution within the films, studied by secondary ion mass spectroscopy (SIMS) and Auger spectroscopy depth profilings, was uniform. The SIMS depth profiling also indicated that C and O concentrations were low in the film. Volatile products trapped at 77 K were analyzed by gas chromatography–mass spectroscopy (GC–MS) and nuclear magnetic resonance (NMR). Isobutylene, acetonitrile, hydrogen cyanide, and ammonia were detected in the condensable mixtures. Possible reaction pathways were proposed to speculate the origin of these molecules.
Conductivity relaxation studies on sol-gel-derived Li2O–SiO2 glasses by a heterogeneous conductor model
Authors: Saha S.K. , Chakravorty D.
Published: September 2007

Conductivity relaxation spectra of sol-gel-derived glasses in the system Li2O (1 – x) SiO2, with x varying from 0.15 to 0.25, have been analyzed using a heterogeneous conductor model. The latter comprises diagonal layers of phases, one of them being rich in alkali ions and the other deficient in the same. Satisfactory agreement between the experimental data and the theoretical curves has been obtained over the frequency range 100 Hz to 1 MHz. The Kohlrausch–Williams–Watts (KWW) exponent, β, has been calculated from the conductivity spectra for all the glasses. The sol-gel-derived glasses have β values smaller than those obtained in the case of the corresponding melt-quenched ones. The β value is found to be correlated to the conductivity fluctuation in the alkali-rich phase.
Theory of the time-lag diffusion method for the case of an outgassing solid
Authors: Baird James K. , Chen Jenn-Shing
Published: September 2007

In the ordinary application of the time-lag method to the measurement of the diffusion coefficient of a gas passing through a plane sheet of an inert solid, the gas is pressurized on one side of the sheet and evacuated on the other. After decay of transients, the cumulative amount,Q(t), of gas diffused through the sheet in time,t, assumes the “time-lag” form,Q(t)=A(t – L). Measurements of the slope,A, and the intercept,L, can be used to determine the diffusion coefficient and the solubility of the gas in the solid. We have rederived this law for the case of a solid that is actively evolving this same gas at an arbitrary rate and have used it to predict the rate of outgassing of the solid upon standing. Practical applications of the theory include radioactive decay of minerals, rejection of plasticizers by plastics, and the decomposition of solid rocket propellants.
Thermodynamic stability and superconductivity of the Bi–Sr–Ca(Y)–Cu–Li–O system
Authors: Xianhui Chen , Chun Lin , Bin Lu , Yitai Qian , Liezhao Cao , Zhaojia Chen , Zuyao Chen
Published: September 2007

In order to understand the effects of the thermodynamic stability on superconductivity, the thermodynamic stability of the Bi2Sr2CaCu2-xLixOy system and the Bi2Sr2YCu2-xLixOy system was studied. It is found that the broad ranges (0 ≤ x ≤ 0.6) of solid solution may be thermodynamically stable at high temperatures for the Bi2Sr2CaCu2-xLixOy), system and the Bi2Sr2YCu2-xLixOy), system. At equilibrium at low temperatures, however, solid solubility is eliminated, and only the end member compounds and possibly intermediate compounds of fixed composition remain for 0.3 ≤ x ≤ 0.6. It suggests that Li can destroy the thermodynamic stability. Superconductivity is improved by Li-doping, which may be related to the thermodynamic instability.
The effect of cracks on the superconducting transport current in thin films: The analogy with two-dimensional elasticity and plasticity
Authors: Clarke David R. , DeGraef Marc
Published: September 2007

Simulations of arrays of resistively shunted Josephson junctions containing a crack of uncoupled junctions indicate that the crack can distort the supercurrent flow and provide a nucleation site at the crack tip for the formation of superconducting vortices at applied currents below the critical current of the homogeneous material. An analogy is established between the supercurrent distribution in two dimensions and the stress field distribution around the crack for antiplane mechanical loading. The analogy is used to show that the supercurrent distribution can be described analytically in terms of a Westergaard function used in elasticity theory. In addition, using a correspondence between the forces acting on a vortex and a crystal dislocation, models for screw dislocation emission from a crack tip are transposed to describe vortex emission from a crack tip. These lead to predictions for the pinning force required to prevent dissipation by vortex emission from the crack tip, as well as for the size of a vortex zone ahead of the crack for different values of the ratio of the applied current to the pinning force.
Evidence for nonclassical nucleation at solid surfaces in diamond deposition from the gas phase
Authors: Tomellini Massimo
Published: September 2007

In the framework of a previously developed kinetic model, a discriminating criterion is established to distinguish between classical and nonclassical nucleation of diamond at solid surfaces. The two-step model gives the non-steady-state nucleation density function in terms of the rate constants for active site → germ, germ → active site, and germ → nucleus kinetic steps. The criterion states that α/β > 6 is a necessary condition for classical nucleation at surfaces to occur, α and β being functions of the rate constants which can be obtained by appropriate analysis of the experimental data. This criterion is applied to recent results on diamond nucleation at silicon surfaces and indicates nonclassical results The expression of the nonequilibrium Zeldovich factor, Z, is also found in the form Z = [1 + K/nd]−1, K and nd being the rate constants for the germ → nucleus and germ → active site steps, respectively. An estimation of the rate constants is reported and the corresponding Zeldovich factor is evaluated to be 0.6 for nucleation at both Si(100) and Si(111) substrates.
On the use of ion scattering to examine the role of hydrogen in the reduction of TiO2
Authors: Wallace W.E. , Zhong Q. , Genzer J. , Composto R.J. , Bonnell D.A.
Published: September 2007

Rutherford backscattering spectrometry (RBS) was used to measure the titanium concentration profile for hydrogen-reduced, vacuum-reduced, and as-received, stoichiometric rutile. These profiles give the degree of reduction, specifically, the extent of oxygen deficiency, as a function of depth below the sample surface. Using forward-recoil spectrometry (FRES), the hydrogen-reduced rutile was found to contain more bulk and near-surface hydrogen than the as-received, stoichiometric rutile. This observation provides additional evidence for a hydrogen-diffusion model for the reduction of rutile in a hydrogen environment.
Correlation between microstructure and mechanical properties in silicon carbide with alumina addition
Authors: Shinozaki S.S. , Hangas J. , Carduner K.R. , Rokosz M.J. , Suzuki K. , Shinohara N.
Published: September 2007

The microstructure of pressureless sintered silicon carbide (SiC) materials with alumina (Al2O3) addition was investigated using analytical electron microscopy and nuclear magnetic resonance. A sintered body with a density of higher than 99% theoretical was obtained with an addition of 5 wt.% Al2O3. The sintered body (SiC–Al2O3) has high strength, high fracture toughness, and high fatigue resistance. Its fracture toughness is approximately 5 MPa-m1/2, which is twice as high as that of pressureless sintered SiC materials with boron and carbon additions (SiC–B–C). The correlation between the microstructure and the mechanical properties is presented here. The starting β–SiC powder is mostly transformed to α–SiC with various polytype distributions during the sintering process. The microstructure has homogeneously distributed, fine, plate-like interlocking gains with a high aspect ratio. Well-developed basal planes form parallel and elongated boundaries, and the crystal structure is mostly the 6H polytype (56%) mixed with thin lamellar 4H.
Effects of temperature on the formation of hydroxyapatite
Authors: Fulmer M.T. , Brown P.W.
Published: September 2007

The formation of hydroxyapatite by an acid-base reaction between solid calcium phosphates at temperatures from 5 to 60 °C was examined. The basic reactant used is Ca4(PO4)2O, while the acidic reactants include CaHPO4, CaHPO4 · H2O, and Ca(H2PO4)2 · H2O. Rates of heat evolution during reaction were determined by isothermal calorimetry. The variations in the proportions of reactants and hydroxyapatite and the formation of intermediate products were assessed by x-ray diffraction. Development of microstructure was observed. Generally, hydroxyapatite formation occurs by rapid initial reaction followed by a period during which reaction occurs slowly. Apparent activation energies calculated for the reaction when CaHPO4 is the acidic reactant show differing values depending on its surface area. When the acidic reactant is Ca(H2PO4)2 · H2O, intermediate products are formed. At low temperatures the intermediate is CaHPO4 · 2H2O, while at higher temperatures it is CaHPO4. Above 38 °C, the rate during the period of slow reaction decreases with increasing temperature. This appears to be related to the retrograde solubilities of the reactants, CaHPO4 · 2H2O and CaHPO4, and of HAp.
Synthesis and characterization of cordierite from acetylacetonate/alkoxide precursors
Authors: David Lawrence D. , Anderson Ronald M. , Dynys Joseph M. , Goldsmith Charles C. , Szule Andrew
Published: September 2007

Syntheses of ultrafine glass powders in the MgO–Al2O3–SiO2 ternary oxide system were effected by drying and calcining sols derived from acetylacetonate-tetraethoxysilane ethanol solutions. A mixture of magnesium and aluminum acetylacetonates, dissolved with Si(OC2H5)4 in an ethanol solution in a 2:4:5 Mg: Al: Si mole ratio, was hydrolyzed, spray-dried, and calcined to yield a glass powder of 70 Å primary particle size, which sintered to densities ≥ 2.5 g/cm3 and which formed μ-cordierite as the only crystalline phase below 1000 °C. Incorporation of dopant quantities of boron and phosphorus, via their alkyl esters in the hydrolysis reaction with Si(OC2H5)4 and the acetylacetonates, afforded powders which crystallized in the α-phase directly after sintering to density = 2.66 g/cm3.
High refractive index films of polymer nanocomposites
Authors: Zimmennann Lorenz , Weibel Martin , Caseri Walter , Suter Ulrich W.
Published: September 2007

Solutions of PbS particles and gelatin were used for the preparation of nanocomposites by a spin-coating process. This allows for the preparation of nanocomposite films with controlled thickness, e.g., between 40 nm and 2 μm for a film containing 45 wt.% PbS. Surface roughness and film thickness were investigated by surface profilometry and scanning electron microscopy (SEM). The refractive index at 632.8 nm can be expressed by a linear function of the volume fraction of PbS in the range of 0 to 55 vol. % PbS. In this range, the refractive index increases from 1.5 to 2.5 with increasing PbS ratio and belongs, therefore, to the highest refractive indices known for polymeric composite materials.
Nucleation of oriented diamond films on nickel substrates
Authors: Yang P.C. , Zhu W. , Glass J.T.
Published: September 2007

A seeding and multistep deposition process has been developed to nucleate and grow diamond films directly on Ni substrates in a hot filament chemical vapor deposition system. High quality diamond films have been deposited without graphite codeposition on both 〈100〉 oriented single-crystal Ni and polycrystalline Ni substrates. Both 〈100〉 and 〈111〉 oriented diamond nuclei have been observed depending upon the underlying substrate orientations. Molten metallic phases were found surrounding the diamond nuclei, and it is speculated that a liquid layer composed of nickel, carbon, and hydrogen also formed on the diamond surface during the growth. The oriented diamond is believed to have been achieved by the reorientation of seeded diamond particles into alignment with the Ni substrate due to interaction between the diamond and Ni lattices.
Mechanical alloying of Al—Ti powder mixtures and their subsequent consolidation
Authors: Itsukaichi T. , Masuyama K. , Umemoto M. , Okane I. , CabañTas-Moreno J.G.
Published: September 2007

Mechanically alloyed mixtures of elemental Al and Ti powders have been characterized in the as-milled condition, as well as after hot consolidation. Ball-milling in an argon atmosphere first induces alloying of the elements, followed by the formation of a certain amount of an amorphous-like phase. This amount increases as the equiatomic composition is approached. However, milling in nonsealed mills usually leads to the production of some form of titanium nitride, particularly in Ti-rich mixtures. In the consolidated products obtained from long-milled powders, intermetallic compounds were found to be the predominant phases. The existence of an amorphous phase in the as-milled powders considerably facilitates their hot consolidation.
Evidence for self-sustained MoSi2 formation during room-temperature high-energy ball milling of elemental powders
Authors: Ma E. , Pagán J. , Cranford G. , Atzmon M.
Published: September 2007

We present evidence indicating that rapid, self-sustained, high-temperature reactions play an important role in the formation of tetragonal MoSi2 during room-temperature high-energy ball milling of elemental powders. Such reactions appear to be ignited by mechanical impact in an intimate, fine-grained, Mo–Si physical mixture formed after an initial milling period. Under certain conditions, limited propagation of self-sustained reactions in these uncompacted powder mixtures renders the compound formation seemingly gradual in bulk-averaged analysis. It is suggested that this type of reaction is an important mechanism in the mechanical alloying of highly exothermic systems. Results are discussed in comparison with similar reactions we observed in ball-milled Al–Ni powders, with self-sustained combustion synthesis previously reported for Mo–Si powders, and with interfacial diffusional reactions in Mo–Si powders or thin-film diffusion couples.
Chaotic dynamics in a model of metal passivation
Authors: Kevin McCoy J. , Parmananda Punit , Rollins Roger W. , Markworth Alan J.
Published: September 2007

The dynamic behavior of a model for the passivation of a metal surface in contact with an aqueous solution is investigated. The model, which is characterized by a three-dimensional state space and five-dimensional parameter space, is obtained by combining elements from passivation models developed by Talbot and Oriani and by Sato. A three-dimensional subspace of parameter space has been studied; the remaining two dimensions are not thought to provide any additional interesting dynamics. The model exhibits remarkably rich dynamics, including the period-doubling, intermittency, and crisis routes to chaos, folds and bubbles in periodic portions of the attractor, and multiple attractors with complex, intertwined basins of attraction.
Electrical resistivity, magnetoresistance, and morphology of vapor-grown carbon fibers prepared in a mixture of benzene and Linz–Donawitz converter gas by floating catalyst method
Authors: Ishioka Munehiro , Okada Toshihiko , Matsubara Kenji , Inagaki Michio , Hishiyama Yoshihiro
Published: September 2007

Vapor-grown carbon fibers (VGCF's) were prepared in a mixture of benzene and Linz–Donawitz converter gas using floating catalytic seeds derived from ferrocene, cobalt acetylacetonate, and thiophene. The diameters of the fibers thus grown were in the range of 2–7 μm. The fibers were heat-treated in argon atmosphere at temperatures between 1700 and 3000 °C. The electrical resistivity at room temperature and magnetoresistance at liquid nitrogen temperature were measured for the as-grown and heat-treated fibers, and morphology of the heat-treated fibers was observed with a scanning electron microscope. The electrical resistivity was nearly similar to that obtained for VGCF's prepared on a substrate in a mixture of benzene and hydrogen. The size effects on the resistivity and magnetoresistance were observed. The magnetoresistance was also found to depend on the heat-treatment time. The magnetoresistance results showed that a transition heat-treatment temperature for the magnetoresistance change from negative to positive was between 2200 and 2300 °C, indicating the graphitizable nature of the present fiber. A characteristic polygonal appearance was observed for the fibers heat-treated above 2500 °C.
Electron-beam-induced structural changes in crystalline C60 and C70
Authors: Seraphin Supapan , Zhou Dan , Jiao Jun
Published: September 2007

High-resolution transmission electron microscopy (HRTEM), electron diffraction, and electron energy-loss spectroscopy (EELS) were used to characterize electron-beam-induced structural transformations in crystalline samples of C60 and C70. During these transformations, the electron-diffraction patterns became progressively more diffuse, with the outer diffraction spots disappearing first, followed in succession by the disappearance of the inner spots. We interpret this course of evolution in the diffraction patterns as evidence of degradation of the crystalline structure via the destruction of individual molecules, as opposed to electron-beam-induced motion of intact fullerene molecules. EELS analyses of the data indicate that the final state of these transformations was amorphous carbon.
Cu precipitation in oxidized wafers with and without a GexSi1−x heteroepitaxial layer
Authors: Kissinger G. , Morgenstern G. , Richter H.
Published: September 2007

Copper silicide precipitation in Czochralski silicon influenced by the existence of a GexSi1−x heteroepitaxial layer has been studied. Samples with and without a 100 nm Ge0.016Si0.984 heteroepitaxial layer were annealed together for different periods of time at 1000 °C in air on copper containing iron plate and after copper plating of the backside, respectively. The absorption or emission of silicon self-interstitials (or vacancies) significantly influences the precipitation behavior of copper silicide. Undersaturation of silicon self-interstitials (or vacancy supersaturation) during oxidation of the heteroepitaxial layers is able to prevent the formation of copper silicide precipitate colonies in the surface region.
The plasma arc production of Si-based ceramic whiskers
Authors: Pickles C.A. , Toguri J.M.
Published: September 2007

Silicon-based ceramic whiskers, such as silicon carbide and silicon nitride, were produced in a plasma arc reactor. Silicon monoxide gas was generated by reacting briquettes of silica and carbon according to the following reaction: SiO2(s or l) + C(s) = SiO(g) + CO(g). The effects of operating variables on the production of silicon monoxide are discussed. The silicon monoxide gas was then back-reacted with carbon monoxide to produce silicon carbide whiskers according to the following reaction: SiO(g) + 3CO(g) = SiC(s) + 2CO2(g). In the presence of nitrogen, the product contained some silicon nitride and silicon oxynitride whiskers. The effects of gas composition and flow rate, crucible mass, and alkali additions on the amount and morphology of the whiskers were determined. It is concluded that the whiskers grew by a vapor-liquid-solid mechanism.
Preparation and properties of uniform coated colloidal particles. VIII. Titanium nitride on silica
Authors: Partch Richard E. , Xie Yuming , Oyama S.T. , Matijević Egon
Published: September 2007

Spherical silica particles, dispersed in ethanol/water solutions, were first coated with titania by hydrolysis of added titanium isopropoxide. The coating thickness could be readily adjusted by the ratio of titanium alkoxide concentration to the amount of silica. The coated cores prepared as above were then treated with ammonia in a temperature-programmed reaction to produce a shell of titanium nitride. After nitridation, the powders of the high specific surface area showed electrical conductivity, which depended on the thickness of the coatings.
Interface control and design for SiC fiber-reinforced titanium aluminide composites
Authors: Chiu Hsing-Pang , Jeng S.M. , Yang J-M.
Published: September 2007

The effectiveness of several coating systems which were used as a diffusion barrier for the SiC fiber-reinforced titanium aluminide composites was investigated. TaC, TiC, TiB2, B, C/graded TiB2, and Ag/Ta were applied to the SiC fiber via chemical vapor deposition and physical vapor deposition. The interfacial compatibility, interfacial stability, thermal residual stress, interfacial bond strength, and the transverse fracture behavior of the composites with coated fibers were characterized and determined. The results show that none of the above coating systems can satisfy the requirements for a strong, tough, and damage-tolerant SiC fiber-reinforced titanium aluminide composite. Several multilayer, multifunctional coating systems are proposed.
Controlled heterogeneous nucleation of melt-textured YBa2Cu3O6+x by addition of Al2O3 particles
Authors: Chen Yan L. , Zhang Lijie , Chan Helen M. , Harmer Martin P.
Published: September 2007

The reaction between alumina and yttrium barium cuprate subjected to a melt-texturing heat treatment was studied. Microstructural examination of quenched, partially transformed samples revealed that at ∼1050 °C (which is above the incongruent melting temperature of YBa2Cu3O6+x) a reaction layer forms at the alumina interface. The reaction products were identified as Ba6Y2Al4O15 and a copper-rich liquid phase. On cooling below the peritectic temperature, aligned domains of YBa2Cu3O6+x (123) were observed to nucleate preferentially at the reaction layer. For samples of melt-textured 123 deliberately seeded with alumina particles, it was found that nucleation and growth of 123 occurred exclusively at the particles. A reaction sequence for the formation of the Ba6Y2Al4O15 is put forward, together with a discussion of the possible nucleation mechanisms for the 123.
Electro-chemo-mechanical treatment: Facilitating steel grinding under electrochemical reduction of active cations
Authors: Vidensky I.V. , Petrova I.V. , Shchukin E.D.
Published: September 2007

Earlier it was shown that some liquid metals may facilitate the cutting of hard materials, being strongly surface-active media with respect to these solids (decreasing their surface energy), e.g., Zn for steels, Cd for titanium alloys, etc. Recently, a similar physico-chemical principle has been proposed and realized in the course of active metals (Zn, Cd) depositing on the surface of a treated sample (ground tempered steel, 50 HRC) by means of reduction of their cations on a sample used as a cathode in a corresponding electrochemical cell, at room temperature, with a very small amount of active substance, and at very low current densities (∼50 mA/cm2); under such conditions, grinding is 1.5–2 times more effective than in the same aqueous solutions without current.
Diffusion of 57Co in amorphous Fe–RE (RE = Dy, Tb, and Ce) and Fe–Si–B alloys
Authors: Yamada Kazumasa , Iijima Yoshiaki , Fukamichi Kazuaki
Published: September 2007

Tracer diffusion of 57Co in amorphous Fe100−xDyx (x = 20–40), Fe75Tb25, Fe67Ce33, and Fe80Si6B14 alloys prepared by dc sputtering has been studied at temperatures of 523 and 573 K. In the Fe–Dy alloys the diffusion coefficient of 57Co shows a maximum at 33 at.% Dy. The magnitude of the diffusion coefficient of 57Co in Fe75Tb25 is nearly equal to that in Fe75Dy25, while those in Fe67Ce33 and Fe80Si6B14 are about one order of magnitude less than the values in Fe67Dy33 and Fe80Dy20. This suggests that the atomic size of the diffusant and the density of the matrix are dominant in the diffusion. Temperature dependence of the diffusion coefficient D of 57Co in the amorphous Fe75Dy25 alloy has been determined in the range from 493–673 K. It shows a linear Arrhenius relationship expressed by D = 5.7 × 10−2 exp(−199 kJ mol−1/RT) m2 s−1. The magnitudes of the pre-exponential factor and the activation energy suggest that the cobalt tracer atoms in the amorphous Fe75Dy25 alloy diffuse by an interstitial-like mechanism.
Pulsed laser deposition of amorphous diamond-like carbon films with ArF (193 nm) excimer laser
Authors: Xiong Fulin , Wang Y.Y. , Leppert V. , Chang R.P.H.
Published: September 2007

We have deposited hydrogen-free diamond-like amorphous carbon films by ArF (193 nm) pulsed laser ablation of graphite. The deposition process is performed with a laser power density of only 5 × 108W/cm2at room temperature without any auxiliary energy source incorporation. The resulting films possess remarkable physical, optical, and mechanical properties that are close to those of diamond and distinct from the graphite target used. The films have a mechanical hardness up to 38 GPa, an optical energy band gap of 2.6 eV, and excellent thermal stability. Analysis of electron energy loss spectroscopy reveals the domination of diamond-type tetrahedral bonding structure in the films with thesp3bond fraction over 95%. Compared with other reported results of pulsed-laser-deposited diamond-like carbon films, our experimental results confirm that the laser wavelength or photon energy plays a crucial role in controlling the properties of the pulsed-laser-deposited diamond-like carbon films.
Photoassisted oxygen doping of C60 films
Authors: Rao A.M. , Wang K-A. , Holden J.M. , Wang Y. , Zhou P. , Eklund P.C. , Eloi C.C. , Robertson J.D.
Published: September 2007

C60 films simultaneously exposed at T = 300 K to oxygen and visible-ultraviolet light are found to exhibit rapid diffusion of oxygen into the deep bulk of the film. The diffusion rate is found to be ∼10 times faster in the presence of light. Vibrational and optical absorption spectroscopy indicate that the photoassisted reaction of O2 with solid C60 involves two coupled reactions: (i) the photoassisted diffusion of molecular oxygen into solid C60 and (ii) oxidation of C60.