bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemistry and structure of mixed magnesium ferric oxide aerogels
Authors: Willey R.J. , Oliver S.A. , Oliveri G. , Busca G.
Published: September 2007

Fine particles of mixed oxides having the composition MgxFe2−2xO3−2x(xbetween 0 and 1.0) were prepared via the sol-gel supercritical drying route (aerogels). Samples were then postprocessed by heating in air at 773 K and 1073 K. The structure and chemistry of the samples were deduced through x-ray diffraction, thermal gravimetric and differential thermal analysis, infrared, and specular UV-visible measurements. Surface areas were measured by the BET method. The volume saturation magnetization and particle volume distribution of MgFe2O4samples were deduced from vibrating sample magnetometer measurements. The samples withx= 0 andx= 0.5 were well-crystallized α–Fe2O3(hematite) and MgFe2O4(magnesioferrite), respectively. Samples with 0 <x< 0.5 consisted of disordered lacunar spinels containing an excess of Fe3+with respect to the spinel stoichiometry and a small amount of Fe2+. Samples heated above 773 K showed progressive segregation of α–Fe2O3. The sample withx= 0.66 was a mixture of a spinel phase and a hydrotalcite-like mixed Fe–Mg hydroxy compound containing methoxy and acetate ions. By heating in air, a monophasic Mg-excess spinel-type structure was obtained. The samples with 0.66 <x< 0.86 were mixtures of a spinel-type phase and another mixed Mg–Fe compound. When heated in air, the samples gave a mixture of MgO (probably containing Fe ions) and MgFe2O4. The saturation magnetization of MgFe2O4samples is comparable to those previously reported, with the as-produced sample having a mean particle radius of 5.5 nm.
Topotactic synthesis of vanadium nitride solid foams
Authors: Oyama Ted S. , Kapoor Rajat , Oyama Hideko T. , Hofmann David J. , Matijević Egon
Published: September 2007

Vanadium nitride has been synthesized with a surface area of 120 m2 g−1 by temperature programmed nitridation of a foam-like vanadium oxide (35 m2 g−1), precipitated from vanadate solutions. The nitridation reaction was established to be topotactic and pseudomorphous by x-ray powder diffraction and scanning electron microscopy. The crystallographic relationship between the nitride and oxide was {200}//{001}. The effect of precursor geometry on the product size and shape was investigated by employing vanadium oxide solids of different morphologies.
A study of the high-Cu Al–Cu–Co decagonal phase
Authors: Grushko B.
Published: September 2007

The decagonal phase was studied by transmission and scanning electron microscopy in an Al62Cu24Co14 alloy annealed at 550–850 °C. The electron diffraction patterns of the decagonal phase exhibited weak quasiperiodic odd-n reflections in the [1-2100] zone axis corresponding to the equilibrated structure. The relative intensities of these reflections were significantly lower in the Al62Cu24Co14 than in the Al68Cu11Co21 decagonal phase. Diffuse scattering observed previously at the same positions can be related to a nonequilibrium state of the decagonal phase.
Measurement of the work function of Y1Ba2Cu3O7−δ under ambient conditions
Authors: DeVries Jon , Wakisaka Steve S. , Spjut Erik R.
Published: September 2007

The photoelectric work function of samples of Y1Ba2Cu3O7−δ of approximately 40 μm diameter was measured under ambient temperature and atmosphere in an electrodynamic balance. The measured values had an average of 4.98 eV and a standard deviation of 0.24 eV. The minimum value was 4.46 eV and the maximum was 5.52 eV.
Temperature and annealing dependence of the longitudinal ultrasonic velocity in aluminum alloys
Authors: Johnson Ward , Mauer F. , Pitchure D. , Norton S.J. , Grinberg Y. , Bendec F.
Published: September 2007

The longitudinal ultrasonic velocities of four commercial aluminum alloys and Al(1.8 wt.% Si) were measured between room temperature and the solidus temperatures. In all of the samples, the velocity deviated significantly from a linear temperature dependence at the highest temperatures. In commercially pure (1100) aluminum, this effect is found to be consistent with reported low-frequency damping and elastic modulus changes that are associated with dislocations or grain boundaries. In the four heat-treatable alloys studied, an additional contribution to the nonlinear temperature dependence arises from the dissolution of precipitates at elevated temperatures. Irreversible velocity changes occur during the first heating, as a result of the recovery from work-hardening and heat treatments which were performed during the production of the material. Small hysteretic changes above ∼ 250 °C are correlated with the precipitation and dissolution of alloying elements. The activation energy for the hysteretic changes in Al(1.8% Si) is found to be 0.82 eV, which is consistent with precipitation limited by silicon diffusion along grain boundaries.
Stabilization of strained multilayers by thin interlayers
Authors: Hirth J.P.
Published: September 2007

Strained multilayers composed of two misfitting layers and a third, thin interlayer are considered. With appropriate intermediate lattice parameters for the interlayer, the latter is shown to stabilize the structure with respect to misfit dislocation formation. Cases of misfit corresponding both to balanced biaxial stress and to pure shear stress in the interface are treated.
Bi-intercalation of H2SO4 into stages 4–6 FeCl3–graphite intercalation compounds
Authors: Mizutani Yasuo , Abe Takeshi , Asano Mitsuru , Harada Toshio
Published: September 2007

Stages 4–6 FeCl3–graphite intercalation compounds (GIC's) have been prepared by an ordinary two-bulb method, and the bi-intercalation processes of H2SO4 into the GIC's have been studied by x-ray diffraction. Since stages 4–6 FeCl3–GIC's have more than three vacant interlayer spaces in the c-axis repeat distance, the bi-intercalation of H2SO4 into the GIC's takes place stepwise. Consequently, various FeCl3–H2SO4–graphite bi-intercalation compounds (GBC's), from the GBC's with one H2SO4 layer to the GBC's saturated with H2SO4, are obtained in the bi-intercalation processes. Their compositions are described as C6.0nFeCl3 · c(6.0/6.5)H2SO4 from the calculation of structure factor for x-ray diffraction, where n is the stage number of the starting FeCl3-GIC's, and c is the number of H2SO4 layers. Stacking sequences of all the FeCl3–H2SO4–GBC's are also determined.
Kinetics of MoSi2 pest during low-temperature oxidation
Authors: Chou T.C. , Nieh T.G.
Published: September 2007

The kinetics of MoSi2 pest, caused by oxidation in air, has been studied. Experimental results indicated that pest disintegration occurred at temperatures between 375 and 500 °C. The volumes of test samples increased with oxidation duration. Analysis of change in sample volume versus oxidation duration revealed that the pest disintegration consisted of two stages, namely nucleation (or incubation) and growth. The onset of the growth stage depended on the test temperature. More importantly, changes in sample volume were found to obey a linear relationship with time during the growth stage. Equations were formulated to demonstrate that the growth kinetics of pest disintegration was proportional to the rates of change in sample volume. The rates of volume change during MoSi2 pest were calculated to be 4.9 × 10−6, 2.8 × 10−5, 3.7 × 10−5, and 5.4 × 10−5 cm3/s at 375, 400, 425, and 450 °C, respectively; the growth kinetics increased with oxidation temperature. The activation energy for the growth stage of pest disintegration was determined to be 27.6 kcal/mole, which agrees well with the activation energy for the low-temperature oxidation of MoSi2.
Friction and wear studies of silicon in sliding contact with thin-film magnetic rigid disks
Authors: Bhushan Bharat , Venkatesan Sreekanth
Published: September 2007

Silicon is an attractive material for the construction of read/write head sliders in magnetic recording applications from the viewpoints of ease of miniaturization and low fabrication cost. In the present investigation we have studied the friction and wear behavior of single-crystal, polycrystalline, ion-implanted, thermally oxidized (wet and dry), and plasma-enhanced chemical vapor deposition (PECVD) oxide-coated silicon pins while sliding against lubricated and unlubricated thin-film disks. For comparison, tests have also been conducted with Al2O3–TiC and Mn–Zn ferrite pins which are currently used as slider materials. With single-crystal silicon the rise in the coefficient of friction with sliding cycles is faster compared to Al2O3–TiC and Mn–Zn ferrite pins. In each case, the rise in friction is associated with the burnishing of the disk surface and transfer of amorphous carbon and lubricant (in the case of lubricated disks) from the disk to the pin. Thermally oxidized (under dry oxygen conditions) single-crystal silicon and PECVD oxide-coated single-crystal silicon exhibit excellent tribological characteristics while sliding against lubricated disks, and we believe this is attributable to the chemical passivity of the oxide coating. In dry nitrogen, the coefficient of friction for single-crystal silicon sliding against lubricated disks behaves differently than in air, decreasing from an initial value of 0.2 to less than 0.05 within 5000 cycles of sliding. We believe that silicon/thin-film disk interface friction and wear is governed by the uniformity and tenacity of the amorphous carbon transfer film and oxygen-enhanced fracture of silicon.
Kinetics and mechanism of stacking fault annihilation and grain growth in porous ceramics of β–SiC
Authors: Seo Won-Seon , Koumoto Kunihito
Published: September 2007

Beta–SiC specimens possessing 15% stacking fault density were annealed at various temperatures for various time periods under an Ar or a N2 atmosphere, and the mechanisms of stacking fault annihilation and grain growth were investigated. The values of the geometric factor in the Avrami–Erofeev equation indicated that the rate of stacking fault annihilation is controlled by the atomic diffusion process. On the other hand, the rate of grain growth was found to be limited by surface diffusivity. Coincidence in the values of activation energy for stacking fault annihilation and grain growth within experimental errors firmly suggested that the annihilation of stacking faults is an apparent phenomenon resulting from the microstructural development in which the grain growth is controlled by surface diffusivity. Incorporation of nitrogen during heating suppressed the surface diffusivity and, hence, the rate of stacking fault annihilation.
Phase development in Si modified sol-gel-derived lead titanate
Authors: Wright John S. , Francis Lorraine Falter
Published: September 2007

Sol-gel methods were used to prepare chemically modified lead titanate (PT) powders. A PT alkoxide solution was synthesized and doped with Si (2–12 mole%) or with equimolar amounts (2–12 mole%) of Si and Pb through the addition of Si and Pb–Si alkoxide solutions, respectively. PT alkoxide solutions were also prepared with excess Pb and Ti (7 and 10 mole%). Gels were prepared through controlled additions of water. Crystalline phase development of gel-derived powders with heat treatment (400–700 °C) was studied using x-ray diffraction (XRD) and differential thermal analysis (DTA). While PT powders without added Si crystallized directly into a perovskite phase, Si modified materials crystallized first into a pyrochlore phase and at a higher temperature transformed into perovskite. The pyrochlore lattice parameter and the temperature for the transformation to perovskite increased with Si content. In all cases, the crystal structure of the final perovskite phase was not affected by the Si addition. The effect of Si on phase development and mechanisms of transformation is discussed.
Oil absorption into cotton-phenolic material
Authors: Bertrand P.A.
Published: September 2007

The kinetics of oil absorption into cotton-phenolic ball-bearing retainer material was determined. The results fit a model of two-step absorption in which the first step can be described by capillary rise and the second step by diffusion of oil from the capillaries into the bulk phenolic resin. The capillaries are associated with the cotton threads and are on the order of several micrometers or less in radius. The model successfully describes data for two cotton-phenolics with different cloth weaves, and five nonpolar liquids (four lubricating oils and heptane) with viscosities that vary over 3 orders of magnitude. The total amount of oil absorbed at the completion of the first step (1.5 to 3.4% v/v in this work) is proportional to the volume of the sample, not its surface area, suggesting that the entire volume of the material stores oil and not just the surface region. If the material is not completely dry when submerged in oil, the capillaries are blocked and only surface absorption of oil can take place. The saturation amount of oil in the resin after diffusion is complete at about 2 to 3% v/v (not including the amount of oil stored in the capillaries). The diffusion coefficient calculated using our model is 3 × 10−12 cm2/s, which is reasonable when compared with available literature data on large and small molecules diffusing into polymers.
Cristobalite growth inhibitor in Pyrex borosilicate glass—gallium oxide
Authors: Jean Jau-Ho , Gupta Tapan K.
Published: September 2007

The effect of gallium oxide on cristobalite formation in Pyrex borosilicate glass has been studied. XRD results show that with 20 vol.% gallium oxide, the precipitation of cristobalite at 800 °C is completely prevented. This result is further evidenced by the linear thermal expansion measurement, in which, in contrast to the system without a sufficient amount of gallium oxide (0–10 vol.%), the thermal expansion coefficient of the composite with 20 vol.% gallium oxide remains unchanged with sintering time at the temperature investigated and, moreover, is very close to the theoretically calculated value.
Superconducting Ba0.6K0.4BiO3: Thin film preparation by RF magnetron sputtering
Authors: Hou C.J. , Steinfink H. , Rabenberg L. , Hilbert Claude , Kroger Harry
Published: September 2007

Superconducting Ba0.6K0.4BiO3 thin films with transition temperatures up to 25 K have been successfully grown on SrTiO3 substrates using RF magnetron sputtering and postgrowth oxygen annealing. Systematic variation of the sputtering process parameters showed that optimum films can be grown on substrates heated to 400 °C in a 10 mTorr atmosphere containing 96% Ar and 4% O2 and using a target containing Ba, K, and Bi in ratios of 0.6:1.2:1.4. High transition temperature superconductivity was observed only in highly oriented, crystalline films having the ideal stoichiometry, Ba0.6K0.4BiO3.
Solid-state reactions of the Ag–Cu–Ti thin film–Al2O3 substrate system
Authors: Suenaga Seiichi , Koyama Miho , Arai Shinji , Nakahashi Masako
Published: September 2007

A new interpretation of the reaction mechanism between active metal thin-film filler and ceramic substrate is proposed. The authors predict the possibility of prebonding reactions, prior to melting of the filler, at the interface of the system described above. To prove this, solid-state reactions of Ag–Cu–Ti thin films on sapphire substrates have been studied with Auger electron spectroscopy (AES) and x-ray diffraction (XRD). Reaction process and products have been clarified at the temperature just below the melting point of the filler. It is evident that Cu3Ti3O (diamond structure of Fd3m) is formed by the reaction between Cu3Ti and O which results from the reduction of sapphire. It seems that Cu3Ti3O contributes to bonding between metals and sapphire as an intermediate phase.
Deformation behavior and high-temperature stability of unidirectionally solidified, twin phase TiAl
Authors: Fair G.H. , Wood J.V. , Nakano T. , Umakoshi Y.
Published: September 2007

Ti−50.8% Al has been cast using unidirectional solidification to produce specimens with a highly directional structure. Each specimen consisted of the intermetallic phases, TiAl and Ti3Al. Both phases existed as single crystal lamellae which were parallel to each other throughout the specimen and aligned along specific crystallographic orientations. Mechanical properties were assessed by deforming specimens such that slip occurred in different directions relative to the lamellae. High-temperature stability was assessed by heating for various times and examining the microstructure. Deformation revealed a highly anisotropic structure since ductility depended upon the principal direction of slip. Poor slip, occurring in a direction through the lamellae, was attributed to the Ti3Al phase where cracking initiated. Heating at 1200 °C resulted in recrystallization of the TiAl phase. The recrystallization started at the ends of the specimens as a result of residual mechanical damage, before spreading along individual lamellae into the bulk.
XRD characterization of highly dispersed metal catalysts on carbon support
Authors: Scardi Paolo , Antonucci Pier Luigi
Published: September 2007

Carbon-supported Pt catalysts were prepared from H2PtCl6 or K2PtCl6 aqueous solutions. Particle size and structure after several thermal activation treatments were studied by X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Cyclic Voltammetry (CV), and the results of the three techniques were compared. As the catalysts were highly dispersed on an amorphous support, a conventional XRD profile analysis for crystallite size determination could not be performed properly, because of the strong overlapping between the broad Pt peaks superposed to the halos of the amorphous phase. Thus, a new procedure of whole XRD pattern fitting, based on the Rietveld method, was used to have reliable data of Pt particle size (surface area) and lattice parameter. All structural and microstructural parameters were refined within the same procedure, also considering the transparency of the carbon supported catalysts and minimizing the effect of the amorphous background. The method can also take into account the presence of bimodal particle size distributions, which is difficult to study by CV or TEM.
Fundamental studies of the influence of boron on the graphite-oxygen reaction using in situ electron microscopy techniques
Authors: Rodriguez N.M. , Baker R.T.K.
Published: September 2007

Controlled atmosphere electron microscopy coupled with in situ electron diffraction has been used to follow the manner by which the addition of boron oxide influences the graphite-oxygen reaction. Continuous observations of the process show that at about 450 °C the boron oxide undergoes a strong interaction with both the graphite edge and the basal plane regions, and this results in a spreading of the oxide to form a uniform thin adherent film over the entire substrate. The coated graphite specimens appear to be impervious to attack by oxygen at temperatures below 815 °C. Above this temperature, however, there is a weakening of the additive-graphite interaction with the “armchair” {11$\overline 1$0} faces, and these regions then become vulnerable to attack by oxygen. At the same time very shallow pits are observed to develop in the basal plane, and this action coincides with the appearance of boron carbide species in the electron diffraction pattern. In a complementary series of experiments, it is found that boron carbide is an extremely active catalyst for the graphite-oxygen reaction even at temperatures as low as 100 °C. The impact of these low pressure studies on the behavior of carbon structures used in aerospace applications is discussed.
Ion bombardment-induced changes in oxide-metal interactions studied by photoelectron spectroscopy
Authors: Mukhopadhyay Sharmila M. , Chen Tim C.S.
Published: September 2007

We have studied the influence of ion bombardment on the surfaces of MgO and Al2O3 single crystals. The two oxides were sputtered with low energy Ar+ ions, and x-ray photoelectron spectroscopy was used to analyze surface compositions and chemical interactions with metallic adsorbates. Expected changes in surface compositions have been calculated mathematically using first principle formulation from existing literature. The resulting effects on XPS spectra have been predicted based on concentration profile models and agree with experimental observations. Changes in surface oxygen activity were studied by evaporating Ni on these surfaces at an extremely slow rate and analyzing what fraction of the first monolayer was oxidized on adsorption. This fraction, used as a measure of surface oxygen activity, was substantially reduced in Al2O3 on sputtering, but remained practically unchanged for MgO. The amount of these changes matches reasonably well with calculated changes in surface oxygen fraction. Based on these results, the relation between ion bombardment and interfacial bonding has been discussed.
Hot corrosion of alumina
Authors: Lawson M.G. , Pettit F.S. , Blachere J.R.
Published: September 2007

The hot corrosion of single crystal and polycrystalline aluminas has been investigated in SO2–SO3–O2 environments and in the presence of molten Na2SO4-based deposits at temperatures of 700 and 1000 °C. The effect of microstructure and impurities on the corrosion has been emphasized. Weight changes and wetting angles were determined, and the evolution of the morphology of the exposed substrates and the reaction products was investigated in detail. The corrosion was small under the conditions of this study and generally increased with the impurity content of the polycrystalline aluminas. Based on the experimental results, particularly those obtained by electron microscopy and microanalysis using the SEM/EPMA (scanning electron microscope–electron probe microanalyzer), mechanisms are proposed for the corrosion of polycrystalline aluminas which emphasize the role of the silicate impurities and the synergy of their corrosion with that of the alumina grains. As a result, the alumina grains were dissolved by acidic fluxing under the acidic and the basic experimental conditions.
Solidification of CaCO3 containing SrCO3 by hydrothermal hot-pressing
Authors: Yamasaki Nakamichi , Weiping Tang , Yanagisawa Kazumichi
Published: September 2007

Powders of calcium carbonate with strontium carbonate were solidified by mechanical compression under hydrothermal conditions. The effects of hydrothermal conditions such as temperature, pressure, and ratio of Ca/Sr on the reaction process and microstructure of the solidified bodies were examined. Solidification of every powder easily proceeded with shrinkage above 200 °C to 300 °C. The compacts of strontium carbonate were more easily solidified, and compressive strength was 190 MPa and its density was 93.5%. The solidification with shrinkage of compacts consisting of mixed powders of calcium and strontium carbonates was accelerated with an increase in strontium carbonate content. It was revealed that the small amount of strontium carbonate powder acts as a bonding material among large calcium carbonate particles.
On the morphological properties of tungsta-titania de-NOxing catalysts
Authors: Cristiani C. , Bellotto M. , Forzatti P. , Bregani F.
Published: September 2007

Tungsta-titania samples with different W loadings up to 15 wt.% and calcined at different temperatures have been prepared and characterized by surface area measurements, mercury porosimetry, x-ray diffraction, microstructural analysis, and laser Raman spectroscopy. It has been found that W inhibits the initial sintering of TiO2 (anatase) and the anatase → rutile transformation. The morphological and structural properties of the samples (surface area, porosity, and phase composition) have been related to the microscopic properties of the materials such as crystallite dimensions and defects concentration. A model for the sintering of TiO2 is discussed. This model is based on the diffusion of surface hydroxyls, formed upon adsorption of water on surface oxygen vacancies. A role for W is proposed in terms of stabilization of both material defects and surface hydroxyls.
Combustion-synthesized functionally gradient refractory materials
Authors: Niedzialek Scott E. , Stangle Gregory C. , Kaieda Yoshinari
Published: September 2007

Functionally Gradient Materials (FGM's) are soon to be used in a variety of important commercial applications; joining and thermal barrier coatings are two of the most widely studied. FGM's of the TiC/NiAl and the TiC/Ni3Al systems were fabricated using a one-step, self-propagating high-temperature synthesis (SHS) and densification method. It was observed that ignition of the starting mixture for these two systems was affected by the initial sample temperature and the external pressure that was applied to the sample during the ignition stage. Quality of the final product (e.g., porosity, grain size, cracking and microcracking, etc.) depends on a number of factors during this one-step operation. Reaction temperature control is important and is necessary to minimize residual porosity of the final product. Particle size of reactant powders, as well as applied pressure, also has an effect on the resulting microstructure. If careful reaction temperature control is achieved, along with optimum reactant powder size and applied pressure, an FGM of minimal porosity is obtained without residual macrocracks. Further, this method can easily be used to fabricate an FGM with a highly precise composition and material properties gradient. Finally, this process results in FGM's of similar quality when compared to those prepared by existing fabrication methods at only a fraction of the cost. Most importantly, it is expected that this process can be scaled up with relative ease.
Fullerenes
Authors: Dresselhaus M.S. , Dresselhaus G. , Eklund P.C.
Published: September 2007

A review of the structure and properties of fullerenes is presented. Emphasis is given to their behavior as molecular solids. The structure and property modifications produced by alkali-metal doping are summarized, including modification to the electronic structure, lattice modes, transport, and optical properties. Particular emphasis is given to the alkali-metal-doped fullerenes because of their importance as superconductors. A review of the structure and properties of fullerene-based graphene tubules is also given, including a model for their one-dimensional electronic band structure. Potential applications for fullerene-based materials are suggested.
High-resolution electron microscopy of epitaxial YBCO/Y2O3/YSZ on Si(001)
Authors: Bardal A. , Eibl O. , Matthée Th. , Friedl G. , Wecker J.
Published: September 2007

The microstructures of YBa2Cu3O7−δ (YBCO) thin films grown on Si with Y-stabilized ZrO2 (YSZ) and Y2O3 buffer layers were characterized by means of high-resolution electron microscopy. At the Si–YSZ interface, a 2.5 nm thick layer of regrown amorphous SiOx is present. The layer is interrupted by crystalline regions, typically 5 to 10 nm wide and 10 to 50 nm apart. Close to the crystalline regions, {111} defects are present in the Si substrate. The typical defect observed is an extrinsic stacking fault plus a perfect dislocation close to the stacking fault which terminates extra {111} planes in the upper part of the Si. These defects are probably formed by condensation of Si self-interstitials created during oxide regrowth. Precipitates are present in the Si close to the Si–YSZ interface and indicate that in-diffusion of Zr has occurred. The YSZ–Y2O3 interface is atomically sharp and essentially planar and contains no second phases. Perfect misfit dislocations with Burgers vector 1/2〈110〉 are present at this interface along with unrelaxed elastic misfit stresses. The Y2O3–YBCO interface is atomically sharp and planar, but contains steps. (001) stacking faults are present in the YBCO above these steps; the faults are, however, healed a few unit cells away from the interface. By HREM analysis of ultrathin specimen areas, the atomic layer of the YBCO closest to the Y2O3 was found to be a barium-oxygen layer.
Preparation and comparison of YBa2Cu4O8 films on MgO(100) and SrTiO3(100) by MOCVD
Authors: Sakai H. , Shiohara Y. , Tanaka S.
Published: September 2007

The thin films (Y/Ba/Cu = 1.0/2.7/4.7) which have a strong intensity corresponding to the c-axis peaks of the 124 phase (YBa2Cu4O8) in x-ray diffraction (XRD) patterns were prepared. The film structure and surface morphology of these films were observed by scanning electron microscopy (SEM), energy dispersive x-ray (EDX), transmission electron microscopy (TEM), and atomic force microscopy (AFM). It was found that the film on MgO(100) clearly has grain boundaries with many a-axis oriented grains of the 123 phase (YBa2Cu3O7−x). However, the film on SrTiO3(100) had a smooth surface, and a-axis oriented grains of the 123 phase couldn't be observed. This difference could not be explained by only the difference in the lattice mismatch.
From imperfect to perfect Bi2Sr2CaCu2Ox (Bi–2212) grains
Authors: Heeb B. , Gauckler L.J. , Heinrich H. , Kostorz G.
Published: September 2007

The 2212 phase formation during annealing of melt textured Bi–2212 (Bi2Sr2CaCu2Ox) was investigated using differential thermal analysis, thermal gravimetric analysis, x-ray diffraction, scanning electron microscopy, energy dispersive x-ray analysis, and high resolution transmission electron microscopy. After zone melting, the material is multiphase consisting of 2212, 2201, Sr1−xCaxCuO2, and the eutectic. The 2212 phase formed is highly perfect with less than 5% intergrowths of 2201 layers; the 2201 phase shows no intergrowth of 2212 at all. In the first period of the annealing, remelting of the eutectic leads to fast oxygen diffusion and a high 2212 formation rate. The 2201 → 2212 transformation proceeds via intermediate states of high defect density. The 2212 grains contain up to 30–70% 2201 intergrowths. Further heat treatments lead to an annihilation of the great majority of intergrown 2201 layers. We propose a model for the formation of 2212 grains with a low planar defect density, based on frequent stacking faults, that allows diffusion of Ca- and Cu-atoms over a short distance. The model provides a schematic description of this solid-state process and correlates it to the characteristic microstructural features of melt-processed Bi–2212.
Annealing of pressure-induced structural damage in superconducting Bi–Pb–Sr–Ca–Cu–O ceramic
Authors: Maartense I. , Sarkar Asok K.
Published: September 2007

Ac susceptibility measurements have been used to monitor the changes in the superconductive properties of sintered and uniaxially pressed samples of Pb-stabilized 2223-phase bismuth cuprate ceramic as the structural damage was annealed in air in a sequence of steps in temperature between 500 and 850 °C. It is concluded that below 600 °C a relaxation of residual stresses is responsible for a 2% shrinkage in sample volume and a small improvement in bulk superconductive transition temperature, Tc. Above 700 °C, a recovery of the original properties occurs through grain regrowth governed by an activation energy of ∼200 kJ/mol. However, in the region between 600 and 700 °C, a decrease in Tc of ∼40 K appears to be the result of plastic flow and amorphization associated with local decomposition of 2223 which reduces the effective grain size and weakens the intergranular superconductive coupling.
Identification of hydrocarbon precursors to diamond in chemical vapor deposition using carbon monoxide reagent
Authors: Johnson Curtis E. , Weimer Wayne A.
Published: September 2007

Diamond films were grown by microwave plasma-assisted chemical vapor deposition using mixtures of 13CH4 and CO. Mass spectrometry was used to identify CO, CH4, and C2H2 as the stable gaseous products in the reactor exhaust gas. By comparing gaseous 13C compositions with that of the diamond films, the efficiency of diamond growth from methane (possibly via the methyl radical) is found to be about two orders of magnitude higher than that for carbon monoxide. Most of the diamond that is formed from the CO reagent results from the conversion of CO to hydrocarbons. The conversion of CO to hydrocarbons is attributed to activation of CO by high-energy electrons in the plasma.
ESR study of activated carbon fibers: preliminary results
Authors: di Vittorio S.L. , Nakayama A. , Enoki T. , Dresselhaus M.S. , Endo M. , Shindo N.
Published: September 2007

We have carried out Electron Spin Resonance (ESR) measurements on activated carbon fibers (ACF) with specific surface areas (SSA) of 3000 and 2000 m2/g. The ESR spectrum of ACF fibers in air is extremely broad (500 to 1000 Gauss), and the spin susceptibility decreases rapidly with decreasing specific surface area. Also measured was the ESR signal of the desorbed fibers in vacuum. As a result of desorption, the broad peak decreases slightly in intensity, and a narrow (≍65 Gauss at room temperature) peak appears. We report results on the temperature dependence of both peaks. The narrow peak is interpreted as due to spins associated with dangling bonds, whereas we attribute the broad peak to the conduction carrier spins which is broadened by the boundary scattering process (T1 contribution) and the dipolar broadening process (T2 contribution) associated with the dangling bond spins.
Intercalation of pentane into the stage 2 to 4 cesium graphitides: Relation between cesium density in the intercalated layers and the stage of the parent binary
Authors: Pillière H. , Goldmann M. , Béguin F.
Published: September 2007

Isotherms (at 300 K and 328 K) and isobars (in the range 300 to 400 K) of n-pentane intercalation in CsC24 and CsC36 were established. With CsC24, three plateaus were identified at 0.52, 0.7, and 1.0 n-pentane/24 C, whereas only two plateaus at 0.8 and 0.97 n-pentane/36 C were found with CsC36. The progress of the reaction between n-pentane and CsC24, CsC36, and CsC56 (stage 2 to 4) was monitored by real-time neutron diffraction. The intercalation of n-pentane in CsC24 results in the simultaneous formation of a second stage ternary and a first stage binary “CsC8”, whereas, from the third stage CsC36 or the fourth stage CsC56, only pure second stage or third stage ternary compounds are formed, respectively. Owing to the formation of binary domains rich in alkali metal (CsC8) or to stage lowering produced by the ternarization, the in-plane cesium density is smaller in the ternary layer than in the starting binary. The electrostatic repulsion between the cesium ions, provoked by the sorption of n-pentane, is believed to be at the origin of the increased coverage. During the intercalation or de-intercalation processes, three-dimensional segregation occurs in each grain. A pleated layer model with canted fronts is presented. It accounts for the various phases present within each grain and for the structural transformations caused by pressure variations. At room temperature, the ternary layer seems to be disordered. The order-disorder transition appearing either by decreasing the temperature or by increasing the n-pentane pressure is correlated to a hindered motion of the intercalated molecules.
Coprecipitation synthesis of doped lanthanum chromite
Authors: De Guire M.R. , Dorris S.E. , Poeppel R.B. , Morissette S. , Balachandran U.
Published: September 2007

Two coprecipitation methods were used to synthesize powder precursors of doped lanthanum chromite (La, Ca)(Cr, Co)O3. The effects of synthesis method and calcination temperature on the composition, sintered density, and microstructure of pressed compacts of (La, Ca)(Cr, Co)O3 were studied by differential thermal analysis/thermogravimetric analysis, x-ray diffraction, scanning electron microscopy, and density measurement. The cation ratios in the precipitated solids were, with few exceptions, within experimental error of the desired compositions for all four components. Powders obtained by both techniques could be sintered to densities exceeding 93% at 1400 °C. The highest densities were obtained with calcining temperatures from 450 to 700 °C. The sintered microstructures exhibited uniform grain sizes averaging 3–5 μm. The Cr(vi) compounds, CaCrO4 and La2CrO6, were observed in all of the calcined powders. The possible role of these phases on chromite densification is discussed.
Selection of structural type and particle size in titanium (IV) oxide
Authors: Vallet-Regí M. , PeñTa J. , Martínez A. , González-Calbet J.M.
Published: September 2007

Titanium (iv) oxide (amorphous, brookite, anatase, and rutile) fine particles have been synthesized by pyrolysis of an aerosol. As-received materials are constituted by hollow balloons of around 1–2 μm formed by very small crystalline nuclei, whose crystal size can be modified by annealing. By heating at 900 °C for 48 h individual crystals of around 1 μm are obtained. Intermediate sizes can be controlled as a function of the thermal treatment.
Study of wave attenuation in concrete
Authors: Berthelot J-M. , Ben Souda M. , Robert J.L.
Published: September 2007

The experimental study of wave attenuation in concrete has been achieved in the case of the propagation of plane waves in concrete rods. Different mortars and concretes have been investigated. A transmitter transducer coupled to one of the ends of the concrete rod generates the propagation of a plane wave in the rod. The receiver transducer, similar to the previous one, is coupled to the other end of the rod. The experimental results lead to an analytical expression for wave attenuation as function of the concrete composition, the propagation distance, and the wave frequency.
Formation of V2O5-based mixed oxides in flames
Authors: Miquel Philippe F. , Hung Cheng-Hung , Katz Joseph L.
Published: September 2007

V2O5–TiO2 and V2O5–Al2O3 mixed oxide powders were synthesized in a hydrogen-oxygen flame using VOCl3, TiCl4, and Al(CH3)3 as precursors. The particle formation processes were investigated as a function of VOCl3 concentration by laser light-scattering and by collecting particles directly onto transmission electron microscopy grids. In the V2O5–TiO2 system, the oxides condense as an intimate mixture at all three VOCl3 concentrations. Spherical particles, 40 to 70 nm in diameter, are obtained. In the V2O5–Al2O3 system, chain-like particles composed of an intimate mixture of V2O5 and Al2O3 form at the lowest VOCl3 concentration. At high VOCl3 concentrations, the chain-like particles have a core-mantle structure (a core mainly of Al2O3 and a mantle mainly of V2O5). The crystalline form and the surface area of these mixed oxides were determined by x-ray diffractometry, FT-IR spectroscopy, and BET analysis by nitrogen desorption. These measurements indicate that amorphous vanadium oxide forms at low VOCl3 concentrations, and V2O5 is obtained at the higher VOCl3 concentrations. The structure of the amorphous vanadium oxide matches that published for vanadium oxide “supported” catalysts.
Erratum
Authors: Kubota Nobuhiko , Sugimoto Tsunemi , Shiohara Yuh , Tanaka Shoji
Published: September 2007

“(110)-oriented Bi–Sr–Ca–Cu–O superconducting thin films prepared by metalorganic chemical vapor deposition” [J. Mater. Res. 8, 978 (1993)]
Nd: YAG laser patterning of plasma-deposited high Tc superconducting thick films
Authors: Varshney Usha , Eichelberger B.D. , Glass J.M. , Churchill R.J. , Kingon A.I.
Published: September 2007

Advances in the development of high Tc superconducting thick film components and devices for microwave, millimeterwave, and submillimeterwave applications have led to the optimization of laser patterning techniques. Plasma-sprayed superconducting thick films of YBaCuO materials on polycrystalline alumina were laser etched using an Nd: YAG laser (λ = 1.06 μm) in the Q-switched mode. Spatial uniformity of the surface elemental distribution of Y, Ba, Cu, and Al was observed in the underlying laser-etched area. An etch rate of 7.5 μm/scan was calculated at an optimized laser fluencc of 1.8 × 104 J/cm2 for a translation rate of 2.54 cm/s, having patterning widths ranging from 5–15 μm with a heat-affected zone of 3 μm. An absorption length of 18.3 μm for the Nd: YAG laser was determined to be suitable for patterning thick films (20–80 μm) for device fabrication. The results are further compared to CO2 (λ = 10.6 μm) laser etching for patterning (250 μm) thick films.
Processing characteristics and properties of BiSrCaCuO superconducting glass ceramics prepared by melt-quenching
Authors: Massalker Y. , Sembira A.N. , Baram J.
Published: September 2007

In the framework of an extensive research program for the production of textured and ductile high Tc BiSrCaCuO (BiSCO) wires and tapes, the influence of processing (by melt-quenching) parameters on the crystallization behavior, the quantitative and qualitative evolution of the crystallized phases, the chemical changes in the bulk, and the superconductive properties of various initial compositions of bulk BiSCO glass ceramics, prepared by melt-quenching, have been studied. The elemental composition of the samples changes drastically during heat treatments, affecting mainly Pb, but Sr and Ca also. The identified crystallographic phases, by XRD, were the low Tc superconducting “2201” (Bi2Sr2CuO6) phase, the high Tc superconducting “2212” and “2223” phases, and the Ca2PbO4, Ca2CuO3, CuO, CaO, Bi2Sr3-xCaxOy, and (Ca, Sr)3Cu5O8 “impurities” compounds. A crystallization sequence from the amorphous state is proposed, involving a reaction at 800 °C between “2223”, CaO, Ca2CuO3, and Bi2SrCaxOy to form “2212” + Ca2PbO4 + CuO and a 2(“2212”) → “2223” + “2201” disproportionation reaction that takes place with the intake of oxygen at a higher temperature. Decomposition of Ca2PbO4, which occurs also at high temperature, causes an increase of “2212”, which favors the increase of “2223” through the disproportionation reaction. The glass transition starts around Tg = 400 °C, and the crystallization reactions from the amorphous state proceed in two steps, at Txl = 465 °C and Tx2 = 504 °C. The Bi2Sr3-xCaxOy “2212” and “2223” phases are among the first to crystallize as early as after a 1 h treatment (in air) at 488 °C. A gain in weight is observed by thermogravimetry, caused by intake of the oxygen necessary for the formation of the high Tc superconducting phases. The oxygen intake starts as early as 600 °C. The Tc onset for the “2223” phase is at 122 K, and at 85.5 °C for the “2212” phase. Coefficients of thermal expansion have been measured and shown to differ according to crystallographic direction of expansion. The resistivity is increased on cooling, indicating semiconducting behavior of the 2223 BiSCO ceramic (semiconductor-to-metal transition temperature: 210–220 K).
Determining fracture facet crystallography using electron backscatter patterns and quantitative tilt fractography
Authors: Slavik D.C. , Wert J.A. , Gangloff R.P.
Published: September 2007

A methodology is presented to characterize the crystallography of individual fracture surface facets. Electron backscatter patterns (EBSP's) from a metallographic section through a facet identify grain orientation, and quantitative tilt fractography identifies facet orientation; these results are combined to establish fracture facet crystallography. For this technique, facet electropolishing is not required, the facet alignment procedure is accurate and quick, and the method can be generalized to different microstructures, test environments, or facet orientations. Method accuracy is illustrated for 25 to 50 μm fatigue crack facets in an unrecrystallized Al–Li–Cu alloy (AA2090) that has 5 μm thick subgrains in elongated grains that are 10 to 200 μm thick. The fine subgrain structure and tortuous fatigue crack profile precludes the use of other diffraction techniques for determining AA2090 facet crystallography. EBSP and tilt fractography results demonstrate that vacuum fatigue cracks in AA2090 are nearly parallel to local {111} planes.
Rapid solidification by unstable combustion synthesis
Authors: Li H.P. , Sekhar J.A.
Published: September 2007

During combustion/micropyretic synthesis, conditions that give rise to rapid solidification and rapidly solidified microstructures may be encountered. In this article, many such conditions are identified for the first time in a Ni–Al system. In addition, the banded structures and aligned dendrites that are encountered in this system are also examined. The various techniques of rapid solidification that may be initiated with combustion synthesis are examined and discussed.
Scanning tunneling microscope observations of metallic glass fracture surfaces
Authors: Kulawansa D.M. , Dickinson J.T. , Langford S.C. , Watanabe Yoshihisa
Published: September 2007

We report scanning tunneling microscope observations of fracture surfaces formed during catastrophic crack growth in three metallic glasses: Ni56Cr18Si22B4, Co69Fe4Ni1Mo2B12Si12, and Fe78B13Si9. Macroscopically, the first two glasses fail along a slip band formed during loading and display a characteristic, μm-scale pattern of vein-like ridges; in contrast, Fe78B13Si9 displays little slip prior to fracture, and its fracture surface shows a μm-scale chevron pattern of steps. STM observations of fracture surfaces of all three materials show nm-scale grooves. The grooves in Co69Fe4Ni1Mo2B12Si12 are especially prominent and display stepped edges which we attribute to the intersection of shear bands with the surface. STM observations of the vein-like features on Ni56Cr18Si22B4 also show stepped edges. We attribute the vein features to the interaction of adjacent crack fingers in which the material between adjacent fingers fails in plane stress. The origin of the grooves is uncertain, but may be due to other shear instabilities along crack fingers.
Topographical changes induced by high dose carbon-bombardment of graphite
Authors: Annis B.K. , Pedraza D.F. , Withrow S.P.
Published: September 2007

Highly oriented pyrolytic graphite has been implanted at room temperature with 165 keV C+-ions at doses from 6 × 1017 to 3 × 1019 ions/m2. Implantation-induced topographical changes of differing size scales were studied by optical, scanning electron, scanning tunneling, and atomic force microscopies. Defects with atomic resolution are seen for the lower dose implants. The formation of a vacancy line is revealed for the first time. At the higher doses a dendrite-like system of deep surface cracks is observed. This cracking develops as a result of the large basal plane contraction produced by irradiation which generates high shearing stresses between the implanted, damaged surface layer and the underlying material. Two independent systems of ridges have been characterized. One appears to follow a crystallographic direction while the other appears as a dense, intricate, generally curvilinear network with short ramifications. Additional experiments in which both the ion energy and dose rate have been varied indicate that ridge evolution progresses with increased energy and fluence, but is independent of dose rate. It is suggested that the ridge networks may form as a result of C transport by diffusion from the heavily damaged near-surface region or of a tectonic-plate-like motion or both. The geometric features of the ridge networks are related to the subsurface radiation damage as well.