bluuebunny/crossref_metadata_2025 huggingface dataset:
Photo-induced magnetic behavior in the amorphous spin-glass material Co3(SbTe3)2
Authors: Wu Biao , Ren Lianwei , O'Connor Charles J. , Tang Jinke , Jung Jin-Seung , Ferré Jacques , Jamet Jean-Pierre
Published: September 2007

A new ternary material Co3(SbTe3)2was prepared by using a rapid precipitation metathesis reaction between the Zintl material K3SbTe3and CoCl2in aqueous solution. The dc specific resistivity of this material is in the region for metallic conductors (p = 2.75 × 10-3Ω-cm). The dc magnetic susceptibility of Co3(SbTe3)2is reported over a 2.2 K-300 K temperature region, and the material is characterized as a spin glass with a freezing temperature of about 5 K. Magnetization data are also reported as both thermal remanent magnetization and isothermal remanent magnetization as a function of magnetizing field and temperature. When cooled well below the glass freezing temperature, the frozen spin glass has been observed to exhibit photomagnetic effects consistent with a disruption of the spin-glass state caused by uv-radiation.
Terrace growth and polytype development in epitaxial β-SiC films on α-SiC (6H and 15R) substrates
Authors: Chien F. R. , Nutt S. R. , Yoo W. S. , Kimoto T. , Matsunami H.
Published: September 2007

Epitaxial β-SiC (3C) films were grown on (0001) 6H-SiC and 15R-SiC substrates by chemical vapor deposition (CVD). TEM characterization revealed that films on both substrates exhibited large areas of atomically flat, coherent interfaces. However, when 3C-SiC films were grown on 6H substrates, double position boundaries (DPB's) were frequently observed, and islands of 6H were occasionally embedded in the predominantly 3C film. In contrast, films of 3C-SiC grown on 15R substrates exhibited relatively few DPB's and only occasional islands of 15R. A model of interlay er interactions in SiC was applied to predict the atomic structures at both 3C/6H and 3C/s15R interfaces, and these predictions were consistent with experimental observations of the interfaces by TEM. The observed interface structures and defect distributions were attributed to a microscopic kinetic mechanism of terrace growth. Consideration of step energies and growth kinetics led to the prediction that DPB's can be avoided by growing 3C-SiC films on 15R-SiC substrates.
Origin of cristobalite formation during sintering of a binary mixture of borosilicate glass and high silica glass
Authors: Gupta Tapan K. , Jean Jau-Ho
Published: September 2007

It was shown previously1that cristobalite precipitates out of a mixture of borosilicate glass (BSG) and high silica glass (HSG) when sintered at temperatures ranging from 800 to 1200 °C. In this paper, both direct and indirect evidences are presented to conclude that the formation of cristobalite originates in HSG. It is proposed that the cristobalite is formed as a result of dissolution of HSG in BSG and precipitation at heterogeneously nucleated sites. The process of dissolution and precipitation continues until the whole HSG particle is consumed.
Environment and time dependent hardness in zirconia
Authors: Pereira A.S. , da Jornada J.A.H.
Published: September 2007

The microhardness of monoclinic ZrO2 single-crystals was measured in different environments: air, water, and toluene. An indentation creep process at room temperature was observed for the measurements in moist media pointing for a water-activated plastic relaxation mechanism. This effect is discussed employing the models previously proposed to explain similar behaviors in ZrO2 and other nonmetallic materials. A possible correlation with the conditions for the nucleation in phase transitions is proposed.
Formation of Bi-2212 superconducting whiskers from melt-quenched BSCCO containing alumina
Authors: Kasuga Toshihiro , Ono Masayasu , Tsuji Kenji , Abe Yoshihiro , Nakamura Koichi , Inukai Eikichi
Published: September 2007

Bi–Sr–Ca–Cu–O (BSCCO) superconducting whiskers (2212 phase) were prepared by heating in air the compacted specimens of the mixtures of melt-quenched Bi2SryCa2Cu4Al1Ox powders and alumina powders. Formation of the whiskers depends on the composition and the applied pressure of the compacts. The optimum composition of the melt-quenched products for preparing the long whiskers is Bi2Sr2Ca2Cu4Al1Ox. Superconducting whiskers <1 mm in length (2212 phase) containing an excess amount of copper were grown numerously from the specimen compacted at 20 MPa; long whiskers (2212 phase) of 1–5 mm in length were obtained from that compacted at 180 MPa. These whiskers showed diamagnetic signals below Tc ≃ 80 K.
Chemisorptive electron emission as a probe of plastic deformation in reactive metals
Authors: Dickinson J.T. , Jensen L.C. , Langford S.C. , Hoagland R.G.
Published: September 2007

The surface area created during tensile deformation and fracture of the reactive metals Ti, Zr, Mg, and Al is probed by real-time measurements of chemisorptive electron emission (CSE) due to oxygen adsorption. CSE is sensitive to the number of fresh metal atoms exposed at the surface as a consequence of plastic deformation. At constant strain rate, Ti, Zr, and Mg all display exponential increases in CSE intensities during loading, reflecting exponential increases in surface area prior to fracture. In Ti and Zr, CSE begins at the onset of unstable necking. In contrast, CSE intensities from Al reflect a nearly constant rate of surface area production during deformation at constant strain rate. Calibration of the Ti CSE intensities per unit surface area allowed determination of the total surface area produced during deformation and fracture. Atomic force microscopy of the necked region in strained Ti samples shows dramatic increases in surface roughness, in near agreement with the CSE results. A model is presented to account for these observations. The utility of CSE measurements as a probe of deformation and ductile fracture is discussed.
Strain aging and breakaway strain amplitude of damping in NiAl and NiAlZr
Authors: Wolfenden A. , Raj S.V. , Kondlapudi S.K.R.
Published: September 2007

Extruded NiAl and NiAlZr alloys often show discontinuous yielding on strain aging in compression at room temperature. Two sets of experiments were conducted to understand the reasons for this yield-point behavior. First, strain-aging experiments were carried out on NiAl alloys containing O to 0.1 at. % Zr. The specimens were all deformed in compression at room temperature at a nominal initial strain rate of 1.1 × 10−4S−1, and the effect of annealing at 700 and 1200 K on the stress-strain curves and the yield strength was studied after an initial prestrain. While annealing at 700 and 1200 K consistently reduced the yield strength of both NiAl and NiAlZr, the effects were quite different. In the case of NiAl, annealing at 1200 K did not result in discontinuous yielding, whereas it generally resulted in a sharp yield point for the Zr containing alloys. Second, the PUCOT (piezoelectric ultrasonic composite oscillator technique) was used to measure the dynamic Young modulus, breakaway strain amplitude, and damping for the alloys. Only small differences were observed in the values of Young's modulus, but the breakaway strain was at least a factor of 2 to 3 lower for NiAl than for NiAlZr. The experimentally determined values of damping were used in the Granato-Lücke model to estimate the binding energy for NiAl. While the binding energy values were found to be in agreement with the calculated values of dislocation kink nucleation and migration energies in this material, to within an order of magnitude, other effects, such as dislocation pinning by quenched-in vacancies, cannot be ruled out. The observations made in this study suggest that the yield-point behavior in NiAl may be due to several factors, such as difficulties in double kink nucleation, and single kink migration, as well as dislocation-vacancy interactions; whereas, the yield-point behavior in the Zr-alloyed material is due at least in part to dislocation-solute interaction.
Formation and structure of tin-iron oxide thin film CO sensors
Authors: Bonzi P. , Depero L.E. , Parmigiani F. , Perego C. , Sberveglieri G. , Quattroni G.
Published: September 2007

Rheotaxial growth and thermal oxidation (RGTO) for depositing thin films is a recognized technique in preparing gas sensitive semiconducting oxides. This paper presents a study performed by x-ray diffraction and scanning Auger microscopy of the mechanisms of growth and formation of the thin films of the new ternary compound Sn1−xFexOy with an iron content in the range O < x < 25 at. %. A structural model of this compound, which is found to be stable over a very large range of Sn/Fe ratios, can be derived by partially substituting Fe3+ ions in Sn4+ sites. This is an easy substitution in view of the similar values shown by the ionic radii (Fe3+ = 0.64 Å, Sn4+ = 0.71 Å) and the Pauling electronegativity (Fe3+ = 1.8, Sn4+ = 1.8) of these two ions. Experimental data, showing that this material is an excellent CO sensor, are reported.
Copper oxides obtained by CO2 laser irradiation of copper salts
Authors: Popescu C. , Fatu D. , Alexandrescu Rodica , Voicu I. , Morjan I. , Popescu Monica , Jianu V.
Published: September 2007

Various copper salts have been irradiated with the help of a continuous wave CO2 laser. In other experiments the same salts have been decomposed under a controlled heating rate, in a thermal balance, and copper oxide crystallites have been detected as the final product of both types of experiment. The measurements of crystallites performed with the help of x-ray diffraction exhibited differences between the two types of oxides: those obtained by laser irradiation as compared with the oxides obtained by thermal heating. The magnitude of this difference seems to depend on the laser quanta absorptivity of the initial salts. The results are consistent with those obtained previously on nickel oxides.
Size dependent thermal vibrations and melting in nanocrystals
Authors: Shi Frank G.
Published: September 2007

A simple model for the size-dependent amplitude of the atomic thermal vibrations of a nanocrystal is presented which leads to the development of a model for the size dependent melting temperature in nanocrystals on the basis of Lindemann's criterion. The two models are in terms of a directly measurable parameter for the corresponding bulk crystal, i.e., the ratio between the amplitude of thermal vibrations for surface atoms and that for interior ones. It is shown that the present model for the melting temperature offers not only a qualitative but even an excellent quantitative agreement with the experimentally observed size-dependent superheating, as well as melting point suppression in both the supported and embedded metallic and semiconductor nanocrystals.
The work-of-fracture of brittle materials: Principle, determination, and applications
Authors: Barinov S.M. , Sakai M.
Published: September 2007

Theoretical and empirical considerations of the work-of-fracture, γwof, of brittle materials are reviewed. The energy principle of the work-of-fracture provides a modified Irwin similarity relationship in the nonlinear fracture mechanics regime. Various microscopic deformation and fracture processes in the crack wake and the crack-face contact regions contribute to the rising R-curve behavior of brittle materials, and then significantly affect the work-of-fracture, resulting in the work-of-fracture that is dependent on the dimension and geometry of test specimens as well as test methods. The requisite for the work-of-fracture to be a material characteristic resistance to failure is discussed in relation to the R-curve behavior. Some examples of the work-of-fracture test results demonstrate the usefulness of the work-of-fracture for designing brittle materials with improved toughness.
The effects of heat treatment on B2O3-contained ZnO varistor
Authors: Huang Jow-Lay , Li Kuo-Bin
Published: September 2007

The effects of annealing on B2O3-contained ZnO varistors with particular emphasis on the degradation behavior, nonlinear coefficient, trap density, donor density, and Schottky energy barrier were investigated. The thermal stability of ZnO varistors was considerably improved by the thermal annealing process.
A mechanism of CVD diamond film growth deduced from the sequential deposition from sputtered carbon and atomic hydrogen
Authors: Olson Darin S. , Kelly Michael A. , Kapoor Sanjiv , Hagstrom Stig B.
Published: September 2007

We describe a growth mechanism of CVD diamond films consisting of a series of surface reactions. It is derived from experimental observations of a sequential deposition process in which incident carbon flux and atomic hydrogen flux were independently varied. In this sequential process, film growth rate increased with atomic hydrogen exposure, and a saturation in the utilization of carbon was observed. These features are consistent with a surface growth process consisting of the following steps: (i) the carburization of the diamond surface, (ii) the deposition of highly disordered carbon on top of this surface, (iii) the etching of disordered carbon by atomic hydrogen, (iv) the conversion of the carburized diamond surface to diamond at growth sites by atomic hydrogen, and (v) the carburization of newly grown diamond surface. The nature of the growth sites on the diamond surface has not been determined experimentally, and the existence of the carburized surface layer has not been demonstrated experimentally. The surface growth mechanism is the only one consistent with the growth observed in conventional diamond reactors and the sequential reactor, while precluding the necessity of gas phase precursors.
Formation and coarsening behavior of Y2BaCuO5 from peritectic decomposition of YBa2Cu3O7−x
Authors: Griffith M.L. , Huffman R.T. , Halloran J.W.
Published: September 2007

Formation and coarsening behavior of the Y2BaCuO5 (211) phase has been examined in samples produced by peritectic decomposition of pure YBa2Cu3O7−x (123), resulting in 211 crystals and the liquid phase [BaCuO2-CuO]. Through various temperature (1020 °C-1060 °C) and time (0.25 h-10 h) studies, the fundamental coarsening behavior was determined. At 1040 °C, 211 crystals coarsen significantly over a 10 h period. The acicular crystals can be modeled by the diffusional ripening law, r–r0 = (Kt)1/3, where r=V1/3. However, the log-normal distributions of the lengths, widths, and volumes for each coarsening run are much wider than general ripening theory would predict. Results from coarsening studies at 1020 °C and 1060 °C for 3 h reveal that the 211 crystal volume increases with increasing superheat. Prior coarsening of the 123 grain size yields much larger 211 particles, suggesting that the 211 crystals must nucleate at the 123 grain boundaries during peritectic decomposition, and this nucleation governs the size of the 211 crystals for short coarsening times. Addition of properitectic 211 (15 mole %) to pure 123 before peritectic decomposition strongly influences the particle habit of the resulting 211 crystals. Without any additions, acicular or needle-like 211 crystals result from the melting of 123. However, when equiaxed properitectic 211 is added to the 123, the resulting 211 is faceted, but still equiaxed. If acicular 211 is added to the starting composition, the resulting 211 is needle shaped. These results will be discussed in terms of 123 melt-texturing and directional solidification processing.
Effects of oxygen pressure on the structure of Y–Ba–Cu–O materials formed by containerless melting and solidification
Authors: Weber J.K.R. , Nordine P.C. , Goretta K.C. , Poeppel R.B.
Published: September 2007

YBa2Cu3Ox ceramic superconductors were levitated, laser-beam melted, and undercooled in pure oxygen, dry air, and pure argon atmospheres. Solidified material from this containerless melting was examined by electron microscopy and x-ray diffraction analysis. The solubility of yttrium oxide in the melt was found to depend on ambient oxygen pressure such that complete dissolution was readily achieved in argon, but not in oxygen. The material formed in argon could be deeply undercooled and resolidified to produce a fine-grained and chemically homogeneous material. Use of ambient oxygen pressure to control the product morphology is discussed.
Effects of oxygen partial pressure on the crystallization of amorphous Bi–Sr–Ca–Cu–O and Bi–Sr–Ca–Cu–O + Ag
Authors: Kramer M.J. , Margulies L. , Arrasmith S.R. , Dennis K.W. , Lang J.C. , McCallum R.W. , Gallagher P.K.
Published: September 2007

The temperatures and pathways of crystallization for amorphous Bi–Sr–Ca–Cu–O are strongly dependent on oxygen partial pressure (Po2), the oxidation state of the glass, and the presence of Ag. Reducing the Po2 from 1 to 0 bar lowers the onset of melting 100 °C, but does not change the onset of crystallization. Decreasing Po2 does inhibit the formation of (CaSr)CuO3 (011). Although Ag appears to be immiscible in Bi–Sr–Ca–Cu–O, finely distributed Ag lowers the onset of melting by 20 to 35 °C, depending on Po2. In oxygen-deficient glass, two exotherms are observed upon heating. The first exotherm at 460 °C is independent of Po2, and Ag and corresponds with the formation of Bi2Sr2CuO6 (Bi-2201). The second exotherm occurs at 486 °C with Ag or 500 °C without Ag. The onset temperature of the second exotherm does not change with Po2, but the exotherm decreases in intensity with decreasing Po2. EXAFS indicates that in the as-quenched amorphous material, Cu is in a reduced state. Annealing the amorphous material in oxygen below the first exotherm (i.e., glass relaxation) increases the oxidation state of the Cu to that necessary for the formation of the Bi2Sr2CaCu2O8 (Bi-2212) without additional oxygen diffusion. This relaxation of the glass increases the crystallization temperature of the Bi-2201 to 500 °C. The crystallization sequence of Bi-2201 to Bi-2212 occurs at all Po2 ≥ 0.1 bar, but the composition of the secondary phases changes in the unrelaxed glass. Relaxing the glass eliminates liquid formation at low temperatures, allowing for rapid formation of the Bi-2212 phase with minimal formation of secondary phases for 0.1 ⋚ Po2 ⋚ 0.2 bar.
Porous alumina ceramics by spray-pyrolyzed powder from aluminum sulfate and aluminum nitrate solutions
Authors: Okada Kiyoshi , Tanaka Akihiro , Hayashi Shigeo , Daimon Keiji , Otsuka Nozomu
Published: September 2007

Porous α-alumina ceramics were prepared using the spray-pyrolyzed powder from aluminum nitrate solution and aluminum sulfate solution. Porosity and pore size distribution of the samples were examined with respect to the forming pressure and firing temperature. The porosity obtained changed from around 80% in the green compacts to 60–70% in the fired bodies fired at 1500 °C. The porosity of 30–40% remained even by firing at 1700 °C. Although there was no significant difference in the porosity of the fired bodies prepared from the sulfate and nitrate solutions, the fired bodies prepared from the nitrate solution showed apparently larger pore size than those from the sulfate solution. Fired bodies with an average pore radius from 0.2 to 0.8 μm can be prepared by this method. The four-point bending strength of the fired bodies, which had a porosity of 57% and an average pore radius of 0.23 μm, was 35 MPa.
Fracture behavior of fused quartz with laser-induced internal flaws
Authors: Li Y.Z. , Harmer M.P. , Chou Y.T.
Published: September 2007

Pulsed-laser light was used to generate internal flaws in fused quartz. The size of the flaw produced was proportional to the amount of laser pulse energy above a threshold value of 2.5 mJ. Specimens of different flaw sizes were tested at room temperature under four-point bending. The bending strength decreased as the flaw size increased, and a Griffith relationship was established between the lateral flaw size and the critical stress at the flaw tip. Characteristic demarcation lines were observed on the fracture surfaces of specimens with a flaw size greater than 0.25 mm. It is suggested that the formation of the demarcation line is caused by a discontinuous change of stress intensity in the dynamic process of the crack propagation.
Heteroepitaxy of diamond on c-BN: Growth mechanisms and defect characterization
Authors: Argoitia Alberto , Angus John C. , Ma Jing S. , Wang Long , Pirouz Pirouz , Lambrecht Walter R.L.
Published: September 2007

Diamond films grown on {100}, {111} boron-terminated, and nitrogen-terminated facets of cubic boron nitride (c-BN) single crystals were characterized by Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The evolution of morphology and microstructure of the diamond films at different stages during the growth process were followed by SEM investigation. The results indicate that diamond growth proceeds by nucleation of oriented three-dimensional islands followed by their coalescence. Cross-sectional TEM specimens were prepared from thick (over 10 μm) continuous diamond films grown on {111} boron-terminated surfaces. Selected-area diffraction and high resolution TEM images show that the diamond film has a parallel orientation relationship with respect to the substrate. Characteristic defects, common to diamond films obtained by chemical vapor deposition on other substrates, are also discussed.
Formation of Ni80P20 solid solution phase under high pressure
Authors: Mao Z.L. , Chen H. , Jung Ai-Lien
Published: September 2007

A Ni-P solid-solution phase was obtained by quenching from the melt under a pressure of 4.5 GPa. It was considered as a metastable high-pressure phase. Metastable phases with the same composition as the melt, such as supersaturated solid solutions and amorphous phases, are easily prepared using a high pressure quenching method.
Preparation of β-SiAION from silica-alumina gel
Authors: Rames P.D. , Rao K.J.
Published: September 2007

Amorphous aluminosilicate gel powders have been subjected to carbothermal reduction and nitridation reaction at high temperature (1673 K). The influence of Al2O3 content in the gel powder on the nature and structure of the product phases has been examined. Between 5% and 9% Al2O3 in the gel powder, it is found that only β-SiAION is formed as the product of CTR/N reaction.
Systematic trends of YBa2Cu3O7−δ thin films post annealed in low oxygen partial pressures
Authors: Hou S.Y. , Phillips Julia M. , Werder D.J. , Tiefel T.H. , Marshall J.H. , Siegal M.P.
Published: September 2007

Systematic studies have been performed on 1000 Å YBa2Cu3O7−δ films produced by the BaF2 process and annealed in an oxygen partial pressure (Po2) range from 740 Torr to 10 mTorr as well as a temperature range from 600 to 1050 °C. The results show that while high quality films can be annealed in a wide range of oxygen partial pressure, they have different characteristics. In general, crystalline quality and Tc are optimized at high Po2 and high annealing temperature, while strong flux pinning and low normal state resistivity are achieved at lower values of both variables. Under optimized low Po2 conditions, an ion channeling Xmin of 6% is obtained on films as thick as 5000 Å. This study will serve as a useful guide to tailoring film properties to the application at hand.
Crystal growth of YBa2Cu3O7 by the SRL-CP method under low oxygen partial pressure atmosphere
Authors: Nakamura M. , Yamada Y. , Shiohara Y.
Published: September 2007

Single crystals of YBa2Cu3O7−x (Y123) were grown by a modified top-seeded crystal pulling method using a BaO-CuO solution with the solid Y2BaCuO5 (Y211) as a solute in an yttria crucible [the so-called solute-rich liquid crystal pulling (SRL-CP) method] under 2% oxygen partial pressure atmosphere [P(O2) = 0.02 atm]. According to the pseudo-binary phase diagrams of Lee and Lee,1 the temperature of Y123 crystal growth was expected to be lower for 0.02 atm oxygen pressure than for 0.21 atm oxygen pressure. The single crystals grown under P(O2) = 0.02 atm and cooled under the same atmosphere after the separation of crystal from a solution had twins near the microcracks on the crystal surface. On the other hand, the single crystals grown under P(O2) = 0.02 atm and cooled under pure nitrogen atmosphere (6N) showed no twin structure. These results indicate that twins did not form during crystal growth but formed due to tetragonal-orthorhombic transition as a consequence of oxygenation at cooling under low oxygen partial pressure.
Preparation and microstructure of SrCuO2/(Sr0.5Ca0.5)CuO2 epitaxial multilayers with the infinite-layer structure
Authors: Satoh Toshifumi , Adachi Hideaki , Ichikawa Yo , Setsune Kentaro , Wasa Kiyotaka
Published: September 2007

Multilayer thin films of SrCuO2/(Sr0.5Ca0.5)CuO2 with an infinite-layer structure have been prepared on (100) SrTiO3 single crystals by multitarget rf magnetron sputtering. The structural analyses of the multilayers were carried out by means of x-ray diffraction (XRD), secondary ion-mass-spectroscopy (SIMS), and transmission electron microscopy (TEM). Heteroepitaxial growth of the SrCuO2 and (Sr0.5Ca0.5)CuO2 layers was confirmed to be with the c-axis perpendicular to the (100) SrTiO3 surface. The XRD and SIMS measurements revealed that the compositional modulations of the multilayer films were successfully constructed as we designed. However, in the TEM images, there existed planar dislocations parallel to the ac- and bc-planes, which went across the boundaries of the SrCuO2 and (Sr0.5Ca0.5)CuO2 layers. The resistivity of the multilayer films showed semiconductor-like behavior with temperature, and there was no relation between electric properties and modulation wavelength.
A study of aluminum speciation in aluminum chloride solutions by small angle x-ray scattering and 27Al NMR
Authors: Singhal Amit , Keefer Keith D.
Published: September 2007

Aluminum ions may exist in one of several different types of polynuclear species depending upon the degree to which it is hydrolyzed. The hydrolysis of aluminum can be explained by a metastable equilibrium among Al(H2O)3+6, Al(OH)(H2O)2+5, Al13O4(OH)24(H2O)7+12, and an oligomer. We have made small angle x-ray scattering and 27Al NMR measurements on 0.3 M aluminum chloride solutions over a range of OH/Al ratios. 27Al NMR shows two sharp peaks corresponding to the monomer Al(H2O)3+6 and the tetrahedrally coordinated aluminum in Al13O4(OH)24(H2O)7+12, and a broad peak due to the octahedrally coordinated aluminum in the oligomer. The observed radius of gyration (Rg) of the species in solution increases from 3.0 to 3.6 A, and the intensity at zero angle (which is proportional to the weight average molecular weight of the species) also increases as the OH/Al ratio increases from 1.0 to 2.2. The oligomer has an Rg of approximately 3.3 A, a hydrolysis ratio close to 2.0, and may be a six-membered ring with a structure similar to that of a gibbsite layer.
Heteroepitaxial growth of β'-SiC films on TiC substrates: Interface structures and defects
Authors: Chien F.R. , Nutt S.R. , Carulli J.M. , Buchan N. , Beetz C.P. , Yoo W.S.
Published: September 2007

Thin epitaxial films of β-SiC were grown by CVD on (100), (111), and (112) TiC substrates. TEM observations of the resulting interfaces revealed that island nucleation prevailed in the early stages of deposition for all three substrate orientations. Films grown on (111) and (112) TiC were monocrystalline, while SiC films deposited on (100) substrates were polycrystalline and not epitaxial, a phenomenon attributed to the poor match of atomic positions in SiC and TiC on their respective (100) planes. The (111) interface was abrupt and atomically flat, while the (112) interface exhibited {111} facets and steps. Simulated images of the stable (111) interface were calculated based on several possible atomic configurations, and the atomic structure of the interface was deduced from comparisons between the simulated images and phase-contrast TEM images.
Defect structure in single crystal titanium carbide
Authors: Zhao Q.H. , Wu J. , Chaddha A.K. , Chen H.S. , Parsons J.D. , Downham D.
Published: September 2007

The defects and crystallinity of as-grown and annealed TiCx(grown by the high-pressure float-zone) were examined by TEM and double crystal x-ray rocking curves. Three types of subboundaries and planar defects within subgrains were observed in as-grown TiCx. Subboundaries are classified by structure as (i) wide-extended, fault-like defects (WEFLD's), (ii) edge dislocation arrays, and (iii) dislocation networks. Planar defects were observed at dislocation nodes of subboundaries and also within subgrains; this is the first reported observation of planar defects within TiCxsubgrains. The misorientation and/or density of subgrains in TiCxwas reduced significantly by annealing at 2300 °C in contact with graphite.
Crystallization of nanosized titania particles prepared by the sol-gel process
Authors: Haro-Poniatowski E. , Rodríguez-Talavera R. , de la Cruz Heredia M. , Cano-Corona O. , Arroyo-Murillo R.
Published: September 2007

Sols of titania were obtained by the sol-gel method and their size profile was followed by dynamical light scattering. In the early stages of the reaction an unstable behavior was detected. After this unstable regime the particle size reaches a steady state where the sols have a constant size while increasing in number. Once the sol concentration reaches its overlap value, the gelation regime takes place. For samples prepared in this way Raman spectra and x-ray diffractometry were used to characterize the kinetics of crystallization of the material.
Order-disorder in A2M3+M5+O6 perovskites
Authors: Woodward P. , Hoffmann R-D. , Sleight A.W.
Published: September 2007

Using x-ray and neutron diffraction data, the degree of order of the octahedral site cations has been determined for the perovskites Sr2AlNbO6 and Sr2AlTaO6, which have been prepared by several different methods and annealed at temperatures up to 1690 °C. The degree of order generally increases with increasing synthesis temperature. The amount of cation ordering is, therefore, primarily controlled by kinetic processes and not by thermodynamic equilibrium considerations. Increased order obtained with increased heating time confirms this general kinetic limitation on the degree of order. However, annealing Sr2AlNbO6 in the highest temperature region resulted in some decrease in order, presumably due to thermodynamic considerations. The cubic edge of both compounds decreases significantly with increasing order. Ordered domains are separated by antiphase boundaries which occur in high concentrations. The cubic cell edge within the ordered domains is significantly smaller than the overall cell edge when the concentration of antiphase boundaries is high. The antiphase boundaries cause significant lattice strain which generally decreases as the concentration of antiphase boundaries decreases. Results on other A2M3+M5+O6 systems are briefly presented.
Carbon fiber-reinforced tin-lead alloy composites
Authors: Ho C.T.
Published: September 2007

Brominated, anodically oxidized, and pristine p-100 carbon fiber reinforced tin-lead alloy composites were fabricated by squeeze casting. The fibers were brominated by bromine vapor for 48 h and then desorbed at 200 °C in air for 12 h. The anodic oxidation treatment of fibers involved electrochemical etching in a dilute sodium hydroxide electrolyte for 3 min, or immersing in nitric acid for 72 h. The composites containing surface-treated carbon fibers had higher tensile and interlaminar shear strength than the ones containing pristine carbon fibers. The composite containing brominated carbon fibers had better tensile strength than the other two surface treatments.
Modeling study of effects of temperature profiling on CVI processing of woven graphite preforms with dimethyldichlorosilane
Authors: King Merrill K.
Published: September 2007

There is currently considerable interest in producing high temperature capability graphitic materials by first weaving graphite fiber tows (yarns) into a “preform” structure, followed by densification via cracking of precursor compounds such as dimethyldichlorosilane within the pores (macro and micro) at high temperature. The model described in this paper addresses this densification process, treating diffusion of gaseous species both within the macropores (spaces between the tows) and the micropores (spaces between individual fibers in the tows) and finite kinetics associated with the cracking of the precursor gas (treated parametrically). The resulting model is used to examine the effects of temperature distributions through cylindrical preforms on ultimate densification distribution. As might be expected (and as observed experimentally), uniform temperature through the preform leads to premature full densification of the pore structure at the periphery of the cylinder (blocking further densification in the interior), leading to severe porosity in the interior regions. Effects of externally imposed nonuniform temperature profiles (possibly via microwave heating) in alleviating this problem are examined, and it is shown that proper profiling can lead to nearly complete uniform densification throughout the preform.
Synthesis and characterization of barium bis(citrato) oxozirconate(IV) tetrahydrate: A new molecular precursor for fine particle BaZrO3
Authors: Rajendran M. , Rao Subba M.
Published: September 2007

Barium metazirconate (BaZrO3) fine powder has been produced by thermally decomposing a molecular precursor, barium bis(citrato)oxozirconate(IV) tetrahydrate at about 700 °C. The precursor, Ba[ZrO(C6H6O7)2] · 4H2O (BZO) has been synthesized and characterized by employing a combination of spectroscopic and thermoanalytical techniques. The precursor undergoes thermal decomposition in three major stages: (i) dehydration to give an anhydrous barium zirconyl citrate, (ii) decomposition of the anhydrous citrate in a multistep process to form an ionic oxycarbonate intermediate, Ba2Zr2O5CO3, and (iii) decomposition of the oxycarbonate to produce BaZrO3 fine powder. The particle size of the resultant BaZrO3 is about 0.2 μm, and the surface area is found to be 4.0 m2 g−1.
Properties of leucite-glass composites prepared by a coprecipitation process
Authors: Yoon Chong K. , Rasmussen Stephen T. , O'Brien William J. , Tien Tseng-Ying
Published: September 2007

The purpose of this investigation was to study the effect of changes in composition and thermal history on some properties of potential leucite-glass composites made by a coprecipitation process. Four compositions from the K2O–Al2O3−SiO2system were investigated for differences in phases, density, thermal expansion, and strength. Strength, density, and thermal expansion increased with increasing leucite content. Strengths of up to 140 MPa were obtained with air-fired specimens. Composites with different thermal expansion coefficients could be obtained by altering SiO2content in a predictable way. Leucite-glass composites offer potential for improved dental porcelains.
Radial variation of elastic properties of SCS-6 silicon carbide fiber
Authors: Sathish Shamachary , Cantrell John H. , Yost William T.
Published: September 2007

The upper and lower bounds of the bulk, shear, and Young's moduli are calculated point-by-point along the radius of SCS-6 silicon carbide fibers using the Hashin-Shtrikman equations from considerations of the Auger spectra along the fiber radius. The calculated values are in agreement with measurements of the average Young modulus obtained over relatively large radial regions of the fiber using scanning acoustic microscopy (SAM). The validity of the Hashin-Shtrikman bounds calculations is enhanced by the agreement found between experimental SAM V(z) curves for fixed z along the fiber radius and the theoretical bounds of such curves obtained from the bounds of the calculated bulk and shear moduli.
Electrical conduction in composites of nanosized iron particles and oxide glasses
Authors: Roy S. , Chakravorty D.
Published: September 2007

Nanocomposites involving iron particles in silica glass matrix have been synthesized by the hot pressing of suitably reduced precursor gel powders. The metal particles have diameters in the range 3.8 to 10.2 nm. An almost four orders of magnitude resistivity range at room temperature has been obtained by such changes in particle diameters. The resistivity in the temperature range 200-340 K shows a fractional temperature dependence with an average value of n ∼ 0.69. The resistivity changes in this temperature region can be explained on the basis of an electron tunneling mechanism.
The growth of silicon nitride crystalline films using microwave plasma enhanced chemical vapor deposition
Authors: Grannen K.J. , Xiong F. , Chang R.P.H.
Published: September 2007

Crystalline thin films of silicon nitride have been grown on a variety of substrates by microwave plasma-enhanced chemical vapor deposition using N2, O2, and CH4 gases at a temperature of 800 °C. X-ray diffraction and Rutherford backscattering measurements indicate the deposits are stoichiometric silicon nitride with varying amounts of the α and β phases. Scanning electron microscopy imaging indicates β-Si3N4 possesses sixfold symmetry with particle sizes in the submicron range. In one experiment, the silicon necessary for growth comes from the single crystal silicon substrate due to etching/sputtering by the nitrogen plasma. The dependence of the grain size on the methane concentration is investigated. In another experiment, an organo-silicon source, methoxytrimethylsilane, is used to grow silicon nitride with controlled introduction of the silicon necessary for growth. Thin crystalline films are deposited at rates of 0.1 μm/h as determined by profilometry. A growth mechanism for both cases is proposed.
Flocculation after injection molding in ceramic suspensions
Authors: Song J.H. , Evans J.R.G.
Published: September 2007

The flocculation of an unstabilized suspension of fine ceramic particles is advanced as the explanation for the formation of cracks in a “liquid” suspension. The development of cracks was observed several minutes after reheating wax-based ceramic moldings above the melting point of the wax and was accompanied by phase separation of the wax from the molding. Calculations of the acceleration of particles under London dispersion forces in a viscous fluid show the “time to impact” as a function of initial separation distance, fluid viscosity, and particle size. This is compared with the intercollision time calculated from classical flocculation theory, and it is shown that for crowded suspensions initial interparticle distances are such that the London force field cannot be neglected. Methods of preventing the flocculation are described.
Evidence of cooperative grain boundary sliding in superplastically deformed γ-TiAl
Authors: Zelin M.G. , Lee W.B. , Mukherjee A.K.
Published: September 2007

A study of the prepolished surface of γ-TiAl specimens superplastically deformed in tension showed evidence of macroscopic surfaces at which grain boundary sliding occurs. In a scanning electron microscope these surfaces are observed as black bands on the deformed surface because of the difference in the appearance of these bands and grain regions situated in between these bands. It has been suggested that cooperative grain boundary sliding, i.e., sliding of grain groups, occurs during superplastic deformation, giving rise to these bands.
Two-zone annealing of Tl0.5Pb0.5(Sr0.8Ba0.2)2Ca2Cu3Oy
Authors: Aselage T.L. , Venturini E.L. , Voigt J.A. , Lamppa D.L. , Van Deusen S.B.
Published: September 2007

Stable conditions have been identified for two-zone processing of the superconducting thallium cuprate Tl0.5Pb0.5(Sr0.8Ba0.2)2Ca2Cu3Oy. With P(O2) of 0.8 atm, P(Tl2O) of 4.4 × 10−3 atm, and a sample temperature of 920 °C, single-phase Tl0.5Pb0.5(Sr0.8Ba0.2)2Ca2Cu3Oy, is produced with a Tc of 115 K, complete diamagnetic shielding, and Meissner fraction greater than 70%. Although a small amount of melting occurs under these conditions, a comparison of the low-field diamagnetic shielding for these samples with samples of Pb- and Sr-free TlBa2Ca2Cu3Oy and Tl2Ba2Ca2Cu3Oy, suggests that such melting is not necessary to produce the triple-CuO2-layer superconductors.
SiC whisker- and C fiber-reinforced calcium phosphate composites
Authors: Hark K. , Sundaresan S. , Vasilos T. , Sung C.
Published: September 2007

SiC whisker/calcium phosphate and C fiber/calcium phosphate composites were fabricated by hot-pressing at 1185 °C and 7.6 MPa in vacuum. The microstructural characteristics, especially interfaces, of these composites were studied using transmission electron microscopy. The mechanical properties of these composites were measured by four-point bending tests. It was found that the mechanical properties of these calcium phosphate-based composites were strongly dependent on their microstructural characteristics.
Effect of additional elements in the glass precursor on the growth of the Bi2Sr2Ca2Cu2Ox whiskers
Authors: Dimesso L. , Matsubara I. , Ogura T. , Funahashi R. , Yamashita H. , Tampieri A.
Published: September 2007

The effect of the doping on the growth and superconducting properties of whiskers in the Bi–Sr–Ca–Cu–X–O system (where X = Na, B, Ga, and Pb, respectively), prepared by heating a glassy melt-quenched plate in flowing oxygen, is reported. The thermal behavior of the glassy samples (studied by DTA, XRD, and SEM techniques) was found to be dependent on the dopant as well as on its concentration. The Bi2Sr2CaCu2Ox (2212 phase) whiskers were grown only from the Ga-doped system annealed at 855 °C. The dependence of the mechanism of the whiskers' growth on the dopant is also discussed.
Microstructure of gas-atomized Al-20 wt. % Si-1 wt. % Ni powders studied by electron microscopy
Authors: Lee Byong-Taek , Chun Byong-Sun , Hiraga Kenji
Published: September 2007

The microstructure of gas-atomized Al-20 wt. % Si-1 wt. % Ni powders was investigated by electron microscopy. Primary Si crystals about 2 μm in size are homogeneously distributed in the Al matrix. Eutectic Si crystals about 50 nm in size are precipitated with the definite crystallographic relationship of 〈110〉Si ‖ 〈110〉Al. Most of the interfaces between Al and Si are semicoherently bonded with close-packed planes of {111}Si and {111}Al. The special crystallographic relationship and interfaces are interpreted by matching between Si and Al lattice spacings.
Effects of the scale of precursor mixing on densification behaviors and phase-transformation kinetics of cordierite gels
Authors: Jang Hyun M. , Lim Byung C.
Published: September 2007

Effects of the scale of precursor mixing on densification behaviors and phase-transformation kinetics of multicomponent aluminosilicate gels were examined using cordierite gels prepared by two different processing routes: a monophasic (single phase) gel having molecular homogeneity, and a triphasic gel in which constitutive precursor phases are uniformly mixed in a nanometer scale (∼50 nm). In the case of the monophasic cordierite gel, homogeneous molecular-scale mixing of precursors induces a direction formation of μ-cordierite without a preceding crystallization, and the densification stops at the onset of crystallization to μ-cordierite at 960 °C. On the other hand, the densification of triphasic cordierite gel was not affected by the crystallization of intermediate spinel phase at 700 °C. The triphasic gel continues to densify until amorphous silica matrix crystallizes to quartz at 1180 °C. The enhanced densification observed in the triphasic gel was attributed to the retarded crystallization of amorphous silica component caused by the increase in diffusion distances.
A comparison of the interphase development and mechanical properties of Nicalon and Tyranno SiC fiber-reinforced ZrTiO4 matrix composites
Authors: Bender B.A. , Jessen T.L.
Published: September 2007

The differences in interphase development and mechanical properties between Nicalon and Tyranno fiber-reinforced ZrTiO4 matrix composites were assessed. The composites were reinforced with either uncoated or BN-coated fibers. The microstructure and interphase development were analyzed using scanning electron microscopy, transmission electron microscopy, and analytical electron microscopy. Mechanical behavior was characterized by strength and toughness measurements. Tyranno composites developed thinner interphases due to the differences in the structure and chemistry of the starting fiber resulting from the addition of Ti to the starting polymer precursor. BN-coated fiber composites developed a thicker carbon interphase layer due to incorporation of O into the as-received BN coating. Tyranno composites were weakened by fiber clustering and were not as tough as Nicalon composites due to a lack of thermal debonding.
An efficient single-mode Nd3+ fiber laser prepared by the sol-gel method
Authors: Wu Fengqing , Machewirth David , Snitzer Elias , Sigel George H.
Published: September 2007

High quality Nd-doped single-mode fibers have been prepared by using a sol-gel process. The longest fluorescence lifetime measured was 520 μs in an Al: SiO2 glass fiber containing 0.47 wt.% neodymium oxide. An efficient neodymium fiber laser with a slope above threshold of 42% was successfully demonstrated with the sol-gel prepared Nd-doped single-mode fibers as a fiber laser oscillator.
Characterization of striations in silicon wafers by a multipass Fabry-Pérot Rayleigh-Brillouin scattering spectrometer
Authors: Taijing Lu , Ng S.C.
Published: September 2007

Swirls and oxidation stacking faults (OSF) ring-bands in the near surface region of Si crystals have been detected and characterized by a 180°backscattering Rayleigh-Brillouin spectrometer using an argon-ion laser as its light source. In FZ Si wafers with swirls, the central region exhibits high scattered light with random undulation, the peripheral region with swirls shows a periodic undulation of scattered light intensity, while the region in-between is a nearly uniform zone of low scattered light intensity. In contrast to this, the CZ Si wafers with OSF ring-bands display a low uniformly scattered light background with a high undulated scattered light zone corresponding to the OSF ring-band. The scattered light intensity and its structure in the OSF ring-band vary with the heat-treatment conditions. The features of scattered light detected by the scattering spectrometer are discussed.
Compression behavior of materials: Part I. Glassy polymers
Authors: Kozey Victor V. , Kumar Satish
Published: September 2007

In this three-part series the compressive behavior of (i) glassy polymers, (ii) high performance polymeric and carbon fibers, and (iii) polymeric matrix composites has been addressed. The glassy polymers exhibit plastic yielding in compression. The dependence of compressive yield strength on factors such as tensile modulus, glass transition temperature, density, and free volume has been examined. Failure theories for yielding in glassy polymers have been reviewed. Compression behavior of high performance fibers and that of the composites is discussed in Parts II and III, respectively.
Quantity and character of grain boundary phase in mixed α′/β′ sialon ceramics
Authors: Watari Koji , Sakaguchi Shuji , Kanzaki Shuzo , Hamasaki Toyohiro , Ishizaki Kozo
Published: September 2007

The quantity and character of intergranular phases in mixed α′/β′ sialon ceramics were examined by low-temperature specific heat and internal friction measurements. Present results of specific heat suggest the existence of a small amount of intergranular glassy phase in mixed α′/β′ sialon ceramics. Furthermore, it can be predicted that the material possesses excellent high strength at elevated temperatures, because the temperature dependence of the internal friction in the tested material is similar to that in the annealed Si3N4ceramics with oxide addition and Si3N4ceramics without sintering aid.
Hydrothermal synthesis and characterization of a new potassium phosphatoantimonate
Authors: An Yonglin , Feng Shouhua , Xu Yihua , Xu Ruren , Yue Yong
Published: September 2007

A new potassium phosphatoantimonate with a composition of 4K2O · 4Sb2O5 · P2O5 · 8H2O, which crystallizes in a monoclinic system with a = 23.952 Å, b = 9.515 Å, c = 8.193 Å, and β = 124.77, was hydrothermally synthesized and characterized by powder x-ray diffraction, chemical composition analysis, x-ray photoelectron spectroscopy, and 31P MAS-NMR techniques. The potassium ion can be exchanged by other monovalent ions.