bluuebunny/crossref_metadata_2025 huggingface dataset:
The extent of phase transformation in silicon hardness indentations
Authors: Callahan Daniel L. , Morris Jonathan C.
Published: September 2007

The extent of phase transformation occurring in silicon during room-temperature indentation experiments has been examined by transmission electron microscopy of low-load microindents. The results show that the entire hardness impression arises from structural transformation and extrusion of a ductile high pressure phase. In particular, there is no dislocation activity or other mechanism of plastic deformation operating outside the clearly demarcated transformation zone. The observable impression consists of an amorphous transformation zone with an adjacent region of plastically extruded material and a layer of polycrystalline silicon at the near-surface transformation interface.
Diffusion solidification model on Y-system superconductors
Authors: Izumi Teruo , Nakamura Yuichi , Shiohara Yuh
Published: September 2007

The unidirectional solidification of the zone melt method was performed in order to clarify the growth mechanism on Y-system superconductors. A sharp faceted interface of YBa2Cu3Oy (123) crystals was obtained in the sample grown at the low growth rate of 1 mm/h. The volume of the 211 phase changed drastically from liquid to 123 crystal. These results lead to the idea that the necessary solute for peritectic reaction is provided through a liquid. Based on this idea, we developed a simple solidification model that is in good agreement with the experimental results.
High temperature ultrasonic characterization of intrinsic and microstructural changes in ceramic YBa2Cu3O7−δ
Authors: Suasmoro S. , Smith D.S. , Lejeune M. , Huger M. , Gault C.
Published: September 2007

Young's modulus of ceramic YBa2Cu3O7−δ was measured between room temperature and 1000 °C, E(T), by an ultrasonic pulse-echo technique. Experimental results are presented for nonaligned ceramics with average grain sizes from 2 to 10 μm and densities from 80% to 95% of the theoretical value. Young's modulus is shown to be strongly sensitive to oxygen content with the orthorhombic phase being significantly stiffer than the tetragonal phase. In addition, the phase transition is denoted by a pronounced minimum in E(T) relating to softening of certain bonds in the unit cell. At high temperature (>900 °C) melting of the second phase gives a steep drop in E(T) while subsequent densification increases E. Finally, cooling of large-grained ceramic below 450 °C opens microcracks due to anisotropic volume changes. This results in hysteresis between the heating and cooling curves for E(T). Hence the technique is used to show that ceramic, with an average grain size of 2 μm and 85% of theoretical density, is well oxygenated and undamaged by its thermal treatment.
Effusion of hydrogen from proton implanted ceramic YBa2Cu3O7−x during annealing in oxygen atmosphere
Authors: Yokota Katsuhiro , Hatanaka Kazuhiro , Kura Takeshi , Katayama Saichi , Ochi Mitsukazu , Murakami Mitsuaki , Chayahara Akiyoshi , Satho Mamoru
Published: September 2007

Hydrogen implanted into ceramic YBa2Cu3O7−x (YBCO) with a dose of 1 × 1017 H+ cm−2 started to effuse as molecular hydrogen from the YBCO to atmosphere at a temperature of 200 °C, effuse predominantly as water by reacting with oxygen at temperatures of 300–700 °C, and again effuse as molecular hydrogen at temperatures above 800 °C. The improvement of the superconducting properties of the proton implanted YBCO occurred at annealing temperatures for which implanted hydrogen effused predominantly as water by reacting with oxygen.
Limit of superconductivity in silver/superconductor metal-matrix composites prepared with Bi–Pb–Sr–Ca–Cu–O ceramic powders
Authors: Sarkar Asok K. , Maartense I. , Peterson T.L.
Published: September 2007

Superconducting composite samples were prepared by sintering a mixture of metallic silver and Bi(Pb)–Sr–Ca–Cu–O powder in air. For Ag contents between 0 and ∼15 vol. %, the bulk critical temperature was depressed as much as 40 K, and the relative proportion of the 2223 phase with respect to the 2212 phase was decreased, compared to the pure sample. However, as the Ag content was increased to between 19 and 70 vol. %, critical temperatures above 100 K were measured by transport and ac susceptibility techniques, and the relative proportions of the phases were restored to that of the pure sample. The limit of bulk superconductivity was reached at 73 vol. % Ag where the bulk critical temperature was ∼85 K. For higher Ag contents bulk superconductivity was not observed. Our results also show that sintering in air of silver-clad tapes containing Bi(Pb)–Sr–Ca–Cu–O powder, either by itself or with silver powder, need not be detrimental to the final superconductive properties.
Phase and lattice parameter relationships in rapidly solidified and heat-treated (Mn0.53Al0.47)100−xCx pseudo-binary alloys
Authors: Lee C.T. , Han K.H. , Kook I.H. , Choo W.K.
Published: September 2007

The phase constitution and the lattice parameter relationships in the rapidly solidified and heat-treated (Mn0.53Al0.47)100−xCx pseudo-binary alloys (x = 0–6) have been investigated by means of x-ray diffraction and transmission electron microscopy. The melt-spun alloys contained a single ∊ phase (cph) with 0.63–4.0 at. % C, and below and beyond this carbon composition range small traces of γ2-MnAl and Al4C3 compounds were formed, respectively. The heat treatment of the melt-spun alloys at 823 K produced a single τ phase (ordered bct, CuAu type I, L10) with 0.63–3.6 at.% C. The c lattice parameter of the ∊ unit cell was observed to increase pronouncedly with the carbon content whereas that of the a-axis revealed no apparent change; the corresponding increase of the unit cell volume was taken to indicate an interstitial dissolution of the carbon atoms in the ∊ lattice. On the other hand, for the τ phase, the c lattice parameter increased markedly with the carbon content while the a parameter decreased slightly, so that a large increase of c/a ratio was produced. The lattice parameter data for the τ phase thus indicated an increase of the unit cell volume with the carbon content, providing new evidence that the carbon atoms dissolve interstitially in the bct lattice. In addition, it was deduced that the higher c/a ratio with increasing carbon content may arise from a preferential site occupation of the carbon atoms at a specific type of octahedral interstitial site lying in the manganese atom layers.
Electronic structure of planar faults in TiAl
Authors: Woodward C. , MacLaren J.M. , Rao S.
Published: September 2007

The mechanical behavior of intermetallic alloys is related to the mobility of the dislocations found in these compounds. Currently the effect of bonding on dislocation core structure and its influence on deformation behavior is not well understood. However, the unusual properties of these materials, such as the anomalous temperature dependence of flow stress observed in TiAl, are derived in part from the aspects of bonding that determine dislocation mobility. Several recent studies have suggested a particular relationship between directional bonding in TiAl and dislocation mobility. To understand better the flow behavior of high temperature intermetallics, and as a step toward bridging the gap between electronic structure and flow behavior, we have calculated the electronic structure of various planar faults in TiAl. The self consistent electronic structure has been determined using a layered Korringa Kohn Rostoker (LKKR) method which embeds the fault region between two semi-infinite perfect crystals. Calculated defect energies in stoichiometric TiAl agree reasonably well with other theoretical estimates, though overestimating the experimental (111) anti-phase boundary (APB) energy, found for Ti46Al54. We approximate the energy of the (111) APB for the Al-rich stoichiometry by calculating the energy of Al antisites near that defect plane. The calculated (111)APB energy decreases by 6% in going from stoichiometric TiAl to Ti46Al54. The overall hierarchy of fault energies is found to be associated with the number of crystal bond states that are disrupted by the introduction of the fault plane. However, the hierarchy of fault energies is inconsistent with the traditionally accepted ordering. Changes in bonding taking place in the vicinity of the planar defects are illustrated through the density of states and charge density plots. A three body atomistic model is introduced to parameterize the bonding observed in TiAl. The L10 lattice (c/a = 1.00), within a second nearest neighbor three body model, yields a (111)APB energy which is the sum of the complex and superlattice-intrinsic stacking fault energies.
Uncoupling crystal growth and nucleation in the deposition of diamond from the gas phase
Authors: Molinari E. , Polini R. , Terranova M.L. , Ascarelli P. , Fontana S.
Published: September 2007

Diamond deposits of well-separated particles have been obtained by the hot filament CVD technique on Si(100) wafers. Particle counting in SEM images and determination of their linear dimensions require a separate study of growth rates and of nucleation densities as a function of time, substrate temperature (500 °C–950 °C), gas phase composition (0.5–2% CH4 in H2), and total pressure (15–76 Torr). It is shown that recent models proposed for the growth process can successfully be applied if proper consideration is given to the high catalytic activity of the growing diamond surface for the heterogeneous recombination of gaseous H-atoms. This fast reaction controls the H-atom concentration at the surface and couples growth rates and nucleation densities via the gas phase.
Structural characterization of heat-treated activated carbon fibers
Authors: Rao A.M. , Fung A.W.P. , Dresselhaus M.S. , Endo M.
Published: September 2007

Raman scattering, x-ray diffraction, and BET measurements are used to study the effect of heat treatment on the microstructure of activated carbon fibers (ACFs) and to correlate the structural changes with the metal-insulator transition observed in the electronic transport properties of heat-treated ACFs. A sequence of events is identified, starting with desorption, followed by micropore collapse plus the stacking of basic structural units in the c-direction, and ending up with in-plane crystallization. The graphitization process closely resembles that depicted by Oberlin's model, except that the final material at high-temperature heat treatment remains turbostratic. Because the metal-insulator transition was observed to occur at heat-treatment temperature THT ≃ 1200 °C, which is well below the THT value (2000 °C) for in-plane crystallization, we conclude that this electronic transition is not due to in-plane ordering but rather to the collapse of the micropore structure in the ACFs. Raman scattering also provides strong evidence for the presence of local two-dimensional graphene structures, which is the basis for the transport phenomena observed in heat-treated ACFs.
Selective nucleation of silicon clusters in CVD
Authors: Shi Frank G. , Seinfeld John H.
Published: September 2007

A nucleation model is developed that includes chemical etching of atoms as an additional loss process besides thermal dissociation that competes with the process of atom addition in forming a cluster. The model has the proper qualitative features to describe observations of the evolution of cluster formation on amorphous silicon substrates in the low pressure CVD of a mixture of SiH2Cl2/HCl/H2.
Space-charge-limited-current conduction in heteroepitaxial 3C–SiC (111) on TiC (111)
Authors: Tan S.H. , Beetz C.P. , Carulli J.M. , Lin B.Y. , Cummings D.F.
Published: September 2007

Unintentionally doped 3C–SiC (111) films were grown on TiC (111) substrates. The films were characterized by electrical measurements employing Pt Schottky contacts, optical microscopy, and transmission electron microscopy (TEM). The observed current-voltage (I-V) characteristics appear to be dominated by space-charge-limited-current (SCLC) conduction in the films. Analysis of the I-V characteristics has resulted in information pertaining to the electrically active defects in the films. These active defects are believed to be associated with stacking faults and point defects present in the films and contribute to traps at ∼0.656 eV below the conduction band edge. The concentration of traps was found to vary with film thickness and surface morphology.
Microstructural evolution of Pb(Zr, Ti)O3 thin films prepared by hybrid metallo-organic decomposition
Authors: Tuttle B.A. , Headley T.J. , Bunker B.C. , Schwartz R.W. , Zender T.J. , Hernandez C.L. , Goodnow D.C. , Tissot R.J. , Michael J. , Carim A.H.
Published: September 2007

We report on the microstructural analyses of chemically prepared Pb(Zr0.53Ti0.47)O3 (PZT 53/47) films. Although several techniques were used to analyze films, transmission electron microscopy (TEM) was emphasized. Phase evolution of these films, fabricated using hybrid metallo-organic decomposition (HMD), was determined by processing films at temperatures ranging from 500 °C to 650 °C. Our films, when observed with an optical microscope, appeared to consist of two distinct phases: (1) a featureless matrix and (2) 1–2 μm diameter “rosettes”. PZT films fired at 500 °C consisted of a pyrochlore containing phase (featureless matrix) and contained no perovskite, whereas films fired at 600 °C were ferroelectric and were approximately 90% perovskite (rosettes) by volume. Our TEM analysis showed that the pyrochlore-containing phase consisted of interpenetrating nanocrystalline pyrochlore and amorphous phases, both with dimensions on the order of 5 nm. For PZT films processed at 650 °C, the perovskite phase was observed in two forms: (1) large (≍2 μm) rosette structures containing 30 nm pores and (2) dense equiaxed particles on the order of 100 nm. We propose that phase evolution—with increasing temperature of HMD PZT 53/47 films—consists of the following steps: (1) phase separation, probably occurring in solution, (2) pyrochlore crystallization, (3) heterogeneous nucleation of perovskite PZT, and (4) homogeneous nucleation of perovskite PZT.
Surface fractal dimensions of some industrial minerals from gas-phase adsorption isotherms
Authors: Lefebvre Yves , Lacelle Serge , Jolicoeur Carmel
Published: September 2007

A high precision gravimetric method was used to investigate the adsorption of nitrogen (N2) and carbon tetrachloride (CCl4), respectively, at 77 and 298 K, onto various industrial minerals. The solids investigated include silica isomorphs, blast furnace slags, an Y-zeolite, and several naturally occurring fibrous minerals. The adsorption isotherms were analyzed to derive surface areas, BET constants (C), and surface fractal parameters (D). The latter were obtained through an approach recently suggested by Avnir and Jaroniec,1 using multilayer adsorption data; D values obtained for the various gas-solid systems investigated cover the range 2.1–3.0. A systematic comparison between D values inferred from N2 and CCl4 adsorption data shows that the CCl4 molecule probes the surface roughness in a more discriminate fashion than N2. In most cases, the differences between D(N2) and D(CCl4) appear to reflect changes due to sample preparation more than intrinsic differences amongst the various solids.
A multinuclear MAS NMR study of the short-range structure of fluorophosphate glass
Authors: Brow R.K. , Osborne Z.A. , Kirkpatrick R.J.
Published: September 2007

We have examined the bonding arrangements in Na–P–O–F and Na–Al–P–O–F glasses using19F,27Al, and31P solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. For the Al-free series of glasses, the19F NMR spectra are dominated by peaks near +90 ppm, representative of F terminating P-chains. The formation of these bonds has little effect on the31P chemical shifts, indicating that F preferentially replaces bridging oxygen on the phosphate tetrahedra, consistent with previous NMR studies of crystalline fluorophosphates and other spectroscopic studies of fluorophosphate glass. For the Na–Al–P–O–F glasses,27Al NMR detects only octahedral Al-sites, the19F NMR spectra include a second peak near −12 ppm due to F bonded to Al, and the31P NMR spectra contain signals due to Q1-sites with one or more Al next-nearest neighbors. The relative intensity of the two19F peaks correlates well with previous spectroscopic studies and shows that a greater fraction of F–P bonds forms when the base glass is remelted in NH4HF2.
Reply to comments of D. H. Zeuch
Authors: Gengxiang Hu , Shipu Chen , Xiaohua Wu , Xiaofu Chen
Published: September 2007

Compression test results of our research on Al3Ti-base alloys are reported. It is evident that the specimen length-to-width ratio we used for compression testing can significantly reflect the difference in ductility of different alloys. Thus the tests fulfilled the aim of our present research.
Metal reinforced thermal plasma diamond coatings
Authors: Tsai C. , Nelson J. , Gerberich W.W. , Heberlein J. , Pfender E.
Published: September 2007

A three-step process to produce wear-resistant, composite diamond coatings is presented. Diamonds are deposited by RF thermal plasma CVD, reinforced by an electroplated metal binder, and regrown to form a continuous film. Microscratching and pin-on-disk wear tests indicate that the three-step composite films are more adherent than plasma deposited diamonds alone.
27Al- and63Cu-NMR in quasicrystal Al65Cu20TM15alloy (TM:Fe, Ru, and Os)
Authors: Shinohara T. , Tsai A.P. , Sato T. , Masumoto T.
Published: September 2007

27Al- and63Cu-NMR measurements have been made on quasicrystalline phases of the Al65Cu20TM15alloy (TM:Fe, Ru, and Os). There is no temperature dependence on peak shift and line width of either27Al- or63Cu-NMR spectra, suggesting that all the quasicrystalline phases are Pauli-paramagnetic. However, the peak shift of both spectra depends upon the constituent transition element in a different way: Al-peak shift increases from Fe through Os while the Cu-peak shift decreases.
X-ray photoelectron spectroscopy of Cu–In–Se–N and Cu–In–Se thin films
Authors: Kohiki Shigemi , Nishitani Mikihiko , Negami Takayuki , Wada Takahiro
Published: September 2007

The Cu, In, and Se core-level electron binding energies of the p-type Cu–In–Se–N thin film were larger than those of the n-type Cu–In–Se thin film. The positive shift of the core-electron binding energies for the Cu–In–Se–N film is consistent with that expected from the conduction types of the films. Holes were positioned in the Cu–Se antibonding orbitals of the Cu–In–Se–N film. The analysis using the Auger parameter revealed that the Cu–Se bonding interaction is stronger for the Cu–In–Se–N film than for the Cu–In–Se film.
Effects of composition on the microstructure of YBa2Cu3O7−x thin films prepared by plasma-enhanced metalorganic chemical vapor deposition
Authors: Lu P. , Zhao J. , Chern C.S. , Li Y.Q. , Kulesha G.A. , Gallois B. , Norris P. , Kear B. , Cosandey F.
Published: September 2007

The microstructures of (A) near stoichiometric, (B) Y-rich, and (C) Y- and Cu-rich YBa2Cu3O7−x thin films have been studied by high-resolution transmission electron microscopy. The films were deposited on (100) LaAlO3 by plasma-enhanced metalorganic chemical vapor deposition. In near stoichiometric films, microstructural features similar to those of thin films deposited by other techniques have been observed. These features which include epitaxial growth with the c-axis perpendicular to the substrate, twin boundaries on (110) planes, and stacking faults on (100) and (001) planes were also present in the off-stoichiometric materials. In Y-rich thin films, yttria (Y2O3) precipitates with an average size of about 5 nm have been identified in the matrix. The precipitates are uniformly distributed, have a high density as large as 1024/m3, and are highly oriented with respect to the matrix. In Y- and Cu-rich thin films, CuO particles up to 1 μm in size were observed on the surfaces of the films. The observed microstructural features were similar to those of the Y-rich materials.
Processing of Bi–Sr–Ca–Cu–O glasses using platinum and alumina crucibles
Authors: Holesinger T.G. , Miller D.J. , Fleshler S. , Chumbley L.S.
Published: September 2007

Reactions with alumina and platinum crucibles were studied during the preparation of Bi2Sr2Ca1Cu2Oy “2212” glasses. In particular, reactions with Al2O3 are of interest since alumina is a potential substrate material in applications of this superconductor. Glasses processed using alumina crucibles were completely homogeneous and free of secondary phases although the material contained 2.26 at. % Al in solution. After heat treatments, Al was found in the form of SrCaAlOy particles located primarily along grain boundaries of the 2212 superconducting phase. Platinum contamination was minimal (<0.02 at. %) and no Pt-containing secondary phases were found in amorphous or annealed samples. Glasses made with Pt crucibles were found to contain small amounts of CaO, Sr14−xCaxCu24O41, and 2201 as second phases. Differential thermal analysis (DTA) suggested that the crystallization processes were essentially the same for all samples although the small amount of Al seemed to slow the kinetics leading to the formation of 2212. Neither Al nor Pt was detected within the 2212 phase. The measured superconducting compositions in each annealed sample were nearly the same with identical transition temperatures of 88 K. Overall differences in stoichiometry were accommodated by changes in the number and composition of the secondary phases present.
Characterization of tungsten-carbon layers deposited on stainless steel by reactive magnetron sputtering
Authors: Pauleau Y. , Gouy-Pailler Ph.
Published: September 2007

Tungsten-carbon coatings have been deposited on stainless steel substrates by reactive magnetron sputtering from Ar–CH4 mixtures. The carbon concentration in the coatings measured by electron microprobe analyses was found to be proportional to the CH4 flow rate. Only the cubic α–W phase with a dilated lattice parameter was identified in W–C coatings having a carbon content lower than 25 at. %. Since the lattice parameter of the α–W phase in these W–C coatings increased with increasing carbon content, these coatings may be assumed to be W–C solid solutions. Only the nonstoichiometric β–WC1−x carbide (cubic phase) was detected in W–C coatings containing 30 to 70 at. % of carbon. The chemical state of the elements was investigated by x-ray photoelectron spectroscopy. The Vickers hardness of the W–C coatings was found to be considerably dependent on the carbon concentration. A maximum microhardness of 26 000 MPa was measured for W–C coatings containing either 14–15 at. % or 40–45 at. % of carbon. The correlation between crystallographic structure and microhardness is analyzed and discussed in this paper.
An atomic simulation of the influence of hydrogen on the fracture behavior of nickel
Authors: Hoagland R.G. , Heinisch H.L.
Published: September 2007

A model exploring the effect of the presence of a single hydrogen interstitial on the crack tip configuration of nickel is described. The model is based on a EAM-type potential developed by Daw, Baskes, Bisson, and Wolfer for describing the Ni–Ni, Ni–H, and H–H interactions, and involves the crack tip region of a semi-infinite crack in an infinite solid. Several types of interactions are observed to occur. In a model oriented such that dislocation emission is difficult, hydrogen is observed to increase the crack tip opening displacement (CTOD), exert a force on the crack tip due to interaction between the dilatancy of the defect and the hydrostatic component of the field of the crack, and increase the local tensile stresses. However, the largest contribution to extending the crack derives from the energy released when a hydrogen interstitial escapes to the crack surface. A hydrogen interstitial is also observed to assist dislocation emission in models with an easy emission orientation.
Effect of addition of molybdenum or niobium on creep-rupture properties of Fe3Al
Authors: McKamey C.G. , Maziasz P.J. , Jones J.W.
Published: September 2007

Recent alloy development efforts have shown that Fe3Al-based alloys can have room temperature tensile ductilities of 10–20% and yield strengths of 500 MPa at temperatures to 600 °C. These property improvements are important for enabling the use of iron-aluminides for structural applications that require their excellent corrosion resistance. New data are presented here from creep-rupture studies on Fe3Al and on Fe3Al-based alloys containing molybdenum or niobium plus zirconium. Binary Fe3Al alloys have low creep resistance, but the addition of 2 at. % Mo or 1% Nb plus 0.1% Zr increases the creep life and reduces the minimum creep rate, with the niobium-containing alloy being the strongest. The improvement in creep life is the result of a combination of factors which include grain boundary strengthening, resistance to dynamic recrystallization during stressing, precipitation strengthening, and changes in the formation and mobility of the dislocation network. Correlation of optical, scanning electron, and transmission electron microscopy data suggests that the intergranular creep failure found in Fe3Al after creep testing at 550–650 °C is related to weak high-angle grain boundaries and to formation of subgrain boundary arrays, which reduce the ability of dislocations to glide or multiply to produce matrix plasticity. The addition of niobium/zirconium results in solid solution strengthening effects, as well as the formation of fine MC precipitates (a small amount of carbon is present as a contaminant from the casting process) which strengthen both the matrix and grain boundaries. The result relative to the binary alloy is increased creep-rupture strength and life coupled with a change to a ductile-dimple transgranular failure mode. This suggests that the mechanisms that cause failure during creep can be controlled by macro- and microalloying effects.
Effect of accelerating voltage on planar and axial channeling in ordered intermetallic compounds
Authors: Munroe P.R. , Baker I.
Published: September 2007

The ternary site occupancy of two alloys, vanadium in NiAl + V and hafnium in nickel-rich, boron doped Ni3Al + Hf, was determined by ALCHEMI, or atom location by channeling enhanced microanalysis. Vanadium exhibited a preference for the aluminum sublattice in NiAl, and hafnium preferentially occupied the aluminum sites in Ni3Al. Spectra were acquired over a range of accelerating voltages from 80 kV to 200 kV. Delocalization effects were observed to increase as the accelerating voltage increased, which thus reduces the accuracy of the ALCHEMI data. For NiAl + V, both planar and axial channeling were performed, and delocalization effects were greater for axial channeling, further reducing the accuracy of the ALCHEMI data.
Intercalation of molecular species into the interstitial sites of fullerene
Authors: Assink Roger A. , Schirber James E. , Loy Douglas A. , Morosin Bruno , Carlson Gary A.
Published: September 2007

Molecular species were found to diffuse readily into the octahedral interstitial sites of the fcc lattice of C60. The 13C NMR spectrum of C60 under magic angle spinning (MAS) conditions consisted of a primary resonance at 143.7 ppm and a minor peak shifted 0.7 ppm downfield. The downfield shift obeys Curie's law and is attributed to the Fermi-contact interaction between paramagnetic oxygen molecules and all 60 carbon atoms of rapidly rotating adjacent C60 molecules. Exposure of C60 to 1 kbar oxygen for 1.75 h at room temperature resulted in a spectrum of seven evenly spaced resonances corresponding to the filling of 0 to 6 of the adjacent octahedral interstitial sites with oxygen molecules. The distribution of site occupancies about a C60 molecule provided evidence that the intercalation process is controlled by diffusion kinetics. Exposure to 0.14 kbar hydrogen gas at room temperature for 16 h filled a substantial fraction of the interstitial sites of C60 and C70 with hydrogen molecules.
Solid solutions in the system Ag2S–Ag2Se
Authors: Pingitore N.E. , Ponce B.F. , Eastman M.P. , Moreno F. , Podpora C.
Published: September 2007

Optical, electron microprobe, and x-ray diffraction analysis of 88 samples of various compositions between Ag2S and Ag2Se synthesized at high temperature in sealed quartz tubing indicates the presence of two solid-solution series in this system at ambient (room) conditions. One series extends from Ag2S to approximately Ag2S0.4Se0.7and has the Ag2S-III-type structure (monoclinic). The second series ranges from Ag2S0.3Se0.7to Ag2Se and is characterized by the Ag2Se-II-type structure (orthorhombic). Members of both series, in appropriate proportions, characterize the apparent compositional gap between the two solid solutions. Gradual shifts in the locations of the x-ray diffraction peaks along the compositional gradient of each solid solution revealed an expansion of thed-spacing as the larger Se ion was substituted for S in the Ag2S-III-type structure and a contraction as S was substituted for Se in the Ag2Se-II-type structure. The reported discrete phase, Ag4SSe (aguilarite, orthorhombic), appears to be simply a member of the monoclinic Ag2S-III-type solid solution.
Microstructure of LaB6-base thick film resistors
Authors: Li Z.G. , Carcia P.F. , Donohue P.C.
Published: September 2007

The microstructure of LaB6-base thick film resistors was investigated by cross-sectional transmission electron microscopy. The specimens were prepared by a technique that polished them to a thin wedge, thus avoiding ion-milling and permitting imaging over a distance of tens of microns. The resistor microstructure contained a finely divided electrically conductive phase of TaB2 and nonconducting crystals of CaTa4O11, formed during high temperature processing of glass and LaB6 ingredients of the thick film ink. Using higher surface area ingredients virtually suppressed the formation of CaTa4O11 crystals, and the microstructure became more uniform. Resistors made with higher surface area intermediates also had better voltage withstanding properties.
Effects of intrinsic properties of TiN coatings on acoustic emission behavior at scratch test
Authors: Yamamoto Shuji , Ichimura Hiroshi
Published: September 2007

Acoustic emission (AE) behavior at scratch test has been studied for TiN coatings that deposited in an ion plating system. The AE intensity of the coatings measured by the scratch test depended not only on the adhesion of a coating-substrate interface but also on the intrinsic properties of a deposited film. In this work, a careful separation of coating parameters was investigated to clarify the relationship between the AE intensity and the intrinsic properties of coated films. For identical coating conditions, the AE intensity increased with coating thickness since the elastic energy released during the scratching is proportional to film thickness. It has been found that the coatings prepared at higher substrate temperature and/or at higher substrate bias voltage had a larger crystallite size and lower internal stress. With increasing bias voltage and substrate temperature, the AE intensity of TiN coatings decreased. An explanation for this behavior has been proposed in terms of the internal stress and the toughness of the deposited films.
Effective elastic constants and acoustic properties of single-crystal TiN/NbN superlattices
Authors: Kim Jin O. , Achenbach Jan D. , Shinn Meenam , Barnett Scott A.
Published: September 2007

Using the measured elastic constants of TiN and NbN single crystals with cubic symmetry, the effective elastic constants of single-crystal TiN/NbN superlattice films with tetragonal symmetry, namely c11, c12, c13, c33, c44, and c66 have been calculated for various thickness ratios of the layers. Using a line-focus acoustic microscope, measurements of surface acoustic waves (SAWs) have been carried out on single-crystal TiN/NbN superlattice films grown on the (001) plane of cubic crystal MgO substrates. The phase velocities measured as functions of the angle of propagation display the expected anisotropic nature of cubic crystals. Dispersion curves of SAWs propagating along the symmetry axes have been obtained by measuring wave velocities for various film thicknesses and frequencies. The SAW dispersion curves calculated from the effectiveelastic constants and the effective mass density of the superlattice films show very good agreement with experimental results. The results of this paper exhibit no anomalous dependence of the elastic constants on the superlattice period of TiN/NbN superlattices.
Chemical processing and densification characteristics of lithium aluminosilicate (LAS) gels
Authors: Jang Hyun M. , Kim Kwang S. , Jung Chang J.
Published: September 2007

Two different chemical processing routes were successfully used for the fabrication of lithium aluminosilicate (LAS) specimens having dense and homogeneous microstructure with an essentially pore-free state. These are (i) sol-gel route using the hydrolysis-condensation reaction of metal alkoxides and (ii) mixed colloidal processing route. Lowering Li content in the sol-gel-derived LAS significantly enhanced densification and retarded the crystallization. The β-spodumene (∼0.8 μm) seeding in the sol-gel-derived LAS modified the sequence of phase transformations and lowered the crystallization temperature by 120 °C. Therefore, combining the epitaxial seeding with the sol-gel process, one can bring down the crystallization temperature to the sintering temperature range (∼800 °C). Similarly, the LAS gel prepared by the mixed colloidal processing route exhibited a noticeable shrinkage over a broad temperature range (600–950 °C) and produced a dense sintered body with an essentially pore-free microstructure.
Effects of seeding on phase transformation and mechanical properties in complex-alkoxide-derived cordierite gel powder
Authors: Okuyama Masahiko , Fukui Toshimi , Sakurai Chihiro
Published: September 2007

Cordierite gel powder prepared by the hydrolysis of an alkoxide complex was seeded with crystalline α-cordierite particles. The effects of seeding on the phase transformation and mechanical properties were studied. The unseeded cordierite gel powder crystallized into α-cordierite via μ-cordierite at 1050 °C. The μ-α cordierite transformation caused microcracks which led to poor flexural strengths of about 30 MPa, although the strength of μ-cordierite was 160 MPa. The seeding appeared to promote a direct formation of the α-cordierite from the amorphous state. The development of microcracks due to μ-α cordierite transformation could be prevented by the modified crystallization behavior, resulting in the improved flexural strength of α-cordierite of 190 MPa. The seeding also lowered the crystallization temperature of α-cordierite to 900 °C, which could be attributed to the epitaxial crystallization of the complex-alkoxide-derived cordierite gel powder.
Thin film growth of the 2122-phase of BCSCO superconductor with high degree of crystalline perfection
Authors: Raina K.K. , Narayanan S. , Pandey R.K.
Published: September 2007

Thin films of the 80 K-phase of BiCaSrCu-oxide superconductor having the composition of Bi2Ca1.05Sr2.1Cu2.19Ox and high degree of crystalline perfection have been grown on c-axis oriented twin free single crystal substrates of NdGaO3. This has been achieved by carefully establishing the growth conditions of the liquid phase epitaxy (LPE) experiments. The temperature regime of 850 °C to 830 °C and quenching of the specimens on the termination of the growth period are found to be pertinent for the growth of quasi-single crystalline superconducting BCSCO films on NdGaO3 substrates. The TEM analysis reveals a single crystalline nature of these films which exhibit 100% reflectivity in infrared regions at liquid nitrogen temperature.
Role of pO2 in microstructural development and properties of YBa2Cu3Ox superconductors
Authors: Singh J.P. , Guttschow R.A. , Dusek J.T. , Poeppel R.B.
Published: September 2007

An evaluation of the effects of oxygen partial pressure (pO2) on sintering behavior and the resulting microstructure of YBa2Cu3Ox (YBCO) indicates that sintering kinetics are enhanced at reduced pO2. The density of specimens sintered at 910 °C increased from 79 to 94% theoretical when pO2 was decreased from 0.1 to 0.0001 MPa. It is believed that increase in density with decrease in pO2 is the result of enhanced sintering kinetics, due probably to increased defect concentration, decreased activation energy of the rate-controlling species, and possibly the presence of a small amount of liquid phase. Sintering at 910 °C resulted in a fine-grain microstructure, with an average grain size of ≍4 μm. Such a microstructure results in reduced microcracking. Consequently, strength as high as 191 MPa is achieved. Reduced microcracking may have important implications for developing microstructures with improved critical current density.
TEM studies on the crystal structure of YBa2Cu3O7−x thin films prepared by pulsed laser deposition
Authors: Takahashi H. , Ohata K. , Morishita T.
Published: September 2007

The crystal structures of YBa2Cu3O7−x thin films deposited on MgO(100) substrates have been investigated by cross-sectional transmission electron microscopy (TEM). At a substrate temperature of 670 °C, Cu–O chains become wavy. Stacking faults are formed between the Cu–O chains. Therefore, the domains with c-axis lattice constants of 15 Å, 27 Å, and 39 Å are observed. By applying a biasing voltage of + 300 V, deviation in the c-axis orientation is reduced compared to that at an unbiased condition; even some grain boundaries remain. Near substrate surfaces, deviation of the c-axis direction from the substrate normal increases with a decreasing substrate temperature from 700 °C to 670 °C. Application of a substrate bias voltage of +300 V improved the heteroepitaxial growth of the c-axis oriented YBa2Cu3O7−x.
Preparation of superconducting YBa2Cu4O8 by the sol-gel method
Authors: Fujihara Shinobu , Zhuang Haoren , Yoko Toshinobu , Kozuka Hiromitsu , Sakka Sumio
Published: September 2007

Superconducting YBa2Cu4O8 ceramics were synthesized by the sol-gel method using corresponding metal acetates as starting materials, water as solvent, and tartaric acid under 1 atm oxygen pressure. YBa2Cu4O8 powder was obtained as a single phase by heating the gel at 780 °C for 30 h in flowing oxygen. Tc was 80 K from magnetic susceptibility measurements. Pelletizing and sintering this powder revealed Tc (onset) of 87 K and Tc (end) of 78 K from resistance measurements. YBa2Cu4O8 pellets sintered at 780 °C for 30 h in O2 also consist of a single phase with Tc (onset) of 85 K and Tc (end) of 75 K.
Bi-based high Tc superconducting fibers by melt extraction
Authors: Chang J.J. , Rudkowska G. , Zaluska A. , Rudkowski P. , Ström-Olsen J.O. , Cave J.
Published: September 2007

Bismuth-based high Tc superconductors have been prepared as fibers by a technique of melt extraction. As-made, the fibers are amorphous with diameters ranging from 0.7 μm to 100 μm and lengths of up to 5 cm. The fibers were subsequently transformed into high Tc superconductors by heat treatment in air. Superconducting transitions at 105 K and 82 K were measured in annealed fibers of initial composition Bi1.8Pb0.2Sr2Ca3Cu4Ox by SQUID magnetometry. The volume fractions of superconducting phases were estimated to have lower bounds of 30% for 2212 and 5% for 2223. The crystallization process has been studied by differential scanning calorimetry, electron microscopy, and x-ray diffraction. Crystallization involves first the formation of the Bi-2201 phase and a bcc phase with lattice parameter a = 0.425 nm before finally significant fractions of both the Bi-2212 and Bi-2223 phases are formed.
Transmission electron microscopy of C70 single crystals at room temperature
Authors: Dravid Vinayak P. , Lin Xiwei , Zhang Hong , Liu Shengzhong , Kappes Manfred M.
Published: September 2007

Transmission electron microscopy (TEM) techniques have been employed to study the room temperature solid state form of chromatographically purified C70. Tilting and electron diffraction experiments in three-dimensional reciprocal space, on samples prepared by crystallization from several different solvents, show that C70 crystallites adopt hexagonal close packed (hcp) structure with a = 1.01 ± 0.05 nm and c = 1.70 ± 0.08 nm. The extinctions and observed reflections conform to the P63/mmc space group. High resolution TEM images reveal the molecular order and periodicity associated with C70 crystallites in real space. The experimental results are in agreement with the preliminary computations of crystal structure within acceptable error limits.
Observations on the electrical characterization of the heteroepitaxially grown cubic SiC
Authors: Molnar B.
Published: September 2007

This paper deals with the electrical characterization of thin layers of cubic SiC, grown on (100) Si substrates. The resistivity and Hall coefficient for undoped SiC layers were measured between 10 K and 500 K. The influence of inhomogeneities on the electrical properties of the as-grown films has been established. The Hall data show a clear sign of a transition to impurity band conduction. The donor concentrations studied are in the “intermediate” range. The donor activation energy has been shown to decrease with increasing nitrogen concentration. The nitrogen concentration was measured by SIMS. The variation in nitrogen concentration is also seen in changes in the shape of the ESR spectrum. The presence of nitrogen in the intermediate concentration range is the most likely reason for the conflicting values reported for the donor ionization energy as measured by Hall and PL measurements.
On the growth of Hg2Cl(1–x)2Brx2 single crystals
Authors: Singh N.B. , Metz E.P.A. , Stewart A. , Gottlieb M. , Hopkins R. , Mazelsky R. , Glicksman M.E. , Haley T.E.
Published: September 2007

Single crystals of mixed mercurous halides Hg2X2 (where X = Cl and Br) were grown by the physical vapor transport process. Structural parameters and optical properties were determined to examine the suitability of mixed crystals for acousto-optic devices. Mixed crystals grown in the low concentration region did not show striations or banding. Crystals grown in the high concentration region showed considerable inhomogeneity in the refractive index. Since we had used identical thermal conditions, inhomogeneity was attributed to the solutal convection caused by the difference in densities of mercurous chloride and bromide vapors. A low concentration of dopant did not change the acoustic velocity of the parent components.
The direct observation of structural development during vanadium pentoxide gelation
Authors: Bailey J.K. , Pozarnsky G.A. , Mecartney M.L.
Published: September 2007

The sequence of structural evolution in the gelation of vanadic acid to form vanadium pentoxide gels was studied using cryogenic transmission electron microscopy (cryo-TEM) and scanning tunneling microscopy (STM). Small whiskers form from initially homogeneous solutions, and then grow into crystalline ribbon-like colloidal particles. It is proposed that the whiskers form from polymerization of dioxovanadium cations. The ribbons then grow by continued addition of dioxovanadium cations which are supplied by the decomposition of decavanadate ions. In solution, the ribbon-like particles have dimensions of approximately 25 nm × 3 nm × over 1 μm. These ribbons are flexible perpendicular to the plane of the ribbon. Upon drying, a flat rigid mass of ribbon-like particles is formed. The ribbons examined by STM showed striations 3 nm wide, a value that corresponds with the width of the unit cell proposed by J. Legendre and J. Livage [J. Colloid. and Interf. Sci. 94, 75 (1983)].
Particle size effects on colloidal processing of oxide powders
Authors: Hirata Yoshihiro , Haraguchi Ichiroh , Ishihara Yoshimi
Published: September 2007

Rheological behavior, properties of colloidal solids consolidated by filtration, and their structure change during drying were studied with aqueous suspensions of a mullite powder of nanometer size and two kinds of alumina powders of submicrometer sizes. The rheological properties of non-Newtonian flow suspensions were analyzed by a power law equation of S = K γn, where S is the shear stress, γ the shear rate, and K and n (O ⋚ n ⋚ 1) constants. The critical solids content (Vc) at n = O (indicating colloidal solids) depended greatly on the zeta potential of particles in suspensions, and dominated the densities of dried green compacts (Vg) of submicrometer sized powders. In a nanometer sized powder, the densities of dried green compacts were dependent on both Vc value and the solid contents of suspensions. Phase diagrams of one-component colloidal systems were constructed by plotting the Vc and Vg values against the zeta potential of particles. These phase diagrams indicate that the colloidal solids range (surrounded by Vc and a minimum Vg lines) is narrow for nanometer sized powder and wide for submicrometer sized powder. The solids content range of dried green compacts was very narrow for submicrometer sized powder but relatively wide for nanometer sized powder due to the low flexibility of colloidal structure during drying.
Growth and characterization of Ba2YCu3O7−δ films in reduced oxygen partial pressures using the BaF2 post-annealing process
Authors: Siegal M.P. , Hou S.Y. , Phillips Julia M. , Tiefel T.H. , Marshall J.H.
Published: September 2007

Epitaxial films of Ba2YCu3O7−δ (BYCO), as thin as 250 Å and with Jc's approaching those for the best in situ grown films, can be formed by coevaporating BaF2, Y, and Cu followed by a two-stage anneal. These results extend the work of R. Feenstra et al. [J. Appl. Phys. 69, 6569 (1991)] for film thicknesses >2000 Å. This involves using low oxygen partial pressure [p(O2) = 4.3 Torr] during the high temperature portion of the anneal, which we vary from Ta = 600 to 950 °C. The BYCO melt line is seen to be the upper limit for Ta. The use of lower p(O2) shifts the window for stable BYCO film growth to lower temperature. The lower growth temperature required for the low p(O2) process allows the formation of smooth films with greater microstructural disorder than for films grown in p(O2) = 740 Torr at higher Ta, resulting in higher Jc values by a factor of four. The relationship between the Ta required to grow films with the strongest pinning force and p(O2) is log [p(O2)] ∝ Ta−1, independent of growth method (in situ or ex situ) over a range of five orders of magnitude in p(O2).
Ion beam synthesis of nitride layers in iron
Authors: Vredenberg A.M. , Pérez-Martin C.M. , Custer J.S. , Boerma D.O. , de Wit L. , Saris F.W. , van der Pers N.M. , de Keijser Th.H. , Mittemeijer E.J.
Published: September 2007

Stoichiometric iron nitride layers have been synthesized by high dose, high energy nitrogen implantation into Fe using a two-step implantation process. First, a nitrogen preimplantation at ~100 °C is used to form nitride precipitates. A low temperature is necessary to restrict the nitrogen mobility. Second, 1 MeV implantation at ~300 °C leads to the formation of a stoichiometric γ′–Fe4N layer at the position of the preimplanted N atoms. Growth of this nitride layer proceeds by diffusion of the implanted N through either the α–Fe matrix (for 200 or 500 keV preimplantations) or the nitride layer itself (for 1 MeV preimplantation). During annealing above 350 °C the γ′ layers dissolve in a planar fashion, characterized by an activation energy of 2.3 eV. Phase formation during preimplantation and phase transformations during subsequent annealing or hot implantation can be understood from the thermodynamics for the Fe–N system, while the kinetics of layer growth is influenced by the beam-induced defects. The experiment and model suggest that γ′ is not a thermodynamically stable phase below 310 ± 10 °C and should decompose into α (ferrite) and ∊ nitride.