bluuebunny/crossref_metadata_2025 huggingface dataset:
Microstructural engineering through donor and acceptor doping in the grain and grain boundary of a polycrystalline semiconducting ceramic
Authors: Gupta Tapan K.
Published: September 2007

This paper deals with the concept of microstructural engineering through donor and acceptor dopings within the grain and at the grain boundary of a polycrystalline semiconducting ceramic. These concepts are derived from an analysis of the “prebreakdown” and the “upturn” current-voltage characteristics of a ZnO varistor and from the construction of corresponding defect models as a function of donor and acceptor dopants at the grain and grain boundary. By using Li, Al, and Na as dopants, it is shown that the dopants can be grain or grain boundary specific in the ZnO microstructure and that they can act as donors, acceptors, or both, depending on the nature and concentration of dopants and their location on the host crystal lattice structure. In the case of the ZnO varistor, the grain and grain boundary properties can thus be tuned independently or concurrently by systematic engineering of the entire microstructure through defect dopings that are specific to the grain, grain boundary, or both. Following a detailed analysis of the defect models thus developed for the ZnO varistor, a set of ground rules are proposed for applying these concepts of donor and acceptor dopings at the grain and grain boundary to the general case of microstructural engineering in a polycrystalline semiconducting ceramic.
Pyroelectric characteristics of modified barium titanate ceramics
Authors: Deb K.K. , Hill M.D. , Kelly J.F.
Published: September 2007

BaZrO3and BaHfO3modified polycrystalline BaTiO3are candidate materials for transverse mode and pyroelectric charge coupled device (CCD) arrays. Addition of 3–4% barium zirconate and barium hafnate to barium titanate alters the temperature of phase transformations and with it, dielectric and pyroelectric properties. These additions create a temperature range close to room temperature where the pyroelectric coefficient is extremely high relative to the dielectric permittivity. These materials show a very high figure of merit for dielectric bolometer applications that is competitive with existing materials, while being relatively easy to prepare.
Enhancement of critical current density in YBa2Cu3Oxsuperconductor by mechanical deformation
Authors: Selvamanickam V. , Mironova M. , Salama K.
Published: September 2007

The critical current density of melt-textured YBa2Cu3Oxsuperconductor has been enhanced by mechanical deformation at a high temperature. Hot deformation at 45° to both the slip plane (001) and the slip directions [100]/[010] has resulted in a high density of dislocation loops and stacking faults. The deformed samples are found to exhibit a critical current density(Jc)atHc-axis as high as that atHa-bplane at 1.5 T and 77 K. AJcof 35300 A/cm2has been achieved atHc(1.5 T and 77 K) which is twice as high as that observed in undeformed samples. The enhancedJcin this magnetic field orientation is attributed to pinning by the defects created by mechanical deformation. This pinning mechanism is found to be effective over a wide angle between the magnetic field and thea-bplane and thus results in a marked reduction in the critical current anisotropy.
Extrusion of superconducting wires of YBa2Cu3O7−x
Authors: Ponnusamy D. , Ravi-Chandar K.
Published: September 2007

High Tc superconductors need to be fabricated into desirable configurations such as wires for practical applications. Toward this objective, the plastic extrusion process was applied to the fabrication of wires and rods of YBa2Cu3O7−x. The ceramic oxide powder is combined with several organic additives to form a slurry which is extruded by forcing it through a small aperture. This is subjected to a sequence of heat treatments for drying, binder burn-out, and sintering. The critical current density of these wires ranges from 800 A/cm2 to 1500 A/cm2 depending upon the diameter and the current-carrying capacity is uniform over long lengths of up to 140 cm. This process was also applied to fabrication of larger cross-sectional area rods and hollow cylinders, capable of carrying up to 245 A. The mechanical properties of these wires were also investigated.
Microstructural characterization of reactions in Al–Zr thin film couples
Authors: Mingard K.P. , Cantor B.
Published: September 2007

The mechanisms of reaction in sputter-deposited Al–Zr thin film couples have been investigated using extensive transmission electron microscopy (TEM) observations of cross-sectional specimens prepared by the novel use of an ultramicrotome. The TEM observations greatly facilitated understanding of composition-depth profiles obtained by Rutherford backscattering spectroscopy (RBS). The reaction between Al and Zr in thin films, and the influence of Cu on this reaction, was shown by the TEM to be much more complex than previously reported in studies that largely used RBS. Reaction of Al and Zr in sputter-deposited films occurs in two stages. Initially reaction occurs by growth of an amorphous layer at the Zr interface and is promoted by intermixing of Al and Zr during deposition. Growth of an amorphous layer in an Al-transition metal thin film couple has not been reported previously. Subsequently, the amorphous layer is consumed by growth of microcrystalline Al3Zr from the Zr/reaction layer interface. The transition from growth of amorphous to microcrystalline Al3Zr results in thickening of the reaction layer with an overall growth rate exponent of 1/3. The Al3Zr grows with the metastable L12cubic structure, except in the presence of Cu, when it grows with the tetragonal DO23structure. The fine initial Al and Zr grain sizes limit the influence of Cu on the morphology of the reaction; in all cases, a continuous and uniform reaction layer thickness is observed by TEM.
Atmosphere effects of the amorphization reaction in NiZr by ball milling
Authors: Aoki K. , Memezawa A. , Masumoto T.
Published: September 2007

An intermetallic compound c–NiZr and a mixture of elemental powders of nickel and zirconium [Ni50Zr50 (at. %)] have been mechanically ground (MG) and mechanically alloyed (MA), respectively, using a high-energy ball mill in various atmospheres. The products were characterized by x-ray diffraction, transmission electron microscopy, differential scanning calorimetry, and chemical analysis as a function of milling time. An amorphous a–NiZr alloy was prepared by both MG and MA in an argon atmosphere. By MG of NiZr, an amorphous nitride a–NiZrN0.15 was synthesized in a nitrogen atmosphere, while a crystalline hydride c–NiZrH3 was formed in a hydrogen atmosphere. On the other hand, ZrN and ZrH2 were formed by MA in a nitrogen and a hydrogen atmosphere, respectively. The amorphization reaction was observed between ZrH2 and Ni by further MA in a hydrogen atmosphere, and a mixture of a–NiZrxHy (x < 1) and ZrH2 was obtained. However, no amorphization was observed by MA between ZrN and Ni in a nitrogen atmosphere. The effects of the milling atmosphere on the phase formations during MG and MA are discussed based on the gas absorption rate.
Growth of (110)-oriented diamond films by electron-assisted chemical vapor deposition
Authors: Kumagai Kazuo , Miyata Koichi , Nishimura Kozo , Kobashi Koji
Published: September 2007

Diamond films were deposited on Si substrates by Electron-Assisted Chemical Vapor Deposition (EACVD) using various methane concentrations below 8.1%. It was found that the deposited films were strongly (110)-oriented. This seemed to arise from a high nucleation density of diamond caused by the initial deposition of an amorphous carbon film. A comparison of the graphite etching rate between EACVD and Microwave Plasma CVD (MPCVD) under the standard growth conditions showed that EACVD was able to etch graphite about five times faster than MPCVD. Hence, it was concluded that the differences in the growth rate and morphology between EACVD and MPCVD arise from the different graphite etching rates as well as different chemical species in the reaction gas.
The Y1Ba4Cu2CO3O5.5±δ oxycarbonate—An interesting nonsuperconductor
Authors: Rodriguez Mark A. , Simmins J.J. , Snyder R.L.
Published: September 2007

A compound with the stoichiometry of Y1Ba4Cu2CO3O5.5±δ is formed in the presence of CO2 or carbonate. Analysis techniques including oxygen overpressure, wet chemical analysis, and quenching experiments establish that this material is an oxycarbonate compound. Electrical measurements indicated no superconductive properties in the 10–298 K temperature range. The ease of formation of this compound causes it to commonly appear in synthesis in this region of the phase diagram and accounts for many of the conflicting reports of properties and solid solution formation. XRD shows evidence for the existence of some solid solution in this oxycarbonate. The compound Y1Ba4Cu3O8.5±δ, formed in the absence of CO2, and Y1Ba4Cu2CO3O5.5±δ, formed in the presence of CO2, are the only barium-rich phases seen to exist.
Growth defects in diamond films
Authors: Shechtman D. , Hutchison J.L. , Robins L.H. , Farabaugh E.N. , Feldman A.
Published: September 2007

Growth defects in diamond films grown by plasma-assisted chemical vapor deposition (CVD) were studied by high resolution electron microscopy. Several features of the microstructure were resolved and their importance to the growth of the diamond film was evaluated. The observations included various twin boundaries of the type ∑ = 3, as well as ∑ = 9, ∑ = 27, and ∑ = 81, which form by an interaction of lower order twins. These higher order boundaries are loci of intersection points of growing planes on two adjacent twins and can serve as an indicator for the local crystal growth direction. The central nucleation site for the growing planes in many cases can be traced back to a quintuplet twin point. A twin quintuplet has five re-entrant angles and thus serves as a preferred nucleation site for new planes as the crystal grows.
Fiber strength utilization in carbon/carbon composites
Authors: Zaldivar Rafael J. , Rellick Gerald S. , Yang J.M.
Published: September 2007

The utilization of tensile strength of carbon fibers in unidirectional carbon/carbon (C/C) composites was studied for a series of four mesophase-pitch-based carbon fibers in a carbon matrix derived from a polyarylacetylene (PAA) resin. The fibers had moduli of 35, 75, 105, and 130 Mpsi. Composite processing conditions ranged from the cured-resin state to various heat-treatment temperatures (HTT's) from 1100 to 2750 °C for the C/C's. Room-temperature tensile strength and modulus were measured for the various processing conditions, and were correlated with SEM observations of fracture surfaces, fiber and matrix microstructures, and fiber/matrix interphase structures. Fiber tensile strength utilization (FSU) is defined as the ratio of apparent fiber strength in the C/C to the fiber strength in an epoxy-resin-matrix composite. Carbonization heat treatment to 1100 °C results in a brittle carbon matrix that bonds strongly with the three lower modulus fibers, resulting in matrix-dominated failure at FSU values of 24 to 35%. However, the composite with the 130-Mpsi-modulus filament had an FSU of 79%. It is attributed to a combination of tough fracture within the filament itself and a weaker fiber/matrix interface. Both factors lead to crack deflection and blunting rather than to crack propagation. The presence of a weakened interface is inferred from observations of fiber pullout. Much of the FSU of the three lower modulus fibers is recovered by HTT to 2100 or 2400 °C, principally as a result of interface weakening, which works to prevent matrix-dominated fracture. With HTT to 2750 °C, there is a drop in FSU for all the composites; it is apparently the result of a combination of fiber degradation and reduced matrix stress-transfer capability.
Conversion of single crystal Si(100) to SiC film by C2H2
Authors: Chiu Chien C. , Desu Seshu B.
Published: September 2007

SiC thin films grown from the reaction between acetylene (C2H2) and the Si(100) substrates in a horizontal hot-wall CVD reactor by different procedures were studied using x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The growth of the SiC films was observed from the behavior of Si2p peaks and their plasmons. A SiC thin film with a thickness of 35 Å and having a smooth surface morphology was obtained in C2H2 diluted by H2 at 1050 °C for a period of 60 min. Etch pits and hillocks were observed with increasing reaction time at 1050 °C. For the conversion conducted in C2H2, but in the absence of H2, a SiC monolayer with smooth morphology was obtained at 950 °C for 7 min and defects were observed for longer reaction times at this temperature. Defects were also observed for reaction times as short as 10 s at higher reaction temperatures (e.g., 1000 °C). H2 seems to play a key role in suppressing the formation of defects and the reaction between C2H2 and Si substrate. The formation of defects was correlated to the out-diffusion of Si in the carbonization process.
Synthesis of aluminum hydroxide nanoparticles in spontaneously generated vesicles
Authors: Yaacob Iskandar I. , Bhandarkar Suhas , Bose Arijit
Published: September 2007

Unilamellar vesicles, formed spontaneously by mixing single-tailed cationic (cetyl trimethyl ammonium tosylate, CTAT) and anionic aqueous solutions of (sodium dodecyl benzene sulfonate, SDBS) surfactants have been used as reactors for the aqueous phase precipitation of nanometer sized particles within their inner cores. AlCl3 solution was encapsulated within these vesicles, aluminum ions were replaced with sodium ions in the extravesicular phase, and sodium hydroxide was then added to the extravesicular region. Hydroxyl ions penetrate through the vesicle walls and react with the available aluminum in the intravesicular region to form the product. The morphology and sizes of these particles were examined by transmission electron microscopy, while their phase and crystalline nature were probed by electron and x-ray diffraction. The product particles were nanometer-sized with near spherical morphology. Good control of particle size was achieved by varying the initial concentration of electrolyte. Single particle electron diffraction revealed a symmetric pair of spots, indicating that the particles were either single crystals or polycrystalline with á low number of grain boundaries or defects. Although wide area electron diffraction showed that the product was δ–Al2O3, powder x-ray diffraction revealed that these particles were, in fact, Al(OH)3. It is likely that heating of these nanoparticles by the high energy electron beam in a high vacuum environment causes a phase transformation, resulting in the difference between the electron and x-ray diffraction results. These results represent the first demonstration of precipitation within vesicles produced spontaneously by mixing appropriate ratios of inexpensive single-tailed surfactants, and may potentially make intravesicular precipitation a commercially viable route for making nanometer-sized particles.
The effect of porosity development on the removal of organic vehicle from ceramic or metal moldings
Authors: Matar S.A. , Edirisinghe M.J. , Evans J.R.G. , Twizell E.H.
Published: September 2007

An experimentally verified model, which quantifies the degradation and diffusion of organic vehicle during pyrolysis of a ceramic molding in the shape of an infinite cylinder, has been modified to accommodate the various effects of porosity development as degradation proceeds. Thus, in the first case, a shrinking radius has been taken into account as organic matter recedes. Secondly, the formation of uniformly distributed porosity is considered as organic vehicle is removed. In each of these instances, the model predicts the critical heating rate and temperature at which boiling occurs in the ceramic body, giving rise to internal defects for various cylinder radii.
Raman study of fresnoite-type materials: Polarized single crystal, crystalline powders, and glasses
Authors: Markgraf S.A. , Sharma S.K. , Bhalla A.S.
Published: September 2007

Raman spectra are reported for fresnoite-type [Ba2Ti(Si, Ge)2O8] materials. Single crystal and glasses of the endmember compositions, Ba2TiSi2O8 (BTS) and Ba2TiGe2O8 (BTG), along with crystalline powders along the pseudobinary join, Ba2TiSi−xGe2−xO8, are discussed. The polarized single-crystal Raman spectra of BTS show the vibrational bands of the Si2O7 and TiO5 molecular groups. The v (Si–O–Si) mode is found at 666 cm−1, and is relatively weak. The strongest bands in the BTS single crystal spectra are found at 858 cm−1 and 873 cm−1. These are attributed to the v (SiO3) modes and the stretching of a short Ti–O bond, with mixing between them evident. The BTG single crystal polarized Raman spectra show the vs(Ge–O–Ge) mode at 521 cm−1, and a strong band at 841 cm−1 assigned to the stretch of the short Ti–O bond. The angular dependence of the TO-LO splitting is cataloged for the A1(z) stretching modes. The vibrational characteristics of both BTS and BTG are dominated by the molecular groups, and do not show behavior indicative of a sheet structure composed of TiO5 and (Si, Ge)2O7 groups. Raman investigation of the crystalline powders points to the presence of Si2O7, Ge2O7, and SiGeO7 groups within the nonendmember compositions. The BTG glass spectra show correspondence with the BTG crystalline Raman spectra; the vs(Ge–O–Ge) mode occurs at 518 cm−1 in the glass. Fivefold coordinated titanium may be present in the BTG glass, as revealed by a very strong band at 824 cm−1 in the I spectrum. BTS glass spectra also show some similarity with the BTS crystalline spectrum; the strongest band is found at 836 cm−1 in the I glass spectrum. Through comparison with other titania-silicate glasses, we conclude that the BTS glass has a structure similar to the crystalline phase.
Determination of the dependence of the surface force induced contact radius on particle radius: Cross-linked polystyrene spheres on SiO2/silicon
Authors: Rimai D.S. , Moore R.S. , Bowen R.C. , Smith V.K. , Woodgate P.E.
Published: September 2007

The contact radii arising from surface forces were determined for cross-linked polystyrene particles having radii between 1.5 and 10 μm in contact with SiO2/silicon substrates using scanning electron microscopy. It was observed that the contact radius varied approximately as the particle radius to the 0.42 ± 0.13 power. These results are consistent with the theories that assume plastic response of the materials, such as that proposed by Maugis and Pollock [D. Maugis and H. M. Pollock, Acta Metall. 32, 1323 (1984)] but are inconsistent with the predictions of models which assume only elastic response, such as that of Johnson, Kendall, and Roberts [K. L. Johnson, K. Kendall, and A. D. Roberts, Proc. R. Soc. London A 324, 301 (1971)]. The thermodynamic work of adhesion, calculated from the experimental results, was found to be 0.32 J/m2, which is significantly smaller than that reported previously on a similar system.
Deconvolution of superimposed DTA/DSC peaks using the simplex algorithm
Authors: Speyer Robert F.
Published: September 2007

Mathematical models emulating DSC/DTA traces of fusion and first-order decomposition were developed. A computer program is described that uses the simplex numerical optimization algorithm to determine the coefficients for the model equations that best fit experimental data, by the criterion of least-squared error. Use of the program permitted deconvolution of various superimposed endotherms.
The effect of residual carbon on the superconducting properties of YBa2Cu3O7−x powders
Authors: Masuda Yoshio , Ogawa Rikuro , Kawate Yoshio , Matsubara Kazuo , Tateishi Tsuyoshi , Sakka Sumio
Published: September 2007

Dependence of the contents and the states of residual carbon in YBa2Cu3O7−x powders synthesized by the sol-gel method on heating conditions, and effects of residual carbon on the superconducting properties were investigated. The carbon content of the powder heated at 950 °C for 20 h was reduced to 0.02%. It has been found that a large number of carbonaceous materials in the starting materials that are used in the sol-gel method does not affect the carbon content of synthesized superconducting powders. The volume of diamagnetism was increased and the Tc,onset was raised as the carbon contents were reduced to 0.04%. These properties were further improved by O2–HIP treatment at 500 °C.
A model of thermomechanical fatigue in a lead-base alloy
Authors: Lawson Larry
Published: September 2007

A model for the growth of a grain boundary crack in thermomechanical fatigue is derived for a single-phase or low alloy fcc metal at homologous temperatures near 0.5. Crack growth is hypothesized to proceed through vacancies binding in pairs at the crack tip to impurities or an oxide layer. This model is applied to 97Pb−3Sn, and the results compared with experiment. Good agreement is shown between the model and experiment, especially in predicting the effects of frequency and thermal-mechanical phasing. These effects do not appear to have been previously modeled successfully.
TEM analysis of Co–Gd and Co–Gd multilayer structures
Authors: Bertero G.A. , Hufnagel T.C. , Clemens B.M. , Sinclair R.
Published: September 2007

Sputter-deposited Co–Gd and Cr–Gd multilayer films were studied by high resolution transmission electron microscopy (HRTEM). These two systems are expected to give rise to quite different structures. In good agreement with predictions, the individual Cr and Gd layers were found to be 100% crystalline with sharp interfaces and no noticeable intermixing. Conversely, the Co–Gd multilayer system underwent a solid state amorphization reaction during the deposition itself. This reaction has been found elsewhere to show an asymmetric character with the order of deposition, strong evidence for which is observed in this work.
Patterns of diamond nucleation from the gas phase
Authors: Molinari E. , Polini R. , Tomellini M.
Published: September 2007

On the basis of an analysis of experimental data on nucleation rates performed in the preceding paper in this issue, patterns of the process of diamond nucleation from the gas phase on Si(100) substrates have been drawn. A kinetic scheme is proposed which is apparently able to describe the complex behavior of the system in the temperature range 750 K−1225 K, at (CH4/H2) ratios between 0.5 and 2% and at a total pressure of 100 mbar. The model postulates a distribution function of the nucleation centers over the heats of adsorption of active gaseous species and introduces a cut-off criterion based on the necessary condition that adjacent active sites coexist for times longer than the characteristic time of germ growth. Nucleation centers, active sites, germs, and nuclei have been defined by simple molecular models and their rates of transformation discussed, using available literature data.
Substrate dependence of the crystallinity and mosaic texture of YBa2Cu3O7−δ ultrathin films deposited by laser ablation
Authors: Chin C.C. , Takahashi H. , Morishita T. , Sugimoto T.
Published: September 2007

C-axis oriented ultrathin films with good crystallinity were deposited by laser ablation on both SrTiO3 and NdGaO3 substrates. The full width at the half maximum of the rocking curves of the (005) peak δω of our ultrathin films with thickness d ≤ 150 Å is less than 0.11°, and the χmin of the 150 Å films on SrTiO3 determined by RBS is equal to 8.5%. SrTiO3 and NdGaO3 have different degrees of lattice mismatches of 2% and 0.2%, respectively, with YBa2Cu3O7−δ at the deposition temperature of 700 °C. To study the impact on the growth of YBa2Cu3O7−δ thin films by the lattice mismatch, the thickness dependence of the crystallinity of the films on SrTiO3 and NdGaO3 substrates was compared. δω of the films deposited on SrTiO3 increases slightly from 0.085° to 0.11° as d increases from 50 Å to 150 Å, but δω increases rapidly from d ≍ 150 Å and saturates at 0.35°. In contrast, films on NdGaO3 showed a rather flat thickness dependence of δω which is below 0.1°. We interpreted the rapid increase in δω of the films on SrTiO3 in terms of strain relaxation. Lattice matching is one of the key factors to obtain good crystallinity. The grains of the films on SrTiO3 and MgO are not flat and smooth but have “mosaic texture.” In contrast, the films on NdGaO3 have no mosaic texture. The possible origins of the mosaic texture are also discussed.
New tricks in the rapid quench route to high Tc materials
Authors: Miller A.E. , Nassau K. , Werder D.J.
Published: September 2007

High Tc materials containing Bi form glasses when quenched from their melts. These materials pass through a number of intermediate phases when furnace heated, causing chemical segregation along the way; extended annealing is then required to achieve a chemically uniform product. Chemical uniformity is preserved from the glass directly to a high Tc phase by placing the glass suddenly into a furnace at a temperature in which the final product is stable. In addition, it was found possible to hot press the glass flakes into a compact at a sufficiently low temperature without chemical segregation so that a sudden heating step will achieve the same result in bulk material.
Site selectivities and magnetic moments of V, Cr, and Mn doped in γ–TiAl alloys
Authors: Babu Suresh V. , Pavlovic Arthur S. , Seehra Mohindar S.
Published: September 2007

Magnetic and x-ray diffraction measurements on the system (Ti50Al50)100−xMx (M = V, Cr) are used to determine, respectively, the magnetic moment (associated with the dopants M) and changes in the lattice parameters. In addition, site selectivity by the dopants between the Ti and Al sites in the L10 structure is determined by measuring the changes with x (x = 0.12, 0.33, 0.71, and 1.21 at. % for V and x = 0.17, 0.41, 0.88, 1.14, and 1.91 at. % for Cr) the intensities of the (001) and (110) superlattice peaks. These results are compared with our earlier studies of the Mn-doped system that showed that the Mn substitutes for Ti and carries a moment of 2.3 μB/Mn atom. For V and Cr dopings, μ/atom decreases as x increases, the largest values being 1.01 μB/V and 0.55 μB/Cr at the lowest dopings. At the lowest x, V and Cr occupy the Ti sites, but for higher dopings, Ti and Al sites are occupied with about equal probability and μ approaches constant magnitudes of about 0.3 μB. These results suggest that only Ti occupancy results in a localized moment on the dopant. For Mn and V dopings, the magnitudes of the moments are in good agreement with a recent calculation of Khowash et al.
Microstructural characterization of Al98.5wt. %Si1.0wt. %Cu0.5wt. % on chemical-vapor-deposited W
Authors: Pico Carey A. , Bonifield Tom D.
Published: September 2007

The microstructural and morphological properties of thin (6000 Å) Al98.5wt. %Si1.0wt. %Cu0.5wt. % films on chemical-vapor-deposited tungsten-coated substrates have been characterized as functions of substrate temperature during deposition and a postdeposition sinter. Scanning electron and transmission electron microscopic investigations show these properties can be categorized with respect to the substrate temperature during deposition. The Al98.5wt. %Si1.0wt. %Cu0.5wt. % films deposited on substrates heated at temperatures ≤200 °C are rough and are comprised of rounded grains. For deposition on substrates heated at ≤300 °C, the films are smooth. Large voids and small precipitates (presumably Al2Cu) are present in the films deposited at 400 °C. The films retain their as-deposited texture during a 450 °C sinter. Precipitates and evidence of W interactions occur in the sintered films deposited on the lower temperature substrates. In addition, the shapes of thermal hillocks and mesa-like protrusions that form during the sintering process are influenced by the films' as-deposited morphologies.
The frictional component of the indentation size effect in low load microhardness testing
Authors: Li H. , Ghosh A. , Han Y.H. , Bradt R.C.
Published: September 2007

The role of friction between the microhardness indenter and the test specimen is addressed through the analysis of dry (unlubricated) and lubricated tests on iron by Atkinson and Shi. Quantitative evaluation through a proportional specimen resistance model accurately describes the results. It suggests that friction is a major portion of the observed hardness increase at low test loads, the indentation size effect. The ISE is related to the surface-area-to-volume ratio of the indentation, which is inversely related to the indentation dimension.
Exoelectron emission from unexcited metallic glasses Fe40Ni38Mo4B18 during heat treatment
Authors: Watanabe Yoshihisa , Nakamura Yoshikazu , Kubozoe Tadayoshi
Published: September 2007

In order to elucidate a mechanism for exoelectron emission from metallic glasses during heat treatment, metallic glasses Fe40Ni38Mo4B18, which have a 140 K lower crystallization temperature than the metallic glasses Fe78B13Si9 reported previously, were heated up to 723 K with a constant heating rate of 0.5 K/s under ultrahigh vacuum condition. Although specimens were not excited by ionizing radiations, mechanical treatment, or chemical processes prior to measurements, exoelectrons have been detected for the first heating cycle. However, in the subsequent heating cycles, the counting rate does not exceed the background level. These results agree well with our previous report about metallic glasses Fe78B13Si9, but it is recently found that from the present glasses exoelectrons are emitted from the lower temperature region, and the total number of emitted exoelectrons is larger than that from the previous glasses. An atomic force microscopy (AFM) study reveals many crystallites formed on the surface of the heated specimen, and the spatial density of crystallites on the surface is much higher than that of previous glasses. Both results of exoelectron measurements and AFM observations show that the number of emitted exoelectrons increases with increasing the number of crystallites on the surface. This result implies that exoelectrons are emitted associated with the crystallites formation process on the surface.
Formation and characterization of ohmic contacts to n–AlGaAs using Pd/AuGe/Ag/Au
Authors: Zheng L.R. , Lawrence D.J. , Blanton T.N.
Published: September 2007

Sequential deposition of Pd/AuGe/Ag/Au and rapid thermal annealing at 450 to 550 °C were employed to form a shallow ohmic contact to n–Alx Ga1−x As. Contact resistivity was in the range of (0.7–1) × 10−5 at x = 0.3 to (2–4) × 10−5 ohm-cm2 at x = 0.55. Limited interfacial reaction was verified by ion backscattering and x-ray diffraction techniques and attributed to the stability of Au–Ag solid solutions against underlying substrates. Contacts form at 450 to 500 °C, possibly via solid-state reaction, whereas liquid phase reaction may take place at 550 °C.
Dopant induced modifications in the physical properties of sprayed ZnO:In films
Authors: Goyal D.J. , Agashe Chitra , Takwale M.G. , Bhide V.G. , Mahamuni Shailaja , Kulkarni S.K.
Published: September 2007

Indium-doped zinc oxide (IZO) films were prepared by the spray pyrolysis technique. The effect of gradual incorporation of indium cations on the structural, electrical, and compositional properties of IZO films was studied in detail. It was observed that even a small addition of indium modifies the preferred growth of IZO film from the [002] direction to the [101] direction. Such a modification in growth pattern is a result of more nucleating centers created by indium doping. Indium dopant improves the electrical properties of the films. The carrier concentration depends mainly on the indium dopant level while the mobility is affected by the changes in crystal orientation that take place due to addition of dopants. X-ray photoelectron spectroscopy results show that indium doping does not lead to any stoichiometric changes in the IZO films and the dopant incorporation in the film is linearly proportional to that in the solution.
Energy principle of indentation contact: The application to sapphire
Authors: Nowak Roman , Sakai Mototsugu
Published: September 2007

The recently developed energy principle of indentation mechanics was applied to the continuous indentation test performed on pure sapphire. Three crystallographic planes, M = (10$\overline 1$0), A = (1$\overline 1$10), and C = (0001), have been indented by a symmetrical triangular pyramid (Berkovich). The distinct anisotropic behavior of the indented crystal has been observed for the maximum indentation loads of 1.961 N, 0.686 N, and 0.392 N. The indentation hysteresis loop energy and the related “true hardness parameter” have been determined for various crystallographic orientations, as well as for two different orientations of the indenter. The observed effects have been discussed in terms of the energy principle of indentation with crystallographic considerations. The effective resolved shear stresses for the slip and twinning systems were calculated and applied to the anisotropic indentation behavior. It was concluded that the energy principle is highly recommended for analyzing the data of continuous indentation tests.
Pressurized internal lenticular cracks at healed mica interfaces
Authors: Wan K-T. , Horn R.G. , Courmont S. , Lawn B.R.
Published: September 2007

The equilibrium states of internal penny cracks at interfaces in thin-sheet bodies are investigated. Consideration is given to cracks held open by a center-loading force from an entrapped particle in combination with a uniform pressure from a fixed mass of entrapped gas. A fracture mechanics analysis indicates that under these conditions the cracks are stable, but are amenable to growth from an enhancement in net pressure (increase in internal pressure or decrease in external pressure) or effective particle size. Essential details of the theory are confirmed by experiments on lenticular cracks at healed interfaces in muscovite mica. The results are pertinent to flaw responses in brittle ceramic systems where structural integrity is an issue.
Structure and behavior of metal/ceramic interfaces in Ti alloy/SiC metal matrix composites
Authors: Hall Ernest L. , Ritter Ann M.
Published: September 2007

The structure and mechanical behavior of the fiber/matrix interface in Ti alloy/SCS-6 SiC metal matrix composites were studied. In these composites the interface region consists of a fiber-coating region and a metal reaction zone between the SiC fiber body and the metal matrix. The fiber coating consists of a number of zones or layers which are comprised of cubic SiC particles in a turbostratic carbon matrix. Some ambiguity remains, concerning the number of distinct layers and the size, shape, and density of the SiC particles. The effect of composite fabrication and heat treatment on the coating structure is relatively small. Studies of the metal reaction zone adjacent to the fiber in Ti alloy/SCS-6 SiC MMC's have shown that a number of discrete zones or layers form. Nearest the fiber, a zone of cubic TiC occurs, with increasing grain size with distance from the fiber. Nearest the metal matrix, a zone of Ti5Si3 forms. In high Al content alloys, an intermediate zone forms that consists of Ti2AlC or Ti3AlC. The fiber/matrix interface plays an important role during transverse tensile loading of these composites. The tensile behavior is controlled by debonding at the interface, followed by deformation of the matrix ligaments. Replica observations show that the debonding initiates and propagates within the coating layers, but is not confined to a single layer interface.
Diamond/Al metal matrix composites formed by the pressureless metal infiltration process
Authors: Johnson William B. , Sonuparlak B.
Published: September 2007

Diamond particles are unique fillers for metal matrix composites because of their extremely high modulus, high thermal conductivity, and low coefficient of thermal expansion. Diamond reinforced aluminum metal matrix composites were prepared using a pressureless metal infiltration process. The diamond particulates are coated with SiC prior to infiltration to prevent the formation of Al4C3, which is a product of the reaction between aluminum and diamond. The measured thermal conductivity of these initial diamond/Al metal matrix composites is as high as 259 W/m-K. The effects of coating thickness on the physical properties of the diamond/Al metal matrix composite, including Young's modulus, 4-point bend strength, coefficient of thermal expansion, and thermal conductivity, are presented.
Synthesis of nylon 6-clay hybrid
Authors: Usuki Arimitsu , Kojima Yoshitsugu , Kawasumi Masaya , Okada Akane , Fukushima Yoshiaki , Kurauchi Toshio , Kamigaito Osami
Published: September 2007

It was found that montmorillonite cation exchanged for 12-aminolauric acid (12-montmorillonite) was swollen by ∊-caprolactam to form a new intercalated compound. Caprolactam was polymerized in the interlayer of montmorillonite, a layer silicate, yielding a nylon 6-clay hybrid (NCH). The silicate layers of montmorillonite were uniformly dispersed in nylon 6. The carboxyl end groups of 12-aminolauric acid in 12-montmorillonite initiated polymerization of ∊-caprolactam, and as 12-montmorillonite content became larger, the molecular weight of nylon was reduced. From the result of end-group analysis, carboxyl end groups were more than amino end groups. The difference between the carboxyl and the amino end groups was attributed to ammonium cations (-NH3+) of nylon molecules, because the difference agreed with the anion site concentration of the montmorillonite in NCH. It is suggested that the ammonium cations in nylon 6 interact with the anions in montmorillonite.
Comment on “Transient kinetics of nucleation and crystallization: Part I. Nucleation” [J. Mater. Res. 6, 2091 (1991)]
Authors: Shneidman Vitaly A.
Published: September 2007

Most of the results of a recent paper by Shi and Seinfeld [J. Mater. Res. 6, 2091 (1991)] are documented in literature in a very similar form. Asymptotic formulas of transient nucleation are briefly reviewed and smooth matching of the nucleation and growth regions is discussed.
Etch-delineation of defects in diamond by exposure to an oxidizing flame
Authors: Malta D.P. , Posthill J.B. , Rudder R.A. , Hudson G.C. , Markunas R.J.
Published: September 2007

An experimental study of the etching properties of defects in diamond using propane flame exposure in air is presented. Both natural diamond crystals and polycrystalline diamond films were exposed to a flame for an optimum time of 3–4 s. This process topographically delineates defects in diamond via an accelerated etch rate at defect sites. Using transmission electron microscopy (TEM) to determine the exact nature and density of defects present in the diamond, we have found a direct correlation between topographical delineation observed by scanning electron microscopy (SEM) and the defect structure observed by TEM.
Synthesis of YBa2Cu3O7−xthin films by three-stage oxidation process from metallic Y–Ba–Cu precursors
Authors: Hase Takashi , Kita Ryusuke , Kawaguchi Kenichi , Koga Takeshi , Morishita Tadataka
Published: September 2007

Growth behavior of YBa2Cu3O7-x(YBCO) thin films from metallic Y–Ba–Cu precursors was investigated. The precursors were prepared by coevaporation of the constituent metallic sources under ultra-high vacuum conditions, and they were oxidized in a load lock chamber and a tube furnace. The grain size of YBCO and degree of crystallographic orientation of the films increased with decreasing oxygen partial pressurep(O2) during transformation of the precursor into the crystal close to the thermodynamic stability line of YBa2Cu3O6.0in ap(O2) – 1/Tphase diagram. The growth of YBCO crystal is controlled by the oxygen diffusion along the c-axis direction of the crystal. The diffusion coefficient of oxygen is determined asD= 1.94 × 1010exp(−433 000/RT) (cm2/s).
Effects of silver additions on resistance to thermal shock and delayed failure of YBa2Cu3O7−δ superconductors
Authors: Singh J.P. , Joo J. , Singh D. , Warzynski T. , Poeppel R.B.
Published: September 2007

The effect of silver (Ag) addition on the resistance to thermal shock and delayed failure of YBa2Cu3O7−δ superconducting wires has been evaluated. Resistance to thermal shock was evaluated by measuring the critical-current density (Jc) as a function of the number of thermal cycles for YBCO and its composites with 15 vol. % Ag additions. Composite YBCO-Ag wires show a slower decrease in Jc than does monolithic YBCO as the number of thermal cycles increases. The nature of delayed failure was studied by measuring strength (σf) as a function of loading rate (σ) and evaluating the value of the subcritical crack growth parameter N[σf = Aσ1/(N+1)]. The value of the N parameter was observed to increase slightly from 35 for YBCO to 41 for YBCO-Ag composites. The improved resistance to thermal shock and subcritical crack growth is believed to be due to improvements in mechanical properties (strength and fracture toughness) and thermal conductivity of YBCO as a result of Ag addition.
Microstructure and properties of Cu-rich 123: Part II. Homogeneous copper and high magnetic Jc
Authors: Zhang J.P. , Li D.J. , Boldt C. , Plass R. , Dravid V. , Marks L.D. , Lin C.H. , Eades J.A. , Sodonis A. , Wolbach W. , Chabala J.M. , Levi-Setti R.
Published: September 2007

Copper- and yttrium-rich YBa2Cu3O7 bulk superconductors have been prepared by mixing copper oxide or yttrium oxide in nitric acid and adding the solution to premade stoichiometric YBa2Cu3O7 followed by annealing. In contrast to materials made by mixing oxide powders, both samples contain copper-rich defects spread homogeneously throughout the grains, either small platelet copper oxide precipitates or bundles of planar defects (Cu–O double planes). These materials also show large magnetic hysteresis at 77 K, comparable to the results obtained from decomposed YBa2Cu4O8. This implies that small copper oxide precipitates and bundles of planar defects are strong flux pinners, and indicates a processing route to producing large amounts of strongly intragranular pinned superconductors. However, the materials also show clean grain boundaries, so an equally valid interpretation is that there is a substantial component of intergranular superconductivity in field, enhancing the effective circuit size to a value far larger than the grain size.
Doping effects on coarsening of Y2BaCuO5 phase in liquid
Authors: Izumi Teruo , Nakamura Yuichi , Shiohara Yuh
Published: September 2007

The coarsening rate of Y2BaCuO5 (211) phase in liquid was investigated by observations of samples which were held at 1070 °C for different holding times. The 211 phase formed rod-like shapes, and the diameters increased with the increase in the holding time. The average diameter increased in proportion to t1/3 (t, holding time). This means that coarsening can be explained by the modified Ostwald ripening theory. Platinum-doping suppressed the coarsening rate and alumina-doping promoted it. However, both coarsening rates were also proportional to t1/3. The changes in the rate can be represented by the changes of the DLΓ value (DL, diffusivity in liquid and Γ, Gibbs—Thomson coefficient) using the Ostwald ripening theory; i.e., the value is made lower by platinum-doping and higher by alumina-doping.
The influence of mill energy and temperature on the structure of the TiNi intermetallic after mechanical attrition
Authors: Yamada Kenjiro , Koch Carl C.
Published: September 2007

Mechanical attrition of intermetallic compound TiNi powder was carried out in two different ball mills and as a function of milling temperature. The microstructural changes with milling time were followed by x-ray diffraction, TEM, and DSC. The more energetic Spex shaker mill provided a higher degree of lattice strain and rapidly refined the grain size to the nanometer size regime. Amorphization was observed in the Spex mill with a linear increase in the milling time for amorphization with increasing milling temperature. No amorphization was observed in the less energetic vibratory mill, and the grain size saturated to a constant value of 15 nm after ≥60 h of milling. A critical grain size for the amorphization of 4–5 nm was estimated from the temperature dependent studies in the Spex mill. The grain boundary energy (706 mJ/m2), estimated from the vibratory mill experiments, and the above critical grain sizes (5 nm) for amorphization were used to calculate the enthalpy supplied by the nanocrystalline grain boundaries. The calculated value of 4.1 kJ/mol was comparable to the measured enthalpy of crystallization of 3.2 kJ/mol. It is concluded that the nanocrystalline grain boundary energy is responsible for driving the crystalline-to-amorphous phase transformation induced by mechanical attrition in TiNi.
Domain growth of Dy2O3 buffer layers on SrTiO3
Authors: Catana A. , Locquet J-P.
Published: September 2007

Dy2O3 layers have been grown on SrTiO3 by molecular beam epitaxy. X-ray and electron diffraction patterns clearly show that Dy2O3 grows epitaxially on SrTiO3 with {100} planes parallel to the substrate surface. Transmission electron microscopy reveals that the Dy2O3 film breaks up into small domains (10–40 nm). This leads to the formation of terraces which limits the structural perfection of thin overgrown DyBa2Cu3O7 by introducing steps and small misorientations (within 3°). The resulting surface corrugation does not preclude the growth of epitaxial c-axis DyBa2Cu3O7 films with a Tc0 of 86 K. Crystallographic analysis and image calculations show that the domain growth of Dy2O3 is associated with the formation of 90° rotation twins.
Microstructural characterization, mechanical properties, and corrosion resistance of the CaO–TiO2system
Authors: Maiya P.S. , Russell R. , Poa D.S. , Wagh A.S. , Poeppel R.B.
Published: September 2007

Ceramic materials with chemical compositions of CaO–Xmole % TiO2, whereXranges from 3 to 40, have been prepared for the evaluation of microstructures, mechanical properties, thermal shock resistance, resistance to hydration in air, and resistance to corrosion in molten salt saturated with calcium. When the TiO2content is less than 40.3 mole %, the CaO forms two-phase microstructures consisting of CaO and Ca3Ti2O7, and the composition CaO−40.3 mole % TiO2corresponds to the (single-phase) compound Ca3Ti2O7. The data on mechanical properties suggest that the single-phase material has better flexural strength and fracture toughness than do the two-phase materials. It is also demonstrated that the stabilizing effect of TiO2on CaO imparts resistance to atmospheric hydration. Both scanning electron microscopy and energy-dispersive x-ray analysis of the specimens exposed to molten CaCl2−20 mole % CaF2saturated with calcium at 900 °C show that corrosion resistance increases with an increase in TiO2content, but that corrosion does occur in all materials. On the basis of experimental observations and thermodynamic considerations, the degradation of Ca3Ti2O7is shown to be associated with the reduction of Ca3Ti2O7by calcium to form CaO and metallic titanium followed by the dissolution of the reaction product CaO in the molten salt.
Low-temperature chemical routes to formation and IR properties of lanthanum sesquisulfide (La2S3) ceramics
Authors: Kumta Prashant N. , Risbud Subhash H.
Published: September 2007

Lanthanum sulfide (La2S3) exists in three different allotropic forms in the crystalline state. The low-temperature orthorhombic phase (α) transforms to the intermediate tetragonal phase (β) which further undergoes transformation to the high-temperature cubic phase (γ). The intermediate tetragonal phase (β) is an oxysulfide with possible use as a phosphor, and the cubic phase (γ) of La2S3 shows potential for application as a far IR (8–14 μm) window material. We report metal-organic synthesis routes for the preparation of the β- and γ-phases of La2S3 by processing modifications and choice of precursor chemistry. The alkoxide-derived precursors and the transformed sulfide powders were characterized for morphology, microstructure, and thermal stability. Phase evolution studies were also conducted, and the results are discussed in the context of formation of the β- and γ-phases. Results of densification, microstructure, and infrared transmission characteristics are also presented.