bluuebunny/crossref_metadata_2025 huggingface dataset:
Stoichiometric effects in epitaxial Ba2−xY1−yCu3−zO7−δ thin films on LaAlO3(100)
Authors: Carlson Douglas J. , Siegal Michael P. , Phillips Julia M. , Tiefel T. H. , Marshall J. H.
Published: September 2007

Stoichiometric deviations of up to ±5% in Ba2YCu3O7−δ, thin films grown by coevaporation on LaAlO3(100) substrates are found to cause (1) a decrease of the critical current density (Jc) of up to an order of magnitude, (2) a depression of the critical temperature (Tc) and a broadening of the superconducting transition width (ΔT), (3) a deterioration of the surface morphology, and (4) a decrease in the crystallinity of the films. The data indicate that composition deviations of greater than ±1% result in degradation of film quality. These findings have significant implications for the degree of composition control required during deposition to produce films with optimized properties.
Relative stabilities of L12 and DO22 structures in ternary MAl3-base aluminides
Authors: Carlsson A. E. , Meschter P. J.
Published: September 2007

The effects of additions of Fe and Zn on the relative stabilities of the cubic L12 and tetragonal DO22 structures in TiAl3- and NbAl3-base alloys are evaluated using ab initio electronic band calculations. The Fe or Zn distribution on the aluminum sublattice is modeled by a periodic array which corresponds to alloying additions of 12.5 or 25 at.% Fe or Zn. Addition of 12.5 at.% Fe is sufficient to stabilize the L12 structure in both TiAl3- and NbAl3-base alloys. Interpolation of the structural energy differences suggests that the DO22 and L12 structures have the same energy at ∼4.5 at.% Fe in Ti(Al, Fe)3, in agreement with experiment,1,2 and at ∼11 at.% Fe in Nb(Al, Fe)3. The L12 stabilization effect per atom of Zn in Nb(Al, Zn)3 is approximately half as large as that of Fe. The stable structure is the one for which the Fermi energy lies in a minimum in the density-of-states (DOS) distribution, as in the binary compounds. The results are explained on the basis of a simple model involving charge transfer to alloying additions on the Al sublattice.
Phase formation during reactive molybdenum-silicide formation
Authors: Doland C. M. , Nemanich R. J.
Published: September 2007

Silicide formation due to thermal treatment of thin (5–10 nm) molybdenum films on single-crystal, polycrystalline, and hydrogenated amorphous silicon substrates in the temperature range of 100 to 1000 °C was studied, with an emphasis on the initial interactions. The molybdenum deposition, annealing, and characterization using Raman scattering and Auger electron spectroscopy was carried out in UHV in order to minimize the effects of contaminants. Raman spectroscopy is used to distinguish between tetragonal (t-MoSi2) and hexagonal MoSi2 (h-MoSi2). The Raman spectrum of bulk tetragonal MoSi2 exhibits two prominent lines which are associated with the A1g (325 cm−1) and Eg (440 cm−1) modes. The only silicide phases detected in the thin film experiments were t-MoSi2 and h-MoSi2. While hexagonal MoSi2 does not appear in the bulk phase diagram, it is the first silicide phase formed in thin film reactions at a temperature between 300 and 400 °C. The nucleation temperature of h-MoSi2 was the same for Si〈100〉, Si〈111〉, and amorphous Si. Indirect evidence for disordered intermixing of silicon and molybdenum before nucleation of h-MoSi2 is found. Annealing at approximately 800 °C causes the silicide to transform from the hexagonal phase to the tetragonal phase for all substrates. Contaminants interfere with the formation of h-MoSi2 and also retard the transformation of h-MoSi2 to t-MoSi2. For the thin films considered here, the transformation to t-MoSi2 is accompanied by islanding of the silicide film. A lower interfacial energy between the silicon and silicide for h-MoSi2 has been proposed to explain the nucleation of h-MoSi2 before t-MoSi2.
Investigation of the structure and stability of the Pt/SiC(001) interface
Authors: Bermudez V. M. , Kaplan R.
Published: September 2007

Auger electron spectroscopy and low energy electron diffraction have been applied to the study of the structure and thermal stability of the Pt/β–SiC(001) interface. The morphology of the interface appears to be governed by the competition among surface diffusion, intermixing, and chemical reaction. An ultrathin Pt layer (8 Å thick) deposited on a substrate at low temperature is laterally uniform with some degree of intermixing across the interface. Brief anneals at 1000 °C result in aggregation of the Pt into islands interspersed with essentially bare SiC. Higher temperatures lead to reaction of the aggregated Pt to form Pt silicide and release free C. The reaction is signaled by characteristic changes in the Si LVV and C KLL Auger line shapes and by the appearance in LEED of a (2 ⊠ 2) pattern (believed to arise from ordered PtSi) and of diffraction rings from oriented polycrystalline graphite. Subsequent deposition of Si and annealing leads to regeneration of SiC by reaction with the free C. These results contrast with those for ultrathin Pt on Si(001) and on α-SiC(0001) which are dominated by the rapid indiffusion of Pt during annealing. A detailed model is presented for the growth and annealing dependence of the Pt/β–SiC(001) interface.
Oxygen adsorption and VDR effect in (Sr, Ca)TiO3−x based ceramics
Authors: Nakano Yoshitaka , Ichinose Noboru
Published: September 2007

The relation between the oxygen adsorption and the voltage dependence of the resistor (VDR effect) in (Sr, Ca)TiO3−x based ceramics has been investigated. The nonlinearity of the voltage-current characteristics increased with increasing the barrier height, which is thought to be generated by the oxygen chemisorption. Acceptor type trap levels were detected by means of a zero biased DLTS technique at high temperatures. These interfacial energy levels changed with reoxidizing temperatures, and the change can be explained by the degradation of the chernisorbed oxygen. The high temperature type of the chemisorbed oxygen as O2− and O is relatively stable due to the strong pinning effect of trapped electrons, with reoxidizing anneals of grain surfaces above the oxidation temperature, and it contributes greatly to the VDR effect. It is concluded that energy barriers are caused by the interface states generated by the chemisorbed oxygen on grain surfaces and that they determine the VDR effect.
Preparation of fine, hollow, spherical NiFe2O4 powders
Authors: Gadalla Ahmed M. , Yu Hsuan-Fu
Published: September 2007

Aerosol droplets of a dilute aqueous solution of Ni(II) and Fe(III) nitrates were used to synthesize fine nickel ferrite powders by pyrolysis at temperatures ranging from 450 °C to 810 °C. The aerosol product was characterized by thermal analysis, x-ray diffraction, scanning electron microscopy, and BET surface area determination. At low temperature (480 °C), very active fine, hollow, noncrystalline spheres were formed. As the pyrolysis temperature increased, sintering occurred, larger particles with broader size distribution were obtained, and a higher degree of crystallinity of nickel ferrite was produced. The products at pyrolysis temperatures of 660 °C and above exhibited magnetic properties. The powders prepared from the aerosolization process at low temperatures had high reactivity and crystallized rapidly. The effect of the initial solution concentration was also studied, and dilute solutions produced spheres with fine size and narrow size distribution.
Vacancy diffusion along twist grain boundaries in copper
Authors: Nomura Miki , Lee Sing-Yun , Adams James B.
Published: September 2007

Vacancy diffusion along two different high-angle twist grain boundaries (Σ5 and Σ13) was studied using the Embedded Atom Method (EAM). Vacancy formation energies in all the possible sites were calculated and found to be directly related to the degree of coincidence with the neighboring crystal planes. Optimal migration paths and migration energies were determined and found to be very low. The activation energies for self-diffusion at the boundaries were found to be less than half of the bulk value.
Atmospheric pressure chemical vapor deposition of aluminum nitride thin films at 200–250 °C
Authors: Gordon Roy G. , Hoffman David M. , Riaz Umar
Published: September 2007

The atmospheric pressure chemical vapor deposition of aluminum nitride coatings using hexakis(dimethylamido)dialuminum, Al2(NMe2)6, and ammonia precursors is reported. The films were characterized by ellipsometry, transmission electron microscopy, x-ray photoelectron spectroscopy, and Rutherford backscattering spectrometry. The films were deposited at 200–250 °C with growth rates up to 1000 Å/min. They displayed good adhesion to silicon, vitreous carbon, and glass substrates and were chemically inert, except to concentrated hydrofluoric acid. Rutherford backscattering analysis showed that the N/Al ratio ranged from 1.1 to 1.2. Refractive indexes were 1.8–1.9. The films were smooth and amorphous by transmission electron microscopy.
Recovery and recrystallization of the deformed, orderable alloy (Co78Fe22)3V
Authors: Cahn R.W. , Takeyama M. , Horton J.A. , Liu C.T.
Published: September 2007

An alloy of composition (Co78Fe22)3V, which orders to an L12 superlattice below a critical temperature (Tc) of 910 °C, was rolled to 25–50% reduction in the initially ordered condition and annealed at various temperatures above and below Tc and examined by hardness, tensile testing, optical and electron microscopy and dilatometry, in order to study the progress of recovery and recrystallization. Recrystallization was severely retarded on annealing below Tc; close to Tc, recrystallization was ≈ 300 × slower in the ordered than the disordered range. Although recrystallization started promptly, predominantly at grain boundaries, very rapid recovery-softening of the unrecrystallized regions progressively reduced the driving force for recrystallization and slowed it down drastically. However, at 770°and 500 °C, recovery-softening was replaced by some recovery-hardening (i.e., strain-age hardening). Above Tc, recrystallization was complete in a few seconds and a special annealing method was needed to measure such times accurately. Dilatometric measurements showed that most of the order destroyed by rolling was restored long before recrystallization began, but the restoration was never complete unless the alloy was heated up through Tc and then slow cooled. Electron microscopy showed no sign of any antiphase domains in recrystallized grains except for a few isolated domain boundaries on annealing at 770 °C. A model is proposed to rationalize the incidence of recovery-softening or strain-age hardening at different annealing temperatures.
Properties and microstructure of tungsten films deposited by ion-assisted evaporation
Authors: Roy R.A. , Petkie R. , Boulding A.
Published: September 2007

The modification of film properties in evaporated tungsten was studied as a function of deposition environment. Using concurrent argon ion bombardment of the growing film, the stress varied in the same manner at all ion energies and substrate temperatures. Initial increases in tensile stress are followed by a monotonic trend toward compressive stress, for all sets of films. On the other hand, the qualitative changes in film resistivity with concurrent bombardment were dependent on the ion energy and substrate temperature, showing increases at high temperature and energy and decreases at low temperature and energy. Changes in the microstructure and impurity content in deposited films were found to be strongly linked to stress and resistivity changes. The trend toward compressive stress induced by high levels of ion bombardment is primarily reflected in an increase in (110) orientation. Increased resistivity is related to decreased grain size, increased (110) texture, and increased levels of film argon and oxygen content. By choice of deposition conditions, both the resistivity and stress can be minimized.
Characterization of composite carbon coatings deposited by dc cathodic arc technique
Authors: Rother B. , Siegel J. , Breuer K. , Mühling I. , Deutschmann S. , Vetter J. , Trommer G. , Rau B. , Heiser C.
Published: September 2007

Carbon films deposited by a modified dc cathodic arc technique were characterized by several analytical methods. The coatings consist of two constituents originating from process characteristics of the technique applied. Small fragments of graphite are embedded in a matrix material of an amorphous structure with diamond-like short-range order. The coatings exhibit friction and wear reducing properties.
Phase change materials for thermal stabilization of composite thermistors
Authors: Brodeur Sheri A. , Huebner Wayne , Runt James P. , Newnham Robert E.
Published: September 2007

The objective of this investigation was to develop a triphasic PTC thermistor composite which incorporated a phase capable of absorbing heat at a critical temperature, and thus limiting deleterious effects associated with thermal runaway. The system chosen for study was pentaerythritol incorporated into a carbon black–polyethylene thermistor system. Pentaerythritol exhibits a first order tetragonal to cubic phase transition at 185 °C, with a 1.87 to 3.18 J/°C · g change in specific heat and a 425 J/cm3 heat of transition. Composites with room temperature resistivities as low as 0.1 Ω · m, a PTCR effect of up to six orders of magnitude, and reproducible temperature-cycling behavior were developed. The pentaerythritol introduced thermal delays up to 7 min at 185 °C and substantially increased the electrical and mechanical stability of the composites with temperature and voltage cycling. High fields imparted irreversible effects in these composites as reflected by an increase in the room temperature and high temperature resistivity.
Processing tetramethylammonium-carbonate-coprecipitated slurries to obtain small-particle-size YBa2Cu3O7
Authors: Spencer Nicholas D. , Peders Thorvald S. , Baer Matthew B.
Published: September 2007

The effect of different drying and calcination methods on the ultimate particle size of YBa2Cu3O7 (Y-123) has been investigated. The starting material was a tetramethylammonium (TMA) carbonate-precipitated slurry. Spray-drying the slurry after filtering and reslurrying (to remove residual TMA) was most effective in the ultimate formation of finely divided Y-123. The morphology of the spray-dried powder could be preserved by calcining in very low total pressures of flowing oxygen. When a slurry with 0.16% solids content was spray dried, and this powder calcined at 750 °C in 2 Torr of flowing oxygen, a Y-123 powder of mean particle size 0.74 μm (66% submicron) was obtained.
Fabrication of YBa2Cu3O7−x superconducting bulk with aligned microstructure by sintering technique
Authors: Su S.R. , O'Connor M. , Levinson M.
Published: September 2007

By controlling the pH of mixed nitrate solutions, we have produced single-phase YBa2Cu3O7−x platelet particles with an average size of 2.5 μm. The kinetics of superconducting phase formation and the thermal properties of the related phases were systematically studied. Through this understanding, we have developed sintering techniques which produce YBa2Cu3O7−x with wide areas of highly aligned grains at 1010 °C. By incorporating aligned microsized particles into the matrix of this reactive precursor powder as nucleation sites, samples with large randomly oriented domains can be formed at 980 °C.
Aerosol processing of YBaCuO superconductors in a flame reactor
Authors: Zachariah Michael R. , Huzarewicz Serge
Published: September 2007

Submicron YBa2Cu3O7−δ particles have been produced in an aerosol flame reactor by a spray pyrolysis method in which an aerosol composed of an aqueous solution of Y, Ba, and Cu nitrate salts was introduced into a flame. It was observed that the configuration of the flame is particularly important for the successful application of the method. A premixed flame gave particles with an x-ray diffraction pattern resembling the XRD spectra of materials exposed to high concentrations of water vapor. An over-ventilated diffusion flame configuration produced the desired result: submicron unagglomerated, solid crystalline particles with Tc = 92 K.
Cu deposition on rough ceramic substrate: Physical structure, microstructure, and resistivity
Authors: Bai P. , McDonald J.F. , Lu T-M.
Published: September 2007

Cu films with a thickness around 3.5 μm have been deposited on rough Al2O3 ceramic substrates by the partially ionized beam deposition technique. While the ion bombardment parameters are similar for all depositions, the substrate temperature during deposition is varied from 50 °C to 300 °C. Physical structure of the films is studied by SEM in cross-sectional and surface geometries. X-ray diffraction 2θ scan is performed to obtain information on the microstructure of the films, such as (111) fiber texture and average crystallite size. Resistivity of the films is also measured. It has been found that the physical structure of the films varies from a typical columnar structure at 50 °C to a completely noncolumnar structure at 300 °C. The XRD results show that the films are polycrystalline and have different degrees of 〈111〉 preferred orientation, depending on the substrate temperature. The average crystallite size increases with the increase of substrate temperature. No correlation between the physical structure and microstructure of the films is observed. The resistivity of the films was also seen to change as a function of the substrate temperature. This can be explained by quantitative models in which grain and columnar boundaries in the films are responsible for the decrease in electric conductivity of the films.
Properties of the liquid-vapor interface of fcc metals calculated using the embedded atom method
Authors: Holzman L.M. , Adams J.B. , Foiles S.M. , Hitchon W.N.G.
Published: September 2007

The Embedded Atom Method (EAM) is used to compute density, internal energy, and structure factor for bulk liquids of the fcc metals at several temperatures above and below the melting temperature. The calculated values are found to be in generally good agreement with experiment, although the volume expansion upon melting does differ by up to 50% from the expected result for some of the elements studied. The total energy of a liquid system with surfaces is calculated, and the results are compared with the bulk liquid results to determine the enthalpy and thickness of the liquid-vapor interface. Also, the surface tension is found for Cu near the melting temperature. The EAM values for surface enthalpy and surface tension are found to be smaller than experimental values, which is consistent with results for EAM calculations of the surface energy of crystalline solids.
Diffusion barrier development for fiber-reinforced Ni3Al composites
Authors: Brennan P.C. , Kao W.H. , Katzman H.A. , Yang J-M.
Published: September 2007

An alumina (Al2O3) diffusion barrier coating to inhibit the interfacial reactions between boron-carbide-coated boron (B4C/B) fibers and a nickel-aluminide (Ni3Al) (IC-221) matrix was investigated. The alumina diffusion barrier was deposited on the B4C/B fibers using chemical vapor deposition. Also, Saphikon single-crystal Al2O3 fibers were used to demonstrate the compatibility between Al2O3 and Ni3Al. The detailed microstructures and chemical compositions of the fibers, coating, and matrix before and after various thermal exposures were analyzed using scanning electron microscopy, energy dispersive x-ray analysis, and ion microprobe mass analysis. The interfacial reaction products present after 6 h at 980 °C were characterized, and Ni was found to be the dominant diffusion species. The alumina diffusion barrier shows promise for effectively inhibiting the deleterious reactions between B4C/B fibers and the Ni3Al matrix. The uncoated B4C/B fibers were consumed by the matrix after fabrication alone, whereas the Al2O3 coated fibers demonstrated resistance to the matrix for 25 h at 880 °C and 6 h at 980 °C. The Saphikon fiber-reinforced Ni3Al composites demonstrated excellent compatibility after 50 h at 1000 °C. Zirconium (Zr)-rich precipitates on the order of 2 μm in diameter formed at the fiber interface after this exposure, but no gross reaction was indicated between the fiber and the matrix.
Rapid solidification of Ni3Al by Osprey deposition
Authors: Morris D.G. , Morris M.A.
Published: September 2007

A Ni3Al-based alloy containing Cr is examined after preparation by Osprey deposition. The microstructure consists of solidified spherical regions showing cellular segregation interspersed with regions of finer, equiaxed segregation morphology. The segregation structure is characterized by cell interiors rich in aluminum and poor in chromium, while the cell walls are poor in aluminum and rich in chromium. This segregation pattern is the inverse of that expected, based on earlier melt spinning experience, and is explained in terms of the undercooling of the melt prior to solidification. Very high temperature annealing is required to homogenize the material, despite the use of this rapid solidification process for material fabrication.
Magnetic domains around annealed shear bands and embrittlement of FeBSi metallic glass
Authors: Lakshmanan V. , Li J.C.M.
Published: September 2007

Magnetic domains in the vicinity of shear bands produced by bending were investigated to study the effect of furnace annealing on the domain structures. Domain wall spacings were measured as a function of annealing time at several temperatures. The same specimens were also used to evaluate the degree of embrittlement by measuring the bending fracture strains.
A MAS NMR investigation of aluminosilicate, silicophosphate, and aluminosilicophosphate gels and the evolution of crystalline structures on heating the gels
Authors: Prabakar S. , Rao K.J. , Rao C.N.R.
Published: September 2007

Gels of various composition containing SiO2, Al2O3, and P2O5 have been investigated by employing high resolution magic-angle-spinning (MAS) 27Al, 29Si, and 31P NMR spectroscopy. Changes occurring in the NMR spectra as the gels are progressively heated have been examined to understand the nature of structural changes occurring during the crystallization of the gels. 27Al resonance is sensitive to changes in the coordination number even when the Al concentration is as low as 1 mol%. As the percentage of Al increases, the hydroxyl groups tend to be located on the Al sites while Si remains as SiO4/2 (Q4). Mullite is the major phase formed at higher temperature in the aluminosilicate gels. In the case of the silicophosphate gels, Si is present in the form of Q4 and Q3 species. There is a change in the coordination of Si from four to six as the gel is heated. The formation of six-coordinated Si is facilitated even at lower temperatures (∼673 K) when the P2O5 content is high. The phosphorus atoms present as orthophosphoric acid units in the xerogels change over to metaphosphate-like units as the gel is heated to higher temperatures. In aluminosilicophosphates, Si is present as Q4 and Q3 species while P is present as metaphosphate units; Al in these gels seems to be inducted into the tetrahedral network positions.
X-ray fluorescence technique for detection of SiO2 in renal calculi
Authors: Singh V.R. , Agarwal R.
Published: September 2007

The presence of silicon dioxide (SiO2) in the renal calculi is detected, for the first time, by using an x-ray fluorescence technique.
Theory of capacitive probe method for noncontact characterization of dielectric properties of materials
Authors: Tewary V.K. , Heyliger P.R. , Clark A.V.
Published: September 2007

The capacitive probe method for noncontact characterization and monitoring of dielectric materials is analyzed theoretically. An analytical method based upon the Hilbert transform technique and a numerical method using the finite element technique for calculating the potential distribution and change in admittance of the probe caused by presence of the dielectric material as a function of liftoff (distance between the probe plane and the surface of the dielectric material) are described. The two methods are compared with each other and their relative advantages discussed. The possibility of extracting useful information about the dielectric constant of the material from experimental data is also discussed in the light of the proposed theory.
Reactive coevaporation of YBaCuO superconducting films
Authors: Matijasevic V. , Rosenthal P. , Shinohara K. , Marshall A.F. , Hammond R.H. , Beasley M.R.
Published: September 2007

Growth conditions for YBaCuO thin films are investigated. Films have been made by reactive e-beam coevaporation using three metal sources. In the best cases, as-made films are superconducting with Tc's (R = 0) up to 90 K and Jc's (at 4.2 K) above 107 A/cm2. Oxygen pressure, deposition temperature, as well as compositional dependencies of the films are presented. It is found that in conditions of lower oxygen, pressure films with average composition off the 1–2–3 stoichiometry have higher Tc's. For pressure <10 mTorr, the highest Tc obtained is for Ba/Y deposition ratio ⋚1.4. The morphology and impurity phases of these films are examined. The Ba-deficient films have oriented CuYO2 and CuO as the dominant impurity phases. C-axis lattice parameters (c0) are also examined. It is found that for a given Tc, films made at lower pressure have c0's which are expanded compared to the films made at higher pressures (>100 mTorr). The expanded c0's for these films cannot be reduced by a low temperature oxygen anneal. We suggest that metal-atom point-like defects are quenched into these films and we discuss a particular Ba-for-Y substitution model.
LFZ growth of (Bi, Pb)–Sr–Ca–Cu–O superconducting fibers
Authors: de la Fuente G.F. , Navarro R. , Lera F. , Rillo C. , Bartolomé J. , Badía A. , Beltrán D. , Ibáñez R. , Beltrán A. , Sinn E.
Published: September 2007

Powder x-ray diffraction, d.c. and a.c. susceptibilities, and SEM have been used to study (Bi1−xPbx)2Sr2Ca2Cu3O10−δ fibers grown by the Laser Floating Zone method. The well-oriented, long-grained superconductor fiber properties are shown to be highly dependent on the partial pressure of oxygen in the growth atmosphere, as well as on fiber pulling rate. Slowly grown fibers contain initially the 2212 (80 K) phase; the 2223 (110 K) phase also appears upon annealing in air. Faster growth rates result in fibers that contain a mixture of the 2212 and 2201 phases and, in this case, long annealing procedures are necessary to observe the 2223 phase.
Deposition characteristics and microstructures of the high Tc superconducting Y1Ba2Cu3O7−δ thin films prepared by MOCVD
Authors: Kim Sang Ho , Cho Chang Hyun , No Kwang Soo , Chun John S.
Published: September 2007

Superconducting Y1Ba2Cu3O7−δ films were prepared on the MgO (100) and SrTiO3 (100) single crystals using metal organic chemical vapor deposition (MOCVD) of β-diketone metal chelates of Y(thd)3, Ba(th)2, and Cu(thd)2. The evaporation kinetics of Y(thd)3, Ba(thd)2, and Cu(thd)2 and the ratios of deposited to evaporated mole percents of Y, Ba, and Cu cations were studied. The microstructure of Y1Ba2Cu3O7−δ films deposited using MOCVD was observed using scanning electron microscopy and transmission electron microscopy to investigate surface morphology change with the film composition and transformation twin structures. The experimental results showed that the volatility of Ba(thd)2 did not perceptively increase with decreasing evaporation pressure from 10 Torr to 5 Torr, but that of Y(thd)3 or Cu(thd)2 increased with the pressure decrease. The ratio of deposited to evaporated mole percents of Ba cation was smaller than those of Y and Cu cations. Therefore, Ba must be evaporated more than the stoichiometric amount for Y1Ba2Cu3O7−δ in order to obtain single phase Y1Ba2Cu3O7−δ films. The surface morphology of the Y1Ba2Cu3O7−δ films showed perculiar changes with slight composition changes. The transition onset and zero resistance temperatures of typical stoichiometric film deposited on MgO were 93 K and 91 K, respectively. The Y1Ba2Cu3O7−δ films had about 50 nm grain size, and most grains consisted of transformation twins. The critical current density of a film deposited on SrTiO3 was 105 A/cm2 at 77 K and zero magnetic field.
A powder x-ray diffraction study of a solution treated and ice brine quenched Al–14.25 at.% Li alloy
Authors: Fox A.G. , Fuller S.C. , Whitman C.E. , Radmilovic V.
Published: September 2007

An x-ray diffractogram was generated from a powder sample of solution treated and ice brine quenched Al–14.25 at.% Li alloy. The Bragg reflections obtained were characteristic of a very-nearly fully ordered Al-rich L12 phase based on δ'Al3Li together with two very weak reflections associated with δAlLi. All the lines were significantly broadened due to particle size effects. The average particle size associated with the 100 L12 superlattice line was found to be 4.2 (3) nm and with the fundamental lines, 26.8 nm. A simple structure factor calculation indicated the volume fraction of ordered phase to be around 0.77, assuming that the lack of maximum order was due to the presence of disordered fcc AlLi solid solution. These results suggest that the microstructure of this as-quenched alloy comprises ordered regions of about 4 nm in size in a sea of disordered matrix with a very small amount of δAlLi present. This conclusion is in excellent agreement with recent small angle x-ray and transmission electron microscope studies on similar alloys and suggests that AlLi alloys which are ostensibly disordered at high temperatures go through a disorder-order transformation and then decompose into regions of order and disorder which are associated with a composition spinodal.
First principles investigation of hydrogen embrittlement in FeAl
Authors: Fu C.L. , Painter G.S.
Published: September 2007

The mechanism underlying the hydrogen-induced embrittlement effect in FeAl has been investigated using a local density functional total-energy approach. The bonding characteristics, the bond and cleavage strength between iron and aluminum layers, and the surface energy with and without interstitially absorbed H are calculated from first-principles band-structure and atomic-cluster methods. Our unique combination of techniques permits the simultaneous study of the metallic and localized bonding effects on an equal footing. Results from this study show that FeAl (in the absence of H) is intrinsically highly resistant to cleavage fracture in terms of the high theoretical cleavage strength. Hydrogen locally dilates the Fe–Al lattice, and this is accompanied by a sizable decrease in Fe–Al cleavage (or cohesive) strength. Our results suggest that the underlying mechanism of H-embrittlement in aluminides is a depletion of d-bonding charge on the Fe site resulting from the charge transfer from Fe to H. Results also indicate that the H-embrittlement effect is greater for H adsorbed in Fe-rich sites.
Effects of quench rate on the microstructure in melt-spun Nd–Fe–B alloys
Authors: Zaluska A. , Xu Yan , Altounian Z. , Ström-Olsen J.O. , Allem R. , L'Espérance G.
Published: September 2007

Electron microscope studies have shown significant changes in the microstructure of the Nd14Fe80B6 melt-spun ribbons, depending on the quenching conditions. The average grain size of about 20 nm near the wheel side, increasing to 500–700 nm near the free side, can be observed for ribbons of the thickness of about 25–30 μm, indicating strong internal variations in quench rate across the ribbon thickness. For low quenching rates different morphologies of the crystalline phase Nd2Fe14B are observed. Equiaxed grains near the wheel side of the ribbon become columnar grains or even dendritic grains near the free side. Texturing effects in the equiaxed grains are observed only for grains larger than 30 nm and disappear for very large equiaxed grains (about 500 nm), as well as for dendritic structures. Most evident texture is observed in columnar grains, which crystallized with the c-axis perpendicular to the ribbon plane.
Effect of surfactant on casting metalorganic films and writing copper metal patterns
Authors: Lin C.T. , Lee H.Y. , Souto M.A.
Published: September 2007

A highly effective wetting agent and emulsifier, Triton X-100 detergent is introduced to metalorganic solutions. When applied to a substrate, this detergent/metalorganic solution promotes the formation of a uniform spin-on thin film with good adhesion. The precursor, copper formate film, was spin-deposited with a metalorganic solution prepared by a solvent mixture of methanol and Triton X-100 (50:1 by volume). KrF laser (248 nm) direct writing of high-purity copper patterns on glass and polyimide substrates has been achieved. When the laser beam is imaged through a slit device, periodic copper metal structures with a linewidth of ∼2.5 μm are observed, which may be attributed to result from Fraunhofer diffraction. A general process for casting homogeneous amorphous films on substrates from a solution of molecular or ionic metalorganics dissolved in the neutral or charged micelles is proposed.
Effects and measurement of internal surface stresses in materials with ultrafine microstructures
Authors: Cammarata R.C. , Eby R.K.
Published: September 2007

Variations in lattice parameters for materials that form very thin lamellae are analyzed using a thermodynamic model that incorporates surface stress effects. It is predicted that lattice spacing variations should be proportional to the reciprocal of the lamella thickness, in agreement with experimental data for polyethylene, n-paraffins, and a copolymer of tetrafluoroethylene and hexafluoropropylene. The model is then used to calculate the surface stress associated with lamella interfaces in these crystalline materials. The calculated surface stress has the same order of magnitude as a surface tension, but is negative. The model is extended so that surface stresses associated with grain boundaries can be measured in very fine-grained metals and ceramics.
Synthesis of intermetallic NiAl and Ni3Al fine powders through organometallic precursors
Authors: Abe Osami , Tsuge Akira
Published: September 2007

Fine powders of intermetallic NiAl and Ni3Al were synthesized through organometallic precursors, which were coprecipitated from aqueous solutions of NiCl2 and AlCl3 by the addition of ammonium benzoate and hydradinium monochloride as precipitants. Ni3Al and NiAl were synthesized by a two-step heat treatment of the precursors. The initial step was the thermal decomposition of organic groups to form homogeneous mixtures of Ni3C, amorphous Al2O3, and free C below 1000 °C. The other step was the reaction above 1300 °C to form the intermetallics. Single phase powders of NiAl and Ni3Al with the particle size less than 3 μm were obtained above 1300 and 1400 °C, respectively.
Characterization of InP wafers by use of a system for high resolution photoluminescence imaging
Authors: Vetter Th. , Winnacker A.
Published: September 2007

An apparatus for acquisition of highly resolved macroscopic photoluminescence topograms has been developed, using a laser scanner for PL excitation, and a linear diode array as imaging device. As a typical application, room temperature PL-topograms of various 2″ In P wafers are presented and discussed. Substrate inhomogeneities revealed in these topograms are growth striations, dendrites, and subsurface damage from wafer handling and processing.
Preparation and characterization of sputtered TiB2 films
Authors: Zdaniewski Wieslaw A. , Wu Joseph , Gujrathi Subhash C. , Oxorn Kenneth
Published: September 2007

Smooth, continuous TiB2 films were deposited on TiB2−AlN composites and Si substrates by sputtering in high vacuum. Excellent adhesion between films and substrates was indicated; no film delamination was induced by Vickers indentation at high loads. Analyses by secondary ion mass and x-ray photoelectron spectroscopies showed that the films were of high purity and contained H, C, O, N, and Na elements as trace impurities. Quantitative depth profile by the elastic recoil detection (ERD) nuclear scattering technique using 30 MeV 35Cl beam revealed that the Ti/B atomic ratio was very close to 0.5 while the 10B/11B isotopic ratio was 0.250.
Synthesis of porosity controlled ceramic membranes
Authors: Xu Qunyin , Anderson Marc A.
Published: September 2007

Porosity control in ceramic membranes has been achieved by controlling particle packing densities in sol-gel processing. TiO2 xerogels with two mean pore radii of 0.7 and 1.7 nm and a porosity varying from 30% to 52% have been obtained. ZrO2 xerogels with a mean pore radius of 0.7 nm and a porosity varying from 7% to 34% have also been prepared. The principle of controlling porosity is to make spongy aggregates and to control the degree of aggregation. Experiments have been conducted to show that spongy aggregates can be produced by gradually removing protons from the strongly charged particles. Viscosity techniques have been used to measure the relative volume fraction of the dispersed phase which, in turn, provides information on aggregate structures. Two aggregation models have been proposed to explain different structural aggregates formed through thermal destabilization in the highly charged system and through charge neutralization by gradually removing charge from the particles in the system.
Molecular composite films of MoS2 and styrene
Authors: Divigalpitiya W.M.R. , Frindt R.F. , Morrison S.R.
Published: September 2007

Novel composite films with a monolayer of organic molecules included between single molecular layers of MoS2 can be prepared with a recently described technique. Styrene-included MoS2, before and after polymerization, is discussed as an example that exhibits interesting properties. The sensitivity to humidity of the styrene-included MoS2 system is monitored in situ with x-ray diffraction where reversible incorporation and removal of water molecules are observed. Films with an anisotropy of electrical conductivity of about 108 are obtained by polymerizing the monomer between the layers.
Note on sufficient symmetry conditions for isotropy of the elastic moduli tensor
Authors: Dresselhaus M.S. , Dresselhaus G.
Published: September 2007

Group theoretical methods are used to obtain the form of the elastic moduli matrices and the number of independent parameters for various symmetries. Particular attention is given to symmetry groups for which 3D and 2D isotropy is found for the stress-strain tensor relation. The number of independent parameters is given by the number of times the fully symmetric representation is contained in the direct product of the irreducible representations for two symmetrical second rank tensors. The basis functions for the lower symmetry groups are found from the compatibility relations and are explicitly related to the elastic moduli. These types of symmetry arguments should be generally useful in treating the elastic properties of solids and composites.
Local magnetic field distribution in polycrystalline YBa2Cu3O7 and its influence on bulk critical currents
Authors: Askew Thomas R. , Flippen Richard B. , Leary Kevin J. , Kunchur Milind N.
Published: September 2007

The transport critical current density Jc has been measured in high quality polycrystalline YBa2Cu3O7 samples made from extruded preforms. The hysteretic response of Jc to applied magnetic fields was studied as a function of temperature (77–87 K) and sample morphology for various field sweeps in the 0–5 KOe range. A variety of samples show a basic Jc ∼ H−n behavior where n depends on temperature, but is independent of the other variables. A simple model of the local magnetic field distribution is presented and compared to the data, and the theoretical implications of the power law behavior are considered.
The degradation of YBa2Cu3O7 resulting from exposure to wet and dry steam
Authors: Pham D.K. , Ru-Peng Zhao , Fielding P.E. , Myhra S. , Turner P.S.
Published: September 2007

The severity of degradation of YBa2Cu3O7−x from exposure to an aqueous phase has the following dependence: wet steam (condensed thin film) > bulk fluid phase > dry steam, other variables being constant. Also, the surface reactivity increases with oxygen deficiency. It is found that the early stages of degradation are associated with surface hydroxylation and preferential extraction of Ba. Subsequently, dissolution takes place, followed by solution saturation and sequential precipitation, nucleation, and growth of secondary phases. These observations form the basis of a model which incorporates surface hydroxylation and OH− penetration of the bulk, formation of planar defects and extraction of Ba, lattice dissolution, and mobility and solubility constraints.
Lattice location and hardness of Ta-implanted Ni3Al
Authors: Was Gary S. , Mantl Siegfried , Oliver Warren
Published: September 2007

Implantation of Ta into single crystal Ni3Al was conducted to determine the degree of surface hardening in monolithic alloys in relation to its lattice location. Ta was implanted at 400 keV to doses of 0.07, 0.36, and 2.52 × 1016 cm−2 along the [100] axis of a [100] crystal of Ni3Al at room temperature. Composition versus depth profiles were determined by RBS, and lattice location of Ta was determined by channeling angular yield scans about the [100] axis. The hardness of the surface was measured by ultra-low load indentation. Results show that implantation softens the surface and that the Ta is randomly distributed between Ni and Al sites. Annealing at 1000 °C/1 h significantly reduces the damage and causes preferential occupation of Al sites by Ta, resulting in a slight increase in surface hardness. Further annealing at 1200 °C/0.25 h increases the surface hardness substantially and increases occupation of Al lattice sites to roughly 84%. Results are consistent with a model in which the as-implanted surface is softened by disordering, and subsequent diffusion of Ta to Al sites during thermal treatment causes hardening of the surface.
On the elastic behavior of composition-modulated superlattices
Authors: Jaszczak J.A. , Wolf D.
Published: September 2007

Atomistic computer simulations are used to systematically investigate the role of interfacial disorder on the elastic behavior of composition-modulated superlattices of fcc metals, represented by simple Lennard–Jones potentials. The structures, energies, and average elastic properties of four types of superlattices with various degrees of interfacial disorder are computed as a function of the modulation wavelength along [001]. The four superlattice types studied include perfectly coherent, incoherent, and two types derived from these by introducing relative twists about [001] between alternating layers. A 20% lattice-parameter mismatch between the two modulating materials is assumed. Results are compared with our earlier work on unsupported thin films, grain-boundary superlattices, and incoherent superlattices with a 10% lattice-parameter mismatch. The degree of structural disorder at the interfaces is found to correlate well with the magnitude of the elastic anomalies, which cannot be accounted for by anisotropic lattice-parameter changes alone. The grain-boundary superlattices studied earlier are found to provide a good model limit for the elastic behavior of interfacially disordered dissimilar-material superlattices.
Predicting quasicrystals with quantum structural diagrams
Authors: Tartas Jean , Knystautas Emile J.
Published: September 2007

Binary and ternary quasicrystals have been successfully separated by means of quantum structural diagrams in the light of new experimental data, and new potential candidates for the quasicrystalline state are presented. A general and simple classification of alloy systems, consistent with the experimental data, is proposed, which allows predictions of any type of binary and ternary quasicrystals. Quantum coordinates are analyzed and interpreted, and suggestions are given for an improvement of the present method.
Identification of tetragonal and cubic structures of zirconia using synchrotron x-radiation source
Authors: Srinivasan Ram , De Angelis Robert J. , Ice Gene , Davis Burtron H.
Published: September 2007

X-ray diffraction from a synchrotron source was employed in an attempt to identify the crystal structures in zirconia ceramics produced by the sol-gel method. The particles of chemically precipitated zirconia, after calcination below 600 °C, are very fine, and have a diffracting particle size in the range of 7–15 nm. As the tetragonal and cubic structures of zirconia have similar lattice parameters, it is difficult to distinguish between the two. The tetragonal structure can be identified only by the characteristic splittings of the Bragg profiles from the “c” index planes. However, these split Bragg peaks from the tetragonal phase in zirconia overlap with one another due to particle size broadening. In order to distinguish between the tetragonal and cubic structures of zirconia, three samples were studied using synchrotron radiation source. The results indicated that a sample containing 13 mol% yttria-stabilized zirconia possessed the cubic structure with a0 = 0.51420 ± 0.00012 nm. A sample containing 6.5 mol% yttria stabilized zirconia was found to consist of a cubic phase with a0 = 0.51430 ± 0.00008 nm. Finally, a sample which was precipitated from a pH 13.5 solution was observed to have the tetragonal structure with a0 = 0.51441 ± 0.00085 nm and c0 = 0.51902 ± 0.00086.
Accumulation of structural defects in ion-irradiated Ca2Nd8(SiO4)6O2
Authors: Weber W.J. , Eby R.K. , Ewing R.C.
Published: September 2007

Ion irradiations of the rare-earth orthosilicate, Ca2Nd8(SiO4)6O2, have been carried out using both alpha particles (emitted from a 238PuO2 source) and 3 MeV argon ions. The unit cell exhibits anisotropic expansion under irradiation, consistent with expectations based on the polyhedral connectivity within the structure. A least-squares analysis of the interatomic distances suggests that the unit-cell expansions are primarily due to changes in oxygen-oxygen distances and cation separations between neighboring polyhedra rather than to bonds within polyhedra. The irradiation-induced change in unit-cell volume is proportional to 1 – exp (BD), where B is an annealing rate constant and D is the dose, in agreement with a model for the accumulation of isolated point defects in the structure. The volume expansion saturates at 2.56% and 1.40% for the alpha and argon irradiations, respectively. Analysis of the results suggests that a significant fraction of the defects produced in the argon-ion displacement cascades are lost to in-cascade recombination. Differential scanning calorimetry of powder irradiated with 3 MeV argon ions to 20 ions/nm2 reveals an exothermic recovery peak at 350 °C with an activation energy of 1.3 ± 0.1 eV and average stored energy release of 28.2 J/g. There is no evidence for amorphization of this material under alpha or argon irradiation.
Characterization of interface states in (Sr, Ca)TiO3−x based ceramics by ICTS analysis
Authors: Nakano Yoshitaka
Published: September 2007

Isothermal capacitance transient spectroscopy measurements were performed to study the reoxidizing induced change in interface states in (Sr, Ca)TiO3−x based ceramics. A discrete acceptor type trap level was detected with reoxidation below the oxidation temperature of grain surfaces, but two deep levels were detected with reoxidation above this temperature. These ICTS results are in good agreement with the previous report with the DLTS measurements. These interface states are considered to originate from chemisorbed oxygen (O2−, O−) on grain surfaces. The O− and O2− chemisorption levels contribute to the non-ohmic conduction with reoxidizing anneals below and above the oxidation temperature, respectively.
Experimental evidence for a structural unit model of quasiperiodic grain boundaries in aluminum
Authors: Deymier P.A. , Shamsuzzoha M. , Weinberg J.D.
Published: September 2007

This paper presents a geometrical description of aperiodic grain boundaries within the framework of a quasiperiodic lattice. Experimental evidence is given in support of a structural unit model of quasiperiodic [100]45°twist and [100]45°twist plus tilt grain boundaries in aluminum.