bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural Analysis of Gold Clusters Using Exafs
Authors: Cluskey P.D. , Newport R.J. , Benfield R. E. , Gurman S.J. , Schmid G.
Published: March 2011

Three gold clusters, Au55 (PPh3 )12Cl6, Au55{PPh2(C6H4SO3Na)}12Cl6 and Au11{PPh2(p-ClC6H4)}7I3 (which we shall denote Au55, Au55, and Au11 for convenience) have been studied using EXAFS. Average near-neighbour bond lengths are contracted from the bulk by approximately 0.15Å (±0.02Å) for Au55, and 0.22Å (± 0.02Å) for Au11. Ligand contributions have been successfully modelled in the fitting. Debye-Waller factors for samples at room temperature and at 80K are compared to ascertain the relative degrees of thermal and static disorder present in the systems. Results clearly indicate that static structural disorder predominates over thermally induced vibrational disorder.
Conformations of Crumpled Sheet Polymers
Authors: Nelson David R.
Published: March 2011

Flexible sheet polymers or “membranes” can be regarded as two-dimensional generalizations of linear polymer chains, for which there is a vigorous theoretical and experimental literature. Flexible membranes should exhibit even more richness and complexity, for two basic reasons. The first is that important geometric concepts like intrinsic curvature, orientability and genus, which have no direct analogue in linear polymers, appear naturally in discussions of two-dimensional macromolecules. Our understanding of the interplay between these concepts and the statistical mechanics of surfaces is still in its infancy. [1] The second reason is that surfaces can exist in a variety of different phases. The possibility of a two-dimensional shear modulus in planar membranes shows that we must distinguish between solids and liquids when these objects are allowed to crumple into three dimensions. Hexatic membranes, with extended six-fold bond orientational order, provide yet another important possibility. All three phases have quite distinctive properties. [2, 3] There are no such sharp distinctions for linear polymer chains.
Inorganic-Organic Sol-Gel Processing of Semiconductor Quantum Dots and some Preliminary Self-Diffraction Studies on CdS-PbS
Authors: Spanhel L. , Schmidt H. , Uhrig A. , Klingshirn C.
Published: March 2011

Films and monoliths, containing clusters (sizes < 5 nm) of the binary semiconductor CdS and sandwiched CdS-PbS, were prepared via multifunctional inorganic-organic sol-gel processing. As a sulfur source, hexamethyldisilylthiane was employed. In precursor sols, the metal sulfide clusters are carrying functionalized silanes acting as stabilizing centers as well as inorganic and organic network formers. Hydrolysis and condensation produces an inorganic network yielding viscous liquids useful to prepare optically transparent films or monoliths. The final organic cross-linking at T < 100°C results in materials of variable spectral response, thickness and optical density. In preliminar degenerate four-wave mixing experiments, self-diffraction from laser-induced gratings was observed on unsupported 200 μm thick CdS-PbS doped monoliths. The maximum first order grating efficiency, measured at different wavelengths between 490 and 520 nm, was 0.5 · 10−3 and the corresponding calculated effective third order susceptibility was of the order of 10−9 esu.
Chemistry and Optical Properties of Small Metal Particles in Aqueous Solution
Authors: Henglein Arnim
Published: March 2011

Metal particles in aqueous solution can be surface-modified by chemisorbed molecules and/or by a second deposited metal. The electron density in the metal particles is varied by controlled electron transfer from free radicals.’ These modifications lead to significant electronic changes in the metal particles, the result being changes in the optical properties and in chemical reactivity.

The physicochemical properties also change during the transition from the atomic to the metallic state with increasing particle size. In the case of silver, oligomeric non-metallic clusters can be stabilized in solution for days, time enough to study many of their properties. In other cases (Au, Cu, Pb), the clusters are short-lived; they are investigated by fast kinetic methods such as pulse radiolysis.
Extrusion Textures in NiAl and Reaction Milled NiAl/AlN Composites
Authors: Bieler T. R. , Noebe R. D. , Whittenberger J. D. , Luton M. J.
Published: March 2011

Extrusion textures in monolithic NiAl and an NiAl/AlN particle composite are compared. The NiAl has a generally increasing grain size with extrusion temperature, and a corresponding transition from <110> to <111> fiber texture. The results suggests that <110> is more closely related to the deformation texture, while the <111> is a recrystallization orientation formed by preferential growth. The composite exhibits <311> fiber texture, and this is consistent with slip in <100> directions. The effect of the AlN particles is to prevent the orientation changes observed in the monolithic NiAl during recrystallization.
In-Situ Synthesis of Particle Dispersed Nanocrystalline NiAl By Cryomilling
Authors: Huang Benlih , Vallone J. , Klein C. F. , Luton M. J.
Published: March 2011

The present research was carried out to study the cryogenic synthesis NiAl from elemental powders with the view of forming particle dispersed NiAl. It was found that nanocrystalline NiAl with a crystallite size in the range 5nm to 10nm is produced during cryomilling. Additionally, the nanocrystalline NiAl maintains the fine crystallite size when annealed at high temperatures. It is thought that the resistance of the microstructure to coarsening is due to the presence of nano-scale particles of AIN formed by reaction between milling medium, liquid nitrogen, and aluminum. The transformation of Ni and Al to ordered NiAl was studied using x-ray diffraction. This showed that cryomilling not only produces a solid solution but also can induce in-situ ordering of stoichiometric NiAl. The B2 NiAl structure forms during milling after the elemental powder blend is cryomilled for about 40 hours. It also forms when the “mechanical” solid solution is annealed at room temperature for about twelve hours.
An Evaluation of the Creep Properties of an Al2O3/Ni3A1 Composite and the Effect of Disorder on Mechanical Properties
Authors: Brennan P. C. , Kao W. H. , Yang J.-M.
Published: March 2011

Ordered Ni3A1 alloys and their composites are attractive materials for elevated-temperature structural applications due to their many favorable properties. The addition of alloying elements can significantly lower the Ni3Al order-disorder transition temperature and also result in the formation of a Ni solid solution. As the percentage of Ni solid solution increases, the composite's room-temperature flexural strength increases. The effect of rocessing parameters on the material's microstructure is discussed. The complex matrix microstructure also has a significant effect on the composite's creep properties. Normal power-law creep was exhibited by the composite material when tested in compression.
Evaluation of Coatings for Sapphire Fibers in TiAl Using the Fiber Push-Out Test
Authors: Mackin T. J. , Yang J.
Published: March 2011

Fiber push-out tests were used to evaluate several double coating systems for sapphire fibers in γ -TiAl. The double coatings consisted of an inner debond coating, followed by an outer coating of alumina that serves as a diffusion barrier. The inner coatings were of three types: carbon black, colloidal graphite and porous alumina. By comparing estimates of interface toughness computed from the push-out tests, each double coating is found to permit debonding and sliding of the sapphire fibers. Consequently, each of the double coatings would afford improved fracture toughness for sapphire reinforced γ -TiAl.
Particulate Intermetallic Matrix Composite Made by One-Step Pulsed Laser Deposition
Authors: Chen Li-Chyong , Hall Ernest L. , Lou Karen A.
Published: March 2011

A novel way of producing particulate intermetallic matrix composites based on Nb-Al in one step using pulsed laser deposition (PLD) has been investigated. One unique characteristic, inherent to laser ablation, is the generation of particulates. These particulates condense on the substrate and become part of the film. In some cases, such as in high Tc superconducting or optical films applications, it is believed that the presence of particulates diminishes the quality of the films. In this work, we demonstrate that it can be advantageous in some applications to incorporate these particulates into the films.

Nb-Al films were prepared by laser ablation from a Nb3Al target using a 248nm KrF excimer laser at various fluences and substrate temperatures. Transmission (TEM) and scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and Auger electron spectroscopy (AES) were used to characterize the samples. The resultant films consist of a homogeneous matrix with uniformly distributed inclusions. The size of the particles generated by PLD is in the range of a few tens of nanometers to a few microns. EDX shows that the matrix has a composition of Nb7A13 and the particulates contain barely detectable Al. The mechanism of the depletion of Al in the particulates will be discussed. The merit of having a ductile Nb phase embedded in the intermetallic matrix is evident as the cracks generated during TEM sample thinning process propagate in the brittle matrix and finally are arrested at the ductile inclusions.
Fatigue Crack Growth in a TiAl Alloy with Lamellar Microstructure
Authors: Davidson David L.
Published: March 2011

The mechanisms of fatigue crack advance are examined for a lamellar α2 + γ alloy. Crack growth rates are highly dependent on the orientation of the loading axis to the lamellae direction. Thus, the material has some of the characteristics of a composite. For crack growth perpendicular to the lamellae, the mechanisms of crack advance are similar to those of other titanium alloys, while crack growth parallel to lamellae has other characteristics.
Reinforcement from In-Situ Precipitated Silica in Polysiloxane Elastomers under Various Types of Deformation
Authors: Mark James E. , Wang Shuhong , Xu Ping , Wen Jianye
Published: March 2011

Elastomeric networks prepared by tetrafunctionally end linking hydroxyl-terminated poly(dimethylsiloxane) chains (PDMS) were filled by the in-situ precipitation of silica. The resulting networks were investigated under uniaxial elongation, biaxial extension, shear, and torsion in order to characterize the resulting changes in mechanical properties. Compared with the unfilled networks, the silica-filled materials showed large reinforcing effects. Specifically, their values of the modulus, ultimate strength, and rupture energy increased significantly. The results thus indicate that the PDMS networks filled by the in-situ precipitation of silica have very good mechanical properties in several, rather different deformations. Examples of other deformations of interest are equilibrium swelling, and dynamic cycling for characterization of compression set.
Tc-δ Relations in YB2Cu3O7-δ Thin Films: Effects of Oxygen Pressure During Growth
Authors: Feenstra R. , Norton D. P. , Budai J. D. , Christen D. K. , Lowndes D. H. , Matijasevic V. C. , Eom C. B. , Geballe T. H. , Hellman E. S. , Hartford E. H.
Published: March 2011

The Tc dependence on oxygen content was measured for YBa2Cu3O7-δ films grown with a variety of techniques (solid phase epitaxy, laser ablation, off-axis sputtering, co-evaporation) at oxygen pressures p(O2) ranging from 1.0 atm to 0.1 mTorr. Dissimilar dependences resulted for each film type, with Tc either increasing or decreasing for small increments in δ from maximum oxygen occupancy. Varying systematically with p(O2) during growth, the deviations are attributed to competing effects from hole-doping lattice defects (most likely on the Y-site) on the carrier density of the CuO2 planes and basal plane oxygen capacity, respectively, giving rise to overdoping or underdoping after low temperature oxidation in 1.0 atm of oxygen.
Extreme Sensitivity of Tc to Cool-Down Rate and Substrate Temperature for Epitaxial Fe-DOPED Ybco Films
Authors: Bridges Frank , Boyce J. B. , Johnson R. I.
Published: March 2011

Epitaxial Fe-substituted YBCO films on LaAIO3 substrates prepared by pulsed laser deposition exhibit a large variation in Tc with the substrate temperature and the cool-down rate of the films in an O2 environment, immediately after deposition. For 9% Fe films deposited at 700°C, a rapid cool-down yields a Tc of 60 K while a slow cool-down of 10°C/minute gives 18 K; Tc goes to 0 at 11% Fe for the slow cool-down films. For 9% Fe films deposited at 770°C, Tc > 80K and is much less sensitive to the cool-down rate or additional anneals.
Flux Pinning and Creep in High Temperature Superconductors
Authors: Kes P. H. , Van Der Beek C. J.
Published: March 2011

Various questions concerning pinning and flux creep in high- temperature superconductors are addressed. First the pinning force by point defects in CuO2-layers in Bi:2212 and Y:123 single crystals is computed. Then some possibilities for the pin mechanism of the recently discovered screw dislocations in sputtered Y:123 films are discussed. Finally, we turn to the issue of flux creep in these materials and the relation between activation barrier and current density. The relevance of different creep models is further elucidated by the numerical solution of the non-linear creep equation for infinite slab and cylinder geometries and various initial field and current distributions.
Neutron Depolarisation by YBa2Cu5O6.85, Superconductors
Authors: Wang J. , Kennedy S. J. , N. R. K.
Published: March 2011

We report a study of magnetic flux penetration into a single crystal superconductor, using the technique of 1-dimensional neutron depolarisation. In these experiments we have applied magnetic fields of up to 1.7 kOe to the crystal, perpendicular to the polarisation direction of the neutron beam. By analysing the polarisation of the transmitted neutron beam, after removal of the applied field, we are able to determine the remanent magnetisation in the crystal as well as its decay over periods from 10 to 105 seconds. This technique allows us to study both field and time dependent behaviour.

We are developing a model to explain these observations, using the Bean critical state model to calculate the bulk trapped flux with the assumption of simple spin precession in a uniaxial magnetic environment. We have been able to relate the field dependence of the remanent magnetisation to the lower critical field Hcl and the field at which magnetisation saturation is reached 2H*-Hcl. We also relate the time dependence to the flux pinning energy U which determines the rate of flux relaxation. Measurements of this type could be used to plot the magnetic phase diagram of high temperature superconductors, to study anisotropy and flux creep in such crystals.
Scaling Behavior Near Superconducting Transition in the YBa2Cu3O7 System
Authors: Wu M. K. , Wang M. J. , Chen Y. C. , Chien T. R. , Chi C. C. , Holtzberg F.
Published: March 2011

We have carried out a systematic study of the detailed I-V characteristic in magnetic fields of the YBa2Cu3O7 (YBCO) single crystals and c-axis oriented thin films. For all the samples studied, there exists a temperature at which the I-V satisfies simple power law. The I-V characteristic also follows the scaling behavior predicted by the phase transition model based on the random disorder. The derived critical exponents suggest the occurrence of 3D to 2D crossover of the flux motion. The critical field at which the crossover occurs depends on the sample thickness.
Ac Permeability Measurements in 770 MeV 129Xe17+ and 560 MeV 16O6+ Irradiated YBa2Cu3O7
Authors: Bechtold J. , Jin X. C. , Xue Y. Y. , Rusakova I. A. , Hungerford E. V. , Feng H. , Hor P. H. , Chu C. W.
Published: March 2011

High-energy heavy-ion irradiation has been shown to be effective in introducing flux-pinning defects in Y123 single crystals. Contrary to electron, low energy proton, and fast neutron irradiation, high-energy heavy-ion irradiation produces defects beneficial to flux pinning through the electronic (rather than nuclear) stopping mechanism. This type of stopping results in tubular defects many microns long that can increase the range of irreversible magnetic behavior to higher fields. In contrast, dense arrays of small defects (size < 100 Ang.) produce enhanced critical currents at low fields but do not extend the range of irreversible behavior significantly. We have used two ion-energy combinations in which the electronic stopping powers vary by nearly two orders of magnitude. The difference in stopping powers results in very different defect types, ranging from well oriented and dense (Xe) to much less well oriented and shorter (O) tubular defects. A novel analysis of the ac μ data indicates that the strong pinning introduced by the Xe irradiation breaks the vortex glass picture in favor of an individual pinning model, while O irradiation does not.
The Influence of Transport Currents on the Harmonic Generation in High Tc Superconductors
Authors: Wang J. , Yue Y. , Taylor K. N. R. , Russell G. J. , Matthews D. N. , Dou S. X.
Published: March 2011

This paper details the experimental results for the influence of transport currents on the ac magnetisation of high-Tc superconductors. It has been found that the variation of the third harmonic amplitude V3 with an ac field can still be described by square and cubic laws for low and high ac fields, respectively, the same relationships as observed in the absence of transport currents. The only significant difference is that the magnitude of V3 in the low field region increases linearly with transport current. Consequently, the crossover point (from square law to cubic law) moves to higher ac fields and the variation of V3 with transport current is less pronounced in the high field region. The results are explained in terms of an effective increase in Hg, which is a parameter in Kim critical state model, with increase of transport current.
Properties of the Melt Processed 2212 Bscco in Silver
Authors: Yang Ming , Belenli I. , Grovenor C. M. R. , Goringe M. J. , Jenkins R. , Jones H.
Published: March 2011

Introduction:

This study is a part of an on-going research programme in Oxford to produce Bi-based ceramic superconductor wires. It aims at optimising processing parameters in order to achieve desired properties of the material. It has been shown that the 2212 BSCCO material can be successfully produced using the melt processing technique. High critical current density has been obtained in the form of wire and thin film/1,2/. Although several studies have been recently reported regarding the crystallisation of the 2212 phase/3/ and the effect of the oxygen stoichiometry on the properties of the material/4,5/, there is limited knowledge in the literature about the details of the processing of the 2212 material enclosed in silver. The present study is undertaken to elaborate the microstructural evolution mechanism of the 2212 phase in silver when it is cooled from the melted state, and to establish the effect of the cooling rate from the annealing temperature on the electric properties.
Cuprates: Back to the Fermi Surface
Authors: Newns D. M. , Tsuei C. C. , Pattnaik P. C. , Kane C. L. , Chi C. C. , Krishnamurthy H. R. , Daumling M.
Published: March 2011

The cuprate superconductor problem is approached from the conventional metallic standpoint, valid at dopings near the Tc maximum and beyond. There is strong evidence that the Tc maximum corresponds to EF lying at a van Hove singularity, a special situation leading to Marginal Fermi Liquid behavior, and also to a minimum in the isotope shift, as observed. The superconducting properties are discussed in the light of phononic and electronic pairing models.
YBCO-Based Multilayer Structures on Large-Area Substrates
Authors: Gavaler J. R. , Talvacchio J.
Published: March 2011

We have deposited YBa2Cu3O7 (YBCO) films on LaAlO3 substrates up to two inches in diameter using 90° off-axis magnetron sputtering from a single stoichiometric target. With this same method, epitaxial bilayers and trilayers of insulating SrTiO3, LaAlO3, MgO, metallic PrBa2Cu3O7 (PrBCO), or superconducting La2−xSrxCuO4 (LSCO) were deposited in combinations with YBCO. In the resulting structures with c-axis orientations, the YBCO had Tc's of 90K or higher over the entire surface area. Maximum resistivity values of 6 × 109 Ω-cm in YBCO/insulator/YBCO structures were obtained in vertical transport measurements with SrTiO3. An important cause for degraded electrical properties in the structures incorporating epitaxial insulators was the presence of copper oxide particles on the YBCO surface. Methods used to minimize the number of these particles are discussed.
Recrystallization of Amorphous or Nanocrystalline NdBa2Cu3O7−x and GdBa2Cu3O7−x
Authors: Folkerts T. J. , Yoo S. I. , Xu Youwen , Kramer M. J. , Dennis K. W. , Mccallum R. W.
Published: March 2011

Using a novel melt-spinning technique, we have produced highly disordered NdBa2Cu3O7−x and GdBa2Cu3Oy−x materials. Samples which were melt-spun in an O2 environment consist of nanocrystals with the tetragonal REBa2Cu3O7−x structure: samples which were processed in an N2 environment consist of an amorphous matrix with small amounts of crystalline BaCu2O2, as shown by x-ray diffraction and electron microscopy. High temperature XRD studies indicate that the BaCu2O2 is eliminated during heating to 500°C in O2 and that the REBa2Cu3O7−x Phase recrystallizes directly from the amorphous matrix at temperatures below 800°C. Preliminary magnetization measurements show that higher temperature heat treatments are needed to restore superconductivity.
Anisotropic Electrical Resistivity of YBCO/PBCO Superlattice Films Grown on Miscut Substrates
Authors: Kerchner H. R. , Klabunde C. E. , Christen D. K. , Budai J. D. , Lowndes D. H. , Norton D. , Thomson J. O.
Published: March 2011

Anisotropie electrical resistivity is studied in epitaxial superlattice films of YBa2Cu3O7-δ/PrBa2Cu3O7-ε grown in situ by laser ablation on SrTiO3 surfaces aligned slightly away from the [100] direction. Layer thicknesses of each compound range from two to eight atomic cells. Electrical resistivity of these superlattice films always shows a peak at some cryogenic temperature that decreases with increasing PrBa2Cu3O7-ε layer thickness. Clear evidence is seen of vortex pinning or of supercurrent blocking by the step edges or by twin boundaries. The ratio of electrical resistivities for current directions parallel and perpendicular to these boundaries shows a large step-like change just above the superconductive critical temperature. The possibility is discussed that the Kosterlitz-Thouless transition explains this anisotropy change.
Deposition of Superconducting Tl-Ba-Ca-Cu-O Phases on Metal Foils by Metal-Organic Chemical Vapor Deposition
Authors: Schulzi D. L. , Hano B. , Neumayer D. , Hinds B. J. , Markst T. J. , Degroot D. C. , Schindler J. L. , Hogan T. , Kannewurf C. R.
Published: March 2011

The synthesis of superconducting Tl-Ba-Ca-Cu-O thin films on metal foils (Au and Ag) by metal-organic chemical vapor deposition (MOCVD) has been investigated. Ba-Ca-Cu-O-(F) films are first prepared via MOCVD using fluorinated “second generation” metal-organic precursors. After an intermediate anneal with water vapor-saturated oxygen to promote removal of F, Tl is introduced by annealing in the presence of a mixture of oxides (Tl2O3, BaO, CaO, CuO) of a specific composition. Characterization of the thin films by scanning electron microscopy, EDX, x-ray diffraction, and variable temperature magnetization measurements has been carried out. High temperature superconductor (HTS) films of Tl2Ba2Ca1Cu2O8−x on Au foil exhibit a magnetically derived Tc = 80K and a high degree of texturing with the crystallite c-axes oriented perpendicular to the substrate surface as evidenced by enhanced (000 x-ray diffraction reflections. Thin film coverage on Ag foil becomes non-contiguous during the (Tl2O3, BaO, CaO, CuO) mixture anneal.
Critical Current Densities JC(H,T) and Current-Voltage Characteristics in Melt Processed Bi-2212 with and without Preferential Orientation
Authors: Paul W. , Heeb B. , Baumann Th. , Guidolin M. , Gauckler L. J.
Published: March 2011

In contrast to sintered Bi2Sr2Ca1Cu2Oy (Bi-2212), melt processed material exhibits nonzero intergrain supercurrents at T < 77 K. By means of partial melting and long term annealing we achieved nonoriertted material with Jc(77) - 3500 A/cm2 (1μV/cm criterion) and Tc = 96 K. Zone melting leads to well textured bulk material with the c-axis of the crystallites oriented perpendicular to the pulling direction R, and thus to an anisotropie critical current density. The anisotropy factor is between 2 and 5. At low temperatures the melt textured ceramic exhibits higher critical currents than the nonoriented material. This is not true at high temperatures, since the latter has a higher critical temperature.

At 77 K and small fields both oriented and nonoriented ceramics exhibit no transition in their current-voltage characteristics, but obey the power law E = Eo(j/jo(H)) α(H) over a large range in E (10”−3-10”−3 V/cm). Measurements on the textured ceramic with magnetic fields parallel and perpendicular to the pulling direction demonstrate that the superconducting properties of these materials are only affected by the field components parallel to the c-axes of the crystallites. For fields parallel to the pulling direction the measured values of α(H), Jo(H) and of the reversible magnetization Mrev can be attributed to the misorientation of the Bi-2212 platelets which is in average about 14°.
Microstructures at the Interface Between a-Axis Oriented YBa2Cu3O7−x and NdGaO3(110) Substrate
Authors: Takahashi Hiromi , Homma Norio , Okayama Satoru , Morishita Tadataka
Published: March 2011

The interface between an a-axis oriented YBa2Cu3O7-x film and a NdGaO3(110) substrate has been investigated by cross-section transmission electron microscopy (TEM). The orientational relationship between the a-axis oriented film and substrate is YBa2Cu3O7-x[001] / NdGaO3[001]. This preferentially c-axis aligned direction of the YBa2Cu3O7-x film would be caused by a very small lattice mismatch (0.1%) between b(=a) lattice constant of YBa2Cu3O7-x and of the pseudo-cubic sub-lattices in NdGaO3 at a substrate temperature of 750°C. Two kinds of imperfections have been observed in the crystal lattice of YBa2Cu3O7-x near the interface; One is the deviation of YBa2Cu3O7-x [301] from NdGaO3 [111]. The other type is pair dislocations with a positive and negative Burgers vectors in the YBa2Cu3O7-x (103) planes. These two kinds of defects at the interface would be act to reduce the tensile stress within a distance of about l.lnm from the substrate interface.
Microstrucrure and Properties of In-Situ R. F. Sputtered YBa2Cu3O7−x Thin Films for Microwave Applications
Authors: Reagor David W. , Piza Margarita E. , Houlton Robert J. , Garzon Fernando H. , Hawley Marilyn , Raistrick Ian D.
Published: March 2011

The residual surface resistance of a number of films of YBa2Cu3O7−x, prepared by off-axis sputtering onto MgO substrates, has been measured using a parallel-plate resonator technique. Deposition conditions were kept constant, apart from the substrate temperature. There is no correlation between surface resistance and other important microscopic parameters, such as Tc and c-axis lattice parameter. There is, however, a trend to higher Rs with increasing volume fraction of in-plane misoriented material, although the correlation is not perfect. Furthermore, we have found that most of the misoriented material is localized at the film substrate interface and, therefore, is probably not responsible for most of the RF losses. The data suggest that at higher deposition temperatures, there is an increasing tendency for 45° misoriented material to appear in the films, and a significant fraction of this material may be present closer to the free film surface. Scanning Tunneling Microscopy (STM) qualitatively supports this conclusion.
Current Controlled HTS Switches Fabricated by a Reactive Patterning Technique
Authors: Ma Q. Y. , Hardy W. N. , Laibowitz R. B. , Yang E. S.
Published: March 2011

A current controlled HTS switch (CCSS) has been fabricated using the Si-YBCO reactive patterning technique. The Si films were deposited on MgO substrates by e-beam evaporation and patterned via a lift-off process. The YBCO films were deposited thereafter by pulsed laser deposition to form micron-sized HTS strip lines with different widths and lengths. A typical line has a normal resistance ranging from 10 Ω to 100 KΩ. A CCSS was constructed using such a HTS line as a resistor in parallel with a load resistor. The operation of CCSS is similar to that of a current divider. Applications of CCSS includes HTS logic gates and circuit breakers.
Grain Boundary Structure and Properties of High-Tc Superconductors
Authors: Jagannadham K. , Narayan J.
Published: March 2011

We have modelled the grain boundaries in high-Tc superconducting oxides and determined the critical current density. The tunneling of superconducting pairs across the coalesced regions is used to determine the boundary effects. The length of the coalesced regions, with continuity of the Cu-O planes maintained by relaxation of the atom positions, is determined by minimization of the energy of the configuration. The depression of the order parameter is evaluated using the continuity conditions at the boundary in the proximity effect formulation. The excess charge distribution at the core of the boundary, determined from the solution to the Poisson's equation, is used to determine the scattering of the superconducting pairs. The width of the boundary, evaluated from modelling, determines the transmission coefficient for tunnelingof superconducting pairs. The critical current density is expressed in terms of these four important factors associated with the grain boundary. All the experimental results are explained by the present modelling of the grain boundary effects.
Layered Structure in Melt-Processed BSCCO and YBCO Superconductors
Authors: Jin S. , Chen S. K. , Tiefel T. H.
Published: March 2011

Because of the natural preference for crystal growth in the a-b direction, melt-processed YBCO as well as BSCCO tends to show a local texture with large parallel plates aligned in the direction of CuO2 planes. In YBCO, macroscopically layered and bi-axially textured material can be achieved over extended sample lengths through the use of temperature gradient during melt-texture processing. In BSCCO, a layered structure is relatively easily obtained by subjecting thin ribbon samples in contact with silver to a partial melt processing. While the nature and the mechanism of layer formation may not be the same, the layer configuration in both YBCO and BSCCO is essential for overcoming the grain boundary weak link problem and achieving high transport Jc. In this paper, the process and the mechanism of layer formation will be described, and the implications on superconductor properties will be discussed.
Correlation Between Nonlinear Charge Transfer and Behaviour of Tc in YBa2Cu3O7-y System
Authors: Zhao Y. , Liu H. K. , Dou S. X.
Published: March 2011

The charge transfer in YBa2CU3O7-y system have been analysed by bond valence sum calculation. It is found that there exists an additional effective charge-transfer between the CuO2 planes and the CuO chain which varies nonlinearly with the oxygen stoichiometry and can be expressed as q = 0.151 exp(∼0.007/y4) + 0.35 exp(0.5 – 0.5/y4). This nonlinear charge-transfer is associated with the structural transitions of orthorhombic (I)-to-orthorhombic(II) and orthorhombic(II)-to-tetragonal symmetry, and thus related to the oxygen ordering in the CuO chains. A new relationship between the mobile hole number P(2) the localised hole number P(l) and the total hole number P has been established as P(2) = P - P(1), P(1) = 0.5 - y - q. The results also show that the 90 K and 60 K plateaus in Tc as a function of oxygen stoichiometry correspond to the plateaus in the curve of q versus y, indicating that both the nonlinear variations of Tc and the charge-transfer with the oxygen stoichiometry are intrinsically correlated.
Fabrication and Characterization of Nb/MgO/Bi2Sr2CaCu2Ox Tunnel Junctions Using a Single Crystal
Authors: Usuki T. , Yamano K. , Shimaoka K. , Takahashi K. , Yoshisato Y. , Nakano S.
Published: March 2011

Nb/MgO/Bi2Sr2CaCu2Ox (BSCCO) tunnel junctions using both amorphous and crystalline MgO barriers were fabricated using a BSCCO single crystal in order to compare their superconducting states of interface. Both MgO thin films, 4–8 nm thick and with full coverage, were deposited onto BSCCO single crystals using the MBE method. The junctions using amorphous and crystalline MgO films were found to have gap opening structures at a bias voltage of about 38 mV and 25 mV at 5.2 K, respectively. The gap opening of the junction using an amorphous MgO barrier occurred below 86 K, corresponding very closely with the Tc for the BSCCO single crystal. Its characteristics showed a BCS-like temperature dependence.
Rheed Intensity Monitored Growth of Bi-Sr-Ca-Cu-O Superconductors
Authors: Sakai Shigeki , Kasai Yuji , Bodin Peter , Matsuhata Hirofumi
Published: March 2011

To make Bi2Sr2CaCu2O8 superconducting films with top insulating SrTiO3, we use the molecular beam epitaxy technique (MBE) with in situ monitoring by reflection high-energy electron diffraction (RHEED). A new (RHEED)image enhancing technique, difference reflection high-energy electron diffraction (DRHEED) gave striking information on the growth process of each layer in the Bi:2212 compound as well as the rough and flat transition occurring during co-evaporated deposition of SrTi03.
Annealing Effects on Tc in the Tl-Ca-Ba-Cu-O System
Authors: Tsai M.-J. , Wu S. F. , Lu S. W. , Huang Y. T.
Published: March 2011

Tl-2223 phase with a composition of Tl1.7Ca2.3Ba2Cu3O10 was prepared via a solid-state reaction route and systematically annealed under various conditions, including flowing oxygen, sealed quartz in vacuo, high oxygen pressures and hot isostatic press in an attempt to enhance the superconducting properties in the Tl-Ca-Ba-Cu-0 system. An almost monophasic sample was obtained by sintering at 910 °C for 3 hours in flowing oxygen. Most anneals enhance the Tc by 2 – 7 K, depending on the environment. When the sample is post-annealed at 820 °C for 16 hours in flowing oxygen it results in an optimal Tc of 127 K, as confirmed by magnetic susceptibility and electrical resistance measurements.
Observation of Macroscopic Quantum Effects in YBCO Thin Film Microbridge Rings
Authors: Huang Y. , Jiang H. , Widom A. , Vittoria C.
Published: March 2011

Thin film rings with microbridge were constructed using high Tc Y-Ba-Cu-O super-conducting thin films which were made by laser ablation on the single crystal MgO(100) substrates. The thickness of the films is 5000 Å. The weak links were biased with an AC induced current. The resulting electron pair charges on the weak links were monitored by an output voltage. The charge stored on the link (as a function of the input bias voltage) is consistent with that of a “macroscopic quantum pendulum”
Microwave Absorption in YBCO/PrBCO Superlattices
Authors: Carlos W. E. , Kaplan R. , Lowndes D. H. , Norton D. P.
Published: March 2011

Non-resonant microwave absorption is employed to probe YBCO/PrBCO superlattices and compare the response to that of a YBCO film. Near the transition temperatures, the response of the superlattice samples and the YBCO film have similar amplitudes and orientation dependencies. At lower temperatures, the response of the superlattices is much stronger than that of the YBCO film and, while both responses are hysteretic at low temperatures, the widths of the hysteresis have opposite orientation dependencies, which we attribute to the role of the PrBCO layers.
YBa2Cu3O7−x Grain Boundary Weak Links on MgO Bicrystal
Authors: Lu H. B. , Tu S. L. , Wang J. J. , Lin J. , Huang T. W. , Yang S. J. , Hsu S. E.
Published: March 2011

The characteristics of YBa2Cu3O7−x (YBCO) thin films by laser ablation on MgO bicrystals have been investigated. The bicrystals were fabricated by hot pressing two single crystals with the configuration of [001] tilt boundaries. The YBCO films were epitaxial grown with C-axis normal to the both adjacent grains of bicrystals. The FWHM about (005) reflection was 0.4–0.5 degree, indicating the high degree of the oriention for the film with small mosaic spread. Our preliminary study showed that the typical value of Jc on either side of the bicrystal boundary was 0.4–10×106 A/cm2 at 15K, while that across the 10° tilt boundaries was 0.3–9×105 A/cm2 at 15K. These results implied that the artificial grain boundaries effectively weakened the supercurrent, and therefore, the weak-link properties of artificial boundaries were more easily controllable than those of naturally occuring grain boundaries.
Effect of the Weak Links on the Surface Resistance of YBa2Cu3Ox Bulk Material
Authors: Wosik J. , Xie L. M. , Halbritter J. , Chau R. , Samaan A. , Wolfe J. C. , Selvamanickam V. , Saláma K.
Published: March 2011

Dc magnetic field, temperature, and magnetic history dependencies of the millimeter-wave surface resistance have been measured in high quality grain-aligned and in polycrystalline YBa2Cu3Ox bulk material. The measurements were carried at 75 GHz using the endplate of the cavity replacement method. The data is interpreted in terms of the presence of intra- and inter-grain types of weak links. The values of characteristic critical fields of the bulk and the weak link junctions are determined and discussed.
Optical Energy Transfer and Photochromism in Doped Sol-Gel Glasses
Authors: L'Espérance Drew , Pelo John M. , Chronister Eric L.
Published: March 2011

Organically doped sol-gel glasses are investigated by optical absorption and time-resolved fluorescence depolarization measurements. Silicate and aluminosilicate glasses have been doped with quinizarin (Q) and aluminum phthalocyanine chloride (CAP). The structure and bonding in these inorganic sol-gel solids is different from that in frozen liquids and organic polymers, and the unique dynamics of these systems are a motivation for this study. Both rotational dynamics and optical energy transfer of doped chromophores in sol-gel glasses are investigated by time-resolved fluorescence depolarization measurements. Since the dipole-dipole energy transfer rate is very sensitive to transfer distance, these measurements are used to probe the spatial distribution of dopant molecules within these solids. We also present photochromic results on aluminum phthalocyanine chloride doped silicate sol-gel glasses and discuss possible uses as an optical power limiting material.
Stressed In Situ X-Ray Diffraction Studies of a Ni-Ti Shape Memory Alloy
Authors: Hwang J. Y. , Yang C. F.
Published: March 2011

The B2 ⌊ R ⌊ B19′ phase transformations in a Ni-50.7 at.% Ti alloy were investigated under a series of uniaxial tensile stresses. A custom-build X-ray sample holder with hot, cold and stressing stages was used for structure analysis at temperatures up to 140 °C under a variety of tensile stresses (up to 250 MPa). Reorientation and growth of favorably oriented variant domains of R phase and martensite under stress were observed. In the stress-assisted phase transformations, the favorable (020)M and (300)R planes are expected to align preferably parallel to (011)B2 planes, and the unfavorable (111)M and (112)R planes perpendicular to the (011)B2 planes. In addition, the results of semi-quantitative analyses of the relative amounts of parent phase and martensite formed under a specific stress and temperature condition, x=x(σ,T), are presented.
Electronic Determination of the Modulus of Elasticity and Intrinsic Stress of Thin Films using Capacitive Bridges
Authors: Wang Sha , Crary Selden , Najafi Khalil
Published: March 2011

We extend the method of Najafi and Suzuki [1] for the electronic determination of the modulus of elasticity and intrinsic stress of thin films using capacitive bridge structures. New theoretical concepts are introduced. The extended method does not require that the test structures be exercised to the point of snap-action.
Materials Development of Thin Film TiNi Ferroelastic-Ferroelectric Heterostructures
Authors: Jardine A. Peter
Published: March 2011

Active and adaptive thin film materials for “smart” materials will likely be a heterostructure of several types of thin films. In this investigation, deposition and processing of thin film Shape Memory Effect TiNi and thin film ferroelectric BaTiO3 and SrTiO3 are discussed. Growth conditions as well as the thermodynamic conditions for thin films are described. It was found that amorphous thin films of TiNi and BaTiO3 can be crystallized simultaneously by air annealing at 600°C.
Analysis of Microfabricated Textured Multicrystalline Beams: I. Homogenization Approach
Authors: Mirfendereski Dariush , Ferrari Mauro , Der Kiureghian Armen
Published: March 2011

This paper discusses a deterministic approach to the stress and deformation analysis of miniaturized structures. Analytical Voigt-Reuss-Hill averages for the elements of the fourth-ranked elasticity tensor representing a polycrystal are evaluated for a planar problem and the variations of elastic moduli with respect to the degree of anisotropy and the mean and coefficient of variation of the preferred orientation direction are studied. The applicability of the averaging methods to the characterization of materials used in micro-electro-mechanical systems are then assessed.
Analysis of Microfabricated Textured Multicrystalline Beams: II. Probabilistic Approach
Authors: Mirfendereski Dariush , Der Kiureghian Armen , Ferrari Mauro
Published: March 2011

This paper reports on detailed approaches to the probabilistic modelling and analysis of multicrystalline microfabricated beams in the context of micro-electromechanical systems (mems) using simulation techniques. Random, textured multicrystalline beam structures are simulated and analyzed using the finite element method, employing randomly oriented orthotropic material properties to model each individual crystal grain.

The simulation studies are used to assess the influence of size effects and texture on overall beam characteristics, such as stiffness. Results of these studies are then compared with the current approaches based on deterministic analysis and employing simplifying assumptions such as homogeneous, isotropic material properties, and with results obtained from the homogenization approach of Part I.
Photoconductivity and Photovoltage Generation in Novel Photocrosslinkable Nonlinear Optical Polymers
Authors: Li Lian , Jeng R. J. , Kamath M. , Kumar J. , Tripathy S. K.
Published: March 2011

Photoconductivity in photocrosslinkable second-order nonlinear optical (NLO) polymeric materials and photovoltage generation in a poled guest-host polymer are reported. These NLO polymers have been functionalized with crosslinkable cinnamoyl groups and show relatively stable second-order optical nonlinearities at room temperature when poled and crosslinked. Without introducing any photosensitizer or charge carrier transport agents, photoconductivity was obtained in these polymer systems. Optical excitations of the NLO chromophores lead to photocarrier generation and it is conjectured that these NLO chromophores also play a role in the carrier transport. In addition to photoconductivity, photovoltage generation was observed in the dye doped polymer upon poling. This poled polymer also exhibits a polarization dependent photovoltage when illuminated by light in the absorption region of the NLO chromophore.
Studies on Ferroelectric Liquid Crystal Tolan Derivatives Designed for Nonlinear Optical Applications
Authors: Walba David M. , Rego James A. , Clark Noel A. , Shao Renfan
Published: March 2011

The structure and properties of a series of ferroelectric liquid crystal (FLC) guests possessing a tolan unit in the core are presented. The data shows that in racemic smectic C hosts, the o-nitroalkoxytolan unit can be oriented along the polar axis in the FLC thin films, affording interesting new materials for NLO applications. The data are consistent with basic expectations regarding the structure of the FLC host-guest system. There are, however, interesting deviations from expectation suggesting that for some guests there is a large and unexplained effect of host structure on the properties of doped FLC phases.
Electronic Transport in Organic Electrophotographic Receptors
Authors: Abkowitz Martin A. , Stolka Milan
Published: March 2011

All organic photoreceptors are now widely deployed in electrophotographic copiers and printers and represent the most important and commercially successful application of electronic organic materials. The need to optimize polymers for this application has stimulated fundamental investigations of charge generation, injection and transport processes in the disordered organic solid state. Electronic transport in a wide variety of glassy polymeric insulators has been studied by the time of flight drift mobility technique. It is typically found to proceed by a field driven chain of thermally activated tunneling events among active, compositionally identical, but energetically inequivalent sites. The inequivalece of site energies is attributed to the combined effect of disorder and site relaxation. Polymeric systems which differ widely in composition and morphology are found to exhibit a remarkably recurrent pattern of features in their transport behavior. Theoretical attempts to account for these universal features have been frustrated by their inability to explain the sublinear field dependence of the drift mobility and its variation with temperature. The drift mobility of polymers can be systematically modified by doping. Doping studies have provided the design rules which enable the fabrication of trap free polymers. The latter are an absolute requirement in electrophotography.
Functional Mimics of the Dimanganese Catalase Enzyme
Authors: Pessiki P. J. , Khangulov S. V. , Dismukes G. C. , Barynin V. V.
Published: March 2011

A series of synthetic enzyme mimics of the dimanganese catalase enzyme from bacterial cells has been prepared and the chemical mechanism of their activity has been examined. These are formulated as [LMn2X]Y2, μ2-X = CH3CO2−, ClCH2CO2−, Cl−, OH−; Y = ClO4−, BPh4−, CH3COO−, Cl−, Br−, utilizing the septadentate ligand, HL = N,N,N′,N′-tetrakis(benzimidazole)-1,3-diaminopropan-2-ol. These catalyze the disproportionation of H2O2into O2and H2O by a mechanism indistinguishable from the enzyme mechanism. Like the enzyme three electrons can be removed from Mn to form four oxidation states ranging from Mn2(II,II) to Mn2(III,IV). The electrochemical properties, the energy level spacing between electronic spin states (S =0, 1..5), ligand exchange and disproportionation kinetics can be controlled by the choice of the bridging ligand X. The mechanism involves the cyclic oxidation and reduction of the Mn2(II,II) and Mn3(III,III) species by H2O. An important component of the kinetic barrier in the rate-limiting step appears to be the free energy required for oxidation to the Mn2(III,III) species. This is determined by the Mn ligands. An ENDOR study of the manganese catalase(III,IV) fromT. thermophilushas revealed the Mn coordination environment to be predominantly carboxylate protein residues. This ligand environment greatly lowers the barrier to oxidation, enabling catalysis at rates approaching the diffusion limit.
Atomistic Simulations of Point Defect Properties in Silicon
Authors: Maroudas Dimitrios , Brown Robert A.
Published: March 2011

A systematic analysis based on atomistic simulations is presented for the calculation of energies and equilibrium concentrations of intrinsic point defects in silicon. Calculation of Gibbs free energies is based on the quasi-harmonic approximation for the reference state and the cumulant analysis of the enthalpy distribution function from Monte Carlo simulations in the reference state. Results are presented for the temperature dependence of enthalpies, volumes, and free energies of formation and thermal equilibrium concentrations of vacancies and self-interstitials.