Microstructural Evidence for Diffusion-Driven Tetragonal-to-Orthorhombic Phase Transformation in Y-Ba-Cu-O Superconductor

Microstructures of Y-Ba-Cu-O superconductor were observed using a polarized microscope. Orthorhombic and tetragonal phases were easily distinguished by the optical etching with cross-polarized light. In the specimen cooled rapidly in oxygen, it was possible to see the path of oxygen diffusion by observing the variation of the twin concentration. At the boundary between transformed and untransformed materials, evidence for diffusion-driven transformation could be clearly seen.     

Highly Efficient Overall Water Splitting Through Optimization of Preparation and Operation Conditions of Layered Perovskite Photocatalysts

The layered perovskite materials were found to give the high photocatalytic activity in water splitting reaction under UV irradiation, where the electronic structure of perovskite slab constructing the layered structure (the total cation valency) was the most crucial factor to the high photocatalytic activity. Both the excessive cation valency and the layered structure were required for active photocatalysts, while the slab thickness of layered perovskites had an insignificant effect on water-splitting activity. In order to identify key variables that affected photocatalytic activity and to optimize the performance of (110) layered perovskite, La₂Ti₂O₇ was modified by various methods. The optimum amount of loaded nickel had a great effect and the amount depended on the surface area of the perovskite phase. When an alkaline-earth element such as Ba, Sr, and Ca was doped on La₂Ti₂O₇, the photocatalytic activity was enhanced markedly. Introduction of an alkaline hydroxide into the reaction system as an external additive enhanced the activity further showing extremely high quantum yields close to 50%.     

Synthesis and electrical properties of poly[2-(2-(4-trifluoromethyl)phenyl)ethenyl-1,4-phenylenevinylene] and PPV copolymers

Summary Poly[2-(2-(4-(trifluoromethyl)phenyl)ethenyl)-5-methoxy-1,4-phenylenevinylene] (PFEMPV) and a series of PPV copolymers containing 1,4-phenylenevinylene (PV) units were synthesized through a water-soluble precursor route, and their electrical and third-order nonlinear optical properties were studied. The PFEMPV films could not be doped with I₂, but FeCl₃-doped films showed an electrical conductivity of 5.0x10^-4 S/cm. The conductivities of FeCl₃-doped copolymer films ranged from 2.0x10^-3 to 2.0 S/cm depending on their copolymer compositions. The third-order nonlinear optical susceptibility, ⁽³⁾(–;,,–), was also investigated by the degenerate four wave mixing technique at 602 nm. The ⁽³⁾ value of PFEMPV was 6.9x10^-11 esu. The photoluminescence spectrum of PFEMPV shows its emission maximum at 550 nm.     

Structure-property relationships in rubber-toughened epoxies

Epoxies toughened with two reactive liquid rubbers, an epoxy-terminated butadiene acrylonitrile rubber (ETBN) and an amino-terminated butadiene acrylonitrile rubber (ATBN), were prepared and studied in terms of their structure property relationships. A two-phase structure was formed, consisting of spherical rubber particles dispersed in an epoxy matrix. A broad distribution of rubber particles was observed in all the materials with most of the particles ranging in size from 1 to 4 μm, but some particles exceeding 20 μm were also found. Impact strength, plane strain fracture toughness (K_IC), and fracture energy (G_IC) were increased, while Young's modulus and yield strength decreased slightly with increasing rubber content and volume fraction of the dispersed phase. Both G_IC and K_IC were found to increase with increasing apparent molecular weight between crosslinks and decreasing yield strength. The increased size of the plastic zone at the crack tip associated with decreasing yield strength could be the cause of the increased toughness. An ATBN-toughened system containing the greatest amount of epoxy sub-inclusion in the rubbery phase demonstrated the best fracture toughness in this series. In the present systems, rubber-enhanced shear deformation of the matrix is considered to be the major toughening mechanism. Curing conditions and the miscibility between the liquid rubber and the epoxy resin determine the phase morphology of the resulting two-phase systems. Kerner's equation successfully describes the modulus dependence on volume fraction for the two-phase epoxy materials.     

Fabrication of High-Curie-Point Barium-Lead Titanate PTCR Ceramics

High-Curie-point semiconducting barium-lead titanate positive temperature coefficient of resistivity (PTCR) ceramics of composition Ba_0.897Pb_0.1La_0.003TiO₃ and Ba_0.5Pb_0.5La_0.003TiO₃ were prepared. The starting powders were synthesized by reacting commercial BaTiO₃, PbO, and TiO₂. To avoid the nonstoichiometry due to the volatilization of Pb during the sintering process, a lead atmosphere sintering approach with PbTiO₃ as packing powder was used. The samples being fabricated by this method show a PTCR effect of 3 to 4.5 orders of magnitude above the Curie point. The curie points were about 180°C for Ba_0.897Pb_0.1La_0.003TiO₃ and about 360°C for Ba_0.497Pb_0.5La_0.003TiO₃.     

Pb-Diffusion Barrier Layers for PbTiO3 Thin Films Deposited on Si Substrates by Metal Organic Chemical Vapor Deposition

The interfaces between metal organic chemical vapor deposited PbTiO₃ thin films and various diffusion barrier layers deposited on Si substrates were investigated by transmission electron microscopy. Several diffusion barrier thin films such as polycrystalline TiO₂, amorphous TiO₂, ZrO₂, and TiN were deposited between the PbTiO₃ thin film and Si substrate, because the deposition of PbTiO₃ thin films on bare Si substrates produced Pb silicate layers at the interface irrespective of the deposition conditions. The TiO₂ films were converted to PbTiO₃ by their reaction with diffused Pb and O ions during PbTiO₃ deposition at a gubstrate temperature of 410°C. Further diffusion of Pb and O induces formation of a Pb silicate layer at the interface. ZrO₂ did not seem to react with Pb and O during PbTiO₃ deposition at the same temperature, but the Pb and O ions that diffused through the ZrO₂ layer formed a Pb silicate layer between the ZrO₂ and Si substrate. The TiN films did not seem to react with Pb and O ions during the deposition of PbTiO₃ at 410°C, but reacted with PbTiO₃ to form a lead-deficient pyrochlore during postdeposition rapid thermal annealing at 700°C. However, TiN could effectively block the diffusion of Pb and O ions into the Si substrate and the formation of Pb silicate at the interface.     

Properties of chitosan‐based biopolymer films with various degrees of deacetylation and molecular weights

An increase in the depolymerization of chitosan was found with an increased concentration of sodium perborate. Acetic anhydride was added to reacetylated chitosan in a molar ratio per gulcosamine unit, and the amide I band of IR spectra changed with the addition of acetic anhydride. Sixteen chitosans with various molecular weights (MWs) and degrees of deacetylation (DODs) were prepared. X‐ray diffraction patterns indicated their amorphous and partially crystalline states. Increases in the chitosan MW and DOD increased the tensile strength (TS). TS of the chitosan films ranged from 22 to 61 MPa. However, the elongation (E) of chitosan films did not show any difference with MW. TS of chitosan films decreased with the reacetylation process. However, E of chitosan films was not dependent on DOD. The water vapor permeabilities (WVPs) of the chitosan films without a plasticizer were between 0.155 and 0.214 ng m/m² s Pa. As the chitosan MW increased, the chitosan film WVP increased, but the values were not significantly different. Moreover, the WVP values were not different from low DOD to high DOD. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3476–3484, 2003     

Modeling nonisothermal impregnation of fibrous media with reactive polymer resin

The manufacture of polymer composites through resin transfer molding (RTM) or structural reaction injection molding (SRIM) involves the impregnation of a fibrous reinforcement in a mold cavity with a reactive polymer resin. The design of RTM and SRIM operations requires an understanding of the various parameters, such as materials properties, mold geometry, and mold filling conditions, that affect the resin impregnation process. Modeling provides a potential tool for analyzing the relationships among the important parameters. The present work provides the physical model and finite element formulations for simulating the mold filling stage. Resin flow through the fibers is modeled using two-dimensional Darcian flow. Simultaneous resin reaction and heat transfer among resin, mold walls, and fibers are considered in the model. The proposed technique emphasizes the use of the least squares finite element method to solve the convection dominated mass and energy equations for the resin. Excellent numerical stability of the proposed technique provides a powerful numerical method for the modeling of polymer processing systems characterized by convection dominated transport equations. Results from example numerical studies for SRIM of polyurethane/glass fiber composites were presented to illustrate the application of the proposed model and numerical scheme.     

Calcium binding mode of {gamma}-carboxyglutamic acids in conantokins

Conantokin-T (con-T) and conantokin-G (con-G) are two highlyhomologous peptide toxins found in Conus venom. The former isa 21-residue peptide with four -carboxyglutamic acid (Gla) residues(at positions 3, 4, 10 and 14), while the latter is a 17-residuepeptide with five -carboxyglutamic acid residues (at positions3, 4, 7, 10 and 14). Despite the apparent similarity in numberand relative positions of the -carboxyglutamic acid residues,¹¹³Cd-NMR studies indicated a distinct metal binding behaviorfor con-G and con-T. There appears to be four binding sitesin con-G in contrast to one metal binding site in con-T. Toelucidate the mode of calcium binding by the -carboxyglutamicacid residues in these conantokins, we designed various analogouspeptides with their -carboxyglutamic acid replaced by otheramino acid residues. ¹¹³Cd-NMR experiments on conantokin analoguesreveal that the major difference in the number of metal bindingsites between con-G and con-T is due to the residue at position7. We also performed molecular simulations to calculate therelative binding free energies of several potential bindingsites. Based on our theoretical and experimental results, wepropose a `four-site' binding model for conantokin-G and a `single-site'binding model for conantokin-T.     

Study on the thermoplastic vulcanizate using ultrasonically treated rubber powder

Nowadays, waste EPDM (ethylene propylene diene monomer) increasingly has been causing significant environmental problems with increasing numbers of vehicles. From the perspective of the environment and economics, recycling is the best method to treat waste materials. This study investigated waste EPDM/PP (polypropylene) blends with waste EPDM. Waste EPDM powders were treated ultrasonically, which physically modifies the rubber particles to confer good mechanical properties. Also investigated were the relevance of the mass percentage of the dispersed phase, the influence of the geometry and rotation speeds of the screw used in extrusion, and the melting temperature of PP materials on the morphology and mechanical properties of the blend. The purpose of this study was to develop a valuable thermoplastic elastomer from waste EPDM. This study concentrated on determining the optimum conditions for producing a blend by extrusion, including parameters of screw geometry, screw rotational speed, and operating temperature. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2503–2507, 2003