Influence of coatings on microstructure and mechanical properties of preceramic paper-derived porous alumina substrates

Preceramic papers loaded with inorganic fillers may be used as preforms in a novel manufacturing technique to fabricate lightweight ceramic structures. In order to reduce the porosity caused by burning out cellulosic fibers and organics, porous preceramic paper-derived alumina substrates were post-treated via two different coating routes using silica suspension or methylphenylvinylhydrogen polysiloxane. Sintering of the alumina-filled preceramic papers in air at 1600 °C for 2 h resulted in a non-uniform distributed open porosity ranging from 23 to 26%. After coating and infiltration, all samples were additionally heat treated up to at 1500 °C for 2 h. Thermal analysis (DTA/TG) was applied to determine the pyrolysis temperature of polysiloxane. Microstructure and phase analysis were performed respectively by SEM and XRD. After sintering, water absorption, apparent density and open porosity of test pieces were determined, and mechanical properties of the substrates were evaluated before and after coating. For the samples coated with silica suspension, the mechanical strength remained in the same range of those for uncoated samples, while for the polysiloxane coated samples the mechanical strength steadily increases after repeated impregnation steps, reaching ～350 MPa.     

Phase Constitution and Dynamic Properties of Spark Plasma‐Sintered Alumina–Titanium Composites

Al₂O₃/Ti composites of various metal to ceramic ratios were fabricated by the spark plasma sintering (SPS) technique. The effects of titanium concentration in the initial mixture on phase composition, and on the static and dynamic (planar impact testing) mechanical properties of the SPS‐processed composites were investigated. It was observed that the significant alumina dissolution in titanium takes place during SPS treatment. The composites fabricated from starting alumina/titanium powder mixtures with a mass fraction of titanium less than 0.8 consisted of two phases, alumina and a solid solution of oxygen and aluminum in titanium. For starting mixtures with higher titanium content, the presence of a Ti₃Al intermetallic phase with a relatively low fraction of dissolved oxygen was detected. Changes in phase composition could explain the effect of titanium content in the starting mixture on physical and mechanical properties of the composites. Mechanisms governing the dynamic response of the composites under loading of different intensities are discussed.     

Corrugated glass–ceramics from LZSA cast tapes

The aim of this work was to fabricate corrugated lightweight structures from green tapes of Li₂O–ZrO₂–SiO₂–Al₂O₃ (LZSA) glass–ceramics. The green tapes were cast, thermopressed, corrugated, and sintered at 800 °C for 1 h, yielding low porous and homogenous microstructures confirmed by electron microscopy analysis. Dilatometric analysis demonstrated three linear regions, each one exhibiting a specific thermal expansion coefficient (TEC) in the 25–800 °C temperature range. It was also concluded that 600 °C was the critical temperature above which the LZSA glass–ceramic structure could not be assembled. The fabricated bodies were also characterized macroscopically and geometrically to verify the integrity and the morphology of the structures.     

Authoritarianism, leader acceptance, and group cohesion.

An examination of the validity of the Frommian compromise between leader-centered and member-centered theories of group cohesiveness is made, using a sample of enlisted men of the Air Defense Command. The variables of authoritarianism and leader-acceptance were measured in this sample by means of Likert-type scales. The Frommian hypothesis is supported to some extent, since, as expected, under conditions of military organization, more high authoritarians than low apparently accept or idealize their formally designated leader; and, attractive force of the group is apparently stronger for high than for low scorers on the authoritarianism scale. The strong positive association between leader acceptance and group cohesion, for high authoritarians, that is expected on the Frommian hypothesis is not found. A discussion of the Frommian compromise is then made. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effect of Processing on Mechanical Properties of Platelet-Reinforced Mullite Composites

SiC- and Al₂O₃-platelet-reinforced mullite-matrix composites were fabricated by both conventional powder processing and a pressure filtration route with constant filtration rate. Consolidation during the pressure filtration experiment was monitored as a function of time with respect to actual pressing stages. The packing density of green bodies was strongly affected by the different processing methods. Either fracture toughness (3.2 MPa · m^1/2) or bending strength (344 MPa) increased, depending on surface conditioning of the SiC platelets. Pressure filtration was shown to decrease processing flaws in the platelet-reinforced bodies leading to substantial improvement of the mechanical properties.     

A dry electrochemical cell containing cupric chloride supported by γ-alumina

An electrochemical cell was formed by enclosing a commercial catalyst containing cupric chloride supported by γ-alumina between electrodes of copper and iron. The voltage obtained is a logarithmic function of the vapour pressure of the water sorbed by the support. Voltages of a few hundred millivolts are obtained at relative pressures between 0.1 and 0.8 and temperatures between 25 and 170°C.     

A novel sensor platform based on aptamer-conjugated polypyrrole nanotubes for label-free electrochemical protein detection

We first present a simple yet versatile strategy for the functionalization of polymer nanotubes in a controlled fashion. Carboxylic-acid-functionalized polypyrrole (CPPy) nanotubes were fabricated by using cylindrical micelle templates in a water-in-oil emulsion system, and the functional carboxyl groups were effectively incorporated into the polymer backbone during the polymerization by using pyrrole-3-carboxylic acid (P3CA) as a co-monomer without a sophisticated functionalization process. It was noteworthy that the chemical functionality of CPPy nanotubes was readily controlled in both qualitative and quantitative aspects. On the basis of the controlled functionality of CPPy nanotubes, a field-effect transistor (FET) sensor platform was constructed to detect specific biological entities by using a buffer solution as a liquid-ion gate. The CPPy nanotubes were covalently immobilized onto the microelectrode substrate to make a good electrical contact with the metal electrodes, and thrombin aptamers were bonded to the nanotube surface via covalent linkages as the molecular recognition element. The selective recognition ability of thrombin aptamers combined with the charge transport property of CPPy nanotubes enabled the direct and label-free electrical detection of thrombin proteins. Upon exposure to thrombin, the CPPy nanotube FET sensors showed a decrease in current flow, which was probably attributed to the dipole-dipole or dipole-charge interaction between thrombin proteins and the aptamer-conjugated polymer chains. Importantly, the sensor response was tuned by adjusting the chemical functionality of CPPy nanotubes. The efficacy of CPPy nanotube FET sensors was also demonstrated in human blood serum; this suggests that they may be used for practical diagnosis applications after further optimization.