Amperometric cholesterol biosensors based on hybrid organic–inorganic Langmuir–Blodgett films

Preparation and characterization of a thin-film cholesterol biosensor employing an organic–inorganic hybrid system of cholesterol oxidase (ChOx) and Prussian blue (PB) is described. ChOx was immobilized in Langmuir–Blodgett (LB) films consisting of positively charged octadecyltrimethylammonium (ODTA) and nano-sized PB clusters. Immobilization was performed by simple immersion of ODTA/PB LB films into an aqueous solution of ChOx. Subsequent ChOx absorption into LB films was confirmed by infrared reflection–absorption spectroscopy. Obtained ODTA/PB/ChOx LB films clearly exhibited a response current to cholesterol under an applied potential of 0.0 V (vs. Ag/AgCl). The linearity of current density versus cholesterol concentration was confirmed for the range 0.2–1.2 mmol/L. The present study indicates that simple immersion of ODTA/PB LB films into an enzyme solution is a promising method to produce many types of thin-film biosensors comprising a hybrid system of an oxidative enzyme and PB nano-clusters that work at a very low potential range.     

Activated pressureless infiltration of metal-matrix composites with graded activator content

Metal-matrix composites (MMCs) were produced by activated pressureless infiltration of porous Al₂O₃ compacts with the presence of elemental titanium as an activator and steel (X38CrMoV5-1) as the metal matrix. The quality of infiltration was subsequently investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The results show that poor quality of infiltration is associated with blocking of infiltration channels due to the formation of Ti-rich phases which are accumulated over infiltration depth. To prevent the pores blocking, a layer-graded activator green body in which the activator quantity is decreased with infiltration depth was used. Furthermore, it could be shown that the mechanism based on evaporation/condensation of metal onto activator particles is in agreement with the results of this study.     

Recrystallization mechanism of as-cast AZ91 magnesium alloy during hot compressive deformation

Dynamic recrystallization (DRX) behavior of as-cast AZ91 magnesium alloy during hot compression at 300 °C and the strain rate of 0.2 s⁻¹ was systematically investigated by electron backscattering diffraction (EBSD) analysis. Twin DRX and continuous DRX (CDRX) are observed in grains and near grain boundaries, respectively. Original coarse grains are firstly divided by primary {} tensile twins and {} compression twins, and then {}–{} double twins are rapidly propagated within these primary compression twins with increasing compressive strain. Some twin-walled grains are formed by the mutual crossing of twins or by the formation of the {}–{} double twins and furthermore, subgrains divided by low-grain boundaries in the double twins are also formed. Finally, DRXed grains are formed by the in situ evolution of the subgrains with the growth of low-angle boundaries to high-angle grain boundaries in twins. CDRX around the eutectic Mg₁₇Al₁₂ phases at grain boundaries occurs together with the precipitation of discontinuous Mg₁₇Al₁₂ phase and the fragmentation of the precipitates during compression. The discontinuous fragmented precipitates distribute at the newly formed CDRXed grain boundaries and have remarkable pinning effect on the CDRXed grain growth, resulting in the average grain size of about 1.5 μm.     

Mechanical and elastic properties of transparent nanocrystalline TeO2-based glass-ceramics

Mechanical and elastic properties of transparent TeO₂-based glass-ceramics (15K₂O · 15Nb₂O₅ · 70TeO₂) consisting of nanocrystalline particles (each particle size: 40–50 nm) and showing optical second harmonic generation were evaluated by means of usual Vickers indentation and nanoindentation tests. The precursor glass has Vickers hardness H _v of 2.9 GPa, Young's modulus E of 54.7 GPa, the fracture toughness K _c of 0.25 MPam^1/2 and Poisson's ratio of 0.24. The transparent nanocrystalline glass-ceramic heat-treated at 420°C for 1 h has H _v = 3.8 GPa, E = 75.9 GPa and K _c = 0.34 MPam^1/2, and the opaque glass-ceramic heat-treated at 475°C for 1 h has H _v = 4.5 GPa, E = 82.9 GPa and K _c = 0.68 MPam^1/2, demonstrating that poor mechanical and elastic properties of the precursor TeO₂-based glass are improved through sufficient crystallization. The fracture surface energy, brittleness and elastic recoveries (about 44%) after unloading (the maximum load: 30 mN) of transparent nanocrystalline glass-ceramics are almost the same as those of the precursor glass, implying that the interaction among nanocrystalline particles is not so strong.     

Fretting wear of nitrogen-implanted AISI 304 stainless steel

Fretting wear resistance of nitrogen-implanted AISI 304 stainless steel was measured and compared to that of unimplanted steel. After 5 × 10³ cycles, unimplanted steel revealed severe damage on non-slip area and adhesive-type wear on microslip region whereas nitrogen-implanted stainless steel was still undamaged. The improved fretting wear resistance is explained to be due to the increased load carrying capacity and decreased adhesion of nitrogen-implanted steel.     

Electronic conductivity of catalyst layers of polymer electrolyte membrane fuel cells: Through-plane vs. in-plane

In this work, novel procedures are developed to measure in-plane and through-plane electronic conductivities of catalyst layers (CLs) for polymer electrolyte membrane fuel cells. The developed procedures are used in a parametric study on different CL designs to investigate effects of different composition and fabrication parameters, including ionomer to carbon weight ratio (I/C ratio), dry milling time of the catalyst powder, and drying temperature of the catalyst ink. Results show that CLs have anisotropic electronic conductivity with through-plane values being three orders of magnitude lower than the in-plane values. The reason for this anisotropy is speculated to be alignment of fibrillar nanostructures of ionomer by large shear forces during coating, which could result in better carbon-carbon contact in the in-plane direction. A simple order of magnitude analysis shows the significance of poor through-plane conduction for fuel cell performance.     

Morphological instability of ZrO2 crystallites formed by CVD technique operated under atmospheric pressure

Zirconium oxide (ZrO₂) crystallites were deposited using a chemical-vapor-deposition apparatus operated under atmospheric pressure on a silicon substrate. The X-ray diffraction pattern reveals that the crystallites have a preferred crystallographic orientation along the ZrO₂ <200> direction and ascertains the presence of the monoclinic structure. The sample morphology indicates the existence of the aggregation of the whisker with a growth rate of 0.5 nm/s, 0.7 nm/s and 0.9 nm/s corresponding to the substrate temperature of 650°C, 680°C and 700°C, respectively. High temperatures introduce morphological instability in the whiskers. The appearance of a dendrite structure is an example of such morphological instability. Furthermore, a noticeable cathodoluminescence emission is observed in the U.V. and blue regions at room temperature.     

Raman spectra and structure study of silicate glasses and their liquids

Stress index of tetrahedron （SIT） was defined to describe the topological connectivities among various sili- con-oxygen tetrahedra （SiOT） in anionic clusters of binary silicate crystals, glasses, and melts. It was found that the value of SIT was well correlated with the wavenumber of Raman active symmetric stretching vibration of non-bridging oxygen of SiOT. The spatial fractional dimension of hyperfine structure was introduced while comparative analysis was made with the value of SIT. It can be concluded that the concepts of SIT, vibrational wavenumber, and spatial fractional dimension were inherently and holographically correlated and exhibit isomorphic representations of complex structure of binary silicates. Experimental Raman spectra of binary silicates with different alkali cations were investigated. It was demonstrated that alkali cations have little effect on the vibrational wavenumber of symmetric stretching of non-bridging oxygen （NBO） of SiOT, but remarkably affect its Raman active optical cross section, as was consensus resulted from ab initio calculation. It can also be concluded that the spatial fractional dimension of binary silicate is predominantly determined by the hyperfine structure of the anionic clusters and little affected by alkali cations, although the species of anionic clusters and their distributions were originally assigned by the content of alkali oxides. And Raman optical activity extinct effect of isolated SiOT at high basicity should be considered while being applied to quantitatively analysis.     

Influence of size and distribution of W phase on strength and ductility of high strength Mg-5.1Zn-3.2Y-0.4Zr-0.4Ca alloy processed by indirect extrusion

A high strength Mg-5.1Zn-3.2Y-0.4Zr-0.4Ca(wt%) alloy containing W phase(Mg_3Y_2Zn_3) prepared by permanent mold direct-chill casting is indirectly extruded at 350?C and 400?C, respectively. The extruded alloys show bimodal grain structure consisting of fine dynamic recrystallized(DRXed) grains and unrecrystallized coarse regions containing fine W phase and β2' precipitates. The fragmented W phase particles induced by extrusion stimulate nucleation of DRXed grains, leading to the formation of fine DRXed grains, which are mainly distributed near the W particle bands along the extrusion direction. The alloy extruded at 350?C exhibits yield strength of 373 MPa, ultimate tensile strength of 403 MPa and elongation to failure of 5.1%. While the alloy extruded at 400?C shows lower yield strength of 332 MPa,ultimate tensile strength of 352 MPa and higher elongation to failure of 12%. The mechanical properties of the as-extruded alloys vary with the distribution and size of W phase. A higher fraction of DRXed grains is obtained due to the homogeneous distribution of micron-scale broken W phase particles in the alloy extruded at 400?C, which can lead to higher ductility. In addition, the nano-scale dynamic W phase precipitates distributed in the un DRXed regions are refined at lower extrusion temperature. The smaller size of nano-scale W phase precipitates leads to a higher fraction of un DRXed regions which contributes to higher strength of the alloy extruded at 350?C.     

Structural stabilization of EBC with thermal energy reflection at high temperatures

Structural stability of an environmental barrier coating (EBC) with thermal energy reflection function that has a periodic layered structure consisting of Y₂Ti₂O₇ (YT) and Al₂O₃ is essential to maintenance of the EBC performance at high temperatures. The effect of adding Al to YT layer on the structural stability was investigated using model samples in which Al₂O₃ layer was formed on both the YT and Al-doped YT (AYT) substrate by aerosol deposition (AD). Exposure to heat selectively dissipated the Al₂O₃ layer on the YT substrate near the interface between the layer and the grain boundaries of the substrate. In contrast, the Al₂O₃ layer on the AYT substrate remained intact upon heating when Al was added at the solid solution limit. The Al₂O₃ layer was found to exhibit a pronounced degree of (0001) basal plane texture. An increase in the impact velocity of particles during AD effectively developed the basal fiber texture.