Early oxidation behaviors of Mg–Y alloys at high temperatures

The early oxidation behaviors of Mg–Y alloys (Y = 0.82, 1.09, 4.31 and 25.00 wt.%) oxidized in pure O₂ have been investigated at high temperatures. The results showed that the oxidation behaviors of the Mg–Y alloys (Y = 4.31 and 25.00 wt.%) obeyed a parabolic law, while that of the Mg–Y (Y = 0.82 and 1.09 wt.%) exhibited both parabolic and linear kinetics depending on the oxidation temperature. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses indicated that an oxide film with a single structure composed of MgO and Y₂O₃ had formed. Moreover, the higher the oxidation temperature was, the thicker the oxide film was. Finally, the corresponding oxidation mechanism has been discussed, and the improved oxidation resistance of the Mg–Y alloys can be due to the formation of a continuous Mg-dissolving Y₂O₃ protective film.     

A study on electrochemical growth behavior of the Co–Ni alloy nanowires in anodic aluminum oxide template

This study shows the growth behavior of Co–Ni alloy nanowires in AAO template. Growth of nanowires consists of four different stages namely electronucleation, steady state growth, filling of pores, and coverage of filled nanowires and forming of a film on the template surface. TEM study of nanowires showed that the nanowires possess hemispherical head due to the preferable and more rapid growth phenomenon in central section of nanowires instead of edge sides. Studies on the relations between nanowires composition and ion concentration in solution showed that growth of nanowires is a diffusion-controlled process. The compositional, structural and magnetic properties of nanowires were investigated by means of EDX, TEM and VSM.     

Effect of electrolyte solvent on the morphology of polypyrrole films: Application to the use of polypyrrole in pH sensors

Electrochemical and morphological characteristics of polypyrrole (PPy) films electro-deposited from three different electrolyte solutions (acetonitrile, water and acetonitrile + water) have been investigated using atomic force microscopy and scanning electron microscopy. Experimental parameters including the electrolyte and the deposition time were shown to affect the morphologies of polypyrrole films. After characterization of the polypyrrole film morphologies, these polymer films were successfully tested as sensitive layers in pH sensors since the pH sensor responses were fast, linear and sensitive to pH changes. More, these responses of the pH sensors were dependent on the experimental conditions of the electro-deposition (thickness and solvent).     

Improvement of the electrochemical performance of LiMn2O4 cathode active material by lithium borosilicate (LBS) surface coating for lithium-ion batteries

The LBS coating on the surface of spinel LiMn₂O₄ powder was carried out using the solid-state method, followed by heating at 425 °C for 5 h in air. The powder X-ray diffraction pattern of the LBS-coated spinel LiMn₂O₄ showed that the LBS coating medium was not incorporated in the spinel bulk structure. The SEM result showed that the LBS coating particles were homogeneously distributed on the surface of the LiMn₂O₄ powder particles. The effect of the lithium borosilicate (LBS) coating on the charge-discharge cycling performance of spinel powder (LiMn₂O₄) was studied in the range of 3.5-4.5 V at 1C. The electrochemical results showed that LBS-coated spinel exhibited a more stable cycle performance than bare spinel. The capacity retention of LBS-coated spinel was more than 93.3% after 70 cycles at room temperature, which was maintained at 71.6% after 70 cycles at 55 °C. The improvement of electrochemical performance may be attributed to suppression of Mn²⁺ dissolution into the electrolyte via the LBS glass layer.     

Synthesis of in situ network-like vapor-grown carbon fiber improved LiFePO4 cathode materials by microwave pyrolysis chemical vapor deposition

The low electronic conductivity of LiFePO₄ currently limits its use in lithium ion batteries. In order to solve the problem, in situ network-like vapor-grown carbon fiber (VGCF) improved LiFePO₄ cathode materials have been prepared in one step by microwave pyrolysis chemical vapor deposition. The phase, microstructure and electrochemical performances of the composites were investigated. Compared with the cathodes without in situ VGCF, the initial discharge capacity of the composite electrode increases from 84 mAh g⁻¹ to 123 mAh g⁻¹ at 3.0 C rate, and the charge transfer resistance varies from 420 Ω to 75 Ω. The possible reasons of those are proposed.     

Hydrogen absorption properties of the slurry system composed of liquid C6H6 and F-treated Mg2Ni

The hydrogenation characteristics of the slurry composed of the NH₄F solution treated Mg₂Ni and liquid C₆H₆ were studied. The F-treatment results in a net-shaped MgF₂ surface and higher nickel content in the sub-layer. It is found that the hydride of the NH₄F treated alloy has a much higher activity for the hydrogenation of benzene. The catalytic activity for hydrogenation of the alloy depended strongly on the surface properties of the catalyst. At 483 K and under a hydrogen pressure of 4.0 MPa, the alloy absorbed hydrogen first, transformed into hydride and then the benzene was hydrogenated to cyclohexane with the hydride as the catalyst. The hydrogen absorption capacity of slurry system composed of 20 wt.% treated alloy and benzene reached 6.4 wt.% and the hydrogenation completed in 20 min. Results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) analysis on the crystal structure, surface composition and surface morphology of the untreated and treated alloy are presented and discussed.     

Effect of synthesis conditions and post-deposition treatments on composition and structural morphology of medium-phosphorus electroless Ni-P films

The optimized plating bath composition (mol%) of 0.47NiCl₂:0.23NaH₂PO₂:0.13C₄H₄Na₂O₄:3.47NaCl:95.70H₂O produced Ni-P films with ~ 10 wt.% P at 85 °C over copper and steel substrates. The presence of chloride ions (Cl⁻) in the plating solution was found to facilitate initial deposition without any need of surface activation. Film growth rates as high as 7-8 μm/h were achieved with minimal or no deposition on surfaces other than the substrates. Scanning electron microscope (SEM) and atomic force microscope (AFM) studies revealed cauliflower-like morphology with submicron size grains. Increasing the amount of reducing agent increased film smoothness and refined grain size with corresponding increase in P content. X-ray diffraction studies of the deposit revealed semi-crystalline nature of the film which underwent transition to fully crystalline form upon heat treatment with an associated increase in average HK value from ~ 390 to 807 HK. High temperature provided driving force for surface and volume diffusion that subsequently led to reduced porosity and more homogeneous composition through rearrangement of atoms to form certain Ni_xP_y phases. Acid etching of the deposits transformed shiny films into matte black surfaces due to preferential removal of nickel leading to formation of porous structure with stalagmite-like morphology.     

Nanoscale studies of the oxidation and hydrogenation of graphite surface

Nanoscale studies of the thermal oxidation and hydrogenation of graphite surfaces have been investigated using scanning tunnelling microscopy and scanning tunnelling spectroscopy. These techniques have been used to study the electronic states of the islands located at the edges of thermally oxidized and hydrogen-etched graphite surface. Spectroscopy results have shown the presence of Q_2g⁻(π), Q_2u⁻(π), Q_2u⁻(π^*), Q_2g⁻(π^*), IS and negative differential resistance electronic states recorded over islands of such treated graphite. However, these states vanish on various regions on the thermally oxidized graphite islands. This is ascribed to the disappearance of π/π^* bands when the graphite surface oxidizes and indicates the presence of oxygenated groups on the graphite surface.     

Laser-assisted synthesis of carbon nanofibers: From arrays to thin films and coatings

Carbon nanofiber assemblies in the form of non-aligned films, arrays of vertically aligned nanofibers, aligned nanofiber mats and composite coatings were produced by laser-assisted catalytic chemical vapor deposition. A visible argon ion laser was used to thermally decompose pure ethylene over alumina supported nickel catalysts. Straight, vermicular, beaded, branched and coiled individual nanofibers were observed. The effects of the laser irradiation time on individual nanofiber characteristics, thus on overall nanofiber assembly characteristics were investigated. The arrays, nanostructured films and coatings were examined by scanning electron microscopy. The individual nanofibers were examined by transmission electron microscopy. Nanofiber texture and nanotexture were assessed by lattice fringe analysis of high resolution transmission electron microscopy images. The observed variation in the interfringe distance along the nanofiber wall suggests a pulsed growth mode. This growth mode and the nanofiber shaping mechanism are discussed. Recommendations on how to control nanofiber characteristics such as shape and internal structure are provided.     

A novel photocatalyst and improvement of photocatalytic properties by Cu doping

A new series of photocatalysts, Bi₂Zn_2/3−xCu_xTa_4/3O₇ (Cu-β-BZT) crystals with pyrochlore structure were synthesized by the method of solid-state reaction (SSR). With small amount of Cu doped (0.01 ≤ x ≤ 0.04), the phase structure was kept to be monoclinic pyrochlore as pure β-BZT. The diffuse reflectance spectrum of Cu-β-BZT samples showed a red shift. The method of Cu doping enhanced the photocatalytic activity, and when the value of x is 0.03, the sample showed the highest activity, which is about 10 times higher than that of pure β-BZT under UV light. Especially, the samples of Cu-β-BZT showed photocatalytic activities under visible light irradiation (λ > 400 nm). Effects of the Cu doped on the photocatalytic activities of the catalysts were also discussed.     

Effect of shear heating during injection molding on the morphology of PC/LCP blends

Fiber relaxation of liquid crystalline polymer (LCP) in the mold during injection molding was investigated. A blend of LCP and polycarbonate was used. The LCP used, namely LC5000, is a thermotropic LCP consisting of 80% and 20% of hydroxybenzoic acid and ethylene terephthalate, respectively. The filling of the mold and the temperature profile of the melt in the mold, after the mold has been completely filled, were computed using the finite element/finite difference method (FE/FDM). The morphology of the fibers was greatly influenced by the temperature of the different layers in the sample. This was confirmed by scanning electron microscopy (SEM) examination of the injection-molded specimen. When shear heating caused the temperature of the melt to increase above 280 °C, relaxation of the fibers was rapid. This resulted in a final morphology where the LCP existed in short fibers or ellipsoids. It was concluded that the high shear rate, which is needed for fiber deformation, must be accompanied by fast cooling to minimize the effects of shear heating, so that the fibers formed could be retained.     

The effects of friction-stir process parameters on the fabrication of Ti/SiC nano-composite surface layer

Sound friction-stir processed layers were fabricated on a commercially pure titanium substrate with or without introduction of nano-sized SiC powder to the stir zone under an argon shrouding system using tool rotation and substrate advancing speeds in the range 800-1250 rpm and 35-55 mm/s, respectively. Surface layers exhibited finer grain sizes and greater hardness values compared to those of the as-received substrate. Superior surface enhancements were resulted by uniform dispersion of nano-sized SiC powder in the fabricated surface composite layer after four friction stir process passes. The fabricated Ti/SiC nano-composite surface layer showed a matrix of dynamically restorated ultra fine grains/subgrains with a mean size of ~ 400 nm and a hardness value of ~ 534 HV; this is about 3.3 times greater than that of the as-received substrate.     

Electrografting of 3-methyl thiophene and carbazole random copolymer onto carbon fiber: characterization by FTIR-ATR, SEM, EDX

Electrografting of poly(3-methylthiophene-co-carbazole) onto high tenacity (TENAX HTA 5000) carbon fiber was carried out under preparative constant-current electrolysis conditions by electropolymerizing in non-aqueous media. The surface morphology of the electrografted carbon fibers was determined by scanning electron microscopy (SEM). For characterization of the chemical composition of the random copolymer, FTIR reflectance measurements (FTIR-ATR) were performed. The efficiency of the electrocopolymerization on carbon fiber surfaces under preparative constant-current electrolysis conditions as a function of the experimental conditions was evaluated (effect on thickness and morphology).     

Flame synthesis of carbon nanotubes with high density on stainless steel mesh

Multi-walled carbon nanotubes grew directly on wires of stainless steel mesh in controllable methane diffusion flames. The formation and morphology of carbon nanotubes were dependent on gas composition of the flames. On pre-etched mesh wires with HCl, high density of carbon nanotubes were synthesized with uniform outer diameters of 60 nm and large inner diameters of 50 nm. With the high yield of carbon nanotubes, less carbon impurities were formed in the process. A mechanistic model was proposed in detail to suggest the formation of catalyst directly on bulk surface and explain the whole process of carbon nanotubes growth in this study.     

A combined composition and morphology study of electrodeposited Zn-Co and Zn-Co-Fe alloy coatings

The composition and morphology of electrodeposited Zn-Co and Zn-Co-Fe alloy coatings are studied by a variety of complementary analytical techniques. Morphology of the alloy deposits is shown to change significantly with Co content in the alloy coating. An increase in the Co content in the range of 0.7-9 wt.% Co in Zn-Co and Zn-Co-Fe alloys results in a change in grain shape from angular to nodular and a further increase up to 10 wt.% Co corresponds to a characteristic growth mode. In the range of 10-29 wt.% of Co, the deposit contains two types of grains, i.e. one with low Co content (5-7 wt.%) and another with higher Co content (i.e. 15-35 wt.%). Zn-Co and Zn-Co-Fe alloys with Co contents of or higher than 32 wt.% Co show a homogeneous structure, which can be considered to be nanocrystalline in nature. The presence of two or more phases is not desired in terms of enhanced local corrosion by (micro-)galvanic coupling of phases while the single phase or nanocrystalline coatings provide good corrosion protection properties.     

Roles of aluminium and chromium in sulfidation and oxidation of sputter-deposited Al- and Cr-refractory metal alloys

Our recent results of the sulfidation and oxidation behavior of sputter-deposited Al- and Cr-refractory metal alloys at high temperatures are reviewed, and the roles of the aluminum and chromium in sulfidation and oxidation of these alloys are discussed in this paper. Niobium, molybdenum and tantalum are highly resistant to sulfide corrosion. Their sulfidation resistance is further enhanced by alloying with aluminum. Although Cr-refractory metal alloys also reveal high sulfidation resistance, their sulfidation rates do not become lower than those of the corresponding refractory metals. The sulfide scales formed on the Al-refractory metal and Cr-refractory metal alloys consist of two layers, comprising an outer Al₂S₃ or Cr₂S₃ layer and an inner refractory metal disulfide layer. The inner layer has a columnar structure, and the growth direction of the refractory metal disulfides is perpendicular to 0 0 1 direction. Intercalation of Al³⁺ ions into NbS₂ and a decrease in the sulfur activity at the outer layer/inner layer interface by the presence of the Al₂S₃ layer are probably responsible for the improvement of the sulfidation resistance by the addition of aluminum. The oxidation resistance of niobium and tantalum is improved more effectively by the addition of chromium rather than aluminum. Although preferential oxidation of chromium does not occur, an outer protective Cr₂O₃ layer in the oxide scales is formed on Cr-rich Cr-Nb and Cr-Ta alloys due to outward diffusion of Cr³⁺ ions. In contrast, continuous alumina layer cannot be formed on the Al-Nb and Al-Ta alloys, and the alloys reveal a pest phenomenon at 1073 K, and at higher temperatures rapid oxidation occurs. Concerning the oxidation of molybdenum, the addition of aluminum, which has higher activity for oxidation than chromium, is more effective in improving the oxidation resistance of molybdenum than chromium addition, since preferential oxidation of aluminum suppresses the formation of volatile molybdenum oxide.     

Effects of intermetallic compound on the electrical and mechanical properties of friction welded Cu/Al bimetallic joints during annealing

Al/Cu metal joints applied for the electrical connector was joined by the friction welding method to limit the formation of intermetallic compound under optimum friction welding condition. To guarantee the reliability of the Al/Cu joints in service requirement, the effects of the intermetallic compound layer on the electrical and mechanical properties have been investigated under various annealing conditions. Two kinds of intermetallic compounds layer were formed in the joints interface and identified by AlCu and Al₂Cu. The growth kinetic of these intermetallics during the annealing can be followed by volume diffusion process. The activation energy of Al₂Cu, AlCu and total intermetallic compound (AlCu + Al₂Cu) represented 107.5, 98.42 and 110.22 kJ/mol, respectively. A thicker intermetallic compound layers could seriously degrade the electrical resistivity and tensile strength. The electrical resistivity with 21 μm thickness of intermetallic compound was 45 μΩ cm and increased to be 85 μΩ cm with 107 μm of intermetallic compound. Tensile strength remarkably decreased from 85 MPa to near zero at the annealing condition of 773 K and 129.6 ks and fracture occurred through the intermetallic compound layers.     

Electrochemical impedance spectroscopy and morphological analyses of pyrrole, phenylpyrrole and methoxyphenylpyrrole on carbon fiber microelectrodes

In this study, N-pyrrole (Py), N-phenylpyrrole (PhPy), and 1[4-methoxyphenyl]-1H-pyrrole (MPhPy) homopolymers were synthesized electrochemically onto carbon fiber microelectrodes (CFMEs). The influences of the substituent effect on electrochemical impedance spectroscopy (EIS) were studied comparatively. All the monomers were electrodeposited in 0.05 M tetraethyl ammonium perchlorate (TEAP)/dichloromethane (CH₂Cl₂) solution and characterized by cyclic voltammetry (CV), Fourier transform infrared reflectance spectrophotometry (FTIR-ATR), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The morphological study reveals that the polymers were deposited as a continuous and well adhered film to surface of the CFME. An equivalent electrical circuit for three different monomers on CFMEs was proposed and experimental data were simulated to obtain the numerical values of circuit components. All results support the high quality film deposition that resulted in desired electronic properties due to the electron donating behaviors of substituent group of phenyl and methoxy.     

The effect of Cu addition on the thermoelectric properties of Cu2CdGeSe4

Recently, research in copper based quaternary chalcogenide materials has focused on the study of thermoelectric properties due to the complexity in the crystal structure. In the present work, stoichiometric quaternary chalcogenide compounds Cu_2+xCd_1−xGeSe₄ (x = 0, 0.025, 0.05, 0.075, 0.1, 0.125) were prepared by solid state synthesis. The powder X-ray diffraction patterns of all the samples showed a tetragonal crystal structure with the space group I-42m of the main phase, whereas the samples with x = 0 and x = 0.025 revealed the presence of an orthorhombic phase in addition to the main phase as confirmed by Rietveld analysis. The elemental composition of all the samples characterized by Electron Probe Micro Analyzer showed a slight deviation from the nominal composition. The transport properties were measured in the temperature range of 300 K–723 K. The electrical conductivity of all the samples increased with increasing Cu content due to the enhancement of the hole concentration caused by the substitution of Cd (divalent) by Cu (monovalent). The positive Seebeck coefficient of all the samples in the entire temperature ranges indicates that holes are the majority carriers. The Seebeck coefficient of all the samples decreased with increasing Cu content and showed a reverse trend to the electrical conductivity. The total thermal conductivity of all the samples decreased with increasing temperature which was dominated by the lattice contribution. The maximum figure of merit ZT = 0.42 at 723 K was obtained for the compound Cu_2.1Cd_0.9GeSe₄.     

Reactivity and deuterium retention properties of titanium-beryllium intermetallic compounds

Beryllium intermetallic compounds (beryllides), such as Be₁₂Ti and Be₁₂V, are the most promising advanced neutron multipliers in demonstration (DEMO) fusion power reactors because of higher stability, lower retention, and swelling. The advanced neutron multipliers are being developed by Japan and the EU as a framework of Broader Approach (BA) activities, targeted at broadening the research fields to not only establish fabrication methods but also for their characterization. Our group has proposed a plasma sintering method for the synthesis of beryllides. When the mixed powder was plasma-sintered at the beginning of the experiment, consolidation of the target composition was so insufficient that single-phase beryllides could not be synthesized. In order to obtain single-phase beryllides, an additional homogenization treatment of the sintered beryllides at 1473 K was necessary, resulting in increased porosity. Using the homogenized powder as the starting material, single-phase Be₁₂Ti and Be₁₇Ti₂ intermetallic compounds were successfully synthesized. As experimental results, the hardness of the compounds was relatively low owing to low sintering density. In addition, the oxidation behavior of the beryllides, when exposed to 15% H₂O/Ar at high temperatures, were investigated and the results indicate the presence, gray colored Be oxide formed on the surface in Be₁₂Ti tested at above 1073 K and Be oxide with a small fraction of white colored Ti oxide even tested at 873 K in Be₁₇Ti₂ phase resulting in the bigger increase of the weight gain than Be₁₂Ti. In addition, this oxidation occurring at lower temperature can be reasoned by the assumption that the existence of BeO (=oxygen content) in Be₁₂Ti (1.95%) and Be₁₇Ti₂ (2.95%) may facilitate increased reactivity. Furthermore, thermal expansion of Be₁₇Ti₂ was found out to be bigger than that of Be₁₂Ti because Be₁₇Ti₂ has more complex crystal structure and higher melting temperature. In terms of deuterium desorption and retention properties, the maximum peaks are detected around 600 K in beryllides, while those in beryllium occur at 800 and 980 K. From the comparison of deuterium retention, it was obvious that Be₁₇Ti₂ has a lower retention than Be₁₂Ti, while Be has the highest value.