Sliding wear of silicon carbide modified by etching with chlorine at various temperatures

The effect of reaction temperature on the formation of a carbon layer on the surface of SiC has been investigated. Subsequently, the tribological properties of the formed carbon layers were studied. The experimental procedure involved exposing reaction-bonded SiC balls to a flowing gas mixture of 5% Cl₂, 2.5% H₂, and Ar at a high temperature of 800, 1000, or 1200 °C. A ball-on disk tribometer was used to investigate the friction and wear behavior of the treated specimens. While partially unreacted SiC phases were observed in the layer modified at 800 °C, rhombohedral graphite crystals were formed in the layer modified at 1200 °C. Compared to untreated SiC, the treated SiC materials were found to have relatively low friction coefficients and better wear resistance. Increasing the treatment temperature was found to improve the tribological performance of the resulting surface-modified SiC balls. A possible reason for this tribological improvement has been discussed based on the observed carbon phases.     

Percolation threshold related to field-effect transistors using thin multi-walled carbon nanotubes composites

We fabricated thin-film field-effect transistors (TF-FETs) using thin multi-walled carbon nanotubes (t-MWCNTs) and poly (methyl methacrylate) (PMMA) composites as the active layer. The gate-dependent current–voltage characteristics, the current on/off ratio (I_on/off), and the dc conductivity (σ_dc) were measured as a function of various weight (wt.%) of t-MWCNTs. The typical p-type FET characteristics were observed. We found that the field-effect I_on/off increased rapidly for TF-FETs with a wt.% of t-MWCNTs below 0.6. For the TF-FETs with a wt.% of t-MWCNT above 0.6, the I_on/off was relatively low. From the measured σ_dc as a function of the wt.% of t-MWCNTs, the percolation threshold (p_c) was observed to be approximately 0.6 wt.% for the t-MWCNT composites. We infer that the TF-FET characteristics are closely related to the p_c for the charge conduction of the t-MWCNTs composites.     

Microstructure and mechanical properties of metallic glass/metallic glass composites

We report microstructure and mechanical properties of bulk metallic glass (BMG)/metallic glass composites fabricated by mechanical alloying with subsequent consolidation process. The microstructural investigations of a bulk composite reveal that a submicron-scale layered structure with irregular interfaces consists of three amorphous phases in tornado-like morphology. Based on these results, poor plasticity of the metallic glass composite can be understood possibly due to the irregular interfacial morphology of the submicron-scale heterogeneous amorphous phases throughout the materials.     

Application of electroless plating process for multiscale Ni-La0.8Sr0.2Ga0.8Mg0.2O3-σ SOFC anode fabrication

An electroless plating process of nickel is introduced to solve the drawbacks of impregnation for developing the multiscale anode of a solid oxide fuel cell (SOFC). Impregnation is the conventional fabrication method of the electrode. The process is not favorable for depositing nanoscale metal catalysts due to severe problems including agglomeration of the catalysts while reducing metal oxides. Thus, as an alternative, we propose electroless plating of nickel to fabricate a multiscale nickel-based SOFC anode. A Ni-LSGM (La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-σ) anode is selected. The low chemical compatibility of LSGM with nickel emphasizes the advantage of the electroless plating process. First, nanoscale nickel particles are successfully applied as the main catalyst of the SOFC anode by plating nickel to the surface of the LSGM scaffold substrate near the triple phase boundary region. Thin film X-ray diffraction and image analysis confirm that pure nanoscale nickel particles form on the entire substrate, even at a low temperature (60 °C) without secondary phase formation. Electrochemical impedance spectroscopy analysis is then performed to verify the possibility of implementing an efficient Ni-LSGM anode through nickel electroless plating. As a result, the new Ni-LSGM anode shows ～50 times higher electrochemical performance than that of an impregnated Ni-LSGM anode.     

Effect of unsintered gadolinium-doped ceria buffer layer on performance of metal-supported solid oxide fuel cells using unsintered barium strontium cobalt ferrite cathode

In this study, a Gd_0.1Ce_0.9O_1.95 (GDC) buffer layer and a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathode, fabricated without pre-sintering, are investigated (unsintered GDC and unsintered BSCF). The effect of the unsintered GDC buffer layer, including the thickness of the layer, on the performance of solid oxide fuel cells (SOFCs) using an unsintered BSCF cathode is studied. The maximum power density of the metal-supported SOFC using an unsintered BSCF cathode without a buffer layer is 0.81 W cm⁻², which is measured after 2 h of operation (97% H₂ and 3% H₂O at the anode and ambient air at the cathode), and it significantly decreases to 0.63 W cm⁻² after 50 h. At a relatively low temperature of 800 °C, SrZrO₃ and BaZrO₃, arising from interaction between BSCF and yttria-stabilized zirconia (YSZ), are detected after 50 h. Introducing a GDC interlayer between the cathode and electrolyte significantly increases the durability of the cell performance, supporting over 1000 h of cell usage with an unsintered GDC buffer layer. Comparable performance is obtained from the anode-supported cell when using an unsintered BSCF cathode with an unsintered GDC buffer layer (0.75 W cm⁻²) and sintered GDC buffer layer (0.82 W cm⁻²). When a sintered BSCF cathode is used, however, the performance increases to 1.23 W cm⁻². The adhesion between the BSCF cathode and the cell can be enhanced by an unsintered GDC buffer layer, but an increase in the layer thickness (1-6 μm) increases the area specific resistance (ASR) of the cell, and the overly thick buffer layer causes delamination of the BSCF cathode. Finally, the maximum power densities of the metal-supported SOFC using an unsintered BSCF cathode and unsintered GDC buffer layer are 0.78, 0.64, 0.45 and 0.31 W cm⁻² at 850, 800, 750 and 700 °C, respectively.     

Oxygen-carrier selection and thermal analysis of the chemical-looping process for hydrogen production

The three-reactor chemical-looping process (TRCL) for the production of hydrogen from natural gas is quite attractive for both CO₂ capture and hydrogen production. The TRCL process consists of a fuel reactor, a steam reactor and an air reactor. In the fuel reactor, natural gas is oxidized to CO₂ and H₂O by the lattice oxygen of the oxygen carrier. In the steam reactor, the steam is reduced to hydrogen through oxidation of the reduced oxygen carrier. In the air reactor, the oxygen carrier is fully oxidized by air. In this process, the oxygen carrier is recirculated among the three reactors, which avoids direct contact between fuel, steam and air. In this study, various candidate materials were proposed for the oxygen carrier and support, and a thermal analysis of the process was performed. The oxygen carrier for the process must have the ability to split water into hydrogen in its reduced state, which is a different chemical property from that of the chemical-looping combustion medium. The selection of the oxygen carrier and support require careful consideration of their physical and chemical properties. Fe₂O₃, WO₃ and CeO₂ were selected as oxygen carriers. Thermal analysis indicated an expected hydrogen production of 2.64 mol H₂ per mol CH₄ under thermoneutral process conditions. The results indicated that hydrogen production was affected mainly by the steam-conversion rate. The solid-circulation rate and temperature drop in the fuel reactor were calculated for the selected oxygen carriers with different metal oxide contents and solid-conversion rates.     

Effects of crystal and electronic structures of ANb2O6 (A=Ca,Sr, Ba) metaniobate compounds on their photocatalytic H2 evolution from pure water

Alkali-earth metaniobate compounds, ANb₂O₆ (A = Ca, Sr, Ba), were prepared by the conventional solid-state reaction route and their electronic band structures and photocatalytic activities were investigated. The prepared powders were characterized using X-ray diffraction (XRD), field-emission electron microscopy (FE-SEM), UV–vis diffuse reflectance spectroscopy, and fluorescence spectroscopy. It was found that the particle sizes (∼1 μm) and BET surface areas (∼1 m²/g) of the metaniobate compounds were nearly identical. From the electronic band structure calculations, however, the band-gap energies of these metaniobate compounds were found to be in the order of CaNb₂O₆ > SrNb₂O₆ > BaNb₂O₆. These calculated band-gap energies were consistent with those estimated from the UV–vis diffuse reflectance spectra. Moreover, the conduction-band edge (reduction potential) of SrNb₂O₆ calculated from the electronegativity data was higher than those of CaNb₂O₆ and BaNb₂O₆. The photoluminescence spectra revealed that CaNb₂O₆ exhibited a strong blue luminescence emission (at 300K), while no obvious emissions were observed in either SrNb₂O₆ or BaNb₂O₆. The luminescence behaviors of these metaniobate compounds and their band structure variations originating from their crystal structures play an important role in their photocatalytic activity for the evolution of H₂ from pure water. SrNb₂O₆, which has a higher conduction-band edge potential than the other compounds, exhibited higher photocatalytic activity.     

Effect of particle size on the microstructure and properties of kinetic sprayed nickel coatings

This study aims to examine the effect of particle size on the microstructure and corresponding properties in kinetic sprayed coatings of commercially pure nickel (CP-Ni) in conjunction with finite element modeling (FEM). Prior to the experiments, the adhesion factors (interface temperature, contact time and contact area), rebound factor (relative recovery energy) and the resultant critical velocities of CP-Ni for different particle sizes and temperatures were estimated by FEM. Based on the simulations, three different sized CP-Ni powders were successfully deposited onto mild steel substrates using a powder preheating system. Here we suggest optimized windows of operation for particle sizes of CP-Ni based on the microstructure and properties of the coatings (i.e. deposition efficiency, bond strength and micro-hardness) which are in good correspondence with the simulation results.     

Assessment of the crack growth characteristics at the low corrosion fatigue limit of an A106 Gr b steel pipe weld

In the area of heavy construction, welding processes are vital in the production and maintenance of pipelines and power plants. The fusion welding process generates formidable welding residual stresses and metallurgical change, which increase the crack driving force and reduce the resistance of brittle fracture as well as environmental fracture. This is a serious problem with many alloys as well as A106 Gr B steel pipe. This pipe, used in petrochemical and heavy chemical plants, either degrades due to corrosive environments, e.g., chlorides and sulfides, and/or becomes damaged during service due to the various corrosion damage mechanisms. Thus, in this study, the sulfide corrosion fatigue strength of multi-pass welded A106 Gr B steel pipe was evaluated in a 5.0 wt.% NaCl solution that was saturated with H2S gas at room temperature on the basis of NACE TM 0177-90. The crack growth characteristics of the welded pipe were then assessed at the low limit of sulfide corrosion fatigue strength, which was previously obtained from the sulfide corrosion fatigue (SCF) tests. From the results, in terms of the SCF, all the specimens failed at the heat-affected zone, where a high welding residual stress distributes. It was found that the fatigue crack grew at the low corrosion fatigue limit (σ _{SCFun-notched}), which was 32 % (160MPa) of the ultimate tensile strength (502MPa) of the welded specimens.     

Studies on process parameters for chlorine dioxide production using IrO2 anode in an un-divided electrochemical cell

Chlorine dioxide is potentially a powerful oxidant with environmentally compatible application in several strategic areas relating to pollution control typically for water disinfection, and its sustained production is a key factor for its successful application. Although increased attention has been paid for on-line chlorine dioxide generation by several chemical and electrochemical methods, the details are mostly confined as patents. We studied in this work the electrochemical generation of chlorine dioxide from an un-buffered solution of sodium chlorite and sodium chloride mixture in an un-divided electrochemical cell under constant current mode, with a view to optimize various process parameters, which have a direct bearing on the chlorine dioxide formation efficiency under laboratory conditions. The effect of feed flow rate (10-150 ml min(-1)), feed solution pH (2.3-5.0), concentration of sodium chloride (0-169.4mM), concentration of sodium chlorite (0-7.7 mM), and the applied current (100-1200 mA) on the formation of dissolved ClO(2) gas in solution and the pH of the product-containing solution was investigated by performing single pass experiments, with no circulation, in a cell set-up with Ti/IrO(2) anode and Ti/Pt cathode. The current efficiency and the power consumption were calculated for the optimized conditions.