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A Cr/Pt bilayered catalytic layer on a flat glass substrate is used as a counter electrode in order to improve the energy conversion efficiency of a dye-sensitized solar cell device with an effective structure area of 0.45 cm2 glass/FTO/blocking layer/TiO2/N719(dye)/electrolyte/50 nm Pt/50 nm Cr/glass. For comparison, 100 nm-thick Pt and Cr counter electrodes on flat glass substrates are also prepared using the same procedure. The sheet resistance is examined using a four-point probe. The photovoltaic properties, such as the short circuit current density, open circuit voltage, fill factor, energy conversion efficiency, and impedance, are characterized using a solar simulator and potentiostat. The phases of the bilayered films are examined using X-ray diffraction. Furthermore, the microstructure of the bilayered films is characterized via field emission electron microscopy. The measured energy conversion efficiencies of the dye-sensitized solar cell devices with only Pt and Cr/Pt bilayer counter electrodes are 4.60% and 6.68%, respectively. The interface resistance at the interface between the counter electrode and electrolyte decreases when a Cr/Pt bilayer thin film is applied. The new phases of Pt3Cr enable an increase in the catalytic activity. This suggests that the Cr/Pt bilayer catalytic layers might improve the efficiency of dye-sensitized solar cells compared with the conventional Pt layers.

keyword : solar cells, sputtering, optical properties, solar simulator, bilayer

The Kroll process of magnesium reduction of titanium tetrachloride is used for mass production of zirconium sponges. This study is conducted in a laboratory-scale reactor in order to develop a better understanding of the zirconium sponge formation mechanism in the Kroll reactor with respect to the reaction degrees and reaction time. The MgCl2 produced during the initial stage of the reaction does not sink into the molten magnesium, but remains on the surface of the molten magnesium. As a result, ZrCl4 feed reacts with the Mg exposed on the edge of the molten MgCl2 in the crucible. Therefore, the Zr particles produced at the later reaction stage descend into the molten magnesium at the crucible wall.

keyword : Kroll process, ZrCl4, zirconium sponge, magnesium, reduction

This study focuses on the fabrication of the self-lubricating ceramic composite coating layers with low friction components at high temperatures. Al2O3-15 wt% ZrO2 coatings, that contain solid lubricants such as CaF2, BaF2, h-BN, MoS2, and WS2, are deposited using the atmospheric plasma spray (APS) method at a gun power of 41400 W, and their tribological behaviours are investigated. BaF2, MoS2, and WS2 decompose to β-Ba, Mo, and W, respectively, during the APS coating process. The friction coefficients of the deposited Al2O3-15 wt%ZrO2 composite coating layers are in the range of 0.35 to 0.65. The tribological behaviors of the APSed Al2O3-15 wt% ZrO2-solid lubricant composite coating layers are dominated by the formation of transfer films of wear debris and microcracking during sliding.

keyword : plasma deposition/spray, coating, wear, composites

Ternary CdxZn1-xO thin films are deposited by a sol-gel dip-coating method onto quartz substrates with various amounts of Cd content (x = 0, 0.05, 0.15, and 0.25). The structural and optical properties of the CdxZn1-xO thin films are investigated using x-ray diffraction (XRD), photoluminescence (PL), and (ultravioletvisible) (UV-Vis) spectroscopy. In the XRD patterns, the intensity of the diffraction peaks for ZnO decreases, while the intensity of the diffraction peaks for CdO increases with increases in the Cd content. Above x = 0.15, both diffraction peaks for ZnO and CdO are observed in the XRD pattern, and this indicates that the wurtzite structure for ZnO and the rock-salt structure for CdO coexist in CdZnO thin films. The PL spectra of the films results demonstrate that the near band edge emission peaks in the ultraviolet region shift to lower energy range (red-shift) and the deep level emission peaks at the visible region decreases with increases in the Cd content. The optical properties such as transmittance, optical band gap, and Urbach energy are calculated using the optical data. Using linear fitting of the absorption edge, the band gap energies of the thin films are derived as 3.27, 3.19, 3.12, and 3.03 eV for x = 0, 0.05, 0.15 and 0.25, respectively. In addition, the bowing parameter for the energy band gap of CdxZn1-xO is estimated to be Eg( x) = 3.3 . 1.2 x + x2.

keyword : CdZnO, thin films, sol-gel, x-ray diffraction, optical properties

Three different carbon blacks (Vulcan, Ketjen Black, and 1:1 mixture of the two) are used as cathode catalysts for a lithium/air secondary battery, and their electrochemical properties are investigated. The surface morphology change after discharge is examined using scanning electron microscopy (SEM), and the discharge products are analyzed using Fourier transform-infrared spectroscopy (FT-IR). In order to measure the electrochemical properties, the charge and discharge measurements are conducted at a constant current density of 0.2 mA/cm2, which monitors the voltage change. Electrochemical impedance spectroscopy (EIS) analyses are also performed in order to examine the change in the charge transfer resistance during the charge-discharge process. Compared with both the Ketjen black and Vulcan, the 1:1 mixture of the two carbon blacks exhibits the best performance as a cathode for the Li/air secondary battery, which results from a synergistic effect that exploits-both carbon blacks.

keyword : Energy storage material, Coating, Electrochemistry, Scanning, electron microscopy, SEM, Li/Air Secondary Battery, Cycleability

The high cycle fatigue and fatigue crack propagation behaviors of the recently developed modified A7075-T73 alloy are investigated. This alloy is manufactured using Eco-Mg (including CaO) instead of a magnesium element. The alloy consists of an Al matrix, Al2CuMg, MgZn2, minute amounts of Ca phases and reduced Fe-based intermetallic or oxide. The modified A7075 alloy exhibits superior strengths (YS: 492 MPa, TS: 548 MPa) with a similar elongation (12.8%) compared with the conventional A7075. Furthermore, the high cycle fatigue properties are significantly improved (fatigue limit of 330 MPa at 107 cycles) compared with those of the conventional alloys. The fatigue crack propagation rate of the modified A7075 is similar or somewhat lower (higher resistance) than those of the conventional alloys. The superior tensile and fatigue properties of the modified A7075-T73 alloy are primarily attributed to the grain size refinement, evenly distributed strengthening phases (MgZn2 precipitates), and reduction of detrimental effects of the Fe- intermetallic phases and oxides.

keyword : Alloys, Modified A7075-T73, Extrusion, Fatigue, Scanning Electron Microscopy, SEM

The hydrogen storage properties of pure Mg are investigated at 573 K under 12 bar H2. In addition, in order to increase the hydriding and dehydriding rates of pure Mg, it is ground under hydrogen (reactive mechanical grinding, RMG), and its hydrogen storage properties are investigated. The pure Mg absorbs hydrogen very slowly. At n = 1, the pure Mg absorbs 0.05 wt.% H for 5 min, 0.08 wt.% H for 10 min, and 0.29 wt.% H for 60 min at 573 K under 12 bar H2. The hydriding rate decreases as the number of cycles increases from n = 7. At n = 7, the pure Mg absorbs 0.96 wt.% H for 5 min, 1.29 wt.% H for 10 min, and 2.20 wt.% H for 60 min. At n = 1, the pure Mg after RMG does not absorb hydrogen. The hydriding rate of pure Mg after RMG increases as the number of cycles increases from n = 1 to n = 11. The pure Mg after RMG absorbs 1.91 wt.% H for 5 min, 2.61 wt.% H for 10 min, and 3.65 wt.% H for 60 min at n = 11. The reactive mechanical grinding of the pure Mg and the hydriding-dehydriding cycling of the pure Mg after RMG are believed to create defects on the surface and in the interior of Mg particles and to form cracks in Mg particles.

keyword : hydrogen absorbing materials, mechanical alloying/milling, microstructure, X-ray diffraction, pure magnesium

The optimization of the gas atomization for particle size control and optimal energy efficiency cannot be achieved simply, due to the lack of understanding of the basic atomization process. A numerical analysis is performed in order to optimize the manufacturing process and to understand the behavior of the gas at the end of the nozzle. In this study, the gas flow is analyzed numerically using a computational fluid dynamics software package (Fluent). It is found that a protrusion length of 3 mm is optimal for nozzles with a quick gas velocity and stable aspiration pressure. In addition, the impact on the air gap size is also investigated.

keyword : Metals, Numerical simulation, Gas atomization, Nozzle design

When complex-phase (CP) steel with a tensile strength of more than 1 GPa grade is joined using resistance spot welding (RSW), the optimum Ti content should be chosen in order to satisfy the weldability and mechanical properties. Therefore, in this study, the effect of the Ti content (0~300 ppm) on the tensile- shear load of the RSW in CP steels is investigated. Because the resistivity of the base metal is independent of the Ti content, it does not affect the nugget diameter. Regardless of the Ti content, the specimens under 5t1/2 (t = sheet thickness) are fractured in the interfacial failure mode. In the low welding current condition (lower than 6.4 kA), the measured nugget diameters are smaller than the calculated critical nugget diameter regardless of the amount of Ti addition in order that the fracture mode is interfacial failure. Pull-out failure occurs at the softened zone, which is the boundary between the base metal and heat-affected zone. The tensile-shear load of the specimen failure in the pull-out mode increases as the fractured diameter and hardness of the softened zone increase. The shear load is only dependent on the fractured diameter.

keyword : metals, joining, microstructure, scanning electron microscopy, SEM, tensile-shear load

Abstract: In this study, the effect of the material strength grades of cold rolled dual phase 590, 780, and 980 MPa steels on the weld strength of gas metal arc welding lap joints is investigated. With increasing strength grades of the base metal, the bead geometry is similar for identical welding currents and speeds. However, the failure location changes from a base metal to a fusion zone and the joint efficiency decreases with increasing material strength grades. This is attributed to the changes in the location of minimum hardness that result from the heat-affected zone softening phenomena. Joint efficiency is related to the ratio between the base metal hardness and minimum hardness of the welds. A prediction formula for joint efficiency (η) is obtained in terms of proportion of base metal hardness and minimum hardness (HMIN/HBM) as follows: η = 0.801 (HMIN/HBM) + 20.0.

keyword : metals, welding, mechanical properties, hardness test, joint efficiency