발간논문

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Abstract: The effects of pre-corrosion and applied potentials in artificial seawater on the S-N fatigue behavior of X80 steel were examined. The X80 specimens were pre-corroded in a FeCl3 solution for varying immersion times ranging from 0 to 96 h and subsequently S-N fatigued in air. It was found that the resistance to S-N fatigue decreased abruptly with 1 h immersion, while it became saturated with a further increase in immersion time. The trend observed in this study was relatively well explained by the stress concentration effect from the presence of corrosion damage on the surface. The in-situ S-N fatigue tests were also conducted on X80 steel in artificial seawater under applied potentials of .600, .850 and .1,050 mV (SCE). The fractographic and micrographic analyses were conducted on the fatigue specimens to identify the optimum cathodic protection conditions.

keyword : alloys, rolling, fatigue, scanning electron microscopy (SEM), pre-corrosion

A durability testing method under thermo-mechanical stress for thermal barrier coatings (TBC) specimens was designed by a combination of an electric furnace and a tensile testing machine, which was done on TBCs on NIMONIC 263 substrates by an atmospheric plasma spraying (APS) deposition method. The testing conditions were chosen according to a preliminary experiment that identified the elastic deformation region of the top coating and the substrate during mechanical loading. Surface cracking and a decrease in the thickness of the top coating, which are typical degradation behaviors under conventional thermal shock testing, were observed after the designed thermal fatigue test, and delamination at the top coating-bond coating interface occurred by the mechanical load. Lifetime assessment was conducted by statistical software using lifecycle data which were obtained after the thermal fatigue test.

keyword : ceramics, plasma deposition/spray, fatigue, tensile test, durability test

Nanopowders of MgO, Al2O3 and SiO2 were made by high energy ball milling. The simultaneous synthesis and consolidation of nanostuctured MgAl2O4-Mg3Al2Si3O12 composites from milled 4MgO, 2Al2O3 and 3SiO2 powders was investigated by a high-frequency induction heating process. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition of grain growth. Highly dense nanostructured MgAl2O4-Mg3Al2Si3O12 composites were produced with a simultaneous application of 80 MPa pressure and an induced current within 1 min. The relative density and fracture toughness of the composite were 98% and 3.2 MPam1/2, respectively. The fracture toughness of the MgAl2O4-Mg3Al2Si3O12 composite was higher than that of monolithic MgAl2O4.

keyword : compounds, nanostructured materials, sintering, mechanical properties

In this study, porous Al-Cu sintered body was fabricated without any space-holder material and its properties, such as porosity, compressive yield strength, shore hardness, and permeability, were evaluated. Al-Cu powder mixtures were prepared by low-energy ball milling. The ball-milled powders were granulated and classified into size with <150 μm and 150-300 μm, and then they were compacted and sintered at 600 ℃ for 1 h in a vacuum and N2 atmosphere. The porosities of sintered bodies could be controlled in range of 15-48% by controlling the compaction pressure. The compressive yield strength and hardness were increased with a decreasing porosity. As a result, the mechanical properties and permeability of the optimized porous body were superior to that of commercial one.

keyword : porous materials, sintering, mechanical properties, compression test, permeability

The reduction behavior of molten electric arc furnace (EAF) slag containing (FeO), (MnO) and (P2O5) was investigated using graphite as a reductant. Reaction temperature (1400-1600 ℃) and slag basicity (0.5-2.0) were varied during the experiment. Alumina and magnesia crucibles were used to observe the effect of refractory on the reduction behavior of the molten EAF slag. When an alumina crucible was used, (Al2O3) content in the slag increased significantly by its elution from the crucible. The content of poorly reducible oxides, (CaO), (MgO), and (SiO2), maintained an almost constant level regardless of the reaction temperature, because the increase of (Al2O3) and the decrease in reducible oxides (FeO, MnO, P2O5 etc.) offset one another. The reduction degree of reducible oxides evaluated through chemical composition analysis agreed well with that calculated by off-gas analysis. The reduction degree increased with an increase in the reaction temperature and the slag basicity. The reduction degree in the magnesia crucible was higher than that in the alumina crucible at identical conditions. When a magnesia crucible was used, the reduction reaction was more promoted due to the enhancement of the activity of (FeO) and (MnO), compared to the alumina crucible.

keyword : oxides, recycling, melting, X-ray diffraction, slag, off-gas analysis

Abstract: After separation of valuable metals from the leaching solution of ores or secondary resources, the recovery of inorganic acids from the raffinate is important. Therefore, the possibility of recovering inorganic acids, such as HCl, H2SO4 and HNO3, by extraction with Tris 2-ethylhexyl amine (TEHA) was investigated. The solvent extraction reaction of acid by TEHA was identified by applying a slope analysis method to the extraction data. The extraction efficiency of the acids by TEHA was in the order HNO3 > HCl > H2SO4, which corresponds well to the variation in the electric charge densities of the anions of each acid. The loading capacity of TEHA for HNO3, HCl and H2SO4 was determined and the extraction behavior of the acid seemed to be affected by the acid concentration in the organic phase. It was possible to strip all the HCl and H2SO4 by using distilled water at room temperature, while the stripping percentage of HNO3 at the same conditions was very low.

keyword : solvent extraction, stripping, TEHA, inorganic acid

For automotive parts used at elevated temperatures, such as transmission cases and engine blocks, it is necessary to improve the creep resistance of magnesium alloys. For this reason, various magnesium alloys have been developed for elevated temperature applications. In this study, the microstructure and creep behavior of CaO added Mg-Al-Si alloys (Mg-5.0Al-0.7Si, Mg-5.0Al-0.7Si-0.5CaO, Mg-5.0Al-0.7Si- 0.5CaO-0.3Sr) were investigated. The microstructures were observed by OM, SEM and TEM. To evaluate the mechanical properties, tensile tests were carried out at room and high temperature, and creep tests were performed at 150 ℃ with applied loads from 50 to 80 MPa.

keyword : alloys, casting, creep, tensile test, CaO

This paper describes an inverted bulk heterojunction organic solar cell featuring solution-processed zinc oxide nanoparticles (ZnO NPs) as the electron transport layer. ZnO NPs with an average diameter of ~5 nm are synthesized by a wet chemistry method. The ZnO NP layers are deposited from colloidal ZnO NPs dispersed in a mixed solvent having various volumetric ratios of chlorobenzene (CB) and isopropyl alcohol (IPA). After the coating step, the surface roughness of the coating films was reduced and that was related to the dispersion of the ZnO NP solution. The maximum performance of the solar cell was achieved when the ZnO NPs were suspended within a volumetric ratio (50 vol%:50 vol%) of CB and IPA, with a solar power conversion efficiency of 3.74% at AM 1.5 G, 100 mW/cm2 illumination.

keyword : solar cells, chemical synthesis, electrical properties, AFM, ZnO nanoparticle

Abstract: The properties of the Cr/Pt bilayered catalytic layer with additional low temperature annealing using a dye sensitized solar cell (DSSC) were investigated. A DSSC device with a structure with an effective area of 0.45 cm2 glass/FTO/blocking layer/TiO2/N719 (dye)/electrolyte/50 nm Pt/50 nm Cr/glass was prepared. For comparison, 100 nm-thick Pt and Cr counter electrodes on flat glass substrates were also prepared using the same procedure. The sheet resistance was examined using a four point probe. The photovoltaic properties, such as the short circuit current density, open circuit voltage, fill factor, energy conversion efficiency (ECE), and impedance, were characterized using a solar simulator and potentiostat. The phases of the bilayered films were examined by X-ray diffraction. The microstructure of the bilayered films was characterized by atomic force microscopy. The measured sheet resistance of the counter electrode with annealing increased to. The measured ECE of the dye-sensitized solar cell devices with vacuum annealed Cr/Pt bilayer counter electrodes decreased abruptly. The interface resistance at the interface between the counter electrode and electrolyte increased as the annealing time increased. The increase in catalytic activity of the Cr/Pt bilayer before annealing resulted from the effect of a compressive strain field. After 10 minutes, the strain field was removed by annealing. The new phases of Pt3Cr and CrPt by annealing led to drastically decreased catalytic activity.

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

Abstract: Samples with the compositions of 80 wt% Mg-14 wt% Ni-6 wt% Fe2O3, 80 wt% Mg-14 wt% Ni- 6 wt% Ti and 80 wt% Mg-14 wt% Ni-3 wt% Fe2O3-3 wt% Ti (called Mg-14Ni-6Fe2O3, Mg-14Ni-6Ti and Mg-14Ni-3Fe2O3-3Ti, respectively) were prepared by reactive mechanical grinding. Their hydriding and dehydriding rates were then measured at the first cycle, and the specific BET surface areas, grain sizes, and strains of the samples were analyzed. The quantities of hydrogen absorbed and desorbed for x min are indicated as Ha (x min) and Hd (x min), respectively. The values of Ha (10 min) and Hd (30 min) for Mg-14Ni-6Ti were the largest, followed in order by Mg-14Ni-3Fe2O3-3Ti and Mg-14Ni-6Fe2O3. The particles on the surface of the Mg-14Ni-6Ti were the finest, followed in order by Mg-14Ni-3Fe2O3-3Ti and Mg-14Ni-6Fe2O3. Mg-14Ni-6Ti had the smallest grain size, followed in order by Mg-14Ni-3Fe2O3-3Ti and Mg-14Ni-6Fe2O3. The inverse dependence of the values of Ha (10 min) and Hd (30 min) on the grain size indicates that the values of Ha (10 min) and Hd (30 min) increase as the grain size decreases and thus the grain boundary area increases.

keyword : hydrogen absorbing materials, mechanical alloying/milling, microstructure, X-ray diffraction, grain boundary area