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Global warming and air pollution have forced greater attention to new energy sources to replace fossil fuels. Among several eco-friendly energy sources, polymer electrolyte membrane fuel cells have been increasingly investigated since they have zero emissions, high energy density, and high energy efficiency. Carbon-supported Pt catalyst is generally used for the cathodic catalyst in polymer electrolyte membrane fuel cells. However, Pt/C catalysts corrode under start-up/shut-down conditions. Pt agglomeration, separation, and loss can occur due to the carbon corrosion, which results in a rapid performance loss. Metal oxide is a promising candidate as an alternative support since it shows high stability in the high potential. Of several metal oxides, titanium oxides and tin oxides have been widely investigated. Their performance is comparable to the Pt/C catalyst, and they have shown even higher durability than the Pt/C catalyst in accelerated stress tests simulating start-up/shut-down conditions. In this paper, we summarize the development of metal oxide supports for the Pt catalyst in the five most recent years. In recent studies, the characteristics of metal oxides have been varied using new synthesis methods, annealing temperature, precursors, and dopants, which results in enhanced ORR activity and durability. Advanced metal oxides have shown high durability and exhibited acceptable performance compared to the state-of-the-art Pt/C catalysts. (Received 16 October, 2022; Accepted 3 January, 2023)

keyword : metal oxide, oxygen reduction reaction, proton exchange membrane fuel cells, catalytic activity, stability

To ensure the lifetime of marine constructions and the safety of workers and pedestrians, corrosion protective non-skid coating is an effective solution. However, the conventional polymer-based coating has some limitations. In this study, newly-suggested Al and Al-3%Ti coatings were deposited on high strength low alloyed steel substrate using twin wire arc spraying (TWAS). The static and dynamic friction coefficients of the Al-based coatings under dry and wet conditions were measured using portable friction testers. To evaluate the corrosion behavior under sea water conditions, a cyclic potentiodynamic polarization test (CPDP) and salt solution immersion test (SSIT) were performed with a 3.5% NaCl solution. To confirm the coating degradation, mechanical properties (Vickers hardness and adhesion strength) were compared before and after SSIT. The results showed that the TWAS Al-based coatings were well fabricated on HSLA steel and had the general microstructure of a thermal spray. The coatings provided excellent corrosion protection for the steel substrate and greatly increased the friction coefficient of the surface. The Vickers hardness slightly increased and adhesion strength decreased after SSIT. The microstructure observation revealed that the TWAS coatings had a bimodal structure induced by non-uniform droplet generation at the TWAS tips. After SSIT, some oxides formed on the surface and porous regions of the coatings. This indicated that the TWAS coating successfully provided corrosion protection and non-skid properties. (Received 17 October, 2022; Accepted 3 January, 2023)

keyword : twin wire arc spray, TWAS, mechanical property, friction coefficient, cyclic potentiodynamic polarization, CPDP, salt solution immersion test, SSIT

In this study, the relationship between solidification cracking and epitaxial growth behavior with the high-speed laser surface melting of a directionally solidified 247LC superalloys was fundamentally and metallurgically investigated, to develop a successful welding procedure for the next generation of gas turbine blades. Under typical laser surface melting conditions (scan speed: 50 mm/s, heat input: 40 J/mm), severe solidification cracking phenomena occurs. The key metallurgical factors of solidification cracking have been identified as solidification segregation-assisted pipeline diffusion behavior at the solidification grain boundary, and in the randomly formed polycrystalline melting zone microstructure. In addition, under extremely low heat input and high-speed laser beam scan conditions (scan speed: 1000 mm/s, heat input: 2 J/mm), an effective surface melting zone can be obtained within a single directionally solidified grain under a relatively high-energy beam density (65 J/mm2) using the characteristics of single-mode fiber lasers. Results reveal that the laser melting zone successfully shows a 99.9% epitaxial growth achievement ratio. Because of the superior epitaxial growth ratio within the laser surface melting zone, and the rapid solidification phenomena, formation of a solidification grain boundary and solidification segregation-assisted pipeline diffusion behavior can be suppressed. Finally, a solidification crack-free laser melting zone can thus be achieved. (Received 9 December, 2022; Accepted 28 December, 2022)

keyword : directional solidification, 247LC superalloy, laser surface melting, epitaxial growth, solidification cracking

The solidification and phase transformation behaviors of IN625 with low C and Fe were investigated through directional solidification and quenching experiments. The primary and secondary dendrite arm spacing decreased exponentially as the solidification rate increased, demonstrating good agreement with theoretical equations. The MC carbide formed at a constant temperature gradient regardless of the solidification rate. It is reasonable to assume that the Laves phase solidified in the final stage of solidification, as it was found at the bottom of the mushy zone. The morphologies and sizes of the MC carbide and the Laves phase were closely related to the solidification rate. Increasing the solidification rate at a constant thermal gradient resulted in the development of the MC carbide from blocky to Chinese script, and eventually spotty shapes, along with decreasing size. As a result of this research, it is believed that the weldability of the alloy could be improved at the expense of mechanical properties, especially with low C and Fe contents and a relatively high solidification rate, which are able to generate the finely distributed MC and Laves phase. (Received 23 November, 2022; Accepted 10 January, 2023)

keyword : directional solidification, superalloy, IN 625, phase transformation, laves phase

Permingeatite (Cu₃SbSe₄) is a promising thermoelectric material with narrow bandgap energy and large carrier effective mass. However, doping is required to improve its electrical conductivity and thermoelectric properties. In this study, Cu₃Sb_1-y(Al/In)_ySe₄ doped with B^III-group elements (Al or In) at the Sb sites was synthesized using mechanical alloying followed by sintering through hot pressing. The resulting Cu₃Sb_1-y(Al/In)_ySe₄ contained a single phase of permingeatite with a tetragonal structure and therefore achieved a high relative density of 97.5-99.2%. The substitution of Al/In at the Sb sites produced lattice constants of a = 0.5652-0.5654 nm and c = 1.1249-1.1254 nm. As the Al/In doping content increased, the carrier (hole) concentration increased, reducing the Seebeck coefficient and increasing the electrical and thermal conductivities. Substituting Al³⁺ or In³⁺ at the Sb⁵⁺ site can generate additional carriers, resulting in a high electrical conductivity of (1.4-1.1) × 10⁴ Sm^-1 at 323-623 K for Cu₃Sb_0.92In_0.08Se₄. Cu₃Sb_0.96Al_0.04Se₄ exhibited a maximum power factor of 0.51 mWm^-1K^-2 at 623 K and a minimum thermal conductivity of 0.74 Wm^-1K^-1, resulting in a maximum dimensionless figure of merit, ZT, of 0.42 at 623 K. Cu₃Sb_0.96In_0.04Se₄ obtains a ZT of 0.47 at 623 K, indicating a high power factor of 0.65 mWm^-1K^-2 at 623 K and low thermal conductivity of 0.84 Wm^-1K^-1 at 523 K. (Received 31 October, 2022; Accepted 3 January, 2023)

keyword : thermoelectric, permingeatite, mechanical alloying, hot pressing, doping

Skinnerite (Cu₃SbS₃) has recently attracted attention as a promising thermoelectric material because of its low thermal conductivity. In this study, we performed the solid-state synthesis of Fe-doped skinnerite Cu₃Sb_1-yFe_yS₃ (y = 0.02-0.06) using mechanical alloying and hot pressing, and examined the samples’ thermoelectric properties relative to Fe doping content. All samples contained only a cubic skinnerite phase, as evidenced by X-ray diffraction. As the Fe content increased, the lattice constant decreased from 1.03370 to 1.03310 nm, indicating successful substitution of Fe at the Sb sites. The carrier concentration increased with the Fe doping level, resulting in increased electrical conductivity. The specimens with y = 0.02-0.04 exhibited non-degenerate semiconductor behavior, where the electrical conductivity increased as the temperature increased. In contrast, a specimen with y = 0.06 changed conduction behavior to the degenerate state with minimal temperature dependence. As the Fe content increased, the Seebeck coefficient decreased, and Cu₃Sb_0.98Fe_0.02S₃ exhibited a maximum power factor of 1.16 mWm^-1K^-2 at 623 K. Thermal conductivity values for all specimens were lower than 1.20 Wm^-1K^-1 in the measured temperature range but were higher than undoped skinnerite. The highest thermoelectric performance was achieved by the Cu₃Sb_0.98Fe_0.02S₃ specimen, with a dimensionless figure of merit, ZT, of 0.9 obtained at 623 K. (Received 5 December, 2022; Accepted 3 January, 2023)

keyword : thermoelectric, skinnerite, mechanical alloying, hot pressing, doping

Solar energy is attracting much attention as an eco-friendly source for future energy needs. Herein, NiZnS photocatalysts were synthesized with a hydrothermal method at various sulfur contents. The ZnS material is widely used as a photocatalyst because of its high stability, low toxicity, and excellent charge separation characteristics. Nickel is considered a co-component in the ZnS base to improve hydrogen evolution efficiency, because nickel sulfide has a narrow band gap. Field emission scanning electron microscopy analysis was used to observe particle size and shape. As the sulfur ratio increased, the particle size increased, and relatively uniform particle sizes were obtained at the 2:2 molar ratio of NiZn:S. X-ray diffractometer analysis showed the formation of ZnO crystals at low sulfur contents in the NiZnS photocatalysts. Among the various NiZnS compositions, the NiZn:S ratio of 2:2 resulted in the highest hydrogen production rate (1541.5 μmol/g/h) with stable reproducibility. UV-vis spectroscopy was used to analyze light absorbance, and the band gap changed with different sulfur contents due to the oxygen vacancies in ZnO, as identified by X-ray photoelectron spectroscope. High amounts of thiourea used to introduce the sulfur increased the particle sizes and blocked sunlight coming to NiZnS surfaces, thereby degrading photocatalytic performance. Therefore, changing the sulfur content when fabricating the NiZnS composite photocatalysts affected the crystalline structures and band characteristics of the materials, and it finally resulted in improved light absorption, charge separation, and the hydrogen production rate of the photocatalysts. (Received 26 December, 2022; Accepted 7 January, 2023)

keyword : NiZnS photocatalyst, hydrogen production, water splitting, band gap, oxygen vacancy

A lot of slag was produced by ancient production processes such as smelting, melting, and refining. Iron slag has information that can be used to determine the characteristics of the site, the iron-making process and manufacturing date, but there is a limit to reading information through visual observation and nondestructive analysis. Various slag terms have been used depending on the location and characteristics, but it is necessary to use terms that have exact meaning and unity. Although many scientific analyses have been supplemented by archaeological theses, the iron-making trends of each period have not yet been systematically proven. This study reviewed the type of iron-making process used for 100 ancient slags. The slags were analyzed by XRF, revealing when it was formed and some of the detailed processes. XRD analysis was used to define and classify Tap slag, Bloom slag, and Ceramic-rich slag as relative concepts. From the Three Kingdoms Period to the Joseon Dynasty, the amount of temperature variation in the iron-making furnace decreased and the GAS reaction became uniform. It was observed that the amount of iron oxide remaining decreased and the proportion of ceramic-rich slag increased. The change in iron oxide content kept decreasing while the levels of refining, smelting, and melting were maintained, in that order. The results confirmed that advances in ancient iron-making technology can be interpreted based on a comparative review of the relative changes in iron oxide and silicon oxide content in slag. (Received 23 August, 2022; Accepted 28 December, 2022)

keyword : smelting, melting, refining, tap slag, bloom slag, ceramic-rich slag

The creep properties of Alloy 690, used as a steam generator tube material in nuclear power plants, were evaluated at 650℃, 750℃, and 850℃. The parameters of creep life prediction models were derived using the Larson-Miller (LM), Manson-Haferd (MH), and Orr-Sherby-Dorn (OSD) models, to use as mechanical properties under a virtual severe accident condition like station black out (SBO). The yield strength (YS) and creep property of Alloy 690 were compared with those of Alloy 600, and the effects of the precipitation behavior of Cr carbides on creep properties were analyzed. The YS of Alloy 600 decreased rapidly above the temperature of 750℃, but the YS of Alloy 690 decreased linearly up to the temperature of 850℃ because of the formation of M23C6 carbides. The creep stress exponent (n) of Alloy 690 was between 5 and 6, and this indicated that dislocation creep was the major creep mechanism at the test temperatures. The results of creep tests were well matched with the LM, MH, and OSD models for Alloy 690, and there were no significant differences in accuracy between the models. The stress-rupture test results of Alloy 600 and Alloy 690 using the LM model showed that the decrease in creep strength with rupture time of Alloy 690 was steeper than that of Alloy 600 at high temperatures. This indicated that Alloy 690 was more susceptible to creep degradation under long-term creep conditions. The precipitation of Cr carbides in Alloy 690 increased YS, benefitting creep properties for short-term creep. However, the Cr carbides coarsened significantly under loading conditions at high temperature, and this deteriorated the creep properties for long-term creep. (Received 19 December, 2022; Accepted 19 January, 2023)

keyword : Alloy 690, steam generator tube, creep, creep life prediction model, Cr carbide precipitation