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Previously reported low-Mn ferritic-based lightweight steels are potential candidates for industrial applications, however, they typically exhibit lower strength, with < 1 GPa and lower strength-ductility balance, than medium- and high-Mn austenitic lightweight steels. Herein, we introduce a low-temperature tempering-induced partitioning (LTP) treatment that avoids the strength-ductility dilemma of low-Mn ferritic-based steels. When the LTP process was performed at 330 ℃ for 665 s, the strength of typical ferritic base Fe-2.8Mn5.7Al0.3C (wt%) steel with heterogeneously sized metastable austenite grains embedded in a ferrite matrix, exceeded 1.1 GPa. Notably, the increased strength-ductility balance of the LTP-processed ferritic steel was comparable to that of the high-Mn based austenitic lightweight steel series. Using microscale to nearatomic scale characterization we found that the simultaneous improvement in strength and total elongation could be attributed to size-dependent dislocation movement, and controlled deformation-induced martensitic transformation. (Received May 14 2021; Accepted July 5, 2021)

keyword : ferrite-based lightweight steels, tempering, size-dependent partitioning, dislocation movement

ZrB₂ is considered a candidate material for ultra-high temperature ceramics because of its high thermal conductivity, high melting point, and low coefficient of thermal expansion. Despite these attractive properties, ZrB₂ applications are limited by its low fracture toughness below the brittle-ductile transition temperature. To improve its ductile properties, the approach universally utilized has been to add a second material to form composites, and to fabricate nanostructured materials. In this study a dense nanostructured ZrB₂-Al₂O₃ composite was rapidly sintered using the pulsed current activated heating (PCAH) method within 3 min in one step, from mechanically synthesized powders of ZrB₂ and Al₂O₃. Consolidation was accomplished using an effective combination of current and mechanical pressure. A highly dense ZrB₂- Al₂O₃ composite with a relative density of up to 97.4% was fabricated using the simultaneous application of 70 MPa pressure and a pulsed current. The fracture toughness and hardness of the ZrB₂-Al₂O₃ composite were 3.9 MPa.m^1/2 and 1917 kg/㎟, respectively. The fracture toughness of the composite was higher than that of monolithic ZrB₂. (Received May 7 2021; Accepted July 20, 2021)

keyword : Composite, Synthesis, Nanomaterials, Mechanical properties, ZrB_2-Al2O3

The metallurgical aspects of weld solidification cracking in Ni-based superalloys (with Ti+Al > 5 mass%) have not been widely investigated thus far. Herein, the solidification cracking susceptibility of the CM247LC superalloy and its welds with ERNiFeCr-2 filler wire was quantitatively evaluated using a novel modified Varestraint testing method, for the successful manufacturing of CM247LC superalloy gas turbine blades. It was found that the solidification brittle temperature range (BTR) of the CM247LC superalloy was 400 K. This measurement was obtained with a high-speed thermo-vision camera. The BTR increased to 486 K for the CM247LC/ERNiFeCr-2 welds (dilution ratio: 74%). Theoretical calculations (i.e., the Scheil equation, performed using Thermo-Calc software) were conducted to determine the temperature range in which both solid and liquid phases coexist, together with the microstructural characterization of the solidification cracking surfaces. The greater increase in BTR for the CM247LC/ERNiFeCr-2 welds than that for CM247LC was attributed to the enlargement of the solid-liquid coexistence temperature range. This correlated with the formation of a low-temperature Laves phase during the terminal stage of solidification, and was affected by the diluted Nb and Fe components in the ERNiFeCr-2 filler metal. Based on the experimental and theoretical results, the proposed modified Varestraint testing method for dissimilar welds is expected to be an effective testing process for solidification cracking behavior in the manufacturing of high-soundness CM247LC superalloy welds. (Received June 24 2021; Accepted July 16, 2021)

keyword : gas turbine blade, CM247LC, ERNiFeCr-2, solidification cracking susceptibility, varestraint test

VCl₃ (vanadium (III) chloride) was selected as an additive to Mg to increase the hydrogenation and dehydrogenation rates and the hydrogen storage capacity of Mg. Instead of MgH₂, Mg was used as a starting material since Mg is cheaper than MgH₂. Samples with a composition of 95 wt% Mg + 5 wt% VCl₃ (named Mg-5VCl₃) were prepared by milling in hydrogen atmosphere (reactive milling). In the first cycle (n=1), Mg-5VCl₃ absorbed 5.38 wt% H for 5 min and 5.95 wt% H for 60 min at 573 K in 12 bar hydrogen. The activation of Mg-5VCl₃ was completed after three hydrogenation-dehydrogenation cycles. During milling in hydrogen, β-MgH₂ and γ-MgH₂ were produced. The formed β-MgH₂ and γ-MgH₂ are considered to have made the effects of reactive milling stronger as β-MgH₂ and γ-MgH₂ themselves were being pulverized. The introduced defects and the interfaces between the Mg and the phases formed during the reactive milling and during hydrogenation-dehydrogenation cycling are believed to serve as heterogeneous active nucleation sites for MgH₂ and Mg-H solid solution. The phases generated during hydrogenation-dehydrognation cycling are also believed to prevent the particles from coalescing during hydrogenation-dehydrognation cycling. (Received June 13 2021; Accepted July 28, 2021)

keyword : hydrogen absorbing materials, mechanical alloying, milling, scanning electron microscopy, SEM, X-ray diffraction, VCl_{3 addition}

We report a highly efficient quantum dot light emitting diode (QLEDs) with a radio frequency (RF) sputtered ZnO thin film as an electron transport layer (ETL) instead of the conventional ZnO nanoparticles (NPs) by solution process. ZnO NPs have been used as a key material to improve the performance of QLEDs, but the charge imbalance in ZnO NPs resulting from fast electron injection, and their limited uniformity are significant disadvantages. In this study, ZnO layers were deposited by RF sputtering with various O₂ partial pressures. All of the ZnO films showed preferential growth along the (002) direction, smooth morphology, and good optical transmittance. To test their feasibility for QLEDs, we fabricated devices with RF sputtered ZnO layers as an ETL, which has the inverted structure of ITO/RF sputtered ZnO/QDs/CBP/MoO₃/Al. The optical/electrical characteristics of two devices, comprised of RF sputtered ZnO and ZnO NPs, were compared with each other. QLEDs with the sputtered ZnO ETL achieved a current efficiency of 11.32 cd/A, which was higher than the 8.23 cd/A of the QLEDs with ZnO NPs ETL. Next, to find the optimum ZnO thin film for highly efficient QLEDs, deposition conditions with various O₂ partial pressures were tested, and device performance was investigated. The maximum current efficiency was 13.33 cd/A when the ratio of Ar/O₂ was 4:3. Additional oxygen gas reduced the O vacancies in the ZnO thin film, which resulted in a decrease in electrical conductivity, thereby improving charge balance in the emission layer of the QLEDs. As a result, we provide a way to control the ZnO ETL properties and to improve device performance by controlling O₂ partial pressure. (Received June 14 2021; Accepted July 9, 2021)

keyword : Quantum dot, ZnO, RF sputtering, Electroluminescence

In this study, tetrahedrite compounds doped with Sn were prepared by mechanical alloying and hot pressing, and their charge transport and thermoelectric properties were analyzed. X-ray diffraction analysis revealed that both the synthetic powders and sintered bodies were synthesized as a single tetrahedrite phase without secondary phases. Densely sintered specimens were obtained with relatively high densities of 99.5%-100.0% of the theoretical density, and the component elements were distributed uniformly. Sn was successfully substituted at the Sb site, and the lattice constant increased from 1.0348 to 1.0364 nm. Positive signs of the Hall and Seebeck coefficients confirmed that the Sn-doped tetrahedrites were p-type semiconductors. The carrier concentration decreased from 1.28 × 10¹⁹ to 1.57 × 10¹⁸ cm^-3 as the Sn content decreased because excess electrons were supplied by doping with Sn⁴⁺ at the Sb³⁺ site of the tetrahedrite. The Seebeck coefficient increased with increasing Sn content, and Cu₁₂Sb_3.6Sn_0.4S₁₃ exhibited maximum values of 238-270 μVK^-1 at temperatures of 323-723 K. However, the electrical conductivity decreased as the amount of Sn doping increased. Thus, Cu₁₂Sb_3.9Sn_0.1S₁₃ exhibited the highest electrical conductivity of (2.24-2.40) × 10⁴ Sm^-1 at temperatures of 323-723 K. A maximum power factor of 0.73 mWm^-1K^-2 was achieved at 723 K for Cu₁₂Sb_3.9Sn_0.1S₁₃. Sn substitution reduced both the electronic and lattice thermal conductivities. The lowest thermal conductivity of 0.49-0.60Wm^-1K^-1 was obtained at temperatures of 323-723 K for Cu₁₂Sb_3.6Sn_0.4S₁₃, where the lattice thermal conductivity was dominant at 0.49-0.57 Wm^-1K^-1. As a result, a maximum dimensionless figure of merit of 0.66 was achieved at 723 K for Cu₁₂Sb_3.9Sn_0.1S₁₃. (Received July 12, 2021; Accepted July 28, 2021)

keyword : thermoelectric, tetrahedrite, mechanical alloying, hot pressing

Using ethylene glycol (C₂H₆O₂) as the solvent, ZnS particles were synthesized in high yield at a relatively low temperature of 125 ℃ via the glycothermal method. We report a facile method for preparing spherical self-aggregated ZnS particles from ZnS nanocrystals, using zinc acetate as the Zn²⁺ source and thiourea as a sulfur source, without mineralization or other agents. The crystal phase structure, morphology, size, surface chemical composition, and optical properties of the self-aggregated ZnS particles were characterized using XRD, FE-SEM, TEM, XPS, BET, and UV-Vis absorption. The ZnS particles had a cubic phase zinc blende structure without any other impurities. The average crystallite size of the synthesized primary nanocrystal, estimated from XRD peak width and TEM images, was nearly 4 nm. FE-SEM images showed that all of the ZnS consisted of self-aggregated particles with a spherical morphology and a size of approximately 0.2 μm~0.5 μm, and contained many tiny primary nanocrystals. The prepared ZnS exhibited strong photoabsorption in the UV region. The optical band gap decreased from 3.85 eV to 3.62 eV as the glycothermal reaction temperature was increased, due to improvement in particle size and crystallization. The effects of the glycothermal reaction temperature on the photocatalytic activity of the synthesized self-aggregated ZnS particles were investigated by the photodecomposition of methyl orange (MO) dye under UV illumination (λ = 365 nm). The prepared ZnS exhibited excellent photocatalytic degradation with increasing reaction temperature, of 125 ℃ (5%), 150 ℃ (10%), 175 ℃ (60%), and 200 ℃ (90%) after irradiation for 60 min. It was found that the ZnS particle prepared at 200 ℃ achieved the highest photocatalytic degradation, with nearly 100% MO decomposition after 90 min, by various photogenerated radical scavengers. (Received June 9 2021; Accepted July 6, 2021)

keyword : Self-aggregated ZnS, glycothermal, ethylene glycol, methyl orange, photodecomposition

Due to environmental pollution and global warming, research on new energy sources that can replace fossil fuels is important. A fuel cell is an eco-friendly energy conversion system that discharges water, and uses hydrogen as fuel. Although platinum is a widely used catalyst in PEMFCs, it has commercial limitations because of its low stability and high cost. Pt-based bimetal catalysts are being studied to improve performance and reduce the cost of fuel cell catalysts. Pt-M is excellent in terms of performance, stability, and cost, avoiding the disadvantages of the Pt catalyst. Studies on various bimetallic catalysts have been conducted, and among them, studies on Pt-Ni, Pt-Co, and Pt-Fe have been the most active. This review summarizes reports of fuel cell catalysts using Pt-M from 2014 to 2020. In recent studies, in order to improve the Pt-M performance, there have been attempts to change the pretreatment, the type of support, and the composition of Pt and M. There have also been studies that have applied new synthetic methods, which are different from traditional synthetic methods. Many Pt-M catalysts have shown better performance than commercial Pt/C, and exhibited stable performance in durability tests. (Received May 29 2021; Accepted July 7, 2021)

keyword : Pt-based alloy catalyst, oxygen reduction reaction, proton exchange membrane fuel cells, activity, stability

A porous ZrFe alloy specimen was prepared as a 6 × 3 mm (diameter × thickness) disk. The reaction of the ZrFe alloy was confirmed while the whole system was maintained at a target temperature, which was increased from 150 ℃ to 950 ℃ in a 99.999% low purity nitrogen atmosphere, consisting of 10 ppm of impurity gas. Surface color, pore size, stabilized layer, and phase change were confirmed with optical microscopy, scanning electron microscopy-energy dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Micro-Raman, according to temperature. The surface color of the ZrFe alloy changed from metallic silver to dark gray as the temperature increased. In the EDS and XPS results, nitrogen component was not observed, and oxygen content increased on each surface at the elevated temperatures. In this way, the ZrFe alloy was stabilized in a low purity nitrogen atmosphere, preventing rapid nitride reactions. (Received June 1 2021; Accepted July 16, 2021)

keyword : ZrFe, nitrogen gas, gettering, nitration, oxidation