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The effect of freezing conditions on the pore structure of porous TiO₂ fabricated using a freeze casting process with tert-butyl alcohol slurry was investigated. The raw powder was heat treated to manufacture a porous body with stable shape by suppressing volume change induced by the phase transformation of TiO₂ powder. The XRD analysis result indicated that the anatase phase fraction in the raw powder could be converted to 98.6% rutile phase by heating it at 800℃ for 30 min. Porous TiO₂ ceramics with unidirectionally aligned pore channels were prepared by freezing tert-butyl alcohol slurry containing 10 vol% TiO₂ powder under different freezing temperatures of -20, -30 and -40℃, respectively. After removing the frozen tert-butyl alcohol by sublimation in vacuum, the green samples were sintered at 1100℃ for 4 h in air. The porous body contains directional pores with a size of approximately 30 μm, and a hexagonal pore shape corresponding to the crystal structure of tert-butyl alcohol was observed in the cross-section of the pores. Small pores were observed at the bottom part of the porous body near the Cu plate where the solidification heat was released, but relatively large pores were present at the upper part. Both microstructure observation and pore size distribution indicated that the pore size and the strut thickness increased significantly with increasing freezing temperature. The change in pore characteristics was explained as being mainly due to the difference in the solidification behavior of the slurry and the rearrangement of the solid powder.

keyword : Freezing casting, Porous TiO₂, Tert-butyl alcohol, Freezing temperature, Pore structure

Direct Energy Deposition (DED) is one of the main methods of additive manufacturing in which a feedstock material in the form of powder or wire is delivered to a substrate while an energy source, such as laser beam or electron beam, is simultaneously applied to melt the feedstock. In addition to fabricating complex three-dimensional shapes using computer aided design, the DED laser welding process can be employed to fill complex shaped gaps between two objects. In this study, 3 mm thick Fe-based shape memory alloy plates with a 45° groove were welded by DED using high manganese steel powder filler. The process parameters including scan speed, hatch spacing, and layer thickness were fixed at values of 840 mm/min, 0.3 mm, 0.15 mm, respectively and laser power was varied during the welding process from 150 W to 300 W. Microstructures of the welds showed different shapes and quantities of defects such as cracks, pores and a lack of fusion depending on the laser power used. Laser power of 150 W showed crack-free weldment with the highest mechanical properties. The tensile strength and recovery stress of the welded sample were higher than the base metal, demonstrating the potential high performance of welding Fe-Mn-Si shape memory alloy components using the L-DED.

keyword : Direct energy deposition, Shape memory alloy, High manganese steel, Welding

As environmental pollution has intensified, the importance of recycling spent lithium-ion batteries (LIBs) has significantly increased. Valuable metals contained in LIBs are typically recovered through processes such as leaching, precipitation, and solvent extraction. However, these recycling processes require large amounts of sulfuric acid and caustic soda, resulting in the generation of significant volumes of Na₂SO₄ waste solution. In order to solve this problem, recent research has focused on using bipolar membrane electrodialysis (BMED) technology to split Na₂SO₄ and simultaneously recover H₂SO₄ and NaOH. This study investigated the process of recovering H₂SO₄ and NaOH from Na₂SO₄ solution using BMED in a three-compartment cell under constant current conditions. The study evaluated the effects of current density, initial Na₂SO₄ concentration, and initial H₂SO₄ and NaOH concentrations. Recovery of H₂SO₄ and NaOH, current efficiency, energy consumption, and process time were analyzed to determine optimal conditions. Under optimal conditions (1.30 M Na₂SO₄ solution at 360 A/m²), the recovery of H₂SO₄ and NaOH were 77.35% and 75.18%, respectively. Energy consumption was 1.43 kWh/kg, with current efficiencies for the acid and base of 51.1% and 49.7%, respectively.

keyword : Bipolar membrane electrodialysis, LIBs recycling, Sodium sulfate, Sulfuric acid recovery, Sodium hydroxide recovery

To determine the effect of Polyethylene Glycol(PEG) and Chloride ions(Cl^-) as additives during electroplating, this study used interaction plots of various characteristics of the electroplated Cu foils for analyses. Crystals of about 2 μm in size were observed on the surface of Cu electroplated in the electrolyte without PEG. When 30-100 ppm PEG was added, the crystal size and surface roughness decreased in relation to the amount of PEG. However, there were lots of dented areas, which became fewer as Chloride ions were increased from 20 ppm to 40 ppm. When 300 ppm PEG was added, the dented areas were not observed. Nevertheless, the surface morphology was very irregular and rough. In the group with 30-100 ppm PEG added, the resistivity decreased due to the increase in grain size. Moreover, the tendency of crystals to grow preferentially in the direction of the (111) plane led to an increased resistance to EM(electro-migration). When 40 ppm of chloride ions and 100 ppm of PEG were added, the yield strength increased by 22.4%, the tensile strength increased by 28.7%, and the elongation increased by 293.5% compared to the group without additives. This study showed that a combination of appropriate additives and amounts was required to form copper foils with low surface roughness, excellent electrical properties, and EM resistance and low corrosion rate. Therefore, with 100 ppm added PEG, it is necessary to add 20 ppm of chloride ions to ensure a low corrosion rate and 40 ppm of chloride ions for high mechanical properties.

keyword : PEG, Chloride ion, Interaction plot, Electrical property, Electroplating, Copper foil

Hydrogen sulfide (H₂S) is a highly toxic and dangerous gas with a flammable and corrosive nature, making the development of reliable gas sensors for its detection vital. This study investigated the enhancement in H₂S gas sensing performance of commercial SnO₂ powders after high-energy milling. SnO₂ powders were subjected to high-energy milling for 30, 60, and 90 min and then were characterized using advanced techniques to evaluate their morphology, chemical composition, and crystallinity. The response of a pristine SnO₂ gas sensor, and ones where the SnO₂ was milled for 30, 60 and 90 min, were 2.46, 2.27, 3.01, and 1.98, respectively, to 10 ppm H₂S at 300℃. Thus, the H₂S gas sensing results revealed that the SnO₂ powders milled for 60 min exhibited the highest sensing performance. This improvement in H₂S sensing performance was attributable to the reduced particle sizes achieved through the high-energy milling process, which increased the surface area and created defects on the surface of the SnO₂ particles, thereby enhancing the interaction between the gas molecules and sensor material. The smaller morphological size of the particles and surface defects subsequently promoted the resistance modulation crucial for H₂S gas detection. This study demonstrates that high-energy ball milling can effectively boost the gas-sensing features of SnO₂ powders. The findings can provide guidance for enhancing the gas-sensing capabilities of other resistive sensors.

keyword : SnO₂, High-energy ball milling, Gas sensor, H₂S gas, Sensing mechanism

This study investigates the thermoelectric properties of Fe_1-xCo_xSe₂ (x = 0, 0.25, 0.5, 0.75, and 1) alloys using theoretical modeling approaches. The Single Parabolic Band (SPB) model was employed to calculate band parameters such as density-of-states effective mass (md*), non-degenerate mobility (μ₀), weighted mobility (μ_w), and B-factor. As the alloying content (x) increased, the B-factor improved, leading to higher predicted maximum zT values. For CoSe₂ (x = 1), optimizing the carrier concentration to ~1.3 × 10¹⁹ cm^-3 can potentially enhance zT to ~0.64, a 21-fold increase over the experimental value (~0.03). The Callaway-von Baeyer (CvB) model was used to analyze thermal transport properties, revealing that the increased x strengthens phonon scattering by point defects, resulting in a continuous decrease in lattice thermal conductivity (κ_l). Specifically, a phase transition from orthorhombic to cubic structure occurs as x increases, with a mixed phase observed at x = 0.75 and a complete transition to cubic at x = 1. This structural change significantly impacts both electronic and phonon band structures, leading to a dramatic increase in the B-factor and a substantial reduction in κl for x > 0.75. These findings suggest that CoSe₂ alloying, coupled with the induced phase transition, can significantly improve the thermoelectric performance of FeSe₂-CoSe₂ alloys.

keyword : FeSe₂, CoSe₂, Alloys, Phase transition, Thermoelectric

Carbon materials used as an electrode for aqueous supercapacitors should be synthesized with a porous structure and hydrophilic properties to facilitate the adsorption and desorption of electrolyte ions for charge storage. To enlarge the specific surface area, the porous morphology should contain micropores (diameter < 2 nm). Mesopores (diameter: 2 - 50 nm) should also be present for facile ionic transport. Hydrophilic carbon can be achieved by introducing hydrophilic functional groups on the surface. Here, hydrophilic porous carbon was synthesized by mixing a polyvinylidene chloride (PVDC) resin precursor with copper oxide (CuO) and tetrahydrofuran (THF), followed by heat treatment at 750℃. CuO acted as a template during the heat treatment, creating large mesopores. The generated HCl from PVDC combined with CuO to form CuCl₂, contributing to the micropore formation. THF played a role in introducing hydrophilic functional groups on the carbon surface, to promote the adsorption of aqueous electrolyte ions. The activated carbon synthesized using CuO and THF exhibited a specific capacitance of 90 F g^-1 at a scan rate of 5 mV s^-1 in 0.5 M K₂SO₄ electrolyte. The synthesized activated carbon demonstrated excellent rate capability, retaining 82% of its capacitance at 10 times faster charging rate (50 vs. 5 mV s^-1).

keyword : Aqueous supercapacitor, Hydrophilicity, Functional group, Porous carbon, Template, CuO

Since the recently reported biotoxicity of Ni, the use of Ti-Ni base shape memory alloys as biomaterial has been reduced. For this reason, Ti-Nb base superelastic alloys consisting of only non-toxic elements are being actively studied for potential use as biomaterials. In this study, the effects of Zr content and annealing temperature on the shape memory characteristics of Ti-(14-22)Zr-6Nb-2Mo-2Sn alloys were investigated using tensile tests, X-ray diffraction measurement and Electro Backscatter Diffraction (EBSD). Results showed that the maximum total recovery strain and critical stress for slip increased with increasing Zr content. This is due to the change in lattice constant with the addition of Zr. In all annealed conditions XRD analysis of the Ti-22Zr-6Nb-2Mo-2Sn(at.%) alloy revealed only β phase at room temperature, and outstanding superelasticity was observed. This means that the α/β transus temperature of this alloy is below 650℃. Maximum total recovery strain and critical stress for slip increased with decreasing annealing temperature. A maximum of 592MPa and 5.83% were obtained in the Ti-22Zr-6Nb-2Mo-2Sn(at.%) alloy annealed at 650℃ for 1hr. The {001}β<110>β texture intensity increased with decreasing annealing temperature. The maximum recovery strain in the Ti-22Zr-6Nb-2Mo-2Sn(at.%) alloy annealed at 650℃ for 1hr may have been caused by the well-developed {001}β<110>β type recrystallization texture.

keyword : Shape memory effect, β-type Ti alloy, Biomaterials, Ti-Zr-Nb-Mo-Sn

The purpose of this study is to analyze the components and observe the microstructure of 98 bronze vessels excavated from the Korean Peninsula, where such vessels were widely used, to identify the alloy composition and manufacturing process of bronze artifacts by period. In addition, we attempted to confirm the changing processes of bronze manufacturing technology by reviewing previous studies. The analysis results showed that the bronzeware of the Unified Silla period was mainly produced using a Cu-Sn binary alloy containing 20-26 wt% Sn, which was cast and slowly cooled. Also, most bronze vessels from the Goryeo dynasty were made of a Cu-Sn binary alloy, with an Sn content of 20-24 wt%. In the microstructure, a twinned α phase and β(M) phase were observed, indicating that the container was manufactured using the casting-hot forging-quenching process. Bronze vessels from the Joseon dynasty were made using a binary alloy of Cu-Sn and a ternary alloy of Cu-Sn-Pb. In order to confirm the changes in bronze manufacturing technology over time, we investigated the alloy composition and microstructure of a total of 295 vessels, including previous studies, and were able to confirm the dominant production technology for each period. During the Unified Silla period, the shape of the container was made by casting using a Cu-Sn binary alloy, and the process was completed through slow cooling or quenching. During the Goryeo Dynasty, vessels were made using a Cu-Sn binary alloy through a process of casting, hot forging, and quenching, or by casting a Cu-Sn-Pb ternary alloy into a vessel shape and then slowly cooling it. And in the Joseon Dynasty, the use of ternary alloys of Cu-Sn-Pb increased.

keyword : Bronze vessel, Microstructure, Chemical composition, Manufacturing process, Casting, Forging