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Two welding methods of FCAW (Flux-cored arc welding) + FCAW, and FCAW + GTAW (Gas tungsten arc welding) were compared to identify effective routes for STS 316L/A516-70N clad stainless steel. Since the welding speed of the FCAW + GTAW welding method is slower than that of the FCAW + FCAW welding method, the amount of heat input is large, the size of the heat-affected zone formed is larger, and the amount of angular deformation is also large. The microstructure and deformation of the welding specimens were investigated by computational simulation, and the results were compared with calculated results for actual specimens. The results were very similar, thus confirming the high accuracy of the calculation. After measuring the hardness value of the actual welded specimens, the specimen welded by the FCAW + FCAW welding method was found to have a higher hardness value. The hardness of the welded portion of the FCAW + GTAW specimen tests tended to remain at nearly the same value throughout the specimen, indicating that the FCAW + GTAW hardness profile is desirable. In the computer simulation, butt welding was used to set the optimal heat source conditions for the two welding methods, and the effect of heat formed during welding was confirmed using the thermal analysis results according to the conditions. Based on the butt welding conditions, the residual stress and strain for the two welding methods are discussed, using calculations for the welding of C-seam (circumferential seam) and L-seam (longitudinal seam) used in the manufacture of pressure vessels. (Received 9 August, 2022; Accepted 22 September, 2022)

keyword : sysweld, clad steel, simulation, welding, deformation

In this study, we tried to understand the difference in the mechanical properties (elastic modulus, E & hardness, H) values of CIGS compound semiconductors measured using the Berkovich indenter and the sharper cube corner and conical indenter. Adopted continuous stiffness measurement technique to characterize it along with the indentation depth. The depth was up to 1000 nm, and E and H values w ere obtained according to the depth. The projected contact area of the cube edge and the conical indenter was calculated assuming the two indenters had an ideal shape. In the case of the Berkovich indenter, which is the subject of comparison, both calculated values assuming an ideal shape and experimental values were used. Cube-corner and conical tip showed lower load at the same depth of 1000 nm compared to Berkovich indenter. This means that the same depth of indentation was achieved even at a low load, which could affect the value of the mechanical properties of the thin film. The cube-corner tip and the conical tip have a sharp shape with a projected contact area about 6 times smaller than that of the Berkovich tip. It was observed that not only the effect of grain boundaries of the microstructure of the CIGS thin film, but also the effect of reflecting the characteristics of the Mo substrate had a significant effect on the mechanical properties of the CIGS thin film. (Received 19 August, 2022; Accepted 19 September, 2022)

keyword : semiconducting compounds, CIGS, mechanical behavior, nanoindentation

General austenitic stainless steel has a problem with intergranular corrosion due to volatilizing chromium, which forms chromium carbide in a high temperature environment. By adding titanium as an alloying element, STS 321 stainless steel has excellent creep resistance and intergranular corrosion resistance at high temperatures, because the formation of chromium carbide is suppressed. It is important to find the optimal process conditions for STS 321 stainless steel used in the aerospace field, because high temperature processing is mainly applied, and defects or inhomogeneity of materials that occur during high temperature processing lowers the yield of products. In this study, to investigate the effect of the initial microstructure on the high-temperature deformation behavior of STS 321 stainless steel, a high-temperature compression test was performed on two types of STS321 alloys with different initial microstructures. The temperature range was set at 50°C intervals from 800°C to 1100°C, and the strain rate was set at 10^-1/sec intervals from 1 × 100/ sec to 1 × 10^-3/sec. Based on the experimental results, the thermal activation energy, which differed depending on differences in the initial microstructure, was calculated. In addition, by deriving flow stress and processing maps, the difference in energy dissipation efficiency depending on temperature and strain rate was explained, along with the initial microstructure and high-temperature deformation mechanism. (Received 11 August, 2022; Accepted 8 September, 2022)

keyword : STS 321, dynamic material model, high temperature deformation, formability

The recycling of critical metals like cobalt and nickel from spent lithium-ion batteries (LIBs) is of immense importance in sustainable manufacturing. This work aims to purify Co(II) and Ni(II) present in acidic and ammonia solutions using ion exchange. AG-1-X8 resin modified with thiocyanate anions (R₄N⁺SCN^- resin) was used to separate a small amount of Co(II) from HCl and H₂SO₄ solutions containing Ni(II) and Si(IV) at pH 3, and the effect of factors such as pulp density, shaking time, and Ni(II) concentration on the adsorption of Co(II) was investigated. The difference in the adsorption behaviour of metal ions onto the R₄N⁺SCN^- resin from HCl and H₂SO₄ solutions was insignificant. Increasing the Ni(II) concentration in the feed negatively affected the loading efficiency of Co(II). The complete elution of the metals from the loaded resin was attained with a 5% NH₃ solution. Purolite C100 resin was selected for the purification of Co(II) from a 10% (v/v) NH₃ solution containing a small amount of Ni(II), and the best conditions for the complete adsorption of both Co(II) and Ni(II) were determined. Selective elution of Ni(II) over Co(II) from the loaded resin was achieved by dilute H₂SO₄ solution. The purity of Co(II) in the solution after elution with 2.0 mol/L H₂SO₄ from the Ni(II) free resin was higher than 99.99%. Ion exchange was effective for purifying a weak acidic or ammonia solution in which there is a significant difference in the Co(II) and Ni(II) concentrations. (Received 10 June, 2022; Accepted 11 October, 2022)

keyword : lithium-ion batteries, cobalt, nickel, ion exchange, purification

Bi₂O₂Se oxyselenide has been actively studied as a potential n-type thermoelectric material because of its intrinsically low thermal conductivity and high Seebeck coefficient (S). However, Bi₂O₂Se has very low electrical conductivity (σ), resulting in relatively poor thermoelectric performance. Herein, we investigate the effect of Cu addition on the electrical and thermal transport of n-type polycrystalline Bi₂O₂Se. A series of CuxBi₂O₂Se (x = 0, 0.0025, 0.005, and 0.0075) polycrystalline samples were synthesized by a conventional solidstate reaction. Tetragonal Bi₂O₂Se was successfully synthesized, and its lattice parameters gradually decreased with the addition of Cu. Further, σ decreased and the magnitude of S increased with increasing Cu content, according to the trade-off relationship between these parameters. Consequently, a maximum power factor of 0.106 mW m^-1 K^-2 was achieved for the sample with x = 0.0025 at 300 K, owing to the increase in the magnitude of S. The Hall carrier concentration decreased exponentially with the addition of Cu, which is mainly attributed to the possible enlargement of the band gap of the Cu-added samples. The lattice thermal conductivity decreased with increasing x, which was attributed to point-defect phonon scattering via Cu addition. Therefore, a maximum zT of 0.222 was obtained at 790 K for the Cu_0.0025Bi₂O₂Se (x = 0.0025) sample, which was approximately 8% higher than that of the pristine Bi₂O₂Se sample. (Received 10 June, 2022; Accepted 7 September, 2022)

keyword : thermoelectric, doping, oxyselenide, Bi_2O2Se

Re₂Te₅ is considered a potential thermoelectric material because of its intrinsically low thermal conductivity, due to its complex crystal structure. Herein, a series of Se-doped Re₂Te₅ (Re₂Te_5-xSe_x, x = 0, 0.2, 1, and 2) samples were synthesized, and their electrical and thermal transport properties were investigated. Pure orthorhombic Re₂Te₅ phases were successfully synthesized without any impurities for all compositions, and the continuous decrease in the calculated lattice parameters confirmed the substitution of Se atoms at the Te sites. A maximum power factor of 0.135 mW/mK² was achieved for the sample with x = 0.2 at 880 K, mainly due to the increase in carrier concentration and electrical conductivity. The lattice thermal conductivity significantly decreased for all doped samples, which was attributed to the point defect phonon scattering caused by Se doping. The thermoelectric figure of merit, zT reached a maximum value of 0.20 at 880 K for Re₂Te_4.8Se_0.2 (x = 0.2) sample, which was approximately 22% higher than that of the pristine Re2Te5 sample. The weighted mobility, quality factor, and expected zT were calculated to evaluate the optimization of the power factor and zT. (Received 17 June, 2022; Accepted 7 September, 2022)

keyword : thermoelectric, doping, Re_2Te5

We investigated the effect of Ni doping on the electrical, thermal, and thermoelectric transport properties of FeSe₂ polycrystalline alloys. FeSe₂ alloys exhibit both n-type and p-type conduction at different temperatures and thus their practicality as a thermoelectric material is somewhat limited. In this study, Fe_1-xNixSe₂ polycrystalline samples with x = 0, 0.01, 0.05, 0.075, 0.1, and 0.125 were synthesized, and it was shown that Ni doping in FeSe₂ alloys induces n-type conduction in the entire temperature range. The electrical conductivity is gradually increased with an increase in carrier concentration as Ni doping increases, from 8.52 (x = 0) to 329 S/cm (x = 0.125) at 300 K. The maximum power factor of 0.61 mW/mK² was observed in the Fe_1-xNixSe₂ sample at x = 0.01 at 600 K. Other Ni-doped samples exhibited power factors between 0.31 and 0.34 mW/mK² at 600 K. The thermal conductivity gradually decreased from 6.9 (x = 0) to 4.2 W/mK (x = 0.125) with Ni doping because of the additional point-defect phonon scattering of the Ni substitutes. As a result, a zT of 0.11 was observed in x = 0.01 at 600 K, while the zT of the other Ni-doped samples exhibited 0.056- 0.069 at 600 K. (Received 8 August, 2022; Accepted 7 September, 2022)

keyword : thermoelectric transport, FeSe₂

In recent decades, many research efforts have been devoted to developing high-performance stretchable strain sensors due to their potential for application in various emerging wearable sensor systems. This work presents a facile yet highly efficient way of modulating the sensing performance of a thin metal film/conductive composite hybrid piezoresistor-based stretchable strain sensor by simply controlling the metal film thickness. The hybrid strain sensor can be simply fabricated by sputtering a thin platinum (Pt) film onto a silver nanowire (AgNW)/dragon skin (DS) composite substrate prepared via a facile embed-and-transfer process in a reproducible manner. The density of the network-shaped mechanical crack induced in the Pt film tended to decrease with increasing the Pt thickness, thereby leading to a higher gauge factor of the sensor. The fabricated hybrid strain sensor also exhibited a large stretchability of 150% owing to its electrical robustness under strain, based on the unique morphology, formed of the network-shaped Pt crack and AgNW percolation network embedded in the DS matrix. Thanks to the balanced strain-sensing performance of the hybrid strain sensor in conjunction with large stretchability, the device was successfully demonstrated as a wearable human-activity monitoring solution that can monitor a wide range of human motions in real time. (Received 14 July, 2022; Accepted 16 September, 2022)

keyword : stretchable strain sensor, mechanical crack, conductive composite, metal film thickness

In order to investigate the mechanical properties of the developed wrought γ'-hardened Ni-based superalloy, LESS alloy (Low Eta Sigma Superalloy), and to understand the deformation behavior, tensile tests were carried out from room temperature to 800℃ in comparison with IN 740H alloy. Like other superalloys, the tensile properties of the LESS alloy were found to be dependent on temperature. As the temperature rose between room temperature and 700℃, the ultimate tensile strength (UTS) decreased slightly, and then decreased sharply. The yield strength (YS) of the LESS alloy was found to be about 40% higher than that of IN 740H alloy from room temperature to 700℃. On the basis of microstructure observations, the high yield strength of the LESS alloy is thought to result from the dense grain boundary, where the fine carbides (MC, M₂₃C₆) and the γ' phase form a continuous film structure, which more effectively suppressed dislocation movements during deformation. It was found that the deformation mode of the LESS alloy changed from γ'- shearing to mixed mode (γ'-shearing + climb) at around 700~800℃. Although the main fracture mode of the LESS alloy was ductile dimple fracture, elongation decreased with temperature due to the local brittle region caused by micro-twins. (Received 16 June, 2022; Accepted 23 September, 2022)

keyword : Ni-based superalloy, tensile property, grain boundary morphology, deformation behavior, γ` phase