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The combination of severe plastic deformation, deformational heat, and frictional heat generated during the high-pressure torsion process has a great effect on microstructural evolutions, including grain refinement, dynamic recrystallization and recovery. In particular, the low melting temperature metallic alloys (e.g., aluminum and magnesium) also induce nano-precipitations in the matrix during high-pressure torsion, which results in both unique microstructure and mechanical properties of the nanocrystalline metallic materials. In this study, the mechanical properties and microstructural evolution of high-pressure torsionprocessed aluminum 7068 alloy at room temperature were investigated. In the early deformation stage, both grain refinement and nano-precipitates were formed that increase the strength of ultrafine-grained aluminum alloy, and a maximum strength of 844 MPa was obtained for the 5T sample. However, in the later deformation stage, tensile strength decreases with the increase in shear strain due to grain growth, and coarse precipitates act as crack initiation sites during plastic deformation. In summary, the mechanical properties of highpressure torsion represent the strength-ductility trade-off as the increase of the number of revolutions. Their mechanical properties are superior to those of recent severe plastic deformation-processed aluminum alloys. This result shows that the severe plastic deformation of the HPT process positively contributes to their mechanical properties by inducing multiple strengthening paths, such as grain refinement and nanoprecipitation of the aluminum alloy. (Received 11 July, 2022; Accepted 4 October, 2022)

keyword : aluminum alloy, high-pressure torsion, mechanical property, microstructure

Stainless steel-based composites (SMCs) mixed with 316L stainless steel powder and Fe-Cr-B based M alloys powder ed M) powder were manufactured using the powder injection molding (PIM) process, and their microstructure and mechanical properties were investigated. To examine the effect of M content on the properties of SMCs, the M fraction was controlled to 30 vol.%, 50 vol.%, and 70 vol.%. It was observed that PIM-processed Fe-Cr-B/316L SMCs (30 vol.%, 50 vol.%, 70 vol.%) had γ-austenite and minor α-ferrite as a matrix, and (Cr, Fe)2B was formed as a reinforced phase. In tensile tests at room temperature, the yield strengths of the 30 vol.%, 50 vol.%, 70 vol.% M SMCs were measured to be 519.9 MPa, 574.1 MPa, and 604.8 MPa (peak stress), respectively. From the fracture surface observation results, it was found that the deformation behavior changed from the intergranular fracture of boride to intergranular fracture between the matrix and boride, as the M fraction increased. Based on the above results, the deformation and fracture mechanisms of novel Fe-Cr-B/316L SMCs are also discussed. (Received 19 August, 2022; Accepted 4 October, 2022)

keyword : powder injection molding, stainless steel matrix composite, 316L, Fe-Cr-B, microstructure, mechanical property

The main components of molding sand used in sand casting are sand, clay and water. The composition of the molding sand has a great influence on the properties of the casting. In order to obtain highquality castings, it is important to manage the components that affect the properties of the molding sand such as especially green compression strength and compactability. In this work, green compression strength and compactability are predicted through a machine learning technique using the processing data obtained from a foundry and the properties of molding sand as the input variables. Through the correlation analysis between the input variables and the response variable, we investigated the relationship between the processing data and the properties of the molding sand. The possibility of predicting the characteristics of molding sand with high accuracy was confirmed using a model created through data preprocessing with the real foundry data. If the composition of the molding sand is adjusted in the foundry using the generated model, it is expected that higher quality castings can be produced and the productivity can be increased. (Received 16 September, 2022; Accepted 4 October, 2022)

keyword : machine learning, molding sand, green compression strength, compactability

Nowadays, high-density electronic devices and component mounting have gained popularity. Because of the heat generated from these devices, efficiency of the electronic parts is significantly lowered and life time of these devices is considerably shortened. Therefore, it is very important to efficiently dissipate the heat generated from the device to extend product. In this study, a copper thick film was deposited using a sputtering technique using plasma. In addition, graphene, a two-dimensional nanomaterial, was inserted in the form of a sandwich structure between the deposited copper (Cu) layers of various thicknesses. Through this, a study was conducted to control the residual stress of thick copper foil deposited by sputtering. In this study, the residual stress was measured according to the location where graphene was inserted, the treatment method, and the transfer area. As a result, the highest residual stress reduction effect was observed when the transfer area of graphene was about 70%. Reduction of residual stress was observed by inserting graphene into the deposited copper thick film of various thicknesses. In addition, by increasing the number of layers of graphene inserted, changes in residual stress were observed. (Received 22 July, 2022; Accepted 23 September, 2022)

keyword : copper film, sputtering, plasma, graphene

Colloidal quantum dots (QDs) have attractive optical and electrical characteristics for various applications in the display industry. They have superb properties including color tunability, which is achieved by controlling the particle size and solution processes used for quantum dot light-emitting diodes (QLEDs). Here we demonstrate high efficiency QLEDs that consist of a mixed layer of TiO₂ nanoparticles (NPs) and QDs. ZnO is the key material responsible for the promising results of the electroluminescence devices, which exhibited well-matched energy levels and robustness. In this report, Li-doped TiO₂ NPs were synthesized under ambient conditions as an alternative electron transport layer (ETL) for QLEDs. A stable mixture of TiO₂ NPs and QDs was prepared in chlorobenzene and then applied to QLEDs without a conventional ETL. QLEDs with such a simplified structure produce a maximum luminance of 123,311 cd/m² with a current efficiency of 40.5 cd/A. These results are comparable to those of conventional QLEDs. Additionally, the predicted T50 at 100 cd/m² was 1,420 h, based on the T50 at 1,600 cd/m². These clearly indicate not only the promising results of the TiO₂ NPs as an inorganic ETL, but also the remarkable performance of the simplified device with less layers. The reduction in fabrication steps using this solution-based process is also advantageous for next-generation display technology. (Received 1 September, 2022; Accepted 11 October, 2022)

keyword : quantum dot, TiO₂, QLED, a mixed layer

One of the most popular routes used to improve the thermoelectric performance of materials is to suppress their lattice thermal conductivities. Thermoelectric performance is characterized by a figure-of-merit zT, which is defined as σS²T/(κ_e + κ_l), where the σ, S, T, κ_e, and κ_l are the electrical conductivity, Seebeck coefficient, temperature (in Kelvin), electronic thermal conductivity, and the lattice thermal conductivity, respectively. Among the variables in zT, the κ_l is the only variable that is independent of all other variables. In other words, reduction in κl guarantees zT improvement. Therefore, several different strategies to decrease κl have been introduced and implemented. Among the many κ_l reduction strategies, introducing point defects in the material by forming an alloy is particularly effective. Here, phonon scattering due to point defects in Mo(Se_1-xTe_x)₂ (x = 0.0, 0.25, 0.50, 0.75, 1.0) was studied using both the Debye-Callaway (DC) model and Callaway-von Baeyer (CvB) model. The advantages and disadvantages of using DC or CvB models are thoroughly discussed. When analyzing the effect of phonon scattering due to point defects, the CvB model is simpler and gives more information about the details of phonon scattering. (Received 6 September, 2022; Accepted 26 September, 2022)

keyword : hermoelectric, lattice thermal conductivity, point defect, debye-callaway model, callaway-von baeyer model

In this study, Cu₃Sb_1-ySn_ySe₄ (0 ≤ y ≤ 0.08) permingeatites were synthesized via mechanical alloying followed by hot pressing. The phase transformation, microstructure, charge transport parameters of the permingeatites, and their thermoelectric properties were analyzed. The permingeatites were present in a single phase with a tetragonal structure, and secondary phases were not detected. The permingeatites had relative densities of 97.2%-98.5%. The lattice constants of the a- and c-axis increased when Sn was substituted at the Sb sites. With increasing Sn content, the carrier concentration increased to (2.2-4.1) × 10¹⁹ cm^-3; however, mobility did not change significantly, at 58-66 cm²V^-1s^-1. The undoped Cu₃SbSe₄ behaved as a nondegenerate semiconductor. Its Lorenz number was calculated to be (1.57-1.56) × 10^-8 V²K^-2 at 323-623 K, and a maximum dimensionless figure of merit (ZT) of 0.39 was obtained at the temperature of 623 K, power factor of 0.49 mWm^-1K^-2, and thermal conductivity of 0.76 Wm^-1K^-1. However, the Sn-doped specimens behaved as degenerate semiconductors. Their Lorenz numbers increased to (1.63-1.94) × 10^-8 V²K^-2 at 323-623 K. Cu3Sb0.96Sn0.04Se4 exhibited a remarkably enhanced ZT of 0.71 at a temperature of 623 K, power factor of 1.18 mWm^-1K^-2, and thermal conductivity of 1.01 Wm^-1K^-1. (Received 4 October, 2022; Accepted 31 October, 2022)

keyword : permingeatite, thermoelectric, charge transport

Despite the recent advances in nature-inspired superhydrophobic surfaces with multifunctional properties such as self-healing, self-cleaning, antibacterial material, anti-biofouling, and oil-water separation in various industrial applications, there have been very few reports of robust superhydrophobic surfaces fabricated via a low-cost method. Herein, we report a rationally designed, self-healable, robust, and superhydrophobic nanocomposite film, fabricated by a one-step reaction process from a multi-component solution. These superhydrophobic nanocomposite films can be readily fabricated over a large area with high throughput via simple spray coating of the component solution. As the spray-coating materials, we used functionalized hydrophobic silica nanoparticles (hydrophobic-SiO₂ NPs) to achieve hierarchical textured structures, and low surface energy. Then, thermal crosslinkers and long hydrocarbon silanes were added to improve the stability and hydrophobicity of the fabricated nanocomposite films. Because of the synergetic effect of the hydrophobic NPs and crosslinked polymeric compounds, the spray-coated superhydrophobic nanocomposite films exhibited excellent superhydrophobicity (static water contact angles > 150°), high transparency (>80%), high stability, and self-healing abilities. Tests confirmed the rationally designed, solution-based spray coating method provides a simple route to construct transparent, robust, and self-healing superhydrophobic surfaces, with high throughput and large area, which can be utilized in various industrial applications, including conventional products and advanced non-wetting surfaces. (Received 29 September, 2022; Accepted 31 October, 2022)

keyword : superhydrophobic, hydrophobic nanoparticles, self-healing, nanocomposite film

Macrostructural features in continuously-cast stainless steel slabs such as the distribution of equiaxed and columnar grains, and microporosity, are crucial to the post-processes, and the mechanical properties of the final steel products. Among the methods for controlling grain structure during the continuous casting process, the application of electromagnetic stirring (EMS) during casting is known to alter the grain structure of the slab. In this study, we predicted macrostructural features in a continuous-cast stainless steel slab using computer simulation. We approximated the heat transfer system as a one-dimensional heat conduction model with solidification. The effect of EMS was investigated by introducing an effective thermal conductivity of the melt. We assumed that EMS caused turbulent flow, and enhanced the thermal conductivity. Columnar-to-equiaxed transition (CET) and microporosity were estimated with the models proposed by Hunt and Niyama, respectively. The effect of EMS on microporosity was negligible, and microporosity was inevitable in the centerline of the slab. EMS made a second equiaxed region between the outer wall and the center of the slab. Because the effective thermal conductivity was high due to EMS, the temperature field in the slab was distorted, and the Hunt criterion for CET was satisfied in the region. The CET results were compared and verified with cellular automata simulation. Various process parameters including casting speed, superheating, and the position of the EMS module, were examined to predict the macrostructure of the slab. (Received 25 August, 2022; Accepted 27 September, 2022)

keyword : continuous casting, heat transfer, electromagnetic stirring, columnar-to-equiaxed transition, microporosity