발간논문

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In this study, a high-melting point element, tantalum carbide (TaC) was added to improve the mechanical properties and the oxidation/corrosion resistance of Fe. The Fe-TaC composites were synthesized by high energy ball milling for the mechanical alloying of the non-equilibrium phase (Fe-Ta) and the homogeneous dispersion of TaC. Fe-TaC composite samples were fabricated using 5, 10, and 20 wt.% TaC. The ductile particles (Fe) got harden and the brittle particles (TaC) were uniformly dispersed, while facilitating short-range diffusion in the ductile matrix by the high energy ball milling method. Spark plasma sintering was performed at a sintering temperature of 850 ℃ and pressure of 60 MPa. As the TaC contents increased, the sintering exponential (m) increased. A higher ‘m’ value indicates a lower magnitude of shrinkage, by decreasing the lattice and grain boundary (G/B) diffusion path between the pores and particles. The hardness increased from 128.9±10.4 to 444.2±20.6 kg/mm² as the grain size decreased from 5.13 to 3.99 μm. This enhancement is attributed to the Hall-Petch relationship and dispersion strengthening effect. The mechanical properties of the sintered bodies were studied to evaluate how the different TaC content affect their characteristics. In addition, oxidation resistance increased with increasing TaC contents. It was considered that the local oxidation resistance based on the formation of an oxide layer of TaO and Fe₂O₃. (Received 26 October, 2023; Accepted 28 November, 2023)

keyword : Fe, TaC, spark plasma sintering, high energy ball milling, mechanical property

This study demonstrates that the addition of the rare earth element Erbium (Er) significantly enhances the machinability and tensile properties of titanium (Ti). Pure Ti alloys and Er-added Ti alloys with 0.5-1.1 wt.% Er content were prepared, and their microstructure, machinability, and tensile properties were compared. Two different types of Er secondary phase particles were identified in the microstructure: pure Er and Er-oxide. The amounts of these particles increased with higher Er content. The machinability of the Eradded Ti alloys was significantly improved due to the ability of Er secondary particles to cut machining chips or absorb heat from localized deformation within the Ti matrix. In addition, Er-added Ti alloys exhibited higher strength than pure Ti. The strength enhancement was attributed to grain refinement induced by the Er element. Er secondary phase particles reduced the β grain size during solidification, and they also served as preferential sites for α nucleation during the β → α phase transformation, resulting in a refined microstructure. In addition, the Er secondary phase contributed to the strength enhancement through the well-known precipitation strengthening mechanism. Although ductility decreased with higher Er content due to the increased amount of Er secondary phase particles, 0.5 wt.% Er-added Ti showed no such degradation; its ductility was comparable to that of pure Ti. Er-oxidation was expected to reduce oxygen content within the Ti matrix, enhancing intrinsic Ti ductility; this effect offset the adverse impact on ductility caused by the Er secondary phase particles. Above 0.5 wt.% Er, the adverse effects caused by the Er secondary phase particles overwhelmed the beneficial effect caused by the reduction in oxygen content. The present findings will contribute significantly to the development of highly machinable Ti alloys with superior tensile properties.

keyword : titanium, machinability, microstructure, precipitate, tensile properties

Capacitors not only store and release electricity but selectively conduct alternating current. Among the various types of capacitors, multi-layer ceramic capacitors (MLCCs) have been widely used in automotive, smartphone, and wearable devices because of their compact size and high capacitance capabilities. In this study, we have developed an electrolyte for tin electroplating on multi-layer ceramic capacitors (MLCCs) to address the barium leaching issue at the termination points of the MLCCs. This issue has been effectively mitigated by introducing NaHSO₄ into the conventional tin plating electrolyte as a corrosion inhibitor. This addition facilitates a rapid reaction between the dissolved barium ions and NaHSO₄, resulting in the formation of a thin passivation layer on the surface of the MLCC. The BaSO₄ passivation layer effectively prohibits excessive leaching of barium ions from the glass in MLCCs, thereby maintaining chip insulation resistance and preventing crack formation. However, the chemical reaction of NaHSO₄ and the formation of the passivation layer can lead to the generation of tin hydroxide precipitates due to pH fluctuations. To address this issue, we increase the amount of complexing agent from 100 g/L to 130 g/L. This adjustment enhanced the ability of tin ions to form stronger complexes, thereby enabling stable electrodeposition on the termination of MLCC. Consequently, the final electrolyte for Sn electroplating (denoted as LW-3) simultaneously achieves corrosion resistance and practical working efficiency, resulting in a uniform 5.4 μmthick tin plating layer with outstanding solderability, and high temperature/humidity stability. (Received 24 October, 2023; Accepted 5 December, 2023)

keyword : multi-layer ceramic capacitor, termination, Sn electroplating, corrosion resistance, complexing agent

To develop an eco-friendly titanium (Ti) production process, the calciothermic reduction of titanium dioxide (TiO₂) was investigated. The mechanism involved in the reduction of TiO₂ using calcium (Ca) was examined. The influence of a molten salt, calcium chloride (CaCl₂), on the calciothermic reduction was investigated by conducting experiments at 1123 - 1223 K in an argon (Ar) atmosphere for the duration of 0.5 - 9 h. When CaCl₂ was used as a molten salt, the oxygen (O) concentration in Ti decreased to 0.162 mass% owing to the removal of calcium oxide (CaO) from the surface of the Ti particles. In addition, the use of a CaCl₂ - lithium chloride (LiCl) molten salt decreased the reaction temperature to 1073 K and the O concentration in the obtained Ti reached 0.333 mass%. Furthermore, after the calciothermic reduction of TiO₂ at 1173 K using a perforated crucible, the residual Ca-containing salt was sufficiently separated in-situ from the crucible, and a mixture of Ti and TiH_1.924 was produced via hydrogenation at 1023 K by changing the atmosphere gas from Ar to hydrogen gas (H₂). The results of this study demonstrate the feasibility of the calciothermic reduction of TiO₂ for the eco-friendly production of Ti and TiH₂ powders. (Received 9 October, 2023; Accepted 5 December, 2023)

keyword : titanium dioxide, titanium, calciothermic reduction, calcium chloride, lithium chloride, hydrogen

Bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272) is known to selectively extract Au(III) from dilute to concentrated hydrochloric acid solutions. Therefore, Cyanex 272 can be employed in the separation of Au(III) from the hydrochloric acid leaching solutions of secondary resources containing gold metal. This work identified the extraction reaction of Au(III) by Cyanex 272 from the hydrochloric acid solutions. Au(III) extraction data was obtained by varying the concentrations of HCl (1-9 M), Au(III) (0.1-1 g/L) and Cyanex 272 (0.01-0.1 M). Among the three variables, it was found that HCl concentration had the most pronounced effect on the extraction of Au(III), which increased with HCl concentration. The extraction reaction of Au(III) by Cyanex 272 is proposed by applying slope analysis method to the extraction data. Comparison of Fourier transform infrared spectroscopy results for the fresh and loaded Cyanex 272 revealed no change in the chemical structure of Cyanex 272 after the extraction. Nuclear magnetic resonance spectroscopic data showed a change in the characteristic peaks of the phosphorus atom of Cyanex 272, indicating the formation of a coordinative bond between Au(III) and Cyanex 272. FT-IR and NMR data together with application of slope analysis clearly verified that molecular Cyanex 272 takes parts in the extraction of Au(III). (Received 25 October, 2023; Accepted 19 December, 2023)

keyword : gold, hydrochloric acid, Cyanex 272, FT-IR spectra, NMR spectra

In this study, the architecture of an interfacial switching memristor, which has a metal-insulatormetal structure of Pt/SrTiO3/Nb-SrTiO3 was investigated. The performance of a neural network that uses memristors as its synapse components was also examined with system-level simulations. A finite element solver, COMSOL Multiphysics, was used to simulate synaptic device characteristics, specifically, the conductance change, using a series of pulses for a given architecture. An open-source software, NeuroSim, was used to simulate the ability of the neural network to recognize and identify handwritten digits. Electrostatics, mass transport, and thermionic emission equations were numerically solved in a fully coupled manner to model the Schottky barrier height modulation at the Pt/SrTiO3 contact using the applied bias. The barrier height is a function of the oxygen vacancy concentration in the SrTiO3 near the contact. The gradual change of the oxygen vacancy concentration profile caused by successive pulses results in the gradual change of conductance. Utilizing the simulations, the influences of device structure modification, and more specifically, changing the size of the Schottky contact, on long-term potentiation and depression were analyzed for planar devices. The results show that a smaller Schottky contact yields a higher digit recognition rate. Based on this finding, a three-dimensional device architecture that is vertically stackable was designed. (Received 8 November, 2023; Accepted 4 December, 2023)

keyword : Interfacial switching memristor, Schottky barrier height modulation, Synaptic device, Device simulation

Compound droplets can consist of two or more immiscible substances sharing an interface. Among such droplets, the low-viscosity component of Janus droplets can exhibit peculiar bouncing behavior on nonwettable surfaces. There have been recent advances in droplet control technologies, however the impact dynamics of droplets on complex surfaces, and strategies to control their behavior, have not been extensively studied. This study employs the volume of fluid method to analyze the effects of Janus droplet size and the initial interface angle on the dynamics of the two fluidic components in droplets on superhydrophobic cylinders. Janus droplets are composed of low-viscosity (W-) and high-viscosity liquid (G-component). The dynamic characteristics of Janus droplets are investigated as a function of Weber number (We), initial interface angle, the ratio of the droplet’s diameter to the cylinder’s diameter, and viscosity ratio (α). Numerical models provide a regime map of the separation ratio of Janus droplets based on We and α, and the influence of droplet size on asymmetric bouncing is discussed. This study also examines the threshold We at which separation begins after impact, varying with droplet size and α. In addition, the shape evolutions of the droplets are discussed for various initial interface angles to understand the bouncing behavior and separation efficiency. This study is expected to provide valuable strategies for controlling droplet behavior and separation in applications such as liquid purification, rheology, and solidification. (Received 20 November, 2023; Accepted 13 Decmeber, 2023)

keyword : superhydrophobic surface, compound material, Janus material, interfacial dynamics

The TiAl alloy is attracting attention as a lightweight and heat-resistant material, because of its high specific strength, excellent high-temperature formability, and fatigue strength. However, its applications are limited by its high unit price and low room temperature ductility. To overcome this issue, dissimilarly bonded materials have been extensively employed. This involves joining a brittle metal to a low-cost metal that possesses excellent plasticity, using various dissimilar bonding techniques. In this study, TiAl/HI-TEMP 820/SCM440H materials were fabricated using a vacuum brazing process under different temperature conditions. After the brazing process, the microstructure of the interfacial area revealed seven distinct layers resulting from chemical reactions between the base metals and the filler metal. These reaction layers consisted of a Ni solid solution, intermetallic compounds (Ti₃Al, TiNi₂Al, Ti₂Ni, FeNi), and borides (CrB, TiB₂, FeB). To analyze the effect of brazing temperature on the relationship between the microstructure and mechanical properties at the interface of TiAl/HI-TEMP 820/SCM440H materials, conventional uniaxial tests and nanoindentation tests were performed. The measured nanohardness exhibited a significantly large distribution for each reaction layer, with the highest hardness values observed in the intermetallic compounds and borides layers. Additionally, room temperature tensile tests confirmed that fractures initiated in the highhardness and brittle intermetallic compounds and borides layers. (Received 3 November, 2023; Accepted 20 December, 2023)

keyword : TiAl, Vacuum brazing, Bonding interface, Mechanical properties, Nanoindentation

This review provides an overview of recent advances in the materials and structural design of flexible piezoresistive sensors, which are of significant interest as smart devices in the Fourth Industrial Revolution. Recent research has focused on addressing demand for a wide pressure range and high sensitivity for use in diverse applications. In terms of sensor materials, this review begins by introducing elastomers used for matrix and substrate, which provide exceptional conformability to curved surfaces and mechanical flexibility to withstand stretching and bending. They can also incorporate sensing materials in various forms of nanostructures capable of effectively transmitting electrical signals, exploiting the nanomaterial’s electrical properties and conductivity pathways formation mechanisms under pressure, which define the sensor’s characteristics. Additionally, this review explores structural design considerations, including different types of microstructures that can deform easily even under low pressure, enhancing sensor sensitivity, as well as hierarchical structures capable of gradual changes in contact area to provide a wide range of operating pressures. Moreover, the review discusses time- and cost-efficient microfabrication techniques to facilitate practical utilization of the device. This review then summarizes the applications of high-performance sensors in fields such as health monitoring, including pulse detection and motion tracking, robotics, and human-machine interaction involving components such as touch screens and keypads. (Received 7 November, 2023; Accepted 22 November 2023)

keyword : Flexible piezoresistive sensor, Nanomaterial, Microstructure, Hierarchical structure, Wearable device