발간논문

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Liquid immiscible alloy systems present a unique opportunity for designing composites with sphere-dispersed and/or interconnected microstructures. Herein, we demonstrate that the addition of Pb in Al₈₄TM₁₀RE₆ (TM=Ni,Co,Fe,Ti, RE=La,Ce,Nd,Gd,Y) metallic glasses can result in liquid-liquid phase separation in a liquid immiscibility gap and substantially solidifies into a Pb-rich crystalline phase and Alrich amorphous phase. The Pb-rich phase has a spherical shape and is dispersed in the Al-based metallic glass matrix. The average diameter of the Pb-rich spheres changed from 75 ± 10 nm near the wheel side (fastest cooling region) to 138 ± 32 nm near the center of the ribbon (slowest cooling region), which exhibited a melting temperature of about 599 K, close to that of pure Pb (Tm = 600.5 K), due to the low solubility of Pb and other constituent elements. Interestingly, the dispersed Pb-rich 2^nd phase in the Al-based metallic glass matrix causes delayed crack propagation and enhanced fatigue lifetime due to crack blocking and filling by partial liquifaction. These results provide a guideline for how to design promising Al-based metallic glasses containing sphere-dispersed 2^nd phase with low melting temperature, which is a key step in developing damage-tolerant metallic glass composites with a liquid-based healing mechanism. (Received November 18, 2019; Accepted December 18, 2019)

keyword : Al-based metallic glass, liquid-liquid phase separation, Pb-rich 2^nd phase, crack shielding, liquid-based healing mechanism

The growth of the PEO coating with processing time on an AZ31B magnesium alloy in a dilute alkaline electrolyte containing silicate and the corrosion characteristics according to its thickness were investigated. The oxide film grew a porous outer layer mainly consisting of silicate with the spark discharge phenomenon from the beginning of the reaction and reached a certain limit thickness later in the reaction. This reaction was followed by the inner layer, mainly consisting of MgO, growing rapidly and densifying. Based on the results of the potentiodynamic polarization and chronoamperometric test, it was found that within 1 minute of the processing time, due to the contribution of the dielectric layer and MgF₂ phase layer formed at the interface between the oxide and the substrate, the corrosion resistance was relatively high, but its stability against corrosion was not enough. Moreover, when the thickness of the oxide coating was grown mainly with the silicate outer layer, the increase in the pore size, which acts as a channel of spark discharge, was the leading cause of the decrease in the corrosion resistance. The superior corrosion resistance and the stability of the PEO coating in the later reaction time of the processing are due to the growth and densification of the inner layer of MgO. (Received October 4, 2019; Accepted December 19, 2019)

keyword : plasma electrolytic oxidation (PEO), AZ31B, corrosion resistance, potentiodynamic polarization

This study created alumina structures with the highest hydrophilic properties on 6061 aluminum alloy. The anodization process was applied to make various aluminum oxide structures. To create uniform alumina structures on top of a 6061 aluminum alloy surface, after conducting the first anodization in 0.3 M oxalic acid at 40 V at 0 ℃, the alumina was removed using a mixture of chromic acid and phosphoric acid. Then, secondary and tertiary anodization was performed using the same electrolyte conditions as the primary anodization for 30 minutes at 40 V, respectively. Pore-widening (PW) of oxide film formed after the secondary anodic oxidation was performed for 20, 30, and 40 minutes in 0.1 M phosphoric acid solution. The PW time control allowed various oxide structures to be created, and reduced the area of the outermost surface in contact with water droplets. The smaller the initial area of water droplets, the better the hydrophilic phenomenon. The surface area can be represented as a solid fractional value. Surfaces with solid fraction values of less than or equal to 0.5 were superhydrophilic. This well-controlled anodization process with a pore-widening step can be used to create excellent superhydrophilicity on various metallic substrates, expanding their usefulness and efficacy. (Received October 21, 2019; Accepted December 19, 2019)

keyword : alumina structure, aluminum alloy, anodizing, pore-widening, superhydrophilicity, solid fraction

A block Ni-Cr-Al superalloy foam with dimensions of 300 mm (width direction, WD) × 500 mm (rolling direction, RD) × 60 mm (normal direction, ND) was fabricated using powder alloying, multi-sheet stacking, and hot rolling processes. The structural characteristics, microstructure, and mechanical asymmetry of the block Nibased foam were investigated. Analysis of the structural features showed that the interfaces between the sheets had complex strut interactions, such as contacted (deformed) and intersected struts. The average pore size along the directions was measured to be 2569.6 μm (WD), 2988.1 μm (RD), and 2493.2 μm (ND). The average thickness of the strut was 340.8 μm, and the wall thickness of the strut was 27.7 μm. The elemental distributions in the struts were uniformly controlled, and the block foam consisted of γ (matrix) and γ ' (Ni3Al) phases. Tensile properties in the ND direction showed a yield strength of 0.175 MPa, tensile strength of 0.233 MPa, and elongation of 2.54%, while the tensile properties in the RD direction were 1.27 MPa (YS), 3.01 MPa (UTS), 8.92% (El.) respectively. The foam was observed to have a compressive yield strength of 0.795 MPa in the ND direction, and that of 2.18 MPa in the RD direction were obtained. The asymmetry and anisotropy of these mechanical properties could be explained by the difference in pore sizes along the direction, and the structural characteristics of the sheet interface generated by sheet stacking and rolling. (Received September 10, 2019; Accepted December 13, 2019)

keyword : block Ni-Cr-Al foam, superalloy foam, structural characteristics, powder alloying process, mechanical asymmetry

In this study, an electrospray technique was used for the synthesis of macroporous ceramic particles, such as silica or titania, by a colloidal templating method. For this purpose, a suspension of polystyrene nanospheres was synthesized by dispersion polymerization for use as sacrificial templates. The feed solution containing a ceramic precursor and polymeric beads was injected through a metallic nozzle under a high electric field for nebulization of aerosol droplets as micro-reactors. Under ambient air conditions, the volatile components were evaporated from the droplets, and gelation of the precursor was completed simultaneously. The resulting supra-aggregates were then collected, and calcination was performed to form porous ceramic particles by removing the polymeric templates. As a demonstrative application, the porous particles of silica were deposited as a coating film for superhydrophobic surfaces with a high water contact angle larger than 150°. Furthermore, macroporous titania particles were used as photocatalytic particles in a wastewater system with first-order reaction kinetics. (Received October 21, 2019; Accepted December 14, 2019)

keyword : encapsulation, photocatalysis, porous media, powders, spraying, interfacial phenomena

As global regulations of CO₂ emissions and fuel consumption efficiency become more strict, the automotive powertrain system has become more compact with lower lubrication viscosity. With the current trends in powertrain system improvement, the operating conditions of powertrain components such as gears are becoming more severe. As a result, it is increasingly important to improve the strength and durability of the materials used for the powertrain system by optimizing alloy design and heat treatment. Much research and development has focused on improving components by heat treatment, and especially carburization. Also, many different alloy concepts, such as Fe-Cr-Mo or Fe-Cr-Ni systems have been proposed for automotive components. Among various approaches, Cr and Si , and solid solution hardening, are known to effectively increase hardenability. However, it has been reported in many works that the carburized case depth can also be reduced by increasing the Cr/Si content, due to oxide film formation. The mechanism of Cr/Si oxide film formation during carburization has not yet been clearly determined. In the present work, therefore, the mechanism of surface oxide film formation during carburization was examined. Also, the microstructure, composition and crystal structure of different oxides were examined and compared in detail. (Received September 6, 2019; Accepted December 19, 2019)

keyword : carburization, oxide film, hematite, magnetite

Radio frequency identification (RF-ID) uses electromagnetic fields to automatically identify and track tags attached to objects. In response to the need to suppress the electromagnetic interference between adjacent RF-ID equipment or systems, this study aims to achieve a thin and multi-resonance absorber using metamaterials composed of patterned grids on a grounded dielectric substrate in the frequency region of UHF and microwaves. A computational tool (ANSYS HFSS) was used to model and estimate the reflection coefficient and surface current distribution. The samples used to measure reflection loss were fabricated by the printed circuit board (PCB) method in which a copper film was deposited on both sides of a photosensitized board. The reflection loss was measured using a free space measurement system, composed of a pair of horn antennas and network analyzer. The grid-patterned metamaterial absorbers exhibited dual-band absorption peaks at 0.88 GHz and 2.45 GHz with a small substrate thickness (about 3.7 mm) that can be usefully applied to electromagnetic compatibility in RF-ID system. Magnetic coupling is achieved via antiparallel currents in the grid conductor on the top layer and the ground conductor on the bottom layer. Since the magnetic response is influenced by the dimension of the resonators, it is possible to achieve two resonances by scaling the resonating structures. The metamaterial absorbers also exhibited good oblique incidence performance. A high level of absorption (above 10 dB) was maintained at up to 30 degrees of incidence angle for both TE and TM polarization. (Received September 3, 2019; Accepted December 21, 2019)

keyword : microwave absorbers, radio frequency identification, metamaterials

Multi-layer core-shell nanoparticles (YVO₄:Nd³⁺/mSiO₂/SiO₂) consisting of silica cores (SiO₂), mesoporous silica (mSiO₂) intermediate layers, and Neodymium doped rare-earth phosphor (YVO₄:Nd³⁺) shell layers were successfully synthesized using the stepwise sol-gel method. The morphological structure and optical properties of the functional core-shell nanoparticles were characterized and evaluated by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) analysis. mSiO₂ intermediate layers were utilized as the bridge between the core and shell materials. Their porous surfaces served to anchor the YVO₄:Nd³⁺ crystals. This prevents energy loss during the energy transfer of electrons, resulting in improved optical properties. The use of intermediate layer combinations of mSiO₂/SiO₂ in the coreshell structure also improved cost-effectiveness, because the core is filled with cheap silica, not expensive phosphors. Even though the nanoparticles used only a thin layer of the photoluminescent shell materials, the optical properties, resulting from the energy-transfer emitting mid-infrared light, were remarkably enhanced by increasing the crystallinity of the phosphor. To demonstrate the practical use of the synthesis method, the photoluminescent properties of the core-shell nanoparticles were optimized by adjusting the annealing temperature and scaling to mass production. We believe that our efficient synthetic strategy provides a facile way of obtaining functional, cost-effective core-shell nanoparticles with improved photoluminescent properties. (Received November 18, 2019; Accepted December 13, 2019)

keyword : mesoporous silica, rare-earth phosphor, sol-gel method, core-shell, photoluminescent property

The nanofabrication of modern electronic devices requires advanced nanopatterning technologies. To fabricate desirable nanodevices with excellent device performance, controlling the shape and dimension of the pattern is very important. However, to achieve more facile and faster device fabrication, with better pattern resolution, pattern-tunability, process simplicity, and cost-effectiveness, some remaining challenges still need to be resolved. In this study, we introduce a simple and practical method to generate various patterns using a nanotransfer printing (nTP) process. To obtain functional materials with diverse shapes on a polymer replica pattern, in the nTP process we controlled the angle of deposition before transfer-printing. First, we obtained three different pattern shapes with a thickness of ~ 30 nm on polymethyl methacrylate (PMMA) replica patterns. Then, the deposited functional patterns on the PMMA patterns are successfully transfer-printed onto SiO₂/Si substrates, showing line, L-shape line, and concavo-convex patterns. We observed the pattern shapes of the patterns by scanning electron microscope (SEM) and optical microscope. Moreover, we systemically analyzed how to form patterns of various shapes using one kind of master mold. We expect that this simple approach will be widely used to fabricate various useful patterns for electronic device applications. (Received November 18, 2019; Accepted December 18, 2019)

keyword : nanopattern, angle deposition, nanotransfer printing