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Ferroelectricity can be induced in fluorite-structured oxides such as HfO₂ and ZrO₂, a feature of increasing interest in both academia and industry. Initially, most research focused on solid solution films, but recently, it has been suggested that nanolaminates with independent HfO₂ and ZrO₂ layers may show electrical and physical properties superior to those of solid solution samples. It was reported that the nanolaminate samples could have remanent polarization higher than that of solid solution film or a wider composition window for robust ferroelectricity compared to the solid solution films. In this review, the existing literature on fluorite-structured nanolaminates is comprehensively reviewed. (Received August 12 2021; Accepted September 1, 2021)

keyword : ferroelectric, hafnia, zirconia, nanolaminate, superlattice

In this study we developed a novel (TRIP+TWIP) high entropy alloy (HEA) with high specific strength and large ductility. First, by controlling the atomic constitution of the 3d transition metals (Cr, Mn, Fe, Co, and Ni), we designed a light-weight TRIP-assisted dual-phase HEA with a non-equiatomic composition of Cr₂₂Mn₆Fe₄₀Co₂₆Ni₆, which exhibited 5% lighter density than the Cantor HEA. Secondly, we systematically added Al (a lightweight element (2.7 g/㎤), which has a large atomic size misfit with 3d transition metals, and Ferrite stabilizer) up to 5 at.% in Cr₂₂Mn₆Fe₄₀Co₂₆Ni₆ HEA. With increasing Al content, the phase constitution of the alloy changed from a dual-phase of FCC and HCP (0 to 2.0 at.%) to a FCC single-phase (2.5 to 3.5 at.%), to a dual-phase of FCC and BCC (4.0 to 5.0 at.%). In particular, the (Cr₂₂Mn₆Fe₄₀Co₂₆Ni₆)_97.5Al_2.5 HEA with the FCC single-phase exhibited a large Hall-Petch coefficient and relatively lower thermal conductivity due to its three times larger atomic size mismatch (δ) than the Cantor HEA, which causes the superior solid solution strengthening effect. Furthermore, the (Cr₂₂Mn₆Fe₄₀Co₂₆Ni₆)₉₆Al_4.0 HEA, a boundary composition of BCC precipitation in the FCC phase, exhibited a 10% higher specific strength than the Cantor HEA as well as 50% larger strain, due to the unique TRIP and TWIP complex deformation mechanism. This result shows that the addition of Al in Cr₂₂Mn₆Fe₄₀Co₂₆Ni₆ HEA can result not only in greater chemical complexity due to the multicomponent high entropy compositions, but also microstructural complexity due to the increase in competing crystalline phases. The confusion effect caused by both complexities lets the alloy overcome the trade-off relationship among conflicting intrinsic properties, such as strength versus ductility (or density). Consequently, these results pave the way for a new design strategy of a novel (TRIP+TWIP) HEA with high specific strength and large ductility. (Received August 19 2021; Accepted September 8, 2021)

keyword : High entropy alloy, Al addition, Complexity, Specific strength, Deformation mechanism

Al 7075 alloy was manufactured using the twin-roll strip casting (TRC) process, and the mechanical and wear properties of the fabricated TRC process were investigated. To compare the properties of the alloy manufactured by TRC, another Al 7075 alloy was fabricated by conventional direct chill (DC) casting as a comparative material. Based on initial microstructure observations, the Al 7075 alloy manufactured by the DC process showed relatively elongated grains compared to the Al 7075 alloy by TRC process. In both alloys, η(MgZn₂) phases were present at the grain and grain boundaries. In the Al 7075 alloy manufactured by the DC process, the η(MgZn₂) phases were coarse with a size of ~86 nm and were mainly concentrated in the local area. However, the Al 7075 alloy manufactured by TRC had relatively fine η(MgZn₂) phases size of ~40 nm, and they were evenly distributed throughout the matrix. When the mechanical properties of the two alloys were compared, the TRC process showed higher hardness and strength properties than the DC process. In room temperature wear test results, the TRC process exhibited lower weight loss and wear rates compared to the DC process at all wear loads. In other words, the TRC process resulted in relatively superior wear resistance properties compared to the conventional DC process. The wear behavior of both alloys changed from abrasive wear to adhesive wear as the wear load increased. However, the TRC process maintained abrasive wear up to higher loads. Based on the above results, a correlation between the microstructure and wear mechanism of the Al 7075 alloy manufactured by TRC is also suggested. (Received May 24 2021; Accepted September 17, 2021)

keyword : Al 7075 alloy, Twin-roll strip casting, Direct chill casting, Wear property, Microstructure

Nanopatterning methods for pattern formation of high-resolution nanostructures are essential for the fabrication of various electronic devices, including wearable displays, high-performance semiconductor devices, and smart biosensor systems. Among advanced nanopatterning methods, nanotransfer printing (nTP) has attracted considerable attention due to its process simplicity, low cost, and great pattern resolution. However, to diversify the pattern geometries for wide device applications, more effective and useful nTP based patterning methods must be developed. Here, we introduce a facile and practical nanofabrication method to obtain various three-dimensional (3D) ultra-thin metallic films via thermally assisted nTP (T-nTP). We show how to generate surface-wrinkled 3D nanostructures, such as angular line, concave-valley, and convex-hill structures. We also demonstrate the principle for effectively forming 3D nanosheets by T-nTP, using Si master molds with a low aspect ratio (A/R ≤ 1). In addition, we explain how to obtain a 3D wavy structure when using a mold with high A/R (≥ 3), based on the isotropic deposition process. We also produced a highly ordered 3D Au nanosheet on flexible PET over a large area (> 15 μm). We expect that this T-nTP approach using various Si mold shapes will be applied for the useful fabrication of various metal/oxide nanostructured devices with high surface area. (Received September 10, 2021; Accepted September 24, 2021)

keyword : Nanotransfer printing, Nanopattern, Aspect ratio, Surface wrinkle

The dissimilar welding of titanium and copper by fusion welding is very difficult because the melting points of the materials are very highly different and strong brittle intermetallic compounds (IMCs) can be easily produced in welded zone and heat-affected zone, etc. Friction stir welding was employed as a type of solid-state welding for Ti/Cu dissimilar welding to obtain a sound welded zone and reduce the total process cost. This study investigated how the metal flow of the welded zone changes according to the variation in the rotational speed of the tool, from 450 rpm to 600 rpm. When the rotational speed was too high, the plastic flow of the softened material increased and intermetallic compounds such as TiCu, Ti₂Cu₃, and Ti₂Cu, were generated in the Cu region of the welded zone. The microstructural evolution of AS (Advancing Side) and RS (Retreating Side) were investigated and the soundness of the welded zone and its mechanical properties were evaluated through the microstructural evolution. A high hardness value of 200 Hv or more was exhibited in some points, due to the formation of intermetallic compounds in the RS (Cu) region. Ti/Cu dissimilar friction stir welding at a welding speed of 50 mm/min and an appropriate rotation speed of 500 rpm showed a good welded zone and mechanical properties. (Received June 11 2021; Accepted August 27, 2021)

keyword : Ti, Cu friction stir welding, dissimilar welding, rotation speed, intermetallic compounds

This study evaluated the effect of preheating on early stage melting behavior of a Al-Si coated hot stamped boron steel bolt during projection welding. A large amount of heat was generated in the early stage of projection welding. Because of the large heat generation, a rapid collapse of the projection occurred and a molten coating layer remained on the interface of the welded part. This caused welding defects such as expulsion and porosity. However, preheating helped remove the molten Al-Si coating layer by pushing it out toward the outer edge of the molten pool. This suggests that preheating can effectively minimize or remove the molten coating layer within the weld. Preheating also prevented the rapid collapse of the projection by partially melting the projection, and thus improving the contact area. These phenomena can prevent the concentration of current density at the weld interface and hence decrease heat generation. Finally, the preheating current improved nugget quality by promoting the stable growth of the melted metal and by preventing expulsion and porosity. (Received July 23 2021; Accepted August 25, 2021)

keyword : bolt projection welding, preheating current, Al-Si coated hot-stamping steel, initial melting behavior, SORPAS simulation

In this study, half-Heusler (HH) thermoelectric materials Nb_0.8Hf_0.2FeSb_0.98Sn_0.02 (p-type) and Hf_0.25Zr_0.25Ti_0.5NiSn_0.98Sb_0.02 (n-type) were synthesized using induction melting and spark plasma sintering. For alloying, a conventional induction melting technique was employed rather than arc melting, for mass production compatibility, and the thermoelectric properties of the materials were analyzed. The maximum dimensionless figures of merit (zT_max) were 0.75 and 0.82 for the p- and n-type material at 650 ℃ and 600 ℃, respectively. These materials were then used to fabricate generator modules, wherein two pairs of p- and nlegs without interfacial metal layers were brazed on direct bonded copper (DBC)/Al₂O₃ substrates using a Zrbased alloy. A maximum power of 0.57 W was obtained from the module by applying a temperature gradient of 476 ℃, which corresponds to a maximum power density of 1.58W cm ^-2 when normalized by the area of the material. The maximum electrical conversion efficiency of the module was 3.22% at 476 ℃ temperature gradient. This value was negatively affected by the non-negligible contact resistivity of the brazed interfaces, which ranged from 6.63 × 10 ^-9 Ω ㎡ to 7.54 × 10 ^-9 Ω ㎡ at hot-side temperatures of 190 ℃ and 517 ℃, respectively. The low electrical resistivity of the HH materials makes it especially important to develop a brazing technique for ultralow resistance contacts. (Received August 11 2021; Accepted August 31, 2021)

keyword : thermoelectric, half-Heusler, module, brazing, contact resistivity

Supercapacitor electrode slurry is prepared for mass production by mixing activated carbon powder, conductive agent, and binder, which is then deposited on a substrate using the doctor-blade method. Polyvinylidene fluoride (PVDF) and 1-methyl-2-pyrrolidone (NMP) are used as binder and solvent, respectively, to form the electrode slurry on a metal substrate. In this study, ethyl cellulose (EC) is evaluated as a binder to prepare an electrode on an indium-tin-oxide (ITO) substrate obtaining transparent supercapacitors. Terpineol and isopropyl alcohol (IPA) are compared as suitable solvents for the EC binder. When terpineol is employed as a solvent, the conductive agent is uniformly deposited around the activated carbon powder. An electrode prepared using EC and terpineol exhibits slightly lower specific capacitance and rate performance than that using conventional PVDF and NMP. However, the electrode prepared using EC and terpineol securely adheres to the electrode components, resulting in a robust electrode. In contrast, an electrode prepared using EC and IPA exhibits high charge transfer resistance at the interface of the electrode/electrolyte, leading to a low specific capacitance and rate performance. Thus, ecofriendly EC and terpineol can substitute the conventional PVDF and NMP for depositing activated carbon powder on an ITO substrate, while improving the specific capacitance of manufactured electrodes. (Received September 1 2021; Accepted September 17, 2021)

keyword : ethyl cellulose, terpineol, indium-tin-oxide, specific capacitance, charge transfer resistance, adhesion

Radiation causes damage to the human body, the environment, and electronic equipment. Shielding against neutron and gamma rays is particularly difficult because of their strong ability to penetrate materials. Conventional gamma ray shields are typically made of materials containing Pb. However, they pose problems in that Pb is a heavy metal, and human poisoning and/or pollution can result from the manufacturing, use, and disposal of these materials. In addition, neutron rays are shielded by materials rich in H₂ or concrete. In the case of the latter, the manufacturing cost is high. Thus, it is necessary to develop a new multilayer structure that can shield against both neutron and gamma rays. We set up a simulation model of a multilayered structure consisting of metal hydrides and heavy metals, and then evaluated the simulations using Monte Carlo N-Particle Transport Code. Monte Carlo simulation is an accurate method for simulating the interaction between radiation and materials, and can be applied to the transport of radiation particles to predict values such as flux, energy spectrum, and energy deposition. The results of the study indicated the multilayer structure of ZrH₂, U, and W could shield both neutron and gamma rays, thus showing potential as a new shielding material to replace Pb and concrete. (Received July 6 2021; Accepted September 15, 2021)

keyword : monte carlo, neutron, gamma ray, shielding, multilayer, MCNP