발간논문

Home > KJMM 논문 > 발간논문

Stainless steel is a metal with excellent workability, economy, and corrosion resistance, so it is used in various industrial applications including the marine, machinery, electronic parts, piping, power generation, and nuclear power fields. However, in contaminated environments such as marine and gas pipelines, stainless steel has problems, including surface aging and corrosion. Several surface treatment methods have been proposed to address those problems. This study was conducted to observe the water repellency and corrosion resistance of a superhydrophobic phosphorylated film on the surface of stainless steel. The anodization was carried out using a step-by-step process under voltage in an ethylene glycol electrolyte, at 30, 50, and 70 V for 3 hours, respectively. The distance between the anode electrode and cathode electrode was maintained at 5 cm. A water-repellent surface was achieved using a FDTS (1H, 1H, 2H, 2H-Perfluorodecyltrichlorosilane) solution, a material with low energy, on the surface of the fabricated specimen. A Field Emission Scanning Electron Microscope (FE-SEM) was used to analyze the surface shape of the structure, and water repellency was analyzed using an angle meter. The corrosion behaviors of the electrochemical oxide film were investigated through polarization experiments. (Received January 27, 2021; Accepted February 23, 2021)

keyword : stainless steel, anodization, thin oxide films, superhydrophobic coating, corrosion resistance

Bipolar electrodialysis was used in a process of desalting a lithium sulfate solution, converting it to lithium hydroxide and sulfuric acid, and concentrating and recovering them. The effects of the experimental variables such as applied voltage, the concentration of electrode solution, the concentration of raw material solution, volume ratio, and impurity were confirmed. The optimum conditions were investigated by comparing the conversion(%) of lithium hydroxide and sulfuric acid, the process time, and energy consumption. As the applied voltage was increased, the energy consumption tended to increase, but the processing time decreased significantly. As the concentration of lithium sulfate in the raw material solution increased, the conversion(%) of lithium hydroxide decreased. As the concentration of lithium sulfate increased, the energy consumption did not increase linearly, and energy consumption increased significantly. When a raw material solution of 0.5 M Li₂SO₄ or more is used in the bipolar electrodialysis process, an applied voltage of 25 V is preferable. As the applied voltage increased at a constant process time, the conversion(%) of LiOH and H₂SO₄ increased. Regarding the effect of the electrode solution concentration, when a 5.0 wt% electrode solution was used rather than a 3.0 wt% electrode solution, energy consumption decreased by more than 10%. When the volume of the raw material solution was increased, the processing time required for desalting increased. By using a low concentration raw material solution, it was confirmed that it was simultaneously possible to recover and concentrate lithium hydroxide and sulfuric acid through volume ratio control. When the raw material solution contained Na as an impurity, it was converted to NaOH with a surface LiOH, and it was not possible to separate the lithium and sodium. (Received December 3, 2020; Accepted February 6, 2021)

keyword : bipolar electrodialysis, EDBM, lithium hydroxide, lithium sulfate, lithium ion battery

To manufacture finer solder bumps, the SR and DFR patterns were filled using a Sn electroplating process instead of the microball process currently used in BGA technology, and the solder bump shape was fabricated through a reflow process. The microstructure of the solder bump was investigated by EBSD and TEM measurements. The EBSD results showed that the grain size of the Sn structure became finer after the reflow treatment and a scallop shape of Cu₆Sn₅ was formed on the Cu UBM. However, the Cu₃Sn phase was difficult to measure in the EBSD measurement. The Cu3Sn compound could be investigated with TEM analysis. The Cu₃Sn phase also existed in the Sn region, with a size of several tens of nanometers, due to the eutectic reaction. The volume fraction of the Cu₆Sn₅ phase in the Sn region could be calculated from the TEM image, and the concentration of copper dissolved in the liquid tin during the reflow process could be estimated from the volume fraction. It was possible to observe the Cu₃Sn and Cu₆Sn₅ lattice images through high resolution TEM analysis, but it was difficult to observe the lattice coherency between the two phases because both were polycrystalline. Based on the results of this study, it is expected that solder bumps with a diameter of less than 100 μm can be robustly manufactured through the Sn electroplating process. (Received December 2, 2020; Accepted February 8, 2021)

keyword : electroplating, wetting, soldering, electron backscattering diffraction, EBSD

La/Ce-partially double-filled and Co-charge-compensated (La_1-zCe_z)_0.8Fe_4-xCo_xSb₁₂ skutterudites were synthesized, and their thermoelectric properties were studied by varying the filling ratio and charge compensation. X-ray diffraction analysis revealed that the matrix phase was skutterudite and a secondary phase was determined to the marcasite FeSb₂. However, the formation of marcasite could be inhibited by increasing the Co content. Rare-earth antimonides, including LaSb₂ and CeSb₂, which were formed in fully filled La_1-zCe_zFe_4-xCo_xSb₁₂, were not found after La/Ce partial filling. La/Ce filling and Co substitution were confirmed by the decrease in lattice constants, from 0.9137 to 0.9099 nm, with increasing Ce and Co contents. Electrical conductivity showed negative temperature dependence, indicating metallic or degenerate semiconductor characteristics. Intrinsic conduction resulted in the maximum Seebeck coefficient at temperatures between 723 and 823 K. As the Co-substitution and Ce-filling contents increased, the Seebeck coefficient increased, while electrical and thermal conductivities decreased. This was considered to be due to difference in the valences of La³⁺ and Ce^3+/4+ and the increase in carrier concentration caused by Co charge compensation. However, because they had similar atomic masses and ionic radii, the effects of the La/Ce filling ratio were not significant. Instead, Co charge compensation had the dominant effect on thermoelectric properties. The maximum Seebeck coefficient of 165.4 μVK^-1 was obtained for (La_0.25Ce_0.75)_0.8Fe₃CoSb₁₂ at 823 K, and the highest electrical conductivity of 2.27 × 10⁵ Sm^-1 was achieved for (La_0.75Ce_0.25)_0.8Fe₄Sb₁₂. (La_0.25Ce_0.75)_0.8Fe₃CoSb₁₂ exhibited the lowest thermal conductivity of 2.15 W m^-1K^-1 at 523 K and (La_0.75Ce_0.25)_0.8Fe_3.5Co_0.5Sb₁₂ showed the highest power factor of 2.53 mW m^-1K^-2 at 723 K. The maximum dimensionless figure of merit, ZT_max = 0.71, was achieved at 723 K for (La_0.75Ce_0.25)_0.8Fe₃CoSb₁₂. (Received December 24, 2020; Accepted February 10, 2021)

keyword : thermoelectric, skutterudite, partial double filling, charge compensation

All solid-state batteries (ASSBs) are now anticipated to be an ultimate solution to the persistent safety issues of conventional lithium-ion batteries (LIBs). Contemporary society’s expanding power demands and growing energy consumption require energy storage with greater reliability, safety and capacity, which cannot be easily achieved with current state-of-the-art liquid-electrolyte-based LIBs. In contrast, these conditions are expected to be met by implementing ASSBs with high-performance solid-state electrolytes (SSEs). In this work, we altered the microscopic structure and Li diffusional behaviors of argyrodites (Li_6-xPS_5-xCl_1+x), which were precisely monitored with cooling protocols. It was shown that, at the cooling speed of -3 ℃·h^-1, as the cooling rate decreased, impurities in Li_5.5PS_4.5Cl_1.5 such as LiCl and Li₃PO₄ gradually diminished and eventually disappeared. At the same time, differences in the lattice sizes of Li_5.5PS_4.5Cl_1.5 crystallites gradually decreased, resulting in a single phase Li_5.5PS_4.5Cl_1.5. It was also found that the Cl content of the 4d crystallographic sites increased, eventually contributing to the improvement in ionic conductivity. This work also revealed the effect of cooling rates on the crystallographic atomic arrangements, which became weaker as a decrease in x. The correlations between ionic conductivities and structural features were experimentally verified via XRD and solid-state NMR studies. (Received February 10, 2021; Accepted February 17, 2021)

keyword : lithium-ion batteries, cooling rate, solid-state battery, argyrodite, ionic conductivity

Hybrid Organic-Inorganic Perovskites (HOIP) have received a great deal of attention as a key material for applications like solar cells and light emitting devices because of their many advantages, in spite of their stability and toxicity issues. Attempting to discover and characterize novel HOIPs using just an experimental approach would be prohibitively time-and-cost-consuming. Using theoretical or empirical calculations would greatly help. For these reasons, HOIP has been actively investigated using DFT (Density Functional Theory) calculations, which have significantly reduced research time and cost. However, the input model structure treatment needs to be standardized to avoid unnecessary complications. For this purpose, a sort of optimization of DFT calculation protocols for HOIPs is essential, because DFT calculation results are greatly affected by the input model structure arrangements and exchange-correlation functionals. In this paper, we used DFT to calculate the band gap, formation energy, and effective mass of the well-known cubic perovskite structure, methylammonium lead iodide (CH₃NH₃PbI₃: MAPbI₃) with and without the van der Waals function and SOC (Spin Orbit Coupling) and various geometrical molecule arrangements in the structure. In particular, the initial orientation of the ‘A’ site molecule in the input model structure was intensively investigated in terms of band gap, formation energy and effective mass. It was found that the relaxation-induced final structure was greatly influenced by the initial orientation of the molecule and thereby significantly affected the DFT-calculated result. (Received February 10, 2021; Accepted February 18, 2021)

keyword : hybrid organic-inorganic perovskite, density functional theory, band gap, effective mass, formation energy

ZnO nano/microstructures were grown via a simple thermal evaporation process in air at atmospheric pressure. Mixtures of ZnS and carbon powders were used as the sources. The effects of growth temperature and mass ratio of carbon powder to ZnS on the morphologies and luminescence properties of the ZnO nano/microstructures were investigated in this study. When the growth temperature was 1000 ℃, ZnO nanowires with a hexagonal wurtzite crystal structure started to be formed and were preferentially grown along the [0001] direction. As the temperature increased to 1200 ℃, the crystal growth in the lateral direction perpendicular to the [0001] direction was enhanced, which resulted in a decreasing aspect ratio of the onedimensional ZnO nanowires. When source powders with different mass ratios of ZnS:C=2:1, 1:1 and 1:2 were used to grow ZnO nano/microstructures at 1200 ℃, ZnO microrods with wurtzite crystal structure were formed in all the samples. As the mass ratio of carbon powder to ZnS increased, the aspect ratio of ZnO microrods was reduced, which suggests that the carbon powder enhanced the growth of ZnO microrods in the lateral directions. A strong ultraviolet emission band centered at 380 nm was observed in the ZnO nano/ microstructures synthesized using the source powders with the mass ratios of ZnS:C=1:1 and 1:2, indicating that the ZnO nano/microstructures had a high crystalline quality. (Received February 5, 2021; Accepted February 16, 2021)

keyword : ZnO nano, microstructures, ZnS, C mass ratio, thermal evaporation, morphological variation, ultraviolet emission

Boron Nitride Nanotubes (BNNT) are one of the candidate materials for storing hydrogen by physical adsorption. It has been reported that this hydrogen storage capacity increases as the crystallinity of the nanostructures decreases. Here, BNNT was synthesized using an RF plasma torch system, and the surface of the BNNT was irradiated with nitrogen ions using an ion beam device, and changes in the surface microstructure were subsequently investigated. A multi-walled BNNT with a wall thickness of about 5 nm was synthesized using a 60 kW RF plasma torch. Amorphous impurities generated during the synthesis process were removed by heat treatment and membrane filtering. Then nitrogen ions were irradiated for 40 minutes at energies of 40 keV and 50 keV, respectively, using an ion beam irradiation device. The changes in the microstructure of the BNNT surface following ion beam irradiation were confirmed by HR-TEM, Raman spectrometer and FT-IR spectrometer. The tube walls of the BNNT were disordered by the nitrogen ions irradiation. At 50 keV, the tube walls located in the middle became disordered, which was attributed to an increase in penetration depth due to the higher irradiation energy. The maximum peak in the Raman spectra and FT-IR spectra of the ion irradiated BNNT were also shifted to a lower frequency. Ion irradiation reduced the crystallinity of the nanostructures. The potential improvement in hydrogen storage capacity by nitrogen ion irradiation of BNNT was confirmed. (Received November 2, 2020; Accepted February 20, 2021)

keyword : BNNT, RF plasma torch system, nitrogen ion irradiation, hydrogen storage capacity

Natural sand and organic binders are commonly used in casting processes, but these ingredients produce environmental problems with dust and harmful TVOC(Total Volatile Organic Carbon) gases. Research on the introduction of artificial sand and inorganic binders to solve these environmental problems is being actively conducted mainly in the casting industry. Artificial sand has superior durability and a spherical shape compared to natural sand, and above all, it does not generate dust. In addition, inorganic binders have the advantage that no harmful gas is generated during casting and the used sand can be recycled. This study confirmed whether inorganic binders can be applied when replacing natural sand with artificial sand. First, eco-friendly inorganic binders that do not produce harmful gas were synthesized. Then characteristic analyses were carried out with artificial sand and natural sand. Physical and chemical properties were compared using X-Ray Fluorescence (XRF), Powder Flow Test (PFT) and particle size distribution analyses. The general strength and absorption (absolute humidity 29.9 g/cm³) strength of the sample core was measured using each sand (artificial sand, natural sand) and inorganic binder. Also, X-ray Photoelectron Spectroscope (XPS) analysis confirmed the combination structure. As a result, it was confirmed that artificial sand exhibited mold characteristics with similar strength even with lower inorganic binder content than natural sand. (Received January 5, 2021; Accepted February 8, 2021)

keyword : inorganic binder, sand core, natural sand, artificial sand, strength