• Korean Chemical Engineering Research
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• v.46 no.1
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• pp.99-105
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• 2008

Core-shell structured $BaTiO_3$ powders were produced via nano-coating of $Cr_2O_3$ and $Mn_2O_3$ to barium titanate powder system for MLCCs. From preliminary experiments, the optimal solution reaction condition employing using $KMnO_4$, $K_2Cr_2O_4$ and sulfur was established. Not only powders of $Cr_2O_3$ and $Mn_2O_3$ were synthesized but also their coating on $BaTiO_3$ powders were peformed under the same reaction condition. The coating was carried out in two ways, one-step and two-step, and its results were characterized for comparison. Conclusively speaking, two oxide additives were coated onto the $BaTiO_3$ powder surface with high quality and excellent reaction yield even under mild condition, which indicates that the contents as well as the properties of additive shell layer can be precisely controlled with rather ease.

• Journal of Korean Society of Industrial and Systems Engineering
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• v.46 no.3
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• pp.15-22
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• 2023

BNKT Ceramics, one of the representative Pb free based piezoelectric ceramics, constitutes a perovskite(ABO3) structure. At this time, the perovskite structure (ABO3) is in the form where the corners of the octahedrons are connected, and in the unit cell, two ions, A and B, are cations, A ion is located at the body center, B ion is located at each corner, and an anion O is located at the center of each side. Since Bi, Na, and K sources constituting the A site are highly volatile at a sintering temperature of 1100℃ or higher, it is difficult to maintain uniformity of the composition. In order to solve this problem, there should be suppression of volatilization of the A site material or additional compensation of the volatilized. In this study, the basic composition of BNKT Ceramics was set to Bi_0.5(Na_0.78K_0.22)_0.5TiO₃ (= BNKT), and volatile site (Bi, Na, and K sources) were coated in the form of a shell to compensate additionally for the A site ions. In addition, the physical and electrical properties of BNKT and its coated with shell additives(= @BNK) were compared and analyzed, respectively. As a result of analyzing the crystal structure through XRD, both BNKT(Core) and @BNK(Shell) had perovskite phases, and the crystallinity was almost similar. Although the Curie temperature of the two sintered bodies was almost the same (TC = 290 ~ 300 ℃), it was confirmed that the d33 (piezoelectric coefficient) and Pr (residual polarization) values were different. The experimental results indicated that the additional compensation for a shell additive causes the coarsening, resulting in a decrease in sintering density and Pr(remanent polarization). However, coating shell additives to compensate for A site ion is an effective way to suppress volatilization. Based on these experimental results, it would be the biggest advantage to develop an eco-friendly material (Lead-free) that replaced lead (Pb), which is harmful to the human body. This lead-free piezoelectric material can be applied to a biomedical device or products(ex. earphones (hearing aids), heart rate monitors, ultrasonic vibrators, etc.) and skin beauty improvement products (mask packs for whitening and wrinkle improvement).

• Korean Journal of Materials Research
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• v.24 no.9
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• pp.502-507
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• 2014

Recently, improvement in the conversion efficiency of silicon-based solar cells has been achieved by decreasing emitter doping concentration, because the lightly doped emitter can effectively prevent the recombination of electrons and holes generated by solar light irradiation. This type of emitter is very thin due to the low doping concentration, thus conductive materials (i.e., silver) used for front electrodes can easily penetrate the emitter during a firing process because of their large diffusivity in silicon. This results in junction leakage currents which might reduce cell efficiencies. In this study, $Al_2O_3$-coated Ag powders were synthesized by an ultrasonic spray pyrolysis method and applied to the conductive materials of the front electrode to control the junction leakage current. The $Al_2O_3$ shell obstructs the Ag diffusion into the emitter during the firing process. The powder is spherical with a core-shell structure and the thickness of the $Al_2O_3$ shell is tens of nanometers. Solar cells were fabricated using pure Ag powders or the $Al_2O_3$-coated Ag powder as front electrode materials, and the conversion efficiency and junction leakage current were compared to investigate the role of the $Al_2O_3$ shell during the firing processes.

• Journal of Electrochemical Science and Technology
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• v.12 no.3
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• pp.346-357
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• 2021

In recent years, supercapacitors have been developed rapidly as a rechargeable energy storage device. And the performance of supercapacitors is depending on electrode materials, the preparation method and performance of electrode materials have become the primary goal of scientific development. This study synthesizes Co₃O₄@MnO₂@PPy cathode material with porous pattern core-shell structure by hydrothermal method and electrodeposition. The result samples are characterized by X-ray diffraction transmission/scanning electron microscope, and X-ray photoelectron spectroscopy. Electrochemical evaluation reveals that electrochemical performance is significantly enhanced by PPy depositing. The specific capacitance of Co₃O₄@MnO₂@PPy is 977 F g^-1 at 1 A g^-1, the capacitance retention rate of 105%. Furthermore, the electrochemical performance of Co₃O₄@MnO₂@PPy//AC asymmetric supercapacitor assembles with AC as the negative electrode material is significantly better than that of MnO₂//AC and Co₃O₄@MnO₂//AC. The capacity of Co₃O₄@MnO₂@PPy//AC is 102.78 F g^-1. The capacity retention rate is still 120% for 5000 charge-discharge cycles.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.65 no.6
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• pp.940-945
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• 2016

In this paper, zero sequence equivalent circuit of Yg-Yg three phase core-type transformer is analyzed. Many problems by iron core structure of the three phase transformer due to asymmetric three phase lines, which includes line disconnection, ground fault, COS OFF, and unbalanced load are reported in the distribution system. To verify a feasibility of zero sequence impedance of Yg-Yg type three phase transformer, fault current generation in the three phase core and shell-type Yg-Yg transformer is compared by PSCAD/EMTDC when single line ground fault is occurred. As a result, shell-type transformer does not affect the flow of fault current, but core-type transformer generate an adverse effect by the zero sequence impedance. The adverse effect is explained by the zero sequence equivalent circuit of core-type transformer and Yg-Yg type three phase core-type transformer supplies a zero sequence fault current to the distribution system.

• Journal of Powder Materials
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• v.23 no.1
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• pp.38-42
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• 2016

$BaTiO_3$-coated Fe nanofibers are synthesized via a three-step process. ${\alpha}-Fe_2O_3$ nanofibers with an average diameter of approximately 200 nm are first prepared using an electrospinning process followed by a calcination step. The $BaTiO_3$ coating layer on the nanofiber is formed by a sol-gel process, and a thermal reduction process is then applied to the core-shell nanofiber to selectively reduce the ${\alpha}-Fe_2O_3$ to Fe. The thickness of the $BaTiO_3$ shell is controlled by varying the reaction time. To evaluate the electromagnetic (EM) wave-absorbing abilities of the $BaTiO_3@Fe$ nanofiber, epoxy-based composites containing the nanofibers are fabricated. The composites show excellent EM wave absorption properties where the power loss increases to the high frequency region without any degradation. Our results demonstrate that the $BaTiO_3@Fe$ nanofibers obtained in this work are attractive candidates for electromagnetic wave absorption applications.

• Proceedings of the Materials Research Society of Korea Conference
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• 2009.11a
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• pp.33.1-33.1
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• 2009

To date, nanostructured tungsten oxides with a variety of stoichiometries, such as WO3, WO2.9, W18O49, and WO2, have been prepared, because they are promising candidates for applications such as gas sensors, photocatalysts, electrochromic devices, and field emission devices. Among them, W18O49 and WO2 have been widely studied due to their outstanding chemical sensing, catalytic, and electron emissive properties. Here we report, for the first time, a one-pot solution-phase route to synthesizing a novel composite hierarchical hollow structure without adding catalysts, surfactants, or templates. The products, consisting of a WO2 hollow core sphere surrounded by a W18O49 nanorod shell (yielding a sea urchin-like structure), were generated as discrete structures via Ostwald ripening. To our knowledge, this type of composite hierarchical core/shell structure has not been reported previously. The morphological evolution and the detailed growth mechanism were carefully studied. We also demonstrate that the size of the hollow urchins is readily tunable by controlling the reactant concentrations.Interestingly, although bulk tungsten oxides are weakly paramagnetic or diamagnetic, the as-prepared products show unusual ferromagnetic behavior atroom temperature. The urchin structures also show a very high Brunauer-Emmet-Teller (BET) surface area, suggesting that they may potentially be applied to chemical sensor or effective catalyst technologies.

• Applied Chemistry for Engineering
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• v.29 no.6
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• pp.710-715
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• 2018

In this study, $ZnFe_2O_4@SnO_2@TiO_2$ core-shell nanoparticles (NPs), a photocatalytic material with magnetic properties, were synthesized through a three-step process. Structural properties were investigated using X-ray diffraction (XRD) analysis. It was confirmed that $ZnFe_2O_4$ of the spinel, $SnO_2$ of the tetragonal and $TiO_2$ of the anatase structure were synthesized. The magnetic properties of synthesized materials were studied by a vibrating sample magnetometer (VSM). The saturation magnetization value of $ZnFe_2O_4$, a core material, was confirmed at 33.084 emu/g. As a result of the formation of $SnO_2$ and $TiO_2$ layers, the magnetism due to the increase in thickness was reduced by 33% and 40%, respectively, but sufficient magnetic properties were reserved. The photocatalytic efficiency of synthesized materials was measured using methylene blue (MB). The efficiency of the core material was about 4.2%, and as a result of the formation of $SnO_2$ and $TiO_2$ shell, it increased to 73% and 96%, respectively while maintaining a high photocatalytic efficiency. In addition, the antibacterial activity was validated via the inhibition zone by using E. Coli and S. Aureus. The formation of shells resulted in a wider inhibition zone, which is in good agreement with photocatalytic efficiency measurements.

• Applied Chemistry for Engineering
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• v.23 no.6
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• pp.554-560
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• 2012

Recently, it has been increasing trend to use conductive materials as electronics and communication technology in electronics industry are developing. The noble metal such as Ag, Pt, Pd etc. are mostly used as conductive materials, To reduce production cost, alternative materials with similar characteristics of noble metals are needed. Copper has advantages, i.e its electronic properties are similar to noble metals and low cost than noble metal, but its use has been restricted because of oxidation in air. In this study, the tin film was coated on copper by electroless plating to protect copper from oxidation and to confirm the effects of temperature, pH, amount of $SnCl_2$, and feeding speed in plating conditions. Additionally, we apply $Cu_{core}Sn_{shell}$ powder as conductive filler with low-temperature densification and analysis by SEM, XRD, FIB and 4-Point Probe techniques. As result of the study, tin film was coated well on copper and was protected from oxidation. After low-temperature densification treatment, the meted tin made chemical interconnections with copper. Accordingly, conductivity was increased than before condition. We hope $Cu_{core}Sn_{shell}$ powder to replace noble metals and use in the electronic field.

• Clean Technology
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• v.8 no.4
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• pp.173-180
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• 2002

As a primary material for MLCC, $BaTiO_3$ particles coated with two additives in a core-shell structure were prepared in this study. This composite powder can not only reduce the VOC emission during MLCC manufacture but also increase the density and reliability of electronic products. The additives were $Y_2O_3$ and $MnCO_3$, whose composition information was obtained from domestic companies. It was observed that the surface of $BaTiO_3$ particles was uniformly as well as simultaneously coated by those two materials via urea-decomposition reaction over $70^{\circ}C$ as the reaction temperature. Elemental analysis indicated that the measured content of each additve was quite close to the designated value. The effect of polymeric dispersant such as PVP, on the coating characteristics was not as significant as expected.