• 한국재료학회지
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• 제30권11호
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• pp.573-580
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• 2020

YAG (Yttrium Aluminum Garnet, Y₃Al₅O₁₂) has excellent plasma resistance and recently has been used as an alternative to Y₂O₃ as a chamber coating material in the semiconductor process. However, due to the presence of an impurity phase and difficulties in synthesis and densification, many studies on YAG are being conducted. In this study, YAG powder is synthesized by an organic-inorganic complex solution synthesis method using PVA polymer. The PVA solution is added to the sol in which the metal nitrate salts are dissolved, and the precursor is calcined into a porous and soft YAG powder. By controlling the molecular weight and the amount of PVA polymer, the effect on the particle size and particle shape of the synthesized YAG powder is evaluated. The sintering behavior of the YAG powder compact according to PVA type and grinding time is studied through an examination of its microstructure. Single phase YAG is synthesized at relatively low temperature of 1,000 ℃ and can be pulverized to sub-micron size by ball milling. In addition, sintered YAG with a relative density of about 98 % is obtained by sintering at 1,650 ℃.

• Corrosion Science and Technology
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• 제18권6호
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• pp.312-317
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• 2019

The objective of this study was to investigate the corrosion behavior of aluminum (AA1100) in a mixed solution of 0 ~ 0.1 g/L Na₂S + 0.3 ~ 3 g/L NaCl + 0 ~ 10 mL/L H₂O₂. Potentiodynamic polarization tests were performed. Effects of solution compositions on corrosion potential, corrosion rate, and pitting potential of aluminum were statistically analyzed with a regression model. Results suggested that localized corrosion susceptibility of aluminum was increased in the solution with increasing concentration of NaCl because the pitting potential was lowered linearly with increasing NaCl concentration. On the contrary, H₂O₂ mitigated the galvanic corrosion of aluminum by increasing the corrosion potential. It also mitigated localized corrosion by increasing the pitting potential of aluminum. Na₂S did not exert a noticeable effect on the corrosion of aluminum. These effects of different chemical species at various concentrations were independent of each other. Synergy or offset effect was not observed.

• 한국수소및신에너지학회논문집
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• 제30권3호
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• pp.209-219
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• 2019

Methanol is a liquid fuel which could also be produced from renewable energy sources and has appreciably high energy density. In this work, we investigated the application of $Ce_{0.9}Gd_{0.1}O_{2-x}$ supported Cu and Ni catalysts for hydrogen production via methanol steam reforming. Catalysts were synthesized by solution combustion synthesis. The prepared catalysts with various active materials and Cu loading amounts were tested in a reactor at $200-300^{\circ}C$, 0-5 barg range and steam to methanol molar ratio was 1.5. The catalytic properties of Cu and Ni were compared, and the catalytic performance was shown to depend on the amounts of metal loading and operating conditions such as reaction temperature and pressure.

• 한국세라믹학회지
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• 제42권12호
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• pp.800-807
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• 2005

Cathode material, $(Ba_{0.5}Sr_{0.5})_{0.99}Co_{0.8}Fe_{0.2}O_{3-\delta}$, for low temperature SOFC was prepared by the Glycine-Nitrate synthesis Process (GNP). Characteristics of the synthesized powders were studied with controlling the pH of a precursor solution. Highly acidic precursor solution increased a perovskite forming temperature. It is considered that Ba and Sr cannot complex by carboxylic acid group of glycine, because under highly acidic condition the caboxylic group mainly combined with H+ insead of alkaline earth cations. A lack of bond between cations and glycine resulted in selective precipitation of the elements during evaporation of the precursor solution. In case of using precursor solution with pH %2\~3$, a single perovskite phase was obtained at $1000^{\circ}C$. Polarization resistance of $(Ba_{0.5}Sr_{0.5})_{0.99}Co_{0.8}Fe_{0.2}O_{3-\delta}$ was measured by AC impedance spectroscopy from the two electrode symmetric cell. Area specific resistance of the $(Ba_{0.5}Sr_{0.5})_{0.99}Co_{0.8}Fe_{0.2}O_{3-\delta}$ air electrode at $500^{\circ}C\;and\;600^{\circ}C$ were $0.96{\Omega}{\cdot}cm^2\;and\;0.16{\Omega}{\cdot}cm^2$, respectively.

• 한국재료학회지
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• 제33권3호
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• pp.95-100
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• 2023

This research examines the effect of adding aluminum on the structural, phasic, and magnetic properties of CoCrFe NiMnAl_x high-entropy alloys. To this aim, the arc-melt process was used under an argon atmosphere for preparing cast samples. The phasic, structural, and magnetic properties of the samples were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and vibrational magnetometry (VSM) analyses. Based on the results, the addition of aluminum to the compound caused changes in the crystalline structure, from FCC solid solution in the CoCrFeNiMn sample to CoCrFeNiMnAl BBC solid solution. It was associated with changes in the magnetic property of CoCrFeNiMnAl_x high-entropy alloys, from paramagnetic to ferromagnetic. The maximum saturation magnetization for the CoCrFeNiMnAl casting sample was estimated to be around 79 emu/g. Despite the phase stability of the FCC solid solution with temperature, the solid solution phase formed in the CrCrFeNiMnAl high-entropy compound was not stable, and changed into FCC solid solution with temperature elevation, causing a reduction in saturation magnetization to about 7 emu/g.

• 한국표면공학회지
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• 제54권4호
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• pp.184-193
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• 2021

This study demonstrates formation behavior and morphological changes of PEO (Plasma Electrolytic Oxidation) films on AZ91 Mg alloy as a function of pre-treatment time in 1 M HF solution at 25 ± 1 ℃. The electrochemical behavior and morphological changes of AZ91 Mg alloy in the pre-treatment solution were also investigated with pre-treatment time. The PEO films were formed on the pre-treated AZ91 Mg alloy specimen by the application of anodic current 100 mA/cm² of 300 Hz AC in 0.1 M NaOH + 0.4 M Na₂SiO₃ solution. Vigorous generation of hydrogen bubbles were observed upon immersion in the pre-treatment solution and its generation rate decreased with immersion time. It was also found that 𝛽-Mg₁₇Al₁₂ in AZ91 Mg alloy was dissolved and a protective thin film of MgF₂ was formed on the AZ91 Mg alloy surface during the pre-treatment process in the 1 M HF solution. PEO film did not grow on the AZ91 Mg alloy specimen when the surface was not pre-treated and irregular PEO films with nodular defects were formed for the specimens pre-treated up to 1 min. Uniform PEO films were formed when the AZ91 Mg alloy specimen was pre-treated more than 3 min. The growth rate of PEO films on AZ91 Mg alloy increased significantly with increasing pre-treatment time.

• 한국표면공학회지
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• 제54권6호
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• pp.315-323
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• 2021

In general, surface treatments of electroless Ni-P coating are extensively applied in the industry due to their excellent properties for considerable wear resistance, hardness, corrosion resistance. This study aims to determine the effect of ultrasonic conditions on the morphology, alumina content, roughness, hardness, and corrosion resistance of electroless Ni-P-Al₂O₃ composite coatings. The characteristics were analyzed by Energy-dispersive X-ray spectroscopy (EDX), x-ray diffractions (XRD), and atomic force microscopy (AFM), etc. In this study, the effect of ultrasonic condition uniformly distributed alumina within Ni-P solution resulting in a smoother surface, lower surface roughness. Furthermore, the corrosion resistance behavior of the coating was analyzed using tafel polarization curves in a 3.5 wt.% NaCl solution at 25 ℃. Under ultrasonic, Al₂O₃ content in Ni-P composite solution increased from 0.5 to 5.0 g/L, Al₂O₃ content at 3.0 g/L was showed a significantly enhanced corrosion resistance. These results suggested that ultrasonic condition was an effective method to improve the properties of the composite coating.

• 한국재료학회지
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• 제13권3호
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• pp.180-184
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• 2003

Ce-doped $Y_2$SiO$_{5}$ phosphor particles of spherical morphology, fine size, high crystallinity and high photoluminescence (PL) intensity were prepared by spray pyrolysis. When nitrate precursor solution is adopted, hollow particles were formed by uneven drying rate between surface and inside of droplet. Citric acid and ethylene glycol were introduced as polymeric precursor to control the morphology of particles. When polymeric solution is adopted, polymeric chain is formed by the esterification reaction between carboxyl and hydroxy groups of citric acid and ethylene glycol, and considered as controlling the drying characteristics of droplet. $Y_2$$SiO_{5}$ :Ce phosphor particles prepared from polymeric precursor solution were spherical, filled, fine size and not agglomerate before and after post heat treatment. The optimum doping concentration of cerium was 0.5 mol% of overall solution concentration. The optimum amount of TBOS of high PL intensity and pure crystallinity of X2-type $Y_2$$SiO _{5}$ was 105% of stoichiometric amount. The PL intensity of $Y_2$X$/_{5}$ :Ce phosphor particles prepared using the polymeric precursor solution was 164% of that of the nitrate precursor solution due to homogeneous composition and good morphology.y.

• 대한수학회지
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• 제50권4호
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• pp.771-795
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• 2013

In this paper we consider the Navier-Stokes equations in the half space. Our aim is to construct a mild solution for initial data in $B^{-\alpha}_{{\infty},{\infty}}(\mathbb{R}^n_+)$, 0 < ${\alpha}$ < 1. To do this, we derive the estimate of the Stokes flow with singular initial data in $B^{-\alpha}_{{\infty},q}(\mathbb{R}^n_+)$, 0 < ${\alpha}$ < 1, 1 < $q{\leq}{\infty}$.

• 한국주조공학회지
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• 제42권6호
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• pp.337-346
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• 2022

본 연구에서는 Al-7Si-(0.3~0.5)Mg-(0~0.5)Cu 합금의 용체화 처리 조건 최적화를 위해 545℃ 온도 조건에서 최대 7시간까지 용체화 처리를 수행한 후 광학현미경 및 FE-SEM을 활용한 미세조직 관찰 및 브리넬 경도 측정을 수행하였다. 합금 내 공정 Si 상은 용체화 처리 초반 3시간 동안 급격한 조대화 현상을 나타내었으며, 이후에는 용체화 처리가 진행되어도 Si 상 크기는 크게 변화하지 않았다. 한편 공정 Si 상의 구상화의 경우, 용체화 처리 시간이 7시간에 도달할 때 까지 지속적으로 진행되었다. Cu가 첨가된 합금의 주방상태에서는 Q-Al₅Cu₂Mg₈Si₆ 상과 θ-Al₂Cu 상이 확인되었으나, 545℃에서 3시간 동안 진행된 용체화 처리 이후에는 모두 분해되었다. 주방상태에서 확인된 π-Al₈FeMg₃Si 상은 5시간의 용체화 처리 이후에 사라지거나(0.3wt%Mg) 혹은 7시간의 용체화 처리 이후에도 존재(0.5wt%Mg)하였다. 초정 α상 내 Mg 및 Cu 함량은 용체화 처리 시간이 5시간에 도달할 때 까지 증가하였으며, 이는 Mg 및 Cu를 함유한 금속간 화합물의 용체화 시간에 따른 분해 거동과 일치하였다. Al-7Si-Mg-Cu 합금의 용체화 처리 과정에서 확인한 미세조직 변화를 종합적으로 고려할 때, 본 연구에서 다룬 합금의 석출강화 효과 극대화를 위해서는 545℃ 조건에서 최소 5시간의 용체화 처리가 필요한 것으로 판단되며, 용체화 처리 조건 별로 측정된 브리넬 경도 데이터로부터 동일한 최적 용체화 처리 조건을 도출할 수 있었다.