• 한국세라믹학회지
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• 제53권1호
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• pp.87-92
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• 2016

Due to its stability at high temperature and its layered structure, $Ti_3SiC_2$ MAX phase was considered to the interphase of $SiC_f/SiC$ composite. In this study, $Ti_3SiC_2$ MAX phase powder was deposited on SiC fiber via the electrophoretic deposition (EPD) method. The Zeta potential of the $Ti_3SiC_2$ suspension with and without polyethyleneimine as a dispersant was measured to determine the conditions of the EPD experiments. Using a suspension with 0.03 wt.% ball milled $Ti_3SiC_2$ powder and 0.3 wt.% PEI, $Ti_3SiC_2$ MAX phase was successfully coated on SiC fiber with an EPD voltage of 10 V for 2 h. Most of the coated $Ti_3SiC_2$ powders are composed of spherical particles. Part of the $Ti_3SiC_2$ powders that are platelet shaped are oriented parallel to the SiC fiber surface. From these results we expect that $Ti_3SiC_2$ can be applied to the interphase of $SiC_f/SiC$ composites.

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2011년도 추계학술대회 초록집
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• pp.122.2-122.2
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• 2011

Sulfur hexafluoride ($SF_6$), one of the most potent greenhouse gases, is known as a hydrate former and has been studied at the high pressure up to 1.3 GPa with gas mixtures and with aqueous surfactant. Since we regard $SF_6$ as a potential promoter molecule that can stabilize hydrate structure more effectively compare to the other promoters, further investigation is required to verify the stabilizing ability of $SF_6$ in the hydrate structure. However, the insoluble nature of $SF_6$ in water or gases hinders fine scale analyses. This work discusses the data obtained by using molecular dynamics simulations of structure II (sII) clathrate hydrates containing $SF_6$ and $H_2$. The simulations were performed using the TIP4P/Ice model for water molecule and a previously reported $SF_6$ molecular model (optimized at the pure $SF_6$ single phase system (Olivet and Vega, 2007)), and a $H_2$ molecular model (adapted from the THF+$H_2$ hydrate system (Alavi et al., 2006)). The simulations are performed to observe the stability of $SF_6$ and $H_2$ in the sII clathrate hydrate system with varying temperature and pressure conditions and occupancies of $SF_6$ and $H_2$, which cannot be easily tuned experimentally. We observe that stability of H2 enclathrated in the hydrate structure more affected by the occupancy of $SF_6$ molecules and temperature than pressure, which ranges from 1 to 100 bar.

• 한국지반신소재학회논문집
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• 제13권4호
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• pp.45-56
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• 2014

한대지역은 계절 변화에 따라 동결융해가 반복되는 상부의 활동층과 하부의 영하상태로 항시 유지되는 영구동토층으로 구성되어 있다. 한대지역에서 여름철에는 동결되어 있는 지반을 융해시키기 때문에 지반 강도저하 및 침하가 발생한다. 이러한 강도저하 및 침하는 상부 구조물의 구조적 안정성에 문제를 야기시켜 활동층을 포함하고 있는 한대지역에서는 지반의 온도를 항시 영하상태로 유지할 수 있는 동결지반 안정화 공법이 필요하다. 열사이펀이란 구조체 내부에 충전된 냉매의 자가적인 열순환에 의해 지반의 온도를 항시 영하상태로 유지할 수 있는 지반 안정화 공법 이다. 본 연구에서는 열사이펀의 지반 동결성능을 분석하기 위해 R-134a 냉매를 이용하여 충전율에 따른 열사이펀의 지반 온도제어를 실내실험과 수치해석을 통해 분석하고 열사이펀의 열전도율을 산정하였다.

• 한국분말재료학회지
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• 제18권3호
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• pp.226-237
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• 2011

W-ZrC and W-HfC composite powders were fabricated by the Plasma Alloying & Spheroidization (PAS) method and the powders were sprayed into hybrid coating layers by using Low Vacuum Plasma Spray (LVPS) process, respectively. Microstructure, mechanical properties, and ablation characteristics of the fabricated coating layers were investigated. The LVPS process led to successful production of W-Carbide hybrid coatings, approximately 400 ${\mu}M$ or above in thickness. As the substrate preheating temperature increased from $870^{\circ}C$ to $917^{\circ}C$, the hardness of the W-ZrC coating layer increased due to decreased porosity. Vickers hardness showed higher value (about 108.4 HV) in W-ZrC hybrid coating material compared to that of W-HfC while adhesive strength was found to be similar in both coating layers. The plasma torch test revealed good ablation resistance of the W-Carbide hybrid coating layers. The relatively high performance W-ZrC coating layer at the elevated temperature is thought to be attributed to both the strengthening effect of ZrC particle remained in the layer and the formation of ZrO2 phase with high temperature stability.

• 한국재료학회지
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• 제22권10호
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• pp.513-518
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• 2012

Recently, automobile parts have been required to have high strength and toughness to allow for weight lightening or improved stability. But, traditional micro-alloyed steel cannot be applied in automobile parts. In this study, we considered the influence of quenching temperature and cooling rate for specimens fabricated by vacuum induction furnace. Directly quenched micro-alloyed steel for hot forging can be controlled according to its micro structure and the heat-treatment process. Low carbon steel, as well as alloying elements for improvement of strength and toughness, was used to obtain optimized conditions. After hot forging at $1,200^{\circ}C$, the ideal mechanical properties (tensile strength ${\geq}$ 1,000 MPa, Charpy impact value ${\geq}\;100\;J/cm^2$) can be achieved by using optimized conditions (quenching temperature : 925 to $1,050^{\circ}C$, cooling rate : ${\geq}\;5^{\circ}C/sec$). The difference of impact value according to cooling rate can be influenced by the microstructure. A fine lath martensite micro structure is formed at a cooling rate of over $5^{\circ}C/sec$. On the other hand, the second phase of the M-A constituent microstructure is the cause of crack initiation under the cooling rate of $5^{\circ}C/sec$.

• 보존과학회지
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• 제30권4호
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• pp.395-407
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• 2014

이 연구에서는 벽돌 재료인 토양이 온도별로 소성되면서 겪는 광물학적 변화와 이에 수반된 물리적 변화를 토대로 송산리 고분군 출토 벽돌의 소성온도를 해석하였다. 토양시료를 $500{\sim}1,200^{\circ}C$로 소성하였을 때, 구성광물의 상전이에 의해 광물조성과 미세조직이 변했고 이에 따라 색, 흡수율, 기공률 등의 물리적 특성도 달라졌다. 광물조성은 상안정범위에 따른 광물의 구조붕괴와 재결정으로 인해 온도별로 달라졌지만 색, 흡수율, 미세조직은 $1,000^{\circ}C$ 이하에서 변화 폭이 적었고 $1,000{\sim}1,200^{\circ}C$에서는 급격한 변화가 나타났다. 이를 토대로 송산리 고분군 벽돌시료의 소성온도를 해석한 결과, 송산리 고분군 벽돌은 $1,200^{\circ}C$ 이상의 과소성품, $1,100{\sim}1,200^{\circ}C$의 고온소성품, $900{\sim}1,000^{\circ}C$의 소성품과 소성되지 않은 것으로 판단되는 벽돌이 공존하는 것으로 확인되었다. 향후 이러한 광물학적 및 물리적 변화과정을 토대로 보다 정밀한 소성온도 해석이 가능할 것으로 사료된다.

• 청정기술
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• 제29권1호
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• pp.39-45
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• 2023

To investigate the effects of Mg addition at different aging times and temperatures, Cu/MgO/Al₂O₃ catalysts were synthesized for the low-temperature water gas shift (LT-WGS) reaction. The co-precipitation method was employed to prepare the catalysts with a fixed Cu amount of 30 mol% and varied amounts of Mg/Al. Synthesized catalysts were characterized using XRD, BET, and H₂-TPR analysis. Among the prepared catalysts, the highest CO conversion was achieved by the Cu/MgO/Al₂O₃ catalyst (30/40/30 mol%) with a 60 ℃ aging temperature and a 24 h aging time under a CO₂-rich feed gas. Due to it having the lowest reduction temperature and a good dispersion of CuO, the catalyst exhibited around 65% CO conversion with a gas hourly space velocity (GHSV) of 14,089 h-1 at 300 ℃. However, it has been noted that aging temperatures greater or less than 60 ℃ and aging times longer than 24 h had an adverse impact, resulting in a lower surface area and a higher reduction temperature bulk-CuO phase, leading to lower catalytic activity. The main findings of this study confirmed that one of the main factors determining catalytic activity is the ease of reducibility in the absence of bulk-like CuO species. Finally, the long-term test revealed that the catalytic activity and stability remained constant under a high concentration of CO₂ in the feed gas for 19 h with an average CO conversion of 61.83%.

• 대한화장품학회지
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• 제47권4호
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• pp.273-280
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• 2021

자외선차단 제형은 내수성이 중요하므로 수상이 유상에 분산되는 water-in-oil (W/O) 에멀젼으로 만드는 것이 바람직하다. 이 경우에는 오일 특유의 무겁고 끈적이는 사용감 때문에 이를 보완하는 방법으로 실리콘계 오일을 사용한다. 이러한 오일은 유기 자외선차단제와의 상용성이 낮아 에멀젼의 안정성에 문제가 되고 있다. 본 연구에서는 자외선차단 제형의 W/O 에멀젼에서 다양한 오일들 간의 상용성을 Hansen solubility parameter(HSP)를 이용하여 수치적으로 도출하였다. HSP는 물질의 분산, 극성 및 수소 결합을 나타내는 지표로 물질간의 상용성을 판단하는데 유용하다. 본 연구에서는 HSP가 다른 다양한 오일을 선정하여 W/O 에멀젼을 제조하고 제형의 시간에 따른 점도 변화 및 안정성을 조사하여 HSP 수치와의 상관관계 또는 일치성을 고찰하였다. HSP를 이용하면 상용성이 높은 오일 선택을 용이하게 할 수 있었다. 또한, W/O 에멀젼에서 유상의 상용성이 높으면 점도의 경시변화가 작았으며 freeze-thaw cycle (-15 ℃ ~ 45 ℃) 조건에서 안정성이 향상된 것을 확인하였다. 향후 HSP를 통해 성분 구성을 최적화한다면 사용감이 우수하면서 안정성을 확보한 W/O형 자외선차단 제형 개발에 도움이 될 것이라 기대된다.

• 한국분말재료학회지
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• 제9권6호
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• pp.394-408
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• 2002

Nanostructured high strength metastable Al-, Mg- and Ti-based alloys containing different amorphous, quasicrystalline and nanocrystalline phases are synthesized by non-equilibrium processing techniques. Such alloys can be prepared by quenching from the melt or by powder metallurgy techniques. This paper focuses on one hand on mechanically alloyed and ball milled powders containing different volume fractions of amorphous or nano-(quasi)crystalline phases, consolidated bulk specimens and, on the other hand. on cast specimens containing different constituent phases with different length-scale. As one example. $Mg_{55}Y_{15}Cu_{30}$- based metallic glass matrix composites are produced by mechanical alloying of elemental powder mixtures containing up to 30 vol.% $Y_2O_3$ particles. The comparison with the particle-free metallic glass reveals that the nanosized second phase oxide particles do not significantly affect the glass-forming ability upon mechanical alloying despite some limited particle dissolution. A supercooled liquid region with an extension of about 50 K can be maintained in the presence of the oxides. The distinct viscosity decrease in the supercooled liquid regime allows to consolidate the powders into bulk samples by uniaxial hot pressing. The $Y_2O_3$ additions increase the mechanical strength of the composites compared to the $Mg_{55}Y_{15}Cu_{30}$ metallic glass. The second example deals with Al-Mn-Ce and Al-Cu-Fe composites with quasicrystalline particles as reinforcements, which are prepared by quenching from the melt and by powder metallurgy. $Al_{98-x}Mn_xCe_2$ (x =5,6,7) melt-spun ribbons containing a major quasicrystalline phase coexisting with an Al-matrix on a nanometer scale are pulverized by ball milling. The powders are consolidated by hot extrusion. Grain growth during consolidation causes the formation of a micrometer-scale microstructure. Mechanical alloying of $Al_{63}Cu_{25}Fe_{12}$ leads to single-phase quasicrystalline powders. which are blended with different volume fractions of pure Al-powder and hot extruded forming $Al_{100-x}$$(Al_{0.63}Cu_{0.25}Fe_{0.12})_x$ (x = 40,50,60,80) micrometer-scale composites. Compression test data reveal a high yield strength of ${\sigma}_y{\geq}$700 MPa and a ductility of ${\varepsilon}_{pl}{\geq}$5% for than the Al-Mn-Ce bulk samples. The strength level of the Al-Cu-Fe alloys is ${\sigma}_y{\leq}$550 MPa significantly lower. By the addition of different amounts of aluminum, the mechanical properties can be tuned to a wide range. Finally, a bulk metallic glass-forming Ti-Cu-Ni-Sn alloy with in situ formed composite microstructure prepared by both centrifugal and injection casting presents more than 6% plastic strain under compressive stress at room temperature. The in situ formed composite contains dendritic hcp Ti solid solution precipitates and a few $Ti_3Sn,\;{\beta}$-(Cu, Sn) grains dispersed in a glassy matrix. The composite micro- structure can avoid the development of the highly localized shear bands typical for the room temperature defor-mation of monolithic glasses. Instead, widely developed shear bands with evident protuberance are observed. resulting in significant yielding and homogeneous plastic deformation over the entire sample.

• 반도체디스플레이기술학회지
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• 제6권3호
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• pp.13-17
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• 2007

Effect of Co substitution on crystal structures of two nickel silicides, NiSi and $NiSi_2$, is investigated by using an ab initio calculation. Relaxed NiSi and $NiSi_2$ structures are calculated and the calculated lattice parameters are in good agreement with experimentally determined lattice parameters within about 2%. A Co atom substitutes a Ni and Si site, respectively, to evaluate the preferable site between them. Co prefers Ni site to Si site in both NiSi and $NiSi_2$. The calculated total energy also indicates that the Co substitution to Ni site stabilizes both the NiSi and $NiSi_2$ structures. Co also prefers Ni site in $NiSi_2$ to that in NiSi, indicating that $NiSi_2$ becomes more stable than NiSi with Co substitution. As Co addition to NiSi improves its thermal stability experimentally, this indicates that the energy barrier between the two phases is high enough to prevent the phase transformation from NiSi to $NiSi_2$ up to high temperature.