• 대한수학회보
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• 제56권1호
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• pp.253-263
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• 2019

Let M be a curvature homogeneous or ball-homogeneous non-$coK{\ddot{a}}hler$ almost $coK{\ddot{a}}hler$ 3-manifold. In this paper, we prove that M is locally isometric to a unimodular Lie group if and only if the Reeb vector field ${\xi}$ is an eigenvector field of the Ricci operator. To extend this result, we prove that M is homogeneous if and only if it satisfies ${\nabla}_{\xi}h=2f{\phi}h$, $f{\in}{\mathbb{R}}$.

• 한국분말재료학회지
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• 제4권4호
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• pp.304-309
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• 1997

The synthesis of W-l5wt%Cu nanocomposite powder by hydrogen reduction of ball milled W-Cu oxide mixture was investigated in terms of powder characteristics such as particle size, mixing homogeneity and micropore structure. It is found that the micropores in the ball milled oxide (2-50 nm in size) act as an effective removal path of water vapor, followed by the formation of dry atmosphere at reaction zone. Such thermodynamic condition enhances the nucleation of W phase but suppresses the growth process, being in favor of the formation of W nanoparticles (about 21 nm in size). In addition, the superior mixing homogeneity of starting oxide mixture turned out to Play a significant role for forming extraordinary chemical homogeneity of W-l5wt%Cu nanocomposite powder.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part2
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• pp.824-825
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• 2006

Homogeneous microstructures of the PM compacts are difficult to attain when mixed elemental powders are used. This study examined the microstructures of pressed-and-sintered and MIM products that contain Ni and Mo.Ni-rich areas, which were lean in carbon and were soft and were found easily in regular specimens. Gaps or cracks near the Ni-rich or Mo-rich areas were also frequently observed. This problem worsened when Ni and Mo particles were large and were irregular in shape. By using ball milling treatment and ferroalloy powders, the microstructure homogeneity and mechanical properties were improved. The addition of 0.5wt%Cr further improved the distribution of Ni because Cr reduced the repulsion effect between nickel and carbon. With the elimination of Ni-rich areas, more bainites and martensites were formed and mechanical properties were significantly improved.

• 한국분말재료학회지
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• 제22권3호
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• pp.208-215
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• 2015

Fe-TiC composite powders were fabricated by planetary ball mill processing. Two kinds of powder mixtures were prepared from the starting materials of (a) (Fe, TiC) powders and (b) (Fe, $TiH_2$, Carbon) powders, respectively. Milling speed (300, 500 and 700 rpm) and time (1, 2, and 3 h) were varied. For (Fe, $TiH_2$, Carbon) powders, an in situ reaction synthesis of TiC after the planetary ball mill processing was added to obtain a homogeneous distribution of ultrafine TiC particulates in Fe matrix. Powder characteristics such as particle size, size distribution, shape, and mixing homogeneity were investigated.

• 한국재료학회지
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• 제15권2호
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• pp.85-88
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• 2005

[ $W-15wt.\%$ ] Cu nanocomposite powders are fabricated by ball-milling and subsequent hydrogen-reduction. The compacted parts of $W-15wt.\%Cu$ nanocomposite powders were sintered at $1200^{\circ}C$ for 1 h with various heating rates of 5 and $20^{\circ}C/min$. The homogeneity of the sintered microstructures was evaluated through homogeneity index by the standard deviation of Victor's hardness test. The W-W contiguities were calculated by using Voronoi diagrams. The sintered microstructure with the heating rate of $20^{\circ}C/min$ was more homogeneous and had lower W-W contiguity than that of $5^{\circ}C/min$. The microstructural homogeneity was directly related to the W-W contiguity. Thermal conductivity of the sintered parts with the heating rate of $20^{\circ}C/min$ was higher than that with heating rate of $5^{\circ}C/min$. This phenomenon indicates that the thermal conductivity is affected by the W-W contiguity resulting from the homogeneity of the sintered microstructure.

• 한국분말재료학회지
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• 제5권4호
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• pp.250-257
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• 1998

The synthesis of titanium silicides ($Ti_3Si$, $TiSi_2$, $Ti_5Si_4$, $Ti_5Si_3$ and TiSi) by mechanical alloying has been investigated. Rapid, self-propagating high-temperature synthesis (SHS) reactions were observed to produce the last three phases during room-temperature high-energy ball milling of elemental powders. Such reactions appeared to be ignited by mechanical impact in an intimate, fine powder mixture formed after a critical milling period. During the high-energy ball milling, the repeated impact at contact points leads to a local concentration of energy which may ignite a self-propagating reaction. From in-situ thermal analysis, each critical milling period for the formation of $Ti_5Si_4$, $Ti_5Si_3$ and TiSi was observed to be 22, 35.5 and 53.5 min, respectively. $Ti_3Si$ and $TiSi_2$, however, have not been produced even till the milling period of 360 min due to lack of the homogeneity of the powder mixtures. The formation of titanium silicides by mechanical alloying and the relevant reaction rates appeared to depend upon the critical milling period, the homogeneity of the powder mixtures, and the heat of formation of the products involved.

• Progress in Superconductivity
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• 제10권1호
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• pp.23-28
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• 2008

We report refinement of crystalline boron by an attrition ball milling system and the superconducting properties of the $MgB_2$ pellets prepared from the refined boron. In this work, we have conducted the ball milling with only crystalline boron powder, in order to improve homogeneity and control the grain size of the $MgB_2$ that is formed from it. We observed that the crystalline responses in the ball-milled boron became broader and weaker when the ball-milling time was further increased. On the other hand, the $B_{2}O_{3}$ peak became stronger in the powders, resulting in an increase in the amount of MgO within the $MgB_2$ volume. The main reason for this is a greater oxygen uptake. From the perspective of the superconducting properties, however, the sample prepared from boron that was ball milled for 5 hours showed an improvement of critical current density ($J_c$), even with increased MgO phase, under an external magnetic field at 5 and 20 K.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회A
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• pp.37-42
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• 2007

We present a powder injection molding technique of fabricating cemented tungsten carbide(WC) balls for milling and dispersing nano-powder in this paper. The conventional powder metallurgy approach is investigated to reveal its drawbacks of density non-homogeneity. New procedures of powder injection molding for the homogeneous high-precision WC balls, involving the binding process, powder injection molding process and sintering process, are presented in detail. Each process is investigated empirically and numerically to obtain its engineering information, which can used for process optimization.

• 한국안전학회지
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• 제23권3호
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• pp.65-70
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• 2008

Electroencephalogram(EEG) would be the most objective psychophysiological research technique on human errors though few research has been taken yet. This study aimed to get characteristics of human error while committing simple Odd-Ball tasks by utilizing the power spectrum technique of EEG data. Each experiment was composed of 3 tasks with different rules, and three young undergraduate students participated in this study as paid subjects. The result showed that subject and the interaction of subject and task factors were statistically significant on variation of power of $\alpha$ and $\beta$ bands which implied there would exist groups with homogeneity in their response. And though the variation of band powers due to task factors were not so great as to get statistical significance, it implied that the task requiring decoding process would be more strange to human beings than the task merely requiring psychological recall process.

• 한국분말재료학회지
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• 제9권5호
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• pp.346-351
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• 2002

Pure and fine, two-component titanate powders (barium titanate, calcium titanate etc.) were synthesized by an ethylene glycol method. Titanium isopropoxide and other metal ionic salts were dissolved in liquid-type ethylene glycol without any precipitation. In non-aqueous system, the amount of ethylene glycol affected the solubility and homogeneity of metal cation sources in the solution. At the optimum amount of the polymer, the metal ions were dispersed effectively in solution and a homogeneous polymeric network was formed. Most of the synthesized powders had sub-micron or nano-size primary particles after calcination and the agglomerated calcined powders were easily ground by ball milling process. All synthesized titanate powders had stable crystallization behavior at low temperature and high specific surface area after ball milling. The crystallization behavior and the microstructures of the calcined powders were affected on the ethylene glycol content.