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WO<sub>3</sub>/tert-butyl alcohol 슬러리의 동결건조 조건이 다공체의 미세구조 특성에 미치는 영향
이의선,허연지,석명진,오승탁,Lee, Eui Seon,Heo, Youn Ji,Suk, Myung-Jin,Oh, Sung-Tag 한국분말재료학회 (*구 분말야금학회) 2021 한국분말재료학회지 (KPMI) Vol.28 No.4
The effects of drying temperature on the microstructure of porous W fabricated by the freeze-casting process of tert-butyl alcohol slurry with WO<sub>3</sub> powder was investigated. Green bodies were hydrogen-reduced at 800℃ for 1 h and sintered at 1000℃ for 6 h. X-ray diffraction analysis revealed that WO<sub>3</sub> powders were completely converted to W without any reaction phases by hydrogen reduction. The sintered body showed pores aligned in the direction of tert-butyl alcohol growth, and the porosity and pore size decreased as the amount of WO<sub>3</sub> increased from 5 to 10vol%. As the drying temperature of the frozen body increased from -25℃ to -10℃, the pore size and thickness of the struts increased. The change in microstructural characteristics based on the amount of powder added and the drying temperature was explained by the growth behavior of the freezing agent and the degree of rearrangement of the solid powder during the solidification of the slurry.
공정 계 동결제 슬러리의 동결건조 공정에 의한 Mo 다공체 제조
이규태,서한길,석명진,오승탁,Lee, Gyu-Tae,Seo, Han Gil,Suk, Myung-Jin,Oh, Sung-Tag 한국분말야금학회 2013 한국분말재료학회지 (KPMI) Vol.20 No.4
Freeze drying for porous Mo was accomplished by using $MoO_3$ powder as the source and camphor-naphthalene eutectic system as the sublimable material. Eutectic composition of camphor-naphthalene slurries with the initial $MoO_3$ content of 5 vol%, prepared by milling at $55^{\circ}C$ with a small amount of oligomeric dispersant, was frozen at $-25^{\circ}C$. The addition of dispersant showed improvement of dispersion stability in slurries. Pores were generated subsequently by sublimation of the camphor-naphthalene during drying in air for 48 h. To convert the $MoO_3$ to metallic Mo, the green body was hydrogen-reduced at $750^{\circ}C$, and sintered at $1100^{\circ}C$ for 2 h. The sintered samples, frozen by heated Teflon cylinder, showed large pores with the size of about 40 ${\mu}m$ which were aligned parallel to the sublimable vehicles growth direction. The formation of unidirectionally aligned pores is explained by the rejection and accumulation of solid particles in the serrated solid-liquid interface.
W-Cu 복합분말의 제조를 위한 기계화학적 공정에서 볼 밀링 시간에 따른 환원거동
김대건,이강원,석명진,김영도,Kim, Dae-Gun,Lee, Kang-Won,Suk, Myung-Jin,Kim, Young-Do 한국재료학회 2003 한국재료학회지 Vol.13 No.3
W-Cu composite powders can be prepared by mechanochemical process, where the $WO_3$-CuO composite powders were mechanically synthesized from the elemental oxide powders and subsequently reduced to W-Cu composite powders. In the present work, reduction behavior of$ WO_3$-CuO composite powders that were synthesized at different milling time was examined in terms of hygrometric analysis. In case of $WO_3$-CuO ball-milled for 20 h, the reaction temperature of CuO\longrightarrowCu became lower than in case of 1 h. Also, the reaction of $WO_3$\longrightarrow$WO_{2.9-2.72}$ and $WO_{2.9-2.72}$ \longrightarrow$WO_2$were shifted to lower temperatures and the peaks were changed to much sharper shape. While the reaction of $WO_2$\longrightarrowW in case of ball-milling for 20 h started at lower temperature, the peak temperature was the same as in 1 h ball-milling. The reduced W particle size was somewhat finer fer 20 h ball-milling. It was considered that the refinement of oxide particles caused by ball-milling process leads to such a change in the reduction behavior.
다양한 동결제를 이용하여 동결건조 공정으로 제조한 Cu 다공체의 기공구조 특성
이규휘,오승탁,석명진,정영근,Lee, Gyuhwi,Oh, Sung-Tag,Suk, Myung-Jin,Jeong, Young-Keun 한국분말야금학회 2020 한국분말재료학회지 (KPMI) Vol.27 No.3
The effect of sublimable vehicles on the pore structure of Cu fabricated by freeze drying is investigated. The 5 vol% CuO-dispersed slurries with camphene and various camphor-naphthalene compositions are frozen in a Teflon mold at -25℃, followed by sublimation at room temperature. After hydrogen reduction at 300℃ and sintering at 600 ℃, the green bodies of CuO are completely converted to Cu with various pore structures. The sintered samples prepared using CuO/camphene slurries show large pores that are aligned parallel to the sublimable vehicle growth direction. In addition, a dense microstructure is observed in the bottom section of the specimen where the solidification heat was released, owing to the difference in the solidification behavior of the camphene crystals. The porous Cu shows different pore structures, such as dendritic, rod-like, and plate shaped, depending on the composition of the camphornaphthalene system. The change in pore structure is explained by the crystal growth behavior of primary camphor and eutectic and primary naphthalene.
액상-고체입자 혼합물의 응고 시 응고계면에서의 입자의 거동
이호석,이규희,오승탁,김영도,석명진,Lee, Ho-Suk,Lee, Kyu-Hee,Oh, Sung-Tag,Kim, Young Do,Suk, Myung-Jin 한국분말야금학회 2018 한국분말재료학회지 (KPMI) Vol.25 No.4
A unique porous material with controlled pore characteristics can be fabricated by the freeze-drying process, which uses the slurry of organic material as the sublimable vehicle mixed with powders. The essential feature in this process is that during the solidification of the slurry, the dendrites of the organic material should repel the dispersed particles into the interdendritic region. In the present work, a model experiment is attempted using some transparent organic materials mixed with glass powders, which enable in-situ observation. The organic materials used are camphor-naphthalene mixture (hypo- and hypereutectic composition), salol, camphene, and pivalic acid. Among these materials, the constituent phases in camphor-naphthalene system, i.e. naphthalene plate, camphor dendrite, and camphor-naphthalene eutectic exclusively repel the glass powders. This result suggests that the control of organic material composition in the binary system is useful for producing a porous body with the required pore structure.
구형 PMMA와 WO<sub>3</sub> 분말이 혼합된 Camphene 슬러리의 동결건조에 의한 W 다공체 제조
이한얼,전기철,김영도,석명진,오승탁,Lee, Han-Eol,Jeon, Ki Cheol,Kim, Young Do,Suk, Myung-Jin,Oh, Sung-Tag 한국재료학회 2015 한국재료학회지 Vol.25 No.11
Porous W with spherical and directionally aligned pores was fabricated by the combination of sacrificial fugitives and a freeze-drying process. Camphene slurries with powder mixtures of $WO_3$ and spherical PMMA of 20 vol% were frozen at $-25^{\circ}C$ and dried for the sublimation of the camphene. The green bodies were heat-treated at $400^{\circ}C$ for 2 h to decompose the PMMA; then, sintering was carried out at $1200^{\circ}C$ in a hydrogen atmosphere for 2 h. TGA and XRD analysis showed that the PMMA decomposed at about $400^{\circ}C$, and $WO_3$ was reduced to metallic W at $800^{\circ}C$ without any reaction phases. The sintered bodies with $WO_3$-PMMA contents of 15 and 20 vol% showed large pores with aligned direction and small pores in the internal walls of the large pores. The pore formation was discussed in terms of the solidication behavior of liquid camphene with solid particles. Spherical pores, formed by decomposition of PMMA, were observed in the sintered specimens. Also, microstructural observation revealed that struts between the small pores consisted of very fine particles with size of about 300 nm.
소결 공정 개선을 통한 미세 결정립 몰리브덴 소결체 제조
이태호 ( Tae Ho Lee ),김세훈 ( Se Hoon Kim ),박민서 ( Min Suh Park ),석명진 ( Myung Jin Suk ),김영도 ( Young Do Kim ) 대한금속재료학회(구 대한금속학회) 2011 대한금속·재료학회지 Vol.49 No.11
In this study, the fabrication of ultra fine grained Mo bulk was conducted. MoO3 nanopowders were prepared by a high energy ball-milling process and then reduced at the temperature of 800℃ without holding time in H2 atmosphere. The particle size of Mo nanopowder was ~150 nm and grain size was ~40 nm. The two-step process was employed for the sintering of Mo nanopowder to obtain fine grain size. The densification over 90% could be obtained by the two-step sintering with a grain size of less than 660 nm. For higher density, modified two-step sintering was designed. 95% of theoretical density with the grain size of 730 nm was obtained by the modified two-step sintering.
WO3/Camphene 슬러리의 동결건조 공정에서 동결제의 응고거동이 기공구조에 미치는 영향
오승탁 ( Sung Tag Oh ),이규태 ( Gyu Tae Lee ),이근재 ( Kun Jae Lee ),석명진 ( Myung Jin Suk ) 대한금속재료학회(구 대한금속학회) 2014 대한금속·재료학회지 Vol.52 No.3
This paper describes the effect of the solidification behavior of camphene on the pore structure of porous W fabricated via a freeze-drying process. Camphene slurries with 10 and 15 vol% WO3 were prepared by milling at 50 ℃ with a small amount of dispersant. Porous W with unidirectionally aligned pore channels was prepared by freeze-drying of slurry and heat treatment in a hydrogen atmosphere. The sintered specimen with 10 vol% WO3 showed large pores with a size of about 100 μm which were aligned parallel to the growth direction of camphene. The internal wall of large pores had relatively small pores. In the slurry with 15 vol% WO3, large pores were not observed in the sintered body, and only small pores existed. The difference in the pore size in the two specimens is explained on the basis of the growth characteristics of the camphene dendrite depending on the amount of particles in the slurry.
방전 플라즈마 소결(Spark Plasma Sintering) 방법에 의해 제조된 Nb-Si-B계 합금의 미세조직 특성
김상환,김남우,정영근,오승탁,김영도,이성,석명진,Kim, Sang-Hwan,Kim, Nam-Woo,Jeong, Young-Keun,Oh, Sung-Tag,Kim, Young Do,Lee, Seong,Suk, Myung Jin 한국분말야금학회 2015 한국분말재료학회지 (KPMI) Vol.22 No.6
Microstructural examination of the Nb-Si-B alloys at Nb-rich compositions is performed. The Nb-rich corner of the Nb-Si-B system is favorable in that the constituent phases are Nb (ductile and tough phase with high melting temperature) and $T_2$ phase (very hard intermetallic compound with favorable oxidation resistance) which are good combination for high temperature structural materials. The samples containing compositions near Nb-rich corner of the Nb-Si-B ternary system are prepared by spark plasma sintering (SPS) process using $T_2$ and Nb powders. $T_2$ bulk phase is made in arc furnace by melting the Nb slug and the Si-B powder compact. The $T_2$ bulk phase was subsequently ball-milled to powders. SPS is performed at $1300^{\circ}C$ and $1400^{\circ}C$, depending on the composition, under 30 MPa for 600s, to produce disc-shaped specimen with 15 mm in diameter and 3 mm high. Hardness tests (Rockwell A-scale and micro Vickers) are carried out to estimate the mechanical property.
Nb-T<sub>2</sub> 합금의 파쇄분말을 사용한 Nb-Si-B계 합금의 제조
조민호,김성준,강현지,오승탁,김영도,이성,석명진,Cho, Min-Ho,Kim, Sung-Jun,Kang, Hyun-Ji,Oh, Sung-Tag,Kim, Young Do,Lee, Seong,Suk, Myung Jin 한국분말야금학회 2019 한국분말재료학회지 (KPMI) Vol.26 No.4
Nb-Si-B alloys with Nb-rich compositions are fabricated by spark plasma sintering for high-temperature structural applications. Three compositions are selected: 75 at% Nb (Nb0.7), 82 at% Nb (Nb1.5), and 88 at% Nb (Nb3), the atomic ratio of Si to B being 2. The microstructures of the prepared alloys are composed of Nb and $T_2$ phases. The $T_2$ phase is an intermetallic compound with a stoichiometry of $Nb_5Si_{3-x}B_x$ ($0{\leq}x{\leq}2$). In some previous studies, Nb-Si-B alloys have been prepared by spark plasma sintering (SPS) using Nb and $T_2$ powders (SPS 1). In the present work, the same alloys are prepared by the SPS process (SPS 2) using Nb powders and hypereutectic alloy powders with composition 67at%Nb-22at%Si-11at%B (Nb67). The Nb67 alloy powders comprise $T_2$ and eutectic ($T_2+Nb$) phases. The microstructures and hardness of the samples prepared in the present work have been compared with those previously reported; the samples prepared in this study exhibit finer and more uniform microstructures and higher hardness.