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WO<sub>3</sub>-TiH<sub>2</sub> 혼합분말의 동결건조 및 수소환원에 의한 W-Ti 다공체 제조
강현지,박성현,오승탁,Kang, Hyunji,Park, Sung Hyun,Oh, Sung-Tag 한국분말야금학회 2017 한국분말재료학회지 (KPMI) Vol.24 No.6
Porous W-10 wt% Ti alloys are prepared by freeze-drying a $WO_3-TiH_2$/camphene slurry, using a sintering process. X-ray diffraction analysis of the heat-treated powder in an argon atmosphere shows the $WO_3$ peak of the starting powder and reaction-phase peaks such as $WO_{2.9}$, $WO_2$, and $TiO_2$ peaks. In contrast, a powder mixture heated in a hydrogen atmosphere is composed of the W and TiW phases. The formation of reaction phases that are dependent on the atmosphere is explained by a thermodynamic consideration of the reduction behavior of $WO_3$ and the dehydrogenation reaction of $TiH_2$. To fabricate a porous W-Ti alloy, the camphene slurry is frozen at $-30^{\circ}C$, and pores are generated in the frozen specimens by the sublimation of camphene while drying in air. The green body is hydrogen-reduced and sintered at $1000^{\circ}C$ for 1 h. The sintered sample prepared by freeze-drying the camphene slurry shows large and aligned parallel pores in the camphene growth direction, and small pores in the internal walls of the large pores. The strut between large pores consists of very fine particles with partial necking between them.
MoO<sub>3</sub>-CuO 혼합분말의 볼 밀링 및 수소분위기 열처리에 의한 Mo-Cu 복합분말 제조
강현지,오승탁,Kang, Hyunji,Oh, Sung-Tag 한국분말야금학회 2018 한국분말재료학회지 (KPMI) Vol.25 No.4
The hydrogen reduction behavior of $MoO_3-CuO$ powder mixture for the synthesis of homogeneous Mo-20 wt% Cu composite powder is investigated. The reduction behavior of ball-milled powder mixture is analyzed by XRD and temperature programmed reduction method at various heating rates in Ar-10% $H_2$ atmosphere. The XRD analysis of the heat-treated powder at $300^{\circ}C$ shows Cu, $MoO_3$, and $Cu_2MoO_5$ phases. In contrast, the powder mixture heated at $400^{\circ}C$ is composed of Cu and $MoO_2$ phases. The hydrogen reduction kinetic is evaluated by the amount of peak shift with heating rates. The activation energies for the reduction, estimated by the slope of the Kissinger plot, are measured as 112.2 kJ/mol and 65.2 kJ/mol, depending on the reduction steps from CuO to Cu and from $MoO_3$ to $MoO_2$, respectively. The measured activation energy for the reduction of $MoO_3$ is explained by the effect of pre-reduced Cu particles. The powder mixture, hydrogen-reduced at $700^{\circ}C$, shows the dispersion of nano-sized Cu agglomerates on the surface of Mo powders.
금속산화물 분말의 동결건조 및 수소환원에 의한 Mo-Cu 다공체 제조
강현지,한주연,오승탁,Kang, Hyunji,Han, Ju-Yeon,Oh, Sung-Tag 한국분말재료학회 (*구 분말야금학회) 2019 한국분말재료학회지 (KPMI) Vol.26 No.1
In this study, porous Mo-5 wt% Cu with unidirectionally aligned pores is prepared by freeze drying of camphene slurry with $MoO_3-CuO$ powders. Unidirectional freezing of camphene slurry with dispersion stability is conducted at $-25^{\circ}C$, and pores in the frozen specimens are generated by sublimation of the camphene crystals. The green bodies are hydrogen-reduced at $750^{\circ}C$ and sintered at $1000^{\circ}C$ for 1 h. X-ray diffraction analysis reveals that $MoO_3-CuO$ composite powders are completely converted to a Mo-and-Cu phase without any reaction phases by hydrogen reduction. The sintered bodies with the Mo-Cu phase show large and aligned parallel pores to the camphene growth direction as well as small pores in the internal walls of large pores. The pore size and porosity decrease with increasing composite powder content from 5 to 10 vol%. The change of pore characteristics is explained by the degree of powder rearrangement in slurry and the accumulation behavior of powders in the interdendritic spaces of solidified camphene.
CuO-Al<sub>2</sub>O<sub>3</sub>/camphene 슬러리의 동결건조 공정에 의한 Al<sub>2</sub>O<sub>3</sub> 입자분산 Cu 다공체 제조
강현지,류도형,오승탁,Kang, Hyunji,Riu, Doh-Hyung,Oh, Sung-Tag 한국분말야금학회 2018 한국분말재료학회지 (KPMI) Vol.25 No.1
Porous Cu with a dispersion of nanoscale $Al_2O_3$ particles is fabricated by freeze-drying $CuO-Al_2O_3$/camphene slurry and sintering. Camphene slurries with $CuO-Al_2O_3$ contents of 5 and 10 vol% are unidirectionally frozen at $-30^{\circ}C$, and pores are generated in the frozen specimens by camphene sublimation during air drying. The green bodies are sintered for 1 h at $700^{\circ}C$ and $800^{\circ}C$ in $H_2$ atmosphere. The sintered samples show large pores of $100{\mu}m$ in average size aligned parallel to the camphene growth direction. The internal walls of the large pores feature relatively small pores of ${\sim}10{\mu}m$ in size. The size of the large pores decreases with increasing $CuO-Al_2O_3$ content by the changing degree of powder rearrangement in the slurry. The size of the small pores decreases with increasing sintering temperature. Microstructural analysis reveals that 100-nm $Al_2O_3$ particles are homogeneously dispersed in the Cu matrix. These results suggest that a porous composite body with aligned large pores could be fabricated by a freeze-drying and $H_2$ reducing process.
WO<sub>3</sub>-TiH<sub>2</sub> 혼합분말의 반응처리 및 방전 플라스마 소결에 의한 W-Ti 치밀체 제조
강현지,김헌주,한주연,이윤주,정영근,오승탁,Kang, Hyunji,Kim, Heun Joo,Han, Ju-Yeon,Lee, Yunju,Jeong, Young-Keun,Oh, Sung-Tag 한국재료학회 2018 한국재료학회지 Vol.28 No.9
W-10 wt% Ti alloys that have a homogeneous microstructure are prepared by thermal decomposition of $WO_3-TiH_2$ powder mixtures and spark plasma sintering. The reduction and dehydrogenation behavior of $WO_3$ and $TiH_2$ are analyzed by temperature programmed reduction and a thermogravimetric method, respectively. The X-ray diffraction analysis of the powder mixture, heat-treated in an argon atmosphere, shows W- oxides and $TiO_2$ peaks. Conversely, the powder mixtures heated in a hydrogen atmosphere are composed of W, $WO_2$ and $TiO_2$ phases at $600^{\circ}C$ and W and W-rich ${\beta}$ phases at $800^{\circ}C$. The densified specimen by spark plasma sintering at $1500^{\circ}C$ in a vacuum using hydrogen-reduced $WO_3-TiH_2$ powder mixtures shows a Vickers hardness value of 4.6 GPa and a homogeneous microstructure with pure W, ${\beta}$ and Ti phases. The phase evolution dependent on the atmosphere and temperature is explained by the thermal decomposition and reaction behavior of $WO_3$ and $TiH_2$.
볼 밀링한 CuO-Co<sub>3</sub>O<sub>4</sub> 혼합분말의 수소환원 거동과 미세조직 특성
한주연,이규휘,강현지,오승탁,Han, Ju-Yeon,Lee, Gyuhwi,Kang, Hyunji,Oh, Sung-Tag 한국분말야금학회 2019 한국분말재료학회지 (KPMI) Vol.26 No.5
The hydrogen reduction behavior of the $CuO-SCo_3O_4$ powder mixture for the synthesis of the homogeneous Cu-15at%Co composite powder has been investigated. The composite powder is prepared by ball milling the oxide powders, followed by a hydrogen reduction process. The reduction behavior of the ball-milled powder mixture is analyzed by X-ray diffraction (XRD) and temperature-programmed reduction at different heating rates in an Ar-10%H2 atmosphere. The scanning electron microscopy and XRD results reveal that the hydrogen-reduced powder mixture is composed of fine agglomerates of nanosized Cu and Co particles. The hydrogen reduction kinetics is studied by determining the degree of peak shift as a function of the heating rate. The activation energies for the reduction of the oxide powders estimated from the slopes of the Kissinger plots are 58.1 kJ/mol and 65.8 kJ/mol, depending on the reduction reaction: CuO to Cu and $SCo_3O_4$ to Co, respectively. The measured temperature and activation energy for the reduction of $SCo_3O_4$ are explained on the basis of the effect of pre-reduced Cu particles.