Journal of Powder Materials

Eui Seon Lee

6 Articles

Preparation and Microstructural Characteristics of Ti Nanopowder by Ball Milling and Dehydrogenation of TiH₂ Powder: Ji Young Kim, Eui Seon Lee, Ji Won Choi, Youngmin Kim, Sung-Tag Oh; J Powder Mater. 2024;31(4):324-328. Published online August 30, 2024; DOI: https://doi.org/10.4150/jpm.2024.00199

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Abstract PDF: This study analyzed the influence of ball size and process control agents on the refinement and dehydrogenation behavior of TiH2 powder. Powders milled using ZrO2 balls with diameters of 0.1 mm, 0.3 mm, and 0.3+0.5+1 mm exhibited a bimodal particle size distribution, of which the first mode had the smallest size of 0.23 μm for the 0.3 mm balls. Using ethanol and/or stearic acid as process control agents was effective in particle refinement. Thermogravimetric analysis showed that dehydrogenation of the milled powder started at a relatively low temperature compared to the raw powder, which is interpreted to have resulted from a decrease in particle size and an increase in defects. The dehydrogenation kinetics of the TiH2 powder were evaluated by the magnitude of peak shift with heating rates using thermogravimetric analysis. The activation energy of the dehydrogenation reaction, calculated from the slope of the Kissinger plot, was measured to be 228.6 kJ/mol for the raw powder and 194.5 kJ/mol for the milled powder. TEM analysis revealed that both the milled and dehydrogenated powders showed an angular shape with a size of about 200 nm.

Effect of Ball Milling Conditions on the Microstructure and Dehydrogenation Behavior of TiH₂ Powder: Ji Young Kim, Eui Seon Lee, Ji Won Choi, Youngmin Kim, Sung-Tag Oh; J Powder Mater. 2024;31(2):132-136. Published online April 30, 2024; DOI: https://doi.org/10.4150/jpm.2024.00001

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This study investigated the effects of revolution speed and ball size in planetary milling on the microstructure and dehydrogenation behavior of TiH2 powder. The particle size analysis showed that the large particles present in the raw powder were effectively refined as the revolution speed increased, and when milled at 500 rpm, the median particle size was 1.47 m. Milling with a mixture of balls of two or three sizes was more effective in refining the raw powder than milling with balls of a single size. A mixture of 3-mm and 5-mm-diameter balls was the optimal condition for particle refinement, and the measured median particle size was 0.71 m. The dependence of particle size on revolution speed and ball size was explained by changes in input energy and the number of contact points of the balls. In the milled powder, the endothermic peak measured using differential thermal analysis was observed at a relatively low temperature. This finding was interpreted as the activation of a dehydrogenation reaction, mainly due to the increase in the specific surface area and the concentration of lattice defects.

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Preparation and Microstructural Characteristics of Ti Nanopowder by Ball Milling and Dehydrogenation of TiH2 Powder
Ji Young Kim, Eui Seon Lee, Ji Won Choi, Youngmin Kim, Sung-Tag Oh
Journal of Powder Materials.2024; 31(4): 324. CrossRef

Pressureless Sintering and Microstructure of Pure Tungsten Powders Prepared by Ultrasonic Spray Pyrolysis: Youn Ji Heo, Eui Seon Lee, Sung-Tag Oh, Jongmin Byun; J Powder Mater. 2022;29(3):247-251. Published online June 1, 2022; DOI: https://doi.org/10.4150/KPMI.2022.29.3.247

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This study demonstrates the effect of the compaction pressure on the microstructure and properties of pressureless-sintered W bodies. W powders are synthesized by ultrasonic spray pyrolysis and hydrogen reduction using ammonium metatungstate hydrate as a precursor. Microstructural investigation reveals that a spherical powder in the form of agglomerated nanosized W particles is successfully synthesized. The W powder synthesized by ultrasonic spray pyrolysis exhibits a relative density of approximately 94% regardless of the compaction pressure, whereas the commercial powder exhibits a relative density of 64% under the same sintering conditions. This change in the relative density of the sintered compact can be explained by the difference in the sizes of the raw powder and the densities of the compacted green body. The grain size increases as the compaction pressure increases, and the sintered compact uniaxially pressed to 50 MPa and then isostatically pressed to 300 MPa exhibits a size of 0.71 m. The Vickers hardness of the sintered W exhibits a high value of 4.7 GPa, mainly due to grain refinement.

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Preparation of W-Ni-Cu Alloy Powder by Hydrogen Reduction of Metal Oxides
Youn Ji Heo, Eui Seon Lee, Ji Won Choi, Jongmin Byun, Sung-Tag Oh
Korean Journal of Metals and Materials.2024; 62(5): 334. CrossRef
Influence of the initial powder characteristic on the densified tungsten microstructure by spark plasma sintering and hot isostatic pressing
Ji Young Kim, Eui Seon Lee, Youn Ji Heo, Young-In Lee, Jongmin Byun, Sung-Tag Oh
Powder Metallurgy.2023; 66(5): 644. CrossRef

Fabrication of Porous Tungsten by Freeze Casting and Vacuum Drying of WO₃/Tert-butyl Alcohol Slurry: Youn Ji Heo, Eui Seon Lee, Sung-Tag Oh, Young-Keun Jeong; J Powder Mater. 2022;29(2):118-122. Published online April 1, 2022; DOI: https://doi.org/10.4150/KPMI.2022.29.2.118

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The synthesis of porous W by freeze-casting and vacuum drying is investigated. Ball-milled WO₃ powders and tert-butyl alcohol were used as the starting materials. The tert-butyl alcohol slurry is frozen at –25°C and dried under vacuum at –25 and –10°C. The dried bodies are hydrogen-reduced at 800°C and sintered at 1000°C. The XRD analysis shows that WO₃ is completely reduced to W without any reaction phases. SEM observations reveal that the struts and pores aligned in the tert-butyl alcohol growth direction, and the change in the powder content and drying temperature affects the pore structure. Furthermore, the struts of the porous body fabricated under vacuum are thinner than those fabricated under atmospheric pressure. This behavior is explained by the growth mechanism of tert-butyl alcohol and rearrangement of the powders during solidification. These results suggest that the pore structure of a porous body can be controlled by the powder content, drying temperature, and pressure.

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Fabrication of Porous TiO2 with Aligned Pores Using Tert-Butyl Alcohol Based Freeze Casting
Eui Seon Lee, Sung-Tag Oh
Korean Journal of Metals and Materials.2024; 62(12): 929. CrossRef

Effect of Freeze Drying Condition of WO₃/Tert-Butyl Alcohol Slurry on the Microstructural Characteristics of Porous Body: Eui Seon Lee, Youn Ji Heo, Myung-Jin Suk, Sung-Tag Oh; J Korean Powder Metall Inst. 2021;28(4):331-335. Published online August 1, 2021; DOI: https://doi.org/10.4150/KPMI.2021.28.4.331

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Abstract PDF: The effects of drying temperature on the microstructure of porous W fabricated by the freeze-casting process of tert-butyl alcohol slurry with WO₃ powder was investigated. Green bodies were hydrogen-reduced at 800°C for 1 h and sintered at 1000°C for 6 h. X-ray diffraction analysis revealed that WO₃ powders were completely converted to W without any reaction phases by hydrogen reduction. The sintered body showed pores aligned in the direction of tertbutyl alcohol growth, and the porosity and pore size decreased as the amount of WO₃ increased from 5 to 10v ol%. As the drying temperature of the frozen body increased from -25°C to -10°C, 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.

Effect of Tert-Butyl Alcohol Template on the Pore Structure of Porous Tungsten in Freeze Drying Process: Eui Seon Lee, Youn Ji Heo, Yun Taek Ko, Jin Gyeong Park, Yong-Ho Choa, Sung-Tag Oh; J Korean Powder Metall Inst. 2021;28(3):216-220. Published online June 1, 2021; DOI: https://doi.org/10.4150/KPMI.2021.28.3.216

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Abstract PDF: The effect of tert-butyl alcohol (TBA) as a freezing solvent on the pore structure of a porous tungsten body prepared by freeze-drying is analyzed. TBA slurries with a WO₃ content of 10 vol% are prepared by mixing with a small amount of dispersant and binder at 30°C. The slurries are frozen at -25°C, and pores are formed in the frozen specimens by the sublimation of TBA during drying in air. After hydrogen reduction at 800°C and sintering at 1000°C, the green body of WO₃ is completely converted to porous W with various pore structures. Directional pores from the center of the specimen to the outside are observed in the sintered bodies because of the columnar growth of TBA. A decrease in pore directionality and porosity is observed in the specimens prepared by long-duration drying and sintering. The change in pore structure is explained by the growth of the freezing solvent and densification.

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