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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• 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

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.

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.

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.

The (Ga_1-xZn_x)(N_1-xO_x) solid solution is attracting extensive attention for photocatalytic water splitting and wastewater treatment owing to its narrow and controllable band gap. To optimize the photocatalytic performance of the solid solution, the key points are to decrease its band gap and recombination rate. In this study, (Ga_1-xZn_x)(N_1-xO_x) nanofibers with various Zn fractions are prepared by electrospinning followed by calcination and nitridation. The effect of the composition and crystallinity of electrospun oxide nanofibers on the morphology and optical properties of the obtained solid-solution nanofibers are systematically investigated. The results show that the final shape of the (Ga_1-xZn_x) (N_1-xO_x) material is greatly affected by the crystallinity of the oxide nanofibers before nitridation. The photocatalytic properties of (Ga_1-xZn_x)(N_1-xO_x) with different Ga:Zn atomic ratios are investigated by studying the degradation of rhodamine B under visible light irradiation.

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• Fabrication of Nanowire by Electrospinning Process Using Nickel Oxide Particle Recovered from MLCC
  Haein Shin, Jongwon Bae, Minsu Kang, Kun-Jae Lee
  journal of Korean Powder Metallurgy Institute.2023; 30(6): 502.     CrossRef

Porous Cu-14 wt% Co with aligned pores is produced by a freeze drying and sintering process. Unidirectional freezing of camphene slurry with CuO-Co₃O₄ powders is conducted, and pores in the frozen specimens are generated by sublimation of the camphene crystals. The dried bodies are hydrogen-reduced at 500°C and sintered at 800°C for 1 h. The reduction behavior of the CuO-Co₃O₄ powder mixture is analyzed using a temperature-programmed reduction method in an Ar-10% H₂ atmosphere. The sintered bodies show large and aligned parallel pores in the camphene growth direction. In addition, small pores are distributed around the internal walls of the large pores. The size and fraction of the pores decrease as the amount of solid powder added to the slurry increases. The change in pore characteristics according to the amount of the mixed powder is interpreted to be due to the rearrangement and accumulation behavior of the solid particles in the freezing process of the slurry.

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°C, followed by sublimation at room temperature. After hydrogen reduction at 300°C and sintering at 600 °C, 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.

The hydrogen reduction behavior of the CuO-Co₃O₄ 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 Co₃O₄ to Co, respectively. The measured temperature and activation energy for the reduction of Co₃O₄ are explained on the basis of the effect of pre-reduced Cu particles.

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• Synthesis of Porous Cu-Co using Freeze Drying Process of Camphene Slurry with Oxide Composite Powders
  Gyuhwi Lee, Ju-Yeon Han, Sung-Tag Oh
  Journal of Korean Powder Metallurgy Institute.2020; 27(3): 193.     CrossRef

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 T₂ phases. The T₂ phase is an intermetallic compound with a stoichiometry of Nb₅Si_3-xB_x (0 ≤ x ≤ 2). In some previous studies, Nb-Si-B alloys have been prepared by spark plasma sintering (SPS) using Nb and T₂ 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 T₂ and eutectic (T₂ + 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.

In this study, porous Mo-5 wt% Cu with unidirectionally aligned pores is prepared by freeze drying of camphene slurry with MoO₃-CuO powders. Unidirectional freezing of camphene slurry with dispersion stability is conducted at -25°C, and pores in the frozen specimens are generated by sublimation of the camphene crystals. The green bodies are hydrogen-reduced at 750°C and sintered at 1000°C for 1 h. X-ray diffraction analysis reveals that MoO₃-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.

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• Characteristic Evaluation of WC Hard Materials According to Ni Content Variation by a Pulsed Current Activated Sintering Process
  Hyun-Kuk Park
  Korean Journal of Materials Research.2020; 30(12): 672.     CrossRef
• Effect of α-lath size on the mechanical properties of Ti–6Al–4V using core time hydrogen heat treatment
  Gye-Hoon Cho, Jung-Min Oh, Hanjung Kwon, Jae-Won Lim
  Materials Science and Technology.2020; 36(7): 858.     CrossRef

In this study, freeze drying of a porous Ni with unidirectionally aligned pore channels is accomplished by using a NiO powder and camphene. Camphene slurries with NiO content of 5 and 10 vol% are prepared by mixing them with a small amount of dispersant at 50°C. Freezing of a slurry is performed at -25°C while the growth direction of the camphene is unidirectionally controlled. Pores are generated subsequently by sublimation of the camphene during drying in air for 48 h. The green bodies are hydrogen-reduced at 400°C and then sintered at 800°C and 900°C for 1 h. X-ray diffraction analysis reveals that the NiO powder is completely converted to the Ni phase without any reaction phases. The sintered samples show large pores that align parallel pores in the camphene growth direction as well as small pores in the internal walls of large pores. The size of large and small pores decreases with increasing powder content from 5 to 10 vol%. The influence of powder content on the pore structure is explained by the degree of powder rearrangement in slurry and the accumulation behavior of powders in the interdendritic spaces of solidified camphene.

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• Synthesis of Porous Cu-Co using Freeze Drying Process of Camphene Slurry with Oxide Composite Powders
  Gyuhwi Lee, Ju-Yeon Han, Sung-Tag Oh
  Journal of Korean Powder Metallurgy Institute.2020; 27(3): 193.     CrossRef

The hydrogen reduction behavior of MoO₃-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% H₂ atmosphere. The XRD analysis of the heat-treated powder at 300°C shows Cu, MoO₃, and Cu₂MoO₅ phases. In contrast, the powder mixture heated at 400°C is composed of Cu and MoO₂ 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 MoO₃ to MoO₂, respectively. The measured activation energy for the reduction of MoO₃ is explained by the effect of pre-reduced Cu particles. The powder mixture, hydrogen-reduced at 700°C, shows the dispersion of nano-sized Cu agglomerates on the surface of Mo powders.

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• Fabrication of Porous Mo-Cu by Freeze Drying and Hydrogen Reduction of Metal Oxide Powders
  Hyunji Kang, Ju-Yeon Han, Sung-Tag Oh
  Journal of Korean Powder Metallurgy Institute.2019; 26(1): 1.     CrossRef
• Hydrogen Reduction Behavior and Microstructure Characteristics of Ball-milled CuO-Co₃O₄ Powder Mixtures
  Ju-Yeon Han, Gyuhwi Lee, Hyunji Kang, Sung-Tag Oh
  Journal of Korean Powder Metallurgy Institute.2019; 26(5): 410.     CrossRef

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 camphornaphthalene 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.

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• Freeze Drying Process and Pore Structure Characteristics of Porous Cu with Various Sublimable Vehicles
  Gyuhwi Lee, Sung-Tag Oh, Myung-Jin Suk, Young-Keun Jeong
  Journal of Korean Powder Metallurgy Institute.2020; 27(3): 198.     CrossRef

Porous Cu with a dispersion of nanoscale Al₂O₃ particles is fabricated by freeze-drying CuO-Al₂O₃/camphene slurry and sintering. Camphene slurries with CuO-Al₂O₃ contents of 5 and 10 vol% are unidirectionally frozen at -30°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°C and 800°C in H₂ atmosphere. The sintered samples show large pores of 100 μm in average size aligned parallel to the camphene growth direction. The internal walls of the large pores feature relatively small pores of ~10 μm in size. The size of the large pores decreases with increasing CuO-Al₂O₃ 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 Al₂O₃ 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 H₂ reducing process.

Porous W-10 wt% Ti alloys are prepared by freeze-drying a WO₃-TiH₂/camphene slurry, using a sintering process. X-ray diffraction analysis of the heat-treated powder in an argon atmosphere shows the WO₃ peak of the starting powder and reaction-phase peaks such as WO_2.9, WO₂, and TiO₂ 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 WO₃ and the dehydrogenation reaction of TiH₂. To fabricate a porous W-Ti alloy, the camphene slurry is frozen at -30°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°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.

The effect of the mixing method on the characteristics of hybrid-structure W powder with nano and micro sizes is investigated. Fine WO₃ powders with sizes of ~0.6 μm, prepared by ball milling for 10 h, are mixed with pure W powder with sizes of 12 μm by various mixing process. In the case of simple mixing with ball-milled WO₃ and micro sized W powders, WO₃ particles are locally present in the form of agglomerates in the surface of large W powders, but in the case of ball milling, a relatively uniform distribution of WO₃ particles is exhibited. The microstructural observation reveals that the ball milled WO₃ powder, heat-treated at 750°C for 1 h in a hydrogen atmosphere, is fine W particles of ~200 nm or less. The powder mixture prepared by simple mixing and hydrogen reduction exhibits the formation of coarse W particles with agglomeration of the micro sized W powder on the surface. Conversely, in the powder mixture fabricated by ball milling and hydrogen reduction, a uniform distribution of fine W particles forming nano-micro sized hybrid structure is observed.

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• The Efficiency of Radiation Shielding Sheet to Reduce Radiation Exposure during C-arm Fluoroscopy
  Hosang Jeon, Won Chul Shin, Hee Yun Seol, Yongkan Ki, Kyeong Baek Kim, Ki Seok Choo, Sang Don Lee, Suk-Woong Kang
  Journal of the Korean Fracture Society.2023; 36(4): 111.     CrossRef
• Facile phosphorus-embedding into SnS2 using a high-energy ball mill to improve the surface kinetics of P-SnS2 anodes for a Li-ion battery
  Hongsuk Choi, Seungmin Lee, KwangSup Eom
  Applied Surface Science.2019; 466: 578.     CrossRef
• Hydrogen reduction behavior and microstructural characteristics of WO3 and WO3-NiO powders
  Hyunji Kang, Young-Keun Jeong, Sung-Tag Oh
  International Journal of Refractory Metals and Hard Materials.2019; 80: 69.     CrossRef
• Fabrication of Densified W-Ti by Reaction Treatment and Spark Plasma Sintering of WO3-TiH2 Powder Mixtures
  Hyunji Kang, Heun Joo Kim, Ju-Yeon Han, Yunju Lee, Young-Keun Jeong, Sung-Tag Oh
  Korean Journal of Materials Research.2018; 28(9): 511.     CrossRef

The effects of the heat treatment temperature and of the atmosphere on the dehydrogenation and hydrogen reduction of ball-milled TiH₂-WO₃ powder mixtures are investigated for the synthesis of Ti-W powders with controlled microstructure. Homogeneously mixed powders with refined TiH₂ particles are successfully prepared by ball milling for 24 h. X-ray diffraction (XRD) analyses show that the powder mixture heat-treated in Ar atmosphere is composed of Ti, Ti₂O, and W phases, regardless of the heat treatment temperature. However, XRD results for the powder mixture, heat-treated at 600°C in a hydrogen atmosphere, show TiH₂ and TiH peaks as well as reaction phase peaks of Ti oxides and W, while the powder mixture heat-treated at 900°C exhibits only XRD peaks attributed to Ti oxides and W. The formation behavior of the reaction phases that are dependent on the heat treatment temperature and on the atmosphere is explained by thermodynamic considerations for the dehydrogenation reaction of TiH₂, the hydrogen reduction of WO₃ and the partial oxidation of dehydrogenated Ti.

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• Fabrication of Porous W-Ti by Freeze-Drying and Hydrogen Reduction of WO3-TiH2 Powder Mixtures
  Hyunji Kang, Sung Hyun Park, Sung-Tag Oh
  Journal of Korean Powder Metallurgy Institute.2017; 24(6): 472.     CrossRef

An optimum route to synthesize Ti-Mo system powders is investigated by analyzing the effect of the heat treatment atmosphere on the formation of the reaction phase by dehydrogenation and hydrogen reduction of ball-milled TiH₂-MoO₃ powder mixtures. Homogeneous powder mixtures with refined particles are prepared by ball milling for 24 h. XRD analysis of the heat-treated powder in a hydrogen atmosphere shows TiH₂ and MoO₃ peaks in the initial powders as well as the peaks corresponding to the reaction phase species, such as TiH_0.7, TiO, MoO₂, Mo. In contrast, powder mixtures heated in an argon atmosphere are composed of Ti, TiO, Mo and MoO₃ phases. The formation of reaction phases dependent on the atmosphere is explained by the partial pressure of H2 and the reaction temperature, based on thermodynamic considerations for the dehydrogenation reaction of TiH₂ and the reduction behavior of MoO₃.

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• Effect of titanium addition on mechanical properties of Mo-Si-B alloys
  Won June Choi, Seung Yeong Lee, Chun Woong Park, Jung Hyo Park, Jong Min Byun, Young Do Kim
  International Journal of Refractory Metals and Hard Materials.2019; 80: 238.     CrossRef

This study is performed to fabricate a Ti porous body by freeze drying process using titanium hydride (TiH2) powder and camphene. Then, the Ti porous body is employed to synthesize carbon nanotubes (CNTs) using thermal catalytic chemical vapor deposition (CCVD) with Fe catalyst and methane (CH4) gas to increase the specific surface area. The synthesized Ti porous body has 100 μm-sized macropores and 10-30 μm-sized micropores. The synthesized CNTs have random directions and are entangled with adjacent CNTs. The CNTs have a bamboo-like structure, and their average diameter is about 50 nm. The Fe nano-particles observed at the tip of the CNTs indicate that the tip growth model is applicable. The specific surface area of the CNT-coated Ti porous body is about 20 times larger than that of the raw Ti porous body. These CNT-coated Ti porous bodies are expected to be used as filters or catalyst supports.

The present study demonstrates the effect of freezing conditions on the pore structure of porous Cu-10 wt.% Sn prepared by freeze drying of CuO-SnO₂/camphene slurry. Mixtures of CuO and SnO₂ powders are prepared by ball milling for 10 h. Camphene slurries with 10 vol.% of CuO-SnO₂ are unidirectionally frozen in a mold maintained at a temperature of -30°C for 1 and 24 h, respectively. Pores are generated by the sublimation of camphene at room temperature. After hydrogen reduction and sintering at 650°C for 2 h, the green body of the CuO-SnO₂ is completely converted into porous Cu-Sn alloy. Microstructural observation reveals that the sintered samples have large pores which are aligned parallel to the camphene growth direction. The size of the large pores increases from 150 to 300 μm with an increase in the holding time. Also, the internal walls of the large pores contain relatively small pores whose size increases with the holding time. The change in pore structure is explained by the growth behavior of the camphene crystals and rearrangement of the solid particles during the freezing process.

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• Fabrication of Porous Ni by Freeze Drying and Hydrogen Reduction of NiO/Camphene Slurry
  Jae-Hun Jeong, Sung-Tag Oh, Chang-Yong Hyun
  Journal of Korean Powder Metallurgy Institute.2019; 26(1): 6.     CrossRef
• Fabrication of Al2O3 Dispersed Porous Cu by Freeze Drying of CuO-Al2O3/Camphene Slurry
  Hyunji Kang, Doh-Hyung Riu, Sung-Tag Oh
  journal of Korean Powder Metallurgy Institute.2018; 25(1): 25.     CrossRef
• Porous W-Ni Alloys Synthesized from Camphene/WO3-NiO Slurry by Freeze Drying and Heat Treatment in Hydrogen Atmosphere
  Sung Hyun Park, Seong-Min Park, So-Jeong Park, Bo-Yeong Park, Sung-Tag Oh
  Korean Journal of Materials Research.2018; 28(2): 108.     CrossRef

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 T₂ 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 T₂ and Nb powders. T₂ bulk phase is made in arc furnace by melting the Nb slug and the Si-B powder compact. The T₂ bulk phase was subsequently ballmilled to powders. SPS is performed at 1300°C and 1400°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.

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• Fabrication of Nb-Si-B Alloys Using the Pulverized Nb-T₂ Alloy Powder
  Min-Ho Cho, Sung-Jun Kim, Hyun-Ji Kang, Sung-Tag Oh, Young Do Kim, Seong Lee, Myung Jin Suk
  Journal of Korean Powder Metallurgy Institute.2019; 26(4): 299.     CrossRef
• Mechanical properties of Mo-Nb-Si-B quaternary alloy fabricated by powder metallurgical method
  Jong Min Byun, Su-Ryong Bang, Se Hoon Kim, Won June Choi, Young Do Kim
  International Journal of Refractory Metals and Hard Materials.2017; 65: 14.     CrossRef
• Mechanical properties of Mo-Si-B alloys fabricated by using core-shell powder with dispersion of yttria nanoparticles
  Jong Min Byun, Su-Ryong Bang, Won June Choi, Min Sang Kim, Goo Won Noh, Young Do Kim
  Metals and Materials International.2017; 23(1): 170.     CrossRef
• Fabrication of Ta2O5 Dispersion-Strengthened Mo-Si-B Alloy by Powder Metallurgical Method
  Jong Min Byun, Won June Choi, Su-Ryong Bang, Chun Woong Park, Young Do Kim
  JOM.2017; 69(4): 683.     CrossRef
• Rapid consolidation of nanostuctured WC-FeAl3 by pulsed current activated heating and its mechanical properties
  In-Jin Shon, Seok-Jae Lee
  International Journal of Refractory Metals and Hard Materials.2017; 65: 69.     CrossRef

The effect of sublimable vehicle composition in the camphor-naphthalene system on the pore structure of porous Cu-Ni alloy is investigated. The CuO-NiO mixed slurries with hypoeutectic, eutectic and hypereutectic compositions are frozen into a mold at -25°C. Pores are generated by sublimation of the vehicles at room temperature. After hydrogen reduction at 300°C and sintering at 850°C for 1 h, the green body of CuO-NiO is completely converted to porous Cu-Ni alloy with various pore structures. The sintered samples show large pores which are aligned parallel to the sublimable vehicle growth direction. The pore size and porosity decrease with increase in powder content due to the degree of powder rearrangement in slurry. In the hypoeutectic composition slurry, small pores with dendritic morphology are observed in the sintered Cu-Ni, whereas the specimen of hypereutectic composition shows pore structure of plate shape. The change of pore structure is explained by growth behavior of primary camphor and naphthalene crystals during solidification of camphor-naphthalene alloys.

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• Freeze Drying Process and Pore Structure Characteristics of Porous Cu with Various Sublimable Vehicles
  Gyuhwi Lee, Sung-Tag Oh, Myung-Jin Suk, Young-Keun Jeong
  Journal of Korean Powder Metallurgy Institute.2020; 27(3): 198.     CrossRef
• Interaction of Solid Particles with the Solidifying Front in the Liquid-Particle Mixture
  Ho-Suk Lee, Kyu-Hee Lee, Sung-Tag Oh, Young Do Kim, Myung-Jin Suk
  Journal of Korean Powder Metallurgy Institute.2018; 25(4): 336.     CrossRef

The synthesis of NiTi alloy powders by hydrogen reduction and dehydrogenation process of NiO and TiH₂ powder mixtures is investigated. Mixtures of NiO and TiH₂ powders are prepared by simple mixing for 1 h or ball milling for 24 h. Simple-mixed mixture shows that fine NiO particles are homogeneously coated on the surface of TiH₂ powders, whereas ball milled one exhibits the morphology with mixing of fine NiO and TiH₂ particles. Thermogravimetric analysis in hydrogen atmosphere reveals that the NiO and TiH₂ phase are changed to metallic Ni and Ti in the temperature range of 260 to 290°C and 553 to 639°C, respectively. In the simple-mixed powders by heat-up to 700°C, agglomerates with solid particles and solidified liquid phase are observed, and the size of agglomerates is increased at 1000°C. From the XRD analysis, the presence of liquid phase is explained by the formation and melting of NiTi₂ intermetallic compound due to an exothermic reaction between Ni and Ti. The simple-mixed powders, heated to 1000°C, lead to the formation of NiTi phase but additional Ni-, Ti-rich and Ti-oxide phases. In contrast, the microstructure of ball-milled powders is characterized by the neck-grown particles, forming Ni3Ti, Ti-oxide and unreacted Ni phase.

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• Effect of Heat Treatment Atmosphere on the Microstructure of TiH2-MoO3 Powder Mixtures
  Ki Cheol Jeon, Sung Hyun Park, Na-Yeon Kwon, Sung-Tag Oh
  Journal of Korean Powder Metallurgy Institute.2016; 23(4): 303.     CrossRef

In this study, titanium(Ti) meshes and porous bodies are employed to synthesize carbon nanotubes(CNTs) using methane(CH₄) gas and camphene solution, respectively, by chemical vapor deposition. Camphene is impregnated into Ti porous bodies prior to heating in a furnace. Various microscopic and spectroscopic techniques are utilized to analyze CNTs. It is found that CNTs are more densely and homogeneously populated on the camphene impregnated Ti-porous bodies as compared to CNTs synthesized with methane on Ti-porous bodies. It is elucidated that, when synthesized with methane, few CNTs are formed inside of Ti porous bodies due to methane supply limited by internal structures of Ti porous bodies. Ti-meshes and porous bodies are found to be multi-walled with high degree of structural disorders. These CNTs are expected to be utilized as catalyst supports in catalytic filters and purification systems.

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• Solvent induced surface modifications on hydrogen storage performance of ZnO nanoparticle decorated MWCNTs
  Madhavi Konni, Anima S. Dadhich, Saratchandra Babu Mukkamala
  Sustainable Energy & Fuels.2018; 2(2): 466.     CrossRef
• Influence of nickel nanoparticles on hydrogen storage behaviors of MWCNTs
  Ye-Ji Han, Soo-Jin Park
  Applied Surface Science.2017; 415: 85.     CrossRef

Porous W with controlled pore structure was fabricated by thermal decomposition and hydrogen reduction process of PMMA beads and WO₃ powder compacts. The PMMA sizes of 8 and 50 μm were used as pore forming agent for fabricating the porous W. The WO₃ powder compacts with 20 and 70 vol% PMMA were prepared by uniaxial pressing and sintered for 2 h at 1200°C in hydrogen atmosphere. TGA analysis revealed that the PMMA was decomposed at about 400°C and WO₃ was reduced to metallic W at 800°C. Large pores in the sintered specimens were formed by thermal decomposition of spherical PMMA, and their size was increased with increase in PMMA size and the amount of PMMA addition. Also the pore shape was changed from spherical to irregular form with increasing PMMA contents due to the agglomeration of PMMA in the powder mixing process.

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• Synthesis of Porous Silica Particles Using Sodium Silicate Precursor for Water-Repellent Surfaces
  Young-Sang Cho, Nahee Ku, Young-Seok Kim
  JOURNAL OF CHEMICAL ENGINEERING OF JAPAN.2019; 52(2): 194.     CrossRef

The present study demonstrates the effect of solidification condition on the pore structure in freeze drying process using the slurries of CuO/sublimable vehicles. Camphene and Camphor-45 wt% naphthalene based slurries with 14 vol% CuO powder were frozen into a mold at -25°C, followed by sublimation at room temperature. The green bodies were hydrogen-reduced and sintered at 500°C for 1 h. The porous Cu specimen, frozen the CuO/camphene slurry into the heated mold of the upper part, showed large pores with unidirectional pore channels and small pores in their internal wall. Also, it was observed that the size of large pores was decreasing near the bottom part of specimen. The change of pore structure depending on the freezing condition was explained by the nucleation behavior of camphene crystals and rearrangement of solid powders during solidification. In case of porous Cu prepared from CuO/Camphornaphthalene system, the pore structure exhibited plate shape as a replica of the original structure of crystallized vehicles with hypereutectic composition.

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• Interaction of Solid Particles with the Solidifying Front in the Liquid-Particle Mixture
  Ho-Suk Lee, Kyu-Hee Lee, Sung-Tag Oh, Young Do Kim, Myung-Jin Suk
  Journal of Korean Powder Metallurgy Institute.2018; 25(4): 336.     CrossRef
• Fabrication of Ti Porous body with Improved Specific Surface Area by Synthesis of CNTs
  Hye Rim Choi, Jong Min Byun, Myung-Jin Suk, Sung-Tag Oh, Young Do Kim
  Journal of Korean Powder Metallurgy Institute.2016; 23(3): 235.     CrossRef

In this study, modified catalytic chemical vapor deposition (CCVD) method was applied to control the CNTs (carbon nanotubes) growth. Since titanium (Ti) substrate and iron (Fe) catalysts react one another and form a new phase (Fe₂TiO₅) above 700°C, the decrease of CNT yield above 800°C where methane gas decomposes is inevitable under common CCVD method. Therefore, we synthesized CNTs on the Ti substrate by dividing the tube furnace into two sections (left and right) and heating them to different temperatures each. The reactant gas flew through from the end of the right tube furnace while the Ti substrate was placed in the center of the left tube furnace. When the CNT growth temperature was set 700/950°C (left/right), CNTs with high yield were observed. Also, by examining the micro-structure of CNTs of 700/950°C, it was confirmed that CNTs show the bamboo-like structure.

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• Fabrication of Ti Porous body with Improved Specific Surface Area by Synthesis of CNTs
  Hye Rim Choi, Jong Min Byun, Myung-Jin Suk, Sung-Tag Oh, Young Do Kim
  Journal of Korean Powder Metallurgy Institute.2016; 23(3): 235.     CrossRef
• Synthesis of CNT on a Camphene Impregnated Titanium Porous Body by Thermal Chemical Vapor Deposition
  Hogyu Kim, Hye Rim Choi, Jong Min Byun, Myung-Jin Suk, Sung-Tag Oh, Young Do Kim
  Journal of Korean Powder Metallurgy Institute.2015; 22(2): 122.     CrossRef

This study reports a simple way of fabricating the porous Cu with unidirectional pore channels by freeze drying camphene slurry with Cu oxide coated Cu powders. The coated powders were prepared by calcination of ballmilled powder mixture of Cu and Cu-nitrate. Improved dispersion stability of camphene slurry could be achieved using the Cu oxide coated Cu powders instead of pure Cu powders. Pores in the frozen specimen at -25°C were generated by sublimation of the camphene during drying in air, and the green bodies were sintered at 750°C for 1 h in H₂ atmosphere. XRD analysis revealed that the coated layer of Cu oxide was completely converted to Cu phase without any reaction phases by hydrogen heat treatment. The porous Cu specimen prepared from pure Cu powders showed partly large pores with unidirectional pore channels, but most of pores were randomly distributed. In contrast, large and aligned parallel pores to the camphene growth direction were clearly observed in the sample using Cu oxide coated Cu powders. Pore formation behavior depending on the initial powders was discussed based on the degree of powder rearrangement and dispersion stability in slurry.

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• Effect of Freezing and Sintering Condition of CuO-SnO2/Camphene Slurries on the Pore Structure of Porous Cu-Sn
  Joo-Hyung Kim, Sung-Tag Oh, Chang-Yong Hyun
  Journal of Korean Powder Metallurgy Institute.2016; 23(1): 49.     CrossRef
• Fabrication of Cu-30 vol% SiC Composites by Pressureless Sintering of Polycarbosilane Coated SiC and Cu Powder Mixtures
  Yeon Su Kim, Na-Yeon Kwon, Young-Keun Jeong, Sung-Tag Oh
  Korean Journal of Materials Research.2016; 26(6): 337.     CrossRef
• Synthesis of Aligned Porous Sn by Freeze-Drying of Tin Chloride/camphene Slurry
  수룡 방, 승탁 오
  Korean Journal of Materials Research.2015; 25(1): 27~31.     CrossRef

The 304 stainless steel powders were prepared by high energy ball milling and subsequently sintered by spark plasma sintering, and the microstructural characteristics and micro-hardness were investigated. The initial size of the irregular shaped 304 stainless steel powders was approximately 42 μm. After high energy ball milling at 800 rpm for 5h, the powders became spherical with a size of approximately 2 μm, and without formation of reaction compounds. From TEM analysis, it was confirmed that the as-milled powders consisted of the aggregates of the nano-sized particles. As the sintering temperature increased from 1073K to 1573K, the relative density and micro-hardness of sintered sample increased. The sample sintered at 1573K showed the highest relative density of approximately 95% and a micro-hardness of 550 Hv.

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• Investigating molybdenum’s sulphur scavenging ability for MoS2 formation in preventing pitting corrosion of stainless steels
  Kai Xiang Kuah, Daniel J. Blackwood
  npj Materials Degradation.2023;[Epub]     CrossRef

In order to fabricate the porous Al₂O₃ with dispersion of nano-sized Cu particles, freeze-drying of camphene/ Al₂O₃ slurry and solution chemistry process using Cu-nitrate are introduced. Camphene slurries with 10 vol% Al₂O₃ was frozen at -25°C. Pores were generated by sublimation of the camphene during drying in air. The sintered samples at 1400 and 1500°C showed the same size of large pores which were aligned parallel to the sublimable vehicles growth direction. However, the size of fine pores in the internal walls of large pores decreased with increase in sintering temperature. It was shown that Cu particles with the size of 100 nm were homogeneously dispersed on the surfaces of the large pores. Antibacterial test using fungus revealed that the porous Al₂O₃/1 vol% Cu composite showed antifungal property due to the dispersion of Cu particles. The results are suggested that the porous composites with required pore characteristics and functional property can be fabricated by freeze-drying process and addition of functional nano particles by chemical method.

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• Collection of Industrial Oil Using Nanoparticles and Porous Powders of Silica
  Y.-S. Cho, J.-W. Moon
  Archives of Metallurgy and Materials.2017; 62(2): 1371.     CrossRef
• Fabrication and Mechanical Properties of STS316L Porous Metal for Vacuum Injection Mold
  Se Hoon Kim, Sang Min Kim, Sang Ho Noh, Jin Pyeong Kim, Jae Hyuck Shin, Si-Young Sung, Jin Kwang Jin, Taean Kim
  Journal of Korean Powder Metallurgy Institute.2015; 22(3): 197.     CrossRef

This paper describes the surface modification effect of a Ti substrate for improved dispersibility of the catalytic metal. Etching of a pure titanium substrate was conducted in 50% H₂SO₄, 50°C for 1 h-12 h to observe the surface roughness as a function of the etching time. At 1 h, the grain boundaries were obvious and the crystal grains were distinguishable. The grain surface showed micro-porosities owing to the formation of micro-pits less than 1 μm in diameter. The depths of the grain boundary and micro-pits appear to increase with etching time. After synthesizing the catalytic metal and growing the carbon nano tube (CNT) on Ti substrate with varying surface roughness, the distribution trends of the catalytic metal and grown CNT on Ti substrate are discussed from a micro-structural perspective.

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• Solvent induced surface modifications on hydrogen storage performance of ZnO nanoparticle decorated MWCNTs
  Madhavi Konni, Anima S. Dadhich, Saratchandra Babu Mukkamala
  Sustainable Energy & Fuels.2018; 2(2): 466.     CrossRef
• Influence of nickel nanoparticles on hydrogen storage behaviors of MWCNTs
  Ye-Ji Han, Soo-Jin Park
  Applied Surface Science.2017; 415: 85.     CrossRef
• Spontaneous Formation of Titanium Nitride on the Surface of a Ti Rod Induced by Electro-Discharge-Heat-Treatment in an N2 Atmosphere
  W.H. Lee, Y.H. Yoon, Y.H. Kim, Y.K. Lee, J.Y. Kim, S.Y. Chang
  Archives of Metallurgy and Materials.2017; 62(2): 1281.     CrossRef
• Synthesis of CNT on a Camphene Impregnated Titanium Porous Body by Thermal Chemical Vapor Deposition
  Hogyu Kim, Hye Rim Choi, Jong Min Byun, Myung-Jin Suk, Sung-Tag Oh, Young Do Kim
  Journal of Korean Powder Metallurgy Institute.2015; 22(2): 122.     CrossRef

Cu-Ni alloys with unidirectionally aligned pores were prepared by freeze-drying process of CuO-NiO/camphene slurry. Camphene slurries with dispersion stability by the addition of oligomeric polyester were frozen at -25°C, and pores in the frozen specimens were generated by sublimation of the camphene during drying in air. The green bodies were hydrogen-reduced at 300°C and sintered at 850°C for 1 h. X-ray diffraction analysis revealed that CuO-NiO composite powders were completely converted to Cu-Ni alloy without any reaction phases by hydrogen reduction. The sintered samples showed large and aligned parallel pores to the camphene growth direction, and small pores in the internal wall of large pores. The pore size and porosity decreased with increase in CuO-NiO content from 5 to 10 vol%. The change of pore characteristics was explained by the degree of powder rearrangement in slurry and the accumulation behavior of powders in the interdendritic spaces of solidified camphene.

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• Enhancement in electrical conductivity of pastes containing submicron Ag-coated Cu filler with palmitic acid surface modification
  Eun Byeol Choi, Jong-Hyun Lee
  Applied Surface Science.2017; 415: 67.     CrossRef
• Investigation for Microstructure and Hardness of Welded Zone of Cu-Ni Alloy using W92-Ni-Fe Sintering Tool
  Tae-Jin Yoon, Sang-Won Park, Myung-Chang Kang, Joong-Suk Noh, Sung-Wook Chung, Chung-Yun Kang
  Journal of Korean Powder Metallurgy Institute.2015; 22(3): 181.     CrossRef
• Controlling Structural and Electrical Properties of Pt Nanopowder-Dispersed SiO2Film
  Jae Ho Lee, In Joo Shin, Sung Woo Lee, Hyeong Cheol Kim, Byung Joon Choi
  Journal of Korean Powder Metallurgy Institute.2014; 21(5): 355.     CrossRef