Titanium is the ninth most abundant element in the Earth’s crust and is the fourth most abundant structural metal after aluminum, iron, and magnesium. It exhibits a higher specific strength than steel along with an excellent corrosion resistance, highlighting the promising potential of titanium as a structural metal. However, titanium is difficult to extract from its ore and is classified as a rare metal, despite its abundance. Therefore, the production of titanium is exceedingly low compared to that of common metals. Titanium is conventionally produced as a sponge by the Kroll process. For powder metallurgy (PM), hydrogenation-dehydrogenation (HDH) of the titanium sponge or gas atomization of the titanium bulk is required. Therefore, numerous studies have been conducted on smelting, which replaces the Kroll process and produces powder that can be used directly for PM. In this review, the Kroll process and new smelting technologies of titanium for PM, such as metallothermic, electrolytic, and hydrogen reduction of TiCl₄ and TiO₂ are discussed.

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 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

Due to the rapid development of electricity, electronics, information communication, and biotechnology in recent years, studies are actively being conducted on nanopowders as it is required not only for high strengthening but also for high-function powder with electric, magnetic, and optical properties. Nonetheless, studies on nickel nanopowders are rare. In this study of the synthesis of nickel nanoparticles from LiNiO₂ (LNO), which is a cathode active material, we have synthesized the nanosized nickel powder by the liquid reduction process of NiSO₄ obtained through the leaching and purification of LNO. Moreover, we have studied the reduction reaction rate according to the temperature change of liquid phase reduction and the change of particle size as a function of NaOH addition amount using hydrazine monohydrate (N₂H₄·H₂O) and NaOH.

To meet the current demand in the fields of permanent magnets for achieving a high energy density, it is imperative to prepare nano-to-microscale rare-earth-based magnets with well-defined microstructures, controlled homogeneity, and magnetic characteristics via a bottom-up approach. Here, on the basis of a microstructural study and qualitative magnetic measurements, optimized reduction conditions for the preparation of nanostructured Sm-Co magnets are proposed, and the elucidation of the reduction-diffusion behavior in the binary phase system is clearly manifested. In addition, we have investigated the microstructural, crystallographic, and magnetic properties of the Sm-Co magnets prepared under different reduction conditions, that is, H₂ gas, calcium, and calcium hydride. This work provides a potential approach to prepare high-quality Sm-Co-based nanofibers, and moreover, it can be extended to the experimental design of other magnetic alloys.

To meet the current demand in the fields of permanent magnets for achieving a high energy density, it is imperative to prepare nano-to-microscale rare-earth-based magnets with well-defined microstructures, controlled homogeneity, and magnetic characteristics via a bottom-up approach. Here, on the basis of a microstructural study and qualitative magnetic measurements, optimized reduction conditions for the preparation of nanostructured Sm-Co magnets are proposed, and the elucidation of the reduction-diffusion behavior in the binary phase system is clearly manifested. In addition, we have investigated the microstructural, crystallographic, and magnetic properties of the Sm-Co magnets prepared under different reduction conditions, that is, H₂ gas, calcium, and calcium hydride. This work provides a potential approach to prepare high-quality Sm-Co-based nanofibers, and moreover, it can be extended to the experimental design of other magnetic alloys.

Ti has received considerable attention for aerospace, vehicle, and semiconductor industry applications because of its acid-resistant nature, low density, and high mechanical strength. A common precursor used for preparing Ti materials is TiCl₄. To prepare high-purity TiCl₄, a process based on the removal of VOCl₃ has been widely applied. However, VOCl₃ removal by distillation and condensation is difficult because of the similar physical properties of TiCl₄ and VOCl₃. To circumvent this problem, in this study, we have developed a process for VOCl₃ removal using Cu powder and mineral oil as purifying agents. The effects of reaction time and temperature, and ratio of purifying agents on the VOCl₃ removal efficiency are investigated by chemical and structural measurements. Clear TiCl₄ is obtained after the removal of VOCl₃. Notably, complete removal of VOCl₃ is achieved with 2.0 wt% of mineral oil. Moreover, the refined TiCl₄ is used as a precursor for the synthesis of Ti powder. Ti powder is fabricated by a thermal reduction process at 1,100ºC using an H₂-Ar gas mixture. The average size of the Ti powder particles is in the range of 1-3 μm.

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

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

Metallic tantalum powder is manufactured by reducing tantalum oxide (Ta₂O₅) with magnesium gas at 1,073–1,223 K in a reactor under argon gas. The high thermodynamic stability of magnesium oxide makes the reduction reaction from tantalum oxide into tantalum powder possible. The microstructure after the reduction reaction has the form of a mixture of tantalum and magnesium oxide, and the latter could be entirely eliminated by dissolving in weak hydrochloric acid. The powder size in SEM microstructure for the tantalum powder increases after acid leaching in the range of 50–300 nm, and its internal crystallite sizes are observed to be 11.5 to 24.7 nm with increasing reduction temperatures. Moreover, the optimized reduction temperature is found to be 1,173 K as the minimum oxygen concentration is approximately 1.3 wt.%.

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• A review of tantalum resources and its production
  Xue WEI, Long-gong XIA, Zhi-hong LIU, Le-ru ZHANG, Qi-hou LI
  Transactions of Nonferrous Metals Society of China.2023; 33(10): 3132.     CrossRef
• Valuable metal recovery from waste tantalum capacitors via cryogenic crushing-alkaline calcination-leaching process
  Longgong Xia, Xue Wei, Hongjun Wang, Fengchun Ye, Zhihong Liu
  Journal of Materials Research and Technology.2022; 16: 1637.     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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• Synthesis of Mo-Cu nanocomposite powder by hydrogen reduction of copper nitrate coated MoO3 powder mixture
  Ji Won Choi, Ji Young Kim, Youngmin Kim, Eui Seon Lee, Sung-Tag Oh
  Materials Letters.2024; 377: 137565.     CrossRef
• 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

This study focuses on the fabrication of a WC/Co composite powder from the oxide of WC/Co hardmetal scrap using solid carbon in a hydrogen gas atmosphere for the recycling of WC/Co hardmetal. Mixed powders are manufactured by mechanically milling the oxide powder of WC-13 wt% Co hardmetal scrap and carbon black with varying powder/ball weight ratios. The oxide powder of WC-13 wt% Co hardmetal scrap consists of WO₃ and CoWO₄. The mixed powder mechanically milled at a lower powder/ball weight ratio (high mechanical milling energy) has a more rapid carbothermal reduction reaction in the formation of WC and Co phases compared with that mechanically milled at a higher powder/ball weight ratio (lower mechanical milling energy). The WC/Co composite powder is fabricated at 900°C for 6 h from the oxide of WC/Co hardmetal scrap using solid carbon in a hydrogen gas atmosphere. The fabricated WC/Co composite powder has a particle size of approximately 0.25-0.5 μm.

In this paper, the recovery and nanoparticle synthesis of Ag from low temperature co-fired ceramic (LTCC) by-products are studied. The effect of reaction behavior on Ag leaching conditions from the LTCC by-products is confirmed. The optimum leaching conditions are determined to be: 5 M HNO₃, a reaction temperature of 75°C, and a pulp density of 50 g/L at 60 min. For the selective recovery of Ag, the [Cl]/[Ag] equivalence ratio experiment is performed using added HCl; most of the Ag (more than 99%) is recovered. The XRD and MP-AES results confirm that the powder is AgCl and that impurities are at less than 1%. Ag nanoparticles are synthesized using a chemical reduction process for recycling, NaBH4 and PVP are used as reducing agents and dispersion stabilizers. UV-vis and FE-SEM results show that AgCl powder is precipitated and that Ag nanoparticles are synthesized. Ag nanoparticles of 100% Ag are obtained under the chemical reaction conditions.