Fe-based pigments have attracted much interest owing to their eco-friendliness. In particular, the color of nanosized pigments can be tuned by controlling their size and morphology. This study reports on the effect of length on the coloration of β-FeOOH pigments prepared using an NH₄OH solution. First, rod-type β-FeOOH is prepared by the hydrolysis of FeCl₃·6H₂O and NH₄OH. When the amount of NH₄OH is increased, the length of the rods decreases. Thus, the length of the nanorods can be adjusted from 10 nm to 300 nm. The color of β-FeOOH changes from orangered to yellow depending on the length of β-FeOOH. The color and phase structure of β-FeOOH is characterized by UVvis spectroscopy, CIE Lab color parameter measurements, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and powder X-ray diffraction (XRD).

Quantum dots (QDs) are capable of controlling the typical emission and absorption wavelengths because of the bandgap widening effect of nanometer-sized particles. These phosphor particles have been used in optical devices, photovoltaic devices, advanced display devices, and several biomedical complexes. In this study, we synthesize ZnSe QDs with controlled surface defects by a heating-up method. The optical properties of the synthesized particles are analyzed using UV-visible and photoluminescence (PL) measurements. Calculations indicate nearly monodisperse particles with a size of about 5.1 nm at 260°C (full width at half maximum = 27.7 nm). Furthermore, the study results confirm that successful doping is achieved by adding Eu³⁺ preparing the growth phase of the ZnSe:Eu QDs when heating-up method. Further, we investigate the correlation between the surface defects and the luminescent properties of the QDs.

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• An investigation into the effective surface passivation of quantum dots by a photo-assisted chemical method
  So-Yeong Joo, Hyun-Su Park, Do-yeon Kim, Bum-Sung Kim, Chan Gi Lee, Woo-Byoung Kim
  AIP Advances.2018;[Epub]     CrossRef
• Multimodal luminescence properties of surface-treated ZnSe quantum dots by Eu
  Ji Young Park, Da-Woon Jeong, Kyoung-Mook Lim, Yong-Ho Choa, Woo-Byoung Kim, Bum Sung Kim
  Applied Surface Science.2017; 415: 8.     CrossRef

In this study, TiO₂ powders are synthesized from ammonium hexafluoride titanate (AHFT, (NH₄)₂TiF₆) as a precursor by heat treatment. First, we evaluate the physical properties of AHFT using X-ray diffraction (XRD), particle size analysis (PSA), thermogravimetric analysis (TGA), and field-emission scanning electron microscopy (FESEM). Then, to prepare the TiO₂ powders, is heat-treated at 300-1300°C for 1 h. The ratio of anatase to rutile phase in TiO₂ is estimated by XRD. The anatase phase forms at 500°C and phase transformation to the rutile phase occurs at 1200°C. Increase in the particle size is observed upon increasing the reaction temperature, and the phase ratio of the rutile phase is determined from a comparison with the calculated XRD data. Thus, we show that anatase and rutile TiO₂ powders could be synthesized using AHFT as a raw material, and the obtained data are utilized for developing a new process for producing high-quality TiO₂ powder.

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• Photocatalytic activity of rutile TiO2 powders coupled with anatase TiO2 nanoparticles using surfactant
  Jong Min Byun, Chun Woong Park, Young In Kim, Young Do Kim
  journal of Korean Powder Metallurgy Institute.2018; 25(3): 257.     CrossRef

Noncontact direct-printed conductive silver patterns with an enhanced electrical resistivity are fabricated using a silver ink with a mixture of silver nanoparticles and nanoplates. The microstructure and electrical resistivity of the silver pattern are systematically investigated as a function of the mixing ratio of the nanoparticles and nanoplates. The pattern, which is fabricated using a mixture with a mixing ratio of 3(nanoparticles):7(nanoplates) and sintered at 200°C shows a highly dense and well-sintered microstructure and has a resistivity of 7.60 μΩ·cm. This originates a mutual synergistic effect through a combination of the sinterability of the nanoparticles and the packing ability of the nanoplates. This is a conductive material that can be used to fabricate noncontact direct-printed conductive patterns with excellent electrical conductivity for various flexible electronics applications, including solar cells, displays, RFIDs, and sensors.

This study investigates the thermal shock property of a polycrystalline diamond compact (PDC) produced by a high-pressure, high-temperature (HPHT) sintering process. Three kinds of PDCs are manufactured by the HPHT sintering process using different particle sizes of the initial diamond powders: 8-16 μm (D50 = 4.3 μm), 10-20 μm (D50 = 6.92 μm), and 12-22 μm (D50 = 8.94 μm). The microstructure observation results for the manufactured PDCs reveal that elemental Co and W are present along the interface of the diamond particles. The fractions of Co and WC in the PDC increase as the initial particle size decreases. The manufactured PDCs are subjected to thermal shock tests at two temperatures of 780°C and 830°C. The results reveal that the PDC with a smaller particle size of diamond easily produces microscale thermal cracks. This is mainly because of the abundant presence of Co and WC phases along the diamond interface and the easy formation of Co-based (CoO, Co₃O₄) and W-based (WO₂) oxides in the PDC using smaller diamond particles. The microstructural factors for controlling the thermal shock property of PDC material are also discussed.

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• HPHT sintering and performance investigation of PDC with different interfacial geometry substrates for trimodal diamond particle size
  Jianbo Tu, Xueqi Wang, Haibo Zhang, Baochang Liu
  Ceramics International.2024; 50(11): 19074.     CrossRef

This study focuses on the development of an alkaline leaching hydrometallurgy process for the recovery of tungsten from WC/Co hardmetal sludge, and an examination of the effect of the process parameters on tungsten recovery. The alkaline leaching hydrometallurgy process has four stages, i.e., oxidation of the sludge, leaching of tungsten by NaOH, refinement of the leaching solution, and precipitation of tungsten. The WC/Co hardmetal sludge oxide consists of WO₃ and CoWO₄. The leaching of tungsten is most affected by the leaching temperature, followed by the NaOH concentration and the leaching time. About 99% of tungsten in the WC/Co hardmetal sludge is leached at temperatures above 90°C and a NaOH concentration above 15%. For refinement of the leaching solution, pH control of the solution using HCl is more effective than the addition of Na₂S·9H₂O. The tungsten is precipitated as high-purity H₂WO₄·H₂O by pH control using HCl. With decreasing pH of the solution, the tungsten recovery rate increases and then decrease. About 93% of tungsten in the WC/Co hardmetal sludge is recovered by the alkaline leaching hydrometallurgy process.

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• Fabrication of tungsten oxide powder from WC–Co cemented carbide scraps by oxidation behaviour
  Min Soo Park, Jong-Min Gwak, Kyeong-mi Jang, Gook-Hyun Ha
  Powder Metallurgy.2023; 66(5): 688.     CrossRef

A thick film of Li₇La₃Zr₂O₁₂ (LLZO) solid-state electrolyte is fabricated using the tape casting process and is compared to a bulk specimen in terms of the density, microstructure, and ion conductivity. The final thickness of LLZO film after sintering is 240 μm which is stacked up with four sheets of LLZO green films including polymeric binders. The relative density of the LLZO film is 83%, which is almost the same as that of the bulk specimen. The ion conductivity of a LLZO thick film is 2.81 × 10⁻⁴ S/cm, which is also similar to that of the bulk specimen, 2.54 × 10⁻⁴ S/ cm. However, the microstructure shows a large difference in the grain size between the thick film and the bulk specimen. Although the grain boundary area is different between the thick film and the bulk specimen, the fact that both the ion conductivities are very similar means that no secondary phase exists at the grain boundary, which is thought to originate from nonstoichiometry or contamination.

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• Powder Aerosol Deposition as a Method to Produce Garnet‐Type Solid Ceramic Electrolytes: A Study on Electrochemical Film Properties and Industrial Applications
  Tobias Nazarenus, Yanyan Sun, Jörg Exner, Jaroslaw Kita, Ralf Moos
  Energy Technology.2021;[Epub]     CrossRef
• Synthesize of Nd₂Fe₁₄B Powders from 1-D Nd₂Fe₁₄B Wires using Electrospinning Process
  Nu Si A Eom, Su Noh, Muhammad Aneeq Haq, Bum Sung Kim
  Journal of Korean Powder Metallurgy Institute.2019; 26(6): 477.     CrossRef

Tri-isotropic (TRISO) coatings on zirconia surrogate beads are deposited using a fluidized-bed vapor deposition (FB-CVD) method. The silicon carbide layer is particularly important among the coated layers because it acts as a miniature pressure vessel and a diffusion barrier to gaseous and metallic fission products in the TRISO-coated particles. In this study, we obtain a nearly stoichiometric composition in the SiC layer coated at 1400°C, 1500°C, and 1400°C with 20 vol.% methyltrichlorosilane (MTS), However, the composition of the SiC layer coated at 1300-1350°C shows a difference from the stoichiometric ratio (1:1). The density decreases remarkably with decreasing SiC deposition temperature because of the nanosized pores. The high density of the SiC layer (≥ 3.19 g/cm²) easily obtained at 1500°C and 1400°C with 20 vol.% MTS did not change at an annealing temperature of 1900°C, simulating the reactor operating temperature. The evaluation of the mechanical properties is limited because of the inaccurate values of hardness and Young’s modulus measured by the nano-indentation method.

In this study, ultra-fine soft-magnetic micro-powders are prepared by high-pressure gas atomization of an Fe-based alloy, Fe-Hf-B-Nb-P-C. Spherical powders are successfully obtained by disintegration of the alloy melts under high-pressure He or N2 gas. The mean particle diameter of the obtained powders is 25.7 μm and 42.1 μm for He and N2 gas, respectively. Their crystallographic structure is confirmed to be amorphous throughout the interior when the particle diameter is less than 45 μm. The prepared powders show excellent soft magnetic properties with a saturation magnetization of 164.5 emu/g and a coercivity of 9.0 Oe. Finally, a toroidal core is fabricated for measuring the magnetic permeability, and a μr of up to 78.5 is obtained. It is strongly believed that soft magnetic powders prepared by gas atomization will be beneficial in the fabrication of high-performance devices, including inductors and motors.

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• Optimization of Densification Behavior of a Soft Magnetic Powder by Discrete Element Method and Machine Learning
  Jungjoon Kim, Dongchan Min, Suwon Park, Junhyub Jeon, Seok-Jae Lee, Youngkyun Kim, Hwi-Jun Kim, Youngjin Kim, Hyunjoo Choi
  MATERIALS TRANSACTIONS.2022; 63(10): 1304.     CrossRef
• Optimizing the magnetic properties of Fe-based amorphous powder by adjusting atomic structures from vitrification at different temperatures
  Song-Yi Kim, Hye-Ryeong Oh, Hyeon-Ah Kim, A-Young Lee, Hwi-Jun Kim, Sang-Sun Yang, Yong-Jin Kim, Hyun-Joo Choi, Il-Hyun Kim, Hyun-Gil Kim, Jürgen Eckert, Jong-Ryoul Kim, Min-Ha Lee
  Journal of Applied Physics.2019;[Epub]     CrossRef
• Soft magnetic properties of Fe-based amorphous/nanocrystalline hybrid materials
  Yeonjoo Lee, Jonggyu Jeon, Seungjin Nam, Teasuk Jang, Hwijun Kim, Minwoo Lee, Yongjin Kim, Dongyeol Yang, Kyeongsik Min, Hyunjoo Choi
  Powder Technology.2018; 339: 440.     CrossRef

A lean alloy is defined as a low alloy steel with a minimum amount of the alloying element that maintains the characteristics of the sintered alloy. It is well known that the addition of elements such as Cr, P, Si, or Mn improves the mechanical characteristics of the alloy, but decreases the sinterability. The mother alloy is used to avoid an oxidation reaction with the alloying elements of Cr, P, Si or Mn. The purpose of this study is to determine the change in the mechanical properties of Fe-P-Mo and Fe-P-Mn alloys as a result of the addition of Si. In this article, the Fe-P-Mo and Fe-P-Mn alloys to which Si is added are compacted at 7.0 g/cm³ and then sintered in H₂-N₂ at 1120°C. The P around the macropores and large grains reduces due to the formation of SiO₂ as the Si content increases. This is caused by the increase in strength owing to reducing intergranular fracture by suppressing the reaction with oxygen.

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• The Effects of Si or Sn on the Sintered Properties of Fe-(Mo,Mn)-P Lean alloy
  Woo-Young Jung, Jin-Uk Ok, Dong-Kyu Park, In-Shup Ahn
  Journal of Korean Powder Metallurgy Institute.2018; 25(4): 302.     CrossRef
• Sintering behavior of Fe-(Mo-Mn-P)-xSi alloys according to the Green Density
  Woo-Young Jung, Jin-Uk Ok, Dong-Kyu Park, In-Shup Ahn
  Journal of Korean Powder Metallurgy Institute.2017; 24(5): 400.     CrossRef