Liquid metal extraction (LME), a pyrometallurgical recycling method, is popular owing to its negligible environmental impact. LME mainly targets rare-earth permanent magnets having several rare-earth elements. Mg is used as a solvent metal for LME because of its selective and eminent reactivity with rare-earth elements in magnets. Several studies concerning the formation of Dy-Fe intermetallic compounds and their effects on LME using Mg exist. However, methods for reducing these compounds are unavailable. Fe reacts more strongly with B than with Dy; B addition can be a reducing method for Dy-Fe intermetallic compounds owing to the formation of Fe₂B, which takes Fe from Dy-Fe intermetallic compounds. The FeB alloy is an adequate additive for the decomposition of Fe₂B. To accomplish the former process, Mg must convey B to a permanent magnet during the decomposition of the FeB alloy. Here, the effect of Mg on the transfer of B from FeB to permanent magnet is observed through microstructural and phase analyses. Through microstructural and phase analysis, it is confirmed that FeB is converted to Fe₂B upon B transfer, owing to Mg. Finally, the transfer effect of Mg is confirmed, and the possibility of reducing Dy-Fe intermetallic compounds during LME is suggested.

Stainless steel, a type of steel used for high-temperature parts, may cause damage when exposed to high temperatures, requiring additional coatings. In particular, the Cr₂O₃ product layer is unstable at 1000°C and higher temperatures; therefore, it is necessary to improve the oxidation resistance. In this study, an aluminide (Fe₂Al₅ and FeAl₃) coating layer was formed on the surface of STS 630 specimens through Al diffusion coatings from 500°C to 700°C for up to 25 h. Because the coating layers of Fe₂Al₅ and FeAl₃ could not withstand temperatures above 1200°C, an Al₂O₃ coating layer is deposited on the surface through static oxidation treatment at 500°C for 10 h. To confirm the ablation resistance of the resulting coating layer, dynamic flame exposure tests were conducted at 1350°C for 5–15 min. Excellent oxidation resistance is observed in the coated base material beneath the aluminide layer. The conditions of the flame tests and coating are discussed in terms of microstructural variations.

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• Thermal Stability and Degradation Properties of Aluminide Coated and Uncoated Ti-6Al-4V Alloys Exposed to High Temperature Flame
  C. Hwang, J. Park, J. Oh, D. Han, S. Lee, K. Shin, J. Choi, K. P. Shinde, J. S. Park
  Metals and Materials International.2023; 29(7): 1855.     CrossRef

Rare earth magnets with excellent magnetic properties are indispensable in the electric device, wind turbine, and e-mobility industries. The demand for the development of eco-friendly recycling techniques has increased to realize sustainable green technology, and the supply of rare earth resources, which are critical for the production of permanent magnets, are limited. Liquid metal extraction (LME), which is a type of pyrometallurgical recycling, is known to selectively extract the metal forms of rare earth elements. Although several studies have been carried out on the formation of intermetallic compounds and oxides, the effect of oxide formation on the extraction efficiency in the LME process remains unknown. In this study, microstructural and phase analyses are conducted to confirm the oxidation behavior of magnets pulverized by a jaw crusher. The LME process is performed with pulverized scrap, and extraction percentages are calculated to confirm the effect of the oxide phases on the extraction of Dy during the reaction. During the LME p rocess, Nd i s completely e xtracted a fter 6 h, w hile D y remains as D y₂Fe₁₇ and Dy-oxide. Because the decomposition rate of Dy₂Fe₁₇ is faster than the reduction rate of Dy-oxide, the importance of controlling Dy-oxide on Dy extraction is confirmed.

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• Manipulation of reactivity based on metallic adsorption in magnesium alloy scraps for rare-earth recycling by liquid metal extraction
  Sangmin Park, Yoonhyung Keum, Jaeyun Jeong, Seunghun Cha, Ju-Young Cho, Hyunchul Kim, Jiseong Lee, Taek-Soo Kim, Dae-Kyeom Kim, Myungsuk Song
  Journal of Alloys and Compounds.2025; 1022: 178711.     CrossRef
• A Review of the Current Progress in High-Temperature Recycling Strategies for Recovery of Rare-Earth Elements from Magnet Waste
  Ali Zakeri, Leili Tafaghodi
  Journal of Sustainable Metallurgy.2025; 11(1): 88.     CrossRef
• Selective growth of Nb–Fe–B intermetallic compounds for the direct separation of rare earths based on manipulating liquation
  Sangmin Park, Jaeyun Jeong, Seunghun Cha, Yoonhyung Keum, Ju-Young Cho, Hyungbeen Park, Taek-Soo Kim, Dae-Kyeom Kim, Myungsuk Song
  Materials Today Sustainability.2024; 28: 101042.     CrossRef
• Separation and recovery Nd and Dy from Mg-REEs alloy by vacuum distillation
  Sangmin Park, Dae-Kyeom Kim, Jaeyun Jeong, Jae Hong Shin, Yujin Kang, Rongyu Liu, Taek-Soo Kim, Myungsuk Song
  Journal of Alloys and Compounds.2023; 967: 171775.     CrossRef
• The Supported Boro-Additive Effect for the Selective Recovery of Dy Elements from Rare-Earth-Elements-Based Magnets
  Sangmin Park, Dae-Kyeom Kim, Javid Hussain, Myungsuk Song, Taek-Soo Kim
  Materials.2022; 15(9): 3032.     CrossRef
• Influence of Dysprosium Compounds on the Extraction Behavior of Dy from Nd-Dy-Fe-B Magnet Using Liquid Magnesium
  Sun-Woo Nam, Sang-Min Park, Mohammad Zarar Rasheed, Myung-Suk Song, Do-Hyang Kim, Taek-Soo Kim
  Metals.2021; 11(9): 1345.     CrossRef

High-entropy alloys (HEAs) are generally defined as solid solutions containing at least 5 constituent elements with concentrations between 5 and 35 atomic percent without the formation of intermetallic compounds. Currently, HEAs receive great attention as promising candidate materials for extreme environments due to their potentially desirable properties that result from their unique structural properties. In this review paper, we aim to introduce HEAs and explain their properties and related research by classifying them into three main categories, namely, mechanical properties, thermal properties, and electrochemical properties. Due to the high demand for structural materials in extreme environments, the mechanical properties of HEAs including strength, hardness, ductility, fatigue, and wear resistance are mainly described. Thermal and electrochemical properties, essential for the application of these alloys as structural materials, are also described.

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• Composites of equiatomic Y, La, Ce, Nd, and Gd rare earth oxides: Chemical-shift effects and valence spectra
  Jungsu Bin, Hyunbae Gee, Taesung Park, UiJun Go, Jeoung Han Kim, Youn-Seoung Lee
  Current Applied Physics.2024; 59: 85.     CrossRef
• Sintering Behavior and Mechanical Property of Transition Metal Carbide-Based Cermets by Spark Plasma Sintering
  Jeong-Han Lee, Hyun-Kuk Park, Sung-Kil Hong
  Korean Journal of Materials Research.2022; 32(1): 44.     CrossRef

In the present work, we synthesize nano-sized ZnO, SnO₂, and TiO₂ powders by hydrothermal reaction using metal chlorides. We also examine the energy-storage characteristics of the resulting materials to evaluate the potential application of these powders to dye-sensitized solar cells. The control of processing parameters such as pressure, temperature, and the concentration of aqueous solution results in the formation of a variety of powder morphologies with different sizes. Nano-rod, nano-flower, and spherical powders are easily formed with the present method. Heat treatment after the hydrothermal reaction usually increases the size of the powder. At temperatures above 1000°C, a complete collapse of the shape occurs. With regard to the capacity of DSSC materials, the hydrothermally synthesized TiO₂ results in the highest current density of 9.1 mA/cm² among the examined oxides. This is attributed to the fine particle size and morphology with large specific surface area.

Uniform TiO₂ blocking layers (BLs) are fabricated using ultrasonic spray pyrolysis deposition (USPD) method. To improve the photovoltaic performance of dye-sensitized solar cells (DSSCs), the BL thickness is controlled by using USPD times of 0, 20, 60, and 100 min, creating TiO₂ BLs of 0, 40, 70, and 100 nm, respectively, in average thickness on fluorine-doped tin oxide (FTO) glass. Compared to the other samples, the DSSC containing the uniform TiO₂ BL of 70 nm in thickness shows a superior power conversion efficiency of 7.58±0.20% because of the suppression of electron recombination by the effect of the optimized thickness. The performance improvement is mainly attributed to the increased open-circuit voltage (0.77±0.02 V) achieved by the increased Fermi energy levels of the working electrodes and the improved short-circuit current density (15.67±0.43 mA/cm2) by efficient electron transfer pathways. Therefore, optimized TiO₂ BLs fabricated by USPD may allow performance improvements in DSSCs.

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• Flexible Dye-sensitized Solar Cell Using Titanium Gel at Low Temperature
  Seung Hwan Ji, Hyunsu Park, Doyeon Kim, Do Hyung Han, Hye Won Yun, Woo-Byoung Kim
  Korean Journal of Materials Research.2019; 29(3): 183.     CrossRef
• Surface tailoring of zinc electrodes for energy storage devices with high-energy densities and long cycle life
  Geon-Hyoung An, SeungNam Cha, Jung Inn Sohn
  Applied Surface Science.2019; 467-468: 1157.     CrossRef
• Crystallinity Control Effects on Vanadium Oxide Films for Enhanced Electrochromic Performances
  Kue-Ho Kim, Ju-Won Bae, Tae-Kuen Lee, Hyo-Jin Ahn
  Korean Journal of Materials Research.2019; 29(6): 385.     CrossRef

Transition-metal oxide semiconductors have various band gaps. Therefore, many studies have been conducted in various application fields. Among these, methods for the adsorption of organic dyes and utilization of photocatalytic properties have been developed using various metal oxides. In this study, the adsorption and photocatalytic effects of WO₃ nanomaterials prepared by hydrothermal synthesis are investigated, with citric acid added in the hydrothermal process as a structure-directing agent. The nanostructures of WO₃ are studied using transmission electron microscopy and scanning electron microscopy images. The crystal structure is investigated using X-ray diffraction patterns, and the changes in the dye concentrations adsorbed on WO₃ nanorods are measured with a UV-visible absorption spectrophotometer based on Beer-Lambert’s law. The methylene blue (MB) dye solution is subjected to acid or base conditions to monitor the change in the maximum adsorption amount in relation to the pH. The maximum adsorption capacity is observed at pH 3. In addition to the dye adsorption, UV irradiation is carried out to investigate the decomposition of the MB dye as a result of photocatalytic effects. Significant photocatalytic properties are observed and compared with the adsorption effects for dye removal.

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• Photocatalytic Properties of WO₃ Thin Films Prepared by Electrodeposition Method
  Kwang-Mo Kang, Ji-Hye Jeong, Ga-In Lee, Jae-Min Im, Hyun-Jeong Cheon, Deok-Hyeon Kim, Yoon-Chae Nah
  Journal of Korean Powder Metallurgy Institute.2019; 26(1): 40.     CrossRef
• Photocatalysis of TiO₂/WO₃ Composites Synthesized by Ball Milling
  Su-Yeol Yu, Chunghee Nam
  Journal of Korean Powder Metallurgy Institute.2018; 25(4): 316.     CrossRef

We report on a simple and robust route to the spontaneous assembly of well-ordered magnetic nanoparticle superstructures by irreversible evaporation of a sessile single droplet of a mixture of a ferrofluid (FF) and a nonmagnetic fluid (NF). The resulting assembled superstructures are seen to form well-packed, vertically arranged columns with diameters of 5~0.7 μm, interparticle spacings of 9~2 μm, and heights of 1.3~3 μm. The assembled superstructures are strongly dependent on both the magnitude of magnetic field and the mixing ratio of the mixture. As the magnitude of the externally applied magnetic field and the mixing ratio of the mixture increase gradually, the size and interspacing of the magnetic nanoparticle aggregations decrease. Without an externally applied magnetic field, featureless patterns are observed for the γ-Fe₃O₄ nanoparticle aggregations. The proposed approach may lead to a versatile, cost-effective, fast, and scalable fabrication process based on the field-induced self-assembly of magnetic nanoparticles.

Recently, the grain boundary diffusion process (GBDP), involving heavy rare-earth elements such as Dy and Tb, has been widely used to enhance the coercivity of Nd-Fe-B permanent magnets. For example, a Dy compound is coated onto the surface of Nd-Fe-B sintered magnets, and then the magnets are heat treated. Subsequently, Dy diffuses into the grain boundaries of Nd-Fe-B magnets, forming Dy-Fe-B or Nd-Dy-Fe-B. The dip-coating process is also used widely instead of the GBDP. However, it is quite hard to control the thickness uniformity using dip coating. In this study, first, a DyF₃ paste is fabricated using DyF₃ powder. Subsequently, the fabricated DyF₃ paste is homogeneously coated onto the surface of a Nd-Fe-B sintered magnet. The magnet is then subjected to GBDP to enhance its coercivity. The weight ratio of binder and DyF₃ powder is controlled, and we find that the coercivity enhances with decreasing binder content. In addition, the maximum coercivity is obtained with the paste containing 70 wt% of DyF₃ powder.

We investigate the microstructural and magnetic property changes of DyH₂, Cu + DyH₂, and Al + DyH₂ diffusion-treated NdFeB sintered magnets with the post annealing (PA) temperature. The coercivity of all the diffusiontreated magnets increases with increasing heat treatment temperature except at 910°C, where it decreases slightly. Moreover, at 880°C, the coercivity increases by 3.8 kOe in Cu and 4.7 kOe in Al-mixed DyH₂-coated magnets, whereas this increase is relatively low (3.0 kOe) in the magnet coated with only DyH₂. Both Cu and Al have an almost similar effect on the coercivity improvement, particularly over the heat treatment temperature range of 790-880°C. The diffusivity and diffusion depth of Dy increases in those magnets that are treated with Cu or Al-mixed DyH₂, mainly because of the comparatively easy diffusion path provided by Cu and Al owing to their solubility in the Nd-rich grain boundary phase. The formation of a highly anisotropic (Nd, Dy)₂Fe₁₄B phase layer, which acts as the shell in the core-shell-type structure so as to prevent the reverse domain movement, is the cause of enhanced coercivity of diffusion-treated Nd-Fe-B magnets.

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• 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

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.

Carbon nanofiber (CNF) composites coated with spindle-shaped Fe₂O₃ nanoparticles (NPs) are fabricated by a combination of an electrospinning method and a hydrothermal method, and their morphological, structural, and chemical properties are measured by field-emission scanning electron microscopy, transmission electron microscopy, Xray diffraction, and X-ray photoelectron spectroscopy. For comparison, CNFs and spindle-shaped Fe₂O₃ NPs are prepared by either an electrospinning method or a hydrothermal method, respectively. Dye-sensitized solar cells (DSSCs) fabricated with the composites exhibit enhanced open circuit voltage (0.70 V), short-circuit current density (12.82 mA/cm2), fill factor (61.30%), and power conversion efficiency (5.52%) compared to those of the CNFs (0.66 V, 11.61 mA/cm2, 51.96%, and 3.97%) and spindle-shaped Fe₂O₃ NPs (0.67 V, 11.45 mA/cm2, 50.17%, and 3.86%). This performance improvement can be attributed to a synergistic effect of a superb catalytic reaction of spindle-shaped Fe₂O₃ NPs and efficient charge transfer relative to the one-dimensional nanostructure of the CNFs. Therefore, spindle-shaped Fe₂O₃-NPcoated CNF composites may be proposed as a potential alternative material for low-cost counter electrodes in DSSCs.

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• Ni Nanoparticles-Graphitic Carbon Nanofiber Composites for Pt-Free Counter Electrode in Dye-Sensitized Solar Cells
  Dong-Hyeun Oh, Bon-Ryul Koo, Yu-Jin Lee, HyeLan An, Hyo-Jin Ahn
  Korean Journal of Materials Research.2016; 26(11): 649.     CrossRef

The organic binder-free paste for dye-sensitized solar cell (DSSC) has been investigated using peroxo titanium complex. The crystal structure of TiO₂ nanoparticles, morphology of TiO₂ film and electrical properties are analyzed by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Electrochemical Impedance Spectra (EIS), and solar simulator. The synthesized TiO₂ nanopowders by the peroxo titanium complex at 150, 300, 400°C, and 450°C have anatase phase and average crystal sizes are calculated to be 4.2, 13.7, 16.9, and 20.9 nm, respectively. The DSSC prepared by the peroxo titanium complex binder have higher V_oc and lower J_sc values than that of the organic binder. It can be attributed to improvement of sintering properties of TCO/TiO₂ and TiO₂/TiO₂ interface and to formation of agglomerate by the nanoparticles. As a result, we have investigated the organic binder-free paste and 3.178% conversion efficiency of the DSSC at 450°C.

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• Development of Eco-Friendly Ag Embedded Peroxo Titanium Complex Solution Based Thin Film and Electrical Behaviors of Resistive Random Access Memory
  Won Jin Kim, Jinho Lee, Ryun Na Kim, Donghee Lee, Woo-Byoung Kim
  Korean Journal of Materials Research.2024; 34(3): 152.     CrossRef
• Development of eco-friendly thin film manufacturing process using poeroxo titanium complex solution and potential for resistive random access memory
  Jinho Lee, Ryun Na Kim, Kee-Ryung Park, Woo-Byoung Kim
  Applied Surface Science.2021; 562: 150170.     CrossRef
• Preparation of ultra-thin TiO2 shell by peroxo titanium complex (PTC) solution-based green surface modification, and photocatalytic activity of homo-core/shell TiO2
  Jinho Lee, Jiyong Hwang, Hyunsu Park, Tohru Sekino, Woo-Byoung Kim
  Applied Surface Science.2021; 540: 148399.     CrossRef
• Effects of Annealing Temperature on the Crystal Structure, Morphology, and Optical Properties of Peroxo-Titanate Nanotubes Prepared by Peroxo-Titanium Complex Ion
  Hyunsu Park, Tomoyo Goto, Sunghun Cho, Soo Wohn Lee, Masato Kakihana, Tohru Sekino
  Nanomaterials.2020; 10(7): 1331.     CrossRef
• Study on thermal behavior of Ammonium Hexafluofide Titanate for Synthesis of TiO2 Powders
  Duk-Hee Lee, Jae-Ryang Park, Chan-Gi Lee, Kyung-Soo Park, Hyeon-Mo Kim
  Journal of Korean Powder Metallurgy Institute.2016; 23(5): 353.     CrossRef