Plastic pollution is threatening human health and ecosystems, resulting in one of the biggest challenges that humanity has ever faced. Therefore, this study focuses on the preparation of macroporous carbon from biowaste (MC)-supported manganese oxide (MnO₂) as an efficient, reusable, and robust catalyst for the recycling of poly(ethylene terephthalate) (PET) waste. As-prepared MnO₂/MC composites have a hierarchical pore network and a large surface area (376.16 m²/g) with a narrow size distribution. MnO₂/MC shows a maximum yield (98%) of bis(2-hydroxyethyl)terephthalate (BHET) after glycolysis reaction for 120 min. Furthermore, MnO₂/MC can be reused at least nine times with a negligible decrease in BHET yield. Based on this remarkable catalytic performance, we expect that MnO₂-based heterogeneous catalysts have the potential to be introduced into the PET recycling industry.

In this paper, a new Co₁₀Fe₁₀Mn₃₅Ni₃₅Zn₁₀ high entropy alloy (HEA) is identified as a strong candidate for the single face-centered cubic (FCC) structure screened using the upgraded TCFE2000 thermodynamic CALPHAD database. The Co₁₀Fe₁₀Mn₃₅Ni₃₅Zn₁₀ HEA is fabricated using the mechanical (MA) procedure and pressure-less sintering method. The Co₁₀Fe₁₀Mn₃₅Ni₃₅Zn₁₀ HEA, which consists of elements with a large difference in melting point and atomic size, is successfully fabricated using powder metallurgy techniques. The MA behavior, microstructure, and mechanical properties of the Co₁₀Fe₁₀Mn₃₅Ni₃₅Zn₁₀ HEA are systematically studied to understand the MA behavior and develop advanced techniques for fabricating HEA products. After MA, a single FCC phase is found. After sintering at 900°C, the microstructure has an FCC single phase with an average grain size of 18 μm. Finally, the Co₁₀Fe₁₀Mn₃₅Ni₃₅Zn₁₀ HEA has a compressive yield strength of 302 MPa.

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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
• Fabrication, microstructure and mechanical property of a novel Nb-rich refractory high-entropy alloy strengthened by in-situ formation of dispersoids
  Byungchul Kang, Taeyeong Kong, Ahmad Raza, Ho Jin Ryu, Soon Hyung Hong
  International Journal of Refractory Metals and Hard Materials.2019; 81: 15.     CrossRef

We report a method for preparing rare earth oxides (Re_xO_y) from the recycling process for spent Ni-metal hydride (Ni-MH) batteries. This process first involves a leaching of spent Ni-MH powders with sulfuric acid at 90°C, resulting in rare earth precipitates (i.e., NaRE(SO₄)₂·H₂O, RE = La, Ce, Nd), which are converted into rare earth oxides via two different approaches: i) simple heat treatment in air, and ii) metathesis reaction with NaOH at 70°C. Not only the morphological features but also the crystallographic structures of all products are systematically investigated using field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD); their thermal behaviors are also analyzed. In particular, XRD results show that some of the rare earth precipitates are converted into oxide form (such as La₂O₃, Ce₂O₃, and Nd₂O₃) with heat treatment at 1200°C; however, secondary peaks are also observed. On the other hand, rare earth oxides, Re_xO_y can be successfully obtained after metathesis of rare earth precipitates, followed by heat treatment at 1000°C in air, along with a change of crystallographic structures, i.e., NaRE(SO₄)₂·H₂O → RE(OH)₃ →Re_xO_y.

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• Recycling of Rechargeable Batteries: Insights from a Bibliometrics‐Based Analysis of Emerging Publishing and Research Trends
  Jiao Lin, Xiaodong Zhang, Li Cai, Ersha Fan, Shumeng Wu, Su Ma, Feng Wu, Renjie Chen, Li Li
  Advanced Energy and Sustainability Research.2022;[Epub]     CrossRef
• Synthesis of Rare Earth Oxide from NaREE(SO4)2·H2O by Ion Substitution Reaction
  Dae-Weon Kim, Bo-Ram Kim, Hee-Lack Choi
  Journal of the Korean Society of Mineral and Energy Resources Engineers.2020; 57(5): 405.     CrossRef

The microstructure and mechanical characteristics of SUS630 specimens fabricated using the direct energy deposition (DED) process are investigated. In DED, several process parameters such as laser scan speed, chamber gas flow, powder carrier gas flow, and powder feed rate are kept fixed; the laser power is changed as 150 W, 180 W, and 210 W. As the laser power increases, the surface becomes smooth, the thickness uniformity improves, and the size and number of pores decreases. With the increase in laser power, the hardness deviation decreases and the average hardness increases. The microstructure of the material is columnar; pores are formed preferentially along the columnar interface. The lath-martensite phase governs the overall microstructure. The volumetric fraction of the retained austenite phase is measured to increase with the increase of laser input power.

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• Optimization of Additive Manufacturing of Precipitation Hardening Type STS630 by DED (Direct Energy Deposition) Process
  Yongjae Kwon, SeongSeon Shin, SangEun Joo, JongHoon Lee, JunHo Hwang, HyunDeok Kim
  Journal of Welding and Joining.2021; 39(6): 590.     CrossRef
• Effects of pre- and post-repair heat treatments on microstructure and tensile behaviors of 630 stainless steel repaired by metal additive manufacturing
  Do Sik Shim, Hyub Lee, Yong Son, Wook Jin Oh
  Journal of Materials Research and Technology.2021; 13: 980.     CrossRef
• Solution annealing and precipitation hardening effect on the mechanical properties of 630 stainless steel fabricated via laser melting deposition
  Wook Jin Oh, Yong Son, Seung Yeong Cho, Seung Weon Yang, Gwang Yong Shin, Do Sik Shim
  Materials Science and Engineering: A.2020; 794: 139999.     CrossRef
• Spheroidization of Pure-vanadium Powder using Radio Frequency Thermal Plasma Process
  Nana Kwabena Adomako, Seungmin Yang, Min Gyu Lee, N. S. Reddy, Jeoung-Han Kim
  Journal of Korean Powder Metallurgy Institute.2019; 26(4): 305.     CrossRef
• Joint Properties of Stainless Steel and Titanium Alloys Additive Manufactured on Medium Entropy Alloys
  Chan Woong Park, Nana Kwabena Adomako, Min Gyu Lee, Jeoung Han Kim
  Journal of Korean Powder Metallurgy Institute.2019; 26(4): 319.     CrossRef

F-containing TiO₂ nanopowders are synthesized using simple wet processes (precipitation-based and hydrothermal) from ammonium hexafluorotitanate (AHFT, (NH4)2TiF6) as a precursor to apply as a photocatalyst for the degradation of rhodamine B (RhB). The surface properties of the prepared samples are evaluated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The results confirm that the synthesized anatase TiO₂ has sphere-like shapes, with numerous small nanoparticles containing fluorine on the surface. The photocatalytic activity of F-containing TiO₂ compared with F-free TiO₂ is characterized by measuring the degradation of RhB using a xenon lamp. The photocatalytic degradation of F-containing TiO₂ exhibits improved photocatalytic activity, based on the positive effects of adsorbed F ions on the surface.

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.

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 study, we investigate the deformation behavior of Hf_44.5Cu₂₇Ni1_3.5Nb₅Al₁₀ metallic glass powder under repeated compressive strain during mechanical milling. High-density (11.0 g/cc) Hf-based metallic glass powders are prepared using a gas atomization process. The relationship between the mechanical alloying time and microstructural change under phase transformation is evaluated for crystallization of the amorphous phase. Planetary mechanical milling is performed for 0, 40, or 90 h at 100 rpm. The amorphous structure of the Hf-based metallic glass powders during mechanical milling is analyzed using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Microstructural analysis of the Hf-based metallic glass powder deformed using mechanical milling reveals a layered structure with vein patterns at the fracture surface, which is observed in the fracture of bulk metallic glasses. We also study the crystallization behavior and the phase and microstructure transformations under isothermal heat treatment of the Hf-based metallic glass.

We report the feasibility of TaC production via self-propagating high temperature synthesis, and the influence of the initial green compact density on the final composite particle size. Experiments are carried out from a minimum pressure of 0.3 MPa, the pressure at which the initial green body becomes self-standing, up to 3 MPa, the point at which no further combustion occurs. The green density of the pellets varies from 29.99% to 42.97%, as compared with the theoretical density. The increase in green density decreases the powder size of TaC, and the smallest particle size is observed with 1.5 MPa, at 10.36 μm. Phase analysis results confirm the presence of the TaC phase only. In the range of 0.3-0.5 MPa, traces of unreacted Ta and C residues are detected. However, results also show the presence of only C residue in the matrix within the pressure range of 0.6-3.0 MPa.

The coupling of two semiconducting materials is an efficient method to improve photocatalytic activity via the suppression of recombination of electron-hole pairs. In particular, the coupling between two different phases of TiO₂, i.e., anatase and rutile, is particularly attractive for photocatalytic activity improvement of rutile TiO₂ because these coupled TiO₂ powders can retain the benefits of TiO₂, one of the best photocatalysts. In this study, anatase TiO₂ nanoparticles are synthesized and coupled on the surface of rutile TiO₂ powders using a microemulsion method and heat treatment. Triton X-100, as a surfactant, is used to suppress the aggregation of anatase TiO₂ nanoparticles and disperse anatase TiO₂ nanoparticles uniformly on the surface of rutile TiO₂ powders. Rutile TiO₂ powders coupled with anatase TiO₂ nanoparticles are successfully prepared. Additionally, we compare the photocatalytic activity of these rutile-anatase coupled TiO₂ powders under ultraviolet (UV) light and demonstrate that the reason for the improvement of photocatalytic activity is microstructural.

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• Refractory Metal Oxide–Doped Titanate Nanotubes: Synthesis and Photocatalytic Activity under UV/Visible Light Range
  Min-Sang Kim, Hyun-Joo Choi, Tohru Sekino, Young-Do Kim, Se-Hoon Kim
  Catalysts.2021; 11(8): 987.     CrossRef
• Effect of Surfactant on the Dispersion Stability of Slurry for Semiconductor Silicon CMP
  Hye Won Yun, Doyeon Kim, Do Hyung Han, Dong Wan Kim, Woo-Byoung Kim
  Journal of Korean Powder Metallurgy Institute.2018; 25(5): 395.     CrossRef

Recent developments in the field of energy harvesting technology that convert ambient energy resources into electricity enable the use of self-powered energy systems in wearable and portable electronic devices without the need for additional external power sources. In particular, piezoelectric-effect-based flexible energy harvesters have drawn much attention because they can guarantee power generation from ubiquitous mechanical and vibrational movements. In response to demand for sustainable, permanent, and remote use of real-life personal electronics, many research groups have investigated flexible piezoelectric energy harvesters (f-PEHs) that employ nanoscaled piezoelectric materials such as nanowires, nanoparticles, nanofibers, and nanotubes. In those attempts, they have proven the feasibility of energy harvesting from tiny periodic mechanical deformations and energy utilization of f-PEH in commercial electronic devices. This review paper provides a brief overview of f-PEH devices based on piezoelectric nanomaterials and summarizes the development history, output performance, and applications.

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• Development and Characterization of Hafnium-Doped BaTiO3 Nanoparticle-Based Flexible Piezoelectric Devices
  HakSu Jang, Hyeon Jun Park, Gwang Hyeon Kim, Gyoung-Ja Lee, Jae-Hoon Ji, Donghun Lee, Young Hwa Jung, Min-Ku Lee, Changyeon Baek, Kwi-Il Park
  JOURNAL OF SENSOR SCIENCE AND TECHNOLOGY.2024; 33(1): 34.     CrossRef
• Enhanced Piezoelectric Performance of Composite Fibers Based on Lead-Free BCTZ Ceramics and P(VDF-TrFE) Piezopolymer for Self-Powered Wearable Sensors
  Sung Cheol Park, Chaeyoung Nam, Changyeon Baek, Min-Ku Lee, Gyoung-Ja Lee, Kwi-Il Park
  ACS Sustainable Chemistry & Engineering.2022; 10(43): 14370.     CrossRef
• A Comparison Study of Output Performance of Organic-Inorganic Piezoelectric Nanocomposite Made of Piezoelectric/Non-piezoelectric Polymers and BaTiO₃ Nanoparticles
  Dong Yeol Hyeon, Kwi-Il Park
  Journal of Korean Powder Metallurgy Institute.2019; 26(2): 119.     CrossRef
• Piezoelectric Flexible Energy Harvester Based on BaTiO3 Thin Film Enabled by Exfoliating the Mica Substrate
  Dong Yeol Hyeon, Kwi-Il Park
  Energy Technology.2019;[Epub]     CrossRef
• Piezoelectric Energy Harvesting from Two-Dimensional Boron Nitride Nanoflakes
  Gyoung-Ja Lee, Min-Ku Lee, Jin-Ju Park, Dong Yeol Hyeon, Chang Kyu Jeong, Kwi-Il Park
  ACS Applied Materials & Interfaces.2019; 11(41): 37920.     CrossRef