Journal of Powder Materials

Formation of Uniform SnO₂ Coating Layer on Carbon Nanofiber by Pretreatment in Atomic Layer Deposition: Dong Ha Kim, Doh-Hyung Riu, Byung Joon Choi; J Korean Powder Metall Inst. 2018;25(1):43-47. Published online February 1, 2018; DOI: https://doi.org/10.4150/KPMI.2018.25.1.43

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Carbon nanofibers (CNF) are widely used as active agents for electrodes in Li-ion secondary battery cells, supercapacitors, and fuel cells. Nanoscale coatings on CNF electrodes can increase the output and lifespan of battery devices. Atomic layer deposition (ALD) can control the coating thickness at the nanoscale regardless of the shape, suitable for coating CNFs. However, because the CNF surface comprises stable C–C bonds, initiating homogeneous nuclear formation is difficult because of the lack of initial nucleation sites. This study introduces uniform nucleation site formation on CNF surfaces to promote a uniform SnO₂ layer. We pretreat the CNF surface by introducing H₂O or Al₂O₃ (trimethylaluminum + H₂O) before the SnO₂ ALD process to form active sites on the CNF surface. Transmission electron microscopy and energy-dispersive spectroscopy both identify the SnO₂ layer morphology on the CNF. The Al₂O₃-pretreated sample shows a uniform SnO₂ layer, while island-type SnOx layers grow sparsely on the H₂Opretreated or untreated CNF.

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Atomic layer deposition of ZnO layers on Bi2Te3 powders: Comparison of gas fluidization and rotary reactors
Myeong Jun Jung, Myeongjun Ji, Jeong Hwan Han, Young-In Lee, Sung-Tag Oh, Min Hwan Lee, Byung Joon Choi
Ceramics International.2022; 48(24): 36773. CrossRef
Effects of SnO2 layer coated on carbon nanofiber for the methanol oxidation reaction
Dong Ha Kim, Dong-Yo Shin, Young-Geun Lee, Geon-Hyoung An, Jeong Hwan Han, Hyo-Jin Ahn, Byung Joon Choi
Ceramics International.2018; 44(16): 19554. CrossRef

Fabrication of Al₂O₃ Dispersed Porous Cu by Freeze Drying of CuO-Al₂O₃/Camphene Slurry: Hyunji Kang, Doh-Hyung Riu, Sung-Tag Oh; J Korean Powder Metall Inst. 2018;25(1):25-29. Published online February 1, 2018; DOI: https://doi.org/10.4150/KPMI.2018.25.1.25

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Abstract PDF: 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.

Direct Bonding of Cu/AlN using Cu-Cu₂O Eutectic Liquid: Junsung Hong, Jung-Hoon Lee, You-Na Oh, Kwang-Jun Cho, Doh-Hyung Riu, Sung-Tag Oh, Chang-Yong Hyun; J Korean Powder Metall Inst. 2013;20(2):114-119.; DOI: https://doi.org/10.4150/KPMI.2013.20.2.114

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In the DBC (direct bonding of copper) process the oxygen partial pressure surrounding the AlN/Cu bonding pairs has been controlled by Ar gas mixed with oxygen. However, the direct bonding of Cu with sound interface and good adhesion strength is complicated process due to the difficulty in the exact control of oxygen partial pressure by using Ar gas. In this study, we have utilized the in-situ equilibrium established during the reaction of 2CuOrightarrowCu_2O + 1/2 O_2 by placing powder bed of CuO or Cu_2O around the Cu/AlN bonding pair at 1065sim1085°C. The adhesion strength was relatively better in case of using CuO powder than when Cu_2O powder was used. Microstructural analysis by optical microscopy and XRD revealed that the interface of bonding pair was composed of Cu_2O, Cu and small amount of CuO phase. Thus, it is explained that the good adhesion between Cu and AlN is attributed to the wetting of eutectic liquid formed by reaction of Cu and Cu_2O.

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Low-Temperature Sintering Behavior of Aluminum Nitride Ceramics with Added Copper Oxide or Copper
Jin-Geun Hwang, Kyung-Sik Oh, Tai-Joo Chung, Tae-Heui Kim, Yeong-Kyeun Paek
Journal of the Korean Ceramic Society.2019; 56(1): 104. CrossRef

Preparation of Magnetite Nanoparticles by Two Step Reaction: Dae-Kyu Shin, Doh-Hyung Riu; J Korean Powder Metall Inst. 2008;15(2):148-155.; DOI: https://doi.org/10.4150/KPMI.2008.15.2.148

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Abstract PDF: Nano magnetite particles have been prepared by two step reaction consisting of urea hydrolysis and ammonia addition at certain ranges of pH. Three different concentrations of aqueous solution of ferric (Fe3+) and ferrous (Fe2+) chloride (0.3 M-0.6 M, and 0.9 M) were mixed with 4 M urea solution and heated to induce the urea hydrolysis. Upon reaching at a certain pre-determined pH (around 4.7), 1 M ammonia solution were poured into the heated reaction vessels. In order to understand the relationship between the concentration of the starting solution and the final size of magnetite, in-situ pH measurements and quenching experiments were simultaneous conducted. The changes in the concentration of starting solution resulted in the difference of the threshold time for pH uprise, from I hour to 3 hours, during which the akaganeite (beta-FeOOH) particles nucleated and grew. Through the quenching experiment, it was confirmed that controlling the size of beta-FeOOH and the attaining a proper driving force for the reaction of beta-FeOOH and Fe2+ ion to give Fe_3O_4 are important process variables for the synthesis of uniform magnetite nanoparticles.

Titanate Nanotube Formation and Nanostructure Development from the Reaction of TiO₂ Nanopowder and Alkalihydroxide: Eun-Ju Jin, Doh-Hyung Riu, Seung-Hun Huh, Chang-Yeoul Kim, Hae-Jin Hwang; J Korean Powder Metall Inst. 2008;15(2):125-135.; DOI: https://doi.org/10.4150/KPMI.2008.15.2.125

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[ TiO_2 ] nanotubes for photocatalytic application have been synthesized by hydrothermal method. TiO_2 nanotubes are formed by washing process after reaction in alkalic solution. Nanotubes with different morphology have been fabricated by changing NaOH concentration, temperature and time. TiO_2 nanoparticles were treated inside NaOH aqueous solution in a Teflon vessel at 110°C for 20 h, after which they were washed with HCl aqueous solution and deionized water. Nanotube with the most perfect morphology was formed from 0.1 N HCl washing treatment. TiO_2 nanotube was also obtained when the precursor was washed with other washing solutions such as NH_4OH, NaCl, K_2SO_4, and Na_2SO_3. Therefore, it was suggested that Na+ ion combined inside the precursor compound slowly comes out from the structure, leaving nanosheet morphology of TiO_2 compounds, which in turn become the nanotube in the presence of hydroxyl ion. To stabilize the sheet morphology, the different type of washing treatment solution might be considered such as amine class compounds.

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Coating Property of Hybrid Structured Photo-Electrode to Increase Dye-Sensitized Solar Cells Efficiency
Min-Hee Kim, Hyung-Woo Lee, Young-Keun Jeong
Journal of Korean Powder Metallurgy Institute.2010; 17(6): 449. CrossRef

Microstructural Wear Mechanism of Al_2O_3-5 vol% SiC nanocomposite and Si_3N_4Ceramics: Doh-Hyung Riu, Yoon-Ho Kim, Soo-Wohn Lee, Niihara Koichi; J Korean Powder Metall Inst. 2001;8(3):179-185.

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Abstract PDF: Through the observation of wear scar of two ceramic materials, microstructural wear mechanisms was investigated. As for the Al_2O_3-5 vol% SiC nanocomposite, the grain boundary fracture was suppressed by the presence of SiC nano-particles. The intragranular SiC particles have inhibited the extension of plastic deformation through the whole grain. Part of plastic deformation was accommodated around SiC particles, which made a cavity at the interface between SiC and matrix alumina. On the other hand, gas-pressure sintered silicon nitride showed extensive grain boundary fracture due to the thermal fatigue. The lamination of wear scar was initiated by the dissolution of grain boundary phase. These two extreme cases showed the importance of microstructures in wear behavior.

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