Papers by Author: Jung Goo Lee

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Authors: J.C. Yun, S.S. Jung, Jung Goo Lee, C.J. Choi, Jai Sung Lee
Abstract: The present investigation attempted to optimize the R-D (reduction-diffusion) process for fabricating Sm2Fe17 nanoscale powder from ball-milled powders of samarium oxide and iron oxide using a solid reducing agent of calcium hydrides (CaH2). It was found that the target alloy phase of Sm2Fe17 can be produced by controlling the gas atmosphere in the process of powder preparation to R-D reaction. Powder handling of CaH2 in a protective atmosphere is essential to avoid the formation of Ca(OH)2 which suppresses calcium formation. A switching gas atmosphere of H2 to Ar-H2 during the R-D process at 350oC resulted in a reduction of Fe2O3 and alloying of Sm-Fe, consequently forming nanocrystalline Sm2Fe17.
Authors: C.J. Choi, J.H. Yu, Jung Goo Lee
Abstract: The formation of Fe nanopowders by Plasma Arc discharge process and sintering behavior of Fe nanopowder has been investigated. The effect of processing parameters on the size and microstructure and their pressureless sintering behaviors have been analyzed. Also the addition effect of micron powder to nanopowder on the compaction and sintering was studied. The prepared Fe nanpowder showed nearly spherical shape with the average size of 100 nm. The surface oxide layer of nanopowder played an important role on the initial densification of the green compact. The hardness and sintered density of nano/micron bimodal powder was similar to those of pure nanopowder.
Authors: Ryusuke Nakamura, Daisuke Tokozakura, Jung Goo Lee, Hirotaro Mori, Hideo Nakajima
Abstract: Oxidation behavior of Cu nanoparticles in the formation process of hollow Cu2O spheres was investigated by TEM. The thickness of Cu2O layers on Cu nanoparticles oxidized at 323 K in air was measured as a function of oxidation time. At the initial stage of oxidation until the oxide film with 2.5 nm in thickness is formed, the thickness of oxide films on Cu nanoparticles with the diameter of 10, 20 and 35 nm shows a nearly equal value regardless of diameter of Cu. After the formation of 2.5 nm layer, however, the growth rate of the oxide films on smaller nanoparticles becomes slower than that on larger nanoparticles. This result suggests that the voids formed at the Cu/Cu2O interface prevent Cu atoms from diffusing outward across the interface because the volume ratio of voids to inner Cu in smaller nanoparticles is much larger than that in larger nanoparticles.
Authors: Pu Li, Jung Goo Lee, Xing Long Dong, Chul Jin Choi
Abstract: MnAlC nanoparticles were synthesized by plasma arc-discharge method. Heat treatment of these nanoparticles at temperature from 400 to 600 °C resulted in the formation of the ferromagnetic τ-phase. Most of the nanoparticles had nearly spherical shape, smooth surface and core/shell structure. The shells of the nanoparticles mainly consisted of Al2O3 and a small amount of Mn oxides. Though the saturation magnetization of MnAlC nanoparticles was lower than that of bulk samples due to the effect of nonmagnetic phases (β, γ2 and Mn3AlC) and the oxide shell, the highest coercivity, up to 5.6 kOe in the MnAlC magnets, was achieved when annealed at 500 °C for 30 min.
Authors: Hai Xia Wang, Ping Zhan Si, Wei Jiang, Jin Jun Liu, Jung Goo Lee, C.J. Choi, Hong Liang Ge
Abstract: Novel nano-composite powders composed of hard-magnetic Mn54Al46 and soft-magnetic α-Fe were prepared by high-energy ball milling. The effect of α-Fe and preparation conditions on the structure and magnetic properties of the composite powders has been investigated. The ε-MnAl transforms to γ-MnAl, τ-MnAl, and β-phase under ball milling and annealing. The saturation magnetization and coercivity of the two-phase samples decrease with increasing temperature for the τ-phase decomposes at elevated temperatures. With increasing iron content, the coercivity decrease first and then increase up to 0.33 T when the Fe content is 10 wt%. Further addition of the magnetically soft iron phase would result in a decrease of the coercivity.
Authors: Wei Jiang, Ping Zhan Si, Hai Xia Wang, Qiong Wu, Jin Jun Liu, Jung Goo Lee, C.J. Choi, Hong Liang Ge
Abstract: We report on a novel method for preparation of high purity Sm2Fe17 phase by removing the liquid impurity phase from the Sm-rich precursor at high temperatures. With decreasing Sm addition, the percentage of liquid phase left as impurity is decreasing. Nitrogen absorption to the Sm2Fe17 powders results in a complete formation of Sm2Fe17N3, which shows a weak disproportionation reaction at 773 K. The magnetic performance of the Sm2Fe17N3 from precursors with different Sm content is improving with decreasing Sm content.
Authors: Jong Bin Ahn, Dong Soo Kim, Young Kook Kim, Jung Goo Lee
Abstract: Novel simple route to prepare AlN nanoparticles was proposed in this study. Aluminum nitride powder was synthesized by microwave-assisted urea route. Aluminum chloride was dissolved in ethanol and urea was added to this solution considering molar concentration with Aluminum chloride. The solution was heated under microwave (MW) irradiation from 60°C to 80°C for 5-10 minutes. And then heat treatment was performed in N2 atmosphere with various temperatures and time to obtain AlN particles. While microwave was irradiated to the mixed solution with Aluminum chloride, urea and ethanol, the solvent was eliminated and polymerization reaction was accelerated to formmetal-organic complex. AlN particles were successfully synthesized after heat treatment above 1600°C under N2 or N2-H2 mixed gas atmosphere in the molar ratio (urea/Aluminum chloride) of 6.
Authors: Jung Goo Lee, J.C. Yun, Jai Sung Lee, C.J. Choi
Abstract: Calciothermic reduction-diffusion (CRD) method was employed to prepare the Sm2Fe17 powder. By using CRD method, single-phase Sm2Fe17 powders were successfully made and no α-Fe phase detected. And Subsequent suitable nitrogenation treatment after CRD process enabled us to obtain Sm2Fe17Nx magnetic powders. However, the magnetic performance of the powders was below expectation due to their large particle size. Further study on effective milling process is needed.
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