Papers by Author: Chung Hyo Lee

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Authors: Ji Soon Kim, S.-H. Jung, Young Do Kim, Chung Hyo Lee, Young Soon Kwon
Abstract: Mechanically-alloyed NiAl powder was sintered by Spark-Plasma Sintering (SPS) process. Densification behavior and mechanical property were determined. Above 97% relative density was obtained after sintering at 1150oC for 5min. Crystallite size determined by the Scherrer method was approximately 80 nm. TEM observation revealed a relative larger crystallite size. X-ray diffraction analysis showed that the sintered bodies were composed mainly of NiAl phase together with Ni3Al phase. Sintered NiAl body showed an average Vicker’s hardness of 555Hv, transverse-rupture strength of 1393MPa, 4-point-bending strength of 1100MPa, and fracture toughness of 19.9MPa m-1/2
Authors: Dong Choul Cho, Cheol Ho Lim, D.M. Lee, Seung Y. Shin, Chung Hyo Lee
Abstract: The n-type thermoelectric materials of Bi2Te2.7Se0.3 doped with SbI3 were prepared by spark plasma sintering technique. The powders were ball-milled in an argon and air atmosphere. Then, powders were reduced in H2 atmosphere. Effects of oxygen content on the thermoelectric properties of Bi2Te2.7Se0.3 compounds have been investigated. Seebeck coefficient, electrical resistivity and thermal conductivity of the sintered compound were measured at room temperature. It was found that the effect of atmosphere during the powder production was remarkable and thermoelectric properties of sintered compound were remarkably improved by H2 reduction of starting powder. The obtained maximum figure of merit was 2.4 x 10-3/K.
Authors: Seong Hee Lee, Chung Hyo Lee, Cha Yong Lim
Abstract: Two and six-layer stack accumulative roll bonding (ARB) processes were applied to commercial purity aluminum in order to investigate the effect of the stacking layer number on the mechanical properties. The initial thickness of the aluminum sheets for two and six-layer stack ARB was 1mm and 0.5mm, respectively. Two-layer stack ARB was performed by 50% reduction per cycle. For six-layer stack ARB, the six aluminum sheets were first stacked together and cold-roll-bonded by 50% reduction rolling, and then followed by four-pass rolling so that the final thickness was 0.5mm. The sheet was then cut to the six pieces of same length and the same procedure was repeated to the sheets. The tensile strength of the ARB processed specimens increases with the number of ARB cycles in both two and six layer stack ARB. The tensile strength is lower by the six-layer stack ARB than that by the two-layer stack ARB. The elongation slightly decreases with the number of the ARB cycles, regardless of the stacking layer number. TEM observation reveals that the grain size of the six-layer stack ARB is larger than that of the two-layer stack ARB. The effects of the number of the layers in stacking are explained by the redundant shear deformation.
Authors: Chung Hyo Lee
Abstract: We have applied mechanical alloying technique to produce magnetic nanocomposite material using a mixture of Fe2O3 and Ca powders at room temperature. An optimal ball milling and heat treatment conditions to obtain magnetic α-Fe/CaO composite with fine microstructure were investigated by X-ray diffraction, scanning electron microscope and vibrating sample magnetometer measurements. We have revealed that the magnetic α-Fe /CaO nanocomposite powders can be produced by solid state reduction during ball milling. It is found that α-Fe/CaO nanocomposite powders in which CaO is dispersed in α-Fe matrix with a grain size of 45 nm are obtained by mechanical alloying of Fe2O3 with Ca for 5 hours. The saturation magnetization of ball-milled powders increases with increasing milling time and reaches to a maximum value of 65 emu/g after 7 hours of MA. The average grain size of a-Fe in 5 hours MA powders estimated by diffraction line-width are gradually decreased with increasing milling time, and tend to reach at 45 nm. The magnetic hardening due to the reduction of the α-Fe grain size by MA is also observed.
Authors: Chung Hyo Lee, S.H. Lee, S.Y. Chun, Sang J. Lee, Joo Sun Kim
Abstract: The mechanochemical reaction of hematite with graphite by mechanical alloying (MA) has been investigated at room temperature. The solid state reduction of hematite to Fe3O4 and FeO has been observed after 120 hours of MA by a planetary ball mill. Saturation magnetization is gradually increased with milling time up to 80 h, and then deceased after 120 h of MA, indicating the transformation of Fe3O4 into nonmagnetic FeO through further reduction. Neither the solid state reduction of Fe2O3 by graphite nor a sizable grain refinement is observed in the MA process using a horizontal ball mill.
Authors: Chung Hyo Lee, S.Y. Chun, N. Wakiya, Kazuo Shinozaki, Nobuyasu Mizutani
Authors: Chung Hyo Lee, Young Sup Lee, Dong Choul Cho, Chang Hee Lee
Abstract: The process of Direct Bonding Copper (DBC) is performed by a spinel reaction between CuO and Al2O3. In order to develop DBC on alumina substrate with high bonding strength, alumina substrate was preformed as follows: Cu was sputter-deposited on alumina substrate. Sputter-Deposited Cu (SDC) on alumina substrate was oxidized at 673K for 30min in air atmosphere and then stabilized at 1273K for 30min in N2 gas atmosphere to improve bonding strtrength between preformed alumina substrate and SDC layer. Subsequently, the Cu-foil (300µm) was bonded on preformed-alumina substrate in N2 gas atmosphere at 1342~1345K. It was found that optimum condition of DBC on preformed-alumina substrate could be successfully obtained at 1345K for 30min. Consequently, bonding strength of DBC on alumina substrate was the high value of 80N/cm. Observation and analysis of microstructure for Cu sputtered DBC showed that reaction compounds such as CuAlO2 and CuAl2O4 approved to be formed in the vicinity of interface between Cu and alumina substrate.
Authors: Seong Hee Lee, Tetsuo Sakai, Chung Hyo Lee, Yong Ho Choa
Abstract: Nano-structured aluminum was fabricated by accumulative roll-bonding (ARB) process using different rolling methods. One is the ARB using conventional rolling (CR) in which the speed of two rolls (3.0m/min) was equal to each other. The other is the ARB using differential speed rolling (DSR) in which the speed of two rolls is different to each other. The roll peripheral speed of one roll was 2.0m/min and that of another roll was 3.6m/min. The roll speed ratio was kept at 1.8. The ARB was conducted up to 6 cycles at ambient temperature without lubrication. In both cases, the ultrafine grains were developed in the samples. The grains formed by the DSR-ARB were more equiaxed and finer than those produced by the CR-ARB. Tensile strength of the DSR-ARB processed sample was superior to that of the CR-ARB processed one. The elongation was not affected significantly by the number of ARB cycles in both cases. Texture analysis demonstrated that the shear strain, in the case of DSR-ARB, was introduced into the center of thickness. It was concluded that the DSR-ARB process was more effective for grain refinement and strengthening than the CR-ARB process.
Authors: Chung Hyo Lee, S.H. Lee, S.Y. Chun, Sang Jin Lee, Young Soon Kwon
Abstract: Nanocomposite formation of metal-metal oxide systems by mechanical alloying (MA) has been investigated at room temperature. The systems we chose are the Fe 2O 3-M(M=Al,Ti,Zn,Cu), where pure metals are used as reducing agent. It is found that nanocomposite powders in which Al 2O 3 and TiO 2 are dispersed in Fe matrix with nano-sized grains are obtained by mechanical alloying Fe 2O 3 with Al and Ti, respectively. However, the reduction of Fe 2O 3 with Cu by MA is not occurred. And the system of Fe 2O 3-Zn results in the formation of FeO plus ZnO after 120 h of milling. It is also shown that the magnetic evidence for the solid state reduction by mechanical alloying through changes in saturation magnetization and coercivity.
Authors: Chung Hyo Lee, Seong Hee Lee, Sang Jin Lee, Yong Ho Choa, Ji Soon Kim
Abstract: Nanocomposite formation of metal-metal oxide systems by mechanical alloying (MA) has been investigated at room temperature. The systems we chose are the Fe3O4-M (M=Al, Ti), where pure metals are used as a reducing agent. It is found that nanocomposite powders in which Al2O3 and TiO2 are dispersed in a α-Fe matrix with nano-sized grains are obtained by MA of Fe3O4 with Al and Ti for 25 and 75 hours, respectively. It is suggested that the shorter MA time for the nanocomposite formation in Fe3O4-Al is due to a large negative heat associated with the chemical reduction of magnetite by aluminum. X-ray diffraction results show that the average grain size of α-Fe in Fe-TiO2 nanocomposite powders is in the range of 30 nm. From magnetic measurement, we can also obtain indirect information about the details of the solid-state reduction process during MA.
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