Papers by Author: Jai Sung 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: Kee Bong Yoon, Jai Sung Lee
Authors: H.W. Nam, S.W. Jung, Jai Sung Lee, Kyung Seop Han
Authors: Yun Sung Kang, B.H. Cha, H.G. Kang, Jai Sung Lee
Abstract: Densification behavior of nano-agglomerate powder during pressureless sintering of Fe-Ni nanopowder was investigated in terms of diffusion kinetics and microstructural development. To understand the role of agglomerate boundary for sintering process, densification kinetics of Fe-Ni nano-agglomerate powder with different agglomerate size was investigated. It was found that activation energy for densification process was lower in the small-sized agglomerate powder. The increase in the volume fraction of inter-agglomerate boundary acting as high diffusion path might be responsible for the enhanced diffusion process.
Authors: Hyoung Seop Kim, Min Hong Seo, Won Sun Ryu, Seung Chae Yoon, Hong Rho Lee, Jai Sung Lee, Chang Kyu Rhee
Authors: Bermha Cha, Jin Man Jang, Won Sik Lee, Jung Sik Seo, Se Hyun Ko, Seong Ho Son, Woo Kyung You, Jai Sung Lee
Abstract: Powder Injection molding (PIM) is a cost-effective process for the fabrication of complex shaped parts, and has a great potential in many applications. In this work, an improved wax-based binder was developed for the powder injection molding of TiH2 powder fabricated by recycling of Ti chip. Fine TiH2 powders of about 350 nm in particle size were produced by attrition milling of Ti chip in less than five minutes, resulting from simultaneous self-propagating High temperature Synthesis (SHS) and fracturing. TiH2 feedstock, a mixture of binder and powders, was fabricated with critical powder loading of 68 vol.%. The rheological characteristics of the feedstock were investigated for subsequent processing step. Viscosity of the feedstock showed pseudo-plastic flow behavior and to optimize injection molding parameter, in-mold rheology curve was generated. The results indicated that the recycled TiH2 feedstock can be used for the fabrication of the complex shaped parts with good shape.
Authors: Sergiy V. Divinski, Jai Sung Lee, Christian Herzig
Abstract: The radiotracer technique was applied to measure self- (Fe, Ni) and solute- (Ag) grain boundary diffusion in nanocrystalline Fe-40wt.%Ni alloy. The nanocrystalline material was prepared by pressureless sintering of the nanoalloy powders. The nano-sized crystallites were found to be clustered in micrometer-large agglomerates. Two types of internal interfaces with fundamentally different properties exist in the nanomaterial: the grain boundaries between the nanocrystallites and the interfaces between the agglomerates. A complete and consistent model of the diffusion processes in such material is elaborated. Whereas the nanocrystalline boundaries reveal diffusivities, which are similar to those in coarse-grained material, diffusion along interagglomerate interfaces occurs faster by orders of magnitude. This behavior is explained by a nonrelaxed structure of the inter-agglomerate interfaces.
Authors: S.H. Yun, C.W. Lee, Jai Sung Lee, C.W. Seo, E.K. Lee
Abstract: Coating of -Fe2O3 nanoparticles with SiO2 layer by wet chemical synthesis and its applications to protein separation and purification were investigated. The average particle size of -Fe2O3 core was 20 nm and SiO2 layer thickness was 5 nm. The band of OH- radicals on SiO2 layer was detected between 3600 and 3200 cm-1 in wave numbers, which showed that the surface property of coated nanoparticles was similar to that of conventional fumed silica. Finally, the feasibility of -Fe2O3/SiO2 nanoparticles for magnetic separation media in various bio processes was discussed in terms of structural and functional properties such as pore structure and magnetic properties.
Authors: Sergiy V. Divinski, Jai Sung Lee, Christian Herzig
Authors: Jai Sung Lee, W. K. You, B. H. Cha
Abstract: The key concept of nanopowder agglomerate sintering (NAS) process is based on the optimization of structure design and full density processing of nanopowder into nanostructured micro-components. The kinetics of NAS process is characteristic of being controlled by material transport through hierarchical interface structures of nanopowder agglomerates. Through optimal design of those hierarchical interfaces such as nano grain boundary and agglomerate boundary, thus, full density nanopowder materials can be fabricated by pressureless sintering. In this paper we overview recent studies on the role of hierarchical interfaces for processing of full density nanopowder materials.
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