Jai Sung Lee

Densification Behavior and Microstructural Development of Nano-Agglomerate Powder during Sintering

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.

The Optimization of Hydrogen Reduction Process for Mass Production of Fe-8wt%Ni Nanoalloy Powder

The present investigation has attempted to optimize hydrogen reduction process for the mass production of Fe-8wt%Ni nanoalloy powder from Fe2O3-NiO powder. In-situ hygrometry study was performed to monitor the reduction behavior in real time through measurement of water vapor outflowing rate. It was found that the reduction process can be optimized by taking into account the apparent influence of water vapor trap in the reactor on reduction kinetics which strongly depends on gas flow rate, reactor volume and reduction.

Hierarchical Structured Nanomaterial Fabricated by Nanopowder Process: Nanopowder Agglomerate Sintering

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.

Sintering Behavior of the Powder Injection Molded Nanocrystalline Fe-50wt%Ni

Sintering process of powder injection molded (PIMed) Fe-50wt%Ni nanoalloyed powder was investigated in association with microstructure development and residual impurity effect. Compared to conventional powder metallurgical (PM) processed Fe-Ni nanoalloy powder, the PIM compact showed a homogeneous and uniform densification behavior. This is owing to more homogeneous particle distribution in the PIM resulting from preparation of feedstock which was fabricated by mixing of nano powder with thermoplastic binder. Residual impurities originating from the binder material did not have any apparent influences on sintering behavior. Conclusively, Fe-50wt%Ni nanoalloy powder is effectively applicable to the PIM parts.

Diffusion Properties of Internal Interfaces in Bulk Nanocrystalline Materials: Radiotracer Investigation

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.

Fabrication of SiO2-Coated Magnetic Nanoparticles for Applications to Protein Separation and Purification

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.

A Study on the Optimization of Reduction-Diffusion Process for Synthesis of Sm2Fe17Nx Nanopowder

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.

Processing and Property of Net-Shaped Fe-Based Nanoparticulate Material

This paper overviews our recent investigations on the processing of net-shaped Fe-based nanoparticulate materials and their related material properties such as mechanical and corrosion properties. The key-process for fabricating fully densified net-shaped nanopowder by pressureless sintering is an optimal control of agglomerate size of nanopowder. Enhanced mechanical property of powder injection molded Fe-Ni nanopowder could be explained by grain refinement and uniformity of microstructure.

Strength of Nanostructured Materials Using a Phase Mixture Model

A phase mixture model (PMM) was considered in which materials are treated as a mixture of grain interior phase, grain boundary phase and pores (if the material is porous) for the elasticity and plasticity of nanostructured materials (NSMs). In order to investigate the effects of grain size and porosity on the elastic modulus, a self-consistent method in conjunction with PMM was employed. The calculated results are compared with the experimental measurements in the literature. The elastic modulus of NSMs decreases with a decrease of the grain size and the decrement is relatively large at grain sizes below about 10 nm. The effect of porosity, however, is substantially greater than the grain size effect. For the plasticity of NSMs, grain size effects were introduced both via the dislocation glide mechanism and through the diffusion mechanisms providing mass transfer via grain boundaries. A good agreement between the calculated deformation behavior and experiment was found. The quality of the above predictions with regard to strength, strain hardening, strain sensitivity and ductility behavior testify the adequacy of the model. It is concluded that the model can be used as a convenient tool for simulating the deformation behavior of NSMs.

Synthesis of Nano Metal Powder by Electrochemical Reduction of Iron Oxides

This work has attempted to find a new low temperature reduction process for fabrication of Cu nanopowder from fine CuO powder. For this purpose, we used electrochemical reduction method which is conducted in an electrolyte of NaCl aqueous solution at room temperature. It was found that ball-milled CuO powder (particle size ~100 nm and grain size ~40 nm) was completely reduced under the conditions of 20 V power, 0.5 mol NaCl solution and 2 h reaction time, producing Cu nanopowder (particle size ~80 nm and crystallite size ~25 nm). Simultaneously, we observed that sintering of nanopowders occurred during the reduction process, leading to agglomeration of nanopowder. Based upon the experimental results, the correlation between electrochemical reduction process and its related powder characteristics was discussed in terms of material transport.