Papers by Author: Chul Jin Choi

Abstract: The carbon-coated Fe(C), Co(C) and Ni(C) nanocapsules were prepared by a modified arc-discharge method in methane atmosphere. The nanocapsule powder, 50 % by weight, was mixed uniformly with paraffin wax to form measurement samples. Their electromagnetic (EM) wave reflection loss of the composite samples was calculated using the relative complex permeability and permittivity measured in microwave frequency range of 2-18 GHz. It is indicated that Fe(C), Co(C) and Ni(C) nanocapsules show the broadband characteristics with strong absorption from 2 to 18 GHz for a coating thickness of about 2-3 mm. The excellent EM wave absorption properties are mainly attributed to the proper electromagnetic match in microstructure, strong natural resonance as well as multi-polarization mechanisms, etc. As an inspiration, we found this kind of nanocapsules with a dielectric shell and a ferromagnetic core are very promising for new EM wave absorption materials.

Abstract: WC nanosized powders are synthesized by the plasma arc discharge process and annealing under an inert (Ar) atmosphere. The high temperature used during discharging at local region causes the work-piece and electrode to melt and evaporate. The melted tungsten on the arc discharge electrode and carbon arising from decomposition CH4 gas forms WC1-x nanosized powders. The WC1-x phase is encapsulated in a amorphous carbon shell. The WC1-x nanosized particles are annealed at 1200~1400°C under an Ar atmosphere. The WC1-x nanosized powder is transformed to the W2C phase, and then it becomes WC above at 1400°C.

Abstract: The nano-sized Fe powders were prepared by plasma arc discharge (PAD) process using pure Fe rod. The microstructure of the prepared nanopowders was evaluated and the effect of hydrogen gas in the chamber atmosphere was investigated. In addition, the sintering behavior of nanosized Fe powders was analyzed and compared with those of conventional micron powders. The prepared Fe nanopowders had nearly spherical shapes and consisted of metallic core and oxide shell structures. In PAD nanopowder sintering, the higher volume shrinkage at low sintering temperature was observed due to the reduction of surface oxide. The PAD nanopowders showed 6 times higher densification rate and more significant isotropic shrinkage behavior than those of micron sized Fe powders.

Abstract: Micro powder metal injection molding has received attention as a manufacturing technology for microparts. Small powder size is very useful in achieving detailed structures. STS nanopowders with an average diameter of 100 nm and STS micropowders with an average diameter of 5 micron were utilized to produce feedstock. The mixing behavior of the feedstock was indicated that the nanoparticle feedstock produce highest mixing torque at various powder_loading than the micropowder feedstock. Ares rheometer was utilized to examine visco-elatic flow behavior. The nanoparticles feedstocks showed that elastic properties are dominant in flow behavior and high viscosity. Whereas the micropowders feedstocks, viscous properties are dominant in flow behavior and less viscosity.

Abstract: Fe/SiO2 nanocomposite powders were synthesized by the chemical vapor condensation(CVC) process. Phase and microstructures of the as-prepared powders were investigated with CVC experimental parameters by XRD, TEM and VSM. The analysis of XRD patterns revealed that the Fe/SiO2 powders could be produced above 700
. With increasing reaction temperature, the XRD peaks became clearer. The size of powders was about 50nm at 1100oC. The Fe/SiO2 powders showed an intricate long-stand structure attributed to their magnetic characteristics. TEM results revealed that the Fe powders were covered by SiO2 layer fully or partially depending on the experimental condition. The saturation magnetization and the coercive force of the as-sytheised powders were investigated with the decomposition temperature.

Abstract: Nanosized Fe based powders have been synthesized by both chemical vapor condensation (CVC) and plasma arc discharge (PAD) processes. Fe, Fe-C and Fe-N nanopowders were successfully synthesized. The influence of experimental parameters on microstructures and phase composition of nanopowders was investigated. The prepared powders were nearly spherical in shape and core-shell type in structure. Various kinds of Fe-C and Fe-N composite nanopowders could be synthesized by controlling the carrier gas and precursor decomposition temperature, etc.

Abstract: Effect of washing step in R-D process on the structure and magnetic properties of Nd-Fe-B powder fabricated by the process using a spray-dried precursor was investigated. Although the powder washed in water contained much more residual CaO than that washed in dilute acetic acid, magnetic properties of the former were much better than those of the latter due to less removal of Nd-rich phase. In the powder washed in water Nd2Fe14B particles were enclosed with thin layer of Nd-rich phase, and each particle consisted of one or more subgrains of the size 50 ~ 100 nm or more. Milling the powder before water washing was effective to remove more CaO, thereby increased the remanence of the powder.