Papers by Author: Chang Kyu Rhee

Abstract: The stoichiometric Ti50Ni50 alloy nanopowders were synthesized by levitational gas condensation (LGC) using micron powder feeding system and their particulate properties were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Brunauer- -Emmett-Teller (BET) method. The starting Ti and Ni micron powders ∼ 150μm were incorporated into the micron powder feeding system. The ingot type of Ti-Ni alloy was used as a seed material for levitation and evaporation reactions. The collected powders were finally passivated by oxidation. The x-ray diffraction experiments have shown that the synthesized powders were completely alloyed with 50Ti and 50Ni in at.% and comprised of two different cubic and monoclinic crystalline phases. The TEM results showed that the produced powders were a very fine and uniform with the spherical particle size of 18 to 32 nm. The typical thickness of passivated oxide layer on the particle surface was about 2 to 3 nm. The specific surface area of the Ti-Ni alloy nanopowders was 54.8 m2/g based on a BET method.

Abstract: The microstructure and mechanical properties of Al-21 wt% Si components fabricated by plasma spray forming are analyzed. The microstructure of the plasma spry formed component showed a homogeneous distribution of fine Si particles embedded in the Al matrix. The grain size of α-Al varied between 200 to 500 nm and the size of the Si particles was about 50 to 100 nm in the plasma spray formed component. The room temperature tensile strength of the plasma spray formed component was 215 MPa with 0.5 % elongation, while for cast material, it was 130 MPa. Despite the porosities, the ultra-fine microstructure and homogeneous distribution of Si particles embedded in matrix are the foundation of the increased strength of the plasma spry formed component.

Abstract: In this study the effects of harder ultrafine Al2O3 particles on the mechanical milling of ductile Cu flakes (200mesh, 70μm) have been investigated. The small hard Al2O3 particle in the mixture acted as an effective milling agent not only by inducing a drastic change of the Cu morphology from flattened flakes to equiaxed crystals but also by reducing the milling time required for a uniform dispersion of Al2O3 in the Cu matrix. This was more pronounced as the Al2O3 concentration increased. A critical concentration of the reinforcing Al2O3 required for a shape change was observed at the range of 8 to 9wt. %. All the hard Al2O3 particles were uniformly embedded in the ductile Cu matrix regardless of the concentration of Al2O3. By increasing the milling time, the weldment and fragmentation of the Cu flakes became significant and a uniformly distributed equiaxed composite structure (13wt.% Al2O3) was obtained at above 70 min..

Abstract: Highly dense Y2O3 ceramics have been fabricated by a magnetic pulsed compaction (MPC) which is capable of reaching a sufficiently high pressure (~1GPa) in a very short duration (a few microseconds), and a subsequent pressureless sintering at 1600°C. The Y2O3 green bodies with a relative density of about 68% were achieved by the application of the MPC process due to the effect of an enhanced rearrangement and a high speed movement of the particles, without the help of ceramic binder. Those compacts showed densities greater than 95%, which is very close to the theoretical density, after the subsequent pressureless sintering process at 1600 oC. The shrinkage rates of the diameter for the samples compacted by the MPC process were markedly reduced, when compared to those for the ones by the conventional compaction (CC) process.

Abstract: The electrophoretic deposition process of Ni nano-particles in organic suspension was employed for self-repairing of heat exchanger tubes. For this purpose, Ni nano-particles prepared by levitation-gas condensation were dispersed into the solution of ethanol with the addition of dispersant. The pitted Ni alloy specimen was prepared by applying a potential of 0.9 V (vs. Ag/AgCl) in aqueous 0.1 M NaCl solution. For electrophoretic deposition of Ni nano-particles on the specimen, a constant electric field of 100 V cm-1 was applied to the specimen for 180 s in Nidispersed solution. It was found that as the electrophoretic deposition time increased, the size of the pit remarkably decreased due to the agglomeration of Ni nano-particles at the pit with a higher current value rather than the outer surfaces of the specimen with a lower current value. Moreover, the current density increased with electrophoretic deposition time and reached a constant value. From the above, it is concluded that as the electrophoretic deposition proceeds, the pit becomes smaller in size, and hence the nano-particles more extensively aggregate at the pit by lyosphere distortion.

Abstract: By means of magnetic pulsed compaction and sintering of weakly aggregated alumina based nanopowders the jet forming nozzle samples for hydroabrasive cutting were fabricated. The ceramics were obtained from pure alumina, as well as from alumina, doped by TiO2, MgO and AlMg. The ceramic properties of the channel surface and the nozzle volume were investigated. It was shown that the samples sintered from AlMg doped Al2O3 powder had the best desired mechanical properties and structural characteristics: relative density ~0.97, channel microhardness – 18-20 GPa, channel surface roughness ~0.7 /m, average crystallite size ~1 /m.

Abstract: Fe-doped TiO2 nanocrystalline powders were prepared by mechanical alloying (MA) by varying Fe contents up to 8.0 wt.%. The TEM analyses were carried out to clarify morphologies and position of Fe within the mechanically alloyed powders. The Fe-doped powder consisted of spherical particles, and the average grain size was less than 10 nm. For the Fe-doped TiO2, the color of the powders changed from white to bright yellow with increasing concentration of Fe. The UVvis absorption showed that the UV absorption for the Fe-doped powder shifted to a longer wavelength (red shift) and the photo-efficiency was enhanced. The absorption threshold depends on the concentration of nano-sized Fe dopant. Mössbauer spectrum for 4 wt.% Fe showed the ferromagnetic phase (sextet) and paramagnetic phase (doublet). However, the only paramagnetic phase (doublet) was seen for 8 wt.% Fe. As the Fe concentration increased up to 4 wt.%, the UV-vis absorption and the magnetization were increased. The beneficial effect of Fe doping for photocatalysis and ferromagnetism was observed at the critical dopant concentration of 4 wt.%. Based on the UV absorption and magnetization, the dopant level was localized to the valence band of TiO2.

Abstract: Aluminum hydroxides were synthesized by a simple electrolytic reaction of aluminum plates. The aluminum monohydroxide, boehmite(AlO(OH)), was predominantly formed by the application of an electrical potential above 30V, while the mixture of the bayerite(Al(OH)3) and boehmite(AlO(OH)) phases were formed below 20V. The boehmite has a clear fibrous structure which is controlled on a nanometer scale. On the contrary, the bayerite consists of the typical hourglass or semi-hourglass shaped coarse crystals as a result of an aggregation of the various crystals stacked together. The specific surface area of the boehmite nanofiber was remarkably high, reaching about 300m2/g.