Papers by Author: Chang Kyu Rhee

Abstract: Nano metal Ag nanoparticles were synthesized by pulse electron beam (e-beam) irradiation at room temperature and the atmospheric pressure. The staring materials were AgNO₃, toluene, ethanol and ethylene glycol. The ethanol and ethylene glycol plays a role of prohibiting agglomeration of metal ions. Energy dispersive X-ray spectrometer (EDX) was used to characterize the elements. Transmission electron microscopy (TEM) images were used to determine the shape and diameter. The time of e-beam irradiation affect to the particle size and aggregation. The average particle size was 10 nm and 30 nm for the dose time of 1 and 5 min, respectively. The shape of particles were changed from spherical to disk-like to coral-like, with increasing the irradiation time from 1 to 5 to 10 min.

Abstract: Metallic and ceramic foam catalyst supports and catalysts were manufactured. Comparative studies of the foam-structure catalyst supports and straight-channel catalysts supports were performed. Affect of the catalyst support pore structure upon the catalyst operational performance was analyzed.

Abstract: In this study, joining characteristics of dissimilar Ti and Cu metals have been investigated, when using both an Ag-based eutectic alloy as a filler and an Ag layer present on Ti base metal as a diffusion barrier. The observed microstructures were classified into three characteristic types, depending on the presence of a Ag layer at the Ti interface, e.g. first, the sample retaining thick continuous intermetallic layers, e.g. Ti2Cu, TiCu, Ti3Cu4, Ti2Cu3, and TiCu4 by a significant dissolution of the Ti atoms into the molten filler, in the absence of a Ag coating layer onto the Ti base metal, second, the sample with relatively thin Ti-Cu intermetallic layers by the reduced reaction of Ti with Cu due to a prominent decrease in the Ti dissolution, owing to the role of the Ag coating layer as a diffusion barrier, and finally, the sample without any brittle Ti-Cu intermetallics in the joint by a complete suppression of both the dissolution of the Ti atoms and its reaction with the Cu elements in the molten filler due to the presence of a Ag layer.

Abstract: The microstructure and mechanical properties of Ti joints brazed with a Zr41.2Ti13.8Ni10.0Cu12.5Be22.5 (at.%) amorphous filler were investigated. With a Zr-based amorphous filler, in this study, Ti joints with a homogeneous composition could be obtained by heating to well below the α-β transformation temperature for a short time, so that the undesirable effects of the high temperature heating are considerably diminished. The joints brazed at 790 °C for 10 min consisted of the coarse acicular structure rather than the fine Widmanstätten structure which generally deteriorates the ductility of the joints. The joints with the homogenous coarse acicular structure, i.e. without a residual liquid region, show almost the same mechanical properties as those required for base metals without heating. Although the residual liquid region in the joints deteriorates the ductility of the joints, this region could be successfully removed by a diminution in the quantity of the filler.

Abstract: In the present work, corrosion characteristics of ceramic dispersion strengthened (CDS) high-Cr stainless steel (SS) has been investigated. For the preparation of CDS SS with uniformly distributed reinforcements, TiC particles were mechanically activated with Cu powder by using high-energy ball mill machine, and then Cu-coated TiC particles were mixed with molten SS. Potentiodynamic polarization curves were experimentally measured on unmodified SS and CDS SS in 0.05 M NaCl solution. The values of corrosion potential Ecorr, corrosion current io, pitting potential Epit and polarization resistance Rp were quantitatively determined from the measured potentiodynamic polarization curves. From the result, it is concluded that CDS SS exhibits higher corrosion resistivity than unmodified SS.

Abstract: NiFe2O4 nano particles were synthesized by a simple levitational gas condensation (LGC) using Ni and Fe metal powder as starting materials. The TEM image reveals spherical particles with the range of the particle size from 8 to 22 nm. The Mössbauer spectrum revealed the presence of superparamagnetic phase with abnormal ionic states of Fe2+ and Fe3+

Abstract: In this study, we have successfully developed the technology to grow the nanotube array thin films from dip-coated titania using hydrothermal method. The nanotube array thin film strongly adhered onto the substrate, was formed in short time reaction at 140°C. Even a self-supporting films, consisted of vertically aligned nanotube in large part, were formed after long time reactions at 140°C with 10 μm in thickness. The most probable formation mechanism of TiO2-based nanotube array thin films is discussed.

Abstract: In order to select the most suitable solvent for the stable dispersion of Alloy 625 nanoparticles, we investigated the effects of several organic solvents on the dispersion stability. After preparing suspensions with a dispersant, we characterized the dispersion stability of the suspensions by means of visual inspection and Turbiscan transmission profiles. The dispersion stability of the solvents increased in the following order: ethyl acetate, xylene, toluene, 2-propanol, and ethanol. The polarity of the solvent affected the dispersion stability. The Alloy 625 nanoparticles were optimally dispersed in ethanol with the dispersant, Hypermer KD-2, which was stabilized by the steric effect of the dispersant.

Abstract: The n-type (95%Bi2Te3- 5%Bi2Se3) compound was newly fabricated by gas atomization and hot extrusion, which is considered to be a mass production technique of this alloy. The effect of powder size on thermoelectric properties of 0.04% SbI3 doped 95%Bi2Te3- 5%Bi2Se3 alloy were investigated. Seebeck coefficient (α) and Electrical resistivity (ρ) increased with increasing powder size due to the decrease in carrier concentration by oxygen content. With increasing powder size, the compressive strength of 95%Bi2Te3-5%Bi2Se3 alloy was increased due to the relative high density. The compound with ~300 μm size shows the highest power factor among the four different powder sizes. The rapidly solidified and hot extruded compound using 200~300 μm powder size shows the highest compressive strength.

Abstract: In this study, the colloidal stability and sedimentation behavior of crystalline TiO2 particles (∼300nm) in various organic solvents have been investigated by means of a backscattered light flux profile (Turbiscan) and a zeta potential measurement. The backscattered light flux profiles revealed that the TiO2 nanoparticles were readily sedimented in water, methyl alcohol, and ethyl alcohol due to a flocculation-induced particle growth, while a particle coalescence and a sedimentation of the TiO2 nanoparticles were hardly observed in isopropyl alcohol. The measured ζ potentials verified the differences of the colloidal stabilities of the TiO2 particles in the organic solvents, showing a good correlation with the migration velocity.