Papers by Author: Sung Churl Choi

Abstract: Cordierite-bonded silicon carbide porous ceramics were prepared by a reactive process, in which kaolin, talc, and alumina powders were mixed with silicon carbide powders and graphite powder was used as a pore former. The mixture was heated in air so that graphite was burned out and silicon carbide particles were bonded by reaction-derived cordierite. Open porosity and strength of porous ceramics were strongly dependent on the volume fraction of graphite and cordierite. A three-point bending strength of 13.9 MPa was achieved at a open porosity of 38.5 %, exhibiting coefficient of thermal expansion of 7.28 × 10-6/oC at the range of 25 – 800oC. Increase of cordierite volume in SiC matrix indicated mechanical improvement of the porous ceramics due to the increase of neck area among the SiC particles. Porous ceramics sintered at 1450oC possessed higher open porosity and median pore diameter than those of samples sintered at 1250 and 1350oC, since pore generated by the transport of dissolved talc materials as increasing temperature.

Abstract: Bi-based glass pastes were prepared in the mixed organic solvents (α-terpineol and texanol) with different kinds and concentrations of organic dispersant such as fish oil , phosphate ester and poly methyl methacrylate (PMMA). The rheological stability of glass pastes was characterized by using a rheometer and adsorption behavior of organic additives for the glass powders was carried out by using a thermal gravimetric analyzer (TGA).The Bi-based glass pastes exhibited the most stable rheological behavior at an addition of phosphate ester of 0.5 in mass %, due to the steric repulsive force acting on the surface of glass powers with adsorbing the organic additives. This study allowed significant delay of the sedimentation time and suppressed generation of micropores during practical forming process by a screen printing method.

Abstract: The effects of thermal fatigue conditions on the mechanical and contact damage behavior in thermal barrier coatings (TBCs) are investigated as functions of the bond coat thickness and the preparation method. Three kinds of TBCs with different thickness in the bond coat are prepared by two different methods of APS and HVOF. The static and cyclic thermal fatigues for the TBCs are conducted at temperatures of 950
and 1100
with different dwell times of 10 and 100 hr and 10 cycles at each temperature. Mechanical properties, hardness H and modulus E, in each condition and component are measured by nano-indentation. The contact damage behaviors are investigated by Hertzian indentation, including the cyclic fatigue behavior on the surface of the TBCs. The TGO formation is dependent on both temperature tested and time exposed, showing a similar effect with the cyclic thermal fatigues. The mechanical properties of the TBCs are increased due to the re-sintering of the top coating and the composition change of the bond coat during the thermal fatigues. The contact damage behaviors are affected by the thermal fatigue conditions and the fabrication method, independent of the bond coat thickness.

Abstract: Well-crystallized and nano-sized indium tin oxide (ITO) powders were prepared by a solvothermal process from a mixed solution consisting of indium nitrate and tin chloride. The characteristics of the solvothermally synthesized ITO powders were studied to investigate the contribution of the processing variables on the physico-chemical properties of powders. The results of this study show that the nano-sized blue ITO powders with a single phase could be directly obtained by changing the medium from water to organic solvent.

Abstract: Indium tin oxide (ITO) thin films were deposited on glass substrates via sol-gel spin coating process from a mixed solution of Indium (Ⅲ) acetylacetonate and Tin (Ⅳ) iso-propoxide. Then, ITO thin films were fired at 500°C, and then annealed at 500°C for 30 min with the sequential annealing process; Vacuum
N2
Ar/H2, N2
Ar/H2 and Ar/H2 gas. The effects of the different annealing processes on the surface morphologies and sheet resistance of ITO thin films were investigated. Sheet resistance values of ITO thin films treated under Vacuum
N2
Ar/H2, N2
Ar/H2 and Ar/H2 annealing process were 1.25 kohm/sq., 3.18 kohm/sq. and 4.92 kohm/sq., respectively. Actually, the sequential atmosphere gases and non-oxidizing gas, which were used in annealing process influenced the microstructural features or surface morphologies of ITO thin films: grain size and surface roughness. Thus, it was presumed that the sequential annealing condition influenced the densification behavior in the microstructural evolution of ITO thin films.

Abstract: In order to provide higher driving forces for densification of Indium-Tin oxide(ITO) sputtering targets, nano-sized ITO powders with uniform size distribution were prepared by solvothermal process. In this study, sintered bodies were densified by a spark plasma sintering (SPS) process at 950 oC. The experiments conduct the SPS method in order to achieve the highest density of sputtering targets. The phase and relative density of ITO sputtering targets were identified by X-ray diffractometry_(XRD) and Archimedes method, respectively. The surface microstructure and quanitative analysis of the targets were observed by scanning scanning electron microscopy (SEM), and ITO powders particle were measured by SEM, XRD-LB methods.

Abstract: Electrically conductive porous Si/SiC fiber media were prepared by infiltration of liquid silicon into porous carbon fiber preforms. The series rule of mixture for the effective electrical conductivity was applied to the disc shaped samples to estimate their silicon content, effective electrical conductivity and porosity. The electrical conductivity was estimated by assuming the disc sample as a plate of equivalent geometry, i.e., same thickness, electrode distance and volume. As the volumetric content of silicon in a sample increases from 0.026% to 0.97%, the estimated electrical conductivity increases from 0.17 S/cm to 2.09 S/cm. The porosity of the samples measured by Archimedes principle was in the range of 75~83% and 1~4% less than the one estimated by the series rule of mixture for the effective electrical conductivity.