Papers by Author: Jae Won Kim

Abstract: Porous alumina green bodies were fabricated by a gel-casting method, for which the slurries of alumina (AKP-30) and PMMA (polymethylmetacrylate) bead were mixed and dispersed at 1:1, 1:2, 1:3, and 1:4 volume ratios. PMMA bead as precursor of carbon source was used for growing SiC nano-fiber or nano-whisker during a VLS (vapor-liquid-solid) reaction at 1450°C for 9hrs. PMMA beads are converted to carbon particles after calcination at 1300°C for 9hrs in static argon (Ar) atmosphere. Finally, carbon particles remain in pores of alumina bodies connected by continuous pore channels. Fe solution was infiltrated into the porous alumina bodies, which are absorbed on the carbon particles in pores. Fe precursor functions as a seed to develop the SiC nano-fiber or nano-whisker in the porous alumina bodies. The liquid droplets formed at the end of the SiC fiber or whisker are evident for the typical VLS mechanism. The microstructure of the SiC fiber or whisker grown by the VLS reaction was observed by SEM. The porosity was measured by mercury porosimeter. The formation behavior of SiC fiber or whisker is dependent on volume ratio of carbon converted from PMMA bead.

Abstract: Layered structures of dense porcelain/porous alumina and dense porcelain/porcelainalumina/ porous alumina are designed and their crack propagation behaviors are investigated. As a substrate, the porous alumina, which is prepared by a gel-casting process using the binary slip of alumina powder and PMMA spherical micro-bead, is dried at room temperature for 24 h and then sintered at 1600
for 2 h. Porcelain is coated on the porous alumina substrate and then re-sintered at 987
. Bi- and tri-layered structures are produced by the different dwell times (2 min, 10 min) at re-sintering temperature. There is no delaminating or cracks observed after re-sintering the layered samples. The crack propagation behaviors in the bi- and tri-layered structures are evaluated by micro-indentation. The indentation cracks do not propagate into the porous alumina through interface (porosity; 36~62%) except for another one (porosity; 70%) in the bi-layered samples. In the case of the high porosity bi-layered sample (porosity; 70%), cracks are scattered along the 3-D open-pore channels. However, cracks do not propagate through the interface in the tri-layered samples with a porcelain-alumina buffer layer, because the porcelain-alumina buffer layer plays an important part such as a barrier layer in crack propagation.

Abstract: Silicon carbide (SiC) nano-powders are successfully synthesized by a reaction between carbon nano-powders (carbon black) and SiO gas at 1300°C for 9 hrs in dynamic argon atmosphere (flow rate; 400 cm3 min-1), using the solid-vapor reaction method. The particle size of synthesized SiC nano-powders is below 40 nm and the shape is uniform. Unexpectedly, SiC nano-fibers are also coexisted in the SiC nano-powders. The quantitative and qualitative properties of the SiC nanopowders and nano-fibers are analyzed by scanning electron microscopy (SEM), transmission electron microscope (TEM), and X-ray diffraction (XRD). Carbon residuals removed by heating over 700°C in air are estimated by thermogravimetry analysis (TGA). It is found that the SiC nanopowders are easily produced by direct synthesis via the solid-vapor reaction method. The morphological characteristics of the resulting SiC nano-powders are dependent upon the morphology of carbon black used as precursor.

Abstract: TiO2 coated open-cell mullite ceramics were fabricated with coating TiO2 sol on pores of mullite, and its processing parameters were investigated. Open-cell mullite ceramics were fabricated through a gel-casting process. Two kinds of mullite precursor powders were prepared by dissolution of two kinds of aluminum salts (Al(NO3)3·9H2O; type I and Al2(SO4)3·16H2O; type II) into colloidal silica sols. To produce porous mullite ceramics, both mullite precursor powders and PMMA beads (≈ 5μm) were co-dispersed by electrosteric stabilized mechanism in an aqueous system and then gel-casted. The green bodies were sintered above 1300°C for 3hrs in air. The PMMA was offered as pores in sintered mullite bodies. The prepared open-cell mullite ceramics were dipped in TiO2 precursor solution. The sintered bodies coated with TiO2 sols were re-sintered below 1000°C for 3hrs after drying at room temperature. The TiO2 was successfully coated into the open-cell mullite ceramics. The characteristics of each TiO2 coated porous mullite ceramics were investigated by XRD, SEM, porosimetry, as functions of aluminum salt and pH of sol. It was found that the synthesis behavior and the porosity of the mullite are strongly dependent upon aluminum salt species, resulting in different grain size, morphology, and pore size.

Abstract: ZnO-SiO2 nanoparticles were synthesized using a reverse micelle technique combined with metal alkoxide hydrolysis and condensation. The size of the particles was controlled by manipulating the relative rates of the hydrolysis and condensation reaction of tetraethyl orthosilicate (TEOS) within the micro-emulsion. The average size of synthesized ZnO-SiO2 nanoparticles was in the range of 20-40 nm. The effects of synthesis parameters such as the molar ratio of water to precursor and the molar ratio of water to surfactant are discussed.

Abstract: A modified sol-gel method is proposed for the preparation of silicon carbide nano-particles and nano-whisker. Tetraethoxysilane (TEOS), SiO2 bead, and PMMA as precursor of carbon source were used for preparing a ternary carbonaceous silicon xerogel, and morpholine is employed in the sol-gel process as a catalyst reagent. SiC nano-particles and nano-whisker were obtained from carbothermal reduction and vapor-solid (VS) reaction of the ternary xerogel at 1300°C for 9h in a dynamic argon atmosphere (flow rate; 600 cm3/min), and then purified by removing excess silica, residual carbon, and other impurities. The purified SiC sample was characterized by TG/DTA, XRD, SEM, and HRTEM. The nano-particles and the nano-whisker were synthesized by different growth mechanisms, with an initial nucleation via a VS reaction between SiO vapor and carbon precursor (PMMA) and a partially subsequent growth via a vapor-vapor (VV) reaction between SiO and CO, respectively.

Abstract: Nanosized ITO(indium tin oxide) powders were prepared in ethylene glycol solution under mild temperature and pressure conditions by precipitation from metal nitrates with aqueous potassium hydroxide. The average size and distribution of the synthesized ITO powders were about 20 ㎚ and broad, respectively. The phase of synthesized particles was crystalline reacted at 230 0C for 6 h.. The optical properties of the synthesized ITO powders were transparent.

Abstract: Al addition is known to enhance corrosion resistance and high temperature properties in ferritic and austenitic stainless steels. Due to the addition of Al, formation and characteristic of nitride were studied in Al added 18Cr stainless steels. The phase diagram and segregation were estimated with addition of Al in 18Cr stainless steels by the Thermo-calc program. Formation behavior of the AlN phase was studied by observing the solid/liquid interface through directional solidification and the phase characterization was performed by XRD in the extracted precipitates from the matrix. It was confirmed that the AlN phase formed at the cell boundary from the liquid in 3 and 5 wt.% Al added 18Cr stainless steels.

Abstract: Silicon carbide (SiC) microtubes with villus-like morphology have been successfully fabricated through a reaction between carbon fiber and SiO gas via the pack cementation process. The effect of gas flow nature (argon and/or hydrogen) on silicon carbide microtube synthesis has been studied. The morphology of the microtubes and the production yield are greatly influenced by the nature of gas flow. Mixed ratio of argon to hydrogen gas has a strong influence on the thickness of the microtubes.