Papers by Author: Joo Sun Kim

Abstract: Porous composites containing ceramic fiber have been developed for the fabrication of SOFC seals. They were fabricated using glass powder and alumino-silicate chopped fibers. Effect of mixing ratios of ceramic fiber and glass on the leak rates and strength of the composite seals was investigated. In addition, seal performance of commercial glasses was compared with that of SiO2-BaO-B2O3 glass synthesized in this work. The leak rate of the composite seals containing 55 vol% glass was seven times higher than the one containing 75 vol% glass. The flexural strength of the composite seals was reduced to one fourth of the initial value as the porosity increased from 1 to 29%. The incorporation of alumino-silicate chopped fibers into a sealing glass degraded room temperature strength and increased leak rates due to increase in porosity with increasing fiber content. The viscosity of glass at the seal test temperature is presumed to affect the leak rate of the glass seal.

Abstract: A novel Ni-YSZ anode with interpenetrating phase composite (IPC) structure was developed using NiO-YSZ core-shell composite powder and evaluated in terms of microstructure, electrical conductivity, thermal expansion and flexural strength. In comparison to conventional anode, the anodic performance of IPC anode appeared to be more desirable for improving structural reliability of SOFC unit cells and stacks. This study reveals that the anodic performance of IPC anode can be readily tailored by controlling core-shell composite powder particles.

Abstract: Thermoset molding in wet and dry state was successfully employed to fabricate high strength reaction bonded silicon carbide (RBSC) ceramics. Granule transfer molding (GCM) was developed to prevent segregation of component particles and binder phase in wet state, while granule compression molding was applied in dry state to fabricate green compact with significant variation of compaction ratio. Low-fill density granules with mixing homogeneity were critical for promoting lateral deformation of granules during consolidation. In addition, anodic performance of Ni-YSZ anode was significantly enhanced by replacing solid fugitive phase with viscoelastic fugitive phase used as binder in thermoset molding.

Abstract: In the surface machining of brittle materials, there exists a transition from brittle to ductile modes when the depth of cut is reduced below a critical size using ultrafine abrasive grains. Vitrified grinding wheels containing ultrafine abrasives in the sub-micrometer to nanometer range were fabricated by mechanochemically milling nanoabrasive particles and subsequent viscous sintering of abrasive-binder composites. The grinding characteristics of the nanoabrasive grinding wheels were evaluated for the fine grinding of silicon wafers in terms of a variety of variables. Preliminary wafer grinding results are presented on material removal rate and surface quality of silicon wafers.

Abstract: High strength reaction-bonded silicon carbide ceramics was successfully produced by reducing the amount of residual silicon and the silicon pocket size with carbon black as an additional carbon source. A prototype of wafer carrier was also produced in near-net dimension by planar contact infiltration of molten silicon into a preform joined with six pieces of simple shape by eliminating process shrinkages. Forming shrinkages were decreased to a negligible level by compression molding, while sintering shrinkage was eliminated by reactive infiltration of molten silicon.

Abstract: A feasibility study was conducted, in order to fabricate WC-Co powder granule as high-performance feedstock for thermal spraying process, by spray drying weakly flocculated nano-particulate slurries under various conditions. The defects were evaluated with respect to the amount of additives to prepare spherical WC-Co feedstock powders with few morphological defects and homogeneous microstructures. The characteristics of feedstock powders heat-treated at 850~ 1200ı were analyzed using SEM, EPMA, and Hg-porosimetry. The microstructure investigation revealed that the relative portion of larger pores around 10 µm increased with increasing heattreatment temperature. On the other hand, the relative portion of smaller pores of 1 µm decreased due to the coalescence of WC particles, leading to dense feedstock powders with less defects such as hollow or craters. The hardness of thermal-sprayed coat developed with the feedstock in this study, which had exceptionally high feedstock strength, was superior to that of any other sprayed coat obtained with commercial feedstock powders.

Abstract: Nano-crystalline CeO2 was synthesized by the mechanical milling and subsequent heat-treatment from the mixture of Ce(OH)4 as precursor, and NaCl as diluent. The diluent provided diffusion barrier during milling and heat-treatment, which was easily dissolved out by deionized water. The size of crystallite and the strain variance of CeO2 were depended on the temperature and heat-treatment time: increased with the temperature (400~700oC) and time (1~24 hours) increasing, and saturated near at 20nm in size owing to the densification of diluent. The synthesized nano-crystalline CeO2 powder was applied as an abrasive in CMP (Chemical Mechanical Planarization) slurry. When blanket-type SiO2 and Si3N4 wafers were polished with the slurries, the removal rates (RR) of SiO2 and Si3N4 wafers and selectivities (RRSiO2/RRSi3N4) were influenced by synthetic condition of abrasive, the suspension stability and the pHs of slurries.

Abstract: The mechanochemical reaction of hematite with graphite by mechanical alloying (MA) has been investigated at room temperature. The solid state reduction of hematite to Fe₃O₄ and FeO has been observed after 120 hours of MA by a planetary ball mill. Saturation magnetization is gradually increased with milling time up to 80 h, and then deceased after 120 h of MA, indicating the transformation of Fe₃O₄ into nonmagnetic FeO through further reduction. Neither the solid state reduction of Fe₂O₃ by graphite nor a sizable grain refinement is observed in the MA process using a horizontal ball mill.