Papers by Author: Woo Teck Kwon

Abstract: Foam-type porous silicon carbide (SiC) ceramics without cracks and hollow struts were fabricated using the polymer replica method with polycarbosilane (PCS) and polyurethane (PU) foam as the starting materials. The synthesized porous SiC was analyzed using X-ray diffraction and scanning electron microscopy. The results revealed that a porous SiC ceramic structure was formed with a dense framework at a low temperature of 1200°C. During the heat-treatment process, the PCS experienced an organic–inorganic transformation and then converted to the SiC ceramics. It was determined that the organic–inorganic transformation of PCS, which was the stage of silicon oxycarbide formation, is affected by the curing condition. In this study, the optimum curing condition was determined to be an air atmosphere at 200°C for 7 h.

Abstract: SMC composites consist of chopped glass fiber as a reinforcements, polyester and mineral fillers. Among them, filler is one of the important factors for improving mechanical and thermal properties of composites, but it has not drawn much attention for SMC composites. In this study, the size effect of calcium carbonate as mineral filler on mechanical properties of SMC composites was discussed using five different sizes of commercial calcium carbonates without chopped fiber reinforcement, to focus on the size effect itself. The SMC process was modified to be suitable for a laboratory scale composed of three steps. The mean sizes of the calcium carbonates were 3 – 20 μm, and the specific surface areas were calculated to be 1 – 5 m²/g by BET. Small size of calcium carbonate having high surface area up to 4 m²/g showed high thermal resistance, and showed higher strength comparing to the large fillers because it affected to form a dense packed microstructure.

Abstract: AlN single crystal can be grown by physical vapor transport (PVT) method. The purity and particle size of the source material are important factors for single crystal growth to obtain good quality of AlN single crystal. The aim of this study was purification of AlN powder and growth of its particle size through thermocyclic treatment. The initial particle size was about 1.0 μm and the purity was 98.4% containing 1.6% oxygen and metallic impurities such as Fe, Sn, Ca, Na and Mg etc. which were analyzed based on KSL1612. For purification of AlN powder, it was performed using a thermocycle process with various thermocyclic numbers. After the thermocycle treatment, it is able to obtain large particle size AlN powder as much as 30μm and purity was increased to 99.6% based on chemical analysis.

Abstract: Since mesoporous silica such as MCM-41 and SBA-15 was developed, the study of the properties of high-surface area materials was accelerated. Moreover, the mesoporous silica is used as a template to produce high-surface materials by nanocasting technology. The purpose of this paper is the synthesis of a high surface silicon carbide sphere by the nanocasting technology. In this study, KCC-1 silica sphere was used as a template, and polycarbosilane and poly (phenyl carbosilane) were selected for precursor of silicon carbide. Carbosilane polymer gives advantage of synthesis silicon carbide under low temperature, and hollow spheres were produced. In this study, the polycarbosilane was more effective for the synthesis of SiC hollow spheres by inversion of template structure showing a fibrous morphology on the sphere wall. And it was confirmed that the sphere was composed of nanosized SiC crystals, and has high surface area using TEM, XRD and BET analysis.

Abstract: β-SiC particles were synthesized using silicon powder and carbon black at 1300°C. The effect of grinding the silicon powder raw material into smaller particles on the synthesis and crystallinity of the SiC particles was investigated. The size of the silicon particles decreased and the surface of the silicon particles increasingly oxidized into silicon oxide with increasing grinding time, thus decreasing the yield of SiC because of the higher SiO₂ content in the ground silicon powder, since SiO₂ transforms into SiC at a higher temperature than does silicon. Although the sizes of the silicon particles in the raw materials were different, all the synthesized SiC crystallites showed similar size. Transmission electron microscopy-energy dispersive X-ray spectroscopy analysis showed that although most of the synthesized SiC particles were smaller than 100 nm, the primary SiC particles had aggregated, and some unreacted silicon was observed inside the large aggregated SiC particles.

Abstract: Characterization of silicon kerf from photovoltaic silicon-wafer production was carried out. Also, SiC powder was synthesized using high purity silicon kerf by varying grinding conditions. With increase of grinding time, surface of the silicon was oxidized to form silicon oxide. Also, it was observed that the unreacted silicon oxide and free silicon amount in the SiC powder increases with an increasing grinding times, even though silicon particle size of the starting material is decreased.

Abstract: In an environmental and economic point of view, recycling of silicon wafer sludge is important. The aim of this work is to investigate the recycling method of silicon wafer sludge. Therefore, drying rate of silicon wafer sludge has been studied for separation of liquid and solid from sludge. Silicon and silicon carbide powder obtained from silicon wafer sludge were analyzed by SEM, XFR, XRD and particle size analyzer. The recovered oil was also characterized using GC-MS. From this work, it can be seen that the falling drying rate of silicon wafer sludge is linear equation. Various metal components have been found in recovered solid powder caused by wire sawing processing.

Abstract: Uniform nanosized β-SiC powders have been prepared in a flow reactor by thermal pyrolysis using organometallic precursors with different C/Si mole ratios such as methyltrichlorosilane and triphenylsilane. The temperature was varied from 1200°C to 1400°C. In order to examine the pyrolysis residue after the reaction, the sample was collected and analyzed with XRD, Raman and TEM. The X-ray diffraction result of the pyrolysis residue shows that the diffraction peaks around 35°, 60°, and 73° correspond to the beta SiC phase. All the diffraction peaks were quite broad, which indicated that the SiC powder was in the early stage of crystallization at this heat treatment temperature.

Abstract: Antimony doped tin oxide (ATO) is a typical p-type semiconductor and widely used due to its good properties such as electrical conductivity and transparency in visible range, while reflects infrared light. These features allow ATO to be used as transparent electrodes, heat mirrors and energy storage device, display panel. However, the use of tin oxide ceramics is limited by the low densification during sintering due to the dominance of non-densification mechanism such as surface diffusion or evaporation condensation. So, sintering agents such as Cobalt (II) oxide (CoO), zinc oxide (ZnO), manganese dioxide (MnO₂) used to improve the densification of SnO₂ by forming lattice solid solution or liquid phase. In this study, we studied the effect of the sintering agents. The antimony-doped tin oxide nanoparticles were synthesized by a sol-gel method. After sintering ATO with sintering agents such as MnO₂ and CoO, sintered body characteristics were investigated by XRD, SEM, thermal conductivity, resistivity and interesting characteristic.

Abstract: The formation of organic-inorganic hybrid composite with ceramic platelets and polymeric compound can have the higher strength and higher elasticity than metal, which is a nanocomposite with high strength and light weight. Ceramic platelet such as Al₂O₃ has been used to form organic-inorganic composite material using PMMA as an organic polymer. Bending strength and density of the composites prepared by infiltration and post-warm pressing were measured. FE-SEM and TG analysis were carried out to determine the microstructure of the organic-inorganic composite materials. Bending strengths and densities of the composites prepared by Al₂O₃ ceramic plate and PMMA after post-warm pressing were ~70MPa and ranged from 2.4 to 2.6, respectively.