Papers by Author: Shi Woo Lee

Abstract: Yttria stabilized zirconia (YSZ) films with the thickness of up to 12 μm were prepared on alumina and NiO-YSZ substrates by electron beam physical vapor deposition (EB-PVD). The films showed nano-scaled columnar structures depending on the substrate temperature. Electrical conductivity of the YSZ films on alumina was also investigated at the temperature between 700 and 1000oC in oxidizing atmosphere. High activation energy of the conductivity (>1.03eV) indicated that the conduction via grain boundary controlled the ionic conduction in the films prepared by EB-PVD. La0.6Sr0.4CoO3-δ as a cathode was applied on the YSZ/NiO-YSZ in order to evaluate the performance of the YSZ electrolyte.

Abstract: This paper presents the work on the development of ceramic coating processing. Nano-structured zirconia coating has been developed with functions; substrate temperature and oxygen gas change in chamber by electron beam physical vapor deposition (EB-PVD). The microstructure of the coating layer has been characterized with FE-SEM, and SEM. The crystalline phase of the coating layer has been also characterized with XRD. The zirconia coating by EB-PVD had not monoclinic zirconia phase as shown in XRD pattern and Raman spectra and the thickness of coating were quite homogeneous. The fracture microstructure of the coating layer for a thickness of ~15 μm showed columnar or non-columnar structure and had nano-structure with nano scaled grain as shown in micrograph by FE-SEM.

Abstract: Silicon nitride ceramics were prepared by new nitrided pressureless sintering (NPS) process in this study. The microstructures, strengths and thermal properties of the NPS silicon nitride ceramics containing three types of Al2O3 and Y2O3 sintering additives were investigated. Additionally, we have investigated the effect of silicon metal contents changing with 0, 5, 10, 15 and 20 wt% in each composition. The silicon nitride was successfully densified using NPS process, particularly at the starting composition of 5 wt.% Al2O3, 5 wt.% Y2O3, and 5 wt.% Si addition. The maximum flexural strengths and relative densities of these specimens were 500 MPa and 98%, respectively. The flexural strength of sintered specimens after the thermal shock test between 30oC and 1300oC for 20,000 cycles was maintained with the original laboratory strength of 500MPa by low thermal expansion coefficient, 2.9 × 10-6/oC, and high thermal conductivity, 28 W/m⋅oC.

Abstract: Two different types of carbon fibre bundles were used for filament winding to obtain C/C preforms. C/C-SiC composites were produced from the C/C preforms by a silicon melt infiltration technique. The effect of the type of carbon fibre bundle on the mechanical and thermal properties of the resultant C/C-SiC composites was compared. The spun fiber preform yields C/C-SiC composites of better mechanical properties than the unidirectional continuous fiber preform. The strength of the composites from the SFP was 1.8 times higher than that from the CFP. The flexural strength and the O-ring strength of the composites from the SFP with a density of 2.35 g/cm3 were about 160 MPa and 170 MPa, respectively.

Abstract: A layer consisting of La0.6Sr0.4CoO3-d particles with inner nano-pores is coated on the La0.7Sr0.3Ga0.6Fe0.4O3-d oxide to improve the surface reaction resulting in high oxygen permeability. The La0.6Sr0.4CoO3-d particles with inner nano-pores are synthesized by an ultrasonic spray pyrolysis method. The permeated oxygen contents from air to helium gas were measured using gas chromatography after the synthesized particles were coated on the La0.7Sr0.3Ga0.6Fe0.4O3-d oxide by the conventional screen-printing. The results demonstrated that the reactive free surfaces of the nano-pores are an important factor in determining the oxygen permeation for application such as gas separation membrane and ceramic fuel cell.

Abstract: Contact damage resistances of silicon nitride ceramics with various grain boundary phases are investigated in this study. The grain boundary phases are controlled by the addition of different types of sintering additives, or the crystallization of intergranular phase in a silicon nitride. We control the microstructures of materials to have similar grain sizes and the same phases to each other. Contact testing with spherical indenters is used to characterize the damage response. The implication is that the grain boundary phase can be another controllable factor against contact damage and strength degradation even though it is not critical relative to the effect of grain morphology.

Abstract: The Ministry of Commerce, Industry and Energy has contracted with the KIER including the ceramics company to localize of the ceramic heat exchanger tubes and its components. The basic objective of this paper is to develop a ceramic formulation and process which would yield a cost-effective and long-life Si/SiC tube materials by extrusion and reaction sintering process. Thus,KIER is currently developed and localized heat exchanger tubes with 40mm out diameter, 1500mm length. This paper was tested the mechanical and thermal properties of the localized Si/SiC tubes obtained from reaction sintering. Also, the effect of energy savings of the heat exchanger after exposure to forging furnace with hot corrosive environment was investigated.

Abstract: Surface modification effects have been investigated for perovskite-type mixed ionic-electronic conductors. As the mixed conducting oxides show both ionic and electronic conductivity, these can be applied as oxygen permeable membranes. We have coated surfaces of the perovskite-type mixed conductors, LaSrCoFeO3 and LaSrGaFeO3, with LaSrCoO3 and investigated the effects on oxygen permeability. Enhanced oxygen permeability was achieved when the LaSrGaFeO3 membrane was surface-modified with LaSrCoO3. However, there was no effect on oxygen permeability of LaSrCoFeO3 even when the surface of which was modified. The morphological factors related with electrochemical reactions have also been discussed