Papers by Author: Han Ki Yoon

Abstract: With the rapid development of aerospace and automobile industries, metal matrix composites (MMCs) have attracted much attention because of its excellent performance. In this paper, Ni-Cr/AC8A composites reinforced with porous Ni-Cr preform were manufactured by low pressure infiltration process, infiltration temperatures are 700oC~850oC. The microstructure and phase composition of composites were evaluated using optical microscope, X-ray diffraction (XRD) and electro-probe microanalysis (EPMA), It's found that they're intermetallic compounds generated in the composites. Recently, intermetallic compounds have attracted much attention as high-temperature material. We study the hardness of Ni-Cr/AC8A composites, the results show the Ni-Cr/AC8A composite has high hardness due to the intermetallic compounds exist.

Abstract: . This paper deals with the mechanical characterization of short bamboo fiber reinforced green composites (BFGC). The effects of thermal shock on the mechanical properties of BFGC were investigated. Their flexural strength was evaluated by 3-point bending test, and Vickers hardness and elastic modulus were evaluated by a nano-indentation method. The fracture surfaces and microstructure of BFGC were examined by SEM. The results of evaluated mechanical properties (3-point bending test) after thermal shock indicated that the strength of BFGC decreased below 40°C, however the strength became constant above 40°C. The Vickers hardness and modulus of elasticity evaluated from the nano-indentation tests also represented a similar trend to that of flexural strength. The fracture surface of thermal shocked BFGC after bending tests indicated the pull-outed bamboo fibers. This might be responsible for insufficient bonding between bamboo fiber and resin.

Abstract: ZnO is an n-type semiconductor having a hexagonal wurzite structure. ZnO exhibits good piezoelectric and optical properties, and might be a good candidate for an electroluminescence device like an UV laser diode. Then, these devices are very small, their films are very thin and they are prepared in the limited size and shape, so they are unsuitable for the extensive mechanical testing. In this present work, ZnO thin films are prepared on the glass, GaAs(100), Si(100) and Si(111) substrates at various temperatures by the pulsed laser deposition (PLD) method. ZnO thin films were evaluated by X-ray diffraction (XRD) and mechanical properties such as hardness and elastic modulus were measured through the nano-indenter.

Abstract: In this paper the tensile and fatigue life tests of carbon fiber-reinforced epoxy prepreg (CFRP) were conducted in order to investigate the influence of volume ratio and fiber orientation. CFRP/Al7075 laminate hybrid composite (CARALL) consists of alternating Al7075-T6 sheets and carbon epoxy prepreg (epoxy 121oC #2560). The fiber orientations of CFRP were applied to the extent of 0/90° and ±45°. The CFRP layers are 1ply, 3plies and 5plies in case of 0/90°, and 1ply and 2plies in case of ±45° of carbon fiber direction, respectively. The tensile strength decreased with the volume ratio of CFRP in both the cases of fiber orientation 0/90° and ±45°. The fatigue life is lower in large volume ratio of CFRP than in small volume ratio in both the cases of fiber orientation 0/90° and ±45°.

Abstract: The thin films of indium tin oxide (ITO) are used for a variety of electronic devices such as solar cells, touch panels, liquid crystal displays (LCDs). However, these electronic devices are not strong enough against heavy impact since their ITO thin films are deposited on glass substrates. Therefore, ITO thin films were prepared by the inclination opposite target type DC magnetron sputtering equipment onto the Polyethylene Terephthalate (PET) substrate at room temperature using oxidized ITO with In2O3 and SnO2 in a weight ratio of 9:1. In this study, the transmittance, resistivity and electromagnetic wave shielding effectiveness of the ITO thin films prepared at various sputtering time (20~80min namely film thickness; 130~500nm) are measured. The results show that transmittance of the ITO thin films could show about 70% in the range of a visible ray by the variation of film thickness. It also can be seen that a minimum exists in the resistivity of ITO thin films for the variation of film thickness. Electromagnetic wave shielding effectiveness was increased as film thickness increased.

Abstract: Silicon carbide (SiC) materials have been extensively studied for high-temperature components in fusion blanket system and gas turbines, because they have excellent a hightemperature mechanical properties, high thermal conductivity and wear resistance. However, the brittle characteristics of SiC such as low strain-to fracture still impose a severe limitation on the practical application of SiC materials. Therefore, a study of the sliding wear characteristics and fabrication of SiC ceramic by sintering temperature and additives are need. As the result of abrasion, the friction coefficient of the monolithic SiC sintered at 1800°C was the lowest, and the friction coefficient of that sintered at 1760°C was the highest. The monolithic SiC manufactured at 1800°C showed the highest hardness and the lowest friction coefficient. The friction coefficient of the monolithic SiC sintered by the SiO2 contents of 2wt% was the lowest, and the friction coefficient that sintered by the SiO2 contents of 5wt% was the highest. 1800°C of sintering temperature and 2wt% of SiO2 contents ware shown high hardness, low friction coefficient and wear loss compare with other temperatures and SiO2 contents.

Abstract: Silicon carbide fiber-reinforced silicon carbide matrix composites (SiCf/SiC composites) are attractive materials for use in the blankets and divertors of fusion reactors due to their excellent thermo-mechanical properties and inherently low induced radioactivation. However, the brittle characteristics of SiC such as low fracture toughness and low strain-to fracture impose a severe limitation on the practical applications of SiC materials. SiCf/SiC composites can be considered as a promising candidate in various structural materials, because of their good fracture toughness. In this composite system, the direction of SiC fiber will give an effect to the mechanical properties such as fracture toughness and tensile strength. Therefore, it is important to control a proper direction of SiC fiber for the fabrication of high performance SiCf/SiC composites. .

Abstract: In this study, Nano Infiltration Transient Eutectic Phase SiC ceramics (NITE-SiC) was fabricated by hot press method with nano-SiC powder which a particle size is about 30nm, and alumina, yttria and silica were used for sintering additives. Process condition of NITE-SiC was given temperature of 1800oC, pressure of 20Mpa and holding time of 1Hr. NITE-SiC was optimization for densification and good mechanical properties by preceding researches. As a result, weight loss was occurred heat treatment temperature of all. Surface of heat treated specimens was occurred reacting of oxidation or erosion. All Point was occurred residual stress, but residual Stress was show Relaxant properties by effect of heat treatment.

Abstract: Oxide dispersion strengthened (ODS) materials is leading candidates for blanket/first-wall structures of the fusion reactor. ODS materials for structure application in fusion rector would allow to increase the operating temperature to approximately 650
. Therefore, this work focused on the optimization of metallurgical features to improve high temperature strength and elongation through understanding of contents of Cr and Al. In the study, the three kinds of ODS steels such as 19Cr-ODS (K1), 13Cr-Al-ODS (K2) and 19Cr-Al-ODS (K4) with Y2O3 content of 0.37wt% have been produced. And tensile test were performed on three ODS ferritic/martensitic steel between RT, 300, 400 and 600
Dispersion hardening represents an interesting approach to improve the mechanical properties at elevated temperature, as they are foreseen in the future fusion reactor It has been successfully demonstrated that it is possible to expanse the temperature range for the application of fusion reactor.

Abstract: Nano infiltration transient eutectic ceramic (NITE-SiC) was fabricated by hot pressing method using Al2O3 and Y2O3 as sintering additives. The ratio of the Al2O3/Y2O3 additives was changed between 6:4 and 4:6. The densification and mechanical properties were investigated for the sintered temperature. The starting powder was high purity β -SiC nano-powder with an average particle size of 30nm. The bending strength characterization and densification of NITE-SiC was investigated by the FE-SEM and three point bending test.