Papers by Author: Yun Hae Kim

Abstract: Transparent conductive layers on flexible substrates are important components of today’s optoelectronic technology. They are used in filters for plasma displays, low-e windows, solar cells, etc. At present, in-doped indium oxide (ITO) layers on PET substrate is the predominant transparent conducting oxide film in diverse practical applications. However, ITO is a relatively expensive material because indium is not abundant, but aluminum-doped zinc oxide (AZO) film is emerging as an alternative potential candidate to ITO thin film due to its abundance as a raw material, nontoxic nature, cost-effectiveness, easy fabrication, and good stability in plasma. They have, however, several drawbacks: they exhibit relatively high electrical resistance (sheet resistance, 20-200Ω), considerable emissivity, and significant absorption in the spectral region 1-2μm, in which transition from high transmittance to high reflectance takes place. Furthermore, these films do not block solar thermal radiation (0.7-3μm), which may cause overheating problems to devices such as electro-chromic and photovoltaic devices. On the other hand, ITO/Ag/ITO multilayer films are used to achieve high transparent conducting properties. A thin silver layer of about 10nm thickness is embedded between two ITO layers. The ITO/Ag/ITO film has very low sheet resistance, high optical transparency in the visible range, relatively lower thickness than single-layered ITO film, and better durability than single-layered silver film. In terms of ZnO, which is a wide direct band-gap semiconductor, ZnO has a band-gap energy of 3.37 eV with a binding energy as high as 60 meV at room temperature. ZnO has been applied to various domains for excellent physical and chemical properties, such as piezoelectric sensors, rheostats , gas sensors, semiconductor lasers, and transparent conductive films.

Abstract: The purpose of this study is to determine the correct estimation of the various designs for artificial hip joint of the Carbon/PEEK composites. Validity of the study has been tested with the alternative materials for the metal-based materials for artificial hip joint. Moreover, this work evaluated the FEA according to the fiber ply orientation and the condition of load. The stem shape of two kinds was designed through the normal shape of the femur. Three load cases of 1kN, 2.5kN and 5kN were used for each of FEA model. In the case of general shape, the results by ply configuration showed that the stress of ply configuration I was lower compared to ply configuration II and III. On the other hand, in the case of curved shape, ply configuration II were lower compared to ply configuration I and III. The result was checked that the stress of curved shape was higher than that of the general shape in the load of 2.5kN. It could be confirmed then that a similar phenomenon would occur in the condition of 5kN load. However, in the case of ply configuration II, the stress of the curved shape was lower than the stress of the general shape.

Abstract: The purpose of this study is to determine the correct estimation of the mechanical property between epoxy resin and PEEK sheet as the composites and its validity has been tested with the alternative materials of the metal-based materials for artificial hip joint. Moreover, this work evaluated the mechanical properties according to the moisture absorption and the fractured surfaces of Carbon/Epoxy and Carbon/PEEKcomposites were also evaluated. First, the specimens for the Carbon/PEEK and Carbon/Epoxy composites manufactured with the ASTM standard. The specimens immersed in distilled water at 37°C during 100 days and the coefficient of moisture was measured in according tothe Fick's law. In addition, this work evaluated the fracture energy according to the fiber ply orientation. As a result, the coefficient ofmoisture-absorption of Carbon/PEEK composites was the lowest because the interface coherence with the fiber and resin are the most strong. Also, the fracture energy of the Carbon/PEEK composites was more excellent than the Carbon/Epoxy composites.

Abstract: The purpose of this study is to determine the correct estimation of the mechanical property between epoxy resin and PEEK sheet as the composites and its validity has been tested with the alternative materials of the metal-based materials for artificial hip joint. Moreover, this work evaluated the mechanical properties according to the temperature of heat treatments for sizing removal of carbon fiber and the fractured surfaces of Carbon/Epoxy and Carbon/PEEK composites were also evaluated. First, the sizing removal of carbon fiber were conducted at 300°C for 4 hours and 400°C for 2 hours. The fractured surface in the specimen of tensile test made from PEEK and epoxy resin was observed by SEM. the fracture surface of the tensile test specimen of the Carbon/Epoxy composites heat-treated to 400°C showed that the resin did not bury nearly in the fiber surface and pull out was observed. It is shown that 400°C is suitable for the sizing removal of the carbon fiber. The mechanical test result showed that there was no significant differences in short beam strength. However, the tensile strength and compressive strength of the Carbon/PEEK composites was higher than those of the Carbon/Epoxy composites in the case of the Vacuum Bag process. In addition, this result showed that the sizing material did not have a significant effect on the strength of the Carbon/PEEK composites.

Abstract: Reinforced concrete structures have been increasingly widely used in numerous industrial fields. These structures are often exposed to severely corrosive environments such as sea water, contaminated water, acid rain, and the seashore. Thus, corrosion problems of the steel bars embedded in concrete are very important from a safety and economic point of view. In this study, the effects of cover thickness on the corrosion properties of reinforced steel bar embedded in mortar specimen were investigated using electrochemical methods such as corrosion potentials, polarization curves, cyclic voltammograms, galvanostat and potentiostat. Corrosion potentials shifted in the noble direction, and the value of AC impedance also exhibited a higher value with decreasing cover thickness, furthermore, polarization resistance also increased with decreasing cover thickness. This is probably that the thinner cover thickness, it is easy for the dissolved oxygen and chloride ion to intrude and diffuse to the steel bar, thereby making it easier to corrode on the steel surface compared to thicker cover thickness, which is resulted in forming the corrosive products on the steel surface. Therefore, it is considered that the corrosive products plays a role to provide nobler value of corrosion potential and higher value of impedance. Consequently, it seems that the corrosion resistance of inner steel bar may depend on mainly not cover thickness but the resistance polarization due to corrosive products in the case of immerged for 5 years in this experiment.

Abstract: The double-walled steel vessel filled with insulation powder in the space between two walls was used to minimize heat transfer by radiation and conduction in cryogenic storage tank. And, evaluation of the insulation effect by the thermal conductivity of perlite powder and the effect of heat flux by welding conditions were carried out with insulation analysis of double-wall. The solid supports were also used to bear the weight of the inner container. Furthermore, thermal and structural analysis of the tank was performed to study the effect of vacuum and weldment geometry of the internal supports. If the degree of vacuum were low, the thermal conductivity of perlite increases rapidly and the heat flux should be increased in proportion to the thermal conductivity. Two types of solid supports did not show significant differences in the aspect of working stress or heat flux. Therefore, it is considered that the heat flux is less sensitive against the weldment geometry.

Abstract: Recently, the fuel oil of diesel engines of marine ships is being changed to heavy oil of low quality as the oil price is getting higher and higher. Therefore, the wear and corrosion in all parts of the engine, such as cylinder liner, piston crown, and spindle and seat ring of exhaust valves has predominantly increased. Thus, the repair welding of the piston crown is a unique method to prolong its life in a economical point of view. In this case, filler metals with a high corrosion and wear resistance are mainly being used for repair welding. However, the piston crown on the ships job site is often actually being welded with mild filler metals. Therefore, in this study, mild filler metals, such as E4301, E431316, and E4316, were welded to the SS401 steel as the base metal, and the corrosion properties of their weld metals with and without post weld heat treatment were investigated with some electrochemical methods in 0.1% H₂SO₄ solution. The weld metal welded with E4301 filler metal exhibited the best corrosion resistance among the filler metals in the case of no heat treatment, however, its resistance was considerably decreased due to the post weld heat treatment (annealing:625°C, 2hr). In particular, the weld metal of E4316 exhibited relatively a good corrosion resistance by the post weld heat treatment.

Abstract: Hot dip galvanizing is being widely used to the numerous constructional steels such as a guard rail of high way, various types of structural steel for manufacturing ship and for some other fields etc.. Recently, the cost of zinc is getting higher and higher, thus, a proper manufacturing process should be developed possibly to reduce the cost of production. one of hot dip galvanizing process, that is, flux solution immersion process is very important to control more effective coating. However, flux solution is deteriorated with increasing its using time because dissolved iron from structural steel due to continuous flux treatment was increasingly increased, which is resulted in coating badness and increment of dross. In this study, the effect of additives to elimination of iron dissolved in flux solution was investigated with four types of inner solution, that is, solution of inner barrel was filled with seawater, NaOH, NH4Cl, including parameters of amounts of thiourea(0.2,0.4 ,0.6 and 0.8g) and electrolysis experiment was carried out with applied current density at 20mA/cm2. The best effect for elimination of iron from flux solution is considered that inner barrel with separator should be filled with solution different from original flux solution which contained in iron ion, that is, its inner solution should be composed with seawater added with additives such as thiourea(0.4g), NaOH(0.2 g) and NH4Cl(10g). In particular, we can see that the optimum amount of thiourea to extract effectively iron from flux solution is 0.4g.

Abstract: This paper describes the hull design, structural analysis and dynamic performance analysis of an Underwater Robotic Vehicle (URV). The dynamic performance analysis was made with the help of computer simulations. The main purpose of this URV is to carry out fundamental tests on its station keeping ability, attitude control, and position tracking accuracy. Its configuration is similar in appearance to a general open frame ROV for underwater works. The octagonal frame was designed to sustain obstacle collision and the material, stainless steel 316L, was chosen to maximize the resistance to corrosion in seawater. The navigation of the vehicle is controlled by an on-board computer and underwater sensors. This gives us an ideal platform to develop various algorithms needed for the further development of URV technology.

Abstract: TCO (transparent conducting oxide) films are widely used as photoelectric devices in flat panel displays and solar cells. Until now, ITO (indium-tin oxide) films have been used as TCO films. However, with the increase in the cost of ITO films , researchers have been searching for new materials to use as TCO films .Transparent and conductive aluminum-doped zinc oxide films were prepared by DC magnetron sputtering at different substrate temperatures. The electric and optical properties of these films were studied by Hall measurement and optical spectroscopy, respectively. All of the films that were deposited at temperatures higher than 200 °C substrate temperature demonstrated over 80% transmittance in the range of the visible spectrum. Since the surface mobility of a particle is limited at a low temperature, the growth rate of AZO thin films would be higher than that at a high temperature. And the films showed minimum resistivity of 6.77 × 10 ^-3 Ω•cm at substrate temperature of 200 °C.