Young-Hoon Yun

Silicon surface texturing for enhanced nanocrystalline diamond seeding efficiency

inductively coupled plasmas were employed to texture Si surface as a pretreatment for nanocrystalline diamond film growth. It was found that the plasma texturing provided a very wide process window where normalized roughness values in the range of 2~16 could be obtained. Significantly improved nucleation densities of compared to conventional mechanical abrasion were achieved after seeding for the textured Si substrate.

Microstructures and Electrochemical Behavior of Ti-Mo Alloys for Biomaterials

The Ti alloy with 7 wt% Mo revealed a microstructure that contained only the orthorhombic phase of a fine acicular martensitic structure. The corrosion resistance of the Ti-Mo alloys increased as the Mo content increased. Based on the results obtained from the polarization curve and electrochemical impedance, the Ti-Mo alloys were shown to be corrosion resistant because of the passive films formed on their surfaces. No ion release was detected in SBF (simulated body fluid) solution, while Ti ions were released in 0.1% lactic acid ranging from 0.05 to 0.12 μg/mL for the Ti-Mo alloys. In vitro tests showed that MC3T3-E1 cell proliferation on Ti-7 wt% Mo alloy was rather active compared to other Ti-Mo alloys and commercial-grade pure Ti.

SiC Conversion Coating Prepared from Silica-Graphite Reaction

The β-SiC conversion coatings were successfully synthesized by the SiO(v)-graphite(s) reaction between silica powder and graphite specimen. This paper is to describe the effects on the characteristics of the SiC conversion coatings, fabricated according to two different reaction conditions. FE-SEM, FE-TEM microstructural morphologies, XRD patterns, pore size distribution, and oxidation behavior of the SiC-coated graphite were investigated. In the XRD pattern and SAD pattern, the coating layers showed cubic SiC peak as well as hexagonal SiC peak. The SiC coatings showed somewhat different characteristics with the reaction conditions according to the position arrangement of the graphite samples. The SiC coating on graphite, prepared in reaction zone (2), shows higher intensity of beta-SiC main peak (111) in XRD pattern as well as rather lower porosity and smaller main pore size peak under 1 μm.

A Study to Improve PEMFC Performance by Using Electro Polishing and CrN Coating on Metal Bipolar Plate

Abstract : As an important component of a fuel cell, the bipolar plate comprises a large proportion in the fuel cell’s volume, weight and price. The bipolar plate is the most widely used; however, graphite bipolar plate is large in volume, brittle and therefore easily broken during assembling. In addition, due to its poor machinability, production costs a lot, unless mass production. Compared with the graphite bipolar plate, the metal bipolar plate has good machinability, high electric conductivity and strong mechanical strength; however, it corrodes easily and has a high contact resistance, so in order to prevent corrosion and reduce the contact resistance, the basic metal needs to be processed by use of electro polishing and coating. The water which is produced by electrochemical reactions in the fuel cell must be discharged smoothly. In this study, in order to prevent corrosion the processes of electro polishing and CrN coating were used. According to the presence or absence of these processes, the contact angles can be measured and different metal bipolar plates can be made, these plates can be used for comparing and analyzing the performance of the fuel cell.

Microstructure and plasma resistance of Y 2 O 3 ceramics

Y2O3 ceramic specimens were fabricated from the granular powder, obtained by spray drying process from the slurry. The slurry was prepared by mixing PVA binder, NaOH for Ph control, PEG and Y2O3 powder. The Y2O3 specimen was shaped in size of Ø14 mm and then sintered at 1650 o C. The characteristics, microstructure, densities and plasma resistance of the Y2O3 specimens were investigated with the function of forming pressure and sintering time. Y2O3 specimens were exposed under the CHF3/O2/Ar plasma, the dry etching treatment of specimens was carried out by the physical reaction etching of Ar + ion beam and the chemical reaction etching of F − ion decomposed from CHF3. With increasing sintering time, Y2O3 specimens showed relatively high density and strong resistance in plasma etching test.

Multi-layered Coating Deposited on PEMFC (Proton Exchange Membrane Fuel Cell) Bipolar Plates

The surface region of commercial stainless steel 304 and 316 plates has been modified through deposition of the multi-layered coatings composed of titanium film (0.1 ㎛) and gold film (1-2 ㎛) by an electron beam evaporation method. XRD patterns of the stainless steel plates deposited with conductive metal films showed the peaks of the external gold film and the stainless steel substrate. Surface microstructural morphologies of the stainless steel bipolar plates modified with multi-layered coatings were observed by AFM and FE-SEM images. The stainless steel plates modified with 0.1 ㎛ titanium film and 1 ㎛ gold film showed microstructure of grains of under 100 ㎚ diameter. The external surface of the stainless steel plates deposited with 0.1 ㎛ titanium film and 2 ㎛ gold film represented somewhat grain growth of Au grains in FE-SEM image. The electrical resistance and water contact angle of the stainless steel bipolar plates modified with multi-layered coatings were examined with the thickness of the gold film.

Surface Morphology and Electrical Property of PEMFC (Proton Exchange Membrane Fuel Cell) Bipolar Plates

The multi-films of a metallic film and a transparent conducting oxide (TCO, indium-tin oxide, ITO) film were formed on the stainless steel 316 and 304 plates by a sputtering method and an E-beam method and then the external metallic region of the stainless steel bipolar plates was converted into the metal nitride films through an annealing process. The multi-film formed on the stainless steel bipolar plates showed the XRD patterns of the typical indium-tin oxide, the metallic phase and the metal substrate and the external nitride film. The XRD pattern of the thin film on the bipolar plates modified showed two metal nitride phases of CrN and Cr₂N compound. Surface microstructural morphology of the multi-film deposited bipolar plates was observed by AFM and FE-SEM. The metal nitride film formed on the stainless steel bipolar plates represented a microstructural morphology of fine columnar grains with 10 ㎚ diameter and 60㎚ length in FE-SEM images. The electrical resistivity of the stainless steel bipolar plates modified was evaluated.

Zinc Gallate(ZnGa₂O₄) 박막 형광체의 합성과 발광특성

Zinc gallate (ZnGa₂O₄) thin film phosphors have been formed on ITO glass substrates by a sol-gel spinning coating method. For the formation of the film phosphors, the starting materials of zinc acetate dihydrate, gallium nitrate hydrate and 2-methoxyethanol as a solution were used. The thin films deposited were firstly dried at 100℃ and fired at 500℃ or 600℃ for 30 min and then, annealed at 500℃ or 600℃ for 30 min under an annealing atmosphere of 3% H₂/Ar. The thin films deposited on ITO glass plates showed the (220), (222), (400), (422), (511), and (440) peaks of spinel structure as well as the (311) peak indicating a standard powder diffraction pattern. The surface morphologies of the thin film phosphors were observed with a firing and an annealing condition. The ZnGa₂O₄ film phosphors showed the blue emission spectra around 410 ㎚ as well as the emission spectra in the UV region (360-380 ㎚).

Surface Treatment of ITO (Indium-Tin-Oxide) thin Films Prepared by Sol-Gel Process

ITO (Indium-tin oxide) thin films have been prepared by a sol-gel spinning coating method and fired and annealed in the temperature range of 450-600℃. The XRD patterns of the films indicated the main peak of (222) plane and showed higher crystallinity with increasing an annealing temperature. The surface of the ITO thin films were treated with 0.1 N HCl 20% solution at room temperature. The effects of surface treatment on electrical properties and surface morphologies of the ITO films were investigated with the results of sheet resistance and FE-SEM, AFM images. The samples, subsequently treated with acidic solution for 40 sec showed the sheet resistance of 0.982 ㏀/square. The surface treatment using acidic solution diminished the RMS (root mean square) value and the residual carbon content of the ITO films. It seemed that the acid-cleaning of the ITO thin films lead to the decrease of surface roughness and sheet resistance.