Papers by Author: Won Seung Cho

Abstract: In this study, the machining characteristics of developed AlN-hBN composites were investigated in the end-milling and precision lapping processes. To achieve the objectives, material properties of the developed AlN-hBN composites were evaluated according to the variation of hBN contents. And, required experimental works were performed to investigate the machining characteristics of the composites. First, the machiniability of the composite was evaluated in the end-milling process under various cutting conditions, such as spindle speed, feederate, and depth of cut variations. Also, generated micro cracks caused by the cutting process were investigated via SEM photographs. Next, precision lapping experiments were performed under various conditions, and the results were estimated.

Abstract: In this study, micro patterns were formed on the developed AlN-hBN composites using powder blasting techniques, which have been considered as one of the most appropriate micro machining methods for hard and brittle materials such as ceramics. To achieve the objective, first, material properties of the developed AlN-hBN composites were evaluated according to the variation of h-BN contents. And, a series of required experiments were performed, and the results were analyzed. As the results, it was investigated that the machiniability of the developed AlN-hBN composites increased with the increase of the h-BN contents in the composites. From the experimental results, it was possible to determine optimum blasting conditions for micro pattern making on the developed AlN-hBN composites.

Abstract: This paper reports the fabrication of indium oxide (In2O3) films using a triethylindium and oxygen mixture. The deposition has been carried out on TiAlN substrates (200-350°C). We have established the correlation between the substrate temperature and the structural properties. The films deposited at 300-350°C were polycrystalline, whereas those deposited at 200°C was close to amorphous. XRD analysis and SEM images indicated that the films grown at 350°C had grained structures with the (222) preferred orientation. The room-temperature photoluminescence spectra of the In2O3 films exhibited a visible light emission.

Abstract: The anti-corrosion properties of and catalytic effect of crosslinking on Bi2O3 were investigated in this study. The results of double rubs test with methyl ethyl ketone, cyclic corrosion test and FT-IR with ATR show that Bi2O3 performs anti-corrosion property and catalytic effect of crosslinking of resin.

Abstract: In order to produce Si3N4/hBN composite with low cost, it seems necessary to use nitrided Si3N4 powders since the cost of Si powder is much cheaper than that of Si3N4 powder. The purpose of the present work is to investigate the nitride conditions, and in particular, we focused on the relationship between microstructures and mechanical properties of hot-pressed Si3N4/hBN ceramic composite using nitrided Si3N4 powders. The mixed powders of Si3N4 and hBN were prepared by nitriding Si powders at 1380oC for 24 h, and subsequently sintered by hot-pressing at 1800oC for 2 h in N2 atmosphere. The microstructure and mechanical properties of the Si3N4/hBN composites were investigated. Flexural strength, Young’s modulus, and hardness decreased by the addition of 20 vol% hBN. The addition of BN resulted in a decrease in the modulus as well as an increase in the size of fracture source, both contribute to the observed decrease in mechanical properties. The Si3N4/BN based ceramic composites revealed enhanced crack deflection. The Vickers indentation crack paths in specimens are sinusoidal due to pull-out of grains during crack propagation.

Abstract: Porous alumina bodies were successfully prepared by spark plasma sintering of alumina powders with different amounts of graphite, and by subsequently burning out the graphite. Highly porous bodies were fabricated by spark plasma sintering at 1000°C for 3 min under a pressure of 30 MPa. The heating rate was 80°C/min, and the pulse pattern (on-off) was 12:2. For example, alumina bodies prepared by the addition of 10 ~ 30 vol% graphite showed high porosity of 50 ~ 57%. Porous alumina bodies prepared by the addition of 10 ~ 30 vol% graphite had a high compressive strength of 200 ± 55 MPa, about 35 times higher than those obtained on samples prepared by pressureless sintering, and about 2.5 times higher than those in samples prepared by hot-pressing. The significant improvement in strength relative to values obtained with conventional sintering was attributed to better sintering resulting from the rapid heating between particles.

Abstract: The effects of tungsten content on the nanocrystalline structure of Ni-W alloy electrodeposits were investigated. Direct current electrodeposition was conducted with various W mole fractions in a nickel sulfate solution containing citric acid as a complexing agent. Current efficiency was an almost constant value of 52% up to W mole fraction of 0.58. However, it was sharply decreased owing to severe H2 evolution with increasing cathode overvoltage at a W mole fraction higher than 0.74. The X-ray diffraction peak was decreased and broadened with increasing W content, and an amorphous pattern appeared clearly at 49 wt.% W. It was confirmed that Ni-W alloy deposit was transformed from crystalline to amorphous structure between 41 and 49 wt.% W. This result was also verified through a TEM analysis.

Abstract: The microstructure and thermoelectrical properties of the 4wt% Te doped p-type Bi0.5Sb1.5Te3 compounds, fabricated by using spark plasma sintering in the temperature ranging from 250°C to 350°C, were characterized. The density of the sintered compounds was increased to 99.2% of theoretical density by carrying out the consolidation at 350oC for 2 min. The Seebeck coefficient, thermal conductivity and electrical resistivity were dependent on hydrogen reduction process and sintering temperature. The Seebeck coefficient increased with reduction process while the electrical resisitivity significantly decreased. Also, the electrical resistivity decreased and thermal conductivity increased with sintering temperature. The results suggest that the carrier density and mobility vary with reduction process and sintering temperature. The highest figure of merit of 3.5×10-3/K was obtained for the compounds spark plasma sintered at 350°C for 2 min by using the hydrogen-reduced powders.