Papers by Author: Yo Seung Song

Abstract: The ferrite powders were prepared by high energy ball-milling, and subsequently compacted under a pressure of 490MPa for 3min and sintered at 1273~1673K for 3h. The spherical shaped initial powders with a mean size of approximately 70μm were changed to irregular shaped powders of 230nm in a mean size after milling at 300rpm for 3h. The milled powders were composed of the nanosized particles with a size of approximately 10nm. After compacting, the green density was approximately 70%. The sintered sample with a relative density of approximately 98% was obtained by sintering at 1473K, which showed the saturation magnetization (M_s) of approximately 90emu/g and the coercivtity (H_c) of approximately 19Oe at room temperature. The M_s decreased with increasing temperature, whereas the H_c and initial permeability (μ_i) were unchanged. There was no change in the M_s, H_c and μ_i during holding at 423K.

Abstract: The Mg-6wt%Al-1wt%Zn alloy powders mixture produced by gas atomization were spark plasma sintered (SPSed). The SPSed sample was coated by plasma electrolytic oxidation (PEO) method, and the microstructure, mechanical and corrosion properties were investigated and compared to the normally sintered sample. The SPSed sample exhibited a microstructure of well-bonded grains with approximately 48 ,m in grain size, which was bigger than that of the normally sintered sample. The coatings on the SPSed sample were characterized by dense and porous layers, while, in the case of the normally sintered sample, the coatings had grown through the grain boundary of substrate with increasing the coating time. The surface roughness, thickness and micro-hardness of coatings on the SPSed sample increased gradually with increasing the coating time. The corrosion property of coatings on the SPSed sample had a tendency to improve with increasing the coating time in 3.5%NaCl solution. However, it was worse than that of coatings on the normally sintered sample.

Abstract: A pH sensitive polyacrylonitrile (PAN) fiber, exhibiting soft actuation as a linear actuator, is prepared by electrospinning to investigate the optimum experimental condition of the as-spun fibers and the effect of pH variation on length change of PAN gel fiber. Unbeaded smooth and continuous PAN nanofibers with diameter of ~700 nm are obtained for the 10 wt% PAN fibers at a flow rate of 0.5 mL/h and an electric field of 0.875 kV/cm, suggesting that the value of viscosity is the most significant on the fiber morphology. Diameter and volume changes of PAN filaments are observed as greater than 100% and greater than 720%, respectively for pH activated systems. A typical hysteresis loop is observed for the length change (~38%) of PAN gels with respect to pH variation.

Abstract: Bond coatings, CoNiCrAlY, are prepared on Inconel 738 substrate by vacuum plasma spray (VPS). Mechanical properties of VPSed CoNiCrAlY coatings are investigated via Taguchi method and L18(21×37)
orthogonal arrays to determine the optimal setting and the relationship of experimental variables. Seven parameters were considered as follows: (A) arc current; (B) primary gas flow rate; (C) secondary gas flow rate; (D) stand-off distance; (E) working pressure; (F) carrier gas flow rate (Ar); and (G) powder feeding rate.
The effect of carrier gas flow rate on the porosity is determined to be the highest among the parameters investigated. Higher microhardness values are observed for the VPSed coatings as compared to the coatings prepared by conventional high velocity oxygen fuel probably due to low amount of the porosity.

Abstract: SrZrO3 resistive oxides on Ag sheathed Bi(2223) tapes were prepared by the sol-gel and dip-coating method to investigate the effect of PVP content and thermal stage on microstructure of the films. Although film thickness rose with the addition of PVP, the amount of PVP was not effective to reduction in the crack density of the films. However, longer drying time at low temperature and shorter heat treatment time at 500oC were effective for controlling the microcracks of the SrZrO3 films, indicating that the microcracking of the films was governed primarily by the thermal stage.

Abstract: Hertzian cyclic fatigue properties of the glass-infiltrated alumina and spinel were evaluated using a WC sphere of radius of 3.18 mm in exact in vitro environment (artificial saliva) at contact loads from 200 N to 1000 N to investigate indentation damage and strength degradation. At 200 N, no strength degradation was observed up to 106 contact cycles. As the load increased from 200 N to 1000 N, the reduction in strength was found when the transition from ring to radial cracking occurred. The degree of strength degradation after critical cycling was more pronounced probably owing to the chemical reaction of the artificial saliva with the glass phase along the radial cracks introduced during the large numbers of contact cycles.

Abstract: TiO2 thin films were prepared by RF sputtering on Si wafer to investigate hydrophilicity and bactericidal effects by using AFM, XRD, standard plate count method and wetting angle measurement. Experimentally, the survival rate of E. coli was above 74% and the wetting angle of water for the 30 min UV illuminated films was 4.2o. It was found that the bactericidal efficiency was closely related to the large specific surface area caused by the roughness, however, hydrophilicity was governed by the surface defective sites rather than the roughness.

Abstract: c-BN film was synthesized using ME-ARE on Si substrate. The deposition process was optimized via the Taguchi method. The optimized conditions were as follows: substrate temperature, anode (plasma) current, Ar/N2 ratio, pulse frequency, duty frequency, bias voltage and deposition time were 500°C, 15A, 3, 1 kHz, 50%, -130V and 15 min, respectively. The crosssectional TEM observation revealed that the c-BN films with a thickness of 100nm ~300nm were composed of two layers, a columnar h-BN layer with a thickness of 30nm~40 nm normal to Si substrate and a c-BN structure.