Papers by Author: Y. Choi

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Authors: Man Kim, S.U. Park, D.Y. Kim, Sik Chol Kwon, Y. Choi
Authors: Y. Choi
Abstract: NixZn1-xFe2O4 ferrites with initial unit molar ratio of NiO/ZnO were prepared by self-propagating high temperature synthesis (SHS) at the oxygen partial pressures of 0.25 MPa. Neutron diffraction analysis shows that final product is Ni0.38Zn0.62Fe2O4 (χ2=1.68). The combustion temperature and propagating rate were about 1180 K and 4.2 mm/sec, respectively. The ferrite particle size was less than about 350 nm. The coercive force, maximum magnetization, residual magnetization, susceptibility and curie temperature of Ni0.38Zn0.62Fe2O4 powders prepared by SHS are 2878 A/m, 6.805 Wb/m2Kg, 1.27 Wb/m2Kg , 0.042 m3/kg and 42.20 °C, respectively.
Authors: Y. Choi, Nam Ihn Cho, Hyoung Seop Kim, Sun Ig Hong
Abstract: Effect of equal-channel angular pressing (ECAP) on the corrosion and mechanical properties of Cu-35%Zn alloy were studied. Two types of feed direction were selected. One is parallel pass and the other is 180°degree rotated ECAP pass after each pass. Both ECAP passes made texture in each specimen in which shear band with 45 degree on transverse direction and twins exist. The specimen prepared by parallel ECAP pass has finer shear band. Relative amount of twins to shear band on the microstructure becomes decrease with number of ECAP pass. Microhardness increased from 75 Hv to 210 Hv by ECAP. The corrosion potential and rate of the ECAPed Cu-35%Zn alloys in aerated aqueous 1 M-H2SO4 solution were –92.3 mVSHE and 3.72x10-2 A/cm2 for route- A and –38.6 mVSHE and 5.08x10-2 A/cm2 for route-C, respectively. The corrosion potential and rate of depended on the feed direction and number of pass.
Authors: Y. Choi, M.S. Song, K.L. Lee, Sun Ig Hong
Abstract: The Cu-10vol.% Cr in situ composites were fabricated and their mechanical and corrosion properties were determined. For an equivalent amount of deformation, fcc-bcc composites have a much thinner effective fibre thickness and a much larger interfacial area than do fcc-fcc composite. The yield strength and the modulus of the composite decrease with temperature whereas the failure strain increases with temperature. The observed increase in failure strain with an increase in temperature from 22-600°C is as high as 333 %. Corrosion potential and rate in aerated hydro-sulfuric acid solution are -36.6 mVSHE and 3.525x10-3 A/cm2, respectively. Based on the electrochemical behaviors of the copper and chromium and observation of corrode surface, the corrosion mechanism of the copper-10% chromium composites in aerated hydro-sulfuric acid solution includes deallying mechanism in α-phase and galvanic corrosion at the interface between copper and chromium.
Authors: Y. Choi, Hirofumi Inoue
Abstract: Crystallographic texture of pilgered zirconium alloy tubes was analyzed by neutron diffraction, electron backscatter diffraction (EBSD) and X-ray techniques to study bulk and local texture change with pilgering and heat treatment above re-crystallization temperature. Pilgering resulted in slightly inclining (001) planes to sample normal direction, and aligning effectively (100) planes to the normal of radial direction, respectively. (001) planes of the zirconium tube uniformly exist in radial and tangential directions, however, the (001) planes moved from tangential to radial directions after pilgering followed by heat treatment for 20 hours at 540°C. Some of (001) and (100) planes of pilgered tube were paralleled to longitudinal direction by re-crystallization.
Authors: Y. Choi, Nam Ihn Cho
Abstract: Titinium carbide (TiCx) was produced by self-propagating high temperature synthesis (SHS) method. The morphology and non-stoichiometric number of the SHS product were observed by scanning electron microscopy and neutron diffractometry, respectively. Tubular titanium carbide with hole inside was formed with different non-stoichiometric number (x), which value increased with combustion temperature.
Authors: K.S. Choi, Y. Choi, Hirofumi Inoue
Abstract: Copper-silver quasi-nano filamentary composites prepared by hot isostatic pressing followed by heavy drawing were irradiated in CT hole of a domestic experimental nuclear reactor, HANARO to study the morphological change of the poly-crystalline nano-filaments with irradiation and annealing conditions. The irradiation was carried out for 36 days at 320 o C with the maximum neutron flux of 4.8x10 20 n/cm 2 (E>1.0 MeV). The gamma heating rate was in the range of 2.0-6.45 W/gm. Polycrystalline silver nano-filaments of the composites were equilibrium shape like the faceted sphere shape. Annealing results in the increase of the faceting rate of the irradiated specimen. Nano-indentation results supported that the morphology change of nano-filaments by annealing in this study is more effective on strength change of the irradiated nano-filamentary composites than irradiated hardening.
Authors: Yong Kap Park, In Churl Cho, Y. Choi
Abstract: X-ray irradiation was applied to control the size of metal colloids in a silicate glass to make a high precise photosensitive glass containing K2O, Na2O, ZnO BaO, 0.01-0.05wt.% Au metal as sensitizer, <1.2wt.% Sb2O3 and SnO2 as thermal reducer. Colorimetry showed that X-ray irradiation effectively works to make the color of red system. Transmission electron microscopy revealed that the metal colloids were oxided and had uniform and spherical shape of 10-20 nm. Au colloid made various colors like pink after heating at 400°C, red at 500°C and dark-red at 600°C. Ag colloid made them like yellow-green at 400°C, yellow at 500°C and dark-brown, respectively.
Authors: Min Young Choi, Y. Choi, J.I. Choe, Man Kim, Sik Chol Kwon
Abstract: Chromium chloride (CrCl3) or chromium sulfate(Cr2(SO4)3) content increased trivalent chrome deposition rate. The addition of formic acid and the relative content change of potassium hydroxide decreased the trivalent chrome deposition rate. Industrially applicable trivalent chromium layers with more than 200 µm thick were well prepared at the conditions of pH with the range of 1.3-1.8, electrical current density of 5.0-5.6 A/cm2 and applied voltage of 4.0-5.9V. Average microhardnesses of the chromium layers prepared in the chromium chloride and chromium sulfate solutions are about 803-820 and 700-712 Hv, respectivevly. The wear resistance of the trivalent chromium layers was improved by plating voltage which was related to residual compressive stress on the chomium layer.
Authors: K.S. Choi, Y. Choi, Bong Goo Kim, Y.W. Lee
Abstract: Small specimen with simple geometry has advantage for irradiation tests because the tests have limits for experiments and nuclear wastes after irradiation tests. In this study, a kinetic indentation technique is applied to evaluate mechanical properties of coated layers of a particle fuel of a high temperature gas cooled reactor. Hardness of buffer, IPyC and OPyC are 0.55, 0.874 and 2.726 GPa, respectively. The density, strength and friction coefficient estimated by kinetic indentation method are 1.08, 1.15 and 1.81 g/cm3, and 174, 291 and 606 MPa, and 0.51, 0.45 and 0.4, respectively.
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