Papers by Author: Bo Young Hur

Abstract: This study reported the fabrication of tin oxide (SnO2) nanostructures on Co-coated Si substrates by the thermal heating of Sn powders. The microstructures and morphologies of the resultant nanostructures were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and scanning electron microscopy (SEM). The product mainly comprised the tangled nanowires with average diameters in the range of 50-180 nm. The nanostructures were single-crystalline rutile structure of SnO2. The PL measurement with the Gaussian fitting exhibited visible light emission bands centered at 576 nm and 638 nm, respectively. We have discussed the possible growth mechanism of the nanostructures.

Abstract: The influence of wheel speed and melt temperature on the surface characteristics of slag fibers were examined in mineral fibers synthesized from the steel-making slag, using a modified melt extraction method, i.e. melt spinning. The synthesized fibers were characterized by optical microscope and scanning electron microscopy. It was found that the wheel speed of 700 rpm generated better quality of mineral fibers in terms of diameter and length. This was attributed to the relative extent of contact width between the flowing melt and the rotating wheel. The thickness of the slag fibers were also decreased with increasing the slag melt temperature due mainly to significant decrease in the viscosity of the slag melt. In addition, the lower melt temperature caused an increase in number of shots plus the mineral fibers.

Abstract: In case of the low carbon chromium steel that have widely been used for the carburized gears or rolling bearings, the austenite grain coarsening may be occasionally occurred during carburizing. To restrain this phenomenon, most of hot forged parts have been given to an isothermal heat treatment or normalizing immediately after hot forging and/or prior to carburizing. Therefore, their production cost includes unexpected additional expenses caused by such a non-value added process. To confirm the possibility of an energy saving, as well as attaining a superior fatigue life, in the production of automotive parts to be carburized, the austenite grain coarsening with the manufacturing process of the microalloyed steel, containing Nb and B as a grain refining elements, was investigated. The heat treatment characteristics and rolling contact fatigue behavior of the carburized specimens with the isothermal heat treatment were also investigated. In spite of omitting the isothermal heat treatment after hot forging, the abnormally coarse austenite grains were not found out in the carburized specimens. However, the rolling contact fatigue life of the carburized specimens, in which the isothermal heat treatment was omitted, was shorter than that of isothermally heat-treated specimens.

Abstract: Al-Mg alloy foams containing different Mg contents were synthesized via a conventional melt foaming method. The surface structures of pores formed in resultant foamed alloys was characterized by scanning electron microscopy and x-ray diffractometer. It was found that the pore structures were deteriorated with increasing Mg contents, while the percent porosities did not vary with increasing Mg contents, about 90% and 3~5 mm in pore size. The detailed microstructural examination conducted on increasing Mg containing alloy foams revealed presence of various oxide particles on the surface of individual cells, including MgAl2O4 particles in a form of fine spinel; its small amount for Al-1wt%Mg alloy foam but relative high amount of Al-4wt%Mg one. This suggested the negative effect of MgAl2O4 on the stable pore and thus cell structures in corresponding alloy foams. The possible mechanism associated with MgAl2O4 formation was discussed in the present study.

Abstract: A study was carried out to understand the relationship between the pore structure’s evolution of Al alloy melt foam and its preparation variables, which include decomposition properties of titanium hydride, the stirring foaming time and the holding foaming time. The results show that the foaming process of Al alloy melt can be inferred by the relation curves of decomposition quantity of titanium hydride and time at a specific temperature. The porosity of Al alloy melt during stirring foaming period is kept constant while the pore number increases and pore diameter decreases, which is a new method of fabricating Al alloy foam with a relatively low porosity and small pore size.

Abstract: A study on the tensile, compression and bending test of Aluminum foam Application for auto bumper and rod. We know a more efficiency to adhesive Aluminum foam out side of part in aspect to price, weight and capacity when make auto-component using aluminum foam. We secured basis properties of aluminum foam for applying auto-component by tensile, compressive and bending test. Aluminum foam was equipped to crash core with different shape. The results of maximum loads test are higher by full> cavity> bridge> half type relationship order. And, we could know that a difference of weight of full, cavity, and bridge type is not big.

Abstract: Metal foams are not easy to use as materials as their manufacturing process involves all three, solid, liquid and gaseous phases occurring simultaneously at varying temperatures and moreever the morphology of the solidified foam is quite complex. There has not been any report on the stabilization of metal foams. To compare the stabilisation of metal foams using a different method, powder metallurgy and casting method using SiC and Ca alloy particle are required. Our investigation results showed deeply etched valleys with high oxygen content and secondary phases attached to line-shaped pores with higher SiC content than the matrix. These line-shaped pores - most probably oxide bifilms-form networks decorated with either secondary phases or oxide particle. We suspect that these decorated bifilms play a crucial role in the stabilisation of metal foams.

Abstract: In this paper, the whole temperature programmed decomposition (TPD) spectrum of titanium hydride was acquired by the special designed set-up. After separating and simulating the TPD spectrum by using Spectrum Superposition Method (SSM), Consulting Table Method (CTM) and differential spectrum technique, the kinetics parameters of titanium hydride and corresponding equations were obtained. Using these kinetics equations, the fabrication parameters of Al alloy foam can be determined and foaming process of Al alloy melt can be predicted.

Abstract: The surface tension and the viscosity characteristics of molten metal are the most important factors in casting process and metallic foam manufacturing especially. The surface tension (by the modified ring method) and the viscosity (by the rotational method) of molten Al-Cu alloys have been measured under high purity Ar gas atmosphere. The surface tension and the viscosity of Al-Cu alloys were investigated in the temperature range of 660-800oC, and the effects of the additional elements were investigated at the 660~680
. The result show that the surface tension and viscosity of these alloys decrease with increasing temperature together. The viscosity of Al-Cu alloys near the melting point is about 4.7 to 5.7 [mPa.s]. The effect of additional thickening elements has the tendency that is the surface tension decreased and the viscosity increased. This anomalous behavior has the relation of the preferential adsorption of high activity elements on the surface.

Abstract: The phase and microstructure of four kinds of plasma sprayed TiO2 coatings (P25, ST, NK, KT) were characterized by XRD (X-ray Diffraction), FE-SEM (Field Emission Scanning Electron Microscope) and TEM ( Transmission Electron Microscopy). Their photocatalytic properties were discussed in relation to phase composition and microstructure. The FE-SEM observation results reveal that there are quite a number of nanoparticles on the surfaces of all four kinds of plasma sprayed TiO2 coatings, which provide photocatalytic reactive sites. Based on the observation results of FE-SEM and TEM, the microstructure schematic of plasma sprayed TiO2 coatings using nanoparticles as feedstock is suggested. The best photocatalytic property of P25 coating among the four plasma sprayed TiO2 coatings is attributed to the combination of its fine “cauliflower” structure and the highest anatase content.