Papers by Author: D.H. Kim

Abstract: Polymer/layered silicate nanocomposities were prepared by in situ polymerization with microwave process. The influence of the amount of clay on the structure and thermal properties for the synthesized nanocomposites were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA). It was found that the structure of nanocomposites, an intercalated/exfoliated structure, depended on the clay content.

Abstract: Carbon and boron were mainly considered to strengthen grain boundaries formed during single crystal casting of complex shaped components. However, those elements cause segregation forming the phase with low melting temperature or with brittle nature. To determine the optimum amount of these elements, the effect of boron on solidification behavior was investigated in the C doped single crystal RR 2072 alloy. The solid/liquid interface morphologies and the solidification microstructures were studied at various solidification rates and with B addition by directional solidification.

Abstract: Carbon nanotubes have attracted considerable attention because their high aspect ratio leads to a large electric field enhancement and a low operating voltage. The dependence of environmental gases such as Ar, O2 on the emission current from carbon nanotube emitters was examined in this study. Based on our experiments, the current density is decreased in single-wall carbon nanotubes (SWNTs), but is increased in multi-wall carbon nanotubes (MWNTs) as the vacuum level decreases from 10-7 Torr to 10-4 Torr by the inflow purging gases. The current density subsequently recovered as the vacuum level increased to 10-7 Torr when gas inflow stopped. From those results, we conclude that the MWNTs have completely different degradation characteristics in comparison to SWNTs. The increased current density of an MWNT in the high-pressure range of 10-4 Torr is a significant finding and would be widely applicable to low cost vacuum packages.

Abstract: We improved the sensitivity of existing commercial Pirani-vacuum gauges employing the AC method in the vacuum range above 1 Torr. The signals obtained through the use of the AC method yield information related to the specific heat and heat conductivity of gas. The output signal is obtained by two components: the oscillating temperature amplitude, and its phase. The amplitude increases with the decrease of pressure in the vacuum range from the atmosphere to about 1 Torr, which arises from the decrease of the heat capacity with the decrease of gas density. In contrast, the phase decreases monotonically as pressure decreases and the slop of a dΘ/d(log P) is large at higher than 1 Torr. This provides a good advantage for developing a new Pirani gauge with high sensitivity.

Abstract: Reduced activation ferritic (JLF-1) steel is one of the leading candidates for blanket/first-wall structures in D-T fusion reactors. In fusion applications, structural materials will suffer the effects of repeated temperature changes. Therefore, a database of tensile strength and fracture toughness are very important as the temperature is 400 ° C and it's TIG welded joint. In this paper, fracture toughness J_IC and KIC) and tensile tests were carried out at room temperature (RT) and at elevated temperature (400 ° C). The tensile properties of the TIG welding joint of JLF-1 steel was also investigated. It was shown that the tensile strength and fracture toughness values of the JLF-1 steel are slightly decreased with as increase in temperature. The fracture toughness values of JLF-1 steel at room temperature and at 400°C show excellent JIC values of about 530 kJ/m2 and 340 kJ/m2, respectively.

Abstract: Deformation behavior of Ni-based metallic glass matrix composites reinforced by short brass-fibers, synthesized by warm extrusion of gas atomized powders, has been investigated under the uniaxial compression condition at room temperature. The brass-fibers are well distributed in the metallic glass matrix and enhanced macroscopic plasticity is observed due to the formation of multiple shear bands, initiated from the interface between brass-fiber and metallic glass matrix, as well as their confinement between the brass-fibers, stemming from the constrained plastic deformation of the reinforcing brass phase.