Papers by Author: Yong Ho Park

Abstract: A theoretical model is applied to the analysis of thermomechanical properties of Al-SiCp FGMs in this study. Functionally graded Al-SiCp composites (Al-SiCp FGMs) consisted with 10 layers gradually changing volume fractions of Al and SiCp were fabricated using the pressureless infiltration technique. Al-SiCp FGMs plates of total thickness of 3mm, 5mm and 7mm with fairly uniform distribution and compositional gradient of SiCp reinforcement in the Al matrix throughout the thickness was successfully fabricated. The curvature of Al-SiCp FGM plates was measured to check the internal stress distribution predicted via a theoretical model for the analysis of thermomechanical deformation. The evolution of curvature and also internal stresses in response to temperature variations could be predicted for the different combinations of geometric thickness of FGM plates. Theoretical prediction of thermally induced stress distribution makes it possible to design FGM structures without any critical failure during the usage of them.

Abstract: The thermoelectric properties of Ag2Se and excess Ag alloys synthesized by mechanical alloying process were studied. The absolute S value slightly decreased with the Ag content. Contrary to that, the electrical conductivity (σ) increased with the Ag content. This was due to the increased carrier concentration supplied by the excess Ag clusters. Below 280K, the σ value of the excess Ag alloys is almost constant regardless of temperature, which means that the Ag excess alloys is highly degenerated. At higher temperatures above 290 K, the intrinsic conduction behavior for all materials is observed and the band gap energy for Ag2Se calculated from the slope of curve in this higher temperature range is about 220 meV. The figure of merit value (Z) was increased by the excess Ag, from 0.28/K (Ag2Se) to 0.91/K (Ag2.05Se0.95).

Abstract: Plasma radical nitriding has been performed to harden the surface of SKD 61 steel for 1- 10 hours at temperature range of 450-550°C. A NH radical, which has played a key role to produce a nitrogen diffusion layer without the formation of the brittle compound layer, has been generated in a gas mixture of NH3 and H2 . One of the main advantages of the plasma radical nitriding is to improve the surface hardness by maintaining the roughness of the initial polished surface. The microstructures and material properties of the radical nitrided layer have been characterized in order to investigate the effects of various radical nitriding processing parameters. The hardness and surface roughness of the hardened layer were compared between two processes. In addition, PVD CrN coating has been deposited on both the radical nitrided substrates and conventional nitrided substrates by an arc ion plating (AIP) technique. The effect of two different of plasma nitriding treatments on the adhesive strength of the coating layer on the substrates was also investigated.

Abstract: The direct extrusion process using the mechanically alloyed powder as raw materials was applied to prepare the thermoelectric materials. The mechanically alloyed powders of Ag added Bi1Sb3Te6 were extruded using pulse discharge sintering method at temperatures ranging from 345°C to 425°C. The fractographs of extruded specimens shows that no grain size changes was observed until 385°C, but became coarser with the increase of the extrusion temperature above 405°C. The X-ray diffraction patterns showed that the intensity of (110) plane increased with extrusion temperature up to 385°C and started to drop down above 405°C. The power factor value (PF) also increased with the extrusion temperature up to 385°C and altered above 405°C. These trends agreed with the variation in intensity of (110) plane of XRD patterns. The figure of merit (Z) value of the extruded specimens at 385°C was 3.1 × 10-3 W/K, which was the highest value among the prepared specimens.

Abstract: The effect of Al content on the castability, tensile and creep properties of Mg-xAl-Zn alloy was investigated. The molten Mg-(3~11)Al-Zn alloys of 700 °C were poured into the mould designed to estimate castability. Tensile and creep tests were performed for the alloys. It has been shown that creep resistance decreased while tensile strength improved with increasing Al amount. Microstructure study revealed that morphology of Mg17Al12 is main factor determining the tensile and creep properties of as cast Mg-xAl-Zn alloys. The final purpose of this study is to develop the heat resistant Mg alloys to apply transportation systems such as automobile. Further study on effect of heat treatment and additional alloying elements on the microstructure and mechanical properties of Mg-xAl alloys is going to be performed.

Abstract: A study has been made on the effects of antimony (Sb) and strontium (Sr) additions on the microstructure of Mg-Al-Si alloys. Results showed that the additions of Sb and Sr can modify the morphology of Mg2Si particles from Chinese script shape to refined polygonal shape. Tensile strength and creep resistance were improved and tensile elongation was also increased in the modified alloy with Sb and Sr. The addition of Sr was more effective than Sb modification of AS52 alloy for refining microstructure and thus improving properties.

Abstract: An effect of anisotropy on the thermoelectric properties of Bi1Sb3Te6 added with Au alloys prepared by a mechanical alloying process has been studied. The conduction properties including electrical conductivity and thermal conductivity were increased with Au content. The electrical conductivity and the power factor of the perpendicular direction to the pressing direction were larger than those of the parallel direction to the pressing direction. The intensity of (1 1 0) perpendicular plane was larger than that of the parallel plane. It was suggested that the increase of intensity of the (1 1 0) plane would contribute to improve the thermoelectric performance. Although the power factor and thermal conductivity revealed the anisotropic behavior with direction, the Z value showed almost the equal value regardless of direction. From these results, it appeared that the Z value of the Bi1Sb3Te6 added with Au alloy prepared by powder metallurgy process was almost isotropic.

Abstract: We investigated the effects of Fe3O4 (0~0.1 wt.%) on the thermoelectric properties of Bi1Sb4Te7.5 alloy prepared by mechanical alloying process. The Seebeck coefficient increased with Fe3O4 content, but the power factor decreased with Fe3O4 content because of the decreased electrical conductivity. The thermal conductivity decreased with Fe3O4. The carrier concentration measured by the Hall effect measurement decreased with Fe3O4. The thermal conductivity of 0.1 wt.% Fe3O4 alloy was 0.814 W/Km, 20%lower than that of Fe3O4 free alloy. As a result, the small addition of Fe3O4 improved the Z value owing to the decreased thermal conductivity by adding Fe3O4. The Z value of 0.01 wt.% Bi1Sb4Te7.5 alloy was 3.1×10-3 /K, the highest value among the prepared alloys.

Abstract: The effect of Al content on the castability, tensile and creep properties of Mg-xAl-Zn alloy was investigated. The molten Mg-(3~11)Al-Zn alloys of 700°C were poured into the mould designed to estimate castability. Tensile and creep tests were performed for the alloys. The results show that the castability increased with higher Al content. Creep resistance decreased while tensile strength improved with increasing Al amount. Our microstructure study revealed that morphology of Mg17Al12 is the main factor determining the tensile and creep properties of as cast Mg-xAl-Zn alloys. The final purpose of this study is to develop the heat resistant Mg alloys to be applied in transportation systems such as automobile. Further research will be conductedon the effect of heat treatment and additional alloying elements on the microstructure and properties of Mg-xAl alloys.

Abstract: The effects of antimony (Sb) and strontium (Sr) additions on the microstructure of Mg-Al-Si alloys were studied. The results show that the additions of Sb and Sr can modify the Mg2Si particles from Chinese script shape to refined polygonal shape. Tensile strength and creep resistance are improved, and tensile elongation is also increased in the modified alloy. Sr modification is more effective than Sb modification of AS52 alloy for refining the microstructure and thus improving its properties.