Papers by Author: Byong Sun Chun

Abstract: Mg and Mg alloys are promising materials for light weight high strength applications. In this paper, grain refinement of pure Mg using severe plastic deformation was tried to enhance mechanical properties of the hard-to-deform metallic material. The microstructure and the mechanical properties of Mg processed by equal channel angular pressing (ECAP) at various processing temperatures were investigated experimentally. ECAP of channel angle of 90o and corner angle of 0o was successful without fracture of the samples at 300 oC. The hardness of the ECAP processed Mg decreased with increasing ECAP processing temperature. The effect of temperature on the hardness and microstructure of the ECAP processed Mg were explained by the dislocation glide in the basal plane and non-basal slip systems and the dynamic recrystallization and recovery.

Abstract: The n-type (95%Bi2Te3- 5%Bi2Se3) compound was newly fabricated by gas atomization and hot extrusion, which is considered to be a mass production technique of this alloy. The effect of powder size on thermoelectric properties of 0.04% SbI3 doped 95%Bi2Te3- 5%Bi2Se3 alloy were investigated. Seebeck coefficient (α) and Electrical resistivity (ρ) increased with increasing powder size due to the decrease in carrier concentration by oxygen content. With increasing powder size, the compressive strength of 95%Bi2Te3-5%Bi2Se3 alloy was increased due to the relative high density. The compound with ~300 μm size shows the highest power factor among the four different powder sizes. The rapidly solidified and hot extruded compound using 200~300 μm powder size shows the highest compressive strength.

Abstract: N-type Bi2Te3-Sb2Te3 solid solutions doped with CdCl2 was prepared by melt spinning, crushing and vacuum sintering processes. Microstructure, bending strength and thermoelectric property were investigated as a function of the doping quantity from 0.03wt.% to 0.10wt.% and sintering temperature from 400oC to 500oC, and finally compared with those of conventionally fabricated alloys. The alloy showed a good structural homogeneity as well as bending strength of 3.88Kgf/mm2. The highest thermoelectric figure of merit was obtained by doping 0.03wt.% and sintering at 500oC.

Abstract: FeSi2 compounds were fabricated by rapid solidification and hot pressing, which is considered to be a mass production technique for this alloy. Structural behavior of melt-spun ribbon during heat-treatment and Seebeck coefficient of the hot pressed bulk were systemically investigated and compared with conventionally fabricated alloys. The melt-spun ribbon consists of α-Fe2Si5 and ε-FeSi phase. With increasing annealing time, the phase transition to β-FeSi2 phase occurred more rapidly. 20 min of annealing is sufficient for a homogeneous formation of β-FeSi2 phase in melt-spun ribbon, while it is 100 h in as-cast alloy. In this research, the formation mechanism of β-FeSi2 phase during annealing is a transition of α+ε→β. The microstructure of sintered bulk generally consist of a randomly distributed β-FeSi2 phase with an average grain size of 0.9 μm. The increase of Seebeck coefficient in melt-spun and sintered specimen is due to fine grain size formed by rapid solidification.