Papers by Author: Sung Doo Hwang

Abstract: P-type thermoelectric material Si0.8Ge0.2 was fabricated by mechanical alloying(MA) and Hot-Press Process(HP) The effect of Boron(0.25~2wt%) addition on the thermoelectric properties of p-type Si0.8Ge0.2 alloy was reported. Experiments showed that the electrical conductivity decreased with Boron content at temperature ranging from 500K to 1250K. The carrier concentration measured by the Hall effect measurement also decreased as a function of doping level. With increasing temperature, the Seebeck coefficient and the power factor increased with boron content. Based on measured results, the Figure of Merit (Z) value of 0.5 wt% Si0.8Ge0.2 alloy increased with the small addition of Boron, and reached maximum rapidly; the Z value was 0.9×10-3/K, the highest value among the prepared alloys.

Abstract: AZ31 Mg alloy matrix composites were fabricated by squeeze casting method to improve high temperature properties in this study. The results showed that Mg composites reinforced with Alborex and Ag revealed improved high temperature properties and mechanical properties compared with Mg alloys. High temperature hardness and flexural strength were increased with reinforcement of Alborex, and further increase was obtained with addition of Ag. Also, Alborex reinforced AZ31 containing Ag Mg matrix composite exhibited greater improvement on creep properties. Therefore Mg composites reinforced with Alborex containing Ag were better than those reinforced with Alborex in mechanical properties and high temperature properties.

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 n-type Bi2(Te0.94Se0.06)3 thermoelectric compound was prepared by the direct extrusion process using the powder as raw materials. Hot extruded specimens were annealed at 200°C and 350°C for 2hrs. The electrical conductivity, thermoelectric power and thermal conductivity of hot extruded and annealed specimens were measured at room temperature. The fractographs of the specimens showed that the grain size became coarser and a lot of porosities were generated during annealing process. The power factor value (PF) decreased with increasing the annealing temperature. The electric conductivity and thermal conductivity of the specimens also decreased with the increase of annealing temperature. This may be reasoned that the generated porosities affect the thermal conductivity of the specimens prepared in this study. The figure of merit (Z) value of the annealed specimens at 350°C was improved about 10%. The highest Z value of the specimens annealed at 350°C was 2.0 x 10-3/K among the prepared specimens.