Papers by Author: Chang Ho Lee

Abstract: The direct extrusion process using the powder as raw materials was applied to prepare the thermoelectric materials. The mechanically alloyed powders of Ag added (Bi0.25Sb0.75)2Te3 were extruded by pulse discharge sintering method in the temperature range of 345°C ~ 425°C. High quality products were obtained by hot-extrusion method and their texture and thermoelectric properties were measured. The intensity of (110) plane increased with extrusion temperature up to 385°C and altered in the range of above 405°C
which coincided with the variation of power factor. The measured Power factor ranged from 3.5 ~ 4.0 × 10-3 W/K2·m. The figure of merit (Z) of the material extruded at 385°C was 3.1 × 10-3 /K, the highest value among the prepared materials.

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: 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 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.

Abstract: Effect of Au (0-0.3 wt.%) on the thermoelectric properties of Bi1Sb3Te6 alloy prepared by mechanical alloying process has been investigated. The Seebeck coefficient decreases with Au content. The Seebeck coefficient of Au-free alloy at room temperature reaches a value of 250 µV/K, which is the maximum. Because of the increased power factor, the figure of merit increases with Au content up to 0.03 wt. % and its value at room temperature is about 2.8×10-3/K, the highest value among the prepared alloys. The Hall measurement shows that the carrier concentration increases with Au content, which explains the reason why the power factor has increased. The XRD patterns show that the intensities of (2 0 5), (0 0 18), (0 2 10), (1 0 19), (1 2 5), and (2 1 10) planes increase with Au content. The atomic distance of these complex planes is longer than that of other main planes, and the chemical bonding strength of these complex planes between constitutional elements is weaker than that of other main planes. As a result, a portion of electrons as minor carrier is trapped into accept level of Au atoms and/or clusters. The XPS observation result, which a portion of Au atoms is chemically shifted in contrast to Bi, Sb and Te atoms are not, explains that Au behaves as an acceptor.