Papers by Author: Kyung Wook Jang

Abstract: Group B^I(Cu, Ag)-, B^II(Zn)- and B^III(Al, In)-doped Mg₂Si compounds were synthesized by solid state reaction and mechanical alloying. Electronic transport properties (Hall coefficient, carrier concentration and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, power factor, thermal conductivity and figure of merit) were examined. Mg₂Si powder was synthesized successfully by solid state reaction at 773 K for 6 h and doped by mechanical alloying for 24 h. It was fully consolidated by hot pressing at 1073 K for 1 h. The electrical conductivity increased by doping due to an increase in the carrier concentration. However, the thermal conductivity did not changed significantly by doping, which was due to much larger contribution of the lattice thermal conductivity over the electronic thermal conductivity. Group B^III(Al, In) elements were more effective to enhance the thermoelectric properties of Mg₂Si.

Abstract: Sn-doped CoSb3 skutterudites were prepared by encapsulated induction melting and their electronic transport properties were examined. The Sn dopant generated excess charge carriers, which increased in concentration with increasing Sn doping content. However, the carrier mobility decreased with increasing doping content, indicating a decrease in the hole mean free path by impurity scattering. The Seebeck coefficient decreased and the electrical resistivity decreased slightly with increasing the carrier concentration due to the reduced carrier mobility by impurity scattering. The lattice thermal conductivity was dominant in the Sn-doped CoSb3 skutterudites.

Abstract: In the present study, the fluid flow and the heat transfer with solidification analyses for the Cu thin wire production by OCC (Ohno Continuous Casting) process. The OCC process is widely used to produce cylindrical column castings continuously, a number of researches have been focused on the OCC process. However, few researches on the production of Cu thin wire by the OCC process have been reported, therefore it is necessary to investigate and optimize the process variables of the OCC process when producing the Cu thin wires. A commercial multiphysics software was used to analyse to the flow pattern and the temperature distribution in the OCC system proposed in the present study. Effect of the casting speed, the OCC mold temperature, the melt temperature on the castablility of the thin Cu wire and flow pattern and temperature distribution of the melt were discussed. It is expected that the present study is able to give the design parameters of the OCC system for production of Cu thin wire before the actual OCC system construction.

Abstract: The Arc melting technique was employed to synthesize the type I clathrate of Ba8Al16Si30 compound. Phase transformations during synthesis and homogenization treatment were investigated using X-ray diffraction (XRD) and thermal analysis (TG/DSC). Thermoelectric properties as functions of temperature and homogenization treatment time were evaluated in this study. Maximum ZT was 0.14 at 590K for homogenized at 1173 K for 168hrs specimen and it is strongly expected to show higher value above 600K.

Abstract: Ba8Al16Si30 type I clathrate was produced by arc melting and thermoelectric properties were investigated. The phase transformation behavior of arc-melted type I Ba8Al16Si30 was examined by thermogravimetric analysis, differential scanning calorimetry, hardness test, density measurement, X-ray diffraction and scanning electron microscope analyses. Homogenization was carried out to induce the transformation to a thermoelectric phase at 773K to 973K for 5 hours and 24 hours in the vacuum furnace. Thermoelectric properties in the temperature range between 300K and 600K were measured and evaluated. Electrical conductivity was decreased and Seebeck coefficient was increased with increasing homogenization temperature and time. The arc-melted and the homogenized specimens represented n-type conduction at temperatures examined, and they showed reliable thermoelectric behaviors with increasing homogenization temperature and time.

Abstract: Binary skutterudite CoSb3 compounds were prepared by the arc melting and hot pressing processes and their thermoelectric properties were investigated at 300K-600K. Annealing effect was examined and it was correlated to phase transformation and homogenization. Thermoelectric properties of the arc-melted and hot-pressed CoSb3 were discussed and compared. Undoped CoSb3 prepared by the arc melting showed p-type conduction and metallic behavior at all temperatures examined. However, hot pressed specimens showed n-type conduction, possibly due to Sb evaporation. Thermoelectric properties were remarkably improved by annealing in vacuum and they were closely related to phase transitions.

Abstract: Clathrate Ba8Al16Si30 was produced by arc melting and annealing effects on the microstructure and thermoelectric properties were investigated. The phase transformation behavior of arc-melted Ba8Al16Si30 was examined by thermogravimetric analysis, differential scanning calorimetry, hardness test, X-ray diffraction and scanning electron microscope analyses. Isothermal annealing was carried out to induce the transformation to a thermoelectric phase at 500°C to 700°C for 5 hrs. Thermoelectric properties in the temperature range between 300K and 600K were measured and evaluated. Electrical conductivity was decreased and Seebeck coefficient was increased with increasing isothermal annealing temperature. The arc-melted and the isothermal annealed specimens represented n-type conduction at temperatures examined, and they showed reliable thermoelectric behaviors.

Abstract: Effects of annealing treatments of aluminum substrate for ordered nanopore arrays formed by self-organized anodization have been investigated. To observe the relationship between microstructures of aluminum substrate and nanopore regularity formed after anodization of the aluminum substrate, aluminum specimens were annealed at 300, 400, and 500 °C. The anodic alumina layer was prepared by two-step anodizing process in oxalic acid at 40 V. The ordered arrays of nanopore on anodic alumina were shown to be strongly dependent on the annealing conditions, and nanopore regularity on alumina template increased with increasing annealing temperature and time.