Contact Information
Baker Laboratory Ithaca, NY 14853
yd39@cornell.edu
office:(607) 255-4164
Research Associate
High Pressure, High Temperature (HPHT) Processing of Thermoelectric Materials
Doctor of Philosophy, 2003: Ajou University, Suwon, South Korea
- Kwanjeong Educational Foundation Scholarship for Postdoctoral Research, South Korea, 2004
- Fundamental Science Research Fellowship, Ministry of Education, South Korea, 1998
- Military Service, Korean Army Force, 1992
Project Description:
Thermoelectric materials are of interest because of the prospect of power generation and refrigeration. Power generation is based on the ability of the materials to generate voltage from an applied temperature. For this use, TE materials can generate useful electricity by harvesting waste heat, for example, from automobile exhaust gas. For such applications, PbTe based thermoelectric materials would be suitable because their highest efficiency is in the approximately 400 °C range (the temperature of automobile exhaust gas ranges from 400 °C to 600 °C).
Recently, the China research group has reported large improvements in the thermoelectric properties of PbTe which was prepared under high pressure and high temperature (HPHT) conditions. The synthesis was carried out at 1200 K and the synthesis pressures ranged up to 5 GPa and all measurements were then done on samples at ambient temperature and pressure. A permanent change in the Seebeck coefficient was observed. The Seebeck coefficient, at 300 K, decreased with increasing synthesis pressure, from an absolute value of about 200 μV/K at 3 GPa to about 125 μV/K at 5 GPa. However, they observed a simultaneous decrease in electrical resistivity, which led to an overall ZT of 0.87, at 300 K, a 20 fold enhancement over the room temperature ZT of PbTe.
We have observed a permanent increase in the magnitude of the Seebeck coefficient of PbTe, after HPHT treatment. The change in Seebeck coefficient, at 300 K, varies from 0 to about 50 %, depending on the processing conditions used. However, we observed an increase in the electrical resistivity, by approximately 2 orders of magnitude, of these HPHT processed materials, leading to a decrease in the overall ZT. The Seebeck coefficient of PbTe can be permanently improved by HPHT processing are encouraging. However, in order to improve the ZT, we need to maintain the electrical conductivity of the material. Dopants should be required to control the electrical conductivity of the material. Dopants that change the carrier density include, Ga, In, Na, Ag, and Br, among others, while Sn or Se would be expected to primarily affect the thermal conductivity.
In the first phase of the project, we built and validated a high temperature (up to 400 ºC) measurement capability for TE materials and demonstrated an increase in ZT values for pure PbTe, near 350 °C, from 0.2, for standard sintered, to 0.65, for HPHT sintered. We have also produced HPHT treated PbTe containing intentional bromine (Br) impurities (dopants) showing maximum ZT of 0.8 near 400 °C.