Sim Loo

Thermoelectric properties of the cubic AgPb10SbTe12

AgPb 10 SbTe 12 is one member of the cubic family of materials AgPb m SbTe m+2 , which adopts NaCl structure where Ag, Pb and Sb atoms occupy the Na site and Te atoms occupy the Cl site. Ingots of this compound were prepared by a solid state reaction for thermoelectric measurements. AgPb 10 SbTe 12 is a narrow band gap semiconductor with E g ∼0.26 eV. In order to optimize the ZT of this member, compositions with deficiency of Ag and Bi-substitution were examined and found to exhibit enhanced power factor at 300 K. The Bi-substituted ingot had ZT∼0.39 at 300 K and ZT∼0.68 at 400 K. Carrier concentration and the mobility measurements are reported.

High Temperature Measurement System Design for Thermoelectric Materials In Power Generation Application

Recent interest in thermoelectric materials for power generation applications has initiated the development of a measurement system in our laboratory for characterization of materials in the 80K to 800K temperature range. This system has been specifically designed for measuring thermoelectric power and electrical conductivity as needed for determining the power factor of the measured samples. This is a single sample measurement system based on a continuous flow cryostat. Significant effort has gone into the computer controlled data acquisition and PID controlled temperature stabilization. Investigation of the influence of temperature stability on the measured data will be presented along with important aspects of the system design, development, and testing. Data collected on reference materials and new thermoelectric materials of interest will be presented.

THERMOELECTRIC MODULE FOR LOW TEMPERATURE APPLICATIONS

The possibility of a prototype thermoelectric cooling device for operation near liquid nitrogen temperatures has been explored. In these devices, the figure of merit involves a combination of the properties of the two branches of the module. Here, we investigate the fabrication of a module with a new low temperature material, CsBi 4 Te 6 ( p -type), and the best known low temperature n -type materials Bi 85 Sb 15 . Transport measurements for each of these materials show high performance at low temperatures. Known values for the figure of merit Z max of CsBi 4 Te 6 is 3.5 × 10 −3 K −1 at 225K and for Bi 85 Sb 15 is 6.5 × 10 −3 K −1 at 77K. At 100K these values drop to 2.0×10 −3 K −1 for CsBi 4 Te 6 and 6.0×10 −3 K −1 for Bi 85 Sb 15 . Theoretical simulations based on these data show a cooling of δT = 12K at 100K, which is almost three times the efficiency of a Bi 2 Te 3 module at that temperature. We present transport measurements of elements used in the fabrication of a low temperature thermoelectric module and properties of the resulting module.

Measurement system for doping and alloying trends in new thermoelectric materials

Several new materials in the A/sub 2/Bi/sub 8/Se/sub 13/, (A=K, Rb, Cs), HoNiSb, Ba/Ge/B (B=In, Sn), and AgPbBiQ/sub 3/ (Q=S, Se, Te) systems have shown promising characteristics for thermoelectric applications. To accommodate the large number of samples required for doping and alloying studies, a new measurement system has been designed with a high sample throughput. A second system is then utilized for complete characterization of the most promising samples. This paper presents a description of the systems and measurement techniques involved, with preliminary data on some of the above mentioned compounds.

Synthesis, crystal structure and thermoelectric properties of β-K 2Bi 8Se 13 solid solutions

Solid solution series of the type K 2 Bi 8-x Sb x Se 13 , K 2-x Rb x Bi 8 Se 13 as well as K 2 Bi 8 Se 13-x S x were prepared and the distribution of the atoms (Bi/Sb, K/Rb and Se/S) on different crystallographic sites, the band gaps and their thermoelectric properties were studied. The distribution Se/S appears to be more uniform than the distribution of the Sb and Rb atoms in the β-K 2 Bi 8 Se 13 structure that shows preference in specific sites in the lattice. Band gap is mainly affected by Sb and S substitution. Seebeck coefficient measurements showed n-type character for of all Se/S members. In the Bi/Sb series an enhancement of p-type character was observed. The thermoelectric performance as well as preliminary high temperature measurements suggest the potential of these materials for high temperature applications.

Hall effect measurements on new thermoelectric materials

In the field of thermoelectrics, the figure of merit of new materials is based on the electrical conductivity, thermoelectric power, and thermal conductivity of the sample, however additional insight is gained through knowledge of the carrier concentrations and mobility in the materials. The figure of merit is commonly related to the material properties through the B factor which is directly dependent on the mobility of the carriers as well as the effective mass. To gain additional insight on the new materials of interest for thermoelectric applications, a Hall Effect system has been developed for measuring the temperature dependent carrier concentrations and mobilities. In this paper, the measurement system will be described, and recent results for several new materials will be presented.

Low temperature thermoelectric modules

Optimization of modules based on the new low temperature thermoelectric material CsBi/sub 4/Te/sub 6/ has been investigated. These materials have shown very promising thermoelectric properties for cooling applications in the 100-300 K range, however doping and alloying optimization is essential toward developing high efficiency coolers. Progress in device fabrication through the investigation of contact resistances for various electrodes is reported.

Doping and Alloying Trends in New Thermoelectric Materials

New thermoelectric bulk materials such as CsBi 4 Te 6 have shown superior properties to traditional materials, however, optimal performance requires continuing investigations of doping and alloying trends. A recently modified high throughput measurement system is presented for doping and alloying investigations in several new thermoelectric materials. The modification includes a four-probe configuration for more accurate measurements while maintaining a relatively short sample preparation time. The system is fully computer controlled and provides flexible contacts to accommodate various sample dimensions. Optimal compositions are then identified for further investigations in thermoelectric prototype modules. The most promising materials will be further characterized for electrical conductivity, thermoelectric power, thermal conductivity, and Hall effect measurements as a function of temperature.