Kuei Fang Hsu

Resonant states in the electronic structure of the high performance thermoelectrics AgPbmSbTe2+m: The role of Ag-Sb microstructures

Ab initio electronic structure calculations based on gradient corrected density-functional theory were performed on a class of novel quaternary compounds AgPb(m)SbTe(2+m), which were found to be excellent high temperature thermoelctrics with a large figure of merit ZT approximately 2.2 at 800 K. We find that resonant states appear near the top of the valence and bottom of the conduction bands of bulk PbTe when Ag and Sb replace Pb. These states can be understood in terms of modified Te-Ag(Sb) bonds. The electronic structure near the gap depends sensitively on the microstructural arrangements of Ag-Sb atoms, suggesting that large ZT values may originate from the nature of these ordering arrangements.

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.

CsPb3Bi3Te8 and CsPb4Bi3Te9: low-dimensional compounds and the homologous series CsPbmBi3Te5 + m

Two new thermoelectric materials of quaternary bismuth telluride CsPb3Bi3Te8 and CsPb4Bi3Te9 are reported, which are members of a homologous series featuring anionic slabs [PbmBi3Te5 + m]- (m = 1-4) of monotonically increasing thickness.

Structure and Thermoelectric Properties of New Layered Compounds in the Quaternary System Cs-Pb-Bi-Te

By introducing of various equivalents of PbTe into the layered framework of CsBi 4 Te 6 , the four new compounds CsPbBi 3 Te 6 (1), CsPb 2 Bi 3 Te 7 (2), CsPb 3 Bi 3 Te 8 (3) and CsPb 4 Bi 3 Te 9 (4), were discovered. The compounds adopt layered structures built up of anionic slabs of progressively increasing thickness. The [Pb m Bi 3 Te 5+m ] - (m = 1, 2, 3, 4) slabs in the four structures can be viewed as fragments excised from the PbTe-type structures with 4-, 5-, 6- and 7-monolayers, respectively. As prepared, these materials are off-stoichiometric and n-type conductors. We present preliminary results of the crystal structures and thermoelectric properties of these materials and classify them as members of the new homologous series CsPb m Bi 3 Te 5+m (m = 1 to 4).

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.

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.