Huijun Kong

Transport and mechanical properties of Yb-filled skutterudites

A series of Yb-filled skutterudites were produced and powder samples consolidated using spark plasma sintering (SPS). The effect of different heating cycles on the resulting transport properties of the consolidated samples was explored. Specifically, the effect of sample uniformity on the electrical and thermal transport properties was explored. In addition to the optimal Yb-filling fraction, other factors, such as heating profiles and sintering conditions, were found to play a pivotal role in the performance of these materials. Large quantities of Yb-filled skutterudite material can be generated with high purity and uniformity. Resonant ultrasound spectroscopy was used to determine the elastic modulus and Poissons ratio. Fracture strength was measured for 12 specimens and, taken with information obtained from resonant ultrasound spectroscopy and from thermal expansion and thermal transport characteristics, the thermal shock resistance parameter was evaluated. These parameters will be important for the engineering of thermoelectric modules based on skutterudite materials.

Figure of merit of quaternary (Sb0.75Bi0.25)2−xInxTe3 single crystals

Single crystals of a quaternary system based on (Sb0.75Bi0.25)2Te3 doped with In were prepared using the Bridgman technique. Samples with varying contents of In were characterized by the measurements of electrical conductivity σ⊥c, Hall coefficient RH(B∥c), Seebeck coefficient S(ΔT⊥c), and thermal conductivity κ(ΔT⊥c). The measurements indicate that by incorporating In in (Sb0.75Bi0.25)2Te3 one lowers the concentration of free holes. This effect is explained in terms of a point defect model in the crystal lattice. We also discuss the temperature dependence of the thermoelectric figure of merit Z=σS2∕κ of the samples. It is observed that low concentrations of In atoms in the (Sb0.75Bi0.25)2Te3 crystal lattice result in a substantial increase in the parameter Z in the temperature region 100–300K.

Transport behavior and thermal conductivity reduction in the composite system PbTe-Pb-Sb

We report the synthesis of nanostructured composite PbTe with excess Pb and Sb metal inclusions. Scanning and transmission electron microscopy reveal these inclusions in both the nano- and macroscales. The electrical conductivity and Seebeck coefficient dependence on temperature show unusual trends which depend on the inclusion Pb/Sb ratio. Several ratios showed marked enhancements in power factor at 700 K. The thermal conductivity of these composites is reported.

Thermoelectric properties of rare earth-ruthenium-germanium compounds

We report our studies of the thermoelectric properties of a class of compounds of composition R3Ru4Ge13 (R=Y, Dy, Ho, and Lu). Magnetization measurements show that the R ion is in the trivalent state in each of these compounds. Each of these compounds displays a semiconductorlike rise in electrical resistivity with decreasing temperature. The magnitude of the resistivity is much larger than typical metals and is similar to that of a heavily doped semiconductor or semimetal. The Seebeck coefficient is positive throughout the temperature range of 2–800K with room temperature values of approximately 40μVK−1. The lattice thermal conductivity is remarkably low and exhibits a very flat temperature dependence. This is consistent with the observed internal disorder associated with the cagelike structure of these compounds. Cobalt substitution for ruthenium enhances the semiconductor character but does not improve the thermoelectric properties.

Nanostructuring and its Influence on the Thermoelectric Properties of the AgSbTe 2 -SnTe Quaternary System

The structural and thermoelectric properties of the AgSbTe2-SnTe quaternary system were studied. Powder averaged x-ray diffraction of Ag0.85SnSb1.15Te3 indicates a cubic NaCltype structure in contrast with the single crystal refinements, which point towards tetragonal symmetry. Furthermore, high-resolution electron microscopy imaging revealed the system to be a nano-composite formed by thermodynamically driven compositional fluctuations rather than a solid solution as it was viewed in the past. The lattice thermal conductivity attains very low values, which is in accord with recent theories on thermal transport in heterogeneous systems. The charge transport properties of the system exhibit a rich physical behavior highlighted in the coexistence of an almost metallic carrier concentration (~5×10 21 cm -3 ) with a large thermoelectric power response of ~160 μV/K at 650 K. This is attributed to a heavy hole effective mass that is almost six times that of the electron rest mass.

Phase Segregation and Thermoelectric Properties of AgPb m SbTe m +2 m =2,4,6, and 8

The preparation and characterization of the AgPbmSbTem+2 family of compounds with m=2, 4, 6, and 8 is reported. Phase segregation was observed in all of these materials. The lattice thermal conductivity of these samples is low (<1.1 W/m-K). Powder x-ray diffraction, thermal analysis, and electron microscope investigations of these systems show that ideal solid solutions are not formed. The transport properties of these composite materials are presented and suggest that they could have promising thermoelectric properties when optimized.

Filled Ir x Co 1-x Sb 3 -based skutterudite solid solutions

Forming iso-electronic solid solutions is a standard approach to achieving low lattice thermal conductivity in thermoelectric materials. The phase diagram for IrxCo1-xSb3 solid solutions is calculated by ab initio method, and a limited solid solubility is predicted. We were able to prepare pure and filled CoSb3 doped with Ir in single phase form for the doping concentrations x less than 0.15. The samples maintain very good electrical transport properties due to weak charge carrier scattering from the iso-electronic atoms and, at the same time, the lattice thermal conductivity is reduced significantly. A systematic study of the lattice thermal conductivity of IrxCo1-xSb3 was carried out over a broad range of temperatures from 2 K to 800 K. The behavior of the lattice thermal conductivity as a function of the filling fraction and the concentration of Ir is presented. The data indicate that Ir-doped skutterudite solid solutions are good candidates for high temperature thermoelectric applications.

Thermoelectric properties of the nanostructured NaPb18-xSn xMTe20 (M=Sb, Bi) materials

The thermoelectric properties of materials with compositions NaPb 18-x Sn x MTe 20 (M=Sb, Bi, x=0, 3, 5, 9, 13, 16 and 18) were investigated in the temperature range 300-670K. All compositions exhibited p-type behavior over the measured temperature range. Electronic properties and transport were tuned through the manipulation of the Pb/Sn ratio. Increasing the Sn fraction results in an increase in electrical conductivity and a decrease in thermopower. The compositions NaPb 13 Sn 5 SbTe 20 and NaPb 9 Sn 9 SbTe 20 show a lattice thermal conductivity of ∼1 W/m/K at room temperature.