J. Yang

High figure of merit in Eu-filled CoSb3-based skutterudites

We report measurements of electrical resistivity, thermopower, thermal conductivity, and Hall coefficient of polycrystalline Eu-doped CoSb3-based skutterudites with compositions Eu0.20Co4Sb12, Eu0.43Co4Sb11.59Ge0.31, and Eu0.42Co4Sb11.37Ge0.50. The relatively high mobility of these compounds, as compared to that of La- and Ce-filled skutterudites, may play a role in the large thermoelectric figure of merit (ZT>1 at 700 K) of Eu0.42Co4Sb11.37Ge0.50. We discuss the significant potential of these compounds for thermoelectric applications.

Enhanced thermoelectric figure of merit of CoSb3 via large-defect scattering

We have measured the structural, chemical, and transport properties of a series of CoSb3 skutterudite samples modified by fullerene additions of 0, 0.52, 3.28, 3.90, 4.77, and 6.54 mass%. Fullerene is a 60-atom carbon molecule that forms microsize clusters between the grain boundaries of CoSb3. We observed that the dominant scattering mechanism in the electrical transport changes from impurity scattering to grain-boundary scattering near a C60 content of ∼5–6 mass%, and that thermal conductivity decreases with increasing C60 content. A significant increase in the thermoelectric figure of merit is achieved for 6.54 mass% C60 compared to the pure CoSb3.

Structural and transport properties of Ba8Ga16SixGe30−x clathrates

We report on the structural, chemical, electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall measurements of polycrystalline Si–Ge type-I clathrates with nominal composition of Ba8Ga16SixGe30−x such that a constant Ga-to-group-IV element ratio is maintained while varying the Si-to-Ge ratio. Electrical transport measurements on the n-type specimens show a modest increase in the absolute Seebeck coefficient and a decrease in electrical resistivity with increasing Si content. This may imply a modification in the band structure with Si substitution and reveals a possible approach for optimizing these materials for thermoelectric applications. These data are compared with those of Ba8Ga16Ge30 and Sr8Ga16SixGe30−x.

Predication of an ultrahigh filling fraction for K in CoSb3

The filling fraction limit (FFL) for potassium in skutterudite CoSb3 has been investigated by first-principles methods. The FFL is derived by minimizing the formation energy of a reaction that includes the formation of both the filled skutterudite KyCo4Sb12 and the potential secondary phases KSb and CoSb2 when doping K into CoSb3. Our calculations show that K has an ultrahigh filling fraction up to more than 60% in CoSb3, as compared with those of previously reported fillers such as Sr, Ba, Ca, La, Ce, Eu, and Yb [X. Shi et al., Phys. Rev. Lett. 95, 185503 (2005)]. The extent to which this affects the thermoelectric properties of K-filled skutterudites is discussed.

Assessing the role of filler atoms on the thermal conductivity of filled skutterudites

We investigate the lattice thermal conductivity of a series of La- and Yb-filled skutterudite antimonides by modeling different phonon-scattering parameters using the Debye approximation. We also investigate the contributions of the different phonon-scattering mechanisms in understanding the relative importance of the different scattering mechanisms on filled skutterudites. We find that increasing the filler atom concentration effectively increases both point defect scattering and resonance scattering of phonons. In addition, point defect scattering can have a substantial effect in lowering the thermal conductivity of skutterudites. These results help elucidate the role of the different scattering mechanisms on the thermal conductivity of these materials.

Statistical Analysis of a Round-Robin Measurement Survey of Two Candidate Materials for a Seebeck Coefficient Standard Reference Material.

In an effort to develop a Standard Reference Material (SRM™) for Seebeck coefficient, we have conducted a round-robin measurement survey of two candidate materials—undoped Bi2Te3 and Constantan (55 % Cu and 45 % Ni alloy). Measurements were performed in two rounds by twelve laboratories involved in active thermoelectric research using a number of different commercial and custom-built measurement systems and techniques. In this paper we report the detailed statistical analyses on the interlaboratory measurement results and the statistical methodology for analysis of irregularly sampled measurement curves in the interlaboratory study setting. Based on these results, we have selected Bi2Te3 as the prototype standard material. Once available, this SRM will be useful for future interlaboratory data comparison and instrument calibrations.

Solid solubility of Ir and Rh at the Co sites of skutterudites

Phase diagrams for skutterudite solid solutions made by alloying at the Co site of CoSb3 skutterudite structure are studied by density functional theory. A limited solid solubility is observed for IrxCo1−xSb3 and RhxCo1−xSb3, showing good agreement with experimental reports. A correlation between solubility and material preparation procedure is discussed. A simple method to synthesize skutterudite nanocomposites is proposed. Our calculations also demonstrate that IrxRh1−xSb3 is a compound with ordered structure rather than a random solid solution.

Semiconductors and Thermoelectric Materials

We have reviewed thermal conductivities of conventional and newly developed thermoelectric materials. For most conventional materials and half-Heusler alloys, low lattice thermal conductivities are achieved via alloying between isomorphous compounds without deteriorating the electrical properties. In the cases of filled skutterudites, clathrates, and some of the novel chalcogenides, guest ions can be inserted into the large interstitial voids of the crystal structures. These guest ions rattle inside the voids and therefore interact resonantly with low-frequency acoustic phonons, leading to significant thermal conductivity reduction. Phonon scattering by valence disorder is responsible for the low thermal conductivity observed in Tl9GeTe6. In-depth investigation on the NaCo2O4-based oxides, potential hightemperature thermoelectric materials, is much needed to clarify the exact phonon scattering mechanisms in these compounds.