The Czochralski method was used to grow a 46-mm-long crystal of the Ba8Ga16Ge30 clathrate, which was cut into disks that were evaluated for thermoelectric performance. The Seebeck coefficient and electrical and thermal conductivities all showed evidence of a transition from extrinsic to intrinsic behavior in the range of 600–900K. The corresponding figure of merit (ZT) was found to be a record high of 1.35 at 900K and with an extrapolated maximum of 1.63 at 1100K. This makes the Ba8Ga16Ge30 clathrate an exceptionally strong candidate for medium and high-temperature thermoelectric applications.

The high concentration of grain boundaries provided by nanostructuring is expected to lower the thermal conductivity of thermoelectric materials, which favors an increase in their thermoelectric figure‐of‐merit, ZT. A novel chemical alloying method has been used for the synthesis of nanoengineered‐skutterudite CoSb3. The CoSb3 powders were annealed for different durations to obtain a set of samples with different particle sizes. The samples were then compacted into pellets by uniaxial pressing under various conditions and used for the thermoelectric characterization. The transport properties were investigated by measuring the Seebeck coefficient and the electrical and thermal conductivities in the temperature range 300 K to 650 K. A substantial reduction in the thermal conductivity of CoSb3 was observed with decreasing grain size in the nanometer region. For an average grain size of 140 nm, the thermal conductivity was reduced by almost an order of magnitude compared to that of a single crystalline or highly annealed polycrystalline material. The highest ZT value obtained was 0.17 at 611 K for a sample with an average grain size of 220 nm. The observed decrease in the thermal conductivity with decreasing grain size is quantified using a model that combines the macroscopic effective medium approaches with the concept of the Kapitza resistance. The compacted samples exhibit Kapitza resistances typical of semiconductors and comparable to those of Si–Ge alloys.

Quenched Ge–Sb–Te (GST) compounds exhibit strongly disordered metastable structures whose average structure corresponds to a distorted rocksalt type with trigonal symmetry. Depending on the composition and thermal treatment, the metrics remain more or less pseudocubic. The corresponding stable phases show regular sequences of distorted rocksalt-type blocks that formally result from layer-like cation defect ordering. These thermodynamically stable layered phases can gradually be approached by annealing the metastable (pseudo)cubic compounds that are accessible by quenching high-temperature phases. The relaxation of Te atoms in the vicinity of the defect layers leads to van der Waals gaps rather than defect layers in an undistorted matrix. The partially ordered phases obtained show defect layers with an average distance and arrangement depending on the composition and the thermal treatment of the samples. This variation of the nanostructure influences the lattice thermal conductivity (κL) and thus the thermoelectric figure of merit (ZT). This results in ZT values up to 1.3 at 450 °C for bulk samples of Sb2Te3(GeTe)n (n = 12 and 19). The stability ranges of the various phases have been examined by temperature programmed X-ray powder diffraction and can be understood in conjunction with the changes of the nanostructure involved. The real structure of phases Sb2Te3(GeTe)n (n = 3–19) has been investigated by high-resolution electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM)-high-angle annular dark-field (HAADF) with respect to the stoichiometry and synthesis conditions. The correlation of the nanostructure with the thermoelectric properties opens an interesting perspective for tuning thermoelectric properties.

Partially filled CoSb3 skutterudite compounds are emerging materials for thermoelectric energy conversion at high temperature. CaxCo4Sb12 with different Ca contents has been prepared by the conventional metallurgical route. The temperature dependences of the electrical resistivity, Seebeck coefficient, and thermal conductivity have been measured on these compounds in the 300–800 K temperature range. These measurements have identified Ca as being a true n-type filler atom and offer, for this family of skutterudite, several valuable insights into the potential of Ca to provide good thermoelectric performance.