Presently, the only commercially available power generating thermoelectric (TE) modules are based on bismuth telluride (Bi2Te3) alloys and are limited to a hot side temperature of 250 °C due to the melting point of the solder interconnects and/or generally poor power generation performance above this point. For the purposes of demonstrating a TE generator or TEG with higher temperature capability, we selected skutterudite based materials to carry forward with module fabrication because these materials have adequate TE performance and are mechanically robust. We have previously reported the electrical power output for a 32 couple skutterudite TE module, a module that is type identical to ones used in a high temperature capable TEG prototype. The purpose of this previous work was to establish the expected power output of the modules as a function of varying hot and cold side temperatures. Recent upgrades to the TE module measurement system built at the Fraunhofer Institute for Physical Measurement Techniques allow for the assessment of not only the power output, as previously described, but also the thermal to electrical energy conversion efficiency. Here we report the power output and conversion efficiency of a 32 couple, high temperature skutterudite module at varying applied loading pressures and with different interface materials between the module and the heat source and sink of the test system. We demonstrate a 7% conversion efficiency at the module level when a temperature difference of 460 °C is established. Extrapolated values indicate that 7.5% is achievable when proper thermal interfaces and loading pressures are used.

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In this study, we have synthesized a series of low thermal conductivity diamond-like materials with the general formula Cu 2 Ga x Ge 1-x Se 3 for 0 ≤ x ≤ 0.1, and their transport properties were evaluated to establish their suitability for TE based waste heat recovery applications. We report results for the Seebeck coefficient (S), electrical resistivity (ρ), thermal conductivity (κ), Hall coefficient (R H ), crystal structure, and elastic properties of Cu 2 Ga x Ge 1-x Se 3 for x = 0.01, 0.03, 0.05, 0.07 and 0.1. Powder x-ray diffraction revealed that a small amount of a related cubic polymorph appeared along with the orthorhombic parent phase at high Ga concentrations. This cubic phase is related to the parent phase in that both contain three-dimensional tetrahedral diamond-like substructures. All samples showed positive values of S and R H over the entire temperature range studied, indicative of p-type charge carriers. The largest value of S = 446 μVK -1 was observed at 745 K for undoped Cu 2 GeSe 3 . With increasing Ga content, both S and ρ decreased. Low values of κ were observed for all samples with the lowest value of κ = 0.67 W m -1 K -1 at 745 K for undoped Cu 2 GeSe 3 . This value approaches the theoretical minimum thermal conductivity for these materials at high temperatures. Although this diamondlike material has highly symmetric, lower coordination number tetrahedral bonding, an unusually large Grüneisen parameter (γ), a measure of bonding anharmonicity, was observed for Cu 2 Ga 0.1 Ge 0.9 Se 3 . A value of γ = 1.7 was calculated from the measured values of the elastic properties, heat capacity, and volume thermal expansion. Given the fact that all materials investigated have similar elastic property values and likely comparable coefficients of thermal expansion we surmise that this large Grüneisen parameter is a general feature for this material system. We conclude that this high level of anharmonicity gives rise to enhanced phononphonon scattering that is, in addition to the scattering brought about by the disordered structure, resulting in very low values of thermal conductivity.