In this study, we developed a higher manganese silicide (HMS) that possesses a high dimensionless figure of merit ZT exceeding unity. HMSs containing a larger amount of Re than its solubility limit were prepared by the liquid quenching technique, and the obtained metastable HMSs showed good thermal stability to enable pulse current sintering at 1240 K. The lattice thermal conductivity was effectively reduced with increasing Re concentration, whereas the electron transport properties were not greatly affected. Consequently, the ZT of p-type HMS increased to 1.04 at 6 at. % Re from 0.4 of the Re-free sample.

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In this study, the effect of the grain boundary density on the transport properties of the Re-substituted higher manganese silicide Mn30.4Re6Si63.6 has been investigated. The efficiency of electrical energy conversion from waste heat, mainly in thermoelectric generators, depends on how the thermal conduction is reduced, while the charge-carrier electrons/holes contribute to possess a large magnitude of both the electrical conductivity σ and Seebeck coefficient S. In this work, we tried to obtain such a condition with a novel approach of merging the energy-filtering effect at the grain boundaries to improve the power factor (PF) = S 2σ. The nanostructuring and heavy-element substitution were also employed to greatly scatter the phonon conduction. As a result, enhancement of the PF was observed in the diffused nanostructure of annealed ribbon samples, and the enhancement was correlated with the formation of Schottky barriers at the grain boundary interface. Together with a reduction of the thermal conductivity to very low magnitude 1.27 W m–1 K–1, we obtained a maximum ZT = 1.15 at 873 K for the annealed ribbon samples.

High-performance Si–Ge-based thermoelectric materials were prepared simultaneously using nano-structuring and electronic structure modifications. Density functional theory calculations predicted that Fe atoms in the Si–Ge alloy would constructively modify the electronic structure to significantly increase the Seebeck coefficient, experimentally confirmed as ∣S∣ > 517 ± 20 μV K−1 at 673 K. Dense bulk samples made of nano-grains possessed very small thermal conductivity, κ < 0.80 ± 0.10 W m−1 K−1 at T < 873 K. Very large ZT values exceeding 1.00 were obtained in the temperature range 710 K < T < 873 K, with a maximal value of 1.88 at 873 K.