Improved Thermoelectric Properties of Re-Substituted Higher Manganese Silicides by Inducing Phonon Scattering and an Energy-Filtering Effect at Grain Boundary Interfaces.

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) = S2σ. 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.

Discovery of colossal Seebeck effect in metallic Cu2Se.

Both electrical conductivity σ and Seebeck coefficient S are functions of carrier concentration being correlated with each other, and the value of power factor S2σ is generally limited to less than 0.01 W m-1 K-2. Here we report that, under the temperature gradient applied simultaneously to both parallel and perpendicular directions of measurement, a metallic copper selenide, Cu2Se, shows two sign reversals and colossal values of S exceeding ±2 mV K-1 in a narrow temperature range, 340 K < T < 400 K, where a structure phase transition takes place. The metallic behavior of σ possessing larger magnitude exceeding 600 S cm-1 leads to a colossal value of S2σ = 2.3 W m-1 K-2. The small thermal conductivity less than 2 W m-1 K-1 results in a huge dimensionless figure of merit exceeding 400. This unusual behavior is brought about by the self-tuning carrier concentration effect in the low-temperature phase assisted by the high-temperature phase.