Thermoelectric Properties of Ga/Ag Codoped Type-III Ba₂₄Ge₁₀₀ Clathrates with in Situ Nanostructures

Because of the low thermal conductivity and high electrical conductivity, type-III Ba24Ge100 clathrates are potentially of interest as power generation thermoelectric materials for midto-high temperature operations. Unfortunately, their too high intrinsic carrier concentration results in a quite low Seebeck coefficient. To reduce the carrier concentration, we prepared a series of Ga/Ag codoped type-III Ba24Ge100 clathrate specimens by vacuum melting and subsequently compacted by spark plasma sintering (SPS). Doping Ga-Ag on the sites of Ge reduces the concentration of electrons and, at higher concentrations, also leads to the in situ formation of BaGe2 nanoprecipitates detected by the microstructural analysis. As a result of doping, the Seebeck coefficient increases, the thermal conductivity decreases, and the dimensionless figure of merit ZT reaches a value of 0.34 at 873 K, more than three times the value obtained with undoped Ba24Ge100.