Enhanced thermoelectric performance of optimized Ba, Yb filled and Fe substituted skutterudite compounds

Abstract

It is well known that Ba and Yb are effective pair-filling species in skutterudites. They offer a large mass and size contrast therefore yielding widely different phonon resonant scattering frequencies. These features of Ba and Yb fillers inspired our investigation of the high temperature thermoelectric properties of Fe substituted and Ba/Yb double-filled skutterudite compounds of compositions BaxYbyFe0.4Co3.6Sb12 (0.25 <= x, y <= 0.4 nominal). We measured the Seebeck coefficient, electrical conductivity, thermal conductivity, and the Hall coefficient from 300 K to 800 K. All samples were prepared using the traditional method consisting of melting-annealing followed by spark plasma sintering. The Hall coefficient and the Seebeck coefficient signs indicate that all samples are n-type conductors. The electrical conductivity increases and the Seebeck coefficient decreases with the increasing Ba content, likely due to the increasing carrier density as the amount of Ba increases. It is seen that Ba is more effective than Yb in tuning electronic transport properties while Yb has a greater effect on the lattice thermal conductivity. EMPA and PXRD analyses indicate that increasing the Ba content reduces the filling fraction of Yb, therefore the solubility limit of combined Ba + Yb filling remains about 0.4 (actual) for BaxYbyFe0.4Co3.6Sb12 (0.25 <= x, y <= 0.4 nominal) compositions. The combination of a high power factor (47 mu W/K-2 cm) and a reasonably low thermal conductivity (2.79 W/m K) of Ba0.3Yb0.3Fe0.4Co3.6Sb12 (nominal) leads to a value of the thermoelectric figure of merit ZT similar to 1.35 at 800 K, one of the highest values of ZT for double-filled n-type skutterudite compounds. (C) 2013 Elsevier B.V. All rights reserved.