n-Type skutterudites (R,Ba,Yb)yCo4Sb12 (R=Sr, La, Mm, DD, SrMm, SrDD) approaching ZT≈2.0

The influence of various fillers R (R = Sr, La, Mm, DD, SrMm, SrDD with Mm standing for mischmetal and DD for didymium) on the physical properties of the triple- and multifilled n-type skutterudites with the nominal composition (R _0.33Ba _0.33Yb _0.33) _0.35Co ₄Sb _12.3 is investigated. In comparison to values from previous works of the authors and from the literature it is demonstrated that even small changes in the filling fractions of the fillers as well as in the total filling level influence the power factor and the thermal conductivity. These skutterudites, partially filled with multiple fillers of different chemical nature, can achieve high power factors (>6 mW m ^-1 K ^-2 at 823 K) and at the same time low thermal conductivity, resulting in ZT = 1.4 at 823 K. On the contrary, Yb ₂O ₃ as a secondary phase in (Sr _0.25Ba _0.25Yb _0.50) _0.5Co ₄Sb _12.5 significantly reduces the thermoelectric (TE) performance due to a decreased power factor and an increased lattice thermal conductivity. However, a homogeneous nanoscale distribution of these impurities substantially improves the TE performance, resulting in ZT values of ∼1.6 at 835 K. The ZT value of this skutterudite could be further enhanced to 1.9 (at 835 K) after severe plastic deformation via high-pressure torsion.