p-type High Temperature Thermoelectric Behavior of Dy Filled CoSb₃ and Fe_1.5Co_2.5Sb₁₂ and Their Magnetic Properties

Skutterudites are considered to be an ideal choice of material for medium temperature waste heat conversion into usable electrical power. Their inherent electrical transport characteristics together with an option to reduce their thermal conductivity by filling the voids facilitates enhancement of their figure-of-merit zT. Pure CoSb3 with a hole dominant high temperature transport has been found to retain its behavior even after filling the voids with an electron donating ferromagnetic rare-earth Dy. A systematic increase of Dy from 0 to 0.4 is found to increase the maximum value of the Seebeck coefficient from +122 to +146 μVK–1 for a Dy content of 0.3 before decreasing for 0.4. The electrical conductivity exhibits a degenerate semiconducting behavior with the conductivity decreasing due to Dy-filling. This is due to a decrease in room temperature mobility from 483 to 169 cm2V–1s–1, although hole carrier concentration increases slightly. The low temperature magnetic properties of these alloys also show an interesting behavior: diamagnetic to ferromagnetic to paramagnetic transition with increasing Dy-filling. Thermal conductivity on the other hand decreases drastically due to void filling by Dy to a lowest value of 2.7 Wm–1K–1 for 0.3 Dy at 775 K from 3.8 Wm–1K–1 with an enhancement in zT from 0.14 to 0.21, a 50% increase. Partial substitution of Co with Fe in the DyzCo2.5Fe1.5Sb12 alloys retains the hole transport behavior, enhances the Dy-filling fraction limit, and also leads to significant improvement in thermoelectric properties. The maximum Seebeck coefficient increases to 155 μVK–1 while thermal conductivity decreases further to 1.05 Wm–1K–1, the lowest value for a single-filled skutterudite. The corresponding zT increases to 0.67, an increase of ∼380% compared to Co4Sb12. The Dy-filled Fe-substituted alloys on the other hand show clear ferromagnetic behavior even at room temperature. These studies clearly show that Co or Sb substitution is not essential to have hole transport and that Dy introduces states at the valence band edge, retaining the p-type characteristics.