Enhanced Thermoelectric Performance in Ternary Skutterudite Co(Ge0.5Te0.5)3 via Band Engineering.

We report the phase evolution and thermoelectric properties of a series of Co(Ge0.5Te0.5)3-xSbx (x = 0-0.20) compositions synthesized by mechanical alloying. Pristine ternary Co(Ge0.5Te0.5)3 skutterudite crystallizes in the rhombohedral symmetry (R3̅), and Sb doping induces a structural transition to the cubic phase (ideal skutterudite, Im3̅). The Sb substitution increases the carrier concentration while maintaining a high thermopower even at higher doping levels owing to an increased effective mass. The exceptional electronic properties exhibited by Co(Ge0.5Te0.5)3 upon doping are attributed to the carrier transport from both the primary and secondary conduction bands, as shown by theoretical calculations. The enhanced electrical conductivity and high thermopower increase the power factor by more than 20 times. Because the dominant phonon propagation modes in binary skutterudites are associated with the vibrations of pnictogen rings, twisting the latter through the isoelectronic replacement of Sb4 rings with Ge2Te2 ones, as done in this study, can effectively reduce the thermal conductivity. This leads to an increase in the dimensionless figure-of-merit (zT) by a factor of 30, reaching 0.65 at 723 K for Co(Ge0.5Te0.5)2.9Sb0.1.

Designing a Thermoelectric Copper-Rich Sulfide from a Natural Mineral: Synthetic Germanite Cu22Fe8Ge4S32.

This study shows that the design of copper-rich sulfides by mimicking natural minerals allows a new germanite-type sulfide Cu22Fe8Ge4S32 with promising thermoelectric properties to be synthesized. The Mössbauer spectroscopy and X-ray diffraction analyses provide evidence that the structure of our synthetic compound differs from that of the natural germanite mineral Cu26Fe4Ge4S32 by its much higher Cu+/Cu2+ ratio and different cationic occupancies. The coupled substitution Cu/Fe in the Cu26-xFe4+xGe4S32 series also appears as a promising approach to optimize the thermoelectric properties. The electrical resistivity, which decreases slightly as the temperature increases, shows that these materials exhibit a semiconducting behavior, but are at the border of a metallic state. The magnitudes of the electrical resistivity and Seebeck coefficient increase with x, which suggests that Fe for Cu substitution decreases the hole concentration. The thermal conductivity decreases as the temperature increases leading to a moderately low value of 1.2 W m-1 K-1 and a maximum ZT value of 0.17 at 575 K.