RESUMO
Recently, thermoelectric (TE) materials have been attracting great attention due to their improved capability to convert heat directly into electricity. PbTe-based TE materials are among the most competitive ones; however, lead toxicity limits their potential applications. Thus, the current focus in the field is on the discovery of lead-free analogues. GeTe is considered to be a promising candidate, however, its thermoelectric performance is limited by a non-ideal band structure and intrinsic Ge vacancies. In this work, GeTe was co-doped with Bi, Zn, and In. Initial doping with Bi enhances the performance by tuning the electronic properties and bringing down the thermal conductivity. Subsequent Zn doping permits to maintain the high power factor by increasing carrier mobility and reducing carrier concentration. Additionally, Zn incorporation lowers thermal conductivity and, thus, increases the performance. Subsequent In doping in (Ge0.97Zn0.02In0.01Te)0.97(Bi2Te3)0.03 reduces thermal conductivity even further and makes this material the best performing one. Scanning transmission electron microscopy shows the presence of nano twinning, defect layers, and dislocation bands that contribute to the suppression of the lattice thermal conductivity. A peak zT value of 2.06 and an average zT value of 1.30 have been achieved in (Ge0.97Zn0.02In0.01Te)0.97(Bi2Te3)0.03. These results are among the best state-of-the-art thermoelectric materials.
RESUMO
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 (R3Ì ), and Sb doping induces a structural transition to the cubic phase (ideal skutterudite, Im3Ì ). 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.
