High Pressure Synthesis And Properties Investigation Of P-Type Skutterudites Thermoelectric Materials

Skutterudites as one of the most appealing thermoelectric materials have attracted great research interests in the past years. During the endeavor to improve the thermoelectric properties of skutterudites, the developments in p-type skutterudites relatively lag behind compared with n-type counterparts. For practical applications in thermoelectric modules, p-type legs are as important as n-type ones for better performance. In this thesis, we investigate the thermoelectric properties of p-type skutterudites and try to reveal possible ways for thermoelectric properties enhancement.Co-substituited p-type FeSb3-based skutterudites, Ce_yFe_4-xCoxSb₁₂, were successfully synthesized with a high pressure synthesis（HPS） method. The structure, composition, and thermoelectric properties were investigated. The obtained Cey Fe_4-xCoxSb₁₂ samples show the skutterudite structure of Im 3 symmetry. The actual filling fraction of Ce decreases slightly with elevating Co substitution level, and a notable decrease in the hole concentration is observed. The hole concentration decrease shows a more pronounced effect on the electrical resistivity, leading to a significantly reduced power factor in the sample with the highest Co level. As a result, the thermoelectric performance of Ce_yFe_4-xCoxSb₁₂ deteriorates with higher Co content. The maximal ZT of 0.91 was achieved at 763 K for the optimal Ce0.92Fe₄Sb₁₂ sample. To optimize the thermoelectric properties of Fe-based skutterudites, it is crucial to maintain a relatively higher hole concentration.Fe-substituted CoSb3 skutterudites were successfully synthesized with high pressure synthesis method. The composition, structure, and thermoelectric properties were investigated. Co（4-x） FexSb₁₂ samples have the skutterudite structure of Im 3 symmetry. The hole concentration increases with elevating Fe substitution level. Compared with those ambient-pressure synthesized Fe-substituted CoSb3 samples, our samples exhibit enhanced power factor and suppressed thermal conductivity beneficial from high pressure. The optimal Co3.2Fe0.8Sb₁₂ shows a peak ZT of 0.53 at 823 K, the highest value for Co-rich p-type skutterudites with single-element substitution. Starting from Co3.2Fe0.8Sb₁₂, a criterion for combined elemental substitution and filling can be suggested through simple electron counting: the electrons from filling species must be compensated by holes from extra substitution. Current study on Co（4-x）FexSb₁₂ therefore provides a basis for further thermoelectric performance enhancement of Co-rich p-type skutterudites.With HPS, the substitution level of Sn in CoSb3-based skutterudites, Co4Sb₁₂-xSn, was successfully increased to 3 atom%, which is high enough for the appearance of band resonant state theoretically predicted. We investigated the thermoelectric properties and found an abnormal increase of Seebeck coefficient that may connect with the predicted band resonant state. Further increase of Sn substituting level detoriated the thermoelectric performance due to the appearance of secondary phases. As a result, a maximal ZT of 0.24 is reached for the sample Co4Sb11.6Sn0.4 at 800 K. This is the highest value for Sn-substituted CoSb3 up to now and 20% higher than the best value of the sample fabricated under ambient pressure through melting-annealing.In the end, we discuss the influence on lattice thermal conductivity through double/multiple filling（with the same total filling fraction） with a Debye model. Phonon resonant scattering in elemental filled skutterudites is not simply enhanced by rattling atoms with broader frequencies. Actually, it depends on the factors such as the filling fraction and resonant frequency of each filler, which was further verified by our experimental results on Ce0.92Fe₄Sb₁₂ and Ce0.72Ca0.21Fe₄Sb₁₂.