High Pressure Synthesis And Thermoelectric Performance Of CoSb₃and Li-Filled CoSb₃

Skutterudite materials were named after Skutterud (Norway) where they were discovered. The CoSb3-based materials with skutterudite crystal structure are one of the most promising thermoelectric materials because of the excellent carrier transport properties. In addition, their thermal transport properties can be also greatly optimized after filled with selected appropriate atoms. Great research endeavors have been focused on them due to this phonon glass electron crystal feature.In our experiments, we first fabricated highly densified single-phase n-type polycrystalline CoSb3samples with high pressure sintering method followed by annealing under argon environment. The physical and thermoelectric properties were measured. The results show that at the concentration of majority carrier (electron) is about1019cm-3for the annealed samples, and the samples have high thermal conductivity in the range of9-10Wm-1K-1. For the sample annealed at773K, the power factor reaches1900μWm-2K-2at423K, and the maximum ZT of0.15is achieved at523K. The high thermal conductivity is always the bottleneck in improving thermoelectric properties of CoSb3compared with other thermoelectric materials.To reduce the thermal conductivity, CoSb3bulks with distinct grain size were obtained using ball milling and high pressure sintering method. The thermal conductivity of sample with nanograin was sharply reduced compared the sample sintered without ball milling process. However, the power factor was seriously deteriotated. It is thus hard to improve ZT simply through nanostructural approach.The "phonon glass electron crystal" concept has been applied to synthesis filled CoSb3compounds. The filling atoms in CoSb3void vibrate incoherently and scatter phonon effectively, leading to remarkable reduction of the thermal conductivity without affecting the power factor. However, the conventional prepared methods under ambient pressure are usually time consuming of (generally10days) with limited filling elemental species, which restricted the optimization of their thermoelectric performance.We consider the filling behavior of lithium into CoSb3under high pressure using first principle calculation according to FFL theory. The formation enthalpy AG3has been obtained as a function of filling fraction under different pressure. The results show that the filling fraction of0.25is obtained under12GPa and0.875obtained under20GPa.According to the theoretical calculation results, the compounds of LiyCo4Sb12, with a characteristic Im3symmetry, were synthesized using high pressure synthesis method. LiyCo4Sb12samples possess higher carrier mobility compared with other single elemental filled CoSb3, leading to greatly enhanced power factor. At the same time, Einstein oscillation of Li with smaller ionic radius in CoSb3viod effectively reduced the lattice thermal conductivity. A large ZT of1.3is achieved at700K for Lio.36Co4Sb12, beneficial from the combined contributions mentioned above.We laso checked Pb-filled CoSb3compounds fabricated the same experimental conditions applied to Li-filling. The polycrystalline Pb-filled CoSb3was characterized by X-ray diffraction with the skutterudite Im3symmetry. The Seebeck coefficient and electrical resistivity at room temperature were measured. Further experiments for optimizing thermoelectric properties are suggested according to experimental data.