High Pressure Synthesis And Thermoelectric Properties Of CoSb₃-baesd And Layered Bi₂Se₃ Compounds

Due to the threats from fossil energy depletion and environmental degradation,developments of alternative environmental-friendly energy have become urgently needed.Thermoelectric materials,which are the basis for solid-state thermoelectric devices converting between heat and electricity directly,have received increasing attention.Among them,cage-structured skutterudites and layer-structured Bi₂Se₃ compounds,possessing low thermal conductivity and good electrical transport performance,are considered as promising thermoelectric materials working in the middle and low temperature region.In this work,elemental-filled CoSb₃ and layer-structured Bi₂Se₃ compounds were prepared with a high pressure synthesis?HPS?method,and the thermoelectric properties of HPS samples were investigated.The filling behavior of barium into voids of CoSb₃ under high pressure was investigated by the first principles calculations.The filling fraction limit of Ba into the voids of CoSb₃ relies on the competition between the formation of Ba-filled CoSb₃ and that of the secondary phase BaSb2.It was found that the pressure can significantly promote the formation of Ba_yCo₄Sb₁₂?0 < y ? 1?compounds,and increase FFL of Ba.FFLs are 0.375,0.529,0.707,and 0.913 at 0,1,2,and 3 GPa,respectively,and a fully filling is reached under 4GPa.Inspired by the theoretical investigation result,we synthesized Ba-filled CoSb₃ bulks with a high pressure synthesis following by spark plasma sintering?SPS?.The compostion,crystal structure,and low temperature heat capacity measurements all evidenced that pressure can increase the filling fraction limit of Ba,confirming the result of theoretical invesitgation.Higher filling fraction increases the carrier concentration and enhances the electrical transport performance: The electrical resistivity of Ba0.51Co4Sb12 was reduced to 1.97 ??m at room temperatue,and the power factor reached 5400 Wm-1K-2 at 820 K.Meanwhile,the lattice thermal conductivity of Ba0.51Co4Sb12 was lowered to 0.73 Wm-1K-1 at 883 K.ZT of Ba0.51Co4Sb12 reached 1.0 at 883 K.On the basis of Ba-filled CoSb₃ work,we synthesized Ba/Ce dual-filled CoSb₃ sample.The electrical resistivity of Ba0.42Ce0.06Co4Sb12 was reduced to 2.35 ??m at room temperature and increased to 4.5 ??m at 883 K,contributing to a relatively high power factor?4820 Wm-1K-2 at 883 K?.Meanwhile,the lattice thermal conductivity of dual-filled sample was strongly depressed since different atomic weights and ion radii of Ba and Ce can effectively scatter phonons with different frequency.The minimum lattice thermal conductivity of Ba0.42Ce0.06Co4Sb12 sample is 0.75 Wm-1K-1 at 883 K.ZT of Ba0.42Ce0.06Co4Sb12 reached 1.04 at 883 K.This work indicates HPS is also effective to produce dual or multiple elemental filled CoSb₃ skutterudites.Polycrystalline Bi₂Se₃ were prepared with high pressure synthesis.X-ray diffraction measurements on HPS products revealed that the crystal structure of Bi₂Se₃ is closely related to the synthetic pressure.While the trigonal phase with R3 m symmetry?T-Bi₂Se₃?was achieved at 1 GPa,the orthorhombic phase with Pnma symmetry?O-Bi₂Se₃?was dominant at pressure higher than 3 GPa.This result is consistent with the first-principles calculations.XRD and SEM measurements revealed an anisotropic texture in the T-Bi₂Se₃ sample densified with spark plasma sintering,which was further demonstrated in the thermoelectric properties.The highest ZT of 0.37 was achieved at 560 K along the compressing direction of spark plasma sintering,a relatively high value for undoped Bi₂Se₃.