| Thermoelectric materials as energy convention materials have attracted extensive research interests in recent years due to their environmental friendliness and flexibility. Skutterudites represented by Co Sb3 are typical medium-temperature thermoelectric materials and of good prospects in energy convention applications. Co Sb3 shows moderate electrical conductivity and high Seebeck coefficient. However, the relatively high thermal conductivity limits its application in thermoelectric devices. Experimental results suggest that substitution for framework atoms as well as filling of lattice void can effectively reduce the lattice thermal conductivity of Co Sb3, improve the electrical transport properties, and eventually great enhance the thermoelectric performance. This dissertation is focused on Co Sb3-based compounds. We explored the effect of distinct doping and filling elements on the thermoelectric properties of Co Sb3 materials.Co Sb3 and Co8Sb23 Te structures were builted and the first principle callculations were performed with CASTEP. The electronic structures under different pressures were investigated theoretically. The electrical transport properties of Co8Sb23 Te as a function of chemical potential and carrier concentration were also investigated under different pressures based on Boltzmann transport theory. The results indicate that the Seebeck coefficient maxmum increases with increasing pressures, while the ratio of electrical conductivity to the relaxation time decrease first and then increase with increasing pressure. The ratio of the power factor to the relaxation time at 20 GPa is twice of that at 0 GPa, suggesting high pressure can improve the thermoelectric properties of Co8Sb23 Te.Guided by the theoretical results, single phase Te doped Co Sb3 compounds were fabricated with high pressure synthesis. The effect of Te doping on the thermoelectric properties was investigated. The carrier concentration increased to the optimum with increasing Te content, and the maximal power factor of 4000 mWm-1K-2 were achieved at 650 K for Co4Sb11.5Te0.5, which is comparable to the best value of the single elemental filling Co Sb3. The point defects and extra electrons due to Te substitution can generate extra scattering to phonons, which depress the thermal conductivity effectively, and enhance the thermoelectric performance eventually. The maximal ZT of 1.15 was achieved for Co4Sb11.5Te0.5 at 883 K, which is as high as that of single elemental filled Co Sb3.To further reduce the thermal conductivity, Fe and Te dual-doped Co Sb3 are prepared with the same method as Te doped Co Sb3. The effect of Fe and Te dual-doping on the thermoelectric properties is investigated. The results suggest that for the samples with the same number of electrons, the resistivity of dual-doped samples are higher than that of single Te doped Co Sb3. Nonetheless, high power factor is maintained in dual-doped samples. The presence of Fe can further enhance the point defect scattering and depress the thermal conductivity. The highest ZT value of 1.26 was achieved in Co3.8Fe0.2Sb11.3Te0.7 at 883 K.Sm filled Co Sb3 samples were synthesized for the first time through a melt-annealing method at ambient pressure, and their structure and thermoelectric properties were investigated. Rietveld refinement shows the maximum filling fraction of Sm in Co Sb3 was 0.08. Due to the enhanced power factor and depressed thermal conductivity originated from Sm filling, a ZT value of 0.8 was obtained in Sm0.6Co4Sb12.High pressure synthesis was employed to synthesize Sm filled Co Sb3 compounds. In order to increase Sm filling fraction in Co Sb3 and realize the optimum of carrier concentration, high pressure technical were applied to fabricate Sm filled Co Sb3 compounds. XRD characterization, physical properties measurement, the transport properties of SmyCo4Sb12 was investigated. Results suggest that the maximum filling fraction of Sm break through to 0.20, reaching the optimal fraction of three valence elements. The power factor enhanced to a value of 5000 mWm-1K-2, and the thermal conductivity decreased to 1.33 W/m K at 770 K, and eventually a ZT value of 1.17 is achieved for the sample with Sm filling fraction of 0.2, which is 45% higher than that of sample synthesized at atmosphere pressure. Compared with normal atmosphere pressure synthesis method, high pressure synthesis can effectively increase element filling fraction, and further optimize their thermoelectric properties. |
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