Study On The Thermoelectric Properties Of CoSb₃ Based Skutterudite Materials

Thermoelectric materials can be extensively used in the fileds such as refrigeration and power generation from waste heat and their properties can be evaluated by a dimensionless figure of merit (ZT), which is determined by an electrical conductivity, a Seebeck coefficient, and a thermal conductivity. Thermoelectric material with excellent properties needs a high electrical conductivity, a high Seebeck coefficient and a low thermal conductivity simultaneously. In this work we selected the CoSb₃ based skutterudite materials as our objects and the different strategies including the rear earth atoms filling, the nanosized morphology controlling, the binary-phased nanostructure and introducing disordered structure were employed in order to improve their thermoelectric properties. The nanosized powders were synthesized via a hydro/solvo thermal route and the resultant powders were hot-pressed into bulks in a vacuum circumstance. The microstructures were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), Raman scattering spectrum, differential scanning calorimetry (DSC) and thermogravimetry (TG), and the thermoelectric properties were characterized by electrical conductivity, Seebeck coefficient, power factor, thermal conductivity and figure of merit. Moreover, the thermoelectric properties have been illustrated and predicted in terms of the theoretical calculations.Rare earths filling and nanostructure have been viewed as the effective ways to optimize the thermoelectric properties of materials, however, we have integrated them into the CoSb₃ based skutterudite materials. Firstly, La, Ce and LaCe were filled into the interstitial voids in the structure and the effects of the different filled rare earths atoms on the thermoelectric properties were studied. In comparison with the unfilled CoSb₃, the electrical conductivity of the rare earths filled (La, Ce, LaCe)Fe₃CoSb₁₂ is decreased slightly, while the Seebeck coefficient is enhanced and meanwhile the thermal conductivity can be suppressed, and thus the figure of merit can be increased significantly. The obtained ZT value (ZT=0.19 at 773 K) for the unfilled CoSb₃ in this work is larger than the reported value 0.11 but smaller than the reported maximum value 0.78. A ZT value of 0.82 at 773 K can be obtained for LaCeFe3CoSb₁₂ in our work, which is increased significantly as compared to the unfilled CoSb₃. Secondly, the LaFe3CoSb₁₂ nanopowders with different morphologies have been prepared via a hydro/solvo thermal route by selecting the solvents and the Sb sources, and the effects of the different nanosized morphology on the thermoelectric properties have also been investigated. The results suggest that the mixed morphology with nanorods and nanospheres can enhance the electrical conductivity but also increase the thermal conductivity slightly, and the single nanospheres can reduce the thermal conductivity but meanwhile decrease the electrical conductivity. As a whole, introducing the nanorods into the nanospheres can enhance the power factor and the figure of merit and a high ZT value of 0.54 at 575 K can be achieved for the LaFe3CoSb₁₂ bulk with a mixed morphology, which is increased by 59 per cent as compared to the value of 0.34 for the bulk with single nanospheres.In order to clarify the effects of the filled rare earths atoms on the thermoelectric properties the electronic structure and the lattice dynamics for the unfilled CoSb₃ and the rare earths filled (La, Ce, LaCe)Fe3CoSb₁₂ were calculated by using first-principles method. The results indicate that the rare earths filling can enhance the effective mass of carriers at Fermi level but decrease the density of state, and the covalent bonding strength between the parent atoms can also be weakened. Consequently, the changes in the electronic structure results in a large Seebeck coefficient, a small electrical conductivity and a low thermal conductivity of the rare earths filled (La, Ce, LaCe)Fe3CoSb₁₂. To illustrate the frequency shifting of the Raman vibration modes resulted from the filled rare earths atoms we have calculated the electronic structure and the phonon vibrational frequency at Brillouin zone center for the unfilled CoSb₃ and the filled (La, Ce, LaCe)Fe3CoSb₁₂. We attribute the frequency shifting of Raman modes to the changes of the complicated covalent bonding caused by the filled rare earths atoms. In addition, we have calculated the phonon dispersion and the phonon density of state of CoSb₃, and then the heat capacity, the Debye temperature, the mean free path of phonons and the theoretical thermal conductivity of CoSb₃ have also been calculated. Our results are consistent with other calculation and the experimental data well.To further optimize the thermoelectric properties of LaCeFe₃CoSb₁₂ skutterudites by increasing the electrical conductivity, the binary-phased nano materials composed of the LaCeFe₃CoSb₁₂ skutterudite nanospheres and the semi-metal Sb microribbons were fabricated via a hydro/solvo thermal route. The microsized Sb ribbons can increase the electrical conductivity and the Seebeck coefficient about 1 time respectively, and the LaCeFe₃CoSb₁₂ nanospheres can maintain a relatively low thermal conductivity. Consequently, the figure of merit can be increased from 0.38 for the single-phased LaCeFe₃CoSb₁₂ to 1.87 for the binary-phased LaCeFe₃CoSb13.5. The enhancement in ZT could be attributed to the combined effects of the grain boundary scattering, the filling voids, the atoms doping and the disordered structure resulted from the different introduced Sb phase.We have attempted to construct a material in which the ordered structure and the disordered structure coexist, and we believe that it could solve the conflicts among the electrical conductivity, the Seebeck coefficient and the thermal conductivity and thus increase the figure of merit. So, the strategy mixing the melted and quenched LaCeFe₃CoSb₁₂ powders into the raw LaCeFe₃CoSb₁₂ powders was adopted to introduce the disordered structure into the ordered crystalline. The results indicate that the disordered structure formed at a higher temperature can be preserved partly into the materials at a room temperature successfully by the above way. The introduced disordered structure can lead to a transition from semimetallic to semiconducting. Although the electrical conductivity can be decreased, the Seebeck coefficient can be enhanced and meanwhile the thermal conductivity can be suppressed. Therefore, the figures of merit can be improved significantly.In conclusion, the rare earths filling combined with the nano engineering, the binary-phased nano structure, and the nanostructure combined with the disordered structure should be considered as the effective ways to optimize the thermoelectric properties of CoSb₃ based skutterudite materials. In addition, integrating the theoretical calculation and the experimental experience should be beneficial to better understanding the substantial relationship between the thermoelectric properties and the microstructure.