First-principle Theory Study On Optimization And Mechanism Of Filled Skutterudites

CoSb3 skutterudites is one of the narrow bandgap semiconductor materials with open structure, it has high Seebeck coefficient and high electrical conductivity and can became be a promising thermoelectric material used in medium temperature by some special optimization method. Up to present, the restraining factor of CoSb3 skutterudites' thermoelectric properties is its high thermal conductivity. It is not clear that how to optimize the thermoelectric performance of CoSb3 skutterudites and what is the mechanism in optimization. For this issue, the lattice structures, electronic structures and transport properties of several kinds of atom single filled and combinational filled CoSb3 have been studied and discussed by combining density functional calculations and Boltzmann transport theory in this paper.There are three parts of works in this paper. Firstly, a detailed study have been done on lattice structure, electronic structure and transport properties of the atoms Ba and In single filled CoSb3 with different filling fraction. The results revealed that lattice structur is expanded by atom filling, and the Sb4 ring became foursquare from rectangle. The band gap linear change with the filling fraction, the bandgap of In filled CoSb3 linear increase with the increasing of filling fraction. On the contrary, the bandgap of Ba filled CoSb3 linear decrease with the increasing of filling fraction. The density of states results indicated that there may be orbital hybridization between atom In and CoSb3. Then, the band structure, density of states, Seebeck coefficient of single-filled CoSb3 by seven kinds of atoms (R0.125Co4Sb12) are calculated by the density functional method. It was found that the band structure of the single-filled CoSb3 with a small quantity of R exhibited no significant changes compared with that of CoSb3. The main change is that Fermi level moves upwards towards the conduction band, and there are impurity bands induced by filling atoms at the bottom of the conduction band. Seebeck results reveal that filling with atoms In and Sn could improve the Seebeck coefficient of CoSb3. In addition, the lattice vibration energy of the fillers in CoSb3 have been calculated. According to the results of electronic structures and lattice vibration energy, the combinations (In, Ca), (In, Ba), (Sn, Eu), and (Sn, La) are even more promising for filling CoSb3. Second, the electronic structures and electrical transport properties of a series of RFe4Sb12 skutterudites have been studied by using density functional calculations and boltzmann transport theory. The results reveal that the band structures the six kinds of RFe4Sb12 have an indirect band gap and valence bands cross the Fermi level, which indicate that all six RFe4Sb12 are p-type semiconductors. The orbital contributions on valence band of the filler atoms are about 1%. But in conduction band area, orbital contributions of filler atoms are about 10%, the maximum contribution of 20.9% by the atom Sn. The Seebeck coefficients of the six types of RFe4Sb12 are very similar due to their similar band structure near the Fermi levelIn the third part, The electronic structure and electrical transport properties of the two series of double filled CoSb3 skutterudites have been studied and discussed by the same way. One is the double filled CoSb3 with atoms combination of Ba, Yb and In. The results indicate that the combination of (Ba, In) could greatly improve the thermoelectric properties while the combination of (In, Yb) and (Ba, Yb) would have negative effect on the power factors, due to the fact that the interaction of Yb atoms with CoSb3 would result in a reduction of the electron mobility. The other kind is atom Ca and four rare earth metal atoms Pr, Nd, Eu and Sm double filled CoSb3. according to the band structure results, four rare earth metal atoms Pr, Nd, Eu and Sm could be divided into two groups, one is atoms Eu and Sm, the other group is atoms Pr and Nd. The results of the electrical transport properties reveal that the doubled filled CoSb3 with the atoms in the same group has the similar properties, and the double filled CoSb3 with the atom Ca and Eu or Sm would have better power factors.