Quantum Chemistry Calculation On Thermoelectric Properties Of Ca-Co-O System

In this paper, the thermoelectric properties of Ca3Co2O6, and Ca3Co409 with layered structure are investigated by the quantum chemistry calculation. Their conductivity and good thermoelectric properties are explained. Furthermore, the doped models of Ca3Co2O6 are calculated and effects of substitutional atoms are predicted.The electronic structures of Ca3Co2O6, the Na-doped model Ca1.5Na1.5Co2O6 and the Ni-doped model Ca3CoNiO6 are studied by the quantum chemical software CAmbride Serial Total Energy Package (CASTEP) which is based on Density Function Theory (DFT) and pseudopotential. Effective masses at the top of valence band and at the bottom of conduction band are deduced from the band structure. According to these results, the electronic conductivity, Seebeck coefficient, thermal conductivity and figure of merit of materials are computed. The results are consistent with experimental measurements. The band structure reveals the form of the impurity levels due to the substitutional impurity in semiconductors. Na-doped model shows the character of p-type semiconductor, and Ni-doped model is n-type semiconductor. In both of the two doped models, the calculation results show that their conductivity is higher than that of the non-doped model, while the Seebeck coefficient and thermal conductivity are lower than the non-doped one. Because of the large increase of the conductivity, the figure of merit in Ca1.5Na1.5Co2O6 is enhanced and thermoelectric properties are improved compared with Ca3Co2O6, whereas the figure of merit in Ca3CoNiO6 is much less than the non-doped model because of its large decline of Seebeck coefficient. The calculation result of the figure of merit of Ca3CoNiO6, Ca1.5Na1.5Co2O6 and Ca3CoNiO6 are respectively about 10-5 K-1, 10-4 K-1 and 10-6 K-1.The bond orders, net charges and the composition of molecular orbitals are calculated for the simplified Ca3Co409 model. The results reveal that the bond order of Co(l)-O in Co02 layer is larger than that of Co(2)-O in Ca2CoO3 layer. Furthermore, both of them are larger than those in Ca3Co2O6. The different bond character may cause different thermoelectric properties between Ca3Co4O9 andCa3Co2O6.The error of the calculation methods used in this paper is estimated by comparing the calculation results of Ge, GaP, GaAs arid InP with their experimental results. The error due to ignoring the lattice aberration causing by substitutional atoms is also discussed. The way to increase the accuracy is discussed.