Enhancement Of The Spin Entropy And Thermoelectric Properties In Ca₃Co₄O_9+δ

As a series of thermoelectric meterials which have broad application prospect, cobalt oxide thermoelectric materials have attracted great attention. In Ca3Co4O9+δ (CCO) ceramics, enhanced thermopower originates in the spin of electrons in carriers and the spin entropy plays an important role in improving the thermopower.In this paper, the rare earth element (Ce or Lu) on Ca site and magnetic element (Ni) on Co site co-doping in misfit layered cobalt oxides materials CCO has been investigated systematically. The mechanism of spin entropy controlling and the enhancement of thermoelectric properties by co-doping have been investigated. In addition, we study the effects of CNTs’ doping on the thermoelectric properties of CCO. The results are shown:a) In (Ce, Ni) co-doped CCO materials, the enhancement of spin entropy originates in the decrease of Co4+ concentration derived from Ce3+/Ce4+ partial to substitute Ca2+ and a new model which is provided by Ni2+ and Ni3+ coexistence. The increased spin entropy leads to the enhancement of thermopower. Meanwhile, Ce, Ni ions doping into the lattice of CCO leads the increase of phonon scattering, and the thermal conductivity decrease significantly.b) In (Lu, Ni) co-doped CCO materials, substitutions of Lu3+ to Ca2+ and Ni2+/Ni3+ to Co2+ leads the Co4+ concentration to decrease and enhances the spin entropy. Meanwhile, Ni2+ and Ni3+ coexistence in this series provides a new model for transporting the spin entropy, which contributes to the increase of total spin entropy. Furthermore, phonon scattering is enhanced by Lu and Ni ions doping in CCO system, which leads the lattice thermal conductivity to decrease, and total thermal conductivity as well. The performance of thermoelectric properties gets greatly improved by (Lu, Ni) co-doping.c) As CNTs’ doping in CCO, the thermal conductivity reduction is induced by the CNTs; nanoinclusions, porous structure, and CCO/CNTs; boundaries, which is identified by the dramatic decrease in phonon mean free path. However, CNTs’ doping increases the resistivity of these nanocomposites. Therefore, the addition of CNTs into the CCO matrix does not succeed in improving ZT value and need to be investigated in future.