Electronic Structure And Thermoelectric Properties Of Misfit Layered Cobaltite And Doped Series

Ca3Co4O9and its doped series is one of the most prospect thermoelectric materials inthe field of medium-high temperature nowadays. Researshers of domestic and foreignfocus on doped modification to inhance the thermoelectric conversion efficiency ofCa3Co4O9and have some positive result. Previous studies and literature mainlyconcentrated on the experimental aspect, but experiment cycle span longer, cost moreresources,and the theoretical calculation study can make up for these deficiencies.This paper taking Ca3Co4O9as the research object, using the discrete variationaldensity functional method (DFT-DVM), the density of state, covalent bond andionic bond， and the relationship between the thermoelectric performance ofCa3Co4O9is discussed, explained why Ca3Co4O9have good thermalperformance.Doped Ca3Co4O9system is discussed and calculated on the density ofstate and the change of the covalent bond and ionic bond，and the relationshipbetween the thermoelectric performance change, the influence of different elementsdoped on the thermoelectric properties is predicted.The research results of this paper show that the covalent bond and ionic bond of Co-O about CoO2layer are much more stronger than the Ca2CoO3layer, the valence bandand conduction band near the Fermi level is mainly contributed by O2p and Co3datomic orbital of Ca2CoO3layer, the thermoelectric performance of the Ca3Co4O9system is mainly decided by the interaction of atom Co and O of Ca2CoO3layer, andhave no direct link with CoO2layer. And the conduction mechanism of the Ca3Co4O9system is the same while transition metal elements doped.The valence band andconduction band near the Fermi level is mainly contributed by O2p and Co3datomic orbital of Ca2CoO3layer, and Co3d atomic orbital of CoO2layer while Ladoped, which has certain difference with no doped Ca3Co4O9system about theconduction mechanism of thermoelectric performance.This paper chooses five kinds of transition metal elements (Ti, Cr, Mn, Fe and Cu)and La as dopant, different elements doped Ca3Co4O9system’s density of state,covalent bond and ionic bond strength and the change of the relationship between thethermoelectric performance are studied respectively, the results showed that the improvement of thermoelectric performance about Ti, Cr, Mn and Fe dopedCa3Co4O9system is not obvious, and the improvement of thermoelectric performanceabout Cu doped Ca3Co4O9system is the most obvious, the results show that researchabout doped on Ca3Co4O9system for the improvement of the thermoelectricperformance of the Ca3Co4O9system is reasonable.La doped Ca3Co4O9system, the thermoelectric performance of Ca3Co4O9systempresent progressively worse first and better later with the increase of doped amount,[LaxCa12-xCo14O42](x=1,2,…,6), while x=3, the thermoelectric performance of theCa3Co4O9system come to a minimum, as the amount of La doped increase, thethermoelectric performance of the Ca3Co4O9system gradually getting better to amaximum while x=5, as the amount of La doped continued, the change tendencyof the thermoelectric performance of the Ca3Co4O9system getting worse.Thissuggests that La doped are helpful to improve the thermoelectric performance of theCa3Co4O9system, but not the greater of the doped amount, the better, there is anappropriate amount of doped. The thermoelectric performance of the Ca3Co4O9system achieve the best value while x=5in this doped model.