Effects of Eu and Fe co-doping on thermoelectric properties of misfit-layered Ca3Co4O9+δ

Polycrystalline Ca3-xEuxCo4-yFeyO9+delta samples were synthesized by a solid-state reaction method and consolidated by spark plasma sintering. Effects of Eu/Fe dual doping on thermoelectric properties have been systematically investigated. The results indicate that the alteration of resistivity results from the change of carrier concentration, while the variety of thermopower originates from the spin entropy. The thermal transport process is mostly dominated by the point-defect scattering in this system. The transport properties imply that Ca3-xEuxCo4-yFeyO9+delta undergo a transition from the thermally activated semiconductor into two-dimensional variable range hopping semiconductor at x = 0.05 and y = 0.1. A larger ZT value of 0.027 at 300 K was achieved for Ca2.95Eu0.05Co3.9Fe0.1O9+delta, which is about 60 % larger than that of the pure Ca3Co4O9+delta. This investigation suggests that combining a co-doping approach and SPS technology is an effective path to enhance the ZT value for polycrystalline Ca3Co4O9+delta.