High Temperature Thermoelectric Properties of Y and Fe codopants in Ca₃Co₄O_9+δ

A series of Ca3xYx Co4yFeyO9+δ powders was synthesized by auto-combustion reaction and densified with spark plasma sintering (SPS) processing. The electrical resistivity and the Seebeck coefficient increase with the increasing yttrium content, while the electrical resistivity decreases with the iron content below 0.05. The reasons for the increase in both the electrical resistivity and the Seebeck coefficient with the yttrium substitution are due to the carrier (Co4+) number reduction in CdI2 type CoO2 layer. In contrary, iron substitution below certain content may only occur in rocksalt type Ca2CoO3 layer resulting in the increase of the carrier number. The Y and Fe co-dopants effect on the thermal conductivity is not significant even though the yttrium heavier atomic weight is expected as an effective phonon scattering site. At 800 °C, the improved power factor of 520 μW/m•K2 from x=0.2 and y=0.03 sample was obtained, compared with the power factor of 450 μW/m•K2 from pure Ca3Co4O9+δ, which demonstrates the improvement of Ca3Co4O9+δ thermoelectric properties with co-dopants yttrium and iron.