Effect of Ca substitution by Sr on the thermoelectric properties of Ca3Co4O9 ceramics

(Ca1-xSrx)(3)Co4O9 polycrystalline samples (x=0, 0.001, 0.002, 0.004, 0.005, 0.006, 0.01, 0.02, 0.1, 0.2, 0.3, 0.5 and 1) have been prepared by solid state reaction and sintered by spark plasma sintering. Their thermoelectric properties at high temperature have been studied (650-1000K). The substitution limit is close to 20%. For higher Sr contents, a pseudo one-dimensional phase of the type Sr6Co5O15 is appearing. The substitution of calcium by strontium does not significantly affect the Seebeck coefficient. Concerning electrical conductivity and power factor an optimum value exists for 0.5% substitution. Finally ZT values of samples with less than 20% Sr content are increased due to lower thermal conductivities. The dimensionless figure of merit ZT reaches 0.22 at 1000K for the composition (Ca0.995Sr0.005)(3)Co4O9. (C) 2010 Elsevier B.V. All rights reserved.