High-temperature thermoelectric properties of late rare earth-doped Ca sub(3)Co sub(4)O sub(9+d)

Misfit-layered oxides Ca sub(3-x)Ln sub(x)Co sub(4)O sub(9+d) with Ln = Dy, Er, Ho, and Lu were synthesized using solid state reactions. The resulting samples were hot-pressed (HP) at 1123 K in air for 2 h under a uniaxial pressure of 60 MPa. Thermoelectric properties of Ca sub(3-x)Ln sub(x)Co sub(4)O sub(9+d) were investigated up to 1200 K. Both the Seebeck coefficient and electrical resistivity increase upon Ln substitution for Ca. Among the Ln-doped samples, the magnitude of Seebeck coefficient tends to increase with decreasing ionic radius of Ln super(3+). The Ln-doped samples exhibit a lower thermal conductivity than the non-doped one due to a decrease of their lattice thermal conductivity. The dimensionless figure of merit, ZT, reaches 0.36 at 1073 K for the Ca sub(2.8)Lu sub(0.2)Co sub(4)O sub(9+d) sample, which is about 1.6 times larger than that for the non-doped counterpart.