Thermoelectric properties of the Bi- and Na-substituted Ca3Co4O9 system

Abstract

Bi- and Na-substituted ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$ polycrystalline samples have been prepared using a hot-pressing technique and their thermoelectric properties were carefully studied in air from room temperature to 1000 K. The substitutions of ${\mathrm{Bi}}^{\text{3+}}$ and ${\mathrm{Na}}^{+}$ for ${\mathrm{Ca}}^{\text{2+}},$ as well as ${\mathrm{Bi}}^{\text{3+}}$ and ${\mathrm{Na}}^{+}$ double substitution, cause both the electrical conductivity (σ) and thermoelectric power $\mathrm{(S)}$ to increase simultaneously. The double substitution has also been demonstrated to be effective to decrease the thermal conductivity (κ). The dimensionless figure of merit $\mathrm{ZT}$ ${\mathrm{(=S}}^2\mathrm{\sigma T/\kappa )}$ reaches 0.32 at 1000 K in the double-substituted sample.