Improvement on the high temperature thermoelectric performance of Ga-doped misfit-layered Ca₃Co_4-xGa_xO_9+δ (x = 0, 0.05, 0.1, and 0.2)

Highly densified Ca₃Co_4-xGa_xO_9+δ (0 ≤ x ≤ 0.2) misfit-layered thermoelectric oxides are prepared by solid state reaction methods followed by hot-pressing. Thermoelectric properties of the samples are measured from room temperature to 1200 K. The results show that partial Ga substitution leads to a simultaneous increase of the electrical conductivity and thermopower. The Ga-doped samples have lower thermal conductivity than that of the non-doped sample in the high temperature region (>600 K). The x = 0.05 sample shows a higher figure of merit (Z = 3.37 × 10^-4 K^-1) than that of the non-doped sample (Z = 1.98 × 10^-4 K^-1) at 1073 K, indicating significant improvement of the thermoelectric performance of Ca₃Co₄O_9+δ by partial Ga substitution for Co.