Effect of Ga addition on Ca-deficient Ca3Co4Oy thermoelectric materials

Ca3−xGaxCo4Oy polycrystalline thermoelectric ceramics with small amounts of Ga have been synthesized by the classical solid state method. XRD data have shown that all the Ga has been incorporated into the Ca3Co4O9 and Ca3Co2O6 thermoelectric phases and no Ga-containing secondary phases have been produced. Moreover, for Ga contents ≥0.07, the Ca3Co2O6 phase disappears. Apparent density measurements have revealed that all samples are very similar, with densities around 70% of the theoretical one. Electrical resistivity decreases in all Ga-doped samples, compared with the pure ones, while the Seebeck coefficient is practically the same for all samples, except for the 0.12 and 0.15 Ga substituted ones, at high temperatures which present slightly higher values. Both factors in the 0.15 Ga-substituted samples lead to power factor values around 0.35 mW/K2 m at 800 °C which is about 70% higher than the values obtained for the undoped samples.