Effect of partial substitution of Ca for Al on the microstructure and high-temperature thermoelectric properties of CuAlO2

The sintered CuAl1−xCaxO2 (0 ≤ x ≤ 0.2) bodies contained the CuAl1−xCaxO2 solid solution with a rhombohedral structure, R3¯m, along with second phases such as CuO, CaO, Ca3Al2O6, and CaAl4O7 phases. It was found that, as the amount of Ca in the CuAl1−xCaxO2 samples increased, the grain size increased from 0.94 to 3.01 μm and the porosity decreased from 29.3 to 6.8%. The substitution of Ca up to x = 0.1 for Al in the CuAl1−xCaxO2 samples gave rise to an increase in both the electrical conductivity and the Seebeck coefficient. On the other hand, the higher Ca substitution (x ≥ 0.15) decreased both the electrical conductivity and the Seebeck coefficient. The highest value of power factor (7.82 × 10−5 Wm−1 K−2) was attained for CuAl0.9Ca0.1O2 at 1140 K. We demonstrate that the Ca substitution is highly effective for enhancing high-temperature thermoelectric properties.