A power-generation test for oxide-based thermoelectric modules using p-type Ca ₃Co ₄O ₉ and n-type Ca _0.9Nd _0.1MnO ₃ legs

Metal oxides are considered to be promising thermoelectric (TE) materials, especially for high-temperature power-generation applications, because they have many advantages such as low price, light weight, thermal stability, nontoxicity, and high oxidation resistance. For these reasons, oxide-based TE modules were fabricated using p-type pure Ca ₃Co ₄O ₉ and n-type Ca _0.9Nd _0.1MnO ₃ legs for power generation at temperatures in excess of 1000 K. This study involved the use of Ag sheets with a Ag paste as electrode materials and alumina plates as a substrate for the modules. The p-type pure Ca ₃Co ₄O ₉ legs were manufactured by spark plasma sintering, and the n-type Ca _0.9Nd _0.1MnO ₃ legs were sintered by a conventional process at atmospheric pressure. From a unicouple, a power density as high as 93.2 mW/cm ² under a temperature condition of ΔT = 727 K (T _hot = 1175 K) was obtained. This high power density is believed to be a result of the modified contact of the electrode (notch process) and the optimized material properties (the SPS process and a dopant effect) along with the high ΔT obtained in this study (reduced thermal losses because of good packing of thermal insulation). Areas of concern for future research include the following: (1) the measured open-circuit voltage from the present unicouples was only 94.3% of the theoretical voltage, and (2) the internal resistance value was as high as 490% of the theoretical resistance.