Synthesis and characterization of p-type Ca2.5Ag0.3Nd0.2Co4O9 thermoelectric materials for high-temperature aerospace applications

Abstract In this study, Ca 2.5 Ag 0.3 Nd 0.2 Co 4 O 9 polycrystalline ceramics were synthesized via the sol-gel method for high-temperature thermoelectric applications in aerospace. The stable solution was prepared from precursors, solvent, and chelating agent at 25 o C under ambient conditions. Homogeneous xerogels prepared by the sol-gel method were dried at 200 °C and calcined at 800 °C to form Ca 2.5 Ag 0.3 Nd 0.2 Co 4 O 9 powders. The powders were cold-pressed into pellets at 400 MPa and sintered at 900 °C for 24 h in air to produce bulk samples. After the material is manufactured using the sol-gel process, it is thoroughly characterized thermally, structurally, and morphologically using TG-DTA, FTIR, XRD, XPS, and SEM. Thermoelectric characteristics, including the Seebeck coefficient, electrical resistivity, and thermal conductivity, are systematically measured. TG-DTA, FTIR, and XRD results revealed that thermal treatment up to 800-900 °C under oxidative conditions ensured decomposition of organics and nitrates, promoting formation of the Ca 3 Co 4 O 9 with minor Ca 3 O 4 phase. XRD and XPS analyses confirmed the successful incorporation of Ag and Nd dopants and the material’s p-type semiconducting nature. SEM revealed typical plate-like, flake microstructures. Thermoelectric characterization showed a Seebeck coefficient of 267.79 µV/K, electrical resistivity of 14.09 mΩ·cm, and a maximum power factor of 0.51 mW/mK² at 800 °C. The thermal conductivity remained low (1.09 W/mK at 200 °C; 1.06 W/mK at 800 °C), preserving the temperature gradient and enhancing energy conversion efficiency. These findings demonstrate that Ca 2.5 Ag 0.3 Nd 0.2 Co 4 O 9 ceramics exhibit a stable structure and promising thermoelectric performance at high temperatures, making them strong candidates for energy harvesting in aerospace and industrial environments.