P-type Ca_2.5Ag_0.3Tb_0.2Co₄O₉ semiconducting materials for thermoelectric generators: Synthesis and characterization

This study focuses on the synthesis and characterization of p-type Ca_2.5Ag_0.3Tb_0.2Co₄O₉ semiconducting materials for thermoelectric applications. Ca_2.5Ag_0.3Tb_0.2Co₄O₉ ceramic materials were successfully synthesized via the sol-gel method for thermoelectric applications. The synthesis involved heat treatment steps, including drying, combustion, calcination, and sintering under oxidative conditions. The produced materials were evaluated for their thermal, structural, morphological, and thermoelectric properties using TG-DTA, FTIR, XRD, XPS, SEM, and TM techniques. TG-DTA analysis shows that Ca_2.5Ag_0.3Tb_0.2Co₄O₉ ceramics achieve the desired structural and functional properties with optimal heat treatment at 800 °C. FTIR analysis shows that at 800 °C, organic and nitrate bonds in Ca_2.5Ag_0.3Tb_0.2Co₄O₉ ceramics are completely decomposed, leading to the formation of the metal–oxide phase. XRD analysis shows that Ca₂.₅Ag₀.₃Tb₀.₂Co₄O₉ powders heat-treated at 800 °C exhibit a phase-pure Ca₃Co₄O₉ structure, where Ag and Tb co-doping induces lattice expansion and distortion, thereby influencing thermoelectric properties. XPS analysis confirms that Ca²⁺, Co³⁺, Ag⁺, and Tb³⁺ ions are successfully incorporated into the Ca_2.5Ag_0.3Tb_0.2Co₄O₉ ceramics, and that doping, along with oxygen vacancies, optimizes the thermoelectric properties. The material exhibited a Seebeck coefficient of 238.18 μV/K, electrical resistivity of 12.98 mΩ cm, and a peak power factor of 0.44 mW/m·K² at 800 °C, highlighting its thermoelectric potential. The results indicate that Tb and Ag co-doping enhances the thermoelectric properties, making this material a promising candidate for thermoelectric generators.