Fabrication of Ca_2.7-xAg_0.3Sm_xCo₄O₉ semiconducting materials for thermoelectric integration in aerospace systems

Ca_2.7-xAg_0.3Sm_xCo₄O₉ (x = 0.1, 0.2, and 0.3) thermoelectric oxides were synthesized via the sol–gel method to investigate the effects of Ag–Sm co-doping on structural, microstructural, and thermoelectric properties. Solution chemistry analysis revealed that increasing Sm content significantly influences pH and turbidity, indicating its strong role in sol evolution and precursor stability. TG–DTA analysis enabled optimization of calcination and sintering temperatures by identifying multistep decomposition associated with organic burnout and nitrate removal. X-ray diffraction confirmed the formation of single-phase misfit-layered Ca₃Co₄O₉ for all compositions, while peak broadening indicated reduced crystallite size and dopant-induced lattice distortion. SEM observations revealed plate-like morphologies characteristic of Ca₃Co₄O₉, with composition-dependent grain refinement and porosity. Thermoelectric measurements demonstrated stable p-type conduction over the entire temperature range investigated. Ag–Sm co-doping enhanced the Seebeck coefficient and significantly modified electrical resistivity, leading to a substantial improvement in power factor, particularly for the Ca₂.₆Ag₀.₃Sm₀.₁Co₄O₉ composition, which achieved a maximum power factor of 0.26–0.27 mW/mK² at high temperatures. Although thermal conductivity increased at low and intermediate temperatures due to enhanced carrier transport, phonon scattering became dominant at elevated temperatures. Consequently, the Ca₂.₆Ag₀.₃Sm₀.₁Co₄O₉ sample exhibited the highest thermoelectric performance, with a maximum dimensionless figure of merit (zT) of 0.16–0.17 at 800 °C, nearly twice that of pristine Ca₃Co₄O₉. These results demonstrate that optimized Ag–Sm co-doping is an effective strategy for improving the high-temperature thermoelectric performance of Ca₃Co₄O₉-based oxides.