Synthesis, characterization, and thermoelectric properties of Ca_2.5Ag_0.3Lu_0.2Co₄O₉ materials by sol-gel processing for thermoelectric generators

Layered cobaltite-based oxides are promising p-type thermoelectric materials due to their high thermal stability, oxidation resistance, and decent Seebeck coefficients at elevated temperatures. In this work, Ca_2.5Ag_0.3Lu_0.2Co₄O₉ ceramics were successfully synthesized via a sol-gel method and investigated for their structural, microstructural, and thermoelectric properties. Ca_2.5Ag_0.3Lu_0.2Co₄O₉ ceramics synthesized via the sol-gel method demonstrated a stable, homogeneous solution (pH 1.32, turbidity 10.17 ntu). After thermal treatment, the powders formed a pure layered monoclinic Ca₃Co₄O₉ phase with minor Co₃O₄, preferred (00 l) texture, and dopant-induced lattice strain. Comprehensive characterization, such as TG-DTA, FTIR, XRD, XPS, SEM, and TM, confirmed stepwise removal of organics, nitrates, and water, successful incorporation of Ag⁺ and Lu³⁺, mixed Co³⁺/Co⁴⁺ valence states, oxygen vacancies, and well-bonded M − O frameworks. SEM revealed hierarchical microstructures with micron-sized agglomerates composed of nanoscale platelets (30–400 nm) forming stacked, anisotropic layers, promoting efficient charge transport and phonon scattering. Thermoelectric measurements showed p-type behavior with the Seebeck coefficient increasing from 233.17 μV/K to 272.70 μV/K, and electrical resistivity decreasing from 14.96 to 14.31 mΩ cm. The power factor rising from 0.36 to 0.52 mW/mK² at 800 °C confirmed that the material's structural, microstructural, and electronic features are well-optimized for intermediate-to high-temperature thermoelectric applications.