Synthesis, characterization, and thermoelectric properties of Ca_2.5Ag_0.3Ho_0.2Co₄O₉ materials by sol–gel processing

We report here a new approach for the synthesis of p-type Ca_2.5Ag_0.3Ho_0.2Co₄O₉ ceramic materials utilizing the sol–gel method, which facilitates the formation of homogeneous, finely-grained powders. To determine their suitability, the synthesized materials are subjected to a comprehensive assessment that includes thermal, structural, morphological, and thermoelectric properties. For a thorough examination, these materials were put through a variety of characterization procedures, such as DTA-TG, FTIR, XRD, XPS, SEM, and TM. The process parameters were determined based on a combination of scientific results obtained from DTA-TG, FTIR, XRD, XPS, and SEM analyses. Based on these findings, the process involved drying at 100 °C for 3 h, followed by decomposition at 200 °C for 2 h, oxidation and calcination at 800 °C for 2 h, and sintering at 900 °C for 24 h under oxidizing conditions. This procedure ensured the formation of a complete stoichiometric, plate-like, preferred textured, distorted rock salt-type layered Ca_2.5Ag_0.3Ho_0.2Co₄O₉ semiconductor ceramic phase. Thermoelectric measurements indicated that a doubly doped sample with the Ca_2.5Ag_0.3Ho_0.2Co₄O₉ chemical composition reached the power factor of 0.65 mW/mK² at 800 °C. The electrical resistivity of the p-type Ca_2.5Ag_0.3Ho_0.2Co₄O₉ ceramic material was determined to be 13.18 mΩcm, and the Seebeck coefficient was 292.66 μV/K at 800 °C.