Anisotropic Thermoelectric Performance and Sustainable Thermal Stability in Textured Ca₃Co₄O₉/Ag Nanocomposites

The Ca₃Co₄O₉ (CCO) with layered structure has been considered as a potential candidate for high-temperature thermal energy harvesting application. However, CCO's layered structure imparts anisotropy in transport properties, which results in anisotropic thermoelectric performance. So far, limited attempt has been made to understand the anisotropic thermoelectric performance of CCO, which often results in erroneous estimation of the thermoelectric response. Here, we fabricated highly textured CCO/x wt % Ag (x = 0, 1, 3, 5) nanoinclusion composites using the spark plasma sintering (SPS) technique and systematically investigated correlation between microstructure and anisotropic thermoelectric properties. The thermoelectric response was measured along both in-plane and out-of-plane directions (perpendicular and parallel to the pressure axis). We developed a two-step SPS method to achieve enhanced degree of texturing and increased electrical conductivity along ab-planes. The addition of Ag nanoinclusions was found to increase the overall electrical conductivity and thermoelectric power factor due to improved electrical connections among the grains. The peak ZT value for the CCO/3 wt % Ag composites, measured along both perpendicular and parallel directions, was found to be 0.14 and 0.06 at 640 °C, respectively. Almost the same values of resistivity, power factor, and ZT were maintained after repeated thermal cycling. These results reveal that CCO/3 wt % Ag composites have the desired thermal stability, which will make the thermoelectric module reliable for the intended period of operation.