Advances in Texturing and Thermoelectric Properties of a Calcium Cobaltite Ceramic via Combined Spark Plasma Sintering and Spark Plasma Texturing

Misfit-layered calcium cobaltite [Ca₂CoO_3-δ]_0.62[CoO₂] is an outstanding p-type semiconducting thermoelectric with strong anisotropic properties. Texture engineering is crucial for enhancing its thermoelectric performance in polycrystalline ceramics. The in-plane orientation of the grains improves the Seebeck coefficient and electrical conductivity, while the multi-scale parallel interfaces scatter phonons and reduce thermal conductivity. Here, a tandem process of spark plasma sintering and edge-free spark plasma texturing is used to produce dense and highly textured calcium cobaltite ceramics. The resulting ceramic shows a high degree of texturization, secondary phases, and enhanced electrical conductivity of 246 S cm⁻¹ together with a strongly improved Seebeck coefficient of 224 µV K⁻¹ at 1073 K. High grain ordering leads to carrier mobility of 0.49 cm² V⁻¹ s⁻¹, which has a positive effect on both parameters. With a power factor of 12.4 µW cm⁻¹ K⁻² at 1073 K in air, previous thermoelectric performances of calcium cobaltite are surpassed, regardless of its form: pristine, doped, or composite. By combining the high power factor with a relatively low thermal conductivity, a remarkable figure-of-merit of 0.49 at 1073 K in air is obtained for the textured polycrystalline ceramic, which reaches 60 % of the figure-of-merit of a calcium cobaltite single crystal.