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

Abstract

Misfit‐layered calcium cobaltite [Ca2CoO3‐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.