Enhanced thermoelectric performance of Ca₃Co₄O₉ceramics through grain orientation and interface modulation

The commercial application of Ca3Co4O9 ceramics at high temperatures is greatly restricted due to poor thermoelectric properties. One of the approaches to enhance the performance of oxide thermoelectric ceramics is to control the microstructure of the ceramics to provide texturing. Herein, a high figure of merit of ZT = 0.43 was reported at 1073 K for the highly grain-oriented (Ca0.87La0.03Ag0.1)3Co4O9 ceramics, which were fabricated by tape casting combined with templated grain growth. Multi-scale interfaces, such as the parallel grain boundary and regular zigzag edge, the "core-shell"interface inside the stripe-like grain orientation, and lattice stack default, were observed in the "brick-wall"microstructure. The electron transport properties and thermal conductivity in different directions were investigated during the texture evolution, which was regulated by increasing the sintering temperature. By adjusting the sintering temperature, the sample parallel to the tape-casting direction showed a maximum power factor of 0.64 mW (m-1 K-2) as a result of effective interfacial effects, which was ∼3 times higher than that in the perpendicular direction. The ultralow thermal conductivity of 0.366 W (m-1 K-1) at 1073 K was obtained because phonon scattering was strengthened through the "core-shell"interface and zigzag interface. This work provides a new insight to achieving high-performance thermoelectric oxides using an interesting interface modulation and anisotropic properties through texture engineering.