Texture, mechanical and thermoelectric properties of Ca3Co4O9 ceramics

Ca3Co4O9 thermoelectric (TE) ceramics have been processed using conventional sintering (CS), hot-pressing (HP) and spark plasma sintering (SPS). Microstructure investigations have revealed that HP processing is effective for obtaining highly textured 349 ceramics, with a degree of crystallite orientation showing a texture index of 6 mrd2, and that SPS affords strong densified 349 ceramics, typically 98% of the theoretical X-ray density. Investigations of mechanical and thermal properties were undertaken for a reliable integration of these materials in practical devices. In detail, these properties were assessed by microindentation, nanoindentation and three point bending tests. Hardness, elastic modulus, strength and fracture toughness of the HP and SPS samples were drastically improved, compared with the CS specimen. The thermal expansions were measured and shown, for the samples processed by HP and SPS, different between the directions parallel and perpendicular to the stress axis. Thermoelectric properties were investigated from 5 to 850 K and a remarkable power factor value of 550 μW m−1 K−2 was obtained at 850 K. Mechanical and TE properties were correlated to the microstructure and texture features.