Advanced thermoelectric performance of a textured ceramic composite: Encapsulation of Na_xCoO₂ into a triple-phase matrix

Sodium cobaltite (Na_xCoO₂) is one of the most renowned and thermoelectrically promising p-type cobalt oxide materials, showing exceptional performance in this domain. Nonetheless, its thermal instability in air renders it unsuitable for high-temperature applications such as energy harvesting from industrial waste heat. To utilize the beneficial properties of Na_xCoO₂, microscale Na_xCoO₂ template particles of significantly larger size were effectively embedded within a thermally stable Ca₃Co_4−yO_9+δ–Na_xCoO₂–Bi₂Ca₂Co₂O₉ triple-phase matrix. This approach additionally aimed to enhance the texture and boost the thermoelectric performance of the ceramic composite. Highly textured p-type ceramic composites were fabricated via uniaxial cold-pressing and pressureless sintering in air. The unique hexagonal Na_xCoO₂ template particles, produced through molten-flux synthesis, allowed precise control over their shape and dimensions, while the matrix was synthesized via a sol–gel synthesis. The integrated Na_xCoO₂ particles of the textured composite exhibited increased thermal stability, showing no sign of decomposition at 1173 K in air, whereas the sole template particles decomposed at 1073 K during sintering. A 20 wt% template particle content in the textured composites resulted in a remarkably high and nearly temperature-independent power factor of 8.8 µW cm⁻¹ K², corresponding to an improvement of 13% compared to that of the pure matrix material.