Hierarchically porous Ca₃Co₄O₉ ceramics prepared by freeze casting: Emerging ultra-low thermal conductivity and apparent anisotropy

The freeze-casting method shows great potential for environmentally friendly and industrial scale-up application in thermoelectric materials due to low cost, low toxicity, and pore control. The porous Ca₃Co₄O₉ ceramics with anisotropic thermoelectric performance were fabricated by the freeze-casting method. High porosities, hierarchical pores, and three-dimensional (3D) networks significantly contributed to the suppressed thermal conductivity. An ultralow total thermal conductivity of 0.21 W/(m·K) was obtained. The optimized freeze-casting conditions gave rise to a Seebeck coefficient of 232.3 µV/K. In addition, the ZT value of 0.39 was obtained at 800°C due to the manipulation of hierarchically connected pores and small portions of isolated hexagonal pores. This work provides a new route to form the 3D pores network in oxide thermoelectric ceramics, leading to a decrease in thermal conductivity and optimized thermoelectric performance parallel to the freeze-casting direction.