Cross-scale porous structure design leads to optimized thermoelectric performance and high output power for CaMnO3 ceramics and their uni-leg modules

• Three kinds of cross-scale pores with nano-, micro-, and sub-millimeter scales are designed and successfully synthesized for Ca 0.96 Dy 0.02 Yb 0.02 MnO 3 ceramics by controlling synthesis process. • An ultra-low lattice thermal conductivity κ L of 0.39 W/mK comparable to that of the insulating bricks was obtained for the submillimeter-porous ceramics. • The p a and ϕ as high as 104.94 mW·cm −2 and 0.54 µW·cm −2 ·K −2 are obtained for nano-porous ceramic module, which are far greater than the performance of the uni-leg CaMnO 3 module, and even greater than that of most π-type thermoelectric module composed of CaMnO 3 reported up to now. In this work, three kinds of cross-scale pores with nano-, micro-, and sub-millimeter scales are designed and successfully synthesized in the same composition of Ca 0.96 Dy 0.02 Yb 0.02 MnO 3 , and the effects of pore structures with different scales on the performance of thermoelectric materials and uni-leg power generation modules are studied. The average pore sizes (porosities) of these three porous ceramics are about 500 nm (6.0%), 3 μm (28.0%), and 400 μm (68.8%), respectively. It is a notable result, the thermal conductivity downfall sharply with the increasing of pore size and porosity, and an ultra-low lattice thermal conductivity κ L of 0.39 W/mK is obtained for submillimeter-porous ceramic owing to multiple phonon scattering and boundary effect of sub-millimeter pores. The achieved lowest value of thermal conductivity can meet the level of insulation bricks. Combining the relative higher power factor, the highest zT of 0.19 is obtained for CaMnO 3 thermoelectric sample with micro-scale pores. The highest output power density of p a = 104.94 mW·cm −2 and thermoelectric efficiency factor of ϕ = 0.54 µW·cm −2 ·K −2 are obtained for thermoelectric module fabricated by nano-porous ceramics, due to the lower contact resistance and enhanced thermoelectric properties of the material, which is far greater than that of most reported CaMnO 3 modules. As above finds, the optimal values of κ L , zT, p a and ϕ correspond to the three kinds of pores. So trying to exert all advantages of three kinds of pores synergistically in the future further enhances the thermoelectric performance for porous oxide materials and modules simultaneously.