Electrical conductivity increase by order of magnitude through controlling sintering to tune hierarchical structure of oxide ceramics

Highlights: • Sintering temperature has a significant impact on the hierarchical structure. • Hierarchical structure changes affect the electrical and thermal conductivities. • Processing principle for perovskite thermoelectric ceramics. • Design guidance of the perovskite family oxides. Perovskite calcium manganate CaMnO_3-δ is representative of an essential group of semiconductors with unique physical phenomena including colossal magnetoresistance and thermoelectric properties. For the large-scale applications that require the utilization of oxide ceramics, the polycrystalline materials' physical properties such as conductivity can be controlled and ultimately optimized through tuning the ceramics sintering conditions. In the present study, the impact of the sintering temperature on the structure and thermoelectric performance of CaMnO_3-δ is systematically studied. For the ceramics pellets made of precursors powders synthesized using the chemical sol-gel reaction, the increase in the sintering temperature dramatically increases the electrical conductivity by order of magnitude and simultaneously increases the Seebeck coefficient. Meanwhile, the thermal conductivity increases with the rise of the sintering temperature. Among the samples sintered at different temperatures, the peaking thermoelectric Figure of Merit ZT of pristine CaMnO_3-δ reached 0.28, which is a factor of 2.5 higher than the highest reported ZT for pristine CaMnO_3-δ and approached that of the doped single-phase CaMnO_3-δ. The electrical and thermal properties changes were interpreted based on the oxide ceramics hierarchical structure evolutions, from unit cell level to micron scales, induced by changes of sintering temperatures.