Effects of Lattice Defects and Niobium Doping on Thermoelectric Properties of Calcium Manganate Compounds for Energy Harvesting Applications

We have investigated the thermoelectric (TE) properties of Ruddlesden–Popper (RP) CaO(CaMnO₃)_mn-type compounds, to be applied for TE waste heat recovery at elevated temperatures. We prepared several Nb-doped and undoped CaO(CaMnO₃)_m compounds having different CaO planar densities by controlling the Ca content via solid-state reaction, and characterized the resulting microstructures by x-ray diffraction analysis and high-resolution scanning electron microscopy. The thermal conductivity, electrical conductivity, and TE thermopower of the different compounds were measured in the range from 300 K through 1000 K. We observed a remarkable reduction in thermal conductivity as a result of increasing the CaO planar density for the Nb-doped RP compounds, from a value of 2.9 W m⁻¹ K⁻¹ for m = ∞ down to 1.3 W m⁻¹ K⁻¹ for m = 1 at 1000 K. This trend was, however, accompanied by a corresponding reduction in electrical conductivity from 76 Ω⁻¹ cm⁻¹ to 2.9 Ω⁻¹ cm⁻¹, which is associated with electron scattering. Finally, we propose an approach that enables optimization of the TE performance of these RP compounds.