Effect of dual doping by rare-earth and sodium ions on thermoelectric properties of CaMnO₃ ceramics

The effects of dual doping by trivalent and monovalent cations on the thermoelectric properties of CaMnO_3-δ ceramics were investigated. Na⁺ was selected as the monovalent cation, while three rare-earth (R) trivalent cations of La³⁺, Gd³⁺, and Dy³⁺ were used to study the role of their size and mass in the electrical and thermal transport properties. Dense ceramic samples with a nominal composition Ca_1-x(R_1/2Na_1/2)_xMnO₃ (x = 0.1, 0.2, and 0.3) were fabricated via the conventional solid-state reaction route. Due to the partial evaporation of Na during sintering, the electrical conductivity increased and the Seebeck coefficient decreased with the doping level. Additionally, the dual doping by Na and R ions effectively decreased the thermal conductivity. The comparison of the thermoelectric properties of the samples demonstrated that a smaller and heavier R ion is more effective to obtain a high thermoelectric power factor and a low thermal conductivity. As a result, the sample with R = Dy and x = 0.1 possessed a high dimensionless figure-of-merit (ZT) of 0.11 at 1000 K.