Effect of Y, La, and Yb simultaneous doping on the thermal conductivity and thermoelectric performances of CaMnO3 ceramics

Abstract

Triple doping of CaMnO3 in different stoichiometric proportions has been studied in bulk sintered materials. They were prepared through the classical ceramic route using planetary milling to decrease the precursors particle sizes. It has been found that particle sizes decrease with the amount of dopant, both in the precursors and in the sintered bodies. XRD patterns showed that all samples were nearly single phase, with small amounts of CaMn2O4 phase. SEM and TEM observations revealed a homogeneous distribution of dopants in the CaMnO3 phase, while EDX showed a composition close to the nominal one. However, HREM imaging has shown some dark regions in the crystallites, with different Ca(Y,La,Yb)/Mn ratios found through STEM image-chemical mapping, which points out to the coexistence of CaMnO3 and CaMn2O4–type domains in the crystallites. Electrical resistivity, absolute Seebeck coefficient and thermal conductivity have been drastically decreased with doping. The highest PF values at 800 ºC have been achieved in 0.02(Y,La,Yb) doped samples (∼ 0.37 mW/K2m), which is among the best reported values in literature. On the other hand, lattice thermal conductivity is dramatically decreased with doping due to the phonon scattering produced by the dopants, the decrease in the grain sizes, and the strains present inside the crystallites, reaching the minimum values at 800 ºC in 0.03(Y,La,Yb) doped samples (∼ 0.8 W/K m). Consequently, ZT reaches the maximum values (∼ 0.29) in 0.03(Y,La,Yb) doped samples due to their very low thermal conductivity, being higher than the best ZT reported values in the literature.