In the present world situation, with fossil fuels and energy costs increasing day-by-day, the relatively low energy conversion efficiency of classical systems is an important factor to explain the impressive waste of natural resources. Consequently, in the last decades the search for new renewable energy generation systems, as well as the increase of efficiency of classical systems has been a priority for all industrialized countries. In this situation, thermoelectric (TE) technology can be regarded as one of the most promising methods to produce useful energy from waste and/or renewable heat sources [1]. When used to harvest waste heat, this technology can play an important role in fighting against global warming by increasing energy conversion efficiency, reducing the fossil fuel consumption, and leading to the decrease CO₂ emissions. For these applications, it is necessary to have TE materials with high performances, determined through the dimensionless figure of merit, ZT (= TS²/rk; T absolute temperature, S Seebeck coefficient, r electrical resistivity, and k thermal conductivity) [2].
In this work, the objective is increasing the ZT of n-type CaMnO₃ in order to obtain adequate properties for its use in thermoelectric power generators. For this purpose, triple doping of CaMnO₃ using Y, La, and Yb by Ca in different stoichiometric proportions (Ca_1-3xY_xLa_xYb_xMnO₃ with x = 0, 0.01, 0.02, and 0.03) 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 single phase. SEM and TEM observations revealed a homogeneous distribution of dopants in the CaMnO₃ phase, while EDS showed a composition close to the nominal one. 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. However, ZT reaches the maximum values (~ 0.29) in 0.03(Y,La,Yb) doped samples due to their very low thermal conductivity, being one of the best reported values in the literature.
 
[1] M. H. Elsheikh, D. A. Shnawah, M. F. M. Sabri, S. B. M. Said, M. H. Hassan, M. B. A. Bashir, M. Mohamad, A review on thermoelectric renewable energy: Principle parameters that affect their performance, Renew. Sust. Energy Rev. 30, 337-355 (2014)
[2] D. M. Rowe (2006) In: D. M. Rowe (ed.), Thermoelectrics Handbook: Macro to Nano, 1st edn. CRC Press, Boca Raton, FL.