Exploring the high-temperature electrical performance of Ca3−xLaxCo4O9 thermoelectric ceramics for moderate and low substitution levels

This article belongs to the Special Issue Advances in Thermoelectric Materials: From Particle to Bulk.Aliovalent substitutions in Ca3Co4O9 often result in complex effects on the electrical properties and the solubility, and impact of the substituting cation also depends largely on the preparation and processing method. It is also well-known that the monoclinic symmetry of this material’s composite crystal structure allows for a significant hole transfer from the rock salt-type Ca2CoO3 buffer layers to the hexagonal CoO2 ones, increasing the concentration of holes and breaking the electron–hole symmetry from the latter layers. This work explored the relevant effects of relatively low La-for-Ca substitutions, for samples prepared and processed through a conventional ceramic route, chosen for its simplicity. The obtained results show that the actual substitution level does not exceed 0.03 (x < 0.03) in Ca3−xLaxCo4O9 samples with x = 0.01, 0.03, 0.05 and 0.07 and that further introduction of lanthanum results in simultaneous Ca3Co4O9 phase decomposition and secondary Ca3Co2O6 and (La,Ca)CoO3 phase formation. The microstructural effects promoted by this phase evolution have a moderate influence on the electronic transport. The electrical measurements and determined average oxidation state of cobalt at room temperature suggest that the present La substitutions might only have a minor effect on the concentration of charge carriers and/or their mobility. The electrical resistivity values of the Ca3−xLaxCo4O9 samples with x = 0.01, 0.03 and 0.05 were found to be ~1.3 times (or 24%) lower (considering mean values) than those measured for the pristine Ca3Co4O9 samples, while the changes in Seebeck coefficient values were only moderate. The highest power factor value calculated for Ca2.99La0.01Co4O9 (~0.28 mW/K2m at 800 °C) is among the best found in the literature for similar materials. The obtained results suggest that low rare-earth substitutions in the rock salt-type layers can be a promising pathway in designing and improving these p-type thermoelectric oxides, provided by the strong interplay between the mobility of charge carriers and their concentration, capable of breaking the electron–hole symmetry from the conductive layers.G. Constantinescu acknowledges the support of the TEOsINTE project (Grant agreement ID: 101003375), funded under the H2020-EU.4. Programmes (Funding Scheme: MSCA-IF-EF-ST—Standard EF). Partial support of the project REMOTE (POCI-01-0145-FEDER-031875) is also acknowledged. This work was partially developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 and UIDP/50011/2020, financed by national funds through the Portuguese Foundation for Science and Technology/MCTES. J.C. Diez, A. Sotelo, M.A. Madre and M.A. Torres acknowledge the financial support of the Spanish MINECO-FEDER project MAT2017-82183-C3-1-R and the Aragón Government (Research Group T54-20R). The technical contributions of C. Estepa, and C. Gallego are highly appreciated and acknowledged. Sh. Rasekh acknowledges the support of the Research Employment Contract FCT–CEECIND/02608/2017. P. Amirkhizi acknowledges the financial support of the Portuguese FCT Doctoral Studentship 2020.08051.BD. D. V. Lopes acknowledges the financial support of the research fellowship BI/UI50/9051/2020 provided by the European Commission (project SIDERWIN-DLV-768788—Horizon 2020/SPIRE10).Peer reviewe