TY - JOUR

T1 - Tuning the Thermoelectric Performance of CaMnO3-Based Ceramics by Controlled Exsolution and Microstructuring

AU - Kanas, Nikola

AU - Williamson, Benjamin A.D.

AU - Steinbach, Frank

AU - Hinterding, Richard

AU - Einarsrud, Mari Ann

AU - Selbach, Sverre M.

AU - Feldhoff, Armin

AU - Wiik, Kjell

N1 - Funding Information: Financial support from The Research Council of Norway under the program Nano2021 under Project 228854 “Thermoelectric materials: Nanostructuring for improving the energy efficiency of thermoelectric generators and heat-pumps” (THELMA) conducted by NTNU, UiO, SINTEF, FFI, UiS, and UiA and Project 275810 is gratefully acknowledged. B.A.D.W. and S.M.S. acknowledge support from the Research Council of Norway (Project 275810). Computational resources were provided by UNINETT Sigma2 through Projects NN9264K and ntnu243. We also thank the Antares program (Grant 739570) and Deutsche Forschungsgesellschaft (DFG, German Research Foundation) Grant FE928/17-1. The authors are grateful to Dr. R. Almeev for access to the JSM-7610FPlus scanning electron microscope.

PY - 2022/10/24

Y1 - 2022/10/24

N2 - The thermoelectric properties of CaMnO3-δ/CaMn2O4 composites were tuned via microstructuring and compositional adjustment. Single-phase rock-salt-structured CaO-MnO materials with Ca:Mn ratios larger than unity were produced in reducing atmosphere and subsequently densified by spark plasma sintering in vacuum. Annealing in air at 1340 °C between 1 and 24 h activated redox-driven exsolution and resulted in a variation in microstructure and CaMnO3-δ materials with 10 and 15 vol % CaMn2O4, respectively. The nature of the CaMnO3-δ/CaMn2O4 grain boundary was analyzed by transmission electron microscopy on short- and long-term annealed samples, and a sharp interface with no secondary phase formation was indicated in both cases. This was further complemented by density functional theory (DFT) calculations, which confirmed that the CaMnO3-δ indeed is a line compound. DFT calculations predict segregation of oxygen vacancies from the bulk of CaMnO3-δ to the interface between CaMnO3-δ and CaMn2O4, resulting in an enhanced electronic conductivity of the CaMnO3-δ phase. Samples with 15 vol % CaMn2O4 annealed for 24 h reached the highest electrical conductivity of 73 S·cm-1 at 900 °C. The lowest thermal conductivity was obtained for composites with 10 vol % CaMn2O4 annealed for 8 h, reaching 0.56 W·m-1K-1 at 700 °C. However, the highest thermoelectric figure-of-merit, zT, was obtained for samples with 15 vol % CaMn2O4 reaching 0.11 at temperatures between 800 and 900 °C, due to the enhanced power factor above 700 °C. This work represents an approach to boost the thermoelectric performance of CaMnO3-δ based composites.

AB - The thermoelectric properties of CaMnO3-δ/CaMn2O4 composites were tuned via microstructuring and compositional adjustment. Single-phase rock-salt-structured CaO-MnO materials with Ca:Mn ratios larger than unity were produced in reducing atmosphere and subsequently densified by spark plasma sintering in vacuum. Annealing in air at 1340 °C between 1 and 24 h activated redox-driven exsolution and resulted in a variation in microstructure and CaMnO3-δ materials with 10 and 15 vol % CaMn2O4, respectively. The nature of the CaMnO3-δ/CaMn2O4 grain boundary was analyzed by transmission electron microscopy on short- and long-term annealed samples, and a sharp interface with no secondary phase formation was indicated in both cases. This was further complemented by density functional theory (DFT) calculations, which confirmed that the CaMnO3-δ indeed is a line compound. DFT calculations predict segregation of oxygen vacancies from the bulk of CaMnO3-δ to the interface between CaMnO3-δ and CaMn2O4, resulting in an enhanced electronic conductivity of the CaMnO3-δ phase. Samples with 15 vol % CaMn2O4 annealed for 24 h reached the highest electrical conductivity of 73 S·cm-1 at 900 °C. The lowest thermal conductivity was obtained for composites with 10 vol % CaMn2O4 annealed for 8 h, reaching 0.56 W·m-1K-1 at 700 °C. However, the highest thermoelectric figure-of-merit, zT, was obtained for samples with 15 vol % CaMn2O4 reaching 0.11 at temperatures between 800 and 900 °C, due to the enhanced power factor above 700 °C. This work represents an approach to boost the thermoelectric performance of CaMnO3-δ based composites.

KW - composite CaMnO

KW - electrical conductivity

KW - heterostructuring

KW - oxide thermoelectrics

KW - thermal conductivity

UR - http://www.scopus.com/inward/record.url?scp=85139545348&partnerID=8YFLogxK

U2 - 10.1021/acsaem.2c02012

DO - 10.1021/acsaem.2c02012

M3 - Article

AN - SCOPUS:85139545348

VL - 5

SP - 12396

EP - 12407

JO - ACS Applied Energy Materials

JF - ACS Applied Energy Materials

IS - 10

ER -