Received: 9 March 2023 Revised: 30 April 2023 Accepted: 30 April 2023 DOI: 10.1111/jace.19221 RESEARCH ARTICLE Thermoelectric properties of hot-pressed Ruddlesden-Popper phases CaO(CaMnO 3 ) m Yixuan Shi 1 Zahra Sepahi 1 Leilane R. Macario 1 Cheryl Sturm 1 Nour Mashmoushi 1 Yaron Amouyal 2 Holger Kleinke 1 1 Department of Chemistry and Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario, Canada 2 Department of Materials Science and Engineering, Technion-Israel Institute of Technion, Haifa, Israel Correspondence Holger Kleinke, Department of Chemistry and Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, ON, Canada. Email: kleinke@uwaterloo.ca Funding information Israel Science Foundation (ISF), Grant/Award Number: 1997/18; Gerald Schwartz & Heather Reisman Foundation (Waterloo–Technion) Abstract We investigated the Ruddlesden-Popper series CaO(CaMnO 3 ) m with m = 1, 2, 3, ∞, to study the impact of the varying amounts of CaO layers on their thermoelec- tric properties. Previous studies showed that highly dense samples are difficult to obtain due to the refractory nature of these materials. In this study, we man- aged to obtain dense pellets during a classical hot-pressing step, if and only if the samples were subjected to extended ball-milling prior to pressing, resulting in crystallite sizes of 30–35 nm after hot-pressing. The sample with the largest amount of CaO layers (m = 1) had the lowest electrical and thermal conductiv- ity, and the highest Seebeck coefficient, as predicted. Ultimately the perovskite CaMnO 3 (m = ∞, no CaO layers) exhibited the best thermoelectric properties. KEYWORDS oxides, perovskites, Ruddlesden-Popper phases, semiconductors, thermoelectric materials 1 INTRODUCTION The fossil energy crisis and global warming effect have been challenging mankind for decades. 1,2 Consequently, utilizing thermoelectric (TE) materials to harvest waste heat and convert it into electricity has become one poten- tial method to contribute to more sustainable energy conversion. 3–5 The outstanding merits of calcium man- ganite oxide TE materials include low cost, nontoxicity, and good chemical stability at high temperatures. 6–8 How- ever, the TE applications of CaMnO 3 are confined by its drawbacks, such as moderate thermal conductivity, low electrical conductivity, and high porosity of its pressed pellets. 9–11 The TE materials’ performance is characterized by the dimensionless figure of merit, zT = S 2 σT κ −1 , where the numerator of the equation incorporates S—the Seebeck coefficient, σ—the electrical conductivity, and T—the absolute temperature; the product S 2 σ is denoted as the TE power factor (PF). The denominator includes κ—the total thermal conductivity. 12,13 The total thermal conductivity results from contributions of both electrons (κ e ) and phonons (κ L ) travelling through the lattice, where the electronic thermal conductivity is interrelated to the electrical transport properties of the material via the Wiedemann-Franz rule. Larger zT values of a material directly translate into higher TE conversion efficiency. To enhance the TE performance of CaMnO 3 , optimization of the charge carrier concentration by doping with other elements has been implemented. For example, zT = .25 at 973 K was achieved by co-doping with Dy and Yb to form Ca 0.96 Dy 0.02 Yb 0.02 MnO 3 , 14 and similarly, Ca 0.92 Pr 0.04 Yb 0.04 MnO 3 was reported with zT = .24 at 973 K, 15 and CaMn 0.96 Ta 0.04 O 3 with zT = .21 at 1160 K. 16 Thin films of Nb-doped CaMnO 3 were also studied. 17 However, introduction of rare earth elements increases the cost and toxicity of the material, which contradicts the initial attempt of developing nontoxic and affordable TE materials. Additional methods to increase zT should be attempted, such as reducing lattice thermal 6098 © 2023 The American Ceramic Society. J Am Ceram Soc. 2023;106:6098–6105. wileyonlinelibrary.com/journal/jace