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Thermoelectric Properties of Ca<sub>3</sub>Co<sub>2−<i>x</i></sub>Mn<i><sub>x</sub></i>O<sub>6</sub> (<i>x</i> = 0.05, 0.2, 0.5, 0.75, and 1)
oleh: Nikola Kanas, Sathya Prakash Singh, Magnus Rotan, Temesgen Debelo Desissa, Tor Grande, Kjell Wiik, Truls Norby, Mari-Ann Einarsrud
Format: | Article |
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Diterbitkan: | MDPI AG 2019-02-01 |
Deskripsi
High-temperature instability of the Ca<sub>3</sub>Co<sub>4−<i>y</i></sub>O<sub>9+<i>δ</i></sub> and CaMnO<sub>3−<i>δ</i></sub> direct p-n junction causing the formation of Ca<sub>3</sub>Co<sub>2−<i>x</i></sub>Mn<i><sub>x</sub></i>O<sub>6</sub> has motivated the investigation of the thermoelectric performance of this intermediate phase. Here, the thermoelectric properties comprising Seebeck coefficient, electrical conductivity, and thermal conductivity of Ca<sub>3</sub>Co<sub>2−<i>x</i></sub>Mn<i><sub>x</sub></i>O<sub>6</sub> with <i>x</i> = 0.05, 0.2, 0.5, 0.75, and 1 are reported. Powders of the materials were synthesized by the solid-state method, followed by conventional sintering. The material Ca<sub>3</sub>CoMnO<sub>6</sub> (<i>x</i> = 1) demonstrated a large positive Seebeck coefficient of 668 μV/K at 900 °C, but very low electrical conductivity. Materials with compositions with <i>x</i> < 1 had lower Seebeck coefficients and higher electrical conductivity, consistent with small polaron hopping with an activation energy for mobility of 44 ± 6 kJ/mol and where both the concentration and mobility of hole charge carriers were proportional to 1−<i>x</i>. The conductivity reached about 11 S·cm<sup>−1</sup> at 900 °C for <i>x</i> = 0.05. The material Ca<sub>3</sub>Co<sub>1.8</sub>Mn<sub>0.2</sub>O<sub>6</sub> (<i>x</i> = 0.2) yielded a maximum <i>zT</i> of 0.021 at 900 °C. While this value in itself is not high, the thermodynamic stability and self-assembly of Ca<sub>3</sub>Co<sub>2−<i>x</i></sub>Mn<i><sub>x</sub></i>O<sub>6</sub> layers between Ca<sub>3</sub>Co<sub>4−<i>y</i></sub>O<sub>9+<i>δ</i></sub> and CaMnO<sub>3−<i>δ</i></sub> open for new geometries and designs of oxide-based thermoelectric generators.