Thermoelectric Properties of Ca3Co2−xMnxO6 (x = 0.05, 0.2, 0.5, 0.75, and 1)

Abstract

High-temperature instability of the Ca3Co4−yO9+δ and CaMnO3−δ direct p-n junction causing the formation of Ca3Co2−xMnxO6 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 Ca3Co2−xMnxO6 with x = 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 Ca3CoMnO6 (x = 1) demonstrated a large positive Seebeck coefficient of 668 μV/K at 900 °C, but very low electrical conductivity. Materials with compositions with x < 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−x. The conductivity reached about 11 S·cm−1 at 900 °C for x = 0.05. The material Ca3Co1.8Mn0.2O6 (x = 0.2) yielded a maximum zT of 0.021 at 900 °C. While this value in itself is not high, the thermodynamic stability and self-assembly of Ca3Co2−xMnxO6 layers between Ca3Co4−yO9+δ and CaMnO3−δ open for new geometries and designs of oxide-based thermoelectric generators.

All-Oxide Thermoelectric Module with in Situ Formed Non-Rectifying Complex p–p–n Junction and Transverse Thermoelectric Effect

“…When two materials A and B dissimilar in thermal conductivity and Seebeck coefficient are contacted over an area exposed to a parallel thermal gradient, a voltage is generated in the transversal direction, and an effective transversal Seebeck coefficient of the couple can be expressed according to Goldsmid where S A , S B , K A , and K B represent the Seebeck coefficients and thermal resistances of the two materials. In our case, the thermal conductivity of CCO at 900 °C is 0.94 W m –1 K –1 while that for CCMO is 1.3 W m –1 K –1 and the CMO-composite is 2.0 W m –1 K –1 . This will give rise to different heat flows down the n- and p-type materials, resulting in an increasing transversal temperature gradient down the CCMO interface layer corresponding to the transversal heat flow illustrated in Figure a.…”

“…According to Kanas et al, electrical conductivity of the CCMO phase formed at the interface increases sharply with increasing temperature from 800 to 900 °C and reaches around 0.1 S cm –1 at 900 °C, where the positive Seebeck coefficient furthermore reaches as high as 668 μV K –1 . The three-layered CCO–CCMO–CMO junction exhibits Ohmic behavior with relatively modest interfacial resistance above 700 °C as evidenced in Figure a.…”

“…According to Kanas et al, electrical conductivity of the CCMO phase formed at the interface increases sharply with increasing temperature from 800 to 900 °C and reaches around 0.1 S cm –1 at 900 °C, where the positive Seebeck coefficient furthermore reaches as high as 668 μV K –1 . 34 The three-layered CCO–CCMO–CMO junction exhibits Ohmic behavior with relatively modest interfacial resistance above 700 °C as evidenced in Figure 5a. We tentatively interpret this as an effect of the electron energy levels of CCMO as laying intermediate of those of CCO and CMO, hence decreasing the depletion of charge carriers at the CCO–CCMO and CCMO–CMO interfaces as compared to a hypothetical pristine CCO–CMO p–n interface.…”

Time-Enhanced Performance of Oxide Thermoelectric Modules Based on a Hybrid p–n Junction

“…At high temperatures, CCO and CMO react to form Ca 3 CoMnO 6 (CCMO). CCMO crystallizes in the A 3 BB′O 6 structure, consisting of alternating face-sharing CoO 6 octahedra and trigonal biprismic chains, with Ca ions between the chains. , Electrical conductivity increases exponentially above 800 °C due to the pronounced polaron hopping mechanism, reaching a maximum of 10 –4 S·cm –1 at 900 °C, followed by a high positive Seebeck coefficient . High-temperature stability of Ca 3 Co 2– x Mn x O 6 for 1 < x < 0.75 has been reported …”

“…21,22 Electrical conductivity increases exponentially above 800 °C due to the pronounced polaron hopping mechanism, reaching a maximum of 10 −4 S•cm −1 at 900 °C, followed by a high positive Seebeck coefficient. 23 Hightemperature stability of Ca 3 Co 2−x Mn x O 6 for 1 < x < 0.75 has been reported. 24 In our recent publication, a prototype TE module consisting of p-type CCO and n-type CMO with a p−p−n junction (CCO/CCMO/CMO) demonstrated promising performance (∼23 mW/cm 2 ) attributed to the formation of the nonrectifying p−p−n junction and unexpectedly high open-circuit voltage.…”