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.
Highlights
Ceramics based on cobalt oxides are attractive for many applications [1] because the wide variation in crystal structure, oxygen non-stoichiometry, and valence of Co give rise to a wide range of useful properties [2]
The precursors were first dried at 120 ◦ C for 5 h to remove adsorbed moisture and perform accurate weighing for five different compositions, and each batch was ball-milled in isopropanol for 8 h using zirconia balls in order to obtain homogeneous mixtures
The final powders were pressed into bars (15 × 5 × 2 mm3 ) and pellets by cold isostatic pressing at 200 MPa, and conventionally sintered in ambient air at 1010 ◦ C for 30 h, using heating and cooling rates of 200 ◦ C/h
Summary
Ceramics based on cobalt oxides are attractive for many applications [1] because the wide variation in crystal structure, oxygen non-stoichiometry, and valence of Co give rise to a wide range of useful properties [2]. Hervoches et al [11] investigated the effect of Mn substitution on the Co-site in terms of structural and magnetic properties, and found that the crystal structure belongs to the R3c space group with unit cell dimensions of 9.084 Å < a < 9.134 Å and 10.448 Å < c < 10.583 Å for Ca3 CoMnO6 [11] They confirmed a solid solution in the entire composition interval (0 < x < 1) [11] by using sol-gel synthesis, in contrast to results obtained by solid-state synthesis by Bazuev et al [15], where an inhomogeneity region at about x = 0.5 was observed [15,16].
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
