Thermoelectric properties of Ca3Co4O9 ceramics prepared by an alternative pressure-less sintering/annealing method

• New sintering/annealing route of nearly dense thermoelectric CCO ceramics.
• Sintering upon phase separation at 1200 °C and formation of single-phase CCO at 900 °C.
• Charge and thermal transport properties were studied.
• CCO ceramics exhibit a high power factor 170 μW/(K2m) at 600 °C.

Nearly dense polycrystalline samples of Ca3Co4O9 (CCO) thermoelectric misfit layer cobaltite were prepared by the mixed-oxide route and pressure-less sintering along a two-step sintering protocol. Samples were first sintered at high temperature (e.g. 1200 °C) to reach a high degree of densification coupled with complete CCO decomposition into CaO and CoO phases. Subsequent annealing at 900 °C within the stability range of Ca cobaltite leads to complete re-formation of CCO after 72 h. The samples prepared using this sintering/annealing route exhibit an increased conductivity compared to samples sintered conventionally at 920 °C. This leads to an enhanced power factor of 170 μW/(K2m) at 600 °C. The thermal conductivity of these samples is κ = 2.8 W/mK at 100 °C which is larger as compared to porous and anisotropic conventionally sintered samples. The figure of merit of samples sintered with the proposed sintering/annealing process is ZT = 0.06 at 600 °C. This process enables simple preparation of dense CCO ceramics as p-type component in thermoelectric generators.

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