The effect of Cr substitution on the structure and properties of misfit-layered Ca3Co4−xCrxO9+δ thermoelectric oxides

• The effect of Cr doping in Ca3Co4−xCrxO9+δ on thermoelectric properties was studied.
• XANES and XPS spectra confirm charge states of +3.5 and >+4.5 for Cr ion.
• Thermoelectric variation was due to charge states and ionic radii of Co and Cr ions.
• The highest ZT of 0.19 at 1073 K was obtained for the Ca3Co3.85Cr0.15O9+δ sample.

Misfit-layered Ca3Co4−xCrxO9+δ (0 ⩽ x ⩽ 0.2) powders were synthesized by a simple thermal hydro-decomposition method and then fabricated to form highly dense ceramics using spark plasma sintering. X-ray diffraction, and a field emission scanning electron microscope equipped with an energy dispersive X-ray spectrometer were used to verify the single phase of Ca3Co4O9+δ for the samples with x = 0, 0.05, 0.10 and 0.15. The characteristic plate-like grain structure was observed. The chemical composition of the fabricated samples was found to be close to nominal composition. The thermoelectric measurement showed that Cr doping has an influence on the thermoelectric properties. The resistivity and the Seebeck coefficient increased while the thermal conductivity was suppressed when Cr was added into the system. Explanation of the change in thermoelectric properties was discussed regarding the difference in the charge states and the ionic radii between Co and Cr ions. The highest ZT of 0.19 at 1073 K was obtained for the Ca3Co3.85Cr0.15O9+δ sample. In addition, the paramagnetic property was observed in all samples at room temperature.