Synthesis and structural properties of thermoelectric Ca3-xAgxCo4O9+δ powders.

A series of Ca3-xAgxCo4O9+δ (0 ≤ x ≤ 0.2) powders is prepared by the Pechini sol-gel method. The effect of dispersants on the size and morphology of the Ca2.9Ag0.1Co4O9+δ powders is investigated. The desired powders are obtained after calcinations of the dried powders at 800 °C for 12 h. The structural and morphological properties are studied with the help of XRD patterns, FE-SEM images, FT-IR spectra, Raman spectra, and XPS spectra. It is found that a mixed valence (+3 and +4) of Co exists in a CdI2-type CoO2 layer, while a mixed valence (+2, +3, and +4) exists in the CoO of a rock salt-type Ca2CoO3 layer. The mixed-valence state in the CoO2 layer can improve the high-temperature thermoelectric properties of Ca3Co4O9 systems.