High-Temperature Stability of Hot-Pressed Sr-Doped Ca 3 Co 4 O 9

Bulk textured Ca 2.93 Sr 0.07 Co 4 O 9 samples have been prepared by hot uniaxial pressing followed by thermal treatment at 800°C under air for between 0 h and 1532 h. The microstructural, thermoelectric, and mechanical properties as well as density of all the samples were evaluated as a function of the duration of thermal treatment. Scanning electron microscopy characterization showed that the samples were mainly composed of Sr-doped Ca 3 Co 4 O 9 thermoelectric (TE) phase, accompanied by minor amounts of Sr-free Ca 3 Co 2 O 6 secondary phase. After an initial decrease of density after the first aging treatment, it remained practically constant for longer durations. This behavior is reflected in the mechanical properties, which slightly decreased after 12 h of thermal treatment, compared with the as-hot-pressed state, but remained practically constant after larger durations. However, the TE properties were not affected by the aging process, lying within typical errors independently of the aging duration. Moreover, the power factor values measured at 850°C (> 0.60 mW/K 2 m) lie among the highest obtained so far for this kind of material.