Thermopower and chemical stability of Na0.77CoO2/Ca3Co4O9 composites

Layered cobaltates, including Na0.77CoO2 and Ca3Co4O9, can be viewed as the foundation of p-type oxide thermoelectric materials. Here, we demonstrate the successful fabrication of (1 − x)Na0.77CoO2/xCa3Co4O9 (x = 0, 0.3, 0.5, 0.7, 1) composites by means of spark plasma sintering. A significant enhancement of the Seebeck coefficient (thermopower) has been achieved in the composites, approximately 17% higher than that of Ca3Co4O9 at 680 °C. The increases in Seebeck coefficient and electrical resistivity are mostly likely to be associated with the compressive strain in Ca3Co4O9 grains due to the mismatch of thermal expansion coefficients between Na0.77CoO2 and Ca3Co4O9. Most importantly, the chemical stability of Na0.77CoO2 could be improved by adding up to a 30 vol.% fraction of Ca3Co4O9 without deteriorating its thermoelectric performance. This observation may open a pathway to practical applications of Na0.77CoO2-based thermoelectric materials.