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

Abstract Layered cobaltates, including Na 0.77 CoO 2 and Ca 3 Co 4 O 9 , can be viewed as the foundation of p-type oxide thermoelectric materials. Here, we demonstrate the successful fabrication of (1 − x )Na 0.77 CoO 2 / x Ca 3 Co 4 O 9 ( 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 Ca 3 Co 4 O 9 at 680 °C. The increases in Seebeck coefficient and electrical resistivity are mostly likely to be associated with the compressive strain in Ca 3 Co 4 O 9 grains due to the mismatch of thermal expansion coefficients between Na 0.77 CoO 2 and Ca 3 Co 4 O 9 . Most importantly, the chemical stability of Na 0.77 CoO 2 could be improved by adding up to a 30 vol.% fraction of Ca 3 Co 4 O 9 without deteriorating its thermoelectric performance. This observation may open a pathway to practical applications of Na 0.77 CoO 2 -based thermoelectric materials.