Grain boundary segregation and thermoelectric performance enhancement of bismuth doped calcium cobaltite

The effect of Bi doping on the nanostructure and thermoelectric performance of the polycrystalline calcium cobaltite Ca3−xBixCo4O9 (x = 0, 0.1, 0.2, 0.3 and 0.4) is reported. The samples were prepared using a chemical sol-gel route. Increasing Bi concentrations up to x = 0.3 enhance the grain growth and improve the crystal texture. Through nanostructural and chemical analyses, significant Bi segregation at grain boundary was observed for the first time. From 318 K up to 1073 K, the Seebeck coefficient increases and the electrical resistivity decreases as Bi increases to 0.3, resulting in high power factor of 0.95 mW m−1 K−2 at 318 K, which is so far the highest power factor for Calcium Cobaltite ceramics. Combined with low thermal conductivity of 1.9 W m−1 K−1, Ca2.8Bi0.2Co4O9 shows the peak ZT value of 0.43 at 1073 K. The Bi grain boundary segregation improves the texture development and acts as carrier filter in increasing the Seebeck coefficient.