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

Abstract

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 318K up to 1073K, the Seebeck coefficient increases and the electrical resistivity decreases as Bi increases to 0.3, resulting in high power factor of 0.95mWm−1K−2 at 318K, which is so far the highest power factor for Calcium Cobaltite ceramics. Combined with low thermal conductivity of 1.9 Wm−1K−1, Ca2.8Bi0.2Co4O9 shows the peak ZT value of 0.43 at 1073K. The Bi grain boundary segregation improves the texture development and acts as carrier filter in increasing the Seebeck coefficient.