Effect of precursor calcination temperature on the microstructure and thermoelectric properties of Ca3Co4O9 ceramics

Ca3Co4O9 (CCO) powder precursors were prepared by the chemical sol-gel route and calcined at various temperatures between 923 K (CCO-923 K) and 1,073 K (CCO-1,073 K). The calcination temperature was found to be a critical factor affecting the microstructure and thermoelectric properties of CCO ceramic bulk samples. The grain size increases with calcination temperature. The nano-crystals with size about 100 nm in the powders calcined at 923 K promote large crystal growth and texture development during sintering. Bulk pellets made from CCO-923 K powder have large crystal grains, uniform grain size distribution, and a high degree of crystal alignment. By contrast, pellets made from CCO powders at higher calcination temperatures have a bimodal distribution of large and small grains and a large amount of randomly oriented grains. Transmission electron microscopy analysis shows that each crystal grain (identified in SEM images) consists of bundles of CCO nano-lamellas. The nano-lamellas within one bundle share the same c-axis orientation and have fiber texture. The electrical resistivity of CCO-923 K is weakly dependent on operating temperature. Compared to the CCO-1,073 K sample, the CCO-923 K sample has the highest power factor, a lower thermal conductivity, and higher electrical conductivity.