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.