Effects of In2O3 nanoparticles addition on microstructures and thermoelectric properties of Ca3Co4O9 compounds

The ( x ) In 2 O 3 /(1- x ) Ca 3 Co 4 O 9 ( x  = 0-15 wt%) composites were prepared using the solid reaction method followed by a spark plasma sintering process. There is no reaction occurring, and the natural c -axis grain-oriented structure is observed in all samples by XRD measurements. Scanning electron microscopy and Energy dispersive spectroscopy observations indicate that the In 2 O 3 nanoparticles are present as the ultrafine particles/clusters secondary phase in Ca 3 Co 4 O 9 . Electric properties measurement shows that the electrical conductivity reduced by adding In 2 O 3 particles, while the seebeck coefficient is virtually unaffected, which leads to the low power factor PF value of Ca 3 Co 4 O 9 . Significantly, however, there is an obvious recovery in the electrical conductivity of S15 sample compared to that of S5 and S10 samples, which also results in a rise again in PF of S15. This result may be of significance for further increase In 2 O 3 secondary phase content ( x  > 15 wt%) to enhance the thermoelectric property of Ca 3 Co 4 O 9 .