Ca3Co4O9 (C-349) samples with average grain sizes ranging from 18 nm to 2 mu m were synthesized using the citrate sol-gel method. PEG 400 was used to vary the particle size and porosity, and the thermoelectric (TE) properties were examined from 300 to 1000 K. Nanostructuring and porosity do not appear to significantly affect the Seebeck coefficient; however, a considerable increase in the resistivity over the entire temperature range was observed, leading to lower power factors for the nano-sized samples. The main effect was observed in the thermal conductivity, which was substantially lower for the nano-sized samples with lower density (0.32 W/m K at 1000 K for the 18-nm sample with a density of 66%) due to grain boundary scattering at the interfaces. The porosity further contributed to a reduction of the thermal conductivity, leading to an increase in the density of scattering regions. The best ZT value was 0.29 at 1000 K for the 18-nm-sized sample. (C) 2014 Elsevier B.V. All rights reserved.