Microstructural and thermoelectric properties of WO3-doped Ca3Co4O9 ceramics

Abstract WO 3 -doped ceramics with the composition Ca 3 Co 4−x W x O 9 and x=0, 0.02, 0.05 0.1 and 0.2 were prepared by the classic ceramic procedure from fine oxide powders and sintering in air at 900 °C. The influence of the composition on the microstructure development and the thermoelectric characteristics in the temperature range from 40 °C to 615 °C was studied. The formation of a solid solution with the composition Ca 3 Co 3.88 W 0.12 O 9 was determined and indicates a much lower limit of WO 3 solubility in the Ca 3 Co 4 O 9 than has been reported previously. Doping with WO 3 resulted in more irregularly shaped, sharp-edged and thinner plate-like grains in comparison to the grains in the un-doped Ca 3 Co 4 O 9 ceramics. The un-doped ceramics had an electrical conductivity σ of 31 S/cm, a Seebeck coefficient S of 147 μV/K and a thermal conductivity κ of 1.15 W/mK, giving a figure of merit ZT of 0.06 at 615 °C. The WO 3 -doping decreased the electrical conductivity σ to 18 S/cm, increased the Seebeck coefficient to 157 μV/K and also decreased the thermal conductivity κ to 0.88 W/mK, resulting in an overall reduced ZT of 0.047 for the Ca 3 Co 3.88 W 0.12 O 9 ceramics.