Microstructural and thermoelectric properties of WO3-doped Ca3Co4O9 ceramics

WO3-doped ceramics with the composition Ca3Co4-xWxO9 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 degrees C. The influence of the composition on the microstructure development and the thermoelectric characteristics in the temperature range from 40 degrees C to 615 degrees C was studied. The formation of a solid solution with the composition Ca3CO3.88W0.12O9 was determined and indicates a much lower limit of WO3 solubility in the Ca3Co4O9 than has been reported previously. Doping with WO3 resulted in more irregularly shaped, sharp-edged and thinner plate-like grains in comparison to the grains in the un-doped Ca3Co4O9 ceramics. The un-doped ceramics had an electrical conductivity sigma of 31 S/cm, a Seebeck coefficient S of 147 mu V/K and a thermal conductivity kappa of 1.15 W/mK, giving a figure of merit ZT of 0.06 at 615 degrees C. The WO3-doping decreased the electrical conductivity sigma to 18 S/cm, increased the Seebeck coefficient to 157 mu V/K and also decreased the thermal conductivity kappa to 0.88 W/mK, resulting in an overall reduced ZT of 0.047 for the Ca3Co3.88W0.12O9 ceramics. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.