Microstructure control and thermoelectric figure of merit of Bi2O3-added WO3 ceramics

Microstructure control for Bi2O3-added WO3 ceramics is explored by changing the atomic ratio of Bi/W and the firing temperature. Densified WO3 ceramics with two distinct microstructures are obtained for Bi/W = 0.005; the ceramic fired at 1123 K consists of WO3 fine grains measuring several micrometers in size, whereas those fired at 1173 K and 1223 K consist of large WO3 grains measuring several 10 mu m in size. The grain growth is enhanced by a Bi2W2O9 liquid phase formed among the WO3 particles during the firing. Seebeck coefficient S, electrical conductivity sigma, and thermal conductivity kappa are measured at high temperatures for the three ceramics to elucidate the dimensionless figure of merit ZT, where Z is defined as S2 sigma kappa- 1 and T is the absolute temperature. The ceramics consisting of larger WO3 grains display higher power factors S2 sigma due to the higher electron carrier concentrations. The ZT values are consequently higher for these ceramics because the kappa values are lower, due most likely to the larger phonon scattering by the grain boundaries developed among the larger WO3 grains. The highest ZT is 2.4 x 10-3 at 873 K for the ceramic fired at 1223 K.