Thermoelectric properties of Bi2O3-added WO3 ceramics

Tungsten trioxide (WO3) ceramics were prepared by firing Bi2O3-added WO3 compacts with atomic ratios of Bi/W = 0.00, 0.01, 0.03, or 0.05, in which Bi2O3 was mixed as a sintering agent. Dense ceramics consisting of remarkably grown WO3 grains were obtained for Bi-containing samples with Bi/W = 0.01, 0.03, and 0.05. The grain growth was enhanced by the liquid phase of Bi2W2O9 formed among the WO3 grains while firing. The XRD patterns did not show evidence for Bi inclusion into the WO3 lattice, but the SEM-EDX showed an intensive distribution of Bi into the grain boundaries. Electrical conductivity a and Seebeck coefficient S were measured in a temperature range of 373-1073 K. The temperature dependences indicated that the Bi2O3-added WO3 ceramics were n-type semiconductors. It was considered that the electron carriers were generated from oxygen vacancies included into the WO3 grains. The thermoelectric power factors S-2 sigma for the ceramics ranged from 1.5 x 10(-7) W m(-1) K-2 to 2.8 x 10(-5) W m(-1) K-2, and the highest value occurred at 970 K for the ceramic with Bi/W = 0.01.