Seebeck coefficient and electrical conductivity of BSCF (Ba 0.5Sr 0.5Co xFe 1 − xO 3 − δ, 0 ≤ x ≤ 0.8) as a function of temperature and partial oxygen pressure

Abstract

The electrical conductivity and Seebeck coefficient of sintered BSCF ceramics (Ba 0.5Sr 0.5Co xFe 1 − x O 3 − δ , 0 ≤ x ≤ 0.8) were simultaneously measured as a function of pO 2 (10 − 5 ≤ pO 2 ≤ 1 atm) at 500 °C, 700 °C and 900 °C. All samples exhibited a positive Seebeck coefficient over the range of pO 2 and temperature examined, which indicates the predominance of p-type conduction. In all cases, conductivity increased with increasing pO 2, ranging from a minimum of ~ 0.6 S/cm (x = 0, pO 2 = 10 − 5 atm, T = 900 °C) to a maximum of ~ 36 S/cm (x = 0.8, pO 2 = 1 atm, and T = 900 °C). At low temperatures and high pO 2, conductivity was approximately proportional to pO 2 1/4, which was attributed to the reduction of B-site cations from their tetravalent to trivalent state. At low pO 2 and high temperature, the conductivity exhibited positive deviations from the pO 2 1/4 dependence. At 500 °C and x ≤ 0.6, the Seebeck coefficient (Q) decreased linearly with increasing log pO 2. At 700 and 900 °C, Q vs. log pO 2 curves exhibited maxima at 10 − 3 < pO 2 < 10 − 1 atm, and the maxima shifted to higher pO 2 as x increased. A simple p-type polaron hopping model, assuming negligible contribution from n-type or ionic carriers, was used to extract the carrier concentration of the x = 0.8 sample from the measured thermopower data. The calculated hole mobility for the x = 0.8 sample was less than 0.1 cm 2/V-s, confirming a p-type polaron-hopping model. This analysis, as well as analysis of Jonker plots, suggested that hole mobility decreased with decreasing pO 2.