文摘
The electrical conductivity and Seebeck coefficient of sintered BSCF ceramics (Ba0.5Sr0.5CoxFe1 ?#xA0;xO3 ?#xA0;¦Ä, 0 ?#xA0;x ?#xA0;0.8) were simultaneously measured as a function of pO2 (10?#xA0;5 ?#xA0;pO2 ?#xA0;1 atm) at 500 ¡ãC, 700 ¡ãC and 900 ¡ãC. All samples exhibited a positive Seebeck coefficient over the range of pO2 and temperature examined, which indicates the predominance of p-type conduction. In all cases, conductivity increased with increasing pO2, ranging from a minimum of ~ 0.6 S/cm (x = 0, pO2 = 10?#xA0;5 atm, T = 900 ¡ãC) to a maximum of ~ 36 S/cm (x = 0.8, pO2 = 1 atm, and T = 900 ¡ãC). At low temperatures and high pO2, conductivity was approximately proportional to pO21/4, which was attributed to the reduction of B-site cations from their tetravalent to trivalent state. At low pO2 and high temperature, the conductivity exhibited positive deviations from the pO21/4 dependence. At 500 ¡ãC and x ?#xA0;0.6, the Seebeck coefficient (Q) decreased linearly with increasing log pO2. At 700 and 900 ¡ãC, Q vs. log pO2 curves exhibited maxima at 10?#xA0;3 < pO2 < 10?#xA0;1 atm, and the maxima shifted to higher pO2 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 cm2/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 pO2.