Oxygen stoichiometry, electrical conductivity, and thermopower measurements of BSCF (Ba 0.5Sr 0.5Co xFe 1 − xO 3 − δ, 0≤ x ≤ 0.8) in air

Abstract

The DC electrical conductivity and Seebeck coefficient of sintered BSCF ceramics (Ba 0.5Sr 0.5Co x Fe 1 − x O 3 − δ , 0 ≤ x ≤ 0.8) were simultaneously measured from 200 to 900 °C in air. Oxygen stoichiometry (3 − δ) was determined from iodometry at room temperature and thermogravimetry analysis to 900 °C. The electrical conductivity was thermally activated at low temperatures, exhibiting a maximum of 32 ± 4 S/cm at ∼ 350 °C and then decreasing with increasing temperature above 400 °C. The Seebeck coefficient ranged from a minimum of 37 ± 10 μV/K for all compositions at ∼ 300 °C to 150 μV/K for x = 0 and 67 μV/K for x = 0.8 at 900 °C. Oxygen stoichiometry ranged from 2.65 for x = 0 and 2.54 for x = 0.8 at 100 °C to 2.55 for x = 0 and 2.36 for x = 0.8 at 900 °C. A simple p-type polaron hopping model, assuming negligible contribution from n-type or ionic carriers, was used to extract the carrier concentration and mobilities from the measured data. The calculated mobilities were < 0.1 cm 2/V-s, supporting the p-type polaron hopping model. For a given composition, the activation energy for conduction below ∼ 350 °C was identical to that of the carrier mobility, ranging from 0.2 eV for x = 0 to 0.6 eV for x = 0.8. Above 350 °C, the activation energy for carrier mobility decreased dramatically, ranging from 0.05 eV for x = 0.2 to 0.1 eV for x = 0.8. Unlike the carrier mobilities of the x ≥ 0.2 samples, the carrier mobility for the x = 0 sample did not appear to be thermally activated above 350 °C.