pO2 stability of Ba0.5Sr0.5Co0.8Fe0.2O3-delta

ABSTRACTThe mixed-conducting perovskite oxide Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), given its outstanding oxygen ionic and electronic transport properties, is considered a promising material composition for oxygen transport membranes (OTM) operated at high temperatures.Its long-term stability under operating conditions is, however, still an important issue. Although the incompatibility of BSCF with CO2-containing atmospheres can be avoided by appropriate means (oxyfuel processes in the absence of carbon dioxide), the thermal as well as the chemical stability of BSCF itself are still under thorough investigation.This work is focused on the stability of BSCF in the targeted temperature range for OTM applications (700…900 °C) and in atmospheres with low oxygen contents. Previous studies in literature suggest limited chemical stability below oxygen partial pressures pO2 of around 10-6 bar.By using a coulometric titration method based on a zirconia “oxygen pump” setup, precise control of the oxygen partial pressure pO2 between 1 bar and 10-18 bar was facilitated. Combining electrical measurements on dense ceramic bulk samples performed as a function of pO2 with an XRD phase composition study of single phase BSCF powders subjected to various pO2 treatments, an assessment of the chemical stability of BSCF is facilitated as a function of oxygen partial pressure. It could thus be shown that the pO2 stability limit is considerably lower than previously assumed in literature.</jats:p

Thermal Stability of the Cubic Phase in Ba0.5Sr0.5Co0.8Fe0.2O3- (BSCF)

Ba0.5Sr0.5Co0.8Fe0.2O0-delta (BSCF) is a material with excellent oxygen ionic and electronic transport properties reported by many research groups. In its cubic phase, this mixed ionic-electronic conducting (MIEC) perovskite is a promising candidate for oxygen permeation membranes. For this application, its long-term stability under operating conditions (especially temperature and oxygen partial pressure) is of crucial importance.The present work is focused on the thermal stability of the BSCF cubic phase in the targeted temperature range for applications (700 ... 900 degrees C) in light of previous studies in literature reporting a reversible transition to a hexagonal phase somewhere below 900 degrees C.To this end, single phase cubic BSCF powders were annealed at different temperatures over varying periods of time. Phase composition was subsequently analysed by X-ray diffractometry (XRD) in order to determine both the temperature limit and the time-scale for the formation of the hexagonal phase. Additionally, the long-term behaviour of the electrical conductivity was examined on bulk samples at 700 degrees C, 800 degrees C and 900 degrees C over several hundreds of hours, showing a prolonged decrease at 800 degrees C. The decrease in electrical conductivity at this temperature was also examined on bulk samples with different grain sizes, showing a more pronounced decrease the smaller the average grain size. Coexistence of both phases (cubic and hexagonal) could also be shown for 700 degrees C, however with a different phase equilibrium than at 800 degrees C. (C) 2011 Elsevier B.V. All rights reserved