Oxygen permeation and stability of La0.6Sr0.4TixFe1–xO3–δ (x = 0.2 and 0.3) membrane

An oxygen transport ceramic membrane was prepared with La0.6Sr0.4TixFe1–xO3–δ (x = 0.2 and 0.3) oxides which was synthesized by polymerized complex method. In order to investigate the effects of Ti substitutions into lanthanum–strontium ferrites, material property, thermal expansion coefficient, oxygen permeation, electrical conductivity and phase stability were examined according to Ti contents. The thermal expansion coefficients of LSTF6428 and LSTF6437 were 25.76 × 10–6 K–1 and 21.32 × 10–6 K–1 in the temperature range of 973−1273 K, respectively. The oxygen permeation fluxes through a disk type dense membrane of LSTF6428 and LSTF6437 with 1.6 mm thickness increased exponentially with increasing temperature and were 0.17 mL min–1 cm–2 and 0.13 mL min–1 cm– 2 with P = 0.21 atm (O rich side) and P = 10–5 atm (lean side) at 1223 K, respectively. In the result of phase h 2 l stability test with various conditions such as He, H2/N2 and CO2 atmosphere, La0.6Sr0.4Ti0.3Fe0.7O3–δ was confirmed to be more stable than La0.6Sr0.4Ti0.2Fe0.8O3–δ through XRD and SEM analysis.

Perovskite type ceramic powders (La0.6Sr0.4TixFe1–xO3–δ, x = 0.2 and 0.3) were synthesized by polymerized complex method using citric acid as the chelating agent. The powder and membrane were analyzed with XRD, FT-IR, SEM–EDS and dilatometry. The characterization results showed that the oxides sintered at 1573 K possessed single perovskite structure without secondary phase irrespective of composition and the molar composition of membrane can be controlled by stoichiometric molar ratio fed initially during powder synthesis. Dense titanium-substituted La0.6Sr0.4TixFe1–xO3–δ membrane showed relatively high oxygen permeability at elevated temperature and lower Ti content. Oxygen permeation fluxes of 0.17 and 0.13 mL min –1 cm–2 can be obtained with a 1.6 mm thick disk-shaped membrane at 1223 K with Ph=0.21 atm (O2-rich side) and Pl=10–5 atm (lean side) for LSTF6428 and LSTF6437, respectively. The long-term stability test with various conditions such as He, H2/N2 and CO2 atmosphere showed LSTF6437 had strong reduction resistance and durability for CO2 for long time. In addition, the thermal expansion coefficient of LSTF6437 was 21.32 × 10–6 K –1 in the temperature range of 973–1273 K, which was lower than LSTF6428. Therefore, the Ti substitution effect on phase stability and thermal expansion is positive so that LSTF6437 composition is one of good materials for application of CO2 recycle system for oxy-fuel combustion even if the oxygen permeation flux is lower than LSCF materials.