Oxygen exchange and diffusion coefficients of strontium-doped lanthanum ferrites by electrical conductivity relaxation

Perovskite-type solid oxide solutions La1-xSrxFeO3-δ are candidate materials for use as high temperature electrodes and oxygen separation membranes. Electrical conductivity relaxation experiments were performed on thin specimens of La1-xSrxFeO3-δ (x = 0.1, 0.4) at oxygen partial pressures pO2 = 10-5 to 1 bar in the temperature range 923 to 1,223 K. The transient response of the electrical conductivity after a sudden change of the ambient oxygen partial pressure was analyzed in the frequency domain. The latter procedure allowed diffusion-limited and surface exchange-limited kinetics of re-equilibration to be distinguished. The response of specimens with thicknesses of 350 to 460 microm indicated diffusion-controlled kinetics at pO2 > 0.03 bar. The chemical diffusion coefficients, D, were found invariant with oxygen pressure. At 1,073 K the absolute values were D = 6.5 x 10-6 cm2/s for x = 0.1 and D = 1.1 x 10-5 cm2/s for x = 0.4, with activation energies of about 80 kJ/mol. The equilibration process was governed by surface exchange at pO2 < 0.01 bar. The surface exchange coefficient, kO, was proportional to pO2n, where n = 0.65 to 0.85. This pressure dependency was interpreted in terms of a slow surface process involving an oxygen molecule and a surface oxygen vacancy, and causes the observed sharp transition from diffusion- to exchange-controlled kinetics. The activation energy of kO was estimated at 110 to 135 kJ/mol