Mixed oxygen ionic and electronic conduction in CaFe0.2T0.8O3-δ:: a combined oxygen permeation and electrical conductivity study

A combined oxygen permeation and electrical conductivity study on CaFe0.2Ti0.8O3-delta was conducted in the temperature range of 1100-850 degrees C, aiming at clarifying the oxygen transport mechanism. The oxide shows appreciated oxygen permeability at elevated temperature. An oxygen permeation flux of 6.37 x 10(8) mol cm(-2) s(-1) was observed at 1100 degrees C by applying an oxygen partial pressure difference to a 0.84-mm thick disk-shaped, sample (P-O2(h) = 0.209 and P-O2(l) = 0.004 atm). The apparent activation energy for oxygen permeation remained almost unchanged (170 kT/mol) with the variation of the sample thickness in the range of 2.5-0.8 mm. This indicates that the oxygen permeation kinetics is controlled by the transport of charge carriers in the bulk of sample, and can by promoted by further reduction of the thickness. In combination with electrical conductivity measurements, the partial conductivities of oxygen ions and electron holes, sigma(l) and sigma(e) were determined. The value of sigma(l) is comparable to that for sigma(e) at the high temperature of 1100 degrees C, but the latter became dominant at reduced temperature. This is due to the much higher activation energy associated with the transport of oxygen ions (179 +/- 6 kJ/mol) than that for electron holes (22 +/- 2 kJ/mol). It becomes clear that increasing the partial conductivity of oxygen ions in the oxide should lead to the improvement of the oxygen permeability of the oxide. (C) 1999 Published by Elsevier Science B.V. All rights reserved.