Conductivity Transitions Of La0.7sr0.3mno3 +/-Delta And La0.6sr0.4co0.2fe0.8o3-Delta In Ce0.9gd0.1o2-Delta Matrix For Dual-Phase Oxygen Transport Membranes

Using van der Pauw method, the conductivity of disk samples of La0.7Sr0.3MnO3 +/-delta (LSM) and La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) in a Ce0.9Gd0.1O2-delta (GDC) matrix was accurately quasi-continuously measured over 800 degrees C to -73 degrees C, and the transition points in Arrhenius behavior were systematically obtained from the extremum points of the second derivatives. While LSM-containing samples showed reproducible conductivity trajectories, the LSCF system exhibited unsystematic changes which may be related to the substantial oxidation/reduction reactions accompanying the ferroelastic-paraelastic transitions with a substantial thermal hysteresis at 650 degrees C to 750 degrees C, corresponding to conductivity maxima. A sudden decrease in activation energies on cooling corresponds to the para-to-ferromagnetic, weak insulator-metal transitions and the Curie temperature of LSM appears to gradually decrease in composites to 90 degrees C, while LSCF composites exhibit blurred transitions at approximately -40 degrees C. Relatively insulating paramagnetic phases are characterized by activation energy values similar to 0.2 eV, change to the high temperature phase exhibiting activation energy 0.1 eV for small polaron hopping mechanisms at 300 degrees C to 500 degrees C with increasing GDC content in the LSM composites and by two transitions at similar to 60 degrees C and similar to 245 degrees C for the LSCF composites. LSCF single phase shows distinctly lower transition points which appear to match with the singularly large c lattice parameter whereas the composites exhibit decreasing c with LSCF amount together with increasing lattice parameter of GDC. Van der Pauw conductivity is a feasible and sensitive in situ tool for monitoring the status of oxygen transport membranes.