Conductivity Transitions of La_0.7Sr_0.3MnO_3±<i>δ</i> and La_0.6Sr_0.4Co_0.2Fe_0.8O_{3−<i>δ</i>} in Ce_0.9Gd_0.1O_{2−<i>δ</i>} Matrix for Dual-Phase Oxygen Transport Membranes

Abstract

Using van der Pauw method, the conductivity of disk samples of La0.7Sr0.3MnO3±δ (LSM) and La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) in a Ce0.9Gd0.1O2−δ (GDC) matrix was accurately quasi-continuously measured over 800 °C to −73 °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 °C to 750 °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 °C, while LSCF composites exhibit blurred transitions at approximately −40 °C. Relatively insulating paramagnetic phases are characterized by activation energy values ~0.2 eV, change to the high temperature phase exhibiting activation energy 0.1 eV for small polaron hopping mechanisms at 300 °C to 500 °C with increasing GDC content in the LSM composites and by two transitions at ∼60 °C and ∼245 °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.

Keywords

• dual-phase membrane
• van der Pauw conductivity
• Rietveld analysis
• phase transitions