Conductivity Transitions of La0.7Sr0.3MnO3±δ and La0.6Sr0.4Co0.2Fe0.8O3−δ in Ce0.9Gd0.1O2−δ 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.

Highly Conductive Ceramics with Multiple Types of Mobile Charge Carriers

“…It can be tricky to separate the electronic and ionic contributions when several charge carriers are involved. Pham et al [1] used the van der Pauw method to determine the conductivity of the cathode composite materials La 0.7 Sr 0.3 MnO 3±δ (LSM)/Ce 0.9 Gd 0.1 O 2−δ (GDC) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−δ (LSCF)/GDC over a wide temperature range from 800 • C to −73 • C. The samples containing LSM showed reproducible conductivity trajectories, while the LSCF system exhibited unsystematic changes, which may be related to the substantial oxidation/reduction accompanying ferroelastic-paraelastic transitions at 650-750 • C. Combined with structural analysis, they reached the conclusion that a small amount of GDC on the LSCF crystal structures may control the grain size and therefore affect the elastic properties and oxidation/reduction in a subtle way.…”