| Summary: | Using van der Pauw method, the conductivity of disk samples of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>La</mi><mrow><mn>0.7</mn></mrow></msub><msub><mi>Sr</mi><mrow><mn>0.3</mn></mrow></msub><msub><mi>MnO</mi><mrow><mn>3</mn><mo>±</mo><mi>δ</mi></mrow></msub></mrow></semantics></math></inline-formula> (LSM) and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>La</mi><mrow><mn>0.6</mn></mrow></msub><msub><mi>Sr</mi><mrow><mn>0.4</mn></mrow></msub><msub><mi>Co</mi><mrow><mn>0.2</mn></mrow></msub><msub><mi>Fe</mi><mrow><mn>0.8</mn></mrow></msub><msub><mi mathvariant="normal">O</mi><mrow><mn>3</mn><mo>−</mo><mi>δ</mi></mrow></msub></mrow></semantics></math></inline-formula> (LSCF) in a <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>Ce</mi><mrow><mn>0.9</mn></mrow></msub><msub><mi>Gd</mi><mrow><mn>0.1</mn></mrow></msub><msub><mi mathvariant="normal">O</mi><mrow><mn>2</mn><mo>−</mo><mi>δ</mi></mrow></msub></mrow></semantics></math></inline-formula> (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 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>−</mo><mn>40</mn></mrow></semantics></math></inline-formula> °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 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>∼</mo><mn>60</mn></mrow></semantics></math></inline-formula> °C and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>∼</mo><mn>245</mn></mrow></semantics></math></inline-formula> °C for the LSCF composites. LSCF single phase shows distinctly lower transition points which appear to match with the singularly large <i>c</i> lattice parameter whereas the composites exhibit decreasing <i>c</i> 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.
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