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.
Highlights
The particular composition La0.6 Sr0.4 Co0.2 Fe0.8 O3 ́δ, abbreviated in this work as La0.6Sr0.4Co0.2Fe0.8O3 ́δ (LSCF), is one of the earliest and most demonstrated perovskite-type oxides for use as catalytic membranes for oxidative coupling of methane [1,2]. These mixedconductive membranes exhibit high electronic and ionic conductivities and high oxygen permeability in oxidizing conditions, chemical and mechanical instability under large oxygen chemical potential gradients and reactivity with CO2 and H2 O prompted the development of the dual-phase membranes, consisting of a composite of an oxide ionic-conductor, representatively, a Gd-doped ceria Ce1 ́x Gdx O2 ́δ, and an electronically conducting phase, e.g., La0.7 Sr0.3 MnO3 ̆δ, La0.7Sr0.3MnO3 ̆δ (LSM), which perform heterogeneously ambipolar diffusion for oxygen permeation [3,4]
The finer grain sizes of perovskites in the composites may be ascribed to the grain growth retardation effect by the second-phase GDC particles. (Surface structure was not developed for the smaller perovskite grains in the composites for the same thermal etching treatment.) While GDC:LSM composites show the expected variation in grain size, the grain size of GDC:LSCF composites becomes very small in the compositions of 20%, 25%, and 35% compared to 15% and 55%
Rather unsystematic microstructural evolution in GDC:LSCF composites may be related with the stress/strain interactions suggested by the lattice parameter variations discussed below
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. 500 °C to 600 °C which was largely attributed to the oxygen loss which decreases the carrier concentration, described as an order–disorder transition of oxygen vacancies [14,15], was related to a second order rhombohedral-cubic or ferroelastic-paraelastic phase transition between 650 °C and 750 °C [16,17,18] Such a phase transition was shown to occur for LSM at 850 °C by in situ XRD [19] but a corresponding conductivity transition has not been discussed. In situ conductivity measurements using the van der Pauw method can be a viable tool to monitor the state of OTM materials such as oxidation/reduction state and structural phase transitions for monitoring of transport and mechanical properties
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