LSCF has a cubic perovskite structure. The oxygen vacancies are formed at elevated temperatures above 873 K, while the transition metal ions are reduced for the charge neutrality, accompanied by a sudden volume expansion, i.e., an increase in lattice constant. This implies that the oxygen vacancies could be introduced by applying mechanical strain or stress, as has been discussed in recent reports. In the present study, the oxygen-nonstoichiometry in LSCF was examined under uniaxial compression using coulometric titration technique.
The test cell consisted of polycrystalline LSCF placed between 8 mol%-yttria stabilized zirconia (YSZ) plates with the platinum paste on the interfaces and on the surfaces, i.e., Pt/YSZ/Pt/LSCF/Pt/YSZ/Pt. The side of LSCF was covered with borosilicate glass. A constant voltage (25 mV to 100 mV) was applied to YSZs using a galvanostat in order to control the oxygen partial pressure in the cell (from 0.0678 to 0.0026). After the measured current reached a constant value (e.g., zero in case there is no leakage), the circuit was opened and the decay current was subsequently measured. The change in the oxygen deficiency was calculated from the integration of the measured decay current. Then, the oxygen non-stoichiometries in LSCF under various oxygen partial pressures were examined. The measurement was conducted under uniaxial compressive stress ranging from 0 to 10 MPa, and all the tests were performed at 1073 K in air. In addition to the above experiment, the relationship between the uniaxial strain and the oxygen non-stoichiometry was estimated based on the mechanical property as well as thermo-chemical-mechanical property at 1073 K based on the literature data, assuming that the oxygen deficiency merely depends on the volume change.
The oxygen deficiency increased as the oxygen partial pressure was decreased, and there was almost no leakage from the test cell. The obtained results showed a good agreement with the previous reports. There was no significant effect of the applied stress up to 10 MPa on the measured decay current or the oxygen non-stoichiometry, whereas the stress above 20 MPa caused the fracture of the test cell. According to our estimation based on the mechanical and thermo-chemical-mechanical properties, the oxygen deficiency could be decreased by ~0.006 when the uniaxial compressive stress of 100 MPa is applied, which is being examined using a modified experimental setup.