Electro-chemo-mechanical studies of perovskite-structured mixed ionic-electronic conducting SrSn_1-xFe_xO_3-x/2+δ Part III: Thermal and chemical expansion

The thermal and chemical expansion of a potential solid oxide fuel cell (SOFC) cathode material SrSn_0.65Fe_0.35O_{3–0.35/2+δ} (SSF35) were investigated to assess its thermo-chemo-mechanical stability at SOFC operating temperatures and to establish the correlation between defect concentrations (oxygen vacancies and electrons) and chemical expansion with the aid of the defect chemical model reported in part I of this study. Thermochemical expansion was measured as a function of temperature and oxygen partial pressure. The chemical expansion of SSF35 showed a strong correlation with changes in oxygen nonstoichiometry associated with changes in Fe valence state. Coefficients of both chemical (CCE) and thermal (CTE) expansion were calculated and found to be smaller than that of the closely related mixed conducting perovskite oxide SrTi_0.65Fe_0.35O_{3–0.35/2+δ} (STF35). The thermal expansion coefficient of SSF was found to be close to that of YSZ (most popular solid oxide electrolyte), which makes SSF35 more attractive in terms of overall thermo-chemical stability. The chemical expansion of SSF35 showed decreasing CCE with increasing temperature and decreasing CTE with increasing oxygen deficiency, both opposite to the trends observed for STF35. Distortion in symmetry from the cubic structure seems to be responsible for the smaller coefficients and increasing asymmetry with expansion seems accountable for opposite trends of CCE and CTE compared to the STF counterpart.