Investigation of thermo-chemical expansions in Pr and Gd doped ceria by a novel temperature modulated dilatometry approach

Many metal oxides exhibit thermo-chemical expansion due to change in the chemical potential of oxygen. The latter can result in change of oxidation state of the metal ions and/or loss of oxygen upon temperature and PO₂ changes. These effects lead to mechanical failure in devices such as solid oxide fuel cells that operate at high temperatures and high PO₂ gradients. It is difficult to study thermal and chemical effects separately in a qualitative manner, as both effects occur simultaneously. In this work, we demonstrate separation of the expansion caused by chemical effects from the pure thermal effects at different temperature ranges, using temperature modulated dilatometry. Such separation is possible if different processes occur on significantly different timescales. However, the case in our model materials - doped ceria and for comparison doped zirconia - is more complex, as different chemical effects are important at different temperatures. Values of coefficient of modulated temperature thermal expansion (MCTE) are compared with the coefficient of linear thermal expansion (CTE). The CTE remains independent of heating rate, whereas the MCTE generally depends on the modulation rate. The latter dependence of the MCTE is changed over temperature, as defect associates vanish, and cation valence change kicks in.