Highly enhanced densification and grain growth of Ce 0.9 Gd 0.1 O 1.95-δ (CGO, gadolinium-doped ceria, with 10 mol% Gd) is achieved in low oxygen activity atmospheres. However, the material can suffer mechanical failures during cooling when the re-oxidation process is not controlled due to the large volume changes. In this work, the redox process of CGO is investigated using dilatometry, microscopy, electrochemical impedance spectroscopy and thermodynamic analysis. In addition, the conditions allowing controlled re-oxidation and cooling in order to preserve the mechanical integrity of the CGO component are defined: this can be achieved over a wide temperature range (800−1200 • C) by gradually increasing the oxygen content of the atmosphere. It is found that the electrical conductivity of the CGO, particularly at low temperature (<450 • C) is influenced by the sintering and controlled re-oxidation conditions. An increase in activation energy for conduction at low temperature is observed as the re-oxidation temperature decreases. Moreover it was observed that the ionic conductivity blocking effect, usually associated with grain boundary contributions, is not influenced by the grain size but rather by the chemical history of the material.Doped ceria (CeO 2 ) materials are being investigated for applications such as solid electrolytes for solid oxide fuel cells (SOFC), 1,2 catalytic substrates for chemical looping and hydrolysis at high temperature, 3,4 and oxygen transport membranes, 5,6 due to their good chemical stability, 1 catalytic properties in redox processes, 3,4 and high ionic conductivity at low temperatures (600-800 • C). 1 In ceria, doping in the range of 10-20 mol%, by trivalent lanthanides elements, results in compounds with high oxygen ion transport properties. 7 Ceria (both pure and doped) can be easily reduced under low oxygen partial pressure (P O 2 ) conditions by partial conversion of Ce 4+ ions to Ce 3+ to compensate oxygen vacancies produced when oxygen is lost from the lattice. The reduction process depends on both temperature and P O 2 , and introduces electronic/ionic conduction coupled with lattice expansion which both depend on the thermodynamic conditions. 1 Particularly, chemical expansion occurs as a net result of the larger radius of Ce 3+ ions (1.14 Å) compared to Ce 4+ ions (0.97 Å) and electrostatic interactions between defects (oxygen vacancies) and neighbouring atoms. 8,9 Such volume changes, corresponding to oxygen being incorporated and released from the lattice during redox cycles, are large and depending on the geometry induce large stresses which can easily result in mechanical failure of the ceramic. [10][11][12][13][14] The redox behavior of ceria has been studied in detail, 15-18 specifically with respect to the thermal and mechanical stability. 10,19 X-ray diffraction studies showed that ceria reduced in hydrogen at either 400 • C or 600 • C (level of reduction less than 60%) could be re-oxidized in air at room temperature. However, when ceria was reduced at higher temper...
