Thermodynamic Stability of Gadolinia-Doped Ceria Thin Film Electrolytes for Micro-Solid Oxide Fuel Cells

Next-generation micro-solid oxide fuel cells for portable devices require nanocrystalline thin-film electrolytes in order to allow fuel cell fabrication on chips at a low operation temperature and with high power outputs. In this study, nanocrystalline gadolinia-doped ceria (Ce_0.8Gd_0.2O_{1.9− x} ) thin-film electrolytes are fabricated and their electrical conductivity and thermodynamic stability are evaluated with respect to microstructure. Nanocrystalline gadolinia-doped ceria thin-film material (Ce_0.8Gd_0.2O_{1.9− x} ) exhibits a larger amount of defects due to strain in the film than state-of-the-art microcrystalline bulk material. This strain in the film decreases the ionic conductivity of this ionic O²⁻ conductor. The thermodynamic stability of a nanocrystalline ceria solid solution with 65 nm grain size is reduced compared with microcrystalline material with 3–5 μm grain size. Nanocrystalline spray-pyrolyzed and PLD Ce_0.8Gd_0.2O_{1.9− x} thin films with average grain sizes larger than 70 nm show predominantly ionic conductivity for temperatures lower than 700°C, which is high enough to be potentially used as electrolytes in low to intermediate-temperature micro-solid oxide fuel cells.     

Comprehensive Linkage of Defect and Phase Equilibria Through Ferroelectric Transition Behavior in BaTiO3-Based Dielectrics: Part 2. Defect Modeling Under Low Oxygen Partial Pressure Conditions

Defect and phase equilibria have been investigated through the ferroelectric phase transition behavior of pure and equilibrated nonstoichiometric BaTiO₃ powders. The paraelectric–ferroelectric phase transition temperature ( T _{C – T} ) was found to vary systematically with materials fabricated with different Ba/Ti ratio ( g ^*) and under various oxygen partial pressure (     ) conditions. ¹ The solubility regime, as determined through the T _{C – T} variation, decreased with decreasing     . ² Determining the solubility limits and equilibrating the defect reactions at the solubility limits provide a direct approach to calculate the defect formation energies and provide data to test a new defect model for concurrent defect reactions of partial Schottky and reduction defects. A refined approach introduces a balanced equilibrium between the oxygen vacancy concentrations controlled by the partial Schottky and reduction reactions. In the limiting ambient cases the approach gives the expected results, and also fully explains the solubility trends under low     's. Universally, the theory supports all the experimental data over different temperatures and     's.