Synthesis and Properties of Rare Earth-doped Ceria as Solid Electrolyte of Fuel Cell

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Doped ceria, which has a higher oxygen ion conductivity in yttria-stabilized zirconia, is one of the possible electrolyte for solid oxide fuel cell at low temperatures. This research covers powder preparation, densification and electrical, mechanical and thermal properties of rare earth-doped ceria. Rare earth-doped ceria powders with a composition of Ce0.8R0.2O1.9 (R=Yb,Y,Gd,Sm,Nd and La) were prepared by heating the oxalate coprecipitate where a mixed R/Ce nitrates solution was added to an oxalic solution. This method provided the oxalate solid solutions containing Ce and R, which were calcined to form the oxide solid solutions at 600℃ in air. The sizes of oxalates and oxides depended on the concentration of oxalic acid and showed a minimum at 0.4 M oxalic acid. The primary oxide particles of 24-63 nm were formed in the skeleton particle clusters of median size of 2μm. The lattice parameters of oxide powders increased linearly with increasing ionic radius of doped rare earth. These r … More are earth-doped ceria powders were formed by uniaxial pressing (49 Mpa) and following cold isostatic pressing (294 Mpa). The samples were densified above 98% relative density by the sintering at 1600℃ for 4 h. The grain sizes (4.7-7.6μm) of samples sintered at 1600℃ showed a tendency to become larger with increasing ionic radius of doped-rare earth element. Observation of rare earth-doped ceria by high magnification transmission electron microscopy indicated that no crystalline or amorphous phase existed at grain boundaries and the dopants of rare earth elements were uniformly distributed in bulk and at grain boundaries. The electrical conductivity of rare earth-doped ceria ceramics was dominated by the migration of oxygen ions in bulk and was in the rage of 6.7- 14.0 S・mィイD1-1ィエD1 at 800℃. The activation energies for diffusion of oxygen ions was lower for the bulk (75-93 kJ/mol) than for grain boundary (101-123 kJ/mol). The activation energies decreased as the ionic radius of rare earth element dopant increased from 0.099 to 0. 105 nm. The diffusion coefficients of oxygen ions in rare earth-doped ceria except for Ce0.8Y0.2O1.9 and pure CeO2, which were calculated from the measured bulk conductivity, were in good agreement with those measured by the tracar method. No significant influence of doped rare earth element on the thermal expansion coefficient of CeO2 was measured (11.4-12.2 x10ィイD1-6ィエD1KィイD1-1ィエD1). The four-point flexural test offered 158-178 Gpa of Young's modulus, 53-81 MPa of strength and 9.1 of Weibull modulus for Ce0.8Sm0.2O1.9 with 86-98% relative density. Less