Consolidation and properties of Gd 0.1 Ce 0.9 O 1.95 nanoparticles for solid-oxide fuel cell electrolytes
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J.L. Woodhead Advanced Material Resources (Europe) Ltd., Abingdon OX14 34S, United Kingdom (Received 19 May 2005; accepted 12 September 2005)
Gd-doped ceria solid solutions have been recognized to be leading electrolytes for use in intermediate-temperature fuel cells. In this paper, the preparation, solubility, and densification of Gd0.1Ce0.9O1.95 ceramics derived from carbonate co-precipitation are reported. The dissolution of Gd2O3 in CeO2 lattice was identified to be completed during the co-precipitation process by studying the lattice parameter as a function of temperature. After calcination at 800 °C for 2 h, the nano-sized Gd0.1Ce0.9O1.95 powder (∼33 nm) with a nearly spherical shape and a narrow particle-size distribution was obtained. This calcined powder has high sinterability and maximum densification rate at ∼1000 °C. Sintering at 1300 °C for 4 h yielded over 97% relative density with near maximum. The grain size increased with increases in sintering temperature. The ionic conductivity of these pellets was tested by alternating current impedance spectroscopy to elucidate the contribution of intragranular and intergranular conductivity to the total ionic conductivity. It was found that sintering temperature does not affect intragranular conductivity, though intergranular conductivity was strongly influenced by grain size, grain boundary area, and relativity density. This pellet sintered at 1500 °C for 4 h showed a high ionic conductivity of 5.90 × 10−2 s/cm when measured at 750 °C. The characterization and structural evaluation of the as-received powders were carried out using x-ray diffraction, transmission electron microscopy, Brunauer–Emmett–Teller, and dilatometer and impedance analysis.
I. INTRODUCTION
Gadolinium oxide-doped fluorite structured cerium oxide, 10 mol% Gd2O3–CeO2 (10GDC), is a solid solution formed by replacing the Ce4+ site of the CeO2 lattice by Gd3+ cations. 10GDC has been recognized as a midtemperature (500–800 °C operation temperature) electrolyte material for applications in solid-oxide fuel cells (SOFCs), as it has high ionic conductivity compared to other electrolyte materials in this range of operating temperature.1,2 10GDC powders synthesized via solid-state reaction require very high sintering temperatures (1700– 1800 °C).3 Traditional ball milling of particle to reduce its size will also introduce impurities such as silicon, and this will severely decrease its ionic conductivity since silicon forms an insulation glassy phase in the grain boundaries.4,5 A lower electrolyte sintering temperature is also desired, as the cathode and anode materials are
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0024 J. Mater. Res., Vol. 21, No. 1, Jan 2006
normally sintered at a relatively lower temperature of 1100–1300 °C.6 Therefore, an electrolyte material that can be co-fired together with the anode/cathode at a lower temperature would be desired. In addition, nanostructure can improve the mechanical properties of dense ceramics. Sever
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