Combustion synthesis of nanocrystalline yttria-doped ceria
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Nanocrystalline yttria-doped ceria powder, with the composition Ce0.55Y0.45O1.775, was synthesized by a combustion technique using citric acid as the fuel and the corresponding metal nitrates as oxidants. This process involves mild conditions as the external temperature required to initiate the combustion is only approximately 250 °C. The product was characterized by x-ray diffraction (XRD) to ascertain the phase purity. The crystallite size of these calcined samples, as seen by transmission electron microscopy, was found to be in the range 6 nm to 50 nm. The surface area of the fine powder, as obtained from the Brunauer–Emmett–Teller technique, was about 140 m2/g. The agglomeration behavior as a function of temperature and the lattice thermal expansion studies were carried out using high-temperature XRD. This nanocrystalline powder resulted in nearly theoretical bulk density at a relatively lower temperature, which is attributed to the superior powder properties. The sintered microstructure, as studied by scanning electron microscopy, revealed the presence of fine grains. I. INTRODUCTION
Doped ceria (CeO2) is an important material in view of its potential applications as a solid electrolyte for use in oxygen concentration cells and in the solid oxide fuel cells. The high ionic conductivity coupled with the low activation energy for ionic conduction makes doped ceria an attractive material for use at temperatures below 800 °C, which would allow greater flexibility in the design of electrode and interconnectors.1 Doped ceria is also an important material for applications in controlling the air-to-fuel ratio in automobile exhaust.2 Several alkaline-earth and rare-earth oxides are known to form extensive solid solutions with ceria. Aliovalent substitution of trivalent yttria into the lattice of ceria results in formation of oxygen ion vacancies, giving rise to an increase in the ionic conductivity. Among the trivalent rare-earth–doped ceria, Ce1–xYxO2–x/2 electrode is most abundant and cheaper. Its conductivity approaches that of Ce1–xGdxO2–x/2 and Ce1−xSmxO2−x/2.3 The solubility of YO1.5 into ceria can be as high as 45 to 50 mol% depending on the heat treatment.4,5 Usually, the higher solubility limits are observed if the samples are long annealed followed by quenching as compared to slow cooling conditions. The high ionic conductivity can be extended over a wide range of compositions having highly defective lattices. The phase equilibria for the ceria–yttria system for temperature at 1600 °C reveals that the solubility of Y2O3 is about 23% (i.e., 46 mol% of Y3+).6 This is in a)
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J. Mater. Res., Vol. 19, No. 2, Feb 2004 Downloaded: 20 Mar 2015
close agreement with phase equlibria in this system recently reported by us.4 The high solubility can be attributed to the quite similar ionic radii of Ce4+ and Y3+ [the ionic radii of Ce4+ and Y3+ are 0.90 Å and 0.93 Å, respectively, eightfold coordination (Wi
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