Synthesis and sintering of rare-earth-doped ceria powder by the oxalate coprecipitation method
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Synthesis and sintering of rare-earth-doped ceria powder by the oxalate coprecipitation method Kenji Higashi, Kazutoshi Sonoda, Hiroshi Ono, Soichiro Sameshima, and Yoshihiro Hirata Department of Applied Chemistry and Chemical Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan (Received 15 August 1997; accepted 17 June 1998)
Doped ceria, which has a higher oxygen ion conductivity than yttria-stabilized zirconia, is one of the possible electrolytes for solid oxide fuel cell at low temperatures. This study concerns powder preparation and densification of rare-earth-doped ceria. Rare-earth-doped ceria powders with a composition of Ce0.8 R0.2 O1.9 (R Yb, Y, Gd, Sm, Nd, and La) were prepared by heating the oxalate coprecipitate when a mixed rare earth/cerium nitrate solution was added to an oxalic solution. The oxalate and derived-oxide powders were characterized by x-ray diffraction (XRD), thermogravimetry differential thermal analysis (TG-DTA), particle size analyzer with laser diffraction, inductively coupled plasma (ICP), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). This method provided the oxalate solid solutions containing Ce and R, which were calcined to form the oxide solid solutions at 600 ±C in air. The lattice parameter of oxide powders increased linearly with increasing ionic radius of doped rare earth. The size of platelike particles of oxalates and oxides depended on the concentration of oxalic acid and showed a minimum at 0.4 M oxalic acid. Dry milling of oxide powder with a –Al2 O3 ball was effective in reducing the size and aspect ratios of particles with little contamination of Al2 O3 . These rare-earth-doped ceria powders with various sizes were formed by uniaxial pressing (49 MPa) followed by cold isostatic pressing (294 MPa), and sintered at 900–1600 ±C in air for 4 h. The micrometer-sized-doped CeO2 powders were densified above 95% of the theoretical density at 1200 ±C. The grain size of rare-earth-doped ceria after sintering at 1600 ±C was larger in the samples with the larger rare-earth element.
I. INTRODUCTION
Solid oxide fuel cell (SOFC) using H2 and O2 gases can generate high-efficient electric power at about 1000 ±C without environmental pollution.1 Reducing the operation temperature is useful in increasing the lifetime and the choice of the constituent materials of SOFC.2–5 Rare-earth-doped CeO2 has a higher oxygen ion conductivity than Y2 O3 -stabilized ZrO2 and is a candidate oxide used in low-temperature SOFC.6–13 The final purpose of this research is to prepare highly conductive rare-earth-doped CeO2 with good mechanical properties. To the above purpose, rare-earth-doped (Yb, Y, Gd, Sm, Nd, or La) ceria powders were synthesized by the oxalate coprecipitation method and characterized with x-ray diffraction (XRD), thermogravimetric differential thermal analysis (TG-DTA), inductively coupled plasma (ICP), scanning electron microscopy (SEM), and transmission electron microscopy (T
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