Low-temperature preparation of dense 10 mol%-Y 2 O 3 -doped CeO 2 ceramics using powders synthesized via carbonate copre
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Ji-Guang Li, Takayasu Ikegami, and Yoshiyuki Yajima Advanced Materials Laboratory, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan (Received 19 December 2002; accepted 20 February 2003)
A carbonate coprecipitation method was used for the facile synthesis of highly reactive 10 mol%-Y2O3-doped CeO2 (20YDC) nanopowders, employing nitrates as the starting salts and ammonium hydrogen carbonate (AHC) as the precipitant. The AHC/RE3+ (RE ⳱ Ce + Y) molar ratio (R) and the reaction temperature (T) significantly affect the final yield and precursor properties, including chemical composition and particle morphology. Suitable processing conditions are T ⳱ 60 °C and R ⳱ 2.5 to 10, under which precipitation is complete, and the resultant precursors show ultrafine particle size, spherical particle shape, and good dispersion. The thus-processed precursors are basic carbonates with an approximate formula of Ce0.8Y0.2(OH)CO3 · 2H2O, which directly yield oxide solid solutions upon thermal decomposition at a very low temperature of approximately 400 °C. The 20YDC solid-solution powders calcined at 700 °C show excellent reactivity and were densified to >99% of theoretical via pressureless sintering at a very low temperature of 950 °C for 6 h. I. INTRODUCTION
Ceria (CeO2) is a fluorite-structured oxide (space group Fm3m) that forms extensive solid solutions with a variety of alien cations. Ceria-based materials are currently under active investigations for applications as an oxygen buffer in automotive three-way catalysts,1 as a promoter for gas phase oxidation/reduction reactions,2,3 and as solid electrolytes4,5 or anodes6 in intermediate-temperature solid oxide fuel cells (SOFCs). Among doped-ceria materials, the Y2O3/CeO2 system (YDC) is of special interest due to its relatively high electrical conductivity and the relative abundance of the yttrium element. Due to their technological importance, a variety of techniques were used during recent years to produce CeO2 and YDC powders of higher reactivity. For CeO2, the powders processed via homogeneous precipitation with hexamethylenetetramine (HMT) as the precipitant were demonstrated to possess excellent reactivity, and nearly fully dense ceramics have been fabricated via pressureless sintering at 1250 °C within a short holding period of just 6 min.7,8 More recently, a similar density level was achieved by sintering at 1000 °C for 2 h the a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 5, May 2003
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powders synthesized from organic solutions.9 In the above cases, ultrafine particle/crystallite size and non- or soft-agglomeration were claimed to be responsible for the excellent sinterability. Unlike CeO2, YDC is a double-cation oxide. Therefore YDC powders are typically synthesized via solid-state reaction starting from the individual component oxides, which requires repeated intermittent mechanical mixing and prolonged heating at ele
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