Low-temperature preparation and structural characterization of Pr-doped ceria solid solutions
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e report a method for preparing Pr-doped ceria by pyrolysis at 600 °C of aerosols consisting of aqueous solutions of Ce(III) nitrate and Pr(III) chloride. This procedure involves an important decrease of the temperature of solid solution formation (several hundred degrees) when compared with the ceramic and other previously reported methods, and does not require the addition of fluxes. The prepared samples and their products after thermal treatment were characterized in terms of particle shape and size distribution, composition, and crystalline structure. The color properties of these solid solutions were also evaluated as a function of the heating temperature and composition; the optimum red pigment was obtained for samples with a Pr/Ce mole ratio ⳱ 0.063 heated for 3 h at 600 °C. The oxidation state of the Pr cations and their distribution in the CeO2 matrix were investigated by x-ray absorption near-edge spectroscopy and x-ray photoelectron spectroscopy. A mixture of trivalent and tetravalent Pr was detected in all studies samples. The Pr(IV)/Pr(III) ratio increased as the Pr content or the heating temperature increased. These Pr(III) cations were found to be mainly located in the particles’ outer layers, in which an enrichment of Pr was detected for all samples heated at 600 °C. The further calcination of the pigments at 1200 °C gave rise to a certain diffusion of the Pr cations toward the particle core, which was more significant for the sample with the highest Pr content (Pr/Ce mole ratio ⳱ 0.091).
I. INTRODUCTION
The toxicity of most red high-temperature (>1000 °C) pigments used in the ceramic industry has lead scientists to search for new systems to replace these undesired materials. One of the suggested possibilities consists of doping CeO2 powders with Pr ions, which has been shown to produce stable red pigments.1–3 The preparation of these pigments is usually carried out by the traditional ceramic procedure, which requires calcination of the precursor mixture (CeO2/Pr6O11) at very high temperatures (1300 °C).2,3 To decrease such temperatures, the addition of fluxes to the starting mixture before heating has been proposed.1,3 In particular, a temperature of only 1000 °C was needed when NaF was added whereas a more important decrease (600 °C) was achieved with alkaline hydroxides, chlorides, or nitrates.3 Obviously, the use of most fluxes causes some additional environmental effects mainly associated with the emission of pollutants into the atmosphere (NOx , halides, etc.), which are not desirable.4 Therefore, it would be advantageous to develop alternative preparation procedures that decrease the calcination temperature without using fluxing agents. A method reported to fulfill this requirement was J. Mater. Res., Vol. 17, No. 4, Apr 2002
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based on the coprecipitation of Ce and Pr hydroxides or oxalates in aqueous solutions, which produced red pigments on heating at 1000 °C.5 The pyrolysis of liquid aerosols has been shown to be a very convenient tec
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