Citrate-nitrate gel transformation behavior during the synthesis of combustion-derived NiO-yttria-stabilized zirconia co
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NiO-yttria-stabilized zirconia powder mixtures were prepared from reactive citrate-nitrate gels using the combustion technique. The influence of the fuel/oxidant molar ratio in the precursor on the combustion rate and its thermal characteristics was studied by thermal analysis and evolved gas analysis. It was found that the precursor thermal decomposition properties depended strongly on the citrate/nitrate ratio prior to the combustion. Intermediate precursors and final powder ashes were also analyzed by x-ray diffraction.
I. INTRODUCTION
Mixed and multicomponent ceramic oxides are generally prepared either by mechanical mixing of the preprepared oxides or by solid-state reactions of the component oxides at elevated temperatures.1,2 Such solid-state reactions are diffusion controlled, and the resulting powders may show a certain degree of compositional inhomogeneity. Furthermore, mixed oxides prepared by mechanical mixing and subsequent thermal treatment are often composed of hard agglomerates and are difficult to sinter to high relative densities. In recent years, the need for homogeneous ceramic powders with a controlled particle-size distribution, particularly for some electrical and magnetic applications, has constantly been on the increase. For that reason, several specialized powder preparation techniques such as the sol-gel method, coprecipitation, spray drying, and freeze drying have been tested and used to produce oxides with the required properties.3–12 Although these methods have unique advantages in ensuring high homogeneity of the product oxides, elevated temperature processing is still required to obtain crystalline oxides. Recently, some novel wet chemical methods have been employed in mixed oxide materials preparation. Among the various solution routes, combustion synthesis seems to be a very promising one.13,14 This synthesis, based on a variety of redox systems, can instantaneously produce a large number of single or multicomponent oxide powders that are crystalline and homogeneous and have a narrow particle-size distribution.15–18 In such reactions an oxidizing reagent (e.g., nitrate) and a reducing reagent [e.g., urea, carbohydrazide, glycine, citrate, ethylene diamine tetraacetic acid (EDTA)] are mixed together and then decomposed rapidly via their combustion. J. Mater. Res., Vol. 18, No. 7, Jul 2003
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The combustion process at some point involves decomposition of the redox system, which then proceeds as a self-sustaining front throughout the reactant gel mixture. The thermal decomposition of different gel precursors has been studied by several authors.19–28 From the variety of redox systems, a combustion mixture based on the citrate-nitrate system can be potentially applied for the production of larger quantities of mixed oxides because of its relatively nonviolent combustion. Although complete citrate oxidation and nitrate reduction yields CO2, H2O, and N2,29 the oxidation/decomposition reaction scheme of such a system is normally explain
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