Colloidal and Polymeric Routes

A well-known approach to the synthesis of multicomponent oxide powders is based on Pechini-type methods where the metal ions present in an aqueous solution are complexed with the help of organic ligands to form polybasic chelates [355-357]. This method generally allows for the use of readily available reagents such as oxides, carbonates, or nitrate as cation sources, with the added advantage that it requires no special apparatus or atmosphere control. The commonly used ligands are a-hydroxycarboxylic acid (e.g., citric acid,

HOC(CH2COOH)2-COOH). The polyhydroxy-metal complex, when treated with a polyfunctional alcohol (e.g., ethylene glycol, HOCH2CH2OH), undergoes a polyesterification reaction with the concurrent formation of a water molecule and the formation of a mixed-cation precursor (Fig. 8).

A number of variations of this methodology are used to obtain stoichiometric powders, with the use of different chelating reagents like citric acid, tartaric acid, glycol, acrylic acid, etc. The common idea is to achieve an atomistic distribution of the cations within the polymer resin. The utility of the method relies on the chemical bonding of the cations to the polymer chains (Fig. 8). It is assumed that various cations are homogeneously mixed with little or no segregation of the components, and the rigid transparent glassy mass is a "single" preceramic precursor [356]. The organic molecules used as a complexing aid not only function as chelating agents and a resin vehicle but also provide combustion heat for calcinations; however, excess organic content can have a negative effect by raising the temperature too high during calcination. Moreover, the composition of the polymeric precursor is important for the final agglomerate morphology and particle size because when compared with a dense resin, a foamy, voluminous precursor is expected to produce loosely bound aggregates. Although these methods claim the advantages of molecular-level mixing and the formation of mixed-metal polymeric precursors, evidence is not always available. However, the characterization of a Ba-Ti precursor to BaTiO3 (Pechini method) by 13 C NMR spectroscopy indicates that the coordination of Ba and Ti in the mixed-metal complex remains unchanged at both solution and resin stages. Nevertheless, this approach represents a low-temperature route to multicomponent material. For example, nanocrystalline yttrium aluminum garnet

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