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when nanosized powders are the intended product. Generally, a calcination step is necessary to transform the hydroxide into crystalline oxides. Although the precipitate is not structurally characterized, the association of ions to form mixed-metal hydroxides (e.g., MgAl2(OH)8 for MgAl2O4) offers the advantages of molecular-scale mixing of the components. This strategy is a commercially viable alternative method for obtaining high-quality oxide powders; however, the method is plagued by the problems of contamination due to anionic ligands (halides, nitrates, etc.), which are detrimental to the material properties if not removed during the calcination steps. In addition, an incomplete precipitation of one of the ions can lead to phase segregation or non-stoichiometric products. In most of the binary, ternary, and quaternary systems, a crystallization step is necessary, which is generally achieved by calcinations or, more elegantly, by a hydrothermal procedure in high-pressure autoclaves.

A large number of reports are available on the synthesis of ultrafine oxide powders (Fe3O4 [368], ZrO2 [369], BaTiO3 [340], NiTiO3 [370], MnFe2O4 [371], ZnAl2O4 [372], oxide-oxide composites CeO2/ZrO2 [373], and biomaterial [Ca5(PO4)3(OH)]2 [374]) by coprecipitation reactions. Nanocrystalline yttria-stabilized zirconia (grain size < 100 nm), consolidated and sintered to a nanostructured ceramic has been prepared by coprecipitation reactions [375]. Figure 10 shows the scheme employed to produce BaTiO3 nanopowders from a barium titanyl oxalate precursor, BaTiO(C2O4)4H2O.

The advantages of coprecipitation reactions are (i) the homogeneity of component distribution, (ii) the relatively low reaction temperature, (iii) the fine and uniform particle size with weakly agglomerated particles, and (iv) the low cost. However, these reactions are highly susceptible

Figure 10. A schematic of various steps involved in the preparation of BaTiO3 by a coprecipitation technique.

to the reaction conditions, and, because of incomplete precipitation of the metal ions, control over the stoichiometry of the precursors is rather difficult to achieve. In addition, the coprecipitation reactions are not suited for certain oxides/hydroxides, for instance, in the case of amphoteric systems.

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