Yhz

(Y3Al5O12) could be obtained at 1000 °C, compared with the 1600 °C required for the solid-state reaction [358-360]. In an another example, the refractory mixed oxide calcium aluminate (CaAl2O4) was prepared in high purity at 900 °C, whereas the solid-state reaction between calcia (CaO) or calcium carbonate (CaCO3) requires temperatures in excess of 1400 °C to complete the reaction, which still contains undesirable CaAl4O7, Ca12Al14O33, and unreacted starting materials [361]. Further advantages of the Pechini-type synthesis are the ability to prepare complex compositions, homogeneous mixing of the cations, and easy scalability.

Another strategy for the in-situ formation of single molecular species as precursors utilizes the complexing abilities of triethanol amine (N(C2H4OH)3) and ethylene glycol (C2H4(OH)2). This concept has been put forward as an inexpensive alternative to metal-organic precursors but is similar to the Pechini-type methods; the molecular structure of these alkoxide precursors remains ambiguous because of the lack of appropriate characterization. For example, Laine et al. have synthesized a precursor to Mg-Al spinel [362, 363] by heating Al(OH)3, MgO, and a stoichiomet-ric amount of N(C2H4OH)3 in ethylene glycol. A plausible ionomer-like structure (Fig. 9) based on the mass spectral and NMR data [362] was proposed for the precursor compound wherein magnesium ion is encapsulated by a TEA molecule bridging two alumatrane units [364]. Following this approach, single precursors to mullite (Al6Si2O13) and cordierite (Mg2Al2Si5O18) have been developed and used to synthesize high-purity phases [362].

The other commonly used solution method for the synthesis of multicomponent oxide ceramics is coprecipitation reactions producing a "mixed" precipitate comprising two or more insoluble species that are simultaneously removed from solution. The precursors used in this method are mostly inorganic salts (nitrate, chloride, sulfate, etc.) that are dissolved in water or any other suitable medium to form a homogeneous solution with clusters of ions. The solution is then subjected to pH adjustment or evaporation to force those salts to precipitate as hydroxides, hydrous oxides, or oxalates [365-367]. A careful control of pH is important for achieving a quantitative precipitation because the precipitating species is a product of the neutralization reaction between the inorganic precursor and the base. The crystal growth and their aggregation are influenced by the concentration of salt, temperature, the actual pH, and the rate of pH change. After precipitation, the solid mass is collected, washed, and gradually dried by heating to the boiling point of the medium. The washing and drying procedures applied for coprecipitated hydroxides affect the degree of agglomeration in the final powder and must be considered rM

H2 C

C H2

H2 C

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