Low temperature synthesis of nanocrystalline Y₃Al₅O₁₂ (YAG) and Ce-doped Y₃Al₅O₁₂via different sol–gel methods

Abstract

Nanocrystalline yttrium aluminium garnet (YAG, Y₃Al₅O₁₂) and Ce-doped YAG ceramics were synthesized by two ‘soft chemistry' sol–gel processes using (i) mixtures of inorganic salts or oxides and (ii) mixtures of alkoxides of the respective elements. In the first approach the metal ions, generated by dissolving metal oxides or nitrates in acetic acid and/or water, were complexed by ethylene glycol to obtain the precursors for pure and doped YAG samples. In the alkoxide route monolithic gels were obtained by hydrolysis and condensation of a compositional mixture of Al, Y and Ce alkoxides in a PrⁱOH solution. The molecular level mixing and the tendency of partially hydrolysed alkoxide species to form extended networks of cross-linked metal centers facilitates the structure evolution thereby lowering the crystallization temperature, in the latter case. The X-ray diffraction (XRD) patterns of the ceramic sintered at 700 °C were identical with the stoichiometric YAG composition which is the lowest temperature reported for the synthesis of crystalline and single phase Y₃Al₅O₁₂ while well developed YAG phases in the non-alkoxide synthesis were obtained only at 1000 °C. Cerium doped YAG powders (CeO₂ + Y₃Al₅O₁₂) were synthesized using [NH₄]₂[Ce(NO₃)₆] (8 mol%) and [Ce₃(OBu^t)₉(Bu^tOH)₂] (5 mol%), as dopants. A homogeneous distribution of cerium in the YAG lattice was achieved in both cases. The thermal behaviour, phase transformations, composition and microstructural features in the gels and polycrystalline samples were studied by TG/DTA, XRD, FT-IR, solid-state ²⁷Al MAS NMR spectroscopy, SEM, TEM, energy dispersive X-ray analysis and high resolution electron microscopy studies. The quality of the resulting products (homogeneity, crystallization temperature, grain size, grain size distribution, etc.) and economical aspects (synthetic skill, cost of precursors, etc.) of the two approaches are discussed.

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