Sol-gel synthesis and NMR characterization of ceramics

Abstract

Sol-gel chemistry allows the low temperature synthesis of ceramic powders. The molecular engineering of alkoxide precursors leads to the crystallization of ceramic powders of controlled morphology and structure. Stable sols of tetragonal zirconia have been synthesized via the hydrolysis of chemically modified zirconium alkoxides. Complexing acetylacetone ligands slow down condensation processes and prevent the aggregation of nanosized zirconia particles. The hydrolysis rates of the different molecular precursors have to be matched when multicomponent ceramics such as cordierite are prepared via the sol-gel route. Otherwise phase separation occurs leading to the undesired spinel phase MgAl 2O 4. The complexation of aluminium alkoxides decreases their reactivity toward hydrolysis and condensation. AlOSi bonds can be formed in solution and the tetrahedral coordination of Al III is preserved allowing the crystallization of cordierite at lower temperatures. Multinuclear NMR appears to be a valuable technique for the characterization of all the species formed during the sol-gel synthesis, from the molecular precursor to the ceramic powder.