Early stages of crystallization in gel derived ZrO2 precursors

Three different precursor materials giving rise to contrasted nanostructures, xerogels, aerogels and precipitates, are prepared by a sol gel route in the Zr n-propoxide-acetylacetone-water-n-propan system. Clear homogeneous gels are made by using a proper amount of acetylacetone and water. The gels are dried either by conventional processing (xerogels) or by supercritical evacuation of alcoholic solvent (aerogels). The complexation ratio (R = [acetylacetone]/[Zr(OR)(4)]) is the main parameter controlling the size of ZrO2 primary particles. When R = 0, precipitates are obtained. Xerogels, aerogels and precipitates are characterized and their textures are compared through small angle X-ray scattering measurements. The fractal structure of gels is destroyed by conventional drying and is preserved in aerogels. On the other hand precipitates are described as homogeneous agglomerates of very small primary units. The first crystallization steps are studied by transmission electron microscopy and X-ray diffraction experiments. The contribution of crystallite size and microstrain effects are investigated by Rietveld whole pattern fitting. The crystallization of precipitate powders starts at the agglomerate scale with large crystal like distorted lattices.