Fractal Aspects of Ceramic Synthesis

Talc-like hybrids: influence of the synthesis

Adding water to the synthesis medium leads to the formation of hybrids with low polycondensation, high crystallinity and high thermal stability.

Impact of Solvent Structuring in Water/ tert-Butanol Mixtures on the Assembly of Silica Nanoparticles to Aerogels

Soft matter structuring is a useful tool for the preparation of well-structured inorganic materials. Here, we report a strategy using a structured solvent based on binary mixtures as a directing agent for silica nanoparticles in aerogel elaboration. Binary mixtures involving water/ethanol and water/ tert-butanol have been respectively chosen as representatives of unstructured and structured solvents. The water/alcohol/TEOS systems were effectively characterized as surfactant-free microemulsions. The enhanced solvent structuring, however, disappears upon the reaction with TEOS, and assembly is directed by solvent structuring found in the binary mixtures. For the first time, the influence of solvent composition on the sol-gel reaction was investigated with respect to the reaction rate and the structuring behavior thanks to dynamic light scattering (DLS), small- and wide-angle X-ray scattering (SWAXS), and transmission electron microscopy (TEM) experiments. The silica nanoparticles aggregate in a different manner depending on the solvent composition, which allows the change in the morphology, the degree of interconnection, and the surface area of the resulting material. Silica nanoparticles with a very high surface area of up to 2000 m²/g can be obtained by this approach.

Small-Angle Neutron Scattering and 27Al Nmr Studies on the Microstructure and Composition of Alumina Sol-Gels

We have used small angle neutron scattering, static light scattering and 27Al NMR to examine the structure and composition of alumina sol-gels formed by the hydrolysis of aluminum alkoxides. For LT sols at low acid concentrations, and HT gels over a wide range of acid concentration, 27Al solution NMR suggests, by the dearth of spectral resonances, that high molecular weight species are being formed. Analysis of the small angle neutron scattering data in the Porod regime indicates these sol-gels exhibit a power-law dependence consistent with mass fractal dimensions ranging from 1.45 to 1.8. These fractal dimensions are consistent with models based on diffusion limited cluster aggregation. The fractal dimensions do not differ significantly between LT and HT sols at the same acid concentration. However, for both temperature regimes, the fractal dimension increases with increasing acid concentration, suggesting a progression to a more compact network. Static light scattering measurements indicate the Guiner radii of the cluster aggregates vary from 600 to 2000Å.

Small-Angle Scattering from Disordered Systems

A general discussion of scattering from disordered materials is presented. The parameters which characterize structure are illustrated in real-space and are also identified in the reciprocal-space profile measured in a scattering experiment. Scattering from fractal objects is described in detail. To illustrate the interpretation of scattering data, the growth and structure of fumed silica is discussed. The flame growth of silica is explained in terms of monomer-cluster ballistic polymerization in the early stages and diffusion-limited aerosol aggregation in the latter stages.

Polymers, Fractals, and Ceramic Materials

Concepts borrowed from polymer science have been applied to tailor the properties of inorganic materials, especially those derived from amorphous precursors. Fractal geometry can be used to characterize macromolecular precursors and to relate their structures to kinetic growth processes. Within the silica system, for example, it is possible to manipulate the conditions of solution polymerization to yield a variety of macromolecules from branched chains to smooth colloidal particles.