SMALL-ANGLE X-RAY-SCATTERING STUDY OF CORDIERITE SOL-GEL SYNTHESIS

The structure of ceramic materials in the size range of a few nanometres to a few tens of nanometres is closely related to the structural features of the precursor. Here the nanostructural development of a cordierite (2MgO-2Al2O3-5SiO2) material is studied by SAXS during its processing by a sol-gel route. Homogeneous alcogels were prepared at 298 K by hydrolysis - condensation of a mixture made of Si(OC2H5)4, sec-Al(OC4H9)3, Mg(NO3)2, water and alcohol. The polymeric nature of the mechanism is evidenced. The final size and internal structure of the particles depend on the quantity of water used for hydrolysis. In the gel state, particles of 7 or 8 nm in size exhibiting a mass fractal structure with D = 1.9 are observed. Xerogels resulting from classical drying were powdered. Firing of pellets leads to porous solids. The small-angle scattered intensity grows strongly and the Porod plot shows a linear behaviour with a slope of -3.5. Sintering to full density is very difficult. Aerogels obtained by supercritical drying consist of primary homogeneous units of about 2 nm in size, attached into volume fractal aggregates (D = 1.8). The aerogels exhibit a dramatically improved sintering ability. Full dense glasses are obtained at temperatures less than 1273 K and fine-grained glass ceramics without any nucleating agent.