Cordierite glass ceramics studied by X-ray scattering and electron microscopy

W. Bras
Dutch-Belgian beam lines, Netherlands Organization for Scientific Reserach (NWO), DUBBLE CRG @ ESRF (European Synchrotron Radiation) Facility, 6 rue Jules Horowitz, BP220, F38043 GRENOBLECEDEX, FRANCE

Glass ceramics are materials composed of a glassy matrix in which nano to micro sized crystallites are embedded. They can exhibit rather special materials properties like for instance low thermal expansion, mechanical shock resistance even at elevated temperatures and act as a low band pass optical filter. The material properties are due to a fine interplay between the differences in the properties of the different phases making up this composite material.
There are several ways in which these materials can be produced but the actual process of formation is still not completely understood even though these materials are widely applied in commercial applications.
We have studied the formation of crystallites in a model glass by X-ray scattering and electron microscopy methods. The glass has the cordierite composition (Mg2Al4Si5O18) doped with a small amount (0.34 mol%) of Cr2O3 which acts as a crystallization enhancer and is subjected to a two step heat treatment. The first step allows the formation of crystallization nucleation sites. The subsequent step, at a higher temperature, allows the nuclei to start growing. By utilizing time-resolved simultaneous SAXS and WAXS we have been able to elucidate the full growth process and make predictions about the final structure that were confirmed by electron microscopy. By utilizing the above mentioned techniques we have been able to distinguish between bulk and surface crystallization, determine the crystalline volume fraction, determine the crystallization energy, the shape and size of the crystalline particles and the internal pressure exerted by the glass matrix onto the crystallites.
We have been able to show that the samples grown in this way are still mainly glass but that the surface consists of a high quartz phase. In the bulk cubic Cr-spinel crystallites with a size polydispersity of less then 4% are found.