Nucleation agents in a new type of glass ceramics with low thermal expansion

Reference Presenter Authors
10-035 Christian Thieme Thieme, C.(Fraunhofer Institute of Materials and Systems IMWS); Kracker, M.(Otto-Schott-Institut für Materialforschung); Thieme, K.(Otto-Schott-Institut für Materialforschung); Patzig, C.(Fraunhofer Institute of Materials and Systems IMWS); Höche, T.(Fraunhofer Institute of Materials and Systems IMWS); Rüssel, C.(Otto-Schott-Institut für Materialforschung);

Glass ceramics with low thermal expansion are of high commercial and scientific interest, especially because of their very good thermal shock resistance. Until now, such glass ceramics are commonly synthesized from the base glass system Li2O-Al2O3-SiO2. However, those materials are difficult to prepare due to the very high melting temperatures above 1600 °C. A completely new approach uses a recently developed crystalline solid solution phase with the composition Ba1-xSrxZn2Si2O7 to achieve low thermal expansion.

This phase shows negative and very anisotropic thermal expansion. It can be crystallized from silicate glasses in high concentrations, but it generally tends to surface crystallization. In order to prepare bulk glass ceramics that precipitate the as mentioned phase, the crystallization mechanism has to be changed and a high density number of volume crystals has to be achieved. However, the most common nucleation agents, such as TiO2 and ZrO2 scarcely trigger volume crystallization in this glass system and hence, new nucleation agents were studied and found.

The study presents the role of different nucleation agents for BaO-SrO-ZnO-SiO2 glasses aside the commonly known ones. Microstructure and thermal expansion properties are correlated using techniques such as high resolution scanning transmission electron microscopy in combination with UV-vis-spectroscopy, X-ray diffraction and dilatometry. Furthermore, the appearance and the suppression of micro cracks is discussed also in terms of the microstructure.