WO3 as a nucleating agent for BaO/SrO/ZnO/SiO2 glasses – experiments and simulations

Abstract

Recently, it has been shown that the Ba_1−xSr_xZn₂Si₂O₇ crystal phase has a negative coefficient of thermal expansion. However, technological applications of this material as low thermal expansion glass ceramics are limited by the undesired surface crystallization of the corresponding glass. Surface nucleation, however, can be turned into bulk nucleation by adding nucleating agents. Here, glasses in the base system BaO–SrO–ZnO–SiO₂ with small additions of ZrO₂ and WO₃ were synthesized and their crystallization behavior was investigated using thermal analysis, X-ray diffraction, electron microscopy, and density functional theory simulations. The addition of WO₃ leads to the formation of volume crystals with a scheelite crystal structure (Ba_1−xSr_xWO₄) in high number density. The limited incorporation of Si⁴⁺ ions into these crystals is discussed. Possible crystal lattice sites for Si⁴⁺ were located by density functional theory simulations. In a much lower number density, crystals with a crystal structure similar to the high-temperature polymorph of BaZn₂Si₂O₇ and the Ba_1−xSr_xZn₂Si₂O₇ composition crystallize as well. These crystals can reach a larger size than the scheelite crystals that occur in parallel. The overall microstructure is thus formed by small dendritic crystals with a scheelite structure and huge Ba_1−xSr_xZn₂Si₂O₇ crystals. Both of them seem to grow independently of each other. In spite of the high anisotropy of both phases, the microstructure is revealed to be free of cracks.