Heterogeneous nucleation and crystal growth of Ba1-xSrxZn2Si2O7 solid solutions with negative thermal expansion studied by X-ray microscopy

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This proposal focuses on recently developed solid solution phases with the composition Ba1-xSrxZn2Si2O7 and their crystallization from glasses. These phases show negative thermal expansion in wide concentration ranges and hence, glass-ceramics containing high concentrations of this phase will be suitable as materials with low or even zero thermal expansion. Such glass ceramics exhibit melting temperatures below 1450 °C, making them a real alternative to conventional zero thermal expansion materials from the lithium-aluminum-silicate system (melting at temperatures above 1600 °C). However, glasses from the system BaO-SrO-ZnO-SiO2 show a high tendency to surface crystallization and micro cracking. The latter is caused by the high anisotropy of the coefficient of thermal expansion of the Ba1-xSrxZn2Si2O7 phase, and thus potentially deteriorates the mechanical properties of respective samples. To overcome these issues, volume crystallization has to be triggered and the size of the crystals has to be kept small enough to prevent micro-cracking. For this purpose, different nucleation agents will be added to the glass in order to promote volume crystallization. Standard techniques of microstructural characterization, such as electron microscopy, X-ray diffraction, and UV-Vis spectroscopy shall be applied to study phase formation and crystal growth. This includes information on the growth mechanisms in general, epitaxial growth on the nuclei, and nucleation rates. In addition, cutting-edge X-ray microscopy shall be applied to fathom the limits of the latter method with respect to the sample system under consideration. Especially the detection of micro cracks inside the sample volume facilitated by the three-dimensional nature of information provided by this technique will be very helpful in finding crack origins and thus understanding the mechanisms of crack formation. Furthermore, X-ray microscopy will be used for the first time to determine nucleation rates in three dimensions at a tens-of-nanoscale level, which will lead to much more precise values, compared to those gained by conventional techniques. These nucleation rates can then be used to control the number and also the size of the crystals inside the sample volume. It is the aim of this project to gain a general understanding of the crystal growth mechanisms in the new glass system BaO-SrO-ZnO-SiO2, and to find a correlation between size and orientation of the crystals and the appearance of micro cracks. Furthermore, the mechanisms of crack formation and growth during crystallization of glasses will be studied using the mentioned new glass system as a model. This will significantly contribute to both, the development of Ba1-xSrxZn2Si2O7 containing glass ceramics with low thermal expansion and the development of other glass ceramics containing phases with a high anisotropy of the coefficients of thermal expansion.