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

这项建议侧重于最近开发的组成为Ba1-xSrxZn2Si2O7的固溶体相及其从玻璃中析晶。这些相在很宽的浓度范围内表现出负的热膨胀，因此，含有高浓度这一相的微晶玻璃将适合作为低热膨胀甚至零热膨胀的材料。这种微晶玻璃的熔化温度低于1450°C，使其成为锂-铝-硅酸盐系统传统零热膨胀材料的真正替代品(熔化温度高于1600°C)。而BaO-SRO-ZnO-SiO_2系玻璃具有较高的表面析晶倾向和微裂倾向。后者是由于Ba1-xSrxZn2Si2O7相热膨胀系数的高各向异性造成的，从而潜在地恶化了各样品的力学性能。为了克服这些问题，必须触发体积结晶，并且必须保持晶体的尺寸足够小，以防止微裂纹。为此，将向玻璃中添加不同的成核剂，以促进体积析晶。应采用电子显微镜、X射线衍射和UV-Vis光谱等微结构表征的标准技术来研究相形成和晶体生长。这包括一般的生长机制、在核上的外延生长和成核率的信息。此外，应使用尖端X射线显微镜，以确定后一种方法相对于所考虑的样品系统的限度。特别是利用该技术提供的三维信息，检测样品体积内的微裂纹，将非常有助于寻找裂纹的来源，从而了解裂纹的形成机理。此外，X射线显微镜将首次用于在几十纳米级的三维水平上确定成核率，这将导致比传统技术获得的值更精确。这些成核速率可以用来控制样品体积内晶体的数量和大小。本项目的目的是对BaO-SRO-ZnO-SiO_2系新玻璃的晶体生长机制有一个总体的了解，并找到晶体的大小和取向与微裂纹的出现之间的关系。此外，还将以该新型玻璃系统为模型，研究玻璃析晶过程中裂纹的形成和扩展机制。这对开发低热膨胀Ba1-xSrxZn2Si2O7微晶玻璃和开发其他相组成的高热膨胀各向异性微晶玻璃都具有重要意义。

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.