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

本文主要研究了近年来由Ba1-xSrxZn2Si2O7组成的固溶体相及其在玻璃中的结晶。这些相在很宽的浓度范围内表现为负热膨胀，因此，含有高浓度该相的微晶玻璃将适合作为低热膨胀甚至零热膨胀的材料。这种玻璃陶瓷的熔化温度低于1450°C，使其成为锂铝硅酸盐系统中传统零热膨胀材料（熔化温度高于1600°C）的真正替代品。而BaO-SrO-ZnO-SiO2体系的玻璃则表现出较高的表面结晶和微裂倾向。后者是由Ba1-xSrxZn2Si2O7相的热膨胀系数的高各向异性引起的，从而可能使各自样品的力学性能恶化。为了克服这些问题，必须触发体积结晶，并且晶体的尺寸必须保持足够小以防止微裂纹。为此，在玻璃中加入不同的成核剂，以促进体积结晶。显微结构表征的标准技术，如电子显微镜、x射线衍射和紫外可见光谱，应用于研究相的形成和晶体的生长。这包括一般的生长机制、核外延生长和成核速率的信息。此外，应应用尖端的x射线显微镜来了解后一种方法相对于所考虑的样品系统的局限性。特别是利用该技术所提供的三维信息，对试样内部的微裂纹进行检测，将有助于发现裂纹的起源，从而了解裂纹的形成机制。此外，x射线显微镜将首次用于在几十纳米尺度的三维尺度上确定成核速率，与传统技术相比，这将导致更精确的数值。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。本项目旨在对新型玻璃体系BaO-SrO-ZnO-SiO2的晶体生长机制有一个大致的了解，并找到晶体的尺寸和取向与微裂纹出现之间的关系。此外，还将以上述新型玻璃体系为模型，研究玻璃结晶过程中裂纹的形成和扩展机理。这将极大地促进含有Ba1-xSrxZn2Si2O7的低热膨胀玻璃陶瓷的发展，以及含有其他具有高各向异性热膨胀系数的玻璃陶瓷的发展。

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.