Packing-dependent viscous sintering of glass powder from wet deposition

在金属和陶瓷基材上喷涂、涂刷和浸渍玻璃粉末浆料是牙釉质和牙科陶瓷工业中最先进的涂层技术。影响坯体和烧成玻璃涂层结构的工艺参数大多是经验得出的。对控制粘性烧结的潜在机制的知识驱动的理解到目前为止仍然缺失。特别是由于相对较低的工艺温度、亚微米颗粒组分的粘附性和非球形的锯齿状颗粒，不规则和不均匀的填料可能在很大程度上控制烧结。因此，该项目旨在更深入地了解控制烧结动力学的参数。一方面，利用x射线断层扫描（XCT）、x射线显微镜（XRM）和透射电镜断层扫描（TEM断层扫描）分别探索绿体的三维微观结构和短时间烧制后的变化，以显示微米和亚微米范围内的局部颗粒接触（同时采用hg孔隙度法和BET分析，以覆盖整体开放孔隙度的发展）。为了可视化玻璃粉系统的包装结构，XCT和XRM似乎是首选的方法，因为这些非破坏性技术可以在所需的长度尺度上提供局部和全局信息。另一方面，玻璃颗粒烧结过程中的致密化是用离散的收缩描述进行数值模拟的，以解释颗粒的局部致密化结构。利用三维数据的几何参数进行直接实验验证，将使用三项模型（用于颗粒重排，颈部形成和孔隙球化）。干燥玻璃粉末沉积的微观结构将随着亚微米颗粒分数、颗粒尺寸分布的宽度和玻璃颗粒的热历史（高与低有效温度）而系统地变化。烧结玻璃微珠将作为本项目的参考，研究颗粒形状对填充和重排的影响。

Spraying, brushing and dipping of glass powder slurries on/of metal and ceramic substrates are state-of-the-art coating technologies in the enamel and dental-ceramic industries. Process parameters influencing the structure of the green body and the fired glassy coat are mostly empirically developed. A knowledge-driven understanding of the underlying mechanisms that control viscous sintering is up to now still missing. In particular the effect of non-regular and inhomogeneous packing may largely control sintering due to the relative low process temperatures, adhesion of submicron-particle fractions and jagged particles of non-spherical shape. The project aims therefore to gain a deeper understanding of the parameters that control the sinter kinetics. On one hand, this is tackled by exploring the 3D microstructure of the green body and the changes after short periods of firing using X-ray tomography (XCT), X-ray microscopy (XRM) and TEM tomography to visualize local particle contacts in the micron and sub-micron range, respectively (flanked by Hg-porosimetry and BET analysis to cover the development of the overall open porosity). For visualizing the packing structure of the glass powder systems, XCT and XRM seems the method of choice, as these non-destructive techniques give both local and global information on the desired length scales. On the other hand, the densification during sintering of the glass particles is modelled numerically using discrete descriptions of the shrinkage, to account for the local densifying structures of the particulate. A three term model (for particle rearrangement, neck formation and pore spheroidization) will be used with the benefit of direct experimental validation by utilizing geometric parameters of the 3D data. The microstructure of dried glass powder deposition will be varied systematically with respect to the submicron-particle fraction, broadness of the particle size distribution, and thermal history of the glass particles (high vs. low fictive temperature). Sintered glass beads will work as reference in this project with respect to the effect of particle shape on packing and rearrangement.