Viscous Fingering for growing Crystals with Chemical and Structural Disorder

• Applicant: Dr. Wolfgang Wisniewski
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials ; Glass, Ceramics and Derived Composites ; Synthesis and Properties of Functional Materials
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 562044725

The properties of solid materials are governed by their microstructure, e.g., the occurrence of amor¬phous or crystalline phases and/ or the elemental distribution on the micro- and nanoscale. Single-phase materials can be homogeneous glasses or composed of crystals which grew via one of five crystal growth mechanisms, leading to different microstructures ranging from nm-sized grains to macroscopic single crystals. Among these growth mechanisms, 'Viscous Fingering' (VF) is well-known from liquid-liquid interactions but also recognized as a crystallization mechanism that can cause a unique microstructure with structural and chemical disorder, at least in certain Sr-fresnoite and Sr-alumosilicate glass ceramics. This microstructure has rarely been observed and is characterized by a crystal lattice of orientations persistently changing by several degrees around an overall preferred orientation. The goals of this project are to determine and quantify the, so far unknown, specific conditions required to trigger crystal growth via VF in Sr-fresnoite as well as Sr-alumosilicate glass ceramics and possibly in phosphate glass ceramics. A further aim is to reveal the mechanistic causes for the formation of the unique microstructure and, thus, to lay the groundwork for using the mechanism of VF in the future to generate solid materials with novel properties. In this project, the impact of VF on the (micro)mechanical behavior of the otherwise brittle materials will be especially researched. Because important features of the microstructure are localized and often limited to the nm-scale, the use of high-resolution methods, here transmission electron microscopy, is required. This will be combined with averaging techniques regarding lattice parameters, element coordination and chemical bonds such as XRD and Si-MAS-NMR- as well as Raman spectroscopy. The gained information is used to link the chemical and structural disorder to mechanical properties such as crack propagation clarified in-situ by indentation experiments in the SEM and classical bending tests. The persistent orientation changes in these crystallographically textured materials give raise to the expectation of enhanced fracture toughness while maintaining anisotropic properties which could be aligned with an expected maximum load. The use of materials with the unique VF microstructures thus has great potential for minimizing the amount of material required for specific (load bearing) applications, for reducing the amount of raw materials and thus the amount of energy needed in fabrication and possibly also to control optical properties.

• DFG Programme: Research Grants