Mechanism Analysis during THIck samples solidification of FAceted systems – MATHIFA

|The MATHIFA project employed several complementary methods to study the growth of faceted materials and overcome experimental challenges. The main approach consisted of directional solidification of transparent organic materials, allowing direct observation of the solid-liquid interface in real time. Thin rectangular samples were solidified in a controlled thermal gradient to monitor the formation and evolution of facets, grain boundaries, and structural defects. High-resolution optical imaging provided detailed visualization of interface dynamics, including defects like bubble formation, growth striations, facet splitting, distortions, and twin boundaries.

Advanced visualization software VESTA enabled reconstruction of three-dimensional crystal shapes and determination of crystallographic planes, helping to characterize the orientation and growth directions of facets. Thermal microscopy was used to measure phase transitions, providing quantitative data on facet formation and verifying the accuracy of phase diagrams.

To complement experimental observations, phase-field simulations were conducted to model the dynamics of faceted interfaces. These simulations helped interpret complex behaviors observed experimentally and provide predictive insights for other systems.

Additional procedures were developed to address technical challenges, such as purification of materials, preparation of initial seed crystals, optimization of sample handling, and design of thermal zones to ensure reproducible and reliable observations. Together, these methods and technologies allowed a comprehensive study of faceted growth, combining in situ experiments, and quantitative characterization, to improve our knowledge about faceted growth.

|The MATHIFA project improved understanding of faceted growth, a key phenomenon in materials science, observed in semiconductors, quasicrystals, and organic compounds. Directional solidification experiments on transparent organic materials, mainly salol and the succinonitrile-borneol alloy, enabled real-time observation of the solid-liquid interface and identification of the formation and evolution of facets, grain boundaries, and structural defects.

The observations revealed complex growth dynamics, including bubble formation, growth striations, facet splitting, distortions, and twinning. Analysis of facet edges allowed determination of dominant crystallographic planes and characterization of crystal orientations. Facet growth velocity measurements highlighted the influence of undercooling and local interactions, suggesting the coexistence of growth mechanisms via two-dimensional nucleation and screw dislocations.

Studies on succinonitrile-borneol, complemented by thermal microscopy, mapped interfacial microstructure and verified phase diagrams. Together, these results provide quantitative reference data for modeling and simulations, contributing to a predictive understanding of faceted growth. The work highlights the critical role of solidification parameters on facet morphology and defect formation, which directly influence the final material properties.

|The MATHIFA project opens original perspectives both in fundamental research and practical applications. Real-time observation of faceted growth provides a solid foundation for developing predictive solidification models applicable to industrial materials such as silicon, ceramics, and innovative functional materials. The methods developed can also be adapted to other crystalline systems to optimize microstructure and reduce defects, thereby improving mechanical, electrical, and optical properties.

The results improve understanding of facet competition, twin formation, and defect dynamics, providing crucial information for designing new materials and more efficient industrial processes. Future work will include studies of bulk samples, advanced post-mortem characterization (diffraction, topography), continuation of phase-field simulations for quantitative predictions, and investigation of equilibrium crystal shapes.

These investigations offer the potential to develop materials with controlled structures, design more reliable solidification processes, and reduce costs and experimental trials in industry. They also lay the groundwork for a better understanding of faceted growth mechanisms, which remain largely unexplored.

Many materials produced by melt crystallization present solid-liquid interfaces with a faceted morphology and associated kinetic behavior. Despite the recent progress as concern solidification in thin samples of materials presenting a faceted morphology during growth, the solidification dynamics in 3D still remains not enough understood in depth although 3D is the real-life industrial configuration. This constitutes a major bottleneck for the solidification mechanism modelling and for the optimization of predictive simulation tools capable of controlling elaboration processes and the final properties of strategic materials, such as photovoltaic silicon. The main objective of the MATHIFA project is to make major breakthrough on the analysis of solidification mechanisms in the case of faceted systems in 3D samples. To fulfil this objective, an original experimental approach with in situ characterization of directional solidification of transparent model alloys, which freeze with faceted interface, in cylindrical thick samples will be implemented. Visualization and characterization of the faceted growth front will be done in real time. The studies proposed on MATHIFA project are essential to understand the growth dynamics of faceted microstructures, the mechanisms of crystal-grain competition, and the formation of defects in the solid.