Numerical investigation of the VB growth of CaF2 crystal with supercooled crucible wall

Decoupling the nonlinear transport and phase change processes during the growth of calcium fluoride crystals is essential for the preparation of high-quality crystals. In this work, a global model was adopted to predict the melt-crystal interface, convection in the melt, and temperature distribution in the furnace. Our simulation results show that the crucible shoulder experiences severe supercooling, which can cause spontaneous nucleation on the supercooled crucible wall and a periphery-to-center flow along the concave interface. This can result in the formation of grains in the peripheral crystal and impurity accumulation in the middle region, both of which significantly degrade the crystal quality. To address this issue, we proposed using a shorter-shoulder crucible to increase heat absorption and an insulation coat to reduce heat dispersion. Both these approaches are effective in reducing supercooling, but the use of an insulation coat around the supercooled shoulder can effectively improve the shape of the m-c interface while still maintaining the beneficial transitional role of the crucible shoulder. This article presents a technical approach for controlling the melt-crystal interface in the vertical Bridgman growth of calcium fluoride crystals, and is a valuable resource for future experimental research.